Progressive dopaminergic cell loss with unilateral-to-bilateral progression in a genetic model of Parkinson disease

Experimental models of Parkinson's disease: Challenges and Opportunities

Parkinson's disease (PD) is a widespread neurodegenerative disorder occurs due to the degradation of dopaminergic neurons present in the substantia nigra pars compacta (SNpc). Millions of people are affected by this devastating disorder globally, and the frequency of the condition increases with the increase in the elderly population. A significant amount of progress has been made in acquiring more knowledge about the etiology and the pathogenesis of PD over the past decades. Animal models have been regarded to be a vital tool for the exploration of complex molecular mechanisms involved in PD. Various animals used as models for disease monitoring include vertebrates (zebrafish, rats, mice, guinea pigs, rabbits and monkeys) and invertebrate models (Drosophila, Caenorhabditis elegans). The animal models most relevant for study of PD are neurotoxin induction-based models (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 6-Hydroxydopamine (6-OHDA) and agricultural pesticides (rotenone, paraquat), pharmacological models (reserpine or haloperidol treated rats), genetic models (α-synuclein, Leucine-rich repeat kinase 2 (LRRK2), DJ-1, PINK-1 and Parkin). Several non-mammalian genetic models such as zebrafish, Drosophila and Caenorhabditis elegance have also gained popularity in recent years due to easy genetic manipulation, presence of genes homologous to human PD, and rapid screening of novel therapeutic molecules. In addition, in vitro models (SH-SY5Y, PC12, Lund human mesencephalic (LUHMES) cells, Human induced pluripotent stem cell (iPSC), Neural organoids, organ-on-chip) are also currently in trend providing edge in investigating molecular mechanisms involved in PD as they are derived from PD patients. In this review, we explain the current situation and merits and demerits of the various animal models.

In and out: Benchmarking in vitro, in vivo, ex vivo, and xenografting approaches for an integrative brain disease modeling pipeline

Human cellular models and their neuronal derivatives have afforded unprecedented advances in elucidating pathogenic mechanisms of neuropsychiatric diseases. Notwithstanding their indispensable contribution, animal models remain the benchmark in neurobiological research. In an attempt to harness the best of both worlds, researchers have increasingly relied on human/animal chimeras by xenografting human cells into the animal brain. Despite the unparalleled potential of xenografting approaches in the study of the human brain, literature resources that systematically examine their significance and advantages are surprisingly lacking. We fill this gap by providing a comprehensive account of brain diseases that were thus far subjected to all three modeling approaches (transgenic rodents, in vitro human lineages, human-animal xenografting) and provide a critical appraisal of the impact of xenografting approaches for advancing our understanding of those diseases and brain development. Next, we give our perspective on integrating xenografting modeling pipeline with recent cutting-edge technological advancements.

Aging, Parkinson's Disease, and Models: What Are the Challenges?

Parkinson's disease (PD) is a chronic, neurodegenerative condition characterized by motor symptoms such as bradykinesia, rigidity, and tremor, alongside multiple nonmotor symptoms. The appearance of motor symptoms is linked to progressive dopaminergic neuron loss within the substantia nigra. PD incidence increases sharply with age, suggesting a strong association between mechanisms driving biological aging and the development and progression of PD. However, the role of aging in the pathogenesis of PD remains understudied. Numerous models of PD, including cell models, toxin-induced models, and genetic models in rodents and nonhuman primates (NHPs), reproduce different aspects of PD, but preclinical studies of PD rarely incorporate age as a factor. Studies using patient neurons derived from stem cells via reprogramming methods retain some aging features, but their characterization, particularly of aging markers and reproducibility of neuron type, is suboptimal. Investigation of age-related changes in PD using animal models indicates an association, but this is likely in conjunction with other disease drivers. The biggest barrier to drawing firm conclusions is that each model lacks full characterization and appropriate time-course assessments. There is a need to systematically investigate whether aging increases the susceptibility of mouse, rat, and NHP models to develop PD and understand the role of cell models. We propose that a significant investment in time and resources, together with the coordination and sharing of resources, knowledge, and data, is required to accelerate progress in understanding the role of biological aging in PD development and improve the reliability of models to test interventions.

PARK7/DJ-1 in microglia: implications in Parkinson's disease and relevance as a therapeutic target

Microglia are the immune effector cells of the brain playing critical roles in immune surveillance and neuroprotection in healthy conditions, while they can sustain neuroinflammatory and neurotoxic processes in neurodegenerative diseases, including Parkinson's disease (PD). Although the precise triggers of PD remain obscure, causative genetic mutations, which aid in the identification of molecular pathways underlying the pathogenesis of idiopathic forms, represent 10% of the patients. Among the inherited forms, loss of function of PARK7, which encodes the protein DJ-1, results in autosomal recessive early-onset PD. Yet, although protection against oxidative stress is the most prominent task ascribed to DJ-1, the underlying mechanisms linking DJ-1 deficiency to the onset of PD are a current matter of investigation. This review provides an overview of the role of DJ-1 in neuroinflammation, with a special focus on its functions in microglia genetic programs and immunological traits. Furthermore, it discusses the relevance of targeting dysregulated pathways in microglia under DJ-1 deficiency and their importance as therapeutic targets in PD. Lastly, it addresses the prospect to consider DJ-1, detected in its oxidized form in idiopathic PD, as a biomarker and to take into account DJ-1-enhancing compounds as therapeutics dampening oxidative stress and neuroinflammation.

Human Induced Pluripotent Stem Cell Phenotyping and Preclinical Modeling of Familial Parkinson's Disease

Parkinson's disease (PD) is primarily idiopathic and a highly heterogenous neurodegenerative disease with patients experiencing a wide array of motor and non-motor symptoms. A major challenge for understanding susceptibility to PD is to determine the genetic and environmental factors that influence the mechanisms underlying the variations in disease-associated traits. The pathological hallmark of PD is the degeneration of dopaminergic neurons in the substantia nigra pars compacta region of the brain and post-mortem Lewy pathology, which leads to the loss of projecting axons innervating the striatum and to impaired motor and cognitive functions. While the cause of PD is still largely unknown, genome-wide association studies provide evidence that numerous polymorphic variants in various genes contribute to sporadic PD, and 10 to 15% of all cases are linked to some form of hereditary mutations, either autosomal dominant or recessive. Among the most common mutations observed in PD patients are in the genes LRRK2, SNCA, GBA1, PINK1, PRKN, and PARK7/DJ-1. In this review, we cover these PD-related mutations, the use of induced pluripotent stem cells as a disease in a dish model, and genetic animal models to better understand the diversity in the pathogenesis and long-term outcomes seen in PD patients.

