Ziram causes dopaminergic cell damage by inhibiting E1 ligase of the proteasome

The etiology of Parkinson disease (PD) is unclear but may involve environmental toxins such as pesticides leading to dysfunction of the ubiquitin proteasome system (UPS). Here, we measured the relative toxicity of ziram (a UPS inhibitor) and analogs to dopaminergic neurons and examined the mechanism of cell death. UPS (26 S) activity was measured in cell lines after exposure to ziram and related compounds. Dimethyl- and diethyldithiocarbamates including ziram were potent UPS inhibitors. Primary ventral mesencephalic cultures were exposed to ziram, and cell toxicity was assessed by staining for tyrosine hydroxylase (TH) and NeuN antigen. Ziram caused a preferential damage to TH+ neurons and elevated alpha-synuclein levels but did not increase aggregate formation. Mechanistically, ziram altered UPS function through interfering with the targeting of substrates by inhibiting ubiquitin E1 ligase. Sodium dimethyldithiocarbamate administered to mice for 2 weeks resulted in persistent motor deficits and a mild reduction in striatal TH staining but no nigral cell loss. These results demonstrate that ziram causes selective dopaminergic cell damage in vitro by inhibiting an important degradative pathway implicated in the etiology of PD. Chronic exposure to widely used dithiocarbamate fungicides may contribute to the development of PD, and elucidation of its mechanism would identify a new potential therapeutic target.     

Impairment of the ubiquitin-proteasome system causes dopaminergic cell death and inclusion body formation in ventral mesencephalic cultures

Mutations in alpha-synuclein, parkin and ubiquitin C-terminal hydrolase L1, and defects in 26/20S proteasomes, cause or are associated with the development of familial and sporadic Parkinson's disease (PD). This suggests that failure of the ubiquitin-proteasome system (UPS) to degrade abnormal proteins may underlie nigral degeneration and Lewy body formation that occur in PD. To explore this concept, we studied the effects of lactacystin-mediated inhibition of 26/20S proteasomal function and ubiquitin aldehyde (UbA)-induced impairment of ubiquitin C-terminal hydrolase (UCH) activity in fetal rat ventral mesencephalic cultures. We demonstrate that both lactacystin and UbA caused concentration-dependent and preferential degeneration of dopaminergic neurons. Inhibition of 26/20S proteasomal function was accompanied by the accumulation of alpha-synuclein and ubiquitin, and the formation of inclusions that were immunoreactive for these proteins, in the cytoplasm of VM neurons. Inhibition of UCH was associated with a loss of ubiquitin immunoreactivity in the cytoplasm of VM neurons, but there was a marked and localized increase in alpha-synuclein staining which may represent the formation of inclusions bodies in VM neurons. These findings provide direct evidence that impaired protein clearance can induce dopaminergic cell death and the formation of proteinaceous inclusion bodies in VM neurons. This study supports the concept that defects in the UPS may underlie nigral pathology in familial and sporadic forms of PD.     

Ubiquitin–Proteasome System Impairment and MPTP-Induced Oxidative Stress in the Brain of C57BL/6 Wild-type and GSTP Knockout Mice

The ubiquitin–proteasome system (UPS) is the primary proteolytic complex responsible for the elimination of damaged and misfolded intracellular proteins, often formed upon oxidative stress. Parkinson’s disease (PD) is neuropathologically characterized by selective death of dopaminergic neurons in the substantia nigra (SN) and accumulation of intracytoplasmic inclusions of aggregated proteins. Along with mitochondrial dysfunction and oxidative stress, defects in the UPS have been implicated in PD. Glutathione S-transferase pi (GSTP) is a phase II detoxifying enzyme displaying important defensive roles against the accumulation of reactive metabolites that potentiate the aggression of SN neuronal cells, by regulating several processes including S-glutathionylation, modulation of glutathione levels and control of kinase-catalytic activities. In this work we used C57BL/6 wild-type and GSTP knockout mice to elucidate the effect of both MPTP and MG132 in the UPS function and to clarify if the absence of GSTP alters the response of this pathway to the neurotoxin and proteasome inhibitor insults. Our results demonstrate that different components of the UPS have different susceptibilities to oxidative stress. Importantly, when compared to the wild-type, GSTP knockout mice display decreased ubiquitination capacity and overall increased susceptibility to UPS damage and inactivation upon MPTP-induced oxidative stress.     

