Motor dysfunction and gliosis with preserved dopaminergic markers in human alpha-synuclein A30P transgenic mice

Evidence for prodromal changes in neuronal excitability and neuroinflammation in the hippocampus in young alpha-synuclein (A30P) transgenic mice

Introduction

Neuronal hyperexcitability and neuroinflammation are thought to occur at early stages in a range of neurodegenerative diseases. Neuroinflammation, notably activation of microglia, has been identified as a potential prodromal marker of dementia with Lewy bodies (DLB). Using a transgenic mouse model of DLB that over-expresses human mutant (A30P) alpha-synuclein (hα-syn) we have investigated whether early neuroinflammation is evident in the hippocampus in young pre-symptomatic animals.

Methods

Previous studies have shown early hyperexcitability in the hippocampal CA3 region in male A30P mice at 2-4 months of age, therefore, in the current study we have immunostained this region for markers of neuronal activity (c-Fos), reactive astrocytes (glial fibrillary acidic protein, GFAP), microglia (ionizing calcium binding adapter protein 1, Iba-1) and reactive microglia (inducible nitric oxide synthase, iNOS).

Results

We found an interesting biphasic change in the expression of c-Fos in A30P mice with high expression at 1 month, consistent with early onset of hyperexcitability, but lower expression from 2-4 months in male A30P mice compared to wild-type (WT) controls, possibly indicating chronic hyperexcitability. Neuroinflammation was indicated by significant increases in the % area of GFAP and the number of Iba-1+ cells that expressed iNOS immunoreactivity in the CA3 region in 2-4 months A30P male mice compared to WT controls. A similar increase in % area of GFAP was observed in female A30P mice, however, the Iba-1 count was not different between female WT and A30P mice. In WT mice aged 2-4 months only 4.6% of Iba-1+ cells co-expressed iNOS. In contrast, in age matched A30P mice 87% of cells co-expressed Iba-1 and iNOS. Although there was no difference in GFAP immunoreactivity at 1 month, Iba-1/iNOS co-expression was also increased in a cohort of 1 month old A30P mice.

Discussion

Abnormal hα-syn expression in A30P mice caused early changes in network excitability, as indicated by c-Fos expression, and neuroinflammation which might contribute to disease progression.

Deficits in basal and evoked striatal dopamine release following alpha-synuclein preformed fibril injection: An in vivo microdialysis study

Parkinson's disease (PD) is characterized by the accumulation of misfolded alpha-synuclein (α-syn) protein, forming intraneuronal Lewy body (LB) inclusions. The α-syn preformed fibril (PFF) model of PD recapitulates α-syn aggregation, progressive nigrostriatal degeneration and motor dysfunction; however, little is known about the time course of PFF-induced alterations in basal and evoked dopamine (DA). In vivo microdialysis is well suited for identifying small changes in neurotransmitter levels over extended periods. In the present study, adult male Fischer 344 rats received unilateral, intrastriatal injections of either α-syn PFFs or phosphate-buffered saline (PBS). At 4 or 8 months post-injection (p.i.), animals underwent in vivo microdialysis to evaluate basal extracellular striatal DA and metabolite levels, local KCl-evoked striatal DA release and the effects of systemic levodopa (l-DOPA). Post-mortem analysis demonstrated equivalent PFF-induced reductions in tyrosine hydroxylase (TH) immunoreactive nigral neurons (~50%) and striatal TH (~20%) at both time points. Compared with reduction in striatal TH, reduction in striatal dopamine transporter (DAT) was more pronounced and progressed between the 4- and 8-month p.i. intervals (36% ➔ 46%). Significant PFF-induced deficits in basal and evoked striatal DA, as well as deficits in motor performance, were not observed until 8 months p.i. Responses to l-DOPA did not differ regardless of PBS or PFF treatment. These results suggest that basal and evoked striatal DA are maintained for several months following PFF injection, with loss of both associated with motor dysfunction. Our studies provide insight into the time course and magnitude of PFF-induced extracellular dopaminergic deficits in the striatum.

Solubility of α-synuclein species in the L62 mouse model of synucleinopathy

The accumulation of α-synuclein (α-Syn) into Lewy bodies is a hallmark of synucleinopathies, a group of neurological disorders that include Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Small oligomers as well as larger fibrils of α-Syn have been suggested to induce cell toxicity leading to a degenerative loss of neurones. A richer understanding of α-Syn aggregation in disease, however, requires the identification of the different α-Syn species and the characterisation of their biochemical properties. We here aimed at a more in-depth characterisation of the α-Syn transgenic mice, Line 62 (L62), and examined the deposition pattern and solubility of human and murine α-Syn in these mice using immunohistochemical and biochemical methods. Application of multiple antibodies confirmed mAb syn204 as the most discriminatory antibody for human α-Syn in L62. Syn204 revealed an intense and widespread immunohistochemical α-Syn labelling in parietal cortex and hippocampus, and to a lower level in basal forebrain and hindbrain regions. The labelled α-Syn represented somatic inclusions as well as processes and synaptic endings. Biochemical analysis revealed a Triton-resistant human α-Syn pool of large oligomers, a second pool of small oligomers that was not resistant to solubilization with urea/Triton. A third SDS-soluble pool of intermediate sized aggregates containing a mixture of both, human and mouse α-Syn was also present. These data suggest that several pools of α-Syn can exist in neurones, most likely in different cellular compartments. Information about these different pools is important for the development of novel disease modifying therapies aimed at α-Syn.

Recalibrating the Why and Whom of Animal Models in Parkinson Disease: A Clinician's Perspective

Animal models have been used to gain pathophysiologic insights into Parkinson's disease (PD) and aid in the translational efforts of interventions with therapeutic potential in human clinical trials. However, no disease-modifying therapy for PD has successfully emerged from model predictions. These translational disappointments warrant a reappraisal of the types of preclinical questions asked of animal models. Besides the limitations of experimental designs, the one-size convergence and oversimplification yielded by a model cannot recapitulate the molecular diversity within and between PD patients. Here, we compare the strengths and pitfalls of different models, review the discrepancies between animal and human data on similar pathologic and molecular mechanisms, assess the potential of organoids as novel modeling tools, and evaluate the types of questions for which models can guide and misguide. We propose that animal models may be of greatest utility in the evaluation of molecular mechanisms, neural pathways, drug toxicity, and safety but can be unreliable or misleading when used to generate pathophysiologic hypotheses or predict therapeutic efficacy for compounds with potential neuroprotective effects in humans. To enhance the translational disease-modification potential, the modeling must reflect the biology not of a diseased population but of subtypes of diseased humans to distinguish What data are relevant and to Whom.

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.

