The physiological role of α-synuclein and its relationship to Parkinson's Disease

"Janus-Faced" α-Synuclein: Role in Parkinson's Disease

Parkinson's disease (PD) is a pathological condition characterized by the aggregation and the resultant presence of intraneuronal inclusions termed Lewy bodies (LBs) and Lewy neurites which are mainly composed of fibrillar α-synuclein (α-syn) protein. Pathogenic aggregation of α-syn is identified as the major cause of LBs deposition. Several mutations in α-syn showing varied aggregation kinetics in comparison to the wild type (WT) α-syn are reported in PD (A30P, E46K, H 50Q, G51D, A53E, and A53T). Also, the cell-to-cell spread of pathological α-syn plays a significant role in PD development. Interestingly, it has also been suggested that the pathology of PD may begin in the gastrointestinal tract and spread via the vagus nerve (VN) to brain proposing the gut-brain axis of α-syn pathology in PD. Despite multiple efforts, the behavior and functions of this protein in normal and pathological states (specifically in PD) is far from understood. Furthermore, the etiological factors responsible for triggering aggregation of this protein remain elusive. This review is an attempt to collate and present latest information on α-syn in relation to its structure, biochemistry and biophysics of aggregation in PD. Current advances in therapeutic efforts toward clearing the pathogenic α-syn via autophagy/lysosomal flux are also reviewed and reported.

Oligomerization of Lrrk controls actin severing and α-synuclein neurotoxicity in vivo

BACKGROUND

Mutations in LRRK2 are the most common cause of familial Parkinson's disease and typically cause disease in the context of abnormal aggregation and deposition of α-synuclein within affected brain tissue.

METHODS

We combine genetic analysis of Lrrk-associated toxicity in a penetrant Drosophila model of wild type human α-synuclein neurotoxicity with biochemical analyses and modeling of LRRK2 toxicity in human neurons and transgenic mouse models.

RESULTS

We demonstrate that Lrrk and α-synuclein interact to promote neuronal degeneration through convergent effects on the actin cytoskeleton and downstream dysregulation of mitochondrial dynamics and function. We find specifically that monomers and dimers of Lrrk efficiently sever actin and promote normal actin dynamics in vivo. Oligomerization of Lrrk, which is promoted by dominant Parkinson's disease-causing mutations, reduces actin severing activity in vitro and promotes excess stabilization of F-actin in vivo. Importantly, a clinically protective Lrrk mutant reduces oligomerization and α-synuclein neurotoxicity.

CONCLUSIONS

Our findings provide a specific mechanistic link between two key molecules in the pathogenesis of Parkinson's disease, α-synuclein and LRRK2, and suggest potential new approaches for therapy development.

Conformational Dynamics of α-Synuclein during the Interaction with Phospholipid Nanodiscs by Millisecond Hydrogen-Deuterium Exchange Mass Spectrometry

Both normal and pathological functions of α-synuclein (αSN), an abundant protein in the central and peripheral nervous system, have been linked to its interaction with membrane lipid bilayers. The ability to characterize structural transitions of αSN upon membrane complexation will clarify molecular mechanisms associated with αSN-linked pathologies, including Parkinson's disease (PD), multiple systems atrophy, and other synucleinopathies. In this work, time-resolved electrospray ionization hydrogen/deuterium exchange mass spectrometry (TRESI-HDX-MS) was employed to acquire a detailed picture of αSN's conformational transitions as it undergoes complexation with nanodisc membrane mimics with different headgroup charges (zwitterionic DMPC and negative POPG). Using this approach, αSN interactions with DMPC nanodiscs were shown to be rapid exchanging and to have little impact on the αSN conformational ensemble. Interactions with nanodiscs containing lipids known to promote amyloidogenesis (e.g., POPG), on the other hand, were observed to induce substantial and specific changes in the αSN conformational ensemble. Ultimately, we identify a region corresponding residues 19-28 and 45-57 of the αSN sequence that is uniquely impacted by interactions with "amyloidogenic" lipid membranes, supporting the existing "broken-helix" model for α-synuclein/membrane interactions, but do not detect a "helical extension" that is also thought to play a role in αSN aggregation.

In vivo aggregation of presynaptic alpha-synuclein is not influenced by its phosphorylation at serine-129

Abnormal aggregation of the α-synuclein protein is a key molecular feature of Parkinson's disease and other neurodegenerative diseases. The precise mechanisms that trigger α-synuclein aggregation are unclear, and it is not known what role aggregation plays in disease pathogenesis. Here we use an in vivo zebrafish model to express several different forms of human α-synuclein and measure its aggregation in presynaptic terminals. We show that human α-synuclein tagged with GFP can be expressed in zebrafish neurons, localizing normally to presynaptic terminals and undergoing phosphorylation at serine-129, as in mammalian neurons. The visual advantages of the zebrafish system allow for dynamic in vivo imaging to study α-synuclein, including the use of fluorescence recovery after photobleaching (FRAP) techniques to probe protein mobility. These experiments reveal three distinct terminal pools of α-synuclein with varying mobility, likely representing different subpopulations of aggregated and non-aggregated protein. Human α-synuclein is phosphorylated by an endogenous zebrafish Polo-like kinase activity, and there is a heterogeneous population of neurons containing either very little or extensive phosphorylation throughout the axonal arbor. Both pharmacological and genetic manipulations of serine-129 show that phosphorylation of α-synuclein at this site does not significantly affect its mobility. This suggests that serine-129 phosphorylation alone does not promote α-synuclein aggregation. Together our results show that human α-synuclein can be expressed and measured quantitatively in zebrafish, and that disease-relevant post-translational modifications occur within neurons. The zebrafish model provides a powerful in vivo system for measuring and manipulating α-synuclein function and aggregation, and for developing new treatments for neurodegenerative disease.

Insights into the molecular mechanism of amyloid filament formation: Segmental folding of α-synuclein on lipid membranes

Recent advances in the structural biology of disease-relevant α-synuclein fibrils have revealed a variety of structures, yet little is known about the process of fibril aggregate formation. Characterization of intermediate species that form during aggregation is crucial; however, this has proven very challenging because of their transient nature, heterogeneity, and low population. Here, we investigate the aggregation of α-synuclein bound to negatively charged phospholipid small unilamellar vesicles. Through a combination of kinetic and structural studies, we identify key time points in the aggregation process that enable targeted isolation of prefibrillar and early fibrillar intermediates. By using solid-state nuclear magnetic resonance, we show the gradual buildup of structural features in an α-synuclein fibril filament, revealing a segmental folding process. We identify distinct membrane-binding domains in α-synuclein aggregates, and the combined data are used to present a comprehensive mechanism of the folding of α-synuclein on lipid membranes.

Cannabinoid receptor-1 has an effect on CD200 under rotenone and alpha-synuclein induced stress

Decades after identifying cannabinoids and their beneficial effects on Parkinson's disease (PD), many gaps are still missing. Although, CB₂-dependent actions have been shown as underlying positive effects of cannabinoid treatment, in recent years, another receptor of cannabinoids, CB₁, emerged as a valuable player in cannabinoid-induced neuroprotection. Remarkably, the effects of CB₁ are mainly related to immune cells in the CNS, microglia, and astrocytes. However, oxidative stress, α-syn accumulation, and immune disbalance are essential aspects of both neurons and glial cells. Therefore, in this study, we investigated the effects of the CB₁ on both α-syn and rotenone-treated SH-SY5Y and C8-D1A cells. ACEA and AM-251 were used as CB₁ agonists and antagonists. Cell viability, IL-1β, IL-6, TNF-α levels, and CD200 expressions were determined in culture mediums. Our results demonstrated that preformed fibril form (pFF) of α-syn did not cause any significant change in SH-SY5Y cells compared to C8-D1A cells. Rotenone significantly increased the expression of IL-1β, IL-6, and TNF-α levels in both cells. pFF α-syn and rotenone treatment caused a decrease in CD200 expression. Surprisingly both ACEA and AM-251 alleviated rotenone-induced increase in cytokine levels in both cell lines. Although ACEA prevented pFF α-syn induced increase in cytokine levels and decrease in CD200 expression in C8-D1A cells, AM-251 failed to affect CD200 expression levels. Additionally, ACEA + AM-251 abolished the protective effects of both ACEA and AM-251 against rotenone and α-syn insults in both cell lines. The current study suggests that cannabinoid receptor agonism alleviates rotenone and α-syn-dependent inflammation in neurons and astrocytes.

