Loss of PINK1 function decreases PP2A activity and promotes autophagy in dopaminergic cells and a murine model

PTEN-Induced Putative Kinase 1 Dysfunction Accelerates Synucleinopathy

Background:

Mutations in PTEN-induced putative kinase 1 (PINK1) cause autosomal recessive Parkinson’s disease (PD) and contribute to the risk of sporadic PD. However, the relationship between PD-related PINK1 mutations and alpha-synuclein (α-syn) aggregation—a main pathological component of PD—remains unexplored.

Objective:

To investigate whether α-syn pathology is exacerbated in the absence of PINK1 after α-syn preformed fibril (PFF) injection in a PD mouse model and its effects on neurodegeneration.

Methods:

In this study, 10-week-old Pink1 knockout (KO) and wildtype (WT) mice received stereotaxic unilateral striatal injection of recombinant mouse α-syn PFF. Then, α-syn pathology progression, inflammatory responses, and neurodegeneration were analyzed via immunohistochemistry, western blot analysis, and behavioral testing.

Results:

After PFF injection, the total α-syn levels significantly increased, and pathological α-syn was markedly aggregated in Pink1 KO mice compared with Pink1 WT mice. Then, earlier and more severe neuronal loss and motor deficits occurred. Moreover, compared with WT mice, Pink1 KO mice had evident microglial/astrocytic immunoreactivity and prolonged astrocytic activation, and a higher rate of protein phosphatase 2A phosphorylation, which might explain the greater α-syn aggravation and neuronal death.

Conclusion:

The loss of Pink1 function accelerated α-syn aggregation, accumulation and glial activation, thereby leading to early and significant neurodegeneration and behavioral impairment in the PD mouse model. Therefore, our findings support the notion that PINK1 dysfunction increases the risk of synucleinopathy.

Ceramide Metabolism and Parkinson's Disease-Therapeutic Targets

Ceramide is a bioactive sphingolipid involved in numerous cellular processes. In addition to being the precursor of complex sphingolipids, ceramides can act as second messengers, especially when they are generated at the plasma membrane of cells. Its metabolic dysfunction may lead to or be a consequence of an underlying disease. Recent reports on transcriptomics and electrospray ionization mass spectrometry analysis have demonstrated the variation of specific levels of sphingolipids and enzymes involved in their metabolism in different neurodegenerative diseases. In the present review, we highlight the most relevant discoveries related to ceramide and neurodegeneration, with a special focus on Parkinson's disease.

Unraveling the Mystery of Cold Stress-Induced Myocardial Injury

Exposure to low ambient temperature imposes great challenge to human health. Epidemiological evidence has noted significantly elevated emergency admission and mortality rate in cold climate in many regions, in particular, adverse events in cardiovascular system. Cold stress is becoming one of the important risk factors for cardiovascular death. Through recent advance in echocardiography and myocardial histological techniques, both clinical and experimental experiments have unveiled that cold stress triggers a variety of pathological and pathophysiological injuries, including ventricular wall thickening, cardiac hypertrophy, elevated blood pressure, decreased cardiac function, and myocardial interstitial fibrosis. In order to examine the potential mechanism of action behind cold stress-induced cardiovascular anomalies, ample biochemical and molecular biological experiments have been conducted to denote a role for mitochondrial injury, intracellular Ca²⁺ dysregulation, generation of reactive oxygen species (ROS) and other superoxide, altered gene and protein profiles for apoptosis and autophagy, and increased adrenergic receptor sensitivity in cold stress-induced cardiovascular anomalies. These findings suggest that cold stress may damage the myocardium through mitochondrial injury, apoptosis, autophagy, metabolism, oxidative stress, and neuroendocrine pathways. Although the precise nature remains elusive for cold stress-induced cardiovascular dysfunction, endothelin (ET-_A) receptor, endoplasmic reticulum (ER) stress, transient receptor potential vanilloid, mitochondrial-related protein including NRFs and UCP-2, ROS, Nrf2-Keap1 signaling pathway, Bcl-2/Bax, and lipoprotein lipase (LPL) signaling may all play a pivotal role. For myocardial injury evoked by cold stress, more comprehensive and in-depth mechanisms are warranted to better define the potential therapeutic options for cold stress-associated cardiovascular diseases.

