Parkin ubiquitinates the alpha-synuclein-interacting protein, synphilin-1: implications for Lewy-body formation in Parkinson disease

Parkin expression in the developing mouse

Parkin is an E3 ubiquitin ligase causally involved in the pathogenesis of autosomal recessive juvenile parkinsonism. In this paper, we analysed the formation of alternative splice products and the spatio-temporal expression pattern of parkin during pre- and postnatal mouse development. Using RT-PCR, Northern blot, in situ hybridization, Western blot analysis, and immunohistochemistry we found (i) alternative splice forms of parkin; (ii) an early and widespread expression of parkin mRNA and protein in the CNS and several organs, already at E10/12; (iii) a marked increase in expression level during midgestational development (E15-18) in the CNS, followed by a steady increase until adulthood; (iv) an ubiquitous distribution throughout CNS ontogeny. Our results show that parkin expression is correlated with cell maturation and suggests an important physiological role of parkin in neurons that is at no time limited to the dopaminergic system.

Parkin counteracts symptoms in a Drosophila model of Parkinson's disease

Background

Parkinson's disease, a prevalent neurodegenerative disease, is characterized by the reduction of dopaminergic neurons resulting in the loss of motor control, resting tremor, the formation of neuronal inclusions and ultimately premature death. Two inherited forms of PD have been linked to mutations in the α-synuclein and parkin genes. The parkin protein functions as an ubiquitin ligase targeting specific proteins for degradation. Expression of human α-synuclein in Drosophila neurons recapitulates the loss of motor control, the development of neuronal inclusions, degeneration of dopaminergic neurons and the ommatidial array to provide an excellent genetic model of PD.

Results

To investigate the role of parkin, we have generated transgenic Drosophila that conditionally express parkin under the control of the yeast UAS enhancer. While expression of parkin has little consequence, co-expression of parkin with α-synuclein in the dopaminergic neurons suppresses the α-synuclein-induced premature loss of climbing ability. In addition directed expression of parkin in the eye counteracts the α-synuclein-induced degeneration of the ommatidial array. These results show that parkin suppresses the PD-like symptoms observed in the α-synuclein-dependent Drosophila model of PD.

Conclusion

The highly conserved parkin E3 ubiquitin ligase can suppress the damaging effects of human α-synuclein. These results are consistent with a role for parkin in targeting α-synuclein to the proteasome. If this relationship is conserved in humans, this suggests that up-regulation of parkin should suppress α-synucleinopathic PD. The development of therapies that regulate parkin activity may be crucial in the treatment of PD.

Pael-R is accumulated in Lewy bodies of Parkinson's disease

We examined the distribution of Pael-R, a newly identified substrate for Parkin, in Parkinson's disease (PD) and multiple system atrophy (MSA). Pael-R, Parkin, alpha-synuclein, and ubiquitin accumulated in Lewy bodies (LBs) and neurites. Pael-R was localized in the core of LBs. Parkin and alpha-synuclein accumulated in the halo, neuronal cell bodies, and processes. These findings potentially suggest the involvement of Pael-R in LB formation, and protection role of Parkin in Pael-R-mediated neurotoxicity in PD. The absence of Pael-R and Parkin in glial cytoplasmic inclusions (GCIs) in MSA implies a distinct pathway involved in the formation of LBs and GCIs.

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.

Ubiquitylation of synphilin-1 and alpha-synuclein by SIAH and its presence in cellular inclusions and Lewy bodies imply a role in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by Lewy body formation and death of dopaminergic neurons. Mutations in alpha-synuclein and parkin cause familial forms of PD. Synphilin-1 was shown to interact with alpha-synuclein and to promote the formation of cytosolic inclusions. We now report that synphilin-1 interacts with the E3 ubiquitin-ligases SIAH-1 and SIAH-2. SIAH proteins ubiquitylate synphilin-1 both in vitro and in vivo, promoting its degradation by the ubiquitin-proteasome system. Inability of the proteasome to degrade synphilin-1/SIAH complex leads to a robust formation of ubiquitylated cytosolic inclusions. Ubiquitylation is required for inclusion formation, because a catalytically inactive mutant of SIAH-1, which still binds to synphilin-1, fails to promote inclusions. Like synphilin-1, alpha-synuclein associates with SIAH in intact cells, but the interaction with SIAH-2 was much stronger that with SIAH-1. In vitro experiments show that SIAH-2 monoubiquitylates alpha-synuclein. Further evidence that SIAH proteins may play a role in inclusion formation comes from the demonstration of SIAH immunoreactivity in Lewy bodies of PD patients.

Recent Advances in Research on Parkinson Disease: Synuclein and Parkin

BACKGROUND

Until recently, most research effort on Parkinson disease (PD) was focused on possible environmental causes. With the discovery of mutations in two genes, synuclein and parkin, which are responsible for rare familial forms of the disease, there has been a major change in emphasis.

REVIEW SUMMARY

The first genetic cause of PD to be identified was in the gene for synuclein, resulting in an alanine to threonine substitution at position 53. The likely pathogenetic significance of this mutation was supported by the discovery of a second mutation, and the presence of synuclein in Lewy bodies in sporadic PD cases. The synuclein protein has a tendency to self aggregate, and this tendency is increased in the mutants, and by oxidative injury to the protein. While there is growing evidence in animal models that overexpression of wildtype or mutant synuclein may lead to intracytoplasmic inclusions, and dysfunction of dopamine neurons, no animal models in rodents have yet replicated all important features of the disease. Deletions or point mutations in the gene for parkin cause an autosomal recessive, early onset form of parkinsonism. The parkin protein functions as an E3 ubiquitin-protein ligase, and it is involved in the degradation of cellular proteins by the proteasomal pathway. It is hypothesized that the loss of this function results in the toxic accumulation of its target proteins.

CONCLUSIONS

Research on these inherited forms of PD is pointing towards a common theme, that disturbances of cellular protein handling can lead to the death of dopamine neurons in PD.

Casein Kinase II-mediated Phosphorylation Regulates α-Synuclein/Synphilin-1 Interaction and Inclusion Body Formation

Alpha-synuclein is a phosphoprotein that accumulates as a major component of Lewy bodies in the brains of patients with Parkinson disease. Synphilin-1, which is also present in Lewy bodies, binds with alpha-synuclein and forms cytoplasmic inclusions in transfected cells. Yet the molecular determinants of this protein-protein interaction are unknown. Here we report that casein kinase II (CKII) phosphorylates synphilin-1 and that the beta subunit of this enzyme complex binds to synphilin-1. Additionally, both CKII alpha and beta subunits are present within cytoplasmic inclusions in cells that overexpress synphilin-1. Notably, the interaction between synphilin-1 and alpha-synuclein is markedly dependent on phosphorylation. Inhibition of CKII activity by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole blocks the binding between these two proteins and significantly reduces the percentage of cells that contain eosinophilic cytoplasmic inclusions. Mutation of the major CKII phosphorylation site in alpha-synuclein (S129A) has no significant impact on the binding between alpha-synuclein and synphilin-1 or on the formation of synphilin-1/alpha-synuclein-positive inclusions. These data suggest that the CKII-mediated phosphorylation of synphilin-1 rather than that of alpha-synuclein is critical in modulating their tendency to aggregate into inclusions. These observations collectively indicate that a ubiquitous post-translational modification such as phosphorylation can regulate inclusion body formation in the context of alpha-synuclein and synphilin-1 interaction.

