| Alzheimer's disease (AD) is the most common age-associated neurodegenerative disease. Although the precise mechanism leading to AD pathogenesis still remains unknown, self-association of beta-amyloid into soluble oligomers and insoluble fibrils is believed to play a causal role, motivating the search for compounds that bind and inhibit beta-amyloid aggregation. Peptides are an attractive class of such compounds, with potential advantages over small molecules in affinity and specificity. Generally, self-complementation and peptide library screening have been employed in the search for peptides that bind beta-amyloid. In this dissertation, we introduce an alternative approach of designing such peptides by mimicking the binding epitope of a complementary binding protein. Transthyretin (TTR), a homotetrameric protein present in blood and cerebrospinal fluid, is one of a few naturally existing binding partners of beta-amyloid. Interestingly, TTR not only binds beta-amyloid, but also is protective against beta-amyloid-induced toxicity in transgenic AD mice models. We speculated that peptides that mimic the beta-amyloid-binding domains on TTR may reproduce the protective activity of TTR. We first identified and analyzed the beta-amyloid binding domains on TTR. Leu82 on EF loop and Leu110 on strand G were identified as critical residues for binding by alanine scanning mutagenesis. Region involving strand G was observed to bind more strongly to beta-amyloid and was further analyzed in detail.;Peptides with the same linear sequence as binding domain involving strand G were then generated and tested. Among couple of peptides tested, the 16-mer peptide G16 bound to beta-amyloid, reduced beta-amyloid fibrillar aggregate formation and reduced beta-amyloid-induced toxicity. To further improve the activity, we next generated peptide that not only mimics the sequence but also the structure of the beta-amyloid binding domain. To enforce proper residue alignment, we generated a cyclic structure where N-terminus of strand G and C-terminus of strand H is connected using a beta-turn inducing template. CG3, a 22-mer cyclic peptide containing D-Pro-L-Pro template, strongly suppressed beta-amyloid fibrilllar aggregate formation, and protected neurons against beta-amyloid induced toxicity. CG3 retained more selectivity to beta-amyloid oligomers, like TTR, and was much more effective than G16. |
