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

Cannabinoid CB1 receptors in the basal ganglia and motor response to activation or blockade of these receptors in parkin-null mice

The endocannabinoid transmission becomes overactive in the basal ganglia in Parkinson's disease (PD), as reported in patients and animal models of this disease. In the present study, we examined the status of cannabinoid CB(1) receptors in the basal ganglia of female and male Park-2 knockout mice, a genetic model of PD that progresses with no neuronal death and that may be considered representative of early and presymptomatic parkinsonian deficits. We found an increase in the density of CB(1) receptors in the substantia nigra compared to wild-type animals with no changes in other basal ganglia, although this occurred only in females. Despite this increase, the motor inhibition caused by the acute administration of the cannabinoid agonist Delta(9)-tetrahydrocannabinol to Park-2 knockout female mice was markedly of lesser magnitude compared with the response found in wild-type animals. By contrast, the administration of the CB(1) receptor antagonist SR141716 resulted in a hyperkinetic response in parkin-null mice, response that was almost absent in wild-type animals and that was accompanied by a decrease in tyrosine hydroxylase activity in the caudate-putamen. However, parkin-null male mice exhibited normal levels of CB(1) receptors in the substantia nigra and the remaining basal ganglia, with the only exception of a small decrease in the lateral part of the caudate-putamen. This was associated with an increase in mRNA levels for superoxide dismutase in this structure. In addition, the administration of Delta(9)-tetrahydrocannabinol to parkin-null male mice caused a motor inhibition that was significantly greater than in the case of their wild-type counterparts, and that was accompanied by an increase in tyrosine hydroxylase activity in the caudate-putamen. In summary, extending the data obtained in humans and animal models of basal ganglia neurodegeneration, changes in CB(1) receptors were also observed in parkin-null mice, a model of PD that may be considered representative of early stages of this disease. These changes are associated with differences in behavioral responses to cannabinoid agonists or antagonists between Park-2 knockout and wild-type mice, although parkin-null mice exhibited evident gender-dependent differences for both levels of CB(1) receptors and motor responses to agonists or antagonists.

Common anti-apoptotic roles of parkin and α-synuclein in human dopaminergic cells

Parkin, a product of the gene responsible for autosomal recessive juvenile parkinsonism (AR-JP), is an important player in the pathogenic process of Parkinson's disease (PD). Despite numerous studies including search for the substrate of parkin as an E3 ubiquitin-protein ligase, the mechanism by which loss-of-function of parkin induces selective dopaminergic neuronal death remains unclear. Related to this issue, here we show that antisense knockdown of parkin causes apoptotic cell death of human dopaminergic SH-SY5Y cells associated with caspase activation and accompanied by accumulation of oxidative dopamine (DA) metabolites due to auto-oxidation of DOPA and DA. Forced expression of alpha-synuclein (alpha-SN), another familial PD gene product, prevented accumulation of oxidative DOPA/DA metabolites and cell death caused by parkin loss. Our findings indicate that both parkin and alpha-SN share a common pathway in DA metabolism whose abnormality leads to accumulation of oxidative DA metabolites and subsequent cell death.

The ubiquitin-proteasome pathway in Parkinson's disease and other neurodegenerative diseases

During the past decade, it has become apparent that a set of ostensibly unrelated neurodegenerative diseases, including Parkinson's disease and Huntington's disease, shares striking molecular and cell biology commonalities. Each of the diseases involves protein misfolding and aggregation, resulting in inclusion bodies and other aggregates within cells. These aggregates often contain ubiquitin, which is the signal for proteolysis by the 26S proteasome, and chaperone proteins that are involved in the refolding of misfolded proteins. The link between the ubiquitin-proteasome system and neurodegeneration has been strengthened by the identification of disease-causing mutations in genes coding for several ubiquitin-proteasome pathway proteins in Parkinson's disease. However, the exact molecular connections between these systems and pathogenesis remain uncertain and controversial. In this article, we summarize the state of current knowledge, focusing on important unresolved questions.

The Role of alpha-synuclein assembly and metabolism in the pathogenesis of Lewy body disease

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are members of a family of disorders characterized by the presence of inclusion bodies, or Lewy bodies (LBs), filled with aggregates of alpha-synuclein. These diseases are a leading cause of movement disorders and dementia in the aging population, and it is crucial to understand the factors leading to the accumulation and assembly of these alpha-synuclein aggregates. Previous studies have uncovered much about the factors leading to aggregation and the mechanisms causing neurotoxicity of these inclusion bodies; however, little is known about factors that promote the degradation and prevent the aggregation of alpha-synuclein. The present article provides a review of recent efforts in the investigation of factors involved in alpha-synuclein metabolism and the mechanisms involved in preventing accumulation of alpha-synuclein and degrading this molecule. Understanding these processes might provide targets for the development of novel therapies for disorders such as DLB and PD.

[Neurodegeneration caused by ER stress?--the pathogenetic mechanisms underlying AR-JP]

Mutations of the Parkin gene are responsible for autosomal recessive juvenile parkinsonism (AR-JP), the most common cause of early-onset familial Parkinson's disease. Parkin functions as an E3 ubiquitin ligase, thereby promoting ubiquitination and subsequent proteosomal degradation of its substrate(s). AR-JP is, therefore, thought to be caused by accumulation of an unknown toxic protein(s), which would normally be degraded by a molecular machinery involving Parkin. To date, ten different proteins are reported to be substrates of Parkin. Among these, a G protein-coupled orphan receptor called the Pael receptor (Pael-R), which is highly expressed in dopaminergic neurons, attracts particular attention. When over-expressed in cells, the Pael-R protein became improperly folded and insoluble. Excessive accumulation of insoluble Pael-R led to endoplasmic reticulum (ER) stress-induced cell death. Parkin was observed to ubiquitinate the misfolded Pael-R protein, thereby promoting its degradation and suppressing misfolded Pael-R-induced cell death. Moreover, selective dopaminergic neurodegeneration was observed when human Pael-R was ectopically expressed in Drosophila brain, further supporting the idea that Pael-R accumulation plays a major role in AR-JP. In contrast, neither dopaminergic neurodegeneration nor accumulation of any known Parkin substrates was detected in Parkin knockout mice. The role of Pael-R in AR-JP will be discussed based on recent data.