Methamphetamine produces neuronal inclusions in the nigrostriatal system and in PC12 cells

Mice treated with the psychostimulant methamphetamine (MA) showed the appearance of intracellular inclusions in the nucleus of medium sized striatal neurones and cytoplasm of neurones of the substantia nigra pars compacta but not in the frontal cortex. All inclusions contained ubiquitin, the ubiquitin activating enzyme (E1), the ubiquitin protein ligase (E3-like, parkin), low and high molecular weight heat shock proteins (HSP 40 and HSP 70). Inclusions found in nigral neurones stained for alpha-synuclein, a proteic hallmark of Lewy bodies that are frequently observed in Parkinson's disease and other degenerative disorders. However, differing from classic Lewy bodies, MA-induced neuronal inclusions appeared as multilamellar bodies resembling autophagic granules. Methamphetamine reproduced this effect in cultured PC12 cells, which offered the advantage of a simple cellular model for the study of the molecular determinants of neuronal inclusions. PC12 inclusions, similar to those observed in nigral neurones, were exclusively localized in the cytoplasm and stained for alpha-synuclein. Time-dependent experiments showed that inclusions underwent a progressive fusion of the external membranes and developed an electrodense core. Inhibition of dopamine synthesis by alpha-methyl-p-tyrosine (alphaMpT), or administering the antioxidant S-apomorphine largely attenuated the formation of inclusions in PC12 cells exposed to MA. Inclusions were again observed when alphaMpT-treated cells were loaded with l-DOPA, which restored intracellular dopamine levels.

SEPT5_v2 is a parkin-binding protein

Mutations in parkin are associated with various inherited forms of Parkinson's disease (PD). Parkin is a ubiquitin ligase enzyme that catalyzes the covalent attachment of ubiquitin moieties onto substrate proteins destined for proteasomal degradation. The substrates of parkin-mediated ubiquitination have yet to be completely identified. Using a yeast two-hybrid screen, we isolated the septin, human SEPT5_v2 (also known as cell division control-related protein 2), as a putative parkin-binding protein. SEPT5_v2 is highly homologous to another septin, SEPT5, which was recently identified as a target for parkin-mediated ubiquitination. SEPT5_v2 binds to parkin at the amino terminus and in the ring finger domains. Several lines of evidence have validated the putative link between parkin and SEPT5_v2. Parkin co-precipitates with SEPT5_v2 from human substantia nigra lysates. Parkin ubiquitinates SEPT5_v2 in vitro, and both SEPT5_v1 and SEPT5_v2 accumulate in brains of patients with ARJP, suggesting that parkin is essential for the normal metabolism of these proteins. These findings suggest that an important relationship exists between parkin and septins.

Mechanism of neurodegenerative disease: role of the ubiquitin proteasome system

Many neurodegenerative disorders such as Alzheimer's disease (AD) Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS) and Huntington's disease (HD) are characterized by neuronal damage that may be caused by toxic, abnormal, aggregation-prone proteins. The purpose of this review is threefold: 1) to provide the reader with an overview of the genes involved in the abnormal processing and accumulation of misfolded proteins in neurodegenerative diseases using PD as a model disease; 2) to understand the cellular mechanisms for disposal of abnormal proteins, and the effects of toxic protein accumulation on ubiquitin proteasome system (UPS) and neuronal survival and 3) to discuss the development and challenges of cell culture and animal models for a rational and effective treatment for these disorders.

The C289G and C418R missense mutations cause rapid sequestration of human Parkin into insoluble aggregates

Mutations in the parkin gene are responsible for autosomal recessive parkinsonism. The disease-linked missense mutations are highly concentrated in the RING-IBR-RING domains of Parkin. In this study, we investigated the consequences of several missense parkin gene mutations in cell culture. We have demonstrated that two of these mutations (C289G and C418R), which replace consensus cysteine residues in the RING domains, significantly decrease the solubility of Parkin in cells. Upon overexpression, the presumably misfolded proteins formed cytoplasmic aggregates that concentrated into large perinuclear inclusion bodies when proteasome activity was inhibited. This process required active microtubule-dependent retrograde transport, as previously reported for aggresome formation. These results provide information on the molecular basis of the loss of function caused by mutations of critical residues in Parkin. They also contribute to our understanding of the cellular mechanism underlying the aggregation of mutant Parkin.

Novel monoclonal antibodies demonstrate biochemical variation of brain parkin with age

Autosomal recessive juvenile parkinsonism is a movement disorder associated with the degeneration of dopaminergic neurons in substantia nigra pars compacta. The loss of functional parkin caused by parkin gene mutations is the most common single cause of juvenile parkinsonism. Parkin has been shown to aid in protecting cells from endoplasmic reticulum and oxidative stressors presumably due to ubiquitin ligase activity of parkin that targets proteins for proteasomal degradation. However, studies on parkin have been impeded because of limited reagents specific for this protein. Here we report the generation and characterization of a panel of parkin-specific monoclonal antibodies. Biochemical analyses indicate that parkin is present only in the high salt-extractable fraction of mouse brain, whereas it is present in both the high salt-extractable and RIPA-resistant, SDS-extractable fraction in young human brain. Parkin is present at decreased levels in the high salt-extractable fraction and at increased levels in the SDS-extractable fraction from aged human brain. This shift in the extractability of parkin upon aging is seen in humans but not in mice, demonstrating species-specific differences in the biochemical characteristics of murine versus human parkin. Finally, by using these highly specific anti-parkin monoclonal antibodies, it was not possible to detect parkin in alpha-synuclein-containing lesions in alpha-synucleinopathies, thereby challenging prior inferences about the role of parkin in movement disorders other than autosomal recessive juvenile parkinsonism.

Glycogen synthase kinase-3beta phosphorylates synphilin-1 in vitro

alpha-Synuclein is known to be a major component of Lewy bodies and glial cytoplasmic inclusions in the brains of patients with alpha-synucleinopathies. Synphilin-1, an alpha-synuclein-associated protein, is also present in these inclusions. However, little is known about the post-translational modifications of synphilin-1. In the present study, it is reported that synphilin-1 is phosphorylated by glycogen synthase kinase-3beta in vitro. It is well known that protein phosphorylation is involved in various physiological phenomena, including signal transduction and protein degradation. Therefore, phosphorylation of synphilin-1 may play an important role in the function of this protein in the brain.

