Altered proteasomal function in sporadic Parkinson's disease

Oxidized proteins: mechanisms of removal and consequences of accumulation

Elevated levels of oxidized proteins are reported in diseased tissue from age-related pathologies such as atherosclerosis, neurodegenerative disorders, and cataract. Unlike the precise mechanisms that exist for the repair of nucleic acids, lipids, and carbohydrates, the primary pathway for the repair of oxidized proteins is complete catabolism to their constitutive amino acids. This process can be inefficient as is evidenced by their accumulation. It is generally considered that damaged proteins are degraded by the proteasome; however, this is only true for mildly oxidized proteins, because substrates must be unfolded to enter the narrow catalytic core. Rather, evidence suggests that moderately or heavily oxidized proteins are endocytosed and enter the endosomal/lysosomal system, indicating co-operation between the proteasomes and the lysosomes. Heavily modified substrates are incompletely degraded and accumulate within the lysosomal compartments resulting in the formation of lipofuscin-like, autofluorescent aggregates. Accumulation eventually results in impaired turnover of large organelles such as proteasomes and mitochondria, lysosomal destablization, leakage of proteases into the cytosol and apoptosis. In this review, we summarize reports published since our last assessments of the field of oxidized protein degradation including a role for modified proteins in the induction of apoptosis.

Protein ubiquitination in postsynaptic densities after hypoxia in rat neostriatum is blocked by hypothermia

Synaptic dysfunction has been associated with neuronal cell death following hypoxia. The lack of knowledge on the mechanisms underlying this dysfunction prompted us to investigate the morphological changes in the postsynaptic densities (PSDs) induced by hypoxia. The results presented here demonstrate that PSDs of the rat neostriatum are highly modified and ubiquitinated 6 months after induction of hypoxia in a model of perinatal asphyxia. Using both two dimensional (2D) and three dimensional (3D) electron microscopic analyses of synapses stained with ethanolic phosphotungstic acid (E-PTA), we observed an increment of PSD thickness dependent on the duration and severity of the hypoxic insult. The PSDs showed clear signs of damage and intense staining for ubiquitin. These morphological and molecular changes were effectively blocked by hypothermia treatment, one of the most effective strategies for hypoxia-induced brain injury available today. Our data suggest that synaptic dysfunction following hypoxia may be caused by long-term misfolding and aggregation of proteins in the PSD.

S-glutathionylation of the Rpn2 regulatory subunit inhibits 26 S proteasomal function

Although increased intracellular concentrations of hydrogen peroxide (H2O2) are associated with inhibition of 26 S proteasomal activity, the mechanisms responsible for such effects have not been well delineated. In the present studies, we found that direct exposure of purified 26 S proteasomes to H2O2 had negligible effects on their activity, whereas incubation with glutathione and H2O2 produced >80% decrease in chymotrypsin-like and trypsin-like activities. Rpn1 and Rpn2, which are subunits of the 19 S regulatory particle, undergo S-glutathionylation after exposure of purified 26 S proteasomes to glutathione and H2O2, as well as in HEK 293 cells and neutrophils incubated with H2O2. Increased oxidation of Rpn1 and Rpn2 cysteine thiols was also found in lung extracts from mice in which catalase was inactivated, a condition associated with augmented intracellular concentrations of H2O2 and diminished 26 S proteasomal activity. Although unoxidized Rpn2 enhanced 20 S proteolytic function in vitro, such potentiation was not found when the 20 S core particle was incubated with oxidized Rpn2. The composition of 26 S proteasomes was not altered after exposure to glutathione and H2O2, with similar amounts of Rpn1 and Rpn2 in control or oxidized 26 S proteasomal complexes. These findings identify S-glutathionylation of Rpn2 as a contributory mechanism for H2O2-induced inhibition of 26 S proteasomal function.

Parkin deficiency increases the resistance of midbrain neurons and glia to mild proteasome inhibition: the role of autophagy and glutathione homeostasis

Parkin mutations in humans produce parkinsonism whose pathogenesis is related to impaired protein degradation, increased free radicals and abnormal neurotransmitter release. In this study, we have investigated whether partial proteasomal inhibition by epoxomicin, an ubiquitin proteasomal system (UPS) irreversible inhibitor, further aggravates the cellular effects of parkin suppression in midbrain neurons and glia. We observed that parkin null (PK-KO) midbrain neuronal cultures are resistant to epoxomicin-induced cell death. This resistance is due to increased GSH and DJ-1 protein levels in PK-KO mice. The treatment with epoxomicin increases, in wild type (WT) cultures, the pro-apoptotic Bax/Bcl-2 ratio, the phosphorylation of tau, and the levels of chaperones heat-shock protein 70 and C-terminal Hsc-interacting protein, but none of these effects took place in epoxomicin-treated PK-KO cultures. Poly-ubiquitinated proteins increased more in WT than in PK-KO-treated neuronal cultures. Parkin accumulated in WT neuronal cultures treated with epoxomicin. Markers of autophagy, such as LC3II/I, were increased in naïve PK-KO cultures, and further increased after treatment with epoxomicin, implying that the blockade of the proteasome in PK-KO neurons triggers the enhancement of autophagy. The treatment with l-buthionine-S,R-sulfoximine and the inhibition of autophagy, however, reverted the increase resistance to epoxomicin of the PK-KO cultures. We also found that PK-KO glial cells, stressed by growth in defined medium and depleted of GSH, were more susceptible to epoxomicin induced cell death than WT glia treated similarly. This susceptibility was linked to reduced GSH levels and less heat-shock protein 70 response, and to activation of p-serine/threonine kinase protein signaling pathway as well as to increased poly-ubiquitinated proteins. These data suggest that mild UPS inhibition is compensated by other mechanisms in PK-KO midbrain neurons. However the depletion of GSH, as happens in stressed glia, suppresses the protection against UPS inhibition-induced cell death. Furthermore, GSH inhibition regulated differentially UPS activity and in old PK-KO mice, which have depletion of GSH, UPS activity is decreased in comparison with that of old-WT.

