Lack of additive role of ageing in nigrostriatal neurodegeneration triggered by α-synuclein overexpression

Antimicrobial drugs for Parkinson's disease: Existing therapeutic strategies and novel drugs exploration

Parkinson's disease (PD), the second most common neurodegenerative disorder, is characterized by loss of dopaminergic neurons in the substantia nigra, as well as the abnormal accumulation of misfolded α-synuclein. Clinically, PD is featured by typical motor symptoms and some non-motor symptoms. Up to now, although considerable progress has been made in understanding the pathogenesis of PD, there is still no effective therapeutic treatment for the disease. Thus, exploring new therapeutic strategies has been a topic that needs to be addressed urgently. Noteworthy, with the proposal of the microbiota-gut-brain axis theory, antimicrobial drugs have received significant attention due to their effects on regulating the intestinal microbiota. Nowadays, there is growing evidence showing that some antimicrobial drugs may be promising drugs for the treatment of PD. Data from pre-clinical and clinical studies have shown that some antimicrobial drugs may play neuroprotective roles in PD by modulating multiple biochemical and molecular pathways, including reducing α-synuclein aggregation, inhibiting neuroinflammation, regulating mitochondrial structure and function, as well as suppressing oxidative stress. In this paper, we summarized the effects of some antimicrobial drugs on PD treatment from recent pre-clinical and clinical studies. Then, we further discussed the potential of a few antimicrobial drugs for treating PD based on molecular docking and molecular dynamics simulation. Importantly, we highlighted the potential of clorobiocin as the therapeutic strategy for PD owing to its ability to inhibit α-synuclein aggregation. These results will help us to better understand the potential of antimicrobial drugs in treating PD and how antimicrobial drugs may alleviate or reverse the pathological symptoms of PD.

In vivo bioluminescence imaging of the intracerebral fibroin-controlled AAV-α-synuclein diffusion for monitoring the central nervous system and peripheral expression

Among the several animal models of α-synucleinopathies, the well-known viral vector-mediated delivery of wild-type or mutated (A53T) α-synuclein requires new tools to increase the lesion in mice and follow up in vivo expression. To this end, we developed a bioluminescent expression reporter of the human A53T-α-synuclein gene using the NanoLuc system into an AAV2/9, embedded or not in a fibroin solution to stabilise its expression in space and time. We first verified the expression of the fused protein in vitro on transfected cells by bioluminescence and Western blotting. Next, two groups of C57Bl6Jr mice were unilaterally injected with the AAV-NanoLuc-human-A53T-α-synuclein above the substantia nigra combined (or not) with fibroin. We first show that the in vivo cerebral bioluminescence signal was more intense in the presence of fibroin. Using immunohistochemistry, we find that the human-A53T-α-synuclein protein is more restricted to the ipsilateral side with an overall greater magnitude of the lesion when fibroin was added. However, we also detected a bioluminescence signal in peripheral organs in both conditions, confirmed by the presence of viral DNA corresponding to the injected AAV in the liver using qPCR.

Development and characterization of a non-human primate model of disseminated synucleinopathy

Introduction

The presence of a widespread cortical synucleinopathy is the main neuropathological hallmark underlying clinical entities such as Parkinson's disease with dementia (PDD) and dementia with Lewy bodies (DLB). There currently is a pressing need for the development of non-human primate (NHPs) models of PDD and DLB to further overcome existing limitations in drug discovery.

Methods

Here we took advantage of a retrogradely-spreading adeno-associated viral vector serotype 9 coding for the alpha-synuclein A53T mutated gene (AAV9-SynA53T) to induce a widespread synucleinopathy of cortical and subcortical territories innervating the putamen. Four weeks post-AAV deliveries animals were sacrificed and a comprehensive biodistribution study was conducted, comprising the quantification of neurons expressing alpha-synuclein, rostrocaudal distribution and their specific location.

Results

Intraputaminal deliveries of AAV9-SynA53T lead to a disseminated synucleinopathy throughout ipsi- and contralateral cerebral cortices, together with transduced neurons located in the ipsilateral caudal intralaminar nuclei and in the substantia nigra pars compacta (leading to thalamostriatal and nigrostriatal projections, respectively). Cortical afferent systems were found to be the main contributors to putaminal afferents (superior frontal and precentral gyri in particular).

Discussion

Obtained data extends current models of synucleinopathies in NHPs, providing a reproducible platform enabling the adequate implementation of end-stage preclinical screening of new drugs targeting alpha-synuclein.

Dopaminergic neuron loss in mice due to increased levels of wild-type human α-Synuclein only takes place under conditions of accelerated aging

Understanding the intricate pathogenic mechanisms behind Parkinson's disease (PD) and its multifactorial nature presents a significant challenge in disease modeling. To address this, we explore genetic models that better capture the disease's complexity. Given that aging is the primary risk factor for PD, this study investigates the impact of aging in conjunction with overexpression of wild-type human α-synuclein (α-Syn) in the dopaminergic system. This is achieved by introducing a novel transgenic mouse strain overexpressing α-Syn under the TH-promoter within the senescence-accelerated SAMP8 (P8) genetic background. Behavioral assessments, conducted at both 10 and 16 months of age, unveil motor impairments exclusive to P8 α-SynTg mice, a phenomenon conspicuously absent in α-SynTg mice. These findings suggest a synergistic interplay between heightened α-Syn levels and the aging process, resulting in motor deficits. These motor disturbances correlate with reduced dopamine (DA) levels, increased DA turnover, synaptic terminal loss, and notably, the depletion of dopaminergic neurons in the substantia nigra and noradrenergic neurons in the locus coeruleus. Furthermore, P8 α-SynTg mice exhibit alterations in gut transit time, mirroring early PD symptoms. In summary, P8 α-SynTg mice effectively replicate parkinsonian phenotypes by combining α-Syn transgene expression with accelerated aging. This model offers valuable insights into the understanding of PD and serves as a valuable platform for further research.

Automated procedure to detect subtle motor alterations in the balance beam test in a mouse model of early Parkinson's disease

Parkinson's disease (PD) is the most common motor neurodegenerative disorder, characterised by aggregated α-synuclein (α-syn) constituting Lewy bodies. We aimed to investigate temporal changes in motor impairments in a PD mouse model induced by overexpression of α-syn with the conventional manual analysis of the balance beam test and a novel approach using machine learning algorithms to automate behavioural analysis. We combined automated animal tracking using markerless pose estimation in DeepLabCut, with automated behavioural classification in Simple Behavior Analysis. Our automated procedure was able to detect subtle motor deficits in mouse performances in the balance beam test that the manual analysis approach could not assess. The automated model revealed time-course significant differences for the "walking" behaviour in the mean interval between each behavioural bout, the median event bout duration and the classifier probability of occurrence in male PD mice, even though no statistically significant loss of tyrosine hydroxylase in the nigrostriatal system was found in either sex. These findings are valuable for early detection of motor impairment in early PD animal models. We provide a user-friendly, step-by-step guide for automated assessment of mouse performances in the balance beam test, which aims to be replicable without any significant computational and programming knowledge.

Neuromelanin accumulation drives endogenous synucleinopathy in non-human primates

Although neuromelanin is a dark pigment characteristic of dopaminergic neurons in the human substantia nigra pars compacta, its potential role in the pathogenesis of Parkinson's disease (PD) has often been neglected since most commonly used laboratory animals lack neuromelanin. Here we took advantage of adeno-associated viral vectors encoding the human tyrosinase gene for triggering a time-dependent neuromelanin accumulation within substantia nigra pars compacta dopaminergic neurons in macaques up to similar levels of pigmentation as observed in elderly humans. Furthermore, neuromelanin accumulation induced an endogenous synucleinopathy mimicking intracellular inclusions typically observed in PD together with a progressive degeneration of neuromelanin-expressing dopaminergic neurons. Moreover, Lewy body-like intracellular inclusions were observed in cortical areas of the frontal lobe receiving dopaminergic innervation, supporting a circuit-specific anterograde spread of endogenous synucleinopathy by permissive trans-synaptic templating. In summary, the conducted strategy resulted in the development and characterization of a new macaque model of PD matching the known neuropathology of this disorder with unprecedented accuracy. Most importantly, evidence is provided showing that intracellular aggregation of endogenous α-synuclein is triggered by neuromelanin accumulation, therefore any therapeutic approach intended to decrease neuromelanin levels may provide appealing choices for the successful implementation of novel PD therapeutics.

