Binding mode of Thioflavin T and other molecular probes in the context of amyloid fibrils-current status

Effects of the English (H6R) and Tottori (D7N) familial Alzheimer disease mutations on amyloid beta-protein assembly and toxicity

Mutations in the amyloid beta-protein (Abeta) precursor gene cause autosomal dominant Alzheimer disease in a number of kindreds. In two such kindreds, the English and the Tottori, the mutations produce amyloid beta-proteins containing amino acid substitutions, H6R and D7N, respectively, at the peptide N terminus. To elucidate the structural and biological effects of the mutations, we began by examining monomer conformational dynamics and oligomerization. Relative to their wild type homologues, and in both the Abeta40 and Abeta42 systems, the English and Tottori substitutions accelerated the kinetics of secondary structure change from statistical coil --> alpha/beta --> beta and produced oligomer size distributions skewed to higher order. This skewing was reflected in increases in average oligomer size, as measured using electron microscopy and atomic force microscopy. Stabilization of peptide oligomers using in situ chemical cross-linking allowed detailed study of their properties. Each substitution produced an oligomer that displayed substantial beta-strand (H6R) or alpha/beta (D7N) structure, in contrast to the predominately statistical coil structure of wild type Abeta oligomers. Mutant oligomers functioned as fibril seeds, and with efficiencies significantly higher than those of their wild type homologues. Importantly, the mutant forms of both native and chemically stabilized oligomers were significantly more toxic in assays of cell physiology and death. The results show that the English and Tottori mutations alter Abeta assembly at its earliest stages, monomer folding and oligomerization, and produce oligomers that are more toxic to cultured neuronal cells than are wild type oligomers.

Molecular mechanism of Thioflavin-T binding to amyloid fibrils

Intense efforts to detect, diagnose, and analyze the kinetic and structural properties of amyloid fibrils have generated a powerful toolkit of amyloid-specific molecular probes. Since its first description in 1959, the fluorescent dye Thioflavin-T (ThT) has become among the most widely used "gold standards" for selectively staining and identifying amyloid fibrils both in vivo and in vitro. The large enhancement of its fluorescence emission upon binding to fibrils makes ThT a particularly powerful and convenient tool. Despite its widespread use in clinical and basic science applications, the molecular mechanism for the ability of ThT to recognize diverse types of amyloid fibrils and for the dye's characteristic fluorescence has only begun to be elucidated. Here, we review recent progress in the understanding of ThT-fibril interactions at an atomic resolution. These studies have yielded important insights into amyloid structures and the processes of fibril formation, and they also offer guidance for designing the next generation of amyloid assembly diagnostics, inhibitors, and therapeutics.