An Azobenzene Photoswitch Sheds Light on Turn Nucleation in Amyloid-β Self-Assembly

Abstract

Amyloid-β (Aβ) self-assembly into cross-β amyloid fibrils is implicated in a causative role in Alzheimer’s disease pathology. Uncertainties persist regarding the mechanisms of amyloid self-assembly and the role of metastable prefibrillar aggregates. Aβ fibrils feature a sheet-turn-sheet motif in the constituent β-strands; as such, turn nucleation has been proposed as a rate-limiting step in the self-assembly pathway. Herein, we report the use of an azobenzene β-hairpin mimetic to study the role turn nucleation plays on Aβ self-assembly. [3-(3-Aminomethyl)­phenylazo]­phenylacetic acid (AMPP) was incorporated into the putative turn region of Aβ42 to elicit temporal control over Aβ42 turn nucleation; it was hypothesized that self-assembly would be favored in the <i>cis</i>-AMPP conformation if β-hairpin formation occurs during Aβ self-assembly and that the <i>trans</i>-AMPP conformer would display attenuated fibrillization propensity. It was unexpectedly observed that the <i>trans</i>-AMPP Aβ42 conformer forms fibrillar constructs that are similar in almost all characteristics, including cytotoxicity, to wild-type Aβ42. Conversely, the <i>cis</i>-AMPP Aβ42 congeners formed nonfibrillar, amorphous aggregates that exhibited no cytotoxicity. Additionally, <i>cis</i>-<i>trans</i> photoisomerization resulted in rapid formation of native-like amyloid fibrils and <i>trans–cis</i> conversion in the fibril state reduced the population of native-like fibrils. Thus, temporal photocontrol over Aβ turn conformation provides significant insight into Aβ self-assembly. Specifically, Aβ mutants that adopt stable β-turns form aggregate structures that are unable to enter folding pathways leading to cross-β fibrils and cytotoxic prefibrillar intermediates