This study demonstrated for the first time that (i) HopA might be able to bind to Aβ1–42 directly through four potential amino acid residues and decrease the formation of amyloid plaque; (ii) HopA decreased the interaction between Aβ1–42 and ABAD, which was demonstrated to induce mitochondrial dysfunction as well as oxidative stress both in vivo and in vitro; and (iii) HopA protected synaptic function and attenuated memory deficits in APP/PS1 mice.
The accumulation of Aβ-forming senile plaques is one of the main pathological hallmarks of AD. Aβ aggregation is neurotoxic and may be the primary toxic species in AD (Muirhead et al., 2010). Therefore, prevention of Aβ aggregation and attenuation of its neurotoxicity have been the focus of AD therapies. This study suggested that HopA might bind to Aβ1–42 directly through four negatively charged residues, Glu3, Asp7, Glu22, and Ala42, which was possibly due to its functional groups, such as the hydroxyl group, and aromatic rings (Reinke & Gestwicki, 2007). Clinical trials showed that immunization with Aβ42 could clear the amyloid plaques in patients with AD, but it did not prevent progressive neurodegeneration (Holmes et al., 2008). Actually, intracellular Aβ accumulation seems to be more closely related to AD symptoms onset and progression than extracellular Aβ deposition, which results in amyloid plaque formation (Cavallucci et al., 2012). Our results have demonstrated that HopA not only decreased amyloid load, but also suppressed the interaction of Aβ1–42 and ABAD, which led to partial recovery of the mitochondrial function and decrease in oxidative damage in vitro and in vivo. Aβ–ABAD interaction induces mitochondrial dysfunction and oxidative stress (Lustbader et al., 2004; Seo et al., 2011), which triggers a series of events including increased generation of ROS, DNA fragmentation, lactate dehydrogenase and cytochrome c release, diminished COX IV activity, and decreased ATP levels. Finally, it results in high caspase-3 activity, apoptosis, and severe impairment in spatial learning and memory (Marques et al., 2009). The expression of ABAD was increased in transgenic APP/PS1 mice, which was consistent with previous reports (Yan et al., 1997; He et al., 2002; Lustbader et al., 2004). We speculated that abundant Aβ led to the dysfunction of ABAD, which was induced to maintain the homeostasis in AD brains, and eventually became a cellular cofactor for Aβ-induced cell stress. In addition, our study also demonstrated that HopA could not only bind to the C-terminal of Aβ1–42 directly and decrease the ABAD–Aβ1–42 interaction, but also restored ABAD activity without changing its expression. Interestingly, an ABAD decoy peptide (ABAD-DP) alleviates cognitive dysfunction in Tg mAPP/ABAD mice by restraining the ABAD–Aβ interaction (Yao et al., 2011), which indicates that ABAD–Aβ interaction might be an effective target for the treatment of AD. Furthermore, HopA protected the synaptic function and LTP induction in APP/PS1 mice, which was accompanied by the improvement of spatial memory performance.
In addition, HopA is a revisable inhibitor of AChE with an IC50 value of 4.33 ± 0.17 μm in vitro (Ge et al., 2008) and also exerts an AChE inhibitory effect in APP/PS mice (data not shown). Thus, HopA inhibited both Aβ1–42 aggregation and AChE, which contributed to the therapeutic effects on AD. Besides, AChE is a potent amyloid-promoting factor and promotes the aggregation of Aβ through its peripheral anionic site (PAS; Alvarez et al., 1997). The binding assay and TEM analysis indicated that HopA might bind to Aβ1–42 directly and inhibit the self-induced Aβ1–42 aggregation, but whether HopA could also inhibit the AChE-induced Aβ1–42 aggregation remains to be investigated.
In conclusion, HopA is a novel polyphenol extracted from the bark of the stems of Hopea hainanensis. The compound might target Aβ1–42 directly and decrease the ABAD–Aβ1–42 interaction. Furthermore, HopA alleviates mitochondrial dysfunction induced by Aβ1–42 and its downstream cascades, suggesting that it is a promising candidate for the treatment of AD.