Endogenous dopamine (DA) renders dopaminergic cells vulnerable to challenge of proteasome inhibitor MG132

This study demonstrated that dopaminergic MN9D and PC12 cells were more vulnerable than non-dopaminergic N2A cells to the challenge by proteasome inhibitor MG132, which could be alleviated by reductants and alpha-methyl tyrosine (alpha-MT), a specific tyrosine hydroxylase inhibitor. Furthermore, challenging non-dopaminergic N2A cells with exogenous DA could aggravate MG132-induced cell viability decrease, which could be abrogated by reductants but not by alpha-MT. It was observed that alpha-MT could decrease endogenous DA content in dopaminergic MN9D and PC12 cells while N2A cells could take in exogenous DA into cytosol. The endogenous DA in dopaminergic cells was demonstrated to inhibit proteasome activity in the cells and further sensitize the proteasome to MG132 inhibition. In addition, the endogenous DA was also implicated for the increased level of lipid peroxidation and ubiquitinated proteins as well as inclusion bodies formation when non-dopaminergic cells were challenged with exogenous DA. Taken together it is proposed that endogenous DA in dopaminergic neurons could promote selective dopaminergic neurodegeneration, especially under the conditions of exopathic or idiopathic defects of ubiquitin–proteasome system (UPS), which may be abolished by reductant remedy.     

Endogenous dopamine (DA) renders dopaminergic cells vulnerable to challenge of proteasome inhibitor MG132

This study demonstrated that dopaminergic MN9D and PC12 cells were more vulnerable than non-dopaminergic N2A cells to the challenge by proteasome inhibitor MG132, which could be alleviated by reductants and alpha-methyl tyrosine (alpha-MT), a specific tyrosine hydroxylase inhibitor. Furthermore, challenging non-dopaminergic N2A cells with exogenous DA could aggravate MG132-induced cell viability decrease, which could be abrogated by reductants but not by alpha-MT. It was observed that alpha-MT could decrease endogenous DA content in dopaminergic MN9D and PC12 cells while N2A cells could take in exogenous DA into cytosol. The endogenous DA in dopaminergic cells was demonstrated to inhibit proteasome activity in the cells and further sensitize the proteasome to MG132 inhibition. In addition, the endogenous DA was also implicated for the increased level of lipid peroxidation and ubiquitinated proteins as well as inclusion bodies formation when non-dopaminergic cells were challenged with exogenous DA. Taken together it is proposed that endogenous DA in dopaminergic neurons could promote selective dopaminergic neurodegeneration, especially under the conditions of exopathic or idiopathic defects of ubiquitin-proteasome system (UPS), which may be abolished by reductant remedy.     

Are heat shock proteins therapeutic target for Parkinson's disease?

Heat shock proteins (HSPs), known as molecular chaperone to assist protein folding, have recently become a research focus in Parkinson's disease (PD) because the pathogenesis of this disease is highlighted by the intracellular protein misfolding and inclusion body formation. The present review will focus on the functions of different HSPs and their protective roles in PD. It is postulated that HSPs may serve as protein folding machinery and work together with ubiquitin-proteasome system (UPS) to assist in decomposing aberrant proteins. Failure of UPS is thought to play a key role in the pathogenesis of PD. In addition, HSPs may possess anti-apoptotic effects and keep the homeostasis of dopaminergic neurons against stress conditions. The critical role of HSPs and recent discovery of some novel HSPs inducers suggest that HSPs may be potential therapeutic targets for PD and other neurodegenerative disorders.     

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.     

Neuroinflammation and oxidative stress: Co-conspirators in the pathology of Parkinson’s disease

Parkinson’s disease (PD) is a complex disease, with genetics and environment contributing to the disease onset. Recent studies of causative PD genes have confirmed the involvement of cellular mechanisms engaged in mitochondrial and UPS dysfunction, oxidative stress and apoptosis in the progressive degeneration of the dopaminergic neurons in PD. In addition, clinical, epidemiological and experimental evidence has implicated neuroinflammation in the disease progression. This review will discuss neuroinflammation in PD, with particular focus on the genetic and toxin-based models of the disease. These studies have confirmed elevated oxidative stress and the pro-inflammatory response occurs early in the disease and these processes contribute to and/or exacerbate the nigro-striatal degeneration. In addition, the experimental models discussed here have also provided strong evidence that these pathways are an important link between the familial and sporadic causes of PD. The potential application of anti-inflammatory interventions in limiting the dopaminergic neuronal cell death in these models is discussed with evidence suggesting that the further investigation of their use as part of multi-targeted clinical trials is warranted.     

