PINK1 disables the anti-fission machinery to segregate damaged mitochondria for mitophagy

Mitochondrial fission is essential for the degradation of damaged mitochondria. It is currently unknown how the dynamin-related protein 1 (DRP1)–associated fission machinery is selectively targeted to segregate damaged mitochondria. We show that PTEN-induced putative kinase (PINK1) serves as a pro-fission signal, independently of Parkin. Normally, the scaffold protein AKAP1 recruits protein kinase A (PKA) to the outer mitochondrial membrane to phospho-inhibit DRP1. We reveal that after damage, PINK1 triggers PKA displacement from A-kinase anchoring protein 1. By ejecting PKA, PINK1 ensures the requisite fission of damaged mitochondria for organelle degradation. We propose that PINK1 functions as a master mitophagy regulator by activating Parkin and DRP1 in response to damage. We confirm that PINK1 mutations causing Parkinson disease interfere with the orchestration of selective fission and mitophagy by PINK1.     

A Dimeric PINK1-containing Complex on Depolarized Mitochondria Stimulates Parkin Recruitment

Parkinsonism typified by sporadic Parkinson disease is a prevalent neurodegenerative disease. Mutations in PINK1 (PTEN-induced putative kinase 1), a mitochondrial Ser/Thr protein kinase, or PARKIN, a ubiquitin-protein ligase, cause familial parkinsonism. The accumulation and autophosphorylation of PINK1 on damaged mitochondria results in the recruitment of Parkin, which ultimately triggers quarantine and/or degradation of the damaged mitochondria by the proteasome and autophagy. However, the molecular mechanism of PINK1 in dissipation of the mitochondrial membrane potential (ΔΨm) has not been fully elucidated. Here we show by fluorescence-based techniques that the PINK1 complex formed following a decrease in ΔΨm is composed of two PINK1 molecules and is correlated with intermolecular phosphorylation of PINK1. Disruption of complex formation by the PINK1 S402A mutation weakened Parkin recruitment onto depolarized mitochondria. The most disease-relevant mutations of PINK1 inhibit the complex formation. Taken together, these results suggest that formation of the complex containing dyadic PINK1 is an important step for Parkin recruitment onto damaged mitochondria.     

