Parkin-deficient mice are not more sensitive to 6-hydroxydopamine or methamphetamine neurotoxicity

Redox Modulation of Mitochondrial Proteins in the Neurotoxicant Models of Parkinson's Disease

Significance: Mitochondrial proteins regulate the oxidative phosphorylation, cellular metabolism, and free radical generation. Redox modulation alters the mitochondrial proteins and instigates the damage to dopaminergic neurons. Toxicants contribute to Parkinson's disease (PD) pathogenesis in conjunction with aging and genetic factors. While oxidative modulation of a number of mitochondrial proteins is linked to xenobiotic exposure, little is known about its role in the toxicant-induced PD. Understanding the role of redox modulation of mitochondrial proteins in complex cellular events leading to neurodegeneration is highly relevant. Recent Advances: Many toxicants are shown to inhibit complex I or III and elicit free radical production that alters the redox status of mitochondrial proteins. Implication of redox modulation of the mitochondrial proteins makes them a target to comprehend the underlying mechanism of toxicant-induced PD. Critical Issues: Owing to multifactorial etiology, exploration of onset and progression and treatment outcomes needs a comprehensive approach. The article explains about a few mitochondrial proteins that undergo redox changes along with the promising strategies, which help to alleviate the toxicant-induced redox imbalance leading to neurodegeneration. Future Directions: Although mitochondrial proteins are linked to PD, their role in toxicant-induced parkinsonism is not yet completely known. Preservation of antioxidant defense machinery could alleviate the redox modulation of mitochondrial proteins. Targeted antioxidant delivery, use of metal chelators, and activation of nuclear factor erythroid 2-related factor 2, and combinational therapy that encounters multiple free radicals, could ameliorate the redox modulation of mitochondrial proteins and thereby PD progression.

Parkin-deficient rats are resistant to neurotoxicity of chronic high-dose methamphetamine

Methamphetamine (METH) is a highly addictive and powerful central nervous system psychostimulant with no FDA-approved pharmacotherapy. Parkin is a neuroprotective protein and its loss of function contributes to Parkinson's disease. This study used 3-month-old homozygous parkin knockout (PKO) rats to determine whether loss of parkin protein potentiates neurotoxicity of chronic METH to the nigrostriatal dopamine pathway. PKO rats were chronically treated with 10 mg/kg METH for 10 consecutive days and assessed for neurotoxicity markers in the striatum on the 5th and 10th day of withdrawal from METH. The PKO rats showed higher METH-induced hyperthermia; however, they did not display augmented deficits in dopaminergic and serotonergic neurotoxicity markers, astrocyte activation or decreased mitochondrial enzyme levels as compared to wild-type (WT) rats. Interestingly, saline-treated PKO rats had lower levels of dopamine (DA) as well as mitochondrial complex I and II levels while having increased basal levels of glial fibrillary acidic protein (GFAP), a marker of gliosis. These results indicate PKO display a certain resistance to METH neurotoxicity, possibly mediated by lowered DA levels and downregulated mitochondria.

Parkin interacts with Mitofilin to increase dopaminergic neuron death in response to Parkinson's disease-related stressors

Mitochondrial dysfunction plays a critical role in the pathophysiology of Parkinson's disease (PD). The inner mitochondrial membrane (IMM) protein, Mitofilin or Mic60, has been shown to play a key role in controlling and maintaining mitochondrial cristae morphology, and its dysregulation induces cyto-deleterious effects. Here, we investigated the mechanism underlying Mitofilin degradation in dopaminergic neuron death using N27-A cells, and Human Dopamine Neuronal Primary cells treated with PD stressors, Dopamine (DA) or Rotenone (Rot). We found that both PD stressors increased mitochondrial Parkin translocation and interaction with Mitofilin that promotes Mitofilin degradation via ubiquitination, which is responsible for reduced mitochondrial membrane potential and increased ROS production. These effects were concomitant with abnormal mitochondrial structure and increased neuronal death. DA-induced degradation of Mitofilin enhances mitochondrial calpain activity, increases the release of AIF into the cytosol, and promotes apoptosis via an AIF-PARP dependent mechanism. We found that Rot-treated cells exhibit excessive mitophagy, while DA does not trigger mitophagy. In addition, overexpressing USP30, a mitochondrial deubiquitinase, attenuated cell death induced by Rot, but not by DA-treated cells. Together, our study reveals the impact of Parkin-Mitofilin interaction in PD stressor-induced neurotoxicity, which leads to the degradation of Mitofilin, resulting in mitochondrial structural damage and dysfunction that is responsible for neuronal death by apoptosis via an AIF-PARP pathway.

On the Right Track to Treat Movement Disorders: Promising Therapeutic Approaches for Parkinson's and Huntington's Disease

Movement disorders are neurological conditions in which patients manifest a diverse range of movement impairments. Distinct structures within the basal ganglia of the brain, an area involved in movement regulation, are differentially affected for every disease. Among the most studied movement disorder conditions are Parkinson’s (PD) and Huntington’s disease (HD), in which the deregulation of the movement circuitry due to the loss of specific neuronal populations in basal ganglia is the underlying cause of motor symptoms. These symptoms are due to the loss principally of dopaminergic neurons of the substantia nigra (SN) par compacta and the GABAergic neurons of the striatum in PD and HD, respectively. Although these diseases were described in the 19th century, no effective treatment can slow down, reverse, or stop disease progression. Available pharmacological therapies have been focused on preventing or alleviating motor symptoms to improve the quality of life of patients, but these drugs are not able to mitigate the progressive neurodegeneration. Currently, considerable therapeutic advances have been achieved seeking a more efficacious and durable therapeutic effect. Here, we will focus on the new advances of several therapeutic approaches for PD and HD, starting with the available pharmacological treatments to alleviate the motor symptoms in both diseases. Then, we describe therapeutic strategies that aim to restore specific neuronal populations or their activity. Among the discussed strategies, the use of Neurotrophic factors (NTFs) and genetic approaches to prevent the neuronal loss in these diseases will be described. We will highlight strategies that have been evaluated in both Parkinson’s and Huntington’s patients, and also the ones with strong preclinical evidence. These current therapeutic techniques represent the most promising tools for the safe treatment of both diseases, specifically those aimed to avoid neuronal loss during disease progression.