Paradoxical Mitophagy Regulation by PINK1 and TUFm

Aberrant mitophagy has been implicated in a broad spectrum of disorders. PINK1, Parkin, and ubiquitin have pivotal roles in priming mitophagy. However, the entire regulatory landscape and the precise control mechanisms of mitophagy remain to be elucidated. Here, we uncover fundamental mitophagy regulation involving PINK1 and a non-canonical role of the mitochondrial Tu translation elongation factor (TUFm). The mitochondrion-cytosol dual-localized TUFm interacts with PINK1 biochemically and genetically, which is an evolutionarily conserved Parkin-independent route toward mitophagy. A PINK1-dependent TUFm phosphoswitch at Ser222 determines conversion from activating to suppressing mitophagy. PINK1 modulates differential translocation of TUFm because p-S222-TUFm is restricted predominantly to the cytosol, where it inhibits mitophagy by impeding Atg5-Atg12 formation. The self-antagonizing feature of PINK1/TUFm is critical for the robustness of mitophagy regulation, achieved by the unique kinetic parameters of p-S222-TUFm, p-S65-ubiquitin, and their common kinase PINK1. Our findings provide new mechanistic insights into mitophagy and mitophagy-associated disorders.

The circular RNA ACR attenuates myocardial ischemia/reperfusion injury by suppressing autophagy via modulation of the Pink1/ FAM65B pathway

Dysregulated autophagy is associated with many pathological disorders such as cardiovascular diseases. Emerging evidence has suggested that circular RNAs (circRNAs) have important roles in some biological processes. However, it remains unclear whether circRNAs participate in the regulation of autophagy. Here we report that a circRNA, termed autophagy-related circular RNA (ACR), represses autophagy and myocardial infarction by targeting Pink1-mediated phosphorylation of FAM65B. ACR attenuates autophagy and cell death in cardiomyocytes. Moreover, ACR protects the heart from ischemia/reperfusion (I/R) injury and reduces myocardial infarct sizes. We identify Pink1 as an ACR target to mediate the function of ACR in cardiomyocyte autophagy. ACR activates Pink1 expression through directly binding to Dnmt3B and blocking Dnmt3B-mediated DNA methylation of Pink1 promoter. Pink1 suppresses autophagy and Pink1 transgenic mice show reduced myocardial infarction sizes. Further, we find that FAM65B is a downstream target of Pink1 and Pink1 phosphorylates FAM65B at serine 46. Phosphorylated FAM65B inhibits autophagy and cell death in the heart. Our findings reveal a novel role for the circRNA in regulating autophagy and ACR-Pink1-FAM65B axis as a regulator of autophagy in the heart will be potential therapeutic targets in treatment of cardiovascular diseases.

Multiple pathways for mitophagy: A neurodegenerative conundrum for Parkinson's disease

It has been nearly a decade since the first landmark studies implicating familial recessive Parkinson's disease genes in the regulation of selective mitochondrial autophagy. The PTEN-induced kinase 1 (PINK1) and the E3 ubiquitin ligase Parkin (encoded by the PARK2 gene) act together to mark depolarized mitochondria for degradation. There is now an extensive body of literature detailing key mediators and steps in this pathway, based mostly on work in transformed cell lines. However, the degree to which PINK1-triggered mitophagy contributes to mitochondrial quality control in the mammalian brain, and the extent to which its disruption contributes to Parkinson's disease pathogenesis remain uncertain. In recent years, it has become clear that there are multiple, potentially redundant, pathways of cargo specification for mitophagy. Important mitophagy-independent functions of PINK1 and Parkin are also emerging. This review summarizes key features of three major mitophagy cargo recognition systems: receptor-mediated, ubiquitin-mediated and cardiolipin-mediated. New animal models that may be useful for tracking the delivery of mitochondria into lysosomes in different neuronal populations will be highlighted. Combining these research tools with methods to selectively disrupt specific mitophagy pathways may lead to a better understanding of the potential role of mitophagy in modulating neuronal vulnerability in Parkinson's spectrum (PD/PDD/DLB) and other neurodegenerative diseases.

LicA induces autophagy through ULK1/Atg13 and ROS pathway in human hepatocellular carcinoma cells

