Proteolytic activation of proapoptotic kinase protein kinase Cδ by tumor necrosis factor a death receptor signaling in dopaminergic neurons during neuroinflammation

Background: The mechanisms of progressive dopaminergic neuronal loss in Parkinson's disease (PD) remain poorly understood, largely due to the complex etiology and multifactorial nature of disease pathogenesis. Several lines of evidence from human studies and experimental models over the last decade have identified neuroinflammation as a potential pathophysiological mechanism contributing to disease progression. Tumor necrosis factor a (TNF) has recently emerged as the primary neuroinflammatory mediator that can elicit dopaminergic cell death in PD. However, the signaling pathways by which TNF mediates dopaminergic cell death have not been completely elucidated. Methods: In this study we used a dopaminergic neuronal cell model and recombinant TNF to characterize intracellular signaling pathways activated during TNF-induced dopaminergic neurotoxicity. Etanercept and neutralizing antibodies to tumor necrosis factor receptor 1 (TNFR1) were used to block TNF signaling. We confirmed the results from our mechanistic studies in primary embryonic mesencephalic cultures and in vivo using the stereotaxic lipopolysaccharide (LPS) model of nigral dopaminergic degeneration. Results: TNF signaling in dopaminergic neuronal cells triggered the activation of protein kinase C delta (PKC delta), an isoform of the novel PKC family, by caspase-3 and caspase-8 dependent proteolytic cleavage. Both TNFR1 neutralizing antibodies and the soluble TNF receptor Etanercept blocked TNF-induced PKC delta proteolytic activation. Proteolytic activation of PKC delta was accompanied by translocation of the kinase to the nucleus. Notably, inhibition of PKC delta signaling by small interfering (si)RNA or overexpression of a PKC delta cleavage-resistant mutant protected against TNF-induced dopaminergic neuronal cell death. Further, primary dopaminergic neurons obtained from PKC delta knockout (-/-) mice were resistant to TNF toxicity. The proteolytic activation of PKC delta in the mouse substantia nigra in the neuroinflammatory LPS model was also observed. Conclusions: Collectively, these results identify proteolytic activation of PKC delta proapoptotic signaling as a key downstream effector of dopaminergic cell death induced by TNF. These findings also provide a rationale for therapeutically targeting PKC delta to mitigate progressive dopaminergic degeneration resulting from chronic neuroinflammatory processes.