Parkin's substrates and the pathways leading to neuronal damage

Interactions of VMAT2 with CDCrel-1 and Parkin in Methamphetamine Neurotoxicity

In recent years, methamphetamine (METH) misuse in the US has been rapidly increasing, and there is no FDA-approved pharmacotherapy for METH use disorder (MUD). In addition to being dependent on the drug, people with MUD develop a variety of neurological problems related to the toxicity of this drug. A variety of molecular mechanisms underlying METH neurotoxicity has been identified, including the dysfunction of the neuroprotective protein parkin. However, it is not known whether parkin loss of function within striatal dopaminergic (DAergic) terminals translates into decreased DA storage capacity. This study examined the relationship between parkin, its substrate cell division cycle related-1 (CDCrel-1) associated with synaptic vesicles, and vesicular monoamine transporter-2 (VMAT2) responsible for packaging DA in an in vivo model of METH neurotoxicity. To assess the individual differences in response to METH's neurotoxic effects, a large group of male Sprague Dawley rats were treated with binge METH or saline and sacrificed 1 h or 24 h later. This study is the first to show that CDCrel-1 interacts with VMAT2 in the rat striatum and that binge METH can alter this interaction as well as the levels and subcellular localization of CDCrel-1. The proteomic analysis of VMAT-2-associated proteins revealed the upregulation of several proteins involved in the exocytosis/endocytosis cycle and responses to stress. The results suggest that DAergic neurons are engaged in counteracting METH-induced toxic effects, including attempts to increase endocytosis and autophagy at 1 h after the METH binge, with the responses varying widely between individual rats. Studying CDCrel-1, VMAT2, and other proteins in large groups of outbred rats can help define individual genetic and molecular differences in responses to METH neurotoxicity, which, in turn, may aid treating humans suffering from MUD and its neurological consequences.

Neurotoxic Methamphetamine Doses Alter CDCel-1 Levels and Its Interaction with Vesicular Monoamine Transporter-2 in Rat Striatum

In recent years, methamphetamine METH misuse in the US has been rapidly increasing and there is no FDA-approved pharmacotherapy for METH use disorder (MUD). In addition to being dependent on the drug, people with MUD develop a variety of neurological problems related to the toxicity of this drug. A variety of molecular mechanisms underlying METH neurotoxicity has been identified, including dysfunction of the neuroprotective protein parkin. However, it is not known whether parkin loss of function within striatal dopaminergic (DAergic) terminals translates into a decrease in DA storage capacity. This study examined the relationship between parkin, its substrate cell division cycle related-1 (CDCrel-1), and vesicular monoamine transporter-2 (VMAT2) in METH neurotoxicity in male Sprague Dawley rats. To also assess individual differences in response to METH's neurotoxic effects, a large group of rats was treated with binge METH or saline and sacrificed 1h or 24h later. This study is the first to show that binge METH alters the levels and subcellular localization of CDCrel-1 and that CDCrel-1 interacts with VMAT2 and increases its levels at the plasma membrane. Furthermore, we found wide individual differences in the responses of measured indices to METH. Proteomic analysis of VMAT-2-associated proteins revealed upregulation of several proteins involved in the exocytosis/endocytosis cycle. The results suggest that at 1h after METH binge, DAergic neurons are engaged in counteracting METH-induced toxic effects, including oxidative stress- and hyperthermia-induced inhibition of synaptic vesicle cycling, with the responses varying between individual rats. Studying CDCrel-1, VMAT2, and other proteins in large groups of outbred rats can help define individual genetic and molecular differences in responses to METH neurotoxicity which, in turn, will aid treating humans suffering from METH use disorder and its neurological consequences.

Animal Models of Autosomal Recessive Parkinsonism

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder. The neuropathological hallmark of the disease is the loss of dopamine neurons of the substantia nigra pars compacta. The clinical manifestations of PD are bradykinesia, rigidity, resting tremors and postural instability. PD patients often display non-motor symptoms such as depression, anxiety, weakness, sleep disturbances and cognitive disorders. Although, in 90% of cases, PD has a sporadic onset of unknown etiology, highly penetrant rare genetic mutations in many genes have been linked with typical familial PD. Understanding the mechanisms behind the DA neuron death in these Mendelian forms may help to illuminate the pathogenesis of DA neuron degeneration in the more common forms of PD. A key step in the identification of the molecular pathways underlying DA neuron death, and in the development of therapeutic strategies, is the creation and characterization of animal models that faithfully recapitulate the human disease. In this review, we outline the current status of PD modeling using mouse, rat and non-mammalian models, focusing on animal models for autosomal recessive PD.

Maackiain Ameliorates 6-Hydroxydopamine and SNCA Pathologies by Modulating the PINK1/Parkin Pathway in Models of Parkinson's Disease in Caenorhabditis elegans and the SH-SY5Y Cell Line

The movement disorder Parkinson's disease (PD) is the second most frequently diagnosed neurodegenerative disease, and is associated with aging, the environment, and genetic factors. The intracellular aggregation of α-synuclein and the loss of dopaminergic neurons in the substantia nigra pars compacta are the pathological hallmark of PD. At present, there is no successful treatment for PD. Maackiain (MK) is a flavonoid extracted from dried roots of Sophora flavescens Aiton. MK has emerged as a novel agent for PD treatment that acts by inhibiting monoamine oxidase B. In this study, we assessed the neuroprotective potential of MK in Caenorhabditis elegans and investigated possible mechanism of this neuroprotection in the human SH-SY5Y cell line. We found that MK significantly reduced dopaminergic neuron damage in 6-hydroxydopamine (6-OHDA)-exposed worms of the BZ555 strain, with corresponding improvements in food-sensing behavior and life-span. In transgenic worms of strain NL5901 treated with 0.25 mM MK, the accumulation of α-synuclein was diminished by 27% (p < 0.01) compared with that in untreated worms. Moreover, in worms and the SH-SY5Y cell line, we confirmed that the mechanism of MK-mediated protection against PD pathology may include blocking apoptosis, enhancing the ubiquitin-proteasome system, and augmenting autophagy by increasing PINK1/parkin expression. The use of small interfering RNA to downregulate parkin expression in vivo and in vitro could reverse the benefits of MK in PD models. MK may have considerable therapeutic applications in PD.

