The phosphorylation of α-synuclein: development and implication for the mechanism and therapy of the Parkinson's disease

Importance of glucose and its metabolism in neurodegenerative disorder, as well as the combination of multiple therapeutic strategies targeting α-synuclein and neuroprotection in the treatment of Parkinson's disease

According to recent findings, Phosphoglycerate Kinase 1 (pgk-1) enzyme is linked to Parkinson's disease (PD). Mutations in the PGK-1 gene lead to decreases in the pgk-1 enzyme which causes an imbalance in the levels of energy demand and supply. An increase in glycolytic adenosine triphosphate (ATP) production would help alleviate energy deficiency and sustain the acute energetic need of neurons. Neurodegeneration is caused by an imbalance or reduction in ATP levels. Recent data suggest that medications that increase glycolysis and neuroprotection can be used to treat PD. The current study focuses on treatment options for disorders associated with the pgk-1 enzyme, GLP-1, and A2A receptor which can be utilized to treat PD. A combination of metformin and terazosin, exenatide and meclizine, istradefylline and salbutamol treatments may benefit parkinsonism. The review also looked at potential target-specific new techniques that might assist in satisfying unfulfilled requirements in the treatment of PD.

Intra-striatal infusion of the small molecule alpha-synuclein aggregator, FN075, does not enhance parkinsonism in a subclinical AAV-alpha-synuclein rat model

Numerous challenges hinder the development of neuroprotective treatments for Parkinson's disease, with a regularly identified issue being the lack of clinically relevant animal models. Viral vector overexpression of α-synuclein is widely considered the most relevant model; however, this has been limited by high variability and inconsistency. One potential method of optimisation is pairing it with a secondary insult such as FN075, a synthetic molecule demonstrated to accelerate α-synucleinopathy. Thus, the aim of this study was to investigate if sequential infusion of adeno-associated virus (AAV)-α-synuclein and FN075 into the rat brain can replicate α-synucleinopathy, nigrostriatal pathology and motor dysfunction associated with Parkinson's disease. Rats received a unilateral injection of AAV-α-synuclein (or AAV-green fluorescent protein) into two sites in the substantia nigra, followed 4 weeks later by unilateral injection of FN075 (or vehicle) into the striatum. Animals underwent behavioural testing every 4 weeks until sacrifice at 20 weeks, followed by immunohistochemistry assessment post-mortem. As anticipated, AAV-α-synuclein led to extensive overexpression of human α-synuclein throughout the nigrostriatal pathway, as well as elevated levels of phosphorylated and aggregated forms of the protein. However, the sequential administration of FN075 into the striatum did not exacerbate any of the α-synuclein pathology. Furthermore, despite the extensive α-synuclein pathology, neither administration of AAV-α-synuclein nor FN075, alone or in combination, was sufficient to induce dopaminergic degeneration or motor deficits. In conclusion, this approach did not replicate the key characteristics of Parkinson's disease, and further studies are required to create more representational models for testing of novel compounds and treatments for Parkinson's disease.

Aflatoxin B1 Increases Soluble Epoxide Hydrolase in the Brain and Induces Neuroinflammation and Dopaminergic Neurotoxicity

Parkinson's disease (PD) is an increasingly common neurodegenerative movement disorder with contributing factors that are still largely unexplored and currently no effective intervention strategy. Epidemiological and pre-clinical studies support the close association between environmental toxicant exposure and PD incidence. Aflatoxin B1 (AFB1), a hazardous mycotoxin commonly present in food and environment, is alarmingly high in many areas of the world. Previous evidence suggests that chronic exposure to AFB1 leads to neurological disorders as well as cancer. However, whether and how aflatoxin B1 contributes to the pathogenesis of PD is poorly understood. Here, oral exposure to AFB1 is shown to induce neuroinflammation, trigger the α-synuclein pathology, and cause dopaminergic neurotoxicity. This was accompanied by the increased expression and enzymatic activity of soluble epoxide hydrolase (sEH) in the mouse brain. Importantly, genetic deletion or pharmacological inhibition of sEH alleviated the AFB1-induced neuroinflammation by reducing microglia activation and suppressing pro-inflammatory factors in the brain. Furthermore, blocking the action of sEH attenuated dopaminergic neuron dysfunction caused by AFB1 in vivo and in vitro. Together, our findings suggest a contributing role of AFB1 to PD etiology and highlight sEH as a potential pharmacological target for alleviating PD-related neuronal disorders caused by AFB1 exposure.

Phosphorylation of α-synuclein at T64 results in distinct oligomers and exerts toxicity in models of Parkinson's disease

α-Synuclein accumulates in Lewy bodies, and this accumulation is a pathological hallmark of Parkinson's disease (PD). Previous studies have indicated a causal role of α-synuclein in the pathogenesis of PD. However, the molecular and cellular mechanisms of α-synuclein toxicity remain elusive. Here, we describe a novel phosphorylation site of α-synuclein at T64 and the detailed characteristics of this post-translational modification. T64 phosphorylation was enhanced in both PD models and human PD brains. T64D phosphomimetic mutation led to distinct oligomer formation, and the structure of the oligomer was similar to that of α-synuclein oligomer with A53T mutation. Such phosphomimetic mutation induced mitochondrial dysfunction, lysosomal disorder, and cell death in cells and neurodegeneration in vivo, indicating a pathogenic role of α-synuclein phosphorylation at T64 in PD.

Galectin-3 shapes toxic alpha-synuclein strains in Parkinson's disease

Parkinson's Disease (PD) is a neurodegenerative and progressive disorder characterised by intracytoplasmic inclusions called Lewy bodies (LB) and degeneration of dopaminergic neurons in the substantia nigra (SN). Aggregated α-synuclein (αSYN) is known to be the main component of the LB. It has also been reported to interact with several proteins and organelles. Galectin-3 (GAL3) is known to have a detrimental function in neurodegenerative diseases. It is a galactose-binding protein without known catalytic activity and is expressed mainly by activated microglial cells in the central nervous system (CNS). GAL3 has been previously found in the outer layer of the LB in post-mortem brains. However, the role of GAL3 in PD is yet to be elucidated. In post-mortem samples, we identified an association between GAL3 and LB in all the PD subjects studied. GAL3 was linked to less αSYN in the LB outer layer and other αSYN deposits, including pale bodies. GAL3 was also associated with disrupted lysosomes. In vitro studies demonstrate that exogenous recombinant Gal3 is internalised by neuronal cell lines and primary neurons where it interacts with endogenous αSyn fibrils. In addition, aggregation experiments show that Gal3 affects spatial propagation and the stability of pre-formed αSyn fibrils resulting in short, amorphous toxic strains. To further investigate these observations in vivo, we take advantage of WT and Gal3KO mice subjected to intranigral injection of adenovirus overexpressing human αSyn as a PD model. In line with our in vitro studies, under these conditions, genetic deletion of GAL3 leads to increased intracellular αSyn accumulation within dopaminergic neurons and remarkably preserved dopaminergic integrity and motor function. Overall, our data suggest a prominent role for GAL3 in the aggregation process of αSYN and LB formation, leading to the production of short species to the detriment of larger strains which triggers neuronal degeneration in a mouse model of PD.

