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

The non-A beta component of Alzheimer's disease amyloid (NAC) and its precu rsor alpha-synuclein have been linked to amyloidogenesis in Alzheimer's dis ease (AD), Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Pr eviously we have shown that NAC forms beta-sheet structures and fibrils [El -Agnaf, O.M.A., Bodles, A.M., Guthrie, D.J.S., Harriott, P. & Irvine, G.B. (1998) Eur. J. Biochem. 258, 157-163]. As a measure of their neurotoxic pot ential we have examined the ability of fresh and aged NAC and fragments the reof to inhibit the reduction of the redox dye 3-(4,5-dimethylthiazol-2-yl) -2,5 diphenyltetrazolium bromide by rat pheochromocytoma PC12 cells. Microm olar concentrations of NAC and fragments thereof display varying degrees of toxicity. On immediate dissolution and after an incubation period for 3 da ys at 37 degrees 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) exh ibit beta-sheet secondary structure in aqueous solution, whereas NAC(1-13) and NAC(6-14) do not. NAC(3-18) aggregates, as indicated by concentration o f peptide remaining in solution after 3 days measured by an HPLC assay, and forms fibrils, as determined by electron microscopy. However, although som e 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 a nd remain in solution. These findings indicate that the ability of the cent ral region of NAC to form beta-sheet secondary structures is important for determining the toxicity of the peptide. This contrasts with what has been reported previously for most A beta peptides as their toxicity appears to r equire the peptide to have formed fibrillary aggregates as well as displayi ng beta-sheet. These results suggest that an intermediate, which exhibits b eta-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, P D and DLB.