Impact of altered phosphorylation on loss of function of juvenile Parkinsonism-associated genetic variants of the E3 ligase parkin

Autosomal recessive juvenile Parkinsonism (ARJP) is an inherited neurodegenerative disease in which 50% of affected individuals harbor mutations in the gene encoding the E3 ligase parkin. Parkin regulates the mitochondrial recycling pathway, which is induced by oxidative stress. In its native state, parkin is auto-inhibited by its N-terminal ubiquitin-like (Ubl) domain, which blocks the binding site for an incoming E2∼ubiquitin conjugate, needed for parkin's ubiquitination activity. Parkin is activated via phosphorylation of Ser-65 in its Ubl domain by PTEN-induced putative kinase 1 (PINK1) and a ubiquitin molecule phosphorylated at a position equivalent to Ser-65 in parkin. Here we have examined the underlying molecular mechanism of phosphorylation of parkin's Ubl domain carrying ARJP-associated substitutions and how altered phosphorylation modulates parkin activation and ubiquitination. We found that three substitutions in the Ubl domain (G12R, R33Q, and R42P) significantly decrease PINK1's ability to phosphorylate the Ubl domain. We noted that two basic loss-of-function substitutions (R33Q and R42P) are close to acidic patches in the proposed PINK1–parkin interface, indicating that ionic interactions at this site may be important for efficient parkin phosphorylation. Increased auto-ubiquitination with unique ubiquitin chain patterns was observed for two other Ubl domain substitutions (G12R and T55I), suggesting that these substitutions, along with phosphorylation, increase parkin degradation. Moreover, Ubl domain phosphorylation decreased its affinity for the potential effector protein ataxin-3, which edits ubiquitin chain building by parkin. Overall, our work provides a framework for the mechanisms of parkin's loss-of-function, indicating an interplay between ARJP-associated substitutions and phosphorylation of its Ubl domain.

The synaptic function of parkin

Loss of function mutations in the gene PARK2, which encodes the protein parkin, cause autosomal recessive juvenile parkinsonism, a neurodegenerative disease characterized by degeneration of the dopaminergic neurons localized in the substantia nigra pars compacta. No therapy is effective in slowing disease progression mostly because the pathogenesis of the disease is yet to be understood. From accruing evidence suggesting that the protein parkin directly regulates synapses it can be hypothesized that PARK2 gene mutations lead to early synaptic damage that results in dopaminergic neuron loss over time. We review evidence that supports the role of parkin in modulating excitatory and dopaminergic synapse functions. We also discuss how these findings underpin the concept that autosomal recessive juvenile parkinsonism can be primarily a synaptopathy. Investigation into the molecular interactions between parkin and synaptic proteins may yield novel targets for pharmacologic interventions.

Computational identification and analysis of neurodegenerative disease associated protein kinases in hominid genomes

Protein kinases play an important role in the incidence of neurodegenerative diseases. However their incidence in non-human primates is found to be very low. Small differences among the genomes might influence the disease susceptibilities. The present study deals with finding the genetic differences of protein kinases in humans and their three closest evolutionary partners chimpanzee, gorilla and orangutan for three neurodegenerative diseases namely, Alzheimer's, Parkinson's and Huntington's diseases. In total 47 human protein kinases associated with three neurodegenerative diseases and their orthologs from other three non-human primates were identified and analyzed for any possible susceptibility factors in humans. Multiple sequence alignment and pairwise sequence alignment revealed that, 18 human protein kinases including DYRK1A, RPS6KB1, and GRK6 contained significant indels and substitutions. Further phosphorylation site analysis revealed that eight kinases including MARK2 and LTK contained sites of phosphorylation exclusive to human genomes which could be particular candidates in determining disease susceptibility between human and non-human primates. Final pathway analysis of these eight kinases and their targets revealed that these kinases could have long range consequences in important signaling pathways which are associated with neurodegenerative diseases.

Carnosic Acid Attenuates 6-Hydroxydopamine-Induced Neurotoxicity in SH-SY5Y Cells by Inducing Autophagy Through an Enhanced Interaction of Parkin and Beclin1

Enhanced removal of abnormal protein aggregates or injured organelles through autophagy is related to neuroprotection in Parkinson's disease. In this study, we explored whether the induction of autophagy is associated with the neuroprotection of rosemary carnosic acid (CA) against 6-hydroxydopamine (6-OHDA)-induced neurotoxicity in SH-SY5Y cells. The results indicated that cells treated with CA had increased protein levels of parkin and autophagy-related markers, including phosphatidylinositol 3-kinase p100, Beclin1, autophagy-related gene 7, and microtubule-associated protein 1 light chain 3-II, as well as enhanced formation of autophagic vacuoles. Treatment of cells with 6-OHDA decreased the levels of parkin and the autophagy markers, but CA pretreatment reversed these effects. However, wortmannin (an autophagosome formation blocker) pretreatment attenuated the effect of CA. After CA pretreatment, the induction of cleaved caspase 3, cleaved poly-ADP ribose polymerase, and nuclear condensation by 6-OHDA were alleviated. Both wortmannin and bafilomycin A1 (an autophagosome-lysosome fusion blocker) inhibited the anti-apoptosis effects of CA. Additionally, we performed immunoprecipitation with anti-parkin antibody and found that the interaction of parkin and Beclin1 protein was reduced by 6-OHDA but that this effect was reversed in cells pretreated with CA. Moreover, transfection of parkin siRNA in cells inhibited the ability of CA to alleviate 6-OHDA-decreased autophagy-related markers and nuclear condensation. In conclusion, CA protects against 6-OHDA-induced apoptosis by inducing autophagy through the interaction of parkin and Beclin1. These results provide a future strategy for use of CA in the prevention of Parkinson's disease.