Oxidative stress and synaptic dysfunction in rodent models of Parkinson's disease

Parkinson's disease (PD) is a multifactorial disorder involving a complex interplay between a variety of genetic and environmental factors. In this scenario, mitochondrial impairment and oxidative stress are widely accepted as crucial neuropathogenic mechanisms, as also evidenced by the identification of PD-associated genes that are directly involved in mitochondrial function. The concept of mitochondrial dysfunction is closely linked to that of synaptic dysfunction. Indeed, compelling evidence supports the role of mitochondria in synaptic transmission and plasticity, although many aspects have not yet been fully elucidated. Here, we will provide a brief overview of the most relevant evidence obtained in different neurotoxin-based and genetic rodent models of PD, focusing on mitochondrial impairment and synaptopathy, an early central event preceding overt nigrostriatal neurodegeneration. The identification of early deficits occurring in PD pathogenesis is crucial in view of the development of potential disease-modifying therapeutic strategies.

Modeling Parkinson's disease-related symptoms in alpha-synuclein overexpressing mice

BACKGROUND

Intracellular deposition of alpha-synuclein (α-syn) as Lewy bodies and Lewy neurites is a central event in the pathogenesis of Parkinson's disease (PD) and other α-synucleinopathies. Transgenic mouse models overexpressing human α-syn, are useful research tools in preclinical studies of pathogenetic mechanisms. Such mice develop α-syn inclusions as well as neurodegeneration with a topographical distribution that varies depending on the choice of promoter and which form of α-syn that is overexpressed. Moreover, they display motor symptoms and cognitive disturbances that to some extent resemble the human conditions.

PURPOSE

One of the main motives for assessing behavior in these mouse models is to evaluate the potential of new treatment strategies, including their impact on motor and cognitive symptoms. However, due to a high within-group variability with respect to such features, the behavioral studies need to be applied with caution. In this review, we discuss how to make appropriate choices in the experimental design and which tests that are most suitable for the evaluation of PD-related symptoms in such studies.

METHODS

We have evaluated published results on two selected transgenic mouse models overexpressing wild type (L61) and mutated (A30P) α-syn in the context of their validity and utility for different types of behavioral studies.

CONCLUSIONS

By applying appropriate behavioral tests, α-syn transgenic mouse models provide an appropriate experimental platform for studies of symptoms related to PD and other α-synucleinopathies.

Expression Analysis of Genes Involved in Transport Processes in Mice with MPTP-Induced Model of Parkinson’s Disease

Processes of intracellular and extracellular transport play one of the most important roles in the functioning of cells. Changes to transport mechanisms in a neuron can lead to the disruption of many cellular processes and even to cell death. It was shown that disruption of the processes of vesicular, axonal, and synaptic transport can lead to a number of diseases of the central nervous system, including Parkinson’s disease (PD). Here, we studied changes in the expression of genes whose protein products are involved in the transport processes (Snca, Drd2, Rab5a, Anxa2, and Nsf) in the brain tissues and peripheral blood of mice with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced models of PD. We detected changes in the expressions of Drd2, Anxa2, and Nsf at the earliest modeling stages. Additionally, we have identified conspicuous changes in the expression level of Anxa2 in the striatum and substantia nigra of mice with MPTP-induced models of PD in its early stages. These data clearly suggest the involvement of protein products in these genes in the earliest stages of the pathogenesis of PD.

Parkinson's disease protein PARK7 prevents metabolite and protein damage caused by a glycolytic metabolite

Reactive compounds cause cellular damage that is suspected to contribute to aging and neurodegenerative diseases. Oxidative stress and environmental factors likely contribute to this. Here we report that an enzyme mutated in Parkinson’s disease can prevent damage of metabolites and proteins caused by a metabolite from the central pathway of sugar metabolism. Inactivation of this enzyme in model systems, ranging from flies to human cells, leads to the accumulation of a wide range of damaged metabolites and proteins. Thus, this enzyme represents a highly conserved strategy to prevent damage in cells that metabolize sugars. Overall, we discovered a fundamental link between carbohydrate metabolism and a type of cellular damage that might contribute to the development of Parkinson’s disease.

Cells are continuously exposed to potentially dangerous compounds. Progressive accumulation of damage is suspected to contribute to neurodegenerative diseases and aging, but the molecular identity of the damage remains largely unknown. Here we report that PARK7, an enzyme mutated in hereditary Parkinson’s disease, prevents damage of proteins and metabolites caused by a metabolite of glycolysis. We found that the glycolytic metabolite 1,3-bisphosphoglycerate (1,3-BPG) spontaneously forms a novel reactive intermediate that avidly reacts with amino groups. PARK7 acts by destroying this intermediate, thereby preventing the formation of proteins and metabolites with glycerate and phosphoglycerate modifications on amino groups. As a consequence, inactivation of PARK7 (or its orthologs) in human cell lines, mouse brain, and Drosophila melanogaster leads to the accumulation of these damaged compounds, most of which have not been described before. Our work demonstrates that PARK7 function represents a highly conserved strategy to prevent damage in cells that metabolize carbohydrates. This represents a fundamental link between metabolism and a type of cellular damage that might contribute to the development of Parkinson’s disease.

Assessment of Dopaminergic Neurodegeneration in Mice

Neuron death is a key feature of neurological disorders like Alzheimer's or Parkinson's disease (PD). As a result, analysis of neurodegeneration is often considered a central experiment in the postmortem characterization of preclinical PD animal models. Stereology provides a precise estimate of particles, like neurons, in three-dimensional objects, like the brain, and is the gold standard quantification approach for the assessment of neuron survival in neurodegenerative disease research. Here, we provide a detailed step-by-step guide for the quantification of dopaminergic neurons in the substantia nigra pars compacta, a brain area prone to neuron loss in PD. In addition, we outline the protocol for the analysis of the dopaminergic terminals in the striatum, the projection area of midbrain dopaminergic neurons, as a readout for the integrity of the nigrostriatal projections.

Fidelity of the PINK1 knockout rat to oxidative stress and other characteristics of Parkinson disease

Parkinson disease (PD) is the second most common age-related neurodegenerative disease in the world, and PD significantly impacts the quality of life, especially as in general people are living longer. Because of the numerous and complex features of sporadic PD that progressively develops, it is difficult to build an ideal animal model for PD research. Genetically modified PD rodent animal models are considered as a major tool with which to study the mechanisms and potential therapeutic targets for PD. Up to now, none of the rodent animal models displays all PD characteristics. The Michael J. Fox Foundation for Parkinson's Research (MJFF) funded SAGE Laboratories to generate a PTEN-induced putative kinase-1 (PINK1) knockout (KO) rat model for familial PD using zinc finger nuclease (ZFN) technology. In the current paper, we review all papers from PubMed that report studies with PINK1 KO rats, presenting the research results, and discussing the fidelity of this rat model to PD according to its phenotypes studied by several laboratories. This review will serve as a critical reference for future studies with this rodent model, providing a better understanding of PD etiology, pathology, and potential treatment strategies.