Uncovering the roles of PINK1 and parkin in mitophagy

Parkinson disease (PD) is the second most prevalent neurodegenerative disorder, and thus elucidation of the pathogenic mechanism and establishment of a fundamental cure is essential in terms of public welfare. Fortunately, our understanding of the pathogenesis of two types of recessive familial PDs—early-onset familial PD caused by dysfunction of the PTEN-induced putative kinase 1 (PINK1) gene and autosomal recessive juvenile Parkinsonism (ARJP) caused by a mutation in the Parkin gene—has evolved and continues to expand.Key words: PINK1, parkin, ubiquitin, mitochondria, autophagy, mitophagy, membrane potential, quality controlSince the cloning of PINK1 and Parkin, numerous papers have been published about the corresponding gene products, but the mechanism by which dysfunction of PINK1 and/or Parkin causes PD remain unclear. Parkin encodes a ubiquitin ligase E3, a substrate recognition member of the ubiquitination pathway, whereas PINK1 encodes a mitochondria-targeted serine-threonine kinase that contributes to the maintenance of mitochondrial integrity. Based on their molecular functions, it is clear that Parkin-mediated ubiquitination and PINK1 phosphorylation are key events in disease pathogenesis. The underlying mechanism, however, is not as well defined and claims of pathogenicity, until recently, remained controversial. Although Parkin''s E3 activity was clearly demonstrated in vitro, we were unable to show a clear E3 activity of Parkin in cell/in vivo. In addition, despite a predicted mitochondrial localization signal for PINK1, we were unable to detect PINK1 on mitochondria by either immunoblotting or immunocytochemistry. More confusingly, overexpression of nontagged PINK1 mainly localized to the cytoplasm under steady state conditions.Work by Dr. Youle''s group at the National Institutes of Health in 2008, however, offered new insights. They reported that Parkin associated with depolarized mitochondria and that Parkin-marked mitochondria were subsequently cleared by autophagy. Soon after their publication, we also examined the function of Parkin and PINK1 following a decrease in mitochondrial membrane potential. Our findings, described below (Fig. 1), have contributed to the development of a mechanism explaining pathogenicity.Open in a separate windowFigure 1Model of mitochondrial quality control mediated by PINK1 and Parkin. Under steady-state conditions, the mature 60 kDa PINK1 is constantly cleaved by an unknown protease to a 50 kDa intermediate form that is subsequently degraded, presumably by the proteasome (upper part). The protein, however, is stabilized on depolarized mitochondria because the initial processing event is inhibited by a decrease in mitochondrial membrane potential (lower part). Accumulated PINK1 recruits cytosolic Parkin onto depolarized mitochondria resulting in activation of its E3 activity. Parkin then ubiquitinates a mitochondrial substrate(s). As a consequence, damaged mitochondria are degraded via mitophagy. Ub, ubiquitin.(1) We sought to determine the subcellular localization of endogenous PINK1, and realized that endogenous PINK1 is barely detectable under steady-state conditions. However, a decrease in mitochondrial membrane-potential following treatment with the mitochondrial uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) results in the gradual accumulation of endogenous PINK1 on mitochondria. Importantly, when CCCP is washed out, the accumulated endogenous PINK1 rapidly disappears (within 30 min) both in the presence and absence of cycloheximide. These results support the hypothesis that PINK1 is constantly transported to the mitochondria, but is rapidly degraded in a membrane potential-dependent manner (see below for details). We speculate that PINK1 is stabilized by a decrease in mitochondrial membrane potential and as a result accumulates on depolarized mitochondria.(2) We examined the potential role of PINK1 in the mitochondrial recruitment of Parkin. In control MEFs (PINK1^+/+), Parkin is selectively recruited to the mitochondria following CCCP treatment, and subsequently results in the selective disappearance of the mitochondria via autophagy (called mitophagy). In sharp contrast, Parkin is not translocated to the mitochondria in PINK1 knockout (PINK1^−/−) MEFs following CCCP treatment, and subsequent mitochondrial degradation is also completely impeded. These results suggest that PINK1 is “a Parkin-recruitment factor” that recruits Parkin from the cytoplasm to damaged mitochondria in a membrane potential-dependent manner for mitophagy.(3) We monitored the E3 activity of Parkin using an artificial pseudo-substrate fused to Parkin in cells. Parkin''s E3 activity was repressed under steady-state conditions; however, we find that Parkin ubiquitinates the pseudo-substrate when it is retrieved to the depolarized mitochondria, suggesting that activation of the latent Parkin E3 activity is likewise dependent on a decrease in mitochondrial membrane potential.(4) PINK1 normally exists as either a long (approximately 60 kDa) or a short (approximately 50 kDa) protein. Because the canonical mitochondrial targeting signal (matrix targeting signal) is cleaved after import into the mitochondria, the long form has been designated as the precursor and the short form as the mature PINK1. However, our subcellular localization study of endogenous PINK1 following CCCP treatment shows that the long form is recovered in the mitochondrial fraction, suggesting that it is not the pre-import precursor form. Moreover, by monitoring the degradation process of PINK1 following recovery of membrane potential, we realized that the short form of PINK1 transiently appears soon after CCCP is washed out and then later disappears, suggesting that the processed form of PINK1 is an intermediate in membrane-potential-dependent degradation. In conclusion, these results imply that PINK1 cleavage does not reflect a canonical maturation process accompanying mitochondrial import as initially thought, but rather represents constitutive degradation in healthy mitochondria by a two-step mechanism; i.e., first limited processing and subsequent complete degradation probably via the proteasome.(5) PINK1 accumulation by decrease of membrane potential and subsequent recruitment of Parkin onto mitochondria are presumably etiologically important because they are impeded for the most part by disease-linked mutations of PINK1 or Parkin.These results, together with reports by other groups, strongly suggest that recessive familial PD is caused by dysfunction of quality control for depolarized mitochondria.At present, we do not know whether the aforementioned pathogenic mechanism of recessive familial PD can be generalized to prevalent sporadic PD. However, the clinical symptoms of recessive familial PD caused by dysfunction of PINK1 or Parkin resembles that of idiopathic PD except early-onset pathogenesis, and thus it is plausible that there is a common pathogenic mechanism. We accordingly believe that our results provide solid insight into the molecular mechanisms of PD pathogenesis, not only for familial forms caused by Parkin and PINK1 mutations, but also the major sporadic form of PD.To fully understand the molecular mechanism of PINK1-Parkin-mediated mitophagy, further details need to be addressed including: identifying the protease(s) that processes PINK1 in a mitochondrial membrane-potential dependent manner and that presumably monitors mitochondrial integrity; identifying a physiological substrate(s) of PINK1; determining the molecular mechanism underlying Parkin activation; and identifying the protein(s) linking Parkin-mediated ubiquitination to mitophagy. A detailed mechanism of the aforementioned events will be the focus of future research, however, we feel our conclusion that PINK1 and Parkin function in the removal of depolarized mitochondria is evident and hope that our studies will provide a solid foundation for further studies.     

