Parkin functions as an E2-dependent ubiquitin- protein ligase and promotes the degradation of the synaptic vesicle-associated protein, CDCrel-1

Ubiquitin and Parkinson's disease through the looking glass of genetics

Biochemical alterations found in the brains of Parkinson's disease (PD) patients indicate that cellular stress is a major driver of dopaminergic neuronal loss. Oxidative stress, mitochondrial dysfunction, and ER stress lead to impairment of the homeostatic regulation of protein quality control pathways with a consequent increase in protein misfolding and aggregation and failure of the protein degradation machinery. Ubiquitin signalling plays a central role in protein quality control; however, prior to genetic advances, the detailed mechanisms of how impairment in the ubiquitin system was linked to PD remained mysterious. The discovery of mutations in the α-synuclein gene, which encodes the main protein misfolded in PD aggregates, together with mutations in genes encoding ubiquitin regulatory molecules, including PTEN-induced kinase 1 (PINK1), Parkin, and FBX07, has provided an opportunity to dissect out the molecular basis of ubiquitin signalling disruption in PD, and this knowledge will be critical for developing novel therapeutic strategies in PD that target the ubiquitin system.

Formation of neurodegenerative aggresome and death-inducing signaling complex in maternal diabetes-induced neural tube defects

Diabetes mellitus in early pregnancy increases the risk in infants of birth defects, such as neural tube defects (NTDs), known as diabetic embryopathy. NTDs are associated with hyperglycemia-induced protein misfolding and Caspase-8-induced programmed cell death. The present study shows that misfolded proteins are ubiquitinylated, suggesting that ubiquitin-proteasomal degradation is impaired. Misfolded proteins form aggregates containing ubiquitin-binding protein p62, suggesting that autophagic-lysosomal clearance is insufficient. Additionally, these aggregates contain the neurodegenerative disease-associated proteins α-Synuclein, Parkin, and Huntingtin (Htt). Aggregation of Htt may lead to formation of a death-inducing signaling complex of Hip1, Hippi, and Caspase-8. Treatment with chemical chaperones, such as sodium 4-phenylbutyrate (PBA), reduces protein aggregation in neural stem cells in vitro and in embryos in vivo. Furthermore, treatment with PBA in vivo decreases NTD rate in the embryos of diabetic mice, as well as Caspase-8 activation and cell death. Enhancing protein folding could be a potential interventional approach to preventing embryonic malformations in diabetic pregnancies.

[Atypical ubiquitination of proteins]

Ubiquitination is a type of posttranslational modification of intracellular proteins characterized by covalent attachment of one (monoubiquitination) or several (polyubiquitination) of ubiquitin molecules to target proteins. In the case of polyubiquitination, linear or branched polyubiquitin chains are formed. Their formation involves various lysine residues of monomeric ubiquitin. The best studied is Lys48-polyubiquitination, which targets proteins for proteasomal degradation. In this review we have considered examples of so-called atypical polyubiquitination, which mainly involves other lysine residues (Lys6, Lys11, Lys27, Lys29, Lys33, Lys63) and also N-terminal methionine. The considered examples convincingly demonstrate that polyubiquitination of proteins not necessarily targets proteins for their proteolytic degradation in proteasomes. Atypically polyubiquitinated proteins are involved in regulation of various processes and altered polyubiquitination of certain proteins is crucial for development of serious diseases.

SEPT12 phosphorylation results in loss of the septin ring/sperm annulus, defective sperm motility and poor male fertility

Septins are critical for numerous cellular processes through the formation of heteromeric filaments and rings indicating the importance of structural regulators in septin assembly. Several posttranslational modifications (PTMs) mediate the dynamics of septin filaments in yeast. However, little is known about the role of PTMs in regulating mammalian septin assembly, and the in vivo significance of PTMs on mammalian septin assembly and function remains unknown. Here, we showed that SEPT12 was phosphorylated on Ser198 using mass spectrometry, and we generated SEPT12 phosphomimetic knock-in (KI) mice to study its biological significance. The homozygous KI mice displayed poor male fertility due to deformed sperm with defective motility and loss of annulus, a septin-based ring structure. Immunohistochemistry of KI testicular sections suggested that SEPT12 phosphorylation inhibits septin ring assembly during annulus biogenesis. We also observed that SEPT12 was phosphorylated via PKA, and its phosphorylation interfered with SEPT12 polymerization into complexes and filaments. Collectively, our data indicate that SEPT12 phosphorylation inhibits SEPT12 filament formation, leading to loss of the sperm annulus/septin ring and poor male fertility. Thus, we provide the first in vivo genetic evidence characterizing importance of septin phosphorylation in the assembly, cellular function and physiological significance of septins.

Life and death in the trash heap: The ubiquitin proteasome pathway and UCHL1 in brain aging, neurodegenerative disease and cerebral Ischemia

The ubiquitin proteasome pathway (UPP) is essential for removing abnormal proteins and preventing accumulation of potentially toxic proteins within the neuron. UPP dysfunction occurs with normal aging and is associated with abnormal accumulation of protein aggregates within neurons in neurodegenerative diseases. Ischemia disrupts UPP function and thus may contribute to UPP dysfunction seen in the aging brain and in neurodegenerative diseases. Ubiquitin carboxy-terminal hydrolase L1 (UCHL1), an important component of the UPP in the neuron, is covalently modified and its activity inhibited by reactive lipids produced after ischemia. As a result, degradation of toxic proteins is impaired which may exacerbate neuronal function and cell death in stroke and neurodegenerative diseases. Preserving or restoring UCHL1 activity may be an effective therapeutic strategy in stroke and neurodegenerative diseases.

Parkin promotes proteasomal degradation of synaptotagmin IV by accelerating polyubiquitination

Parkin is an E3 ubiquitin ligase whose mutations cause autosomal recessive juvenile Parkinson's disease (PD). Unlike the human phenotype, parkin knockout (KO) mice show no apparent dopamine neuron degeneration, although they demonstrate reduced expression and activity of striatal mitochondrial proteins believed to be necessary for neuronal survival. Instead, parkin-KO mice show reduced striatal evoked dopamine release, abnormal synaptic plasticity, and non-motor symptoms, all of which appear to mimic the preclinical features of Parkinson's disease. Extensive studies have screened candidate synaptic proteins responsible for reduced evoked dopamine release, and synaptotagmin XI (Syt XI), an isoform of Syt family regulating membrane trafficking, has been identified as a substrate of parkin in humans. However, its expression level is unaltered in the striatum of parkin-KO mice. Thus, the target(s) of parkin and the molecular mechanisms underlying the impaired dopamine release in parkin-KO mice remain unknown. In this study, we focused on Syt IV because of its highly homology to Syt XI, and because they share an evolutionarily conserved lack of Ca²⁺-binding capacity; thus, Syt IV plays an inhibitory role in Ca²⁺-dependent neurotransmitter release in PC12 cells and neurons in various brain regions. We found that a proteasome inhibitor increased Syt IV protein, but not Syt XI protein, in neuron-like, differentiated PC12 cells, and that parkin interacted with and polyubiquitinated Syt IV, thereby accelerating its protein turnover. Parkin overexpression selectively degraded Syt IV protein, but not Syt I protein (indispensable for Ca²⁺-dependent exocytosis), thus enhancing depolarization-dependent exocytosis. Furthermore, in parkin-KO mice, the level of striatal Syt IV protein was increased. Our data indicate a crucial role for parkin in the proteasomal degradation of Syt IV, and provide a potential mechanism of parkin-regulated, evoked neurotransmitter release.