Host-to-graft Propagation of α-synuclein in a Mouse Model of Parkinson's Disease: Intranigral Versus Intrastriatal Transplantation

BACKGROUND

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) and by the accumulation of misfolded α-synuclein (α-syn) in Lewy bodies. Ectopic transplantation of human fetal ventral mesencephalic DA neurons into the striatum of PD patients have provided proof-of-principle for the cell replacement strategy in this disorder. However, 10 to 22 y after transplantation, 1% to 27% of grafted neurons contained α-syn aggregates similar to those observed in the host brain. We hypothesized that intrastriatal grafts are more vulnerable to α-syn propagation because the striatum is not the ontogenic site of nigral DA neurons and represents an unfavorable environment for transplanted neurons. Here, we compared the long-term host-to-graft propagation of α-syn in 2 transplantation sites: the SNpc and the striatum.

METHODS

Two mouse models of PD were developed by injecting adeno-associated-virus2/9-human α-syn A53T into either the SNpc or the striatum of C57BL/6 mice. Mouse fetal ventral mesencephalic DA progenitors were grafted into the SNpc or into the striatum of SNpc or striatum of α-syn injected mice, respectively.

RESULTS

First, we have shown a degeneration of the nigrostriatal pathway associated with motor deficits after nigral but not striatal adeno-associated-virus-hαsyn A53T injection. Second, human α-syn preferentially accumulates in striatal grafts compared to nigral grafts. However, no differences were observed for phosphorylated α-syn, a marker of pathological α-syn aggregates.

CONCLUSIONS

Taken together, our results suggest that the ectopic site of the transplantation impacts the host-to-graft transmission of α-syn.

Functional and neuropathological changes induced by injection of distinct alpha-synuclein strains: A pilot study in non-human primates

The role of alpha-synuclein in Parkinson's disease has been heavily investigated since its discovery as a component of Lewy bodies. Recent rodent data demonstrate that alpha-synuclein strain structure is critical for differential propagation and toxicity. Based on these findings, we have compared, for the first time, in this pilot study, the capacity of two alpha-synuclein strains and patient-derived Lewy body extracts to model synucleinopathies after intra-putaminal injection in the non-human primate brain. Functional alterations triggered by these injections were evaluated in vivo using glucose positron emission tomography imaging. Post-mortem immunohistochemical and biochemical analyses were used to detect neuropathological alterations in the dopaminergic system and alpha-synuclein pathology propagation. In vivo results revealed a decrease in glucose metabolism more pronounced in alpha-synuclein strain-injected animals. Histology showed a decreased number of dopaminergic tyrosine hydroxylase-positive cells in the substantia nigra to different extents according to the inoculum used. Biochemistry revealed that alpha-synuclein-induced aggregation, phosphorylation, and propagation in different brain regions are strain-specific. Our findings show that distinct alpha-synuclein strains can induce specific patterns of synucleinopathy in the non-human primate, changes in the nigrostriatal pathway, and functional alterations that resemble early-stage Parkinson's disease.

Premorbid performances determine the deleterious effects of nigrostriatal degeneration and pramipexole on behavioural flexibility

Subtle cognitive impairment can occur early in the course of Parkinson's disease (PD) and may manifest under different forms of executive dysfunction such as impaired cognitive flexibility. The precise contribution of nigrostriatal dopaminergic neurodegeneration to these non-motor features of the disease is poorly known. Whether such cognitive impairment associated with the disease process may also predate and contribute to the development of neuropsychiatric side-effects following dopamine replacement therapy remains largely unknown. To address these issues, we investigated the respective contributions of nigrostriatal degeneration and chronic treatment with the dopamine D3-preferring agonist pramipexole on behavioral flexibility in a rat model of PD. Flexible, intermediate and inflexible rats were identified based on baseline assessment of behavioral flexibility using an operant set-shifting task. Nigrostriatal degeneration was induced by bilateral viral-mediated expression of A53T mutated human α-synuclein in the substantia nigra pars compacta and behavioral flexibility was assessed after induction of nigrostriatal degeneration, and during chronic pramipexole treatment. Nigrostriatal degeneration impaired behavioral flexibility in flexible but not in inflexible rats. Pramipexole induced a decrease of behavioral flexibility that was exacerbated in lesioned rats and in the most flexible individuals. Furthermore, the deficits induced by pramipexole in lesioned rats affected different components of the task between flexible and inflexible individuals. This study demonstrates that nigrostriatal degeneration and pramipexole unequally impair behavioral flexibility, suggesting that the susceptibility to develop non-motor impairments upon treatment initiation could primarily depend on premorbid differences in behavioral flexibility.

Impact of aging on animal models of Parkinson's disease

Aging is the biggest risk factor for developing Parkinson's disease (PD), the second most common neurodegenerative disorder. Several animal models have been developed to explore the pathophysiology underlying neurodegeneration and the initiation and spread of alpha-synuclein-related PD pathology, and to investigate biomarkers and therapeutic strategies. However, bench-to-bedside translation of preclinical findings remains suboptimal and successful disease-modifying treatments remain to be discovered. Despite aging being the main risk factor for developing idiopathic PD, most studies employ young animals in their experimental set-up, hereby ignoring age-related cellular and molecular mechanisms at play. Consequently, studies in young animals may not be an accurate reflection of human PD, limiting translational outcomes. Recently, it has been shown that aged animals in PD research demonstrate a higher susceptibility to developing pathology and neurodegeneration, and present with a more disseminated and accelerated disease course, compared to young animals. Here we review recent advances in the investigation of the role of aging in preclinical PD research, including challenges related to aged animal models that are limiting widespread use. Overall, current findings indicate that the use of aged animals may be required to account for age-related interactions in PD pathophysiology. Thus, although the use of older animals has disadvantages, a model that better represents clinical disease within the elderly would be more beneficial in the long run, as it will increase translational value and minimize the risk of therapies failing during clinical studies. Furthermore, we provide recommendations to manage the challenges related to aged animal models.

NXP031 prevents dopaminergic neuronal loss and oxidative damage in the AAV-WT-α-synuclein mouse model of Parkinson’s disease

Parkinson’s disease (PD) is a neurodegenerative disease characterized by inclusions of aggregated α-synuclein (α-Syn). Oxidative stress plays a critical role in nigrostriatal degeneration and is responsible for α-Syn aggregation in PD. Vitamin C or ascorbic acid acts as an effective antioxidant to prevent free radical damage. However, vitamin C is easily oxidized and often loses its physiological activity, limiting its therapeutic potential. The objective of this study was to evaluate whether NXP031, a new compound we developed consisting of Aptamin C and Vitamin C, is neuroprotective against α-synucleinopathy. To model α-Syn induced PD, we stereotactically injected AAV particles overexpressing human α-Syn into the substantia nigra (SN) of mice. One week after AAV injection, NXP031 was administered via oral gavage every day for eight weeks. We found that oral administration of NXP031 ameliorated motor deficits measured by the rotarod test and prevented the loss of nigral dopaminergic neurons caused by WT-α-Syn overexpression in the SN. Also, NXP031 blocked the propagation of aggregated α-Syn into the hippocampus by alleviating oxidative stress. These results indicate that NXP031 can be a potential therapeutic for PD.

