Elevated Ubiquitin Phosphorylation by PINK1 Contributes to Proteasomal Impairment and Promotes Neurodegeneration

Ubiquitin (Ub), a key player of protein turnover, can be phosphorylated by PINK1 kinase to generate S65-phosphorylated ubiquitin (pUb). Elevated pUb levels have been observed in aged human brains and human brains with Parkinson’s disease. However, how pUb is involved in neurodegeneration remains elusive. Here we show that elevation of pUb is pervasive in a multitude of neurodegenerative conditions, including Alzheimer’s disease, aging, and ischemic injury. In cultured cells, proteasomal inhibition by MG132 leads to sPINK1 accumulation, the cytosolic fragment of PINK1, thus promoting Ub phosphorylation. Elevated pUb impairs proteasomal degradation by disrupting covalent ubiquitin chain elongation and noncovalent proteasome-substrate interaction. Conversely, pink1 knockout mitigates protein aggregation in both aging and ischemic mouse brains, as well as cells treated with MG132. Using AAV2/9 vector to specifically express sPINK1 in mouse hippocampus neurons, we observed cumulative pUb elevation, accompanied by protein aggregation, proteostasis disturbance, neuronal injury, neuroinflammation, and cognitive impairment. These sPINK1-induced impairments were reversed by co-expressing Ub/S65A phospho-null Ub mutant but exacerbated by Ub/S65E phospho-mimic mutant. As such, pUb elevation can result from declined proteasomal activity in neurodegenerative conditions, while a constant elevation of pUb actively drives neurodegeneration by further inhibiting proteasomal degradation. Our study reveals a new pathogenic pathway of neurodegeneration, highlighting the pUb-mediated feedforward loop as a promising therapeutic target for pharmaceutical intervention.