Proteasome inhibition and Parkinson's disease modeling

A Systematic Review of Various In-vivo Screening Models as well as the Mechanisms Involved in Parkinson's Disease Screening Procedures

BACKGROUND

Parkinson's disease is the second most common neurological ailment. It is also known that it affects practically all other brain components, although only gradually. Animal models are mostly used to test the efficacy of treatment against a specific enzyme and aid in creating a new drug dose.

OBJECTIVE

The purpose of this review is to highlight in vivo Parkinson's disease screening approaches, as well as the mechanism of action of each drug involved in Parkinson's disease development, and discuss the limitations of each model. In addition, it also sheds light on Parkinson's disease genetic models.

METHODS

The data for the publication was gathered from databases, such as PubMed, Bentham Science, Elsevier, Springer Nature, Wiley, and Research Gate, after a thorough examination of diverse research findings linked to Parkinson's disease and its screening models.

RESULTS

Each chemical or drug has a unique mechanism for causing disease, whether through the production of reactive oxygen species or the blockage of the dopamine receptor. Almost every disease symptom, whether physical or behavioral, is covered by each of the constructed models' unique set of indicators and symptoms.

CONCLUSION

Animal models are typically used to assess a medicine's activity against a specific enzyme and aid in the creation of a new drug dose. The process, restrictions, and mechanisms interfering with the screening, as well as the level of animal suffering, must all be thoroughly reviewed before any model for screening for Parkinson's disease can be implemented.

Neuroprotective Effects of Sodium Butyrate and Monomethyl Fumarate Treatment through GPR109A Modulation and Intestinal Barrier Restoration on PD Mice

Research has connected Parkinson's disease (PD) with impaired intestinal barrier. The activation of G-protein-coupled receptor 109A (GPR109A) protects the intestinal barrier by inhibiting the NF-κB signaling pathway. Sodium butyrate (NaB), which is a GPR109A ligand, may have anti-PD effects. The current study's objective is to demonstrate that NaB or monomethyl fumarate (MMF, an agonist of the GPR109A) can treat PD mice induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) via repairing the intestinal barrier. Male C57BL/6J mice were divided into four groups randomly: control, MPTP + vehicle, MPTP + NaB, and MPTP + MMF. Modeling mice received MPTP (20 mg/kg/day, i.p.) for a week, while control mice received sterile PBS. Then, four groups each received two weeks of sterile PBS (10 mL/kg/day, i.g.), sterile PBS (10 mL/kg/day, i.g.), NaB (600 mg/kg/day, i.g.), or MMF (100 mg/kg/day, i.g.). We assessed the expression of tight junction (TJ) proteins (occludin and claudin-1), GPR109A, and p65 in the colon, performed microscopic examination via HE staining, quantified markers of intestinal permeability and proinflammatory cytokines in serum, and evaluated motor symptoms and pathological changes in the substantia nigra (SN) or striatum. According to our results, MPTP-induced defected motor function, decreased dopamine and 5-hydroxytryptamine levels in the striatum, decreased tyrosine hydroxylase-positive neurons and increased activated microglia in the SN, and systemic inflammation were ameliorated by NaB or MMF treatment. Additionally, the ruined intestinal barrier was also rebuilt and NF-κB was suppressed after the treatment, with higher levels of TJ proteins, GPR109A, and decreased intestinal permeability. These results show that NaB or MMF can remedy motor symptoms and pathological alterations in PD mice by restoring the intestinal barrier with activated GPR109A. We demonstrate the potential for repairing the compromised intestinal barrier and activating GPR109A as promising treatments for PD.

Exploring the Role of Ubiquitin-Proteasome System in Parkinson's Disease

Over the last decade, researchers have discovered that  a group of apparently unrelated neurodegenerative disorders, such as Parkinson's disease, have remarkable cellular and molecular biology similarities. Protein misfolding and aggregation are involved in all of the neurodegenerative conditions; as a result, inclusion bodies aggregation starts in the cells. Chaperone proteins and ubiquitin (26S proteasome's proteolysis signal), which aid in refolding misfolded proteins, are frequently found in these aggregates. The discovery of disease-causing gene alterations that code for multiple ubiquitin-proteasome pathway proteins in Parkinson's disease has strengthened the relationship between the ubiquitin-proteasome system and neurodegeneration. The specific molecular linkages between these systems and pathogenesis, on the other hand, are unknown and controversial. We outline the current level of knowledge in this article, focusing on important unanswered problems.

