Molecular insight into the early stage of amyloid-β(1-42) Homodimers aggregation influenced by histidine tautomerism

Aggregated amyloid β-peptide (Aβ) in small oligomeric forms inside the brain causes synaptic function disruption and the development of Alzheimer's disease (AD). Histidine is an important amino acid that may lead to structural changes. Aβ42 monomer chain includes 3 histidine residues that considering two ε and δ tautomers 8 isomers, including (εεε) and (εδδ) could be formed. Molecular dynamics simulation on homodimerization of (εεε) (the most common type of tautomers) and (εδδ) tautomers with different initial configurations using monomer chains from our previous work were performed to uncover the tautomeric behavior of histidine on Aβ42 aggregation in a physiological pH which is still largely unknown and impossible to observe experimentally. We found a higher propensity of forming β-sheet in (εδδ) homodimers and specifically in a greater amount from Aβ42 than from Aβ40. A smaller amount of β-sheet formation was observed for (εεε) homodimers compared with (εδδ). Additionally, interactions in (εδδ) homodimers may indicate the importance of the hydrophobic core and C-/N-terminals during oligomerization. Our findings indicate the important role of the tautomeric effect of histidine and (εδδ) homodimers at the early stage of Aβ aggregation.

Design, synthesis and biological activity of novel tacrine-isatin Schiff base hybrid derivatives

A series of novel tacrine-isatin Schiff base hybrid derivatives (7a-p) were designed, synthesized and evaluated as multi-target candidates against Alzheimer's disease (AD). The biological assays indicated that most of these compounds displayed potent inhibitory activity toward acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) and specific selectivity for AChE over BuChE. It was also found that they act as excellent metal chelators. The compounds 7k and 7m were found to be good inhibitors of AChE-induced amyloid-beta (Aβ) aggregation. Most of the compounds inhibited AChE with the IC₅₀ values, ranging from 0.42 nM to 79.66 nM. Amongst them, 7k, 7m and 7p, all with a 6 carbon linker between tacrine and isatin Schiff base exhibited the strongest inhibitory activity against AChE with IC₅₀ values of 0.42 nM, 0.62 nM and 0.95 nM, respectively. They were 92-, 62- and 41-fold more active than tacrine (IC₅₀ = 38.72 nM) toward AChE. Most of the compounds also showed a potent BuChE inhibition among which 7d with an IC₅₀ value of 0.11 nM for BuChE is the most potent one (56-fold more potent than that of tacrine (IC₅₀ = 6.21 nM)). In addition, most compounds exhibited the highest metal chelating property. Kinetic and molecular modeling studies revealed that 7k is a mixed-type inhibitor, capable of binding to catalytic and peripheral site of AChE. Our findings make this hybrid scaffold an excellent candidate to modify current drugs in treating Alzheimer's disease (AD).

Molecular interactions with redox sites and salt bridges modulate the anti-aggregatory effect of flavonoid, tannin and cardenolide moieties against amyloid-beta (1-42) in silico

In this study, the interactions of flavonoid, tannin and cardenolide moieties as well as their known metabolites were docked against the apolar NMR structure of the aggregatory amyloid-beta fragment (Aβ_1-42). Results showed that the catechin moiety favorably bound Aβ_1-42 peptide at Asp23, Asn27, Ser26 and Glu22 residues, with chalcone similarly binding the middle region of the peptide. Remarkably, hippuric and ferulic acids exhibited hydrophobic interactions with Aβ_1-42 at the latter portion of the peptide, possibly blocking the salt bridges formed by Glu22-Lys28 which stabilizes Phe19-Gly25, as well as the β-sheet Leu34-Gly38 that are known to exist in peptide aggregation. Meanwhile, the metabolites of hydrolyzable tannins, such as urolithin A and gallic acid, exhibited H-bonding interactions with different residues of Aβ_1-42, including Asp1, Asp23 and hydrophobic interactions by gallic acid planar ring to the Hsd6 residue. The coverage was lessened in pyrogallol, suggesting that gallic acid loses its efficacy when further metabolized. Lastly, the different binding poses of the cardenolide moiety interacted with Hsp6 (protonated His) and Tyr10 via hydrophobic interactions. Due to these interactions, the large polycyclic moiety of the ligand would also block further interactions with Hsd6 (prototropic tautomer of His), Asp7, Ser8 and Gly9 that are integral to His6-His13-His14, Arg5-Asp7and Leu34-Gly38 β-sheets, salt bridges in Glu22-Lys28 and turn conformation Phe19-Gly25. Together, these data suggest that the known metabolites of anthocyanins and hydrolyzable tannins contribute the most effective anti-aggregatory interactions with Aβ_1-42, with an unexpected role for cardiac glycosides such as the cardenolie moiety. These bring to light the important role of metabolism in vivo, and suggests further investigation on the effects of these metabolites when concentrated in vivo.

Syntheses and evaluation of novel isoliquiritigenin derivatives as potential dual inhibitors for amyloid-beta aggregation and 5-lipoxygenase

A series of new isoliquiritigenin (ISL) derivatives were synthesized and evaluated as dual inhibitors for amyloid-beta (Aβ) aggregation and 5-lipoxygenase (5-LO). It was found that all these synthetic compounds inhibited Aβ (1-42) aggregation effectively with their IC₅₀ values ranged from 2.2 ± 1.5 μM to 23.8 ± 2.0 μM. These derivatives also showed inhibitory activity to 5-LO with their IC50 values ranged from 6.1 ± 0.1 μM to 35.9 ± 0.3 μM. Their structure-activity relationships (SAR) and mechanisms of inhibitions were studied. This study provided potentially important information for further development of ISL derivatives as multifunctional agents for Alzheimer's disease (AD) treatment.