Does smoking reduce the risk of Parkinson's disease through stimulation of the ubiquitin-proteasome system?

Parkinson's disease is a neurodegenerative condition characterized by tremor, rigidity, bradykinesia, and postural instability. Smoking is an inverse risk factor for Parkinson's disease, although the mechanism for this apparent neuroprotection is not definitively established. Smoking consistently upregulates nicotinic acetylcholine receptor levels in various brain regions known to be involved in Parkinson's disease. The ubiquitin-proteasome system--the system that tags and removes unwanted, misfolded, or damaged proteins from cells--regulates nicotinic receptor levels. The ubiquitin-proteasome system has also been implicated in Parkinson's disease, with aberrant activity identified in both sporadic and familial forms of the disease. The involvement of the ubiquitin-proteasome system in nicotinic receptor regulation and Parkinson's disease pathology suggests a link between the two, which forms the basis of the present hypothesis. Specifically, this paper considers the hypothesis that smoking reduces the risk of Parkinson's disease through the upregulation of nicotinic cholinergic receptors in key brain regions involved in Parkinson's disease. This receptor upregulation is hypothesized to increase activity of the ubiquitin-proteasome system, which is believed to prevent neurodegeneration caused by the accumulation of misfolded or damaged proteins or other consequences of inadequate protein sequestration and/or degradation. This hypothesis is supported by evidence documenting the upregulation of nicotinic receptors in the brains of smokers, neuroprotective effects of nicotine, reduced activity of the ubiquitin-proteasome in Parkinson's disease, and increased activity of the ubiquitin-proteasome system in animals exposed to chronic nicotine. Additional research is needed to test several predictions of the hypothesis, including increased activity of the ubiquitin-proteasome system in key brain regions of smokers.

Aging is not associated with proteasome impairment in UPS reporter mice

Background

Covalent linkage of ubiquitin regulates the function and, ultimately, the degradation of many proteins by the ubiquitin-proteasome system (UPS). Given its essential role in protein regulation, even slight perturbations in UPS activity can substantially impair cellular function.

Methodology/Principal Findings

We have generated and characterized a novel transgenic mouse model which expresses a previously described reporter for UPS function. This UPS reporter contains a degron sequence attached to the C-terminus of green fluorescent protein, and is predominantly expressed in neurons throughout the brain of our transgenic model. We then demonstrated that this reporter system is sensitive to UPS inhibition in vivo.

Conclusions/Significance

Given the obstacles associated with evaluating proteasomal function in the brain, our mouse model uniquely provides the capability to monitor UPS function in real time in individual neurons of a complex organism. Our novel mouse model now provides a useful resource with which to evaluate the impact of aging, as well as various genetic and/or pharmacological modifiers of neurodegenerative disease(s).

Inhibition of lysosomal functions reduces proteasomal activity

Protein accumulation and aggregation are signatures of several major neurodegenerative diseases. Proteasomal- and lysosomal-mediated protein degradation pathways are the two major pathways for intracellular protein degradation. Cross-regulation between these two pathways may be important for protein homeostasis. Pharmacological inhibition of proteasomal activities has been shown to up-regulate the levels of lysosomal enzymes. To determine whether the reverse regulatory mechanism also occurs in the cell, we investigated the effects of inhibition of lysosomal function on proteasomal activities. We found that rather than up-regulating proteasomal activities in response to lysosomal disruptors, reduced lysosomal function reduces proteasomal functions, indicating a lack of compensatory up-regulation of proteasomal functions. Inhibition of lysosomal or proteasomal activities led to higher levels of chaperone heat shock cognate protein Hsc70, suggesting an attempt to compensate protein degradation deficiency by enhancing chaperone-mediated autophagy.