Fluc-EGFP reporter mice reveal differential alterations of neuronal proteostasis in aging and disease

The cellular protein quality control machinery is important for preventing protein misfolding and aggregation. Declining protein homeostasis (proteostasis) is believed to play a crucial role in age‐related neurodegenerative disorders. However, how neuronal proteostasis capacity changes in different diseases is not yet sufficiently understood, and progress in this area has been hampered by the lack of tools to monitor proteostasis in mammalian models. Here, we have developed reporter mice for in vivo analysis of neuronal proteostasis. The mice express EGFP‐fused firefly luciferase (Fluc‐EGFP), a conformationally unstable protein that requires chaperones for proper folding, and that reacts to proteotoxic stress by formation of intracellular Fluc‐EGFP foci and by reduced luciferase activity. Using these mice, we provide evidence for proteostasis decline in the aging brain. Moreover, we find a marked reaction of the Fluc‐EGFP sensor in a mouse model of tauopathy, but not in mouse models of Huntington’s disease. Mechanistic investigations in primary neuronal cultures demonstrate that different types of protein aggregates have distinct effects on the cellular protein quality control. Thus, Fluc‐EGFP reporter mice enable new insights into proteostasis alterations in different diseases.

Seven Solutions for Neuroprotection in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by loss of dopaminergic neurons in the substantia nigra and accumulation of iron and alpha-synuclein; it follows a characteristic pattern throughout the nervous system. Despite decades of successful preclinical neuroprotective studies, no drug has then shown efficacy in clinical trials. Considering this dilemma, we have reviewed and organized solutions of varying importance that can be exclusive or additive, and we outline approaches to help generate successful development of neuroprotective drugs for PD: (1) select patients in which the targeted mechanism is involved in the pathological process associated with the monitoring of target engagement, (2) combine treatments that target multiple pathways, (3) establish earliest interventions and develop better prodromal biomarkers, (4) adopt rigorous methodology and specific disease-relevant designs for disease-modifying clinical trials, (5) customize drug with better brain biodistribution, (6) prioritize repurposed drugs as a first line approach, and (7) adapt preclinical models to the targeted mechanisms with translational biomarkers to increase their predictive value. © 2020 International Parkinson and Movement Disorder Society.

Reporter-Based Screens for the Ubiquitin/Proteasome System

Instant and adequate handling of misfolded or otherwise aberrant proteins is of paramount importance for maintaining protein homeostasis in cells. The ubiquitin/proteasome system (UPS) is a central player in protein quality control as it operates in a seek-and-destroy mode, thereby facilitating elimination of faulty proteins. While proteasome inhibition is in clinical use for the treatment of hematopoietic malignancies, stimulation of the UPS has been proposed as a potential therapeutic strategy for various neurodegenerative disorders. High-throughput screens using genetic approaches or compound libraries are powerful tools to identify therapeutic intervention points and novel drugs. Unlike assays that measure specific activities of components of the UPS, reporter substrates provide us with a more holistic view of the general functional status of the UPS in cells. As such, reporter substrates can reveal new ways to obstruct or stimulate this critical proteolytic pathway. Here, we discuss various reporter substrates for the UPS and their application in the identification of key players and the pursuit for novel therapeutics.

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.

No Country for Old Worms: A Systematic Review of the Application of C. elegans to Investigate a Bacterial Source of Environmental Neurotoxicity in Parkinson's Disease

While progress has been made in discerning genetic associations with Parkinson's disease (PD), identifying elusive environmental contributors necessitates the application of unconventional hypotheses and experimental strategies. Here, we provide an overview of studies that we conducted on a neurotoxic metabolite produced by a species of common soil bacteria, Streptomyces venezuelae (S. ven), indicating that the toxicity displayed by this bacterium causes stress in diverse cellular mechanisms, such as the ubiquitin proteasome system and mitochondrial homeostasis. This dysfunction eventually leads to age and dose-dependent neurodegeneration in the nematode Caenorhabditis elegans. Notably, dopaminergic neurons have heightened susceptibility, but all of the neuronal classes eventually degenerate following exposure. Toxicity further extends to human SH-SY5Y cells, which also degenerate following exposure. Additionally, the neurons of nematodes expressing heterologous aggregation-prone proteins display enhanced metabolite vulnerability. These mechanistic analyses collectively reveal a unique metabolomic fingerprint for this bacterially-derived neurotoxin. In considering that epidemiological distinctions in locales influence the incidence of PD, we surveyed soils from diverse regions of Alabama, and found that exposure to ~30% of isolated Streptomyces species caused worm dopaminergic neurons to die. In addition to aging, one of the few established contributors to PD appears to be a rural lifestyle, where exposure to soil on a regular basis might increase the risk of interaction with bacteria producing such toxins. Taken together, these data suggest that a novel toxicant within the Streptomyces genus might represent an environmental contributor to the progressive neurodegeneration that is associated with PD.