α-Synuclein Radiotracer Development and In Vivo Imaging: Recent Advancements and New Perspectives

α-Synucleinopathies including idiopathic Parkinson's disease, dementia with Lewy bodies and multiple systems atrophy share overlapping symptoms and pathological hallmarks. Selective neurodegeneration and Lewy pathology are the main hallmarks of α-synucleinopathies. Currently, there is no imaging biomarker suitable for a definitive early diagnosis of α-synucleinopathies. Although dopaminergic deficits detected with single-photon emission computed tomography (SPECT) and positron emission tomography (PET) radiotracers can support clinical diagnosis by confirming the presence of dopaminergic neurodegeneration, dopaminergic imaging cannot visualize the preceding disease process, nor distinguish α-synucleinopathies from tauopathies with dopaminergic neurodegeneration, especially at early symptomatic disease stage when clinical presentation is often overlapping. Aggregated α-synuclein (αSyn) could be a suitable imaging biomarker in α-synucleinopathies, because αSyn aggregation and therefore, Lewy pathology is evidently an early driver of α-synucleinopathies pathogenesis. Additionally, several antibodies and small molecule compounds targeting aggregated αSyn are in development for therapy. However, there is no way to directly measure if or how much they lower the levels of aggregated αSyn in the brain. There is clearly a paramount diagnostic and therapeutic unmet medical need. To date, aggregated αSyn and Lewy pathology inclusion bodies cannot be assessed ante-mortem with SPECT or PET imaging because of the suboptimal binding characteristics and/or physicochemical properties of current radiotracers. The aim of this narrative review is to highlight the suitability of aggregated αSyn as an imaging biomarker in α-synucleinopathies, the current limitations with and lessons learned from αSyn radiotracer development, and finally to propose antibody-based ligands for imaging αSyn aggregates as a complementary tool rather than an alternative to small molecule ligands. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

Striatal synaptic bioenergetic and autophagic decline in premotor experimental parkinsonism

Synaptic impairment might precede neuronal degeneration in Parkinson’s disease. However, the intimate mechanisms altering synaptic function by the accumulation of presynaptic α-synuclein in striatal dopaminergic terminals before dopaminergic death occurs, have not been elucidated. Our aim is to unravel the sequence of synaptic functional and structural changes preceding symptomatic dopaminergic cell death. As such, we evaluated the temporal sequence of functional and structural changes at striatal synapses before parkinsonian motor features appear in a rat model of progressive dopaminergic death induced by overexpression of the human mutated A53T α-synuclein in the substantia nigra pars compacta, a protein transported to these synapses. Sequential window acquisition of all theoretical mass spectra proteomics identified deregulated proteins involved first in energy metabolism and later, in vesicle cycling and autophagy. After protein deregulation and when α-synuclein accumulated at striatal synapses, alterations to mitochondrial bioenergetics were observed using a Seahorse XF96 analyser. Sustained dysfunctional mitochondrial bioenergetics was followed by a decrease in the number of dopaminergic terminals, morphological and ultrastructural alterations, and an abnormal accumulation of autophagic/endocytic vesicles inside the remaining dopaminergic fibres was evident by electron microscopy. The total mitochondrial population remained unchanged whereas the number of ultrastructurally damaged mitochondria increases as the pathological process evolved. We also observed ultrastructural signs of plasticity within glutamatergic synapses before the expression of motor abnormalities, such as a reduction in axospinous synapses and an increase in perforated postsynaptic densities. Overall, we found that a synaptic energetic failure and accumulation of dysfunctional organelles occur sequentially at the dopaminergic terminals as the earliest events preceding structural changes and cell death. We also identify key proteins involved in these earliest functional abnormalities that may be modulated and serve as therapeutic targets to counterbalance the degeneration of dopaminergic cells to delay or prevent the development of Parkinson’s disease.

Assessment of Repetitive and Compulsive Behaviors Induced by Pramipexole in Rats: Effect of Alpha-Synuclein-Induced Nigrostriatal Degeneration

Treatment with dopamine agonists in Parkinson’s disease (PD) is associated with debilitating neuropsychiatric side-effects characterized by impulsive and compulsive behaviors. The vulnerability to develop such impairments is thought to involve interactions between individual vulnerability traits, types of antiparkinsonian medications, and the neurodegenerative process. We investigated the effect of the dopamine D3/D2 agonist pramipexole (PPX) and selective nigrostriatal degeneration achieved by viral-mediated expression of alpha-synuclein on the expression of repetitive and compulsive-like behaviors in rats. In a task assessing spontaneous food hoarding behavior, PPX increased the time spent interacting with food pellets at the expense of hoarding. This disruption of hoarding behavior was identical in sham and lesioned rats. In an operant post-training signal attenuation task, the combination of nigrostriatal lesion and PPX decreased the number of completed trials and increased the number of uncompleted trials. The lesion led to an increased compulsive behavior after signal attenuation, and PPX shifted the overall behavioral output towards an increased proportion of compulsive lever-presses. Given the magnitude of the behavioral effects and the lack of strong interaction between PPX and nigral degeneration, these results suggest that extra-nigral pathology may be critical to increase the vulnerability to develop compulsive behaviors following treatment with D3/D2 agonists.

A metabolic biomarker predicts Parkinson's disease at the early stages in patients and animal models

BackgroundCare management of Parkinson's disease (PD) patients currently remains symptomatic, mainly because diagnosis relying on the expression of the cardinal motor symptoms is made too late. Earlier detection of PD therefore represents a key step for developing therapies able to delay or slow down its progression.MethodsWe investigated metabolic markers in 3 different animal models of PD, mimicking different phases of the disease assessed by behavioral and histological evaluation, and in 3 cohorts of de novo PD patients and matched controls (n = 129). Serum and brain tissue samples were analyzed by nuclear magnetic resonance spectroscopy and data submitted to advanced multivariate statistics.ResultsOur translational strategy reveals common metabolic dysregulations in serum of the different animal models and PD patients. Some of them were mirrored in the tissue samples, possibly reflecting pathophysiological mechanisms associated with PD development. Interestingly, some metabolic dysregulations appeared before motor symptom emergence and could represent early biomarkers of PD. Finally, we built a composite biomarker with a combination of 6 metabolites. This biomarker discriminated animals mimicking PD from controls, even from the first, nonmotor signs and, very interestingly, also discriminated PD patients from healthy subjects.ConclusionFrom our translational study, which included 3 animal models and 3 de novo PD patient cohorts, we propose a promising biomarker exhibiting a high accuracy for de novo PD diagnosis that may possibly predict early PD development, before motor symptoms appear.FundingFrench National Research Agency (ANR), DOPALCOMP, Institut National de la Santé et de la Recherche Médicale, Université Grenoble Alpes, Association France Parkinson.

Rodent models for gait network disorders in Parkinson's disease - a translational perspective

Gait impairments in Parkinson's disease remain a scientific and therapeutic challenge. The advent of new deep brain stimulation (DBS) devices capable of recording brain activity from chronically implanted electrodes has fostered new studies of gait in freely moving patients. The hope is to identify gait-related neural biomarkers and improve therapy using closed-loop DBS. In this context, animal models offer the opportunity to investigate gait network activity at multiple biological scales and address unresolved questions from clinical research. Yet, the contribution of rodent models to the development of future neuromodulation therapies will rely on translational validity. In this review, we summarize the most effective strategies to model parkinsonian gait in rodents. We discuss how clinical observations have inspired targeted brain lesions in animal models, and whether resulting motor deficits and network oscillations match recent findings in humans. Gait impairments with hypo-, bradykinesia and altered limb rhythmicity were successfully modelled in rodents. However, clear evidence for the presence of freezing of gait was missing. The identification of reliable neural biomarkers for gait impairments has remained challenging in both animals and humans. Moving forward, we expect that the ongoing investigation of circuit specific neuromodulation strategies in animal models will lead to future optimizations of gait therapy in Parkinson's disease.

Preventive Vitamin A Supplementation Improves Striatal Function in 6-Hydroxydopamine Hemiparkinsonian Rats

Background

The mechanisms leading to a loss of dopaminergic (DA) neurons from the substantia nigra pars compacta (SNc) in Parkinson's disease (PD) have multifactorial origins. In this context, nutrition is currently investigated as a modifiable environmental factor for the prevention of PD. In particular, initial studies revealed the deleterious consequences of vitamin A signaling failure on dopamine-related motor behaviors. However, the potential of vitamin A supplementation itself to prevent neurodegeneration has not been established yet.

Objective

The hypothesis tested in this study is that preventive vitamin A supplementation can protect DA neurons in a rat model of PD.

Methods

The impact of a 5-week preventive supplementation with vitamin A (20 IU/g of diet) was measured on motor and neurobiological alterations induced by 6-hydroxydopamine (6-OHDA) unilateral injections in the striatum of rats. Rotarod, step test and cylinder tests were performed up to 3 weeks after the lesion. Post-mortem analyses (retinol and monoamines dosages, western blots, immunofluorescence) were performed to investigate neurobiological processes.

Results

Vitamin A supplementation improved voluntary movements in the cylinder test. In 6-OHDA lesioned rats, a marked decrease of dopamine levels in striatum homogenates was measured. Tyrosine hydroxylase labeling in the SNc and in the striatum was significantly decreased by 6-OHDA injection, without effect of vitamin A. By contrast, vitamin A supplementation increased striatal expression of D2 and RXR receptors in the striatum of 6-OHDA lesioned rats.