Interaction and Redox Chemistry between Iron, Dopamine, and Alpha-Synuclein C-Terminal Peptides

α-Synuclein (αS), dopamine (DA), and iron have a crucial role in the etiology of Parkinson's disease. The present study aims to investigate the interplay between these factors by analyzing the DA/iron interaction and how it is affected by the presence of the C-terminal fragment of αS (Ac-αS_119-132) that represents the iron-binding domain. At high DA:Fe molar ratios, the formation of the [Fe^III(DA)₂]^- complex prevents the interaction with αS peptides, whereas, at lower DA:Fe molar ratios, the peptide is able to compete with one of the two coordinated DA molecules. This interaction is also confirmed by HPLC-MS analysis of the post-translational modifications of the peptide, where oxidized αS is observed through an inner-sphere mechanism. Moreover, the presence of phosphate groups in Ser129 (Ac-αS^pS_119-132) and both Ser129 and Tyr125 (Ac-αS^pY^pS_119-132) increases the affinity for iron(III) and decreases the DA oxidation rate, suggesting that this post-translational modification may assume a crucial role for the αS aggregation process. Finally, αS interaction with cellular membranes is another key aspect for αS physiology. Our data show that the presence of a membrane-like environment induced an enhanced peptide effect over both the DA oxidation and the [Fe^III(DA)₂]^- complex formation and decomposition.

α-Synuclein decoy peptide protects mice against α-synuclein-induced memory loss

AIMS

We previously found that a decoy peptide derived from the C-terminal sequence of α-Synuclein (αSyn) prevents cytotoxic αSyn aggregation caused by fatty acid-binding protein 3 (FABP3) in vitro. In this study, we continued to utilize αSyn-derived peptides to further validate their effects on αSyn neurotoxicity and behavioral impairments in αSyn preformed fibrils (PFFs)-injected mouse model of Parkinson's disease (PD).

METHODS

Mice were injected with αSyn PFFs in the bilateral olfactory bulb (OB) and then were subjected to behavioral analysis at 2-week intervals post-injection. Peptides nasal administration was initiated one week after injection. Changes in phosphorylation of αSyn and neuronal damage in the OB were measured using immunostaining at week 4. The effect of peptides on the interaction between αSyn and FABP3 was examined using co-immunoprecipitation.

RESULTS

αSyn PFF-injected mice showed significant memory loss but no motor function impairment. Long-term nasal treatment with peptides effectively prevented memory impairment. In peptide-treated αSyn PFF-injected mice, the peptides entered the OB smoothly through the nasal cavity and were mainly concentrated in neurons in the mitral cell layer, significantly suppressing the excessive phosphorylation of αSyn and reducing the formation of αSyn-FABP3 oligomers, thereby preventing neuronal death. The addition of peptides also blocked the interaction of αSyn and FABP3 at the recombinant protein level, and its effect was strongest at molar concentrations comparable to those of αSyn and FABP3.

CONCLUSIONS

Our findings suggest that the αSyn decoy peptide represents a novel therapeutic approach for reducing the accumulation of toxic αSyn-FABP3 oligomers in the brain, thereby preventing the progression of synucleinopathies.

The role of altered protein acetylation in neurodegenerative disease

Acetylation is a key post-translational modification (PTM) involved in the regulation of both histone and non-histone proteins. It controls cellular processes such as DNA transcription, RNA modifications, proteostasis, aging, autophagy, regulation of cytoskeletal structures, and metabolism. Acetylation is essential to maintain neuronal plasticity and therefore essential for memory and learning. Homeostasis of acetylation is maintained through the activities of histone acetyltransferases (HAT) and histone deacetylase (HDAC) enzymes, with alterations to these tightly regulated processes reported in several neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS). Both hyperacetylation and hypoacetylation can impair neuronal physiological homeostasis and increase the accumulation of pathophysiological proteins such as tau, α-synuclein, and Huntingtin protein implicated in AD, PD, and HD, respectively. Additionally, dysregulation of acetylation is linked to impaired axonal transport, a key pathological mechanism in ALS. This review article will discuss the physiological roles of protein acetylation and examine the current literature that describes altered protein acetylation in neurodegenerative disorders.

Hypoxic stress accelerates the propagation of pathological alpha-synuclein and degeneration of dopaminergic neurons

AIMS

The etiology of Parkinson's disease (PD) is complex and the mechanism is unclear. It has become a top priority to find common factors that induce and affect PD pathology. We explored the key role of hypoxia in promoting the pathological propagation of α-synuclein (α-syn) and the progression of PD.

METHODS

We performed PD modeling by conducting intracranial stereotaxic surgery in the unilateral striatum of mice. We then measured protein aggregation in vitro. The rotarod and pole tests were employed next to measure the damage of the phenotype. Pathological deposition and autophagy were also observed by immunofluorescence staining and protein levels measured by western blotting.

RESULTS

We demonstrated that short-term hypoxia activated phosphorylated (p)-α-syn in mice. We confirmed that p-α-syn was more readily formed aggregates than α-syn in vitro. Furthermore, we found that hypoxia promoted the activation and propagation of endogenous α-syn, contributing to the earlier degeneration of dopaminergic neurons in the substantia nigra and the deposition of p-α-syn in our animal model. Finally, autophagy inhibition contributed to the above pathologies.

CONCLUSION

Hypoxia was shown to accelerate the pathological progression and damage phenotype in PD model mice. The results provided a promising research target for determining common interventions for PD in the future.

Metformin: Activation of 5′ AMP-activated protein kinase and its emerging potential beyond anti-hyperglycemic action

Metformin is a plant-based drug belonging to the class of biguanides and is known to treat type-2 diabetes mellitus (T2DM). The drug, combined with controlling blood glucose levels, improves the body’s response to insulin. In addition, trials have identified the cardioprotective potential of metformin in the diabetic population receiving the drug. Activation of 5′ AMP-activated protein kinase (AMPK) is the major pathway for these potential beneficial effects of metformin. Historically, much emphasis has been placed on the potential indications of metformin beyond its anti-diabetic use. This review aims to appraise other potential uses of metformin primarily mediated by the activation of AMPK. We also discuss various mechanisms, other than AMPK activation, by which metformin could produce beneficial effects for different conditions. Databases including PubMed/MEDLINE and Embase were searched for literature relevant to the review’s objective. Reports from both research and review articles were considered. We found that metformin has diverse effects on the human body systems. It has been shown to exert anti-inflammatory, antioxidant, cardioprotective, metabolic, neuroprotective, anti-cancer, and antimicrobial effects and has now even been identified as effective against SARS-CoV-2. Above all, the AMPK pathway has been recognized as responsible for metformin’s efficiency and effectiveness. Owing to its extensive potential, it has the capability to become a part of treatment regimens for diseases apart from T2DM.

