Synucleins antagonize endoplasmic reticulum function to modulate dopamine transporter trafficking

The hexosamine biosynthetic pathway rescues lysosomal dysfunction in Parkinson's disease patient iPSC derived midbrain neurons

Disrupted glucose metabolism and protein misfolding are key characteristics of age-related neurodegenerative disorders including Parkinson's disease, however their mechanistic linkage is largely unexplored. The hexosamine biosynthetic pathway utilizes glucose and uridine-5'-triphosphate to generate N-linked glycans required for protein folding in the endoplasmic reticulum. Here we find that Parkinson's patient midbrain cultures accumulate glucose and uridine-5'-triphosphate, while N-glycan synthesis rates are reduced. Impaired glucose flux occurred by selective reduction of the rate-limiting enzyme, GFPT2, through disrupted signaling between the unfolded protein response and the hexosamine pathway. Failure of the unfolded protein response and reduced N-glycosylation caused immature lysosomal hydrolases to misfold and accumulate, while accelerating glucose flux through the hexosamine pathway rescued hydrolase function and reduced pathological α-synuclein. Our data indicate that the hexosamine pathway integrates glucose metabolism with lysosomal activity, and its failure in Parkinson's disease occurs by uncoupling of the unfolded protein response-hexosamine pathway axis. These findings offer new methods to restore proteostasis by hexosamine pathway enhancement.

An Update on the Critical Role of α-Synuclein in Parkinson's Disease and Other Synucleinopathies: from Tissue to Cellular and Molecular Levels

The aggregation of alpha-synuclein (α-Syn) plays a critical role in the development of Parkinson's disease (PD) and other synucleinopathies. α-Syn, which is encoded by the SNCA gene, is a lysine-rich soluble amphipathic protein normally expressed in neurons. Located in the cytosolic domain, this protein has the ability to remodel itself in plasma membranes, where it assumes an alpha-helix conformation. However, the protein can also adopt another conformation rich in cross-beta sheets, undergoing mutations and post-translational modifications, then leading the protein to an unusual aggregation in the form of Lewy bodies (LB), which are cytoplasmic inclusions constituted predominantly by α-Syn. Pathogenic mechanisms affecting the structural and functional stability of α-Syn - such as endoplasmic reticulum stress, Golgi complex fragmentation, disfunctional protein degradation systems, aberrant interactions with mitochondrial membranes and nuclear DNA, altered cytoskeleton dynamics, disrupted neuronal plasmatic membrane, dysfunctional vesicular transport, and formation of extracellular toxic aggregates - contribute all to the pathogenic progression of PD and synucleinopathies. In this review, we describe the collective knowledge on this topic and provide an update on the critical role of α-Syn aggregates, both at the cellular and molecular levels, in the deregulation of organelles affecting the cellular homeostasis and leading to neuronal cell death in PD and other synucleinopathies.

Roles for α-Synuclein in Gene Expression

α-Synuclein (α-Syn) is a small cytosolic protein associated with a range of cellular compartments, including synaptic vesicles, the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes. In addition to its physiological role in regulating presynaptic function, the protein plays a central role in both sporadic and familial Parkinson’s disease (PD) via a gain-of-function mechanism. Because of this, several recent strategies propose to decrease α-Syn levels in PD patients. While these therapies may offer breakthroughs in PD management, the normal functions of α-Syn and potential side effects of its depletion require careful evaluation. Here, we review recent evidence on physiological and pathological roles of α-Syn in regulating activity-dependent signal transduction and gene expression pathways that play fundamental role in synaptic plasticity.

Baicalein alleviates depression-like behavior in rotenone- induced Parkinson's disease model in mice through activating the BDNF/TrkB/CREB pathway

BACKGROUND

Parkinson's disease (PD) is the second most prevalent neurodegenerative disorder in the world. In addition to motor symptoms, a variety of non-motor symptoms seriously affect the life quality of PD patients. Baicalein, a flavonoid extracted from the herb Scutellaria baicalensis Georgi, exhibits anti-PD activity through alleviation of its motor symptoms. However, its effects on non-motor symptoms were barely reported. This study aimed to investigate the therapeutic effects of baicalein on PD-related depression.

METHODS

After a 2-week injection of rotenone, mice with PD-related depression behavior were selected, divided into three groups, and administrated saline, baicalein, or madopar orally for four weeks. Behavior, neuroinflammation, neurotransmitters, and synaptic plasticity were evaluated.