Conclusions

Vitamin A supplementation partially alleviates motor alterations and improved striatal function, revealing a possible beneficial preventive approach for PD.

Animal models of Parkinson's disease: a guide to selecting the optimal model for your research

Parkinson's disease (PD) is a complex, multisystem disorder characterised by α-synuclein (SNCA) pathology, degeneration of nigrostriatal dopaminergic neurons, multifactorial pathogenetic mechanisms and expression of a plethora of motor and non-motor symptoms. Animal models of PD have already been instructive in helping us unravel some of these aspects. However, much remains to be discovered, requiring continued interrogation by the research community. In contrast with the situation for many neurological disorders, PD benefits from of a wide range of available animal models (pharmacological, toxin, genetic and α-synuclein) but this makes selection of the optimal one for a given study difficult. This is especially so when a study demands a model that displays a specific combination of features. While many excellent reviews of animal models already exist, this review takes a different approach with the intention of more readily informing this decision-making process. We have considered each feature of PD in turn - aetiology, pathology, pathogenesis, motor dysfunctions and non-motor symptoms (NMS) - highlighting those animal models that replicate each. By compiling easily accessible tables and a summary figure, we aim to provide the reader with a simple, go-to resource for selecting the optimal animal model of PD to suit their research needs.

Monitoring of a progressive functional dopaminergic deficit in the A53T-AAV synuclein rats by combining 6-[18F]fluoro-L-m-tyrosine imaging and motor performances analysis

With the emergence of disease-modifying therapies for Parkinson's disease, reliable longitudinal markers are needed to quantify pathology and demonstrate disease progression. We developed the A53T-AAV rat model of synucleinopathy by combining longitudinal measures over 12 weeks. We first characterized the progression of the motor and dopaminergic deficits. Then, we monitored the disease progression using the [18F]FMT Positron Emission Tomography (PET) radiotracer. The nigral injection of A53T-AAV led to an increase in phosphorylated α-synuclein on S129, a progressive accumulation of α-synuclein aggregates, and a decrease of dopaminergic function associated with a deterioration of motor activity. The longitudinal monitoring of A53T-AAV rats with [18F]FMT PET showed a progressive reduction of the Kc outcome parameter in the caudate putamen from the lesioned side. Interestingly, the progressive reduction in the [18F]FMT PET signal correlated with defects in the stepping test. In conclusion, we established a progressive rat model of α-synuclein pathology which monitors the deficit longitudinally using both the [18F]FMT PET tracer and behavioral parameters, 2 features that have strong relevance for translational approaches.

Impaired brain insulin signalling in Parkinson's disease

AIMS

Brain insulin resistance (i.e., decreased insulin/insulin-like growth factor-1 [IGF-1] signalling) may play a role in the pathophysiology of Parkinson's disease (PD), and several anti-diabetic drugs have entred clinical development to evaluate their potential disease-modifying properties in PD. A measure of insulin resistance is the amount of the downstream messenger insulin receptor substrate-1 that is phosphorylated at serine residues 312 (IRS-1pS312) or 616 (IRS-1pS616). We assessed IRS-1pS312 and IRS-1pS616 expression in post-mortem brain tissue of PD patients and a preclinical rat model based on viral-mediated expression of A53T mutated human α-synuclein (AAV2/9-h-α-synA53T).

METHODS

IRS-1pS312 and IRS-1pS616 staining intensity were determined by immunofluorescence in both neurons and glial cells in the substantia nigra pars compacta (SNc) and putamen of PD patients and controls without known brain disease. We further explored a possible relation between α-synuclein aggregates and brain insulin resistance in PD patients. Both insulin resistance markers were also measured in the SNc and striatum of AAV2/9-h-α-synA53T rats.

RESULTS

We found higher IRS-1pS312 staining intensity in nigral dopaminergic neurons and a trend for higher IRS-1pS312 staining intensity in putaminal neurons of PD patients. We observed no differences for IRS-1pS616 staining intensity in neurons or IRS-1pS312 staining intensity in glial cells. IRS-1pS312 showed high co-localisation within the core of nigral Lewy bodies. Like PD patients, AAV2/9-h-α-synA53T rats showed higher IRS-1pS312 staining intensity in the SNc and striatum than controls, whereas IRS-1pS616 was not different between groups.

CONCLUSIONS

Our results provide evidence for brain insulin resistance in PD and support the rationale for repurposing anti-diabetic drugs for PD treatment.

Regional locus coeruleus degeneration is uncoupled from noradrenergic terminal loss in Parkinson's disease

Previous studies have reported substantial involvement of the noradrenergic system in Parkinson's disease. Neuromelanin-sensitive MRI sequences and PET tracers have become available to visualize the cell bodies in the locus coeruleus and the density of noradrenergic terminal transporters. Combining these methods, we investigated the relationship of neurodegeneration in these distinct compartments in Parkinson's disease. We examined 93 subjects (40 healthy controls and 53 Parkinson's disease patients) with neuromelanin-sensitive turbo spin-echo MRI and calculated locus coeruleus-to-pons signal contrasts. Voxels with the highest intensities were extracted from published locus coeruleus coordinates transformed to individual MRI. To also investigate a potential spatial pattern of locus coeruleus degeneration, we extracted the highest signal intensities from the rostral, middle, and caudal third of the locus coeruleus. Additionally, a study-specific probabilistic map of the locus coeruleus was created and used to extract mean MRI contrast from the entire locus coeruleus and each rostro-caudal subdivision. Locus coeruleus volumes were measured using manual segmentations. A subset of 73 subjects had 11C-MeNER PET to determine noradrenaline transporter density, and distribution volume ratios of noradrenaline transporter-rich regions were computed. Parkinson's disease patients showed reduced locus coeruleus MRI contrast independently of the selected method (voxel approaches: p < 0.0001, p < 0.001; probabilistic map: p < 0.05), specifically on the clinically-defined most affected side (p < 0.05), and reduced locus coeruleus volume (p < 0.0001). Reduced MRI contrast was confined to the middle and caudal locus coeruleus (voxel approach-rostral: p = 0.48, middle: p < 0.0001, and caudal: p < 0.05; probabilistic map-rostral: p = 0.90, middle: p < 0.01, and caudal: p < 0.05). The noradrenaline transporter density was lower in Parkinson's disease patients in all examined regions (group effect p < 0.0001). No significant correlation was observed between locus coeruleus MRI contrast and noradrenaline transporter density. In contrast, the individual ratios of noradrenaline transporter density and locus coeruleus MRI contrast were lower in Parkinson's disease patients in all examined regions (group effect p < 0.001). Our multimodal imaging approach revealed pronounced noradrenergic terminal loss relative to cellular locus coeruleus degeneration in Parkinson's disease; the latter followed a distinct spatial pattern with the middle-caudal portion being more affected than the rostral part. The data shed first light on the interaction between the axonal and cell body compartments and their differential susceptibility to neurodegeneration in Parkinson's disease, which may eventually direct research toward potential novel treatment approaches.

Adeno-Associated Virus Expression of α-Synuclein as a Tool to Model Parkinson's Disease: Current Understanding and Knowledge Gaps

Parkinson's disease (PD) is the second most common neurodegenerative disorder in the aging population and is characterized by a constellation of motor and non-motor symptoms. The abnormal aggregation and spread of alpha-synuclein (α-syn) is thought to underlie the loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc), leading to the development of PD. It is in this context that the use of adeno-associated viruses (AAVs) to express a-syn in the rodent midbrain has become a popular tool to model SNc DA neuron loss during PD. In this review, we summarize results from two decades of experiments using AAV-mediated a-syn expression in rodents to model PD. Specifically, we outline aspects of AAV vectors that are particularly relevant to modeling a-syn dysfunction in rodent models of PD such as changes in striatal neurochemistry, a-syn biochemistry, and PD-related behaviors resulting from AAV-mediated a-syn expression in the midbrain. Finally, we discuss the emerging role of astrocytes in propagating a-syn pathology, and point to future directions for employing AAVs as a tool to better understand how astrocytes contribute to a-syn pathology during the development of PD. We envision that lessons learned from two decades of utilizing AAVs to express a-syn in the rodent brain will enable us to develop an optimized set of parameters for gaining a better understanding of how a-syn leads to the development of PD.

