Atomic resolution map of the soluble amyloid beta assembly (Aβ_n) “toxic surfaces” that facilitate the early pathogenic events in Alzheimer's disease (AD).

Self-association of amyloid β (Aβ) peptides is a hallmark of Alzheimer's disease and serves as a general prototype for amyloid formation. A key endogenous inhibitor of Aβ self-association is human serum albumin (HSA), which binds ∼90% of plasma Aβ. However, the exact molecular mechanism by which HSA binds Aβ monomers and protofibrils is not fully understood. Here, using dark-state exchange saturation transfer NMR and relaxation experiments complemented by morphological characterization, we mapped the HSA-Aβ interactions at atomic resolution by examining the effects of HSA on Aβ monomers and soluble high-molecular weight oligomeric protofibrils. We found that HSA binds both monomeric and protofibrillar Aβ, but the affinity of HSA for Aβ monomers is lower than for Aβ protofibrils (K_d values are submillimolar rather than micromolar) yet physiologically relevant because of the ∼0.6–0.7 mm plasma HSA concentration. In both Aβ protofibrils and monomers, HSA targets key Aβ self-recognition sites spanning the β strands found in cross-β protofibril structures, leading to a net switch from direct to tethered contacts between the monomeric Aβ and the protofibril surface. These HSA-Aβ interactions are isoform-specific, because the HSA affinity of Aβ monomers is lower for Aβ(1–42) than for Aβ(1–40). In addition, the HSA-induced perturbations of the monomer/protofibrils pseudo-equilibrium extend to the C-terminal residues in the Aβ(1–42) isoform but not in Aβ(1–40). These results provide an unprecedented view of how albumin interacts with Aβ and illustrate the potential of dark-state exchange saturation transfer NMR in mapping the interactions between amyloid-inhibitory proteins and amyloidogenic peptides.

Protein misfolding as well as the subsequent self-association and deposition of amyloid aggregates is implicated in the progression of several neurodegenerative disorders including Alzheimer’s and Parkinson’s diseases. Modulators of amyloidogenic aggregation serve as essential tools to dissect the underlying molecular mechanisms and may offer insight on potential therapeutic solutions. These modulators include green tea catechins, which are potent inhibitors of amyloid aggregation. Although catechins often exhibit poor pharmacokinetic properties and bioavailability, they are still essential tools for identifying the drivers of amyloid aggregation and for developing other aggregation modulators through structural mimicry. As an illustration of such strategies, here we review how catechins have been used to map the toxic surfaces of oligomeric amyloid-like species and develop catechin-based phenolic compounds with enhanced anti-amyloid activity.

The self-association of the amyloid beta (Aβ) peptide into toxic oligomers is implicated in the early events leading to Alzheimer's disease (AD). Blocking the formation of Aβ oligomers and their interactions with the extracellular and cellular environment through small molecules and biopharmaceuticals is therefore a promising preventive strategy for AD. However, given the heterogeneity and transient nature of the Aβ oligomeric species, detailed structural and kinetic characterizations of such oligomers and oligomer:inhibitor complexes have proven to be challenging. Here, we discuss recent advancements in solution NMR that have been instrumental in overcoming these limitations and we provide two representative examples of Aβ inhibitors from our work to demonstrate the applications of such experiments, i.e. EGCG and human serum albumin.