The proteasomal inhibition model of Parkinson's disease: “Boon or bust”?

Pathways of protein synthesis and degradation in PD pathogenesis

Since the discovery of protein aggregates in the brains of individuals with Parkinson's disease (PD) in the early 20th century, the scientific community has been interested in the role of dysfunctional protein metabolism in PD etiology. Recent advances in the field have implicated defective protein handling underlying PD through genetic, in vitro, and in vivo studies incorporating many disease models alongside neuropathological evidence. Here, we discuss the existing body of research focused on understanding cellular pathways of protein synthesis and degradation, and how aberrations in either system could engender PD pathology with special attention to α-synuclein-related consequences. We consider transcription, translation, and post-translational modification to constitute protein synthesis, and protein degradation to encompass proteasome-, lysosome- and endoplasmic reticulum-dependent mechanisms. Novel findings connecting each of these steps in protein metabolism to development of PD indicate that deregulation of protein production and turnover remains an exciting area in PD research.

Alterations in the E3 ligases Parkin and CHIP result in unique metabolic signaling defects and mitochondrial quality control issues

E3 ligases are essential scaffold proteins, facilitating the transfer of ubiquitin from E2 enzymes to lysine residues of client proteins via isopeptide bonds. The specificity of substrate binding and the expression and localization of E3 ligases can, however, endow these proteins with unique features with variable effects on mitochondrial, metabolic and CNS function. By comparing and contrasting two E3 ligases, Parkin and C-terminus of HSC70-Interacting protein (CHIP) we seek to highlight the biophysical properties that may promote mitochondrial dysfunction, acute stress signaling and critical developmental periods to cease in response to mutations in these genes. Encoded by over 600 human genes, RING-finger proteins are the largest class of E3 ligases. Parkin contains three RING finger domains, with R1 and R2 separated by an in-between region (IBR) domain. Loss-of-function mutations in Parkin were identified in patients with early onset Parkinson's disease. CHIP is a member of the Ubox family of E3 ligases. It contains an N-terminal TPR domain and forms unique asymmetric homodimers. While CHIP can substitute for mutated Parkin and enhance survival, CHIP also has unique functions. The differences between these proteins are underscored by the observation that unlike Parkin-deficient animals, CHIP-null animals age prematurely and have significantly impaired motor function. These properties make these E3 ligases appealing targets for clinical intervention. In this work, we discuss how biophysical and metabolic properties of these E3 ligases have driven rapid progress in identifying roles for E3 ligases in development, proteostasis, mitochondrial biology, and cell health, as well as new data about how these proteins alter the CNS proteome.

The Proteasome Inhibition Model of Parkinson's Disease

The pathological hallmarks of Parkinson's disease are the progressive loss of nigral dopaminergic neurons and the formation of intracellular inclusion bodies, termed Lewy bodies, in surviving neurons. Accumulation of proteins in large insoluble cytoplasmic aggregates has been proposed to result, partly, from a failure in the function of intracellular protein degradation pathways. Evidence in support for such a hypothesis emerged in the beginning of the years 2000 with studies demonstrating structural and functional deficits in the ubiquitin-proteasome pathway in post-mortem nigral tissue of patients with Parkinson's disease. These fundamental findings have inspired the development of a new generation of animal models based on the use of proteasome inhibitors to disturb protein homeostasis and trigger nigral dopaminergic neurodegeneration. In this review, we provide an updated overview of the current approaches in employing proteasome inhibitors to model Parkinson's disease, with particular emphasis on rodent studies. In addition, the mechanisms underlying proteasome inhibition-induced cell death and the validity criteria (construct, face and predictive validity) of the model will be critically discussed. Due to its distinct, but highly relevant mechanism of inducing neuronal death, the proteasome inhibition model represents a useful addition to the repertoire of toxin-based models of Parkinson's disease that might provide novel clues to unravel the complex pathogenesis of this disorder.