CK2 and protein kinases of the CK1 superfamily as targets for neurodegenerative disorders

Casein kinases are involved in a variety of signaling pathways, and also in inflammation, cancer, and neurological diseases. Therefore, they are regarded as potential therapeutic targets for drug design. Recent studies have highlighted the importance of the casein kinase 1 superfamily as well as protein kinase CK2 in the development of several neurodegenerative pathologies, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. CK1 kinases and their closely related tau tubulin kinases as well as CK2 are found to be overexpressed in the mammalian brain. Numerous substrates have been detected which play crucial roles in neuronal and synaptic network functions and activities. The development of new substances for the treatment of these pathologies is in high demand. The impact of these kinases in the progress of neurodegenerative disorders, their bona fide substrates, and numerous natural and synthetic compounds which are able to inhibit CK1, TTBK, and CK2 are discussed in this review.

Validation of Top-Down Proteomics Data by Bottom-Up-Based N-Terminomics Reveals Pitfalls in Top-Down-Based Terminomics Workflows

Bottom-up proteomics (BUP)-based N-terminomics techniques have become standard to identify protein N-termini. While these methods rely on the identification of N-terminal peptides only, top-down proteomics (TDP) comes with the promise to provide additional information about post-translational modifications and the respective C-termini. To evaluate the potential of TDP for terminomics, two established TDP workflows were employed for the proteome analysis of the nematode Caenorhabditis elegans. The N-termini of the identified proteoforms were validated using a BUP-based N-terminomics approach. The TDP workflows used here identified 1658 proteoforms, the N-termini of which were verified by BUP in 25% of entities only. Caveats in both the BUP- and TDP-based workflows were shown to contribute to this low overlap. In BUP, the use of trypsin prohibits the detection of arginine-rich or arginine-deficient N-termini, while in TDP, the formation of artificially generated termini was observed in particular in a workflow encompassing sample treatment with high acid concentrations. Furthermore, we demonstrate the applicability of reductive dimethylation in TDP to confirm biological N-termini. Overall, our study shows not only the potential but also current limitations of TDP for terminomics studies and also presents suggestions for future developments, for example, for data quality control, allowing improvement of the detection of protein termini by TDP.

Actions of Metformin in the Brain: A New Perspective of Metformin Treatments in Related Neurological Disorders

Metformin is a first-line drug for treating type 2 diabetes mellitus (T2DM) and one of the most commonly prescribed drugs in the world. Besides its hypoglycemic effects, metformin also can improve cognitive or mood functions in some T2DM patients; moreover, it has been reported that metformin exerts beneficial effects on many neurological disorders, including major depressive disorder (MDD), Alzheimer’s disease (AD) and Fragile X syndrome (FXS); however, the mechanism underlying metformin in the brain is not fully understood. Neurotransmission between neurons is fundamental for brain functions, and its defects have been implicated in many neurological disorders. Recent studies suggest that metformin appears not only to regulate synaptic transmission or plasticity in pathological conditions but also to regulate the balance of excitation and inhibition (E/I balance) in neural networks. In this review, we focused on and reviewed the roles of metformin in brain functions and related neurological disorders, which would give us a deeper understanding of the actions of metformin in the brain.

Glial Fibrillary Acidic Protein: A Biomarker and Drug Target for Alzheimer’s Disease

Glial fibrillary acidic protein (GFAP) is an intermediate filament structural protein involved in cytoskeleton assembly and integrity, expressed in high abundance in activated glial cells. GFAP is neuroprotective, as knockout mice are hypersensitive to traumatic brain injury. GFAP in cerebrospinal fluid is a biomarker of Alzheimer’s disease (AD), dementia with Lewy bodies, and frontotemporal dementia (FTD). Here, we present novel evidence that GFAP is markedly overexpressed and differentially phosphorylated in AD hippocampus, especially in AD with the apolipoprotein E [ε4, ε4] genotype, relative to age-matched controls (AMCs). Kinases that phosphorylate GFAP are upregulated in AD relative to AMC. A knockdown of these kinases in SH-SY5Y-APP_Sw human neuroblastoma cells reduced amyloid accrual and lowered protein aggregation and associated behavioral traits in C. elegans models of polyglutamine aggregation (as observed in Huntington’s disease) and of Alzheimer’s-like amyloid formation. In silico screening of the ChemBridge structural library identified a small molecule, MSR1, with stable and specific binding to GFAP. Both MSR1 exposure and GF AP-specific RNAi knockdown reduce aggregation with remarkably high concordance of aggregate proteins depleted. These data imply that GFAP and its phosphorylation play key roles in neuropathic aggregate accrual and provide valuable new biomarkers, as well as novel therapeutic targets to alleviate, delay, or prevent AD.

Skin alpha-synuclein deposit patterns: A predictor of Parkinson's disease subtypes

Parkinson's disease (PD) is a neurodegenerative disease characterized pathologically by the formation of Lewy bodies comprised mainly of α-synuclein. Assessment of skin synuclein has the potential as an excellent diagnostic method with high sensitivity, specificity, and reproducibility that is also convenient and acceptable to patients. In this review, we summarize findings regarding the characteristics of cutaneous nerve p-α-syn or α-syn deposits and their correlations with clinical phenotypes in PD patients with and without orthostatic hypotension and LRRK2, GBA, and SNCA gene mutations. This review can serve as a reference for the diagnosis and classification of PD based on α-syn deposit patterns and to deeply explore its pathogenesis.

Funding statement

The work was partly supported by the National Natural Science Key Foundation of China (No. 81830040 and No 82130042) and the Program of Excellent Talents in Medical Science of Jiangsu Province (No. JCRCA2016006) .

Modelling α-Synuclein Aggregation and Neurodegeneration with Fibril Seeds in Primary Cultures of Mouse Dopaminergic Neurons

To model α-Synuclein (αS) aggregation and neurodegeneration in Parkinson’s disease (PD), we established cultures of mouse midbrain dopamine (DA) neurons and chronically exposed them to fibrils 91 (F91) generated from recombinant human αS. We found that F91 have an exquisite propensity to seed the aggregation of endogenous αS in DA neurons when compared to other neurons in midbrain cultures. Until two weeks post-exposure, somal aggregation in DA neurons increased with F91 concentrations (0.01–0.75 μM) and the time elapsed since the initiation of seeding, with, however, no evidence of DA cell loss within this time interval. Neither toxin-induced mitochondrial deficits nor genetically induced loss of mitochondrial quality control mechanisms promoted F91-mediated αS aggregation or neurodegeneration under these conditions. Yet, a significant loss of DA neurons (~30%) was detectable three weeks after exposure to F91 (0.5 μM), i.e., at a time point where somal aggregation reached a plateau. This loss was preceded by early deficits in DA uptake. Unlike αS aggregation, the loss of DA neurons was prevented by treatment with GDNF, suggesting that αS aggregation in DA neurons may induce a form of cell death mimicking a state of trophic factor deprivation. Overall, our model system may be useful for exploring PD-related pathomechanisms and for testing molecules of therapeutic interest for this disorder.

Neuroprotective effect of paeoniflorin in the mouse model of Parkinson's disease through α-synuclein/protein kinase C δ subtype signaling pathway

OBJECTIVES

Paeoniflorin, an active component of Radix Paeoniae Alba, has a neuroprotective effect in Parkinson's animal models. However, its mechanism of action remains to be determined.

