Oligomers of the 42-mer amyloid-β protein (Aβ42), rather than fibrils, cause memory loss and synaptic dysfunction in the pathogenesis of Alzheimer’s disease (AD). The aggregation mechanism of Aβ42 is explained by the nucleation-dependent polymerization model that consists of nucleation and elongation phases. During the nucleation phase, Aβ42 monomer gradually forms low-molecular-weight intermediates, called “nuclei”, which are closely related to the formation of toxic oligomers of Aβ42 (12 ~ 24-mer: 2 or 3 × n). Uncaria rhynchophylla is one ingredient of “Yokukansan”, a Kampo medicine, which is widely available for treating AD symptoms. Previously, extracts of U. rhynchophylla were found to delay the aggregation of Aβ42, but the active principles have yet to be identified. Here we identified uncarinic acid A ~ D as inhibitors of Aβ42 aggregation in the acetone extracts of the dried branches of U. rhynchophylla. Moreover, they acted as specific inhibitors of the nucleation phase of Aβ42 aggregation.1) Uncarinic acid C (3) was semi-synthesized from saponin A (10), an abundant by-product of rutin purified from Uncaria elliptica. Subsequent structure-activity studies on 3 indicated that both the C-27 ferulate and C-28 carboxylic acid group were necessary to its preventive activity. Ion mobility-mass spectrometry suggested that 3 could target the dimer ~ tetramer by its direct interaction with Aβ42 through the salt bridge between C-28 carboxylic acid group and Lys16,28 side chain, resulting in the suppression of further oligomerization. The 1H–15N SOFAST-HMQC NMR of Aβ42 in the presence of 3 revealed the significant perturbation of chemical shifts of His13,14, Lys16,28, and Arg5, which were close to the intermolecular β-sheet region (Gln15~Ala21). Given the potent inhibition of Aβ42-induced neurotoxicity by 3, the present findings may aid the development of toxic oligomer-specific inhibitors for AD therapy. 1) Yoshioka, T. et al., J. Nat. Prod. 2016, 79, 2521-2529.