Adeno-Associated Viral Vectors as Versatile Tools for Parkinson's Research, Both for Disease Modeling Purposes and for Therapeutic Uses

It is without any doubt that precision medicine therapeutic strategies targeting neurodegenerative disorders are currently witnessing the spectacular rise of newly designed approaches based on the use of viral vectors as Trojan horses for the controlled release of a given genetic payload. Among the different types of viral vectors, adeno-associated viruses (AAVs) rank as the ones most commonly used for the purposes of either disease modeling or for therapeutic strategies. Here, we reviewed the current literature dealing with the use of AAVs within the field of Parkinson’s disease with the aim to provide neuroscientists with the advice and background required when facing a choice on which AAV might be best suited for addressing a given experimental challenge. Accordingly, here we will be summarizing some insights on different AAV serotypes, and which would be the most appropriate AAV delivery route. Next, the use of AAVs for modeling synucleinopathies is highlighted, providing potential readers with a landscape view of ongoing pre-clinical and clinical initiatives pushing forward AAV-based therapeutic approaches for Parkinson’s disease and related synucleinopathies.

Lewy body-associated proteins: victims, instigators, or innocent bystanders? The case of AIMP2 and alpha-synuclein

Lewy bodies (LBs), one of the neuropathological defining hallmarks of Parkinson's disease (PD), are composed of a complex mixture of alpha-synuclein (aSyn) filaments and hundreds of proteins, lipids, and membranous organelles. However, these proteins' role in aSyn aggregation and the biogenesis of LBs remains poorly understood. Previous studies have focused on investigating the role of these proteins as modifiers of aSyn aggregation, inclusion formation, and toxicity; very often, one protein at a time. In a recent study, Ham et al. suggest that one of these proteins, aminoacyl tRNA synthase complex-interacting multifunctional protein 2 (AIMP2), plays a primary role in the initiation of aSyn aggregation and is essential for aSyn inclusion formation and toxicity in cells and several models of synucleinopathies (Ham et al., 2020). Based on in vitro aggregation studies, they proposed a model in which AIMP2 self-associates to form amyloid-like aggregates that interact with monomeric aSyn and catalyze/seed the formation of aSyn fibrils and, eventually, LB-like inclusions. Herein, we present a critical analysis of their results and conclusions, review previous studies on AIMP2 aggregation, and reexamine the role of AIMP2 in regulating aSyn inclusion formation and clearance and aSyn-induced neurodegeneration in Parkinson's disease. We conclude by presenting lesson learned and recommendations on experimental factors and approaches that should be considered in future studies aimed at investigating the potential of targeting LBs-associated proteins, including AIMP2, for developing therapies to treat PD and other synucleinopathies.

White matter hyperintensities in patients with Parkinson's disease: A systematic review and meta-analysis

INTRODUCTION

Mechanisms driving neurodegeneration in Parkinson's disease (PD) are unclear and neurovascular dysfunction may be a contributing factor. White matter hyperintensities (WMH) are commonly found on brain MRI in patients with PD. It is controversial if they are more prevalent or more severe in PD compared with controls. This systematic review aims to answer this question.

METHODS

A systematic search of electronic databases was conducted for studies of WMH in patients with PD. A qualitative synthesis was done for studies reporting WMH prevalence or WMH scores on a visual rating scale (VRS). In studies reporting total WMH volume, the difference between patients with PD and controls was pooled using random effects meta-analysis.

RESULTS

Among 3860 subjects from 24 studies, 2360 were cases and 1500 controls. Fifteen studies reported WMH scores and four studies reported the prevalence of WMH. On VRS, five studies reported no difference in WMH scores, three found higher WMH scores in PD compared to controls, three reported increased WMH scores either in periventricular or deep white matter, and four reported higher scores only in PD with dementia. In studies reporting WMH volume, there was no difference between patients with PD and controls (pooled standardized mean difference = 0.1, 95%CI: -0.1-0.4, I² = 81%).

CONCLUSION

WMH are not more prevalent or severe in patients with PD than in age-matched controls. PD dementia may have more severe WMH compared to controls and PD with normal cognition. Prospective studies using standardized methods of WMH assessment are needed.

Chaperone-mediated autophagy prevents collapse of the neuronal metastable proteome

Components of the proteostasis network malfunction in aging, and reduced protein quality control in neurons has been proposed to promote neurodegeneration. Here, we investigate the role of chaperone-mediated autophagy (CMA), a selective autophagy shown to degrade neurodegeneration-related proteins, in neuronal proteostasis. Using mouse models with systemic and neuronal-specific CMA blockage, we demonstrate that loss of neuronal CMA leads to altered neuronal function, selective changes in the neuronal metastable proteome, and proteotoxicity, all reminiscent of brain aging. Imposing CMA loss on a mouse model of Alzheimer's disease (AD) has synergistic negative effects on the proteome at risk of aggregation, thus increasing neuronal disease vulnerability and accelerating disease progression. Conversely, chemical enhancement of CMA ameliorates pathology in two different AD experimental mouse models. We conclude that functional CMA is essential for neuronal proteostasis through the maintenance of a subset of the proteome with a higher risk of misfolding than the general proteome.

A New Rise of Non-Human Primate Models of Synucleinopathies

Synucleinopathies are neurodegenerative diseases characterized by the presence of α-synuclein-positive intracytoplasmic inclusions in the central nervous system. Multiple experimental models have been extensively used to understand better the mechanisms involved in the pathogenesis of synucleinopathy. Non-human primate (NHP) models are of interest in neurodegenerative diseases as they constitute the highest relevant preclinical model in translational research. They also contribute to bringing new insights into synucleinopathy’s pathogenicity and help in the quest and validation of therapeutical strategies. Here, we reviewed the different NHP models that have recapitulated key characteristics of synucleinopathy, and we aimed to highlight the contribution of NHP in mechanistic and translational approaches for synucleinopathies.

Comparison of the expression and toxicity of AAV2/9 carrying the human A53T α-synuclein gene in presence or absence of WPRE

Woodchuck Hepatitis Virus Post-transcriptional Regulatory Element (WPRE) is thought to enhance transgene expression of target genes delivered by adeno-associated viral (AAV) vectors. This study assessed the protein expression of α-synuclein, phosphorylated α-synuclein at Serine 129, extent of nigrostriatal degeneration as well as subsequent behavioral deficits induced by unilateral intranigral stereotactic injection in male adult C57BL/6J mice of an AAV2/9 expressing A53T human α-synuclein under the control of the synapsin promoter in presence or absence of the WPRE. The presence of WPRE enabled to achieve greater nigrostriatal degeneration and synucleinopathy which was concomitant with worsened forelimb use asymmetry. This work refines a mouse Parkinson's disease model in which anatomo-pathology is related to behavioral deficits.

Bidirectional gut-to-brain and brain-to-gut propagation of synucleinopathy in non-human primates

In Parkinson's disease, synucleinopathy is hypothesized to spread from the enteric nervous system, via the vagus nerve, to the CNS. Here, we compare, in baboon monkeys, the pathological consequences of either intrastriatal or enteric injection of α-synuclein-containing Lewy body extracts from patients with Parkinson's disease. This study shows that patient-derived α-synuclein aggregates are able to induce nigrostriatal lesions and enteric nervous system pathology after either enteric or striatal injection in a non-human primate model. This finding suggests that the progression of α-synuclein pathology might be either caudo-rostral or rostro-caudal, varying between patients and disease subtypes. In addition, we report that α-synuclein pathological lesions were not found in the vagal nerve in our experimental setting. This study does not support the hypothesis of a transmission of α-synuclein pathology through the vagus nerve and the dorsal motor nucleus of the vagus. Instead, our results suggest a possible systemic mechanism in which the general circulation would act as a route for long-distance bidirectional transmission of endogenous α-synuclein between the enteric and the central nervous systems. Taken together, our study provides invaluable primate data exploring the role of the gut-brain axis in the initiation and propagation of Parkinson's disease pathology and should open the door to the development and testing of new therapeutic approaches aimed at interfering with the development of sporadic Parkinson's disease.