METHODS

In this study, we hypothesized that the neuroprotective effect of paeoniflorin occurs through the α-synuclein/protein kinase C δ subtype (PKC-δ) signaling pathway. We tested our hypothesis in the 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP)-induced mouse model of Parkinson's disease. We evaluated the effects of paeoniflorin on the expression levels of signal components of the α-synuclein/PKC-δ pathway, cellular apoptosis and motor performance.

RESULTS

Our results demonstrated that paeoniflorin restored the motor performance impairment caused by MPTP, inhibited apoptosis, and protected the ultrastructure of neurons. Paeoniflorin treatment also resulted in the dose-dependent upregulation of an antiapoptotic protein, B-cell lymphoma-2, at the mRNA and protein levels, similar to the effects of the positive control, selegiline. In contrast, paeoniflorin treatment downregulated the expression of pro-apoptotic proteins BCL2-Associated X2, α-synuclein, and PKC-δ at the mRNA and protein levels, as well as the level of the activated form of nuclear factor kappa B (p-NF-κB p65).

CONCLUSIONS

Thus, our results showed that paeoniflorin exerts its neuroprotective effect by regulating the α-synuclein/PKC-δ signaling pathway to reduce neuronal apoptosis.

The Small Molecule Alpha-Synuclein Aggregator, FN075, Enhances Alpha-Synuclein Pathology in Subclinical AAV Rat Models

Animal models of Parkinson's disease, in which the human α-synuclein transgene is overexpressed in the nigrostriatal pathway using viral vectors, are widely considered to be the most relevant models of the human condition. However, although highly valid, these models have major limitations related to reliability and variability, with many animals exhibiting pronounced α-synuclein expression failing to demonstrate nigrostriatal neurodegeneration or motor dysfunction. Therefore, the aim of this study was to determine if sequential intra-nigral administration of AAV-α-synuclein followed by the small α-synuclein aggregating molecule, FN075, would enhance or precipitate the associated α-synucleinopathy, nigrostriatal pathology and motor dysfunction in subclinical models. Rats were given unilateral intra-nigral injections of AAV-α-synuclein (either wild-type or A53T mutant) followed four weeks later by a unilateral intra-nigral injection of FN075, after which they underwent behavioral testing for lateralized motor functionality until they were sacrificed for immunohistological assessment at 20 weeks after AAV administration. In line with expectations, both of the AAV vectors induced widespread overexpression of human α-synuclein in the substantia nigra and striatum. Sequential administration of FN075 significantly enhanced the α-synuclein pathology with increased density and accumulation of the pathological form of the protein phosphorylated at serine 129 (pS129-α-synuclein). However, despite this enhanced α-synuclein pathology, FN075 did not precipitate nigrostriatal degeneration or motor dysfunction in these subclinical AAV models. In conclusion, FN075 holds significant promise as an approach to enhancing the α-synuclein pathology in viral overexpression models, but further studies are required to determine if alternative administration regimes for this molecule could improve the reliability and variability in these models.

Mechanistic roles for altered O-GlcNAcylation in neurodegenerative disorders

Neurodegenerative diseases such as Alzheimer's and Parkinson's remain highly prevalent and incurable disorders. A major challenge in fully understanding and combating the progression of these diseases is the complexity of the network of processes that lead to progressive neuronal dysfunction and death. An ideal therapeutic avenue is conceivably one that could address many if not all of these multiple misregulated mechanisms. Over the years, chemical intervention for the up-regulation of the endogenous posttranslational modification (PTM) O-GlcNAc has been proposed as a potential strategy to slow down the progression of neurodegeneration. Through the development and application of tools that allow dissection of the mechanistic roles of this PTM, there is now a growing body of evidence that O-GlcNAc influences a variety of important neurodegeneration-pertinent mechanisms, with an overall protective effect. As a PTM that is appended onto numerous proteins that participate in protein quality control and homeostasis, metabolism, bioenergetics, neuronal communication, inflammation, and programmed death, O-GlcNAc has demonstrated beneficence in animal models of neurodegenerative diseases, and its up-regulation is now being pursued in multiple clinical studies.

Investigation of α-Synuclein Species in Plasma Exosomes and the Oligomeric and Phosphorylated α-Synuclein as Potential Peripheral Biomarker of Parkinson's Disease

α-Synuclein (α-syn), especially its abnormal oligomeric and phosphorylated form, plays a critical role in the pathogenesis of Parkinson's disease (PD). Plasma exosomal α-syn species have been shown to be a promising PD biomarker. However, whether different α-syn species in plasma exosomes (the oligomeric α-syn and the Ser129 phosphorylated α-syn (p-α-syn)) which represent the PD pathogenesis in the brain could be specific peripheral PD biomarker haven't been well elucidated. In this study, we successfully extracted and identified the human plasma exosomes, and the CNS-derived exosomes were detected. The different aggregation status, localization and degradation characteristics of α-syn and p-α-syn in the plasma exosomes between PD patients and healthy controls were further analyzed. The results suggested that α-syn and p-α-syn in the plasma exosomes of PD patients showed poor solubility after protease K (PK) treatment. Aggregated α-syn and p-α-syn existed both inside and on the membrane surface of plasma exosomes. The Receiver operating characteristic (ROC) performance of α-syn oligomer/total α-syn in exosomes was moderately helpful in PD diagnosis (AUC = 0.71, sensitivity = 60.5%, specificity = 59.4%), and the ratio of p-α-syn oligomer/total p-α-syn showed similar result (AUC = 0.69, sensitivity = 60.0%, specificity = 59.5%). This study indicates that the oligomeric α-syn/total α-syn and oligomeric p-α-syn/total p-α-syn ratio in plasma exosomes may be applied to assist the PD diagnosis, which needs further research.

Alpha-Synuclein Post-translational Modifications: Implications for Pathogenesis of Lewy Body Disorders

Lewy Body Disorders (LBDs) lie within the spectrum of age-related neurodegenerative diseases now frequently categorized as the synucleinopathies. LBDs are considered to be among the second most common form of neurodegenerative dementias after Alzheimer's disease. They are progressive conditions with variable clinical symptoms embodied within specific cognitive and behavioral disorders. There are currently no effective treatments for LBDs. LBDs are histopathologically characterized by the presence of abnormal neuronal inclusions commonly known as Lewy Bodies (LBs) and extracellular Lewy Neurites (LNs). The inclusions predominantly comprise aggregates of alpha-synuclein (aSyn). It has been proposed that post-translational modifications (PTMs) such as aSyn phosphorylation, ubiquitination SUMOylation, Nitration, o-GlcNacylation, and Truncation play important roles in the formation of toxic forms of the protein, which consequently facilitates the formation of these inclusions. This review focuses on the role of different PTMs in aSyn in the pathogenesis of LBDs. We highlight how these PTMs interact with aSyn to promote misfolding and aggregation and interplay with cell membranes leading to the potential functional and pathogenic consequences detected so far, and their involvement in the development of LBDs.