Inactivation of parkin by oxidative stress and C-terminal truncations: a protective role of molecular chaperones

Loss of parkin function is linked to autosomal recessive juvenile parkinsonism. Here we show that proteotoxic stress and short C-terminal truncations induce misfolding of parkin. As a consequence, wild-type parkin was depleted from a high molecular weight complex and inactivated by aggregation. Similarly, the pathogenic parkin mutant W453Stop, characterized by a C-terminal deletion of 13 amino acids, spontaneously adopted a misfolded conformation. Mutational analysis indicated that C-terminal truncations exceeding 3 amino acids abolished formation of detergent-soluble parkin. In the cytosol scattered aggregates of misfolded parkin contained the molecular chaperone Hsp70. Moreover, increased expression of chaperones prevented aggregation of wild-type parkin and promoted folding of the W453Stop mutant. Analyzing parkin folding in vitro indicated that parkin is aggregation-prone and that its folding is dependent on chaperones. Our study demonstrates that C-terminal truncations impede parkin folding and reveal a new mechanism for inactivation of parkin.

Case-control study of the ?-synuclein interacting protein gene and Parkinson's disease

We conducted a case-control study of the alpha-synuclein-interacting protein gene (SNCAIP, also known as synphilin-1) and Parkinson's disease (PD). A total of 319 PD cases and 195 controls were genotyped for four SNCAIP variants, including a microsatellite repeat in intron 4 and three restriction fragment length polymorphisms (RFLP) proximal to the 5' terminal of exons 1, 4, and 6. None of the variants were found associated with PD overall. Global score statistics were not significant for four, three, and two loci haplotypes. All four loci were in linkage disequilibrium for cases, controls, or both groups combined (P < 0.0001). Recursive partitioning showed no interactions between variants of the SNCAIP gene and variants of the alpha-synuclein gene (SNCA) or the parkin (PARK2) gene.

Epidemiology of neurodegeneration

Alzheimer's disease, Parkinson's disease, and motor neuron disease share a propensity to occur with increasing age and as either a sporadic or a familial disorder. A number of behavioral and environmental risk factors have been proposed for each disorder, but most associations lack consistency and specificity. Over the last decade the remarkable frequency of these disorders has become apparent, and the identification of mutations in genes has provided the means to understand their pathogenesis. Better and more accurate means to characterize and diagnose these diseases has greatly facilitated analytic epidemiology. The analysis of behavioral and genetic factors that may lower disease risk has led to clinical trials that are either in progress or being planned with the aim of preventing these disorders.

Inhibition of proteasomal activity causes inclusion formation in neuronal and non-neuronal cells overexpressing Parkin

Association between protein inclusions and neurodegenerative diseases, including Parkinson's and Alzheimer's diseases, and polyglutamine disorders, has been widely documented. Although ubiquitin is conjugated to many of these aggregated proteins, the 26S proteasome does not efficiently degrade them. Mutations in the ubiquitin-protein ligase Parkin are associated with autosomal recessive juvenile Parkinsonism. Although Parkin-positive inclusions are not detected in brains of autosomal recessive juvenile Parkinsonism patients, Parkin is found in Lewy bodies in sporadic disease. This suggests that loss of Parkin ligase activity via mutation, or sequestration to Lewy bodies, is a contributory factor to sporadic disease onset. We now demonstrate that decreased proteasomal activity causes formation of large, noncytotoxic inclusions within the cytoplasm of both neuronal and nonneuronal cells overexpressing Parkin. This is not a general phenomenon as there is an absence of similar inclusions when HHARI, a structural homolog of Parkin, is overexpressed. The inclusions colocalize with ubiquitin and with proteasomes. Furthermore, Parkin inclusions colocalize with gamma-tubulin, acetylated alpha-tubulin, and cause redistribution of vimentin, suggesting aggresome-like properties. Our data imply that lower proteasomal activity, previously observed in brain tissue of Parkinson's disease patients, leads to Parkin accumulation and a concomitant reduction in ligase activity, thereby promoting Lewy body formation.

Parkin-deficient mice exhibit nigrostriatal deficits but not loss of dopaminergic neurons

Loss-of-function mutations in parkin are the major cause of early-onset familial Parkinson's disease. To investigate the pathogenic mechanism by which loss of parkin function causes Parkinson's disease, we generated a mouse model bearing a germline disruption in parkin. Parkin-/- mice are viable and exhibit grossly normal brain morphology. Quantitative in vivo microdialysis revealed an increase in extracellular dopamine concentration in the striatum of parkin-/- mice. Intracellular recordings of medium-sized striatal spiny neurons showed that greater currents are required to induce synaptic responses, suggesting a reduction in synaptic excitability in the absence of parkin. Furthermore, parkin-/- mice exhibit deficits in behavioral paradigms sensitive to dysfunction of the nigrostriatal pathway. The number of dopaminergic neurons in the substantia nigra of parkin-/- mice, however, is normal up to the age of 24 months, in contrast to the substantial loss of nigral neurons characteristic of Parkinson's disease. Steady-state levels of CDCrel-1, synphilin-1, and alpha-synuclein, which were identified previously as substrates of the E3 ubiquitin ligase activity of parkin, are unaltered in parkin-/- brains. Together these findings provide the first evidence for a novel role of parkin in dopamine regulation and nigrostriatal function, and a non-essential role of parkin in the survival of nigral neurons in mice.

Molecular pathways of neurodegeneration in Parkinson's disease

Parkinson's disease (PD) is a complex disorder with many different causes, yet they may intersect in common pathways, raising the possibility that neuroprotective agents may have broad applicability in the treatment of PD. Current evidence suggests that mitochondrial complex I inhibition may be the central cause of sporadic PD and that derangements in complex I cause alpha-synuclein aggregation, which contributes to the demise of dopamine neurons. Accumulation and aggregation of alpha-synuclein may further contribute to the death of dopamine neurons through impairments in protein handling and detoxification. Dysfunction of parkin (a ubiquitin E3 ligase) and DJ-1 could contribute to these deficits. Strategies aimed at restoring complex I activity, reducing oxidative stress and alpha-synuclein aggregation, and enhancing protein degradation may hold particular promise as powerful neuroprotective agents in the treatment of PD.