Mitochondrial Quality Control via the PGC1α-TFEB Signaling Pathway Is Compromised by Parkin Q311X Mutation But Independently Restored by Rapamycin

Following its activation by PINK1, parkin is recruited to depolarized mitochondria where it ubiquitinates outer mitochondrial membrane proteins, initiating lysosomal-mediated degradation of these organelles. Mutations in the gene encoding parkin, PARK2, result in both familial and sporadic forms of Parkinson's disease (PD) in conjunction with reductions in removal of damaged mitochondria. In contrast to what has been reported for other PARK2 mutations, expression of the Q311X mutation in vivo in mice appears to involve a downstream step in the autophagic pathway at the level of lysosomal function. This coincides with increased PARIS expression and reduced expression of a reciprocal signaling pathway involving the master mitochondrial regulator peroxisome proliferator-activated receptor-gamma coactivator (PGC1α) and the lysosomal regulator transcription factor EB (TFEB). Treatment with rapamycin was found to independently restore PGC1α-TFEB signaling in a manner not requiring parkin activity and to abrogate impairment of mitochondrial quality control and neurodegenerative features associated with this in vivo model. Losses in PGC1α-TFEB signaling in cultured rat DAergic cells expressing the Q311X mutation associated with reduced mitochondrial function and cell viability were found to be PARIS-dependent and to be independently restored by rapamycin in a manner requiring TFEB. Studies in human iPSC-derived neurons demonstrate that TFEB induction can restore mitochondrial function and cell viability in a mitochondrially compromised human cell model. Based on these data, we propose that the parkin Q311X mutation impacts on mitochondrial quality control via PARIS-mediated regulation of PGC1α-TFEB signaling and that this can be independently restored via upregulation of TFEB function.

SIGNIFICANCE STATEMENT

Mutations in PARK2 are generally associated with loss in ability to interact with PINK1, impacting on autophagic initiation. Our data suggest that, in the case of at least one parkin mutation, Q311X, detrimental effects are due to inhibition at the level of downstream lysosomal function. Mechanistically, this involves elevations in PARIS protein levels and subsequent effects on PGC1α-TFEB signaling that normally regulates mitochondrial quality control. Treatment with rapamycin independently restores PGC1α-TFEB signaling in a manner not requiring parkin activity and abrogates subsequent mitochondrial impairment and neuronal cell loss. Taken in total, our data suggest that the parkin Q311X mutation impacts on mitochondrial quality control via PARIS-mediated regulation of PGC1α-TFEB signaling and that this can be independently restored via rapamycin.

Parkin loss leads to PARIS-dependent declines in mitochondrial mass and respiration

Mutations in parkin lead to early-onset autosomal recessive Parkinson's disease (PD) and inactivation of parkin is thought to contribute to sporadic PD. Adult knockout of parkin in the ventral midbrain of mice leads to an age-dependent loss of dopamine neurons that is dependent on the accumulation of parkin interacting substrate (PARIS), zinc finger protein 746 (ZNF746), and its transcriptional repression of PGC-1α. Here we show that adult knockout of parkin in mouse ventral midbrain leads to decreases in mitochondrial size, number, and protein markers consistent with a defect in mitochondrial biogenesis. This decrease in mitochondrial mass is prevented by short hairpin RNA knockdown of PARIS. PARIS overexpression in mouse ventral midbrain leads to decreases in mitochondrial number and protein markers and PGC-1α-dependent deficits in mitochondrial respiration. Taken together, these results suggest that parkin loss impairs mitochondrial biogenesis, leading to declining function of the mitochondrial pool and cell death.

Parkin-mediated ubiquitination of mutant glucocerebrosidase leads to competition with its substrates PARIS and ARTS

Background

Parkinson’s disease (PD) is a movement neurodegenerative disorder characterized by death of dopaminergic neurons in the substantia nigra pars compacta of the brain that leads to movement impairments including bradykinesia, resting tremor, postural instability and rigidity. Mutations in several genes have been associated with familial PD, such as parkin, pink, DJ-1, LRKK2 and α-synuclein. Lately, mutations in the GBA gene were recognized as a major cause for the development of PD.

Mutations in the GBA gene, which encodes for lysosomal β-glucocerebrosidase (GCase), lead to Gaucher disease (GD), an autosomal recessive sphingolipidosis characterized by accumulation of glucosylceramide, mainly in monocyte-derived cells. It is a heterogeneous disease, with Type 1 patients that do not present any primary neurological signs, and Type 2 or Type 3 patients who suffer from a neurological disease. The propensity of type 1 GD patients and carriers of GD mutations to develop PD is significantly higher than that of the non-GD population.

We have shown in the past that parkin and mutant GCase, expressed in heterologous systems, interact with each other, and that normal but not mutant parkin mediates K48-dependent proteasomal degradation of mutant GCase variants.

Methods

We tested possible competition between mutant GCase and PARIS or ARTS on the E3 ubiquitin ligase parkin, using coimmunoprecipitation assays and quantitative real-time PCR.

Results

We show that endogenous mutant GCase variants associate with parkin and undergo parkin-dependent degradation. Mutant GCase competes with the known parkin substrates PARIS and ARTS, whose accumulation leads to apoptosis. Dopaminergic cells expressing mutant GCase are more susceptible to apoptotic stimuli than dopaminergic cells expressing normal GCase, present increased cleavage of caspase 3 and caspase 9 levels and undergo cell death.

Conclusions

Our results imply that presence of mutant GCase leads to accumulation of parkin substrates like PARIS and ARTS, which may cause apoptotic death of cells.

Parkin plays a role in sporadic Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is a chronic progressive neurologic disorder, which affects approximately one million men and women in the US alone. PD represents a heterogeneous disorder with common clinical manifestations and, for the most part, common neuropathological findings.

OBJECTIVE

This short article reviews the role of the ubiquitin E3 ligase in sporadic PD.

METHODS

The role of parkin in sporadic PD was reviewed by querying PubMed.

RESULTS

Parkin is inactivated in sporadic PD via S-nitrosylation, oxidative and dopaminergic stress, and phosphorylation by the stress-activated kinase c-Abl, leading to the accumulation of AIMP2 and PARIS (ZNF746).

CONCLUSION

Strategies aimed at maintaining parkin in a catalytically active state or interfering with the toxicity of AIMP2 and PARIS (ZNF746) offer new therapeutic opportunities.

Parthanatos mediates AIMP2-activated age-dependent dopaminergic neuronal loss

The defining pathogenic feature of Parkinson’s disease is the age dependent loss of dopaminergic neurons. Mutations and inactivation of parkin, an ubiquitin E3 ligase, cause Parkinson’s disease through accumulation of pathogenic substrates. Here we show that transgenic overexpression of the parkin substrate, aminoacyl-tRNA synthetase complex interacting multifunctional protein-2 (AIMP2) leads to a selective, age-dependent progressive loss of dopaminergic neurons via activation of poly(ADP-ribose) polymerase-1 (PARP1). AIMP2 accumulation in vitro and in vivo results in PARP1 overactivation and dopaminergic cell toxicity via direct association of these proteins in the nucleus providing a new path to PARP1 activation other than DNA damage. Inhibition of PARP1 through gene deletion or drug inhibition reverses behavioral deficits and protects in vivo against dopamine neuron death in AIMP2 transgenic mice. These data indicate that brain permeable PARP inhibitors could be effective in delaying or preventing disease progression in Parkinson’s disease.