Leveraging the preformed fibril model to distinguish between alpha-synuclein inclusion- and nigrostriatal degeneration-associated immunogenicity

Neuroinflammation has become a well-accepted pathologic hallmark of Parkinson's disease (PD). However, it remains unclear whether inflammation, triggered by α-syn aggregation and/or degeneration, contributes to the progression of the disease. Studies examining neuroinflammation in PD are unable to distinguish between Lewy body-associated inflammation and degeneration-associated inflammation, as both pathologies are present simultaneously. Intrastriatal and intranigral injections of alpha-synuclein (α-syn) preformed fibrils (PFFs) results in two distinct pathologic phases: Phase 1: The accumulation and peak formation of α-syn inclusions in nigrostriatal system and, Phase 2: Protracted dopaminergic neuron degeneration. In this review we summarize the current understanding of neuroinflammation in the α-syn PFF model, leveraging the distinct Phase 1 aggregation phase and Phase 2 degeneration phase to guide our interpretations. Studies consistently demonstrate an association between pathologic α-syn aggregation in the substantia nigra (SN) and activation of the innate immune system. Further, major histocompatibility complex-II (MHC-II) antigen presentation is proportionate to inclusion load. The α-syn aggregation phase is also associated with peripheral and adaptive immune cell infiltration to the SN. These findings suggest that α-syn like aggregates are immunogenic and thus have the potential to contribute to the degenerative process. Studies examining neuroinflammation during the neurodegenerative phase reveal elevated innate, adaptive, and peripheral immune cell markers, however limitations of single time point experimental design hinder interpretations as to whether this neuroinflammation preceded, or was triggered by, nigral degeneration. Longitudinal studies across both the aggregation and degeneration phases of the model suggest that microglial activation (MHC-II) is greater in magnitude during the aggregation phase that precedes degeneration. Overall, the consistency between neuroinflammatory markers in the parkinsonian brain and in the α-syn PFF model, combined with the distinct aggregation and degenerative phases, establishes the utility of this model platform to yield insights into pathologic events that contribute to neuroinflammation and disease progression in PD.

Microglial AIM2 alleviates antiviral-related neuro-inflammation in mouse models of Parkinson's disease

Inflammasome involvement in Parkinson's disease (PD) has been intensively investigated. Absent in melanoma 2 (AIM2) is an essential inflammasome protein known to contribute to the development of several neurological diseases. However, a specific role for AIM2 in PD has not been reported. In this study, we investigated the effect of AIM2 in the N-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP)-induced PD model by use of various knockout and bone marrow chimeric mice. The mechanism of action for AIM2 in PD was assessed by RNA-sequencing and in vitro primary microglial transfection. Results were validated in the A30P transgenic mouse model of PD. In the MPTP mouse model, AIM2 activation was found to negatively regulate neuro-inflammation independent of the inflammasome. Microglial AIM2 deficiency exacerbated behavioral and pathological features of both MPTP-induced and transgenic PD mouse models. Mechanistically, AIM2 reduced cyclic GMP-AMP synthase (cGAS)-mediated antiviral-related inflammation by inhibition of AKT-interferon regulatory factor 3 (IRF3) phosphorylation. These results demonstrate microglial AIM2 to inhibit the antiviral-related neuro-inflammation associated with PD and provide for a foundation upon which to identify new therapeutic targets for treatment of the disease.

Prevention of L-Dopa-Induced Dyskinesias by MPEP Blockade of Metabotropic Glutamate Receptor 5 Is Associated with Reduced Inflammation in the Brain of Parkinsonian Monkeys

Proinflammatory markers were found in brains of Parkinson’s disease (PD) patients. After years of L-Dopa symptomatic treatment, most PD patients develop dyskinesias. The relationship between inflammation and L-Dopa-induced dyskinesias (LID) is still unclear. We previously reported that MPEP (a metabotropic glutamate receptor 5 antagonist) reduced the development of LID in de novo MPTP-lesioned monkeys. We thus investigated if MPEP reduced the brain inflammatory response in these MPTP-lesioned monkeys and the relationship to LID. The panmacrophage/microglia marker Iba1, the phagocytosis-related receptor CD68, and the astroglial protein GFAP were measured by Western blots. The L-Dopa-treated dyskinetic MPTP monkeys had increased Iba1 content in the putamen, substantia nigra, and globus pallidus, which was prevented by MPEP cotreatment; similar findings were observed for CD68 contents in the putamen and globus pallidus. There was a strong positive correlation between dyskinesia scores and microglial markers in these regions. GFAP contents were elevated in MPTP + L-Dopa-treated monkeys among these brain regions and prevented by MPEP in the putamen and subthalamic nucleus. In conclusion, these results showed increased inflammatory markers in the basal ganglia associated with LID and revealed that MPEP inhibition of glutamate activity reduced LID and levels of inflammatory markers.

Body-first Parkinson's disease and variant Creutzfeldt-Jakob disease - similar or different?

In several neurodegenerative disorders, proteins that typically exhibit an α-helical structure misfold into an amyloid conformation rich in β-sheet content. Through a self-templating mechanism, these amyloids are able to induce additional protein misfolding, facilitating their propagation throughout the central nervous system. This disease mechanism was originally identified for the prion protein (PrP), which misfolds into PrP^Sc in a number of disorders, including variant Creutzfeldt-Jakob disease (vCJD) and bovine spongiform encephalopathy (BSE). More recently, the prion mechanism of disease was expanded to include other proteins that rely on this self-templating mechanism to cause progressive degeneration, including α-synuclein misfolding in Parkinson's disease (PD). Several studies now suggest that PD patients can be subcategorized based on where in the body misfolded α-synuclein originates, either the brain or the gut, similar to patients developing sporadic CJD or vCJD. In this review, we discuss the human and animal model data indicating that α-synuclein and PrP^Sc misfolding originates in the gut in body-first PD and vCJD, and summarize the data identifying the role of the autonomic nervous system in the gut-brain axis of both diseases.

Targeting alpha-synuclein via the immune system in Parkinson's disease: Current vaccine therapies

Parkinson's disease (PD) is the second most common neurodegenerative disorder and is defined pathologically by the abnormal accumulation of the presynaptic protein alpha-synuclein (aSyn) in the form of Lewy bodies and Lewy neurites and loss of midbrain dopaminergic neurons in the substantia nigra pars compacta. Because of aSyn's involvement in both sporadic and familial forms of PD, it has become a key target for the development of novel therapeutics. Aberrant aSyn is associated with multiple mechanisms of neuronal dysfunction and degeneration including inflammation, impaired mitochondrial function, altered protein degradation systems, and oxidative stress. Inflammation, in particular, has emerged as a potential significant contributor early in the disease making it an attractive target for disease modification and neuroprotection. Thus, immunotherapies targeting aSyn are currently being investigated in pre-clinical and clinical trials. The focus of this review is to highlight the role of aSyn in neuroinflammation and discuss the current status of aSyn-directed immunotherapies in pre-clinical and clinical trials for PD.

Evidence of distinct α-synuclein strains underlying disease heterogeneity

Synucleinopathies are a group of neurodegenerative disorders caused by the misfolding and self-templating of the protein α-synuclein, or the formation of α-synuclein prions. Each disorder differs by age of onset, presenting clinical symptoms, α-synuclein inclusion morphology, and neuropathological distribution. Explaining this disease-specific variability, the strain hypothesis postulates that each prion disease is encoded by a distinct conformation of the misfolded protein, and therefore, each synucleinopathy is caused by a unique α-synuclein structure. This review discusses the current data supporting the role of α-synuclein strains in disease heterogeneity. Several in vitro and in vivo models exist for evaluating strain behavior, however, as the focus of this article is to compare strains across synucleinopathy patients, our discussion predominantly focuses on the two models most commonly used for this purpose: the α-syn140*A53T-YFP cell line and the TgM83^+/- mouse model. Here we define each strain based on biochemical stability, ability to propagate in α-syn140-YFP cell lines, and incubation period, inclusion morphology and distribution, and neurological signs in TgM83^+/- mice.