Soluble α-synuclein-antibody complexes activate the NLRP3 inflammasome in hiPSC-derived microglia

Parkinson's disease is characterized by accumulation of α-synuclein (αSyn). Release of oligomeric/fibrillar αSyn from damaged neurons may potentiate neuronal death in part via microglial activation. Heretofore, it remained unknown if oligomeric/fibrillar αSyn could activate the nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) family pyrin domain-containing 3 (NLRP3) inflammasome in human microglia and whether anti-αSyn antibodies could prevent this effect. Here, we show that αSyn activates the NLRP3 inflammasome in human induced pluripotent stem cell (hiPSC)-derived microglia (hiMG) via dual stimulation involving Toll-like receptor 2 (TLR2) engagement and mitochondrial damage. In vitro, hiMG can be activated by mutant (A53T) αSyn secreted from hiPSC-derived A9-dopaminergic neurons. Surprisingly, αSyn-antibody complexes enhanced rather than suppressed inflammasome-mediated interleukin-1β (IL-1β) secretion, indicating these complexes are neuroinflammatory in a human context. A further increase in inflammation was observed with addition of oligomerized amyloid-β peptide (Aβ) and its cognate antibody. In vivo, engraftment of hiMG with αSyn in humanized mouse brain resulted in caspase-1 activation and neurotoxicity, which was exacerbated by αSyn antibody. These findings may have important implications for antibody therapies aimed at depleting misfolded/aggregated proteins from the human brain, as they may paradoxically trigger inflammation in human microglia.

Effects of α-Synuclein-Associated Post-Translational Modifications in Parkinson's Disease

α-Synuclein (α-syn), a small highly conserved presynaptic protein containing 140 amino acids, is thought to be the main pathological hallmark in related neurodegenerative disorders. Although the normal function of α-syn is closely involved in the regulation of vesicular neurotransmission in these diseases, the underlying mechanisms of post-translational modifications (PTMs) of α-syn in the pathogenesis of Parkinson's disease (PD) have not been fully characterized. The pathological accumulation of misfolded α-syn has a critical role in PD pathogenesis. Recent studies of factors contributing to α-syn-associated aggregation and misfolding have expanded our understanding of the PD disease process. In this Review, we summarize the structure and physiological function of α-syn, and we further highlight the major PTMs (namely phosphorylation, ubiquitination, nitration, acetylation, truncation, SUMOylation, and O-GlcNAcylation) of α-syn and the effects of these modifications on α-syn aggregation, which may elucidate mechanisms for PD pathogenesis and lay a theoretical foundation for clinical treatment of PD.

Discovery of a Novel Acetylcholinesterase Inhibitor by Fragment-Based Design and Virtual Screening

Despite extensive and intensive research efforts in recent decades, there is still no effective treatment for neurodegenerative diseases. On this background, the use of drugs inhibiting the enzyme acetylcholinesterase (AChE) remains an eternal evergreen in the symptomatic treatment of mild to moderate cognitive impairments. Even more, the cholinergic hypothesis, somewhat forgotten in recent years due to the shift in focus on amyloid cascade, is back to life, and the search for new, more effective AChE inhibitors continues. We generated a fragment-based library containing aromatic moieties and linkers originating from a set of novel AChE inhibitors. We used this library to design 1220 galantamine (GAL) derivatives following the model GAL (binding core) - linker (L) - aromatic fragment (Ar). The newly designed compounds were screened virtually for blood–brain barrier (BBB) permeability and binding to AChE. Among the top 10 best-scored compounds, a representative lead molecule was selected and tested for anti-AChE activity and neurotoxicity. It was found that the selected compound was a powerful non-toxic AChE inhibitor, 68 times more active than GAL, and could serve as a lead molecule for further optimization and development.

Novel targeted therapies for Parkinson's disease

Parkinson’s disease (PD) is the second more common neurodegenerative disease with increasing incidence worldwide associated to the population ageing. Despite increasing awareness and significant research advancements, treatment options comprise dopamine repleting, symptomatic therapies that have significantly increased quality of life and life expectancy, but no therapies that halt or reverse disease progression, which remain a great, unmet goal in PD research. Large biomarker development programs are undertaken to identify disease signatures that will improve patient selection and outcome measures in clinical trials. In this review, we summarize PD-related mechanisms that can serve as targets of therapeutic interventions aiming to slow or modify disease progression, as well as previous and ongoing clinical trials in each field, and discuss future perspectives.

Dystonia genes functionally converge in specific neurons and share neurobiology with psychiatric disorders

Dystonia is a neurological disorder characterized by sustained or intermittent muscle contractions causing abnormal movements and postures, often occurring in absence of any structural brain abnormality. Psychiatric comorbidities, including anxiety, depression, obsessive-compulsive disorder and schizophrenia, are frequent in patients with dystonia. While mutations in a fast-growing number of genes have been linked to Mendelian forms of dystonia, the cellular, anatomical, and molecular basis remains unknown for most genetic forms of dystonia, as does its genetic and biological relationship to neuropsychiatric disorders. Here we applied an unbiased systems-biology approach to explore the cellular specificity of all currently known dystonia-associated genes, predict their functional relationships, and test whether dystonia and neuropsychiatric disorders share a genetic relationship. To determine the cellular specificity of dystonia-associated genes in the brain, single-nuclear transcriptomic data derived from mouse brain was used together with expression-weighted cell-type enrichment. To identify functional relationships among dystonia-associated genes, we determined the enrichment of these genes in co-expression networks constructed from 10 human brain regions. Stratified linkage-disequilibrium score regression was used to test whether co-expression modules enriched for dystonia-associated genes significantly contribute to the heritability of anxiety, major depressive disorder, obsessive-compulsive disorder, schizophrenia, and Parkinson's disease. Dystonia-associated genes were significantly enriched in adult nigral dopaminergic neurons and striatal medium spiny neurons. Furthermore, 4 of 220 gene co-expression modules tested were significantly enriched for the dystonia-associated genes. The identified modules were derived from the substantia nigra, putamen, frontal cortex, and white matter, and were all significantly enriched for genes associated with synaptic function. Finally, we demonstrate significant enrichments of the heritability of major depressive disorder, obsessive-compulsive disorder and schizophrenia within the putamen and white matter modules, and a significant enrichment of the heritability of Parkinson's disease within the substantia nigra module. In conclusion, multiple dystonia-associated genes interact and contribute to pathogenesis likely through dysregulation of synaptic signalling in striatal medium spiny neurons, adult nigral dopaminergic neurons and frontal cortical neurons. Furthermore, the enrichment of the heritability of psychiatric disorders in the co-expression modules enriched for dystonia-associated genes indicates that psychiatric symptoms associated with dystonia are likely to be intrinsic to its pathophysiology.

α-Synuclein plasma membrane localization correlates with cellular phosphatidylinositol polyphosphate levels

The Parkinson's disease protein α-synuclein (αSyn) promotes membrane fusion and fission by interacting with various negatively charged phospholipids. Despite postulated roles in endocytosis and exocytosis, plasma membrane (PM) interactions of αSyn are poorly understood. Here, we show that phosphatidylinositol 4,5-bisphosphate (PIP₂) and phosphatidylinositol 3,4,5-trisphosphate (PIP₃), two highly acidic components of inner PM leaflets, mediate PM localization of endogenous pools of αSyn in A2780, HeLa, SK-MEL-2, and differentiated and undifferentiated neuronal SH-SY5Y cells. We demonstrate that αSyn binds to reconstituted PIP₂ membranes in a helical conformation in vitro and that PIP₂ synthesizing kinases and hydrolyzing phosphatases reversibly redistribute αSyn in cells. We further delineate that αSyn-PM targeting follows phosphoinositide-3 kinase (PI3K)-dependent changes of cellular PIP₂ and PIP₃ levels, which collectively suggests that phosphatidylinositol polyphosphates contribute to αSyn's function(s) at the plasma membrane.

