Role of alpha-synuclein in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonism in mice

Is There a Place for Lewy Bodies before and beyond Alpha-Synuclein Accumulation? Provocative Issues in Need of Solid Explanations

In the last two decades, alpha-synuclein (alpha-syn) assumed a prominent role as a major component and seeding structure of Lewy bodies (LBs). This concept is driving ongoing research on the pathophysiology of Parkinson's disease (PD). In line with this, alpha-syn is considered to be the guilty protein in the disease process, and it may be targeted through precision medicine to modify disease progression. Therefore, designing specific tools to block the aggregation and spreading of alpha-syn represents a major effort in the development of disease-modifying therapies in PD. The present article analyzes concrete evidence about the significance of alpha-syn within LBs. In this effort, some dogmas are challenged. This concerns the question of whether alpha-syn is more abundant compared with other proteins within LBs. Again, the occurrence of alpha-syn compared with non-protein constituents is scrutinized. Finally, the prominent role of alpha-syn in seeding LBs as the guilty structure causing PD is questioned. These revisited concepts may be helpful in the process of validating which proteins, organelles, and pathways are likely to be involved in the damage to meso-striatal dopamine neurons and other brain regions involved in PD.

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.

Insights on the Correlation between Mitochondrial Dysfunction and the Progression of Parkinson's Disease

The aetiology of a progressive neuronal Parkinson's disease has been discussed in several studies. However, due to the multiple risk factors involved in its development, such as environmental toxicity, parental inheritance, misfolding of protein, ageing, generation of reactive oxygen species, degradation of dopaminergic neurons, formation of neurotoxins, mitochondria dysfunction, and genetic mutations, its mechanism of involvement is still discernible. Therefore, this study aimed to review the processes or systems that are crucially implicated in the conversion of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) into its lethal form, which directly blockades the performance of mitochondria, leading to the formation of oxidative stress in the dopaminergic neurons of substantia nigra pars compacta (SNpc) and resulting in the progression of an incurable Parkinson's disease. This review also comprises an overview of the mutated genes that are frequently associated with mitochondrial dysfunction and the progression of Parkinson's disease. Altogether, this review would help future researchers to develop an efficient therapeutic approach for the management of Parkinson's disease via identifying potent prognostic and diagnostic biomarkers.

α-Synuclein in synaptic function and dysfunction

α-Synuclein is a neuronal protein that is enriched in presynaptic terminals. Under physiological conditions, it binds to synaptic vesicle membranes and functions in neurotransmitter release, although the molecular details remain unclear, and it is controversial whether α-synuclein inhibits or facilitates neurotransmitter release. Pathologically, in synucleinopathies including Parkinson's disease (PD), α-synuclein forms aggregates that recruit monomeric α-synuclein and spread throughout the brain, which triggers neuronal dysfunction at molecular, cellular, and organ levels. Here, we present an overview of the effects of α-synuclein on SNARE-complex assembly, neurotransmitter release, and synaptic vesicle pool homeostasis, and discuss how the observed divergent effects of α-synuclein on neurotransmitter release can be reconciled. We also discuss how gain-of-function versus loss-of-function of α-synuclein may contribute to pathogenesis in synucleinopathies.

Functional and Pathological Effects of α-Synuclein on Synaptic SNARE Complexes

α-Synuclein is an abundant protein at the neuronal synapse that has been implicated in Parkinson's disease for over 25 years and characterizes the hallmark pathology of a group of neurodegenerative diseases now known as the synucleinopathies. Physiologically, α-synuclein exists in an equilibrium between a synaptic vesicle membrane-bound α-helical multimer and a cytosolic largely unstructured monomer. Through its membrane-bound state, α-synuclein functions in neurotransmitter release by modulating several steps in the synaptic vesicle cycle, including synaptic vesicle clustering and docking, SNARE complex assembly, and homeostasis of synaptic vesicle pools. These functions have been ascribed to α-synuclein's interactions with the synaptic vesicle SNARE protein VAMP2/synaptobrevin-2, the synaptic vesicle-attached synapsins, and the synaptic vesicle membrane itself. How α-synuclein affects these processes, and whether disease is due to loss-of-function or gain-of-toxic-function of α-synuclein remains unclear. In this review, we provide an in-depth summary of the existing literature, discuss possible reasons for the discrepancies in the field, and propose a working model that reconciles the findings in the literature.

Synuclein Proteins in MPTP-Induced Death of Substantia Nigra Pars Compacta Dopaminergic Neurons

Parkinson’s disease (PD) is one of the key neurodegenerative disorders caused by a dopamine deficiency in the striatum due to the death of dopaminergic (DA) neurons of the substantia nigra pars compacta. The initially discovered A53T mutation in the alpha-synuclein gene was linked to the formation of cytotoxic aggregates: Lewy bodies in the DA neurons of PD patients. Further research has contributed to the discovery of beta- and gamma-synucleins, which presumably compensate for the functional loss of either member of the synuclein family. Here, we review research from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity models and various synuclein-knockout animals. We conclude that the differences in the sensitivity of the synuclein-knockout animals compared with the MPTP neurotoxin are due to the ontogenetic selection of early neurons followed by a compensatory effect of beta-synuclein, which optimizes dopamine capture in the synapses. Triple-knockout synuclein studies have confirmed the higher sensitivity of DA neurons to the toxic effects of MPTP. Nonetheless, beta-synuclein could modulate the alpha-synuclein function, preventing its aggregation and loss of function. Overall, the use of knockout animals has helped to solve the riddle of synuclein functions, and these proteins could be promising molecular targets for the development of therapies that are aimed at optimizing the synaptic function of dopaminergic neurons.

Lysosomal exocytosis releases pathogenic α-synuclein species from neurons in synucleinopathy models

Considerable evidence supports the release of pathogenic aggregates of the neuronal protein α-Synuclein (αSyn) into the extracellular space. While this release is proposed to instigate the neuron-to-neuron transmission and spread of αSyn pathology in synucleinopathies including Parkinson’s disease, the molecular-cellular mechanism(s) remain unclear. To study this, we generated a new mouse model to specifically immunoisolate neuronal lysosomes, and established a long-term culture model where αSyn aggregates are produced within neurons without the addition of exogenous fibrils. We show that neuronally generated pathogenic species of αSyn accumulate within neuronal lysosomes in mouse brains and primary neurons. We then find that neurons release these pathogenic αSyn species via SNARE-dependent lysosomal exocytosis. The released aggregates are non-membrane enveloped and seeding-competent. Additionally, we find that this release is dependent on neuronal activity and cytosolic Ca²⁺. These results propose lysosomal exocytosis as a central mechanism for the release of aggregated and degradation-resistant proteins from neurons.

