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

Alpha synuclein aggregation: is it the toxic gain of function responsible for neurodegeneration in Parkinson's disease?

Protein aggregation appears to be the common denominator in a series of distinct neurodegenerative diseases yet its role in the associated neuronal pathology in these various conditions remains elusive. In Parkinson's disease, localization of alpha synuclein aggregates within intracellular Lewy body occlusions represent a major hallmark of this disorder and suggest that such aggregation may play a causative role in the resulting dopaminergic cell loss. In this Viewpoint article, recent data is reviewed related to how alpha synuclein aggregation may occur, what cellular events might be responsible, and how this may interfere with normal cellular function(s). It appears likely that while aggregation of alpha synuclein may interfere with its normal function in the cell, this is not the primary cause of the related neurodegeneration.

Parkin and the molecular pathways of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by the selective demise of specific neuronal populations leading to impairment of motor functions. Recent genetic studies have uncovered several genes involved in inherited forms of the disease. These gene products are implicated in the biochemical pathways underlying the etiology of sporadic PD. Mutations in the parkin gene causal of autosomal recessive juvenile parkinsonism highlight that ubiquitin-mediated proteolysis may play an important role in the pathobiology of PD.

Co-association of parkin and alpha-synuclein

Parkin and alpha-synuclein are two proteins that are associated with the pathophysiology of Parkinson's disease (PD). Parkin is present in Lewy bodies and axonal spheroids in brains affected by PD, and mutations in parkin cause hereditary forms of Parkinsonism. Alpha-synuclein is a major component of Lewy bodies and is associated with rare cases of PD. We now show that parkin binds to alpha-synuclein, including conditions associated with alpha-synuclein aggregation. Parkin and alpha-synuclein complexes were observed in BE-M17 cells under basal conditions, in BE- M17 cells under oxidative conditions and in brains from control or PD donors. Double staining of PD brains shows parkin and alpha-synuclein co-localize to the same pathological structures (both Lewy bodies and axonal spheroids). These results suggest that parkin interacts with alpha-synuclein and could contribute to the pathophysiology of PD more generally than was previously considered.

The genomic structure and promoter region of the human parkin gene

Parkin has been identified as a causative gene of the autosomal recessive juvenile parkinsonism (AR-JP). In this study, we determined the genomic structure of the Parkin gene and identified a core promoter region based on the DNA sequence of 1.4 Mb. The 5'-flanking region contained no apparent TATA or CAAT box elements but several putative cis-elements for various transcription factors. The GC- and CpG-rich regions were observed not only in the 5'-flanking sequence but also in the 5'-part of the first intron of Parkin. We identified an exact starting point of Parkin transcription. A core promoter region was determined by transfecting a series of deletion constructs with a dual luciferase reporter system into human neuroblastoma cells. Furthermore, we located a neighboring novel gene in a head-to-head direction with Parkin with only a 198-bp interval.

E3 ubiquitin-protein ligase activity of Parkin is dependent on cooperative interaction of RING finger (TRIAD) elements

The parkin gene codes for a 465-amino acid protein which, when mutated, results in autosomal recessive juvenile parkinsonism (AR-JP). Symptoms of AR-JP are similar to those of idiopathic Parkinson's disease, with the notable exception being the early onset of AR-JP. We have cloned and expressed human Parkin in Escherichia coli and have examined Parkin-mediated ubiquitination in an in vitro ubiquitination assay using purified recombinant proteins. We found that Parkin has E3 ubiquitin ligase activity in this system, demonstrating for the first time that the E3 activity is an intrinsic function of the Parkin protein and does not require posttranslational modification or association with cellular proteins other than an E2 (human Ubc4 E2 was utilized in this ubiquitination assay). Mutagenesis of individual elements of the conserved RING TRIAD domain indicated that at least two elements were required for ubiquitin ligase activity and suggested a functional cooperation between the RING finger elements. Since the activity assays were conducted with recombinant proteins purified from E. coli, this is the first time TRIAD element interaction has been demonstrated as an intrinsic feature of Parkin E3 activity.