Chemotherapy is the best choice for the vast majority of hepatocellular carcinoma patients at late stage, but few effective chemotherapy drugs are available in clinic. Licochalcone A (LicA) is a new chemotherapy drug inducing apoptosis as Bcl-2 inhibitor, but few studies report on LicA-induced autophagy. This study investigated the phenomenon and mechanisms of LicA-induced autophagy looking for a targeted combination drug. Human hepatocellular carcinoma cells (HCCs) were treated with LicA, to detect markers of autophagy and to investigate the mechanisms. In order to investigate the role of reactive oxygen species (ROS) in LicA-induced autophagy, ROS, glutathione (GSH) and O₂⁻ were measured in LicA treated HCCs, and antioxidant N-Acetyl-L-cysteine (NAC) was cotreated with LicA in HCCs, then mechanisms of ROS-induced autophagy was investigated in LicA or LicA combined with NAC treated HCCs. Finally, the LicA-induced apoptosis was detected in LicA combined with NAC treated HCCs. We first report that LicA can induce autophagy through ULK1/Atg13 and ROS pathway in HCCs, suppression of LicA-induced ROS through antioxidant NAC can enhance LicA-induced apoptosis, promoting the function of LicA killing HCCs. LicA can activate the ULK1/Atg13 complex which is upstream of autophagy, additionally, LicA also can promote ROS generation, ROS trigger the expression level of TSC1/2 complex, PRAS40, CTMP, PP2A, PDK1 and Rubicon change, these molecules are upstream of autophagy. In conclusion, LicA can induce autophagy through ULK1/Atg13 and ROS pathway in HCCs, LicA combined with NAC can enhance LicA-induced apoptosis. Our results may provide a novel design for clinical hepatocellular carcinoma therapy trials.

Pink1 attenuates propofol-induced apoptosis and oxidative stress in developing neurons

BACKGROUND

The underlying mechanisms of propofol-induced neurotoxicity in developing neurons are still not completely understood. We examined the role of PTEN-induced kinase 1 (Pink1), an antioxidant protein, in propofol-induced apoptosis in developing neurons.

MATERIALS AND METHODS

Primary hippocampal neurons isolated from neonatal Sprague-Dawley rats were exposed to propofol 20 μM for 2, 4, 6 and 12 h. Subsequently, neurons underwent overexpression and knockdown of Pink1, followed by propofol exposure (20 μM, 6 h). Neuron apoptosis was detected by terminal transferase deoxyuridine triphosphate-biotin nick-end labeling (TUNEL). Reactive oxygen species (ROS) production in neurons was detected by using a 2,7-dichlorodihydro-fluorescein diacetate probe and target protein or mRNA levels were analyzed by Western blotting or real-time polymerase chain reaction.

RESULTS

Propofol treatment time-dependently increased the number of TUNEL-positive neurons and the expression levels of cleaved caspase-3 and B-cell lymphoma 2 (BcL-2) associated X protein, but decreased expression levels of BcL-2. Furthermore, propofol treatment time-dependently reduced the expression levels of Pink1 mRNA and protein. ROS production and the markers of oxidative stress, 2,4-dinitrophenol and 4-hydroxynonenal, were increased by propofol treatment. However, these propofol-induced changes were significantly restored by Pink1 overexpression.

CONCLUSIONS

Pink1 plays an important role in neuronal apoptosis induced by propofol. Our results may provide some new insights in propofol-induced neurotoxicity in developing neurons.

PINK1 suppresses alpha-synuclein-induced neuronal injury: a novel mechanism in protein phosphatase 2A activation

Alpha-synuclein (α-Syn) and phosphatase and tensin homolog deleted on chromosome ten (PTEN)-induced putative kinase (PINK) 1 are proteins found in Lewy bodies, which are a pathological hallmark of Parkinson's disease (PD). PINK1 overexpression suppresses α-Syn-induced phenotypes and increases lifespan and health in an animal model of PD. It has been suggested that the two proteins regulate protein phosphatase (PP) 2A activity, but the underlying mechanisms and neuroprotective action of PP2A against PD-associated pathology are unknown. We found that α-Syn overexpression in SK-N-SH neuroblastoma cells and primary cortical neurons caused mitochondrial dysfunction and cell injury via phosphorylation of PP2A at Tyr307 and inhibition of its activity. Concomitant overexpression of PINK1 reversed this effect and restored the activity. The level of phospho-activated Src was increased in cells overexpressing α-Syn, which was reversed by co-expressing PINK1, suggesting that the latter suppressed α-Syn-induced PP2A inactivation by inhibiting Src activity. Calmodulin/Src complex formation was also enhanced in α-Syn-overexpressing cells, which was reversed by co-expression of PINK1 as a result of reduced mitochondrial Ca²⁺ releasing. Interestingly, the protective effects of PINK1 in α-Syn induced models were abolished by treatment with the PP2A inhibitor okadaic acid, indicating that PP2A is a target of PINK1. These findings indicate that PINK1 protects against α-Syn-induced neurodegeneration by promoting the dissociation of the calmodulin/Src complex and inhibiting Src, thereby enhancing PP2A activity. This was supported by the observation that PP2A activity was decreased in PD patients, which was negatively correlated with Hoehn and Yahr scores. Our results provide novel insight into the mechanisms underlying neurodegeneration in PD as well as possible avenues for therapeutic intervention in the treatment of this disease.