Parkin Stabilizes Microtubules through Strong Binding Mediated by Three Independent Domains

Mutations of parkin, a protein-ubiquitin isopeptide ligase (E3), appear to be the most frequent cause of familial Parkinson's disease (PD). Our previous studies have demonstrated that parkin binds strongly to alpha/beta tubulin heterodimers and microtubules. Here we show that the strong binding between parkin and tubulin, as well as that between parkin and microtubules, was mediated by three independent domains: linker, RING1, and RING2. These redundant strong interactions made it virtually impossible to separate parkin from microtubules by high concentrations of salt (3.8 m) or urea (0.5 m). Parkin co-purified with tubulin and was found in highly purified tubulin preparation. Expression of either full-length parkin or any of its three microtubule-binding domains significantly attenuated colchicine-induced microtubule depolymerization. The abilities of parkin to bind to and stabilize microtubules were not affected by PD-linked mutations that abrogate its E3 ligase activity. Thus, the tubulin/microtubule-binding activity of parkin and its E3 ligase activity are independent. The strong binding between parkin and tubulin/microtubules through three redundant interaction domains may not only stabilize microtubules but also guarantee the anchorage of this E3 ligase on microtubules. Because many misfolded proteins are transported on microtubules, the localization of parkin on microtubules may provide an important environment for its E3 ligase activity toward misfolded substrates.

α-Synuclein and Parkin Contribute to the Assembly of Ubiquitin Lysine 63-linked Multiubiquitin Chains

Mutations in alpha-synuclein, Parkin, and UCH-L1 cause heritable forms of Parkinson disease. Unlike alpha-synuclein, for which no precise biochemical function has been elucidated, Parkin functions as a ubiquitin E3 ligase, and UCH-L1 is a deubiquitinating enzyme. The E3 ligase activity of Parkin in Parkinson disease is poorly understood and is further obscured by the fact that multiubiquitin chains can be formed through distinct types of linkages that regulate diverse cellular processes. For instance, ubiquitin lysine 48-linked multiubiquitin chains target substrates to the proteasome, whereas ubiquitin lysine 63-linked chains control ribosome function, protein sorting and trafficking, and endocytosis of membrane proteins. It is notable in this regard that ubiquitin lysine 63-linked chains promote the degradation of membrane proteins by the lysosome. Because both Parkin and alpha-synuclein can regulate the activity of the dopamine transporter, we investigated whether they influenced ubiquitin lysine 63-linked chain assembly. These studies revealed novel biochemical activities for both Parkin and alpha-synuclein. We determined that Parkin functions with UbcH13/Uev1a, a dimeric ubiquitin-conjugating enzyme, to assemble ubiquitin lysine 63-linked chains. Our results and the results of others indicate that Parkin can promote both lysine 48- and lysine 63-linked ubiquitin chains. alpha-Synuclein also stimulated the assembly of lysine 63-linked ubiquitin chains. Because UCH-L1, a ubiquitin hydrolase, was recently reported to form lysine 63-linked conjugates, it is evident that three proteins that are genetically linked to Parkinson disease can contribute to lysine 63 multiubiquitin chain formation.

Ubiquitin-proteasome system and Parkinson's diseases

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by nigrostriatal dopaminergic degeneration and development of cytoplasmic inclusions known as Lewy bodies. To date, the mechanisms involved in PD pathogenesis are not clearly understood. Clues from genetic studies including identification of mutations in genes for alpha-synuclein, parkin, and ubiquitin carboxy hydrolase L1 associated with familial PD and the presence of proteinaceous cytoplasmic inclusions in spared dopaminergic nigral neurons in sporadic cases of PD have suggested an important role for ubiquitin-proteasome system (UPS) and aberrant protein degradation. In vivo and in vitro studies have linked parkin, alpha-synuclein, and oxidative stress to a compromised UPS and PD pathogenesis suggesting novel therapeutic targets.

Ubiquitin, proteasome and parkin

The ubiquitin-proteasome system (UPS) is important for intracellular proteolysis, and is responsible for a diverse array of biologically important cellular processes, such as cell-cycle progression, signaling cascades and developmental programs. This system is also involved in the protein quality control, which maintains the health of the cell. Thus, the UPS provides a clue for understanding of the molecular mechanisms underlying various neurodegenerative diseases. In the last decade, we witnessed a tremendous progress in uncovering the mechanisms of Parkinson's disease (PD). Of the several genes that can cause familial PD, parkin, the causative gene of autosomal recessive juvenile parkinsonism (ARJP), is of a special interest because it encodes an ubiquitin-protein ligase, which covalently attaches ubiquitin to target proteins, designating them for destruction by the proteasome. This review summarizes recent studies on the UPS pathway with a special reference to parkin, focusing on how parkin is linked to the pathogenesis of ARJP.

The ubiquitin system: pathogenesis of human diseases and drug targeting

With the many processes and substrates targeted by the ubiquitin pathway, it is not surprising to find that aberrations in the system underlie, directly or indirectly, the pathogenesis of many diseases. While inactivation of a major enzyme such as E1 is obviously lethal, mutations in enzymes or in recognition motifs in substrates that do not affect vital pathways or that affect the involved process only partially may result in a broad array of phenotypes. Likewise, acquired changes in the activity of the system can also evolve into certain pathologies. The pathological states associated with the ubiquitin system can be classified into two groups: (a) those that result from loss of function-mutation in a ubiquitin system enzyme or in the recognition motif in the target substrate that lead to stabilization of certain proteins, and (b) those that result from gain of function-abnormal or accelerated degradation of the protein target. Studies that employ targeted inactivation of genes coding for specific ubiquitin system enzymes and substrates in animals can provide a more systematic view into the broad spectrum of pathologies that may result from aberrations in ubiquitin-mediated proteolysis. Better understanding of the processes and identification of the components involved in the degradation of key regulatory proteins will lead to the development of mechanism-based drugs that will target specifically only the involved proteins.