Molecular mechanisms of neurodegeneration in Parkinson's disease: clues from Mendelian syndromes

The recent identification and characterization of gene products responsible for familial forms of Parkinson disease (PD) have provided significant insights into the pathogenesis of PD. Collectively, these studies point towards ubiquitin-proteasome system (UPS) dysfunction as an underlying mechanism responsible for dopaminergic cell death in PD. Emerging evidence further indicates a complex interplay between UPS derangements and other PD pathogenetic factors, all interwoven in an integrated network leading to dopaminergic cell death in PD. Taken together, these findings suggest that neuronal degeneration in PD is a result of a cascade of events, rather than a primary pathogenic event. Here, we review the clues uncovered from various Mendelian-inherited forms of PD that have helped shaped our understanding of the molecular mechanisms underlying PD pathogenesis.     

Role of the β<Subscript>3</Subscript>-adrenergic receptor subtype in catecholamine-induced myocardial remodeling

Alpha-synuclein (α-synuclein) aggregation and impairment of the Ubiquitin proteasome system (UPS) are implicated in Parkinson’s disease (PD) pathogenesis. While zinc (Zn) induces dopaminergic neurodegeneration resulting in PD phenotype, its effect on protein aggregation and UPS has not yet been deciphered. The current study investigated the role of α-synuclein aggregation and UPS in Zn-induced Parkinsonism. Additionally, levodopa (l-Dopa) response was assessed in Zn-induced Parkinsonian model to establish its closeness with idiopathic PD. Male Wistar rats were treated with zinc sulfate (Zn; 20 mg/kg; i.p.) twice weekly for 12 weeks along with respective controls. In few subsets, animals were subsequently treated with l-Dopa for 21 consecutive days following Zn exposure. A significant increase in total and free Zn content was observed in the substantia nigra of the brain of exposed groups. Zn treatment caused neurobehavioral anomalies, striatal dopamine decline, and dopaminergic neuronal cell loss accompanied with a marked increase in α-synuclein expression/aggregation and Ubiquitin-conjugated protein levels in the exposed groups. Zn exposure substantially reduced UPS-associated trypsin-like, chymotrypsin-like, and caspase-like activities along with the expression of SUG1 and β-5 subunits of UPS in the nigrostriatal tissues of exposed groups. l-Dopa treatment rescued from Zn-induced neurobehavioral deficits and restored dopamine levels towards normalcy; however, Zn-induced dopaminergic neuronal loss, reduction in tyrosine hydroxylase expression, and increase in oxidative stress were unaffected. The results suggest that Zn caused UPS impairment, resulting in α-synuclein aggregation subsequently leading to dopaminergic neurodegeneration, and that Zn-induced Parkinsonism exhibited positive l-Dopa response similar to sporadic PD.     

Impaired ubiquitin-proteasome system activity in the synapses of Huntington's disease mice

Huntington's disease (HD) is caused by the expansion of a polyglutamine tract in the N-terminal region of huntingtin (htt) and is characterized by selective neurodegeneration. In addition to forming nuclear aggregates, mutant htt accumulates in neuronal processes as well as synapses and affects synaptic function. However, the mechanism for the synaptic toxicity of mutant htt remains to be investigated. We targeted fluorescent reporters for the ubiquitin-proteasome system (UPS) to presynaptic or postsynaptic terminals of neurons. Using these reporters and biochemical assays of isolated synaptosomes, we found that mutant htt decreases synaptic UPS activity in cultured neurons and in HD mouse brains that express N-terminal or full-length mutant htt. Given that the UPS is a key regulator of synaptic plasticity and function, our findings offer insight into the selective neuronal dysfunction seen in HD and also establish a method to measure synaptic UPS activity in other neurological disease models.     