RESULTS

Our results showed that 4-week baicalein treatment significantly alleviated the depression-like behavior in the rotenone-induced mice model. Repeated baicalein treatment reduced α-synuclein aggregation, inhibited neuroinflammation, and maintained neurotransmitters homeostasis. Moreover, we found that baicalein treatment could remarkably protect the synaptic plasticity and activate the BDNF/TrkB/CREB pathway in the PD-related depression mice model. As traditional dopamine replacement therapy unleashed few effects on depression-like symptom amelioration and synaptic function protection, baicalein might be a more appropriate choice for PD-related depression.

CONCLUSIONS

The current results suggested that baicalein could act as a treatment for PD-related depression.

Vesicle trafficking and lipid metabolism in synucleinopathy

The neuronal protein α-synuclein (αS) is central to the pathogenesis of Parkinson's disease and other progressive brain diseases such as Lewy body dementia and multiple system atrophy. These diseases, collectively referred to as 'synucleinopathies', have long been considered purely proteinopathies: diseases characterized by the misfolding of a protein into small and large aggregates mainly consisting of that protein (in this case: α-synuclein). However, recent morphological insights into Lewy bodies, the hallmark neuropathology of human synucleinopathies, suggests these lesions are also rich in vesicles and other membranous organelles. Moreover, αS physiology and pathology are both strongly associated with various aspects of intracellular vesicle trafficking and lipid biology. αS physiologically binds to synaptic and other small vesicles, and several functions of αS in regulating vesicle biology have been proposed. Familial PD-linked αS excess and missense mutations have been shown to impair vesicle trafficking and alter lipid homeostasis. On the other hand, vesicle trafficking and lipid-related genes have emerged as Parkinson's risk factors, suggesting a bidirectional relationship. The answer to the question "Does abnormal αS accumulation cause impaired vesicle trafficking and lipid dyshomeostasis or is αS aggregation the consequence of such impairments?" may be "Both". Here, we review current knowledge of the αS-lipid and αS-vesicle trafficking interplay, with a special focus on Parkinson's disease and Lewy body dementia.

Development of an α-synuclein knockdown peptide and evaluation of its efficacy in Parkinson's disease models

Convincing evidence supports the premise that reducing α-synuclein levels may be an effective therapy for Parkinson's disease (PD); however, there has been lack of a clinically applicable α-synuclein reducing therapeutic strategy. This study was undertaken to develop a blood-brain barrier and plasma membrane-permeable α-synuclein knockdown peptide, Tat-βsyn-degron, that may have therapeutic potential. The peptide effectively reduced the level of α-synuclein via proteasomal degradation both in cell cultures and in animals. Tat-βsyn-degron decreased α-synuclein aggregates and microglial activation in an α-synuclein pre-formed fibril model of spreading synucleinopathy in transgenic mice overexpressing human A53T α-synuclein. Moreover, Tat-βsyn-degron reduced α-synuclein levels and significantly decreased the parkinsonian toxin-induced neuronal damage and motor impairment in a mouse toxicity model of PD. These results show the promising efficacy of Tat-βsyn-degron in two different animal models of PD and suggest its potential use as an effective PD therapeutic that directly targets the disease-causing process.

Prolyl Oligopeptidase Regulates Dopamine Transporter Oligomerization and Phosphorylation in a PKC- and ERK-Independent Manner

Prolyl oligopeptidase (PREP) is a serine protease that binds to alpha-synuclein (aSyn) and induces its aggregation. PREP inhibitors have been shown to have beneficial effects in Parkinson's disease models by enhancing the clearance of aSyn aggregates and modulating striatal dopamine. Additionally, we have shown that PREP regulates phosphorylation and internalization of dopamine transporter (DAT) in mice. In this study, we clarified the mechanism behind this by using HEK-293 and PREP knock-out HEK-293 cells with DAT transfection. We tested the effects of PREP, PREP inhibition, and alpha-synuclein on PREP-related DAT regulation by using Western blot analysis and a dopamine uptake assay, and characterized the impact of PREP on protein kinase C (PKC) and extracellular signal-regulated kinase (ERK) by using PKC assay and Western blot, respectively, as these kinases regulate DAT phosphorylation. Our results confirmed our previous findings that a lack of PREP can increase phosphorylation and internalization of DAT and decrease uptake of dopamine. PREP inhibition had a variable impact on phosphorylation of ERK dependent on the metabolic state of cells, but did not have an effect on phosphorylation or function of DAT. PREP modifications did not affect PKC activity either. Additionally, a lack of PREP elevated a DAT oligomerization that is associated with intracellular trafficking of DAT. Our results suggest that PREP-mediated phosphorylation, oligomerization, and internalization of DAT is not dependent on PKC or ERK.

