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

Abstract

Parkinson's disease (PD), a progressive neurodegenerative disease, results in abnormal accumulation of insoluble alpha-synuclein (α-Syn) in dopaminergic neurons. Here we examined tauopathic changes and the α-Syn/p-GSK-3β/proteasome pathway in postmortem striata and inferior frontal gyri (IFG) from patients with PD and PD with dementia (PDD). In both PD and PDD, α-Syn levels were high, especially the insoluble form of this protein; in PDD, insoluble α-Syn levels were persistently higher than PD across both brain regions. Levels of p-GSK-3β 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 α-Syn, and decreased proteasomal activity, and is probably due to lack of increase in p-GSK-3β 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.

Introduction

α-Synuclein (α-Syn) is ubiquitously expressed in brain and is highly enriched in presynaptic nerve terminals, where its chief physiological function is the regulation of synaptic levels of monoamine neurotransmitters through modulation of vesicular release (Murphy et al., 2000) and their cognate transporters (Wersinger and Sidhu, 2005, Wersinger et al., 2006a, Wersinger et al., 2006b). Overexpression of α-Syn, through its gene duplication and triplication, is linked to idiopathic Parkinson's disease (PD), while its A30P and A53T mutants cause the autosomal dominant forms of familial PD (Hofer et al., 2004, Krüger et al., 1998, Polymeropoulos et al., 1997). In pathological states, α-Syn becomes misfolded, aggregates and accumulates in neuronal inclusion bodies, Lewy bodies (LBs), seen in PD and other synucleinopathies (Goedert and Spillantini, 1998, Hofer et al., 2004). Tau, a microtubule binding protein, is most commonly linked to Alzheimer's disease (AD) and other tauopathies, where, after hyperphosphorylation, it accumulates in neurons as neurofibrillary tangles [NFTs] (Joachim et al., 1987).

Despite differences in clinical features, pathological and experimental evidence increasingly indicates that there is considerable overlap between tauopathies and synucleinopathies, reinforcing the notion that these diseases may be mechanistically linked. Thus, in AD, 50–60% of patients with NFTs have α-Syn-containing LBs (Lippa et al., 1998, Iseki et al., 1999, Arai et al., 2001, Szpak et al., 2001, Burns et al., 2005, Griffin et al., 2006) and in AD patients with clinically detected extrapyramidal signs, 50% of the patients showed extensive α-Syn pathology colocalized in the substantia nigra with p-Tau (Mori et al., 2002). In PD and in dementia with LBs, costaining of p-Tau has been observed (Duda et al., 2002, Yamaguchi et al., 2005) in 30–40% of the LBs in the nucleus basalis of Meynert and locus coerulus and in 10–30% of LBs in the medulla (Yamaguchi et al., 2005). Extensive overlap in α-Syn and p-Tau pathology has been noted in patients with the A53T mutation (Yamazaki et al., 2000, Kotzbauer et al., 2004), in the Parkinsonism–Dementia complex of Guam (Forman et al., 2002), in dementia with LBs (Yancopoulou et al., 2005), and in familial frontotemporal dementia and progressive aphasia (Hishikawa et al., 2003).

Despite this wealth of pathological information, the molecular and cellular interplay between α-Syn and p-Tau leading to their pathological codeposition is not understood. Under normal physiological conditions, α-Syn is highly soluble. Under pathological conditions, which include oxidative stress and overexpression, the protein becomes insoluble, self-aggregates, and accumulates into LBs (Nemes et al., 2004, Lippa et al., 1998, Iseki et al., 1999, Mori et al., 2002). Similar to α-Syn, Tau is a highly soluble protein that becomes insoluble by pathological hyperphosphorylation at specific sites with ensuing conformational changes and accumulation of the protein into NFTs. Molecular evidence suggests a direct interaction between these proteins, and when incubated together in vitro, α-Syn serves as a seed to accelerate the aggregation of Tau (Kotzbauer et al.,2004).

Using the MPTP mouse neurotoxin model of PD, as well as MPP⁺ cellular models of PD, we recently demonstrated that increases in α-Syn can initiate and sustain Tau hyperphosphorylation both in vivo and in vitro, with coprecipitation of these proteins (Duka et al., 2006, Duka and Sidhu, 2006, Kozikowski et al., 2006, Duka et al., 2009). The hyperphosphorylation of Tau was absolutely dependent on the presence of α-Syn, as indicated by lack of any p-Tau formation in MPTP-treated α-Syn−/− mice or in neuronal cells lacking α-Syn (Duka et al., 2006, Duka et al., 2009). Moreover, we found that upon oxidative stress by MPTP or MPP⁺, α-Syn induced p-Tau formation through specific activation and recruitment of p-GSK-3β, activated by autophosphorylation at Tyr216 (Kozikowski et al., 2006, Duka et al., 2009). GSK-3β is a proline-directed serine/threonine kinase, ubiquitously expressed in mammalian tissues, and epitopes phosphorylated by GSK-3β are among the pathological phosphorylation sites of Tau seen in NFTs and paired helical filaments (PHFs) of Alzheimer's disease, and GSK-3β is a major kinase implicated in Tau hyperphosphorylation (Baum et al., 1995).

To examine the clinical relevance of our previous findings, the current studies were undertaken, and these are the first neurochemical examination of the α-Syn/p-Tau/GSK-3β pathway in human postmortem striata and IFG from PD and PD with dementia (PDD). The results show higher neurodegeneration in PDD compared to PD, with tauopathy restricted to striata of both diseases, along with decreased proteasomal activity in the IFG of PDD. Our studies show that unlike Alzheimer's disease where tauopathy is a more global event, tauopathy in brains of PD and PDD has a restricted expression and is limited to dopaminergic neurons of the nigrostriatal region, possibly due to increased dopamine-linked oxidative stress of the latter region.