Microglial exosomes facilitate α-synuclein transmission in Parkinson's disease

Accumulation of neuronal α-synuclein is a prominent feature in Parkinson's disease. More recently, such abnormal protein aggregation has been reported to spread from cell to cell and exosomes are considered as important mediators. The focus of such research, however, has been primarily in neurons. Given the increasing recognition of the importance of non-cell autonomous-mediated neurotoxicity, it is critical to investigate the contribution of glia to α-synuclein aggregation and spread. Microglia are the primary phagocytes in the brain and have been well-documented as inducers of neuroinflammation. How and to what extent microglia and their exosomes impact α-synuclein pathology has not been well delineated. We report here that when treated with human α-synuclein preformed fibrils, exosomes containing α-synuclein released by microglia are fully capable of inducing protein aggregation in the recipient neurons. Additionally, when combined with microglial proinflammatory cytokines, these exosomes further increased protein aggregation in neurons. Inhibition of exosome synthesis in microglia reduced α-synuclein transmission. The in vivo significance of these exosomes was demonstrated by stereotaxic injection of exosomes isolated from α-synuclein preformed fibrils treated microglia into the mouse striatum. Phosphorylated α-synuclein was observed in multiple brain regions consistent with their neuronal connectivity. These animals also exhibited neurodegeneration in the nigrostriatal pathway in a time-dependent manner. Depleting microglia in vivo dramatically suppressed the transmission of α-synuclein after stereotaxic injection of preformed fibrils. Mechanistically, we report here that α-synuclein preformed fibrils impaired autophagy flux by upregulating PELI1, which in turn, resulted in degradation of LAMP2 in activated microglia. More importantly, by purifying microglia/macrophage derived exosomes in the CSF of Parkinson's disease patients, we confirmed the presence of α-synuclein oligomer in CD11b+ exosomes, which were able to induce α-synuclein aggregation in neurons, further supporting the translational aspect of this study. Taken together, our study supports the view that microglial exosomes contribute to the progression of α-synuclein pathology and therefore, they may serve as a promising therapeutic target for Parkinson's disease.

Current Status of and Perspectives on the Application of Marmosets in Neurobiology

The common marmoset (Callithrix jacchus), a small New World primate, is receiving substantial attention in the neuroscience and biomedical science fields because its anatomical features, functional and behavioral characteristics, and reproductive features and its amenability to available genetic modification technologies make it an attractive experimental subject. In this review, I outline the progress of marmoset neuroscience research and summarize both the current status (opportunities and limitations) of and the future perspectives on the application of marmosets in neuroscience and disease modeling.

From iPS Cells to Rodents and Nonhuman Primates: Filling Gaps in Modeling Parkinson's Disease

Parkinson's disease (PD) is primarily known as a movement disorder because of typical clinical manifestations associated with the loss of dopaminergic neurons in the substantia nigra. However, it is now widely recognized that PD is a much more complex condition, with multiple and severe nonmotor features implicating additional brain areas and organs in the disease process. Pathologically, typical forms of PD are characterized by the accumulation of α-synuclein-rich protein inclusions known as Lewy bodies and Lewy neurites, although other types of protein inclusions are also often present in the brain. Familial forms of PD have provided a wealth of information about molecular pathways leading to neurodegeneration, but only to add to the complexity of the problem and uncover new knowledge gaps. Therefore, modeling PD in the laboratory has become increasingly challenging. Here, we discuss knowledge gaps and challenges in the use of laboratory models for the study of a disease that is clinically heterogeneous and multifactorial. We propose that the combined use of patient-derived cells and animal models, along with current technological tools, will not only expand our molecular and pathophysiological understanding of PD, but also assist in the identification of therapeutic strategies targeting relevant pathogenic pathways. © 2020 International Parkinson and Movement Disorder Society.

CLR01 protects dopaminergic neurons in vitro and in mouse models of Parkinson's disease

Parkinson’s disease (PD) affects millions of patients worldwide and is characterized by alpha-synuclein aggregation in dopamine neurons. Molecular tweezers have shown high potential as anti-aggregation agents targeting positively charged residues of proteins undergoing amyloidogenic processes. Here we report that the molecular tweezer CLR01 decreased aggregation and toxicity in induced pluripotent stem cell-derived dopaminergic cultures treated with PD brain protein extracts. In microfluidic devices CLR01 reduced alpha-synuclein aggregation in cell somas when axonal terminals were exposed to alpha-synuclein oligomers. We then tested CLR01 in vivo in a humanized alpha-synuclein overexpressing mouse model; mice treated at 12 months of age when motor defects are mild exhibited an improvement in motor defects and a decreased oligomeric alpha-synuclein burden. Finally, CLR01 reduced alpha-synuclein-associated pathology in mice injected with alpha-synuclein aggregates into the striatum or substantia nigra. Taken together, these results highlight CLR01 as a disease-modifying therapy for PD and support further clinical investigation.

CLR01 is a molecular tweezer that inhibits protein aggregation. Here the authors show that CLR01 protects dopaminergic neurons in vitro and in vivo in human neurons and in mouse models showing potential as a disease-modifying therapy for Parkinson’s disease.

Identification of distinct pathological signatures induced by patient-derived α-synuclein structures in nonhuman primates

Dopaminergic neuronal cell death, associated with intracellular α-synuclein (α-syn)-rich protein aggregates [termed "Lewy bodies" (LBs)], is a well-established characteristic of Parkinson's disease (PD). Much evidence, accumulated from multiple experimental models, has suggested that α-syn plays a role in PD pathogenesis, not only as a trigger of pathology but also as a mediator of disease progression through pathological spreading. Here, we have used a machine learning-based approach to identify unique signatures of neurodegeneration in monkeys induced by distinct α-syn pathogenic structures derived from patients with PD. Unexpectedly, our results show that, in nonhuman primates, a small amount of singular α-syn aggregates is as toxic as larger amyloid fibrils present in the LBs, thus reinforcing the need for preclinical research in this species. Furthermore, our results provide evidence supporting the true multifactorial nature of PD, as multiple causes can induce a similar outcome regarding dopaminergic neurodegeneration.

Overexpression of alpha-synuclein promotes both cell proliferation and cell toxicity in human SH-SY5Y neuroblastoma cells

Alpha-Synuclein (aSyn) is a chameleon-like protein. Its overexpression and intracellular deposition defines neurodegenerative α-synucleinopathies including Parkinson's disease. Whether aSyn up-regulation is the cause or the protective reaction to α-synucleinopathies remains unresolved. Remarkably, the accumulation of aSyn is involved in cancer. Here, the neuroblastoma SH-SY5Y cell line was genetically engineered to overexpress aSyn at low and at high levels. aSyn cytotoxicity was assessed by the MTT and vital-dye exclusion methods, observed at the beginning of the sub-culture of low-aSyn overexpressing neurons when cells can barely proliferate exponentially. Conversely, high-aSyn overexpressing cultures grew at high rates while showing enhanced colony formation compared to low-aSyn neurons. Cytotoxicity of aSyn overexpression was indirectly revealed by the addition of pro-oxidant rotenone. Pretreatment with partially reduced graphene oxide, an apoptotic agent, increased toxicity of rotenone in low-aSyn neurons, but, it did not in high-aSyn neurons. Consistent with their enhanced proliferation, high-aSyn neurons showed elevated levels of SMP30, a senescence-marker protein, and the mitosis Ki-67 marker. High-aSyn overexpression conferred to the carcinogenic neurons heightened tumorigenicity and resistance to senescence compared to low-aSyn cells, thus pointing to an inadequate level of aSyn stimulation, rather than the aSyn overload itself, as one of the factors contributing to α-synucleinopathy.

Modeling Parkinson's Disease With the Alpha-Synuclein Protein

Alpha-synuclein (α-Syn) is a key protein involved in Parkinson's disease (PD) pathology. PD is characterized by the loss of dopaminergic neuronal cells in the substantia nigra pars compacta and the abnormal accumulation and aggregation of α-Syn in the form of Lewy bodies and Lewy neurites. More precisely, the aggregation of α-Syn is associated with the dysfunctionality and degeneration of neurons in PD. Moreover, mutations in the SNCA gene, which encodes α-Syn, cause familial forms of PD and are the basis of sporadic PD risk. Given the role of the α-Syn protein in the pathology of PD, animal models that reflect the dopaminergic neuronal loss and the widespread and progressive formation of α-Syn aggregates in different areas of the brain constitute a valuable tool. Indeed, animal models of PD are important for understanding the molecular mechanisms of the disease and might contribute to the development and validation of new therapies. In the absence of animal models that faithfully reproduce human PD, in recent years, numerous animal models of PD based on α-Syn have been generated. In this review, we summarize the main features of the α-Syn pre-formed fibrils (PFFs) model and recombinant adeno-associated virus vector (rAAV) mediated α-Syn overexpression models, providing a detailed comparative analysis of both models. Here, we discuss how each model has contributed to our understanding of PD pathology and the advantages and weakness of each of them.