Neurons and Glia Interplay in α-Synucleinopathies

Accumulation of the neuronal presynaptic protein alpha-synuclein within proteinaceous inclusions represents the key histophathological hallmark of a spectrum of neurodegenerative disorders, referred to by the umbrella term a-synucleinopathies. Even though alpha-synuclein is expressed predominantly in neurons, pathological aggregates of the protein are also found in the glial cells of the brain. In Parkinson's disease and dementia with Lewy bodies, alpha-synuclein accumulates mainly in neurons forming the Lewy bodies and Lewy neurites, whereas in multiple system atrophy, the protein aggregates mostly in the glial cytoplasmic inclusions within oligodendrocytes. In addition, astrogliosis and microgliosis are found in the synucleinopathy brains, whereas both astrocytes and microglia internalize alpha-synuclein and contribute to the spread of pathology. The mechanisms underlying the pathological accumulation of alpha-synuclein in glial cells that under physiological conditions express low to non-detectable levels of the protein are an area of intense research. Undoubtedly, the presence of aggregated alpha-synuclein can disrupt glial function in general and can contribute to neurodegeneration through numerous pathways. Herein, we summarize the current knowledge on the role of alpha-synuclein in both neurons and glia, highlighting the contribution of the neuron-glia connectome in the disease initiation and progression, which may represent potential therapeutic target for a-synucleinopathies.

Periphery and brain, innate and adaptive immunity in Parkinson's disease

Parkinson’s disease (PD) is a neurodegenerative disorder where alpha-synuclein plays a central role in the death and dysfunction of neurons, both, in central, as well as in the peripheral nervous system. Besides the neuronal events observed in patients, PD also includes a significant immune component. It is suggested that the PD-associated immune response will have consequences on neuronal health, thus opening immunomodulation as a potential therapeutic strategy in PD. The immune changes during the disease occur in the brain, involving microglia, but also in the periphery with changes in cells of the innate immune system, particularly monocytes, as well as those of adaptive immunity, such as T-cells. This realization arises from multiple patient studies, but also from data in animal models of the disease, providing strong evidence for innate and adaptive immune system crosstalk in the central nervous system and periphery in PD. Here we review the data showing that alpha-synuclein plays a crucial role in the activation of the innate and adaptive immune system. We will also describe the studies suggesting that inflammation in PD includes early changes in innate and adaptive immune cells that develop dynamically through time during disease, contributing to neuronal degeneration and symptomatology in patients. This novel finding has contributed to the definition of PD as a multisystem disease that should be approached in a more integratory manner rather than a brain-focused classical approach.

The Path to Progress Preclinical Studies of Age-Related Neurodegenerative Diseases: A Perspective on Rodent and hiPSC-Derived Models

Alzheimer's disease (AD) and Parkinson's disease (PD) are the two most prevalent age-related neurodegenerative diseases, and currently no effective clinical treatments exist for either, despite decades of clinical trials. The failure to translate preclinical findings into effective treatments is indicative of a problem in the current evaluation pipeline for potential therapeutics. At present, there are no useful animal models for AD and PD research that reflect the entire biology of the diseases, specifically, the more common non-Mendelian forms. Whereas the field continues to seek suitable rodent models for investigating potential therapeutics for these diseases, rodent models have still been used primarily for preclinical studies. Here, we advocate for a paradigm shift toward the application of human-induced pluripotent stem cell (hiPSC)-derived systems for PD and AD modeling and the development of improved human-based models in a dish for drug discovery and preclinical assessment of therapeutic targets.

Ethanol extract from Gynostemma pentaphyllum ameliorates dopaminergic neuronal cell death in transgenic mice expressing mutant A53T human alpha-synuclein

Gynostemma (G.) pentaphyllum (Cucurbitaceae) contains various bioactive gypenosides. Ethanol extract from G. pentaphyllum (GP-EX) has been shown to have ameliorative effects on the death of dopaminergic neurons in animal models of Parkinson’s disease (PD) induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine- and 6-hydroxydopamine. PD patients exhibit multiple symptoms, so PD-related research should combine neurotoxin models with genetic models. In the present study, we investigated the ameliorative effects of GP-EX, including gypenosides, on the cell death of dopaminergic neurons in the midbrain of A53T α-synuclein transgenic mouse models of PD (A53T). Both GP-EX and gypenosides at 50 mg/kg per day were orally administered to the A53T mice for 20 weeks. α-Synuclein-immunopositive cells and α-synuclein phosphorylation were increased in the midbrain of A53T mice, which was reduced following treatment with GP-EX. Treatment with GP-EX modulated the reduced phosphorylation of tyrosine hydroxylase, extracellular signal-regulated kinase (ERK1/2), Bcl-2-associated death promoter (Bad) at Ser112, and c-Jun N-terminal kinase (JNK1/2) due to α-synuclein overexpression. In the A53T group, GP-EX treatment prolonged the latency of the step-through passive avoidance test and shortened the transfer latency of the elevated plus maze test. Gypenosides treatment exhibited the effects and efficacy similar to those of GP-EX. Taken together, GP-EX, including gypenosides, has ameliorative effects on dopaminergic neuronal cell death due to the overexpression of α-synuclein by modulating ERK1/2, Bad at Ser112, and JNK1/2 signaling in the midbrain of A53T mouse model of PD. Further studies are needed to investigate GP-EX as a treatment for neurodegenerative synucleinopathies, including PD. This study was approved by the Animal Ethics Committee of Chungbuk National University (approval No. CBNUA-956-16-01) on September 21, 2016.

Lipid and immune abnormalities causing age-dependent neurodegeneration and Parkinson's disease

This article describes pathogenic concepts and factors, in particular glycolipid abnormalities, that create cell dysfunction and synaptic loss in neurodegenerative diseases. By phenocopying lysosomal storage disorders, such as Gaucher disease and related disorders, age- and dose-dependent changes in glycolipid cell metabolism can lead to Parkinson's disease and related dementias. Recent results show that perturbation of sphingolipid metabolism can precede or is a part of abnormal protein handling in both genetic and idiopathic Parkinson's disease and Lewy body dementia. In aging and genetic predisposition with lipid disturbance, α-synuclein's normal vesicular and synaptic role may be detrimentally shifted toward accommodating and binding such lipids. Specific neuronal glycolipid, protein, and vesicular interactions create potential pathophysiology that is amplified by astroglial and microglial immune mechanisms resulting in neurodegeneration. This perspective provides a new logic for therapeutic interventions that do not focus on protein aggregation, but rather provides a guide to the complex biology and the common sequence of events that lead to age-dependent neurodegenerative disorders.

α-Synuclein Trafficking in Parkinson's Disease: Insights From Fly and Mouse Models

Protein aggregation and accumulation are common pathological hallmarks in neurodegenerative diseases. To efficiently clear and eliminate such aggregation becomes an important cellular strategy for cell survival. Lewy bodies inclusion and aggregation of α-Synuclein (α-Syn) during the pathogenesis of Parkinson's disease (PD) serve as a good example and are potentially linked to other pathological PD features such as progressive dopaminergic neuron cell death, behavioral defects, and nonmotor symptoms like anosmia, cognitive impairment, and depression. Years of research have revealed a variety of mechanisms underlying α-Syn aggregation, clearance, and spread. Particularly, vesicular routes associated with the trafficking of α-Syn, leading to its aggregation and accumulation, have been shown to play vital roles in PD pathogenesis. How α-Syn proteins propagate among cells in a prion-like manner, either from or to neurons and glia, via means of uptake or secretion, are questions under active investigation and have been of central interest in the field of PD study. This review covers components and pathways of possible vesicular routes involved in α-Syn trafficking. Events including but not limited to exocytosis and endocytosis will be discussed within the context of an overall cellular trafficking theme. Recent advances on α-Syn trafficking mechanisms and their significance in mediating PD pathogenesis will be thoroughly reviewed, ending with a discussion on the advantages and limitations of different animal PD models.