Inhibitory effect of thiacremonone on MPTP-induced dopaminergic neurodegeneration through inhibition of p38 activation

Neuroinflammation is implicated for dopaminergic neurodegeneration. Sulfur compounds extracted from garlic have been shown to have anti-inflammatory properties. Previously, we have investigated that thiacremonone, a sulfur compound isolated from garlic has anti-inflammatory effects on several inflammatory disease models. To investigate the protective effect of thiacremonone against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced behavioral impairment and dopaminergic neurodegeneration, 8 week old ICR mice were given thiacremonone (10 mg/kg) in drinking water for 1 month and received intraperitoneal injection of MPTP (15 mg/kg, four times with 2 h interval) during the last 7 days of treatment. Our data showed that thiacremonone decreased MPTP-induced behavioral impairments (Rotarod test, Pole test, and Gait test), dopamine depletion and microglia and astrocytes activations as well as neuroinflammation. Higher activation of p38 was found in the substantia nigra and striatum after MPTP injection, but p38 activation was reduced in thiacremonone treated group. In an in vitro study, thiacremonone (1, 2, and 5 μg/ml) effectively decreased MPP+ (0.5 mM)-induced glial activation, inflammatory mediators generation and dopaminergic neurodegeneration in cultured astrocytes and microglial BV-2 cells. Moreover, treatment of p38 MAPK inhibitor SB203580 (10 μM) further inhibited thiacremonone induced reduction of neurodegeneration and neuroinflammation. These results indicated that the anti-inflammatory compound, thiacremonone, inhibited neuroinflammation and dopaminergic neurodegeneration through inhibition of p38 activation.

Parkinson's disease: experimental models and reality

Parkinson's disease (PD) is a chronic, progressive movement disorder of adults and the second most common neurodegenerative disease after Alzheimer's disease. Neuropathologic diagnosis of PD requires moderate-to-marked neuronal loss in the ventrolateral substantia nigra pars compacta and α-synuclein (αS) Lewy body pathology. Nigrostriatal dopaminergic neurodegeneration correlates with the Parkinsonian motor features, but involvement of other peripheral and central nervous system regions leads to a wide range of non-motor features. Nigrostriatal dopaminergic neurodegeneration is shared with other parkinsonian disorders, including some genetic forms of parkinsonism, but many of these disorders do not have Lewy bodies. An ideal animal model for PD, therefore, should exhibit age-dependent and progressive dopaminergic neurodegeneration, motor dysfunction, and abnormal αS pathology. Rodent models of PD using genetic or toxin based strategies have been widely used in the past several decades to investigate the pathogenesis and therapeutics of PD, but few recapitulate all the major clinical and pathologic features of PD. It is likely that new strategies or better understanding of fundamental disease processes may facilitate development of better animal models. In this review, we highlight progress in generating rodent models of PD based on impairments of four major cellular functions: mitochondrial oxidative phosphorylation, autophagy-lysosomal metabolism, ubiquitin-proteasome protein degradation, and endoplasmic reticulum stress/unfolded protein response. We attempt to evaluate how impairment of these major cellular systems contribute to PD and how they can be exploited in rodent models. In addition, we review recent cell biological studies suggesting a link between αS aggregation and impairment of nuclear membrane integrity, as observed during cellular models of apoptosis. We also briefly discuss the role of incompetent phagocytic clearance and how this may be a factor to consider in developing new rodent models of PD.

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.

α-Synuclein and nonhuman primate models of Parkinson's disease

Accumulation of α-synuclein (α-syn) leading to the formation of insoluble intracellular aggregates named Lewy bodies is proposed to have a significant role in Parkinson's disease (PD) pathology. Nonhuman primate (NHP) models of PD have proven essential for understanding the neurobiological basis of the disease and for the preclinical evaluation of first-in-class and invasive therapies. In addition to neurotoxin, aging and intracerebral gene transfer models, a new generation of models using inoculations of α-syn formulations, as well as transgenic methods is emerging. Understanding of their advantages and limitations will be essential when choosing a platform to evaluate α-syn-related pathology and interpreting the test results of new treatments targeting α-syn aggregation. In this review we aim to provide insight on this issue by critically analyzing the differences in endogenous α-syn, as well as α-syn pathology in PD and PD NHP models.