Dynamic changes in presynaptic and axonal transport proteins combined with striatal neuroinflammation precede dopaminergic neuronal loss in a rat model of AAV alpha-synucleinopathy

Little is known about key pathological events preceding overt neuronal degeneration in Parkinson's disease (PD) and alpha-synucleinopathy. Recombinant adeno-associated virus 2-mediated delivery of mutant (A53T) human alpha-synuclein into the substantia nigra (SN) under a neuron-specific synapsin promoter resulted in protracted neurodegeneration with significant dopaminergic (DA) neuron loss by 17 weeks. As early as 4 weeks, there was an increase in a dopamine metabolite, DOPAC and histologically, DA axons in the striatum were dystrophic with degenerative bulbs. Before neuronal loss, significant changes were identified in levels of proteins relevant to synaptic transmission and axonal transport in the striatum and the SN. For example, striatal levels of rabphilin 3A and syntaxin were reduced. Levels of anterograde transport motor proteins (KIF1A, KIF1B, KIF2A, and KIF3A) were decreased in the striatum, whereas retrograde motor proteins (dynein, dynamitin, and dynactin1) were increased. In contrast to reduced levels in the striatum, KIF1A and KIF2A levels were elevated in the SN. There were dramatic changes in cytoskeletal protein levels, with actin levels increased and alpha-/gamma-tubulin levels reduced. In addition to these alterations, a neuroinflammatory response was observed at 8 weeks in the striatum, but not in the SN, demonstrated by increased levels of Iba-1, activated microglia and increased levels of proinflammatory cytokines, including IL-1beta, IFN-gamma and TNF-alpha. These results demonstrate that changes in proteins relevant to synaptic transmission and axonal transport coupled with neuroinflammation, precede alpha-synuclein-mediated neuronal death. These findings can provide ideas for antecedent biomarkers and presymptomatic interventions in PD.

Nigral degeneration with inclusion body formation and behavioral changes in rats after proteasomal inhibition

OBJECTIVE

We were interested in studying nigral degeneration with inclusion body formation and behavioral changes in rats after proteasomal inhibition.

METHODS

Observation of progressive behavioral and pathological changes in rats following a unilateral nigral injection of lactacystin, a selective proteasome inhibitor.

RESULTS

After administration at a concentration of 10 microg (2 microl) of lactacystin, when tyrosine hydroxylase (TH) immunostaining decreased gradually in the substantia nigra pars compacta (SNc) and corpus striatum, alpha-synuclein-immunopositive inclusion appeared extensively in the surviving neurons. We also observed the degeneration of diverse cellular organelles by transmission electron microscopy. The effect of cellular organelle degeneration on behavior, a clinical index, was striking and was statistically significant. Over the 3 weeks following the administration of lactacystin, a highly significant decrease in TH immunostaining was observed and alpha-synuclein-immunopositive inclusions gradually appeared. Interestingly, there was a strong correlation in behavioral changes and the increase in alpha-synuclein-immunopositive inclusions whereas the decrease in TH immunostaining did not seem to induce any behavioral changes.

CONCLUSIONS

Our results reveal that unilateral nigral proteasome inhibition induces degeneration in the SNc and corpus striatum as well as behavioral changes demonstrating strong time dependence. Behavioral changes were driven by the formation of alpha-synuclein inclusions, but not by decreased TH neurons.

Functional models of Parkinson's disease: a valuable tool in the development of novel therapies

Functional models of Parkinson's disease (PD) have led to effective treatment for the motor symptoms. Toxin-based models, such as the 6-hydroxydopamine-lesioned rat and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primate, have resulted in novel dopaminergic therapies and new therapeutic strategies. They have also been used to study processes underlying motor complications, particularly dyskinesia, and for developing pharmacological approaches to dyskinesia avoidance and suppression. Symptomatic models of PD based on nigrostriatal degeneration have a high degree of predictability of clinical effect of dopaminergic drugs on motor symptoms in humans. However, the effects of nondopaminergic drugs in these models do not translate effectively into clinical efficacy. Newer experimental models of PD have attempted to reproduce the pathogenic process and to involve all areas of the brain pathologically affected in humans. In addition, models showing progressive neuronal death have been sought but so far unsuccessfully. Pathogenic modeling has been attempted using a range of toxins, as well as through the use of transgenic models of gene defects in familial PD and mutant rodent strains. However, there are still no accepted progressive models of PD that mimic the processes known to occur during cell death and that result in the motor deficits, pathology, biochemistry, and drug responsiveness as seen in humans. Nevertheless, functional models of PD have led to many advances in treating the motor symptoms of the disorder, and we have been fortunate to have them available. They are an important reason the treatment of PD is so much better compared with treatments for related illnesses.

A critical evaluation of the ubiquitin-proteasome system in Parkinson's disease

The evidence for impairment in the ubiquitin proteasome system (UPS) in Parkinson's disease (PD) is mounting and becoming increasingly more convincing. However, it is presently unclear whether UPS dysfunction is a cause or result of PD pathology, a crucial distinction which impedes both the understanding of disease pathogenesis and the development of effectual therapeutic approaches. Recent findings discussed within this review offer new insight and provide direction for future research to conclusively resolve this debate.