Glucocerebrosidase mutations and the pathogenesis of Parkinson disease

Parkinson disease (PD) is the second most common neurodegenerative disease after Alzheimer disease with a lifetime risk in the UK population of almost 5%. An association between PD and Gaucher disease (GD) derived from the observation that GD patients and their heterozygous carrier relatives were at increased risk of PD. GD is an autosomal recessive lysosomal storage disorder caused by homozygous mutations in the gene encoding glucocerebrosidase (GBA). Approximately 5%-10% of PD patients have GBA mutations, making these mutations numerically the most important genetic predisposing risk factor for the development of PD identified to date. GBA mutations result in a phenotype that is virtually indistinguishable clinically, pharmacologically, and pathologically from sporadic PD, except GBA mutations result in a slightly earlier age of onset and more frequent cognitive impairment among PD patients. The mechanisms by which GBA mutations result in PD are not yet understood. Both reduced glucocerebrosidase enzyme (GCase) activity with lysosomal dysfunction, and unfolded protein response (UPR) with endoplasmic reticulum-associated degradation (ERAD) and stress are considered contributory.

Exploring the link between glucocerebrosidase mutations and parkinsonism

Clinical, genetic and pathological studies demonstrate that mutations in glucocerebrosidase (GBA), which encodes the lysosomal enzyme deficient in Gaucher disease (GD), are risk factors for Parkinson disease (PD) and related disorders. Some patients with GD and Gaucher carriers develop parkinsonism. Furthermore, subjects with PD have an increased frequency of GBA mutations. GBA-mutation carriers exhibit diverse parkinsonian phenotypes and have glucocerebrosidase-positive Lewy bodies. Although the mechanism for this association is unknown, we present several theories, including protein aggregation, prion transmission, lipid accumulation and impaired autophagy, mitophagy or trafficking. Each model has inherent limitations, and a second-hit mutation might be essential. Elucidation of the basis for this link will have important consequences for studying these diseases and should provide insights into lysosomal pathways and potential treatment strategies.

Different effects of intranigral and intrastriatal administration of the proteasome inhibitor lactacystin on typical neurochemical and histological markers of Parkinson's disease in rats

Impairment of the ubiquitin-proteasome system, responsible for clearing of misfolded and unwanted proteins, has been implicated in the loss of nigrostriatal dopaminergic neurons characteristic of Parkinson's disease (PD). Recently, proteasome inhibitors have been used to model parkinsonian-like changes in animals. In the present study, the effects of intrastriatal and intranigral injections of the selective proteasome inhibitor lactacystin on key markers of PD were examined in Wistar rats. Comparisons of these two different routes of lactacystin administration revealed that only a unilateral, intranigral injection of lactacystin at a dose of 0.5, 1, 2.5 and 5 μg/2 μl produced after 7 days distinct decreases in the concentrations of dopamine (DA) and its metabolites (DOPAC, 3-MT, HVA) in the ipsilateral striatum. The used doses of lactacystin (except for 0.5 μg/2 μl) significantly accelerated DA catabolism, i.e. the total, oxidative MAO-dependent and COMT-catalyzed pathways, as assessed by HVA/DA, DOPAC/DA and 3-MT/DA ratios, respectively, in the ipsilateral striatum. Such alterations were not observed in the striatal DA content and catabolism either 7, 14 or 21 days after a unilateral, intrastriatal high-dose lactacystin injection (5 and 10 μg/2 μl). Intranigrally administered lactacystin (1 μg/2 μl) caused a marked decline of tyrosine hydroxylase (TH) and α-synuclein protein levels in that structure. Neither TH nor α-synuclein protein levels in the substantia nigra (SN) were affected by high lactacystin doses injected intrastriatally. Moreover, stereological counting of TH-immunoreactive neurons and autoradiographic analysis of [(3)H]GBR 12,935 binding to dopamine transporter confirmed a loss of nigrostriatal dopaminergic neurons after an intranigral lactacystin (1 and 2.5 μg/2 μl) injection. An appearance of cardinal neurochemical and histological changes of parkinsonian type only after intranigral lactacystin injection indicates that DA cell bodies in the SN, but not DA terminals in the striatum are susceptible to proteasome inhibition.

