Molecular mechanisms of PKC[delta] in neurotoxin-induced apoptotic death of nigral dopaminergic neurons: relevance to the pathogenesis of Parkinson's disease
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We investigated the molecular role of PKCdelta in neurotoxic models of Parkinson's disease (PD). Both the protein and mRNA levels of PKCdelta were selectively higher in the dopaminergic neurons in the substantia nigra (SN) region of rodent brains. siRNA designed against PCKdelta effectively silenced the expression of PKCdelta and reduced dieldrin (a neurotoxic pesticide) or MPP+ (Parkinsonian toxin)-induced apoptotic death in rat mesencephalic dopaminergic neuronal (N27) cells and degeneration of primary mesencephalic dopaminergic neurons. Following dieldrin exposure in N27 cells, we determined that the full-length PKCdelta is first cleaved in the cytoplasm, and the activated PKCdelta catalytic fragment subsequently translocates into the nucleus. Blockade of proteolytic activation of PKCdelta by PKCdelta-CRM also protected dopaminergic neurons from MPP+-induced degeneration. Moreover, using PKCdelta-CF, PKCdelta-CRM, PKCdelta-DeltaNLS constructs, we demonstrated that activated cleaved PKCdelta mediates Ser14 phosphorylation of histone H2B followed by its nuclear translocation in apoptotic dopaminergic neuronal cells. Additionally, we also demonstrated that the N- and the C-terminals of PKCdelta are in close proximity in the tertiary structure, as demonstrated by fluorescence resonance energy transfer (FRET). By employing the C1 or C2-like domain deletion mutant of PKCdelta), we showed that deletion of the C2-like domain, but not the C1 domain, induces strong nuclear localization of PKCdelta. Moreover, the C2-like domain alone was fused to the N-terminal of a NES mutated form of Rev protein to generate the Rev chimera, which is still primarily localized in the nucleus. Thus, the N-terminal C2-like domain apparently regulates the subcellular localization of PKCdelta by masking the C-terminal NLS through the position obstacle in the tertiary structure, but not through NES or interaction with anchoring proteins. Taken together, our studies show that (i) the novel PKC isoform family member PKCdelta is highly expressed in nigral dopaminergic neurons, (ii) PKCdelta mainly localizes in the cytoplasm by the position obstacle effect of the C2-like domain to the NLS, (iii) neurotoxin exposure induces proteolytic activation of full-length PKCdelta and subsequent nuclear translocation of the PKCdelta cleaved fragment to cause histone H2B phosphorylation and apoptotic cell death, and (iv) RNAi mediated suppression of PKCdelta protects dopaminergic neurons against neurotoxic insults.