Induction of alpha-synuclein aggregation by intracellular nitrative insult

Initiation and propagation of α-synuclein aggregation in the nervous system

The two main pathological hallmarks of Parkinson’s disease are loss of dopamine neurons in the substantia nigra pars compacta and proteinaceous amyloid fibrils composed mostly of α-synuclein, called Lewy pathology. Levodopa to enhance dopaminergic transmission remains one of the most effective treatment for alleviating the motor symptoms of Parkinson’s disease (Olanow, Mov Disord 34:812–815, 2019). In addition, deep brain stimulation (Bronstein et al., Arch Neurol 68:165, 2011) to modulate basal ganglia circuit activity successfully alleviates some motor symptoms. MRI guided focused ultrasound in the subthalamic nucleus is a promising therapeutic strategy as well (Martinez-Fernandez et al., Lancet Neurol 17:54–63, 2018). However, to date, there exists no treatment that stops the progression of this disease. The findings that α-synuclein can be released from neurons and inherited through interconnected neural networks opened the door for discovering novel treatment strategies to prevent the formation and spread of Lewy pathology with the goal of halting PD in its tracks. This hypothesis is based on discoveries that pathologic aggregates of α-synuclein induce the endogenous α-synuclein protein to adopt a similar pathologic conformation, and is thus self-propagating. Phase I clinical trials are currently ongoing to test treatments such as immunotherapy to prevent the neuron to neuron spread of extracellular aggregates. Although tremendous progress has been made in understanding how Lewy pathology forms and spreads throughout the brain, cell intrinsic factors also play a critical role in the formation of pathologic α-synuclein, such as mechanisms that increase endogenous α-synuclein levels, selective expression profiles in distinct neuron subtypes, mutations and altered function of proteins involved in α-synuclein synthesis and degradation, and oxidative stress. Strategies that prevent the formation of pathologic α-synuclein should consider extracellular release and propagation, as well as neuron intrinsic mechanisms.

Pathways of protein synthesis and degradation in PD pathogenesis

Since the discovery of protein aggregates in the brains of individuals with Parkinson's disease (PD) in the early 20th century, the scientific community has been interested in the role of dysfunctional protein metabolism in PD etiology. Recent advances in the field have implicated defective protein handling underlying PD through genetic, in vitro, and in vivo studies incorporating many disease models alongside neuropathological evidence. Here, we discuss the existing body of research focused on understanding cellular pathways of protein synthesis and degradation, and how aberrations in either system could engender PD pathology with special attention to α-synuclein-related consequences. We consider transcription, translation, and post-translational modification to constitute protein synthesis, and protein degradation to encompass proteasome-, lysosome- and endoplasmic reticulum-dependent mechanisms. Novel findings connecting each of these steps in protein metabolism to development of PD indicate that deregulation of protein production and turnover remains an exciting area in PD research.

Protein Quality Control in Neurodegeneration and Neuroprotection

Proteostasis is essential for regulating the integrity of the proteome. Disruption of proteostasis under some rigorous conditions leads to the aggregation and accumulation of misfolded toxic proteins, which plays a central role in the pathogenesis of protein conformational disorders. The protein quality control (PQC) system serves as a multi-level security system to shield cells from abnormal proteins. The intrinsic PQC systems maintaining proteostasis include the ubiquitin-proteasome system (UPS), chaperon-mediated autophagy (CMA), and autophagy-lysosome pathway (ALP) that serve to target misfolded proteins for unfolding, refolding, or degradation. Alterations of PQC systems in neurons have been implicated in the pathogenesis of various neurodegenerative disorders. This chapter provides an overview of PQC pathways to set a framework for discussion of the role of PQC in neurodegenerative disorders. Additionally, various pharmacological approaches targeting PQC are summarized.

Protective effect of alpha mangostin on rotenone induced toxicity in rat model of Parkinson's disease

Parkinson's disease (PD) is a progressive, late-onset, and degenerative disorder that affects the central nervous system with an unknown etiology. Due to its incredible complexity in disease nature, many of the existing treatment approaches show a vain recovery in Parkinson's patients. Therefore, an in search of disease-modifying therapeutics for an effective recovery is essential. Alpha mangostin is an important polyphenolic xanthone reported for its neuroprotective effect against rotenone-induced α-synuclein aggregation and loss of tyrosine hydroxylase positive (TH⁺)-neurons in SH-SY5Y cells. Hence, the current study aims to test its protective effect in managing the in-vivo rat model of PD. To justify this aim, adult male Sprague Dawley rats (250 ± 20 g) were subjected to chronic treatment of rotenone (2 mg/kg/day, s.c.) for 21 days. In parallel alpha mangostin treatment (10 mg/kg, i.p) was administered along with rotenone for 21 days. Chronic rotenone treatment for 21 days increased lipid peroxidation, nitrite concentration, and decreased glutathione levels. Further, depletion of TH⁺-dopaminergic neuron expression in substantia nigra pars compacta (SNc), and the development of motor and behavioral deficits in rotenone treated animals like cognitive impairment, muscle incoordination, and neuromuscular weakness were observed. Moreover, western blot studies ascertained the reduced normal alpha-synuclein levels and increased phosphorylated α-synuclein levels in comparison to the vehicle-treated group. Treatment with alpha mangostin significantly restored the locomotor activity, memory deficits, and improved the levels of antioxidant enzymes. It also significantly reduced the levels of phosphorylated α-synuclein which in turn gave protection against TH⁺-dopaminergic neuronal loss in SNc, suggesting it's anti-oxidant and anti-aggregatory potential against α-synuclein. In conclusion through our current results, we could suggest that alpha mangostin has a potential neuroprotective effect against rotenone-induced PD and might be used as a neuroprotective agent. Further mechanistic studies on preclinical and clinical levels are required to be conducted with alpha mangostin to avail and foresee it as a potential agent in the treatment and management of PD.

Mechanisms of alpha-synuclein toxicity: An update and outlook

Alpha-synuclein (aSyn) was identified as the main component of inclusions that define synucleinopathies more than 20 years ago. Since then, aSyn has been extensively studied in an attempt to unravel its roles in both physiology and pathology. Today, studying the mechanisms of aSyn toxicity remains in the limelight, leading to the identification of novel pathways involved in pathogenesis. In this chapter, we address the molecular mechanisms involved in synucleinopathies, from aSyn misfolding and aggregation to the various cellular effects and pathologies associated. In particular, we review our current understanding of the mechanisms involved in the spreading of aSyn between different cells, from the periphery to the brain, and back. Finally, we also review recent studies on the contribution of inflammation and the gut microbiota to pathology in synucleinopathies. Despite significant advances in our understanding of the molecular mechanisms involved, we still lack an integrated understanding of the pathways leading to neurodegeneration in PD and other synucleinopathies, compromising our ability to develop novel therapeutic strategies.

Structurally distinct α-synuclein fibrils induce robust parkinsonian pathology

OBJECTIVE

Alpha-synuclein (α-syn) is a major component of Lewy bodies, which are the pathological hallmark in Parkinson's disease, and its genetic mutations cause familial forms of Parkinson's disease. Patients with α-syn G51D mutation exhibit severe clinical symptoms. However, in vitro studies showed low propensity for α-syn with the G51D mutation. We studied the mechanisms associated with severe neurotoxicity of α-syn G51D mutation using a murine model generated by G51D α-syn fibril injection into the brain.

METHODS

Structural analysis of wild-type and G51D α-syn-fibrils were performed using Fourier transform infrared spectroscopy. The ability of α-syn fibrils forming aggregates was first assessed in in vitro mammalian cells. An in vivo mouse model with an intranigral injection of α-syn fibrils was then used to evaluate the propagation pattern of α-syn and related cellular changes.