Proteasome inhibition modeling nigral neuron degeneration in Parkinson's disease

Impairment of the ubiquitin proteasome system (UPS) has been proposed to play an important role in the pathogenesis of Parkinson's disease (PD). Mice with UPS impairment in the nigra have been used for investigating mechanisms underlying neurodegeneration and for testing pre-clinical drugs to treat PD. However, the pathological, biochemical and behavioral features of UPS impairment animal model of PD have not been fully evaluated. For this purpose, we developed a UPS impairment model of nigral dopamine (DA) neuron degeneration by microinjection with proteasome inhibitors lactacystin, PSI or MG-132 into the medial forebrain bundle (iMFB) of C57BL/6 mice and then systematically examined the animal's locomotor activities, and various pathological and biochemical markers of PD. We found that lactacystin iMFB induced a sustained DA neuron degeneration, which can be reproduced by PSI iMFB and MG-132 iMFB. In the animal model, DA neuron degenerated preferentially in the substantia nigra, accompanied by profound inhibition of proteasomal activity, activation of caspase 3, elevated insoluble ubiquitin conjugates and α-synuclein positive inclusion-like granules, activated glia, and decreased motor activities. Thus, this model recapitulates many neuropathological and behavioral features of PD, rendering it likely suitable for studying the mechanisms of nigral DA neuron degeneration and for testing the potential anti-PD medications.

Proteasome inhibition in medaka brain induces the features of Parkinson's disease

Recent findings suggest that a defect in the ubiquitin-proteasome system plays an important role in the pathogenesis of Parkinson's disease (PD). A previous report (McNaught et al. 2004) demonstrated that rats systemically injected with proteasome inhibitors exhibited PD-like clinical symptoms and pathology. However, because these findings have not been consistently replicated, this model is not commonly used to study PD. We used medaka fish to test the effect of systemic administration of proteasome inhibitors because of the high level of accessibility of the cerebrospinal fluid in fish. We injected lactacystin or epoxomicin into the CSF of medaka. With proteasome inhibition in the medaka brain, selective dopaminergic and noradrenergic cell loss was observed. Furthermore, treated fish exhibited reduced spontaneous movement. Treatment with proteasome inhibitors also induced the formation of inclusion bodies resembling Lewy bodies, which are characteristic of PD. Treatment with 6-OHDA also induced dopaminergic cell loss but did not produce inclusion bodies. These findings in medaka are consistent with previous results reporting that non-selective proteasome inhibition replicates the cardinal features of PD: locomotor dysfunction, selective dopaminergic cell loss, and inclusion body formation.

New insights into the role of mitochondrial dysfunction and protein aggregation in Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative movement disorder that affects increasing number of elderly in the world population. The disease is caused by a selective degeneration of dopaminergic neurons in the substantia nigra pars compacta with the molecular mechanism underlying this neurodegeneration still not fully understood. However, various studies have shown that mitochondrial dysfunction and abnormal protein aggregation are two of the major contributors for PD. In fact this notion has been supported by recent studies on genes that are linked to familial PD (FPD). For instance, FPD linked gene products such as PINK1 and parkin have been shown to play critical roles in the quality control of mitochondria, whereas α-synuclein has been found to be the major protein aggregates accumulated in PD patients. These findings suggest that further understanding of how dysfunction of these pathways in PD will help develop new approaches for the treatment of this neurodegenerative disorder.

A randomized, placebo-controlled trial of latrepirdine in Huntington disease

OBJECTIVES

To evaluate the safety and tolerability of latrepirdine in Huntington disease (HD) and explore its effects on cognition, behavior, and motor symptoms.

DESIGN

Double-blind, randomized, placebo-controlled trial.

SETTING

Multicenter outpatient trial.

PARTICIPANTS

Ninety-one participants with mild to moderate HD enrolled at 17 US and UK centers from July 18, 2007, through July 16, 2008.

INTERVENTION

Latrepirdine, 20 mg 3 times daily (n = 46), or matching placebo (n = 45) for a 90-day treatment period.

MAIN OUTCOME MEASURES

The primary outcome variable was tolerability, defined as the ability to complete the study at the assigned drug dosage. Secondary outcome variables included score changes from baseline to day 90 on the Unified Huntington's Disease Rating Scale (UHDRS), the Mini-Mental State Examination (MMSE), and the Alzheimer Disease Assessment Scale-cognitive subscale (ADAS-cog).

RESULTS

Latrepirdine was well tolerated (87% of the patients given latrepirdine completed the study vs 82% in the placebo group), and adverse event rates were comparable in the 2 groups (70% in the latrepirdine group and 80% in the placebo group). Treatment with latrepirdine resulted in improved mean MMSE scores compared with stable performance in the placebo group (treatment effect, 0.97 points; 95% confidence interval, 0.10-1.85; P = .03). No significant treatment effects were seen on the UHDRS or the ADAS-cog.