Inhibition of Protein Ubiquitination by Paraquat and 1-Methyl-4-Phenylpyridinium Impairs Ubiquitin-Dependent Protein Degradation Pathways

Intracytoplasmic inclusions of protein aggregates in dopaminergic cells (Lewy bodies) are the pathological hallmark of Parkinson's disease (PD). Ubiquitin (Ub), alpha (α)-synuclein, p62/sequestosome 1, and oxidized proteins are the major components of Lewy bodies. However, the mechanisms involved in the impairment of misfolded/oxidized protein degradation pathways in PD are still unclear. PD is linked to mitochondrial dysfunction and environmental pesticide exposure. In this work, we evaluated the effects of the pesticide paraquat (PQ) and the mitochondrial toxin 1-methyl-4-phenylpyridinium (MPP(+)) on Ub-dependent protein degradation pathways. No increase in the accumulation of Ub-bound proteins or aggregates was observed in dopaminergic cells (SK-N-SH) treated with PQ or MPP(+), or in mice chronically exposed to PQ. PQ decreased Ub protein content, but not its mRNA transcription. Protein synthesis inhibition with cycloheximide depleted Ub levels and potentiated PQ-induced cell death. The inhibition of proteasomal activity by PQ was found to be a late event in cell death progression and had neither effect on the toxicity of either MPP(+) or PQ, nor on the accumulation of oxidized sulfenylated, sulfonylated (DJ-1/PARK7 and peroxiredoxins), and carbonylated proteins induced by PQ. PQ- and MPP(+)-induced Ub protein depletion prompted the dimerization/inactivation of the Ub-binding protein p62 that regulates the clearance of ubiquitinated proteins by autophagy. We confirmed that PQ and MPP(+) impaired autophagy flux and that the blockage of autophagy by the overexpression of a dominant-negative form of the autophagy protein 5 (dnAtg5) stimulated their toxicity, but there was no additional effect upon inhibition of the proteasome. PQ induced an increase in the accumulation of α-synuclein in dopaminergic cells and membrane-associated foci in yeast cells. Our results demonstrate that the inhibition of protein ubiquitination by PQ and MPP(+) is involved in the dysfunction of Ub-dependent protein degradation pathways.

Hydrogen Sulfide and Neuroinflammation

The innate and adaptive immune system plays an important role in diverse forms of central nervous system (CNS) pathologies including neurodegenerative diseases and peripheral nerve injury. Evidence for an innate inflammatory response in Alzheimer's disease (AD) was described 20 years ago, and subsequent studies have documented roles of inflammation in Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), and a growing number of other CNS pathologies. Although inflammation may not be the initiating factor for neurodegenerative pathologies, experimental data suggests that persistent inflammatory responses involving microglia and astrocytes, as well as blood monocyte-derived macrophages, clearly contribute to disease progression. High levels of hydrogen sulfide exert toxic effects to CNS. On the other hand, low and physiological levels of H2S may have beneficial effects on number of tissues including CNS. For example, a number of studies have reported that H2S exerts anti-inflammatory and anti-apoptotic effects in CNS. In this chapter, studies related to the role of H2S in neuroinflammation and neurodegeneration will be reviewed and discussed. In particular, we will focus on the role of H2S in neuroinflammation associated with PD.

The ubiquitin-proteasome system in neurodegenerative diseases: precipitating factor, yet part of the solution

The ubiquitin-proteasome system (UPS) has been implicated in neurodegenerative diseases based on the presence of deposits consisting of ubiquitylated proteins in affected neurons. It has been postulated that aggregation-prone proteins associated with these disorders, such as α-synuclein, β-amyloid peptide, and polyglutamine proteins, compromise UPS function, and delay the degradation of other proteasome substrates. Many of these substrates play important regulatory roles in signaling, cell cycle progression, or apoptosis, and their inadvertent stabilization due to an overloaded and improperly functioning UPS may thus be responsible for cellular demise in neurodegeneration. Over the past decade, numerous studies have addressed the UPS dysfunction hypothesis using various model systems and techniques that differ in their readout and sensitivity. While an inhibitory effect of some disease proteins on the UPS has been demonstrated, increasing evidence attests that the UPS remains operative in many disease models, which opens new possibilities for treatment. In this review, we will discuss the paradigm shift that repositioned the UPS from being a prime suspect in the pathophysiology of neurodegeneration to an attractive therapeutic target that can be harnessed to accelerate the clearance of disease-linked proteins.