A product of the human gene adjacent to parkin is a component of Lewy bodies and suppresses Pael receptor-induced cell death

Parkin, a RING-type ubiquitin ligase, is the product of the gene responsible for autosomal recessive juvenile parkinsonism. A reverse strand gene located upstream of the parkin gene in the human genome has been identified. The gene product, termed Glup/PACRG, forms a large molecular chaperone complex containing heat shock proteins 70 and 90 and chaperonin components. Glup suppressed cell death induced by accumulation of unfolded Pael receptor (Pael-R), a substrate of Parkin. On the other hand, Glup facilitated the formation of inclusions consisting of Pael-R, molecular chaperones, protein degradation molecules, and Glup itself, when proteasome is inhibited. Glup knockdown attenuated the formation of Pael-R inclusions, which resulted in the promotion of cell death with extensive vacuolization. Moreover, Glup turned out to be a component of Lewy bodies in Parkinson's disease cases. These data suggest that Glup may play an important role in the formation of Lewy bodies and protection of dopaminergic neurons against Parkinson's disease.

Dopamine-dependent neurodegeneration in rats induced by viral vector-mediated overexpression of the parkin target protein, CDCrel-1

Mutations in the parkin gene are linked to autosomal-recessive juvenile parkinsonism (AR-JP). Parkin functions as a ubiquitin protein ligase in the degradation of several proteins, including the neuron-specific septin CDCrel-1. AR-JP-associated parkin mutations inhibit ubiquitination and degradation of CDCrel-1 and other parkin target proteins. Here we show that recombinant adeno-associated virus-mediated CDCrel-1 gene transfer to the substantia nigra of rats results in a rapid onset (6-10 days) of nigral and striatal CDCrel-1 expression that is followed by a progressive loss of nigral dopaminergic neurons and a decline of the striatal dopamine levels. In contrast, neurons of the globus pallidus are spared from CDCrel-1 toxicity. Furthermore, CDCrel-1 inhibits the release of dopamine from stably-transfected PC12 cells, and pharmacological inhibition of tyrosine hydroxylase and dopamine synthesis in rats prevents CDCrel-1-induced nigral neurodegeneration. These results show that CDCrel-1 overexpression exerts dopamine-dependent neurotoxicity and suggest that inhibition of dopamine secretion by CDCrel-1 may contribute to the development of AR-JP.

Monitoring the ubiquitin/proteasome system in conformational diseases

Controlled proteolysis of regulatory or aberrant proteins by the ubiquitin/proteasome system is indispensable for cell viability. Conformational diseases such as Alzheimer's, Parkinson's and Huntington's disease are characterised by the accumulation of misfolded or aggregation-prone proteins. Since these proteins are typical substrates of the ubiquitin/proteasome system, it is not surprising that various models propose impairment of this system as a contributing factor to the pathology of conformational disorders. The complex nature of the ubiquitin/proteasome system and its universal role in cell physiology however turns evaluation of these attractive hypotheses into a major challenge. Several reporter substrates for the ubiquitin/proteasome system have recently been developed to facilitate functional studies of the system in living cells. In this review, we will discuss these new tools as well as the proteins associated with conformational disease that have been studied with these reporters.

Parkin is recruited to the centrosome in response to inhibition of proteasomes

Parkin is a protein-ubiquitin E3 ligase linked to Parkinson's disease. Although several substrates of parkin have been identified, the subcellular location for parkin to recognize and ubiquitinate its targets is unclear. Here we report that parkin was accumulated in the centrosome when SH-SY5Y or transfected HEK293 cells were treated with the proteasome inhibitor lactacystin. The specific recruitment of parkin was dependent on concentration and duration of the treatment, and was accompanied by the centrosomal accumulation of ubiquitinated proteins and CDCrel-1, a substrate of parkin. The recruitment of parkin was apparently mediated through its binding to gamma-tubulin, which has been shown to accumulate in the centrosome in response to misfolded proteins. Furthermore, the effect was abrogated by the microtubule-depolymerizing drug colchicine or the microtubule-stabilizing drug taxol, which indicates that the intact microtubule network is required for the centrosomal recruitment of parkin. Taken together, our data suggest that the lactacystin-induced accumulation of parkin in the centrosome plays a significant role in the ubiquitination of misfolded substrates accumulated there. This process may provide a subcellular locale for parkin to ubiquitinate and degrade protein aggregates critically involved in the pathogenesis of Parkinson's disease.

The Ubiquitin Proteasome System in Neurodegenerative Diseases

The ubiquitin-proteasome system targets numerous cellular proteins for degradation. In addition, modifications by ubiquitin-like proteins as well as proteins containing ubiquitin-interacting and -associated motifs modulate many others. This tightly controlled process involves multiple specific and general enzymes of the system as well as many modifying and ancillary proteins. Thus, it is not surprising that ubiquitin-mediated degradation/processing/modification regulates a broad array of basic cellular processes. Moreover, aberrations in the system have been implicated, either as a primary cause or secondary consequence, in the pathogenesis of both inherited and acquired neurodegenerative diseases. Recent findings indicate that the system is involved in the pathogenesis of Parkinson's, Alzheimer's, Huntington's, and Prion diseases as well as amyotrophic lateral sclerosis. This raises hopes for a better understanding of the pathogenetic mechanisms involved in these diseases and for the development of novel, mechanism-based therapeutic modalities.

Endoplasmic reticulum-associated protein degradation

Proteins that fail to fold properly as well as constitutive or regulated short-lived proteins of the endoplasmatic reticulum (ER) are subjected to proteolysis by cytosolic 26 S proteasomes. This process, termed ER-associated protein degradation (ERAD), has also been implicated in the generation of some important human disorders, for example, cystic fibrosis. To become accessible to the proteasome, ERAD substrates must first be retrogradely transported from the ER into the cytosol, in a process termed dislocation. Surprisingly, protein dislocation from the ER seems to require at least some components that also mediate import into this compartment. Moreover, polyubiquitination of ERAD substrates at the ER membrane as well as the cytoplasmic Cdc48p/Npl4p/Ufd1p complex were shown to contribute to this export reaction. In this article we will summarize our current knowledge on ERAD and discuss the possible function of certain components involved in this process.

Siah-1 facilitates ubiquitination and degradation of synphilin-1

Parkinson's disease is a common neurodegenerative disorder characterized by loss of dopaminergic neurons and appearance of Lewy bodies, cytoplasmic inclusions that are highly enriched with ubiquitin. Synphilin-1, alpha-synuclein, and Parkin represent the major components of Lewy bodies and are involved in the pathogenesis of Parkinson's disease. Synphilin-1 is an alpha-synuclein-binding protein that is ubiquitinated by Parkin. Recently, a mutation in the synphilin-1 gene has been reported in patients with sporadic Parkinson's disease. Although synphilin-1 localizes close to synaptic vesicles, its function remains unknown. To investigate the proteins that interact with synphilin-1, the present study performed a yeast two-hybrid screening and identified a novel interacting protein, Siah-1 ubiquitin ligase. Synphilin-1 and Siah-1 proteins were endogenously expressed in the central nervous system and were found to coimmunoprecipitate each other in rat brain homogenate. Confocal microscopic analysis revealed colocalization of both proteins in cells. Siah-1 was found to interact with the N terminus of synphilin-1 through its substrate-binding domain and to specifically ubiquitinate synphilin-1 via its RING finger domain. Siah-1 facilitated synphilin-1 degradation via the ubiquitin-proteasome pathway more efficiently than Parkin. Siah-1 was found to not facilitate ubiquitination and degradation of wild type or mutant alpha-synuclein. Synphilin-1 inhibited high K+-induced dopamine release from PC12 cells. Siah-1 was found to abrogate the inhibitory effects of synphilin-1 on dopamine release. Such findings suggest that Siah-1 might play a role in regulation of synphilin-1 function.