Age- and brain region-dependent α-synuclein oligomerization is attributed to alterations in intrinsic enzymes regulating α-synuclein phosphorylation in aging monkey brains

We previously reported that the levels of α-syn oligomers, which play pivotal pathogenic roles in age-related Parkinson's disease (PD) and dementia with Lewy bodies, increase heterogeneously in the aging brain. Here, we show that exogenous α-syn incubated with brain extracts from older cynomolgus monkeys and in Lewy body pathology (LBP)-susceptible brain regions (striatum and hippocampus) forms higher amounts of phosphorylated and oligomeric α-syn than that in extracts from younger monkeys and LBP-insusceptible brain regions (cerebellum and occipital cortex). The increased α-syn phosphorylation and oligomerization in the brain extracts from older monkeys and in LBP-susceptible brain regions were associated with higher levels of polo-like kinase 2 (PLK2), an enzyme promoting α-syn phosphorylation, and lower activity of protein phosphatase 2A (PP2A), an enzyme inhibiting α-syn phosphorylation, in these brain extracts. Further, the extent of the age- and brain-dependent increase in α-syn phosphorylation and oligomerization was reduced by inhibition of PLK2 and activation of PP2A. Inversely, phosphorylated α-syn oligomers reduced the activity of PP2A and showed potent cytotoxicity. In addition, the activity of GCase and the levels of ceramide, a product of GCase shown to activate PP2A, were lower in brain extracts from older monkeys and in LBP-susceptible brain regions. Our results suggest a role for altered intrinsic metabolic enzymes in age- and brain region-dependent α-syn oligomerization in aging brains.

Autophagy and KRT8/keratin 8 protect degeneration of retinal pigment epithelium under oxidative stress

Contribution of autophagy and regulation of related proteins to the degeneration of retinal pigment epithelium (RPE) in age-related macular degeneration (AMD) remain unknown. We report that upregulation of KRT8 (keratin 8) as well as its phosphorylation are accompanied with autophagy and attenuated with the inhibition of autophagy in RPE cells under oxidative stress. KRT8 appears to have a dual role in RPE pathophysiology. While increased expression of KRT8 following autophagy provides a cytoprotective role in RPE, phosphorylation of KRT8 induces pathologic epithelial-mesenchymal transition (EMT) of RPE cells under oxidative stress, which is mediated by MAPK1/ERK2 (mitogen-activated protein kinase 1) and MAPK3/ERK1. Inhibition of autophagy further promotes EMT, which can be reversed by inhibition of MAPK. Thus, regulated enhancement of autophagy with concurrent increased expression of KRT8 and the inhibition of KRT8 phosphorylation serve to inhibit oxidative stress-induced EMT of RPE cells as well as to prevent cell death, suggesting that pharmacological manipulation of KRT8 upregulation through autophagy with combined inhibition of the MAPK1/3 pathway may be attractive therapeutic strategies for the treatment of AMD.

Blood Plasma of Patients with Parkinson's Disease Increases Alpha-Synuclein Aggregation and Neurotoxicity

A pathological hallmark of Parkinson's disease (PD) is formation of Lewy bodies in neurons of the brain. This has been attributed to the spread of α-synuclein (α-syn) aggregates, which involves release of α-syn from a neuron and its reuptake by a neighboring neuron. We found that treatment with plasma from PD patients induced more α-syn phosphorylation and oligomerization than plasma from normal subjects (NS). Compared with NS plasma, PD plasma added to primary neuron cultures caused more cell death in the presence of extracellular α-syn. This was supported by the observations that phosphorylated α-syn oligomers entered neurons, rapidly increased accumulated thioflavin S-positive inclusions, and induced a series of metabolic changes that included activation of polo-like kinase 2, inhibition of glucocerebrosidase and protein phosphatase 2A, and reduction of ceramide levels, all of which have been shown to promote α-syn phosphorylation and aggregation. We also analyzed neurotoxicity of α-syn oligomers relative to plasma from different patients. Neurotoxicity was not related to age or gender of the patients. However, neurotoxicity was positively correlated with H&Y staging score. The modification in the plasma may promote spreading of α-syn aggregates via an alternative pathway and accelerate progression of PD.