CONCLUSIONS

Short-term administration of latrepirdine is well tolerated in patients with HD and may have a beneficial effect on cognition. Further investigation of latrepirdine is warranted in this population with HD.

Adaptation to chronic MG132 reduces oxidative toxicity by a CuZnSOD-dependent mechanism

To study whether and how cells adapt to chronic cellular stress, we exposed PC12 cells to the proteasome inhibitor MG132 (0.1 microM) for 2 weeks and longer. This treatment reduced chymotrypsin-like proteasome activity by 47% and was associated with protection against both 6-hydroxydopamine (6-OHDA; 100 microM) and higher dose MG132 (40 microM). Protection developed slowly over the course of the first 2 weeks of exposure and was chronic thereafter. There was no change in total GSH levels after MG132. Buthionine sulfoximine (100 microM) reduced GSH levels by 60%, but exacerbated 6-OHDA toxicity to the same extent in both MG132-treated and control cells and failed to reduce MG132-induced protection. Chronic MG132 resulted in elevated antioxidant proteins CuZn superoxide dismutase (SOD; +55%), MnSOD (+21%), and catalase (+15%), as well as chaperone heat-shock protein 70 (+42%). Examination of SOD enzyme activity revealed higher levels of CuZnSOD (+40%), with no change in MnSOD. We further assessed the mechanism of protection by reducing CuZnSOD levels with two independent siRNA sequences, both of which successfully attenuated protection against 6-OHDA. Previous reports suggested that artificial over-expression of CuZnSOD in dopaminergic cells is protective. Our data complement such observations, revealing that dopaminergic cells are also able to use endogenous CuZnSOD in self-defensive adaptations to chronic stress, and that they can even do so in the face of extensive GSH loss.

Proteasomal abnormalities in cortical Lewy body disease and the impact of proteasomal inhibition within cortical and cholinergic systems

Dementia with Lewy bodies (DLB) accounts for 15-20% of the millions of people worldwide with dementia. In the current work we investigate the association between proteasome dysfunction and the development of cortical Lewy body pathology. Analysis of post-mortem cortical tissue indicated levels of the alpha-subunit of the 20S proteasome were significantly reduced in DLB cortex, but not Alzheimer's, in comparison to control and this reduction correlated with both the severity and duration of dementia. Application of proteasome inhibitors to rodent cortical primary neurones in vitro and by direct injection onto rodent cholinergic forebrain neurons in vivo gave rise to dose dependent neuronal death and in rodent cortex -- marked cholinergic deficits accompanied by the accumulation of inclusions that stained positive for alpha-synuclein and ubiquitin. These findings suggest that proteasomal abnormalities are present within cortical Lewy body disease and the experimental inhibition of proteasomal function mirrors the neuropathological changes seen within the disorder.

Neurodegeneration and peroxidases

Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative diseases that affect different parts of the central nervous system. However, a review of the literature indicates that certain biochemical reactions involved in neurodegeneration in these three diseases are quite similar and could be partly identical. This article critically examines the similarities and, based on data from our own and other laboratories, proposes a novel explanation for neurodegeneration in these three diseases. We identified about 20 commonalities that exist in the neurodegenerative process of each disease. We hypothesize that there are two enzyme-catalyzed pathways that operate in affected neurons: an oxidative pathway leading to destruction of various neuronal proteins and lipids, and an apoptotic pathway which the body normally uses to remove unwanted and dysfunctional cells. Data from many laboratories indicate that oxidative reactions are primarily responsible for neurodegeneration, whereas apoptosis may well be a secondary response to the presence of neurons that have already been severely damaged by oxidative reactions. Attempts to inhibit apoptosis for the purpose of attenuating progression of these diseases may therefore be only of marginal benefit. Specific oxidative reactions within affected neurons led us to propose that one or more heme peroxidases may be the catalyst(s) involved in oxidation of proteins and lipids. Support for this proposal is provided by the recent finding that amyloi-beta peptide may act as a peroxidase in AD. Possible participation of the peroxidase activity of cytochrome c, herein designated as cytochrome c(px) to distinguish it from yeast cytochrome c peroxidase, is discussed. Of special interest is our recent finding that many compounds that cause attenuation of neurodegeneration are inhibitors of the peroxidase activity of cytochrome c. Several inhibitors were subsequently identified as suicide substrates. Such inhibitors could be ideally suited for targeted clinical approaches aimed at arresting progression of neurodegeneration. Finally, it is possible that immobilized yet still active peroxidase(s) may be present in protein aggregates in AD, PD, and ALS. This activity could be the catalyst for the slow, self-perpetuating and irreversible degeneration of affected neurons that occurs over long periods of time in these neurodegenerative diseases.