Dementia with Lewy bodies-associated ß-synuclein mutations V70M and P123H cause mutation-specific neuropathological lesions

Beta (ß)-synuclein (ß-Syn) has long been considered to be an attenuator for the neuropathological effects caused by the Parkinson's disease-related alpha (α)-synuclein (α-Syn) protein. However, recent studies demonstrated that overabundant ß-Syn can form aggregates and induce neurodegeneration in central nervous system (CNS) neurons in vitro and in vivo, albeit at a slower pace as compared with α-Syn. Here, we demonstrate that ß-Syn mutants V70M, detected in a sporadic case of dementia with Lewy bodies (DLB), and P123H, detected in a familial case of DLB, robustly aggravate the neurotoxic potential of ß-Syn. Intriguingly, the two mutations trigger mutually exclusive pathways. ß-Syn V70M enhances morphological mitochondrial deterioration and degeneration of dopaminergic and non-dopaminergic neurons, but it has no influence on neuronal network activity. Conversely, ß-Syn P123H silences neuronal network activity, but it does not aggravate neurodegeneration. ß-Syn wild type (WT), V70M and P123H formed proteinase K-resistant intracellular fibrils within neurons, albeit with less stable C-termini as compared with α-Syn. Under cell-free conditions, ß-Syn V70M demonstrated a much slower pace of fibril formation as compared with WT ß-Syn, and P123H fibrils present with a unique phenotype characterized by large numbers of short, truncated fibrils. Thus, it is possible that V70M and P123H cause structural alterations in ß-Syn, which are linked to their distinct neuropathological profiles. The extent of the lesions caused by these neuropathological profiles is almost identical to that of overabundant α-Syn and is thus likely to be directly involved into the etiology of DLB. Overall, this study provides insights into distinct disease mechanisms caused by mutations of ß-Syn.

Effects of Excess Brain-Derived Human α-Synuclein on Synaptic Vesicle Trafficking

α-Synuclein is a presynaptic protein that regulates synaptic vesicle trafficking under physiological conditions. However, in several neurodegenerative diseases, including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, α-synuclein accumulates throughout the neuron, including at synapses, leading to altered synaptic function, neurotoxicity, and motor, cognitive, and autonomic dysfunction. Neurons typically contain both monomeric and multimeric forms of α-synuclein, and it is generally accepted that disrupting the balance between them promotes aggregation and neurotoxicity. However, it remains unclear how distinct molecular species of α-synuclein affect synapses where α-synuclein is normally expressed. Using the lamprey reticulospinal synapse model, we previously showed that acute introduction of excess recombinant monomeric or dimeric α-synuclein impaired distinct stages of clathrin-mediated synaptic vesicle endocytosis, leading to a loss of synaptic vesicles. Here, we expand this knowledge by investigating the effects of native, physiological α-synuclein isolated from the brain of a neuropathologically normal human subject, which comprised predominantly helically folded multimeric α-synuclein with a minor component of monomeric α-synuclein. After acute introduction of excess brain-derived human α-synuclein, there was a moderate reduction in the synaptic vesicle cluster and an increase in the number of large, atypical vesicles called "cisternae." In addition, brain-derived α-synuclein increased synaptic vesicle and cisternae sizes and induced atypical fusion/fission events at the active zone. In contrast to monomeric or dimeric α-synuclein, the brain-derived multimeric α-synuclein did not appear to alter clathrin-mediated synaptic vesicle endocytosis. Taken together, these data suggest that excess brain-derived human α-synuclein impairs intracellular vesicle trafficking and further corroborate the idea that different molecular species of α-synuclein produce distinct trafficking defects at synapses. These findings provide insights into the mechanisms by which excess α-synuclein contributes to synaptic deficits and disease phenotypes.

α-Synuclein Oligomers Induce Glutamate Release from Astrocytes and Excessive Extrasynaptic NMDAR Activity in Neurons, Thus Contributing to Synapse Loss

Synaptic and neuronal loss are major neuropathological characteristics of Parkinson's disease. Misfolded protein aggregates in the form of Lewy bodies, comprised mainly of α-synuclein (αSyn), are associated with disease progression, and have also been linked to other neurodegenerative diseases, including Lewy body dementia, Alzheimer's disease, and frontotemporal dementia. However, the effects of αSyn and its mechanism of synaptic damage remain incompletely understood. Here, we show that αSyn oligomers induce Ca²⁺-dependent release of glutamate from astrocytes obtained from male and female mice, and that mice overexpressing αSyn manifest increased tonic release of glutamate in vivo In turn, this extracellular glutamate activates glutamate receptors, including extrasynaptic NMDARs (eNMDARs), on neurons both in culture and in hippocampal slices of αSyn-overexpressing mice. Additionally, in patch-clamp recording from outside-out patches, we found that oligomerized αSyn can directly activate eNMDARs. In organotypic slices, oligomeric αSyn induces eNMDAR-mediated synaptic loss, which can be reversed by the drug NitroSynapsin. When we expose human induced pluripotent stem cell-derived cerebrocortical neurons to αSyn, we find similar effects. Importantly, the improved NMDAR antagonist NitroSynapsin, which selectively inhibits extrasynaptic over physiological synaptic NMDAR activity, protects synapses from oligomeric αSyn-induced damage in our model systems, thus meriting further study for its therapeutic potential.SIGNIFICANCE STATEMENT Loss of synaptic function and ensuing neuronal loss are associated with disease progression in Parkinson's disease (PD), Lewy body dementia (LBD), and other neurodegenerative diseases. However, the mechanism of synaptic damage remains incompletely understood. α-Synuclein (αSyn) misfolds in PD/LBD, forming Lewy bodies and contributing to disease pathogenesis. Here, we found that misfolded/oligomeric αSyn releases excessive astrocytic glutamate, in turn activating neuronal extrasynaptic NMDA receptors (eNMDARs), thereby contributing to synaptic damage. Additionally, αSyn oligomers directly activate eNMDARs, further contributing to damage. While the FDA-approved drug memantine has been reported to offer some benefit in PD/LBD (Hershey and Coleman-Jackson, 2019), we find that the improved eNMDAR antagonist NitroSynapsin ameliorates αSyn-induced synaptic spine loss, providing potential disease-modifying intervention in PD/LBD.

Synergistic Effects of Milk-Derived Exosomes and Galactose on α-Synuclein Pathology in Parkinson's Disease and Type 2 Diabetes Mellitus

Epidemiological studies associate milk consumption with an increased risk of Parkinson's disease (PD) and type 2 diabetes mellitus (T2D). PD is an α-synucleinopathy associated with mitochondrial dysfunction, oxidative stress, deficient lysosomal clearance of α-synuclein (α-syn) and aggregation of misfolded α-syn. In T2D, α-syn promotes co-aggregation with islet amyloid polypeptide in pancreatic β-cells. Prion-like vagal nerve-mediated propagation of exosomal α-syn from the gut to the brain and pancreatic islets apparently link both pathologies. Exosomes are critical transmitters of α-syn from cell to cell especially under conditions of compromised autophagy. This review provides translational evidence that milk exosomes (MEX) disturb α-syn homeostasis. MEX are taken up by intestinal epithelial cells and accumulate in the brain after oral administration to mice. The potential uptake of MEX miRNA-148a and miRNA-21 by enteroendocrine cells in the gut, dopaminergic neurons in substantia nigra and pancreatic β-cells may enhance miRNA-148a/DNMT1-dependent overexpression of α-syn and impair miRNA-148a/PPARGC1A- and miRNA-21/LAMP2A-dependent autophagy driving both diseases. MiRNA-148a- and galactose-induced mitochondrial oxidative stress activate c-Abl-mediated aggregation of α-syn which is exported by exosome release. Via the vagal nerve and/or systemic exosomes, toxic α-syn may spread to dopaminergic neurons and pancreatic β-cells linking the pathogenesis of PD and T2D.

Behavioral Deficits and Brain α-Synuclein and Phosphorylated Serine-129 α-Synuclein in Male and Female Mice Overexpressing Human α-Synuclein

BACKGROUND

Alpha-synuclein (α-syn) is involved in pathology of Parkinson's disease, and 90% of α-syn in Lewy bodies is phosphorylated at serine 129 (pS129 α-syn).

OBJECTIVE

To assess behavior impairments and brain levels of α-syn and pS129 α-syn in mice overexpressing human α-syn under Thy1 promoter (Thy1-α-syn) and wild type (wt) littermates.

METHODS

Motor and non-motor behaviors were monitored, brain human α-syn levels measured by ELISA, and α-syn and pS129 α-syn mapped by immunohistochemistry.

RESULTS

Male and female wt littermates did not show differences in the behavioral tests. Male Thy1-α-syn mice displayed more severe impairments than female counterparts in cotton nesting, pole tests, adhesive removal, finding buried food, and marble burying. Concentrations of human α-syn in the olfactory regions, cortex, nigrostriatal system, and dorsal medulla were significantly increased in Thy1-α-syn mice, higher in males than females. Immunoreactivity of α-syn was not simply increased in Thy1-α-syn mice but had altered localization in somas and fibers in a few brain areas. Abundant pS129 α-syn existed in many brain areas of Thy1-α-syn mice, while there was none or only a small amount in a few brain regions of wt mice. The substantia nigra, olfactory regions, amygdala, lateral parabrachial nucleus, and dorsal vagal complex displayed different distribution patterns between wt and transgenic mice, but not between sexes.