RESULTS

We found that G51D α-syn fibrils have higher β-sheet contents than wild-type α-syn fibrils. The addition of G51D α-syn fibrils to mammalian cells overexpressing α-syn resulted in the formation of phosphorylated α-syn inclusions at a higher rate. Similarly, an injection of G51D α-syn fibrils into the substantia nigra of a mouse brain induced more widespread phosphorylated α-syn pathology. Notably, the mice injected with G51D α-syn fibrils exhibited progressive nigral neuronal loss accompanied with mitochondrial abnormalities and motor impairment.

CONCLUSION

Our findings indicate that the structural difference of G51D α-syn fibrils plays an important role in the rapidly developed and more severe neurotoxicity of G51D mutation-linked Parkinson's disease. © 2019 International Parkinson and Movement Disorder Society.

Protein misassembly and aggregation as potential convergence points for non-genetic causes of chronic mental illness

Chronic mental illnesses (CMI), such as schizophrenia or recurrent affective disorders, are complex conditions with both genetic and non-genetic elements. In many other chronic brain conditions, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and frontotemporal dementia, sporadic instances of the disease are more common than gene-driven familial cases. Yet, the pathology of these conditions can be characterized by the presence of aberrant protein homeostasis, proteostasis, resulting in misfolded or aggregated proteins in the brains of patients that predominantly do not derive from genetic mutations. While visible deposits of aggregated protein have not yet been detected in CMI patients, we propose the existence of more subtle protein misassembly in these conditions, which form a continuum with the psychiatric phenotypes found in the early stages of many neurodegenerative conditions. Such proteinopathies need not rely on genetic variation. In a similar manner to the established aberrant neurotransmitter homeostasis in CMI, aberrant homeostasis of proteins is a functional statement that can only partially be explained by, but is certainly complementary to, genetic approaches. Here, we review evidence for aberrant proteostasis signatures from post mortem human cases, in vivo animal work, and in vitro analysis of candidate proteins misassembled in CMI. The five best-characterized proteins in this respect are currently DISC1, dysbindin-1, CRMP1, TRIOBP-1, and NPAS3. Misassembly of these proteins with inherently unstructured domains is triggered by extracellular stressors and thus provides a converging point for non-genetic causes of CMI.

Oxidative stress in vagal neurons promotes parkinsonian pathology and intercellular α-synuclein transfer

Specific neuronal populations display high vulnerability to pathological processes in Parkinson's disease (PD). The dorsal motor nucleus of the vagus nerve (DMnX) is a primary site of pathological α-synuclein deposition and may play a key role in the spreading of α-synuclein lesions within and outside the CNS. Using in vivo models, we show that cholinergic neurons forming this nucleus are particularly susceptible to oxidative challenges and accumulation of reactive oxidative species (ROS). Targeted α-synuclein overexpression within these neurons triggered an oxidative stress that became significantly more pronounced after exposure to the ROS-generating agent paraquat. A more severe oxidative stress resulted in enhanced production of oxidatively modified forms of α-synuclein, increased α-synuclein aggregation into oligomeric species and marked degeneration of DMnX neurons. Enhanced oxidative stress also affected neuron-to-neuron protein transfer, causing an increased spreading of α-synuclein from the DMnX toward more rostral brain regions. In vitro experiments confirmed a greater propensity of α-synuclein to pass from cell to cell under pro-oxidant conditions, and identified nitrated α-synuclein forms as highly transferable protein species. These findings substantiate the relevance of oxidative injury in PD pathogenetic processes, establish a relationship between oxidative stress and vulnerability to α-synuclein pathology and define a new mechanism, enhanced cell-to-cell α-synuclein transmission, by which oxidative stress could promote PD development and progression.

Glial HMOX1 expression promotes central and peripheral α-synuclein dysregulation and pathogenicity in parkinsonian mice

α-Synuclein is a key player in the pathogenesis of Parkinson disease (PD). Expression of human heme oxygenase-1 (HO-1) in astrocytes of GFAP.HMOX1 transgenic (TG) mice between 8.5 and 19 months of age results in a parkinsonian phenotype characterized by neural oxidative stress, nigrostriatal hypodopaminergia associated with locomotor incoordination, and overproduction of α-synuclein. We identified two microRNAs (miR-), miR-153 and miR-223, that negatively regulate α-synuclein in the basal ganglia of male and female GFAP.HMOX1 mice. Serum concentrations of both miRNAs progressively declined in the wild-type (WT) and GFAP.HMOX1 mice between 11 and 19 months of age. Moreover, at each time point surveyed, circulating levels of miR-153 were significantly lower in the TG animals compared to WT controls, while α-synuclein protein concentrations were elevated in erythrocytes of the GFAP.HMOX1 mice at 19 months of age relative to WT values. Primary WT neurons co-cultured with GFAP.HMOX1 astrocytes exhibited enhanced protein oxidation, mitophagy and apoptosis, aberrant expression of genes regulating the dopaminergic phenotype, and an imbalance in gene expression profiles governing mitochondrial fission and fusion. Many, but not all, of these neuronal abnormalities were abrogated by small interfering RNA (siRNA) knockdown of α-synuclein, implicating α-synuclein as a potent, albeit partial, mediator of HO-1's neurodystrophic effects in these parkinsonian mice. Overexpression of HO-1 in stressed astroglia has previously been documented in the substantia nigra of idiopathic PD and may promote α-synuclein production and toxicity by downmodulating miR-153 and/or miR-223 both within the CNS and in peripheral tissues.

In vitro models of synucleinopathies: informing on molecular mechanisms and protective strategies

Alpha-synuclein (α-Syn) is a central player in Parkinson's disease (PD) and in a spectrum of neurodegenerative diseases collectively known as synucleinopathies. The protein was first associated with PD just over 20 years ago, when it was found to (i) be a major component of Lewy bodies and (ii) to be also associated with familial forms of PD. The characterization of α-Syn pathology has been achieved through postmortem studies of human brains. However, the identification of toxic mechanisms associated with α-Syn was only achieved through the use of experimental models. In vitro models are highly accessible, enable relatively rapid studies, and have been extensively employed to address α-Syn-associated neurodegeneration. Given the diversity of models used and the outcomes of the studies, a cumulative and comprehensive perspective emerges as indispensable to pave the way for further investigations. Here, we subdivided in vitro models of α-Syn pathology into three major types: (i) models simulating α-Syn fibrillization and the formation of different aggregated structures in vitro, (ii) models based on the intracellular expression of α-Syn, reporting on pathogenic conditions and cellular dysfunctions induced, and (iii) models using extracellular treatment with α-Syn aggregated species, reporting on sites of interaction and their downstream consequences. In summary, we review the underlying molecular mechanisms discovered and categorize protective strategies, in order to pave the way for future studies and the identification of effective therapeutic strategies. This article is part of the Special Issue "Synuclein".

How is alpha-synuclein cleared from the cell?

The levels and conformers of alpha-synuclein are critical in the pathogenesis of Parkinson's Disease and related synucleinopathies. Homeostatic mechanisms in protein degradation and secretion have been identified as regulators of alpha-synuclein at different stages of its intracellular trafficking and transcellular propagation. Here we review pathways involved in the removal of various forms of alpha-synuclein from both the intracellular and extracellular environment. Proteasomes and lysosomes are likely to play complementary roles in the removal of intracellular alpha-synuclein species, in a manner that depends on alpha-synuclein post-translational modifications. Extracellular alpha-synuclein is cleared by extracellular proteolytic enzymes, or taken up by neighboring cells, especially microglia and astrocytes, and degraded within lysosomes. Exosomes, on the other hand, represent a vehicle for egress of excess burden of the intracellular protein, potentially contributing to the transfer of alpha-synuclein between cells. Dysfunction in any one of these clearance mechanisms, or a combination thereof, may be involved in the initiation or progression of Parkinson's disease, whereas targeting these pathways may offer an opportunity for therapeutic intervention. This article is part of the Special Issue "Synuclein".