Parkinson disease: etiology, pathogenesis and future of gene therapy

Parkinson disease (PD) is a progressive neurological disorder with a prevalence of 1-2% in people over the age of 50. It has a world-wide distribution and has no gender preference. The neurological hallmark of PD is the presence of Lewy bodies and is characterized by the degeneration of nigrostriatal dopaminergic neurons. The causes of PD are unknown but considerable evidence suggests a multifactorial etiology involving genetic and environmental factors. A molecular genetic approach identified three genes and at least two additional loci in rare familial forms of PD. Two of these genes are involved in the ubiquitin mediated pathway of protein degradation and the third one is a highly expressed protein in the synaptic terminal and is called alpha-synuclein. In animal models, it has been shown that use of the household pesticide which is known to contain rotenone, causes PD. Thus, a combined action of genetic and environmental factors is responsible for the pathogenesis of PD. Although use of levodopa or dopamine agonists can substantially reduce clinical symptoms, and transplantation of fetal nerve tissue still remains as an alternative therapy (although it has been recently shown to be having no overall benefit), directed delivery of glial cell derived neurotrophic factor (known to have trophic effects on dopaminergic neurons) may also be a beneficial therapeutic option for PD patients.

Parkin and Parkinson's disease

Parkin is the causative gene for an autosomal recessive form of Parkinson's disease. The gene was discovered in 1998. The parkin gene is a novel gene containing 12 exons spanning over 1.5 Mb and encodes a protein of 465 amino acids with a molecular mass of approximately 52,000 M(r). Various deletion mutations and point mutations have been discovered in patients with autosomal recessive Parkinson's disease. The substantia nigra and the locus coeruleus selectively undergo neurodegeneration without forming Lewy bodies. The parkin gene product, Parkin protein, has a unique structure with a ubiquitin-like domain in the amino-terminus and a RING finger motif in the carboxy terminus. The function of Parkin was not known until recently. During the year 2000, great progress was made in defining its function. First of all, Parkin was found to be a ubiquitin-protein ligase (E3), a component of the ubiquitin system, which is an important adenosine triphosphate-dependent protein degradation machinery. In addition, CDCrel-1, a synaptic vesicle associated protein, was found to be a substrate for Parkin as an E3. Although many studies still need to be performed to elucidate the molecular mechanism of the selective nigral neurodegeneration in this form of familial Parkinson's disease, it will not be too long before this is accomplished. In this review article, we evaluate the developments in this area published since 1 February 2000.

Proteasomal inhibition leads to formation of ubiquitin/alpha-synuclein-immunoreactive inclusions in PC12 cells

Proteasomal dysfunction has been recently implicated in the pathogenesis of several neurodegenerative diseases, including Parkinson's disease and diffuse Lewy body disease. We have developed an in vitro model of proteasomal dysfunction by applying pharmacological inhibitors of the proteasome, lactacystin or ZIE[O-tBu]-A-leucinal (PSI), to dopaminergic PC12 cells. Proteasomal inhibition caused a dose-dependent increase in death of both naive and neuronally differentiated PC12 cells, which could be prevented by caspase inhibition or CPT-cAMP. A percentage of the surviving cells contained discrete cytoplasmic ubiquitinated inclusions, some of which also contained synuclein-1, the rat homologue of human alpha-synuclein. However the total level of synuclein-1 was not altered by proteasomal inhibition. The ubiquitinated inclusions were present only within surviving cells, and their number was increased if cell death was prevented. We have thus replicated, in this model system, the two cardinal pathological features of Lewy body diseases, neuronal death and the formation of cytoplasmic ubiquitinated inclusions. Our findings suggest that inclusion body formation and cell death may be dissociated from one another.

Parkin is associated with cellular vesicles

We recently identified a novel gene, parkin, as a pathogenic gene for autosomal recessive juvenile parkinsonism. Parkin encodes a 52-kDa protein with a ubiquitin-like domain and two RING-finger motifs. To provide a insight into the function of parkin, we have examined its intracellular distribution in cultured cells. We found that parkin was localized in the trans-Golgi network and the secretory vesicles in U-373MG or SH-SY5Y cells by immunocytochemical analyses. In the subsequent subcellular fractionation studies of rat brain, we showed that parkin was copurified with the synaptic vesicles (SVs) when we used low ionic conditions throughout the procedure. An immunoelectromicroscopic analysis indicated that parkin was present on the SV membrane. Parkin was readily released from SVs into the soluble phase by increasing ionic strength at neutral pH, but not by a non-ionic detergent. To elucidate its responsible region for membrane association, we transfected with green fluorescent protein-tagged deletion mutants of parkin into COS-1 cells followed by subcellular fractionation. We demonstrated the ability of parkin to bind to the membranes through a broad region except for the ubiquitin-like domain. The significance of SV localization of parkin is discussed.