Alterations of Nigral Dopamine Levels in Parkinson's Disease after Environmental Enrichment and PACAP Treatment in Aging Rats

The neuroprotective effects of environmental enrichment and PACAP (pituitary adenylate cyclase-activating polypeptide) are well-described in Parkinson’s disease. The aim of our study is to investigate the beneficial effects of these factors in aging parkinsonian rats. Newborn Wistar rats were divided into standard and enriched groups according to their environmental conditions. Standard animals were raised under regular conditions. During the first five postnatal weeks, enriched pups were placed in larger cages with different objects. Aging animals received (1) saline, (2) 6-hydroxidopamine (6-OHDA), or (3) 6-OHDA + PACAP injections into the left substantia nigra (s.n.). On the seventh postoperative day, the left and right s.n. were collected. The s.n. of young and aging unoperated animals were also examined in our experiment. We determined the dopamine (DA) levels by the HPLC-MS technique, while the sandwich ELISA method was used to measure the Parkinson disease protein 7 (PARK7) protein levels. In healthy animals, we found an age-related decrease of DA levels. In aging parkinsonian-enriched rats, the operation did not result in a significant DA loss. PACAP treatment could prevent the DA loss in both the standard and enriched groups. All injured PACAP-treated rats showed remarkably higher protective PARK7 levels. The protective effect of PACAP correlated with the increase of the DA and PARK7 levels.

Classic and evolving animal models in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease with motor and non-motor symptoms. PD is characterized by the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and deficiency of dopamine in the striatal region. The primary objective in PD research is to understand the pathogenesis, targets, and development of therapeutic interventions to control the progress of the disease. The anatomical and physiological resemblances between humans and animals gathered the researcher's attention towards the use of animals in PD research. Due to varying age of onset, symptoms, and progression rate, PD becomes heterogeneous which demands the variety of animal models to study diverse features of the disease. Parkinson is a multifactorial disorder, selection of models become important as not a single model shows all the biochemical features of the disease. Currently, conventional pharmacological, neurotoxin-induced, genetically modified and cellular models are available for PD research, but none of them recapitulate all the biochemical characteristics of the disease. In this review, we included the updated knowledge on the main features of currently available in vivo and in vitro models as well as their strengths and weaknesses.

Machine learning discriminates a movement disorder in a zebrafish model of Parkinson's disease

Animal models of human disease provide an in vivo system that can reveal molecular mechanisms by which mutations cause pathology, and, moreover, have the potential to provide a valuable tool for drug development. Here, we have developed a zebrafish model of Parkinson's disease (PD) together with a novel method to screen for movement disorders in adult fish, pioneering a more efficient drug-testing route. Mutation of the PARK7 gene (which encodes DJ-1) is known to cause monogenic autosomal recessive PD in humans, and, using CRISPR/Cas9 gene editing, we generated a Dj-1 loss-of-function zebrafish with molecular hallmarks of PD. To establish whether there is a human-relevant parkinsonian phenotype in our model, we adapted proven tools used to diagnose PD in clinics and developed a novel and unbiased computational method to classify movement disorders in adult zebrafish. Using high-resolution video capture and machine learning, we extracted novel features of movement from continuous data streams and used an evolutionary algorithm to classify parkinsonian fish. This method will be widely applicable for assessing zebrafish models of human motor diseases and provide a valuable asset for the therapeutics pipeline. In addition, interrogation of RNA-seq data indicate metabolic reprogramming of brains in the absence of Dj-1, adding to growing evidence that disruption of bioenergetics is a key feature of neurodegeneration.This article has an associated First Person interview with the first author of the paper.

Gait Progression Over 6 Years in Parkinson's Disease: Effects of Age, Medication, and Pathology

Background: Gait disturbance is an early, cardinal feature of Parkinson's disease (PD) associated with falls and reduced physical activity. Progression of gait impairment in Parkinson's disease is not well characterized and a better understanding is imperative to mitigate impairment. Subtle gait impairments progress in early disease despite optimal dopaminergic medication. Evaluating gait disturbances over longer periods, accounting for typical aging and dopaminergic medication changes, will enable a better understanding of gait changes and inform targeted therapies for early disease. This study aimed to describe gait progression over the first 6 years of PD by delineating changes associated with aging, medication, and pathology. Methods: One-hundred and nine newly diagnosed PD participants and 130 controls completed at least two gait assessments. Gait was assessed at 18-month intervals for up to 6 years using an instrumented walkway to measure sixteen spatiotemporal gait characteristics. Linear mixed-effects models assessed progression. Results: Ten gait characteristics significantly progressed in PD, with changes in four of these characteristics attributable to disease progression. Age-related changes also contributed to gait progression; changes in another two characteristics reflected both aging and disease progression. Gait impairment progressed irrespective of dopaminergic medication change for all characteristics except step width variability. Conclusions: Discrete gait impairments continue to progress in PD over 6 years, reflecting a combination of, and potential interaction between, disease-specific progression and age-related change. Gait changes were mostly unrelated to dopaminergic medication adjustments, highlighting limitations of current dopaminergic therapy and the need to improve interventions targeting gait decline.

Back and to the Future: From Neurotoxin-Induced to Human Parkinson's Disease Models

Parkinson's disease (PD) is an age-related neurodegenerative disorder characterized by motor symptoms such as tremor, slowness of movement, rigidity, and postural instability, as well as non-motor features like sleep disturbances, loss of ability to smell, depression, constipation, and pain. Motor symptoms are caused by depletion of dopamine in the striatum due to the progressive loss of dopamine neurons in the substantia nigra pars compacta. Approximately 10% of PD cases are familial arising from genetic mutations in α-synuclein, LRRK2, DJ-1, PINK1, parkin, and several other proteins. The majority of PD cases are, however, idiopathic, i.e., having no clear etiology. PD is characterized by progressive accumulation of insoluble inclusions, known as Lewy bodies, mostly composed of α-synuclein and membrane components. The cause of PD is currently attributed to cellular proteostasis deregulation and mitochondrial dysfunction, which are likely interdependent. In addition, neuroinflammation is present in brains of PD patients, but whether it is the cause or consequence of neurodegeneration remains to be studied. Rodents do not develop PD or PD-like motor symptoms spontaneously; however, neurotoxins, genetic mutations, viral vector-mediated transgene expression and, recently, injections of misfolded α-synuclein have been successfully utilized to model certain aspects of the disease. Here, we critically review the advantages and drawbacks of rodent PD models and discuss approaches to advance pre-clinical PD research towards successful disease-modifying therapy. © 2020 The Authors.