Morinda citrifolia and Its Active Principle Scopoletin Mitigate Protein Aggregation and Neuronal Apoptosis through Augmenting the DJ-1/Nrf2/ARE Signaling Pathway

Given the role of oxidative stress in PD pathogenesis and off-target side effects of currently available drugs, several natural phytochemicals seem to be promising in the management of PD. Here, we tested the hypothesis that scopoletin, an active principle obtained from Morinda citrifolia (MC), efficiently quenches oxidative stress through DJ-1/Nrf2 signaling and ameliorates rotenone-induced PD. Despite reducing oxidative stress, the administration of MC extract (MCE) has lessened protein aggregation as evident from decreased levels of nitrotyrosine and α-synuclein. In vitro studies revealed that scopoletin lessened rotenone-induced apoptosis in SH-SY5Y cells through preventing oxidative injury. Particularly, scopoletin markedly upregulated DJ-1, which then promoted the nuclear translocation of Nrf2 and transactivation of antioxidant genes. Furthermore, we found that scopoletin prevents the nuclear exportation of Nrf2 by reducing the levels of Keap1 and thereby enhancing the neuronal defense system. Overall, our findings suggest that scopoletin acts through DJ-1-mediated Nrf2 signaling to protect the brain from rotenone-induced oxidative stress and PD. Thus, we postulate that scopoletin could be a potential drug to treat PD.

El papel de las especies reactivas de oxígeno y nitrógeno en algunas enfermedades neurodegenerativas

Se analiza el importante papel de las especies reactivas de las moléculas de oxígeno y nitrógeno generadas a partir del metabolismo celular fisiológico en los procesos neurodegenerativos con el fin de tener indicios sólidos sobre los posibles tratamientos y prevenir el daño progresivo de enfermedades neurodegenerativas.

Nitrated alpha-synuclein in minor salivary gland biopsies in Parkinson's disease

Alpha-synuclein (α-SYN) is found in peripheral autonomic neuronal network apart from brain in Parkinson's disease (PD). Nitrated α-SYN is an undesirable modification associated with oxidative and nitrative damage and has been found extensively in brain, gastrointestinal(GI) tract and blood cells in PD. We aim to investigate the presence of nitrated α-SYN in minor salivary gland biopsy in PD. Patients with PD and age-matched controls underwent minor salivary gland biopsy. Motor impairment was assessed by Hoehn-Yahr (H-Y) stage and Unified Parkinson's Disease Rating Scale (UPDRS) Part III in off-state. 11C-methyl-N-2b-carbomethoxy-3b-(4-fluorophenyl) tropane (11C-CFT) DAT-PET scan was performed in all subjects. Immunohistochemical staining for nitrated α-SYN was performed in the minor salivary gland tissues. The minor salivary gland tissues of 8 PD cases and 7 controls with early stage (H-Y stage 1-2) were detected. All PD patients showed asymmetrical and reduction of ¹¹C-CFT uptake in the caudate, anterior and posterior putamen, while all control subjects showed normal DAT-PET scan. Positive nitrated α-SYN immunostaining was observed in all PD patients (8/8,100%) but not in control subjects (0/7). The results were consistent well with that of DAT-PET. These nitrated alpha-synuclein positive structures were mainly located in the periacinar stroma in PD patients. Our result suggests that nitrated α-SYN exists in the early stage and is probably a promising biomarker for PD. Minor salivary gland is an ideal site for α-SYN nitration detection. Despite of the small number of subjects, attention should be given to α-SYN nitration in PD and more investigations on nitrated α-SYN in different sites and large sample using should be explored in future.

Modulation of α-Synuclein Aggregation by Cytochrome c Binding and Hetero-dityrosine Adduct Formation

The aggregation of α-synuclein (α-Syn) has been implicated strongly in Parkinson's disease (PD). The intrinsically disordered nature of α-Syn makes this protein prone to self-association or heteroassociation with another protein or lipid. While conformational fluctuation and free radical chemistry have been shown to play important roles in its ability toward self- and heteroassociation, any systematic understanding of their contributions is missing. Here, we report an in vitro investigation of the interaction between α-Syn and cytochrome c in the oxidized (cyt c III) and reduced forms (cyt c II), in which cyt c III was found to induce a large compaction of α-Syn and inhibit the aggregation by favoring a hetero-dityrosine bond formation. In contrast, the presence of cyt c II did not result in any compaction and its presence was found to facilitate α-Syn aggregation. The variation in the charge distribution of the surface residues of cyt c III and cyt c II is expected to play a decisive role in their interaction with α-Syn.

Alpha-Synuclein Nitration and Its Implications in Parkinson's Disease

Parkinson's disease is pathologically characterized by the degeneration of dopaminergic neurons in the substantia nigra and the accumulation of neuronal cytoplasmic inclusions known as Lewy bodies, which are primarily composed of α-synuclein. Post-translational modifications of α-synuclein induced by nitrative stress have been linked to neurodegeneration. Here, we review the concept of α-synuclein nitration and its biological consequences. We also discuss the pathological roles of nitrated α-synuclein and their potential clinical implications in Parkinson's disease.

The unlikely partnership between LRRK2 and α-synuclein in Parkinson's disease

Our understanding of the mechanisms underlying Parkinson's disease, the once archetypical nongenetic neurogenerative disorder, has dramatically increased with the identification of α-synuclein and LRRK2 pathogenic mutations. While α-synuclein protein composes the aggregates that can spread through much of the brain in disease, LRRK2 encodes a multidomain dual-enzyme distinct from any other protein linked to neurodegeneration. In this review, we discuss emergent datasets from multiple model systems that suggest these unlikely partners do interact in important ways in disease, both within cells that express both LRRK2 and α-synuclein as well as through more indirect pathways that might involve neuroinflammation. Although the link between LRRK2 and disease can be understood in part through LRRK2 kinase activity (phosphotransferase activity), α-synuclein toxicity is multilayered and plausibly interacts with LRRK2 kinase activity in several ways. We discuss common protein interactors like 14-3-3s that may regulate α-synuclein and LRRK2 in disease. Finally, we examine cellular pathways and outcomes common to both mutant α-synuclein expression and LRRK2 activity and points of intersection. Understanding the interplay between these two unlikely partners in disease may provide new therapeutic avenues for PD.

α-Synuclein in salivary extracellular vesicles as a potential biomarker of Parkinson's disease

BACKGROUND

Detection of α-synuclein (α-syn) in biological fluids such as saliva may serve as potential biomarker of PD. α-syn pertaining to extracellular vesicles (EVs) has been recently studied in plasma, but not in other biological fluids such as saliva.

AIM

1) To investigate the presence of exosomes in PD saliva; 2) to explore the value of α-syn in salivary EVs as potential biomarker in PD.

METHODS

Saliva samples were obtained from 74 PD and 60 healthy controls (HCs). The EVs were extracted from saliva by XYCQ EV Enrichment KIT. Western blot and Nanosight 300 were used to validate the existence of exosomes in EVs and to analyze the size of salivary EVs. Salivary EVs α-syn levels, including total α-syn (α-syn_Total), oligomeric α-syn (α-syn_Olig) and phosphorylated-ser129 α-syn (α-syn_PS129), were measured by Electrochemiluminescence (ECL) assays. Diagnostic value and clinical relevance of salivary EVs α-syn were assessed by Receiver Operator Characteristic (ROC) curve and Spearman correlation.

