Characterisation of Amyloid- B and its Interactions by NMR Exchange Techniques

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Alzheimer's disease is a fatal neurological condition and the leading cause of dementia worldwide. It is characterised by deposits of aggregated peptide, amyloid-B (AB), which form plaques in the brain. AB transitions into aggregated species through three states: monomeric (where the peptide is intrinsically disordered), oligomeric (small aggregates, identified as the primary candidate for toxicity) and highly ordered fibular structures. Most current drug discovery strategies aim to target the aggregated species including oligomers and fibrils. Amyloid precursor protein is proteolytically cleaved into Ab peptides. The 40-residue variant, Ab40, is the predominant form found in biological fluids. In contrast, the 42-residue variant, Ab42, is more commonly found in Alzheimer's disease plaques and has a higher propensity for aggregation. The most striking difference in aggregation rates between the two peptides is found in the first step of aggregation called primary nucleation, suggesting that there are fundamental differences in monomeric AB40 and AB42. It is well known that these peptides are intrinsically disordered in their monomeric forms, but structural studies of the two peptides have found very little chemical or structural differences in their monomeric states. Many suggest that the difference in aggregation propensity is driven by the additional C-terminal hydrophobic amino acids in AB42 or that the aggregation rates differ due to varying structures of oligomeric and fibrillar species. In my 3-month rotation I used CLEAN-EX NMR methods to understand the difference in solvent accessibility between AB40 and AB42. My preliminary data suggests that there is a difference in the solvation rates between the two peptides, not only at the expected C-terminal end of the peptide, but also at an upstream residue (K16) which is surprisingly more protected in AB42. This observation suggests that the additional amino acids in AB42 may be causing some long-range effects that affect the overall structure of the peptide.During my PhD, I will first repeat these CLEAN-EX experiments to confirm my findings and then extend this approach to delve deeper into the monomeric properties of AB. It has been shown that post translationally modifying AB40 on residue K16 (and other residues) results in a different aggregation profile for the peptide and it would be interesting to see if this difference can be detected at the monomer level by CLEAN-EX experiments. Another avenue I plan to explore involves the binding of small, drug-like molecules to AB. I will first conduct aggregation assays identify molecules that alter the aggregation profile of AB and then follow that up with NMR experiments to identify if there are differences in solvation between the bound and unbound states of the monomer. Overall, there are a lot of unanswered questions about the structure of monomeric AB and solvent-based NMR methods would be an interesting and hopefully lucrative method of investigating these questions.