DJ-1 (Park7) affects the gut microbiome, metabolites and the development of innate lymphoid cells (ILCs)

The proper communication between gut and brain is pivotal for the maintenance of health and, dysregulation of the gut-brain axis can lead to several clinical disorders. In Parkinson’s disease (PD) 85% of all patients experienced constipation many years before showing any signs of motor phenotypes. For differential diagnosis and preventive treatment, there is an urgent need for the identification of biomarkers indicating early disease stages long before the disease phenotype manifests. DJ-1 is a chaperone protein involved in the protection against PD and genetic mutations in this protein have been shown to cause familial PD. However, how the deficiency of DJ-1 influences the risk of PD remains incompletely understood. In the present study, we provide evidence that DJ-1 is implicated in shaping the gut microbiome including; their metabolite production, inflammation and innate immune cells (ILCs) development. We revealed that deficiency of DJ-1 leads to a significant increase in two specific genera/species, namely Alistipes and Rikenella. In DJ-1 knock-out (DJ-1^-/-) mice the production of fecal calprotectin and MCP-1 inflammatory proteins were elevated. Fecal and serum metabolic profile showed that malonate which influences the immune system was significantly more abundant in DJ-1^−/− mice. DJ-1 appeared also to be involved in ILCs development. Further, inflammatory genes related to PD were augmented in the midbrain of DJ-1^−/− mice. Our data suggest that metabolites and inflammation produced in the gut could be used as biomarkers for PD detection. Perhaps, these metabolites and inflammatory mediators could be involved in triggering inflammation resulting in PD pathology.

One Step Into the Future: New iPSC Tools to Advance Research in Parkinson's Disease and Neurological Disorders

Studying Parkinson’s disease (PD) in the laboratory presents many challenges, the main one being the limited availability of human cells and tissue from affected individuals. As PD is characterized by a loss of dopaminergic (DA) neurons in the brain, it is nearly impossible for researchers to access and extract these cells from living patients. Thus, in the past PD research has focused on the use of patients’ post-mortem tissues, animal models, or immortalized cell lines to dissect cellular pathways of interest. While these strategies deepened our knowledge of pathological mechanisms in PD, they failed to faithfully capture key mechanisms at play in the human brain. The emergence of induced pluripotent stem cell (iPSC) technology is revolutionizing PD research, as it allows for the differentiation and growth of human DA neurons in vitro, holding immense potential not only for modelling PD, but also for identifying novel therapies. However, to reproduce the complexity of the brain’s environment, researchers are recognizing the need to further develop and refine iPSC-based tools. In this review, we provide an overview of different systems now available for the study of PD, with a particular emphasis on the potential and limitations of iPSC as research tools to generate more relevant models of PD pathophysiology and advance the drug discovery process.

Noradrenergic pathways of locus coeruleus in Parkinson's and Alzheimer's pathology

Noradrenergic system of brain supplies the neurotransmitter noradrenalin throughout the brain through widespread efferent projections and play pivotal role in cognitive activities and could be involve in motor and non-motor symptoms of Parkinson's disease (PD) pathology. Profound loss of noradrenergic pathways has been reported in both Parkinson's and Alzheimer's disease (AD) pathology however their employment in therapeutics is still scarce. Therefore the present review is providing the various aspects for involvement on noradrenergic pathways in PD and AD pathology as well as the imaging of locus coeruleus as indicative diagnostic marker for disease. The present review is describing about the role of tiny nucleus locus coeruleus located noradrenergic pathways in said pathologies and discussing the past research as well as lacunas in this regard.

Disease model organism for Parkinson disease: Drosophila melanogaster

Parkinson’s disease (PD) is a common neurodegenerative disorder characterized by selective and progressive loss of dopaminergic neurons. Genetic and environmental risk factors are associated with this disease. The genetic factors are composed of approximately 20 genes, such as SNCA, parkin, PTEN-induced kinase1 (pink1), leucine-rich repeat kinase 2 (LRRK2), ATP13A2, MAPT, VPS35, and DJ-1, whereas the environmental factors consist of oxidative stress-induced toxins such as 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP), rotenone, and paraquat. The analyses of their functions and mechanisms have provided important insights into the disease process, which has demonstrated that these factors cause oxidative damage and mitochondrial dysfunction. The most invaluable studies have been performed using disease model organisms, such as mice, fruit flies, and worms. Among them, Drosophila melanogaster has emerged as an excellent model organism to study both environmental and genetic factors and provide insights to the pathways relevant for PD pathogenesis, facilitating development of therapeutic strategies. In this review, we have focused on the fly model organism to summarize recent progress, including pathogenesis, neuro-protective compounds, and newer approaches.

Characterization of Motor and Non-Motor Behavioral Alterations in the Dj-1 (PARK7) Knockout Rat

Parkinson's disease is a neurodegenerative disorder that encompasses a constellation of motor and non-motor symptoms. The etiology of the disease is still poorly understood because of complex interactions between environmental and genetic risk factors. Using animal models to assess these risk factors may lead to a better understanding of disease manifestation. In this study, we assessed the Dj-1 knockout (KO) genetic rat model in a battery of motor and non-motor behaviors. We tested the Dj-1 KO rat, as well as age-matched wild-type (WT) control rats, in several sensorimotor tests at 2, 4, 7, and 13 months of age. The Dj-1-deficient rats were found to rear and groom less, and to have a shorter stride length than their WT counterparts, but to take more forelimb and hindlimb steps. In non-motor behavioral tasks, performed at several different ages, we evaluated the following: olfactory function, anxiety-like behavior, short-term memory, anhedonia, and stress coping behavior. Non-motor testing was conducted as early as 4.5 months and as late as 17 months of age. We found that Dj-1 KO animals displayed deficits in short-term spatial memory as early as 4.5 months of age during place preference testing, as well as impaired coping strategies in the forced swim test, which are consistent with a parkinsonian-like phenotype. In some instances, effects of chronic stress were evaluated in the Dj-1-deficient rats, as an initial test of an environmental challenge combined with a genetic disposition for PD. Although some of the results were mixed with differential effects across several of the behaviors, the combination of the changes we observed indicates that the Dj-1 KO rat may be a promising model for the assessment of the prodromal stage of Parkinson's disease, but further evaluation is necessary.

Parkinson's disease protein DJ-1 regulates ATP synthase protein components to increase neuronal process outgrowth

Familial Parkinson’s disease (PD) protein DJ-1 mutations are linked to early onset PD. We have found that DJ-1 binds directly to the F₁F_O ATP synthase β subunit. DJ-1’s interaction with the β subunit decreased mitochondrial uncoupling and enhanced ATP production efficiency while in contrast mutations in DJ-1 or DJ-1 knockout increased mitochondrial uncoupling, and depolarized neuronal mitochondria. In mesencephalic DJ-1 KO cultures, there was a progressive loss of neuronal process extension. This was ameliorated by a pharmacological reagent, dexpramipexole, that binds to ATP synthase, closing a mitochondrial inner membrane leak and enhancing ATP synthase efficiency. ATP synthase c-subunit can form an uncoupling channel; we measured, therefore, ATP synthase F₁ (β subunit) and c-subunit protein levels. We found that ATP synthase β subunit protein level in the DJ-1 KO neurons was approximately half that found in their wild-type counterparts, comprising a severe defect in ATP synthase stoichiometry and unmasking c-subunit. We suggest that DJ-1 enhances dopaminergic cell metabolism and growth by its regulation of ATP synthase protein components.