Post translational modification of Parkin

Mutations in the gene encoding for the E3 ubiquitin ligase Parkin are associated to a rare form of familiar autosomal recessive Parkinsonism. Despite decades of research on the Parkin protein, whose structure has been recently solved, little is known about the specific signalling pathways that lead to Parkin activation. Parkin activity spans from mitochondria quality control to tumor suppression and stress protection; it is thus tempting to hypothesize that the broad impact of Parkin on cellular physiology might be the result of different post translational modifications that can be controlled by balanced opposing events. Sequence alignment of Parkin from different species indicates high homology between domains across Parkin orthologs and identifies highly conserved amino acid residues that, if modified, impinge on Parkin functions. In this review, we summarize findings on post translational modifications that have been shown to affect Parkin activity and stability.

REVIEWERS

This article was reviewed by Prof. Dr. Konstanze F. Winklhofer and by Prof. Thomas Simmen. Both reviewers have been nominated by Professor Luca Pellegrini.

Expression of sept3, sept5a and sept5b in the Developing and Adult Nervous System of the Zebrafish (Danio rerio)

Septins are a highly conserved family of small GTPases that form cytoskeletal filaments. Their cellular functions, especially in the nervous system, still remain largely enigmatic, but there are accumulating lines of evidence that septins play important roles in neuronal physiology and pathology. In order to further dissect septin function in the nervous system a detailed temporal resolved analysis in the genetically well tractable model vertebrate zebrafish (Danio rerio) is crucially necessary. To close this knowledge gap we here provide a reference dataset describing the expression of selected septins (sept3, sept5a and sept5b) in the zebrafish central nervous system. Strikingly, proliferation zones are devoid of expression of all three septins investigated, suggesting that they have a role in post-mitotic neural cells. Our finding that three septins are mainly expressed in non-proliferative regions was further confirmed by double-stainings with a proliferative marker. Our RNA in situ hybridization (ISH) study, detecting sept3, sept5a and sept5b mRNAs, shows that all three septins are expressed in largely overlapping regions of the developing brain. However, the expression of sept5a is much more confined compared to sept3 and sept5b. In contrast, the expression of all the three analyzed septins is largely similar in the adult brain.

PINK1 and BECN1 relocalize at mitochondria-associated membranes during mitophagy and promote ER-mitochondria tethering and autophagosome formation

Mitophagy is a highly specialized process to remove dysfunctional or superfluous mitochondria through the macroautophagy/autophagy pathway, aimed at protecting cells from the damage of disordered mitochondrial metabolism and apoptosis induction. PINK1, a neuroprotective protein mutated in autosomal recessive Parkinson disease, has been implicated in the activation of mitophagy by selectively accumulating on depolarized mitochondria, and promoting PARK2/Parkin translocation to them. While these steps have been characterized in depth, less is known about the process and site of autophagosome formation upon mitophagic stimuli. A previous study reported that, in starvation-induced autophagy, the proautophagic protein BECN1/Beclin1 (which we previously showed to interact with PINK1) relocalizes at specific regions of contact between the endoplasmic reticulum (ER) and mitochondria called mitochondria-associated membranes (MAM), from which the autophagosome originates. Here we show that, following mitophagic stimuli, autophagosomes also form at MAM; moreover, endogenous PINK1 and BECN1 were both found to relocalize at MAM, where they promoted the enhancement of ER-mitochondria contact sites and the formation of omegasomes, that represent autophagosome precursors. PARK2 was also enhanced at MAM following mitophagy induction. However, PINK1 silencing impaired BECN1 enrichment at MAM independently of PARK2, suggesting a novel role for PINK1 in regulating mitophagy. MAM have been recently implicated in many key cellular events. In this light, the observed prevalent localization of PINK1 at MAM may well explain other neuroprotective activities of this protein, such as modulation of mitochondrial calcium levels, mitochondrial dynamics, and apoptosis.

The Mammalian Septin Interactome

Septins are GTP-binding and membrane-interacting proteins with a highly conserved domain structure involved in various cellular processes, including cytoskeleton organization, cytokinesis, and membrane dynamics. To date, 13 different septin genes have been identified in mammals (SEPT1 to SEPT12 and SEPT14), which can be classified into four distinct subgroups based on the sequence homology of their domain structure (SEPT2, SEPT3, SEPT6, and SEPT7 subgroup). The family members of these subgroups have a strong affinity for other septins and form apolar tri-, hexa-, or octameric complexes consisting of multiple septin polypeptides. The first characterized core complex is the hetero-trimer SEPT2-6-7. Within these complexes single septins can be exchanged in a subgroup-specific manner. Hexamers contain SEPT2 and SEPT6 subgroup members and SEPT7 in two copies each whereas the octamers additionally comprise two SEPT9 subgroup septins. The various isoforms seem to determine the function and regulation of the septin complex. Septins self-assemble into higher-order structures, including filaments and rings in orders, which are typical for different cell types. Misregulation of septins leads to human diseases such as neurodegenerative and bleeding disorders. In non-dividing cells such as neuronal tissue and platelets septins have been associated with exocytosis. However, many mechanistic details and roles attributed to septins are poorly understood. We describe here some important mammalian septin interactions with a special focus on the clinically relevant septin interactions.

A Rab5 endosomal pathway mediates Parkin-dependent mitochondrial clearance

Damaged mitochondria pose a lethal threat to cells that necessitates their prompt removal. The currently recognized mechanism for disposal of mitochondria is autophagy, where damaged organelles are marked for disposal via ubiquitylation by Parkin. Here we report a novel pathway for mitochondrial elimination, in which these organelles undergo Parkin-dependent sequestration into Rab5-positive early endosomes via the ESCRT machinery. Following maturation, these endosomes deliver mitochondria to lysosomes for degradation. Although this endosomal pathway is activated by stressors that also activate mitochondrial autophagy, endosomal-mediated mitochondrial clearance is initiated before autophagy. The autophagy protein Beclin1 regulates activation of Rab5 and endosomal-mediated degradation of mitochondria, suggesting cross-talk between these two pathways. Abrogation of Rab5 function and the endosomal pathway results in the accumulation of stressed mitochondria and increases susceptibility to cell death in embryonic fibroblasts and cardiac myocytes. These data reveal a new mechanism for mitochondrial quality control mediated by Rab5 and early endosomes.