Parkinson's disease laterality: a 11C-PE2I PET imaging study

Asymmetry of striatal dopaminergic deficits and motor symptoms is a typical characteristic of idiopathic Parkinson's disease (PD). This study aims to characterise the trend of asymmetry in moderate-stage PD. We performed a 19-month longitudinal study in 27 patients with PET-CT imaging and appropriate clinical assessments. 11C-PE2I non-displaceable binding potential (BPND) was calculated bilaterally for the striatum at baseline and follow-up to estimate the in vivo density of striatal dopamine transporters (DAT). Changes in striatal 11C-PE2I BPND over time were more prominent in the ipsilateral as compared to contralateral side. Changes in MDS-UPDRS-III (motor component of the Movement Disorders Society Unified PD Rating Scale) were not different between the clinically most and least affected body sides. Our data support that the asymmetry in striatal dopaminergic degeneration becomes less prominent in moderate-stage PD. In contrast, during the above period, the asymmetry of motor symptoms was maintained between the clinically most and least affected body sides.

Alpha-Synuclein FRET Biosensors Reveal Early Alpha-Synuclein Aggregation in the Endoplasmic Reticulum

Endoplasmic reticulum (ER) dysfunction is important for alpha-synuclein (αS) acquired toxicity. When targeted to the ER in SH-SY5Y cells, transient or stable expression of αS resulted in the formation of compact αS-positive structures in a small subpopulation of cells, resembling αS inclusions. Thus, because of the limitations of immunofluorescence, we developed a set of αS FRET biosensors (AFBs) able to track αS conformation in cells. In native conditions, expression in i36, a stable cell line for ER αS, of intermolecular AFBs, reporters in which CFP or YFP has been fused with the C-terminal of αS (αS-CFP/αS-YFP), resulted in no Förster resonance energy transfer (FRET), whereas expression of the intramolecular AFB, a probe obtained by fusing YFP and CFP with αS N- or C- termini (YFP-αS-CFP), showed a positive FRET signal. These data confirmed that αS has a predominantly globular, monomeric conformation in native conditions. Differently, under pro-aggregating conditions, the intermolecular AFB was able to sense significantly formation of αS oligomers, when AFB was expressed in the ER rather than ubiquitously, suggesting that the ER can favor changes in αS conformation when aggregation is stimulated. These results show the potential of AFBs as a new, valuable tool to track αS conformational changes in vivo.

LRRK2 and α-Synuclein: Distinct or Synergistic Players in Parkinson's Disease?

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, characterized by prominent degeneration of dopaminergic neurons in the substantia nigra and aggregation of the protein α-synuclein within intraneuronal inclusions known as Lewy bodies. Ninety percent of PD cases are idiopathic while the remaining 10% are associated with gene mutations that affect cellular functions ranging from kinase activity to mitochondrial quality control, hinting at a multifactorial disease process. Mutations in LRRK2 and SNCA (the gene coding for α-synuclein) cause monogenic forms of autosomal dominant PD, and polymorphisms in either gene are also associated with increased risk of idiopathic PD. Although Lewy bodies are a defining neuropathological feature of PD, an appreciable subset of patients with LRRK2 mutations present with a clinical phenotype indistinguishable from idiopathic PD but lack Lewy pathology at autopsy, suggesting that LRRK2-mediated PD may occur independently of α-synuclein aggregation. Here, we examine whether LRRK2 and α-synuclein, as mediators of neurodegeneration in PD, exist in common or distinct pathways. Specifically, we review evidence from preclinical models and human neuropathological studies examining interactions between the two proteins. Elucidating the degree of interplay between LRRK2 and α-synuclein will be necessary for treatment stratification once effective targeted disease-modifying therapies are developed.

Synucleinopathies: Where we are and where we need to go

Synucleinopathies are a group of disorders characterized by the accumulation of inclusions rich in the a-synuclein (aSyn) protein. This group of disorders includes Parkinson's disease, dementia with Lewy bodies (DLB), multiple systems atrophy, and pure autonomic failure (PAF). In addition, genetic alterations (point mutations and multiplications) in the gene encoding for aSyn (SNCA) are associated with familial forms of Parkinson's disease, the most common synucleinopathy. The Synuclein Meetings are a series that has been taking place every 2 years for about 12 years. The Synuclein Meetings bring together leading experts in the field of Synuclein and related human conditions with the goal of discussing and advancing the research. In 2019, the Synuclein meeting took place in Ofir, a city in the outskirts of Porto, Portugal. The meeting, entitled "Synuclein Meeting 2019: Where we are and where we need to go", brought together >300 scientists studying both clinical and molecular aspects of synucleinopathies. The meeting covered a many of the open questions in the field, in a format that prompted open discussions between the participants, and underscored the need for additional research that, hopefully, will lead to future therapies for a group of as of yet incurable disorders. Here, we provide a summary of the topics discussed in each session and highlight what we know, what we do not know, and what progress needs to be made in order to enable the field to continue to advance. We are confident this systematic assessment of where we stand will be useful to steer the field and contribute to filling knowledge gaps that may form the foundations for future therapeutic strategies, which is where we need to go.