Mitophagy of damaged mitochondria occurs locally in distal neuronal axons and requires PINK1 and Parkin

To minimize oxidative damage to the cell, malfunctioning mitochondria need to be removed by mitophagy. In neuronal axons, mitochondrial damage may occur in distal regions, far from the soma where most lysosomal degradation is thought to occur. In this paper, we report that PINK1 and Parkin, two Parkinson’s disease–associated proteins, mediate local mitophagy of dysfunctional mitochondria in neuronal axons. To reduce cytotoxicity and mimic physiological levels of mitochondrial damage, we selectively damaged a subset of mitochondria in hippocampal axons. Parkin was rapidly recruited to damaged mitochondria in axons followed by formation of LC3-positive autophagosomes and LAMP1-positive lysosomes. In PINK1^−/− axons, damaged mitochondria did not accumulate Parkin and failed to be engulfed in autophagosomes. Similarly, initiation of mitophagy was blocked in Parkin^−/− axons. Our findings demonstrate that the PINK1–Parkin-mediated pathway is required for local mitophagy in distal axons in response to focal damage. Local mitophagy likely provides rapid neuroprotection against oxidative stress without a requirement for retrograde transport to the soma.     

PINK1 controls mitochondrial localization of Parkin through direct phosphorylation

PTEN-induced putative kinase 1 (PINK1) and Parkin, encoded by their respective genes associated with Parkinson’s disease (PD), are linked in a common pathway involved in the protection of mitochondrial integrity and function. However, the mechanism of their interaction at the biochemical level has not been investigated yet. Using both mammalian and Drosophila systems, we here demonstrate that the PINK1 kinase activity is required for its function in mitochondria. PINK1 regulates the localization of Parkin to the mitochondria in its kinase activity-dependent manner. In detail, Parkin phosphorylation by PINK1 on its linker region promotes its mitochondrial translocation, and the RING1 domain of Parkin is critical for this occurrence. These results demonstrate the biochemical relationship between PINK1, Parkin, and the mitochondria and thereby suggest the possible mechanism of PINK-Parkin-associated PD pathogenesis.     

Convergence of Parkin,PINK1, and α-Synuclein on Stress-induced Mitochondrial Morphological Remodeling

