Cdk5 phosphorylation‐dependent C9orf72 degradation promotes neuronal death in Parkinson's disease models

Abstract Aims Chromosome 9 open reading frame 72 (C9orf72) is one of the most dazzling molecules in neurodegenerative diseases, albeit that its role in Parkinson's disease (PD) remains unknown. This article aimed to explore the potential mechanism of C9orf72 involved in the pathogenesis of PD. Methods The expression and phosphorylation levels of C9orf72 were examined by Western blotting, RT‐PCR, and immunoprecipitation using PD models. Multiple bioinformatics software was used to predict the potential phosphorylation sites of C9orf72 by Cdk5, followed by verification of whether Cdk5‐inhibitor ROSCOVITINE could reverse the degradation of C9orf72 in PD. By constructing the sh‐C9orf72‐knockdown adenovirus and overexpressing the FLAG‐C9orf72 plasmid, the effects of C9orf72 knockdown and overexpression, respectively, were determined. A short peptide termed Myr‐C9orf72 was used to verify whether interfering with Cdk5 phosphorylation at the Ser9 site of the C9orf72 protein could alleviate autophagy disorder, neuronal death, and movement disorder in PD models. Results The expression level of the C9orf72 protein was significantly reduced, albeit the mRNA expression was not changed in the PD models. Moreover, the phosphorylation level was enhanced, and its reduction was mainly degraded by the ubiquitin‐proteasome pathway. The key nervous system kinase Cdk5 directly phosphorylated the S9 site of the C9orf72 protein, which promoted the degradation of the C9orf72 protein. The knockdown of C9orf72 aggravated autophagy dysfunction and increased neuronal loss and motor dysfunction in substantia nigra neurons of PD mice. The overexpression of C9orf72 alleviated autophagy dysfunction in PD neurons. Specifically, interference with Cdk5 phosphorylation at the S9 site of C9orf72 alleviated autophagy dysfunction, neuronal death, and motor dysfunction mediated by C9orf72 protein degradation during PD. Conclusions Cumulatively, our findings illustrate the importance of the role of C9orf72 in the regulation of neuronal death during PD progression via the Cdk5‐dependent degradation.