Alpha-synuclein/synapsin III pathological interplay boosts the motor response to methylphenidate

Loss of dopaminergic nigrostriatal neurons and fibrillary α-synuclein (α-syn) aggregation in Lewy bodies (LB) characterize Parkinson's disease (PD). We recently found that Synapsin III (Syn III), a phosphoprotein regulating dopamine (DA) release with α-syn, is another key component of LB fibrils in the brain of PD patients and acts as a crucial mediator of α-syn aggregation and toxicity. Methylphenidate (MPH), a monoamine reuptake inhibitor (MRI) efficiently counteracting freezing of gait in advanced PD patients, can bind α-syn and controls α-syn-mediated DA overflow and presynaptic compartmentalization. Interestingly, MPH results also efficient for the treatment of attention deficits and hyperactivity disorder (ADHD), a neurodevelopmental psychiatric syndrome associated with Syn III and α-syn polymorphisms and constituting a risk factor for the development of LB disorders. Here, we studied α-syn/Syn III co-deposition and longitudinal changes of α-syn, Syn III and DA transporter (DAT) striatal levels in nigrostriatal neurons of a PD model, the human C-terminally truncated (1-120) α-syn transgenic (SYN120 tg) mouse, in comparison with C57BL/6J wild type (wt) and C57BL/6JOlaHsd α-syn null littermates. Then, we analyzed the locomotor response of these animals to an acute administration of MPH (d-threo) and other MRIs: cocaine, that we previously found to stimulate Syn III-reliant DA release in the absence of α-syn, or the selective DAT blocker GBR-12935, along aging. Finally, we assessed whether these drugs modulate α-syn/Syn III interaction by fluorescence resonance energy transfer (FRET) and performed in silico studies engendering a heuristic model of the α-syn conformations stabilized upon MPH binding. We found that only MPH was able to over-stimulate a Syn III-dependent/DAT-independent locomotor activity in the aged SYN120 tg mice showing α-syn/Syn III co-aggregates. MPH enhanced full length (fl) α-syn/Syn III and even more (1-120) α-syn/Syn III interaction in cells exhibiting α-syn/Syn III inclusions. Moreover, in silico studies confirmed that MPH may reduce α-syn fibrillation by stabilizing a protein conformation with increased lipid binding predisposition. Our observations indicate that the motor-stimulating effect of MPH can be positively fostered in the presence of α-syn/Syn III co-aggregation. This evidence holds significant implications for PD and ADHD therapeutic management.

Motor learning and metaplasticity in striatal neurons: relevance for Parkinson's disease

Nigro-striatal dopamine transmission is central to a wide range of neuronal functions, including skill learning, which is disrupted in several pathologies such as Parkinson's disease. The synaptic plasticity mechanisms, by which initial motor learning is stored for long time periods in striatal neurons, to then be gradually optimized upon subsequent training, remain unexplored. Addressing this issue is crucial to identify the synaptic and molecular mechanisms involved in striatal-dependent learning impairment in Parkinson's disease. In this study, we took advantage of interindividual differences between outbred rodents in reaching plateau performance in the rotarod incremental motor learning protocol, to study striatal synaptic plasticity ex vivo. We then assessed how this process is modulated by dopamine receptors and the dopamine active transporter, and whether it is impaired by overexpression of human α-synuclein in the mesencephalon; the latter is a progressive animal model of Parkinson's disease. We found that the initial acquisition of motor learning induced a dopamine active transporter and D1 receptors mediated long-term potentiation, under a protocol of long-term depression in striatal medium spiny neurons. This effect disappeared in animals reaching performance plateau. Overexpression of human α-synuclein reduced striatal dopamine active transporter levels, impaired motor learning, and prevented the learning-induced long-term potentiation, before the appearance of dopamine neuronal loss. Our findings provide evidence of a reorganization of cellular plasticity within the dorsolateral striatum that is mediated by dopamine receptors and dopamine active transporter during the acquisition of a skill. This newly identified mechanism of cellular memory is a form of metaplasticity that is disrupted in the early stage of synucleinopathies, such as Parkinson's disease, and that might be relevant for other striatal pathologies, such as drug abuse.