Release of α-synuclein aggregates by neurons instigates spread of pathology in synucleinopathies, but the mechanism remains unclear. Here the authors show that neuronally generated α-synuclein aggregates accumulate within neuronal lysosomes and are released via SNARE-dependent lysosomal exocytosis.

Bacopa Protects against Neurotoxicity Induced by MPP+ and Methamphetamine

The neurotoxins methamphetamine (METH) and 1-methyl-4-phenylpyridinium (MPP⁺) damage catecholamine neurons. Although sharing the same mechanism to enter within these neurons, METH neurotoxicity mostly depends on oxidative species, while MPP⁺ toxicity depends on the inhibition of mitochondrial activity. This explains why only a few compounds protect against both neurotoxins. Identifying a final common pathway that is shared by these neurotoxins is key to prompting novel remedies for spontaneous neurodegeneration. In the present study we assessed whether natural extracts from Bacopa monnieri (BM) may provide a dual protection against METH- and MPP⁺-induced cell damage as measured by light and electron microscopy. The protection induced by BM against catecholamine cell death and degeneration was dose-dependently related to the suppression of reactive oxygen species (ROS) formation and mitochondrial alterations. These were measured by light and electron microscopy with MitoTracker Red and Green as well as by the ultrastructural morphometry of specific mitochondrial structures. In fact, BM suppresses the damage of mitochondrial crests and matrix dilution and increases the amount of healthy and total mitochondria. The present data provide evidence for a natural compound, which protects catecholamine cells independently by the type of experimental toxicity. This may be useful to counteract spontaneous degenerations of catecholamine cells.

Treating Parkinson's Disease via Activation of BDNF/TrkB Signaling Pathways and Inhibition of Delta-Secretase

Parkinson's disease (PD) is the second most common neurodegenerative disease with motor disorders as the key clinical features. BDNF/TrkB neurotrophic signalings are progressively reduced, whereas δ-secretase, a protease that cleaves α-synuclein (α-Syn) at N103 and promotes its aggregation and neurotoxicity, is gradually escalated in PD patient brains, associated with dopaminergic neuronal loss in the Substantia Nigra. Here, we show that stimulation of deficient BDNF/TrkB signalings with its small molecular agonist CF3CN displays the promising therapeutic effect, and blockade of δ-secretase with an optimal specific inhibitor #11A exhibits marked therapeutic effect, and combination of both demonstrates additive restorative efficacy in MPTP-induced human SNCA transgenic PD mice. Upon oral administration, CF3CN robustly activates TrkB-mediated neurotrophic pathway in the brains of SNCA mice and decreases α-Syn N103 cleavage by δ-secretase, and #11A strongly blocks δ-secretase and reduces α-Syn N103 fragmentation, increasing TH-positive dopaminergic neurons. The mixture of CF3CN and #11A shows the maximal TH and dopamine levels with demonstrable BDNF as compared to negligible BDNF in vehicle-treated MPTP/SNCA mice, leading to the climaxed motor functions. Notably, both compounds possess the appropriate in vivo PK profiles. Hence, our findings support that CF3CN and #11A are promising therapeutic pharmaceutical agents for treating PD.

Synaptic vesicle binding of α-synuclein is modulated by β- and γ-synucleins

α-synuclein, β-synuclein, and γ-synuclein are abundantly expressed proteins in the vertebrate nervous system. α-synuclein functions in neurotransmitter release by binding to and clustering synaptic vesicles and chaperoning SNARE-complex assembly. Pathologically, aggregates originating from soluble pools of α-synuclein are deposited into Lewy bodies in Parkinson's disease and related synucleinopathies. The functions of β-synuclein and γ-synuclein in presynaptic terminals remain poorly studied. Using in vitro liposome binding studies, circular dichroism spectroscopy, immunoprecipitation, and fluorescence resonance energy transfer (FRET) experiments on isolated synaptic vesicles in combination with subcellular fractionation of brains from synuclein mouse models, we show that β-synuclein and γ-synuclein have a reduced affinity toward synaptic vesicles compared with α-synuclein, and that heteromerization of β-synuclein or γ-synuclein with α-synuclein results in reduced synaptic vesicle binding of α-synuclein in a concentration-dependent manner. Our data suggest that β-synuclein and γ-synuclein are modulators of synaptic vesicle binding of α-synuclein and thereby reduce α-synuclein's physiological activity at the neuronal synapse.

In Pancreatic Adenocarcinoma Alpha-Synuclein Increases and Marks Peri-Neural Infiltration

α-Synuclein (α-syn) is a protein involved in neuronal degeneration. However, the family of synucleins has recently been demonstrated to be involved in the mechanisms of oncogenesis by selectively accelerating cellular processes leading to cancer. Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal human cancers, with a specifically high neurotropism. The molecular bases of this biological behavior are currently poorly understood. Here, α-synuclein was analyzed concerning the protein expression in PDAC and the potential association with PDAC neurotropism. Tumor (PDAC) and extra-tumor (extra-PDAC) samples from 20 patients affected by PDAC following pancreatic resections were collected at the General Surgery Unit, University of Pisa. All patients were affected by moderately or poorly differentiated PDAC. The amount of α-syn was compared between tumor and extra-tumor specimen (sampled from non-affected neighboring pancreatic areas) by using in situ immuno-staining with peroxidase anti-α-syn immunohistochemistry, α-syn detection by using Western blotting, and electron microscopy by using α-syn-conjugated immuno-gold particles. All the methods consistently indicate that each PDAC sample possesses a higher amount of α-syn compared with extra-PDAC tissue. Moreover, the expression of α-syn was much higher in those PDAC samples from tumors with perineural infiltration compared with tumors without perineural infiltration.