Pesticides that inhibit the ubiquitin-proteasome system: effect measure modification by genetic variation in SKP1 in Parkinson׳s disease

Cytoplasmic inclusions known as Lewy bodies, a hallmark of Parkinson's disease (PD) pathology, may protect against cytotoxic proteins. Since the ubiquitin-proteasome system (UPS) degrades cytotoxic proteins, dysfunction in the UPS may contribute to PD etiology. Our goal in this study was to screen pesticides for proteasome inhibition and investigate (i) whether ambient exposures to pesticides that inhibit the UPS increase PD risk and (ii) whether genetic variation in candidate genes of the UPS pathway modify those increased risks. We assessed 26S UPS activity in SK-N-MC(u) cells by fluorescence. We recruited idiopathic PD cases (n=360) and population-based controls (n=816) from three counties in California with considerable commercial agriculture. We determined ambient pesticide exposure by our validated GIS-based model utilizing residential and workplace address histories. We limited effect measure modification assessment to Caucasians (287 cases, 453 controls). Eleven of 28 pesticides we screened inhibited 26S UPS activity at 10 µM. Benomyl, cyanazine, dieldrin, endosulfan, metam, propargite, triflumizole, and ziram were associated with increased PD risk. We estimated an odds ratio of 2.14 (95% CI: 1.42, 3.22) for subjects with ambient exposure to any UPS-inhibiting pesticide at both residential and workplace addresses; this association was modified by genetic variation in the s-phase kinase-associated protein 1 gene (SKP1; interaction p-value=0.005). Our results provide evidence that UPS-inhibiting pesticides play a role in the etiology of PD and suggest that genetic variation in candidate genes involved in the UPS pathway might exacerbate the toxic effects of pesticide exposures.

Adaptive changes in autophagy after UPS impairment in Parkinson's disease

AIM

Ubiquitin-proteasome system (UPS) and autophagosome-lysosome pathway (ALP) are the most important machineries responsible for protein degradation in Parkinson's disease (PD). The aim of this study is to investigate the adaptive alterations in autophagy upon proteasome inhibition in dopaminergic neurons in vitro and in vivo.

METHODS

Human dopaminergic neuroblastoma SH-SY5Y cells were treated with the proteasome inhibitor lactacystin (5 μmol/L) for 5, 12, or 24 h. The expression of autophagy-related proteins in the cells was detected with immunoblotting. UPS-impaired mouse model of PD was established by microinjection of lactacystin (2 μg) into the left hemisphere of C57BL/6 mice that were sacrificed 2 or 4 weeks later. The midbrain tissues were dissected to assess alterations in autophagy using immunofluorescence, immunoblotting and electron microscopy assays.

RESULTS

Both in SH-SY5Y cells and in the midbrain of UPS-impaired mouse model of PD, treatment with lactacystin significantly increased the expression levels of LC3-I/II and Beclin 1, and reduced the levels of p-mTOR, mTOR and p62/SQSTM1. Furthermore, lactacystin treatment in UPS-impaired mouse model of PD caused significant loss of TH-positive neurons in the substantia nigra, and dramatically increased the number of autophagosomes in the left TH-positive neurons.

CONCLUSION

Inhibition of UPS by lactacystin in dopaminergic neurons activates another protein degradation system, the ALP, which includes both the mTOR signaling pathway and Beclin 1-associated pathway.

Morphological and metabolic changes in the nigro-striatal pathway of synthetic proteasome inhibitor (PSI)-treated rats: a MRI and MRS study

Systemic administration of a Synthetic Proteasome Inhibitor (PSI) in rats has been described as able to provide a model of Parkinson's disease (PD), characterized by behavioral and biochemical modifications, including loss of dopaminergic neurons in the substantia nigra (SN), as assessed by post-mortem studies. With the present study we aimed to assess in-vivo by Magnetic Resonance (MR) possible morphological and metabolic changes in the nigro-striatal pathway of PSI-treated rats. 10 animals were subcutaneously injected with PSI 6.0 mg/kg dissolved in DMSO 100%. Injections were made thrice weekly over the course of two weeks. 5 more animals injected with DMSO 100% with the same protocol served as controls. The animals underwent MR sessions before and at four weeks after the end of treatment with either PSI or vehicle. MR Imaging was performed to measure SN volume and Proton MR Spectroscopy ((1)H-MRS) was performed to measure metabolites changes at the striatum. Animals were also assessed for motor function at baseline and at 4 and 6 weeks after treatment. Dopamine and dopamine metabolite levels were measured in the striata at 6 weeks after treatment. PSI-treated animals showed volumetric reduction of the SN (p<0.02) at 4 weeks after treatment as compared to baseline. Immunofluorescence analysis confirmed MRI changes in SN showing a reduction of tyrosine hydroxylase expression as compared to neuron-specific enolase expression. A reduction of N-acetyl-aspartate/total creatine ratio (p = 0.05) and an increase of glutamate-glutamine-γ amminobutirrate/total creatine were found at spectroscopy (p = 0.03). At 6 weeks after treatment, PSI-treated rats also showed motor dysfunction compared to baseline (p = 0.02), accompanied by dopamine level reduction in the striatum (p = 0.02). Treatment with PSI produced morphological and metabolic modifications of the nigro-striatal pathway, accompanied by motor dysfunction. MR demonstrated to be a powerful mean to assess in-vivo the nigro-striatal pathway morphology and metabolism in the PSI-based PD animal model.

