Humanin Blocks the Aggregation of Amyloid-ß Induced by Acetylcholinesterase, an Effect Abolished in the Presence of IGFBP-3.

It is known that the humanin (HN) peptide binding to amyloid-ß (Aß) protects against its cytotoxic effects, while acetylcholinesterase (AChE) binding to Aß increases its aggregation and cytotoxicity. HN is also known to bind the insulin-like growth factor binding protein-3 (IGFBP-3). Here, we examined the regulation of Aß conformations by HN, AChE, and IGFBP-3 both in vitro and in the conditioned media from A549 and H1299 lung cancer cells. Our in vitro results showed the following: IGFBP-3 binds HN and blocks it from binding Aß in the absence or presence of AChE; HN and AChE can simultaneously bind Aß but not when in the presence of IGFBP-3; HN is unable to reduce the aggregation of Aß in the presence of IGFBP-3; and HN abolishes the aggregation of Aß induced by the addition of AChE in the absence of IGFBP-3. In the media, AChE and HN can simultaneously bind Aß. While both AChE and HN are detected when using 6E10 Aß antibodies, only AChE is detected when using the Aß 17-24 antibody 4G8, the anti-oligomer A11, and the anti-amyloid fibril LOC antibodies. No signal was observed for IGFBP-3 with any of the anti-amyloid antibodies used. Exogenously added IGFBP-3 reduced the amount of HN found in a complex when using 6E10 antibodies and correlated with a concomitant increase in the amyloid oligomers. Immunodepletion of HN from the media of the A549 and H1299 cells increased the relative abundance of the oligomer vs the total amount of Aß, the A11-positive prefibrillar oligomers, and to a lesser extent the LOC-positive fibrillar oligomers, and was also correlated with diminished cell viability and increased apoptosis.

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

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