Cooperation of molecular chaperones with the ubiquitin/proteasome system

Molecular chaperones and energy-dependent proteases have long been viewed as opposing forces that control protein biogenesis. Molecular chaperones are specialized in protein folding, whereas energy-dependent proteases such as the proteasome mediate efficient protein degradation. Recent data, however, suggest that molecular chaperones directly cooperate with the ubiquitin/proteasome system during protein quality control in eukaryotic cells. Modulating the intracellular balance of protein folding and protein degradation may open new strategies for the treatment of human diseases that involve chaperone pathways such as cancer and diverse amyloid diseases.

Alterations in the solubility and intracellular localization of parkin by several familial Parkinson's disease-linked point mutations

Mutations in the parkin gene, which encodes a ubiquitin ligase, are currently recognized as the main contributor to familial forms of Parkinson's disease (PD). A simple assumption about the effects of PD-linked mutations in parkin is that they impair or ablate the enzyme activity. However, a number of recent studies, including ours, have indicated that many disease-linked point mutants of parkin retain substantial catalytic activity. To understand how the plethora of mutations on parkin contribute to its dysfunction, we have conducted a systematic analysis of a significant number of parkin point mutants (22 in total), which represent the majority of parkin missense/nonsense mutations reported to date. We found that more than half of these mutations, including many located outside of the parkin RING fingers, produce alteration in the solubility of parkin which influences its detergent extraction property. This mutation-mediated alteration in parkin solubility is also associated with its propensity to form intracellular, aggresome-like, protein aggregates. However, they do not represent sites where parkin substrates become sequestered. As protein aggregation sequesters the functional forms away from their normal sites of action, our results suggest that alterations in parkin solubility and intracellular localization may underlie the molecular basis of the loss of function caused by several of its mutations.

RING Finger Ubiquitin-Protein Isopeptide Ligase Nrdp1/FLRF Regulates Parkin Stability and Activity

Parkin is a ubiquitin-protein isopeptide ligase. It has been suggested that loss of function in parkin causes accumulation and aggregation of its substrates, leading to death of dopaminergic neurons in Parkinson disease. Using the yeast two-hybrid screen, we isolated a RING finger protein that interacted with the N terminus of parkin in a Drosophila cDNA library. Interaction between human parkin and the mammalian RING finger protein homologue Nrdp1/FLRF, a ubiquitin-protein isopeptide ligase that ubiquitinates ErbB3 and ErbB4, was validated by in vitro binding assay, co-immunoprecipitation, and immunofluorescence co-localization. Significantly, pulse-chase experiments showed that cotransfection of Nrdp1 and parkin reduced the half-life of parkin from 5 to 2.5 h. Consistent with these findings, we further observed that degradation of CDCrel-1, a parkin substrate, was facilitated by overexpression of parkin protein. However, co-transfection of Nrdp1 with parkin reversed the effects of parkin on CDCrel-1 degradation. We conclude that Nrdp1 is a parkin modifier that accelerates degradation of parkin, resulting in a reduction of parkin activity.

BAG5 inhibits parkin and enhances dopaminergic neuron degeneration

Loss-of-function mutations in the parkin gene, which encodes an E3 ubiquitin ligase, are the major cause of early-onset Parkinson's disease (PD). Decreases in parkin activity may also contribute to neurodegeneration in sporadic forms of PD. Here, we show that bcl-2-associated athanogene 5 (BAG5), a BAG family member, directly interacts with parkin and the chaperone Hsp70. Within this complex, BAG5 inhibits both parkin E3 ubiquitin ligase activity and Hsp70-mediated refolding of misfolded proteins. BAG5 enhances parkin sequestration within protein aggregates and mitigates parkin-dependent preservation of proteasome function. Finally, BAG5 enhances dopamine neuron death in an in vivo model of PD, whereas a mutant that inhibits BAG5 activity attenuates dopaminergic neurodegeneration. This contrasts with the antideath functions ascribed to BAG family members and suggests a potential role for BAG5 in promoting neurodegeneration in sporadic PD through its functional interactions with parkin and Hsp70.

How does parkin ligate ubiquitin to Parkinson's disease?

Recessive mutations in the human PARKIN gene are the most common cause of hereditary parkinsonism, which arises from the degeneration of dopaminergic neurons in the substantia nigra. However, the molecular mechanisms by which the loss of parkin causes dopaminergic neurodegeneration are not well understood. Parkin is an enzyme that ubiquitinates several candidate substrate proteins and thereby targets them for proteasomal degradation. Hypothesis-driven searches have led to the discovery of aggregation-prone protein substrates of parkin. Moreover, the enzyme is upregulated when under unfolded protein stress. Thus, loss-of-function mutations of parkin might impair the removal of potentially toxic protein aggregates. However, the limited neuropathological information that is available from parkin-proven patients, as well as the recent knockout of the parkin gene in fruit flies and mice, may indicate a more complex disease mechanism, possibly involving the misfolding of parkin itself or of additional substrates. The risk factors that predispose dopaminergic neurons to degenerate on parkin failure are yet to be identified.

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.

Unaltered α-synuclein blood levels in juvenile Parkinsonism with a parkin exon 4 deletion

We recently reported here that SNCA triplication results in a doubling in the amount of alpha-synuclein protein in blood from cases with hereditary Lewy body disease. This observation shows that alpha-synuclein levels in blood accurately reflect gene dosage, which we assume drives pathogenesis in these individuals. A previous report has suggested that parkin can affect alpha-synuclein metabolism in human brain. Here we have tested whether there is also an increase of alpha-synuclein in autosomal recessive juvenile Parkinsonism (ARJP). We find there is not and discuss this result in terms of the putative relationships between alpha-synuclein and parkin.

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.

Parkinson's disease: from causes to mechanisms

Parkinson's disease (PD) is a common age-related, progressive neurodegenerative disease of unknown etiology. Environmental factors have long been suspected to participate in the pathogenesis of PD due to the existence of neurotoxins that preferentially damage the dopaminergic nigrostriatal pathway. In the past few years, novel insights into the degenerative process have been provided by the discovery of genes responsible for rare monogenic parkinsonian syndromes. Compelling evidence is accumulating, suggesting that the products of several of these genes can interact with environmental toxins and intervene in molecular pathways controlling the functional integrity of mitochondria.