Time course and magnitude of alpha-synuclein inclusion formation and nigrostriatal degeneration in the rat model of synucleinopathy triggered by intrastriatal α-synuclein preformed fibrils

Animal models that accurately recapitulate the accumulation of alpha-synuclein (α-syn) inclusions, progressive neurodegeneration of the nigrostriatal system and motor deficits can be useful tools for Parkinson's disease (PD) research. The preformed fibril (PFF) synucleinopathy model in rodents generally displays these PD-relevant features, however, the magnitude and predictability of these events is far from established. We therefore sought to optimize the magnitude of α-syn accumulation and nigrostriatal degeneration, and to understand the time course of both. Rats were injected unilaterally with different quantities of α-syn PFFs (8 or 16 μg of total protein) into striatal sites selected to concentrate α-syn inclusion formation in the substantia nigra pars compacta (SNpc). Rats displayed an α-syn PFF quantity-dependent increase in the magnitude of ipsilateral SNpc inclusion formation at 2 months and bilateral loss of nigral dopamine neurons at 6 months. Unilateral 16 μg PFF injection also resulted in modest sensorimotor deficits in forelimb adjusting steps associated with degeneration at 6 months. Bilateral injection of 16 μg α-syn PFFs resulted in symmetric bilateral degeneration equivalent to the ipsilateral nigral degeneration observed following unilateral 16 μg PFF injection (~50% loss). Bilateral PFF injections additionally resulted in alterations in several gait analysis parameters. These α-syn PFF parameters can be applied to generate a reproducible synucleinopathy model in rats with which to study pathogenic mechanisms and vet potential disease-modifying therapies.

Multiple system atrophy prions retain strain specificity after serial propagation in two different Tg(SNCA*A53T) mouse lines

Previously, we reported that intracranial inoculation of brain homogenate from multiple system atrophy (MSA) patient samples produces neurological disease in the transgenic (Tg) mouse model TgM83+/-, which uses the prion protein promoter to express human α-synuclein harboring the A53T mutation found in familial Parkinson's disease (PD). In our studies, we inoculated MSA and control patient samples into Tg mice constructed using a P1 artificial chromosome to express wild-type (WT), A30P, and A53T human α-synuclein on a mouse α-synuclein knockout background [Tg(SNCA+/+)Nbm, Tg(SNCA*A30P+/+)Nbm, and Tg(SNCA*A53T+/+)Nbm]. In contrast to studies using TgM83+/- mice, motor deficits were not observed by 330-400 days in any of the Tg(SNCA)Nbm mice after inoculation with MSA brain homogenates. However, using a cell-based bioassay to measure α-synuclein prions, we found brain homogenates from Tg(SNCA*A53T+/+)Nbm mice inoculated with MSA patient samples contained α-synuclein prions, whereas control mice did not. Moreover, these α-synuclein aggregates retained the biological and biochemical characteristics of the α-synuclein prions in MSA patient samples. Intriguingly, Tg(SNCA*A53T+/+)Nbm mice developed α-synuclein pathology in neurons and astrocytes throughout the limbic system. This finding is in contrast to MSA-inoculated TgM83+/- mice, which develop exclusively neuronal α-synuclein aggregates in the hindbrain that cause motor deficits with advanced disease. In a crossover experiment, we inoculated TgM83+/- mice with brain homogenate from two MSA patient samples or one control sample first inoculated, or passaged, in Tg(SNCA*A53T+/+)Nbm animals. Additionally, we performed the reverse experiment by inoculating Tg(SNCA*A53T+/+)Nbm mice with brain homogenate from the same two MSA samples and one control sample first passaged in TgM83+/- animals. The TgM83+/- mice inoculated with mouse-passaged MSA developed motor dysfunction and α-synuclein prions, whereas the mouse-passaged control sample had no effect. Similarly, the mouse-passaged MSA samples induced α-synuclein prion formation in Tg(SNCA*A53T+/+)Nbm mice, but the mouse-passaged control sample did not. The confirmed transmission of α-synuclein prions to a second synucleinopathy model and the ability to propagate prions between two distinct mouse lines while retaining strain-specific properties provides compelling evidence that MSA is a prion disease.

Immune system responses in Parkinson's disease: Early and dynamic

The neuropathological hallmarks of Parkinson's disease (PD) are the degeneration and death of dopamine-producing neurons in the ventral midbrain, the widespread intraneuronal aggregation of alpha-synuclein (α) in Lewy bodies and neurites, neuroinflammation, and gliosis. Signs of microglia activation in the PD brain postmortem as well as during disease development revealed by neuroimaging, implicate immune responses in the pathophysiology of the disease. Intensive research during the last two decades has advanced our understanding of the role of these responses in the disease process, yet many questions remain unanswered. A transformative finding in the field has been the confirmation that in vivo microglia are able to respond directly to pathological a-syn aggregates but also to neuronal dysfunction due to intraneuronal a-syn toxicity well in advance of neuronal death. In addition, clinical research and disease models have revealed the involvement of both the innate and adaptive immune systems. Indeed, the data suggest that PD leads not only to a microglia response, but also to a cellular and humoral peripheral immune response. Together, these findings compel us to consider a more holistic view of the immunological processes associated with the disease. Central and peripheral immune responses aimed at maintaining neuronal health will ultimately have consequences on neuronal survival. We will review here the most significant findings that have contributed to the current understanding of the immune response in PD, which is proposed to occur early, involve peripheral and brain immune cells, evolve as neuronal dysfunction progresses, and is likely to influence disease progression.

The usual suspects, dopamine and alpha-synuclein, conspire to cause neurodegeneration

Parkinson's disease (PD) is primarily a movement disorder driven by the loss of dopamine-producing neurons in the substantia nigra (SN). Early identification of the oxidative properties of dopamine implicated it as a potential source of oxidative stress in PD, yet few studies have investigated dopamine neurotoxicity in vivo. The discovery of PD-causing mutations in α-synuclein and the presence of aggregated α-synuclein in the hallmark Lewy body pathology of PD revealed another important player. Despite extensive efforts, the precise role of α-synuclein aggregation in neurodegeneration remains unclear. We recently manipulated both dopamine levels and α-synuclein expression in aged mice and found that only the combination of these 2 factors caused progressive neurodegeneration of the SN and an associated motor deficit. Dopamine modified α-synuclein aggregation in the SN, resulting in greater abundance of α-synuclein oligomers and unique dopamine-induced oligomeric conformations. Furthermore, disruption of the dopamine-α-synuclein interaction rescued dopaminergic neurons from degeneration in transgenic Caenorhabditis elegans models. In this Perspective, we discuss these findings in the context of known α-synuclein and dopamine biology, review the evidence for α-synuclein oligomer toxicity and potential mechanisms, and discuss therapeutic implications. © 2019 International Parkinson and Movement Disorder Society.