Role of the ubiquitin proteasome system in Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative movement disorder. Although a subject of intense research, the etiology of PD remains poorly understood. Recently, several lines of evidence have implicated an intimate link between aberrations in the ubiquitin proteasome system (UPS) and PD pathogenesis. Derangements of the UPS, which normally functions as a type of protein degradation machinery, lead to alterations in protein homeostasis that could conceivably promote the toxic accumulation of proteins detrimental to neuronal survival. Not surprisingly, various cellular and animal models of PD that are based on direct disruption of UPS function reproduce the most prominent features of PD. Although persuasive, new developments in the past few years have in fact raised serious questions about the link between the UPS and PD. Here I review current thoughts and controversies about their relationship and discuss whether strategies aimed at mitigating UPS dysfunction could represent rational ways to intervene in the disease. Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; http://www.targetedproteinsdb.com).

Systemic administration of a proteasome inhibitor does not cause nigrostriatal dopamine degeneration

Proteasomal dysfunction has been suggested to contribute to the degeneration of nigrostriatal dopamine neurons in Parkinson's disease. A recent study reported that systemic treatment of rats with the proteasome inhibitor Z-lle-Glu(OtBu)-Ala-Leu-al (PSI) causes a slowly progressive degeneration of nigrostriatal dopamine neurons, the presence of inclusion bodies in dopamine neurons, and motor impairment. We examined in vitro and in vivo the effects of PSI on nigrostriatal dopamine neurons. Mass spectrometric analysis was employed to verify the authenticity of the PSI compound. PSI was non-specifically toxic to neurons in ventral mesencephalic organotypic slice cultures, indicating that impairment of proteasome function in vitro is toxic. Moreover, systemic administration of PSI transiently decreased brain proteasome activity. Systemic treatment of rats with PSI did not, however, result in any biochemical or anatomical evidence of lesions of nigrostriatal dopamine neurons, nor were any changes in locomotor activity observed. These data suggest that systemic administration of proteasome inhibitors to normal adult rats does not reliably cause an animal model of parkinsonism.

Clinical trials for neuroprotection in Parkinson??s disease: overcoming angst and futility?

PURPOSE OF REVIEW

To summarize recently published results of neuroprotection trials for Parkinson's disease, and discuss them in the context of evolving concepts in clinical study design and animal models.

RECENT FINDINGS

Despite compelling preclinical evidence from laboratory models suggesting potential neuroprotective benefits, the antioxidant, antiapoptotic, antiexcitotoxic, immunomodulatory and neurotrophic agents studied to date have not shown clear benefit in human studies. The futility study design, an alternative approach focused on efficiently excluding less promising compounds, has been adopted recently to investigate four candidate neuroprotectants. A delayed-start trial design has also been introduced in a study of the monoamine oxidase inhibitor rasagiline, demonstrating a possible neuroprotective effect as well as its clear symptomatic benefit. In parallel with these clinical innovations, preclinical research initiatives are identifying new animal models that more closely resemble the clinical course and pathology of Parkinson's disease.

SUMMARY

Angst over disappointing results of neuroprotection trials in Parkinson's disease has engendered efforts to refine animal models at one end of the therapeutics pipeline, and to optimize clinical trial design at the other. Building on new insights into the genetics, epidemiology and pathogenesis of Parkinson's disease, these recent improvements in 'translational infrastructure' will enhance the prospects of achieving the critical goal of slowing the progression of disability.

Multiple hit hypotheses for dopamine neuron loss in Parkinson's disease

Parkinson's disease arises from genetic and possibly neurotoxic causes that produce massive cell death of the neuromelanin-containing dopaminergic neurons of the substantia nigra. Loss of these neurons is essential for the diagnostic parkinsonian features. Although many genetic mutations have been suggested as causes or risk factors for Parkinson's disease, the low penetrance of some mutations and the low disease concordance in relatives suggests that there must be interactions between multiple factors. We suggest that 'multiple hits' that combine toxic stress, for example, from dopamine oxidation or mitochondrial dysfunction, with an inhibition of a neuroprotective response, such as loss of function of parkin or stress-induced autophagic degradation, underlie selective neuronal death. We discuss the properties of substantia nigra dopamine neurons that might make them particular targets of such multiple hits.