GDNF/RET signaling in dopamine neurons in vivo

The glial cell line-derived neurotrophic factor (GDNF) and its canonical receptor Ret can signal both in tandem and separately to exert many vital functions in the midbrain dopamine system. It is known that Ret has effects on maintenance, physiology, protection and regeneration in the midbrain dopamine system, with the physiological functions of GDNF still somewhat unclear. Notwithstanding, Ret ligands, such as GDNF, are considered as promising candidates for neuroprotection and/or regeneration in Parkinson's disease, although data from clinical trials are so far inconclusive. In this review, we discuss the current knowledge of GDNF/Ret signaling in the dopamine system in vivo as well as crosstalk with pathology-associated proteins and their signaling in mammals.

Parkin targets NOD2 to regulate astrocyte endoplasmic reticulum stress and inflammation

Loss of substantia nigra dopaminergic neurons results in Parkinson disease (PD). Degenerative PD usually presents in the seventh decade whereas genetic disorders, including mutations in PARK2, predispose to early onset PD. PARK2 encodes the parkin E3 ubiquitin ligase which confers pleotropic effects on mitochondrial and cellular fidelity and as a mediator of endoplasmic reticulum (ER) stress signaling. Although the majority of studies investigating ameliorative effects of parkin focus on dopaminergic neurons we found that astrocytes are enriched with parkin. Furthermore, astrocytes deficient in parkin display stress-induced elevation of nucleotide-oligomerization domain receptor 2 (NOD2), a cytosolic receptor integrating ER stress and inflammation. Given the neurotropic and immunomodulatory role of astrocytes we reasoned that parkin may regulate astrocyte ER stress and inflammation to control neuronal homeostasis. We show that, in response to ER stress, parkin knockdown astrocytes exhibit exaggerated ER stress, JNK activation and cytokine release, and reduced neurotropic factor expression. In coculture studied we demonstrate that dopaminergic SHSY5Y cells and primary neurons with the presence of parkin depleted astrocytes are more susceptible to ER stress and inflammation-induced apoptosis than wildtype astrocytes. Parkin interacted with, ubiquitylated and diminished NOD2 levels. Additionally, the genetic induction of parkin ameliorated inflammation in NOD2 expressing cells and knockdown of NOD2 in astrocytes suppressed inflammatory defects in parkin deficient astrocytes and concurrently blunted neuronal apoptosis. Collectively these data identify a role for parkin in modulating NOD2 as a regulatory node in astrocytic control of neuronal homeostasis.

PICK1 inhibits the E3 ubiquitin ligase activity of Parkin and reduces its neuronal protective effect

Parkin functions as a multipurpose E3 ubiquitin ligase, and Parkin loss of function is associated with both sporadic and familial Parkinson's disease (PD). We report that the Bin/Amphiphysin/Rvs (BAR) domain of protein interacting with PRKCA1 (PICK1) bound to the really interesting new gene 1 (RING1) domain of Parkin and potently inhibited the E3 ligase activity of Parkin by disrupting its interaction with UbcH7. Parkin translocated to damaged mitochondria and led to their degradation in neurons, whereas PICK1 robustly inhibited this process. PICK1 also impaired the protective function of Parkin against stresses in SH-SY5Y cells and neurons. The protein levels of several Parkin substrates were reduced in young PICK1-knockout mice, and these mice were resistant to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-mediated toxicity. Taken together, the results indicate that PICK1 is a potent inhibitor of Parkin, and the reduction of PICK1 enhances the protective effect of Parkin.

Comparative analysis of Parkinson's disease-associated genes in mice reveals altered survival and bioenergetics of Parkin-deficient dopamine neurons

Many mutations in genes encoding proteins such as Parkin, PTEN-induced putative kinase 1 (PINK1), protein deglycase DJ-1 (DJ-1 or PARK7), leucine-rich repeat kinase 2 (LRRK2), and α-synuclein have been linked to familial forms of Parkinson's disease (PD). The consequences of these mutations, such as altered mitochondrial function and pathological protein aggregation, are starting to be better understood. However, little is known about the mechanisms explaining why alterations in such diverse cellular processes lead to the selective loss of dopamine (DA) neurons in the substantia nigra (SNc) in the brain of individuals with PD. Recent work has shown that one of the reasons for the high vulnerability of SNc DA neurons is their high basal rate of mitochondrial oxidative phosphorylation (OXPHOS), resulting from their highly complex axonal arborization. Here, we examined whether axonal growth and basal mitochondrial function are altered in SNc DA neurons from Parkin-, Pink1-, or DJ-1-KO mice. We provide evidence for increased basal OXPHOS in Parkin-KO DA neurons and for reduced survival of DA neurons that have a complex axonal arbor. The surviving smaller neurons exhibited reduced vulnerability to the DA neurotoxin and mitochondrial complex I inhibitor MPP+, and this reduction was associated with reduced expression of the DA transporter. Finally, we found that glial cells play a role in the reduced resilience of DA neurons in these mice and that WT Parkin overexpression rescues this phenotype. Our results provide critical insights into the complex relationship between mitochondrial function, axonal growth, and genetic risk factors for PD.

Transgenic Mice Overexpressing the Divalent Metal Transporter 1 Exhibit Iron Accumulation and Enhanced Parkin Expression in the Brain

Exposure to divalent metals such as iron and manganese is thought to increase the risk for Parkinson's disease (PD). Under normal circumstances, cellular iron and manganese uptake is regulated by the divalent metal transporter 1 (DMT1). Accordingly, alterations in DMT1 levels may underlie the abnormal accumulation of metal ions and thereby disease pathogenesis. Here, we have generated transgenic mice overexpressing DMT1 under the direction of a mouse prion promoter and demonstrated its robust expression in several regions of the brain. When fed with iron-supplemented diet, DMT1-expressing mice exhibit rather selective accumulation of iron in the substantia nigra, which is the principal region affected in human PD cases, but otherwise appear normal. Alongside this, the expression of Parkin is also enhanced, likely as a neuroprotective response, which may explain the lack of phenotype in these mice. When DMT1 is overexpressed against a Parkin null background, the double-mutant mice similarly resisted a disease phenotype even when fed with iron- or manganese-supplemented diet. However, these mice exhibit greater vulnerability toward 6-hydroxydopamine-induced neurotoxicity. Taken together, our results suggest that iron accumulation alone is not sufficient to cause neurodegeneration and that multiple hits are required to promote PD.