Mutations in PARKIN (PARK2), an ubiquitin ligase, cause early onset Parkinson disease. Parkin was shown to bind, ubiquitinate, and target depolarized mitochondria for destruction by autophagy. This process, mitophagy, is considered crucial for maintaining mitochondrial integrity and suppressing Parkinsonism. Here, we report that under moderate mitochondrial stress, parkin does not translocate to mitochondria to induce mitophagy; rather, it stimulates mitochondrial connectivity. Mitochondrial stress-induced fusion requires PINK1 (PARK6), mitofusins, and parkin ubiquitin ligase activity. Upon exposure to mitochondrial toxins, parkin binds α-synuclein (PARK1), and in conjunction with the ubiquitin-conjugating enzyme Ubc13, stimulates K63-linked ubiquitination. Importantly, α-synuclein inactivation phenocopies parkin overexpression and suppresses stress-induced mitochondria fission, whereas Ubc13 inactivation abrogates parkin-dependent mitochondrial fusion. The convergence of parkin, PINK1, and α-synuclein on mitochondrial dynamics uncovers a common function of these PARK genes in the mitochondrial stress response and provides a potential physiological basis for the prevalence of α-synuclein pathology in Parkinson disease.     

α-Synuclein Shows High Affinity Interaction with Voltage-dependent Anion Channel,Suggesting Mechanisms of Mitochondrial Regulation and Toxicity in Parkinson Disease

Participation of the small, intrinsically disordered protein α-synuclein (α-syn) in Parkinson disease (PD) pathogenesis has been well documented. Although recent research demonstrates the involvement of α-syn in mitochondrial dysfunction in neurodegeneration and suggests direct interaction of α-syn with mitochondria, the molecular mechanism(s) of α-syn toxicity and its effect on neuronal mitochondria remain vague. Here we report that at nanomolar concentrations, α-syn reversibly blocks the voltage-dependent anion channel (VDAC), the major channel of the mitochondrial outer membrane that controls most of the metabolite fluxes in and out of the mitochondria. Detailed analysis of the blockage kinetics of VDAC reconstituted into planar lipid membranes suggests that α-syn is able to translocate through the channel and thus target complexes of the mitochondrial respiratory chain in the inner mitochondrial membrane. Supporting our in vitro experiments, a yeast model of PD shows that α-syn toxicity in yeast depends on VDAC. The functional interactions between VDAC and α-syn, revealed by the present study, point toward the long sought after physiological and pathophysiological roles for monomeric α-syn in PD and in other α-synucleinopathies.     

PINK1/Parkin direct mitochondria to autophagy

Mutations in PTEN-induced putative kinase 1 (PINK1) and PARK2/Parkin cause autosomal recessive forms of Parkinson disease. In mammalian cells, cytosolic Parkin is selectively recruited to depolarized mitochondria, followed by a stimulation of mitochondrial autophagy. We show that Parkin translocation to mitochondria is mediated by PINK1, even in cells with normal mitochondrial membrane potential (ΔΨ_m). Once at the mitochondria, Parkin is in close proximity to PINK1, but Parkin does not catalyze PINK1 ubiquitination nor does PINK1 phosphorylate Parkin. However, co-overexpression of Parkin and PINK1 collapses the normal tubular mitochondrial network into large mitochondrial perinuclear clusters, many of which are surrounded by autophagic vacuoles. Our results suggest that Parkin and PINK1 modulate mitochondrial trafficking to the perinuclear region, a subcellular area associated with autophagy. Mutations in either Parkin or PINK1 impair this process and, consequently, mitochondrial turnover may be altered, inducing accumulation of defective mitochondria and, ultimately, causing neurodegeneration in Parkinson disease.     

Mitochondrial Ca2+ oscillation induces mitophagy initiation through the PINK1-Parkin pathway

Dysregulation of the PINK1/Parkin-mediated mitophagy is essential to Parkinson’s disease. Although important progress has been made in previous researches, the biochemical reagents that induce global and significant mitochondrial damage may still hinder deeper insights into the mechanisms of mitophagy. The origin of PINK1/Parkin pathway activation in mitophagy remains elusive. In this study, we develop an optical method, ultra-precise laser stimulation (UPLaS) that delivers a precise and noninvasive stimulation onto a submicron region in a single mitochondrial tubular structure. UPLaS excites localized mitochondrial Ca²⁺ (mitoCa²⁺) oscillations with tiny perturbation to mitochondrial membrane potential (MMP) or mitochondrial reactive oxygen species. The UPLaS-induced mitoCa²⁺ oscillations can directly induce PINK1 accumulation and Parkin recruitment on mitochondria. The Parkin recruitment by UPLaS requires PINK1. Our results provide a precise and noninvasive technology for research on mitophagy, which stimulates target mitochondria with little damage, and reveal mitoCa²⁺ oscillation directly initiates the PINK1-Parkin pathway for mitophagy without MMP depolarization.Subject terms: Mitophagy, Calcium signalling     