Neuroprotective mechanisms of plant extracts against MPTP induced neurotoxicity: Future applications in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease after Alzheimer's disease, affecting about seven to 10 million patients worldwide. The major pathological features of PD are loss of dopaminergic neurons in the nigrostriatal pathway and accumulation of alpha-synuclein molecules, forming Lewy bodies. Until now, there is no effective cure for PD, and investigators are searching for neuroprotective strategies to stop or slow the disease progression. The MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) induced neurotoxicity of the nigrostriatal pathway has been used to initiate PD in animal models. Multiple experimental studies showed the ability of several plant extracts to protect against MPTP induced neurotoxicity through activation of catalase, superoxide dismutase, and glutathione reductase enzymes, which reduce the cellular concentration of free radicals, preventing intracellular Ca++ release and subsequent apoptosis signaling. Other neuroprotective mechanisms of plant extracts include promoting autophagy of alpha-synuclein molecules and exerting an antiapoptotic activity via inhibition of proteolytic poly (ADP-ribose) polymerase and preventing caspase cleavage. The variety of neuroprotective mechanisms of natural plant extracts may allow researchers to target PD progression in different pathological stages and may be through multiple pathways. Further investigations are required to translate these neuroprotective mechanisms into safe and effective treatments for PD.

Parkin and PINK1 functions in oxidative stress and neurodegeneration

Loss-of-function mutations in the genes encoding Parkin and PINK1 are causally linked to autosomal recessive Parkinson's disease (PD). Parkin, an E3 ubiquitin ligase, and PINK1, a mitochondrial-targeted kinase, function together in a common pathway to remove dysfunctional mitochondria by autophagy. Presumably, deficiency for Parkin or PINK1 impairs mitochondrial autophagy and thereby increases oxidative stress due to the accumulation of dysfunctional mitochondria that release reactive oxygen species. Parkin and PINK1 likely have additional functions that may be relevant to the mechanisms by which mutations in these genes cause neurodegeneration, such as regulating inflammation, apoptosis, or dendritic morphogenesis. Here we briefly review what is known about functions of Parkin and PINK1 related to oxidative stress and neurodegeneration.

Parkin deficiency exacerbate ethanol-induced dopaminergic neurodegeneration by P38 pathway dependent inhibition of autophagy and mitochondrial function

Parkinson's disease (PD) is a neurodegenerative disease characterized by selective degeneration of dopaminergic neurons in the substantia nigra. Parkin (which encoded by Park2), an E3 ubiquitin ligase, is the most frequently mutated gene that has casually been linked to autosomal recessive early onset familial PD. We tested the effect of Park2 on ethanol-induced dopaminergic neurodegeneration in Park2 knockout (KO) transgenic mice after chronic ethanol feeding. Male Park2 wild type (WT) and KO mice (8 weeks old) were fed on a Lieber-DeCarli diet containing 6.6% ethanol for 2 weeks, and compared their responses. We found that knockout of Park2 exacerbates ethanol-induced behavioral impairment as well as dopamine depletion. In the mechanism study, we found that knockout of Park2 increased reactive oxygen species (ROS) production, mitophagy formation, mitochondrial dysfunction, and expression of pro-apoptotic proteins, but decreased expression of pro-autophagic proteins. Knockout of Park2 also increased ethanol-induced activation of p38 mitogen-activated protein kinase. In addition, ROS production, mitophagy formation, mitochondrial dysfunction, and expression of pro-apoptotic proteins were increased, but expression of pro-autophagic proteins were decreased by a treatment of ethanol (100 μM) in Park2 siRNA-transfacted PC12 cells (5 μM). Moreover, the exacerbating effects of Park2 deletion on ethanol-induced ROS generation, mitophagy, mitochondrial dysfunction as well as cell death were reduced by p38 specific inhibitor (SB203580) in in vitro (10 μM) and in vivo 10 mg/kg). Park2 deficiency exacerbates ethanol-induced dopaminergic neuron damage through p38 kinase dependent inhibition of autophagy and mitochondrial function.

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EtOH consumption can induce the ROS formation through activation of p38 MAPK.

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ROS can cause the neurodegeneration through inhibition of the autophagy system.

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Park2 knock down amplifies EtOH-induced decrement of autophagy.

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Park2 knock down amplifies EtOH-induced mitochondrial dysfunction.

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Park2 has a neuroprotective effect against ROS mediated damage of neuron.

Mammalian Septin Function in Hemostasis and Beyond

Interest in the biology of mammalian septin proteins has undergone a birth in recent years. Originally identified as critical for yeast budding throughout the 1970s, the septin family is now recognized to extend from yeast to humans and is associated with a variety of events ranging from cytokinesis to vesicle trafficking. An emerging theme for septins is their presence at sites where active membrane or cytoplasmic partitioning is occurring. Here, we briefly review the mammalian septin protein family and focus on a prototypic human and mouse septin, termed SEPT5, that is expressed in the brain, heart, and megakaryocytes. Work from neurobiology laboratories has linked SEPT5 to the exocytic complex of neurons, with implications that SEPT5 regulates neurotransmitter release. Striking similarities exist between neurotransmitter release and the platelet-release reaction, which is a critical step in platelet response to vascular injury. Work from our laboratory has characterized the platelet phenotype from mice containing a targeted deletion of SEPT5. Most strikingly, platelets from SEPT5null animals aggregate and release granular contents in response to subthreshold levels of agonists. Thus, the characterization of a SEPT5-deficient mouse has linked SEPT5 to the Platelet exocytic process and, as such, illustrates it as an important protein for regulating platelet function. Recent data suggest that platelets contain a wide repertoire of different septin proteins and assemble to form macromolecular septin complexes. The mouse platelet provides an experimental framework to define septin function in hemostasis, with implications for neurobiology and beyond.

Specificity and disease in the ubiquitin system

Post-translational modification (PTM) of proteins by ubiquitination is an essential cellular regulatory process. Such regulation drives the cell cycle and cell division, signalling and secretory pathways, DNA replication and repair processes and protein quality control and degradation pathways. A huge range of ubiquitin signals can be generated depending on the specificity and catalytic activity of the enzymes required for attachment of ubiquitin to a given target. As a consequence of its importance to eukaryotic life, dysfunction in the ubiquitin system leads to many disease states, including cancers and neurodegeneration. This review takes a retrospective look at our progress in understanding the molecular mechanisms that govern the specificity of ubiquitin conjugation.

BNIP3 Protein Suppresses PINK1 Kinase Proteolytic Cleavage to Promote Mitophagy

Mutations in PINK1 (PTEN-induced putative kinase 1) cause early onset familial Parkinson's disease (PD). PINK1 accumulates on the outer membrane of damaged mitochondria followed by recruiting parkin to promote mitophagy. Here, we demonstrate that BCL2/adenovirus E1B 19-kDa interacting protein 3 (BNIP3), a mitochondrial BH3-only protein, interacts with PINK1 to promote the accumulation of full-length PINK1 on the outer membrane of mitochondria, which facilitates parkin recruitment and PINK1/parkin-mediated mitophagy. Inactivation of BNIP3 in mammalian cells promotes PINK1 proteolytic processing and suppresses PINK1/parkin-mediated mitophagy. Hypoxia-induced BNIP3 expression results in increased expression of full-length PINK1 and mitophagy. Consistently, expression of BNIP3 in Drosophila suppresses muscle degeneration and the mitochondrial abnormality caused by PINK1 inactivation. Together, the results suggest that BNIP3 plays a vital role in regulating PINK1 mitochondrial outer membrane localization, the proteolytic process of PINK1 and PINK1/parkin-mediated mitophagy under physiological conditions. Functional up-regulation of BNIP3 may represent a novel therapeutic strategy to suppress the progression of PD.

