Effects of cinnamon extract on glycogen synthase kinase 3 phosphorylation of tau

Abstract

Recently correlations have been suggested between abnormal glucose metabolism and the development of Alzheimer's disease. These suggested links stem from the similar portions of the insulin signaling cascade in muscle, adipose tissue, and brain. Although the downstream targets of the insulin signaling pathway differ in these three tissues, the initial signals remain the same. Compounds in cinnamon have been shown to reduce blood glucose levels by functioning in what is typically referred to as an “insulin mimic” fashion. Cinnamon has shown increases in expression levels of some glucose transporters (GLUTs) as well as an increase in GLUT translocation to the surface of the cell membrane. Observations of both of these effects suggests that cinnamon effects multiple cellular processes, potentially insulin signaling and microtubule mediated translocation of the GLUTs. Studies have also demonstrated that cinnamon extract inhibits tau aggregation, a process highly affected by the phosphorylation of tau protein. It is believes that the aggregation of tau affects the formation and function of the microtubules, thus eliminating the ability to effectively translocate damaged proteins included amyloid beta aggregates. Research has suggested that glycogen synthase kinase 3 (GSK3) is one of the enzymes responsible for phosphorylation of the tau protein found in microtubules. In order to determine is the compounds in cinnamon alter the GSK3 dependent phosphorylation of the tau protein in vivo, HEK293 cells were transfected with GSK3 and tau using transfection reagent. The cells were grown in the presence and absence of cinnamon extract for 48 hours. Following growth and treatment, cells were lysed and fractionated, run on an SDS‐Page gel, transferred and Western blotted with antibodies reactive with phosphorylated tau and non‐phosphorylated tau. In order to determine the effects of cinnamon treatment on tau aggregation, protein isolates of the cells were collected, immune‐precipitated in order to isolate tau protein. The isolated protein was then solubilized, disaggregated by short incubations in a sonicating bath, and then allowed to re‐aggregate with addition of heparin or other aggregation agent. Aggregation was monitored at 350 nm spectrophotometrically over several hours. For comparison, recombinant tau was reacted with heparin in the presence or absence of cinnamon extract and monitored spectrophotometrically. Results suggest that cinnamon extract plays a role in minimizing GSK3 phosphorylation of tau, perhaps by limiting activation of GSK3 by protein kinase A dependent phosphorylation.