We show here for the first time that an elevated state of tauopathy exists in the brain stem of the PDGF-α-Syn overexpressing mice, where high levels of p-Ser202, p-Ser262 and p-Ser396/404. This was accompanied by lower levels of tauopathy in the hippocampus. In cerebellum and frontal cortex, increases were seen for only p-Ser396/404, but not for p-Ser202 or p-Ser262. The severity of tauopathy, in general, is linked to parallel increases in α-Syn. Thus, in the brain stem, where we observed the most severe tauopathy, we also observed the highest increases in α-Syn levels. By contrast, other brain regions that showed lower increases in α-Syn levels were also characterized by lower levels of tauopathy. Our earlier studies have shown a tight linkage between α-Syn and p-Tau levels, and we have shown a mandatory requirement for α-Syn in tauopathy. Thus, in α-Syn knock-out animals or in cells lacking α-Syn , we fail to observe any tauopathic changes upon treatment with PD-inducing neurotoxins. In AD, it is long believed that the amyloid protein, β-amyloid, triggers the cascade of events that leads to generation of p-Tau. Similarly in the context of PD, emerging evidence from our laboratory has identified α-Syn to be the amyloid protein triggering tauopathy.
There have been only limited studies identifying the sites of hyperphosphorylation of Tau in PD brains. In one of these, Tau was found to be hyperphosphorylated at Ser396/404 in synaptic fractions from frontal cortex of PD postmortem striata . Studies from our own laboratory have shown that Tau in striata of PD patients is hyperphosphorylated at Ser202, Ser262 and Ser396/404, where increases of 23, 34 and 81% were observed in PD compared to control, non-diseased striata , testifying to the pathophysiological relevance of these sites. There are more than 45 sites of hyperphosphorylation of Tau identified in AD  and the molecular consequences of only a few of these sites are known. Tau is primarily located along axons and, in general, hyperphosphorylation of Tau reduces its affinity for microtubules [MTs], leading to their destabilization, with eventual degeneration of neurons. Hyperphosphorylation at the microtubule MT binding domain [residues 244-368] of Tau is especially crucial in regulating MT stabilization, and phosphorylation at Ser262 detaches Tau from MTs, resulting in their destabilization . Hyperphosphorylation at Ser396/404 promotes self assembly of Tau to form aggregates of Tau . Moreover, in vitro studies using Tau peptides showed that phosphorylation of Tau at Ser262 and Ser356 modified both the negative charge and the local conformation near the phosphorylation sites, reducing the affinity of the peptides to bind to MTs . Therefore, our finding of very high levels of p-Ser262 in the brain stem may explain the increased levels of soluble α-tubulin seen in this brain region. By contrast, the other brain regions had either no [frontal cortex and cerebellum] or low increases in p-Ser262 levels [hippocampus], and, therefore, also had no changes in soluble α-tubulin.
The pattern of tauopathy observed in this mouse model of PD closely parallels the Braak synucleinopathic staging scheme of Parkinson's disease initially proposed by Braak et al. . Thus, it has been proposed that lesions initially occur in the glossopharyngeal and vagal nerves and in the anterior olfactory nucleus, progressing to the brain stem, and thereafter pursuing an ascending course of pathology . Cortical involvement follows, beginning with the anteromedial temporal mesocortex, followed by the neocortex, especially the prefrontal region. The concept proposed by Braak and colleagues  that lower brainstem synucleinopathy represents "early PD", subsequently progressing within the human lifetime to involve the mesencephalon, suggests that the brain stem may be more severely affected than dopaminergic neurons, and this is borne out by the higher degree of tauopathy seen in the brain stem compared to the striatum in the PDGF-α-Syn overexpressing mice. Indeed, our findings of higher levels of soluble α-tubulin in the brain stem of the transgenic mouse as compared to the wild type also occurs in parallel with the higher levels of synucleinopathy seen in this brain region, suggesting that the brain stem is undergoing more degeneration than any other brain region tested.
Unlike our previous findings, the current study does not indicate any involvement of p-GSK-3β in the accumulation of p-Tau in brain regions other than the striatum. Previously, we had found that α-Syn could recruit and activate p-GSK-3β in a specific manner in the striatum [32, 35] and that such activation of p-GSK-3β was dependent on autoxidation of dopamine . The current study suggests that in non-dopaminergic neurons, other kinases may instead become activated. Thus, our data suggest that p-ERK and p-JNK, but not p-p38MAPK, become activated. To our knowledge, this is the first report of a possible involvement of p-ERK and p-JNK in the genesis of tauopathy in PD. Thus, increased levels of p-ERK are seen in brain stem, hippocampus and frontal cortex, but not in cerebellum, indicating the possible participation of this kinase in the first three brain regions mentioned. Indeed, in both PD striata and in cellular as well as animal models of PD, ongoing studies in our laboratory suggest an important and central role for p-ERK in the pathogenesis of PD [Duka & Sidhu, Unpublished Observations]. Since p-JNK is activated in cerebellum, it is likely that the increase in p-Ser396/404 seen here proceeds through p-JNK. By contrast, the increase in tauopathy seen in hippocampus is likely to occur entirely through p-ERK and not p-JNK, since p-JNK is not activated in this region. In brain stem, however, the high levels of both p-ERK and p-JNK may together contribute to the very high levels of tauopathy seen in this region. It should be noted, however, that our results do not eliminate the possibility that other kinases may also participate in hyperphosphorylation of Tau, either in concert with, or entirely independent of, p-ERK and p-JNK.
It is now well established that overexpression of α-Syn, through its gene duplication and triplication, is linked to idiopathic Parkinson's disease [43, 9], although the mechanism[s] by which PD occurs in such populations remains elusive. The PDGF-α-Syn overexpressing mouse model closely mimics sporadic PD  and our data show that at least part of the mechanism may be due to tauopathic changes in not only the striatum of these mice  but also in other brain regions, such as the brain stem, hippocampus, frontal cortex and cerebellum. Therapy aimed at reducing the overall tauopathic burden may be especially useful in alleviating PD.