Structural insights into the catalysis and regulation of E3 ubiquitin ligases

Covalent attachment (conjugation) of one or more ubiquitin molecules to protein substrates governs numerous eukaryotic cellular processes, including apoptosis, cell division and immune responses. Ubiquitylation was originally associated with protein degradation, but it is now clear that ubiquitylation also mediates processes such as protein-protein interactions and cell signalling depending on the type of ubiquitin conjugation. Ubiquitin ligases (E3s) catalyse the final step of ubiquitin conjugation by transferring ubiquitin from ubiquitin-conjugating enzymes (E2s) to substrates. In humans, more than 600 E3s contribute to determining the fates of thousands of substrates; hence, E3s need to be tightly regulated to ensure accurate substrate ubiquitylation. Recent findings illustrate how E3s function on a structural level and how they coordinate with E2s and substrates to meticulously conjugate ubiquitin. Insights regarding the mechanisms of E3 regulation, including structural aspects of their autoinhibition and activation are also emerging.

Parkin elimination of mitochondria is important for maintenance of lens epithelial cell ROS levels and survival upon oxidative stress exposure

Age-related cataract is associated with oxidative stress and death of lens epithelial cells (LECs) whose survival is dependent on functional mitochondrial populations. Oxidative stress-induced depolarization/damage of LEC mitochondria results in increased reactive oxygen species (ROS) levels and cell death suggesting the need for a LEC mechanism to remove mitochondria depolarized/damaged upon oxidative stress exposure to prevent ROS release and LEC death. To date, a mechanism(s) for removal of depolarized/damaged LEC mitochondria has yet to be identified and the importance of eliminating oxidative stress-damaged mitochondria to prevent LEC ROS release and death has not been established. Here, we demonstrate that Parkin levels increase in LECs exposed to H₂O₂-oxidative stress. We establish that Parkin translocates to LEC mitochondria depolarized upon oxidative stress exposure and that Parkin recruits p62/SQSTM1 to depolarized LEC mitochondria. We demonstrate that translocation of Parkin results in the elimination of depolarized/damaged mitochondria and that Parkin clearance of LEC mitochondria is dependent on its ubiquitin ligase activity. Importantly, we demonstrate that Parkin elimination of damaged LEC mitochondria results in reduced ROS levels and increased survival upon oxidative stress exposure. These results establish that Parkin functions to eliminate LEC mitochondria depolarized/damaged upon oxidative stress exposure and that elimination of damaged mitochondria by Parkin is important for LEC homeostasis and survival. The data also suggest that mitochondrial quality control by Parkin could play a role in lens transparency.

Interdependence of laforin and malin proteins for their stability and functions could underlie the molecular basis of locus heterogeneity in Lafora disease

Lafora disease (LD), an autosomal recessive and fatal form of neurodegenerative disorder, is characterized by the presence of polyglucosan inclusions in the affected tissues including the brain. LD can be caused by defects either in the EPM2A gene coding for the laforin protein phosphatase or the NHLRC1 gene coding for the malin ubiquitin ligase. Since the clinical symptoms of LD patients representing the two genetic groups are very similar and since malin is known to interact with laforin, we were curious to examine the possibility that the two proteins regulate each other's function. Using cell biological assays we demonstrate here that (i) malin promotes its own degradation via autoubiquitination, (ii) laforin prevents the auto-degradation of malin by presenting itself as a substrate and (iii) malin preferentially degrades the phosphatase-inactive laforin monomer. Our results that laforin and malin regulate each other's stability and activity offers a novel and attractive model to explain the molecular basis of locus heterogeneity observed in LD.

Molecular insights into RBR E3 ligase ubiquitin transfer mechanisms

RING-in-between-RING (RBR) ubiquitin (Ub) ligases are a distinct class of E3s, defined by a RING1 domain that binds E2 Ub-conjugating enzyme and a RING2 domain that contains an active site cysteine similar to HECT-type E3s. Proposed to function as RING/HECT hybrids, details regarding the Ub transfer mechanism used by RBRs have yet to be defined. When paired with RING-type E3s, E2s perform the final step of Ub ligation to a substrate. In contrast, when paired with RBR E3s, E2s must transfer Ub onto the E3 to generate a E3~Ub intermediate. We show that RBRs utilize two strategies to ensure transfer of Ub from the E2 onto the E3 active site. First, RING1 domains of HHARI and RNF144 promote open E2~Ubs. Second, we identify a Ub-binding site on HHARI RING2 important for its recruitment to RING1-bound E2~Ub. Mutations that ablate Ub binding to HHARI RING2 also decrease RBR ligase activity, consistent with RING2 recruitment being a critical step for the RBR Ub transfer mechanism. Finally, we demonstrate that the mechanism defined here is utilized by a variety of RBRs.

miR-27a and miR-27b regulate autophagic clearance of damaged mitochondria by targeting PTEN-induced putative kinase 1 (PINK1)

Background

Loss-of-function mutations in PINK1 and PARKIN are the most common causes of autosomal recessive Parkinson’s disease (PD). PINK1 is a mitochondrial serine/threonine kinase that plays a critical role in mitophagy, a selective autophagic clearance of damaged mitochondria. Accumulating evidence suggests mitochondrial dysfunction is one of central mechanisms underlying PD pathogenesis. Therefore, identifying regulatory mechanisms of PINK1 expression may provide novel therapeutic opportunities for PD. Although post-translational stabilization of PINK1 upon mitochondrial damage has been extensively studied, little is known about the regulation mechanism of PINK1 at the transcriptional or translational levels.

Results

Here, we demonstrated that microRNA-27a (miR-27a) and miR-27b suppress PINK1 expression at the translational level through directly binding to the 3′-untranslated region (3′UTR) of its mRNA. Importantly, our data demonstrated that translation of PINK1 is critical for its accumulation upon mitochondrial damage. The accumulation of PINK1 upon mitochondrial damage was strongly regulated by expression levels of miR-27a and miR-27b. miR-27a and miR-27b prevent mitophagic influx by suppressing PINK1 expression, as evidenced by the decrease of ubiquitin phosphorylation, Parkin translocation, and LC3-II accumulation in damaged mitochondria. Consequently, miR-27a and miR-27b inhibit lysosomal degradation of the damaged mitochondria, as shown by the decrease of the delivery of damaged mitochondria to lysosome and the degradation of cytochrome c oxidase 2 (COX2), a mitochondrial marker. Furthermore, our data demonstrated that the expression of miR-27a and miR-27b is significantly induced under chronic mitophagic flux, suggesting a negative feedback regulation between PINK1-mediated mitophagy and miR-27a and miR-27b.

Conclusions

We demonstrated that miR-27a and miR-27b regulate PINK1 expression and autophagic clearance of damaged mitochondria. Our data further support a novel negative regulatory mechanism of PINK1-mediated mitophagy by miR-27a and miR-27b. Therefore, our results considerably advance our understanding of PINK1 expression and mitophagy regulation and suggest that miR-27a and miR-27b may represent potential therapeutic targets for PD.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0121-4) contains supplementary material, which is available to authorized users.