CONCLUSION

The severer abnormal behaviors in male than female Thy1-α-syn mice may be related to higher brain levels of human α-syn, in the absence of sex differences in the altered brain immunoreactivity patterns of α-syn and pS129 α-syn.

Pathologically Decreased CSF Levels of Synaptic Marker NPTX2 in DLB Are Correlated with Levels of Alpha-Synuclein and VGF

Background: Dementia with Lewy bodies (DLB) is a neurodegenerative disease where synaptic loss and reduced synaptic integrity are important neuropathological substrates. Neuronal Pentraxin 2(NPTX2) is a synaptic protein that drives the GABAergic inhibitory circuit. Our aim was to examine if NPTX2 cerebral spinal fluid (CSF) levels in DLB patients were altered and how these levels related to other synaptic protein levels and to cognitive function and decline. Methods: NPTX2, VGF, and α-synuclein levels were determined in CSF of cognitive healthy (n = 27), DLB (n = 48), and AD (n = 20) subjects. Multiple cognitive domains were tested, and data were compared using linear models. Results: Decreased NPTX2 levels were observed in DLB (median = 474) and AD (median = 453) compared to cognitive healthy subjects (median = 773). Strong correlations between NPTX2, VGF, and α-synuclein were observed dependent on diagnosis. Combined, these markers had a high differentiating power between DLB and cognitive healthy subjects (AUC = 0.944). Clinically, NPTX2 levels related to global cognitive function and cognitive decline in the visual spatial domain. Conclusion: NPTX2 CSF levels were reduced in DLB and closely correlated to decreased VGF and α-synuclein CSF levels. CSF NPTX2 levels in DLB related to decreased functioning in the visual spatial domain.

A dual role for α-synuclein in facilitation and depression of dopamine release from substantia nigra neurons in vivo

We report a long-sought& in vivo& physiological role for& α-synuclein (α-syn) in dopamine signaling. The results indicate that& α-syn is critical for activity-dependent dopamine plasticity, and that short repeated burst activity produces rapid presynaptic facilitation, while prolonged burst activity slowly depresses evoked dopamine release. We propose that the rapid facilitation is due to an enhanced fusion of synaptic vesicles at active zones during exocytosis while the depression is due to synaptic exhaustion. These results identify a& dynamic role of& α-syn, and are critical for defining& molecular mechanisms and therapeutic targets for various neurological disorders, where the firing properties of neurons are severely altered.

α-Synuclein is expressed at high levels at presynaptic terminals, but defining its role in the regulation of neurotransmission under physiologically relevant conditions has proven elusive. We report that, in vivo, α-synuclein is responsible for the facilitation of dopamine release triggered by action potential bursts separated by short intervals (seconds) and a depression of release with longer intervals between bursts (minutes). These forms of presynaptic plasticity appear to be independent of the presence of β- and γ-synucleins or effects on presynaptic calcium and are consistent with a role for synucleins in the enhancement of synaptic vesicle fusion and turnover. These results indicate that the presynaptic effects of α-synuclein depend on specific patterns of neuronal activity.

Protein Conformational Dynamics upon Association with the Surfaces of Lipid Membranes and Engineered Nanoparticles: Insights from Electron Paramagnetic Resonance Spectroscopy

Detailed study of conformational rearrangements and dynamics of proteins is central to our understanding of their physiological functions and the loss of function. This review outlines the applications of the electron paramagnetic resonance (EPR) technique to study the structural aspects of proteins transitioning from a solution environment to the states in which they are associated with the surfaces of biological membranes or engineered nanoobjects. In the former case these structural transitions generally underlie functional protein states. The latter case is mostly relevant to the application of protein immobilization in biotechnological industries, developing methods for protein purification, etc. Therefore, evaluating the stability of the protein functional state is particularly important. EPR spectroscopy in the form of continuous-wave EPR or pulse EPR distance measurements in conjunction with protein spin labeling provides highly versatile and sensitive tools to characterize the changes in protein local dynamics as well as large conformational rearrangements. The technique can be widely utilized in studies of both protein-membrane and engineered nanoobject-protein complexes.

Monitoring alpha-synuclein oligomerization and aggregation using bimolecular fluorescence complementation assays: What you see is not always what you get

Bimolecular fluorescence complementation (BiFC) was introduced a decade ago as a method to monitor alpha‐synuclein (α‐syn) oligomerization in intact cells. Since then, several α‐syn BiFC cellular assays and animal models have been developed based on the assumption that an increase in the fluorescent signal correlates with increased α‐syn oligomerization or aggregation. Despite the increasing use of these assays and models in mechanistic studies, target validation and drug screening, there have been no reports that (1) validate the extent to which the BiFC fluorescent signal correlates with α‐syn oligomerization at the biochemical level; (2) provide a structural characterization of the oligomers and aggregates formed by the BiFC. To address this knowledge gap, we first analysed the expression level and oligomerization properties of the individual constituents of α‐syn‐Venus, one of the most commonly used BiFC systems, in HEK‐293 & SH‐SY5Y cells from three different laboratories using multiple biochemical approaches and techniques. Next, we investigated the biochemical and aggregation properties of α‐syn upon co‐expression of both BiFC fragments. Our results show that (1) the C‐terminal‐Venus fused to α‐syn (α‐syn‐Vc) is present in much lower abundance than its counterpart with N‐terminal‐Venus fused to α‐syn (Vn‐α‐syn); (2) Vn‐α‐syn exhibits a high propensity to form oligomers and higher‐order aggregates; and (3) the expression of either or both fragments does not result in the formation of α‐syn fibrils or cellular inclusions. Furthermore, our results suggest that only a small fraction of Vn‐α‐syn is involved in the formation of the fluorescent BiFC complex and that some of the fluorescent signal may arise from the association or entrapment of α‐syn‐Vc in Vn‐α‐syn aggregates. The fact that the N‐terminal fragment exists predominantly in an aggregated state also indicates that one must exercise caution when using this system to investigate α‐syn oligomerization in cells or in vivo. Altogether, our results suggest that cellular and animal models of oligomerization, aggregation and cell‐to‐cell transmission based on the α‐syn BiFC systems should be thoroughly characterized at the biochemical level to ensure that they reproduce the process of interest and measure what they are intended to measure.

Disorders of synaptic vesicle fusion machinery

The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative disorders, revealing that the presynaptic machinery governing synaptic vesicle fusion is compromised in many of these neurological disorders. This builds upon decades of research showing that disturbance to neurotransmitter release via toxins can cause acute neurological dysfunction. In this review, we focus on disorders of synaptic vesicle fusion caused either by toxic insult to the presynapse or alterations to genes encoding the key proteins that control and regulate fusion: the SNARE proteins (synaptobrevin, syntaxin-1 and SNAP-25), Munc18, Munc13, synaptotagmin, complexin, CSPα, α-synuclein, PRRT2 and tomosyn. We discuss the roles of these proteins and the cellular and molecular mechanisms underpinning neurological deficits in these disorders.

The Pathology of Parkinson's Disease and Potential Benefit of Dietary Polyphenols

Parkinson's disease (PD) is a progressive neurodegenerative disorder that is characterized by a loss of dopaminergic neurons, leading to bradykinesia, rigidity, tremor at rest, and postural instability, as well as non-motor symptoms such as olfactory impairment, pain, autonomic dysfunction, impaired sleep, fatigue, and behavioral changes. The pathogenesis of PD is believed to involve oxidative stress, disruption to mitochondria, alterations to the protein α-synuclein, and neuroinflammatory processes. There is currently no cure for the disease. Polyphenols are secondary metabolites of plants, which have shown benefit in several experimental models of PD. Intake of polyphenols through diet is also associated with lower PD risk in humans. In this review, we provide an overview of the pathology of PD and the data supporting the potential neuroprotective capacity of increased polyphenols in the diet. Evidence suggests that the intake of dietary polyphenols may inhibit neurodegeneration and the progression of PD. Polyphenols appear to have a positive effect on the gut microbiome, which may decrease inflammation that contributes to the disease. Therefore, a diet rich in polyphenols may decrease the symptoms and increase quality of life in PD patients.