Parkinson's disease: Mechanisms, translational models and management strategies

Parkinson's disease is a progressive neurodegenerative disorder. The classical motor symptoms include resting tremors, bradykinesia, rigidity and postural instability and are accompanied by the loss of dopaminergic neurons and Lewy pathology. Diminished neurotransmitter level, oxidative stress, mitochondrial dysfunction and perturbed protein homeostasis over time worsen the disease manifestations in elderly people. Current management strategies aim to provide symptomatic relief and to slow down the disease progression. However, no pharmacological breakthrough has been made to protect dopaminergic neurons and associated motor circuitry components. Deep brain stimulation, stem cells-derived dopaminergic neurons transplantation, gene editing and gene transfer remain promising approaches for the potential management of neurodegenerative disease. Toxin or genetically induced rodent models replicating Parkinson's disease pathology are of high predictive value for translational research. This review addresses the current understanding, management strategies and the Parkinson's disease models for translational research. Preclinical research may provide powerful tools to quest the potential therapeutic and neuroprotective compounds for dopaminergic neurons and hence possible cure for the Parkinson's disease.

Optogenetic Stimulation of the M2 Cortex Reverts Motor Dysfunction in a Mouse Model of Parkinson's Disease

Neuromodulation of deep brain structures (deep brain stimulation) is the current surgical procedure for treatment of Parkinson's disease (PD). Less studied is the stimulation of cortical motor areas to treat PD symptoms, although also known to alleviate motor disturbances in PD. We were able to show that optogenetic activation of secondary (M2) motor cortex improves motor functions in dopamine-depleted male mice. The stimulated M2 cortex harbors glutamatergic pyramidal neurons that project to subcortical structures, critically involved in motor control, and makes synaptic contacts with dopaminergic neurons. Strikingly, optogenetic activation of M2 neurons or axons into the dorsomedial striatum increases striatal levels of dopamine and evokes locomotor activity. We found that dopamine neurotransmission sensitizes the locomotor behavior elicited by activation of M2 neurons. Furthermore, combination of intranigral infusion of glutamatergic antagonists and circuit specific optogenetic stimulation revealed that behavioral response depended on the activity of M2 neurons projecting to SNc. Interestingly, repeated M2 stimulation combined with l-DOPA treatment produced an unanticipated improvement in working memory performance, which was absent in control mice under l-DOPA treatment only. Therefore, the M2-basal ganglia circuit is critical for the assembly of the motor and cognitive function, and this study demonstrates a therapeutic mechanism for cortical stimulation in PD that involves recruitment of long-range glutamatergic projection neurons.SIGNIFICANCE STATEMENT Some patients with Parkinson's disease are offered treatment through surgery, which consists of delivering electrical current to regions deep within the brain. This study shows that stimulation of an area located on the brain surface, known as the secondary motor cortex, can also reverse movement disorders in mice. Authors have used a brain stimulation technique called optogenetics, which allowed targeting a specific type of surface neuron that communicates with the deep part of the brain involved in movement control. The study also shows that a combination of this stimulation with drug treatment might be useful to treat memory impairment, a kind of cognitive problem in Parkinson's disease.

DJ-1 modulates the unfolded protein response and cell death via upregulation of ATF4 following ER stress

The unfolded protein response (UPR) triggered by endoplasmic reticulum (ER) stress is a feature of many neurodegenerative diseases including Alzheimer’s disease, Huntington’s disease and Parkinson’s disease (PD). Although the vast majority of PD is sporadic, mutations in a number of genes including PARK7 which encodes the protein DJ-1 have been linked to early-onset, familial PD. In this regard, both PD of sporadic and genetic origins exhibit markers of ER stress-induced UPR. However, the relationship between pathogenic mutations in PARK7 and ER stress-induced UPR in PD pathogenesis remains unclear. In most contexts, DJ-1 has been shown to protect against neuronal injury. However, we find that DJ-1 deficiency ameliorates death in the context of acute ER stress in vitro and in vivo. DJ-1 loss decreases protein and transcript levels of ATF4, a transcription factor critical to the ER response and reduces the levels of CHOP and BiP, its downstream effectors. The converse is observed with DJ-1 over-expression. Importantly, we find that over-expression of wild-type and PD-associated mutant form of PARK7^L166P, enhances ER stress-induced neuronal death by regulating ATF4 transcription and translation. Our results demonstrate a previously unreported role for wild-type and mutant DJ-1 in the regulation of UPR and provides a potential link to PD pathogenesis.

Depleting Trim28 in adult mice is well tolerated and reduces levels of α-synuclein and tau

Alzheimer's and Parkinson's disease are late onset neurodegenerative diseases that will require therapy over decades to mitigate the effects of disease-driving proteins such tau and α-synuclein (α-Syn). Previously we found that TRIM28 regulates the levels and toxicity of α-Syn and tau (Rousseaux et al., 2016). However, it was not clear how TRIM28 regulates α-Syn and it was not known if its chronic inhibition later in life was safe. Here, we show that TRIM28 may regulate α-Syn and tau levels via SUMOylation, and that genetic suppression of Trim28 in adult mice is compatible with life. We were surprised to see that mice lacking Trim28 in adulthood do not exhibit behavioral or pathological phenotypes, and importantly, adult reduction of TRIM28 results in a decrease of α-Syn and tau levels. These results suggest that deleterious effects from TRIM28 depletion are limited to development and that its inhibition adulthood provides a potential path for modulating α-Syn and tau levels.

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.

Mitochondrial dysfunction underlies cognitive defects as a result of neural stem cell depletion and impaired neurogenesis

Mitochondrial dysfunction is a common feature of many genetic disorders that target the brain and cognition. However, the exact role these organelles play in the etiology of such disorders is not understood. Here, we show that mitochondrial dysfunction impairs brain development, depletes the adult neural stem cell (NSC) pool and impacts embryonic and adult neurogenesis. Using deletion of the mitochondrial oxidoreductase AIF as a genetic model of mitochondrial and neurodegenerative diseases revealed the importance of mitochondria in multiple steps of the neurogenic process. Developmentally, impaired mitochondrial function causes defects in NSC self-renewal, neural progenitor cell proliferation and cell cycle exit, as well as neuronal differentiation. Sustained mitochondrial dysfunction into adulthood leads to NSC depletion, loss of adult neurogenesis and manifests as a decline in brain function and cognitive impairment. These data demonstrate that mitochondrial dysfunction, as observed in genetic mitochondrial and neurodegenerative diseases, underlies the decline of brain function and cognition due to impaired stem cell maintenance and neurogenesis.