Reduced intestinal lipid absorption and body weight-independent improvements in insulin sensitivity in high-fat diet-fed Park2 knockout mice

Mitochondrial dysfunction is associated with many human diseases and results from mismatch of damage and repair over the life of the organelle. PARK2 is a ubiquitin E3 ligase that regulates mitophagy, a repair mechanism that selectively degrades damaged mitochondria. Deletion of PARK2 in multiple in vivo models results in susceptibility to stress-induced mitochondrial and cellular dysfunction. Surprisingly, Park2 knockout (KO) mice are protected from nutritional stress and do not develop obesity, hepatic steatosis or insulin resistance when fed a high-fat diet (HFD). However, these phenomena are casually related and the physiological basis for this phenotype is unknown. We therefore undertook a series of acute HFD studies to more completely understand the physiology of Park2 KO during nutritional stress. We find that intestinal lipid absorption is impaired in Park2 KO mice as evidenced by increased fecal lipids and reduced plasma triglycerides after intragastric fat challenge. Park2 KO mice developed hepatic steatosis in response to intravenous lipid infusion as well as during incubation of primary hepatocytes with fatty acids, suggesting that hepatic protection from nutritional stress was secondary to changes in energy balance due to altered intestinal triglyceride absorption. Park2 KO mice showed reduced adiposity after 1-wk HFD, as well as improved hepatic and peripheral insulin sensitivity. These studies suggest that changes in intestinal lipid absorption may play a primary role in protection from nutritional stress in Park2 KO mice by preventing HFD-induced weight gain and highlight the need for tissue-specific models to address the role of PARK2 during metabolic stress.

Microtubule: A Common Target for Parkin and Parkinson’s Disease Toxins

Parkinson’s disease (PD) is characterized by the selective loss of nigral dopaminergic (DA) neurons, which have long axons enriched with microtubules. Depolymerization of microtubules by PD toxins such as rotenone disrupts vesicular transport. The ensuing accumulation of vesicles in the cell body leads to increased cytosolic concentration of dopamine due to leakage of the vesicles. Elevated oxidative stress induced by dopamine oxidation may thus trigger the selective demise of DA neurons. Many strategies have been developed to protect DA neurons by stabilizing microtubules either directly or through intracellular signaling cascades. On the other hand, parkin, one of the most frequently mutated genes in PD, encodes for a protein-ubiquitin E3 ligase that strongly binds to microtubules. Parkin stabilizes microtubules through three domains that provide strong and independent interactions with tubulin and microtubules. These interactions anchor parkin on microtubules and may facilitate its E3 ligase activity on misfolded proteins transported along microtubules. Thus, parkin and rotenone, two prominent genetic and environmental factors linked to PD, act in an opposing manner on the same molecular target in the cell, microtubules, whose destruction underlies the selective vulnerability of dopaminergic neurons.

Parkin loss-of-function pathology: Premature neuronal senescence induced by high levels of reactive oxygen species?

Parkinson's and Alzheimer's diseases (PD and AD, respectively) are considered to be diseases of advanced brain ageing, which seems to involve high levels of reactive oxygen species (ROS). AD neurodegeneration is initially apparent in the hippocampus; as AD progresses, many more brain regions are affected. PD-associated neurodegeneration is relatively limited to dopaminergic neurons of the substantia nigra pars compacta (SNpc), especially in cases in which patients inherit particular disease-causing mutations. Thus, the task of elucidating mechanisms by which loss of function of one particular protein triggers death of a subset of neurons may be more approachable. Understanding the mechanisms of neurodegeneration in these forms of PD may not only shed light on avenues leading toward therapeutic strategies in PD and other neurodegenerative diseases, but also on those leading toward understanding natural ageing. Neurodegeneration in PD patients harboring homozygous loss-of-function mutations in the PARK2 gene may result from unbalanced levels of ROS, which are mostly produced in mitochondria and can irreparably damage macromolecules and trigger apoptosis. This review discusses mitochondrial sources of ROS, how ROS can trigger apoptosis, mechanisms by which Parkin loss-of-function may cause neurodegeneration by increasing ROS levels, and concludes with hypotheses regarding selective SNpc dopaminergic neuron vulnerability.

Ubiqutination via K27 and K29 chains signals aggregation and neuronal protection of LRRK2 by WSB1

A common genetic form of Parkinson's disease (PD) is caused by mutations in LRRK2. We identify WSB1 as a LRRK2 interacting protein. WSB1 ubiquitinates LRRK2 through K27 and K29 linkage chains, leading to LRRK2 aggregation and neuronal protection in primary neurons and a Drosophila model of G2019S LRRK2. Knocking down endogenous WSB1 exacerbates mutant LRRK2 neuronal toxicity in neurons and the Drosophila model, indicating a role for endogenous WSB1 in modulating LRRK2 cell toxicity. WSB1 is in Lewy bodies in human PD post-mortem tissue. These data demonstrate a role for WSB1 in mutant LRRK2 pathogenesis, and suggest involvement in Lewy body pathology in sporadic PD. Our data indicate a role in PD for ubiquitin K27 and K29 linkages, and suggest that ubiquitination may be a signal for aggregation and neuronal protection in PD, which may be relevant for other neurodegenerative disorders. Finally, our study identifies a novel therapeutic target for PD.

Polyubiquitination of apurinic/apyrimidinic endonuclease 1 by Parkin

Apurinic/apyrimidinic endonuclease 1 (APE1) is an essential protein crucial for repair of oxidized DNA damage not only in genomic DNA but also in mitochondrial DNA. Parkin, a tumor suppressor and Parkinson's disease (PD) associated gene, is an E3 ubiquitin ligase crucial for mitophagy. Although DNA damage is known to induce mitochondrial stress, Parkin's role in regulating DNA repair proteins has not been elucidated. In this study, we examined the possibility of Parkin-dependent ubiquitination of APE1. Ectopically expressed APE1 was degraded by Parkin and PINK1 via polyubiquitination in mouse embryonic fibroblast cells. PD-causing mutations in Parkin and PINK1 abrogated APE1 ubiquitination. Interaction of APE1 with Parkin was observed by co-immunoprecipitation, proximity ligation assay, and co-localization in the cytoplasm. N-terminal deletion of 41 amino acid residues in APE1 significantly reduced the Parkin-dependent APE1 degradation. These results suggested that Parkin directly ubiquitinated N-terminal Lys residues in APE1 in the cytoplasm. Modulation of Parkin and PINK1 activities under mitochondrial or oxidative stress caused moderate but statistically significant decrease of endogenous APE1 in human cell lines including SH-SY5Y, HEK293, and A549 cells. Analyses of glioblastoma tissues showed an inverse relation between the expression levels of APE1 and Parkin. These results suggest that degradation of endogenous APE1 by Parkin occur when cells are stressed to activate Parkin, and imply a role of Parkin in maintaining the quality of APE1, and loss of Parkin may contribute to elevated APE1 levels in glioblastoma. © 2016 Wiley Periodicals, Inc.