Pale body-like inclusion formation and neurodegeneration following depletion of 26S proteasomes in mouse brain neurones are independent of α-synuclein

Parkinson's disease (PD) is characterized by the progressive degeneration of substantia nigra pars compacta (SNpc) dopaminergic neurones and the formation of Lewy bodies (LB) in a proportion of the remaining neurones. α-synuclein is the main component of LB, but the pathological mechanisms that lead to neurodegeneration associated with LB formation remain unclear. Three pivotal elements have emerged in the development of PD: α-synuclein, mitochondria and protein degradation systems. We previously reported a unique model, created by conditional genetic depletion of 26S proteasomes in the SNpc of mice, which mechanistically links these three elements with the neuropathology of PD: progressive neurodegeneration and intraneuronal inclusion formation. Using this model, we tested the hypothesis that α-synuclein was essential for the formation of inclusions and neurodegeneration caused by 26S proteasomal depletion. We found that both of these processes were independent of α-synuclein. This provides an important insight into the relationship between the proteasome, α-synuclein, inclusion formation and neurodegeneration. We also show that the autophagy-lysosomal pathway is not activated in 26S proteasome-depleted neurones. This leads us to suggest that the paranuclear accumulation of mitochondria in inclusions in our model may reflect a role for the ubiquitin proteasome system in mitochondrial homeostasis and that neurodegeneration may be mediated through mitochondrial factors linked to inclusion biogenesis.

Medaka fish Parkinson's disease model

The teleost fish has been widely used in creating neurodegenerative models. Here we describe the teleost medaka fish Parkinson's disease (PD) models we developed using toxin treatment and genetic engineering. 1-Methyl-4-phenyl-1,2,3,4-tetrahydropyridine (MPTP), 6-hydroxydopamine (6-OHDA), proteasome inhibitors, lysosome inhibitors and tunicamycin treatment in our model fish replicated some salient features of PD: selective dopamine cell loss and reduced spontaneous movement with the last three toxins producing inclusion bodies ubiquitously in the brain. Despite the ubiquitous distribution of the inclusion bodies, the middle diencephalic dopaminergic neurons were particularly vulnerable to these toxins, supporting the idea that this dopamine cluster is similar to the human substantia nigra. PTEN-induced putative kinase 1 (PINK1) homozygous mutants also showed reduced spontaneous swimming movements. These data indicate that medaka fish can serve as a new model animal of PD. In this review we summarize our previous data and discuss future prospects.

Natural toxins implicated in the development of Parkinson's disease

Experimental models of Parkinson's disease (PD) are of great importance for improving the design of future clinical trials. Various neurotoxic models are available, including 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), paraquat and rotenone. However, no model is considered perfect; each has its own limitations. Based on epidemiological data, a new trend of using environmental toxins in PD modeling seems attractive and has dominated public discussions of the disease etiology. A search for new environmental toxin-based models would improve our knowledge of the pathology of the condition. Here, we discuss some toxins of natural origin (e.g. cycad-derived toxins, epoxomicin, Nocardia asteroides bacteria, Streptomyces venezuelae bacteria, annonacin and DOPAL) that possibly represent a contributory environmental component to PD.

Protein degradation pathways in Parkinson's disease: curse or blessing

Protein misfolding, aggregation and deposition are common disease mechanisms in many neurodegenerative diseases including Parkinson's disease (PD). Accumulation of damaged or abnormally modified proteins may lead to perturbed cellular function and eventually to cell death. Thus, neurons rely on elaborated pathways of protein quality control and removal to maintain intracellular protein homeostasis. Molecular chaperones, the ubiquitin-proteasome system (UPS) and the autophagy-lysosomal pathway (ALP) are critical pathways that mediate the refolding or removal of abnormal proteins. The successive failure of these protein degradation pathways, as a cause or consequence of early pathological alterations in vulnerable neurons at risk, may present a key step in the pathological cascade that leads to spreading neurodegeneration. A growing number of studies in disease models and patients have implicated dysfunction of the UPS and ALP in the pathogenesis of Parkinson's disease and related disorders. Deciphering the exact mechanism by which the different proteolytic systems contribute to the elimination of pathogenic proteins, like α-synuclein, is therefore of paramount importance. We herein review the role of protein degradation pathways in Parkinson's disease and elaborate on the different contributions of the UPS and the ALP to the clearance of altered proteins. We examine the interplay between different degradation pathways and provide a model for the role of the UPS and ALP in the evolution and progression of α-synuclein pathology. With regards to exciting recent studies we also discuss the putative potential of using protein degradation pathways as novel therapeutic targets in Parkinson's disease.

