Derek P. Narendra

Parkin is recruited selectively to impaired mitochondria and promotes their autophagy

Loss-of-function mutations in Park2, the gene coding for the ubiquitin ligase Parkin, are a significant cause of early onset Parkinsons disease. Although the role of Parkin in neuron maintenance is unknown, recent work has linked Parkin to the regulation of mitochondria. Its loss is associated with swollen mitochondria and muscle degeneration in Drosophila melanogaster, as well as mitochondrial dysfunction and increased susceptibility to mitochondrial toxins in other species. Here, we show that Parkin is selectively recruited to dysfunctional mitochondria with low membrane potential in mammalian cells. After recruitment, Parkin mediates the engulfment of mitochondria by autophagosomes and the selective elimination of impaired mitochondria. These results show that Parkin promotes autophagy of damaged mitochondria and implicate a failure to eliminate dysfunctional mitochondria in the pathogenesis of Parkinsons disease.

PINK1 Is Selectively Stabilized on Impaired Mitochondria to Activate Parkin

Mutations in PINK1 or Parkin lead to familial parkinsonism. The authors suggest that PINK1 and Parkin form a pathway that senses damaged mitochondria and selectively targets them for degradation.

Mechanisms of mitophagy

Autophagy not only recycles intracellular components to compensate for nutrient deprivation but also selectively eliminates organelles to regulate their number and maintain quality control. Mitophagy, the specific autophagic elimination of mitochondria, has been identified in yeast, mediated by autophagy-related 32 (Atg32), and in mammals during red blood cell differentiation, mediated by NIP3-like protein X (NIX; also known as BNIP3L). Moreover, mitophagy is regulated in many metazoan cell types by parkin and PTEN-induced putative kinase protein 1 (PINK1), and mutations in the genes encoding these proteins have been linked to forms of Parkinsons disease.

Proteasome and p97 mediate mitophagy and degradation of mitofusins induced by Parkin

The Parkin ubiquitin ligase marks the mitofusins Mfn1 and Mfn2 for proteasome-dependent degradation, promoting disposal of damaged mitochondria by preventing their fusion with healthy organelles.

Mitochondrial membrane potential regulates PINK1 import and proteolytic destabilization by PARL

Differential localization to the inner and outer mitochondrial membranes regulates PINK1 stability and function.

p62/SQSTM1 is required for Parkin-induced mitochondrial clustering but not mitophagy; VDAC1 is dispensable for both

Mitochondria sustain damage with aging, and the resulting mitochondrial dysfunction has been implicated in a number of diseases including Parkinson disease. We recently demonstrated that the E3 ubiquitin ligase Parkin, which is linked to recessive forms of parkinsonism, causes a dramatic increase in mitophagy and a change in mitochondrial distribution, following its translocation from the cytosol to mitochondria. Investigating how Parkin induces these changes may offer insight into the mechanisms that lead to the sequestration and elimination of damaged mitochondria. We report that following Parkin’s translocation from the cytosol to mitochondria, Parkin (but not a pathogenic mutant) promotes the K63-linked polyubiquitination of mitochondrial substrate(s) and recruits the ubiquitin- and LC3-binding protein, p62/SQSTM1, to mitochondria. After its recruitment, p62/SQSTM1 mediates the aggregation of dysfunctional mitochondria through polymerization via its PB1 domain, in a manner analogous to its aggregation of polyubiquitinated proteins. Surprisingly and in contrast to what has been recently reported for ubiquitin-induced pexophagy and xenophagy, p62 appears to be dispensable for mitophagy. Similarly, mitochondrial-anchored ubiquitin is sufficient to recruit p62 and promote mitochondrial clustering, but does not promote mitophagy. Although VDAC1 (but not VDAC2) is ubiquitinated following mitochondrial depolarization, we find VDAC1 cannot fully account for the mitochondrial K63-linked ubiquitin immunoreactivity observed following depolarization, as it is also observed in VDAC1/3-/- mouse embryonic fibroblasts. Additionally, we find VDAC1 and VDAC3 are dispensable for the recruitment of p62, mitochondrial clustering and mitophagy. These results demonstrate that mitochondria are aggregated by p62, following its recruitment by Parkin in a VDAC1-independent manner. They also suggest that proteins other than p62 are likely required for mitophagy downstream of Parkin substrates other than VDAC1.