Is Parkinson's disease a lysosomal disorder?

Common forms of Parkinson's disease have long been described as idiopathic, with no single penetrant genetic factor capable of influencing disease aetiology. Recent genetic studies indicate a clear association of variants within several lysosomal genes as risk factors for idiopathic Parkinson's disease. The emergence of novel variants suggest that the aetiology of idiopathic Parkinson's disease may be explained by the interaction of several partially penetrant mutations that, while seemingly complex, all appear to converge on cellular clearance pathways. These newly evolving data are consistent with mechanistic studies linking α-synuclein toxicity to lysosomal abnormalities, and indicate that idiopathic Parkinson's disease resembles features of Mendelian lysosomal storage disorders at a genetic and biochemical level. These findings offer novel pathways to exploit for the development of disease-altering therapies for idiopathic Parkinson's disease that target specific components of the lysosomal system.

The dopamine transporter: An unrecognized nexus for dysfunctional peripheral immunity and signaling in Parkinson's Disease

The second-most common neurodegenerative disease, Parkinson's Disease (PD) has three hallmarks: dysfunctional dopamine transmission due, at least in part, to dopamine neuron degeneration; intracellular inclusions of α-synuclein aggregates; and neuroinflammation. The origin and interplay of these features remains a puzzle, as does the underlying mechanism of PD pathogenesis and progression. When viewed in the context of neuroimmunology, dopamine also plays a role in regulating peripheral immune cells. Intriguingly, plasma dopamine levels are altered in PD, suggesting collateral dysregulation of peripheral dopamine transmission. The dopamine transporter (DAT), the main regulator of dopaminergic tone in the CNS, is known to exist in lymphocytes and monocytes/macrophages, but little is known about peripheral DAT biology or how DAT regulates the dopaminergic tone, much less how peripheral DAT alters immune function. Our review is guided by the hypothesis that dysfunctional peripheral dopamine signaling might be linked to the dysfunctional immune responses in PD and thereby suggests a potential bidirectional communication between central and peripheral dopamine systems. This review seeks to foster new perspectives concerning PD pathogenesis and progression.

Subcellular localization of alpha-synuclein aggregates and their interaction with membranes

For more than a decade numerous evidence has been reported on the mechanisms of toxicity of α-synuclein (αS) oligomers and aggregates in α-synucleinopathies. These species were thought to form freely in the cytoplasm but recent reports of αS multimer conformations when bound to synaptic vesicles in physiological conditions, have raised the question about where αS aggregation initiates. In this review we focus on recent literature regarding the impact on membrane binding and subcellular localization of αS toxic species to understand how regular cellular function of αS contributes to pathology. Notably αS has been reported to mainly associate with specific membranes in neurons such as those of synaptic vesicles, ER/Golgi and the mitochondria, while toxic species of αS have been shown to inhibit, among others, neurotransmission, protein trafficking and mitochondrial function. Strategies interfering with αS membrane binding have shown to improve αS-driven toxicity in worms and in mice. Thus, a selective membrane binding that would result in a specific subcellular localization could be the key to understand how aggregation and pathology evolves, pointing out to αS functions that are primarily affected before onset of irreversible damage.

Mutant α-Synuclein Overexpression Induces Stressless Pacemaking in Vagal Motoneurons at Risk in Parkinson's Disease

α-Synuclein overexpression (ASOX) drives the formation of toxic aggregates in neurons vulnerable in Parkinson's disease (PD), including dopaminergic neurons of the substantia nigra (SN) and cholinergic neurons of the dorsal motor nucleus of the vagus (DMV). Just as these populations differ in when they exhibit α-synucleinopathies during PD pathogenesis, they could also differ in their physiological responses to ASOX. An ASOX-mediated hyperactivity of SN dopamine neurons, which was caused by oxidative dysfunction of Kv4.3 potassium channels, was recently identified in transgenic (A53T-SNCA) mice overexpressing mutated human α-synuclein. Noting that DMV neurons display extensive α-synucleinopathies earlier than SN dopamine neurons while exhibiting milder cell loss in PD, we aimed to define the electrophysiological properties of DMV neurons in A53T-SNCA mice. We found that DMV neurons maintain normal firing rates in response to ASOX. Moreover, Kv4.3 channels in DMV neurons exhibit no oxidative dysfunction in the A53T-SNCA mice, which could only be recapitulated in wild-type mice by glutathione dialysis. Two-photon imaging of redox-sensitive GFP corroborated the finding that mitochondrial oxidative stress was diminished in DMV neurons in the A53T-SNCA mice. This reduction in oxidative stress resulted from a transcriptional downregulation of voltage-activated (Cav) calcium channels in DMV neurons, which led to a reduction in activity-dependent calcium influx via Cav channels. Thus, ASOX induces a homeostatic remodeling with improved redox signaling in DMV neurons, which could explain the differential vulnerability of SN dopamine and DMV neurons in PD and could promote neuroprotective strategies that emulate endogenous homeostatic responses to ASOX (e.g., stressless pacemaking) in DMV neurons.

