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Damage to mitochondria can lead to the depolarization of the inner mitochondrial membrane, thereby sensitizing impaired mitochondria for selective elimination by autophagy. However, fusion of uncoupled mitochondria with polarized mitochondria can compensate for damage, reverse membrane depolarization, and obviate mitophagy. Parkin, an E3 ubiquitin ligase that is mutated in monogenic forms of Parkinson's disease, was recently found to induce selective autophagy of damaged mitochondria. Here we show that ubiquitination of mitofusins Mfn1 and Mfn2, large GTPases that mediate mitochondrial fusion, is induced by Parkin upon membrane depolarization and leads to their degradation in a proteasome- and p97-dependent manner, p97, a AAA+ ATPase, accumulates on mitochondria upon uncoupling of Parkin-expressing cells, and both p97 and proteasome activity are required for Parkin-mediated mitophagy. After mitochondrial fission upon depolarization, Parkin prevents or delays refusion of mitochondria, likely by the elimination of mitofusins. Inhibition of Drp1-mediated mitochondrial fission, the proteasome, or p97 prevents Parkin-induced mitophagy. [ABSTRACT FROM AUTHOR]

PTEN-induced novel kinase 1 (PINK1) mutations are associated with autosomal recessive parkinsonism. Previous studies have shown that PINK1 influences both mitochondrial function and morphology although it is not clearly established which of these are primary events and which are secondary. Here, we describe a novel mechanism linking mitochondrial dysfunction and alterations in mitochondrial morphology related to PINK1. Cell lines were generated by stably transducing human dopaminergic M17 cells with lentiviral constructs that increased or knocked down PINK1. As in previous studies, PINK1 deficient cells have lower mitochondrial membrane potential and are more sensitive to the toxic effects of mitochondrial complex I inhibitors. We also show that wild-type PINK1, but not recessive mutant or kinase dead versions, protects against rotenone-induced mitochondrial fragmentation whereas PINK1 deficient cells show lower mitochondrial connectivity. Expression of dynamin-related protein 1 (Drp1) exaggerates PINK1 deficiency phenotypes and Drp1 RNAi rescues them. We also show that Drp1 is dephosphorylated in PINK1 deficient cells due to activation of the calciumdependent phosphatase calcineurin. Accordingly, the calcineurin inhibitor FK506 blocks both Drp1 dephosphorylation and loss of mitochondrial integrity in PINK1 deficient cells but does not fully rescue mitochondrial membrane potential. We propose that alterations in mitochondrial connectivity in this system are secondary to functional effects on mitochondrial membrane potential. [ABSTRACT FROM AUTHOR]

Summary: In mammals, fusion of the mitochondrial outer membrane is controlled by two DRPs, MFN1 and MFN2, that function in place of a single outer membrane DRP, Fzo1 in yeast. We addressed the significance of two mammalian outer membrane fusion DRPs using an in vitro mammalian mitochondrial fusion assay. We demonstrate that heterotypic MFN1-MFN2 trans complexes possess greater efficacy in fusion as compared to homotypic MFN1 or MFN2 complexes. In addition, we show that the soluble form of the proapoptotic Bcl2 protein, Bax, positively regulates mitochondrial fusion exclusively through homotypic MFN2 trans complexes. Together, these data demonstrate functional and regulatory distinctions between MFN1 and MFN2 and provide insight into their unique physiological roles. [ABSTRACT FROM AUTHOR]