Mutation in E1, the ubiquitin activating enzyme, reduces Drosophila lifespan and results in motor impairment

Neurodegenerative diseases cause tremendous suffering for those afflicted and their families. Many of these diseases involve accumulation of mis-folded or aggregated proteins thought to play a causal role in disease pathology. Ubiquitinated proteins are often found in these protein aggregates, and the aggregates themselves have been shown to inhibit the activity of the proteasome. These and other alterations in the Ubiquitin Pathway observed in neurodegenerative diseases have led to the question of whether impairment of the Ubiquitin Pathway on its own can increase mortality or if ongoing neurodegeneration alters Ubiquitin Pathway function as a side-effect. To address the role of the Ubiquitin Pathway in vivo, we studied loss-of-function mutations in the Drosophila Ubiquitin Activating Enzyme, Uba1 or E1, the most upstream enzyme in the Ubiquitin Pathway. Loss of only one functional copy of E1 caused a significant reduction in adult lifespan. Rare homozygous hypomorphic E1 mutants reached adulthood. These mutants exhibited further reduced lifespan and showed inappropriate Ras activation in the brain. Removing just one functional copy of Ras restored the lifespan of heterozygous E1 mutants to that of wild-type flies and increased the survival of homozygous E1 mutants. E1 homozygous mutants also showed severe motor impairment. Our findings suggest that processes that impair the Ubiquitin Pathway are sufficient to cause early mortality. Reduced lifespan and motor impairment are seen in the human disease X-linked Infantile Spinal Muscular Atrophy, which is associated with mutation in human E1 warranting further analysis of these mutants as a potential animal model for study of this disease.

Modification by ubiquitin-like proteins: significance in apoptosis and autophagy pathways

Ubiquitin-like proteins (Ubls) confer diverse functions on their target proteins. The modified proteins are involved in various biological processes, including DNA replication, signal transduction, cell cycle control, embryogenesis, cytoskeletal regulation, metabolism, stress response, homeostasis and mRNA processing. Modifiers such as SUMO, ATG12, ISG15, FAT10, URM1, and UFM have been shown to modify proteins thus conferring functions related to programmed cell death, autophagy and regulation of the immune system. Putative modifiers such as Domain With No Name (DWNN) have been identified in recent times but not fully characterized. In this review, we focus on cellular processes involving human Ubls and their targets. We review current progress in targeting these modifiers for drug design strategies.

Macroautophagy and the proteasome are differently involved in the degradation of alpha-synuclein wild type and mutated A30P in an in vitro inducible model (PC12/TetOn)

Many data suggest that alpha synuclein (α-syn) aggregation is involved in Parkinson's disease (PD) neurotoxicity and is accelerated by the pathogenetic point mutation A30P. The triplication of α-syn gene has been linked to early-onset familial PD, suggesting that the cellular dosage of α-syn is an important modulator of its toxicity. To verify this point, we developed an inducible model of α-syn expression (both wild type [WT] and mutated A30P) in rat PC12/TetOn cells. At low expression level, both α-syn(WT) and (A30P) did not aggregate, were not toxic, and displayed a protective action against oxidative stress triggered by hydrogen peroxide (H₂O₂). By increasing α-syn expression, its antioxidant function was no longer detectable as for the A30P form, but again no aggregation and cell death were present both for the WT and the mutated protein. To clarify why α-syn did not accumulate at high expression level, we inhibited macroautophagy by 3-methyladenine (3-MA) and the proteasome by MG132. In presence of 3-MA, α-syn(WT) accumulated, A11 anti-oligomer antibody-positive aggregates were detectable, and cell toxicity was evident, while proteasome inhibition did not increase α-syn(WT) accumulation. Macroautophagy or proteasome inhibition slightly increased α-syn(A30P) toxicity, with no detectable aggregation. This model can provide useful details about α-syn function, aggregation, and degradation pathways.

The Unfolded Protein Response, Degradation from Endoplasmic Reticulum and Cancer

The endoplasmic reticulum (ER) is an essential organelle involved in many cellular functions including protein folding and secretion, lipid biosynthesis and calcium homeostasis. Proteins destined for the cell surface or for secretion are made in the ER, where they are folded and assembled into multi-subunit complexes. The ER plays a vital role in cellular protein quality control by extracting and degrading proteins that are not correctly folded or assembled into native complexes. This process, known as ER-associated degradation (ERAD), ensures that only properly folded and assembled proteins are transported to their final destinations. Besides its role in protein folding and transport in the secretory pathway, the ER regulates the biosynthesis of cholesterol and other membrane lipids. ERAD is an important means to ensure that levels of the responsible enzymes are appropriately maintained. The ER is also a major organelle for oxygen and nutrient sensing as cells adapt to their microenvironment. Stresses that disrupt ER function leads to accumulation of unfolded proteins in the ER, a condition known as ER stress. Cells adapt to ER stress by activating an integrated signal transduction pathway called the unfolded protein response (UPR) (1). The UPR represents a survival response by the cells to restore ER homeostasis. If ER stress persists, cells activate mechanisms that result in cell death. Chronic ER stress is increasingly being recognized as a factor in many human diseases such as diabetes, neurodegenerative disorders and cancer. In this review we discuss the roles of the UPR and ERAD in cancer and suggest directions for future research.