Chronic exposure to a glyphosate-containing pesticide leads to mitochondrial dysfunction and increased reactive oxygen species production in Caenorhabditis elegans

Glyphosate-containing herbicides are among the most widely-used in the world. Although glyphosate itself is relatively non-toxic, growing evidence suggests that commercial herbicide formulations may lead to increased oxidative stress and mitochondrial inhibition. In order to assess these mechanisms in vivo, we chronically (24h) exposed Caenorhabditis elegans to various concentrations of the glyphosate-containing herbicide TouchDown (TD). Following TD exposure, we evaluated the function of specific mitochondrial electron transport chain complexes. Initial oxygen consumption studies demonstrated inhibition in mid- and high-TD concentration treatment groups compared to controls. Results from tetramethylrhodamine ethyl ester and ATP assays indicated reductions in the proton gradient and ATP levels, respectively. Additional studies were designed to determine whether TD exposure resulted in increased reactive oxygen species (ROS) production. Data from hydrogen peroxide, but not superoxide or hydroxyl radical, assays showed statistically significant increases in this specific ROS. Taken together, these data indicate that exposure of Caenorhabditis elegans to TD leads to mitochondrial inhibition and hydrogen peroxide production.

TRIM28 regulates the nuclear accumulation and toxicity of both alpha-synuclein and tau

Several neurodegenerative diseases are driven by the toxic gain-of-function of specific proteins within the brain. Elevated levels of alpha-synuclein (α-Syn) appear to drive neurotoxicity in Parkinson's disease (PD); neuronal accumulation of tau is a hallmark of Alzheimer's disease (AD); and their increased levels cause neurodegeneration in humans and model organisms. Despite the clinical differences between AD and PD, several lines of evidence suggest that α-Syn and tau overlap pathologically. The connections between α-Syn and tau led us to ask whether these proteins might be regulated through a shared pathway. We therefore screened for genes that affect post-translational levels of α-Syn and tau. We found that TRIM28 regulates α-Syn and tau levels and that its reduction rescues toxicity in animal models of tau- and α-Syn-mediated degeneration. TRIM28 stabilizes and promotes the nuclear accumulation and toxicity of both proteins. Intersecting screens across comorbid proteinopathies thus reveal shared mechanisms and therapeutic entry points.

DOI:http://dx.doi.org/10.7554/eLife.19809.001

Behind many neurodegenerative diseases are specific proteins that abnormally accumulate inside neurons and damage the cells. In Parkinson’s disease, the protein alpha-synuclein accumulates; in Alzheimer’s disease, the protein tau is one of the toxic culprits; and in other neurodegenerative diseases, alpha-synuclein and tau both accumulate. Genetic studies suggest that accumulation of the two proteins may be linked, but little is known about the factors that regulate the levels of these proteins inside neurons.

Rousseaux et al. set out to identify how these proteins are regulated in the hope of finding new ways of targeting them and reducing their toxicity. Screening a subset of human genes led to one that encodes a protein called TRIM28, which regulates the levels of both alpha-synuclein and tau. When the TRIM28 protein was depleted in human and mouse cells, the levels of alpha-synuclein and tau also went down. This effect was specific because the levels of other proteins with the potential to cause neurodegeneration remained unaffected.

Models of neurodegenerative disease in fruit flies and mice were then used to explore how TRIM28 affects the levels of tau and alpha-synuclein in animals. In each case, the proteins’ levels dropped when TRIM28 was suppressed and this in turn protected the neurons from damage. Rousseaux et al. went on to show that TRIM28 affected how alpha-synuclein and tau were cleared in cells. Overexpressing TRIM28 revealed that it could encourage both alpha-synuclein and tau to accumulate in the nucleus of cells over time.

Finally, Rousseaux et al. compared post-mortem brain tissue from people who had neurodegenerative conditions that are driven by or associated with tau and alpha-synuclein with tissue from those who did not. The cell nuclei in the diseased tissue had much more TRIM28 associated with alpha-synuclein and tau than those in the healthy tissues.

Overall, the findings show that TRIM28 promotes the accumulation and damaging effects of both alpha-synuclein and tau. The next steps will be to understand how TRIM28 does this. It will also be important to determine if this effect can be targeted, whilst leaving others roles of TRIM28 intact, in order to explore it as a potential target to treat or prevent neurodegenerative diseases.

DOI:http://dx.doi.org/10.7554/eLife.19809.002

Evaluation of Models of Parkinson's Disease

Parkinson's disease is one of the most common neurodegenerative diseases. Animal models have contributed a large part to our understanding and therapeutics developed for treatment of PD. There are several more exhaustive reviews of literature that provide the initiated insights into the specific models; however a novel synthesis of the basic advantages and disadvantages of different models is much needed. Here we compare both neurotoxin based and genetic models while suggesting some novel avenues in PD modeling. We also highlight the problems faced and promises of all the mammalian models with the hope of providing a framework for comparison of various systems.

DJ-1 mutation decreases astroglial release of inflammatory mediators

Mutations in DJ-1, reactive gliosis and concomitant inflammatory processes are implicated in the pathogenesis and progression of Parkinson's disease (PD). To study the physiological consequences of DJ-1 mutation in the context of neuroinflammatory insult, primary cortical astrocytes were isolated from DJ-1 knockout mice. Astrocytes were exposed to 1μg/mL lipopolysaccharide (LPS) for 24h following 2h pre-exposure to inhibitors of MEK (U0126), JNK (JNK inhibitor II) or p38 (SB203580). Real-time PCR was used to assess the LPS-induced expression of pro-inflammatory mediators cyclooxygenase 2 (COX2), inducible nitric oxide synthetase (NOS2), and tumor necrosis factor α (TNFα). LPS-induced expression of COX2 decreased similarly in DJ-1(+/+) and DJ-1(-/-) astrocytes in response to inhibition of p38, but was unaffected by inhibition of MEK or JNK. No significant alterations in NOS2 expression were observed in any inhibitor-treated cells. The inhibitors did not affect expression of TNFα; however, DJ-1(-/-) astrocytes had consistently lower expression compared to DJ-1(+/+) counterparts. Secretion of TNFα and prostaglandin E2 (PGE2) into the culture medium was significantly decreased in DJ-1(-/-) astrocytes, and inhibition of p38 decreased this secretion in both genotypes. In conclusion, DJ-1(-/-) astrocytes may provide decreased neuroprotection to surrounding neurons due to alterations in pro-inflammatory mediator expression.