Multiple system atrophy: An update

Multiple system atrophy (MSA) is a sporadic neurodegenerative disorder that usually manifests in the early sixth decade of life and progresses relentlessly with a mean survival of 9 years. Clinically, MSA is dominated by autonomic/urogenital failure, which may be associated with either levodopa (L-dopa) -unresponsive parkinsonism in 80% of cases (MSA-P subtype) or with cerebellar ataxia in 20% of cases (MSA-C subtype). Pathologically, MSA is characterized by a neuronal multisystem degeneration and abnormal glial cytoplasmic inclusions containing alpha-synuclein aggregates. Pharmacological treatment of motor features is disappointing except for a transient L-dopa response in a minority of MSA-P patients. In contrast, autonomic and urogenital features of MSA should be identified early on, because they can be treated effectively in many instances. Neuroprotective strategies are presently unavailable, however, two multicentre European trials have been launched to evaluate the effects of riluzole and human recombinant growth hormone on disease progression in MSA. Clearly, further randomised, controlled trials are required to identify effective symptomatic or neuroprotective agents in MSA. Several in vivo models have become available to allow a careful preselection of candidate agents. Several research groups have been formed in Europe (EMSA-SG, NNIPPS) and United States (NAMSA-SG), providing a framework for coordinated trial activity in MSA.

Parkinson's disease: mechanisms and models

Parkinson's disease (PD) results primarily from the death of dopaminergic neurons in the substantia nigra. Current PD medications treat symptoms; none halt or retard dopaminergic neuron degeneration. The main obstacle to developing neuroprotective therapies is a limited understanding of the key molecular events that provoke neurodegeneration. The discovery of PD genes has led to the hypothesis that misfolding of proteins and dysfunction of the ubiquitin-proteasome pathway are pivotal to PD pathogenesis. Previously implicated culprits in PD neurodegeneration, mitochondrial dysfunction and oxidative stress, may also act in part by causing the accumulation of misfolded proteins, in addition to producing other deleterious events in dopaminergic neurons. Neurotoxin-based models (particularly MPTP) have been important in elucidating the molecular cascade of cell death in dopaminergic neurons. PD models based on the manipulation of PD genes should prove valuable in elucidating important aspects of the disease, such as selective vulnerability of substantia nigra dopaminergic neurons to the degenerative process.

Dorfin localizes to the ubiquitylated inclusions in Parkinson's disease, dementia with Lewy bodies, multiple system atrophy, and amyotrophic lateral sclerosis

In many neurodegenerative diseases, the cytopathological hallmark is the presence of ubiquitylated inclusions consisting of insoluble protein aggregates. Lewy bodies in Parkinson's disease and dementia with Lewy bodies disease, glial cell inclusions in multiple system atrophy, and hyaline inclusions in amyotrophic lateral sclerosis (ALS) are representative of these inclusions. The elucidation of the components of these inclusions and the mechanisms underlying inclusion formation is important in uncovering the pathogenesis of these disorders. We hypothesized that Dorfin, a perinuclearly located E3 ubiquitin ligase, participates in the formation of ubiquitylated inclusions in a wide range of neurodegenerative diseases. Here, we report that affinity-purified anti-Dorfin antibody labeled ubiquitylated inclusions of Parkinson's disease, dementia with Lewy bodies disease, multiple system atrophy, and sporadic and familial ALS. A double-immunofluorescence study revealed that Dorfin shows a distribution pattern parallel to that of ubiquitin. Furthermore, by a filter trap assay, we detected that Dorfin is present in the ubiquitylated high-molecular weight structures derived from these diseases. These results suggest that Dorfin plays a crucial role in the formation of ubiquitylated inclusions of alpha-synucleinopathy and ALS. However, because we failed to show the direct binding of alpha-synuclein with Dorfin, future investigations into the binding partner(s) of Dorfin will be needed to deepen our understanding of the pathophysiology of alpha-synucleinopathy and ALS.

A palmitoylated RING finger ubiquitin ligase and its homologue in the brain membranes

Ubiquitin (Ub) ligation is implicated in active protein metabolism and subcellular trafficking and its impairment is involved in various neurologic diseases. In rat brain, we identified two novel Ub ligases, Momo and Sakura, carrying double zinc finger motif and RING finger domain. Momo expression is enriched in the brain gray matter and testis, and Sakura expression is more widely detected in the brain white matter as well as in many peripheral organs. Both proteins associate with the cell membranes of neuronal and/or glial cells. We examined their Ub ligase activity in vivo and in vitro using viral expression vectors carrying myc-tagged Momo and Sakura. Overexpression of either Momo or Sakura in mixed cortical cultures increased total polyubiquitination levels. In vitro ubiquitination assay revealed that the combination of Momo and UbcH4 and H5c, or of Sakura and UbcH4, H5c and H6 is required for the reaction. Deletion mutagenesis suggested that the E3 Ub ligase activity of Momo and Sakura depended on their C-terminal domains containing RING finger structure, while their N-terminal domains influenced their membrane association. In agreement, Sakura associating with the membrane was specifically palmitoylated. Although the molecular targets of their Ub ligation remain to be identified, these findings imply a novel function of the palmitoylated E3 Ub ligase(s).

Dorfin localizes to Lewy bodies and ubiquitylates synphilin-1

Parkinson's disease (PD) is a neurodegenerative disease characterized by loss of nigra dopaminergic neurons. Lewy bodies (LBs) are a characteristic neuronal inclusion in PD brains. In this study, we report that Dorfin, a RING finger-type ubiquityl ligase for mutant superoxide dismutase-1, was localized with ubiquitin in LBs. Recently, synphilin-1 was identified to associate with alpha-synuclein and to be a major component of LBs. We found that overexpression of synphilin-1 in cultured cells led to the formation of large juxtanuclear inclusions, but showed no cytotoxicity. Dorfin colocalized in these large inclusions with ubiquitin and proteasomal components. In contrast to full-length synphilin-1, overexpression of the central portion of synphilin-1, including ankyrin-like repeats, a coiled-coil domain, and an ATP/GTP-binding domain, predominantly led to the formation of small punctate aggregates scattered throughout the cytoplasm and showed cytotoxic effects. Dorfin and ubiquitin did not localize in these small aggregates. Overexpression of the N or C terminus of synphilin-1 did not lead to the formation of any aggregates. Dorfin physically bound and ubiquitylated synphilin-1 through its central portion, but did not ubiquitylate wild-type or mutant alpha-synuclein. These results suggest that the central domain of synphilin-1 has an important role in the formation of aggregates and cytotoxicity and that Dorfin may be involved in the pathogenic process of PD and LB formation by ubiquitylation of synphilin-1.

