Non-Conjugated Small Molecule FRET for Differentiating Monomers from Higher Molecular Weight Amyloid Beta Species

BACKGROUND Systematic differentiation of amyloid (Aβ) species could be important for diagnosis of Alzheimer's disease (AD). In spite of significant progress, controversies remain regarding which species are the primary contributors to the AD pathology, and which species could be used as the best biomarkers for its diagnosis. These controversies are partially caused by the lack of reliable methods to differentiate the complicated subtypes of Aβ species. Particularly, differentiation of Aβ monomers from toxic higher molecular weight species (HrMW) would be beneficial for drug screening, diagnosis, and molecular mechanism studies. However, fast and cheap methods for these specific aims are still lacking. PRINCIPAL FINDINGS We demonstrated the feasibility of a non-conjugated FRET (Förster resonance energy transfer) technique that utilized amyloid beta (Aβ) species as intrinsic platforms for the FRET pair assembly. Mixing two structurally similar curcumin derivatives that served as the small molecule FRET pair with Aβ40 aggregates resulted in a FRET signal, while no signal was detected when using Aβ40 monomer solution. Lastly, this FRET technique enabled us to quantify the concentrations of Aβ monomers and high molecular weight species in solution. SIGNIFICANCE We believe that this FRET technique could potentially be used as a tool for screening for inhibitors of Aβ aggregation. We also suggest that this concept could be generalized to other misfolded proteins/peptides implicated in various pathologies including amyloid in diabetes, prion in bovine spongiform encephalopathy, tau protein in AD, and α-synuclein in Parkinson disease.