Pine Bark Polyphenolic Extract Attenuates Amyloid-β and Tau Misfolding in a Model System of Alzheimer's Disease Neuropathology

Abstract

Plant-derived polyphenolic compounds possess diverse biological activities, including strong anti-oxidant, anti-inflammatory, anti-microbial, and anti-tumorigenic activities. There is a growing interest in the development of polyphenolic compounds for preventing and treating chronic and degenerative diseases, such as cardiovascular disorders, cancer, and neurological diseases including Alzheimer's disease (AD). Two neuropathological changes of AD are the appearance of neurofibrillary tangles containing tau and extracellular amyloid deposits containing amyloid-β protein (Aβ). Our laboratory and others have found that polyphenolic preparations rich in proanthocyanidins, such as grape seed extract, are capable of attenuating cognitive deterioration and reducing brain neuropathology in animal models of AD. Oligopin is a pine bark extract composed of low molecular weight proanthocyanidins oligomers (LMW-PAOs), including flavan-3-ol units such as catechin (C) and epicatechin (EC). Based on the ability of its various components to confer resilience to the onset of AD, we tested whether oligopin can specifically prevent or attenuate the progression of AD dementia preclinically. We also explored the underlying mechanism(s) through which oligopin may exert its biological activities. Oligopin inhibited oligomer formation of not only Aβ1-40 and Aβ1-42, but also tau in vitro. Our pharmacokinetics analysis of metabolite accumulation in vivo resulted in the identification of Me-EC-O-β-Glucuronide, Me-(±)-C-O-β-glucuronide, EC-O-β-glucuronide, and (±)-C-O-β-glucuronide in the plasma of mice. These metabolites are primarily methylated and glucuronidated C and EC conjugates. The studies conducted provide the necessary impetus to design future clinical trials with bioactive oligopin to prevent both prodromal and residual forms of AD.

Keywords: Alzheimer’s disease; amyloid β-peptide; polyphenols; tau.

Fig. 1.

Oligopin attenuates oligomerization of Aβ peptides in vitro. SDS-PAGE of Aβ_1–40 and Aβ_1–42 in the presence or absence of oligopin following PICUP. (a) 25 μM Aβ_1–40 and (c) Aβ_1–42 were cross-linked in the presence or absence of 25 (1:1) or 100 (1:4) μM oligopin and the bands in subsequent SDS gels were visualized using silver staining. Lane 1: molecular weight; Lanes 2 and 6: UnXL, un-cross-linked Aβ_1–40 or Aβ_1–42; Lanes 3 and 7: XL, cross-linked Aβ_1–40 or Aβ_1–42; Lanes 5 and 9: Aβ_1–40 or Aβ_1–42 aggregated in the presence of 25 or 100 μM oligopin; the molecular weight of oligopin is based on the composition of dimer, trimer and tetramer of oligopin. A compound related to oligopin was used as a control to asses attenuation of Aβ_1–40 or Aβ_1–42 purposes relative to oligopin (lanes 4 and 8). For each lane that contains crosslinked samples (XL) in a and c, the ratios of the integrated densities of each oligomer to the monomer are shown in panels b and d, respectively. The gels are representative of three independent experiments.

Fig. 2.

Oligopin prevents oligomerization of full-length tau protein in vitro. (a) SDS-PAGE of tau protein in the presence or absence of 25 (1:1) or 100 (1:4) μM oligopin following PICUP. This panel is composed of two independent results. Lanes 1: molecular weight; Lanes 2 and 6: UnXL, un-cross-linked tau protein; Lanes 3 and 7: XL, cross-linked tau protein; Lanes 5 and 9: tau protein aggregated in the presence of 25 or 100 μ M oligopin. A compound related to oligopin was used as a control to assess attenuation of tau oligomerization (lanes 4 and 8). For each lane in a, the ratios of the integrated densities of each oligomer to the monomer in the lane are shown. The gel is representative of three independent experiments.

Fig. 3.

Oligopin does not influence oligomerization of GST in vitro. (a) SDS-PAGE of GST in the presence or absence of 25 (1:1) or 100 (1:4) μM oligopin, following PICUP. Lane 1: molecular weight; Lane 2: UnXL, un-cross-linked GST; Lane 3: XL, cross-linked GST; Lanes 4 and 5: GST protein aggregated in the presence of 25 or 100 μM oligopin, respectively. For each lane that contains crosslinked samples, the ratios of the integrated densities of each oligomer to the monomer are shown in panel b. The gels are representative of three independent experiments.

Fig. 4.

Plasma pharmacokinetics C and EC metabolites following 10 days repeated dosing of oligopin. A, B) Plasma pharmacokinetic profile of methylated and glucuronidated C and EC metabolites following acute on repeated dosing of mice by treatment with 200 mg/kg/day oligopin following acute on repeated dosing. LC-MS/MS separation of major C and EC metabolites detected in extracts of mice plasma collected after 10-day treatment. Multiple reaction monitoring trace is shown for C/EC-O-β-glucuronide (465.1 ± 289.1 m/z) and MeO-C/EC-O-β-glucuronide (479.1 ± 303.1 m/z). Data represent mean ± SD, n = 5 mice per group.

Fig. 5.

Brain levels of C and EC metabolites following 10 days repeated dosing of oligopin. Brain content of C and EC metabolites following acute or repeated dosing of rats by treatment with 200 mg/kg/day oligopin. The error bars represent standard errors of the means.

Fig. 6.

Inhibitory effects of LMW-PAOs on Aβ_1–40, Aβ_1–42, and tau oligomerizations. The monomer of Aβ_1–40, Aβ_1–42, and tau may aggregate to form intermediate aggregates such as oligomers and finally fibrils. LMW-PAOs mainly prevents off-pathway oligomers of Aβ_1–40, Aβ_1–42, and tau.