Are ubiquitination pathways central to Parkinson's disease?

Parkinson's disease (PD) is the most common neurodegenerative movement disorder. The major motor disabilities of PD are associated with the extensive loss of dopaminergic neurons in the substantia nigra pars compacta. The physiological changes and biochemical pathways involved in the selective demise of these neurons are still unclear. Recent studies have demonstrated that alterations or reductions in ubiquitin-mediated proteasome function can be causal of at least some forms of parkinsonism, and multiple lines of evidence suggest that this mechanism of protein degradation may play an important role in the etiology of PD.

Models of Parkinson's disease

Parkinson's disease (PD) is a heterogenous disease likely to be caused by more than one specific aetiological factor. In rare familial cases of PD with similar clinical features to the idiopathic form of the disease, the underlying genetic cause has been identified. These PD-associated genes have been manipulated to create animal and cell culture models of the disease that have helped to further our understanding of the pathogenesis of PD, particularly concerning causes of the selective loss of dopaminergic neurons at the molecular level. In addition, these models will aid the future development of rational therapeutic strategies. This study briefly reviews toxin-induced models and the genetics of PD. It focuses on recently developed animal models of PD, as well as in vitro approaches to model the disease.

Ubiquitination of alpha-synuclein is not required for formation of pathological inclusions in alpha-synucleinopathies

alpha-Synucleinopathies, including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, are neurodegenerative disorders in which abnormal inclusions containing alpha-synuclein accumulate in selectively vulnerable neurons and glia. In this report, immunohistochemistry demonstrates ubiquitin in subsets of alpha-synuclein inclusions in dementia with Lewy bodies and multiple system atrophy. Biochemistry demonstrates that alpha-synuclein in the sodium dodecyl sulfate-soluble fractions of diseased brains is ubiquitinated, with mono- and di-ubiquitinated species predominating over polyubiquitinated forms. Similar immunohistochemical and biochemical characteristics were observed in an A53T mutant human alpha-synuclein transgenic mouse model of neurodegenerative alpha-synucleinopathies. Furthermore, in vitro ubiquitination of alpha-synuclein fibrils recapitulated the pattern of alpha-synuclein ubiquitination observed in human disease and the A53T alpha-synuclein mouse model. These results suggest that ubiquitination of alpha-synuclein is not required for inclusion formation and follows the fibrillization of alpha-synuclein.

Neuroprotective mechanisms of antiparkinsonian dopamine D2-receptor subfamily agonists

Numerous studies have shown that endogenous and/or environmental neurotoxins and oxidative stress may participate in the pathogenesis of Parkinson's disease (PD), but the detailed mechanisms are still unclear. While dopamine (DA) replacement therapy with L-DOPA (levodopa) improves PD symptoms, it does not inhibit the degeneration of DA neurons in the substantia nigra. Recently, bromocriptine, pramipexole and several other agonists of the dopamine D2-receptor subfamily (including D2, D3 and D4-subtypes) have been shown to have neuroprotective effects in parkinsonian models in vitro and in vivo. Their neuroprotective effects may be mediated directly and/or indirectly by antioxidant effects, mitochondrial stabilization or induction of the antiapoptotic Bcl-2 family.

Caspase-1 and caspase-8 cleave and inactivate cellular parkin

Lesions in the parkin gene cause early onset Parkinson's disease by a loss of dopaminergic neurons, thus demonstrating a vital role for parkin in the survival of these neurons. Parkin is inactivated by caspase cleavage, and the major cleavage site is after Asp126. Caspases responsible for parkin cleavage were identified by several experimental paradigms. Transient coexpression of caspases and wild type parkin in HEK-293 cells identified caspase-1, -3, and -8 as efficient inducers of parkin cleavage whereas caspase-2, -7, -9, and -11 did not induce cleavage. A D126A parkin mutation abrogates cleavage induced by caspase-1 and -8, but not by caspase-3. In anti-Fas-treated Jurkat T cells, parkin cleavage was inhibited by caspase inhibitors hFlip and CrmA (but not by X-linked inhibitor of apoptosis (XIAP)), indicating that caspase-8 (but not caspase-3) is responsible for the parkin cleavage in this model. Moreover, induction of apoptosis in caspase-3-deficient MCF7 cells, either by caspase-1 or -8 overexpression or by tumor necrosis factor-alpha treatment, led to parkin cleavage. These results demonstrate that caspase-1 and -8 can directly cleave parkin and suggest that death receptor activation and inflammatory stress can cause loss of the ubiquitin ligase activity of parkin, thus causing accumulation of toxic parkin substrates and triggering dopaminergic cell death.

Association of the cytoskeletal GTP-binding protein Sept4/H5 with cytoplasmic inclusions found in Parkinson's disease and other synucleinopathies

alpha-Synuclein-positive cytoplasmic inclusions are a pathological hallmark of several neurodegenerative disorders including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Here we report that Sept4, a member of the septin protein family, is consistently found in these inclusions, whereas five other septins (Sept2, Sept5, Sept6, Sept7, and Sept8) are not found in these inclusions. Sept4 and alpha-synuclein can also be co-immunoprecipitated from normal human brain lysates. When co-expressed in cultured cells, FLAG-tagged Sept4 and Myc-tagged alpha-synuclein formed detergent-insoluble complex, and upon treatment with a proteasome inhibitor, they formed Lewy body-like cytoplasmic inclusions. The tagged Sept4 and alpha-synuclein synergistically accelerated cell death induced by the proteasome inhibitor, and this effect was further enhanced by expression of another Lewy body-associated protein, synphilin-1, tagged with the V5 epitope. Moreover, co-expression of the three proteins (tagged Sept4, alpha-synuclein, and synphilin-1) was sufficient to induce cell death. These data raise the possibility that Sept4 is involved in the formation of cytoplasmic inclusions as well as induction of cell death in alpha-synuclein-associated neurodegenerative disorders.

