Natural Compounds as Inhibitors of Aβ Peptide Aggregation: Chemical Requirements and Molecular Mechanisms

Abstract

Alzheimer's disease (AD) is one of the most common neurodegenerative disorders, with no cure and preventive therapy. Misfolding and extracellular aggregation of Amyloid-β (Aβ) peptides are recognized as the main cause of AD progression, leading to the formation of toxic Aβ oligomers and to the deposition of β-amyloid plaques in the brain, representing the hallmarks of AD. Given the urgent need to provide alternative therapies, natural products serve as vital resources for novel drugs. In recent years, several natural compounds with different chemical structures, such as polyphenols, alkaloids, terpenes, flavonoids, tannins, saponins and vitamins from plants have received attention for their role against the neurodegenerative pathological processes. However, only for a small subset of them experimental evidences are provided on their mechanism of action. This review focuses on those natural compounds shown to interfere with Aβ aggregation by direct interaction with Aβ peptide and whose inhibitory mechanism has been investigated by means of biophysical and structural biology experimental approaches. In few cases, the combination of approaches offering a macroscopic characterization of the oligomers, such as TEM, AFM, fluorescence, together with high-resolution methods could shed light on the complex mechanism of inhibition. In particular, solution NMR spectroscopy, through peptide-based and ligand-based observation, was successfully employed to investigate the interactions of the natural compounds with both soluble NMR-visible (monomer and low molecular weight oligomers) and NMR-invisible (high molecular weight oligomers and protofibrils) species. The molecular determinants of the interaction of promising natural compounds are here compared to infer the chemical requirements of the inhibitors and the common mechanisms of inhibition. Most of the data converge to indicate that the Aβ regions relevant to perturb the aggregation cascade and regulate the toxicity of the stabilized oligomers, are the N-term and β1 region. The ability of the natural aggregation inhibitors to cross the brain blood barrier, together with the tactics to improve their low bioavailability are discussed. The analysis of the data ensemble can provide a rationale for the selection of natural compounds as molecular scaffolds for the design of new therapeutic strategies against the progression of early and late stages of AD.

Keywords: Alzheimer; NMR; amyloid-β protein; natural compound; protein ligand interactions; self-association.

FIGURE 1

Schematic representation of Aβ self-association cascade. Aβ monomers initially combine to form a nucleus through primary nucleation process. Nuclei are defined as aggregates for which monomer addition is faster than its dissociation (Arosio et al., 2015). Addition of monomers to the nucleus, through the elongation process, results in the formation of oligomers, that are transient soluble intermediates that further elongate into fibrils. Fibrils can be disrupted through monomer-independent processes, such as fragmentation, with a rate depending only upon the concentration of existing fibrils. Fibril elongation by monomer addition and secondary nucleation depends on both the concentration of monomers and that of the existing fibrils (Linse, 2017; Scheidt et al., 2019). Once a critical concentration of mature fibrils has formed, the surfaces of existing fibrils catalyze the nucleation of new aggregates from the monomeric state (secondary nucleation). Secondary nucleation reaction overtakes primary nucleation as the major source of new diffusible oligomers (positive feedback) (Cohen et al., 2013). Color code: monomers are colored in yellow; nuclei and soluble transient oligomers are colored in pink; fibrils are colored in blue. The number of circles are for illustration purposes only and do not represent the actual number of subunits in the different species.

FIGURE 2

Aβ residues affected by the addition of the natural compounds, as deduced from ¹H-¹⁵N HSQC experiments, are mapped on the structure of Aβ42 tetramer (PDB ID: 6RHY). The picture summarizes data reported for: resveratrol (Fu et al., 2014) (A), curcumin (Fu et al., 2014) (B), Myricetin (C) (Ono et al., 2012), uncarinic Acid C (Murakami et al., 2018) (D), and Oleuropein (Galanakis et al., 2011) (E). Four different colors are employed to distinguish the four Aβ subunits forming the tetramer. Chains (A–D) of the deposited PDB structure are colored in green, cyana, purple, and yellow, respectively. Residues perturbed by the addition of the natural compounds are highlighted as closed surfaces on the tetramer cartoon.

FIGURE 3

Proposed model for the molecular determinants of Aβ assembly toxicity. (A) Catechin-free oligomers (canonical Abn) insert and colocalize efficiently into the membrane due to their significant solvent exposure of hydrophobic surfaces (yellow glow surrounding Aβn). The catechin-remodeled oligomers, with less exposed hydrophobic sites, only insert into the membrane partially. (B) Both laminated and non-laminated cross-β-sheet structures can insert into the membrane, which indicates that cross-β-sheet structures are not required for membrane insertion. (C) The interaction of monomeric Aβ with toxic and remodeled oligomers is different within a membrane environment. EGCG-remodeled oligomers (maroon) show a significant disengagement of contacts with the b1 region and an opposite enhancement in the contacts with the N-terminal region, compared to untreated (black) oligomers. The EC-remodeled (green) oligomers exhibit a pattern at the N-terminus and b1 regions intermediate to the canonical and EGCG-remodeled oligomers, while a further enhancement in C-terminal contacts relative to both canonical and EGCG-remodeled Abn is observed. (D) In the Aβ40 fibril structure (PDB code: 2LMN) the residues that correlate with toxicity (blue) in the N-terminal and b1 regions can be found in the exterior of the fibril structure, while the b2 region not linked with toxicity is inaccessible to the environment (Ahmed et al., 2019). Published by The Royal Society of Chemistry.