Inhibition of β-Amyloid(40) Fibrillogenesis and Disassembly of β-Amyloid(40) Fibrils by Short β-Amyloid Congeners Containing N-Methyl Amino Acids at Alternate Residues

A potential goal in the prevention or therapy of Alzheimer's disease is to decrease or eliminate neuritic plaques composed of fibrillar β-amyloid (Aβ). In this paper we describe N-methyl amino acid containing congeners of the hydrophobic “core domain” of Aβ that inhibit the fibrillogenesis of full-length Aβ. These peptides also disassemble preformed fibrils of full-length Aβ. A key feature of the inhibitor peptides is that they contain N-methyl amino acids in alternating positions of the sequence. The most potent of these inhibitors, termed Aβ16−22m, has the sequence NH2-K(Me-L)V(Me-F)F(Me-A)E-CONH2. In contrast, a peptide, NH2-KL(Me-V)(Me-F)(Me-F)(Me-A)-E-CONH2, with N-methyl amino acids in consecutive order, is not a fibrillogenesis inhibitor. Another peptide containing alternating N-methyl amino acids but based on the sequence of a different fibril-forming protein, the human prion protein, is also not an inhibitor of Aβ40 fibrillogenesis. The nonmethylated version of the inhibitor peptide, NH2-KLVFFAE-CONH2 (Aβ16−22), is a weak fibrillogenesis inhibitor. Perhaps contrary to expectations, the Aβ16−22m peptide is highly soluble in aqueous media, and concentrations in excess of 40 mg/mL can be obtained in buffers of physiological pH and ionic strength, compared to only 2 mg/mL for Aβ16−22. Analytical ultracentrifugation demonstrates that Aβ16−22m is monomeric in buffer solution. Whereas Aβ16−22 is susceptible to cleavage by chymotrypsin, the methylated inhibitor peptide Aβ16−22m is completely resistant to this protease. Circular dichroic spectroscopy of Aβ16−22m indicates that this peptide is a β-strand, albeit with an unusual minimum at 226 nm. In summary, the inhibitor motif is that of alternating N-methyl and nonmethylated amino acids in a sequence critical for Aβ40 fibrillogenesis. These inhibitors appear to act by binding to growth sites of Aβ nuclei and/or fibrils and preventing the propagation of the network of hydrogen bonds that is essential for the formation of an extended β-sheet fibril.