Using Small Peptide Segments of Amyloid-β and Humanin to Examine their Physical Interactions

Background: Amyloid fibrils in Alzheimer's disease are composed of amyloid-beta (A beta) peptides of variant lengths. Humanin (HN), a 24 amino acid residue neuroprotective peptide, is known to interact with the predominant A beta isoform in the brain, A beta (1-40). Methods: Here, we constructed smaller segments of A beta and HN and identified residues in HN important for both HN-HN and HN-A beta interactions. Peptides corresponding to amino acid residues 5-15 of HN, HN (5-15), HN (5-15, L11S), where Leu11 was replaced with Ser, and residues 17-28 of A beta, A beta (17-28), were synthesized and tested for their ability to block formation of the complex between HN and A beta (1-40). Results: Co-immunoprecipitation and binding kinetics showed that HN (5-15) was more efficient at blocking the complex between HN and A beta (1-40) than either HN (5-15, L11S) or A beta (17-28). Binding kinetics of these smaller peptides with either full-length HN or A beta (1-40) showed that HN (5-15) was able to bind either A beta (1-40) or HN more efficiently than HN (5-15, L11S) or A beta (17-28). Compared to full-length HN, however, HN (5-15) bound A beta (1-40) with a weaker affinity suggesting that while HN (5-15) binds A beta, other residues in the full length HN peptide are necessary for maximum interactions. Conclusion: L11 was more important for interactions with A beta (1-40) than with HN. A beta (17-28) was relatively ineffective at binding to either A beta (1-40) or HN. Moreover, HN, and the smaller HN (5-15), HN (5-15 L11S), and A beta (17-28) peptides, had different effects on regulating A beta (1-40) aggregation kinetics.