Dimerization of Parkinson's disease-causing DJ-1 and formation of high molecular weight complexes in human brain

Mutations in the DJ-1 gene have been implicated in the PARK7-linked autosomal recessive form of Parkinson's disease (PD). The molecular properties of DJ-1WT, DJ-1L166P, and a newly identified disease-causing mutant DJ-1M26I were explored after they were transiently expressed in mammalian cells. Treatment of intact, living cells with the chemical crosslinker disuccinimidyl suberate (DSS) revealed that DJ-1WT and mutant DJ-1M26I were present as stable homodimers; DJ-1L166P in particular tended to form high-order complexes as well. In contrast to DJ-1L166P that is quickly degraded by the proteasome, DJ-1M26I was found to be an efficiently expressed and stable variant of DJ-1, suggesting that these mutations have distinct biochemical effects on DJ-1. We further provide evidence that in human brain, under nondenaturing conditions, DJ-1 is present in high molecular weight (HMW) complexes of approximately 250-700 kDa containing parkin, another PD-associated protein.

Pro-apoptotic protein glyceraldehyde-3-phosphate dehydrogenase promotes the formation of Lewy body-like inclusions

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has long been recognized as a classical glycolytic protein; however, previous studies by our group and others have demonstrated that GAPDH is a general mediator initiating one or more apoptotic cascades. Our most recent findings have elucidated that an expression of a pro-apoptotic protein GAPDH is critically regulated at the promoter region of the gene. Apoptotic signals for its subsequent aggregate formation and nuclear translocation are controlled by the respective functional domains harboured within its cDNA component. In this study, coexpression of GAPDH with either wild-type or mutant (A53T) alpha-synuclein and less likely with beta-synuclein in transfected COS-7 cells was found to induce Lewy body-like cytoplasmic inclusions. Unlike its full-length construct, the deleted mutant GAPDH construct (C66) abolished these apoptotic signals, disfavouring the formation of inclusions. The generated inclusions were ubiquitin- and thioflavin S-positive appearing fibrils. Furthermore, GAPDH coimmunoprecipitated with wild-type alpha-synuclein in this paradigm. Importantly, immunohistochemical examinations of post mortem materials from patients with sporadic Parkinson's disease revealed the colocalized profiles immunoreactive against these two proteins in the peripheral zone of Lewy bodies from the affected brain regions (i.e. locus coeruleus). Moreover, a quantitative assessment showed that about 20% of Lewy bodies displayed both antigenicities. These results suggest that pro-apoptotic protein GAPDH may be involved in the Lewy body formation in vivo, probably associated with the apoptotic death pathway.

Lewy body Parkinson's disease in a large pedigree with 77Parkin mutation carriers

We report the clinical, genetic, and neuropathological findings of a seven generation-spanning pedigree with 196 individuals, 25 of whom had levodopa-responsive parkinsonism. Genetic analyses indicated Parkin mutations in 77 subjects. Among the 25 patients, 5 carried compound heterozygous mutations and met criteria for definite Parkinson's disease (PD) according to UK PD Society Brain Bank guidelines; 8 subjects carried only a heterozygous Parkin mutation. The mutational status of five deceased patients was unknown, and seven PD patients had no Parkin mutation. Survival analyses showed a significant difference in the age-at-onset distribution between patients with compound heterozygous mutations and the groups of heterozygous carriers and subjects without detectable Parkin mutations. Autopsy of a 73-year-old patient, who carried two mutant Parkin alleles (delExon7 + del1072T), showed PD-type cell loss, reactive gliosis, and alpha-synuclein-positive Lewy bodies in the substantia nigra and locus ceruleus. Surviving neurons were reactive with antibodies to the N terminus of Parkin but not the In-Between-RING ("IBR") domain, which had been deleted by both mutations. This large Parkin pedigree represents a unique opportunity to prospectively study the role of heterozygous Parkin mutations as a PD risk factor, to identify additional contributors to the expression of late-onset PD in heterozygous carriers, and to reexamine the role of Parkin in inclusion formation.

Phosphorylated tau and the neurodegenerative foldopathies

Many studies have implicated phosphorylated tau in the Alzheimer disease process. However, the cellular fate of phosphorylated tau has only recently been described. Recent work has shown that tau phosphorylation at substrate sites for the kinases Cdk5 and GSK3-beta can trigger the binding of tau to the chaperones Hsc70 and Hsp27. The binding of phosphorylated tau to Hsc70 implied that the complex may be a substrate for the E3 ligase CHIP and this possibility was experimentally verified. The presence of this system in cells suggests that phosphorylated tau may hold toxic dangers for cell viability, and the response of the cell is to harness a variety of protective mechanisms. These include binding to chaperones, which may prevent more toxic conformations of the protein, ubiquitination which will direct the protein to the proteasome, segregation of tau aggregates from the cellular machinery, and recruitment of Hsp27 which will confer anti-apoptotic properties to the cell.

Reduction in endogenous parkin levels renders glial cells sensitive to both caspase-dependent and caspase-independent cell death

Mutations in the parkin gene give rise to a familial form of Parkinson's disease, autosomal recessive juvenile Parkinsonism (AR-JP). Although the exact mechanisms are unclear, it is thought that these 'loss-of-function' mutations contribute to the pathological process by interfering with parkin's E3 ubiquitin ligase activity. In order to mimic the in vivo loss-of-function, we produced tet-inducible glial cell lines that, in the presence of doxycycline, were able either to under- or to over-express the parkin protein. Using this cell-culture system, we found that the induced alteration of parkin levels in glial cell lines caused different responses compared with their un-induced counterparts under conditions of stress (staurosporine, hydrogen peroxide and dopamine). In particular, reduction in the levels of parkin within the transfected cells rendered them more susceptible to both apoptotic and necrotic cell death. Interestingly, blocking the cell death pathway with caspase inhibitors rescued the cells under-expressing parkin from only some of the stress-induced death. These findings implicate a pathogenic role of glial cells in the pathogenesis of AR-JP caused by mutations in the parkin gene.

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.

Mitochondrial injury: a hot spot for parkinsonism and Parkinson's disease?