RESULTS

Alix and CD9 positive EVs, representing the presence of exosomes, was detected in PD salivary samples. Size of salivary EVs was about 30-400 nm. The levels of α-syn_Olig and α-syn_Olig/α-syn_Total in the salivary EVs were higher in PD than in HCs (10.39 ± 1.46 pg/ng vs.1.37 ± 0.24 pg/ng, p＜0.001;1.70 ± 0.52 pg/ng vs.0.67 ± 0.26 pg/ng, p＜0.001). There was no significant difference in α-syn_Total、α-syn_PS129、 α-syn_PS129/α-syn_Total ratio between PD and HCs (P = 0.723, 0.634, 0.734, respectively). α-syn_Olig 2.05 pg/ng distinguished PD from HCs with sensitivity 92% and specificity 86%; α-syn_Olig /α-syn_Total 0.18 pg/ng differentiated PD from HCs with sensitivity 81% and specificity 71%. No significant correlation between salivary EVs α-syn_Olig, α-syn_Olig/α-syn_Total and disease severity was found.

CONCLUSIONS

Exosomes are present in PD saliva. The α-syn_Olig and α-syn_Olig/α-syn_Total ratio in salivary EVs may serve as potential diagnostic biomarkers for PD.

Disruptive membrane interactions of alpha-synuclein aggregates

Alpha synuclein (αS) is a ~14 kDa intrinsically disordered protein. Decades of research have increased our knowledge on αS yet its physiological function remains largely elusive. The conversion of monomeric αS into oligomers and amyloid fibrils is believed to play a central role of the pathology of Parkinson's disease (PD). It is becoming increasingly clear that the interactions of αS with cellular membranes are important for both αS's functional and pathogenic actions. Therefore, understanding interactions of αS with membranes seems critical to uncover functional or pathological mechanisms. This review summarizes our current knowledge of how physicochemical properties of phospholipid membranes affect the binding and aggregation of αS species and gives an overview of how post-translational modifications and point mutations in αS affect phospholipid membrane binding and protein aggregation. We discuss the disruptive effects resulting from the interaction of αS aggregate species with membranes and highlight current approaches and hypotheses that seek to understand the pathogenic and/or protective role of αS in PD.

A Split Transcriptional Repressor That Links Protein Solubility to an Orthogonal Genetic Circuit

Monitoring the aggregation of proteins within the cellular environment is key to investigating the molecular mechanisms underlying the formation of off-pathway protein assemblies associated with the development of disease and testing therapeutic strategies to prevent the accumulation of non-native conformations. It remains challenging, however, to couple protein aggregation events underlying the cellular pathogenesis of a disease to genetic circuits and monitor their progression in a quantitative fashion using synthetic biology tools. To link the aggregation propensity of a target protein to the expression of an easily detectable reporter, we investigated the use of a transcriptional AND gate system based on complementation of a split transcription factor. We first identified two-fragment tetracycline repressor (TetR) variants that can be regulated via ligand-dependent induction and demonstrated that split TetR variants can function as transcriptional AND gates in both bacteria and mammalian cells. We then adapted split TetR for use as an aggregation sensor. Protein aggregation was detected by monitoring complementation between a larger TetR fragment that serves as a "detector" and a smaller TetR fragment expressed as a fusion to an aggregation-prone protein that serves as a "sensor" of the target protein aggregation status. This split TetR represents a novel genetic component that can be used for a wide range of applications in bacterial as well as mammalian synthetic biology and a much needed cell-based sensor for monitoring a protein's conformational status in complex cellular environments.

Immunology of West Nile Virus Infection and the Role of Alpha-Synuclein as a Viral Restriction Factor

West Nile virus (WNV) is a single-stranded RNA flavivirus and is a major cause of viral encephalitis worldwide. Experimental models of WNV infection in mice are commonly used to define acute neuroinflammatory responses in the brain. Alpha-synuclein (Asyn) is a protein of primarily neuronal origin and is a major cause of Parkinson's disease (PD), a disorder characterized by loss of dopaminergic neurons. Both WNV and PD pathologies are largely mediated by inflammation of the central nervous system (neuroinflammation) and have overlapping inflammatory pathways. In this review, we highlight the roles of the immune system in both diseases while comparing and contrasting both protective and pathogenic roles of immune cells and their effector proteins. Additionally, we review the current literature showing that Asyn is an important mediator of the immune response with diverging roles in PD (pathogenic) and WNV disease (neuroprotective).

Nrf2 activation attenuates the early suppression of mitochondrial respiration due to the α-synuclein overexpression

BACKGROUND

α-synuclein (SNCA) accumulation in the substantia nigra is one of the characteristic pathologies of Parkinson's disease (PD). A53T missense mutations in the SNCA gene has been proved to enhance the expression of SNCA and accelerate the onset of PD. Mitochondrial dysfunction in SNCA aggregation has been under debate for decades but the causal relationship remains uncertain. At a later stage of PD, the cellular dysfunctions are complicated and multiple factors are tangled. Our aim here is to investigate the mitochondrial functional changes and clarify the main causal mechanism at earlier-stage of PD.

METHODS

We used the mutant A53T SNCA-expressed neuro 2a (N2a) cells without detectable cell death to investigate: 1) whether SNCA overexpression impairs the mitochondrial respiration and biogenesis. 2) The role of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) signal in SNCA-induced mitochondria dysfunction.

RESULTS

Accompanying with the increment of SNCA, reactive oxygen species (ROS) accumulation was increased. The maximal respiratory capacity was suppressed. Meanwhile, mitochondrial complex 1 activity and the activity of nicotinamide adenine dinucleotide (NADH) cytochrome C reductase (NCCR) were decreased. Moreover, the mitochondrial DNA (mtDNA) copy number was decreased. On the other hand, the nuclear peroxisome proliferator-activated receptor-gamma coactivator 1α (PGC-1α), Nrf2, and the cytosolic mitochondrial transcription factor A (TFAM) were increased at an early stage and declined thereafter. Above factors triggered by SNCA were reversed by tBHQ, a Nrf2 activator.

CONCLUSION

These results suggested that at an early stage, SNCA-overexpressed increase mtROS accumulation, mitochondrial dysfunction and mtDNA decrement. Nrf2, PGC-1α and TFAM were upregulated to compromise mitochondrial dysfunction. tBHQ effectively reversed the SNCA-induced mitochondrial dysfunction.

Salidroside Promotes the Pathological α-Synuclein Clearance Through Ubiquitin-Proteasome System in SH-SY5Y Cells

Parkinson's disease (PD) is characterized by the loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) and the presence of Lewy bodies (LBs) in the surviving SNc neurons. LBs formation is caused by the accumulation of α-synuclein (α-syn) or phosphorylated α-syn at serine-129 (pSer129-α-syn), which is implicated in the pathological progression of PD. Salidroside (Sal), the main active ingredient of the root of Rhodiola rosea L., has been reported to have potent neuroprotective properties in our previous investigations. Here, we investigated the effects of Sal on 6-OHDA and overexpresssion of WT/A30P-α-syn-induced pathological α-syn increase and the mechanism behind it in SH-SY5Y cells. We found Sal displays neuroprotective effects against 6-hydroxydopamine (6-OHDA)-induced cytotoxicity. Sal decreased the pSer129-α-syn level mainly by maintaining the normal function of ubiquitin-proteasome system (UPS). Furthermore, Sal promoted the clearance of α-syn and protected the cell viability mainly through recovered the 20S proteasome activity in WT/A30P-α-syn-transfected cells. These data provide new mechanistic insights into the neuroprotective effects of Sal and Sal may be a promising therapy to slow neurodegeneration in PD. Highlights: Sal protects cells and decreases the pSer129-α-syn protein level in 6-OHDA-induced impairmental and dysfunctional SH-SY5Y cells. Sal promotes the clearance of α-syn and protects the cell viability mainly through recovering the 20S proteasome activity in WT/A30P-α-syn plasmids transfected cells. Maintaining the normal function of the UPS may be one of the important mechanisms of Sal in neuroprotective effects.