Parkin facilitates the elimination of expanded polyglutamine proteins and leads to preservation of proteasome function

Parkin, the most commonly mutated gene in familial Parkinson's disease, encodes an E3 ubiquitin ligase. A number of candidate substrates have been identified for parkin ubiquitin ligase action including CDCrel-1, o-glycosylated alpha-synuclein, Pael-R, and synphilin-1. We now show that parkin promotes the ubiquitination and degradation of an expanded polyglutamine protein. Overexpression of parkin reduces aggregation and cytotoxicity of an expanded polyglutamine ataxin-3 fragment. Using a cellular proteasome indicator system based on a destabilized form of green fluorescent protein, we demonstrate that parkin reduces proteasome impairment and caspase-12 activation induced by an expanded polyglutamine protein. Parkin forms a complex with the expanded polyglutamine protein, heat shock protein 70 (Hsp70) and the proteasome, which may be important for the elimination of the expanded polyglutamine protein. Hsp70 enhances parkin binding and ubiquitination of expanded polyglutamine protein in vitro suggesting that Hsp70 may help to recruit misfolded proteins as substrates for parkin E3 ubiquitin ligase activity. We speculate that parkin may function to relieve endoplasmic reticulum stress by preserving proteasome activity in the presence of misfolded proteins. Loss of parkin function and the resulting proteasomal impairment may contribute to the accumulation of toxic aberrant proteins in neurodegenerative diseases including Parkinson's disease.

The cast of molecular characters in Parkinson's disease: felons, conspirators, and suspects

Parkinson's Disease (PD) is a common neurodegenerative disorder characterized by the progressive loss of dopamine neurons and the accumulation of Lewy bodies and neurites. Recent advances indicate that PD is due in some individuals to genetic mutations in alpha-synuclein, parkin, and ubiquitin C-terminal hydrolase L1 (UCHL1). All three PD-linked gene products are related directly or indirectly to the functioning of the cellular ubiquitin proteasomal system (UPS), suggesting that UPS dysfunction may be important in PD pathogenesis. Indeed, emerging evidence indicates that derangements of the UPS may be one of the underlying mechanisms of PD pathogenesis. The function of parkin as an ubiquitin protein ligase positions it as an important player in both familial and idiopathic PD. We recently demonstrated that parkin mediates a nondegradative form of ubiquitination on synphilin-1 that could contribute to synphilin-1's aggregation in PD. Our results implicate parkin involvement in the formation of Lewy bodies associated with sporadic PD. This review discusses the role of the UPS, as well as the modus operandi of the three PD candidate felons (alpha-synuclein, parkin, and UCHL1) along with their conspirators in bringing about dopaminergic cell death in PD.

alpha-synuclein aggregation: a link between mitochondrial defects and Parkinson's disease?

Protein aggregation is a shared feature of many human neurodegenerative diseases and appears to be an inevitable consequence of excessive accumulation of misfolded proteins. Recent studies suggest that accumulation of fibrillar alpha-synuclein aggregates is associated with Parkinson's disease and other Lewy body diseases. Furthermore, the missense mutations in alpha-synuclein that are responsible for some early-onset familial types of the disease promote the aggregation process of this protein. Therefore, the mechanism underlying the cellular alpha-synuclein aggregation is of great importance in understanding the pathogenic process of these diseases. This review summarizes recent advances in our understanding of the mechanisms underlying alpha-synuclein aggregation and how the mitochondrial dysfunction plays a role in this process. Protein misfolding and aggregation in vivo can be suppressed and promoted by several factors, such as molecular chaperones, protein degradation systems, and free radicals. Many of these factors are under the control of normal mitochondrial function, prompting the speculation that mitochondrial dysfunction might cause the accumulation of protein aggregates. Recent studies indeed show that mitochondrial defects can lead to the aggregation of alpha-synuclein. In addition, potentially toxic effects of alpha-synuclein have been linked to the aggregated forms rather than the monomers, both in vitro and in cultured cells. Therefore, it is postulated that aggregation of alpha-synuclein might be one of many possible links that connect mitochondrial dysfunction to neurodegeneration.

Residual substantia nigra neuromelanin in Parkinson's disease is cross-linked to alpha-synuclein

The pigmentation of substantia nigra pars compacta dopaminergic neurons is due to the presence of neuromelanin, an irregular macromolecular pigment belonging to the family of melanins. Depletion of neuromelanin in Parkinson's disease is typically indicated by loss of brown color in this area. Unlike that from controls, the pigment extracted from substantia nigra of parkinsonian patients seems to be mainly composed by highly cross-linked, protease-resistant proteic material and the neuromelanin macromolecule appears to be a minor presence. In the present paper we describe the isolation by SDS-PAGE of this proteic component after cleavage of the melanin backbone under solubilizing conditions. A single band is observed, which has been identified as alpha-synuclein by western blotting. As expected, the same process performed on a control specimen did not show occurrence of any major proteic component. Nevertheless, extraction from a 91 years old control with Lewy bodies displayed minor alpha-synuclein immunoreactive aggregates, whereas inclusion of free alpha-synuclein was not observed at all. Results reported here support the view that alpha-synuclein accumulates within substantia nigra neurons and is entrapped in pigment granules during neuromelanin biosynthesis, i.e. before the melanin depletion characteristic of Parkinson's disease starts.

Parkin's substrates and the pathways leading to neuronal damage

Mutations in the Parkin gene are associated with Parkinson s disease (PD). The gene product has been shown to be an E3 protein-ubiquitin ligase, catalyzing the addition of ubiquitin to target proteins prior to their destruction via the proteasome. This activity is thus key in regulating the turnover of substrate proteins. A predictive hypothesis for how this results in PD is that the misregulation of proteasomal degradation of Parkin s substrates is deleterious to neurons. Several different laboratories have identified alternate candidate proteins. In this review, the likelihood of each of the proposed substrates for parkin being robust will be evaluated. The distribution and abundance of the proteins will be examined for clues as to which are the pathologically important substrates for parkin. The possibility that loss of regulation of turnover of one or more of these substrates contributes to the selective neurodegeneration seen in PD is also discussed.

Parkin binds to alpha/beta tubulin and increases their ubiquitination and degradation

In addition to inhibiting the mitochondrial respiratory chain, toxins known to cause Parkinson's disease (PD), such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and rotenone, also strongly depolymerize microtubules and increase tubulin degradation. Microtubules are polymers of tubulin alpha/beta heterodimers, whose correct folding requires coordinated actions of cellular chaperonins and cofactors. Misfolded tubulin monomers are highly toxic and quickly degraded through a hitherto unknown mechanism. Here we report that parkin, a protein-ubiquitin E3 ligase linked to PD, was tightly bound to microtubules in taxol-mediated microtubule coassembly assays. In lysates from the rat brain or transfected human embryonic kidney (HEK) 293 cells, alpha-tubulin and beta-tubulin were strongly coimmunoprecipitated with parkin at 4 degrees C in the presence of colchicine, a condition in which tubulin exits as alpha/beta heterodimers. At the subcellular level, parkin exhibited punctate immunostaining along microtubules in rat brain sections, cultured primary neurons, glial cells, and cell lines. This pattern of subcellular localization was abolished in cells treated with the microtubule-depolymerizing drug colchicine. The binding between parkin and tubulin apparently led to increased ubiquitination and accelerated degradation of alpha- and beta-tubulins in HEK293 cells. Similarly ubiquitinated tubulins were also observed in rat brain lysates. Furthermore, parkin mutants found in PD patients did not ubiquitinate or degrade either tubulin. Taken together, our results show that parkin is a novel tubulin-binding protein, as well as a microtubule-associated protein. Its ability to enhance the ubiquitination and degradation of misfolded tubulins may play a significant role in protecting neurons from toxins that cause PD.