Loss of PINK1 inhibits apoptosis by upregulating α-synuclein in inflammation-sensitized hypoxic-ischemic injury in the immature brains

The incidence of preterm birth is rising worldwide. Among preterm infants, many face a lifetime of neurologic impairments. Recent studies have revealed that systemic inflammation can sensitize the immature brain to hypoxic-ischemic (HI) injury. Therefore, it is important to identify the mechanisms involved in inflammation-sensitized HI injury in immature brains. PTEN-induced putative kinase 1 (PINK1) is a regulatory protein that is highly expressed in the brain. We have previously found that PINK1 gene knockout can protect matured brains from HI injury in postnatal day 10 mice. However, the mechanisms are unknown. In this study, we employed an inflammation-sensitized HI injury model using postnatal day 3 mice to study the roles and mechanisms that PINK1 plays in the immature brains. Lipopolysaccharide (LPS) was injected intraperitoneally into the mice before HI treatment to set up the model. We found that PINK1-knockout mice had fewer brain infarcts and less cell apoptosis than did the wild-type mice. Furthermore, we found that α-synuclein was markedly higher in the PINK1-knockout mice than in the wild-type mice, and inhibition of α-synuclein through small interfering RNA (siRNA) reversed the protective effect in the PINK1-knockout mice. Collectively, these findings indicate that loss of PINK1 plays a novel role in the protection of inflammation-sensitized HI brain damage.

Deregulation of the protein phosphatase 2A, PP2A in cancer: complexity and therapeutic options

The complexity of the phosphatase, PP2A, is being unravelled and current research is increasingly providing information on the association of deregulated PP2A function with cancer initiation and progression. It has been reported that decreased activity of PP2A is a recurrent observation in many types of cancer, including colorectal and breast cancer (Baldacchino et al. EPMA J. 5:3, 2014; Cristobal et al. Mol Cancer Ther. 13:938-947, 2014). Since deregulation of PP2A and its regulatory subunits is a common event in cancer, PP2A is a potential target for therapy (Baldacchino et al. EPMA J. 5:3, 2014). In this review, the structural components of the PP2A complex are described, giving an in depth overview of the diversity of regulatory subunits. Regulation of the active PP2A trimeric complex, through phosphorylation and methylation, can be targeted using known compounds, to reactivate the complex. The endogenous inhibitors of the PP2A complex are highly deregulated in cancer, representing cases that are eligible to PP2A-activating drugs. Pharmacological opportunities to target low PP2A activity are available and preclinical data support the efficacy of these drugs, but clinical trials are lacking. We highlight the importance of PP2A deregulation in cancer and the current trends in targeting the phosphatase.

PINK1 deficiency enhances autophagy and mitophagy induction

Parkinson's disease (PD) is a neurodegenerative disorder with poorly understood etiology. Increasing evidence suggests that age-dependent compromise of the maintenance of mitochondrial function is a key risk factor. Several proteins encoded by PD-related genes are associated with mitochondria including PTEN-induced putative kinase 1 (PINK1), which was first identified as a gene that is upregulated by PTEN. Loss-of-function PINK1 mutations induce mitochondrial dysfunction and, ultimately, neuronal cell death. To mitigate the negative effects of altered cellular functions cells possess a degradation mechanism called autophagy for recycling damaged components; selective elimination of dysfunctional mitochondria by autophagy is termed mitophagy. Our study indicates that autophagy and mitophagy are upregulated in PINK1-deficient cells, and is the first report to demonstrate efficient fluxes by one-step analysis. We propose that autophagy is induced to maintain cellular homeostasis under conditions of non-regulated mitochondrial quality control.

Increased oligomerization and phosphorylation of α-synuclein are associated with decreased activity of glucocerebrosidase and protein phosphatase 2A in aging monkey brains

Aging is associated with an increased risk for Parkinson's disease and dementia with Lewy bodies, in which α-synuclein (α-syn) oligomerization plays key pathogenic roles. Here, we show that oligomeric α-syn levels increase with age in the brain of cynomolgus monkeys and are accompanied by a decrease in the expression and activity of glucocerebrosidase (GCase), a lysosomal enzyme whose dysfunction is linked to accumulation of oligomeric α-syn. Besides, levels of α-syn phosphorylated at serine 129 (pS129 α-syn), a modification that promotes α-syn oligomerization also increase with age in the brain and is associated with a reduction in the activity of protein phosphatase 2A (PP2A), an enzyme that facilitates α-syn dephosphorylation. The inverse relationship between levels of oligomeric α-syn and pS129 α-syn and activity of GCase and PP2A was more evident in brain regions susceptible to neurodegeneration (i.e., the striatum and hippocampus) than those that are less vulnerable (i.e., cerebellum and occipital cortex). In vitro experiments showed that GCase activity was more potently inhibited by oligomeric than by monomeric α-syn in the lysosome-enriched fractions isolated from brain tissues and cultured neuronal cells. Inhibition of GCase activity induced an elevation of oligomeric α-syn levels, which was shown to increase pS129 α-syn levels and reduce PP2A activity in cultured neuronal cells. The alterations in oligomeric and pS129 α-syns and their association with GCase and PP2A in aging brains may explain the vulnerability of certain brain regions to neurodegeneration in Parkinson's disease and dementia with Lewy bodies.