The Role of Free Radicals in Autophagy Regulation: Implications for Ageing

Reactive oxygen and nitrogen species (ROS and RNS, resp.) have been traditionally perceived solely as detrimental, leading to oxidative damage of biological macromolecules and organelles, cellular demise, and ageing. However, recent data suggest that ROS/RNS also plays an integral role in intracellular signalling and redox homeostasis (redoxtasis), which are necessary for the maintenance of cellular functions. There is a complex relationship between cellular ROS/RNS content and autophagy, which represents one of the major quality control systems in the cell. In this review, we focus on redox signalling and autophagy regulation with a special interest on ageing-associated changes. In the last section, we describe the role of autophagy and redox signalling in the context of Alzheimer's disease as an example of a prevalent age-related disorder.

Reduction of RPT6/S8 (a Proteasome Component) and Proteasome Activity in the Cortex is Associated with Cognitive Impairment in Lewy Body Dementia

Lewy body dementia is the second most common neurodegenerative dementia and is pathologically characterized by α-synuclein positive cytoplasmic inclusions, with varying amounts of amyloid-β (Aβ) and hyperphosphorylated tau (tau) aggregates in addition to synaptic loss. A dysfunctional ubiquitin proteasome system (UPS), the major proteolytic pathway responsible for the clearance of short lived proteins, may be a mediating factor of disease progression and of the development of α-synuclein aggregates. In the present study, protein expression of a key component of the UPS, the RPT6 subunit of the 19S regulatory complex was determined. Furthermore, the main proteolytic-like (chymotrypsin- and PGPH-) activities have also been analyzed. The middle frontal (Brodmann, BA9), inferior parietal (BA40), and anterior cingulate (BA24) gyrus' cortex were selected as regions of interest from Parkinson's disease dementia (PDD, n = 31), dementia with Lewy bodies (DLB, n = 44), Alzheimer's disease (AD, n = 16), and control (n = 24) brains. Clinical and pathological data available included the MMSE score. DLB, PDD, and AD were characterized by significant reductions of RPT6 (one-way ANOVA, p < 0.001; Bonferroni post hoc test) in prefrontal cortex and parietal cortex compared with controls. Strong associations were observed between RPT6 levels in prefrontal, parietal cortex, and anterior cingulate gyrus and cognitive impairment (p = 0.001, p = 0.001, and p = 0.008, respectively). These findings highlight the involvement of the UPS in Lewy body dementia and indicate that targeting the UPS may have the potential to slow down or reduce the progression of cognitive impairment in DLB and PDD.

Lack of PINK1 alters glia innate immune responses and enhances inflammation-induced, nitric oxide-mediated neuron death

Neuroinflammation is involved in the pathogenesis of Parkinson’s disease (PD) and other neurodegenerative disorders. We show that lack of PINK1- a mitochondrial kinase linked to recessive familial PD – leads to glia type-specific abnormalities of innate immunity. PINK1 loss enhances LPS/IFN-γ stimulated pro-inflammatory phenotypes of mixed astrocytes/microglia (increased iNOS, nitric oxide and COX-2, reduced IL-10) and pure astrocytes (increased iNOS, nitric oxide, TNF-α and IL-1β), while attenuating expression of both pro-inflammatory (TNF-α, IL-1β) and anti-inflammatory (IL-10) cytokines in microglia. These abnormalities are associated with increased inflammation-induced NF-κB signaling in astrocytes, and cause enhanced death of neurons co-cultured with inflamed PINK1^−/− mixed glia and neuroblastoma cells exposed to conditioned medium from LPS/IFN-γ treated PINK1^−/− mixed glia. Neuroblastoma cell death is prevented with an iNOS inhibitor, implicating increased nitric oxide production as the cause for enhanced death. Finally, we show for the first time that lack of a recessive PD gene (PINK1) increases α-Synuclein-induced nitric oxide production in all glia types (mixed glia, astrocytes and microglia). Our results describe a novel pathogenic mechanism in recessive PD, where PINK1 deficiency may increase neuron death via exacerbation of inflammatory stimuli-induced nitric oxide production and abnormal innate immune responses in glia cells.

The olfactory bulb as the entry site for prion-like propagation in neurodegenerative diseases

Olfactory deficits are present in numerous neurodegenerative disorders and are accompanied by pathology in related brain regions. In several of these disorders, olfactory disturbances appear early and are considered as prodromal symptoms of the disease. In addition, pathological protein aggregates affect olfactory regions prior to other regions, suggesting that the olfactory system might be particularly vulnerable to neurodegenerative diseases. Exposed to the external environment, the olfactory epithelium and olfactory bulb allow pathogen and toxin penetration into the brain, a process that has been proposed to play a role in neurodegenerative diseases. Determining whether the olfactory bulb could be a starting point of pathology and of pathology spread is crucial to understanding how neurodegenerative diseases evolve. We argue that pathological changes following environmental insults contribute to the initiation of protein aggregation in the olfactory bulb, which then triggers the spread of the pathology within the brain by a templating mechanism in a prion-like manner. We review the evidence for the early involvement of olfactory structures in neurodegenerative diseases and the relationship between neuropathology and olfactory function. We discuss the vulnerability and putative underlying mechanisms by which pathology could be initiated in the olfactory bulb, from the entry of pathogens (promoted by increased permeability of the olfactory epithelium with aging or inflammation) to the sensitivity of the olfactory system to oxidative stress and inflammation. Finally, we review changes in protein expression and neural excitability triggered by pathogenic proteins that can promote pathogenesis in the olfactory bulb and beyond.

SUMOylation and ubiquitination reciprocally regulate α-synuclein degradation and pathological aggregation

α-Synuclein accumulation is a pathological hallmark of Parkinson's disease (PD). Ubiquitinated α-synuclein is targeted to proteasomal or lysosomal degradation. Here, we identify SUMOylation as a major mechanism that counteracts ubiquitination by different E3 ubiquitin ligases and regulates α-synuclein degradation. We report that PIAS2 promotes SUMOylation of α-synuclein, leading to a decrease in α-synuclein ubiquitination by SIAH and Nedd4 ubiquitin ligases, and causing its accumulation and aggregation into inclusions. This was associated with an increase in α-synuclein release from the cells. A SUMO E1 inhibitor, ginkgolic acid, decreases α-synuclein levels by relieving the inhibition exerted on α-synuclein proteasomal degradation. α-Synuclein disease mutants are more SUMOylated compared with the wild-type protein, and this is associated with increased aggregation and inclusion formation. We detected a marked increase in PIAS2 expression along with SUMOylated α-synuclein in PD brains, providing a causal mechanism underlying the up-regulation of α-synuclein SUMOylation in the disease. We also found a significant proportion of Lewy bodies in nigral neurons containing SUMO1 and PIAS2. Our observations suggest that SUMOylation of α-synuclein by PIAS2 promotes α-synuclein aggregation by two mutually reinforcing mechanisms. First, it has a direct proaggregatory effect on α-synuclein. Second, SUMOylation facilitates α-synuclein aggregation by blocking its ubiquitin-dependent degradation pathways and promoting its accumulation. Therefore, inhibitors of α-synuclein SUMOylation provide a strategy to reduce α-synuclein levels and possibly aggregation in PD.

The Proteasome Inhibition Model of Parkinson's Disease

The pathological hallmarks of Parkinson's disease are the progressive loss of nigral dopaminergic neurons and the formation of intracellular inclusion bodies, termed Lewy bodies, in surviving neurons. Accumulation of proteins in large insoluble cytoplasmic aggregates has been proposed to result, partly, from a failure in the function of intracellular protein degradation pathways. Evidence in support for such a hypothesis emerged in the beginning of the years 2000 with studies demonstrating structural and functional deficits in the ubiquitin-proteasome pathway in post-mortem nigral tissue of patients with Parkinson's disease. These fundamental findings have inspired the development of a new generation of animal models based on the use of proteasome inhibitors to disturb protein homeostasis and trigger nigral dopaminergic neurodegeneration. In this review, we provide an updated overview of the current approaches in employing proteasome inhibitors to model Parkinson's disease, with particular emphasis on rodent studies. In addition, the mechanisms underlying proteasome inhibition-induced cell death and the validity criteria (construct, face and predictive validity) of the model will be critically discussed. Due to its distinct, but highly relevant mechanism of inducing neuronal death, the proteasome inhibition model represents a useful addition to the repertoire of toxin-based models of Parkinson's disease that might provide novel clues to unravel the complex pathogenesis of this disorder.