Animal models for preclinical Parkinson's research: An update and critical appraisal

Animal models of Parkinson's disease (PD) are essential to investigate pathogenic pathways at the whole-organism level. Moreover, they are necessary for a preclinical investigation of potential new therapies. Different pathological features of PD can be induced in a variety of invertebrate and vertebrate species using toxins, drugs, or genetic perturbations. Each model has a particular utility and range of applicability. Invertebrate PD models are particularly useful for high throughput-screening applications, whereas mammalian models are needed to explore complex motor and non-motor features of the human disease. Here, we provide a comprehensive review and critical appraisal of the most commonly used mammalian models of PD, which are produced in rats and mice. A substantial loss of nigrostriatal dopamine neurons is necessary for the animal to exhibit a hypokinetic motor phenotype responsive to dopaminergic agents, thus resembling clinical PD. This level of dopaminergic neurodegeneration can be induced using specific neurotoxins, environmental toxicants, or proteasome inhibitors. Alternatively, nigrostriatal dopamine degeneration can be induced via overexpression of α-synuclein using viral vectors or transgenic techniques. In addition, protein aggregation pathology can be triggered by inoculating preformed fibrils of α-synuclein in the substantia nigra or the striatum. Thanks to the conceptual and technical progress made in the past few years a vast repertoire of well-characterized animal models are currently available to address different aspects of PD in the laboratory.

The process of Lewy body formation, rather than simply α-synuclein fibrillization, is one of the major drivers of neurodegeneration

Parkinson's disease (PD) is characterized by the accumulation of misfolded and aggregated α-synuclein (α-syn) into intraneuronal inclusions named Lewy bodies (LBs). Although it is widely believed that α-syn plays a central role in the pathogenesis of PD, the processes that govern α-syn fibrillization and LB formation remain poorly understood. In this work, we sought to dissect the spatiotemporal events involved in the biogenesis of the LBs at the genetic, molecular, biochemical, structural, and cellular levels. Toward this goal, we further developed a seeding-based model of α-syn fibrillization to generate a neuronal model that reproduces the key events leading to LB formation, including seeding, fibrillization, and the formation of inclusions that recapitulate many of the biochemical, structural, and organizational features of bona fide LBs. Using an integrative omics, biochemical and imaging approach, we dissected the molecular events associated with the different stages of LB formation and their contribution to neuronal dysfunction and degeneration. In addition, we demonstrate that LB formation involves a complex interplay between α-syn fibrillization, posttranslational modifications, and interactions between α-syn aggregates and membranous organelles, including mitochondria, the autophagosome, and endolysosome. Finally, we show that the process of LB formation, rather than simply fibril formation, is one of the major drivers of neurodegeneration through disruption of cellular functions and inducing mitochondria damage and deficits, and synaptic dysfunctions. We believe that this model represents a powerful platform to further investigate the mechanisms of LB formation and clearance and to screen and evaluate therapeutics targeting α-syn aggregation and LB formation.

Managing Parkinson's disease: moving ON with NOP

The opioid-like neuropeptide nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP receptor) contribute to Parkinson's disease (PD) and motor complications associated with levodopa therapy. The N/OFQ-NOP receptor system is expressed in cortical and subcortical motor areas and, notably, in dopaminergic neurons of the substantia nigra compacta. Dopamine depletion, as in rodent models of PD results in up-regulation of N/OFQ transmission in the substantia nigra and down-regulation of N/OFQ transmission in the striatum. Consistent with this, NOP receptor antagonists relieve motor deficits in PD models by reinstating the physiological balance between excitatory and inhibitory inputs impinging on nigro-thalamic GABAergic neurons. NOP receptor antagonists also counteract the degeneration of nigrostriatal dopaminergic neurons, possibly by attenuating the excitotoxicity or modulating the immune response. Conversely, NOP receptor agonists attenuate levodopa-induced dyskinesia by attenuating the hyperactivation of striatal D1 receptor signalling in neurons of the direct striatonigral pathway. The N/OFQ-NOP receptor system might represent a novel target in the therapy of PD.

Autophagy and LRRK2 in the Aging Brain

Autophagy is a highly conserved process by which long-lived macromolecules, protein aggregates and dysfunctional/damaged organelles are delivered to lysosomes for degradation. Autophagy plays a crucial role in regulating protein quality control and cell homeostasis in response to energetic needs and environmental challenges. Indeed, activation of autophagy increases the life-span of living organisms, and impairment of autophagy is associated with several human disorders, among which neurodegenerative disorders of aging, such as Parkinson’s disease. These disorders are characterized by the accumulation of aggregates of aberrant or misfolded proteins that are toxic for neurons. Since aging is associated with impaired autophagy, autophagy inducers have been viewed as a strategy to counteract the age-related physiological decline in brain functions and emergence of neurodegenerative disorders. Parkinson’s disease is a hypokinetic, multisystemic disorder characterized by age-related, progressive degeneration of central and peripheral neuronal populations, associated with intraneuronal accumulation of proteinaceous aggregates mainly composed by the presynaptic protein α-synuclein. α-synuclein is a substrate of macroautophagy and chaperone-mediated autophagy (two major forms of autophagy), thus impairment of its clearance might favor the process of α-synuclein seeding and spreading that trigger and sustain the progression of this disorder. Genetic factors causing Parkinson’s disease have been identified, among which mutations in the LRRK2 gene, which encodes for a multidomain protein encompassing central GTPase and kinase domains, surrounded by protein-protein interaction domains. Six LRRK2 mutations have been pathogenically linked to Parkinson’s disease, the most frequent being the G2019S in the kinase domain. LRRK2-associated Parkinson’s disease is clinically and neuropathologically similar to idiopathic Parkinson’s disease, also showing age-dependency and incomplete penetrance. Several mechanisms have been proposed through which LRRK2 mutations can lead to Parkinson’s disease. The present article will focus on the evidence that LRRK2 and its mutants are associated with autophagy dysregulation. Studies in cell lines and neurons in vitro and in LRRK2 knock-out, knock-in, kinase-dead and transgenic animals in vivo will be reviewed. The role of aging in LRRK2-induced synucleinopathy will be discussed. Possible mechanisms underlying the LRRK2-mediated control over autophagy will be analyzed, and the contribution of autophagy dysregulation to the neurotoxic actions of LRRK2 will be examined.

Exosome release and neuropathology induced by α-synuclein: new insights into protective mechanisms of Drp1 inhibition

Targeting alpha-synuclein (α-syn) as a therapeutic strategy for Parkinson’s disease (PD) has been intensively pursued largely due to its well-recognized pathogenic role. Since its discovery as the first familial link to PD over two decades ago, this protein has been associated with multiple neurotoxic mechanisms, such as mitochondrial dysfunction and impaired autophagic flux. We report here that blocking dynamin-related protein 1 (Drp1) improved both mitochondrial function and autophagic flux in experimental models of α-syn. Using rat dopaminergic neuronal cells with inducible wild-type human α-syn, we observed excessive mitochondrial fragmentation and increased Drp1 levels 48 h after gene induction. Functionally, these cells exhibited lower mitochondrial membrane potential, reduced ATP production rate and mitochondrial spare respiratory capacity, as well as increased levels of mitochondrial reactive oxygen species. To evaluate the protective role of Drp1 inhibition, we used three complementary approaches: gene silencing mediated by siRNA, overexpression of Drp1-dominant negative and the small molecule mitochondrial division inhibitor-1 (mdivi-1). Both morphological and functional defects induced by α-syn were attenuated by these strategies. Importantly, Drp1 inhibition reduced proteinase K-resistant α-syn aggregates. Based on that observation, we investigated the involvement of autophagy. Through a combination of stable autophagy reporter cells and immunoreactivity for LC3 and p62 in neuronal cells with either α-syn overexpression or treatment of human α-syn preformed fibrils (PFF), we observed that Drp1 inhibition abolished autophagic impairment induced by α-syn. Consistent with its role in improving autophagy function, Drp1 inhibition reduced exosome release and spread of α-syn pathology from neurons to neurons and from microglia to neurons. In summary, this study highlights new insights that Drp1 inhibition confers neuroprotection through both mitochondrial and autophagy-lysosomal pathways, further strengthening the therapeutic potential of targeting Drp1.