Systemic proteasome inhibition triggers neurodegeneration in a transgenic mouse model expressing human α-synuclein under oligodendrocyte promoter: implications for multiple system atrophy

Multiple system atrophy (MSA) is a progressive late onset neurodegenerative α-synucleinopathy with unclear pathogenesis. Recent genetic and pathological studies support a central role of α-synuclein (αSYN) in MSA pathogenesis. Oligodendroglial cytoplasmic inclusions of fibrillar αSYN and dysfunction of the ubiquitin–proteasome system are suggestive of proteolytic stress in this disorder. To address the possible pathogenic role of oligodendroglial αSYN accumulation and proteolytic failure in MSA we applied systemic proteasome inhibition (PSI) in transgenic mice with oligodendroglial human αSYN expression and determined the presence of MSA-like neurodegeneration in this model as compared to wild-type mice. PSI induced open field motor disability in transgenic αSYN mice but not in wild-type mice. The motor phenotype corresponded to progressive and selective neuronal loss in the striatonigral and olivopontocerebellar systems of PSI-treated transgenic αSYN mice. In contrast no neurodegeneration was detected in PSI-treated wild-type controls. PSI treatment of transgenic αSYN mice was associated with significant ultrastructural alterations including accumulation of fibrillar human αSYN in the cytoplasm of oligodendroglia, which resulted in myelin disruption and demyelination characterized by increased g-ratio. The oligodendroglial and myelin pathology was accompanied by axonal degeneration evidenced by signs of mitochondrial stress and dysfunctional axonal transport in the affected neurites. In summary, we provide new evidence supporting a primary role of proteolytic failure and suggesting a neurodegenerative pathomechanism related to disturbed oligodendroglial/myelin trophic support in the pathogenesis of MSA.

Adeno-associated virus type 2 vector-mediated glial cell line-derived neurotrophic factor gene transfer induces neuroprotection and neuroregeneration in a ubiquitin-proteasome system impairment animal model of Parkinson's disease

BACKGROUND

The impairment of the ubiquitin-proteasome system (UPS) is a cellular mechanism underlying the neurodegenerative process in Parkinson's disease (PD). A mouse model induced by the selective proteasome inhibitor lactacystin targeting on substantia nigra has been demonstrated to be valuable in investigating etiopathogenesis and neuroprotection for PD.

OBJECTIVE

In the present study, we used adeno-associated virus type 2 vector (AAV2) encoding glial cell line-derived neurotrophic factor (GDNF) injected into the striatum of this animal model to test the effectiveness and possible mechanisms of GDNF gene therapy.

RESULTS

Our results showed that AAV2-mediated GDNF gene therapy significantly attenuated lactacystin-induced loss of nigral dopamine (DA) neurons and striatal DA levels. Furthermore, we found that GDNF protein is mostly expressed in astrocytes in the subventricular zone (SVZ) and dentate gyrus (DG). AAV2-mediated GDNF therapy can induce neurogenesis in the SVZ and DG, and increase the number of nigral newborn DA neurons.

CONCLUSION

These data indicate that AAV2-mediated GDNF gene therapy can protect the nigral DA neurons from the UPS impairment-induced degeneration, which may partly result from the nigral DA neuron regeneration in the brain, and such experimental results may have implications for the treatment of PD.

A novel "molecular tweezer" inhibitor of α-synuclein neurotoxicity in vitro and in vivo

Aggregation of α-synuclein (α-syn) is implicated as being causative in the pathogenesis of Parkinson's disease, multiple system atrophy, and dementia with Lewy bodies. Despite several therapies that improve symptoms in these disorders, none slow disease progression. Recently, a novel "molecular tweezer" (MT) termed CLR01 has been described as a potent inhibitor of assembly and toxicity of multiple amyloidogenic proteins. Here we investigated the ability of CLR01 to inhibit assembly and toxicity of α-syn. In vitro, CLR01 inhibited the assembly of α-syn into β-sheet-rich fibrils and caused disaggregation of pre-formed fibrils, as determined by thioflavin T fluorescence and electron microscopy. α-Syn toxicity was studied in cell cultures and was completely mitigated by CLR01 when α-syn was expressed endogenously or added exogenously. To determine if CLR01 was also protective in vivo, we used a novel zebrafish model of α-syn toxicity (α-syn-ZF), which expresses human, wild-type α-syn in neurons. α-Syn-ZF embryos developed severe deformities due to neuronal apoptosis and most of them died within 48 to 72 h. CLR01 added to the water significantly improved zebrafish phenotype and survival, suppressed α-syn aggregation in neurons, and reduced α-syn-induced apoptosis. α-Syn expression was found to inhibit the ubiquitin proteasome system in α-syn-ZF neurons, resulting in further accumulation of α-syn. Treatment with CLR01 almost completely mitigated the proteasome inhibition. The data suggest that CLR01 is a promising therapeutic agent for the treatment of Parkinson's disease and other synucleinopathies.