Molecular interaction between parkin and PINK1 in mammalian neuronal cells

Parkinson's disease (PD) is characterized by the deterioration of dopaminergic neurons in the pars compacta of substantia nigra and the formation of intraneuronal protein inclusions. The etiology of PD is not known, but the recent identification of several mutation genes in familial PD has provided a rich understanding of the molecular mechanisms of PD pathology. Mutations in PTEN-induced putative kinase 1 (PINK1) and parkin are linked to early-onset autosomal recessive forms of familial PD. Here we show molecular and functional interactions between parkin and PINK1. Parkin selectively binds to PINK1 and upregulates PINK1 levels. In addition, PINK1 reduces the solubility of parkin, which induces the formation of microtubule-dependent cytoplasmic aggresomes. Our findings reveal that parkin and PINK1 affect each other's stability, solubility and tendency to form aggresomes, and have important implications regarding the formation of Lewy bodies.

Carnosine and its possible roles in nutrition and health

The dipeptide carnosine has been observed to exert antiaging activity at cellular and whole animal levels. This review discusses the possible mechanisms by which carnosine may exert antiaging action and considers whether the dipeptide could be beneficial to humans. Carnosine's possible biological activities include scavenger of reactive oxygen species (ROS) and reactive nitrogen species (RNS), chelator of zinc and copper ions, and antiglycating and anticross-linking activities. Carnosine's ability to react with deleterious aldehydes such as malondialdehyde, methylglyoxal, hydroxynonenal, and acetaldehyde may also contribute to its protective functions. Physiologically carnosine may help to suppress some secondary complications of diabetes, and the deleterious consequences of ischemic-reperfusion injury, most likely due to antioxidation and carbonyl-scavenging functions. Other, and much more speculative, possible functions of carnosine considered include transglutaminase inhibition, stimulation of proteolysis mediated via effects on proteasome activity or induction of protease and stress-protein gene expression, upregulation of corticosteroid synthesis, stimulation of protein repair, and effects on ADP-ribose metabolism associated with sirtuin and poly-ADP-ribose polymerase (PARP) activities. Evidence for carnosine's possible protective action against secondary diabetic complications, neurodegeneration, cancer, and other age-related pathologies is briefly discussed.

Protective effect against Parkinson's disease-related insults through the activation of XBP1

The accumulation of misfolded and unfolded proteins in endoplasmic reticulum (ER) induces ER stress, activating the unfolded protein response (UPR). Recent evidence has suggested the relationship between UPR and dopaminergic neuronal cell death in Parkinson's disease (PD); however, it remains unclear whether it makes sense to modulate UPR, to mitigate the progression of PD. In this study, we investigated a role of the IRE1 alpha-XBP1 pathway in the survival of dopaminergic cells, under stress induced by PD-related insults. The exogenous expression of the active-form XBP1 (XBP1s) protein had protective effects against cell death induced by 1-methyl-4-phenylpyridinium (MPP+) and proteasome inhibitors. Moreover, adenoviral XBP1s expression significantly suppressed the degeneration of dopaminergic neurons in the mouse model of PD, as induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). These results demonstrate that the enhancement of XBP1 could be a novel PD therapeutic strategy.

Update on the pathogenesis of Parkinson's disease

Parkinson's disease is a primarily sporadic occurring neurodegenerative disorder of advanced age. However, in the last few years several genes have been identified that lead to a hereditary parkinsonian disorder with autosomal dominant or autosomal recessive inheritance. This includes autosomal dominant mutations in the alpha-synuclein, ubiquitin-C-terminal hydrolase-L1 (UCH-L1) and the leucine-rich repeat kinase (LRRK)2 genes and autosomal recessively inherited mutations in Parkin, PINK1, DJ-1 and the ATP13A2 genes. By taking the biochemical function of these genes and mutations into account, three underlying pathogenetic pathways can be identified: (i) altered protein quality control, (ii) oxidative stress and mitochondrial dysfunction, and (iii) disturbed kinase activity. It remains an open question whether alterations of these pathways lead to different entities of Parkinson's disease or whether they finally converge at a point that is the common pathogenetic denominator of Parkinson's disease. Finally cell death is executed by excitotoxicity, apoptosis and autophagy and appears to be facilitated by neuroinflammatory processes.

Aggresome-forming TTRAP mediates pro-apoptotic properties of Parkinson's disease-associated DJ-1 missense mutations

Mutations in PARK7 DJ-1 have been associated with autosomal-recessive early-onset Parkinson's disease (PD). This gene encodes for an atypical peroxiredoxin-like peroxidase that may act as a regulator of transcription and a redox-dependent chaperone. Although large gene deletions have been associated with a loss-of-function phenotype, the pathogenic mechanism of several missense mutations is less clear. By performing a yeast two-hybrid screening from a human fetal brain library, we identified TRAF and TNF receptor-associated protein (TTRAP), an ubiquitin-binding domain-containing protein, as a novel DJ-1 interactor, which was able to bind the PD-associated mutations M26I and L166P more strongly than wild type. TTRAP protected neuroblastoma cells from apoptosis induced by proteasome impairment. In these conditions, endogenous TTRAP relocalized to a detergent-insoluble fraction and formed cytoplasmic aggresome-like structures. Interestingly, both DJ-1 mutants blocked the TTRAP protective activity unmasking a c-jun N-terminal kinase (JNK)- and p38-MAPK (mitogen-activated protein kinase)-mediated apoptosis. These results suggest an active role of DJ-1 missense mutants in the control of cell death and position TTRAP as a new player in the arena of neurodegeneration.