β-synuclein potentiates synaptic vesicle dopamine uptake and rescues dopaminergic neurons from MPTP-induced death in the absence of other synucleins

Synucleins, a family of three proteins highly expressed in neurons, are predominantly known for the direct involvement of α-synuclein in the etiology and pathogenesis of Parkinson's and certain other neurodegenerative diseases, but their precise physiological functions are still not fully understood. Previous studies have demonstrated the importance of α-synuclein as a modulator of various mechanisms implicated in chemical neurotransmission, but information concerning the involvement of other synuclein family members, β-synuclein and γ-synuclein, in molecular processes within presynaptic terminals is limited. Here, we demonstrated that the vesicular monoamine transporter 2-dependent dopamine uptake by synaptic vesicles isolated from the striatum of mice lacking β-synuclein is significantly reduced. Reciprocally, reintroduction, either in vivo or in vitro, of β-synuclein but not α-synuclein or γ-synuclein improves uptake by triple α/β/γ-synuclein-deficient striatal vesicles. We also showed that the resistance of dopaminergic neurons of the substantia nigra pars compacta to subchronic administration of the Parkinson's disease-inducing prodrug 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine depends on the presence of β-synuclein but only when one or both other synucleins are absent. Furthermore, proteomic analysis of synuclein-deficient synaptic vesicles versus those containing only β-synuclein revealed differences in their protein compositions. We suggest that the observed potentiation of dopamine uptake by β-synuclein might be caused by different protein architecture of the synaptic vesicles. It is also feasible that such structural changes improve synaptic vesicle sequestration of 1-methyl-4-phenylpyridinium, a toxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, which would explain why dopaminergic neurons expressing β-synuclein and lacking α-synuclein and/or γ-synuclein are resistant to this neurotoxin.

Inhibition of Autophagy In Vivo Extends Methamphetamine Toxicity to Mesencephalic Cell Bodies

Methamphetamine (METH) is a widely abused psychostimulant and a stress-inducing compound, which leads to neurotoxicity for nigrostriatal dopamine (DA) terminals in rodents and primates including humans. In vitro studies indicate that autophagy is a strong modulator of METH toxicity. In detail, suppressing autophagy increases METH toxicity, while stimulating autophagy prevents METH-induced toxicity in cell cultures. In the present study, the role of autophagy was investigated in vivo. In the whole brain, METH alone destroys meso-striatal DA axon terminals, while fairly sparing DA cell bodies within substantia nigra pars compacta (SNpc). No damage to either cell bodies or axons from ventral tegmental area (VTA) is currently documented. According to the hypothesis that ongoing autophagy prevents METH-induced DA toxicity, we tested whether systemic injection of autophagy inhibitors such as asparagine (ASN, 1000 mg/Kg) or glutamine (GLN, 1000 mg/Kg), may extend METH toxicity to DA cell bodies, both within SNpc and VTA, where autophagy was found to be inhibited. When METH (5 mg/Kg × 4, 2 h apart) was administered to C57Bl/6 mice following ASN or GLN, a frank loss of cell bodies takes place within SNpc and a loss of both axons and cell bodies of VTA neurons is documented. These data indicate that, ongoing autophagy protects DA neurons and determines the refractoriness of cell bodies to METH-induced toxicity.

Mitochondrial CHCHD2: Disease-Associated Mutations, Physiological Functions, and Current Animal Models

Rare mutations in the mitochondrial protein coiled-coil-helix-coiled-coil-helix domain containing 2 (CHCHD2) are associated with Parkinson's disease (PD) and other Lewy body disorders. CHCHD2 is a bi-organellar mediator of oxidative phosphorylation, playing crucial roles in regulating electron flow in the mitochondrial electron transport chain and acting as a nuclear transcription factor for a cytochrome c oxidase subunit (COX4I2) and itself in response to hypoxic stress. CHCHD2 also regulates cell migration and differentiation, mitochondrial cristae structure, and apoptosis. In this review, we summarize the known disease-associated mutations of CHCHD2 in Asian and Caucasian populations, the physiological functions of CHCHD2, how CHCHD2 mutations contribute to α-synuclein pathology, and current animal models of CHCHD2. Further, we discuss the necessity of continued investigation into the divergent functions of CHCHD2 and CHCHD10 to determine how mutations in these similar mitochondrial proteins contribute to different neurodegenerative diseases.

The Role of α-Synuclein in Methamphetamine-Induced Neurotoxicity

Methamphetamine (METH), a highly addictive psychostimulant, is the second most widely used illicit drug. METH produces damage dopamine neurons and apoptosis via multiple inter-regulating mechanisms, including dopamine overload, hyperthermia, oxidative stress, mitochondria dysfunction, endoplasmic reticulum stress, protein degradation system dysfunction, and neuroinflammation. Increasing evidence suggests that chronic METH abuse is associated with neurodegenerative changes in the human brain and an increased risk of Parkinson's disease (PD). METH use and PD may share some common steps in causing neurotoxicity. Accumulation of α-synuclein, a presynaptic protein, is the pathological hallmark of PD. Intriguingly, α-synuclein upregulation and aggregation are also found in dopaminergic neurons in the substantia nigra in chronic METH users. This suggests α-synuclein may play a role in METH-induced neurotoxicity. The mechanism of α-synuclein cytotoxicity in PD has attracted considerable attention; however, how α-synuclein affects METH-induced neurotoxicity has not been reviewed. In this review, we summarize the relationship between METH use and PD, interdependent mechanisms that are involved in METH-induced neurotoxicity and the significance of α-synuclein upregulation in response to METH use. The identification of α-synuclein overexpression and aggregation as a contributor to METH-induced neurotoxicity may provide a novel therapeutic target for the treatment of the deleterious effect of this drug and drug addiction.