Parkin deficiency disrupts calcium homeostasis by modulating phospholipase C signalling

Mutations in the E3 ubiquitin ligase parkin cause early-onset, autosomal-recessive juvenile parkinsonism (AJRP), presumably as a result of a lack of function that alters the level, activity, aggregation or localization of its substrates. Recently, we have reported that phospholipase Cgamma1 is a substrate for parkin. In this article, we show that parkin mutants and siRNA parkin knockdown cells possess enhanced levels of phospholipase Cgamma1 phosphorylation, basal phosphoinositide hydrolysis and intracellular Ca2+ concentration. The protein levels of Ca2+-regulated protein kinase Calpha were decreased in AJRP parkin mutant cells. Neomycin and dantrolene both decreased the intracellular Ca2+ levels in parkin mutants in comparison with those seen in wild-type parkin cells, suggesting that the differences were a consequence of altered phospholipase C activity. The protection of wild-type parkin against 6-hydroxydopamine (6OHDA) toxicity was also established in ARJP mutants on pretreatment with dantrolene, implying that a balancing Ca2+ release from ryanodine-sensitive stores decreases the toxic effects of 6OHDA. Our findings suggest that parkin is an important factor for maintaining Ca2+ homeostasis and that parkin deficiency leads to a phospholipase C-dependent increase in intracellular Ca2+ levels, which make cells more vulnerable to neurotoxins, such as 6OHDA.

Parkin expression profile in dopamine d3 receptor knock-out mice brains

Patients affected by autosomic recessive juvenile parkinsonism (ARJP) exhibit parkin gene mutations with brain decrease in dopamine D2/D3 binding sites. To date, there are no data indicating whether the reduction in dopamine D3 receptors (DRD3) may be associated with the expression of specific parkin variants. In the present study we investigated parkin expression profile in DRD3 knock-out mice brains. RT-PCR analysis was performed to assess qualitative changes in parkin isoforms' distribution pattern and in exons' expression both in wild type controls and dopamine D3 receptor's knock-out mice. Real-time PCR was performed to quantify single exons mRNA. Results demonstrated that exons 1, 2, 4, 6, 7, 8, were more expressed in wild type compared to dopamine D3 receptor KO mice brains while some other (3, 9, 10) were lower expressed. The expression levels of exons 5, 11 and 12 did not change in both animal groups. Our analysis was confirmed by western blot, which showed that parkin protein levels were influenced by the absence of DRD3.

The PINK1/Parkin pathway regulates mitochondrial morphology

Loss-of-function mutations in the PTEN-induced kinase 1 (PINK1) or parkin genes, which encode a mitochondrially localized serine/threonine kinase and a ubiquitin-protein ligase, respectively, result in recessive familial forms of Parkinsonism. Genetic studies in Drosophila indicate that PINK1 acts upstream of Parkin in a common pathway that influences mitochondrial integrity in a subset of tissues, including flight muscle and dopaminergic neurons. The mechanism by which PINK1 and Parkin influence mitochondrial integrity is currently unknown, although mutations in the PINK1 and parkin genes result in enlarged or swollen mitochondria, suggesting a possible regulatory role for the PINK1/Parkin pathway in mitochondrial morphology. To address this hypothesis, we examined the influence of genetic alterations affecting the machinery that governs mitochondrial morphology on the PINK1 and parkin mutant phenotypes. We report that heterozygous loss-of-function mutations of drp1, which encodes a key mitochondrial fission-promoting component, are largely lethal in a PINK1 or parkin mutant background. Conversely, the flight muscle degeneration and mitochondrial morphological alterations that result from mutations in PINK1 and parkin are strongly suppressed by increased drp1 gene dosage and by heterozygous loss-of-function mutations affecting the mitochondrial fusion-promoting factors OPA1 and Mfn2. Finally, we find that an eye phenotype associated with increased PINK1/Parkin pathway activity is suppressed by perturbations that reduce mitochondrial fission and enhanced by perturbations that reduce mitochondrial fusion. Our studies suggest that the PINK1/Parkin pathway promotes mitochondrial fission and that the loss of mitochondrial and tissue integrity in PINK1 and parkin mutants derives from reduced mitochondrial fission.

The PINK1/Parkin pathway regulates mitochondrial morphology

Loss-of-function mutations in the PTEN-induced kinase 1 (PINK1) or parkin genes, which encode a mitochondrially localized serine/threonine kinase and a ubiquitin-protein ligase, respectively, result in recessive familial forms of Parkinsonism. Genetic studies in Drosophila indicate that PINK1 acts upstream of Parkin in a common pathway that influences mitochondrial integrity in a subset of tissues, including flight muscle and dopaminergic neurons. The mechanism by which PINK1 and Parkin influence mitochondrial integrity is currently unknown, although mutations in the PINK1 and parkin genes result in enlarged or swollen mitochondria, suggesting a possible regulatory role for the PINK1/Parkin pathway in mitochondrial morphology. To address this hypothesis, we examined the influence of genetic alterations affecting the machinery that governs mitochondrial morphology on the PINK1 and parkin mutant phenotypes. We report that heterozygous loss-of-function mutations of drp1, which encodes a key mitochondrial fission-promoting component, are largely lethal in a PINK1 or parkin mutant background. Conversely, the flight muscle degeneration and mitochondrial morphological alterations that result from mutations in PINK1 and parkin are strongly suppressed by increased drp1 gene dosage and by heterozygous loss-of-function mutations affecting the mitochondrial fusion-promoting factors OPA1 and Mfn2. Finally, we find that an eye phenotype associated with increased PINK1/Parkin pathway activity is suppressed by perturbations that reduce mitochondrial fission and enhanced by perturbations that reduce mitochondrial fusion. Our studies suggest that the PINK1/Parkin pathway promotes mitochondrial fission and that the loss of mitochondrial and tissue integrity in PINK1 and parkin mutants derives from reduced mitochondrial fission.

Ubiquitin ligase Hrd1 enhances the degradation and suppresses the toxicity of polyglutamine-expanded huntingtin

E3 ubiquitin ligases catalyze the conjugation of ubiquitin onto proteins, which acts as a signal for targeting proteins for degradation by the proteasome. Hrd1 is an endoplasmic reticulum (ER) membrane-spanning E3 with its catalytic active RING finger facing the cytosol. We speculated that this topology might allow Hrd1 to ubiquitinate misfolded proteins in the cytosol. We tested this idea by using polyglutamine (polyQ)-containing huntingtin (htt) protein as a model substrate. We found that the protein levels of Hrd1 were increased in cells overexpressing the N-terminal fragment of htt containig an expanded polyQ tract (httN). Forced expression of Hrd1 enhanced the degradation of httN in a RING finger-dependent manner, whereas silencing of endogenous Hrd1 expression by RNA interference stabilized httN. Degradation of httN was found to be p97/VCP-dependent, but independent of Ufd1 and Npl4, all of which are thought to form a complex with Hrd1 during ER-associated degradation. Consistent with its role as an E3 for httN, we demonstrate that Hrd1 interacts with and ubiquitinates httN. Subcellular fractionation and confocal microscopy revealed that Hrd1recruits HttN to the ER and co-localizes with juxtanuclear aggregates of httN in cells. Interaction of Hrd1 with httN was found to be independent of the length of the polyglutamine tract. However, httN with expanded polyglutamine tracts appeared to be a preferred substrate for Hrd1. Functionally, we found that Hrd1 protects cells against the httN-induced cell death. These results suggest that Hrd1 is a novel htt-interacting protein that can target pathogenic httN for degradation and is able to protect cells against httN-induced cell death.