Mitochondria and ubiquitin-proteasomal system interplay: relevance to Parkinson's disease

The cellular mechanisms that may underlie the death of dopaminergic neurons in Parkinson's disease are ubiquitin-proteasomal system (UPS) impairment, mitochondrial dysfunction, and oxidative stress. The goal of this work was to elucidate the correlation between mitochondrial dysfunction and UPS impairment, focusing on the role of oxidative stress. Our data revealed that mitochondria-DNA-depleted cells (rho0) are compromised at the mitochondrial and UPS levels and also show an alteration of the oxidative status. In parental cells (rho+), MPP(+) induced a clear inhibition of complex I activity, as well as an increase in ubiquitinylated protein levels, which was not observed in cells treated with lactacystin. Moreover, MPP(+) induced a decreased in the 20S chymotrypsin-like and peptidyl-glutamyl peptide hydrolytic-like proteolytic activities after 24 h of exposure. ROS production was increased in rho+ cells treated with MPP(+) or lactacystin, at early treatment periods. MPP(+) induced an increase in carbonyl group formation in rho+ cells. The results suggest that a mitochondrial alteration leads to an imbalance in the cellular oxidative status, inducing a proteasomal deregulation, which may exacerbate protein aggregation, and consequently degenerative events.

Progress in the pathogenesis and genetics of Parkinson's disease

Recent progresses in the pathogenesis of sporadic Parkinson's disease (PD) and genetics of familial PD are reviewed. There are common molecular events between sporadic and familial PD, particularly between sporadic PD and PARK1-linked PD due to alpha-synuclein (SNCA) mutations. In sporadic form, interaction of genetic predisposition and environmental factors is probably a primary event inducing mitochondrial dysfunction and oxidative damage resulting in oligomer and aggregate formations of alpha-synuclein. In PARK1-linked PD, mutant alpha-synuclein proteins initiate the disease process as they have increased tendency for self-aggregation. As highly phosphorylated aggregated proteins are deposited in nigral neurons in PD, dysfunctions of proteolytic systems, i.e. the ubiquitin-proteasome system and autophagy-lysosomal pathway, seem to be contributing to the final neurodegenerative process. Studies on the molecular mechanisms of nigral neuronal death in familial forms of PD will contribute further on the understanding of the pathogenesis of sporadic PD.

The ubiquitin-proteasome system in spongiform degenerative disorders

Spongiform degeneration is characterized by vacuolation in nervous tissue accompanied by neuronal death and gliosis. Although spongiform degeneration is a hallmark of prion diseases, this pathology is also present in the brains of patients suffering from Alzheimer’s disease, diffuse Lewy body disease, human immunodeficiency virus (HIV) infection, and Canavan’s spongiform leukodystrophy. The shared outcome of spongiform degeneration in these diverse diseases suggests that common cellular mechanisms must underlie the processes of spongiform change and neurodegeneration in the central nervous system. Immunohistochemical analysis of brain tissues reveals increased ubiquitin immunoreactivity in and around areas of spongiform change, suggesting the involvement of ubiquitin–proteasome system dysfunction in the pathogenesis of spongiform neurodegeneration. The link between aberrant ubiquitination and spongiform neurodegeneration has been strengthened by the discovery that a null mutation in the E3 ubiquitin–protein ligase mahogunin ring finger-1 (Mgrn1) causes an autosomal recessively inherited form of spongiform neurodegeneration in animals. Recent studies have begun to suggest that abnormal ubiquitination may alter intracellular signaling and cell functions via proteasome-dependent and proteasome-independent mechanisms, leading to spongiform degeneration and neuronal cell death. Further elucidation of the pathogenic pathways involved in spongiform neurodegeneration should facilitate the development of novel rational therapies for treating prion diseases, HIV infection, and other spongiform degenerative disorders.

Animal models of Parkinson's disease and L-dopa induced dyskinesia: how close are we to the clinic?

BACKGROUND

Several different animal models are currently used to research the neurodegenerative movement disorder Parkinson's disease (PD).

RESULTS

Models based on the genetic deficits associated with a small percentage of sufferers demonstrate the pathological accumulation of alpha-synuclein characteristic of the disease but have few motor deficits and little neurodegeneration. Conversely, toxin-based models recreate the selective nigrostriatal cell death and show extensive motor dysfunction. However, these toxin models do not reproduce the extra-nigral degeneration that also occurs as part of the disease and lack the pathological hallmark of Lewy body inclusions.

DISCUSSION

Recently, several therapies that appeared promising in the MPTP-treated non-human primate and 6-OHDA-lesioned rat models have entered clinical trials, with disappointing results. We review the animal models in question and highlight the features that are discordant with PD, discussing if our search for pharmacological treatments beyond the dopamine system has surpassed the capacity of these models to adequately represent the disease.