Reduced apurinic/apyrimidinic endonuclease 1 activity and increased DNA damage in mitochondria are related to enhanced apoptosis and inflammation in the brain of senescence- accelerated P8 mice (SAMP8)

The senescence- accelerated mouse prone 8 (SAMP8) is a well- characterized animal model of senescence that shows early age- related neurodegeneration with impairment in learning and memory skills when compared with control senescence- resistant mice (SAMR1). In the current study, we investigated whether such impairment could be partly due to changes in mitochondrial DNA (mtDNA) repair capacity and mitochondrial DNA damage in the brain of SAMP8 mice. Besides we studied whether these potential changes were related to modifications in two major processes likely involved in aging and neurodegeneration: apoptosis and inflammation. We observed that the specific activity of one of the main mtDNA repair enzymes, the mitochondrial APE1, showed an age- related reduction in SAMP8 animals, while in SAMR1 mice mitochondrial APE1 increased with age. The reduction in mtAPE1 activity in SAMP8 animals was associated with increased levels of the DNA oxidative damage marker 8oxodG in mtDNA. Our results also indicate that these changes were related to a premature increase in apoptotic events and inflammation in the brain of SAMP8 mice when compared to SAMR1 counterparts. We suggest that the premature neurodegenerative phenotype observed in SAMP8 animals might be due, at least in part, to changes in the processing of mtDNA oxidative damage, which would lead to enhancement of apoptotic and inflammatory processes.

Animal models of Parkinson's disease: An updated overview

Parkinson's disease (PD) is a progressive neurodegenerative disorder whose etiology, besides a minority of genetic cases, is still largely unknown. Animal models have contributed to elucidate PD etiology and pathogenesis, as well as its cellular and molecular mechanisms, leading to the general hypothesis that this neurological disorder is due to complex interactions between environmental and genetic factors. However, the full understanding of PD is still very far from being achieved, and new potential treatments need to be tested to further improve patients' quality of life and, possibly, slow down the neurodegenerative process. In this context, animal models of PD are required to address all these issues. "Classic" models are based on neurotoxins that selectively target catecholaminergic neurons (such as 6-hydroxydopamine, 1-methyl-1,2,3,6-tetrahydropiridine, agricultural pesticides, etc.), while more recent models employ genetic manipulations that either introduce mutations similar to those find in familial cases of PD (α-synuclein, DJ-1, PINK1, Parkin, etc.) or selectively disrupt nigrostriatal neurons (MitoPark, Pitx3, Nurr1, etc.). Each one of these models has its own advantages and limitations, thus some are better suited for studying PD pathogenesis, while others are more pertinent to test therapeutic treatments. Here, we provide a critical and updated review of the most used PD models.

Overexpression of DJ-1 protects against C2-ceramide-induced neuronal death through activation of the PI3K/AKT pathway and inhibition of autophagy

Parkinson's disease (PD) is the second most frequent neurodegenerative disorder. It is characterized by selective degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc). Early-onset familial forms of PD are associated with mutations in several genes, including parkin, pink1 and dj-1. DJ-1 encodes a protein whose neuroprotective function has not been completely clarified yet. We aim to understand the neuroprotective mechanisms of DJ-1, in particular, DJ-1's involvement in the regulation of the PI3K/PTEN/AKT/mTOR pathway and neuronal autophagy in a neurotoxic context induced by C2-ceramide, by using CAD cells, a murine cathecolaminergic cell line. We demonstrated that C2-ceramide induces CAD cell death associated with decreased phosphorylation of PTEN at Ser380, AKT at Ser473, and mTOR at Ser2448; and increased of autophagic flux (increased LC3-II and autophagosome formation). Additionally, we showed that overexpression of DJ-1 protects against C2-ceramide-induced neuronal death and it is not associated with change in the phosphorylation of mTOR at Ser2448. In conclusion, these data suggest that DJ-1 reinforces the PI3K/AKT survival pathway and inhibits autophagy, probably by a mechanism independent from mTOR.

The Polg Mutator Phenotype Does Not Cause Dopaminergic Neurodegeneration in DJ-1-Deficient Mice

Mutations in the DJ-1 gene cause autosomal recessive parkinsonism in humans. Several mouse models of DJ-1 deficiency have been developed, but they do not have dopaminergic neuron cell death in the substantia nigra pars compacta (SNpc). Mitochondrial DNA (mtDNA) damage occurs frequently in the aged human SNpc but not in the mouse SNpc. We hypothesized that the reason DJ-1-deficient mice do not have dopaminergic cell death is due to an absence of mtDNA damage. We tested this hypothesis by crossing DJ-1-deficient mice with mice that have similar amounts of mtDNA damage in their SNpc as aged humans (Polg mutator mice). At 1 year of age, we counted the amount of SNpc dopaminergic neurons in the mouse brains using both colorimetric and fluorescent staining followed by unbiased stereology. No evidence of dopaminergic cell death was observed in DJ-1-deficient mice with the Polg mutator mutation. Furthermore, we did not observe any difference in dopaminergic terminal immunostaining in the striatum of these mice. Finally, we did not observe any changes in the amount of GFAP-positive astrocytes in the SNpc of these mice, indicative of a lack of astrogliosis. Altogether, our findings demonstrate the DJ-1-deficient mice, Polg mutator mice, and DJ-1-deficient Polg mutator mice have intact nigrastriatal pathways. Thus, the lack of mtDNA damage in the mouse SNpc does not underlie the absence of dopaminergic cell death in DJ-1-deficient mice.

Therapeutic strategies in Parkinson's disease: what we have learned from animal models

Parkinson's disease (PD), the second most common neurodegenerative disorder, is characterized by a loss of dopaminergic neurons in the substantia nigra, as well as in other brain areas. The currently available dopamine replacement therapy provides merely symptomatic benefit and is ineffective because habituation and side effects arise relatively quickly. Studying the genetic forms of PD in animal models provides novel insight that allows targeting of specific aspects of this heterogenic disease more specifically. Among others, two important cellular deficits are associated with PD; these deficits relate to (1) synaptic transmission and vesicle trafficking, and (2) mitochondrial function, relating respectively to the dominant and recessive mutations in PD-causing genes. With increased knowledge of PD, the possibility of identifying an efficient, long-lasting treatment is becoming more conceivable, but this can only be done with an increased knowledge of the specific affected cellular mechanisms. This review discusses how discoveries in animal models of PD have clarified the therapeutic potential of pathways disrupted in PD, with a specific focus on synaptic transmission, vesicle trafficking, and mitochondrial function.