Regeneration of dopaminergic neurons after 6-hydroxydopamine-induced lesion in planarian brain

Planarians have robust regenerative ability dependent on X-ray-sensitive pluripotent stem cells, called neoblasts. Here, we report that planarians can regenerate dopaminergic neurons after selective degeneration of these neurons caused by treatment with a dopaminergic neurotoxin (6-hydroxydopamine; 6-OHDA). This suggests that planarians have a system to sense the degeneration of dopaminergic neurons and to recruit stem cells to produce dopaminergic neurons to recover brain morphology and function. We confirmed that X-ray-irradiated planarians do not regenerate brain dopaminergic neurons after 6-OHDA-induced lesioning, suggesting that newly generated dopaminergic neurons are indeed derived from pluripotent stem cells. However, we found that the majority of regenerated dopaminergic neurons were 5-bromo-2'-deoxyuridine-negative cells. Therefore, we carefully analyzed when proliferating stem cells became committed to become dopaminergic neurons during regeneration by a combination of 5-bromo-2'-deoxyuridine pulse-chase experiments, immunostaining/in situ hybridization, and 5-fluorouracil treatment. The results strongly suggested that G(2) -phase stem cells become committed to dopaminergic neurons in the mesenchymal space around the brain, after migration from the trunk region following S-phase. These new findings obtained from planarian regeneration provide hints about how to conduct cell-transplantation therapy for future regenerative medicine.     

The role of ubiquitin linkages on α-synuclein induced-toxicity in a Drosophila model of Parkinson's disease

Parkinson's disease (PD) is a common movement disorder marked by the loss of dopaminergic (DA) neurons in the brain stem and the presence of intraneuronal inclusions designated as Lewy bodies (LB). The cause of neurodegeneration in PD is not clear, but it has been suggested that protein misfolding and aggregation contribute significantly to the development of the disease. Misfolded and aggregated proteins are cleared by ubiquitin proteasomal system (UPS) and autophagy lysosomal pathway (ALP). Recent studies suggested that different types of ubiquitin linkages can modulate these two pathways in the process of protein degradation. In this study, we found that co-expression of ubiquitin can rescue neurons from α-syn-induced neurotoxicity in a Drosophila model of PD. This neuroprotection is dependent on the formation of lysine 48 polyubiquitin linkage which is known to target protein degradation via the proteasome. Consistent with our results that we observed in vivo , we found that ubiquitin co-expression in the cell can facilitate cellular protein degradation by the proteasome in a lysine 48 polyubiquitin-dependent manner. Taken together, these results suggest that facilitation of proteasomal protein degradation can be a potential therapeutic approach for PD.     

Ultrastructural characterization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced cell death in embryonic dopaminergic neurons

Developing neuronal populations undergo significant attrition by natural cell death. Dopaminergic neurons in the substantia nigra pars compacta undergo apoptosis during synaptogenesis. Following this time window, destruction of the anatomic target of dopaminergic neurons results in dopaminergic cell death but the morphology is no longer apoptotic. We describe ultrastructural changes that appear unique to dying embryonic dopaminergic neurons. In primary cultures of mesencephalon, death of dopaminergic neurons is triggered by activation of glutamate receptors sensitive to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and differs ultrastructurally from both neuronal apoptosis or typical excitotoxicity. AMPA causes morphological changes selectively in dopaminergic neurons, without affecting other neurons in the same culture dishes. Two hours after the onset of treatment swelling of Golgi complexes is apparent. At 3 h, dopaminergic neurons display loss of membrane asymmetry (coinciding with commitment to die), as well as nuclear membrane invagination, irregular aggregation of chromatin, and mitochondrial swelling. Nuclear changes continue to worsen until loss of cytoplasmic structures and cell death begins to occur after 12 h. These changes are different from those described in neurons undergoing either apoptosis or excitotoxic death, but are similar to ultrastructural changes observed in spontaneous death of dopaminergic neurons in the natural mutant weaver mouse.     

Proteasome activator enhances survival of Huntington's disease neuronal model cells

In patients with Huntington's disease (HD), the proteolytic activity of the ubiquitin proteasome system (UPS) is reduced in the brain and other tissues. The pathological hallmark of HD is the intraneuronal nuclear protein aggregates of mutant huntingtin. We determined how to enhance UPS function and influence catalytic protein degradation and cell survival in HD. Proteasome activators involved in either the ubiquitinated or the non-ubiquitinated proteolysis were overexpressed in HD patients' skin fibroblasts or mutant huntingtin-expressing striatal neurons. Following compromise of the UPS, overexpression of the proteasome activator subunit PA28gamma, but not subunit S5a, recovered proteasome function in the HD cells. PA28gamma also improved cell viability in mutant huntingtin-expressing striatal neurons exposed to pathological stressors, such as the excitotoxin quinolinic acid and the reversible proteasome inhibitor MG132. These results demonstrate the specific functional enhancements of the UPS that can provide neuroprotection in HD cells.     

