Toxicity of non-aβ component of Alzheimer's disease amyloid, and N- terminal fragments thereof, correlates to formation of β-sheet structure and fibrils

Abstract

The non-Aβ component of Alzheimer's disease amyloid (NAC) and its precursor α-synuclein have been linked to amyloidogenesis in Alzheimer's disease (AD), Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Previously we have shown that NAC forms β-sheet structures and fibrils [E1- Agnaf, O.M.A., Bodles, A.M., Guthrie, D.J.S., Harriott, P. and Irvine, G.B. (1998) Eur. J. Biochem. 258, 157-163]. As a measure of their neurotoxic potential we have examined the ability of fresh and aged NAC and fragments thereof to inhibit the reduction of the redox dye 3-(4,5-dimethylthiazol-2- yl)-2,5 diphenyltetrazolium bromide by rat pheochromocytoma PC12 cells. Micromolar concentrations of NAC and fragments thereof display varying degrees of toxicity. On immediate dissolution and after an incubation period for 3 days at 37 °C the full-length peptide and fragments NAC(3-18) and NAC(1-18) scrambled sequence [NAC(1-18 s)] were toxic, whereas fragments NAC(1-13) and NAC(6-14) were not. CD indicates that NAC(3-18) and NAC(1-18 s) exhibit β-sheet secondary structure in aqueous solution, whereas NAC(1-13) and NAC(6-14) do not. NAC(3-18) aggregates, as indicated by concentration of peptide remaining in solution after 3 days measured by an HPLC assay, and forms fibrils, as determined by electron microscopy. However, although some fibrils were detected for NAC(1-18 s) it does not come out of solution to a significant degree. Fragments NAC(1-13) and NAC(6-14) form few fibrils and remain in solution. These findings indicate that the ability of the central region of NAC to form β-sheet secondary structures is important for determining the toxicity of the peptide. This contrasts with what has been reported previously for most Aβ peptides as their toxicity appears to require the peptide to have formed fibrillary aggregates as well as displaying β-sheet. These results suggest that an intermediate, which exhibits β-sheet structure, may be responsible for the toxic properties of NAC and provides further evidence for the role of NAC in the pathogenesis of AD, PD and DLB.