Mutations in the mitochondrial encoded protein PTEN-induced putative kinase 1 (PINK1) cause autosomal recessive Parkinson disease (PD). In mammalian cells, mutant PINK1 has been reported to promote fission or inhibit fusion in mitochondria; however, the mechanism by which this process occurs remains elusive. Using an ecdysone-inducible expression system in mammalian dopaminergic neuronal cells, we report here that human mutant PINK1 (L347P and W437X) mediates an overall fission effect by increasing the ratio of mitochondrial fission over fusion proteins, leading to excessive dysfunctional fragmented mitochondria. Knocking down endogenous Pink1 produces similar effects. In contrast, overexpressing human wild type PINK1 produces a pro-fusion effect by increasing the ratio of mitochondrial fusion/fission proteins without resulting in functionally compromised mitochondria. Parkin knockdown blocks the imbalance in fission/fusion proteins. Furthermore, overexpressing parkin and ubiquitin increases degradation of the mitochondrial fission hFis1 protein, suggesting PINK1 and parkin maintain proper mitochondrial function and integrity via the fission/fusion machinery. Through genetic manipulations and treatment with the small molecule mitochondrial division inhibitor (mdivi-1), which inhibits DLP1/Drp1, both structural and functional mitochondrial defects induced by mutant PINK1 were attenuated, highlighting a potential novel therapeutic avenue for Parkinson disease.The discoveries of mutations in the mitochondrial protein PTEN-induced putative kinase 1 (PINK1) 2 as a cause of autosomal recessive PD (1-3) have fueled the longstanding interest in the role of mitochondrial dysfunction in PD. Currently about 50 PINK1 mutations have been reported (4), making it the second most common causative gene for autosomal recessive PD after parkin, an E3 ubiquitin ligase that has been shown to function downstream of PINK1 and to affect mitochondrial morphology (5-8). Although PINK1 mutations are spread throughout the gene, they are most commonly found in the region encoding the functional serine/threonine kinase domain at the C terminus, leading to loss of PINK1 kinase activity. Recently this kinase domain has been shown to face the cytoplasm (9), providing evidence of spatial proximity to allow this mitochondrial protein to directly interact with cytosolic parkin.Current consensus is that PINK1 is a protective protein. Supporting this role, PINK1 overexpression confers resistance to staurosporine, MPP ϩ , and rotenone toxicity in cultured cells (10, 11) as well as to MPTP-induced dopaminergic neuronal loss in mice (12). Conversely, reducing PINK1 levels by RNAi in cultured cells leads to enhanced cell death in the presence of MPP ϩ and rotenone (12, 13). Mitochondrial defects leading to degeneration of flight muscles and loss of dopaminergic neurons have also been reported in Pink1-deficient Drosophila (5, 14). In recent years, PINK1 has gained significant attention for its role in mitochondrial dynamics (fission, fusion, and migrati...

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