Deposition of Phosphorylated α-Synuclein and Activation of GSK-3β and PP2A in the PS19 Mouse Model of Tauopathy

The simultaneous accumulation of multiple pathological proteins, such as hyperphosphorylated tau (hp-tau) and phosphorylated α-synuclein (p-αSyn), has been reported in the brains of patients with various neurodegenerative diseases. We previously demonstrated that hp-tau-dependent p-αSyn accumulation was associated with the activation of GSK-3β in the brains of P301L tau transgenic mice. To confirm the effects of another mutant tau on p-αSyn accumulation in vivo, we herein examined the brains of PS19 mice that overexpress human P301S mutant tau. Immunohistochemically, hp-tau and p-αSyn aggregates were detected in the same neuronal cells in the cerebrum and brain stem of aged PS19 mice. A semiquantitative analysis showed a positive correlation between hp-tau and p-αSyn accumulation. Furthermore, an activated form of GSK-3β was detected within cells containing both hp-tau and p-αSyn aggregates in PS19 mice. Western blotting showed a decrease in inactivated PP2A levels in PS19 mice. The present results suggest that the overexpression of human P301S mutant tau induces p-αSyn accumulation that is accompanied by not only GSK-3β, but also PP2A activation in PS19 mice, and highlight the synergic effects between tau and αSyn in the pathophysiology of neurodegenerative diseases that show the codeposition of tau and αSyn.

Neurons and Glia Interplay in α-Synucleinopathies

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

LncRNA OIP5-AS1 reduces α-synuclein aggregation and toxicity by targeting miR-126 to activate PLK2 in human neuroblastoma SH-SY5Y cells

It has been reported that many long noncoding RNAs (lncRNA) are abnormally expressed in Parkinson's disease (PD). However, the knowledge about the role of dysregulated lncRNA in the pathological process of PD and the potential molecular regulation mechanism is still limited. Our immunofluorescence data show that miR-126 enhances the aggregation and toxicity of synuclein, while lncRNA OIP5-AS1 reduces the aggregation and toxicity of MPP + induced α-synuclein by targeting miR-126. Luciferase experiments have found that miR-126 regulates α-synuclein by targeting PLK2. Western blot and IP experimental analysis showed that this process is achieved by regulating PLK2/α-synuclein autophagy. In conclusion, our data indicate that OIP5-AS1 promotes the autophagy of PLK2-α-synuclein by targeting the miR-126 axis with pathogenic factors, thus reducing the aggregation toxicity of α-synuclein, which It will help better to understand the mechanism of dopaminergic neuron loss in PD and provide novel treatment options.

Determinants of seeding and spreading of α-synuclein pathology in the brain

In Parkinson's disease (PD), fibrillar forms of α-synuclein are hypothesized to propagate through synaptically coupled networks, causing Lewy pathology (LP) and neurodegeneration. To more rigorously characterize the determinants of spreading, preformed α-synuclein fibrils were injected into the mouse pedunculopontine nucleus (PPN), a brain region that manifests LP in PD patients and the distribution of developing α-synuclein pathology compared to that ascertained by anterograde and retrograde connectomic mapping. Within the PPN, α-synuclein pathology was cell-specific, being robust in PD-vulnerable cholinergic neurons but not in neighboring noncholinergic neurons. While nearly all neurons projecting to PPN cholinergics manifested α-synuclein pathology, the kinetics, magnitude, and persistence of the propagated pathology were unrelated to the strength of those connections. Thus, neuronal phenotype governs the somatodendritic uptake of pathological α-synuclein, and while the afferent connectome restricts the subsequent spreading of pathology, its magnitude and persistence is not a strict function of the strength of coupling.

Multiplicity of α-Synuclein Aggregated Species and Their Possible Roles in Disease

α-Synuclein amyloid aggregation is a defining molecular feature of Parkinson's disease, Lewy body dementia, and multiple system atrophy, but can also be found in other neurodegenerative disorders such as Alzheimer's disease. The process of α-synuclein aggregation can be initiated through alternative nucleation mechanisms and dominated by different secondary processes giving rise to multiple amyloid polymorphs and intermediate species. Some aggregated species have more inherent abilities to induce cellular stress and toxicity, while others seem to be more potent in propagating neurodegeneration. The preference for particular types of polymorphs depends on the solution conditions and the cellular microenvironment that the protein encounters, which is likely related to the distinct cellular locations of α-synuclein inclusions in different synucleinopathies, and the existence of disease-specific amyloid polymorphs. In this review, we discuss our current understanding on the nature and structure of the various types of α-synuclein aggregated species and their possible roles in pathology. Precisely defining these distinct α-synuclein species will contribute to understanding the molecular origins of these disorders, developing accurate diagnoses, and designing effective therapeutic interventions for these highly debilitating neurodegenerative diseases.

Phosphorylation and oligomerization of α-synuclein associated with GSK-3β activation in the rTg4510 mouse model of tauopathy

Neurodegenerative diseases are characterized by the accumulation of specific phosphorylated protein aggregates in the brain, such as hyperphosphorylated tau (hp-tau) in tauopathies and phosphorylated α-synuclein (p-αSyn) in α-synucleinopathies. The simultaneous accumulation of different proteins is a common event in many neurodegenerative diseases. We herein describe the detection of the phosphorylation and dimerization of αSyn and activation of GSK-3β, a major kinase known to phosphorylate tau and αSyn, in the brains of rTg4510 mice that overexpress human P301L mutant tau. Immunohistochemistry showed p-αSyn aggregates in rTg4510 mice, which were suppressed by doxycycline-mediated decreases in mutant tau expression levels. A semi-quantitative analysis revealed a regional correlation between hp-tau and p-αSyn accumulation in rTg4510 mice. Furthermore, proteinase K-resistant αSyn aggregates were found in the region with excessive hp-tau accumulation in rTg4510 mice, and these aggregates were morphologically different from proteinase K-susceptible p-αSyn aggregates. Western blotting revealed decreases in p-αSyn monomers in TBS- and sarkosyl-soluble fractions and increases in ubiquitinated p-αSyn dimers in sarkosyl-soluble and insoluble fractions in rTg4510 mice. Furthermore, an activated form of GSK-3β was immunohistochemically detected within cells containing both hp-tau and p-αSyn aggregates. A semi-quantitative analysis revealed that increased GSK-3β activity strongly correlated with hp-tau and p-αSyn accumulation in rTg4510 mice. Collectively, the present results suggest that the overexpression of human P301L mutant tau promoted the phosphorylation and dimerization of endogenous αSyn by activating GSK-3β in rTg4510 mice. This synergic effect between tau, αSyn, and GSK-3β may be involved in the pathophysiology of several neurodegenerative diseases that show the accumulation of both tau and αSyn.