Microglia Response During Parkinson's Disease: Alpha-Synuclein Intervention

The discovery of the central role played by the protein alpha-synuclein in Parkinson's disease and other Lewy body brain disorders has had a great relevance in the understanding of the degenerative process occurring in these diseases. In addition, during the last two decades, the evidence suggesting an immune response in Parkinson's disease patients have multiplied. The role of the immune system in the disease is supported by data from genetic studies and patients, as well as from laboratory animal models and cell cultures. In the immune response, the microglia, the immune cell of the brain, will have a determinant role. Interestingly, alpha-synuclein is suggested to have a central function not only in the neuronal events occurring in Parkinson's disease, but also in the immune response during the disease. Numerous studies have shown that alpha-synuclein can act directly on immune cells, such as microglia in brain, initiating a sterile response that will have consequences for the neuronal health and that could also translate in a peripheral immune response. In parallel, microglia should also act clearing alpha-synuclein thus avoiding an overabundance of the protein, which is crucial to the disease progression. Therefore, the microglia response in each moment will have significant consequences for the neuronal fate. Here we will review the literature addressing the microglia response in Parkinson's disease with an especial focus on the protein alpha-synuclein. We will also reflect upon the limitations of the studies carried so far and in the therapeutic possibilities opened based on these recent findings.

Lewy body-like alpha-synuclein inclusions trigger reactive microgliosis prior to nigral degeneration

BACKGROUND

Converging evidence suggests a role for microglia-mediated neuroinflammation in Parkinson's disease (PD). Animal models of PD can serve as a platform to investigate the role of neuroinflammation in degeneration in PD. However, due to features of the previously available PD models, interpretations of the role of neuroinflammation as a contributor to or a consequence of neurodegeneration have remained elusive. In the present study, we investigated the temporal relationship of neuroinflammation in a model of synucleinopathy following intrastriatal injection of pre-formed alpha-synuclein fibrils (α-syn PFFS).

METHODS

Male Fischer 344 rats (N = 114) received unilateral intrastriatal injections of α-syn PFFs, PBS, or rat serum albumin with cohorts euthanized at monthly intervals up to 6 months. Quantification of dopamine neurons, total neurons, phosphorylated α-syn (pS129) aggregates, major histocompatibility complex-II (MHC-II) antigen-presenting microglia, and ionized calcium-binding adaptor molecule-1 (Iba-1) immunoreactive microglial soma size was performed in the substantia nigra. In addition, the cortex and striatum were also examined for the presence of pS129 aggregates and MHC-II antigen-presenting microglia to compare the temporal patterns of pSyn accumulation and reactive microgliosis.

RESULTS

Intrastriatal injection of α-syn PFFs to rats resulted in widespread accumulation of phosphorylated α-syn inclusions in several areas that innervate the striatum followed by significant loss (~ 35%) of substantia nigra pars compacta dopamine neurons within 5-6 months. The peak magnitudes of α-syn inclusion formation, MHC-II expression, and reactive microglial morphology were all observed in the SN 2 months following injection and 3 months prior to nigral dopamine neuron loss. Surprisingly, MHC-II immunoreactivity in α-syn PFF injected rats was relatively limited during the later interval of degeneration. Moreover, we observed a significant correlation between substantia nigra pSyn inclusion load and number of microglia expressing MHC-II. In addition, we observed a similar relationship between α-syn inclusion load and number of microglia expressing MHC-II in cortical regions, but not in the striatum.

CONCLUSIONS

Our results demonstrate that increases in microglia displaying a reactive morphology and MHC-II expression occur in the substantia nigra in close association with peak numbers of pSyn inclusions, months prior to nigral dopamine neuron degeneration, and suggest that reactive microglia may contribute to vulnerability of SNc neurons to degeneration. The rat α-syn PFF model provides an opportunity to examine the innate immune response to accumulation of pathological α-syn in the context of normal levels of endogenous α-syn and provides insight into the earliest neuroinflammatory events in PD.

Parkinson's disease: experimental models and reality

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

Stomaching the Possibility of a Pathogenic Role for Helicobacter pylori in Parkinson's Disease

While a small subset of Parkinson's disease cases have genetic causes, most cases are sporadic and may have an environmental contributor that has largely remained enigmatic. Remarkably, gastrointestinal symptoms in PD patients serve as a prodrome for the eventual motor dysfunctions. Herein, we review studies exploring a possible link between the gastric human pathogen Helicobacter pylori and PD. We provide plausible and testable hypotheses for how this organism might contribute to PD: 1) a toxin(s) produced by the bacteria; 2) disruption of the intestinal microbiome; 3) local inflammation that crosses the gut-brain axis, leading to neuroinflammation; and 4) manipulation of the pharmacokinetics of the PD drug levodopa by H. pylori, even in those not receiving exogenous levodopa. Key findings are: 1) people with PD are 1.5-3-fold more likely to be infected with H. pylori than people without PD; 2) H. pylori-infected PD patients display worse motor functions than H. pylori-negative PD patients; 3) eradication of H. pylori improves motor function in PD patients over PD patients whose H. pylori was not eradicated; and 4) eradication of H. pylori improves levodopa absorption in PD patients compared to that of PD patients whose H. pylori was not eradicated. Evidence is accumulating that H. pylori has a link with PD, but the mechanism is unclear. Future work should explore the effects of H. pylori on development of PD in defined PD animal models, focusing on the roles of H. pylori toxins, inflammation, levodopa absorption, and microbiome dysbiosis.

Current understanding of the molecular mechanisms in Parkinson's disease: Targets for potential treatments

Gradual degeneration and loss of dopaminergic neurons in the substantia nigra, pars compacta and subsequent reduction of dopamine levels in striatum are associated with motor deficits that characterize Parkinson’s disease (PD). In addition, half of the PD patients also exhibit frontostriatal-mediated executive dysfunction, including deficits in attention, short-term working memory, speed of mental processing, and impulsivity. The most commonly used treatments for PD are only partially or transiently effective and are available or applicable to a minority of patients. Because, these therapies neither restore the lost or degenerated dopaminergic neurons, nor prevent or delay the disease progression, the need for more effective therapeutics is critical. In this review, we provide a comprehensive overview of the current understanding of the molecular signaling pathways involved in PD, particularly within the context of how genetic and environmental factors contribute to the initiation and progression of this disease. The involvement of molecular chaperones, autophagy-lysosomal pathways, and proteasome systems in PD are also highlighted. In addition, emerging therapies, including pharmacological manipulations, surgical procedures, stem cell transplantation, gene therapy, as well as complementary, supportive and rehabilitation therapies to prevent or delay the progression of this complex disease are reviewed.

Are rodent models of Parkinson's disease behaving as they should?

In recent years our understanding of Parkinson's disease has expanded both in terms of pathological hallmarks as well as relevant genetic influences. In parallel with the aetiological discoveries a multitude of PD animal models have been established. The vast majority of these are rodent models based on environmental, genetic and mechanistic insight. A major challenge in many of these models is their ability to only recapitulate some of the complex disease features seen in humans. Although symptom alleviation and clinical signs are of utmost importance in therapeutic research many of these models lack comprehensive behavioural testing. While non-motor symptoms become increasingly important as early diagnostic markers in PD, they are poorly characterized in rodents. In this review we look at well-established and more recent animal models of PD in terms of behavioural characterization and discuss how they can best contribute to progression in Parkinson's research.

Exogenous α-Synuclein Monomers Alter Dopamine Metabolism in Murine Brain

Alpha-synuclein (ASN) is a small presynaptic protein which is the major component of Lewy bodies-the histological hallmark of Parkinson's disease. Among many functions, ASN plays an important role in regulation of dopaminergic system by controlling dopamine concentration at nerve terminals. An abnormal structure or excessive accumulation of ASN in the brain can induce neurotoxicity leading to the dopaminergic neurodegeneration. To date, several transgenic mouse lines overexpressing ASN have been generated and there are several studies using injections of ASN fibrils into the murine brain. However, still is little known about the effects of exogenously applied ASN monomers on dopaminergic neurotransmission. In this study we investigated the influence of cerebral injection of human ASN on dopaminergic system activity. We have demonstrated that a single injection of ASN monomers into the substantia nigra pars compacta or striatum is sufficient to affect dopaminergic neurotransmission in murine nigro-striatal system.