Parkin promotes proteasomal degradation of p62: implication of selective vulnerability of neuronal cells in the pathogenesis of Parkinson's disease

Mutations or inactivation of parkin, an E3 ubiquitin ligase, are associated with familial form or sporadic Parkinson’s disease (PD), respectively, which manifested with the selective vulnerability of neuronal cells in substantia nigra (SN) and striatum (STR) regions. However, the underlying molecular mechanism linking parkin with the etiology of PD remains elusive. Here we report that p62, a critical regulator for protein quality control, inclusion body formation, selective autophagy and diverse signaling pathways, is a new substrate of parkin. P62 levels were increased in the SN and STR regions, but not in other brain regions in parkin knockout mice. Parkin directly interacts with and ubiquitinates p62 at the K13 to promote proteasomal degradation of p62 even in the absence of ATG5. Pathogenic mutations, knockdown of parkin or mutation of p62 at K13 prevented the degradation of p62. We further showed that parkin deficiency mice have pronounced loss of tyrosine hydroxylase positive neurons and have worse performance in motor test when treated with 6-hydroxydopamine hydrochloride in aged mice. These results suggest that, in addition to their critical role in regulating autophagy, p62 are subjected to parkin mediated proteasomal degradation and implicate that the dysregulation of parkin/p62 axis may involve in the selective vulnerability of neuronal cells during the onset of PD pathogenesis.

Structural studies of parkin and sacsin: Mitochondrial dynamics in neurodegenerative diseases

Neurodegenerative diseases are prevalent, chronic diseases emanating from the dysfunction or death of neurons. The disrupted mitochondrial dynamics observed in a large number of neurodegenerative diseases suggests a common etiology with the possibility of therapies targeting multiple diseases. This review highlights the contributions of structural studies of disease-related proteins to the understanding of neurodegenerative disease pathogenesis and especially the cellular events leading to disruptions in mitochondrial dynamics and function. The examples used are parkin and sacsin, two proteins linked respectively to autosomal-recessive early-onset PD and autosomal-recessive spastic ataxia of Charlevoix-Saguenay. Structural studies of parkin and sacsin explain the pathogenicity of a large number of disease-associated mutations and reveal insights into their cellular functions related to mitochondrial dynamics.

Neuroinflammation in Parkinson's disease animal models: a cell stress response or a step in neurodegeneration?

The motor symptoms of Parkinson's disease are due to the progressive degeneration of dopaminergic neurons in the substantia nigra. Multiple neuroinflammatory processes are exacerbated in Parkinson's disease, including glial-mediated reactions, increased expression of proinflammatory substances, and lymphocytic infiltration, particularly in the substantia nigra. Neuroinflammation is also implicated in the neurodegeneration and consequent behavioral symptoms of many Parkinson's disease animal models, although it is not clear whether these features emulate pathogenic steps in the genuine disorder or if some inflammatory features provide protective stress responses. Here, we compare and summarize findings on neuroinflammatory responses and effects on behavior in a wide range of toxin-based, inflammatory and genetic Parkinson's disease animal models.

An update on the rotenone models of Parkinson's disease: their ability to reproduce the features of clinical disease and model gene-environment interactions

Parkinson's disease (PD) is the second most common neurodegenerative disorder that is characterized by two major neuropathological hallmarks: the degeneration of dopaminergic neurons in the substantia nigra (SN) and the presence of Lewy bodies in the surviving SN neurons, as well as other regions of the central and peripheral nervous system. Animal models have been invaluable tools for investigating the underlying mechanisms of the pathogenesis of PD and testing new potential symptomatic, neuroprotective and neurorestorative therapies. However, the usefulness of these models is dependent on how precisely they replicate the features of clinical PD with some studies now employing combined gene-environment models to replicate more of the affected pathways. The rotenone model of PD has become of great interest following the seminal paper by the Greenamyre group in 2000 (Betarbet et al., 2000). This paper reported for the first time that systemic rotenone was able to reproduce the two pathological hallmarks of PD as well as certain parkinsonian motor deficits. Since 2000, many research groups have actively used the rotenone model worldwide. This paper will review rotenone models, focusing upon their ability to reproduce the two pathological hallmarks of PD, motor deficits, extranigral pathology and non-motor symptoms. We will also summarize the recent advances in neuroprotective therapies, focusing on those that investigated non-motor symptoms and review rotenone models used in combination with PD genetic models to investigate gene-environment interactions.

Parkin-mediated K63-polyubiquitination targets ubiquitin C-terminal hydrolase L1 for degradation by the autophagy-lysosome system

Ubiquitin C-terminal hydrolase L1 (UCH-L1) is a key neuronal deubiquitinating enzyme which is mutated in Parkinson disease (PD) and in childhood-onset neurodegenerative disorder with optic atrophy. Furthermore, reduced UCH-L1 protein levels are associated with a number of neurodegenerative diseases, whereas up-regulation of UCH-L1 protein expression is found in multiple types of cancer. However, very little is known about how UCH-L1 protein level is regulated in cells. Here, we report that UCH-L1 is a novel interactor and substrate of PD-linked E3 ubiquitin-protein ligase parkin. We find that parkin mediates K63-linked polyubiquitination of UCH-L1 in cooperation with the Ubc13/Uev1a E2 ubiquitin-conjugating enzyme complex and promotes UCH-L1 degradation by the autophagy-lysosome pathway. Targeted disruption of parkin gene expression in mice causes a significant decrease in UCH-L1 ubiquitination with a concomitant increase in UCH-L1 protein level in brain, supporting an in vivo role of parkin in regulating UCH-L1 ubiquitination and degradation. Our findings reveal a direct link between parkin-mediated ubiquitin signaling and UCH-L1 regulation, and they have important implications for understanding the roles of these two proteins in health and disease.

MUL1 acts in parallel to the PINK1/parkin pathway in regulating mitofusin and compensates for loss of PINK1/parkin

Parkinson's disease (PD) genes PINK1 and parkin act in a common pathway that regulates mitochondrial integrity and quality. Identifying new suppressors of the pathway is important for finding new therapeutic strategies. In this study, we show that MUL1 suppresses PINK1 or parkin mutant phenotypes in Drosophila. The suppression is achieved through the ubiquitin-dependent degradation of Mitofusin, which itself causes PINK1/parkin mutant-like toxicity when overexpressed. We further show that removing MUL1 in PINK1 or parkin loss-of-function mutant aggravates phenotypes caused by loss of either gene alone, leading to lethality in flies and degeneration in mouse cortical neurons. Together, these observations show that MUL1 acts in parallel to the PINK1/parkin pathway on a shared target mitofusin to maintain mitochondrial integrity. The MUL1 pathway compensates for loss of PINK1/parkin in both Drosophila and mammals and is a promising therapeutic target for PD.