Failure of acute administration with proteasome inhibitor to provide a model of Parkinson's disease in mice

We investigated to determine whether acute administration of proteasome inhibitor can cause dopaminergic cell loss in mice, in comparison with that of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The four intraperitoneally administrations of MPTP at 1-h intervals to mice decreased significantly the concentration of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the striatum after 5 days, in comparison with vehicle-treated animals. In contrast, the three subcutaneously administrations of carbobenzoxy-L-gamma-t-butyl-L-glutamyl-L-alanyl-L-leucinal (PSI) did not show significant changes in the concentration of dopamine, DOPAC and HVA in the striatum after 5 days, in comparison with vehicle-treated animals. Our Western blot analysis also showed that the four administrations of MPTP at 1-h intervals to mice produced a significant reduction of anti-tyrosine hydroxylase antibody (TH) protein levels in the striatum after 5 days after. In PSI-treated mice. In contrast, no significant change of TH protein levels was observed in the striatum 5 days after the final treatment with PSI. Furthermore, a significant decrease of TH protein levels was observed in the striatum of MPTP-treated mice, as compared with PSI-treated animals. The present study demonstrates that the acute treatment with proteasome inhibitor PSI did not cause the dopaminergic neurotoxicity in mice, as compared with acute treatment with MPTP. Thus, our findings suggest that acute proteasome inhibition is not a reliable model for Parkinson's disease.

alpha-Synuclein protofibrils inhibit 26 S proteasome-mediated protein degradation: understanding the cytotoxicity of protein protofibrils in neurodegenerative disease pathogenesis

The impaired ubiquitin-proteasome activity is believed to be one of the leading factors that contribute to Parkinson disease pathogenesis partially by causing alpha-synuclein aggregation. However, the relationship between alpha-synuclein aggregation and the impaired proteasome activity is yet unclear. In this study, we examined the effects of three soluble alpha-synuclein species (monomer, dimer, and protofibrils) on the degradation activity of the 26 S proteasome by reconstitution of proteasomal degradation using highly purified 26 S proteasomes and model substrates. We found that none of the three soluble alpha-synuclein species impaired the three distinct peptidase activities of the 26 S proteasome when using fluorogenic peptides as substrates. In striking contrast, alpha-synuclein protofibrils, but not monomer and dimer, markedly inhibited the ubiquitin-independent proteasomal degradation of unstructured proteins and ubiquitin-dependent degradation of folded proteins when present at 5-fold molar excess to the 26 S proteasome. Together these results indicate that alpha-synuclein protofibrils have a pronounced inhibitory effect on 26 S proteasome-mediated protein degradation. Because alpha-synuclein is a substrate of the proteasome, impaired proteasomal activity could further cause alpha-synuclein accumulation/aggregation, thus creating a vicious cycle and leading to Parkinson disease pathogenesis. Furthermore we found that alpha-synuclein protofibrils bound both the 26 S proteasome and substrates of the 26 S proteasome. Accordingly we propose that the inhibitory effect of alpha-synuclein protofibrils on 26 S proteasomal degradation might result from impairing substrate translocation by binding the proteasome or sequestrating proteasomal substrates by binding the substrates.

Nurr1 deficiency predisposes to lactacystin-induced dopaminergic neuron injury in vitro and in vivo

Parkinson's disease (PD) has been proposed to result from a combination of genetic susceptibility and environmental exposure. Dysfunction of the ubiquitin-proteasome system (UPS) has been implicated in neuron degeneration and in pathogenesis of PD. Nurr1, a member of nuclear receptor superfamily, is a potential susceptibility gene for PD. In this in vitro and in vivo study, we investigated whether Nurr1 deficiency may predispose to environmental proteasome inhibitors-induced neuron injury. We found that lactacystin, an irreversible proteasome inhibitor, caused greater injury to SH-SY5Y cells that Nurr1 expression has been suppressed by small interference RNA (siRNA). On the contrary, the Nurr1 overexpressed SH-SY5Y cells by Nurr1 expression vector transfection rescued the lactacystin-induced injury. In vivo, stereotactic microinjection with lactacystin into right median forebrain bundle (MFB) of mice caused significant inhibition of the proteasome activity in both Nurr1 knock out heterozygous (Nurr1 +/-) mice and their littermate wild-type (Nurr1 +/+) mice. At same time, we found that there was a severer loss of tyrosine hydroxylase (TH)-positive neurons in substantia nigra (SN) and greater reduction of striatal dopamine (DA) levels in Nurr1 +/- mice as compared with that in Nurr1 +/+ mice. Furthermore, lactacystin-induced increase of cleaved PARP, cleaved caspase3 and p53 and decrease of bcl-2 in SN was significantly enhanced in Nurr1 +/- mice. These findings suggest that reduction in Nurr1 expression increases susceptibility to DAergic neuron injury induced by UPS impairment.

