The Role of Cysteine String Protein {alpha} Phosphorylation at Serine 10 and 34 by Protein Kinase C{gamma} for Presynaptic Maintenance

Protein kinase C (PKC) knock-out (KO) animals exhibit symptoms of Parkinson's disease (PD), including dopaminergic neuronal loss in the substantia nigra. However, the PKC substrates responsible for the survival of dopaminergic neurons in vivo have not yet been elucidated. Previously, we found 10 potent substrates in the striatum of PKC-KO mice. Here, we focused on cysteine string protein α (CSPα), a protein from the heat shock protein (HSP) 40 cochaperone families localized on synaptic vesicles. We found that in cultured cells, PKC phosphorylates CSPα at serine (Ser) 10 and Ser34. Additionally, apoptosis was found to have been enhanced by the overexpression of a phosphorylation-null mutant of CSPα, CSPα(S10A/S34A). Compared with wild-type (WT) CSPα, the CSPα(S10A/S34A) mutant had a weaker interaction with HSP70. However, in sharp contrast, a phosphomimetic CSPα(S10D/S34D) mutant, compared with WT CSPα, had a stronger interaction with HSP70. In addition, total levels of synaptosomal-associated protein (SNAP) 25, a main downstream target of the HSC70/HSP70 chaperone complex, were found to have decreased by the CSPα(S10A/S34A) mutant through increased ubiquitination of SNAP25 in PC12 cells. In the striatum of 2-year-old male PKC-KO mice, decreased phosphorylation levels of CSPα and decreased SNAP25 protein levels were observed. These findings indicate the phosphorylation of CSPα by PKC may protect the presynaptic terminal from neurodegeneration. The PKC–CSPα–HSC70/HSP70–SNAP25 axis, because of its role in protecting the presynaptic terminal, may provide a new therapeutic target for the treatment of PD.

SIGNIFICANCE STATEMENT Cysteine string protein α (CSPα) is a protein belonging to the heat shock protein (HSP) 40 cochaperone families localized on synaptic vesicles, which maintain the presynaptic terminal. However, the function of CSPα phosphorylation by protein kinase C (PKC) for neuronal cell survival remains unclear. The experiments presented here demonstrate that PKC phosphorylates CSPα at serine (Ser) 10 and Ser34. CSPα phosphorylation at Ser10 and Ser34 by PKC protects the presynaptic terminal by promoting HSP70 chaperone activity. This report suggests that CSPα phosphorylation, because of its role in modulating HSP70 chaperone activity, may be a target for the treatment of neurodegeneration.