Proteasome Subunits Involved in Neurodegenerative Diseases

The ubiquitin-proteasome system is the major pathway for the maintenance of protein homeostasis. Its inhibition causes accumulation of ubiquitinated proteins; this accumulation has been associated with several of the most common neurodegenerative diseases. Several genetic factors have been identified for most neurodegenerative diseases, however, most cases are considered idiopathic, thus making the study of the mechanisms of protein accumulation a relevant field of research. It is often mentioned that the biggest risk factor for neurodegenerative diseases is aging, and several groups have reported an age-related alteration of the expression of some of the 26S proteasome subunits and a reduction of its activity. Proteasome subunits interact with proteins that are known to accumulate in neurodegenerative diseases such as α-synuclein in Parkinson's, tau in Alzheimer's, and huntingtin in Huntington's diseases. These interactions have been explored for several years, but only until recently, we are beginning to understand them. In this review, we discuss the known interactions, the underlying patterns, and the phenotypes associated with the 26S proteasome subunits in the etiology and progression of neurodegenerative diseases where there is evidence of proteasome involvement. Special emphasis is made in reviewing proteasome subunits that interact with proteins known to have an age-related altered expression or to be involved in neurodegenerative diseases to explore key effectors that may trigger or augment their progression. Interestingly, while the causes of age-related reduction of some of the proteasome subunits are not known, there are specific relationships between the observed neurodegenerative disease and the affected proteasome subunits.

The Role of Lipids in the Initiation of α-Synuclein Misfolding

The aggregation of α-synuclein (α-syn) is inseparably connected to Parkinson’s disease (PD). It is now well-established that certain forms of α-syn aggregates, oligomers and fibrils, can exert neurotoxicity in synucleinopathies. With the exception of rare familial forms, the vast majority of PD cases are idiopathic. Understanding the earliest molecular mechanisms that cause initial α-syn misfolding could help to explain why PD affects only some individuals and others not. Factors that chaperone the transition of α-syn’s physiological to pathological function are of particular interest, since they offer opportunities for intervention. The relationship between α-syn and lipids represents one of those factors. Membrane interaction is crucial for normal cellular function, but lipids also induce the aggregation of α-syn, causing cell toxicity. Also, disease-causing or risk-factor mutations in genes related to lipid metabolism like PLA2G6, SCARB2 or GBA1 highlight the close connection between PD and lipids. Despite the clear link, the ambivalent interaction has not been studied sufficiently so far. In this review, we address how α-syn interacts with lipids and how they can act as key factor for orchestrating toxic conversion of α-syn. Furthermore, we will discuss a scenario in which initial α-syn aggregation is determined by shifts in lipid/α-syn ratio as well as by dyshomeostasis of membrane bound/unbound state of α-syn.

Curcumin Inhibits the Primary Nucleation of Amyloid-Beta Peptide: A Molecular Dynamics Study

The amyloid plaques are a key hallmark of neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease. Amyloidogenesis is a complex long-lasting multiphase process starting with the formation of nuclei of amyloid peptides: a process assigned as a primary nucleation. Curcumin (CU) is a well-known inhibitor of the aggregation of amyloid-beta (Aβ) peptides. Even more, CU is able to disintegrate preformed Aβ firbils and amyloid plaques. Here, we simulate by molecular dynamics the primary nucleation process of 12 Aβ peptides and investigate the effects of CU on the process. We found that CU molecules intercalate among the Aβ chains and bind tightly to them by hydrogen bonds, hydrophobic, π–π, and cation–π interactions. In the presence of CU, the Aβ peptides form a primary nucleus of a bigger size. The peptide chains in the nucleus become less flexible and more disordered, and the number of non-native contacts and hydrogen bonds between them decreases. For comparison, the effects of the weaker Aβ inhibitor ferulic acid (FA) on the primary nucleation are also examined. Our study is in good agreement with the observation that taken regularly, CU is able to prevent or at least delay the onset of neurodegenerative disorders.

A Critical LRRK at the Synapse? The Neurobiological Function and Pathophysiological Dysfunction of LRRK2

Since the discovery of LRRK2 mutations causal to Parkinson's disease (PD) in the early 2000s, the LRRK2 protein has been implicated in a plethora of cellular processes in which pathogenesis could occur, yet its physiological function remains elusive. The development of genetic models of LRRK2 PD has helped identify the etiological and pathophysiological underpinnings of the disease, and may identify early points of intervention. An important role for LRRK2 in synaptic function has emerged in recent years, which links LRRK2 to other genetic forms of PD, most notably those caused by mutations in the synaptic protein α-synuclein. This point of convergence may provide useful clues as to what drives dysfunction in the basal ganglia circuitry and eventual death of substantia nigra (SN) neurons. Here, we discuss the evolution and current state of the literature placing LRRK2 at the synapse, through the lens of knock-out, overexpression, and knock-in animal models. We hope that a deeper understanding of LRRK2 neurobiology, at the synapse and beyond, will aid the eventual development of neuroprotective interventions for PD, and the advancement of useful treatments in the interim.

The Role of Alpha-Synuclein and Other Parkinson's Genes in Neurodevelopmental and Neurodegenerative Disorders

Neurodevelopmental and late-onset neurodegenerative disorders present as separate entities that are clinically and neuropathologically quite distinct. However, recent evidence has highlighted surprising commonalities and converging features at the clinical, genomic, and molecular level between these two disease spectra. This is particularly striking in the context of autism spectrum disorder (ASD) and Parkinson's disease (PD). Genetic causes and risk factors play a central role in disease pathophysiology and enable the identification of overlapping mechanisms and pathways. Here, we focus on clinico-genetic studies of causal variants and overlapping clinical and cellular features of ASD and PD. Several genes and genomic regions were selected for our review, including SNCA (alpha-synuclein), PARK2 (parkin RBR E3 ubiquitin protein ligase), chromosome 22q11 deletion/DiGeorge region, and FMR1 (fragile X mental retardation 1) repeat expansion, which influence the development of both ASD and PD, with converging features related to synaptic function and neurogenesis. Both PD and ASD display alterations and impairments at the synaptic level, representing early and key disease phenotypes, which support the hypothesis of converging mechanisms between the two types of diseases. Therefore, understanding the underlying molecular mechanisms might inform on common targets and therapeutic approaches. We propose to re-conceptualize how we understand these disorders and provide a new angle into disease targets and mechanisms linking neurodevelopmental disorders and neurodegeneration.

Alterations in the nigrostriatal system following conditional inactivation of α-synuclein in neurons of adult and aging mice

The etiology and pathogenesis of Parkinson's disease (PD) are tightly linked to the gain-of-function of α-synuclein. However, gradual accumulation of α-synuclein aggregates in dopaminergic neurons of substantia nigra pars compacta (SNpc) leads to the depletion of the functional pool of soluble α-synuclein, and therefore, creates loss-of-function conditions, particularly in presynaptic terminals of these neurons. Studies of how this late-onset depletion of a protein involved in many important steps of neurotransmission contributes to PD progression and particularly, to worsening the nigrostriatal pathology at late stages of the disease are limited and obtained data, are controversial. Recently, we produced a mouse line for conditional knockout of the gene encoding α-synuclein, and here we used its tamoxifen-inducible pan-neuronal inactivation to study consequences of the adult-onset (from the age of 6 months) and late-onset (from the age of 12 months) α-synuclein depletion to the nigrostriatal system. No significant changes of animal balance/coordination, the number of dopaminergic neurons in the SNpc and the content of dopamine and its metabolites in the striatum were observed after adult-onset α-synuclein depletion, but in aging (18-month-old) late-onset depleted mice we found a significant reduction of major dopamine metabolites without changes to the content of dopamine itself. Our data suggest that this might be caused, at least partially, by reduced expression of aldehyde dehydrogenase ALDH1a1 and could lead to the accumulation of toxic intermediates of dopamine catabolism. By extrapolating our findings to a potential clinical situation, we suggest that therapeutic downregulation of α-synuclein expression in PD patients is a generally safe option as it should not cause adverse side effects on the functionality of their nigrostriatal system. However, if started in aged patients, this type of therapy might trigger slight functional changes of the nigrostriatal system with potentially unwanted additive effect to already existing pathology.