SIGNIFICANCE STATEMENT

Overexpression of mutant α-synuclein causes Parkinson's disease, presumably by driving neurodegeneration in vulnerable neuronal target populations. However, the extent of α-synuclein pathology (e.g., Lewy bodies) is not directly related to the degree of neurodegeneration across various vulnerable neuronal populations. Here, we show that, in contrast to dopamine neurons in the substantia nigra, vagal motoneurons do not enhance their excitability and oxidative load in response to chronic mutant α-synuclein overexpression. Rather, by downregulating their voltage-activated calcium channels, vagal motoneurons acquire a stressless form of pacemaking that diminishes mitochondrial and cytosolic oxidative stress. Emulating this endogenous adaptive response to α-synuclein overexpression could lead to novel strategies to protect dopamine neurons and perhaps delay the onset of Parkinson's disease.

α-Synuclein increases β-amyloid secretion by promoting β-/γ-secretase processing of APP

α-Synuclein misfolding and aggregation is often accompanied by β-amyloid deposition in some neurodegenerative diseases. We hypothesised that α-synuclein promotes β-amyloid production from APP. β-Amyloid levels and APP amyloidogenic processing were investigated in neuronal cell lines stably overexpressing wildtype and mutant α-synuclein. γ-Secretase activity and β-secretase expression were also measured. We show that α-synuclein expression induces β-amyloid secretion and amyloidogenic processing of APP in neuronal cell lines. Certain mutations of α-synuclein potentiate APP amyloidogenic processing. γ-Secretase activity was not enhanced by wildtype α-synuclein expression, however β-secretase protein levels were induced. Furthermore, a correlation between α-synuclein and β-secretase protein was seen in rat brain striata. Iron chelation abolishes the effect of α-synuclein on neuronal cell β-amyloid secretion, whereas overexpression of the ferrireductase enzyme Steap3 is robustly pro-amyloidogenic. We propose that α-synuclein promotes β-amyloid formation by modulating β-cleavage of APP, and that this is potentially mediated by the levels of reduced iron and oxidative stress.

α-synuclein toxicity in neurodegeneration: mechanism and therapeutic strategies

Alterations in α-synuclein dosage lead to familial Parkinson's disease (PD), and its accumulation results in synucleinopathies that include PD, dementia with Lewy bodies (DLB) and multiple system atrophy (MSA). Furthermore, α-synuclein contributes to the fibrilization of amyloid-b and tau, two key proteins in Alzheimer's disease, which suggests a central role for α-synuclein toxicity in neurodegeneration. Recent studies of factors contributing to α-synuclein toxicity and its disruption of downstream cellular pathways have expanded our understanding of disease pathogenesis in synucleinopathies. In this Review, we discuss these emerging themes, including the contributions of aging, selective vulnerability and non-cell-autonomous factors such as α-synuclein cell-to-cell propagation and neuroinflammation. Finally, we summarize recent efforts toward the development of targeted therapies for PD and related synucleinopathies.

Lysosomal trafficking defects link Parkinson's disease with Gaucher's disease

Lysosomal dysfunction has been implicated in multiple diseases, including lysosomal storage disorders such as Gaucher's disease, in which loss-of-function mutations in the GBA1 gene encoding the lysosomal hydrolase β-glucocerebrosidase result in lipid substrate accumulation. In Parkinson's disease, α-synuclein accumulates in Lewy bodies and neurites contributing to neuronal death. Previous clinical and genetic evidence has demonstrated an important link between Parkinson's and Gaucher's disease, as GBA1 mutations and variants increase the risk of Parkinson's and Parkinson's patients exhibit decreased β-glucocerebrosidase activity. Using human midbrain neuron cultures, we have found that loss of β-glucocerebrosidase activity promotes α-synuclein accumulation and toxicity, whereas α-synuclein accumulation further contributes to decreased lysosomal β-glucocerebrosidase activity by disrupting β-glucocerebrosidase trafficking to lysosomes. Moreover, α-synuclein accumulation disrupts trafficking of additional lysosomal hydrolases, further contributing to lysosomal dysfunction and neuronal dyshomeostasis. Importantly, promoting β-glucocerebrosidase activity reduces α-synuclein accumulation and rescues lysosomal and neuronal dysfunction, suggesting that β-glucocerebrosidase may be an important therapeutic target for advancing drug discovery in synucleinopathies including Parkinson's disease. © 2016 International Parkinson and Movement Disorder Society.