Parkinson's disease-associated PINK1 G309D mutation increases abnormal phosphorylation of Tau

Mutations in PINK1 gene have been considered the second most common cause of Autosomal Recessive Parkinsonism (ARP). So far, different homozygous PINK1 mutations have been identified in different ARP patients. Abnormal hyperphosphorylation of tau leads to the loss of its biological activity. Multiple lines of evidence have demonstrated that hyperphosphorylated tau is associated with Alzheimer's disease and Parkinson's disease (PD). However, the effects of PD associated PINK1 mutations in tau phosphorylation are unknown. In this study, we investigated the effect of G309D PINK1 mutation in tau phosphorylation. Cells transfected with mutant G309D PINK1 exhibited a significant increase in the phosphorylation of tau protein at the PHF-1 (ser396/404) site. The levels of CDK5, an important activator of tau phosphorylation, did not change in mutant G309D PINK1 transfected cells, suggesting that CDK5 is not involved in tau phosphorylation induced by mutant G309D PINK1. Notably, we found that mutant G309D PINK1 significantly reduced phosphorylation of GSK3β at serine 9, suggesting that alterations in GSK3β activity play an essential role in mutant G309D PINK1-induced tau phosphorylation at the PHF-1 site. PP2A activity maintained consistent in mutant G309D PINK1 transfected cells, suggesting that the increased tau hyperphosphorylation is not ascribed to reduction in PP2A activity.

Beyond mitophagy: cytosolic PINK1 as a messenger of mitochondrial health

SIGNIFICANCE

Disruptions in mitochondrial homeostasis are implicated in human diseases across the lifespan. Recessive mutations in PINK1, which encodes the mitochondrially targeted PTEN-induced putative kinase 1 (PINK1), cause an autosomal recessive form of Parkinson's disease. As with all kinases, PINK1 participates in multiple functional pathways, and its dysregulation has been implicated in a growing number of diseases.

RECENT ADVANCES

In addition to its heavily studied role in mitophagy, PINK1 regulates mitochondrial respiratory function, reactive oxygen species generation, and mitochondrial transport. Moreover, recent studies implicate processed PINK1 in cytosolic signaling cascades that promote cell survival and neuron differentiation. Cytosolic PINK1 is also capable of suppressing autophagy and mitophagy. We propose a working hypothesis that PINK1 is released by functional mitochondria as a signal to coordinate cell growth and differentiation in response to mitochondrial status.

CRITICAL ISSUES

PINK1 biology needs to be better understood in primary neurons, as the stability and subcellular localization of PINK1 is differentially regulated in different cell types. Delineating factors that regulate its mitochondrial import/export, processing by different peptidases, kinase activity, subcellular localization, and degradation will be important for defining relevant downstream kinase targets.

FUTURE DIRECTIONS

It is becoming clear that different subcellular pools of PINK1 mediate distinct functions. Future studies will undoubtedly expand on the spectrum of cellular functions regulated by PINK1. Continued study of cytosolic PINK1 may offer novel insights into how functional mitochondria communicate their status with the rest of the cell.

Bcl-2 phosphorylation triggers autophagy switch and reduces mitochondrial damage in limb remote ischemic conditioned rats after ischemic stroke

Autophagy, an important intracellular degradation pathway, has been reported to clear impaired mitochondria and reduce mitochondria-mediated injury in ischemic disease. Our study and other recent investigations have shown that AKT-dependent autophagy contributes to the neuroprotection afforded by limb remote ischemic conditioning (RIC) in experimental stroke. However, how AKT triggers RIC-based autophagy and whether RIC-afforded autophagy is beneficial for mitochondrial function after cerebral ischemia remains unclear. The disruption of the Bcl-2/Beclin1 complex has been reported to trigger autophagy formation in the condition of Bcl-2 phosphorylation at Ser70. We investigated whether Bcl-2 phosphorylation triggers RIC-based autophagy and thereby confers RIC-induced neuroprotection against mitochondrial injury, using a transient cerebral ischemic rat model. We demonstrated that rats undergoing RIC treatment 30 min after the onset of ischemia (I-30) and at reperfusion (R-0) significantly upregulated Bcl-2 phosphorylation. Immunoprecipitation revealed that RIC increased dissociation of the Bcl-2/Beclin1 complex, leading to a higher level of autophagy than in ischemia/reperfusion rats. Furthermore, AKT activation was shown to play a critical role in regulating Bcl-2-mediated autophagy, as an AKT inhibitor (LY294002, AKTi) administered 30 min prior to ischemia significantly suppressed Bcl-2 phosphorylation and Bcl-2/Beclin1 complex dissociation, thereby reducing autophagy in RIC rats. Blocking Bcl-2 phosphorylation-dependent autophagy with AKTi suppressed RIC-afforded protection on mitochondrial potential and mitochondrial-dependent cell death effector pathway. These findings indicate that Bcl-2 phosphorylation and thereby Bcl-2/Beclin1 complex disruption play a crucial role in triggering autophagy and reducing mitochondrial damage in RIC rats after cerebral ischemia and require the involvement of the AKT activation.