The contribution of alpha synuclein to neuronal survival and function - Implications for Parkinson's disease

The aggregation of alpha synuclein (α-syn) is a neuropathological feature that defines a spectrum of disorders collectively termed synucleinopathies, and of these, Parkinson's disease (PD) is arguably the best characterized. Aggregated α-syn is the primary component of Lewy bodies, the defining pathological feature of PD, while mutations or multiplications in the α-syn gene result in familial PD. The high correlation between α-syn burden and PD has led to the hypothesis that α-syn aggregation produces toxicity through a gain-of-function mechanism. However, α-syn has been implicated to function in a diverse range of essential cellular processes such as the regulation of neurotransmission and response to cellular stress. As such, an alternative hypothesis with equal explanatory power is that the aggregation of α-syn results in toxicity because of a toxic loss of necessary α-syn function, following sequestration of functional forms α-syn into insoluble protein aggregates. Within this review, we will provide an overview of the literature linking α-syn to PD and the knowledge gained from current α-syn-based animal models of PD. We will then interpret these data from the viewpoint of the α-syn loss-of-function hypothesis and provide a potential mechanistic model by which loss of α-syn function could result in at least some of the neurodegeneration observed in PD. By providing an alternative perspective on the etiopathogenesis of PD and synucleinopathies, this may reveal alternative avenues of research in order to identify potential novel therapeutic targets for disease modifying strategies. The correlation between α-synuclein burden and Parkinson's disease pathology has led to the hypothesis that α-synuclein aggregation produces toxicity through a gain-of-function mechanism. However, in this review, we discuss data supporting the alternative hypothesis that the aggregation of α-synuclein results in toxicity because of loss of necessary α-synuclein function at the presynaptic terminal, following sequestration of functional forms of α-synuclein into aggregates.

Fasudil attenuates aggregation of α-synuclein in models of Parkinson's disease

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder, yet disease-modifying treatments do not currently exist. Rho-associated protein kinase (ROCK) was recently described as a novel neuroprotective target in PD. Since alpha-synuclein (α-Syn) aggregation is a major hallmark in the pathogenesis of PD, we aimed to evaluate the anti-aggregative potential of pharmacological ROCK inhibition using the isoquinoline derivative Fasudil, a small molecule inhibitor already approved for clinical use in humans. Fasudil treatment significantly reduced α-Syn aggregation in vitro in a H4 cell culture model as well as in a cell-free assay. Nuclear magnetic resonance spectroscopy analysis revealed a direct binding of Fasudil to tyrosine residues Y133 and Y136 in the C-terminal region of α-Syn. Importantly, this binding was shown to be biologically relevant using site-directed mutagenesis of these residues in the cell culture model. Furthermore, we evaluated the impact of long-term Fasudil treatment on α-Syn pathology in vivo in a transgenic mouse model overexpressing human α-Syn bearing the A53T mutation (α-Syn^A53T mice). Fasudil treatment improved motor and cognitive functions in α-Syn^A53T mice as determined by Catwalk^TM gait analysis and novel object recognition (NOR), without apparent side effects. Finally, immunohistochemical analysis revealed a significant reduction of α-Syn pathology in the midbrain of α-Syn^A53T mice after Fasudil treatment. Our results demonstrate that Fasudil, next to its effects mediated by ROCK-inhibition, directly interacts with α-Syn and attenuates α-Syn pathology. This underscores the translational potential of Fasudil as a disease-modifying drug for the treatment of PD and other synucleinopathies.

Clioquinol Improves Cognitive, Motor Function, and Microanatomy of the Alpha-Synuclein hA53T Transgenic Mice

The abnormal accumulation of alpha-synuclein (α-syn) has been linked to a number of neurodegenerative disorders, the most noteworthy of which is Parkinson's disease. Alpha-synuclein itself is not toxic and fulfills various physiological roles in the central nervous system. However, specific types of aggregates have been shown to be toxic, and metals have been linked to the assembly of these toxic aggregates. In this paper, we have characterized a transgenic mouse that overexpresses the A53T mutation of human α-syn, specifically assessing cognition, motor performance, and subtle anatomical markers that have all been observed in synucleinopathies in humans. We hypothesized that treatment with the moderate-affinity metal chelator, clioquinol (CQ), would reduce the interaction between metals and α-syn to subsequently improve the phenotype of the A53T animal model. We showed that CQ prevents an iron-synuclein interaction, the formation of urea-soluble α-syn aggregates, α-syn-related substantia nigra pars compacta cell loss, reduction in dendritic spine density of hippocampal and caudate putamen medium spiny neurons, and the decline in motor and cognitive function. In conclusion, our data suggests that CQ is capable of mitigating the pathological metal/α-syn interactions, suggesting that the modulation of metal ions warrants further study as a therapeutic approach for the synucleinopathies.

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.

Alpha-Synuclein affects neurite morphology, autophagy, vesicle transport and axonal degeneration in CNS neurons

Many neuropathological and experimental studies suggest that the degeneration of dopaminergic terminals and axons precedes the demise of dopaminergic neurons in the substantia nigra, which finally results in the clinical symptoms of Parkinson disease (PD). The mechanisms underlying this early axonal degeneration are, however, still poorly understood. Here, we examined the effects of overexpression of human wildtype alpha-synuclein (αSyn-WT), a protein associated with PD, and its mutant variants αSyn-A30P and -A53T on neurite morphology and functional parameters in rat primary midbrain neurons (PMN). Moreover, axonal degeneration after overexpression of αSyn-WT and -A30P was analyzed by live imaging in the rat optic nerve in vivo. We found that overexpression of αSyn-WT and of its mutants A30P and A53T impaired neurite outgrowth of PMN and affected neurite branching assessed by Sholl analysis in a variant-dependent manner. Surprisingly, the number of primary neurites per neuron was increased in neurons transfected with αSyn. Axonal vesicle transport was examined by live imaging of PMN co-transfected with EGFP-labeled synaptophysin. Overexpression of all αSyn variants significantly decreased the number of motile vesicles and decelerated vesicle transport compared with control. Macroautophagic flux in PMN was enhanced by αSyn-WT and -A53T but not by αSyn-A30P. Correspondingly, colocalization of αSyn and the autophagy marker LC3 was reduced for αSyn-A30P compared with the other αSyn variants. The number of mitochondria colocalizing with LC3 as a marker for mitophagy did not differ among the groups. In the rat optic nerve, both αSyn-WT and -A30P accelerated kinetics of acute axonal degeneration following crush lesion as analyzed by in vivo live imaging. We conclude that αSyn overexpression impairs neurite outgrowth and augments axonal degeneration, whereas axonal vesicle transport and autophagy are severely altered.

shRNA targeting α-synuclein prevents neurodegeneration in a Parkinson's disease model

Multiple convergent lines of evidence implicate both α-synuclein (encoded by SCNA) and mitochondrial dysfunction in the pathogenesis of sporadic Parkinson's disease (PD). Occupational exposure to the mitochondrial complex I inhibitor rotenone increases PD risk; rotenone-exposed rats show systemic mitochondrial defects but develop specific neuropathology, including α-synuclein aggregation and degeneration of substantia nigra dopaminergic neurons. Here, we inhibited expression of endogenous α-synuclein in the adult rat substantia nigra by adeno-associated virus-mediated delivery of a short hairpin RNA (shRNA) targeting the endogenous rat Snca transcript. Knockdown of α-synuclein by ~35% did not affect motor function or cause degeneration of nigral dopaminergic neurons in control rats. However, in rotenone-exposed rats, progressive motor deficits were substantially attenuated contralateral to α-synuclein knockdown. Correspondingly, rotenone-induced degeneration of nigral dopaminergic neurons, their dendrites, and their striatal terminals was decreased ipsilateral to α-synuclein knockdown. These data show that α-synuclein knockdown is neuroprotective in the rotenone model of PD and indicate that endogenous α-synuclein contributes to the specific vulnerability of dopaminergic neurons to systemic mitochondrial inhibition. Our findings are consistent with a model in which genetic variants influencing α-synuclein expression modulate cellular susceptibility to environmental exposures in PD patients. shRNA targeting the SNCA transcript should be further evaluated as a possible neuroprotective therapy in PD.

