GSK-3β dysregulation contributes to parkinson's-like pathophysiology with associated region-specific phosphorylation and accumulation of tau and α-synuclein

Aberrant posttranslational modifications (PTMs) of proteins, namely phosphorylation, induce abnormalities in the biological properties of recipient proteins, underlying neurological diseases including Parkinson's disease (PD). Genome-wide studies link genes encoding alpha-synuclein (alpha-Syn) and Tau as two of the most important in the genesis of PD. Although several kinases are known to phosphorylate alpha-Syn and Tau, we focused our analysis on GSK-3 beta because of its accepted role in phosphorylating Tau and to increasing evidence supporting a strong biophysical relationship between alpha-Syn and Tau in PD. Therefore, we investigated transgenic mice, which express a point mutant (S9A) of human GSK-3 beta. GSK-3 beta-S9A is capable of activation through endogenous natural signaling events, yet is unable to become inactivated through phosphorylation at serine-9. We used behavioral, biochemical, and in vitro analysis to assess the contributions of GSK-3 beta to both alpha-Syn and Tau phosphorylation. Behavioral studies revealed progressive age-dependent impairment of motor function, accompanied by loss of tyrosine hydroxylase-positive (TH+ DA-neurons) neurons and dopamine production in the oldest age group. Magnetic resonance imaging revealed deterioration of the substantia nigra in aged mice, a characteristic feature of PD patients. At the molecular level, kinase-active p-GSK-3 beta-Y216 was seen at all ages throughout the brain, yet elevated levels of p-alpha-Syn-S129 and p-Tau (S396/404) were found to increase with age exclusively in TH+ DA-neurons of the midbrain. p-GSK-3 beta-Y216 colocalized with p-Tau and p-alpha-Syn-S129. In vitro kinase assays showed that recombinant human GSK-3 beta directly phosphorylated alpha-Syn at a single site, Ser129, in addition to its known ability to phosphorylate Tau. Moreover, alpha-Syn and Tau together cooperated with one another to increase the magnitude or rate of phosphorylation of the other by GSK-3 beta. Together, these data establish a novel upstream role for GSK-3 beta as one of several kinases associated with PTMs of key proteins known to be causal in PD.