Parkinson's disease: animal models and dopaminergic cell vulnerability

Parkinson's disease (PD) is a neurodegenerative disorder that affects about 1.5% of the global population over 65 years of age. A hallmark feature of PD is the degeneration of the dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and the consequent striatal DA deficiency. Yet, the pathogenesis of PD remains unclear. Despite tremendous growth in recent years in our knowledge of the molecular basis of PD and the molecular pathways of cell death, important questions remain, such as: (1) why are SNc cells especially vulnerable; (2) which mechanisms underlie progressive SNc cell loss; and (3) what do Lewy bodies or α-synuclein reveal about disease progression. Understanding the variable vulnerability of the dopaminergic neurons from the midbrain and the mechanisms whereby pathology becomes widespread are some of the primary objectives of research in PD. Animal models are the best tools to study the pathogenesis of PD. The identification of PD-related genes has led to the development of genetic PD models as an alternative to the classical toxin-based ones, but does the dopaminergic neuronal loss in actual animal models adequately recapitulate that of the human disease? The selection of a particular animal model is very important for the specific goals of the different experiments. In this review, we provide a summary of our current knowledge about the different in vivo models of PD that are used in relation to the vulnerability of the dopaminergic neurons in the midbrain in the pathogenesis of PD.

Molecular determinants of selective dopaminergic vulnerability in Parkinson's disease: an update

Numerous disorders of the central nervous system (CNS) are attributed to the selective death of distinct neuronal cell populations. Interestingly, in many of these conditions, a specific subset of neurons is extremely prone to degeneration while other, very similar neurons are less affected or even spared for many years. In Parkinson’s disease (PD), the motor manifestations are primarily linked to the selective, progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). In contrast, the very similar DA neurons in the ventral tegmental area (VTA) demonstrate a much lower degree of degeneration. Elucidating the molecular mechanisms underlying the phenomenon of differential DA vulnerability in PD has proven extremely challenging. Moreover, an increasing number of studies demonstrate that considerable molecular and electrophysiologic heterogeneity exists among the DA neurons within the SNpc as well as those within the VTA, adding yet another layer of complexity to the selective DA vulnerability observed in PD. The discovery of key pathways that regulate this differential susceptibility of DA neurons to degeneration holds great potential for the discovery of novel drug targets and the development of promising neuroprotective treatment strategies. This review provides an update on the molecular basis of the differential vulnerability of midbrain DA neurons in PD and highlights the most recent developments in this field.

Rgs6 is required for adult maintenance of dopaminergic neurons in the ventral substantia nigra

Parkinson disease (PD) is characterized by the preferential, but poorly understood, vulnerability to degeneration of midbrain dopaminergic (mDA) neurons in the ventral substantia nigra compacta (vSNc). These sensitive mDA neurons express Pitx3, a transcription factor that is critical for their survival during development. We used this dependence to identify, by flow cytometry and expression profiling, the negative regulator of G-protein signaling Rgs6 for its restricted expression in these neurons. In contrast to Pitx3^−/− mDA neurons that die during fetal (vSNc) or post-natal (VTA) period, the vSNc mDA neurons of Rgs6^−/− mutant mice begin to exhibit unilateral signs of degeneration at around 6 months of age, and by one year cell loss is observed in a fraction of mice. Unilateral cell loss is accompanied by contralateral degenerating neurons that exhibit smaller cell size, altered morphology and reduced dendritic network. The degenerating neurons have low levels of tyrosine hydroxylase (TH) and decreased nuclear Pitx3; accordingly, expression of many Pitx3 target gene products is altered, including Vmat2, Bdnf, Aldh1a1 (Adh2) and Fgf10. These low TH neurons also express markers of increased dopamine signaling, namely increased DAT and phospho-Erk1/2 expression. The late onset degeneration may reflect the protective action of Rgs6 against excessive DA signaling throughout life. Rgs6-dependent protection is thus critical for adult survival and maintenance of the vSNc mDA neurons that are most affected in PD.

The locomotor deficits associated with Parkinson disease result from the death of a specific subset of dopamine neurons in the ventral part of the midbrain. The reason for the greater sensitivity to degeneration of those, relative to other, neurons is not clear. Prior work showed that the Pitx3 transcription factor is specifically expressed in these neurons where it has a survival role during development. The present work identified a cell signaling component, Rgs6, that is also restricted to the sensitive neurons in the midbrain and that exerts a protective function, particularly late in life. While the loss of Rgs6 function may predispose or contribute to Parkinson disease, its stimulation may provide a novel therapeutic avenue to treat Parkinson disease.

Efficient bi-allelic gene knockout and site-specific knock-in mediated by TALENs in pigs

Pigs are ideal organ donors for xenotransplantation and an excellent model for studying human diseases, such as neurodegenerative disease. Transcription activator-like effector nucleases (TALENs) are used widely for gene targeting in various model animals. Here, we developed a strategy using TALENs to target the GGTA1, Parkin and DJ-1 genes in the porcine genome using Large White porcine fibroblast cells without any foreign gene integration. In total, 5% (2/40), 2.5% (2/80), and 22% (11/50) of the obtained colonies of fibroblast cells were mutated for GGTA1, Parkin, and DJ-1, respectively. Among these mutant colonies, over 1/3 were bi-allelic knockouts (KO), and no off-target cleavage was detected. We also successfully used single-strand oligodeoxynucleotides to introduce a short sequence into the DJ-1 locus. Mixed DJ-1 mutant colonies were used as donor cells for somatic cell nuclear transfer (SCNT), and three female piglets were obtained (two were bi-allelically mutated, and one was mono-allelically mutated). Western blot analysis showed that the expression of the DJ-1 protein was disrupted in KO piglets. These results imply that a combination of TALENs technology with SCNT can efficiently generate bi-allelic KO pigs without the integration of exogenous DNA. These DJ-1 KO pigs will provide valuable information for studying Parkinson's disease.

A new split-GFP-based probe reveals DJ-1 translocation into the mitochondrial matrix to sustain ATP synthesis upon nutrient deprivation

The Parkinson's disease-related protein DJ-1 has a role in the protection against oxidative stress and maintenance of mitochondria structure. Whether this action depends on its localization and activity within the mitochondria is not clear. Here we develop an approach to resolve intra-mitochondrial distribution of DJ-1 and monitor its translocation under specific conditions. By a new split-green fluorescent protein (GFP)-based tool, we can observe that a small DJ-1 fraction is located within the mitochondrial matrix and that it consistently increases upon nutrient depletion. We also find that the targeting of DJ-1 to the mitochondrial matrix enhances mitochondrial and cytosolic adenosine triphosphate levels. Intriguingly, DJ-1 pathogenic mutants fail to improve bioenergetics and translocate within the mitochondrial matrix, suggesting that the DJ-1 protective role requires both these actions. By this new split-GFP-based tool, we can resolve mitochondrial compartmentalization of proteins which are not constitutively resident in mitochondria but translocate to them in response to specific stimuli.