Mechanisms of alpha-synuclein action on neurotransmission: cell-autonomous and non-cell autonomous role

Mutations and duplication/triplication of the alpha-synuclein (αSyn)-coding gene have been found to cause familial Parkinson’s disease (PD), while genetic polymorphisms in the region controlling the expression level and stability of αSyn have been identified as risk factors for idiopathic PD, pointing to the importance of wild-type (wt) αSyn dosage in the disease. Evidence that αSyn is present in the cerebrospinal fluid and interstitial brain tissue and that healthy neuronal grafts transplanted into PD patients often degenerate suggests that extracellularly-released αSyn plays a role in triggering the neurodegenerative process. αSyn’s role in neurotransmission has been shown in various cell culture models in which the protein was upregulated or deleted and in knock out and transgenic animal, with different results on αSyn’s effect on synaptic vesicle pool size and mobilization, αSyn being proposed as a negative or positive regulator of neurotransmitter release. In this review, we discuss the effect of αSyn on pre- and post-synaptic compartments in terms of synaptic vesicle trafficking, calcium entry and channel activity, and we focus on the process of exocytosis and internalization of αSyn and on the spreading of αSyn-driven effects due to the presence of the protein in the extracellular milieu.

The relation between α-synuclein and microglia in Parkinson's disease: Recent developments

Recent research suggests a complex role for microglia not only in Parkinson's disease but in other disorders involving alpha-synuclein aggregation, such as multiple system atrophy. In these neurodegenerative processes, the activation of microglia is a common pathological finding, which disturbs the homeostasis of the neuronal environment otherwise maintained, among others, by microglia. The term activation comprises any deviation from what otherwise is considered normal microglia status, including cellular abundance, morphology or protein expression. The microglial response during disease will sustain survival or otherwise promote cell degeneration. The novel concepts of alpha-synuclein being released and uptaken by neighboring cells, and their importance in disease progression, positions microglia as the main cell that can clear and handle alpha-synuclein efficiently. Microglia's behavior will therefore be a determinant on the disease's progression. For this reason we believe that the better understanding of microglia's response to alpha-synuclein pathological accumulation across brain areas and disease stages is essential to develop novel therapeutic tools for Parkinson's disease and other alpha-synucleinopathies. In this review we will revise the most recent findings and developments with regard to alpha-synuclein and microglia in Parkinson's disease.

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

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

Mutual exacerbation of peroxisome proliferator-activated receptor γ coactivator 1α deregulation and α-synuclein oligomerization

OBJECTIVE

Aggregation of α-synuclein (α-syn) and α-syn cytotoxicity are hallmarks of sporadic and familial Parkinson disease (PD), with accumulating evidence that prefibrillar oligomers and protofibrils are the pathogenic species in PD and related synucleinopathies. Peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), a key regulator of mitochondrial biogenesis and cellular energy metabolism, has recently been associated with the pathophysiology of PD. Despite extensive effort on studying the function of PGC-1α in mitochondria, no studies have addressed whether PGC-1α directly influences oligomerization of α-syn or whether α-syn oligomers impact PGC-1α expression.

MATERIALS AND METHODS

We tested whether pharmacological or genetic activation of PGC-1α or PGC-11α knockdown could modulate the oligomerization of α-syn in vitro by using an α-syn -fragment complementation assay.

RESULTS

In this study, we found that both PGC-1α reference gene (RG-PGC-1α) and the central nervous system (CNS)-specific PGC-1α (CNS-PGC-1α) are downregulated in human PD brain, in A30P α-syn transgenic animals, and in a cell culture model for α-syn oligomerization. Importantly, downregulation of both RG-PGC-1α and CNS-PGC-1α in cell culture or neurons from RG-PGC-1α-deficient mice leads to a strong induction of α-syn oligomerization and toxicity. In contrast, pharmacological activation or genetic overexpression of RG-PGC-1α reduced α-syn oligomerization and rescued α-syn-mediated toxicity.

INTERPRETATION

Based on our results, we propose that PGC-1α downregulation and α-syn oligomerization form a vicious circle, thereby influencing and/or potentiating each other. Our data indicate that restoration of PGC-1α is a promising approach for development of effective drugs for the treatment of PD and related synucleinopathies.

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.

Models of α-synuclein aggregation in Parkinson's disease

Parkinson's disease (PD) is not only characterized by motor disturbances but also, by cognitive, sensory, psychiatric and autonomic dysfunction. It has been proposed that some of these symptoms might be related to the widespread pathology of α-synuclein (α-syn) aggregation in different nuclei of the central and peripheral nervous system. However, the pathogenic formation of α-syn aggregates in different brain areas of PD patients is poorly understood. Most experimental models of PD are valuable to assess specific aspects of its pathogenesis, such as toxin-induced dopaminergic neurodegeneration. However, new models are required that reflect the widespread and progressive formation of α-syn aggregates in different brain areas. Such α-syn aggregation is induced in only a few animal models, for example perikaryon inclusions are found in rats administered rotenone, aggregates with a neuritic morphology develop in mice overexpressing either mutated or wild-type α-syn, and in Smad3 deficient mice, aggregates form extensively in the perikaryon and neurites of specific brain nuclei. In this review we focus on α-syn aggregation in the human disorder, its genetics and the availability of experimental models. Indeed, evidences show that dopamine (DA) metabolism may be related to α-syn and its conformational plasticity, suggesting an interesting link between the two pathological hallmarks of PD: dopaminergic neurodegeneration and Lewy body (LB) formation.

Overexpression of human E46K mutant α-synuclein impairs macroautophagy via inactivation of JNK1-Bcl-2 pathway

Parkinson's disease (PD) is pathologically characterized by selective loss of dopaminergic neurons in the midbrain and the existence of intracellular protein inclusions termed Lewy bodies, largely composed of α-synuclein. Genetic studies have revealed that rare point mutations in the gene encoding α-synuclein including A30P, A53T, and E46K are associated with familial forms of PD, indicating a pathological role for mutant α-synuclein in PD etiology. However, the mechanisms underlying the neuronal toxicity of mutant α-synuclein are still to be elucidated. Growing evidence has suggested a deleterious effect of mutant α-synuclein on the autophagy-lysosome pathway. In this study, we discovered that overexpression of human E46K mutant α-synuclein impaired macroautophagy in mammalian cells. Our data showed that overexpression of E46K mutant α-synuclein impaired autophagy at an early stage of autophagosome formation via the c-Jun N-terminal kinase 1 (JNK1)-Bcl-2 but not the mammalian target of rapamycin (mTOR) pathway. Overexpressed E46K mutant α-synuclein inhibited JNK1 activation, leading to a reduced Bcl-2 phosphorylation and increased association between Bcl-2 and Beclin1, further disrupting the formation of Beclin1/hVps34 complex, which is essential for autophagy initiation. Furthermore, overexpression of E46K mutant α-synuclein increased the vulnerability of differentiated PC12 cells to rotenone treatment, which would be partly due to its inhibitory effects on autophagy. Our findings may shed light on the potential roles of mutant α-synuclein in the pathogenesis of PD.