Increased Expression of the Multimerin-1 Gene in α-Synuclein Knokout Mice

Multimerin-1 (Mmrn-1) is a soluble protein, also known as elastin microfibril interfacer 4 (EMILIN-4), found in platelets and in the endothelium of blood vessels. Its function and role in pathology are still not fully understood. Genetic modifications in alpha-synuclein gene (Snca) locus that mapped 160 Kb apart from Mmrn-1 in mouse genome, could weigh with regulatory elements of Mmrn-1 gene. We have studied the Mmrn-1 expression in brain cortex of three mouse lines with Snca knock-out: B6(Cg)-Snca^tm1.2Vlb/J, B6;129-Snca^tm1Sud/J, and B6;129X1-Snca^tm1Rosl/J. The 35-fold increase for Mmrn-1 mRNA level have been found in B6;129X1-Snca^tm1Rosl/J mice that carry in their genome foreign sequences including bacterial gene neo and a strong promoter of a mouse phosphoglycerate kinase (Pgk1) oriented towards Mmrn-1 gene. This effect on regulatory elements of Mmrn-1 gene as a result of modifications in Snca locus should be taken into consideration when using B6;129X1-Snca^tm1Rosl/J line, that is widely applied for study of neurodegeneration mechanisms.

Immunohistochemical localization of alpha-synuclein in the retina of some nocturnal and diurnal animals

Although alpha-synuclein has been reported to participate in neurodegenerative diseases, the actual normal biological function of alpha-synuclein remains unclear. I investigated the correlation of alpha-synuclein expression with nocturnal and diurnal activity for various species. Hematoxylin and eosin staining, periodic acid-Schiff's reaction (PAS) and immunohistochemistry of alpha-synuclein expression were performed for the retinas of diurnal, nocturnal, nocturnal with diurnal activity species. I found different intensity of alpha-synuclein expression in the retinal layers. I found alpha-synuclein expression in the outer segment of the photoreceptor layer in the diurnal studied species and absence of alpha-synuclein expression in the compartments of photoreceptor layer in the retina of nocturnal species. I found localization of alpha-synuclein in the inner and outer segments of photoreceptors of the retina of nocturnal with diurnal activity species. The retinas of diurnal animals exhibited glycogen in the paraboloid structure in the inner segment of the photoreceptor layer. The retinas of nocturnal and nocturnal with diurnal activity species were devoid of glycogen in the photoreceptor layer. I conclude that the function of alpha-synuclein is more related to diurnal than to nocturnal species.

Alpha-synuclein oligomerization and dopaminergic degeneration occur synchronously in the brain and colon of MPTP-intoxicated parkinsonian monkeys

Dopaminergic (DAergic) degeneration and abnormal α-synuclein (α-syn) expression, phosphorylation and aggregation are observed in both the nigrostriatal system (NSS) and enteric nervous system (ENS) of patients with Parkinson's disease (PD). Whether these alterations in α-syn and DAergic neurons occur synchronously in the two nervous systems or follow a process that spreads from the gut to the brain remains a subject of debate. Here, in MPTP-intoxicated cynomolgus monkeys, we showed a parallel DAergic degeneration in the colon as well as in the substantia nigra and striatum (SN/STR), as indicated by reduced expression of tyrosine hydroxylase (TH) and dopamine transporter (DAT). In addition, we observed a simultaneous increase in the concentrations of total, phosphorylated, and oligomeric α-syn in the colon and SN/STR. Moreover, we identified that the above changes in α-syn were associated with an increase in the expression of polo-like kinase 2 (PLK2), an enzyme that promotes α-syn phosphorylation, and a decrease in the activity of protein phosphatase 2A (PP2A), an enzyme that facilitates α-syn dephosphorylation. Because the colonic ENS can be readily analyzed using routine biopsies, the shared pathological features between the colonic ENS and the brain NSS found in this study provide useful information for assessing and understanding the neuropathology in PD patients using colonic biopsies.

Motor Neurons Pathology After Chronic Exposure to MPTP in Mice

The neurotoxin 1-methyl,4-phenyl-1,2,3,6-tetrahydropiridine (MPTP) is widely used to produce experimental parkinsonism in rodents and primates. Among different administration protocols, continuous or chronic exposure to small amounts of MPTP is reported to better mimic cell pathology reminiscent of Parkinson's disease (PD). Catecholamine neurons are the most sensitive to MPTP neurotoxicity; however, recent studies have found that MPTP alters the fine anatomy of the spinal cord including motor neurons, thus overlapping again with the spinal cord involvement documented in PD. In the present study, we demonstrate that chronic exposure to low amounts of MPTP (10 mg/kg daily, × 21 days) significantly reduces motor neurons in the ventral lumbar spinal cord while increasing α-synuclein immune-staining within the ventral horn. Spinal cord involvement in MPTP-treated mice extends to Calbindin D28 KDa immune-reactive neurons other than motor neurons within lamina VII. These results were obtained in the absence of significant reduction of dopaminergic cell bodies in the Substantia Nigra pars compacta, while a slight decrease was documented in striatal tyrosine hydroxylase immune-staining. Thus, the present study highlights neuropathological similarities between dopaminergic neurons and spinal motor neurons and supports the pathological involvement of spinal cord in PD and experimental MPTP-induced parkinsonism. Remarkably, the toxic threshold for motor neurons appears to be lower compared with nigral dopaminergic neurons following a chronic pattern of MPTP intoxication. This sharply contrasts with previous studies showing that MPTP intoxication produces comparable neuronal loss within spinal cord and Substantia Nigra.

Plant poisoning leads to alpha-synucleinopathy and neuromelanopathy in kangaroos

The pathogenesis of synucleinopathies, common neuropathological lesions normally associated with some human neurodegenerative disorders such as Parkinson's disease, dementia with Lewy bodies and multiple system atrophy, remains poorly understood. In animals, ingestion of the tryptamine-alkaloid-rich phalaris pastures plants causes a disorder called Phalaris staggers, a neurological syndrome reported in kangaroos. The aim of the study was to characterise the clinical and neuropathological changes associated with spontaneous cases of Phalaris staggers in kangaroos. Gross, histological, ultrastructural and Immunohistochemical studies were performed to demonstrate neuronal accumulation of neuromelanin and aggregated α-synuclein. ELISA and mass spectrometry were used to detect serum-borne α-synuclein and tryptamine alkaloids respectively. We report that neurons in the central and enteric nervous systems of affected kangaroos display extensive accumulation of neuromelanin in the perikaryon without affecting neuronal morphology. Ultrastructural studies confirmed the typical structure of neuromelanin. While we demonstrated strong staining of α-synuclein, restricted to neurons, intracytoplasmic Lewy bodies inclusions were not observed. α-synuclein aggregates levels were shown to be lower in sera of the affected kangaroos compared to unaffected herd mate kangaroos. Finally, mass spectrometry failed to detect the alkaloid toxins in the sera derived from the affected kangaroos. Our preliminary findings warrant further investigation of Phalaris staggers in kangaroos, potentially a valuable large animal model for environmentally-acquired toxic synucleinopathy.