Linking F-box protein 7 and parkin to neuronal degeneration in Parkinson's disease (PD)

Mutations of F-box protein 7 (FBXO7) and Parkin, two proteins in ubiquitin-proteasome system (UPS), are both implicated in pathogenesis of dopamine (DA) neuron degeneration in Parkinson's disease (PD). Parkin is a HECT/RING hybrid ligase that physically receives ubiquitin on its catalytic centre and passes ubiquitin onto its substrates, whereas FBXO7 is an adaptor protein in Skp-Cullin-F-box (SCF) SCF(FBXO7) ubiquitin E3 ligase complex to recognize substrates and mediate substrates ubiquitination by SCF(FBXO7) E3 ligase. Here, we discuss the overlapping pathophysiologic mechanisms and clinical features linking Parkin and FBXO7 with autosomal recessive PD. Both proteins play an important role in neuroprotective mitophagy to clear away impaired mitochondria. Parkin can be recruited to impaired mitochondria whereas cellular stress can promote FBXO7 mitochondrial translocation. PD-linked FBXO7 can recruit Parkin into damaged mitochondria and facilitate its aggregation. WT FBXO7, but not PD-linked FBXO7 mutants can rescue DA neuron degeneration in Parkin null Drosophila. A better understanding of the common pathophysiologic mechanisms of these two proteins could unravel specific pathways for targeted therapy in PD.

SEPT8 modulates β-amyloidogenic processing of APP by affecting the sorting and accumulation of BACE1

Dysfunction and loss of synapses are early pathogenic events in Alzheimer's disease. A central step in the generation of toxic amyloid-β (Aβ) peptides is the cleavage of amyloid precursor protein (APP) by β-site APP-cleaving enzyme (BACE1). Here, we have elucidated whether downregulation of septin (SEPT) protein family members, which are implicated in synaptic plasticity and vesicular trafficking, affects APP processing and Aβ generation. SEPT8 was found to reduce soluble APPβ and Aβ levels in neuronal cells through a post-translational mechanism leading to decreased levels of BACE1 protein. In the human temporal cortex, we identified alterations in the expression of specific SEPT8 transcript variants in a manner that correlated with Alzheimer's-disease-related neurofibrillary pathology. These changes were associated with altered β-secretase activity. We also discovered that the overexpression of a specific Alzheimer's-disease-associated SEPT8 transcript variant increased the levels of BACE1 and Aβ peptides in neuronal cells. These changes were related to an increased half-life of BACE1 and the localization of BACE1 in recycling endosomes. These data suggest that SEPT8 modulates β-amyloidogenic processing of APP through a mechanism affecting the intracellular sorting and accumulation of BACE1.

The ubiquitin signal and autophagy: an orchestrated dance leading to mitochondrial degradation

The quality of mitochondria, essential organelles that produce ATP and regulate numerous metabolic pathways, must be strictly monitored to maintain cell homeostasis. The loss of mitochondrial quality control systems is acknowledged as a determinant for many types of neurodegenerative diseases including Parkinson's disease (PD). The two gene products mutated in the autosomal recessive forms of familial early-onset PD, Parkin and PINK1, have been identified as essential proteins in the clearance of damaged mitochondria via an autophagic pathway termed mitophagy. Recently, significant progress has been made in understanding how the mitochondrial serine/threonine kinase PINK1 and the E3 ligase Parkin work together through a novel stepwise cascade to identify and eliminate damaged mitochondria, a process that relies on the orchestrated crosstalk between ubiquitin/phosphorylation signaling and autophagy. In this review, we highlight our current understanding of the detailed molecular mechanisms governing Parkin-/PINK1-mediated mitophagy and the evidences connecting Parkin/PINK1 function and mitochondrial clearance in neurons.

Parkin Regulation and Neurodegenerative Disorders

Parkin is a unique, multifunctional ubiquitin ligase whose various roles in the cell, particularly in neurons, are widely thought to be protective. The pivotal role that Parkin plays in maintaining neuronal survival is underscored by our current recognition that Parkin dysfunction represents not only a predominant cause of familial parkinsonism but also a formal risk factor for the more common, sporadic form of Parkinson’s disease (PD). Accordingly, keen research on Parkin over the past decade has led to an explosion of knowledge regarding its physiological roles and its relevance to PD. However, our understanding of Parkin is far from being complete. Indeed, surprises emerge from time to time that compel us to constantly update the paradigm of Parkin function. For example, we now know that Parkin’s function is not confined to mere housekeeping protein quality control (QC) roles but also includes mitochondrial homeostasis and stress-related signaling. Furthermore, emerging evidence also suggest a role for Parkin in several other major neurodegenerative diseases including Alzheimer’s disease (AD) and Amyotrophic Lateral Sclerosis (ALS). Yet, it remains truly amazing to note that a single enzyme could serve such multitude of functions and cellular roles. Clearly, its activity has to be tightly regulated. In this review, we shall discuss this and how dysregulated Parkin function may precipitate neuronal demise in various neurodegenerative disorders.

Parkin New Cargos: a New ROS Independent Role for Parkin in Regulating Cell Division

Cell cycle progression requires the destruction of key cell cycle regulators by the multi-subunit E3 ligase called the anaphase promoting complex (APC/C). As the cell progresses through the cell cycle, the APC/C is sequentially activated by two highly conserved co-activators called Cdc20 and Cdh1. Importantly, APC/C^Cdc20 is required to degrade substrates in G2/M whereas APC^Cdh1 drives the cells into G1. Recently, Parkin, a monomeric E3 ligase that is required for ubiquitin-mediated mitophagy following mitochondrial stress, was shown to both bind and be activated by Cdc20 or Cdh1 during the cell cycle. This mitotic role for Parkin does not require an activating phosphorylation by its usual kinase partner PINK. Rather, mitotic Parkin activity requires phosphorylation on a different serine by the polo-like kinase Plk1. Interestingly, although Parkin^Cdc20 and Parkin^Cdh1 activity is independent of the APC/C, it mediates degradation of an overlapping subset of substrates. However, unlike the APC/C, Parkin is not necessary for cell cycle progression. Despite this, loss of Parkin activity accelerates genome instability and tumor growth in xenograft models. These findings provide a mechanism behind the previously described, but poorly understood, tumor suppressor role for Parkin. Taken together, studies suggest that the APC/C and Parkin have similar and unique roles to play in cell division, possibly being dependent upon the different subcellular address of these two ligases.

Can the disease course in Parkinson's disease be slowed?

BACKGROUND

The diagnosis of Parkinson's disease (PD), which is needed for useful symptomatic therapy, is based on clinical criteria. However, it became quite clear in recent years that the same features can occur through different etiopathogenic mechanisms. Even a pathological diagnosis of PD, based on the demonstration of α-synuclein deposits in a typical distribution, can result from different causes and, vice versa, nigral cell loss can occur without α-synuclein deposition.

DISCUSSION

Thus far, attempts to influence the progression of PD have failed. However, since the clinical manifestations of PD can be the result of diverse mechanisms, a single intervention may not be able to slow the course of the disease in all patients. Indeed, targeting the underlying pathogenic processes, which differ among cases, may be more effective. PD may develop as a consequence of mitochondrial damage, which itself may result from a variety of genetic or environmental factors. Correction of the ensuing oxidative stress may theoretically be useful in these PD patients, but will not affect the progression of the disease among other PD patients in whom an identical clinical syndrome derives from defects in other pathways such as the ubiquitin-proteasome system and lysosomal dysfunction, among others. Precision medicine can now be used to identify the underlying pathogenic mechanisms in individual patients, paving the way to the development of real disease modification through a pathway-oriented approach, aimed at the underlying biologic processes of disease occurrence and evolution.