Genética de las enfermedades neurodegenerativas más prevalentes

A large number of mutations and polymorphisms associated with neurodegenerative disorders have been described during the last years. These findings have been helpful to improve our knowledge about the pathogenesis of these disorders. In this review we describe the genetic alterations and variants that cause or predispose to develop several neurodegenerative disorders, such as Huntington's disease, Alzheimer's disease, frontotemporal dementia, Parkinson's disease and other parkinsonisms. We also comment on the possible pathogenic mechanism of these mutations, clinical features and the usefulness of this information for the diagnosis and management of these disorders.

Accumulation of the authentic parkin substrate aminoacyl-tRNA synthetase cofactor, p38/JTV-1, leads to catecholaminergic cell death

Autosomal-recessive juvenile parkinsonism (AR-JP) is caused by loss-of-function mutations of the parkin gene. Parkin, a RING-type E3 ubiquitin ligase, is responsible for the ubiquitination and degradation of substrate proteins that are important in the survival of dopamine neurons in Parkinson's disease (PD). Accordingly, the abnormal accumulation of neurotoxic parkin substrates attributable to loss of parkin function may be the cause of neurodegeneration in parkin-related parkinsonism. We evaluated the known parkin substrates identified to date in parkin null mice to determine whether the absence of parkin results in accumulation of these substrates. Here we show that only the aminoacyl-tRNA synthetase cofactor p38 is upregulated in the ventral midbrain/hindbrain of both young and old parkin null mice. Consistent with upregulation in parkin knock-out mice, brains of AR-JP and idiopathic PD and diffuse Lewy body disease also exhibit increased level of p38. In addition, p38 interacts with parkin and parkin ubiquitinates and targets p38 for degradation. Furthermore, overexpression of p38 induces cell death that increases with tumor necrosis factor-alpha treatment and parkin blocks the pro-cell death effect of p38, whereas the R42P, familial-linked mutant of parkin, fails to rescue cell death. We further show that adenovirus-mediated overexpression of p38 in the substantia nigra in mice leads to loss of dopaminergic neurons. Together, our study represents a major advance in our understanding of parkin function, because it clearly identifies p38 as an important authentic pathophysiologic substrate of parkin. Moreover, these results have important implications for understanding the molecular mechanisms of neurodegeneration in PD.

Proteasome regulation of oxidative stress in aging and age-related diseases of the CNS

Proteasome-mediated protein degradation is responsible for a large percentage of bulk protein turnover, particularly the degradation of short-lived and oxidized proteins. Increasing evidence suggests that proteasome inhibition occurs during the aging of the central nervous system (CNS), and in a variety of age-related disorders of the CNS. The focus of this review is to discuss the role of the proteasome as a regulator of oxidative stress, with preservation of proteasome function playing an important role in preventing oxidative stress, and proteasome inhibition playing an important role as a mediator of oxidative stress. In particular, this review will describe experimental evidence that proteasome inhibition is sufficient to induce mitochondrial dysfunction, increase reactive oxygen species generation, elevate RNA and DNA oxidation, and promote protein oxidation. Taken together, these data indicate that the proteasome is an important regulator of oxidative damage in the CNS, and suggest that proteasome inhibition may serve as an important switch for the induction of oxidative stress in the CNS. Additionally we discuss the likelihood that the 20S proteasome and 26S proteasome may play different roles in regulating oxidative stress and neurotoxicity in the aging CNS, and in age-related disorders of the CNS.

Neurotoxicity and behavioral deficits associated with Septin 5 accumulation in dopaminergic neurons

Septin 5, a parkin substrate, is a vesicle- and membrane-associated protein that plays a significant role in inhibiting exocytosis. The regulatory function of Septin 5 in dopaminergic (DAergic) neurons of substantia nigra (SN), maintained at relatively low levels, has not yet been delineated. As loss of function mutations of parkin are the principal cause of a familial Parkinson's disease, a prevailing hypothesis is that the loss of parkin activity results in accumulation of Septin 5 which confers neuron-specific toxicity in SN-DAergic neurons. In vitro and in vivo models were used to support this hypothesis. In our well-characterized DAergic SN4741 cell model, acute accumulation of elevated levels of Septin 5, but not synphilin-1 (another parkin substrate), resulted in cytotoxic cell death that was markedly reduced by parkin co-transfection. A transgenic mouse model expressing a dominant negative parkin mutant accumulated moderate levels of Septin 5 in SN-DAergic neurons. These mice acquired a progressive l-DOPA responsive motor dysfunction that developed despite a 25% higher than normal level of striatal dopamine (DA) and no apparent loss of DAergic neurons. The phenotype of this animal, increased striatal dopamine and reduced motor function, was similar to that observed in parkin knockout animals, suggesting a common DAergic alteration. These data suggest that a threshold level of Septin 5 accumulation is required for DAergic cell loss and that l-DOPA-responsive motor deficits can occur even in the presence of elevated DA.

Molecular genetic pathways in Parkinson's disease: a review

Major progress has been made in the last decade in understanding the genetic basis of PD (Parkinson's disease) with five genes unequivocally associated with disease. As a result, multiple pathways have been implicated in the pathogenesis of PD, including proteasome impairment and mitochondrial dysfunction. Although Mendelian genetics has been successful in establishing a genetic predisposition for familial PD, this has not been reiterated in the sporadic form. In fact no genetic factors have been unequivocally associated with increased risk for sporadic PD. The difficulty in identifying susceptibility factors in PD has not only been because of numerous underpowered studies, but we have been unable to dissect out the genetic component in a multifactorial disease. This review aims to summarize the genetic findings within PD.