Proteasome inhibitor model of Parkinson's disease in mice is confounded by neurotoxicity of the ethanol vehicle

Defects in the ubiquitin-proteasome system have been implicated in Parkinson's Disease (PD). Recently, a rat model of PD was developed using a synthetic proteasome inhibitor (PSI), (Z-lle-Glu(OtBu)-Ala-Leu-al). We attempted to transfer this model to mouse studies, where genetics can be more readily investigated due to the availability of genetically modified mice. We treated C57BL/6 (B6) mice with six intraperitoneal injections of 6 mg/kg PSI in 50 mul of 70% ethanol over a 2-week-period. We found significant decreases in nigrostriatal dopamine in PSI-treated mice compared with saline-treated mice. However, we observed similar decreases in the ethanol-treated vehicle control group. Administration of ethanol alone led to significant long-term alterations in dopamine levels. Ethanol significantly eclipses the effects of PSI in the dopamine system, and therefore is a confounding vehicle for this model.

Nonhuman Primate Models of Parkinson's Disease

Nonhuman primate (NHP) models of Parkinson's disease (PD) play an essential role in the understanding of PD pathophysiology and the assessment of PD therapies. NHP research enabled the identification of environmental risk factors for the development of PD. Electrophysiological studies in NHP models of PD identified the neural circuit responsible for PD motor symptoms, and this knowledge led to the development of subthalamic surgical ablation and deep brain stimulation. Similar to human PD patients, parkinsonian monkeys are responsive to dopamine replacement therapies and present complications associated with their long-term use, a similarity that facilitated the assessment of new symptomatic treatments, such as dopaminergic agonists. New generations of compounds and novel therapies that use directed intracerebral delivery of drugs, cells, and viral vectors benefit from preclinical evaluation in NHP models of PD. There are several NHP models of PD, each with characteristics that make it suitable for the study of different aspects of the disease or potential new therapies. Investigators who use the models and peer scientists who evaluate their use need information about the strengths and limitations of the different PD models and their methods of evaluation. This article provides a critical review of available PD monkey models, their utilization, and how they compare to emerging views of PD as a multietiologic, multisystemic disease. The various models are particularly useful for representing different aspects of PD at selected time points. This conceptualization provides clues for the development of new NHP models and facilitates the clinical translation of findings. As ever, successful application of any model depends on matching the model to the scientific question to be answered. Adequate experimental designs, with multiple outcome measures of clinical relevance and an appropriate number of animals, are essential to minimize the limitations of models and increase their predictive clinical validity.

p53-dependent neuronal cell death in a DJ-1-deficient zebrafish model of Parkinson's disease

Mutations in DJ-1 lead to early onset Parkinson's disease (PD). The aim of this study was to elucidate further the underlying mechanisms leading to neuronal cell death in DJ-1 deficiency in vivo and determine whether the observed cell loss could be prevented pharmacologically. Inactivation of DJ-1 in zebrafish, Danio rerio, resulted in loss of dopaminergic neurons after exposure to hydrogen peroxide and the proteasome inhibitor MG132. DJ-1 knockdown by itself already resulted in increased p53 and Bax expression levels prior to toxin exposure without marked neuronal cell death, suggesting subthreshold activation of cell death pathways in DJ-1 deficiency. Proteasome inhibition led to a further increase of p53 and Bax expression with widespread neuronal cell death. Pharmacological p53 inhibition either before or during MG132 exposure in vivo prevented dopaminergic neuronal cell death in both cases. Simultaneous knockdown of DJ-1 and the negative p53 regulator mdm2 led to dopaminergic neuronal cell death even without toxin exposure, further implicating involvement of p53 in DJ-1 deficiency-mediated neuronal cell loss. Our study demonstrates the utility of zebrafish as a new animal model to study PD gene defects and suggests that modulation of downstream mechanisms, such as p53 inhibition, may be of therapeutic benefit.