SIAH proteins regulate the degradation and intra‐mitochondrial aggregation of PINK1: Implications for mitochondrial pathology in Parkinson's disease

Parkinson''s disease (PD) is characterized by degeneration of neurons, particularly dopaminergic neurons in the substantia nigra. PD brains show accumulation of α‐synuclein in Lewy bodies and accumulation of dysfunctional mitochondria. However, the mechanisms leading to mitochondrial pathology in sporadic PD are poorly understood. PINK1 is a key for mitophagy activation and recycling of unfit mitochondria. The activation of mitophagy depends on the accumulation of uncleaved PINK1 at the outer mitochondrial membrane and activation of a cascade of protein ubiquitination at the surface of the organelle. We have now found that SIAH3, a member of the SIAH proteins but lacking ubiquitin‐ligase activity, is increased in PD brains and cerebrospinal fluid and in neurons treated with α‐synuclein preformed fibrils (α‐SynPFF). We also observed that SIAH3 is aggregated together with PINK1 in the mitochondria of PD brains. SIAH3 directly interacts with PINK1, leading to their intra‐mitochondrial aggregation in cells and neurons and triggering a cascade of toxicity with PINK1 inactivation along with mitochondrial depolarization and neuronal death. We also found that SIAH1 interacts with PINK1 and promotes ubiquitination and proteasomal degradation of PINK1. Similar to the dimerization of SIAH1/SIAH2, SIAH3 interacts with SIAH1, promoting its translocation to mitochondria and preventing its ubiquitin‐ligase activity toward PINK1. Our results support the notion that the increase in SIAH3 and intra‐mitochondrial aggregation of SIAH3‐PINK1 may mediate α‐synuclein pathology by promoting proteotoxicity and preventing the elimination of dysfunctional mitochondria. We consider it possible that PINK1 activity is decreased in sporadic PD, which impedes proper mitochondrial renewal in the disease.     

Phosphorylation of Mitochondrial Polyubiquitin by PINK1 Promotes Parkin Mitochondrial Tethering

The kinase PINK1 and the E3 ubiquitin (Ub) ligase Parkin participate in mitochondrial quality control. The phosphorylation of Ser65 in Parkin''s ubiquitin-like (UBl) domain by PINK1 stimulates Parkin activation and translocation to damaged mitochondria, which induces mitophagy generating polyUb chain. However, Parkin Ser65 phosphorylation is insufficient for Parkin mitochondrial translocation. Here we report that Ser65 in polyUb chain is also phosphorylated by PINK1, and that phosphorylated polyUb chain on mitochondria tethers Parkin at mitochondria. The expression of Tom70^MTS-4xUb SE, which mimics phospho-Ser65 polyUb chains on the mitochondria, activated Parkin E3 activity and its mitochondrial translocation. An E3-dead form of Parkin translocated to mitochondria with reduced membrane potential in the presence of Tom70^MTS-4xUb SE, whereas non-phospho-polyUb mutant Tom70^MTS-4xUb SA abrogated Parkin translocation. Parkin binds to the phospho-polyUb chain through its RING1-In-Between-RING (IBR) domains, but its RING0-linker is also required for mitochondrial translocation. Moreover, the expression of Tom70^MTS-4xUb SE improved mitochondrial degeneration in PINK1-deficient, but not Parkin-deficient, Drosophila. Our study suggests that the phosphorylation of mitochondrial polyUb by PINK1 is implicated in both Parkin activation and mitochondrial translocation, predicting a chain reaction mechanism of mitochondrial phospho-polyUb production by which rapid translocation of Parkin is achieved.     

