Elevated tauopathy and alpha-synuclein pathology in postmortem Parkinson's disease brains with and without dementia

Parkinson's disease (PD), a progressive neurodegenerative disease, results in abnormal accumulation of insoluble alpha-synuclein (alpha-Syn) in dopaminergic neurons. Here we examined tauopathic changes and the alpha-Syn/p-GSK-3beta/proteasome pathway in postmortem striata and inferior frontal gyri (IFG) from patients with PD and PD with dementia (PDD). In both PD and PDD, alpha-Syn levels were high, especially the insoluble form of this protein; in PDD, insoluble alpha-Syn levels were persistently higher than PD across both brain regions. Levels of p-GSK-3beta phosphorylated at Tyr 216, which hyperphosphorylates Tau to produce toxic pathological forms of p-Tau, were higher in striata of both PD and PDD compared to controls, but were unaltered in IFG. While proteasomal activity was unchanged in striatum of PD and PDD, such activity was diminished in the IFG of both PD and PDD. A decrease in 19S subunit of the proteasomes was seen in IFG of PDD, while lower levels of 20S subunits were seen in striatum and IFG of both PD and PDD patients. Parkin levels were similar in PD and PDD, suggesting lack of involvement of this protein. Most interestingly, tauopathic changes were noted only in striatum of PD and PDD, with increased hyperphosphorylation seen at Ser262 and Ser396/404; increases in Ser202 levels were seen only in PD but not in PDD striatum. We were unable to detect any tauopathy in IFG in either PD or PDD despite increased levels of alpha-Syn, and decreased proteasomal activity, and is probably due to lack of increase in p-GSK-3beta in IFG. Unlike Alzheimer's disease where tauopathy is more globally observed in diverse brain regions, our data demonstrates restricted expression of tauopathy in brains of PD and PDD, probably limited to dopaminergic neurons of the nigrostriatal region.