Systemic peptide mediated delivery of an siRNA targeting α-syn in the CNS ameliorates the neurodegenerative process in a transgenic model of Lewy body disease

Neurodegenerative disorders of the aging population are characterized by progressive accumulation of neuronal proteins such as α-synuclein (α-syn) in Parkinson's Disease (PD) and Amyloid ß (Aß) and Tau in Alzheimer's disease (AD) for which no treatments are currently available. The ability to regulate the expression at the gene transcription level would be beneficial for reducing the accumulation of these proteins or regulating expression levels of other genes in the CNS. Short interfering RNA molecules can bind specifically to target RNAs and deliver them for degradation. This approach has shown promise therapeutically in vitro and in vivo in mouse models of PD and AD and other neurological disorders; however, delivery of the siRNA to the CNS in vivo has been achieved primarily through intra-cerebral or intra-thecal injections that may be less amenable for clinical translation; therefore, alternative approaches for delivery of siRNAs to the brain is needed. Recently, we described a small peptide from the envelope protein of the rabies virus (C2-9r) that was utilized to deliver an siRNA targeting α-syn across the blood brain barrier (BBB) following intravenous injection. This approach showed reduced expression of α-syn and neuroprotection in a toxic mouse model of PD. However, since receptor-mediated delivery is potentially saturable, each allowing the delivery of a limited number of molecules, we identified an alternative peptide for the transport of nucleotides across the BBB based on the apolipoprotein B (apoB) protein targeted to the family of low-density lipoprotein receptors (LDL-R). We used an 11-amino acid sequence from the apoB protein (ApoB11) that, when coupled with a 9-amino acid arginine linker, can transport siRNAs across the BBB to neuronal and glial cells. To examine the value of this peptide mediated oligonucleotide delivery system for PD, we delivered an siRNA targeting the α-syn (siα-syn) in a transgenic mouse model of PD. We found that ApoB11 was effective (comparable to C2-9r) at mediating the delivery of siα-syn into the CNS, co-localized to neurons and glial cells and reduced levels of α-syn protein translation and accumulation. Delivery of ApoB11/siα-syn was accompanied by protection from degeneration of selected neuronal populations in the neocortex, limbic system and striato-nigral system and reduced neuro-inflammation. Taken together, these results suggest that systemic delivery of oligonucleotides targeting α-syn using ApoB11 might be an interesting alternative strategy worth considering for the experimental treatment of synucleinopathies.

Long-term RNAi knockdown of α-synuclein in the adult rat substantia nigra without neurodegeneration

α-Synuclein plays a central role in the pathogenesis of Parkinson's disease (PD); interventions that decrease its expression appear neuroprotective in PD models. Successful translation of these observations into effective therapies will be dependent on the safety of suppressing α-synuclein expression in the adult brain. We investigated long-term α-synuclein knockdown in the adult rat CNS. 8-month old animals received either AAV-sh[Snca] (an RNA interference vector targeting the Snca mRNA transcript) or AAV-sh[Ctrl] (a control vector) unilaterally into the substantia nigra. No signs of systemic toxicity or motor dysfunction were observed in either experimental group over 12 months. Viral transgene expression persisted to 12 months post-inoculation, at which point Snca mRNA expression in substantia nigra dopaminergic neurons of animals that received AAV-sh[Snca] was decreased by ≈90%, and α-synuclein immunoreactivity by >70% relative to the control side. Stereological quantification of Nissl-labeled neurons showed no evidence of neurodegeneration in the substantia nigra 12 months after inoculation with either vector, and we observed abundant dopaminergic neurons with minimal α-synuclein immunoreactivity that appeared otherwise unremarkable in the AAV-sh[Snca] group. Despite the absence of neurodegeneration, some loss of TH expression was evident in nigral neurons after transduction with either vector, presumably a non-specific consequence of vector delivery, cellular transduction, or expression of shRNA or GFP. We conclude that long-term α-synuclein knockdown in the substantia nigra does not cause significant functional deficits in the ascending dopaminergic projection, or neurodegeneration. These findings are encouraging that it may be feasible to target α-synuclein expression therapeutically in PD.

The Potential of L-Type Calcium Channels as a Drug Target for Neuroprotective Therapy in Parkinson's Disease

The motor symptoms of Parkinson's disease (PD) mainly arise from degeneration of dopamine neurons within the substantia nigra. As no disease-modifying PD therapies are available, and side effects limit long-term benefits of current symptomatic therapies, novel treatment approaches are needed. The ongoing phase III clinical study STEADY-PD is investigating the potential of the dihydropyridine isradipine, an L-type Ca²⁺ channel (LTCC) blocker, for neuroprotective PD therapy. Here we review the clinical and preclinical rationale for this trial and discuss potential reasons for the ambiguous outcomes of in vivo animal model studies that address PD-protective dihydropyridine effects. We summarize current views about the roles of Cav1.2 and Cav1.3 LTCC isoforms for substantia nigra neuron function, and their high vulnerability to degenerative stressors, and for PD pathophysiology. We discuss different dihydropyridine sensitivities of LTCC isoforms in view of their potential as drug targets for PD neuroprotection, and we conclude by considering how these aspects could guide further drug development.

mTOR Modulates Methamphetamine-Induced Toxicity through Cell Clearing Systems

Methamphetamine (METH) is abused worldwide, and it represents a threat for public health. METH exposure induces a variety of detrimental effects. In fact, METH produces a number of oxidative species, which lead to lipid peroxidation, protein misfolding, and nuclear damage. Cell clearing pathways such as ubiquitin-proteasome (UP) and autophagy (ATG) are involved in METH-induced oxidative damage. Although these pathways were traditionally considered to operate as separate metabolic systems, recent studies demonstrate their interconnection at the functional and biochemical level. Very recently, the convergence between UP and ATG was evidenced within a single organelle named autophagoproteasome (APP), which is suppressed by mTOR activation. In the present research study, the occurrence of APP during METH toxicity was analyzed. In fact, coimmunoprecipitation indicates a binding between LC3 and P20S particles, which also recruit p62 and alpha-synuclein. The amount of METH-induced toxicity correlates with APP levels. Specific markers for ATG and UP, such as LC3 and P20S in the cytosol, and within METH-induced vacuoles, were measured at different doses and time intervals following METH administration either alone or combined with mTOR modulators. Western blotting, coimmunoprecipitation, light microscopy, confocal microscopy, plain transmission electron microscopy, and immunogold staining were used to document the effects of mTOR modulation on METH toxicity and the merging of UP with ATG markers within APPs. METH-induced cell death is prevented by mTOR inhibition, while it is worsened by mTOR activation, which correlates with the amount of autophagoproteasomes. The present data, which apply to METH toxicity, are also relevant to provide a novel insight into cell clearing pathways to counteract several kinds of oxidative damage.