The Role of Glutathione in Dopaminergic Neuronal Survival

Abstract: An increased production of reactive oxygen species is thought to be critical to the pathogenesis of Parkinson's disease. At autopsy, patients with either presymptomatic or symptomatic Parkinson's disease have a decreased level of glutathione in the substantia nigra pars compacta. This change represents the earliest index of oxidative stress in Parkinson's disease discovered to this point. This study compares the sensitivity of dopaminergic and nondopaminergic neurons in dissociated mesencephalic cultures to the depletion of glutathione. We have found that dopaminergic neurons are more resistant to the toxicity of glutathione depletion than nondopaminergic neurons. The possibility that dopaminergic neurons have a higher baseline glutathione level than nondopaminergic neurons is suggested by measurements of levels of cellular glutathione in a parallel system of immortalized embryonic dopaminergic and nondopaminergic cell lines. We also examined the role of glutathione in 1-methyl-4-phenylpyridinium toxicity. Decreasing the glutathione level of dopaminergic neurons potentiates their susceptibility to 1-methyl-4-phenylpyridinium toxicity, although 1-methyl-4-phenylpyridinium does not deplete glutathione from primary mesencephalic cultures. Our data suggest that although a decreased glutathione content is not likely to be the sole cause of dopaminergic neuronal loss in Parkinson's disease, decreased glutathione content may act in conjunction with other factors such as 1-methyl-4-phenylpyridinium to cause the selective death of dopaminergic neurons.     

Protein turnover and inclusion body formation

In a recent study, we investigated the relationship between inclusion body (IB) formation and the activity of the ubiquitin-

proteasome system (UPS) in a primary neuron model of Huntington disease. We followed individual neurons over the

course of days and monitored the level of mutant huntingtin (htt) (which causes Huntington disease), IB formation, UPS function,

and neuronal toxicity. The accumulation of UPS substrates and neuronal toxicity increased with increasing levels of proteasome

inhibition. The UPS was more impaired in neurons that subsequently formed IBs than in those that did not; however, after IBs

formed, UPS function improved. These findings suggest that IB formation is a protective cellular response mediated in part by

increased degradation of intracellular protein.     

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).     

Neuroprotective effects of Astragaloside IV in 6-hydroxydopamine-treated primary nigral cell culture

Parkinson's disease (PD) is caused by a progressive degeneration of dopaminergic neurons in the substantia nigra. Oxidative stress and neural degeneration are suggested to be involved in the pathogenesis of Parkinson's disease. In the present study, Astragaloside IV (AS-IV) extracted from the dried root of Astragalus membranaceus, a well-known Chinese medicine used for the treatment of neurodegenerative diseases, was investigated for its capacity to protect dopaminergic neurons in experimental Parkinson's disease. By examining the effect of AS-IV on 6-hydroxydopamine (6-OHDA)-induced loss of dopaminergic neurons in primary nigral culture, we found that AS-IV pretreatment significantly and dose-dependently attenuated 6-OHDA-induced loss of dopaminergic neurons. Neuronal fiber length studies showed that massive neuronal cell death with degenerated neurons was observed in those cultures incubated with 6-OHDA, whereas in AS-IV co-treatments most dopaminergic neurons were seen to be intact and sprouting. In flow cytometric analysis, AS-IV resulted in a marked and dose-dependent rescue in tyrosine hydrolase (TH)-immunopositive cells from 6-OHDA-induced degeneration of dopaminergic neurons. Double immunofluorescence revealed that AS-IV treatment alone at concentrations of 100 and 200 μM increased the level of TH and NOS (nitrite oxide synthase) immunoreactivities; however, the protective effect of AS-IV on TH and NOS immunopositive cells in 6-OHDA treated nigral cell cultures was only seen at a concentration of 100 μM. These findings show that AS-IV can protect dopaminergic neurons against 6-OHDA-induced degeneration. Besides the neuroprotective effect, AS-IV alone promoted neurite outgrowth and increased TH and NOS immunoreactive of dopaminergic neurons. The neuroprotective and neurosprouting effects of AS-IV are specific for dopaminergic neurons and it has therapeutic potential in the treatment of PD.