Proteasomal inhibition causes loss of nigral tyrosine hydroxylase neurons

Dysfunction of the ubiquitin-proteasomal system (UPS) has been implicated in the pathogenesis of Parkinson's disease. The systemic administration of UPS inhibitors has been reported to induce nigrostriatal cell death and model Parkinson's disease pathology in rodents. We administered a synthetic, specific UPS inhibitor (PSI) subcutaneously to rats and quantified substantia nigral tyrosine hydroxylase-positive dopaminergic neurons by stereology. PSI caused a 15% decrease in UPS activity at 2 weeks and a 42% reduction in substantia nigra pars compacta tyrosine hydroxylase-positive neurons at 8 weeks. Systemic inhibition of the UPS warrants further evaluation as a means to model Parkinson's disease.

Proteasome inhibitor-induced model of Parkinson's disease

We recently reported that systemic administration of a proteasome inhibitor induced a progressive levodopa-responsive, bradykinetic syndrome in rats with imaging, pathological, and biochemical features that strikingly resemble what is found in PD. This model has the potential to be a useful tool for studying the mechanism of cell death in Parkinson's disease and for testing putative neuroprotective agents. Since publication of these findings, several laboratories have sought to reproduce the model; some have been successful in replicating our findings, but others have not. The reason for this variability is not known, but resolution is critically important given the potential utility of this model. We have begun to examine various factors that alone or in combination might explain these differences, and we present in this article preliminary results from these studies.

Reproducible nigral cell loss after systemic proteasomal inhibitor administration to rats

Systemic administration of proteasomal inhibitors to rats has been proposed as producing progressive nigral dopaminergic cell loss and impairment of motor function, although this has proved difficult to reproduce. We report reproducible loss of tyrosine hydroxylase-positive cells in substantia nigra and decrease in locomotor activity by proteasomal inhibitor injection in rats up to 10 months after treatment. Dopaminergic cell death was accompanied by the appearance of ubiquitin and alpha-synuclein-positive inclusions in the substantia nigra in these rats. Neuronal loss was also observed in the locus ceruleus, raphe nuclei, and dorsal motor nucleus of the vagus, verifying that proteasomal inhibition produces a relevant model of Parkinson's disease.

Lack of nigrostriatal pathology in a rat model of proteasome inhibition

Systemic administration of ubiquitin-proteasome system inhibitors to rodents has been reported to induce certain behavioral and neuropathological features of Parkinson's disease. The goal of this study was to replicate these observations by administering a proteasome inhibitor (PSI) to rats using McNaught and colleagues' protocol. No alterations in locomotor activity or striatal dopamine and its metabolites were observed. Differences in nigral dopaminergic cell number between proteasome inhibitor- and vehicle-treated rats and inclusion bodies were not found. Extending the time of survival after administration and using different solvents failed to alter results, indicating this proteasome inhibitor does not consistently produce the selective toxicity and pathological hallmarks characterizing Parkinson's disease.

Failure of proteasome inhibitor administration to provide a model of Parkinson's disease in rats and monkeys

McNaught and colleagues reported recently that systemic administration of proteasome inhibitors PSI (Z-Ileu-Glu(OtBu)-Ala-Leu-CHO) or epoxomicin recapitulated many of the degenerative changes seen in Parkinson's disease including loss of striatal dopamine and cell loss in the substantia nigra, locus ceruleus, dorsal motor nucleus of the X cranial nerve, and nucleus basalis of Meynert. Intracytoplasmic inclusions resembling Lewy bodies were also described. All experiments administering PSI to rats using identical procedures and multiple attempts failed to induce any of the previously described changes. Furthermore, administration of PSI or epoxomicin to monkeys in an attempt to extend the model to a primate species failed. Currently, systemic proteasome inhibition is not a reliable model for Parkinson's disease.

Proteasome inhibition and Parkinson's disease modeling

Impaired proteasome function is a potential mechanism for dopaminergic neuron degeneration. To model this molecular defect, we administered systemically the reversible lipophilic proteasome inhibitor, carbobenzoxy-L-isoleucyl-gamma-t-butyl-L-glutamyl-L-alanyl-L-leucinal (PSI), to rodents. In contrast to a previous report, this approach failed to cause any detectable behavioral or neuropathological abnormality in either rats or mice. Although theoretically appealing, this specific model of Parkinson's disease appears to exhibit poor reproducibility.