Effects and Possible Mechanisms of Some Phytochemicals on Drosophila Models of Alzheimers Disease

Abstract

Alzheimer's disease (AD) is the most common type of presenile and senile dementia in developed and developing countries. The human β-amyloid (Aβ) cleaving enzyme (BACE-1) is a key enzyme responsible for amyloid plaque production, which implicates the progress and symptoms of AD. In this study, a fluorescence resonance energy transfer (FRET)-based enzyme assay was used to identify the BACE-1 inhibitory constituents from methanol extracts from the rhizomes of turmeric, Curcuma longa L. (Zingiberaceae), and the whole blue licorice (Korean mint) plant, Agastache rugosa (Fisch. & C.A. Mey.) O. Kuntze (Lamiaceae). The active constituents were determined to be the curcuminoids, diarylalkyls curcumin (CCN), demethoxycurcumin (DMCCN) and bisdemethoxycurcumin (BDMCCN) from C. longa rhizomes, and the O-methylated flavone, acacetin, and the oleanane triterpenoids, maslinic acid and oleanolic acid from whole A. rugosa plants. BDMCCN exhibited the strongest inhibitory activity toward BACE-1 with IC50 17 μM, which was 20 and 13-fold more potent than those of CCN and DMCCN, respectively. Quantitative structure–activity relationship of the curcuminoids indicates that structural characteristics, such as degrees of saturation, types of carbon skeleton and functional group, and hydrophobicity rather than molecular weight appear to play a role in determining inhibitory potency of curcuminoids on BACE-1. Acacetin was a 4.0-fold and 5.5-fold more potent inhibitor of BACE-1 than oleanolic acid and maslinic acid, respectively. Overall, these compounds were significantly less potent inhibitors of BACE-1 than a positive control, the cell-permeable isophthalamide, BACE-1 inhibitor IV. To assess the neuro-protective ability of the curcuminoids and acacetin, the Drosophila melanogaster models of AD were constructed and characterized by phenotypes, histological analysis, and reverse transcription polymerase chain reaction (RT-PCR). Drosophila model system overexpressed BACE-1 and its substrate amyloid precursor protein (APP) in compound eyes and entire neurons. Overexpression of APP/BACE-1 resulted in the progressive and measurable defects in morphology of eyes and locomotion. The feeding, climbing activity, lifespan, and morphological changes in fly eyes were also evaluated. Remarkably, supplementing diet with either BDMCCN, CCN, and acacetin rescued APP/BACE-1-expressing flies and kept them from developing both eye morphology (dark deposits, ommatidial collapse and fusion, and the absence of ommatidial bristles) and behavioral (motor abnormalities) defects. Acacetins mechanisms of action on transgenic Drosophila model of AD were also determined. The real-time RT-PCR analysis revealed that acacetin reduced both the human APP and BACE-1 mRNA levels in the transgenic flies, suggesting that it plays an important role in the transcriptional regulation of human BACE-1 and APP. Western blot analysis revealed that acacetin reduced Aβ production by interfering with BACE-1 activity and APP synthesis, resulting in a decrease in the levels of the APP carboxy-terminal fragments and the APP intracellular domain. Therefore, the protective effect of acacetin on Aβ production is mediated by transcriptional regulation of BACE-1 and APP, resulting in decreased APP protein expression and BACE-1 activity. Acacetin also inhibited APP synthesis, resulting in a decrease in the number of amyloid plaques. In conclusion, C. longa rhizome-derived curcuminoids and whole A. rugosa plant- derived acacetin are potential therapeutics or lead compounds for the prevention or treatment of AD. The anti-AD action of these compounds provides an indication of at least one of the pharmacological actions of C. longa and A. rugosa. Detailed tests are needed to understand how to improve the anti-AD potency and stability of the compounds isolated from C. longa and A. rugosa for eventual commercial development.