Prospective role of mitochondrial apoptotic pathway in mediating GMG-ITC to reduce cytotoxicity in H2O2-induced oxidative stress in differentiated SH-SY5Y cells

The antioxidant and neuroprotective activity of Glucomoringin isothiocyanate (GMG-ITC) have been reported in in vivo and in vitro models of neurodegenerative diseases. However, its neuroprotective role via mitochondrial-dependent pathway in a noxious environment remains unknown. The main objective of the present study was to unveil the mitochondrial apoptotic genes' profile and prospectively link with neuroprotective activity of GMG-ITC through its ROS scavenging. The results showed that pre-treatment of differentiated SH-SY5Y cells with 1.25 μg/mL purified isolated GMG-ITC, significantly reduced reactive oxygen species (ROS) production level, compared to H₂O₂ control group, as evidenced by flow cytometry-based evaluation of ROS generation. Presence of GMG-ITC prior to development of oxidative stress condition, downregulated the expression of cyt-c, p53, Apaf-1, Bax, CASP3, CASP8 and CASP9 genes with concurrent upregulation of Bcl-2 gene in mitochondrial apoptotic signalling pathway. Protein Multiplex revealed significant decreased in cyt-c, p53, Apaf-1, Bax, CASP8 and CASP9 due to GMG-ITC pre-treatment in oxidative stress condition. The present findings speculated that pre-treatment with GMG-ITC may alleviate oxidative stress condition in neuronal cells by reducing ROS production level and protect the cells against apoptosis via neurodegenerative disease potential pathways.

Transcription factor EB overexpression prevents neurodegeneration in experimental synucleinopathies

The synucleinopathies Parkinson’s disease (PD) and Multiple system atrophy (MSA) — characterized by α-synuclein intracytoplasmic inclusions into, respectively, neurons and oligodendrocytes — are associated with impairment of the autophagy-lysosomal pathways (ALP). Increased expression of the master regulator of ALP, transcription factor EB (TFEB), is hypothesized to promote the clearance of WT α-synuclein and survival of dopaminergic neurons. Here, we explore the efficacy of targeted TFEB overexpression either in neurons or oligodendrocytes to reduce the pathological burden of α-synuclein in a PD rat model and a MSA mouse model. While TFEB neuronal expression was sufficient to prevent neurodegeneration in the PD model, we show that only TFEB oligodendroglial overexpression leads to neuroprotective effects in the MSA model. These beneficial effects were associated with a decreased accumulation of α-synuclein into oligodendrocytes through recovery of the ALP machinery. Our study demonstrates that the cell type where α-synuclein aggregates dictates the target of TFEB overexpression in order to be protective, paving the way for adapted therapies.

Targeted overexpression of transcription factor EB (TFEB) decreases accumulation of α-synuclein and prevents neurodegeneration in animal models of synucleinopathies.

RGS9-2 rescues dopamine D2 receptor levels and signaling in DYT1 dystonia mouse models

Dopamine D2 receptor signaling is central for striatal function and movement, while abnormal activity is associated with neurological disorders including the severe early-onset DYT1 dystonia. Nevertheless, the mechanisms that regulate D2 receptor signaling in health and disease remain poorly understood. Here, we identify a reduced D2 receptor binding, paralleled by an abrupt reduction in receptor protein level, in the striatum of juvenile Dyt1 mice. This occurs through increased lysosomal degradation, controlled by competition between β-arrestin 2 and D2 receptor binding proteins. Accordingly, we found lower levels of striatal RGS9-2 and spinophilin. Further, we show that genetic depletion of RGS9-2 mimics the D2 receptor loss of DYT1 dystonia striatum, whereas RGS9-2 overexpression rescues both receptor levels and electrophysiological responses in Dyt1 striatal neurons. This work uncovers the molecular mechanism underlying D2 receptor downregulation in Dyt1 mice and in turn explains why dopaminergic drugs lack efficacy in DYT1 patients despite significant evidence for striatal D2 receptor dysfunction. Our data also open up novel avenues for disease-modifying therapeutics to this incurable neurological disorder.

Spreading of α-Synuclein and Tau: A Systematic Comparison of the Mechanisms Involved

Alzheimer's disease (AD) and Parkinson's disease (PD) are age-associated neurodegenerative disorders characterized by the misfolding and aggregation of alpha-synuclein (aSyn) and tau, respectively. The coexistence of aSyn and tau aggregates suggests a strong overlap between tauopathies and synucleinopathies. Interestingly, misfolded forms of aSyn and tau can propagate from cell to cell, and throughout the brain, thereby templating the misfolding of native forms of the proteins. The exact mechanisms involved in the propagation of the two proteins show similarities, and are reminiscent of the spreading characteristic of prion diseases. Recently, several models were developed to study the spreading of aSyn and tau. Here, we discuss the mechanisms involved, the similarities and differences between the spreading of the two proteins and that of the prion protein, and the different cell and animal models used for studying these processes. Ultimately, a deeper understanding of the molecular mechanisms involved may lead to the identification of novel targets for therapeutic intervention in a variety of devastating neurodegenerative diseases.

α-Synuclein Expression Is Preserved in Substantia Nigra GABAergic Fibers of Young and Aged Neurotoxin-Treated Rhesus Monkeys

α-Synuclein (α-syn) is a small presynaptic protein distributed ubiquitously in the central and peripheral nervous system. In normal conditions, α-syn is found in soluble form, while in Parkinson's disease (PD) it may phosphorylate, aggregate, and combine with other proteins to form Lewy bodies. The purpose of this study was to evaluate, in nonhuman primates, whether α-syn expression is affected by age and neurotoxin challenge. Young adult (n = 5, 5-10 years old) and aged (n = 4, 23-25 years old) rhesus monkeys received a single unilateral carotid artery injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Three months post-MPTP the animals were necropsied by transcardiac perfusion, and their brains extracted and processed with immunohistochemical methods. Quantification of tyrosine hydroxylase (TH)-positive substantia nigra (SN) neurons showed a significant 80-89% decrease in the side ipsilateral to MPTP administration in young and old animals. Optical density of TH- immunoreactivity (-ir) in the caudate and putamen presented a 60-70% loss compared with the contralateral side. α-Syn-ir was present in both ipsi- and contra- lateral MPTP-treated nigra, caudate, and putamen, mostly in fibers; its intracellular distribution was not affected by age. Comparison of α-syn-ir between MPTP-treated young and aged monkeys revealed significantly higher optical density for both the ipsi- and contralateral caudate and SN in the aged animals. TH and α-syn immunofluorescence confirmed the loss of nigral TH-ir dopaminergic neurons in the MPTP-treated side of intoxicated animals, but bilateral α-syn expression. Colabeling of GAD67 and α-syn immunofluorescence showed that α-syn expression was present mainly in GABAergic fibers. Our results demonstrate that, 3 months post unilateral intracarotid artery infusion of MPTP, α-syn expression in the SN is largely present in GABAergic fibers, regardless of age. Bilateral increase of α-syn expression in SN fibers of aged, compared with young rhesus monkeys, suggests that α-syn-ir may increase with age, but not after neurotoxin-induced dopaminergic nigral cell loss.

Local transgene expression and whole-body transgenesis to model brain diseases in nonhuman primate

Animal model is an essential tool in the life sciences research, notably in understanding the pathogenesis of the diseases and for further therapeutic intervention success. Rodents have been the most frequently used animals to model human disease since the establishment of gene manipulation technique. However, they remain inadequate to fully mimic the pathophysiology of human brain disease, partially due to huge differences between rodents and humans in terms of anatomy, brain function, and social behaviors. Nonhuman primates are more suitable in translational perspective. Thus, genetically modified animals have been generated to investigate neurologic and psychiatric disorders. The classical transgenesis technique is not efficient in that model; so, viral vector-mediated transgene delivery and the new genome-editing technologies have been promoted. In this review, we summarize some of the technical progress in the generation of an ad hoc animal model of brain diseases by gene delivery and real transgenic nonhuman primate.