Insulin Resistance Promotes Parkinson's Disease through Aberrant Expression of α-Synuclein, Mitochondrial Dysfunction, and Deregulation of the Polo-Like Kinase 2 Signaling

Background: Insulin resistance (IR), considered a hallmark of diabetes at the cellular level, is implicated in pre-diabetes, results in type 2 diabetes, and negatively affects mitochondrial function. Diabetes is increasingly associated with enhanced risk of developing Parkinson's disease (PD); however, the underlying mechanism remains unclear. This study investigated the probable culpability of IR in the pathogenesis of PD. Methods: Using MitoPark mice in vivo models, diabetes was induced by a high-fat diet in the in vivo models, and IR was induced by protracted pulse-stimulation with 100 nM insulin treatment of neuronal cells, in vitro to determine the molecular mechanism(s) underlying altered cellular functions in PD, including mitochondrial dysfunction and α-synuclein (SNCA) aberrant expression. Findings: We observed increased SNCA expression in the dopaminergic (DA) neurons of both the wild-type and diabetic MitoPark mice, coupled with enhanced degeneration of DA neurons in the diabetic MitoPark mice. Ex vivo, in differentiated human DA neurons, IR was associated with increased SNCA and reactive oxygen species (ROS) levels, as well as mitochondrial depolarization. Moreover, we demonstrated concomitant hyperactivation of polo-like kinase-2 (PLK2), and upregulated p-SNCA (Ser129) and proteinase K-resistant SNCA proteins level in IR SH-SY5Y cells, however the inhibition of PLK2 reversed IR-related increases in phosphorylated and total SNCA. Similarly, the overexpression of peroxisome proliferator-activated receptor-γ coactivator 1-alpha (PGC)-1α suppressed ROS production, repressed PLK2 hyperactivity, and resulted in downregulation of total and Ser129-phosphorylated SNCA in the IR SH-SY5Y cells. Conclusions: These findings demonstrate that IR-associated diabetes promotes the development and progression of PD through PLK2-mediated mitochondrial dysfunction, upregulated ROS production, and enhanced SNCA signaling, suggesting the therapeutic targetability of PLK2 and/or SNCA as potential novel disease-modifying strategies in patients with PD.

Identification of potential natural neuroprotective molecules for Parkinson's disease by using chemoinformatics and molecular docking

Parkinson's disease is a progressive nervous system disorder characterized by motor, cognitive, sensory, psychiatric, and autonomic disturbances. While there is currently no cure for Parkinson's Disease, medication can offer relief from its symptoms for many years. Although these medications are considered safe, they can present acute or chronic side effects and can become less effective over time. Thus, new medications are highly needed. In this regard, α-synuclein is a protein of great interest to Parkinson's researchers because it is a major constituent of Lewy bodies, which are protein clumps being the pathological hallmark of Parkinson's disease. However, current medications are not focused on the inhibition of α-synuclein oligomerization, and therefore, therapeutics preventing the formation of these bodies through the inhibition of α-synuclein oligomerization may play a role in the fight against this and other synucleinopathies. In this study, we used chemoinformatics tools and molecular docking simulations to analyze molecules that have been experimentally tested and bound to α-synuclein, causing neuroprotective or neurotoxic activity, and whose results have been used to select potential natural neuroprotective molecules. We identified 6 potential natural neuroprotective molecules that are similar in their chemical structure to neuroprotective molecules and have a high number of hydrogen bonds with α-synuclein. We expect that these molecules may lead to the design or discovery of new effective treatments for Parkinson's disease.

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

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

Emerging neuroprotective effect of metformin in Parkinson's disease: A molecular crosstalk

Parkinson's disease (PD) is a devastating neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and Lewy pathology. PD is a major concern of today's aging population and has emerged as a global health burden. Despite the rapid advances in PD research over the past decades, the gold standard therapy provides only symptomatic relief and fails to halt disease progression. Therefore, exploring novel disease-modifying therapeutic strategies is highly demanded. Metformin, which is currently used as a first-line therapy for type 2 diabetes mellitus (T2DM), has recently demonstrated to exert a neuroprotective role in several neurodegenerative disorders including PD, both in vitro and in vivo. In this review, we explore the neuroprotective potential of metformin based on emerging evidence from pre-clinical and clinical studies. Regarding the underlying molecular mechanisms, metformin has been shown to inhibit α-synuclein (SNCA) phosphorylation and aggregation, prevent mitochondrial dysfunction, attenuate oxidative stress, modulate autophagy mainly via AMP-activated protein kinase (AMPK) activation, as well as prevent neurodegeneration and neuroinflammation. Overall, the neuroprotective effects of metformin in PD pathogenesis present a novel promising therapeutic strategy that might overcome the limitations of current PD treatment.

TLR4 deficiency has a protective effect in the MPTP/probenecid mouse model of Parkinson's disease

Parkinson’s disease (PD) is a multifactorial disorder characterized by progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) and the presence of Lewy bodies (LBs) consisting of misfolded α-synuclein protein. The etiology of PD is still not clear but systemic inflammation is proved to trigger and exacerbate DA neurons degeneration. Toll-like receptor 4 (TLR4) is a pattern-recognition receptor (PRR) and plays a major role in promoting the host immune. TLR4-mediated signal pathways induce the release of many inflammatory cytokines. It is reasonable to hypothesize that TLR4 is the mediator in microglia contributing to the damage of DA neurons in the SNpc. In this study, we evaluated the role of TLR4 in the chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)/probenecid mouse model. Both TLR4-deficient and wild-type (WT) mice were injected with probenecid (250 mg/kg, i.p.) followed by injection of MPTP (25 mg/kg, s.c.) every 4 days for 10 times. From D43 to D47, the behavioral performance in pole test and wire hang test was assessed. Then the mice were euthanized, and SN and striatum were dissected out for biochemical tests. We showed that compared with MPTP-treated WT mice, TLR4 deficiency significantly attenuated MPTP-induced motor deficits and TH-protein expression reduction in SNpc and striatum, suppressed MPTP-induced α-synuclein abnormality and neuroinflammation mediated through oxidative stress, glial activation, NF-κB and the NLRP3 inflammasome signaling pathways. These findings highlight the neuroprotective effect of TLR4-pathways in the chronic MPTP-induced PD mouse model.

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.

Intrinsically disordered proteins in various hypotheses on the pathogenesis of Alzheimer's and Parkinson's diseases

Amyloid-β (Aβ) and α-synuclein (αS) are two intrinsically disordered proteins (IDPs) at the centers of the pathogenesis of Alzheimer's and Parkinson's diseases, respectively. Different hypotheses have been proposed for explanation of the molecular mechanisms of the pathogenesis of these two diseases, with these two IDPs being involved in many of these hypotheses. Currently, we do not know, which of these hypothesis is more accurate. Experiments face challenges due to the rapid conformational changes, fast aggregation processes, solvent and paramagnetic effects in studying these two IDPs in detail. Furthermore, pathological modifications impact their structures and energetics. Theoretical studies using computational chemistry and computational biology have been utilized to understand the structures and energetics of Aβ and αS. In this chapter, we introduce Aβ and αS in light of various hypotheses, and discuss different experimental and theoretical techniques that are used to study these two proteins along with their weaknesses and strengths. We suggest that a promising solution for studying Aβ and αS at the center of varying hypotheses could be provided by developing new techniques that link quantum mechanics, statistical mechanics, thermodynamics, bioinformatics to machine learning. Such new developments could also lead to development in experimental techniques.

