Protein Kinase C-delta in dopaminergic system and experimental models of Parkinson's disease

Parkinson’s disease (PD) is a major neurodegenerative disorder characterized by progressive and substantial loss of dopaminergic neurons in the substantia nigra compacta (SNc). Currently, no available drugs prevent the progressive loss of nigral dopaminergic neurons. The mechanisms underlying the dopaminergic degenerative process observed in PD are not well understood, which has hampered development of successful neuroprotective drugs. Tyrosine hydroxylase (TH) is the rate-limiting enzyme in dopamine synthesis. Severely reduced TH positive neurons and TH fibers in dopaminergic terminal fields of PD patients and the successful application of L-DOPA therapy for Parkinson's disease suggest that this enzyme has a primary role in the progression of this disease. Previously, we found that the caspase-3 mediated proteolytic activation of Protein Kinase C δ (PKCδ), a member of the novel PKC isoform family, plays a critical role in oxidative stress-induced dopaminergic cell death in cell culture models of PD. In the present study, we report a novel interaction between PKCδ and TH, in which the kinase modulates dopamine synthesis by negatively regulating TH activity via protein phosphatase 2A (PP2A). We observed that PKCδ is highly expressed in nigral dopaminergic neurons and co-localizes with TH in the mouse brain. Interestingly, suppression of PKCδ activity with the kinase inhibitor rottlerin, PKCδ-siRNA, or with PKCδ dominant negative mutant effectively increased a number of key biochemical events in the dopamine pathway, including TH-ser40 phosphorylation, TH enzymatic activity, and dopamine synthesis in neuronal cell culture models. Additionally, we found that PKCδ not only physically associates with the PP2A catalytic subunit (PP2Ac) but also phosphorylates the phosphatase to increase its activity. Notably, inhibition of PKCδ reduced the dephosphorylation activity of PP2A and thereby increased TH-ser40 phosphorylation, TH activity, and dopamine synthesis. To further validate our findings, we used the PKCδ knockout (PKCδ -/-) mouse model. Consistent with other results, we found greater TH-ser40 phosphorylation and reduced PP2A activity in the substantia nigra of PKCδ -/mice than wild-type mice. Importantly, this was accompanied by an increased dopamine level in the striatum of PKCδ -/mice. Taken together, these results suggest that PKCδ phosphorylates PP2Ac to enhance its activity, and thereby reduces