Alpha-synuclein modulates dopamine neurotransmission

Alpha-synuclein is a small, highly charged protein encoded by the synuclein or SNCA gene that is predominantly expressed in central nervous system neurons. Although its physiological function remains enigmatic, alpha-synuclein is implicated in movement disorders such as Parkinson's disease, multiple system atrophy, and in neurodegenerative diseases such as Dementia with Lewy bodies. Here we have focused on reviewing the existing literature pertaining to wild-type alpha-synuclein structure, its properties, and its potential involvement in regulation of dopamine neurotransmission.

Long-term controlled GDNF over-expression reduces dopamine transporter activity without affecting tyrosine hydroxylase expression in the rat mesostriatal system

The dopamine (DA) transporter (DAT) is a plasma membrane glycoprotein expressed in dopaminergic (DA-) cells that takes back DA into presynaptic neurons after its release. DAT dysfunction has been involved in different neuro-psychiatric disorders including Parkinson's disease (PD). On the other hand, numerous studies support that the glial cell line-derived neurotrophic factor (GDNF) has a protective effect on DA-cells. However, studies in rodents show that prolonged GDNF over-expression may cause a tyrosine hydroxylase (TH, the limiting enzyme in DA synthesis) decline. The evidence of TH down-regulation suggests that another player in DA handling, DAT, may also be regulated by prolonged GDNF over-expression, and the possibility that this effect is induced at GDNF expression levels lower than those inducing TH down-regulation. This issue was investigated here using intrastriatal injections of a tetracycline-inducible adeno-associated viral vector expressing human GDNF cDNA (AAV-tetON-GDNF) in rats, and doxycycline (DOX; 0.01, 0.03, 0.5 and 3mg/ml) in the drinking water during 5weeks. We found that 3mg/ml DOX promotes an increase in striatal GDNF expression of 12× basal GDNF levels and both DA uptake decrease and TH down-regulation in its native and Ser40 phosphorylated forms. However, 0.5mg/ml DOX promotes a GDNF expression increase of 3× basal GDNF levels with DA uptake decrease but not TH down-regulation. The use of western-blot under non-reducing conditions, co-immunoprecipitation and in situ proximity ligation assay revealed that the DA uptake decrease is associated with the formation of DAT dimers and an increase in DAT-α-synuclein interactions, without changes in total DAT levels or its compartmental distribution. In conclusion, at appropriate GDNF transduction levels, DA uptake is regulated through DAT protein-protein interactions without interfering with DA synthesis.

Alpha-synuclein Toxicity in the Early Secretory Pathway: How It Drives Neurodegeneration in Parkinsons Disease

Alpha-synuclein is a predominant player in the pathogenesis of Parkinson's Disease. However, despite extensive study for two decades, its physiological and pathological mechanisms remain poorly understood. Alpha-synuclein forms a perplexing web of interactions with lipids, trafficking machinery, and other regulatory factors. One emerging consensus is that synaptic vesicles are likely the functional site for alpha-synuclein, where it appears to facilitate vesicle docking and fusion. On the other hand, the dysfunctions of alpha-synuclein are more dispersed and numerous; when mutated or over-expressed, alpha-synuclein affects several membrane trafficking and stress pathways, including exocytosis, ER-to-Golgi transport, ER stress, Golgi homeostasis, endocytosis, autophagy, oxidative stress, and others. Here we examine recent developments in alpha-synuclein's toxicity in the early secretory pathway placed in the context of emerging themes from other affected pathways to help illuminate its underlying pathogenic mechanisms in neurodegeneration.

Influence of Lentiviral β-Synuclein Overexpression in the Hippocampus of a Transgenic Mouse Model of Alzheimer's Disease on Amyloid Precursor Protein Metabolism and Pathology

BACKGROUND

β-Synuclein (β-Syn) is a member of the highly homologous synuclein protein family. The most prominent family member, α-synuclein (α-Syn), abnormally accumulates in so-called Lewy bodies, one of the major pathological hallmarks of α-synucleinopathies. Notably, parts of the peptide backbone, called the nonamyloid component, are also found in amyloid plaques. However, β-Syn seems to have beneficial effects by reducing α-Syn aggregation, and amyloid antiaggregatory activity has been described.