Defective autophagy in Parkinson's disease: lessons from genetics

Parkinson's disease (PD) is the most prevalent neurodegenerative movement disorder. Genetic studies over the past two decades have greatly advanced our understanding of the etiological basis of PD and elucidated pathways leading to neuronal degeneration. Recent studies have suggested that abnormal autophagy, a well conserved homeostatic process for protein and organelle turnover, may contribute to neurodegeneration in PD. Moreover, many of the proteins related to both autosomal dominant and autosomal recessive PD, such as α-synuclein, PINK1, Parkin, LRRK2, DJ-1, GBA, and ATPA13A2, are also involved in the regulation of autophagy. We propose that reduced autophagy enhances the accumulation of α-synuclein, other pathogenic proteins, and dysfunctional mitochondria in PD, leading to oxidative stress and neuronal death.

Downregulation of Pink1 influences mitochondrial fusion-fission machinery and sensitizes to neurotoxins in dopaminergic cells

It is now well established that mitochondria are organelles that, far from being static, are subject to a constant process of change. This process, which has been called mitochondrial dynamics, includes processes of both fusion and fission. Loss of Pink1 (PTEN-induced putative kinase 1) function is associated with early onset recessive Parkinson's disease and it has been proposed that mitochondrial dynamics might be affected by loss of the mitochondrial kinase. Here, we report the effects of silencing Pink1 on mitochondrial fusion and fission events in dopaminergic neuron cell lines. Cells lacking Pink1 were more sensitive to cell death induced by C2-Ceramide, which inhibits proliferation and induces apoptosis. In the same cell lines, mitochondrial morphology was fragmented and this was enhanced by application of forskolin, which stimulates the cAMP pathway that phosphorylates Drp1 and thereby inactivates it. Cells lacking Pink1 had lower Drp1 and Mfn2 expression. Based on these data, we propose that Pink1 may exert a neuroprotective role in part by limiting mitochondrial fission.

Alpha-synuclein overexpression increases phospho-protein phosphatase 2A levels via formation of calmodulin/Src complex

Alpha-synuclein (α-Syn) is the principal protein component of Lewy bodies, a pathological hallmark of Parkinson's disease (PD). This protein may regulate protein phosphatase 2A (PP2A) activity, although the molecular mechanisms for α-Syn-mediated regulation of PP2A and the potential neuroprotective actions of PP2A against PD-associated pathology remain largely unexplored. We found that α-Syn gene overexpression in SK-N-SH cells and primary neurons led to PP2A/C phosphorylation at Y307, a known target of Src kinase, and consequent phosphatase inhibition. In addition, phospho-activated Src (p-Y416 Src, pSrc) was higher in SK-N-SH cells and primary neurons overexpressing α-Syn. Thus, α-Syn may promote Src activation and PP2A inactivation, leading to hyperphosphorylation of proteins. Immunoprecipitation revealed higher calmodulin/Src complex formation in α-Syn-overexpressing cells and α-Syn transgenic mice. A TUNEL apoptosis assay and an MTT cell viability assay demonstrated that the PP2A activator C2-ceramide protected neurons against α-Syn-induced cell injury. Buffering the Ca(2+) elevations induced by α-Syn overexpression ameliorated the cytotoxicity of α-Syn. Our findings define a potential molecular mechanism for α-Syn-mediated regulation of PP2A through formation of the calmodulin/Src complex, activation of Src, and Src-mediated phospho-inhibition of PP2A. Overexpression of α-Syn may lead to neurodegeneration in PD in part by suppressing the endogenous neuroprotective activity of PP2A.

