382121 Green and Black Tea Polyphenols Mechanistically Inhibit Amyloid-β Aggregation in Alzheimer's Disease
Alzheimer’s disease (AD) is currently the 6th leading cause of death and is the only disease among the top 10 that cannot be prevented, cured or treated. The amyloid cascade hypothesis states that the cause of the disease is the deposition of plaques comprised of insoluble amyloid-β (Aβ) fibrils in the brain. Naturally occurring Aβ monomer can aggregate via a nucleation dependent pathway in which Aβ monomer forms nuclei that conjoin to create soluble aggregates. These soluble aggregates are then lengthened through elongation and thickened through association to form insoluble fibrils. Research has shown that Aβ aggregates are neurotoxic. Consequently, inhibition of Aβ aggregation is one therapeutic strategy for AD. Epidemiological studies have correlated consumption of green tea, and to a lesser extent black tea, with a reduced incidence of AD. This study sought to explain this correlation by identifying the Aβ aggregation inhibitory capabilities of key polyphenol components in green and black tea.
Polyphenols studied include epicatechin, epigallocatechin and epigallocatechin gallate found in green tea and theaflavin and theaflavin monogallate found in black tea. Four different experiments were used to target unique steps along the aggregation pathway: a monomer aggregation assay to monitor the overall aggregation; an oligomerization assay to monitor the nucleation step; an association assay to monitor the late stage lateral binding of soluble aggregates; and an elongation assay to monitor the late stage lengthening of soluble aggregates. Monomer aggregation is induced via agitation in aqueous Tris-HCl buffer and is monitored using intensity of dynamic light scattering (DLS) and dot blot analysis with aggregate conformation specific antibody LOC. Oligomerization is initiated via the dilution of DMSO-solubilized Aβ in PBS and is monitored via SDS-PAGE and Western blotting to observe changes in oligomer size. Association and elongation are initiated by incubating SEC-purified Aβ soluble aggregates with salt or protein monomer, respectively, and are monitored for increases in aggregate size using DLS.
All catechin compounds exhibited moderate to slight inhibition of the overall aggregation, while the theaflavin compounds exhibited moderate to complete inhibition. To gain further insight into the process of inhibition, the mechanistic steps of aggregation, oligomerization, association and elongation, were examined individually. Theaflavins but not catechins were found to inhibit oligomerization. Conversely, catechins were more effective inhibitors of association than theaflavins. Both catechins and theaflavins showed similar inhibitory capabilities toward elongation. Together, these results demonstrate that while catechins and theaflavins can both inhibit Aβ aggregation, the mechanisms of action of these compounds is distinct.
Catechins and theaflavins show different inhibitory capabilities at varying mechanistic steps of the Aβ monomer aggregation pathway. Catechins affect only the later stages of aggregation, suggesting that the catechins may bind a specific structure present in aggregates. Conversely, theaflavins show inhibitory capabilities at every stage of the Aβ monomer aggregation pathway, alluding to a sequence specific recognition. Furthermore, better inhibitory capabilities, for both polyphenol categories, can be correlated with the number of gallate groups.