The ubiquitin proteasome system acutely regulates presynaptic protein turnover and synaptic efficacy

BACKGROUND

The ubiquitin proteasome system (UPS) mediates regulated protein degradation and provides a mechanism for closely controlling protein abundance in spatially restricted domains within cells. We hypothesized that the UPS may acutely determine the local concentration of key regulatory proteins at neuronal synapses as a means for locally modulating synaptic efficacy and the strength of neurotransmission communication.

RESULTS

We investigated this hypothesis at the Drosophila neuromuscular synapse by using an array of genetic and pharmacological tools. This study demonstrates that UPS components are present in presynaptic boutons and that the UPS functions locally in the presynaptic compartment to rapidly eliminate a conditional transgenic reporter of proteasome activity. We assayed a panel of synaptic proteins to determine whether the UPS acutely regulates the local abundance of native synaptic targets. Both acute pharmacological inhibition of the proteasome (<1 hr) and targeted genetic perturbation of proteasome function in the presynaptic neuron cause the specific accumulation of the essential synaptic vesicle-priming protein DUNC-13. Most importantly, acute pharmacological inhibition of the proteasome (<1 hr) causes a rapid strengthening of neurotransmission (an approximately 50% increase in evoked amplitude) because of increased presynaptic efficacy. The proteasome-dependent regulation of presynaptic protein abundance, both of the exogenous reporter and native DUNC-13, and the modulation of presynaptic neurotransmitter release occur on an intermediate, rapid (tens of minutes) timescale.

CONCLUSIONS

Taken together, these studies demonstrate that the UPS functions locally within synaptic boutons to acutely control levels of presynaptic protein and that the rate of UPS-dependent protein degradation is a primary determinant of neurotransmission strength.

The cellular protein level of parkin is regulated by its ubiquitin-like domain

Parkin is a ubiquitin-protein isopeptide ligase (E3) involved in ubiquitin/proteasome-mediated protein degradation. Mutations in the parkin gene cause a loss-of-function and/or alter protein levels of parkin. As a result, the toxic build-up of parkin substrates is thought to lead to autosomal recessive juvenile Parkinsonism. To identify a role for the ubiquitin-like domain (ULD) of parkin, we created a number of hemagglutinin (HA)-tagged parkin constructs using mutational and structural information. Western blotting and immunocytochemistry showed a much stronger expression level for HA-parkin residues 77-465 (without ULD) than HA-parkin full-length (with ULD). The deletion of ULD in Drosophila parkin also caused a sharp increase in expression of the truncated form, suggesting that the function of the ULD of parkin is conserved across species. By progressive deletion analysis of parkin ULD, we found that residues 1-6 of human parkin play a crucial role in controlling the expression levels of this gene. HA-parkin residues 77-465 showed ubiquitination in vivo, demonstrating that the ULD is not critical for parkin auto-ubiquitination; ubiquitination seemed to cluster on the central domain of parkin (residues 77-313). These effects were specific for the ULD of parkin and not transfection-, toxic-, epitope tag-, and/or vector-dependent. Taken together, these data suggest that the 76 most NH(2)-terminal residues (ULD) dramatically regulate the protein levels of parkin.

Proteolytic stress: a unifying concept for the etiopathogenesis of Parkinson's disease

The etiopathogenesis of Parkinson's disease (PD) has been elusive. Recently, several lines of evidence have converged to suggest that defects in the ubiquitin-proteasome system and proteolytic stress underlie nigral pathology in both familial and sporadic forms of the illness. In support of this concept, mutations in alpha-synuclein that cause the protein to misfold and resist proteasomal degradation cause familial PD. Similarly, mutations in two enzymes involved in the normal function of the ubiquitin-proteasome system, parkin and ubiquitin C-terminal hydrolase L1, are also associated with hereditary PD. Furthermore, structural and function defects in 26/20S proteasomes with accumulation and aggregation of potentially cytotoxic abnormal proteins have been identified in the substantia nigra pars compacta of patients with sporadic PD. Thus, a defect in protein handling appears to be a common factor in sporadic and the various familial forms of PD. This hypothesis may also account for the vulnerability of the substantia nigra pars compacta in PD, why the disorder is age related, and the nature of the Lewy body. It has also facilitated the development of experimental models that recapitulate the behavioral and pathological features of PD, and hopefully will lead to the development of novel neuroprotective therapies for the disorder.

Parkin: a multipurpose neuroprotective agent?

An autosomal recessive juvenile-onset form of Parkinson's disease (AR-JP) is caused by loss-of-function mutations of the parkin gene, which encodes a ubiquitin-protein ligase. Three recent reports demonstrate that parkin can protect neurons from diverse cellular insults, including alpha-synuclein toxicity, proteasomal dysfunction, Pael-R accumulation, and kainate-induced excitotoxicity. These findings suggest a central role for parkin in maintaining dopaminergic neuronal integrity and strengthen the link between AR-JP and the more common sporadic form of Parkinson's disease.

Mitochondrial pathology and apoptotic muscle degeneration in Drosophila parkin mutants

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by loss of dopaminergic neurons in the substantia nigra. Several lines of evidence strongly implicate mitochondrial dysfunction as a major causative factor in PD, although the molecular mechanisms responsible for mitochondrial dysfunction are poorly understood. Recently, loss-of-function mutations in the parkin gene, which encodes a ubiquitin-protein ligase, were found to underlie a familial form of PD known as autosomal recessive juvenile parkinsonism (AR-JP). To gain insight into the molecular mechanism responsible for selective cell death in AR-JP, we have created a Drosophila model of this disorder. Drosophila parkin null mutants exhibit reduced lifespan, locomotor defects, and male sterility. The locomotor defects derive from apoptotic cell death of muscle subsets, whereas the male sterile phenotype derives from a spermatid individualization defect at a late stage of spermatogenesis. Mitochondrial pathology is the earliest manifestation of muscle degeneration and a prominent characteristic of individualizing spermatids in parkin mutants. These results indicate that the tissue-specific phenotypes observed in Drosophila parkin mutants result from mitochondrial dysfunction and raise the possibility that similar mitochondrial impairment triggers the selective cell loss observed in AR-JP.

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.