An unfolded putative transmembrane polypeptide, which can lead to endoplasmic reticulum stress, is a substrate of Parkin

A putative G protein-coupled transmembrane polypeptide, named Pael receptor, was identified as an interacting protein with Parkin, a gene product responsible for autosomal recessive juvenile Parkinsonism (AR-JP). When overexpressed in cells, this receptor tends to become unfolded, insoluble, and ubiquitinated in vivo. The insoluble Pael receptor leads to unfolded protein-induced cell death. Parkin specifically ubiquitinates this receptor in the presence of ubiquitin-conjugating enzymes resident in the endoplasmic reticulum and promotes the degradation of insoluble Pael receptor, resulting in suppression of the cell death induced by Pael receptor overexpression. Moreover, the insoluble form of Pael receptor accumulates in the brains of AR-JP patients. Here, we show that the unfolded Pael receptor is a substrate of Parkin, the accumulation of which may cause selective neuronal death in AR-JP.

Acute and chronic haloperidol treatments increase parkin mRNA levels in the rat brain

Mutations in the parkin gene cause autosomal recessive juvenile parkinsonism. We examined the effects of acute and chronic treatment with haloperidol on parkin mRNA expression in the rat brain by reverse transcription-polymerase chain reaction. Acute haloperidol treatment (2 mg/kg) increased parkin mRNA levels in the striatum and nucleus accumbens but not in the medial prefrontal cortex and substantia nigra. Four-week-treatment with haloperidol decanoate (25 mg eq/kg) produced a significant increase in parkin mRNA levels in the striatum without affecting to those in the medial prefrontal cortex, nucleus accumbens and substantia nigra. These results suggest that Parkin may be involved in the haloperidol-induced synaptic plasticity, since Parkin regulates the turnover of the synaptic protein, CDCrel-1.

Proteases and cellular regulation in plants

Protein degradation is accomplished by a diverse collection of proteases. Recent studies have illustrated the importance of proteolysis in the control of many aspects of cellular regulation from photosynthesis to photomorphogenesis. In addition, new results point to a role for proteolysis in programmed cell death, circadian rhythm, and defense response in plants.

N-Terminally extended human ubiquitin-conjugating enzymes (E2s) mediate the ubiquitination of RING-finger proteins, ARA54 and RNF8

We have previously cloned cDNAs encoding the N-terminally extended class III human ubiquitin-conjugating enzymes (E2s), UBE2E2 and UBE2E3, the biological functions of which are not known. In this study, we performed yeast two-hybrid screening for protein(s) interacting with UBE2E2, and two RING-finger proteins, ARA54 and RNF8, were identified. Both ARA54, a ligand-dependent androgen receptor coactivator, and RNF8 interacted with class III E2s (UBE2E2, UbcH6, and UBE2E3), but not with other E2s (UbcH5, UbcH7, UbcH10, hCdc34, and hBendless) in the yeast two-hybrid assay. The use of various deletion mutants of UBE2E2 and RING-finger proteins and two RING point mutants, ARA54 C(220)S and RNF8 C(403)S, in which the RING structure is disrupted, showed that the UBC domain of UBE2E2 and the RING domain of these RING-finger proteins were involved in this association. Wild-type ARA54 and RNF8, expressed in insect Sf9 cells, catalyzed E2-dependent autoubiquitination in vitro, whereas the point mutated proteins showed markedly reduced activity. Ubiquitination of wild-type ARA54 and RNF8, expressed in COS-7 cells, was also observed, and a proteasome inhibitor, MG132, prevented the degradation of these wild-type proteins, but was much less effective in protecting the RING mutants. Transfection of COS-7 cells with a green fluorescent protein chimera showed that RNF8 was localized in the nucleus, and ARA54 in both the cytoplasm and nucleus. Our results suggest that ARA54 and RNF8 possibly act as Ub-ligases (E3) in the ubiquitination of certain nuclear protein(s).

Themes and variations on ubiquitylation

Ubiquitylation--the conjugation of proteins with a small protein called ubiquitin--touches upon all aspects of eukaryotic biology, and its defective regulation is manifest in diseases that range from developmental abnormalities and autoimmunity to neurodegenerative diseases and cancer. A few years ago, we could only have dreamt of the complex arsenal of enzymes dedicated to ubiquitylation. Why has nature come up with so many ways of doing what seems to be such a simple job?