Genetics of parkinsonism

Parkinson's disease (PD) is the second most common neurodegenerative disease after Alzheimer's disease. Some debate still exists as to whether PD is predominantly environmental or genetic in etiology. The genetic hypothesis of PD etiology has been driven recently by the identification of a number of PD loci. This review deals with each of these loci, discussing the latest data and evidence available. Of particular interest are the recently described mutations in the PINK1 (PARK6) and LRRK2 (PARK8) genes. We also consider the impact of these latest developments on our understanding of sporadic PD and on our everyday practice with PD patients.

Genetic and genomic studies of Drosophila parkin mutants implicate oxidative stress and innate immune responses in pathogenesis

Loss-of-function mutations of the parkin gene, which encodes a ubiquitin-protein ligase, are a common cause of autosomal recessive juvenile parkinsonism (ARJP). Previous work has led to the identification of a number of Parkin substrates that implicate specific pathways in ARJP pathogenesis, including endoplasmic reticulum (ER) stress and cell cycle activation. To test the involvement of previously implicated pathways, as well as to identify novel pathways in ARJP pathogenesis, we are using genetic and genomic approaches to study Parkin function in the fruit fly Drosophila melanogaster. In previous work, we demonstrated that Drosophila parkin null mutants exhibit mitochondrial pathology and flight muscle degeneration. To further explore the mechanisms responsible for pathology in parkin mutants, we analyzed the transcriptional alterations that occur during muscle degeneration and performed a genetic screen for parkin modifiers. Results of these studies indicate that oxidative stress response components are induced in parkin mutants and that loss-of-function mutations in oxidative stress components enhance the parkin mutant phenotypes. Genes involved in the innate immune response are also induced in parkin mutants. In contrast, our studies did not reveal evidence for cell cycle or ER stress pathway induction in parkin mutants. These results suggest that oxidative stress and/or inflammation may play a fundamental role in the etiology of ARJP.

Parkin Phosphorylation and Modulation of Its E3 Ubiquitin Ligase Activity

Mutations in the PARKIN gene are the most common cause of hereditary parkinsonism. The parkin protein comprises an N-terminal ubiquitin-like domain, a linker region containing caspase cleavage sites, a unique domain in the central portion, and a special zinc finger configuration termed RING-IBR-RING. Parkin has E3 ubiquitin-protein ligase activity and is believed to mediate proteasomal degradation of aggregation-prone proteins. Whereas the effects of mutations on the structure and function of parkin have been intensely studied, post-translational modifications of parkin and the regulation of its enzymatic activity are poorly understood. Here we report that parkin is phosphorylated both in human embryonic kidney HEK293 cells and human neuroblastoma SH-SY5Y cells. The turnover of parkin phosphorylation was rapid, because inhibition of phosphatases with okadaic acid was necessary to stabilize phosphoparkin. Phosphoamino acid analysis revealed that phosphorylation occurred mainly on serine residues under these conditions. At least five phosphorylation sites were identified, including Ser101, Ser131, and Ser136 (located in the linker region) as well as Ser296 and Ser378 (located in the RING-IBR-RING motif). Casein kinase-1, protein kinase A, and protein kinase C phosphorylated parkin in vitro, and inhibition of casein kinase-1 caused a dramatic reduction of parkin phosphorylation in cell lysates. Induction of protein folding stress in cells reduced parkin phosphorylation, and unphosphorylated parkin had slightly but significantly elevated autoubiquitination activity. Thus, complex regulation of the phosphorylation state of parkin may contribute to the unfolded protein response in stressed cells.

Lentiviral vector delivery of parkin prevents dopaminergic degeneration in an alpha-synuclein rat model of Parkinson's disease

Parkinson's disease (PD) is characterized by a progressive loss of midbrain dopamine neurons and the presence of cytoplasmic inclusions called Lewy bodies. Mutations in several genes including alpha-synuclein and parkin have been linked to familial PD. The loss of parkin's E3-ligase activity leads to dopaminergic neuronal degeneration in early-onset autosomal recessive juvenile parkinsonism, suggesting a key role of parkin for dopamine neuron survival. To evaluate the potential neuroprotective role of parkin in the pathogenesis of PD, we tested whether overexpression of wild-type rat parkin could protect against the toxicity of mutated human A30P alpha-synuclein in a rat lentiviral model of PD. Animals overexpressing parkin showed significant reductions in alpha-synuclein-induced neuropathology, including preservation of tyrosine hydroxylase-positive cell bodies in the substantia nigra and sparing of tyrosine hydroxylase-positive nerve terminals in the striatum. The parkin-mediated neuroprotection was associated with an increase in hyperphosphorylated alpha-synuclein inclusions, suggesting a key role for parkin in the genesis of Lewy bodies. These results indicate that parkin gene therapy may represent a promising candidate treatment for PD.

Parkin transcript variants in rat and human brain

Alternative splicing has an important role in expanding protein diversity. We have identified complementary DNA species from adult rat and fetal human brain encoding seven new splice variants of parkin, a gene mutated in autosomal recessive juvenile parkinsonism (ARJP). Alternative splicing affects almost all previously characterized exons, plus 3 new exons of 72, 156, and 180 nucleotides. This creates the potential to express hundreds of different isoforms. The encoded parkin isoforms have different amino acid composition, post-translational modifications, and, most important, molecular architectures. They diverge for the presence or absence of the ubiquitin-like domain, one or two C3HC4 ring fingers, the in-between ring fingers (IBR) domain, and a thiol proteases active site, which has not been previously characterized. Distinct expression patterns occur in primary cultures of neuronal and glial cells. Extensive splicing of parkin produces regional and structural diversity and may have important implications for the pathogenetic mechanisms underlying ARJP.

PINK, PANK, or PARK? A clinicians' guide to familial parkinsonism

We provide a pragmatic guide for clinicians, and detail the recent developments in the genetics of Parkinson's disease that have shaped our current understanding and management of this disease and other parkinsonian disorders. These developments have been rapid, and in total over 20 genes have been identified, three of which were discovered in the past year. Although there are undoubtedly more genes to be found, the major challenge for the future is to determine how they function and whether they interact. These genes help us to understand the heterogeneity of parkinsonism, and also inform on the molecular and clinical features of individual parkinsonisms. However, their discovery also requires us to raise issues about genetic testing and genetic counselling.