The recent identification of genes (parkin, DJ-1, and PINK1) involved in recessive autosomal parkinsonism, and the indications that these proteins may have protective effects on the mitochondria, has led to the reemergence of the notion that mitochondrial dysfunction might play a central role in the etiology of sporadic Parkinson's disease (PD). This idea has previously been supported by biochemical analyses showing reduced mitochondrial activity in PD patients and in animal models of PD generated by the selective inhibition of mitochondria activity. However, the involvement of DJ-1 or PINK1 loss of function in classical idiopathic PD, characterized by pathological inclusions composed of aggregated alpha-synuclein protein, has still not been evaluated. More detailed studies of the possible interactions between parkin, DJ-1, PINK1, and alpha-synuclein and their effects on mitochondria are needed to more adequately define the biological pathways that may convergently or independently lead to parkinsonism.

Association of DJ-1 and parkin mediated by pathogenic DJ-1 mutations and oxidative stress

The identification of rare monogenic forms of Parkinson's disease (PD) has provided tremendous insight into the molecular pathogenesis of this disorder. Heritable mutations in alpha-synuclein, parkin, DJ-1 and PINK1 cause familial forms of PD. In the more common sporadic form of PD, oxidative stress and derangements in mitochondrial complex-I function are considered to play a prominent role in disease pathogenesis. However, the relationship of DJ-1 with other PD-linked genes and oxidative stress has not been explored. Here, we show that pathogenic mutant forms of DJ-1 specifically but differentially associate with parkin, an E3 ubiquitin ligase. Chemical cross-linking shows that pathogenic DJ-1 mutants exhibit impairments in homo-dimer formation, suggesting that parkin may bind to monomeric DJ-1. Parkin fails to specifically ubiquitinate and enhance the degradation of L166P and M26I mutant DJ-1, but instead promotes their stability in cultured cells. The interaction of parkin with L166P DJ-1 may involve a larger protein complex that contains CHIP and Hsp70, perhaps accounting for the lack of parkin-mediated ubiquitination. Oxidative stress also promotes an interaction between DJ-1 and parkin, but this does not result in the ubiquitination or degradation of DJ-1. Parkin-mediated alterations in DJ-1 protein stability may be pathogenically relevant as DJ-1 levels are dramatically increased in the detergent-insoluble fraction from sporadic PD/DLB brains, but are reduced in the insoluble fraction from parkin-linked autosomal recessive juvenile-onset PD brains. These data potentially link DJ-1 and parkin in a common molecular pathway at multiple levels that may have important implications for understanding the pathogenesis of inherited and sporadic PD.

The neurological mutant quaking(viable) is Parkin deficient

The mouse mutant quaking(viable) ( qk(v)) has been studied for almost four decades as a model for dysmyelination of the central nervous system (CNS). The genetic lesion associated with the qk(v) phenotype is a large deletion of approximately 1 Megabase on mouse Chromosome (Chr) 17. This deficiency alters the expression of transcripts from the qkI locus in oligodendrocytes, resulting in improper myelination of the CNS in animals homozygous for the deletion. To determine whether other genes within the deletion contribute to the quaking(viable) phenotype, we physically mapped and sequenced the deleted interval. We determined that the mouse Parkin gene, as well as the Parkin co-regulated gene ( Pacrg), lies within the qk(v) deletion. We determined that qk(v) mutants completely lack the expression of the Parkin gene product. Loss-of-function mutations in the human PARKIN gene cause autosomal juvenile Parkinson's disease (AR-JP). Our studies show that the deletion of Parkin in qk(v) brains does not result in the loss of dopaminergic neurons typical of AR-JP patients. Also, alpha-synuclein, a target of Parkin-dependent ubiquitination, does not accumulate in qk(v) mutant brains. Despite the lack of AR-JP-like neuropathology in qk(v) mice, this mutant may constitute a readily available model for the study of the cellular function of Parkin. This is the first report of a gene distinct from qkI affected by the qk(v) deletion. The discovery of the multigenic nature of this classical mouse mutation calls for the re-evaluation of its phenotypic characterization.

Microarray expression analysis of gad mice implicates involvement of Parkinson's disease associated UCH-L1 in multiple metabolic pathways

Parkinson's disease (PD) is thought to be caused by environmental and genetic factors. Mutations in four genes, alpha-synuclein, parkin, DJ-1, and UCH-L1, have been identified in autosomal inherited forms of PD. The pathogenetic cause for the loss of neuronal cells in PD patients, however, remains to be determined. Due to the rarity of mutations in humans with PD, the analysis of animal models might help to further gain insights into the pathogenesis of familial PD. For UCH-L1, deficiency has been described in gad mice leading to axonal degeneration and formation of spheroid bodies in nerve terminals. Here, we investigated the gene expression pattern of the brain of 3-month-old Uch-l1-deficient gracile axonal dystrophy (gad) mice by microarray analysis. A total of 146 genes were differentially regulated by at least a 1.4-fold change with 103 being up-regulated and 43 being down-regulated compared with age and sex matched wildtype littermate mice. The gene products with altered expression are involved in protein degradation, cell cycle, vesicle transport, cellular structure, signal transduction, and transcription regulation. Most of the genes were modestly regulated, which is in agreement that severe alteration of these pathways might be lethal. Among the genes most significantly down-regulated is the brain-derived neurotrophic factor which might be one aspect of the pathogenesis in gad mice. Interestingly, several subunits of the transcription factor CCAAT/enhancer binding protein are up-regulated, which plays a central role in most altered pathways.

Screening for mutations in synaptotagmin XI in Parkinson's disease

Parkinson's disease (PD) is characterized by selective degeneration of neurons in the substantia nigra and subsequent dysfunction of dopaminergic neurotransmission. Genes identified in familial forms of PD encode proteins that are linked to the ubiquitin-proteasome system indicating the pathogenic relevance of disturbed protein degradation in PD. Some of them, i.e. alpha-synuclein, parkin and synphilin-1, have been implicated in presynaptic neurotransmission based on their localization in synaptic vesicles. Synaptotagmin XI is linked to the pathogenesis of PD based on its identification as a substrate of the ubiquitin-E3-ligase parkin. Moreover synaptotagmin XI is involved in the maintainance of synaptic function and represents a component of Lewy bodies (LB) in brains of PD patients. Therefore, we performed a detailed mutation analysis of the synaptotagmin XI gene in a large sample of 393 familial and sporadic PD patients. We did not find any disease causing mutations arguing against a major role of mutations in the synaptotagmin XI gene in the pathogenesis of PD.