Translation Imaging in Parkinson's Disease: Focus on Neuroinflammation

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and the appearance of α-synuclein insoluble aggregates known as Lewy bodies. Neurodegeneration is accompanied by neuroinflammation mediated by cytokines and chemokines produced by the activated microglia. Several studies demonstrated that such an inflammatory process is an early event, and contributes to oxidative stress and mitochondrial dysfunctions. α-synuclein fibrillization and aggregation activate microglia and contribute to disease onset and progression. Mutations in different genes exacerbate the inflammatory phenotype in the monogenic compared to sporadic forms of PD. Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) with selected radiopharmaceuticals allow in vivo imaging of molecular modifications in the brain of living subjects. Several publications showed a reduction of dopaminergic terminals and dopamine (DA) content in the basal ganglia, starting from the early stages of the disease. Moreover, non-dopaminergic neuronal pathways are also affected, as shown by in vivo studies with serotonergic and glutamatergic radiotracers. The role played by the immune system during illness progression could be investigated with PET ligands that target the microglia/macrophage Translocator protein (TSPO) receptor. These agents have been used in PD patients and rodent models, although often without attempting correlations with other molecular or functional parameters. For example, neurodegeneration and brain plasticity can be monitored using the metabolic marker 2-Deoxy-2-[¹⁸F]fluoroglucose ([¹⁸F]-FDG), while oxidative stress can be probed using the copper-labeled diacetyl-bis(N-methyl-thiosemicarbazone) ([Cu]-ATSM) radioligand, whose striatal-specific binding ratio in PD patients seems to correlate with a disease rating scale and motor scores. Also, structural and functional modifications during disease progression may be evaluated by Magnetic Resonance Imaging (MRI), using different parameters as iron content or cerebral volume. In this review article, we propose an overview of in vivo clinical and non-clinical imaging research on neuroinflammation as an emerging marker of early PD. We also discuss how multimodal-imaging approaches could provide more insights into the role of the inflammatory process and related events in PD development.

α-Synuclein Regulates Iron Homeostasis via Preventing Parkin-Mediated DMT1 Ubiquitylation in Parkinson's Disease Models

Iron metabolism imbalance plays a key role in the neurodegeneration of Parkinson's disease (PD), thus iron homeostasis should be tightly controlled by iron transporters. α-Synuclein (α-Syn) serves as a ferrireductase and iron-binding protein, which is supposed to be linked with iron metabolism, but little is known about how α-Syn affects iron homeostasis in PD. Our previous findings that up-regulation of divalent metal transporter 1 (DMT1) accounted for the nigral iron accumulation in PD raised the question whether α-Syn disturbed iron homeostasis by modulating DMT1 expression. Using α-Syn overexpressed SH-SY5Y cells and mutant human A53T α-Syn transgenic mice, we found that α-Syn could up-regulate DMT1 protein levels, followed by enhanced ferrous iron influx and subsequent aggravated oxidative stress injury. Mechanistic studies identified that α-Syn-induced p38 mitogen-activated protein kinase (MAPK) activation phosphorylated parkin at Ser131, which inactivated parkin's E3 ubiquitin ligase activity and further reduced DMT1 ubiquitylation level. Our findings revealed that α-Syn affected brain iron homeostasis through modulating DMT1 protein stability and altering cellular iron uptake, which might provide direct evidence for the involvement of α-Syn in iron metabolism dysfunction and provide insight into PD-associated nigral iron deposition.

α-Synuclein-112 Impairs Synaptic Vesicle Recycling Consistent With Its Enhanced Membrane Binding Properties

Synucleinopathies are neurological disorders associated with α-synuclein overexpression and aggregation. While it is well-established that overexpression of wild type α-synuclein (α-syn-140) leads to cellular toxicity and neurodegeneration, much less is known about other naturally occurring α-synuclein splice isoforms. In this study we provide the first detailed examination of the synaptic effects caused by one of these splice isoforms, α-synuclein-112 (α-syn-112). α-Syn-112 is produced by an in-frame excision of exon 5, resulting in deletion of amino acids 103-130 in the C-terminal region. α-Syn-112 is upregulated in the substantia nigra, frontal cortex, and cerebellum of parkinsonian brains and higher expression levels are correlated with susceptibility to Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple systems atrophy (MSA). We report here that α-syn-112 binds strongly to anionic phospholipids when presented in highly curved liposomes, similar to α-syn-140. However, α-syn-112 bound significantly stronger to all phospholipids tested, including the phosphoinositides. α-Syn-112 also dimerized and trimerized on isolated synaptic membranes, while α-syn-140 remained largely monomeric. When introduced acutely to lamprey synapses, α-syn-112 robustly inhibited synaptic vesicle recycling. Interestingly, α-syn-112 produced effects on the plasma membrane and clathrin-mediated synaptic vesicle endocytosis that were phenotypically intermediate between those caused by monomeric and dimeric α-syn-140. These findings indicate that α-syn-112 exhibits enhanced phospholipid binding and oligomerization in vitro and consequently interferes with synaptic vesicle recycling in vivo in ways that are consistent with its biochemical properties. This study provides additional evidence suggesting that impaired vesicle endocytosis is a cellular target of excess α-synuclein and advances our understanding of potential mechanisms underlying disease pathogenesis in the synucleinopathies.

Reduced complement of dopaminergic neurons in the substantia nigra pars compacta of mice with a constitutive "low footprint" genetic knockout of alpha-synuclein

Previous studies of the alpha-synuclein null mutant mice on the C57Bl6 genetic background have revealed reduced number of dopaminergic neurons in their substantia nigra pars compacta (SNpc). However, the presence in genomes of the studied mouse lines of additional genetic modifications that affect expression of genes located in a close proximity to the alpha-synuclein-encoding Snca gene makes these data open to various interpretations. To unambiguously demonstrate that the absence of alpha-synuclein is the primary cause of the observed deficit of dopaminergic neurons, we employed a recently produced constituent alpha-synuclein knockout mouse line B6(Cg)-Snca^tm1.2Vlb/J. The only modification introduced to the genome of these mice is a substitution of the first coding exon and adjusted short intronic fragments of the Snca gene by a single loxP site. We compared the number of dopaminergic neurons in the SNpc of this line, previously studied B6(Cg)-Snca^tm1Rosl/J line and wild type littermate mice. A similar decrease was observed in both knockout lines when compared with wild type mice. In a recently published study we revealed no loss of dopaminergic neurons following conditional inactivation of the Snca gene in neurons of adult mice. Taken together, these results strongly suggest that alpha-synuclein is required for efficient survival or maturation of dopaminergic neurons in the developing SNpc but is dispensable for survival of mature SNpc dopaminergic neurons.

Alpha-Synuclein and LRRK2 in Synaptic Autophagy: Linking Early Dysfunction to Late-Stage Pathology in Parkinson's Disease

The lack of effective disease-modifying strategies is the major unmet clinical need in Parkinson’s disease. Several experimental approaches have attempted to validate cellular targets and processes. Of these, autophagy has received considerable attention in the last 20 years due to its involvement in the clearance of pathologic protein aggregates and maintenance of neuronal homeostasis. However, this strategy mainly addresses a very late stage of the disease, when neuropathology and neurodegeneration have likely “tipped over the edge” and disease modification is extremely difficult. Very recently, autophagy has been demonstrated to modulate synaptic activity, a process distinct from its catabolic function. Abnormalities in synaptic transmission are an early event in neurodegeneration with Leucine-Rich Repeat Kinase 2 (LRRK2) and alpha-synuclein strongly implicated. In this review, we analyzed these processes separately and then discussed the unification of these biomolecular fields with the aim of reconstructing a potential “molecular timeline” of disease onset and progression. We postulate that the elucidation of these pathogenic mechanisms will form a critical basis for the design of novel, effective disease-modifying therapies that could be applied early in the disease process.

Effect of Parkin on methamphetamine-induced α-synuclein degradation dysfunction in vitro and in vivo

INTRODUCTION

Methamphetamine (METH) is a psychostimulant drug with complicated neurotoxicity, and abuse of METH is very common. Studies have shown that METH exposure causes alpha-synuclein (α-syn) accumulation. However, the mechanism of α-syn accumulation has not been determined.

METHODS

In this study, we established cell and animal models of METH intoxication to evaluate how METH affects α-syn expression. In addition, to explore METH-induced neurotoxicity, we measured the level of Parkin and the phosphorylation levels of α-syn, Polo-like kinase 2 (PLK2), the proteasome activity marker CD3δ, and the apoptosis-related proteins Caspase-3 and PARP. Parkin is a key enzyme in the ubiquitin-proteasome system. In addition, the effect of Parkin on METH-induced neurotoxicity was investigated by overexpressing it in vitro and in vivo.

RESULTS

METH exposure increased polyubiquitin and α-syn expression, as did MG132. Furthermore, the level of Parkin and the interaction between Parkin and α-syn decreased after METH exposure. Importantly, the increases in α-syn expression and neurotoxicity were relieved by Parkin overexpression.

CONCLUSIONS

By establishing stable cell lines and animal models that overexpress Parkin, we confirmed Parkin as an important factor in METH-induced α-syn degradation dysfunction in vitro and in vivo. Parkin may be a promising target for the treatment of METH-induced neurotoxicity.