The PINK1-Parkin pathway promotes both mitophagy and selective respiratory chain turnover in vivo

The accumulation of damaged mitochondria has been proposed as a key factor in aging and the pathogenesis of many common age-related diseases, including Parkinson disease (PD). Recently, in vitro studies of the PD-related proteins Parkin and PINK1 have found that these factors act in a common pathway to promote the selective autophagic degradation of damaged mitochondria (mitophagy). However, whether Parkin and PINK1 promote mitophagy under normal physiological conditions in vivo is unknown. To address this question, we used a proteomic approach in Drosophila to compare the rates of mitochondrial protein turnover in parkin mutants, PINK1 mutants, and control flies. We found that parkin null mutants showed a significant overall slowing of mitochondrial protein turnover, similar to but less severe than the slowing seen in autophagy-deficient Atg7 mutants, consistent with the model that Parkin acts upstream of Atg7 to promote mitophagy. By contrast, the turnover of many mitochondrial respiratory chain (RC) subunits showed greater impairment in parkin than Atg7 mutants, and RC turnover was also selectively impaired in PINK1 mutants. Our findings show that the PINK1-Parkin pathway promotes mitophagy in vivo and, unexpectedly, also promotes selective turnover of mitochondrial RC subunits. Failure to degrade damaged RC proteins could account for the RC deficits seen in many PD patients and may play an important role in PD pathogenesis.

Heavy metal scavenger metallothionein mitigates deep hypothermia-induced myocardial contractile anomalies: role of autophagy

Low-ambient temperature environment exposure increased the risk of cardiovascular morbidity and mortality, although the underlying mechanism remains unclear. This study was designed to examine the impact of cardiac overexpression of metallothionein, a cysteine-rich heavy metal scavenger, on low temperature (4°C)-induced changes in myocardial function and the underlying mechanism involved, with a focus on autophagy. Cold exposure (4°C for 3 wk) promoted oxidative stress and protein damage, increased left ventricular end-systolic and -diastolic diameter, and suppressed fractional shortening and whole heart contractility, the effects of which were significantly attenuated or ablated by metallothionein. Levels of the autophagy markers LC3B-II, beclin-1, and Atg7 were significantly upregulated with unchanged autophagy adaptor protein p62. Fluorescent immunohistochemistry revealed abundant LC3B puncta in cold temperature-exposed mouse hearts. Coimmunoprecipitation revealed increased dissociation between Bcl2 and Beclin-1. Cold exposure reduced phosphorylation of the autophagy inhibitory signaling molecules Akt and mTOR, increased ULK1 phosphorylation, and dampened eNOS phosphorylation (without changes in their total protein expression). These cold exposure-induced changes in myocardial function, autophagy, and autophagy signaling cascades were significantly alleviated or mitigated by metallothionein. Inhibition of autophagy using 3-methyladenine in vivo reversed cold exposure-induced cardiomyocyte contractile defects. Cold exposure-induced cardiomyocyte dysfunction was attenuated by the antioxidant N-acetylcysteine and the lysosomal inhibitor bafilomycin A1. Collectively, these findings suggest that metallothionein protects against cold exposure-induced cardiac anomalies possibly through attenuation of cardiac autophagy.

A dual role for receptor-interacting protein kinase 2 (RIP2) kinase activity in nucleotide-binding oligomerization domain 2 (NOD2)-dependent autophagy

Autophagy is triggered by the intracellular bacterial sensor NOD2 (nucleotide-binding, oligomerization domain 2) as an anti-bacterial response. Defects in autophagy have been implicated in Crohn's disease susceptibility. The molecular mechanisms of activation and regulation of this process by NOD2 are not well understood, with recent studies reporting conflicting requirements for RIP2 (receptor-interacting protein kinase 2) in autophagy induction. We examined the requirement of NOD2 signaling mediated by RIP2 for anti-bacterial autophagy induction and clearance of Salmonella typhimurium in the intestinal epithelial cell line HCT116. Our data demonstrate that NOD2 stimulates autophagy in a process dependent on RIP2 tyrosine kinase activity. Autophagy induction requires the activity of the mitogen-activated protein kinases MEKK4 and p38 but is independent of NFκB signaling. Activation of autophagy was inhibited by a PP2A phosphatase complex, which interacts with both NOD2 and RIP2. PP2A phosphatase activity inhibited NOD2-dependent autophagy but not activation of NFκB or p38. Upon stimulation of NOD2, the phosphatase activity of the PP2A complex is inhibited through tyrosine phosphorylation of the catalytic subunit in a process dependent on RIP2 activity. These findings demonstrate that RIP2 tyrosine kinase activity is not only required for NOD2-dependent autophagy but plays a dual role in this process. RIP2 both sends a positive autophagy signal through activation of p38 MAPK and relieves repression of autophagy mediated by the phosphatase PP2A.