The Japanese Mutant Aβ (ΔE22-Aβ₁₋₃₉) Forms Fibrils Instantaneously, with Low-Thioflavin T Fluorescence: Seeding of Wild-Type Aβ₁₋₄₀ into Atypical Fibrils by ΔE22-Aβ₁₋₃₉

Abstract

The ΔE693 (Japanese) mutation of the β-amyloid precursor protein leads to production of ΔE22-Aβ peptides such as ΔE22-Aβ₁₋₃₉. Despite reports that these peptides do not form fibrils, here we show that, on the contrary, the peptide forms fibrils essentially instantaneously. The fibrils are typical amyloid fibrils in all respects except that they cause only low levels of thioflavin T (ThT) fluorescence, which, however, develops with no lag phase. The fibrils bind ThT, but with a lower affinity and a smaller number of binding sites than wild-type (WT) Aβ₁₋₄₀. Fluorescence depolarization confirms extremely rapid aggregation of ΔE22-Aβ₁₋₃₉. Size exclusion chromatography (SEC) indicates very low concentrations of soluble monomer and oligomer, but only in the presence of some organic solvent, e.g., 2% (v/v) DMSO. The critical concentration is approximately 1 order of magnitude lower for ΔE22-Aβ₁₋₃₉ than for WT Aβ₁₋₄₀. Several lines of evidence point to an altered structure for ΔE22-Aβ₁₋₃₉ compared to that of WT Aβ₁₋₄₀ fibrils. In addition to differences in ThT binding and fluorescence, PITHIRDS-CT solid-state nuclear magnetic resonance (NMR) measurements of ΔE22-Aβ₁₋₃₉ are not compatible with the parallel in-register β-sheet generally observed for WT Aβ₁₋₄₀ fibrils. X-ray fibril diffraction showed different <i>D</i> spacings: 4.7 and 10.4 Å for WT Aβ₁₋₄₀ and 4.7 and 9.6 Å for Δ<i>E</i>22-Aβ₁₋₃₉. Equimolar mixtures of ΔE22-Aβ₁₋₃₉ and WT Aβ₁₋₄₀ also produced fibrils extremely rapidly, and by the criteria of ThT fluorescence and electron microscopic appearance, they were the same as fibrils made from pure ΔE22-Aβ₁₋₃₉. X-ray diffraction of fibrils formed from 1:1 molar mixtures of ΔE22-Aβ₁₋₃₉ and WT Aβ₁₋₄₀ showed the same <i>D</i> spacings as fibrils of the pure mutant peptide, not the wild-type peptide. These findings are consistent with extremely rapid nucleation by ΔE22-Aβ₁₋₃₉, followed by fibril extension by WT Aβ₁₋₄₀, and “conversion” of the wild-type peptide to a structure similar to that of the mutant peptide, in a manner reminiscent of the prion conversion phenomenon.