Genetic variants and animal models in SNCA and Parkinson disease

Parkinson disease (PD; MIM 168600) is the second most common progressive neurodegenerative disorder characterized by a variety of motor and non-motor features. To date, at least 20 loci and 15 disease-causing genes for parkinsonism have been identified. Among them, the α-synuclein (SNCA) gene was associated with PARK1/PARK4. Point mutations, duplications and triplications in the SNCA gene cause a rare dominant form of PD in familial and sporadic PD cases. The α-synuclein protein, a member of the synuclein family, is abundantly expressed in the brain. The protein is the major component of Lewy bodies and Lewy neurites in dopaminergic neurons in PD. Further understanding of its role in the pathogenesis of PD through various genetic techniques and animal models will likely provide new insights into our understanding, therapy and prevention of PD.

Environmental neurotoxic challenge of conditional alpha-synuclein transgenic mice predicts a dopaminergic olfactory-striatal interplay in early PD

The olfactory bulb (OB) is one of the first brain regions in Parkinson's disease (PD) to contain alpha-synuclein (α-syn) inclusions, possibly associated with nonmotor symptoms. Mechanisms underlying olfactory synucleinopathy, its contribution to progressive aggregation pathology and nigrostriatal dopaminergic loss observed at later stages, remain unclear. A second hit, such as environmental toxins, is suggestive for α-syn aggregation in olfactory neurons, potentially triggering disease progression. To address the possible pathogenic role of olfactory α-syn accumulation in early PD, we exposed mice with site-specific and inducible overexpression of familial PD-linked mutant α-syn in OB neurons to a low dose of the herbicide paraquat. Here, we found that olfactory α-syn per se elicited structural and behavioral abnormalities, characteristic of an early time point in models with widespread α-syn expression, including hyperactivity and increased striatal dopaminergic marker. Suppression of α-syn reversed the dopaminergic phenotype. In contrast, paraquat treatment synergistically induced degeneration of olfactory dopaminergic cells and opposed the higher reactive phenotype. Neither neurodegeneration nor behavioral abnormalities were detected in paraquat-treated mice with suppressed α-syn expression. By increasing calpain activity, paraquat induced a pathological cascade leading to inhibition of autophagy clearance and accumulation of calpain-cleaved truncated and insoluble α-syn, recapitulating biochemical and structural changes in human PD. Thus our results underscore the primary role of proteolytic failure in aggregation pathology. In addition, we provide novel evidence that olfactory dopaminergic neurons display an increased vulnerability toward neurotoxins in dependence to presence of human α-syn, possibly mediating an olfactory-striatal dopaminergic network dysfunction in mouse models and early PD.

Mitochondrial permeability transition pore regulates Parkinson's disease development in mutant α-synuclein transgenic mice

Parkinson's disease (PD) is a movement disorder caused by neurodegeneration in neocortex, substantia nigra and brainstem, and synucleinopathy. Some inherited PD is caused by mutations in α-synuclein (αSyn), and inherited and idiopathic PD is associated with mitochondrial perturbations. However, the mechanisms of pathogenesis are unresolved. We characterized a human αSyn transgenic mouse model and tested the hypothesis that the mitochondrial permeability transition pore (mPTP) is involved in the disease mechanisms. C57BL/6 mice expressing human A53T-mutant αSyn driven by a thymic antigen-1 promoter develop a severe, age-related, fatal movement disorder involving ataxia, rigidity, and postural instability. These mice develop synucleinopathy and neocortical, substantia nigra, and cerebello-rubro-thalamic degeneration involving mitochondriopathy and apoptotic and non-apoptotic neurodegeneration. Interneurons undergo apoptotic degeneration in young mice. Mutant αSyn associated with dysmorphic neuronal mitochondria and bound voltage-dependent anion channels. Genetic ablation of cyclophilin D, an mPTP modulator, delayed disease onset, and extended lifespans of mutant αSyn mice. Thus, mutant αSyn transgenic mice on a C57BL/6 background develop PD-like phenotypes, and the mPTP is involved in their disease mechanisms.

The function of α-synuclein

Human genetics has indicated a causal role for the protein α-synuclein in the pathogenesis of familial Parkinson's disease (PD), and the aggregation of synuclein in essentially all patients with PD suggests a central role for this protein in the sporadic disorder. Indeed, the accumulation of misfolded α-synuclein now defines multiple forms of neural degeneration. Like many of the proteins that accumulate in other neurodegenerative disorders, however, the normal function of synuclein remains poorly understood. In this article, we review the role of synuclein at the nerve terminal and in membrane remodeling. We also consider the prion-like propagation of misfolded synuclein as a mechanism for the spread of degeneration through the neuraxis.

Region-specific deficits in dopamine, but not norepinephrine, signaling in a novel A30P α-synuclein BAC transgenic mouse

Parkinson's disease (PD) is a neurodegenerative disorder classically characterized by the death of dopamine (DA) neurons in the substantia nigra pars compacta and by intracellular Lewy bodies composed largely of α-synuclein. Approximately 5–10% of PD patients have a familial form of Parkinsonism, including mutations in α-synuclein. To better understand the cell-type specific role of α-synuclein on DA neurotransmission, and the effects of the disease-associated A30P mutation, we generated and studied a novel transgenic model of PD. We expressed the A30P mutant form of human α-synuclein in a spatially-relevant manner from the 111 kb SNCA genomic DNA locus on a bacterial artificial chromosome (BAC) insert on a mouse null (Snca −/−) background. The BAC transgenic mice expressed α-synuclein in tyrosine hydroxylase-positive neurons and expression of either A30P α-synuclein or wildtype α-synuclein restored the sensitivity of DA neurons to MPTP in resistant Snca −/− animals. A30P α-synuclein mice showed no Lewy body-like aggregation, and did not lose catecholamine neurons in substantia nigra or locus coeruleus. However, using cyclic voltammetry at carbon-fiber microelectrodes we identified a deficit in evoked DA release in the caudate putamen, but not in the nucleus accumbens, of SNCA-A30P Snca −/− mice but no changes to release of another catecholamine, norepinephrine (NE), in the NE-rich ventral bed nucleus of stria terminalis. SNCA-A30P Snca −/− mice had no overt behavioral impairments but exhibited a mild increase in wheel-running. In summary, this refined PD mouse model shows that A30P α-synuclein preferentially perturbs the dopaminergic system in the dorsal striatum, reflecting the region-specific change seen in PD.

•

SNCA A30P BAC transgenic mice recapitulate endogenous α-synuclein expression pattern

•

SNCA A30P BAC transgenic mice demonstrate a region specific deficit in evoked DA, but not NE, release.

•

Expression of A30P or WT α-syn restored the sensitivity of DA neurons to MPTP.

•

A30P BAC mice had no Lewy body-like aggregation or neuronal loss in SNpc or LC.

•

Early changes in DA neurotransmission in the absence of aggregation