Differential Aggregation and Phosphorylation of Alpha Synuclein in Membrane Compartments Associated With Parkinson Disease

The aggregation of α-synuclein (α-syn) is a major factor behind the onset of Parkinson’s disease (PD). Sublocalization of this protein may be relevant for the formation of multimeric α-syn oligomeric configurations, insoluble aggregates that form Lewy bodies in PD brains. Processing of this protein aggregation is regulated by associations with distinct lipid classes. For instance, instability of lipid raft (LR) microdomains, membrane regions with a particular lipid composition, is an early event in the development of PD. However, the relevance of membrane microdomains in the regulation and trafficking of the distinct α-syn configurations associated with PD remains unexplored. In this study, using 6- and 14-month-old healthy and MPTP-treated animals as a model of PD, we have investigated the putative molecular alterations of raft membrane microstructures, and their impact on α-syn dynamics and conformation. A comparison of lipid analyses of LR microstructures and non-raft (NR) fractions showed alterations in gangliosides, cholesterol, polyunsaturated fatty acids (PUFA) and phospholipids in the midbrain and cortex of aged and MPTP-treated mice. In particular, the increase of PUFA and phosphatidylserine (PS) during aging correlated with α-syn multimeric formation in NR. In these aggregates, α-syn was phosphorylated in pSer129, the most abundant post-transductional modification of α-syn promoting toxic aggregation. Interestingly, similar variations in PUFA and PS content correlating with α-syn insoluble accumulation were also detected in membrane microstructures from the human cortex of incidental Parkinson Disease (iPD) and PD, as compared to healthy controls. Furthermore, structural changes in membrane lipid microenvironments may induce rearrangements in raft-interacting proteins involved in other neuropathologies. Therefore, we also investigated the dynamic of other protein markers involved in cognition and memory impairment such as metabotropic glutamate receptor 5 (mGluR5), ionotropic NMDA receptor (NMDAR2B), prion protein (PrPc) and amyloid precursor protein (APP), whose activity depends on membrane lipid organization. We observed a decline of these protein markers in LR fractions with the progression of aging and pathology. Overall, our findings demonstrate that lipid alterations in membranous compartments promoted by brain aging and PD-like injury may have an effect on α-syn aggregation and segregation in abnormal multimeric structures.

Retinal α-synuclein deposits in Parkinson's disease patients and animal models

Despite decades of research, accurate diagnosis of Parkinson's disease remains a challenge, and disease-modifying treatments are still lacking. Research into the early (presymptomatic) stages of Parkinson's disease and the discovery of novel biomarkers is of utmost importance to reduce this burden and to come to a more accurate diagnosis at the very onset of the disease. Many have speculated that non-motor symptoms could provide a breakthrough in the quest for early biomarkers of Parkinson's disease, including the visual disturbances and retinal abnormalities that are seen in the majority of Parkinson's disease patients. An expanding number of clinical studies have investigated the use of in vivo assessments of retinal structure, electrophysiological function, and vision-driven tasks as novel means for identifying patients at risk that need further neurological examination and for longitudinal follow-up of disease progression in Parkinson's disease patients. Often, the results of these studies have been interpreted in relation to α-synuclein deposits and dopamine deficiency in the retina, mirroring the defining pathological features of Parkinson's disease in the brain. To better understand the visual defects seen in Parkinson's disease patients and to propel the use of retinal changes as biomarkers for Parkinson's disease, however, more conclusive neuropathological evidence for the presence of retinal α-synuclein aggregates, and its relation to the cerebral α-synuclein burden, is urgently needed. This review provides a comprehensive and critical overview of the research conducted to unveil α-synuclein aggregates in the retina of Parkinson's disease patients and animal models, and thereby aims to aid the ongoing discussion about the potential use of the retinal changes and/or visual symptoms as biomarkers for Parkinson's disease.

Exploring the Roles of Post-Translational Modifications in the Pathogenesis of Parkinson's Disease Using Synthetic and Semisynthetic Modified α-Synuclein

Alpha-synuclein (α-syn), a small soluble protein containing 140 amino acids, is associated with the recycling pool of synaptic vesicles in presynaptic terminals. The misfolding and aggregation of α-syn is closely related to a group of neurodegenerative diseases, including Parkinson's disease (PD), which is one of the most common progressive neurodegenerative diseases. Varieties of the post-translational modifications (PTMs) of α-syn, including phosphorylation, ubiquitination, and glycosylation, have been detected in soluble and aggregated α-syn in vivo. These PTMs can have either positive or negative effects on α-syn aggregation and toxicity, which may play critical roles in PD pathogenesis. Herein, we review the advances in synthetic and semisynthetic chemistry to generate homogeneous α-syn variants with site-specific modifications. Using these modified α-syn, we gain insight into the consequences of PTMs on α-syn aggregation and other biophysical properties, which can help elucidate the role of PTMs in the pathogenesis of PD and develop potential therapies to PD.

P38 Mitogen-activated Protein Kinase and Parkinson's Disease

Parkinson's disease, the second major neurodegenerative disease, has created a great impact on the elder people. Although the mechanisms underlying Parkinson's disease are not fully understood, considerable evidence suggests that neuro-inflammation, oxidative stress, mitochondrial dysfunction, cell proliferation, differentiation and apoptosis are involved in the disease. p38MAPK, an important member of the mitogen-activated protein family, controls several important functions in the cell, suggesting a potential pathogenic role in PD. This review provides a brief description of the role and mechanism of p38MAPK in Parkinson's disease.

Toxicity, recovery, and resilience in a 3D dopaminergic neuronal in vitro model exposed to rotenone

To date, most in vitro toxicity testing has focused on acute effects of compounds at high concentrations. This testing strategy does not reflect real-life exposures, which might contribute to long-term disease outcome. We used a 3D-human dopaminergic in vitro LUHMES cell line model to determine whether effects of short-term rotenone exposure (100 nM, 24 h) are permanent or reversible. A decrease in complex I activity, ATP, mitochondrial diameter, and neurite outgrowth were observed acutely. After compound removal, complex I activity was still inhibited; however, ATP levels were increased, cells were electrically active and aggregates restored neurite outgrowth integrity and mitochondrial morphology. We identified significant transcriptomic changes after 24 h which were not present 7 days after wash-out. Our results suggest that testing short-term exposures in vitro may capture many acute effects which cells can overcome, missing adaptive processes, and long-term mechanisms. In addition, to study cellular resilience, cells were re-exposed to rotenone after wash-out and recovery period. Pre-exposed cells maintained higher metabolic activity than controls and presented a different expression pattern in genes previously shown to be altered by rotenone. NEF2L2, ATF4, and EAAC1 were downregulated upon single hit on day 14, but unchanged in pre-exposed aggregates. DAT and CASP3 were only altered after re-exposure to rotenone, while TYMS and MLF1IP were downregulated in both single-exposed and pre-exposed aggregates. In summary, our study shows that a human cell-based 3D model can be used to assess cellular adaptation, resilience, and long-term mechanisms relevant to neurodegenerative research.