α-Synuclein stimulation of monoamine oxidase-B and legumain protease mediates the pathology of Parkinson's disease

Dopaminergic neurodegeneration in Parkinson's disease (PD) is associated with abnormal dopamine metabolism by MAO-B (monoamine oxidase-B) and intracellular α-Synuclein (α-Syn) aggregates, called the Lewy body. However, the molecular relationship between α-Syn and MAO-B remains unclear. Here, we show that α-Syn directly binds to MAO-B and stimulates its enzymatic activity, which triggers AEP (asparagine endopeptidase; legumain) activation and subsequent α-Syn cleavage at N103, leading to dopaminergic neurodegeneration. Interestingly, the dopamine metabolite, DOPAL, strongly activates AEP, and the N103 fragment of α-Syn binds and activates MAO-B. Accordingly, overexpression of AEP in SNCA transgenic mice elicits α-Syn N103 cleavage and accelerates PD pathogenesis, and inhibition of MAO-B by Rasagiline diminishes α-Syn-mediated PD pathology and motor dysfunction. Moreover, virally mediated expression of α-Syn N103 induces PD pathogenesis in wild-type, but not MAO-B-null mice. Our findings thus support that AEP-mediated cleavage of α-Syn at N103 is required for the association and activation of MAO-B, mediating PD pathogenesis.

Silencing Alpha Synuclein in Mature Nigral Neurons Results in Rapid Neuroinflammation and Subsequent Toxicity

Human studies and preclinical models of Parkinson’s disease implicate the involvement of both the innate and adaptive immune systems in disease progression. Further, pro-inflammatory markers are highly enriched near neurons containing pathological forms of alpha synuclein (α-syn), and α-syn overexpression recapitulates neuroinflammatory changes in models of Parkinson’s disease. These data suggest that α-syn may initiate a pathological inflammatory response, however the mechanism by which α-syn initiates neuroinflammation is poorly understood. Silencing endogenous α-syn results in a similar pattern of nigral degeneration observed following α-syn overexpression. Here we aimed to test the hypothesis that loss of α-syn function within nigrostriatal neurons results in neuronal dysfunction, which subsequently stimulates neuroinflammation. Adeno-associated virus (AAV) expressing an short hairpin RNA (shRNA) targeting endogenous α-syn was unilaterally injected into the substantia nigra pars compacta (SNc) of adult rats, after which nigrostriatal pathology and indices of neuroinflammation were examined at 7, 10, 14 and 21 days post-surgery. Removing endogenous α-syn from nigrostriatal neurons resulted in a rapid up-regulation of the major histocompatibility complex class 1 (MHC-1) within transduced nigral neurons. Nigral MHC-1 expression occurred prior to any overt cell death and coincided with the recruitment of reactive microglia and T-cells to affected neurons. Following the induction of neuroinflammation, α-syn knockdown resulted in a 50% loss of nigrostriatal neurons in the SNc and a corresponding loss of nigrostriatal terminals and dopamine (DA) concentrations within the striatum. Expression of a control shRNA did not elicit any pathological changes. Silencing α-syn within glutamatergic neurons of the cerebellum did not elicit inflammation or cell death, suggesting that toxicity initiated by α-syn silencing is specific to DA neurons. These data provide evidence that loss of α-syn function within nigrostriatal neurons initiates a neuronal-mediated neuroinflammatory cascade, involving both the innate and adaptive immune systems, which ultimately results in the death of affected neurons.

A Focus on the Beneficial Effects of Alpha Synuclein and a Re-Appraisal of Synucleinopathies

Alpha synuclein (α-syn) belongs to a class of proteins which are commonly considered to play a detrimental role in neuronal survival. This assumption is based on the occurrence of a severe neuronal degeneration in patients carrying a multiplication of the α-syn gene (SNCA) and in a variety of experimental models, where overexpression of α-syn leads to cell death and neurological impairment. In these conditions, a higher amount of normally structured α-syn produces a damage, which is even worse compared with that produced by α-syn owning an abnormal structure (as occurring following point gene mutations). In line with this, knocking out the expression of α-syn is reported to protect from specific neurotoxins such as 1-methyl, 4-phenyl 1,2,3,6-tetrahydropyridine (MPTP). In the present review we briefly discuss these well-known detrimental effects but we focus on findings showing that, in specific conditions α-syn is beneficial for cell survival. This occurs during methamphetamine intoxication which is counteracted by endogenous α-syn. Similarly, the dysfunction of the chaperone cysteine-string protein- alpha leads to cell pathology which is counteracted by over-expressing α-syn. In line with this, an increased expression of α-syn protects against oxidative damage produced by dopamine. Remarkably, when the lack of α-syn is combined with a depletion of β- and γ- synucleins, alterations in brain structure and function occur. This review tries to balance the evidence showing a beneficial effect with the bulk of data reporting a detrimental effect of endogenous α-syn. The specific role of α-syn as a chaperone protein is discussed to explain such a dual effect.

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.