Genetics of neurological disorders

Neurological diseases are defined as an inappropriate function of the peripheral or central nervous system due to impaired electrical impulses throughout the brain and/or nervous system that may present with heterogeneous symptoms according to the parts of the system involved in these pathologic processes. Growing evidence on genetic components of neurological disease have been collected during recent years. Genetic studies have opened the way for understanding the underlying pathology of many neurological disorders. The outcome of current intense research into the genetics of neurological disorders will hopefully be the introduction of new diagnostic tools and the discovery of potential targets for new and more effective medications and preventive measures.

Protective effect of TAT‐delivered α‐synuclein: relevance of the C‐terminal domain and involvement of HSP70

Alpha-synuclein (alpha-syn) is a 140-amino acid presinaptic protein whose mutations A30P and A53T have been linked to familiar Parkinson's disease (PD). Many data suggest that alpha-syn aggregation is the key event that triggers alpha-syn-mediated neurotoxicity. Nevertheless, other lines of evidence proposed a protective role of alpha-syn against oxidative stress (a major feature of PD), even if the exact mechanism of this protective action and the role of the pathogenetic mutations to this respect have not been elucidated yet. To address these points, we developed an in vitro model of oxidative stress by exposing PC12 cells to hydrogen peroxide (H2O2) (150 microM) for 72 h, and we evaluated alpha-syn-mediated protection delivering increasing amounts of alpha-syn (wild type [WT] or mutated) inside cells using the fusion proteins TAT-alpha-syn (WT, A30P, and A53T). We found that nanomolar amounts of TAT-alpha-syn-mediated protected against oxidative stress and other cellular injuries (6-hydroxydopamine and serum deprivation), whereas micromolar amounts of the fusion proteins were intrinsically toxic to cells. The protective effect was independent from the presence of the mutations A30P and A53T, but no protection occurred when cells were challenged with the proteasome inhibitors lactacystin and MG132. We verified that the protection mechanism required the presence of the C-terminal domain of alpha-syn, as nanomolar amounts of the C-terminal truncated fusion protein TAT-alpha-syn (WT[1-97]) failed in preventing H2O2 toxicity. To further characterize the molecular mechanisms at the basis of alpha-syn protection, we investigated the possible involvement of the chaperone protein HSP70 that is widely implicated in neuroprotection. We found that, at nanomolar concentrations, TAT-alpha-syn was able to increase HSP70 protein level, whereas at the micromolar scale, TAT-alpha-syn decreased HSP70 at the protein level. These effects on HSP70 were independent from the presence of alpha-syn pathogenetic mutations but required the alpha-syn C-terminal domain. The implications for alpha-syn-mediated neurotoxicity and for PD pathogenesis and progression are discussed.

Molecular pathways to neurodegeneration

The molecular bases underlying the pathogenesis of neurodegenerative diseases are gradually being disclosed. One problem that investigators face is distinguishing primary from secondary events. Rare, inherited mutations causing familial forms of these disorders have provided important insights into the molecular networks implicated in disease pathogenesis. Increasing evidence indicates that accumulation of aberrant or misfolded proteins, protofibril formation, ubiquitin-proteasome system dysfunction, excitotoxic insult, oxidative and nitrosative stress, mitochondrial injury, synaptic failure, altered metal homeostasis and failure of axonal and dendritic transport represent unifying events in many slowly progressive neurodegenerative disorders.

Parkin and alpha-synuclein: opponent actions in the pathogenesis of Parkinson's disease

Dominant mutations in the gene for alpha-synuclein, a small presynaptic protein, can cause Parkinson's disease. Although there is still substantial debate about the precise mechanisms, alpha-synuclein is toxic to vulnerable neurons, probably as a result of its tendency to aggregate. Opposing this is another gene product that, when mutated, causes a recessive form of parkinsonism, parkin. Parkin has been recently shown to protect cells against alpha-synuclein toxicity. However, the precise details of the mechanism are unclear. This review will discuss the concept that there are multiple neuronal functions that are targeted by mutant alpha-synuclein, and in many cases, there is evidence that parkin can protect cells against damage to the same systems. The authors will also discuss ways in which to test some of these ideas, by using newly identified genes such as DJ-1 that cause similar phenotypes.

Mitochondrial dysfunction and oxidative damage in parkin-deficient mice

Loss-of-function mutations in parkin are the predominant cause of familial Parkinson's disease. We previously reported that parkin-/- mice exhibit nigrostriatal deficits in the absence of nigral degeneration. Parkin has been shown to function as an E3 ubiquitin ligase. Loss of parkin function, therefore, has been hypothesized to cause nigral degeneration via an aberrant accumulation of its substrates. Here we employed a proteomic approach to determine whether loss of parkin function results in alterations in abundance and/or modification of proteins in the ventral midbrain of parkin-/- mice. Two-dimensional gel electrophoresis followed by mass spectrometry revealed decreased abundance of a number of proteins involved in mitochondrial function or oxidative stress. Consistent with reductions in several subunits of complexes I and IV, functional assays showed reductions in respiratory capacity of striatal mitochondria isolated from parkin-/- mice. Electron microscopic analysis revealed no gross morphological abnormalities in striatal mitochondria of parkin-/- mice. In addition, parkin-/- mice showed a delayed rate of weight gain, suggesting broader metabolic abnormalities. Accompanying these deficits in mitochondrial function, parkin-/- mice also exhibited decreased levels of proteins involved in protection from oxidative stress. Consistent with these findings, parkin-/- mice showed decreased serum antioxidant capacity and increased protein and lipid peroxidation. The combination of proteomic, genetic, and physiological analyses reveal an essential role for parkin in the regulation of mitochondrial function and provide the first direct evidence of mitochondrial dysfunction and oxidative damage in the absence of nigral degeneration in a genetic mouse model of Parkinson's disease.

Genes and parkinsonism

Genetic studies in families with mendelian inheritance of Parkinson's disease (PD) have reported the cloning of several disease-associated genes. These studies of rare familial forms of the disease have cast doubt on our understanding of the role of genetics in typical PD and have complicated the classification of the disorder. However, this genetic information might help us to construct a hypothesis for the pathogenetic processes that underlie PD. In this review we describe the molecular genetics of PD as currently understood to help explain the pathways that underlie neurodegeneration.