DOI:http://dx.doi.org/10.7554/eLife.01958.001

Parkinson's disease is the second most common neurodegenerative disorder. Symptoms include tremors, rigidity, and slowness, as well as dementia and depression. While most cases of Parkinson's disease have no known genetic cause, mutations in either of two genes—PINK1 or parkin—are known to lead to the disease.

PINK1 and parkin belong to a single pathway that regulates the structure and function of mitochondria, the organelles that generate energy inside cells. Identifying inhibitors of this pathway is critically important for development of future therapies. In addition, previous studies showed that mice with mutations in PINK1 or parkin, as opposed to those in humans and flies, display subtle signs of Parkinson's disease: the fact that these are weak suggests that other unknown proteins or cellular pathways might compensate for loss of the genes.

Yun et al. have now identified one such protein by showing that an increase in the level of a Protein called MUL1 counteracts the deleterious effects due to the loss of PINK1 or parkin in fruit flies. MUL1 is a mitochondrial protein that regulates another protein called mitofusin; the role of mitofusin is to promote the fusion of mitochondria. Conversely, removing MUL1 from PINK1 or parkin mutant worsens the symptoms because MUL1 is no longer present to compensate for the defects.

Yun et al. also show that MUL1 operates through a pathway that is independent of PINK1/parkin. Moreover, this pathway is found in both flies and mouse neurons, which suggest that it has been conserved during evolution. The work of Yun et al. also suggests that MUL1 as a potential therapeutic target for Parkinson's disease.

DOI:http://dx.doi.org/10.7554/eLife.01958.002

Time course of dopamine neuron loss and glial response in the 6-OHDA striatal mouse model of Parkinson's disease

The 6-hydroxydopamine (6-OHDA) neurotoxic lesion of the midbrain dopamine (DA) system is one of the most widely used techniques for modelling Parkinson's disease in rodents. The majority of studies using this approach, however, largely limit their analysis to lesioning acutely, and looking at behavioural deficits and the number of surviving tyrosine hydroxylase (TH)-stained cells in the midbrain. Here we have analysed additional characteristics that occur following intrastriatal delivery of 6-OHDA, providing better understanding of the neurodegenerative process. Female C57/Black mice were given lesions at 10 weeks old, and killed at several different time points postoperatively (3 and 6 h, 1, 3, 6, 9 and 12 days). While the detrimental effect of the toxin on the TH+ fibres in the striatum was immediate, we found that the loss of TH+ dendritic fibres, reduction in cell size and intensity of TH expression, and eventual reduction in the number of TH+ neurons in the substantia nigra was delayed for several days post-surgery. We also investigated the expression of various transcription factors and proteins expressed by midbrain DA neurons following lesioning, and observed changes in the expression of Aldh1a1 (aldehyde dehydrogenase 1 family, member A1) as the neurodegenerative process evolved. Extracellularly, we looked at microglia and astrocytes in reaction to the 6-OHDA striatal lesion, and found a delay in their response and proliferation in the substantia nigra. In summary, this work highlights aspects of the neurodegenerative process in the 6-OHDA mouse model that can be applied to future studies looking at therapeutic interventions.

Behavioral and neurotransmitter abnormalities in mice deficient for Parkin, DJ-1 and superoxide dismutase

Parkinson’s disease (PD) is a progressive neurodegenerative disease characterized by loss of neurons in the substantia nigra that project to the striatum and release dopamine. The cause of PD remains uncertain, however, evidence implicates mitochondrial dysfunction and oxidative stress. Although most cases of PD are sporadic, 5-10% of cases are caused by inherited mutations. Loss-of-function mutations in Parkin and DJ-1 were the first to be linked to recessively inherited Parkinsonism. Surprisingly, mice bearing similar loss-of-function mutations in Parkin and DJ-1 do not show age-dependent loss of nigral dopaminergic neurons or depletion of dopamine in the striatum. Although the normal cellular functions of Parkin and DJ-1 are not fully understood, we hypothesized that loss-of-function mutations in Parkin and DJ-1 render cells more sensitive to mitochondrial dysfunction and oxidative stress. To test this hypothesis, we crossed mice deficient for Parkin and DJ-1 with mice deficient for the mitochondrial antioxidant protein Mn-superoxide dismutase (SOD2) or the cytosolic antioxidant protein Cu-Zn-superoxide dismutase (SOD1). Aged Parkin^-/-DJ-1^-/- and Mn-superoxide dismutase triple deficient mice have enhanced performance on the rotorod behavior test. Cu/Zn-superoxide dismutase triple deficient mice have elevated levels of dopamine in the striatum in the absence of nigral cell loss. Our studies demonstrate that on a Parkin/DJ-1 null background, mice that are also deficient for major antioxidant proteins do not have progressive loss of dopaminergic neurons but have behavioral and striatal dopamine abnormalities.

