Synthesis of chemically-tethered amyloid-β segment trimer possessing amyloidogenic properties.

As amyloid-β (Aβ) undergoes dynamic aggregation, it is impossible to isolate ('hook') the transient Aβ oligomer in an assembly state-pure form (e.g., sole Aβ dimer, trimer, tetramer, etc.). Obtaining such a pure Aβ oligomer would allow us to establish an in vitro system to perform a more detailed investigation of the pathogenic properties of the oligomer. A chemically-tethered Aβ oligomer, constructed only by covalent bonds, could satisfy this demand. Here we designed a chemically-tethered trimer of a pathogenic Aβ fragment (Aβ25-35) (1) and successfully generated it in situ from its precursor (4), a water-soluble and non-aggregative O-acyl isopeptide of 1, in neutral aqueous media. Chemically-tethered 1 possessed stronger amyloidogenic properties, that is, potential for β-sheet structure, fibril formation, and cytotoxicity, than the corresponding monomer Aβ25-35 (6). Trimerization of Aβ25-35 sequence might affect both the aggregative properties and cytotoxicity, based on the present results. This work opens the door for chemical synthesis of oligomers bigger than trimers in an assembly state-pure form, allowing for identification of the most toxic Aβ oligomer.