Alzheimer's Aβ42 and Aβ40 peptides form interlaced amyloid fibrils

Abstract

Deposition of amyloid β (Aβ) in the brain is a pathological hallmark of Alzheimer's disease. There are two major isoforms of Aβ: the 42-residue Aβ42 and the 40-residue Aβ40. The only difference between Aβ42 and Aβ40 is that Aβ42 has two extra residues at the C-terminus. The amyloid plaques in Alzheimer's brains consist of mostly Aβ42 and some plaques contain only Aβ42, even though Aβ40 concentration is several-fold more than Aβ42. Using electron paramagnetic resonance, we studied the formation of amyloid fibrils using a mixture of Aβ42 and Aβ40 in vitro. We show that Aβ42 and Aβ40 form mixed fibrils in an interlaced manner, although Aβ40 is not as efficient as Aβ42 in terms of being incorporated into Aβ42 fibrils. Our results suggest that both Aβ42 and Aβ40 would be present in amyloid plaques if in vivo aggregation of Aβ were similar to the in vitro process. Therefore, there must be some mechanisms that lead to the preferential deposition of Aβ42 at the extracellular space. Identifying such mechanisms may open new avenues for therapeutic interventions to treat Alzheimer's disease.

Keywords: electron paramagnetic resonance; protein aggre-gation; senile plaques; spin labeling.

Figure 1. Effect of spin dilution on the EPR lineshape of spin-labeled fibrils

Red balls represent spin labels. (A) In a parallel in-register β-sheet structure of amyloid fibrils, the spin label side chains pack closely against each other and lead to strong spin exchange interactions between spin labels. As a result, the EPR spectrum shows a characteristic single-line feature. (B) When the fibrils are formed by spin-labeled Aβ and unlabeled Aβ (i.e. spin dilution), the inter-spin label spacing is more than ∼7 Å, and thus spin exchange interactions are weak, leading to the normal three-line spectrum.

Figure 2. Characterization of spin labeled Aβ42 fibrils

(A) Chemical structure of spin label R1 used in this work. (B) Transmission electron microscopy images show similar morphologies for Aβ42 L17R1 and its mixture with either Aβ40 or Aβ42 wild type peptides.

Figure 3. EPR analysis of spin-labeled Aβ42 fibrils

(A) EPR spectra of fully labeled Aβ42 fibrils and spin-diluted with either Aβ42 or Aβ40 wild-type proteins. The experimental spectra are shown in black and the best fits from spectral simulations are shown in red. Individual spectral components are shown in magenta and blue. (B) A bar graph showing the population of the single-line and three-line components from spectral simulations in the absence and presence of spin dilutions. For the 1:1 spin dilutions, results are expressed as mean ± SD (**p < 0.01, Student's t-test).

Figure 4. A schematic drawing shows distributions of different fibril populations resulting from spin dilution of spin-labeled Aβ42 with unlabeled Aβ40

With 1:3 dilution, two fibril populations are present: interlaced fibrils and Aβ40 fibrils. With 1:1 dilution, all three fibril populations (labeled Aβ42, interlaced, and unlabeled Aβ40) are present.