Surprising behavioral and neurochemical enhancements in mice with combined mutations linked to Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder behind Alzheimer's disease. There are currently no therapies proven to halt or slow the progressive neuronal cell loss in PD. A better understanding of the molecular and cellular causes of PD is needed to develop disease-modifying therapies. PD is an age-dependent disease that causes the progressive death of dopamine-producing neurons in the brain. Loss of substantia nigra dopaminergic neurons results in locomotor symptoms such as slowness of movement, tremor, rigidity and postural instability. Abnormalities in other neurotransmitters, such as serotonin, may also be involved in both the motor and non-motor symptoms of PD. Most cases of PD are sporadic but many families show a Mendelian pattern of inherited Parkinsonism and causative mutations have been identified in genes such as Parkin, DJ-1, PINK1, alpha-synuclein and leucine rich repeat kinase 2 (LRRK2). Although the definitive causes of idiopathic PD remain uncertain, the activity of the antioxidant enzyme glutathione peroxidase 1 (Gpx1) is reduced in PD brains and has been shown to be a key determinant of vulnerability to dopaminergic neuron loss in PD animal models. Furthermore, Gpx1 activity decreases with age in human substantia nigra but not rodent substantia nigra. Therefore, we crossed mice deficient for both Parkin and DJ-1 with mice deficient for Gpx1 to test the hypothesis that loss-of-function mutations in Parkin and DJ-1 cause PD by increasing vulnerability to Gpx1 deficiency. Surprisingly, mice lacking Parkin, DJ-1 and Gpx1 have increased striatal dopamine levels in the absence of nigral cell loss compared to wild type, Gpx1(-/-), and Parkin(-/-)DJ-1(-/-) mutant mice. Additionally, Parkin(-/-)DJ-1(-/-) mice exhibit improved rotarod performance and have increased serotonin in the striatum and hippocampus. Stereological analysis indicated that the increased serotonin levels were not due to increased serotonergic projections. The results of our behavioral, neurochemical and immunohistochemical analyses reveal that PD-linked mutations in Parkin and DJ-1 cause dysregulation of neurotransmitter systems beyond the nigrostriatal dopaminergic circuit and that loss-of-function mutations in Parkin and DJ-1 lead to adaptive changes in dopamine and serotonin especially in the context of Gpx1 deficiency.

Parkin-knockout mice did not display increased vulnerability to intranasal administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

The loss of nigral dopaminergic neurons in Parkinson's disease (PD) is believed to result from interactions between genetic susceptibility and environmental factors. Although loss-of-function mutations in the parkin gene cause early-onset familial PD, the hybrid 129Sv-C57BL/6 parkin-deficient mice did not display spontaneous degeneration of the nigrostriatal pathway or enhanced vulnerability to neurotoxicity induced by 6-hydroxydopamine (6-OHDA) or intraperitoneal 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication. We aimed to re-evaluate the role of parkin in a pure C57BL/6 background after an acute intranasal (i.n.) MPTP administration, a new route of toxin delivery to the brain that mimics environmental exposure to neurotoxins. We found that the deficiency of parkin gene modifies the D-amphetamine-induced locomotion in saline-treated animals. Intranasal MPTP induced Parkinsonism in parkin⁺/⁺ mice, through depletion of striatal dopamine, decreased number of dopaminergic neurons in the substantia nigra, and decreased D-amphetamine-induced hyperlocomotion. Additionally, the deletion of the parkin gene in a pure C57BL/6 background did not lead to increased vulnerability to i.n. MPTP-induced neurotoxicity. Moreover, the i.n. MPTP induced nigral astrogliosis predominantly in the pars reticulata in wild type and parkin⁻/⁻ mice. Taken together, these results showed that the absence of parkin did not modify the vulnerability of nigrostriatal dopaminergic pathway after i.n. MPTP intoxication, suggesting that independently of mouse strain, the endogenous parkin is not required for protection of this system. These findings also suggest that the development of familial parkin-linked PD is not associated with exposure to environmental factors that specifically affects the dopaminergic system.

Overexpression of parkin in the rat nigrostriatal dopamine system protects against methamphetamine neurotoxicity

Methamphetamine (METH) is a central nervous system psychostimulant with a high potential for abuse. At high doses, METH causes a selective degeneration of dopaminergic terminals in the striatum, sparing other striatal terminals and cell bodies. We previously detected a deficit in parkin after binge METH in rat striatal synaptosomes. Parkin is an ubiquitin-protein E3 ligase capable of protecting dopamine neurons from diverse cellular insults. Whether the deficit in parkin mediates the toxicity of METH and whether parkin can protect from toxicity of the drug is unknown. The present study investigated whether overexpression of parkin attenuates degeneration of striatal dopaminergic terminals exposed to binge METH. Parkin overexpression in rat nigrostriatal dopamine system was achieved by microinjection of adeno-associated viral transfer vector 2/6 encoding rat parkin (AAV2/6-parkin) into the substantia nigra pars compacta. The microinjections of AAV2/6-parkin dose-dependently increased parkin levels in both the substantia nigra pars compacta and striatum. The levels of dopamine synthesizing enzyme, tyrosine hydroxylase, remained at the control levels; therefore, tyrosine hydroxylase immunoreactivity was used as an index of dopaminergic terminal integrity. In METH-exposed rats, the increase in parkin levels attenuated METH-induced decreases in striatal tyrosine hydroxylase immunoreactivity in a dose-dependent manner, indicating that parkin can protect striatal dopaminergic terminals against METH neurotoxicity.

Expression of human E46K-mutated α-synuclein in BAC-transgenic rats replicates early-stage Parkinson's disease features and enhances vulnerability to mitochondrial impairment

Parkinson's disease (PD), the second most common neurodegenerative disorder, is etiologically heterogeneous, with most cases thought to arise from a combination of environmental factors and genetic predisposition; about 10% of cases are caused by single gene mutations. While neurotoxin models replicate many of the key behavioral and neurological features, they often have limited relevance to human exposures. Genetic models replicate known disease-causing mutations, but are mostly unsuccessful in reproducing major features of PD. In this study, we created a BAC (bacterial artificial chromosome) transgenic rat model of PD expressing the E46K mutation of α-synuclein, which is pathogenic in humans. The mutant protein was expressed at levels ~2-3-fold above endogenous α-synuclein levels. At 12 months of age, there was no overt damage to the nigrostriatal dopamine system; however, (i) alterations in striatal neurotransmitter metabolism, (ii) accumulation and aggregation of α-synuclein in nigral dopamine neurons, and (iii) evidence of oxidative stress suggest this model replicates several preclinical features of PD. Further, when these animals were exposed to rotenone, a mitochondrial toxin linked to PD, they showed heightened sensitivity, indicating that α-synuclein expression modulates the vulnerability to mitochondrial impairment. We conclude that these animals are well-suited to examination of gene-environment interactions that are relevant to PD.