The PINK1/Parkin-mediated mitophagy is compromised by PD-associated mutations

Mitochondrial dysfunction is an early sign of many neurodegenerative diseases. Very recently, two Parkinson disease (PD) associated genes, PINK1 and Parkin, were shown to mediate the degradation of damaged mitochondria via selective autophagy (mitophagy). PINK1 kinase activity is needed for prompt and efficient Parkin recruitment to impaired mitochondria. PD-associated Parkin mutations interfere with the process of mitophagy at distinct steps. Here we show that whole mitochondria are turned over via macroautophagy. Moreover, disease-associated PINK1 mutations also compromise the selective degradation of depolarized mitochondria. This may be due to the decreased physical binding activity of PD-linked PINK1 mutations to Parkin. Thus, PINK1 mutations abrogate autophagy of impaired mitochondria upstream of Parkin. In addition to compromised PINK1 kinase activity, reduced binding of PINK1 to Parkin leads to failure in Parkin mitochondrial translocation, resulting in the accumulation of damaged mitochondria, which may contribute to disease pathogenesis.     

Parkinson's disease-associated kinase PINK1 regulates Miro protein level and axonal transport of mitochondria

Mutations in Pten-induced kinase 1 (PINK1) are linked to early-onset familial Parkinson's disease (FPD). PINK1 has previously been implicated in mitochondrial fission/fusion dynamics, quality control, and electron transport chain function. However, it is not clear how these processes are interconnected and whether they are sufficient to explain all aspects of PINK1 pathogenesis. Here we show that PINK1 also controls mitochondrial motility. In Drosophila, downregulation of dMiro or other components of the mitochondrial transport machinery rescued dPINK1 mutant phenotypes in the muscle and dopaminergic (DA) neurons, whereas dMiro overexpression alone caused DA neuron loss. dMiro protein level was increased in dPINK1 mutant but decreased in dPINK1 or dParkin overexpression conditions. In Drosophila larval motor neurons, overexpression of dPINK1 inhibited axonal mitochondria transport in both anterograde and retrograde directions, whereas dPINK1 knockdown promoted anterograde transport. In HeLa cells, overexpressed hPINK1 worked together with hParkin, another FPD gene, to regulate the ubiquitination and degradation of hMiro1 and hMiro2, apparently in a Ser-156 phosphorylation-independent manner. Also in HeLa cells, loss of hMiro promoted the perinuclear clustering of mitochondria and facilitated autophagy of damaged mitochondria, effects previously associated with activation of the PINK1/Parkin pathway. These newly identified functions of PINK1/Parkin and Miro in mitochondrial transport and mitophagy contribute to our understanding of the complex interplays in mitochondrial quality control that are critically involved in PD pathogenesis, and they may explain the peripheral neuropathy symptoms seen in some PD patients carrying particular PINK1 or Parkin mutations. Moreover, the different effects of loss of PINK1 function on Miro protein level in Drosophila and mouse cells may offer one explanation of the distinct phenotypic manifestations of PINK1 mutants in these two species.     

Parkinson Phenotype in Aged PINK1-Deficient Mice Is Accompanied by Progressive Mitochondrial Dysfunction in Absence of Neurodegeneration

Background

Parkinson''s disease (PD) is an adult-onset movement disorder of largely unknown etiology. We have previously shown that loss-of-function mutations of the mitochondrial protein kinase PINK1 (PTEN induced putative kinase 1) cause the recessive PARK6 variant of PD.

Methodology/Principal Findings

Now we generated a PINK1 deficient mouse and observed several novel phenotypes: A progressive reduction of weight and of locomotor activity selectively for spontaneous movements occurred at old age. As in PD, abnormal dopamine levels in the aged nigrostriatal projection accompanied the reduced movements. Possibly in line with the PARK6 syndrome but in contrast to sporadic PD, a reduced lifespan, dysfunction of brainstem and sympathetic nerves, visible aggregates of α-synuclein within Lewy bodies or nigrostriatal neurodegeneration were not present in aged PINK1-deficient mice. However, we demonstrate PINK1 mutant mice to exhibit a progressive reduction in mitochondrial preprotein import correlating with defects of core mitochondrial functions like ATP-generation and respiration. In contrast to the strong effect of PINK1 on mitochondrial dynamics in Drosophila melanogaster and in spite of reduced expression of fission factor Mtp18, we show reduced fission and increased aggregation of mitochondria only under stress in PINK1-deficient mouse neurons.