Targeting Mitochondrial Dysfunction: Role for PINK1 and Parkin in Mitochondrial Quality Control

Mitochondria, which convert energy for the cell, accumulate damage with age, and the resulting mitochondrial dysfunction has been linked to the development of degenerative diseases and aging. To curb the accumulation of damaged mitochondria, the cell has elaborated a number of mitochondrial quality control processes. We describe recent work suggesting that Parkin and PTEN-induced putative kinase 1 (PINK1), two gene products linked to familial forms of parkinsonism, may constitute one of the cells mitochondrial quality control pathways-identifying impaired mitochondria and selectively trimming them from the mitochondrial network by mitophagy. In particular, we discuss the regulation of PINK1 protein expression and Parkin localization by the bioenergetic status of individual mitochondria; the mechanism by which PINK1 recruits Parkin to the outer mitochondrial membrane; and Parkins promotion of mitophagy through its ubiquitination of outer mitochondrial membrane proteins. This recent work suggests that Parkin and PINK1 may be among the first mammalian proteins identified with a direct role in regulating mitophagy, and implicate a failure of mitophagy in the pathogenesis of Parkinsons disease.

Parkin overexpression selects against a deleterious mtDNA mutation in heteroplasmic cybrid cells

Mitochondrial genomes with deleterious mutations can replicate in cells along with wild-type genomes in a state of heteroplasmy, and are a cause of severe inherited syndromes, such as mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke (MELAS), neuropathy, ataxia, retinitis pigmentosa-maternally inherited Leigh syndrome (NARP-MILS), and Lebers hereditary optic neuropathy (LHON). The cytosolic E3 ligase, Parkin, commonly mutated in recessive familial parkinsonism, translocates to depolarized mitochondria and induces their autophagic elimination, suggesting that Parkin may signal the selective removal of defective mitochondria within the cell. We report that long-term overexpression of Parkin can eliminate mitochondria with deleterious COXI mutations in heteroplasmic cybrid cells, thereby enriching cells for wild-type mtDNA and restoring cytochrome c oxidase activity. After relieving cybrid cells of Parkin overexpression, a more favorable wild-type to mutant mitochondrial genome ratio is stably maintained. These data support the model that Parkin functions in a mitochondrial quality control pathway. Additionally, they suggest that transiently increasing levels of Parkin expression might ameliorate certain mitochondrial diseases.

Mitochondrial Quality Control Mediated by PINK1 and Parkin: Links to Parkinsonism

Mutations in Parkin or PINK1 are the most common cause of recessive familial parkinsonism. Recent studies suggest that PINK1 and Parkin form a mitochondria quality control pathway that identifies dysfunctional mitochondria, isolates them from the mitochondrial network, and promotes their degradation by autophagy. In this pathway the mitochondrial kinase PINK1 senses mitochondrial fidelity and recruits Parkin selectively to mitochondria that lose membrane potential. Parkin, an E3 ligase, subsequently ubiquitinates outer mitochondrial membrane proteins, notably the mitofusins and Miro, and induces autophagic elimination of the impaired organelles. Here we review the recent rapid progress in understanding the molecular mechanisms of PINK1- and Parkin-mediated mitophagy and the identification of Parkin substrates suggesting how mitochondrial fission and trafficking are involved. We also discuss how defects in mitophagy may be linked to Parkinsons disease.

Parkin-induced mitophagy in the pathogenesis of Parkinson disease

Knockout of the ubiquitin ligase Parkin, the gene product of the Parkinson associated Park2, leads to loss of mitochondrial integrity and function in Drosophila melanogaster. Although Parkin is primarily cytosolic, we have found that Parkin is selectively recruited to dysfunctional mitochondria with low membrane potential and subsequently promotes their autophagy. Here we report that Parkin recruitment is voltage-dependent and independent of changes in ATP or pH. These findings suggest that Parkin promotes mitophagy of dysfunctional mitochondria following loss of mitochondrial membrane potential and implicate the targeted elimination of mitochondria in the pathogenesis of Parkinson disease.