Interaction of cinnamaldehyde and epicatechin with tau: implications of beneficial effects in modulating Alzheimer's disease pathogenesis.

Abnormal modifications in tau such as hyperphosphorylation, oxidation, and glycation interfere with its interaction with microtubules leading to its dissociation and self-aggregation into neurofibrillary tangles, a hallmark of Alzheimer's disease (AD). Previously we reported that an aqueous extract of cinnamon has the ability to inhibit tau aggregation in vitro and can even induce dissociation of tangles isolated from AD brain. In the present study, we carried out investigations with cinnamaldehyde (CA) and epicatechin (EC), two components of active cinnamon extract. We found that CA and the oxidized form of EC (ECox) inhibited tau aggregation in vitro and the activity was due to their interaction with the two cysteine residues in tau. Mass spectrometry of a synthetic peptide, SKCGS, representing the actual tau sequence, identified the thiol as reacting with CA and ECox. Use of a cysteine double mutant of tau showed the necessity of cysteine for aggregation inhibition by CA. The interaction of CA with tau cysteines was reversible and the presence of CA did not impair the biological function of tau in tubulin assembly in vitro. Further, these compounds protected tau from oxidation caused by the reactive oxygen species, H2O2, and prevented subsequent formation of high molecular weight species that are considered to stimulate tangle formation. Finally, we observed that EC can sequester highly reactive and toxic byproducts of oxidation such as acrolein. Our results suggest that small molecules that form a reversible interaction with cysteines have the potential to protect tau from abnormal modifications.