Genetics of Parkinson's disease

The etiology of most cases of Parkinson's disease (PD) remains unknown. In recent years, however, research has successfully focused on genetic factors contributing to the degeneration of dopaminergic neurons. Causative mutations have been identified in several monogenically inherited forms of the disease. Although these genetic forms of PD are usually rare, the gene discoveries are likely to identify molecular pathways that are also relevant in the sporadic disorder. These studies have led to the identification of (i) the central role of α-synuclein aggregation, secondary to either point mutations or an amplification of the α-synuclein gene; and (ii) the relevance of defects in the proteasomal protein degradation pathway in the molecular pathogenesis of recessive parkin-linked forms of PD. The recent discoveries of two additional recessive forms associated with mutations in the genes DJ-1 and PINK1 have brought the mitochondrial energy metabolism and the cell's defence against toxic free radicals into the focus of research.

How do Parkin mutations result in neurodegeneration?

The gene product responsible for autosomal recessive juvenile Parkinsonism, Parkin, has been observed to have ubiquitin ligase activity. This finding has changed the direction of studies on Parkinson's disease by suggesting that abnormal protein turnover might be involved in its pathogenesis. A number of potentially neurotoxic Parkin-specific substrates have been identified. Further investigation of Parkin knockout mice will hopefully provide new evidence in the search for Parkin's substrates and further clarify their role in Parkinson's disease.

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.

Pathogenetic mechanisms of parkin in Parkinson's disease

CONTEXT

The cause and pathogenesis of Parkinson's disease remain unknown; mitochondrial dysfunction, oxidative damage, environmental factors, and genetic predisposition might all be involved. Identification of the causative genes for familial Parkinson's diseases allow study of the pathogenesis of the disease at the molecular level.

STARTING POINT

Katja Hedrich and colleagues studied 75 Serbian patients with early-onset Parkinson's disease for DJ-1 mutations (Neurology 2004; 62: 389-94). One patient was a compound heterozygote and another had a heterozygous exon deletion. DJ-1 mutations seem to be rare in this European population. By contrast, parkin mutations have been found in about 50% of familial cases and in 10-20% of cases without a positive family history.

WHERE NEXT

The fact that parkin is a ubiquitin ligase gives special meaning to the molecular mechanism of neurodegeneration in general. In Parkinson's disease, Lewy bodies are immunoreactive for ubiquitin. Accumulation of abnormal proteins has also been seen in other neurodegenerative disorders. Disturbance of protein degradation by the ubiquitin-proteasome system might have a critical role in neurodegeneration. Although alpha-synuclein mutations are infrequent, alpha-synuclein accumulates in Lewy bodies, and alpha-synuclein fibrils impair the 26S proteasome function. UCH-L1 is also an abundant deubiquitylating enzyme, and its mutation is linked to PARK5. Furthermore, DJ-1 might interact with SUMO-1 (small ubiquitin-like modifier), which can counteract ubiquitin and stabilise proteins against degradation by the 26S proteasome. Uncovering the mechanisms of protein degradation should add importantly to understanding the neurodegenerative process in these neurodegenerative diseases.

The role of synphilin-1 in synaptic function and protein degradation

The name synphilin-1 comes from its identification as an alpha-synuclein-interacting protein (SNCAIP) in yeast two-hybrid screens. Since alpha-synuclein ( PARK1) was the first gene identified as causing inherited forms of Parkinson's disease (PD), synphilin-1 was quickly implicated in neurodegeneration in PD. Recently, the first genetic evidence for the direct contribution of synphilin-1 in the pathogenesis of PD has been defined with the identification of an R621C mutation as a susceptibility factor for PD in two German patients. Extensive in vitro studies have determined the physiological functions of synphilin-1, identified novel synphilin-1-interacting proteins, and linked synphilin-1 to ubiquitin-mediated protein degradation. The present article provides an overview of the current concepts of the role of synphilin-1 in synaptic function and protein degradation and in the molecular mechanisms leading to neurodegeneration in PD.

Genetic contributions to Parkinson's disease

Sporadic Parkinson's disease (PD) is a common neurodegenerative disorder, characterized by the loss of midbrain dopamine neurons and Lewy body inclusions. It is thought to result from a complex interaction between multiple predisposing genes and environmental influences, although these interactions are still poorly understood. Several causative genes have been identified in different families. Mutations in two genes [alpha-synuclein and nuclear receptor-related 1 (Nurr1)] cause the same pathology, and a third locus on chromosome 2 also causes this pathology. Other familial PD mutations have identified genes involved in the ubiquitin-proteasome system [parkin and ubiquitin C-terminal hydroxylase L1 (UCHL1)], although such cases do not produce Lewy bodies. These studies highlight critical cellular proteins and mechanisms for dopamine neuron survival as disrupted in Parkinson's disease. Understanding the genetic variations impacting on dopamine neurons may illuminate other molecular mechanisms involved. Additional candidate genes involved in dopamine cell survival, dopamine synthesis, metabolism and function, energy supply, oxidative stress, and cellular detoxification have been indicated by transgenic animal models and/or screened in human populations with differing results. Genetic variation in genes known to produce different patterns and types of neurodegeneration that may impact on the function of dopamine neurons are also reviewed. These studies suggest that environment and genetic background are likely to have a significant influence on susceptibility to Parkinson's disease. The identification of multiple genes predisposing to Parkinson's disease will assist in determining the cellular pathway/s leading to the neurodegeneration observed in this disease.

