Red Ginseng Inhibits Tau Aggregation and Promotes Tau Dissociation In Vitro

Abstract

Tau, a microtubule-associated protein expressed in mature neurons, interacts with tubulin to promote the assembly and stabilization of microtubules. However, abnormally hyperphosphorylated tau dissociates from microtubules and self-aggregates. Tau aggregates, including paired helical filaments and neurofibrillary tangles, promote neuronal dysfunction and death and are the defining neuropathological feature of tauopathies. Therefore, suppressing tau aggregation or stimulating the dissociation of tau aggregates has been proposed as an effective strategy for treating neurodegenerative diseases associated with tau pathology such as Alzheimer's disease (AD) and frontotemporal dementia. Interestingly, ginsenosides extracted from Panax ginseng reduced the hippocampal and cortical expression of phosphorylated tau in a rat model of AD. However, no studies have been conducted into the effect of red ginseng (RG) and its components on tau pathology. Here, we evaluated the effect of Korean red ginseng extract (KRGE) and its components on the aggregation and disassociation of tau. Using the thioflavin T assay, we monitored the change in fluorescence produced by the aggregation or disassociation of tau K18, an aggregation-prone fragment of tau₄₄₁ containing the microtubule-binding domain. Our analysis revealed that KRGE not only inhibited tau aggregation but also promoted the dissociation of tau aggregates. In addition, the KRGE fractions, such as saponin, nonsaponin, and nonsaponin fraction with rich polysaccharide, also inhibited tau aggregation and promoted the dissociation of tau aggregates. Our observations suggest that RG could be a potential therapeutic agent for the treatment of neurodegenerative diseases associated with tauopathy.

Figure 1

The effect of Korean red ginseng extract (KRGE) on the aggregation and dissociation of tau K18. (a) The aggregation of tau K18 fragments was monitored using the thioflavin T (ThT) assay for 72 hours. (b) The fold changes of the aggregated tau were analyzed by assessing the fluorescence intensity of ThT and normalized to tau K18 monomer. (c) The fluorescence of ThT in the presence and absence of KRGE and methylene blue (MB; 100 μM). (d) The dissociation of preformed tau aggregates was analyzed by assessing the fluorescence intensity of ThT in the presence and absence of KRGE. Values represent the mean ± standard error of the mean of three independent experiments. Statistical significance was determined by one-way analysis of variance followed by Tukey's multiple comparison test. ^∗p < 0.05; ^∗∗p < 0.01; ^∗∗∗p < 0.001.

Figure 2

The inhibitory effect of Korean red ginseng extract (KRGE) fractions on tau K18 aggregation. Tau aggregation was analyzed by assessing the fluorescence intensity of thioflavin T (ThT) in the presence and absence of (a) the saponin fraction (SF), (b) the nonsaponin fraction (NSF), and (c) the NSF with rich polysaccharide (NFP). Methylene blue (MB; 100 μM), a known inhibitor of tau aggregation, was used as a positive control. Values represent the mean ± standard error of the mean of three independent experiments. Statistical significance was determined by one-way analysis of variance followed by Tukey's multiple comparison test. ^∗∗p < 0.01; ^∗∗∗p < 0.001.

Figure 3

The effect of KRGE and KRGE fractions on the aggregation of tau K18 using the transmission electron microscopy (TEM). (a) Images of tau K18 aggregates in the absence or presence of 500 μg/mL KRGE or KRGE fractions (scale bar = 200 nm). (b) The length of the tau K18 aggregates was quantified in TEM images. Values represent the mean ± standard error of the mean. Statistical significance was determined by one-way analysis of variance followed by Tukey's multiple comparison test. ^∗∗p < 0.01; ^∗∗∗p < 0.001.

Figure 4

The stimulatory effect of Korean red ginseng extract (KRGE) fractions on tau K18 dissociation. Tau dissociation was analyzed by assessing the fluorescence intensity of thioflavin T (ThT) in the presence and absence of (a) the saponin fraction (SF), (b) the nonsaponin fraction (NSF), and (c) the NSF with rich polysaccharide (NFP). Values represent the mean ± standard error of the mean of three independent experiments. Statistical significance was determined by one-way analysis of variance followed by Tukey's multiple comparison test. ^∗p < 0.05; ^∗∗p < 0.01; ^∗∗∗p < 0.001.

Figure 5

A schematic illustration of the structure of tau K18 and the modulatory effects of KRGE fractions on tau K18 aggregation/dissociation. KRGE and its constituents exhibited modulatory effects on tau aggregation and dissociation. KRGE: Korean red ginseng extract; NFT: neurofibrillary tangle; PHF: paired helical filament.

Figure 6

Tau pathomechanisms and proposed mechanism underlying the modulatory effect of KRGE and its fractions on tau K18 aggregation/dissociation. KRGE: Korean red ginseng extract; NFT: neurofibrillary tangle; PHF: paired helical filament.