Animal models of Parkinson's disease: vertebrate genetics

Parkinson's disease (PD) is a complex genetic disorder that is associated with environmental risk factors and aging. Vertebrate genetic models, especially mice, have aided the study of autosomal-dominant and autosomal-recessive PD. Mice are capable of showing a broad range of phenotypes and, coupled with their conserved genetic and anatomical structures, provide unparalleled molecular and pathological tools to model human disease. These models used in combination with aging and PD-associated toxins have expanded our understanding of PD pathogenesis. Attempts to refine PD animal models using conditional approaches have yielded in vivo nigrostriatal degeneration that is instructive in ordering pathogenic signaling and in developing therapeutic strategies to cure or halt the disease. Here, we provide an overview of the generation and characterization of transgenic and knockout mice used to study PD followed by a review of the molecular insights that have been gleaned from current PD mouse models. Finally, potential approaches to refine and improve current models are discussed.

Oxidative stress in genetic mouse models of Parkinson's disease

There is extensive evidence in Parkinson's disease of a link between oxidative stress and some of the monogenically inherited Parkinson's disease-associated genes. This paper focuses on the importance of this link and potential impact on neuronal function. Basic mechanisms of oxidative stress, the cellular antioxidant machinery, and the main sources of cellular oxidative stress are reviewed. Moreover, attention is given to the complex interaction between oxidative stress and other prominent pathogenic pathways in Parkinson's disease, such as mitochondrial dysfunction and neuroinflammation. Furthermore, an overview of the existing genetic mouse models of Parkinson's disease is given and the evidence of oxidative stress in these models highlighted. Taken into consideration the importance of ageing and environmental factors as a risk for developing Parkinson's disease, gene-environment interactions in genetically engineered mouse models of Parkinson's disease are also discussed, highlighting the role of oxidative damage in the interplay between genetic makeup, environmental stress, and ageing in Parkinson's disease.

Gene-environment interactions in Parkinson's disease: specific evidence in humans and mammalian models

Interactions between genetic factors and environmental exposures are thought to be major contributors to the etiology of Parkinson's disease. While such interactions are poorly defined and incompletely understood, recent epidemiological studies have identified specific interactions of potential importance to human PD. In this review, the most current data on gene-environment interactions in PD from human studies are critically discussed. Animal models have also highlighted the importance of genetic susceptibility to toxicant exposure and data of potential relevance to human PD are discussed. Goals and needs for the future of the field are proposed.

Mitochondrial dysfunction in Parkinson's disease: molecular mechanisms and pathophysiological consequences

Neurons are critically dependent on mitochondrial integrity based on specific morphological, biochemical, and physiological features. They are characterized by high rates of metabolic activity and need to respond promptly to activity-dependent fluctuations in bioenergetic demand. The dimensions and polarity of neurons require efficient transport of mitochondria to hot spots of energy consumption, such as presynaptic and postsynaptic sites. Moreover, the postmitotic state of neurons in combination with their exposure to intrinsic and extrinsic neuronal stress factors call for a high fidelity of mitochondrial quality control systems. Consequently, it is not surprising that mitochondrial alterations can promote neuronal dysfunction and degeneration. In particular, mitochondrial dysfunction has long been implicated in the etiopathogenesis of Parkinson's disease (PD), based on the observation that mitochondrial toxins can cause parkinsonism in humans and animal models. Substantial progress towards understanding the role of mitochondria in the disease process has been made by the identification and characterization of genes causing familial variants of PD. Studies on the function and dysfunction of these genes revealed that various aspects of mitochondrial biology appear to be affected in PD, comprising mitochondrial biogenesis, bioenergetics, dynamics, transport, and quality control.

Neurotoxic in vivo models of Parkinson's disease recent advances

Animal models have been invaluable to Parkinson's disease (PD) research. Of these, neurotoxin models have historically been the most widely utilized. The goal of this chapter is to give a brief historical description of classic PD models and then to identify the most recent important advances in modeling human PD in animals. Indeed, significant advances in modeling additional features of PD and expansion to new species have occurred in both older and newer models. The roles these new advances in modeling may have in future PD research are examined in this chapter.

Transgenic animal models of neurodegeneration based on human genetic studies

The identification of genes linked to neurodegenerative diseases such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD) and Parkinson's disease (PD) has led to the development of animal models for studying mechanism and evaluating potential therapies. None of the transgenic models developed based on disease-associated genes have been able to fully recapitulate the behavioral and pathological features of the corresponding disease. However, there has been enormous progress made in identifying potential therapeutic targets and understanding some of the common mechanisms of neurodegeneration. In this review, we will discuss transgenic animal models for AD, ALS, HD and PD that are based on human genetic studies. All of the diseases discussed have active or complete clinical trials for experimental treatments that benefited from transgenic models of the disease.

The role of parkin in familial and sporadic Parkinson's disease

Mutations in parkin are the second most common known cause of Parkinson's disease (PD). Parkin is an ubiquitin E3 ligase that monoubiquitinates and polyubiquitinates proteins to regulate a variety of cellular processes. Loss of parkin's E3 ligase activity is thought to play a pathogenic role in both inherited and sporadic PD. Here, we review parkin biology and pathobiology and its role in the pathogenesis of PD.

Identification and characterization of a novel endogenous murine parkin mutation

Various mutations in the PARK2 gene which encodes the protein, parkin, are causal of a disease entity-termed autosomal recessive juvenile parkinsonism. Parkin can function as an E3 ubiquitin-protein ligase, mediating the ubiquitination of specific targeted proteins and resulting in proteasomal degradation. Parkin is thought to lead to parkinsonism as a consequence of a loss in its function. In this study, immunoblot analyses of brain extracts from Balb/c, C57BL/6, C3H, and 129S mouse strains demonstrated significant variations in immunoreactivity with anti-parkin monoclonal antibodies (PRK8, PRK28, and PRK109). This resulted partly from differences in the steady-state levels of parkin protein across mouse strains. There was also a complete loss of immunoreactivity for PRK8 and PRK28 antibodies in C3H mice due to was because of a homologous nucleotide mutation resulting in an E398Q amino acid substitution. In cultured cells, parkin harboring this mutation had a greater tendency to aggregate, exhibited reduced interaction with the E2 ubiquitin-conjugating enzymes, UbcH7 and UbcH8, and demonstrated loss-of-function in promoting the proteosomal degradation of a specific putative substrate, synphilin-1. In situ, C3H mice displayed age-dependent increased levels of brain cortical synphilin-1 compared with C57BL/6, suggesting that E398Q parkin in these mice is functionally impaired and that C3H mice may be a suitable model of parkin loss-of-function similar to patients with missense mutations.