Analysis of gene expression in Parkinson's disease: possible involvement of neurotrophic support and axon guidance in dopaminergic cell death

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra. We have studied alterations in gene expression in the substantia nigra, the caudate nucleus and putamen of four PD patients and four matched controls using custom designed Agilent microarrays. To gain insight into changes in gene expression during early stages of dopaminergic neurodegeneration, we selectively investigated the relatively spared parts of the PD substantia nigra, and correlated gene expression changes with alterations in neuronal density. We identified changes in the expression of 287 transcripts in the substantia nigra, 16 transcripts in the caudate nucleus and four transcripts in the putamen. For selected transcripts, transcriptional alterations were confirmed with qPCR on a larger set of seven PD cases and seven matched controls. We detected concerted changes in functionally connected groups of genes. In the PD substantia nigra, we observed strong evidence for a reduction in neurotrophic support and alterations in axon guidance cues. As the changes occur in relatively spared parts of the PD substantia nigra, they suggest novel disease mechanisms involving neurotrophic support and axon guidance in early stages of cellular stress events, ultimately leading to dopaminergic cell death in PD.

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.

Systemic proteasomal inhibitor exposure enhances dopamine turnover and decreases dopamine levels but does not affect MPTP-induced striatal dopamine depletion in mice

The validation of an in vivo proteasomal inhibitor (PSI) model to translate ubiquitin-proteasomal-system dysfunction involved in the pathogenesis of Parkinson's disease (PD) into a commonly accepted animal model is ongoing. Here we first report the effects of systemic administration of the proteasomal inhibitor Z-lle-Glu(OtBu)-Ala-Leu-CHO (3 mg/kg, s.c., six times over 2 weeks) alone to extend the rat model to mice. Second we investigate the consequences of PSI pretreatment 42 weeks before an acute treatment with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in C57bl/6 mice. HPLC postmortem neurochemistry showed a significant increase in dopamine turnover and decrease of striatal dopamine levels, only 14 weeks after PSI treatment, but no enhancement of dopamine turnover or differences in striatal dopamine levels when comparing MPTP with MPTP plus PSI treatment. Behavioral analysis (rotarod, open field activity) did not indicate that PSI affects this type of motor behavior. Systemic PSI administration in mice appears not to be a valid animal model under the experimental conditions used. Potential solutions are discussed.

Proteasomal inhibition and apoptosis regulatory changes in human isolated lymphocytes: The synergistic role of dopamine

Abnormal deposition of protein aggregates and increased susceptibility to apoptotic cell death may result from defects in the activity of the ubiquitin-proteasome system (UPS); neurotoxicity related to UPS defects seems to require dopamine to be fully expressed. The aim of this study was to investigate the pro-apoptotic effects caused by proteasomal activity inhibition, as well as the synergistic effect of dopaminergic stimulation in human lymphocytes isolated from healthy volunteers. Cells were incubated 20 h at 37 degrees C, with: (1) lactacystin, (2) increasing concentrations of dopamine or (3) mixture of dopamine and lactacystin. Activities of proteasome 20S and pro-apoptotic caspases-3 and -9 and levels of anti-apoptotic Bcl-2 were measured with fluorimetric or immunochemical assays, while a "DNA diffusion" assay was used to determine the apoptosis. Incubation of lymphocytes with lactacystin, which caused reduction of proteasomal activity, was associated with activation of caspases. A clear, dose-dependent reduction of proteasomal activity was also seen in the presence of increasing doses of dopamine, which was accompanied by a slight dose-dependent increase of caspases activities and Bcl-2 levels. Both effects on proteasome and caspase activities were enhanced when cells were simultaneously exposed to lactacystin and elevated concentrations of dopamine. Apoptosis was detected in all treated samples, but not in controls, without significant differences among the treatment groups; however, the association of dopamine and lactacystin induced a clear reduction in the number of cells being analyzed, pointing to marked cytotoxicity. Our data confirm the potentiation of cytotoxicity related to proteasome inhibition, in the presence of dopaminergic stimulation.

Impairment of the ubiquitin-proteasome system associated with extracellular transthyretin aggregates in familial amyloidotic polyneuropathy

The ubiquitin-proteasome system (UPS) has been associated with neurodegenerative disorders of intracellular protein aggregation. We have studied the UPS in familial amyloidotic polyneuropathy (FAP), a neurodegenerative disorder caused by extracellular deposition of mutant transthyretin (TTR). The studies were conducted in TTR-synthesizing and non-synthesizing tissues from affected individuals, in transgenic mouse models for FAP, and in neuronal or Schwannoma cell lines cultured with TTR aggregates. In human FAP tissues presenting extracellular TTR aggregates, ubiquitin-protein conjugates were up-regulated, the proteasome levels were decreased and parkin and alpha-synuclein expression were both decreased. A similar response was detected in mouse models for TTR V30M or L55P. On the other hand, the liver, which normally synthesizes variant TTR V30M, did not show this response. Furthermore, transgenic mice immunized to decrease TTR deposition showed a significant reduction in ubiquitin levels and an increase in parkin and alpha-synuclein levels in comparison to control mice. Studies performed in cell lines with aggregates in the medium resulted in increased ubiquitin and decreased parkin levels. The overall results are indicative of TTR deposition as an external stimulus to an intracellular UPS response in FAP.

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).