Combinational losses of synucleins reveal their differential requirements for compensating age-dependent alterations in motor behavior and dopamine metabolism

Synucleins are involved in multiple steps of the neurotransmitter turnover, but the largely normal synaptic function in young adult animals completely lacking synucleins suggests their roles are dispensable for execution of these processes. Instead, they may be utilized for boosting the efficiency of certain molecular mechanisms in presynaptic terminals, with a deficiency of synuclein proteins sensitizing to or exacerbating synaptic malfunction caused by accumulation of mild alterations, which are commonly associated with aging. Although functional redundancy within the family has been reported, it is unclear whether the remaining synucleins can fully compensate for the deficiency of a lost family member or whether some functions are specific for a particular member. We assessed several structural and functional characteristics of the nigrostriatal system of mice lacking members of the synuclein family in every possible combination and demonstrated that stabilization of the striatal dopamine level depends on the presence of α-synuclein and cannot be compensated by other family members, whereas β-synuclein is required for efficient maintenance of animal's balance and coordination in old age.

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.

Insights from late-onset familial parkinsonism on the pathogenesis of idiopathic Parkinson's disease

Disease-modifying therapies that slow or halt the progression of Parkinson's disease are an unmet clinical need. Many hypotheses have been put forward to explain the pathogenesis of the disease, but none has led to the development of disease-modifying drugs. Here we focus on familial forms of late-onset parkinsonism that most closely resemble idiopathic Parkinson's disease and present a synthesis of emerging molecular advances. Genetic discoveries and mechanistic investigations have highlighted early alterations to synaptic function, endosomal maturation, and protein sorting that might lead to an intracellular proteinopathy. We propose that these cellular processes constitute one pathway to pathogenesis and suggest that neuroprotection, as an adjunct to current symptomatic treatments, need not remain an elusive goal.

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

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

A novel resource for studying function and dysfunction of α-synuclein: mouse lines for modulation of endogenous Snca gene expression

Pathological modification of α-synuclein is believed to be an important event in pathogenesis of Parkinson’s disease and several other neurodegenerative diseases. In normal cells this protein has been linked to many intracellular processes and pathways. However, neither normal function of α-synuclein in neuronal and certain other types of cells nor its exact role in the disease pathogenesis is well understood, which is largely due to limitations of animal models used for studying this protein. We produced and validated several novel mouse lines for manipulating expression of the endogenous Snca gene coding for α-synuclein. These include a line for conditional Cre-recombinase-driven inactivation of the gene; a line for conditional Flp-driven restoration of a neo-cassete-blocked α-synuclein expression; a new line with a “clean” constituent knockout of the gene as well as a line carrying this knockout locus and Rosa26-stop-lacZ reporter locus linked at the same mouse chromosome 6. Altogether these lines represent a set of new useful tools for studies of α-synuclein normal function and the role of this protein in disease pathogenesis.

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.

Interaction between Neuromelanin and Alpha-Synuclein in Parkinson's Disease

Parkinson's disease (PD) is a very common neurodegenerative disorder characterized by the accumulation of α-synuclein (α-syn) into Lewy body (LB) inclusions and the loss of neuronmelanin (NM) containing dopamine (DA) neurons in the substantia nigra (SN). Pathological α-syn and NM are two prominent hallmarks in this selective and progressive neurodegenerative disease. Pathological α-syn can induce dopaminergic neuron death by various mechanisms, such as inducing oxidative stress and inhibiting protein degradation systems. Therefore, to explore the factors that trigger α-syn to convert from a non-toxic protein to toxic one is a pivotal question to clarify the mechanisms of PD pathogenesis. Many triggers for pathological α-syn aggregation have been identified, including missense mutations in the α-syn gene, higher concentration, and posttranslational modifications of α-Syn. Recently, the role of NM in inducing α-syn expression and aggregation has been suggested as a mechanism for this pigment to modulate neuronal vulnerability in PD. NM may be responsible for PD and age-associated increase and aggregation in α-syn. Here, we reviewed our previous study and other recent findings in the area of interaction between NM and α-syn.

Mitochondrial Dysfunction and α-Synuclein Synaptic Pathology in Parkinson's Disease: Who's on First?

Parkinson's disease (PD) is the most common neurodegenerative movement disorder. Its characteristic neuropathological features encompass the loss of dopaminergic neurons of the nigrostriatal system and the presence of Lewy bodies and Lewy neurites. These are intraneuronal and intraneuritic proteinaceous insoluble aggregates whose main constituent is the synaptic protein α-synuclein. Compelling lines of evidence indicate that mitochondrial dysfunction and α-synuclein synaptic deposition may play a primary role in the onset of this disorder. However, it is not yet clear which of these events may come first in the sequel of processes leading to neurodegeneration. Here, we reviewed data supporting either that α-synuclein synaptic deposition precedes and indirectly triggers mitochondrial damage or that mitochondrial deficits lead to neuronal dysfunction and α-synuclein synaptic accumulation. The present overview shows that it is still difficult to establish the exact temporal sequence and contribution of these events to PD.

Tea polyphenols alleviate motor impairments, dopaminergic neuronal injury, and cerebral α-synuclein aggregation in MPTP-intoxicated parkinsonian monkeys

Tea polyphenols (TPs) are bioactive flavanol-related catechins that have been shown to protect dopaminergic (DAergic) neurons against neurotoxin-induced injury in mouse Parkinson's disease (PD) models. However, the neuroprotective efficacy of TP has not been investigated in nonhuman PD primates, which can more accurately model the neuropathology and motor impairments of human PD patients. Here, we show that oral administration of TP alleviates motor impairments and DAergic neuronal injury in the substantia nigra in N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated PD monkeys, indicating an association between protection against motor deficits and preservation of DAergic neurons. We also show a significant inhibition of MPTP-induced accumulation of neurotoxic α-synuclein (α-syn) oligomers in the striatum and other brain regions, which may contribute to the neuroprotection and improved motor function conferred by TP. The association between reduced α-syn oligomerization and neuroprotection was confirmed in cultured DAergic cells. The most abundant and bioactive TP in the mixture used in vivo, (-)-epigallocatechin-3-gallate, reduced intracellular levels of α-syn oligomers in neurons treated with α-syn oligomers, 1-methyl-4-phenylpyridiniumion, or both, accompanied by increased cell viability. The present study provides the first evidence that TP can alleviate motor impairments, DAergic neuronal injury, and α-syn aggregation in nonhuman primates.

Loss of functional alpha-synuclein: a toxic event in Parkinson's disease?