Conclusion

Thus, aging Pink1^−/− mice show increasing mitochondrial dysfunction resulting in impaired neural activity similar to PD, in absence of overt neuronal death.     

PINK1 Is Selectively Stabilized on Impaired Mitochondria to Activate Parkin

Loss-of-function mutations in PINK1 and Parkin cause parkinsonism in humans and mitochondrial dysfunction in model organisms. Parkin is selectively recruited from the cytosol to damaged mitochondria to trigger their autophagy. How Parkin recognizes damaged mitochondria, however, is unknown. Here, we show that expression of PINK1 on individual mitochondria is regulated by voltage-dependent proteolysis to maintain low levels of PINK1 on healthy, polarized mitochondria, while facilitating the rapid accumulation of PINK1 on mitochondria that sustain damage. PINK1 accumulation on mitochondria is both necessary and sufficient for Parkin recruitment to mitochondria, and disease-causing mutations in PINK1 and Parkin disrupt Parkin recruitment and Parkin-induced mitophagy at distinct steps. These findings provide a biochemical explanation for the genetic epistasis between PINK1 and Parkin in Drosophila melanogaster. In addition, they support a novel model for the negative selection of damaged mitochondria, in which PINK1 signals mitochondrial dysfunction to Parkin, and Parkin promotes their elimination.     

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 Parkinson''s disease.Parkinson''s disease (PD) is the second most common neurodegenerative disorder and is characterized by cardinal motor symptoms: slowness of movement, rigidity, rest tremor, and postural instability (Ropper et al. 2009). Although these symptoms initially respond to drugs that modulate dopamine metabolism or surgeries that alter basal ganglia circuitry, the disease eventually progresses. With a modest exception (Olanow et al. 2009), no therapy has been shown to alter the disease course.The pathogenesis of sporadic Parkinson''s disease is likely complex involving altered metabolism of the protein α-synuclein, lysosomal dysfunction, and a dysregulated inflammatory response (reviewed in Shulman et al. 2011). Several lines of evidence also point to mitochondrial dysfunction as a central player in the pathogenesis of PD. Complex I dysfunction is associated with sporadic PD and is sufficient to induce parkinsonism (reviewed in Schapira 2008). The inhibitors of complex I, MPTP (Langston et al. 1983) and rotenone (Betarbet et al. 2000), replicate the symptoms of PD, and rotenone recapitulates key pathognomonic features of PD, such as the α-synuclein-rich inclusion bodies (Betarbet et al. 2000). The cause of mitochondrial dysfunction in sporadic PD is not entirely clear, but laser capture microdissection of substantia nigra neurons from patients with PD reveal a higher burden of mitochondrial DNA deletions relative to age-matched controls (Bender et al. 2006). That such deletions are sufficient to cause parkinsonism is suggested by the occurrence of parkinsonism in patients with rare mutations in their mtDNA replication machinery (e.g., the catalytic subunit of the mtDNA polymerase POLG [Luoma et al. 2004] or the mtDNA helicase Twinkle [Baloh et al. 2007]). The defective mtDNA replicative machinery generates high levels of mtDNA deletions throughout the body that are qualitatively similar to those observed in the substantia nigra in patients with sporadic PD (Reeve et al. 2008). Thus, mitochondrial dysfunction is both associated with sporadic PD and sufficient to cause the parkinsonian syndrome.As is discussed in this review, recent insights from certain genetic forms of PD—resulting from mutations in Parkin or PINK1—support the model that mitochondrial damage is a central driver of PD pathogenesis. Additionally, they provide a rationale for targeting mitochondrial quality control pathways in patients with PD.     

