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

The Delta E693 (Japanese) mutation of the beta-amyloid precursor protein leads to production of Delta E22-A beta peptides such as Delta E22-A beta(1-39). 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) A beta(1-40). Fluorescence depolarization confirms extremely rapid aggregation of Delta E22-A beta(1-39). 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 Delta E22-A beta(1-39) than for WT A beta(1-40). Several lines of evidence point to an altered structure for Delta E22-A beta(1-39) compared to that of WT A beta(1-40) fibrils. In addition to differences in ThT binding and fluorescence, PITHIRDS-CT solid-state nuclear magnetic resonance (NMR) measurements of Delta E22-A beta(1-39) are not compatible with the parallel in-register beta-sheet generally observed for WT A beta(1-40) fibrils. X-ray fibril diffraction showed different D spacings: 4.7 and 10.4 A for WT A beta(1-40) and 4.7 and 9.6 angstrom for Delta E22-A beta(1-39). Equimolar mixtures of Delta E22-A beta(1-39) and WT A beta(1-40) 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 Delta E22-A beta(1-39). X-ray diffraction of fibrils formed from 1:1 molar mixtures of Delta E22-A beta(1-39) and WT A beta(1-40) showed the same D spacings as fibrils of the pure mutant peptide, not the wild-type peptide. These findings are consistent with extremely rapid nucleation by Delta E22-A beta(1-39), followed by fibril extension by WT A beta(1-40), 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.