The ER retention protein RER1 promotes alpha-synuclein degradation via the proteasome

Abnormal accumulation of α-synuclein (αSyn) has been linked to endoplasmic-reticulum (ER) stress, defective intracellular protein/vesicle trafficking, and cytotoxicity. Targeting factors involved in ER-related protein processing and trafficking may, therefore, be a key to modulating αSyn levels and associated toxicity. Recently retention in endoplasmic reticulum 1 (RER1) has been identified as an important ER retrieval/retention factor for Alzheimer's disease proteins and negatively regulates amyloid-β peptide levels. Here, we hypothesized that RER1 might also play an important role in retention/retrieval of αSyn and mediate levels. We expressed RER1 and a C-terminal mutant RER1Δ25, which lacks the ER retention/retrieval function, in HEK293 and H4 neuroglioma cells. RER1 overexpression significantly decreased levels of both wild type and A30P, A53T, and E46K disease causal mutants of αSyn, whereas the RER1Δ25 mutant had a significantly attenuated effect on αSyn. RER1 effects were specific to αSyn and had little to no effect on either βSyn or the Δ71-82 αSyn mutant, which both lack the NAC domain sequence critical for synuclein fibrillization. Tests with proteasomal and macroautophagy inhibitors further demonstrate that RER1 effects on αSyn are primarily mediated through the ubiquitin-proteasome system. RER1 also appears to interact with the ubiquitin ligase NEDD4. RER1 in human diseased brain tissues co-localizes with αSyn-positive Lewy bodies. Together, these findings provide evidence that RER1 is a novel and potential important mediator of elevated αSyn levels. Further investigation of the mechanism of RER1 and downstream effectors on αSyn may yield novel therapeutic targets for modulation in Parkinson disease and related synucleinopathies.

Astroglia as a cellular target for neuroprotection and treatment of neuro-psychiatric disorders

Astrocytes are key homeostatic cells of the central nervous system. They cooperate with neurons at several levels, including ion and water homeostasis, chemical signal transmission, blood flow regulation, immune and oxidative stress defense, supply of metabolites and neurogenesis. Astroglia is also important for viability and maturation of stem-cell derived neurons. Neurons critically depend on intrinsic protective and supportive properties of astrocytes. Conversely, all forms of pathogenic stimuli which disturb astrocytic functions compromise neuronal functionality and viability. Support of neuroprotective functions of astrocytes is thus an important strategy for enhancing neuronal survival and improving outcomes in disease states. In this review, we first briefly examine how astrocytic dysfunction contributes to major neurological disorders, which are traditionally associated with malfunctioning of processes residing in neurons. Possible molecular entities within astrocytes that could underpin the cause, initiation and/or progression of various disorders are outlined. In the second section, we explore opportunities enhancing neuroprotective function of astroglia. We consider targeting astrocyte-specific molecular pathways which are involved in neuroprotection or could be expected to have a therapeutic value. Examples of those are oxidative stress defense mechanisms, glutamate uptake, purinergic signaling, water and ion homeostasis, connexin gap junctions, neurotrophic factors and the Nrf2-ARE pathway. We propose that enhancing the neuroprotective capacity of astrocytes is a viable strategy for improving brain resilience and developing new therapeutic approaches.

The small heat shock proteins αB-crystallin (HSPB5) and Hsp27 (HSPB1) inhibit the intracellular aggregation of α-synuclein

Protein homeostasis, or proteostasis, is the process of maintaining the conformational and functional integrity of the proteome. Proteostasis is preserved in the face of stress by a complex network of cellular machinery, including the small heat shock molecular chaperone proteins (sHsps), which act to inhibit the aggregation and deposition of misfolded protein intermediates. Despite this, the pathogenesis of several neurodegenerative diseases has been inextricably linked with the amyloid fibrillar aggregation and deposition of α-synuclein (α-syn). The sHsps are potent inhibitors of α-syn aggregation in vitro. However, the limited availability of a robust, cell-based model of α-syn aggregation has, thus far, restricted evaluation of sHsp efficacy in the cellular context. As such, this work sought to establish a robust model of intracellular α-syn aggregation using Neuro-2a cells. Aggregation of α-syn was found to be sensitive to inhibition of autophagy and the proteasome, resulting in a significant increase in the proportion of cells containing α-syn inclusions. This model was then used to evaluate the capacity of the sHsps, αB-c and Hsp27, to prevent α-syn aggregation in cells. To do so, we used bicistronic expression plasmids to express the sHsps. Unlike traditional fluorescent fusion constructs, these bicistronic expression plasmids enable only individual transfected cells expressing the sHsps (via expression of the fluorescent reporter) to be analysed, but without the need to tag the sHsp, which can affect its oligomeric structure and chaperone activity. Overexpression of both αB-c and Hsp27 significantly reduced the intracellular aggregation of α-syn. Thus, these findings suggest that overexpressing or boosting the activity of sHsps may be a way of preventing amyloid fibrillar aggregation of α-syn in the context of neurodegenerative disease.

Polo-like kinase 2 modulates α-synuclein protein levels by regulating its mRNA production

Variations in the α-synuclein-encoding SNCA gene represent the greatest genetic risk factor for Parkinson's disease (PD), and duplications/triplications of SNCA cause autosomal dominant familial PD. These facts closely link brain levels of α-synuclein with the risk of PD, and make lowering α-synuclein levels a therapeutic strategy for the treatment of PD and related synucleinopathies. In this paper, we corroborate previous findings on the ability of overexpressed Polo-like kinase 2 (PLK-2) to decrease cellular α-synuclein, but demonstrate that the process is independent of PLK-2 phosphorylating S129 in α-synuclein because a similar reduction is achieved with the non-phosphorable S129A mutant α-synuclein. Using a specific PLK-2 inhibitor (compound 37), we demonstrate that endogenous PLK-2 phosphorylates S129 only in some cells, but increases α-synuclein protein levels in all tested cell cultures and brain slices. PLK-2 is found to regulate the transcription of α-synuclein mRNA from both the endogenous mouse SNCA gene and transgenic vectors that only contain the open reading frame. Moreover, we are the first to show that regulation of α-synuclein by PLK-2 is of physiological importance since 10days' inhibition of endogenous PLK-2 in wt C57BL/6 mice increases endogenous α-synuclein protein levels. Our findings collectively demonstrate that PLK-2 regulates α-synuclein levels by a previously undescribed transcription-based mechanism. This mechanism is active in cells and brain tissue, opening up for alternative strategies for modulating α-synuclein levels and thereby for the possibility of modifying disease progression in synucleinopaties.