Parkin deficiency increases vulnerability to inflammation-related nigral degeneration

The loss of nigral dopaminergic (DA) neurons in idiopathic Parkinson's disease (PD) is believed to result from interactions between genetic susceptibility and environmental factors. Evidence that inflammatory processes modulate PD risk comes from prospective studies that suggest that higher plasma concentrations of a number of proinflammatory cytokines correlate with an increased risk of developing PD and chronic nonsteroidal anti-inflammatory drug regimens reduce the incidence of PD. Although loss-of-function mutations in the parkin gene cause early-onset familial PD, Parkin-deficient (parkin-/-) mice do not display nigrostriatal pathway degeneration, suggesting that a genetic factor is not sufficient, and an environmental trigger may be needed to cause nigral DA neuron loss. To test the hypothesis that parkin-/- mice require an inflammatory stimulus to develop nigral DA neuron loss, low-dose lipopolysaccaride (LPS) was administered intraperitoneally for prolonged periods. Quantitative real-time PCR and immunofluorescence labeling of inflammatory markers indicated that this systemic LPS treatment regimen triggered persistent neuroinflammation in wild-type and parkin-/- mice. Although inflammatory and oxidative stress responses to the inflammation regimen did not differ significantly between the two genotypes, only parkin-/- mice displayed subtle fine-motor deficits and selective loss of DA neurons in substantia nigra. Therefore, our studies suggest that loss of Parkin function increases the vulnerability of nigral DA neurons to inflammation-related degeneration. This new model of nigral DA neuron loss may enable identification of early biomarkers of degeneration and aid in preclinical screening efforts to identify compounds that can halt or delay the progressive degeneration of the nigrostriatal pathway.

Oxidative stress in Parkinson's disease: a mechanism of pathogenic and therapeutic significance

Parkinson's disease (PD) is a common adult-onset neurodegenerative disorder. Typically PD is a sporadic neurological disorder, and over time affected patients see their disability growing and their quality of life declining. Oxidative stress has been hypothesized to be linked to both the initiation and the progression of PD. Preclinical findings from both in vitro and in vivo experimental models of PD suggest that the neurodegenerative process starts with otherwise healthy neurons being hit by some etiological factors, which sets into motion a cascade of deleterious events. In these models initial molecular alterations in degenerating dopaminergic neurons include increased formation of reactive oxygen species, presumably originating from both inside and outside the mitochondria. In the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD, time-course experiments suggest that oxidative stress is an early event that may directly kill some of the dopaminergic neurons. In this model it seems that oxidative stress may play a greater role in the demise of dopaminergic neurons indirectly by activating intracellular, cell death-related, molecular pathways. As the neurodegenerative process evolves in the MPTP mouse model, indices of neuroinflammation develop, such as microglial activation. The latter increases the level of oxidative stress to which the neighboring compromised neurons are subjected to, thereby promoting their demise. However, these experimental studies have also shown that oxidative stress is not the sole deleterious factor implicated in the death of dopaminergic neurons. Should a similar multifactorial cascade underlie dopaminergic neuron degeneration in PD, then the optimal therapy for this disease may have to rely on a cocktail of agents, each targeting a different critical component of this hypothesized pathogenic cascade. If correct, this may be a reason why neuroprotective trials using a single agent, such as an antioxidant, have thus far generated disappointing results.

Behavioral genetic contributions to the study of addiction-related amphetamine effects

Amphetamines, including methamphetamine, pose a significant cost to society due to significant numbers of amphetamine-abusing individuals who suffer major health-related consequences. In addition, methamphetamine use is associated with heightened rates of violent and property-related crimes. The current paper reviews the existing literature addressing genetic differences in mice that impact behavioral responses thought to be relevant to the abuse of amphetamine and amphetamine-like drugs. Summarized are studies that used inbred strains, selected lines, single-gene knockouts and transgenics, and quantitative trait locus (QTL) mapping populations. Acute sensitivity, neuroadaptive responses, rewarding and conditioned effects are among those reviewed. Some gene mapping work has been accomplished, and although no amphetamine-related complex trait genes have been definitively identified, translational work leading from results in the mouse to studies performed in humans is beginning to emerge. The majority of genetic investigations have utilized single-gene knockout mice and have concentrated on dopamine- and glutamate-related genes. Genes that code for cell support and signaling molecules are also well-represented. There is a large behavioral genetic literature on responsiveness to amphetamines, but a considerably smaller literature focused on genes that influence the development and acceleration of amphetamine use, withdrawal, relapse, and behavioral toxicity. Also missing are genetic investigations into the effects of amphetamines on social behaviors. This information might help to identify at-risk individuals and in the future to develop treatments that take advantage of individualized genetic information.

Parkin-mediated K63-linked polyubiquitination targets misfolded DJ-1 to aggresomes via binding to HDAC6

Sequestration of misfolded proteins into pericentriolar inclusions called aggresomes is a means that cells use to minimize misfolded protein-induced cytotoxicity. However, the molecular mechanism by which misfolded proteins are recruited to aggresomes remains unclear. Mutations in the E3 ligase parkin cause autosomal recessive Parkinson's disease that is devoid of Lewy bodies, which are similar to aggresomes. Here, we report that parkin cooperates with heterodimeric E2 enzyme UbcH13/Uev1a to mediate K63-linked polyubiquitination of misfolded DJ-1. K63-linked polyubiquitination of misfolded DJ-1 serves as a signal for interaction with histone deacetylase 6, an adaptor protein that binds the dynein-dynactin complex. Through this interaction, misfolded DJ-1 is linked to the dynein motor and transported to aggresomes. Furthermore, fibroblasts lacking parkin display deficits in targeting misfolded DJ-1 to aggresomes. Our findings reveal a signaling role for K63-linked polyubiquitination in dynein-mediated transport, identify parkin as a key regulator in the recruitment of misfolded DJ-1 to aggresomes, and have important implications regarding the biogenesis of Lewy bodies.

rAAV-mediated nigral human parkin over-expression partially ameliorates motor deficits via enhanced dopamine neurotransmission in a rat model of Parkinson's disease