OBJECTIVE

The aim of the study was to analyze if wild-type β-Syn can counteract functional and pathological changes in a murine Alzheimer model over different time periods.

METHODS

At the onset of pathology, lentiviral particles expressing human β-Syn were injected into the hippocampus of transgenic mice overexpressing human amyloid precursor protein with Swedish and London mutations (APPSL). An empty vector served as the control. Behavioral analyses were performed 1, 3 and 6 months after injection followed by biochemical and histological examinations of brain samples.

RESULTS

β-Syn expression was locally concentrated and rather modest, but nevertheless changed its effect on APP expression and plaque load in a time- and concentration-dependent manner. Interestingly, the phosphorylation of glycogen synthase kinase 3 beta was enhanced in APPSL mice expressing human β-Syn, but an inverse trend was observed in wild-type animals.

CONCLUSION

The initially reported beneficial effects of β-Syn could be partially reproduced, but locally elevated levels of β-Syn might also cause neurodegeneration. To enlighten the controversial pathological mechanism of β-Syn, further examinations considering the relationship between concentration and exposure time of β-Syn are needed.

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.

α-Synuclein-mediated inhibition of ATF6 processing into COPII vesicles disrupts UPR signaling in Parkinson's disease

The unfolded protein response (UPR) monitors the folding environment within the endoplasmic reticulum (ER). Accumulation of misfolded proteins within the ER activates the UPR resulting in the execution of adaptive or non-adaptive signaling pathways. α-Synuclein (α-syn) whose accumulation and aggregation define the pathobiology of Parkinson's disease (PD) has been shown to inhibit ER-Golgi transit of COPII vesicles. ATF6, a protective branch of the UPR, is processed via COPII mediated ER-Golgi transit following its activation via ER stress. Using cellular PD models together with biochemical reconstitution assays, we showed that α-syn inhibited processing of ATF6 directly through physical interactions and indirectly through restricted incorporation into COPII vesicles. Impaired ATF6 signaling was accompanied by decreased ER-associated degradation (ERAD) function and increased pro-apoptotic signaling. The mechanism by which α-syn inhibits ATF6 signaling expands our understanding of the role ER stress and the UPR play in neurodegenerative diseases such as PD.

From yeast to patient neurons and back again: powerful new discovery platform

No disease-modifying therapies are available for synucleinopathies, including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple systems atrophy (MSA). The lack of therapies has been impeded by a paucity of validated drug targets and problematic cell-based model systems. New approaches are therefore needed to identify genes and compounds that directly target the underlying cellular pathologies elicited by the pathological protein, α-synuclein (α-syn). This small, lipid-binding protein impinges on evolutionarily conserved processes such as vesicle trafficking and mitochondrial function. For decades, the genetically tractable, single-cell eukaryote, budding yeast, has been used to study nearly all aspects of cell biology. More recently, yeast has revealed key insights into the underlying cellular pathologies caused by α-syn. The robust cellular toxicity caused by α-syn expression facilitates unbiased high-throughput small-molecule screening. Critically, one must validate the discoveries made in yeast in disease-relevant neuronal models. Here, we describe two recent reports that together establish yeast-to-human discovery platforms for synucleinopathies. In this exemplar, genes and small molecules identified in yeast were validated in patient-derived neurons that present the same cellular phenotypes initially discovered in yeast. On validation, we returned to yeast, where unparalleled genetic approaches facilitated the elucidation of a small molecule's mode of action. This approach enabled the identification and neuronal validation of a previously unknown "druggable" node that interfaces with the underlying, precipitating pathologies caused by α-syn. Such platforms can provide sorely needed leads and fresh ideas for disease-modifying therapy for these devastating diseases.

Parallel mechanisms for direct and indirect membrane protein trafficking by synucleins

More than 2 decades of work have yet to conclusively determine the physiological role of the synuclein proteins, even though these abundant brain constituents are participants in a broad array of cellular processes. Among proposed physiological roles is a functional interaction between the synuclein proteins and monoamine transporters contributing to transporter trafficking through direct protein–protein interactions. Recent work shows that an antagonistic effect of the synuclein proteins on the secretory functions of the endoplasmic reticulum and the Golgi apparatus appears to simultaneously influence trafficking of the dopamine transporter and other membrane proteins. Here, we highlight these new findings in view of the broader literature identifying the role of synucleins in protein trafficking and suggest emerging themes for ongoing and future work in the field of synuclein biology.