Synthetic alpha-synuclein fibrils cause mitochondrial impairment and selective dopamine neurodegeneration in part via iNOS-mediated nitric oxide production

Intracellular accumulation of α-synuclein (α-syn) are hallmarks of synucleinopathies, including Parkinson's disease (PD). Exogenous addition of preformed α-syn fibrils (PFFs) into primary hippocampal neurons induced α-syn aggregation and accumulation. Likewise, intrastriatal inoculation of PFFs into mice and non-human primates generates Lewy bodies and Lewy neurites associated with PD-like neurodegeneration. Herein, we investigate the putative effects of synthetic human PFFs on cultured rat ventral midbrain dopamine (DA) neurons. A time- and dose-dependent accumulation of α-syn was observed following PFFs exposure that also underwent phosphorylation at serine 129. PFFs treatment decreased the expression levels of synaptic proteins, caused alterations in axonal transport-related proteins, and increased H2AX Ser139 phosphorylation. Mitochondrial impairment (including modulation of mitochondrial dynamics-associated protein content), enhanced oxidative stress, and an inflammatory response were also detected in our experimental paradigm. In attempt to unravel a potential molecular mechanism of PFFs neurotoxicity, the expression of inducible nitric oxide synthase was blocked; a significant decline in protein nitration levels and protection against PFFs-induced DA neuron death were observed. Combined exposure to PFFs and rotenone resulted in an additive toxicity. Strikingly, many of the harmful effects found were more prominent in DA rather than non-DA neurons, suggestive of higher susceptibility to degenerate. These findings provide new insights into the role of α-syn in the pathogenesis of PD and could represent a novel and valuable model to study DA-related neurodegeneration.

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.

Alpha-synuclein at the intracellular and the extracellular side: functional and dysfunctional implications

Alpha-synuclein (α-syn) is an abundant neuronal protein whose physiological function, even if still not completely understood, has been consistently related to synaptic function and vesicle trafficking. A group of disorders known as synucleinopathies, among which Parkinson’s disease (PD), is deeply associated with the misfolding and aggregation of α-syn, which can give rise to proteinaceous inclusion known as Lewy bodies (LB). Proteostasis stress is a relevant aspect in these diseases and, currently, the presence of oligomeric α-syn species rather than insoluble aggregated forms, appeared to be associated with cytotoxicity. Many observations suggest that α-syn is responsible for neurodegeneration by interfering with multiple signaling pathways. α-syn protein can directly form plasma membrane channels or modify with their activity, thus altering membrane permeability to ions, abnormally associate with mitochondria and cause mitochondrial dysfunction (i.e. mitochondrial depolarization, Ca2+ dys-homeostasis, cytochrome c release) and interfere with autophagy regulation. The picture is further complicated by the fact that single point mutations, duplications and triplication in α-syn gene are linked to autosomal dominant forms of PD. In this review we discuss the multi-faced aspect of α-syn biology and address the main hypothesis at the basis of its involvement in neuronal degeneration.

LRRK2 at the interface of autophagosomes, endosomes and lysosomes

Over the past 20 years, substantial progress has been made in identifying the underlying genetics of Parkinson's disease (PD). Of the known genes, LRRK2 is a major genetic contributor to PD. However, the exact function of LRRK2 remains to be elucidated. In this review, we discuss how familial forms of PD have led us to hypothesize that alterations in endomembrane trafficking play a role in the pathobiology of PD. We will discuss the major observations that have been made to elucidate the role of LRRK2 in particular, including LRRK2 animal models and high-throughput proteomics approaches. Taken together, these studies strongly support a role of LRRK2 in vesicular dynamics. We also propose that targeting these pathways may not only be beneficial for developing therapeutics for LRRK2-driven PD, but also for other familial and sporadic cases.

α-synuclein aggregation and its modulation

Parkinson's disease (PD) is a neurological disorder marked by the presence of cytoplasmic inclusions, Lewy bodies (LBs) and Lewy neurites (LNs) as well as the degeneration of dopamine producing neurons in the substantia nigra region of the brain. The LBs and LNs in PD are mainly composed of aggregated form of a presynaptic protein, α-synuclein (α-Syn). However, the mechanisms of α-Syn aggregation and actual aggregated species responsible for the degeneration of dopaminergic neurons have not yet been resolved. Despite the fact that α-Syn aggregation in LBs and LNs is crucial and mutations of α-Syn are associated with early onset PD, it is really a challenging task to establish a correlation between α-Syn aggregation rate and PD pathogenesis. Regardless of strong genetic contribution, PD is mostly sporadic and familial forms of the disease represent only a minor part (<10%) of all cases. The complexity in PD further increases due to the involvement of several cellular factors in the pathogenesis of the disease as well as the environmental factors associated with the risk of developing PD. Therefore, effect of these factors on α-Syn aggregation pathway and how these factors modulate the properties of wild type (WT) as well as mutated α-Syn should be collectively taken into account. The present review specifically provides an overview of recent research on α-Syn aggregation pathways and its modulation by several cellular factors potentially relevant to PD pathogenesis. We also briefly discuss about effect of environmental risk factors on α-Syn aggregation.

Sorting out release, uptake and processing of alpha-synuclein during prion-like spread of pathology

Parkinson's disease is a progressive neurological disorder that is characterized by the formation of intracellular protein inclusion bodies composed primarily of a misfolded and aggregated form of the protein α-synuclein. There is growing evidence that supports the prion-like hypothesis of α-synuclein progression. This hypothesis postulates that α-synuclein is a prion-like pathological agent and is responsible for the progression of Parkinson pathology in the brain. Potential misfolding or aggregation of α-synuclein that might occur in the peripheral nervous system as a result of some insult, environmental or genetic (or more likely a combination of both) that might spread into the midbrain, eventually causing degeneration of the neurons in the substantia nigra. As the disease progresses further, it is likely that α-synuclein pathology continues to spread throughout the brain, including the cortex, leading to deterioration of cognition and higher brain functions. While it is unknown why α-synuclein initially misfolds and aggregates, a great deal has been learned about how the cell handles aberrant α-synuclein assemblies. In this review, we focus on these mechanisms and discuss them in an attempt to define the role that they might play in the propagation of misfolded α-synuclein from cell-to-cell. The prion-like hypothesis of α-synuclein pathology suggests a method for the transmission of misfolded α-synuclein from one neuron to another. This hypothesis postulates that misfolded α-synuclein becomes aggregation prone and when released and taken up by neighboring cells, seeds further misfolding and aggregation. In this review we examine the cellular mechanisms that are involved in the processing of α-synuclein and how these may contribute to the prion-like propagation of α-synuclein pathology. This article is part of a special issue on Parkinson disease.

Parkin and PINK1 Patient iPSC-Derived Midbrain Dopamine Neurons Exhibit Mitochondrial Dysfunction and α-Synuclein Accumulation

Parkinson's disease (PD) is characterized by the selective loss of dopamine neurons in the substantia nigra; however, the mechanism of neurodegeneration in PD remains unclear. A subset of familial PD is linked to mutations in PARK2 and PINK1, which lead to dysfunctional mitochondria-related proteins Parkin and PINK1, suggesting that pathways implicated in these monogenic forms could play a more general role in PD. We demonstrate that the identification of disease-related phenotypes in PD-patient-specific induced pluripotent stem cell (iPSC)-derived midbrain dopamine (mDA) neurons depends on the type of differentiation protocol utilized. In a floor-plate-based but not a neural-rosette-based directed differentiation strategy, iPSC-derived mDA neurons recapitulate PD phenotypes, including pathogenic protein accumulation, cell-type-specific vulnerability, mitochondrial dysfunction, and abnormal neurotransmitter homeostasis. We propose that these form a pathogenic loop that contributes to disease. Our study illustrates the promise of iPSC technology for examining PD pathogenesis and identifying therapeutic targets.

Peroxynitrite: From interception to signaling

Peroxynitrite is a strong oxidant and nitrating species that mediates certain biological effects of superoxide and nitrogen monoxide. These biological effects include oxidative damage to proteins as well as the formation of 3-nitrotyrosyl moieties in proteins. As a consequence, such proteins may lose their activity, gain altered function, or become prone to proteolytic degradation - resulting in modulation of cellular protein turnover and in the modulation of signaling cascades. In analogy to hydrogen peroxide, peroxynitrite may be scavenged by selenoproteins like glutathione peroxidase-1 (GPx-1) or by selenocompounds with a GPx-like activity, such as ebselen; in further analogy to H2O2, peroxiredoxins have also been established as contributors to peroxynitrite reduction. This review covers three aspects of peroxynitrite biochemistry, (i) the interaction of selenocompounds/-proteins with peroxynitrite, (ii) peroxynitrite-induced modulation of cellular proteolysis, and (iii) peroxynitrite-induced modulation of cellular signaling.