Loss of parkin promotes lipid rafts-dependent endocytosis through accumulating caveolin-1: implications for Parkinson's disease

Background

Parkinson’s disease (PD) is characterized by progressive loss of midbrain dopaminergic neurons, resulting in motor dysfunctions. While most PD is sporadic in nature, a significant subset can be linked to either autosomal dominant or recessive mutations. PARK2, encoding the E3 ubiquitin ligase, parkin, is the most frequently mutated gene in autosomal recessive early onset PD. It has recently been reported that PD-associated gene products such as PINK1, α-synuclein, LRRK2, and DJ-1, as well as parkin associate with lipid rafts, suggesting that the dysfunction of these proteins in lipid rafts may be a causal factor of PD. Therefore here, we examined the relationship between lipid rafts-related proteins and parkin.

Results

We identified caveolin-1 (cav-1), which is one of the major constituents of lipid rafts at the plasma membrane, as a substrate of parkin. Loss of parkin function was found to disrupt the ubiquitination and degradation of cav-1, resulting in elevated cav-1 protein level in cells. Moreover, the total cholesterol level and membrane fluidity was altered by parkin deficiency, causing dysregulation of lipid rafts-dependent endocytosis. Further, cell-to-cell transmission of α-synuclein was facilitated by parkin deficiency.

Conclusions

Our results demonstrate that alterations in lipid rafts by the loss of parkin via cav-1 may be a causal factor of PD, and cav-1 may be a novel therapeutic target for PD.

Drug Discovery Opportunities at the Endothelin B Receptor-Related Orphan G Protein-Coupled Receptors, GPR37 and GPR37L1

Orphan G protein-coupled receptors (GPCRs) represent a largely untapped resource for the treatment of a variety of diseases, despite sophisticated advances in drug discovery. Two promising orphan GPCRs are the endothelin B receptor-like proteins, GPR37 [ET(B)R-LP, Pael-R] and GPR37L1 [ET(B)R-LP-2]. Originally identified through searches for homologs of endothelin and bombesin receptors, neither GPR37 nor GPR37L1 were found to bind endothelins or related peptides. Instead, GPR37 was proposed to be activated by head activator (HA) and both GPR37 and GPR37L1 have been linked to the neuropeptides prosaposin and prosaptide, although these pairings are yet to be universally acknowledged. Both orphan GPCRs are widely expressed in the brain, where GPR37 has received the most attention for its link to Parkinson’s disease and parkinsonism, while GPR37L1 deletion leads to precocious cerebellar development and hypertension. In this review, the existing pharmacology and physiology of GPR37 and GPR37L1 is discussed and the potential therapeutic benefits of targeting these receptors are explored.

Roles of LIM kinases in central nervous system function and dysfunction

LIM kinase 1 (LIMK1) and LIM kinase 2 (LIMK2) regulate actin dynamics by phosphorylating cofilin. In this review, we outline studies that have shown an involvement of LIMKs in neuronal function and we detail some of the pathways and molecular mechanisms involving LIMKs in neurodevelopment and synaptic plasticity. We also review the involvement of LIMKs in neuronal diseases and emphasize the differences in the regulation of LIMKs expression and mode of action. We finally present the existence of a cofilin-independent pathway also involved in neuronal function. A better understanding of the differences between both LIMKs and of the precise molecular mechanisms involved in their mode of action and regulation is now required to improve our understanding of the physiopathology of the neuronal diseases associated with LIMKs.

The associations between Parkinson's disease and cancer: the plot thickens

Epidemiological studies support a general inverse association between the risk of cancer development and Parkinson’s disease (PD). In recent years however, increasing amount of eclectic evidence points to a positive association between PD and cancers through different temporal analyses and ethnic groups. This positive association has been supported by several common genetic mutations in SNCA, PARK2, PARK8, ATM, p53, PTEN, and MC1R resulting in cellular changes such as mitochondrial dysfunction, aberrant protein aggregation, and cell cycle dysregulation. Here, we review the epidemiological and biological advances of the past decade in the association between PD and cancers to offer insight on the recent and sometimes contradictory findings.

Parkin Regulates Mitosis and Genomic Stability through Cdc20/Cdh1

Mutations in the E3 ubiquitin ligase Parkin have been linked to familial Parkinson's disease. Parkin has also been implicated in mitosis through mechanisms that are unclear. Here we show that Parkin interacts with anaphase promoting complex/cyclosome (APC/C) coactivators Cdc20 and Cdh1 to mediate the degradation of several key mitotic regulators independent of APC/C. We demonstrate that ordered progression through mitosis is orchestrated by two distinct E3 ligases through the shared use of Cdc20 and Cdh1. Furthermore, Parkin is phosphorylated and activated by polo-like kinase 1 (Plk1) during mitosis. Parkin deficiency results in overexpression of its substrates, mitotic defects, genomic instability, and tumorigenesis. These results suggest that the Parkin-Cdc20/Cdh1 complex is an important regulator of mitosis.

Structural studies of parkin and sacsin: Mitochondrial dynamics in neurodegenerative diseases

Neurodegenerative diseases are prevalent, chronic diseases emanating from the dysfunction or death of neurons. The disrupted mitochondrial dynamics observed in a large number of neurodegenerative diseases suggests a common etiology with the possibility of therapies targeting multiple diseases. This review highlights the contributions of structural studies of disease-related proteins to the understanding of neurodegenerative disease pathogenesis and especially the cellular events leading to disruptions in mitochondrial dynamics and function. The examples used are parkin and sacsin, two proteins linked respectively to autosomal-recessive early-onset PD and autosomal-recessive spastic ataxia of Charlevoix-Saguenay. Structural studies of parkin and sacsin explain the pathogenicity of a large number of disease-associated mutations and reveal insights into their cellular functions related to mitochondrial dynamics.

F-box protein 7 mutations promote protein aggregation in mitochondria and inhibit mitophagy

The mutations of F-box protein 7 (FBXO7) gene (T22M, R378G and R498X) are associated with a severe form of autosomal recessive juvenile-onset Parkinson's disease (PD) (PARK 15). Here we demonstrated that wild-type (WT) FBXO7 is a stress response protein and it can play both cytoprotective and neurotoxic roles. The WT FBXO7 protein is vital to cell mitophagy and can facilitate mitophagy to protect cells, whereas mutant FBXO7 inhibits mitophagy. Upon stress, the endogenous WT FBXO7 gets up-regulated, concentrates into mitochondria and forms FBXO7 aggregates in mitochondria. However, FBXO7 mutations aggravate deleterious FBXO7 aggregation in mitochondria. The FBXO7 aggregation and toxicity can be alleviated by Proline, glutathione (GSH) and coenzyme Q10, whereas deleterious FBXO7 aggregation in mitochondria can be aggravated by prohibitin 1 (PHB1), a mitochondrial protease inhibitor. The overexpression of WT FBXO7 could lead to FBXO7 protein aggregation and dopamine neuron degeneration in transgenic Drosophila heads. The elevated FBXO7 expression and aggregation were identified in human fibroblast cells from PD patients. FBXO7 can also form aggregates in brains of PD and Alzheimer's disease. Our study provides novel pathophysiologic insights and suggests that FBXO7 may be a potential therapeutic target in FBXO7-linked neuron degeneration in PD.

