In vitro screening techniques for Thai herbal extracts on alzheimer's disease prevention

Abstract

Alzheimer's disease (AD) is a neurodegenerative disorder that is commonly found in elderly people. The major causative factor of AD is an extracellular accumulation and aggregation of amyloid β-peptide (Aβ) which results from the imbalance between its production and clearance in the brain. Thus, the potential strategies in the battle against AD focus on the discovery of multifunctional agents, especially from the natural products, which can act against multiple pathogenic steps of Aβ. Therefore, the screening system was set up in this study and nine Thai herbs; Caesalpinia sappan Linn., 1753 (CS), Thunbergia laurifolia Lindl., 1867 (TL), Rhinacanthus nasutus (Linn.) Kurz., 1870 (RN), Tabernaemontana divaricata (Linn.) R.Br. ex Roem. & Schult., 1819 (TD), Cyperus rotundus Linn., 1753 (CR), Terminalia chebula Retz., 1788 (TC), Azadirachta indica A. Juss., 1830 (AI), Piper retrofractum Vahl., 1804 (PR), and Asparagus racemosus Willd., 1799 (AR) were selected for the screening based on their properties for remedy the cognitive symptoms in ethnopharmacopoeia and have been researched in the fields of neurological disorders. The stems, leaves, roots or fruits of these herbs were extracted and then performed the 2 consecutive steps of the in vitro test; biochemical- and cell-based assay. Anti-Aβ aggregation, Aβ fibril disaggregation and antioxidant potencies of the extracts were investigated by the biochemical-based assay. The results indicated that, except AR, all eight herbal extracts had potential to inhibit Aβ aggregation and stimulate Aβ fibril disaggregation, and also exhibited high antioxidant activity. Hence, the next step of the investigation using cell-based assay was conducted, and the protective effect of these herbal extracts on Aβ-induced neuronal toxicity in SH-SY5Y neuroblastoma cell was determined. Among the eight herbal extracts, only RN, TD, TC and PR effectively protected against Aβ-induced neurotoxicity. Therefore, these four herbal extracts were then employed to investigate the modulating potential on the expression of genes associated with Aβ production (App, Bace1 and Adam10) and clearance (Ide and Nep) in SH-SY5Y cells. The result showed that PR had a potential to suppress Aβ production by decreasing β-secretase Bace1 and increasing α-secretase Adam10 mRNA expression levels, respectively, while TC had a potential to enhance the degradation of Aβ by increasing mRNA expression level of Ide. Altogether, this study indicates that PR and TC possibly particular candidate herbal medicines for developing the multifunctional anti-AD agents for human use in the future. In addition, this screening system has been suggested to be applied as a standardized tool to screen the high potential Thai herbs on AD therapy at a commercial and pharmaceutical scale.