Proteolysis of α-synuclein fibrils in the lysosomal pathway limits induction of inclusion pathology

Progression of α-synuclein inclusion pathology may occur through cycles of release and uptake of α-synuclein aggregates, which induce additional intracellular α-synuclein inclusion pathology. This process may explain (i) the presence of α-synuclein inclusion pathology in grafted cells in human brains, and (ii) the slowly progressive nature of most human α-synucleinopathies. It also provides a rationale for therapeutic targeting of extracellular aggregates to limit pathology spread. We investigated the cellular mechanisms underlying intraneuronal α-synuclein aggregation following exposure to exogenous preformed α-synuclein amyloid fibrils. Exogenous α-synuclein fibrils efficiently attached to cell membranes and were subsequently internalized and degraded within the endosomal/lysosomal system. However, internalized α-synuclein amyloid fibrils can apparently overwhelm the endosomal/lysosomal machinery leading to the induction of intraneuronal α-synuclein inclusions comprised of endogenous α-synuclein. Furthermore, the efficiency of inclusion formation was relatively low in these studies compared to studies using primary neuronal-glial cultures over-expressing α-synuclein. Our study indicates that under physiologic conditions, endosomal/lysosomal function acts as an endogenous barrier to the induction of α-synuclein inclusion pathology, but when compromised, it may lower the threshold for pathology induction/transmission. Cover Image for this issue: doi: 10.1111/jnc.13787.

C-Terminal Tyrosine Residue Modifications Modulate the Protective Phosphorylation of Serine 129 of α-Synuclein in a Yeast Model of Parkinson's Disease

Parkinson´s disease (PD) is characterized by the presence of proteinaceous inclusions called Lewy bodies that are mainly composed of α-synuclein (αSyn). Elevated levels of oxidative or nitrative stresses have been implicated in αSyn related toxicity. Phosphorylation of αSyn on serine 129 (S129) modulates autophagic clearance of inclusions and is prominently found in Lewy bodies. The neighboring tyrosine residues Y125, Y133 and Y136 are phosphorylation and nitration sites. Using a yeast model of PD, we found that Y133 is required for protective S129 phosphorylation and for S129-independent proteasome clearance. αSyn can be nitrated and form stable covalent dimers originating from covalent crosslinking of two tyrosine residues. Nitrated tyrosine residues, but not di-tyrosine-crosslinked dimers, contributed to αSyn cytotoxicity and aggregation. Analysis of tyrosine residues involved in nitration and crosslinking revealed that the C-terminus, rather than the N-terminus of αSyn, is modified by nitration and di-tyrosine formation. The nitration level of wild-type αSyn was higher compared to that of A30P mutant that is non-toxic in yeast. A30P formed more dimers than wild-type αSyn, suggesting that dimer formation represents a cellular detoxification pathway in yeast. Deletion of the yeast flavohemoglobin gene YHB1 resulted in an increase of cellular nitrative stress and cytotoxicity leading to enhanced aggregation of A30P αSyn. Yhb1 protected yeast from A30P-induced mitochondrial fragmentation and peroxynitrite-induced nitrative stress. Strikingly, overexpression of neuroglobin, the human homolog of YHB1, protected against αSyn inclusion formation in mammalian cells. In total, our data suggest that C-terminal Y133 plays a major role in αSyn aggregate clearance by supporting the protective S129 phosphorylation for autophagy and by promoting proteasome clearance. C-terminal tyrosine nitration increases pathogenicity and can only be partially detoxified by αSyn di-tyrosine dimers. Our findings uncover a complex interplay between S129 phosphorylation and C-terminal tyrosine modifications of αSyn that likely participates in PD pathology.

Parkinson’s disease is characterized by loss of dopaminergic neurons in midbrain and the presence of αSyn protein inclusions. Human αSyn mimics the disease pathology in yeast resulting in cytotoxicity and aggregate formation. αSyn is abundantly phosphorylated at serine S129 and possesses four tyrosines (Y39, Y125, Y133, and Y136) that can be posttranslationally modified by nitration or phosphorylation. The consequence of each of these possible modifications is still unclear. Nitration as consequence of oxidative stress is a hallmark for neurodegenerative diseases. Here, we addressed the molecular mechanism, how tyrosine posttranslational modifications affect αSyn cytotoxicity. Tyrosine nitration can contribute to αSyn toxicity or can be part of a cellular salvage pathway when di-tyrosine-crosslinked dimers are formed. The Y133 residue, which can be either phosphorylated or nitrated, determines whether S129 is protectively phosphorylated and αSyn inclusions are cleared. This interplay with S129 phosphorylation demonstrates a dual role for C-terminal tyrosine residues. Yeast flavohemoglobin Yhb1 and its human counterpart neuroglobin NGB protect cells against cytotoxicity and aggregate formation. These novel insights into the molecular pathways responsible for αSyn cytotoxicity indicate NGB as a potential target for therapeutic intervention in PD.

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.

N-Acetyl Cysteine May Support Dopamine Neurons in Parkinson's Disease: Preliminary Clinical and Cell Line Data

Backgound

The purpose of this study was to assess the biological and clinical effects of n-acetyl-cysteine (NAC) in Parkinson’s disease (PD).

Methods

The overarching goal of this pilot study was to generate additional data about potentially protective properties of NAC in PD, using an in vitro and in vivo approach. In preparation for the clinical study we performed a cell tissue culture study with human embryonic stem cell (hESC)-derived midbrain dopamine (mDA) neurons that were treated with rotenone as a model for PD. The primary outcome in the cell tissue cultures was the number of cells that survived the insult with the neurotoxin rotenone. In the clinical study, patients continued their standard of care and were randomized to receive either daily NAC or were a waitlist control. Patients were evaluated before and after 3 months of receiving the NAC with DaTscan to measure dopamine transporter (DAT) binding and the Unified Parkinson’s Disease Rating Scale (UPDRS) to measure clinical symptoms.

Results

The cell line study showed that NAC exposure resulted in significantly more mDA neurons surviving after exposure to rotenone compared to no NAC, consistent with the protective effects of NAC previously observed. The clinical study showed significantly increased DAT binding in the caudate and putamen (mean increase ranging from 4.4% to 7.8%; p<0.05 for all values) in the PD group treated with NAC, and no measurable changes in the control group. UPDRS scores were also significantly improved in the NAC group (mean improvement of 12.9%, p = 0.01).

Conclusions

The results of this preliminary study demonstrate for the first time a potential direct effect of NAC on the dopamine system in PD patients, and this observation may be associated with positive clinical effects. A large-scale clinical trial to test the therapeutic efficacy of NAC in this population and to better elucidate the mechanism of action is warranted.

Trial Registration

ClinicalTrials.gov NCT02445651

Yeast as a model for the identification of novel survival-promoting compounds applicable to treat degenerative diseases

Programmed cell death (PCD) plays an important role in development and normal metabolic functioning of organisms. Excessive cell death is the cause of many degenerative diseases, like neurodegenerative disorders and Wilson's disease, for which current therapies remain insufficient. Current therapies are mainly focused on decreasing the disease symptoms following cell death, rather than blocking the cell death process itself. The latter can be obtained by either decreasing the presence of the toxic trigger (like protein aggregation in case of many commonly known neurodegenerative diseases) or by blocking death-inducing signaling cascade(s). Given the high conservation in PCD processes between yeast and mammalian cells, in this review, we will focus on yeast as a model organism to study PCD-related diseases as well as on its use for drug discovery purposes. More specifically, we will provide a comprehensive overview of new compounds, which were identified in yeast-based drug screens, that either decrease the amount of toxic trigger or inhibit PCD signaling cascades under PCD-inducing conditions.