Parkinson's Disease in Saudi Patients: A Genetic Study

Parkinson’s disease (PD) is one of the major causes of parkinsonism syndrome. Its characteristic motor symptoms are attributable to dopaminergic neurons loss in the midbrain. Genetic advances have highlighted underlying molecular mechanisms and provided clues to potential therapies. However, most of the studies focusing on the genetic component of PD have been performed on American, European and Asian populations, whereas Arab populations (excluding North African Arabs), particularly Saudis remain to be explored. Here we investigated the genetic causes of PD in Saudis by recruiting 98 PD-cases (sporadic and familial) and screening them for potential pathogenic mutations in PD-established genes; SNCA, PARKIN, PINK1, PARK7/DJ1, LRRK2 and other PD-associated genes using direct sequencing. To our surprise, the screening revealed only three pathogenic point mutations; two in PINK1 and one in PARKIN. In addition to mutational analysis, CNV and cDNA analysis was performed on a subset of patients. Exon/intron dosage alterations in PARKIN were detected and confirmed in 2 cases. Our study suggests that mutations in the ORF of the screened genes are not a common cause of PD in Saudi population; however, these findings by no means exclude the possibility that other genetic events such as gene expression/dosage alteration may be more common nor does it eliminate the possibility of the involvement of novel genes.

The three 'P's of mitophagy: PARKIN, PINK1, and post-translational modifications

In this review, Durcan and Fon discuss how post-translational modifications are at the heart of how PARKIN and PINK1 function in mitochondrial quality control. They also ask how our current understanding of these proteins may impact the development of future therapies for Parkinson's disease.

Two Parkinson's disease (PD)-associated proteins, the mitochondrial kinase PINK1 and the E3-ubiquitin (Ub) ligase PARKIN, are central to mitochondrial quality control. In this pathway, PINK1 accumulates on defective mitochondria, eliciting the translocation of PARKIN from the cytosol to mediate the clearance of damaged mitochondria via autophagy (mitophagy). Throughout the different stages of mitophagy, post-translational modifications (PTMs) are critical for the regulation of PINK1 and PARKIN activity and function. Indeed, activation and recruitment of PARKIN onto damaged mitochondria involves PINK1-mediated phosphorylation of both PARKIN and Ub. Through a stepwise cascade, PARKIN is converted from an autoinhibited enzyme into an active phospho-Ub-dependent E3 ligase. Upon activation, PARKIN ubiquitinates itself in concert with many different mitochondrial substrates. The Ub conjugates attached to these substrates can in turn be phosphorylated by PINK1, which triggers further cycles of PARKIN recruitment and activation. This feed-forward amplification loop regulates both PARKIN activity and mitophagy. However, the precise steps and sequence of PTMs in this cascade are only now being uncovered. For instance, the Ub conjugates assembled by PARKIN consist predominantly of noncanonical K6-linked Ub chains. Moreover, these modifications are reversible and can be disassembled by deubiquitinating enzymes (DUBs), including Ub-specific protease 8 (USP8), USP15, and USP30. However, PINK1-mediated phosphorylation of Ub can impede the activity of these DUBs, adding a new layer of complexity to the regulation of PARKIN-mediated mitophagy by PTMs. It is therefore evident that further insight into how PTMs regulate the PINK1–PARKIN pathway will be critical for our understanding of mitochondrial quality control.

Phosphorylation of ubiquitin at Ser65 affects its polymerization, targets, and proteome-wide turnover

Ubiquitylation is an essential post-translational modification that regulates numerous cellular processes, most notably protein degradation. Ubiquitin can itself be phosphorylated at nearly every serine, threonine, and tyrosine residue. However, the effect of this modification on ubiquitin function is largely unknown. Here, we characterized the effects of phosphorylation of yeast ubiquitin at serine 65 in vivo and in vitro. We find this post-translational modification to be regulated under oxidative stress, occurring concomitantly with the restructuring of the ubiquitin landscape into a highly polymeric state. Phosphomimetic mutation of S65 recapitulates the oxidative stress phenotype, causing a dramatic accumulation of ubiquitylated proteins and a proteome-wide reduction of protein turnover rates. Importantly, this mutation impacts ubiquitin chain disassembly, chain linkage distribution, ubiquitin interactions, and substrate targeting. These results demonstrate that phosphorylation is an additional mode of ubiquitin regulation with broad implications in cellular physiology.

Nigral overexpression of alpha-synuclein in the absence of parkin enhances alpha-synuclein phosphorylation but does not modulate dopaminergic neurodegeneration

BACKGROUND

Alpha-synuclein is a key protein in the pathogenesis of Parkinson's disease. Mutations in the parkin gene are the most common cause of early-onset autosomal recessive Parkinson's disease, probably through a loss-of-function mechanism. However, the molecular mechanism by which loss of parkin function leads to the development of the disease and the role of alpha-synuclein in parkin-associated Parkinson's disease is still not elucidated. Conflicting results were reported about the effect of the absence of parkin on alpha-synuclein-mediated neurotoxicity using a transgenic approach. In this study, we investigated the effect of loss of parkin on alpha-synuclein neuropathology and toxicity in adult rodent brain using viral vectors. Therefore, we overexpressed human wild type alpha-synuclein in the substantia nigra of parkin knockout and wild type mice using two different doses of recombinant adeno-associated viral vectors.

RESULTS

No difference was observed in nigral dopaminergic cell loss between the parkin knockout mice and wild type mice up to 16 weeks after viral vector injection. However, the level of alpha-synuclein phosphorylated at serine residue 129 in the substantia nigra was significantly increased in the parkin knockout mice compared to the wild type mice while the total expression level of alpha-synuclein was similar in both groups. The increased alpha-synuclein phosphorylation was confirmed in a parkin knockdown cell line.

CONCLUSIONS

These findings support a functional relationship between parkin and alpha-synuclein phosphorylation in rodent brain.

The KM-parkin-DB: A Sub-set MutationView Database Specialized for PARK2 (PARKIN) Variants

We previously isolated PARKIN (PARK2) as a gene responsible for a unique sort of Parkinson disease, namely Autosomal Recessive Juvenile Parkinsonism (ARJP). In this study, we surveyed all the available literature describing PARK2 gene/Parkin protein mutations found in Parkinson disease patients. Only carefully evaluated data were deposited in the graphical database MutationView (http://mutview.dmb.med.keio.ac.jp) to construct KM-parkin-DB, an independent sub-set database. Forty-four articles were selected for data curation regarding clinical information such as ethnic origins, manifested symptoms, onset age, and hereditary patterns as well as mutation details including base changes and zygosity. A total of 366 cases were collected from 39 ethnic origins and 96 pathogenic mutations were found. PARK2 gene mutations were found also in some general Parkinson disease patients. The majority (63%) of mutations in PARK2 were restricted to two particular domains (UBL and RING1) of the Parkin protein. In these domains, two major mutations, a large deletion (DelEx3) and a point mutation (p.Arg275Trp), were located.