Several lines of evidence suggests that mitochondrial dysfunction is strongly implicated in the etiology of the neurodegenerative disease. PINK1-PRKN/Parkin axis plays a crucial role in mitochondrial quality control through PINK1-dependent phosphorylation of both Parkin and its ubiquitin.

Deubiquitinating enzymes (DUBs) functionally reversing PRKN ubiquitination are critical in controlling the magnitude of PRKN-mediated mitophagy process. However, potential DUBs that directly target PRKN and antagonize its pro-mitophagy effect remain to be identified and characterized.

In a recent study published in Autophagy, a research group led by Prof. ZHAO Yongliang from Beijing Institute of Genomics of the Chinese Academy of Sciences revealed that USP33, one of DUB family, deubiquitinates PRKN on depolarized mitochondria, and USP33 silencing protects SH-SY5Y human neuroblastoma cells from the neurotoxin MPTP-induced apoptotic cell death.

The researchers first demonstrated that USP33/VDU1 is localized at the outer membrane of mitochondria and interacts with USP33, and their interaction is substantially enhanced on depolarized mitochondria.

Further cellular and in vitro assays showed that USP33 deubiquitinates PRKN mainly at Lys435 and prefers to remove K6, K11, K48 and K63-linked ubiquitin conjugates from PRKN. K48- and K63-linked Ub chains mainly target the proteins for proteasome degradation and diverse signaling pathways, respectively. USP33 deficiency enhanced both K48- and K63-linked PRKN ubiquitination, but only K63-linked PRKN ubiquitination was significantly increased under mitochondrial depolarization.

Moreover, the researchers found that USP33 knockdown promotes the recruitment of PRKN to depolarized mitochondria, which leads to an enhanced mitophagy versus a significantly decreased apoptotic cell death induced by a neurotoxin MPTP.

These findings revealed a novel regulatory mechanism of deubiquitinase USP33 in controlling the magnitude of PRKN-mediated mitophagy process, and offered the important insights into dynamic balance of ubiquitination and deubiquitination in the maintenance of mitochondrial self-homeostasis.

In particular, it provided important basis for the potential biological significance of mitophagy process and a potentially valuable strategy for treatment of the neurodegenerative disease.

Model depicting the antagonizing effect of USP33 on PRKN-mediated mitophagy. (Image by ZHAO Yongliang's lab)