Mapping the Binding Interface of PET Tracer Molecules and Alzheimer Disease Aβ Fibrils by Using MAS Solid‐State NMR Spectroscopy

Abstract

Positron emission tomography (PET) tracer molecules like thioflavin T specifically recognize amyloid deposition in brain tissue by selective binding to hydrophobic or aromatic surface grooves on the β‐sheet surface along the fibril axis. The molecular basis of this interaction is, however, not well understood. We have employed magic angle spinning (MAS) solid‐state NMR spectroscopy to characterize Aβ‐PET tracer complexes at atomic resolution. We established a titration protocol by using bovine serum albumin as a carrier to transfer hydrophobic small molecules to Aβ(1‐40) fibrillar aggregates. The same Aβ(1‐40) amyloid fibril sample was employed in subsequent titrations to minimize systematic errors that potentially arise from sample preparation. In the experiments, the small molecules 13C‐methylated Pittsburgh compound B (PiB) as well as a novel Aβ tracer based on a diarylbithiazole (DABTA) scaffold were employed. Classical 13C‐detected as well as proton‐detected spectra of protonated and perdeuterated samples with back‐substituted protons, respectively, were acquired and analyzed. After titration of the tracers, chemical‐shift perturbations were observed in the loop region involving residues Gly25‐Lys28 and Ile32‐Gly33, thus suggesting that the PET tracer molecules interact with the loop region connecting β‐sheets β1 and β2 in Aβ fibrils. We found that titration of the PiB derivatives suppressed fibril polymorphism and stabilized the amyloid fibril structure.

Solid-state NMR spectroscopy

“…Chemical shifts of13 C,15 N and 1 H are correlated to obtain sequence-specific resonance assignment. b | Intra-residue hCANH and inter-residue hCA(CO)NH correlation spectra of Aβ fibrils318 . c | 1 H-19 F rotational echo double resonance (REDOR) spectra for measuring internuclear distances up to 1.5 nm.…”

Maximizing efficiency of dipolar recoupling in solid-state NMR using optimal control sequences

“…While the ramp-CP is very sensitive to proper calibration, the presented tm-SPICE schemes require virtually no optimization on the spectrometer, as they are very robust toward mis-setting of rf amplitudes. Recently, tm-SPICE was implemented as a magnetization transfer element to yield sequential assignments for various amyloid fibrils (43)(44)(45). We anticipate that this method will replace the conventional ramp-CP block, which requires tedious optimization.…”

Toward Novel [18F]Fluorine-Labeled Radiotracers for the Imaging of α-Synuclein Fibrils

“…The scaffold for easy description was conveniently divided into the right hetero-/aryl part (Ar R ), the middle bisthiazole part (M BT ), and the left hetero/aryl part (Ar L ) (Figure 1). The DABTA framework, however, has already been introduced by Niu et al (2020), but as Aβ ligands, which lack functional group(s) unique to the tracers that will be reported in this article.…”

“…To begin with, a series of hetero-/aryl DABTAs with different functional groups initially were synthesized and screened in competition binding assays in which it was discovered that the inclusion of a methylenedioxy moiety (Figure 1) in the structure of the DABTAs significantly improved binding affinity to α-syn and selectivity over Aβ plaque and tau fibrils (PubChem, 2022). In the absence of this methylenedioxy moiety and in the presence of certain functional groups, the DABTAs might lose their high affinity for α-syn with improvement in affinity to the other proteinopathies (PubChem, 2022;Niu et al, 2020). For further development of the DABTAs as α-syn tracers, it made sense to leave the methylenedioxy moiety constant in (Ar R ) and modify (Ar L ) and the aromatic ring in (Ar R ) to further increase binding affinity and selectivity of the tracers, as well as in vivo pharmacokinetics of the DABTAs.…”