Lewy-body formation is an aggresome-related process: a hypothesis

Parkinson's disease (PD) is an age-related neurodegenerative disorder that is associated with the formation of intracytoplasmic protein aggregates (Lewy-body inclusions) in neurons of the substantia nigra pars compacta and other brain areas. These inclusions were discovered over 90 years ago, but the mechanism underlying their formation and their relevance to the neurodegenerative process are unknown. Recent studies have begun to shed light on the biogenesis of Lewy bodies and suggest that they are related to aggresomes. Aggresomes are cytoprotective proteinaceous inclusions formed at the centrosome that segregate and facilitate the degradation of excess amounts of unwanted and possibly cytotoxic proteins. The concept of Lewy bodies as aggresome-related inclusions fits well with ongoing discoveries suggesting that altered protein handling might contribute to the neurodegenerative process in familial and sporadic forms of PD.

Parkin-associated Parkinson's disease

Mutations in the PARK2 gene coding for parkin cause autosomal recessive juvenile parkinsonism (AR-JP), a familial form of Parkinson's disease (PD). Parkin functions as an E3 ubiquitin ligase, and loss of this ubiquitin ligase activity appears to be the mechanism underlying pathogenesis of AR-JP. Recently, the spectrum of genetic, clinical, and pathological findings on AR-JP has been significantly expanded. Moreover, a considerable number of parkin interactors and/or substrates have been identified and characterized, and animal models of parkin deficiency have been generated. In this review, we provide an overview of the most relevant findings and discuss their implications for the pathogenesis of AR-JP and sporadic PD.

Synucleins and their relationship to Parkinson’s disease

Parkinson's disease (PD) is one of the most common neurodegenerative motor disorders, marked by chronic progressive loss of neurons in the substantia nigra. It has long been believed that PD is caused by environmental factors. The discovery of genetic factors involved in PD has improved the understanding of the pathology of the disease. The first gene found to be mutated in PD encodes for the presynaptic protein alpha-synuclein. alpha-Synuclein is a major component of Lewy bodies and Lewy neurites, which represent the morphological hallmarks of the disease. The mechanisms by which alpha-synuclein is involved in nigral cell death remain poorly understood. Moreover, the factors triggering the formation of alpha-synuclein-positive inclusion bodies remain enigmatic. Indeed, even the normal cellular functions of alpha-synuclein and of the other synucleins (beta-synuclein and gamma-synuclein) are still unknown. Several lines of evidence suggest that they play a role in the regulation of vesicular turnover under normal nonpathological conditions.

Alpha-synuclein and Parkinson's disease

Alpha-synuclein (alpha-syn) is a small soluble protein expressed primarily at presynaptic terminals in the central nervous system. Interest in alpha-syn has increased dramatically after the discovery of a relationship between its dysfunction and several neurodegenerative diseases, including Parkinson's disease (PD). The physiological functions of alpha-syn remain to be fully defined, although recent data suggest a role in regulating membrane stability and neuronal plasticity. Various trigger factors, either environmental or genetic, can lead to a cascade of events involving misfolding or loss of normal function of alpha-syn. In dopaminergic neurons, this may promote a vicious cycle in which elevation in cytoplasmic dopamine, oxidative stress, alpha-syn dysfunction, and disruption of vesicle function lead to dopaminergic cell loss and PD. Alpha-syn dysfunction appears to be a common feature of all forms of PD. The mechanism by which alpha-syn induces neuronal cell toxicity may invoke multiple pathways, such as aggregation or interaction with other proteins and molecules, including synphilin-1, chaperone 14-3-3 protein, and dopamine itself. This complexity has hindered the development of models to study PD. The available animal models of PD, each present distinct advantages and limits. Findings to date suggest that alpha-syn-based models represent a paradigm, which is closest to the human pathology.

Loss of locus coeruleus neurons and reduced startle in parkin null mice

Parkinson's disease (PD) is the most common neurodegenerative movement disorder and is characterized pathologically by degeneration of catecholaminergic neurons of the substantia nigra pars compacta and locus coeruleus, among other regions. Autosomal-recessive juvenile Parkinsonism (ARJP) is caused by mutations in the PARK2 gene coding for parkin and constitutes the most common familial form of PD. The majority of ARJP-associated parkin mutations are thought to be loss of function-mutations; however, the pathogenesis of ARJP remains poorly understood. Here, we report the generation of parkin null mice by targeted deletion of parkin exon 7. These mice show a loss of catecholaminergic neurons in the locus coeruleus and an accompanying loss of norepinephrine in discrete regions of the central nervous system. Moreover, there is a dramatic reduction of the norepinephrine-dependent startle response. The nigrostriatal dopaminergic system does not show any impairment. This mouse model will help gain a better understanding of parkin function and the mechanisms underlying parkin-associated PD.

Parkin protects human dopaminergic neuroblastoma cells against dopamine-induced apoptosis

Parkinson's disease (PD) is characterized by the selective degeneration of dopaminergic (DA) neurons in substantia nigra pars compacta (SNpc). A combination of genetic and environmental factors contributes to such a specific loss. Among the five PD-linked genes identified so far, parkin, a protein-ubiquitin E3 ligase, appears to be the most prevalent genetic factor in PD. Although a variety of substrates have been identified for parkin, none of them is selectively expressed in nigral DA neurons. It remains unclear how accumulation of these substrates in the absence of functional parkin may cause the selective death of DA neurons in SNpc. Here, we show that overexpression of parkin protected human DA neuroblastoma cell line (SH-SY5Y) against apoptosis induced by DA or 6-OHDA, but not by H(2)O(2) or rotenone. Parkin significantly attenuated dopamine-induced activation of c-Jun N-terminal kinase (JNK) and caspase-3. It also decreased the level of reactive oxygen species (ROS) and protein carbonyls in the cell. Inhibiting DA uptake through dopamine transporter or treating the cell with antioxidants significantly reduced oxidative stress and dopamine toxicity. Furthermore, PD-linked mutations of parkin significantly abrogated the protective effect of wild-type parkin, as well as its ability to suppress ROS and protein carbonylation. These results suggest that parkin protects against dopamine toxicity by decreasing oxidative stress and ensuing activation of apoptotic programs such as the JNK/caspase pathway. This protective function of parkin, which is greatly attenuated by its PD-linked mutations, may be uniquely important for the survival of DA neurons, as they are constantly threatened by oxyradicals produced during dopamine oxidation.