Role of Alterations in Protein Kinase p38γ in the Pathogenesis of the Synaptic Pathology in Dementia With Lewy Bodies and α-Synuclein Transgenic Models

Progressive accumulation of the pre-synaptic protein α-synuclein (α-syn) has been strongly associated with the pathogenesis of neurodegenerative disorders of the aging population such as Alzheimer's disease (AD), Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy. While the precise mechanisms are not fully understood, alterations in kinase pathways including that of mitogen activated protein kinase (MAPK) p38 have been proposed to play a role. In AD, p38α activation has been linked to neuro-inflammation while alterations in p38γ have been associated with tau phosphorylation. Although p38 has been studied in AD, less is known about its role in DLB/PD and other α-synucleinopathies. For this purpose, we investigated the expression of the p38 family in brains from α-syn overexpressing transgenic mice (α-syn Tg: Line 61) and patients with DLB/PD. Immunohistochemical analysis revealed that in healthy human controls and non-Tg mice, p38α associated with neurons and astroglial cells and p38γ localized to pre-synaptic terminals. In DLB and α-syn Tg brains, however, p38α levels were increased in astroglial cells while p38γ immunostaining was redistributed from the synaptic terminals to the neuronal cell bodies. Double immunolabeling further showed that p38γ colocalized with α-syn aggregates in DLB patients, and immunoblot and qPCR analysis confirmed the increased levels of p38α and p38γ. α1-syntrophin, a synaptic target of p38γ, was present in the neuropil and some neuronal cell bodies in human controls and non-Tg mice. In DLB and and Tg mice, however, α1-syntrophin was decreased in the neuropil and instead colocalized with α-syn in intra-neuronal inclusions. In agreement with these findings, in vitro studies showed that α-syn co-immunoprecipitates with p38γ, but not p38α. These results suggest that α-syn might interfere with the p38γ pathway and play a role in the mechanisms of synaptic dysfunction in DLB/PD.

Nuclear Factor-κB Dysregulation and α-Synuclein Pathology: Critical Interplay in the Pathogenesis of Parkinson's Disease

The loss of dopaminergic neurons of the nigrostriatal system underlies the onset of the typical motor symptoms of Parkinson's disease (PD). Lewy bodies (LB) and Lewy neurites (LN), proteinaceous inclusions mainly composed of insoluble α-synuclein (α-syn) fibrils are key neuropathological hallmarks of the brain of affected patients. Compelling evidence supports that in the early prodromal phases of PD, synaptic terminal and axonal alterations initiate and drive a retrograde degeneration process culminating with the loss of nigral dopaminergic neurons. This notwithstanding, the molecular triggers remain to be fully elucidated. Although it has been shown that α-syn fibrillary aggregation can induce early synaptic and axonal impairment and cause nigrostriatal degeneration, we still ignore how and why α-syn fibrillation begins. Nuclear factor-κB (NF-κB) transcription factors, key regulators of inflammation and apoptosis, are involved in the brain programming of systemic aging as well as in the pathogenesis of several neurodegenerative diseases. The NF-κB family of factors consists of five different subunits (c-Rel, p65/RelA, p50, RelB, and p52), which combine to form transcriptionally active dimers. Different findings point out a role of RelA in PD. Interestingly, the nuclear content of RelA is abnormally increased in nigral dopamine (DA) neurons and glial cells of PD patients. Inhibition of RelA exert neuroprotection against (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) MPTP and 1-methyl-4-phenylpyridinium (MPP+) toxicity, suggesting that this factor decreases neuronal resilience. Conversely, the c-Rel subunit can exert neuroprotective actions. We recently described that mice deficient for c-Rel develop a PD-like motor and non-motor phenotype characterized by progressive brain α-syn accumulation and early synaptic changes preceding the frank loss of nigrostriatal neurons. This evidence supports that dysregulations in this transcription factors may be involved in the onset of PD. This review highlights observations supporting a possible interplay between NF-κB dysregulation and α-syn pathology in PD, with the aim to disclose novel potential mechanisms involved in the pathogenesis of this disorder.

Synuclein Meeting 2019: where we are and where we need to go

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 is taking place in Ofir, a city in the outskirts of Porto, Portugal. The meeting is entitled 'Synuclein Meeting 2019: Where we are and where we need to go'. It has now been 22 years since the initial report of the genetic and pathological association between alpha-synuclein and Parkinson's disease (PD). The field has grown and matured, and major advances have been made. We are witnessing exciting times, with the first clinical trials being conducted that target synuclein, and bring the hope of novel therapies for patients with PD and their families. However, we still face many challenges and need to address fundamental questions for the field to progress to where we need to go: having biomarkers and effective therapies for PD and other synucleinopathies. In this context, we have designed the Synuclein Meeting 2019 with a different format. The program will include sessions in the format of a round-table discussion, to break away from the more rigid format of regular scientific meetings based on oral presentations. Our goal was to create opportunities for discussing the major questions in the field of synuclein and related human disorders, and challenge dogmatic ideas that require a critical revision in light of the most recent knowledge. In this issue, we assembled a series of comprehensive overviews of major topics, questions, and challenges in the field, that will be discussed in the meeting. We are confident that this special issue will be an instrumental reference for inspiring novel paths for future discoveries in the synuclein field and generate other discussions in the scientific community. This is the Preface for the Special Issue "Synuclein". Cover Image for this issue: doi: 10.1111/jnc.14520.

SUMOylation in development and neurodegeneration

In essentially all eukaryotes, proteins can be modified by the attachment of small ubiquitin-related modifier (SUMO) proteins to lysine side chains to produce branched proteins. This process of 'SUMOylation' plays essential roles in plant and animal development by altering protein function in spatially and temporally controlled ways. In this Primer, we explain the process of SUMOylation and summarize how SUMOylation regulates a number of signal transduction pathways. Next, we discuss multiple roles of SUMOylation in the epigenetic control of transcription. In addition, we evaluate the role of SUMOylation in the etiology of neurodegenerative disorders, focusing on Parkinson's disease and cerebral ischemia. Finally, we discuss the possibility that SUMOylation may stimulate survival and neurogenesis of neuronal stem cells.

Disentangling aggregation-prone proteins: a new method for isolating α-synuclein species: An Editorial Highlight for "A simple, versatile and robust centrifugation-based filtration protocol for the isolation and quantification of α-synuclein monomers, oligomers and fibrils: Towards improving experimental reproducibility in α-synuclein research" on page 103

Protein aggregation plays a central role in numerous neurodegenerative diseases. The key proteins in these diseases are of significant importance, but their investigation can be challenging due to unique properties of protein misfolding and oligomerization. Alpha-synuclein protein (α-Syn) is the predominant component of Lewy Bodies in Parkinson's disease (PD) and is a member of this class of proteins. Many α-Syn studies are limited by the inability to separate various monomeric, oligomeric, and fibrillar forms of the protein from heterogeneous mixtures. This Editorial Highlight summarizes the impact of a study published in the current issue of Journal of Neurochemistry, in which Lashuel and colleagues developed a simple, rapid centrifugation- and filter-based method for separating, isolating, and quantifying different forms of α-Syn. The researchers used electron microscopy, SDS-PAGE, circular dichroism, and protein assays to carefully validate the method and quantitate α-Syn yields and loss. The publication of this new method will not only aid in future studies of α-Syn, but will likely extend to other proteins that underlie a variety of neurodegenerative diseases.

Hsc70 Ameliorates the Vesicle Recycling Defects Caused by Excess α-Synuclein at Synapses

α-Synuclein overexpression and aggregation are linked to Parkinson’s disease (PD), dementia with Lewy bodies (DLB), and several other neurodegenerative disorders. In addition to effects in the cell body, α-synuclein accumulation occurs at presynapses where the protein is normally localized. While it is generally agreed that excess α-synuclein impairs synaptic vesicle trafficking, the underlying mechanisms are unknown.

α-Synuclein overexpression and aggregation are linked to Parkinson’s disease (PD), dementia with Lewy bodies (DLB), and several other neurodegenerative disorders. In addition to effects in the cell body, α-synuclein accumulation occurs at presynapses where the protein is normally localized. While it is generally agreed that excess α-synuclein impairs synaptic vesicle trafficking, the underlying mechanisms are unknown. We show here that acute introduction of excess human α-synuclein at a classic vertebrate synapse, the lamprey reticulospinal (RS) synapse, selectively impaired the uncoating of clathrin-coated vesicles (CCVs) during synaptic vesicle recycling, leading to an increase in endocytic intermediates and a severe depletion of synaptic vesicles. Furthermore, human α-synuclein and lamprey γ-synuclein both interact in vitro with Hsc70, the chaperone protein that uncoats CCVs at synapses. After introducing excess α-synuclein, Hsc70 availability was reduced at stimulated synapses, suggesting Hsc70 sequestration as a possible mechanism underlying the synaptic vesicle trafficking defects. In support of this hypothesis, increasing the levels of exogenous Hsc70 along with α-synuclein ameliorated the CCV uncoating and vesicle recycling defects. These experiments identify a reduction in Hsc70 availability at synapses, and consequently its function, as the mechanism by which α-synuclein induces synaptic vesicle recycling defects. To our knowledge, this is the first report of a viable chaperone-based strategy for reversing the synaptic vesicle trafficking defects associated with excess α-synuclein, which may be of value for improving synaptic function in PD and other synuclein-linked diseases.