Proteome analysis of substantia nigra and striatal tissue in the mouse MPTP model of Parkinson's disease

The dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) replicates many of the pathological hallmarks of Parkinson's disease (PD) in mice via selective destruction of dopamine neurons of the substantia nigra and striatum. Although MPTP has been widely used to study downstream effects following the degeneration of dopaminergic neurons, the underlying mechanisms of MPTP action remain poorly understood. To determine the underlying mechanisms of MPTP action at the protein level, a 2-DE-based proteomics approach was used to evaluate the changes in protein expression in substantia nigra and striatal tissue in C57BL/6 mice after MPTP administration. We identified nine proteins that were markedly altered and are likely to be involved in mitochondrial function, heat shock protein activity, and which contribute enzyme activities for energy metabolism and protein degradation.

Assessment of the direct and indirect effects of MPP+ and dopamine on the human proteasome: implications for Parkinson's disease aetiology

Mitochondrial impairment, glutathione depletion and oxidative stress have been implicated in the pathogenesis of Parkinson's disease (PD), linked recently to proteasomal dysfunction. Our study analysed how these factors influence the various activities of the proteasome in human SH-SY5Y neuroblastoma cells treated with the PD mimetics MPP+ (a complex 1 inhibitor) or dopamine. Treatment with these toxins led to dose- and time-dependent reductions in ATP and glutathione and also chymotrypsin-like and post-acidic like activities; trypsin-like activity was unaffected. Antioxidants blocked the effects of dopamine, but not MPP+, suggesting that oxidative stress was more important in the dopamine-mediated effects. With MPP+, ATP depletion was a prerequisite for loss of proteasomal activity. Thus in a dopaminergic neuron with complex 1 dysfunction both oxidative stress and ATP depletion will contribute independently to loss of proteasomal function. We show for the first time that addition of MPP+ or dopamine to purified samples of the human 20S proteasome also reduced proteasomal activities; with dopamine being most damaging. As with toxin-treated cells, chymotrypsin-like activity was most sensitive and trypsin-like activity the least sensitive. The observed differential sensitivity of the various proteasomal activities to PD mimetics is novel and its significance needs further study in human cells.

The bipyridyl herbicide paraquat induces proteasome dysfunction in human neuroblastoma SH-SY5Y cells

Paraquat (PQ) is suspected to be an environmental risk factor for Parkinson's disease (PD). A strong correlation between exposure to paraquat and the occurrence of PD was reported in Canada, Taiwan, and the United States. This correlation is supported by in vivo work showing that paraquat produces dopaminergic pathogenesis. In particular, paraquat forms abnormal protein aggregates in dopaminergic neurons of mice. However, it is not clear how paraquat produces this pathology. Given that proteasome dysfunction induces aberrant protein aggregation, it was hypothesized that paraquat induces proteasome dysfunction. To explore this possibility, proteasome activity and some factors possibly contributing to proteasome dysfunction were investigated in dopaminergic SY5Y cells treated with paraquat. Furthermore, levels of alpha-synuclein and ubiquitin-conjugated proteins were measured to test whether paraquat induces protein accumulation in SY5Y cells. Results showed that at a concentration of paraquat that reduced viability by about 60% at 48 h (0.5 mM) loss of proteasome activity occurred. In addition, the cells showed decreased ATP levels and reduced mitochondrial complex V activity. These changes were significant 24 h after treatment with paraquat. Furthermore, paraquat-treated cells showed decreased protein levels of proteasome 19S subunits, but not 20S alpha or beta subunits, suggesting that the effects observed were not the result of general cytotoxicity. Paraquat also increased levels of alpha-synuclein and ubiquitinated proteins, suggesting that paraquat-induced proteasome dysfunction leads to aberrant protein accumulation. Taken together, these findings support the hypothesis that paraquat impairs proteasome function in SY5Y cells.

Alterations of cerebral cortex and hippocampal proteasome subunit expression and function in a traumatic brain injury rat model

Following cellular stress or tissue injury, the proteasome plays a critical role in protein degradation and signal transduction. The present study examined the beta-subunit expression of constitutive proteasomes (beta1, beta2, and beta5), immunoproteasomes (beta1i, beta2i, and beta5i) and the 11S proteasome activator, PA28alpha, in the rat CNS after traumatic brain injury (TBI). Concomitant measures assessed changes in proteasome activities. Quantitative real time PCR results indicated that beta1 and beta2 mRNA levels were not changed, while beta5 mRNA levels were significantly decreased in injured CNS following TBI. However, beta1i, beta2i, beta5i, and PA28alpha mRNA levels were significantly increased in the injured CNS. Western blotting studies found that beta1, beta2, beta5, beta2i, and beta5i subunit protein levels remained unchanged in the injured CNS, but beta1i and PA28alpha protein levels were significantly elevated in ipsilateral cerebral cortex and hippocampus. Proteasome activity assays found that peptidyl glutamyl peptide hydrolase-like and chymotrypsin-like activity were significantly reduced in the CNS after TBI, and that trypsin-like proteasome activity was increased in the injured cerebral cortex. Our results demonstrated that both proteasome composition and function in the CNS were affected by trauma. Treatments that preserve proteasome function following CNS injury may be beneficial as an approach to cerebral neuroprotection.