Genes, proteins, and neurotoxins involved in Parkinson’s disease

Parkinson's disease (PD) is a common neurodegenerative disorder. The etiology of PD is likely due to combinations of environmental and genetic factors. In addition to the loss of neurons, including dopaminergic neurons in the substantia nigra pars compacta, a further morphologic hallmark of PD is the presence of Lewy bodies and Lewy neurites. The formation of these proteinaceous inclusions involves interaction of several proteins, including alpha-synuclein, synphilin-1, parkin and UCH-L1. Animal models allow to get insight into the mechanisms of several symptoms of PD, allow investigating new therapeutic strategies and, in addition, provide an indispensable tool for basic research. In animals PD does not arise spontaneously, thus, characteristic and specific functional changes have to be mimicked by application of neurotoxic agents or by genetic manipulations. In this review we will focus on genes and gene loci involved in PD, the functions of proteins involved in the formation of cytoplasmatic inclusions, their interactions, and their possible role in PD. In addition, we will review the current animal models of PD.

Parkin and relatives: the RBR family of ubiquitin ligases

Mutations in the parkin gene cause autosomal-recessive juvenile parkinsonism. Parkin encodes a ubiquitin-protein ligase characterized by having the RBR domain, composed of two RING fingers plus an IBR/DRIL domain. The RBR family is defined as the group of genes whose products contain an RBR domain. RBR family members exist in all eukaryotic species for which significant sequence data is available, including animals, plants, fungi, and several protists. The integration of comparative genomics with structural and functional data allows us to conclude that RBR proteins have multiple roles, not only in protein quality control mechanisms, but also as indirect regulators of transcription. A recently formulated hypothesis, based on a case of gene fusion, suggested that RBR proteins may be often part of cullin-containing ubiquitin ligase complexes. Recent data on Parkin protein agrees with that hypothesis. We discuss the involvement of RBR proteins in several neurodegenerative diseases and cancer.

Cross-linking of ubiquitin, HSP27, parkin, and alpha-synuclein by gamma-glutamyl-epsilon-lysine bonds in Alzheimer's neurofibrillary tangles

The accumulation of misfolded proteins in intracellular inclusions is a generic feature of neurodegenerative disorders. Although heavily ubiquitylated, the aggregated proteins are not degraded by the proteasomes. A possible reason for this phenomenon may be a modification of deposited proteins by transglutaminases forming gamma-glutamyl-epsilon-lysine (GGEL) cross-links between distinct proteins. Here, we show that the frequency of GGEL cross-links is an order of magnitude higher in Alzheimer's brain cortex than in age-matched or younger controls. This difference is due to the accumulation of GGEL cross-links in ubiquitin-immunopositive protein particles present in both Alzheimer's brains and those from aged individuals. The highly cross-linked protein aggregates show immunoreactivity to antibodies against tau and neurofilament proteins, and partially also to alpha-synuclein, indicating that these structures are inherent in Alzheimer's neurofibrillary tangles and Lewy bodies. Using mass sequence analysis, we identified the same six pairs of peptide sequences cross-linked in both senile and Alzheimer's specimens: Gln31 and Gln190 of HSP27 protein are cross-linked with Lys29 and Lys48 of ubiquitin and HSP27 therefore may cross-link two (poly)ubiquitin chains. One lysine residue of parkin and one of alpha-synuclein were also found to be cross-linked. The data suggest that cross-linking of (poly)ubiquitin moieties via HSP27 may have a role in the stabilization of the intraneuronal protein aggregates by interference with the proteasomal elimination of unfolded proteins.

Drosophila parkinmutants have decreased mass and cell size and increased sensitivity to oxygen radical stress

Mutations in the gene parkin in humans (PARK2) are responsible for a large number of familial cases of autosomal-recessive Parkinson disease. We have isolated a Drosophila homolog of human PARK2 and characterized its expression and null phenotype. parkin null flies have 30% lower mass than wild-type controls which is in part accounted for by a reduced cell size and number. In addition, these flies are infertile, show significantly reduced longevity, and are unable to jump or fly. Rearing mutants on paraquat, which generates toxic free radicals in vivo, causes a further reduction in longevity. Furthermore, loss of parkin results in progressive degeneration of most indirect flight muscle (IFM) groups soon after eclosion, accompanied by apoptosis. However, parkin mutants have normal neuromuscular junction recordings during the third larval instar stage, suggesting that larval musculature is intact and that parkinis required only in pupal and adult muscle. parkin flies do not show an age-dependent dopaminergic neuron loss in the brain, even after aging adults for 3 weeks. Nevertheless, degeneration of IFMs demonstrates the importance of parkin in maintaining specific cell groups, perhaps those with a high-energy demand and the concomitant production of high levels of free radicals. parkin mutants will be a valuable model for future analysis of the mechanisms of cell and tissue degeneration.

Triad3A, an E3 ubiquitin-protein ligase regulating Toll-like receptors

Activation of Toll-like receptors (TLRs) results in a proinflammatory response needed to combat infection. Thus, limiting TLR signaling is essential for preventing a protective response from causing injury to the host. Here we describe how a RING finger protein, Triad3A, acts as an E3 ubiquitin-protein ligase and enhances ubiquitination and proteolytic degradation of some TLRs. Triad3A overexpression promoted substantial degradation of TLR4 and TLR9 with a concomitant decrease in signaling, but did not affect TLR2 expression or signaling. Conversely, a reduction in endogenous Triad3A by small interfering RNA increased TLR expression and enhanced TLR activation. Thus, ubiquitination by Triad3A represents one pathway by which the intensity and duration of TLR signaling is controlled.

S-nitrosylation of parkin regulates ubiquitination and compromises parkin's protective function

Parkin is an E3 ubiquitin ligase involved in the ubiquitination of proteins that are important in the survival of dopamine neurons in Parkinson's disease (PD). We show that parkin is S-nitrosylated in vitro, as well as in vivo in a mouse model of PD and in brains of patients with PD and diffuse Lewy body disease. Moreover, S-nitrosylation inhibits parkin's ubiquitin E3 ligase activity and its protective function. The inhibition of parkin's ubiquitin E3 ligase activity by S-nitrosylation could contribute to the degenerative process in these disorders by impairing the ubiquitination of parkin substrates.