The discovery that alpha-synuclein (α-syn) is the primary component of the neuropathological hallmarks of Parkinson's disease (PD) and the identification of α-syn mutations in numerous inherited forms of PD has positioned α-syn at the top of the list of important factors in the pathogenesis of PD. Based on the pathological accumulation of α-syn in the brains of patients, the field is currently focused on therapeutic strategies that aim to reduce or eliminate α-syn. However, recent evidence suggests α-syn is a critical protein in neuron (i.e. dopamine neurons) survival and that maintaining a certain level of biologically functional α-syn is an important consideration in targeting α-syn for therapies. Despite the widespread interest in α-syn, the normal biological functions remain elusive, but a large body of work is focused on addressing this issue. In this review, we will discuss the current evidence related to α-syn function, α-syn folding and aggregation, and α-syn's role in disease. Finally, we will propose a relatively novel hypothesis on the pathogenesis of PD that hinges upon the premises that functional α-syn is critical to cell survival and that a reduction in biologically functional α-syn, whether through aggregation or reduced expression, may lead to the neurodegeneration in 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.

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.

α-Synuclein in Parkinson's disease: pathogenic function and translation into animal models

Parkinson's disease is a common neurodegenerative disease characterised by the loss of dopaminergic neurons in the substantia nigra pars compacta and the formation of α-synuclein aggregates found in Lewy bodies throughout the brain. Several α-synuclein transgenic mouse models have been generated, as well as viral-mediated overexpression of wild-type and mutated α-synuclein to mimic the disease and to delineate the pathogenic pathway of α-synuclein-mediated toxicity and neurodegeneration. In this review, we will recapitulate what we have learned about the function of α-synuclein and α-synuclein-mediated toxicity through studies of transgenic animal models, inducible animal models and viral-based models.

Adult neurogenesis in Parkinson's disease

Parkinson's disease (PD), the second most common neurodegenerative disorder, affects 1-2 % of humans aged 60 years and older. The diagnosis of PD is based on motor symptoms such as bradykinesia, rigidity, tremor, and postural instability associated with the striatal dopaminergic deficit that is linked to neurodegenerative processes in the substantia nigra (SN). In the past, cellular replacement strategies have been evaluated for their potential to alleviate these symptoms. Adult neurogenesis, the generation of new neurons within two proliferative niches in the adult brain, is being intensively studied as one potential mode for cell-based therapies. The subventricular zone provides new neurons for the olfactory bulb functionally contributing to olfaction. The subgranular zone of the hippocampus produces new granule neurons for the dentate gyrus, required for memory formation and proper processing of anxiety provoking stimuli. Recent years have revealed that PD is associated with non-motor symptoms such as hyposmia, anhedonia, lack of novelty seeking behavior, depression, and anxiety that are not directly associated with neurodegenerative processes in the SN. This broad spectrum of non-motor symptoms may partly rely on proper olfactorial processing and hippocampal function. Therefore, it is conceivable that some non-motor deficits in PD are related to defective adult neurogenesis. Accordingly, in animal models and postmortem studies of PD, adult neurogenesis is severely affected, although the exact mechanisms and effects of these changes are not yet fully understood or are under debate due to conflicting results. Here, we review the current concepts related to the dynamic interplay between endogenous cellular plasticity and PD-associated pathology.

Systematic mutagenesis of α-synuclein reveals distinct sequence requirements for physiological and pathological activities

α-Synuclein is an abundant presynaptic protein that binds to phospholipids and synaptic vesicles. Physiologically, α-synuclein functions as a SNARE-protein chaperone that promotes SNARE-complex assembly for neurotransmitter release. Pathologically, α-synuclein mutations and α-synuclein overexpression cause Parkinson's disease, and aggregates of α-synuclein are found as Lewy bodies in multiple neurodegenerative disorders ("synucleinopathies"). The relation of the physiological functions to the pathological effects of α-synuclein remains unclear. As an initial avenue of addressing this question, we here systematically examined the effect of α-synuclein mutations on its physiological and pathological activities. We generated 26 α-synuclein mutants spanning the entire molecule, and analyzed them compared with wild-type α-synuclein in seven assays that range from biochemical studies with purified α-synuclein, to analyses of α-synuclein expression in cultured neurons, to examinations of the effects of virally expressed α-synuclein introduced into the mouse substantia nigra by stereotactic injections. We found that both the N-terminal and C-terminal sequences of α-synuclein were required for its physiological function as SNARE-complex chaperone, but that these sequences were not essential for its neuropathological effects. In contrast, point mutations in the central region of α-synuclein, referred to as nonamyloid β component (residues 61-95), as well as point mutations linked to Parkinson's disease (A30P, E46K, and A53T) increased the neurotoxicity of α-synuclein but did not affect its physiological function in SNARE-complex assembly. Thus, our data show that the physiological function of α-synuclein, although protective of neurodegeneration in some contexts, is fundamentally distinct from its neuropathological effects, thereby dissociating the two activities of α-synuclein.

Impact of intravenous immunoglobulin on the dopaminergic system and immune response in the acute MPTP mouse model of Parkinson's disease

Intravenous immunoglobulin (IVIg) is a blood-derived product, used for the treatment of immunodeficiency and autoimmune diseases. Since a range of immunotherapies have recently been proposed as a therapeutic strategy for Parkinson’s disease (PD), we investigated the effects of an IVIg treatment in a neurotoxin-induced animal model of PD. Mice received four injections of MPTP (15 mg/kg) at 2-hour intervals followed by a 14-day IVIg treatment, which induced key immune-related changes such as increased regulatory T-cell population and decreased CD4⁺/CD8⁺ ratio. The MPTP treatment induced significant 80% and 84% decreases of striatal dopamine concentrations (P < 0.01), as well as 33% and 40% reductions in the number of nigral dopaminergic neurons (P < 0.001) in controls and IVIg-treated mice, respectively. Two-way analyses of variance further revealed lower striatal tyrosine hydroxylase protein levels, striatal homovanillic acid concentrations and nigral dopaminergic neurons (P < 0.05) in IVIg-treated animals. Collectively, our results fail to support a neurorestorative effect of IVIg on the nigrostriatal system in the MPTP-treated mice and even suggest a trend toward a detrimental effect of IVIg on the dopaminergic system. These preclinical data underscore the need to proceed with caution before initiating clinical trials of IVIg in PD patients.