What genetics tells us about the causes and mechanisms of Parkinson's disease

Parkinson's disease (PD) is a common motor disorder of mysterious etiology. It is due to the progressive degeneration of the dopaminergic neurons of the substantia nigra and is accompanied by the appearance of intraneuronal inclusions enriched in α-synuclein, the Lewy bodies. It is becoming increasingly clear that genetic factors contribute to its complex pathogenesis. Over the past decade, the genetic basis of rare PD forms with Mendelian inheritance, representing no more than 10% of the cases, has been investigated. More than 16 loci and 11 associated genes have been identified so far; genome-wide association studies have provided convincing evidence that polymorphic variants in these genes contribute to sporadic PD. The knowledge acquired of the functions of their protein products has revealed pathways of neurodegeneration that may be shared between inherited and sporadic PD. An impressive set of data in different model systems strongly suggest that mitochondrial dysfunction plays a central role in clinically similar, early-onset autosomal recessive PD forms caused by parkin and PINK1, and possibly DJ-1 gene mutations. In contrast, α-synuclein accumulation in Lewy bodies defines a spectrum of disorders ranging from typical late-onset PD to PD dementia and including sporadic and autosomal dominant PD forms due to mutations in SCNA and LRRK2. However, the pathological role of Lewy bodies remains uncertain, as they may or may not be present in PD forms with one and the same LRRK2 mutation. Impairment of autophagy-based protein/organelle degradation pathways is emerging as a possible unifying but still fragile pathogenic scenario in PD. Strengthening these discoveries and finding other convergence points by identifying new genes responsible for Mendelian forms of PD and exploring their functions and relationships are the main challenges of the next decade. It is also the way to follow to open new promising avenues of neuroprotective treatment for this devastating disorder.

Inhaled hydrogen sulfide prevents neurodegeneration and movement disorder in a mouse model of Parkinson's disease

Parkinson's disease is one of the major neurodegenerative disorders. Neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) can cause Parkinson's disease-like symptoms and biochemical changes in humans and animals. Hydrogen sulfide (H(2)S) has been shown to protect neurons. The goal of this study was to examine the effects of inhaled H(2)S in a mouse model of Parkinson's disease induced by MPTP. Male C57BL/6J mice received MPTP at 80 mg/kg and breathed air with or without 40 ppm H(2)S for 8 h/day for 7 days. Administration of MPTP induced movement disorder and decreased tyrosine hydroxylase (TH)-containing neurons in the substantia nigra and striatum in mice that breathed air. Inhalation of H(2)S prevented the MPTP-induced movement disorder and the degeneration of TH-containing neurons. Inhaled H(2)S also prevented apoptosis of the TH-containing neurons and gliosis in nigrostriatal region after administration of MPTP. The neuroprotective effect of inhaled H(2)S after MPTP administration was associated with upregulation of genes encoding antioxidant proteins, including heme oxygenase-1 and glutamate-cysteine ligase. These observations suggest that inhaled H(2)S prevents neurodegeneration in a mouse model of Parkinson's disease induced by MPTP, potentially via upregulation of antioxidant defense mechanisms and inhibition of inflammation and apoptosis in the brain.

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.

PINK1: one protein, multiple neuroprotective functions

Mutations in the PINK1 gene are the second most frequent cause of autosomal recessive parkinsonism after Parkin, and can represent a risk factor towards sporadic Parkinson’s disease. The PINK1 protein product has been implicated in several functions, mostly aimed at protecting neuronal cells against different types of stress. Growing evidence indicates that PINK1 interplays with other proteins mutated in familial forms of Parkinson’s disease, such as Parkin, DJ-1 and α-synuclein. These interactions contribute to the definition of a complex scenario in which the mainteinance of mitochondrial homeostasis and the clearance of misfolded proteins and damaged organelles represent key neuroprotective processes. In this review, we summarize the current knowledge on PINK1 characteristics and functions, and analyze the network of functional relationships that link PINK1 to other Parkinson’s disease-related proteins.