Characterization of PINK1 (PTEN-induced Putative Kinase 1) Mutations Associated with Parkinson Disease in Mammalian Cells and Drosophila

Mutations in PINK1 (PTEN-induced putative kinase 1) are tightly linked to autosomal recessive Parkinson disease (PD). Although more than 50 mutations in PINK1 have been discovered, the role of these mutations in PD pathogenesis remains poorly understood. Here, we characterized 17 representative PINK1 pathogenic mutations in both mammalian cells and Drosophila. These mutations did not affect the typical cleavage patterns and subcellular localization of PINK1 under both normal and damaged mitochondria conditions in mammalian cells. However, PINK1 mutations in the kinase domain failed to translocate Parkin to mitochondria and to induce mitochondrial aggregation. Consistent with the mammalian data, Drosophila PINK1 mutants with mutations in the kinase domain (G426D and L464P) did not genetically interact with Parkin. Furthermore, PINK1-null flies expressing the transgenic G426D mutant displayed defective phenotypes with increasing age, whereas L464P mutant-expressing flies exhibited the phenotypes at an earlier age. Collectively, these results strongly support the hypothesis that the kinase activity of PINK1 is essential for its function and for regulating downstream Parkin functions in mitochondria. We believe that this study provides the basis for understanding the molecular and physiological functions of various PINK1 mutations and provides insights into the pathogenic mechanisms of PINK1-linked PD.     

Human A53T α-Synuclein Causes Reversible Deficits in Mitochondrial Function and Dynamics in Primary Mouse Cortical Neurons

Parkinson’s disease (PD) is the second most common neurodegenerative disease. A key pathological feature of PD is Lewy bodies, of which the major protein component is α-synuclein (α-syn). Human genetic studies have shown that mutations (A53T, A30P, E46K) and multiplication of the α-syn gene are linked to familial PD. Mice overexpressing the human A53T mutant α-syn gene develop severe movement disorders. However, the molecular mechanisms of α-syn toxicity are not well understood. Recently, mitochondrial dysfunction has been linked with multiple neurodegenerative diseases including Parkinson’s disease. Here we investigated whether mitochondrial motility, dynamics and respiratory function are affected in primary neurons from a mouse model expressing the human A53T mutation. We found that mitochondrial motility was selectively inhibited in A53T neurons while transport of other organelles was not affected. In addition, A53T expressing neurons showed impairment in mitochondrial membrane potential and mitochondrial respiratory function. Furthermore, we found that rapamycin, an autophagy inducer, rescued the decreased mitochondrial mobility. Taken together, these data demonstrate that A53T α-syn impairs mitochondrial function and dynamics and the deficit of mitochondrial transport is reversible, providing further understanding of the disease pathogenesis and a potential therapeutic strategy for PD.     

PINK1 Is Dispensable for Mitochondrial Recruitment of Parkin and Activation of Mitophagy in Cardiac Myocytes

Myocyte function and survival relies on the maintenance of a healthy population of mitochondria. The PINK1/Parkin pathway plays an important role in clearing defective mitochondria via autophagy in cells. However, how the PINK1/Parkin pathway regulates mitochondrial quality control and whether it coordinates with other mitophagy pathways are still unclear. Therefore, the objective of this study was to investigate the effect of PINK1-deficiency on mitochondrial quality control in myocytes. Using PINK1-deficient (PINK1-/-) mice, we found that Parkin is recruited to damaged cardiac mitochondria in hearts after treatment with the mitochondrial uncoupler FCCP or after a myocardial infarction even in the absence of PINK1. Parkin recruitment to depolarized mitochondria correlates with increased ubiquitination of mitochondrial proteins and activation of mitophagy in PINK1-/- myocytes. In addition, induction of mitophagy by the atypical BH3-only protein BNIP3 is unaffected by lack of PINK1. Overall, these data suggest that Parkin recruitment to depolarized cardiac mitochondria and subsequent activation of mitophagy is independent of PINK1. Moreover, alternative mechanisms of Parkin activation and pathways of mitophagy remain functional in PINK1-/- myocytes and could compensate for the PINK1 deficiency.