We hypothesized that over-expressing the E3 ligase, parkin, whose functional loss leads to Parkinson's disease, in the nigrostriatal tract might be protective in the unilateral 6-hydroxydopamine (6-OHDA) rat lesion model. Recombinant adeno-associated virus (rAAV) encoding human parkin or green fluorescent protein (GFP) was injected into the rat substantia nigra 6 weeks prior to a four-site striatal 6-OHDA lesion. Vector-mediated parkin over-expression significantly ameliorated motor deficits as measured by amphetamine-induced rotational behavior and spontaneous behavior in the cylinder test but forelimb akinesia as assessed by the stepping test was unaffected. rAAV-mediated human parkin was expressed in the nigrostriatal tract, the substantia pars reticulata, and the subthalamic nucleus. However, in lesioned animals, there was no difference between nigral parkin and GFP-transduction on lesion-induced striatal tyrosine hydroxylase (TH) innervation or nigral TH positive surviving neurons. A second lesion experiment was performed to determine if striatal dopamine (DA) neurotransmission was enhanced as measured biochemically. In this second group of parkin and GFP treated rats, behavioral improvement was again observed. In addition, striatal TH and DA levels were slightly increased in the parkin-transduced group. In a third experiment, we evaluated parkin and GFP transduced rats 6 weeks after vector injection without DA depletion. When challenged with amphetamine, parkin treated rats tended to display asymmetries biased away from the treated hemisphere. Nigral parkin over-expression induced increases in both striatal TH and DA levels. Therefore, while parkin over-expression exerted no protective effect on the nigrostriatal DA system, parkin appeared to enhance the efficiency of nigrostriatal DA transmission in intact nigral DA neurons likely due to the observed increases in TH.

Non-motor behavioural impairments in parkin-deficient mice

Mutations in the parkin gene are the major cause of early-onset familial Parkinson's disease (PD). We previously reported the generation and analysis of a knockout mouse carrying a deletion of exon 3 in the parkin gene. F1 hybrid pa+/- mice were backcrossed to wild-type C57Bl/6 for three more generations to establish a pa-/-(F4) mouse line. The appearance of tyrosine hydroxylase-positive neurons was normal in young and aged pa-/- (F4) animals. Loss of parkin function in mice did not enhance vulnerability of dopaminergic neurons to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity. However, the pa-/- (F4) mice displayed impaired exploration and habituation to a new environment and exhibited thigmotaxis behaviour in the open field and Morris water maze. Abnormal anxiety-related behaviour of pa-/- (F4) mice was also observed in the light/dark exploration test paradigm. Dopamine metabolism was enhanced in the striatum of pa-/- (F4) mice, as revealed by increased homovanillic acid (HVA) content and a reduced ratio of dihydroxyphenylacetic acid (DOPAC)/HVA. The alterations found in the dopaminergic system could be responsible for the behavioural impairments of pa-/- (F4) mice. Consistent with a recent observation of cognitive dysfunction in parkin-linked patients with PD, our findings provide evidence of a physiological role of parkin in non-motor behaviour, possibly representing a disease stage that precedes dopaminergic neuron loss.

Model fusion, the next phase in developing animal models for Parkinson's disease

Within the past 25 years, discoveries of environmental and monogenetic forms of parkinsonism have shaped the direction of Parkinson's disease (PD) research and development of experimental systems to study PD. In this review, we outline a remarkable array of in vivo models available, with particular emphasis on their benefits and pitfalls and the contribution each has made to enhance our understanding of pathological mechanisms involved in PD. Further, we discuss the increasingly popular approach of "model fusion" to create a new generation of animal systems in which to study gene-environment interactions, and the usefulness of such models in capturing the most common events underlying PD.

MPTP and DSP-4 susceptibility of substantia nigra and locus coeruleus catecholaminergic neurons in mice is independent of parkin activity

Mutations in the parkin gene cause autosomal recessive familial Parkinson's disease (PD). Parkin-deficient mouse models fail to recapitulate nigrostriatal dopaminergic neurodegeneration as seen in PD, but produce deficits in dopaminergic neurotransmission and noradrenergic-dependent behavior. Since sporadic PD is thought to be caused by a combination of genetic susceptibilities and environmental factors, we hypothesized that neurotoxic insults from catecholaminergic toxins would render parkin knockout mice more vulnerable to neurodegeneration. Accordingly, we investigated the susceptibility of catecholaminergic neurons in parkin knockout mice to the potent dopaminergic and noradrenergic neurotoxins 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) respectively. We report that nigrostriatal dopaminergic neurons in parkin knockout mice do not show increased susceptibility to the parkinsonian neurotoxin, MPTP, in acute, subacute and chronic dose regimens of the neurotoxin. Additionally, parkin knockout mice do not show increased vulnerability to the noradrenergic neurotoxin, DSP-4, regarding levels of norepinephrine in cortex, brain stem and spinal cord. These findings suggest that absence of parkin in mice does not increase susceptibility to the loss of catecholaminergic neurons upon exposure to both dopaminergic and noradrenergic neurotoxins.

Microarrays in Parkinson's disease: a systematic approach

Neurological disease (ND) is one of the greatest challenges facing our population, from medical, financial, and social perspectives. The application of new research approaches to understand the underlying pathogenesis of ND is critical. In this article, we review the use of microarray analysis in Parkinson's disease (PD). Microarrays have tremendous power, simultaneously querying the expression of tens of thousands of genes from a given biological sample. Coupled with impressive advances in statistical tools for analyzing large, complex data sets, well-designed microarray experiments are poised to make a big impact in the field of ND. Parkinson's disease is a devastating neurodegenerative disease well suited to a systems-based microarray analysis. Genetic and environmental rodent models of PD emulate many of the cardinal features of human PD, providing the unique opportunity to compare gene expression profiles from different etiologies of the same disease. The elucidation of important gene expression patterns during disease will make possible identification of genetic susceptibility markers, biomarkers of disease progression, and new therapeutic targets.

Parkinson's disease: a rethink of rodent models

Parkinson's disease (PD) is a multifactorial disease with a complex etiology that results from genetic risk factors, environmental exposures and most likely a combination of both. Rodent models of parkinsonism aim to reproduce key pathogenic features of the syndrome including movement disorder induced by the progressive loss of dopaminergic neurons in the substantia nigra, accompanied by the formation of alpha-synuclein containing Lewy body inclusions. Despite the creation of many excellent models, both chemically induced and genetically engineered, there is none that accurately demonstrates these features. Recent pathological staging studies in man have also emphasized the significant non-CNS component of PD that has yet to be tackled. Herein, we summarize rodent models of PD and what they offer to the field, and suggest future challenges and opportunities.