Neuron-autonomous susceptibility to induced synuclein aggregation is exacerbated by endogenous Lrrk2 mutations and ameliorated by Lrrk2 genetic knock-out

Neuronal aggregates containing α-synuclein are a pathological hallmark of several degenerative diseases; including Parkinson’s disease, Parkinson’s disease with dementia and dementia with Lewy bodies. Understanding the process of α-synuclein aggregation, and discovering means of preventing it, may help guide therapeutic strategy and drug design. Recent advances provide tools to induce α-synuclein aggregation in neuronal cultures. Application of exogenous pre-formed fibrillar α-synuclein induces pathological phosphorylation and accumulation of endogenous α-synuclein, typical of that seen in disease. Genomic variability and mutations in α-synuclein and leucine-rich repeat kinase 2 proteins are the major genetic risk factors for Parkinson’s disease. Reports demonstrate fibril-induced α-synuclein aggregation is increased in cells from leucine-rich repeat kinase 2 pathogenic mutant (G2019S) overexpressing mice, and variously decreased by leucine-rich repeat kinase 2 inhibitors. Elsewhere in vivo antisense knock-down of leucine-rich repeat kinase 2 protein has been shown to protect mice from fibril-induced α-synuclein aggregation, whereas kinase inhibition did not. To help bring clarity to this issue, we took a purely genetic approach in a standardized neuron-enriched culture, lacking glia. We compared fibril treatment of leucine-rich repeat kinase 2 germ-line knock-out, and G2019S germ-line knock-in, mouse cortical neuron cultures with those from littermates. We found leucine-rich repeat kinase 2 knock-out neurons are resistant to α-synuclein aggregation, which predominantly forms within axons, and may cause axonal fragmentation. Conversely, leucine-rich repeat kinase 2 knock-in neurons are more vulnerable to fibril-induced α-synuclein accumulation. Protection and resistance correlated with basal increases in a lysosome marker in knock-out, and an autophagy marker in knock-in cultures. The data add to a growing number of studies that argue leucine-rich repeat kinase 2 silencing, and potentially kinase inhibition, may be a useful therapeutic strategy against synucleinopathy.

Parkinson's disease: proteinopathy or lipidopathy?

Lipids play a more significant role in Parkinson’s disease and its related brain disorders than is currently recognized, supporting a “lipid cascade”. The 14 kDa protein α-synuclein (αS) is strongly associated with Parkinson’s disease (PD), dementia with Lewy bodies (DLB), other synucleinopathies such as multiple system atrophy, and even certain forms of Alzheimer’s disease. Rigorously deciphering the biochemistry of αS in native systems is the key to developing treatments. αS is highly expressed in the brain, the second most lipid-rich organ, and has been proposed to be a lipid-binding protein that physiologically interacts with phospholipids and fatty acids (FAs). αS-rich cytoplasmic inclusions called Lewy bodies and Lewy neurites are the hallmark lesions of synucleinopathies. Excess αS–membrane interactions may trigger proteinaceous αS aggregation by stimulating its primary nucleation. However, αS may also exert its toxicity prior to or independent of its self-aggregation, e.g., via excessive membrane interactions, which may be promoted by certain lipids and FAs. A complex αS-lipid landscape exists, which comprises both physiological and pathological states of αS. As novel insights about the composition of Lewy lesions occur, new lipid-related PD drug candidates emerge, and genome-wide association studies (GWAS) increasingly validate new hits in lipid-associated pathways, it seems timely to review our current knowledge of lipids in PD and consider the roles for these pathways in synucleinopathies.Fig. 1

Association between SNCA rs356220 polymorphism and Parkinson's disease: A meta-analysis

Several studies have investigated the correlation between single nucleotide polymorphism (SNP) rs356220 in the α-synuclein (SNCA) gene and Parkinson's disease (PD) with inconsistent results. Herein, a meta-analysis was conducted to ascertain the association of the SNCA rs356220 polymorphism with the risk of PD. Six eligible articles involving 5333 PD cases and 5477 controls were included in this meta-analysis. The pooled odds ratios (OR) and 95 % confidence interval (CI) were calculated to estimate the association. The fixed or random effect was selected based on the homogeneity among studies. Heterogeneity was detected by I². We performed sensitivity analysis to test the stablility of the results. Publication bias was evaluated by Funnel plot and Begg's test. The pooled results showed a significant association between SNCA rs356220 gene polymorphism and PD susceptibility in the codominant (FEM: OR = 1.31, 95 % CI = 1.24-1.39), dominant (FEM: OR = 1.38, 95 % CI = 1.27-1.49) and recessive (FEM: OR = 1.52, 95 % CI = 1.38-1.68) models. Furthermore, in the subgroup analysis stratified by ethnicity, increased risk of PD was identified in both Caucasian and Asian populations. Overall, the present meta-analysis provided evidence supporting that SNCA rs356220 polymorphism might act as a genetic susceptibility factor for PD.

Innate and adaptive immune responses in Parkinson's disease

Parkinson's disease (PD) has classically been defined as a movement disorder, in which motor symptoms are explained by the aggregation of alpha-synuclein (α-syn) and subsequent death of dopaminergic neurons of the substantia nigra pars compacta (SNpc). More recently, the multisystem effects of the disease have been investigated, with the immune system being implicated in a number of these processes in the brain, the blood, and the gut. In this review, we highlight the dysfunctional immune system found in both human PD and animal models of the disease, and discuss how genetic risk factors and risk modifiers are associated with pro-inflammatory immune responses. Finally, we emphasize evidence that the immune response drives the pathogenesis and progression of PD, and discuss key questions that remain to be investigated in order to identify immunomodulatory therapies in PD.

Heterogeneity of dopamine release sites in health and degeneration

Despite comprising only ~ 0.001% of all neurons in the human brain, ventral midbrain dopamine neurons exert a profound influence on human behavior and cognition. As a neuromodulator, dopamine selectively inhibits or enhances synaptic signaling to coordinate neural output for action, attention, and affect. Humans invariably lose brain dopamine during aging, and this can be exacerbated in disease states such as Parkinson's Disease. Further, it is well established in multiple disease states that cell loss is selective for a subset of highly sensitive neurons within the nigrostriatal dopamine tract. Regional differences in dopamine tone are regulated pre-synaptically, with subcircuits of projecting dopamine neurons exhibiting distinct molecular and physiological signatures. Specifically, proteins at dopamine release sites that synthesize and package cytosolic dopamine, modulate its release and reuptake, and alter neuronal excitability show regional differences that provide linkages to the observed sensitivity to neurodegeneration. The aim of this review is to outline the major components of dopamine homeostasis at neurotransmitter release sites and describe the regional differences most relevant to understanding why some, but not all, dopamine neurons exhibit heightened vulnerability to neurodegeneration.

Intracellular and Intercellular Mitochondrial Dynamics in Parkinson's Disease

The appearance of alpha-synuclein-positive inclusion bodies (Lewy bodies) and the loss of catecholaminergic neurons are the primary pathological hallmarks of Parkinson's disease (PD). However, the dysfunction of mitochondria has long been recognized as a key component in the progression of the disease. Dysfunctional mitochondria can in turn lead to dysregulation of calcium homeostasis and, especially in dopaminergic neurons, raised mean intracellular calcium concentration. As calcium binding to alpha-synuclein is one of the important triggers of alpha-synuclein aggregation, mitochondrial dysfunction will promote inclusion body formation and disease progression. Increased reactive oxygen species (ROS) resulting from inefficiencies in the electron transport chain also contribute to the formation of alpha-synuclein aggregates and neuronal loss. Recent studies have also highlighted defects in mitochondrial clearance that lead to the accumulation of depolarized mitochondria. Transaxonal and intracytoplasmic translocation of mitochondria along the microtubule cytoskeleton may also be affected in diseased neurons. Furthermore, nanotube-mediated intercellular transfer of mitochondria has recently been reported between different cell types and may have relevance to the spread of PD pathology between adjacent brain regions. In the current review, the contributions of both intracellular and intercellular mitochondrial dynamics to the etiology of PD will be discussed.