Re-evaluation of protein kinase CK2 pleiotropy: new insights provided by a phosphoproteomics analysis of CK2 knockout cells

CK2 denotes a ubiquitous and pleiotropic protein kinase whose holoenzyme is composed of two catalytic (α and/or α') and two regulatory β subunits. The CK2 consensus sequence, S/T-x-x-D/E/pS/pT is present in numerous phosphosites, but it is not clear how many of these are really generated by CK2. To gain information about this issue, advantage has been taken of C2C12 cells entirely deprived of both CK2 catalytic subunits by the CRISPR/Cas9 methodology. A comparative SILAC phosphoproteomics analysis reveals that, although about 30% of the quantified phosphosites do conform to the CK2 consensus, only one-third of these are substantially reduced in the CK2α/α'(-/-) cells, consistent with their generation by CK2. A parallel study with C2C12 cells deprived of the regulatory β subunit discloses a role of this subunit in determining CK2 targeting. We also find that phosphosites notoriously generated by CK2 are not fully abrogated in CK2α/α'(-/-) cells, while some phosphosites unrelated to CK2 are significantly altered. Collectively taken our data allow to conclude that the phosphoproteome generated by CK2 is not as ample and rigidly pre-determined as it was believed before. They also show that the lack of CK2 promotes phosphoproteomics perturbations attributable to kinases other than CK2.

Old wines in new bottles: Repurposing opportunities for Parkinson's disease

Parkinson's disease (PD) is a chronic progressive neurological disorder characterized by accumulation of Lewy bodies and profound loss of substantia nigra dopaminergic neurons. PD symptomatology is now recognized to include both cardinal motor as well as clinically significant non-motor symptoms. Despite intensive research, the current understanding of molecular mechanisms underlying neurodegeneration in PD is limited and has hampered the development of novel symptomatic and disease modifying therapies. The currently available treatment options are only partially or transiently effective and fail to restore the lost dopaminergic neurons or retard disease progression. Given the escalating drug development costs, lengthening timelines and declining R&D efficiency, industry and academia are increasingly focusing on ways to repurpose existing molecules as an accelerated route for drug discovery. The field of PD therapeutics is witnessing vigorous repurposing activity supported by big data analytics, computational models, and high-throughput drug screening systems. Here we review the mechanisms, efficacy, and safety of several emerging drugs currently aspiring to be repositioned for PD pharmacotherapy.

Synergistic protective effect of paeoniflorin and β-ecdysterone against rotenone-induced neurotoxicity in PC12 cells

There are several factors, like oxidative stress and neurons loss, involving neurodegenerative diseases such as Parkinson's disease (PD). The combination of antioxidant and anti-apoptotic agent is becoming a promising approach to fight against PD. This study evaluates the hypothesis that paeoniflorin (PF) and β-ecdysterone (β-Ecd) synergize to protect PC12 cells against toxicity induced by PD-related neurotoxin rotenone. The combination of PF and β-Ecd, hereafter referred to as the PF/β-Ecd, at suboptimal concentrations increased the viability of rotenone-exposed PC12 cells in a synergistic manner. PF and β-Ecd cooperate to attenuate the rotenone-induced apoptosis by decrease in Bax expression, caspase-9 activity, and caspase-3 activity. PF or PF/β-Ecd, but not β-Ecd, inhibited rotenone-triggered protein kinase C-δkinase C-δ (PKCδ) upregulation and nuclear factor κB (NF-κB) activation. β-Ecd or PF/β-Ecd, but not PF, enhanced serine/threonine protein kinase (Akt) activation, promoted nuclear factor E2-related factor 2 (Nrf2) nuclear accumulation, suppressed reactive oxygen species (ROS) production. Neuroprotection of PF/β-Ecd could be completely blocked by PKCδ inhibitor rottlerin plus Akt specific inhibitor LY294002. Dual blockade of the PKCδ/NF-κB pathway by PF and activation of Akt/Nrf2 pathway by β-Ecd results in a synergistic neuroprotective effect against rotenone-induced neurotoxicity in vitro. These findings provide the rationale for determining the in vivo activity of combined therapy with PF and β-Ecd against PD.

Astroglial and microglial contributions to iron metabolism disturbance in Parkinson's disease

Understandings of the disturbed iron metabolism in Parkinson's disease (PD) are largely from the perspectives of neurons. Neurodegenerative processes in PD trigger universal and conserved astroglial dysfunction and microglial activation. In this review, we start with astroglia and microglia in PD with an emphasis on their roles in spreading α-synuclein pathology, and then focus on their contributions in iron metabolism under normal conditions and the diseased state of PD. Elevated iron in the brain regions affects glial features, meanwhile, glial effects on neuronal iron metabolism are largely dependent on their releasing factors. These advances might be valuable for better understanding and modulating iron metabolism disturbance in PD.

Post-translational modifications clustering within proteolytic domains decrease mutant huntingtin toxicity

Huntington's disease (HD) is caused in large part by a polyglutamine expansion within the huntingtin (Htt) protein. Post-translational modifications (PTMs) control and regulate many protein functions and cellular pathways, and PTMs of mutant Htt are likely important modulators of HD pathogenesis. Alterations of selected numbers of PTMs of Htt fragments have been shown to modulate Htt cellular localization and toxicity. In this study, we systematically introduced site-directed alterations in individual phosphorylation and acetylation sites in full-length Htt constructs. The effects of each of these PTM alteration constructs were tested on cell toxicity using our nuclear condensation assay and on mitochondrial viability by measuring mitochondrial potential and size. Using these functional assays in primary neurons, we identified several PTMs whose alteration can block neuronal toxicity and prevent potential loss and swelling of the mitochondria caused by mutant Htt. These PTMs included previously described sites such as serine 116 and newly found sites such as serine 2652 throughout the protein. We found that these functionally relevant sites are clustered in protease-sensitive domains throughout full-length Htt. These findings advance our understanding of the Htt PTM code and its role in HD pathogenesis. Because PTMs are catalyzed by enzymes, the toxicity-modulating Htt PTMs identified here may be promising therapeutic targets for managing HD.

A Novel Bibenzyl Compound (20C) Protects Mice from 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine/Probenecid Toxicity by Regulating the α-Synuclein-Related Inflammatory Response

The novel bibenzyl compound 2-[4-hydroxy-3-(4- hydroxyphenyl) benzyl]-4-(4- hydroxyphenyl) phenol (20C) plays a neuroprotective role in vitro, but its effects in vivo have not yet been elucidated. In this study, we estimated the efficacy of 20C in vivo using a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/probenecid (MPTP/p) mouse model from behavior, dopamine, and neuron and then the possible mechanisms for these effects were further investigated. The experimental results showed that 20C improved behavioral deficits, attenuated dopamine depletion, reduced dopaminergic neuron loss, protected the blood-brain barrier (BBB) structure, ameliorated α-synuclein dysfunction, suppressed glial activation, and regulated both nuclear factor-κB (NF-κB) signaling and the NOD-like receptor protein (NLRP) 3 inflammasome pathway. Our results indicated that 20C may prevent neurodegeneration in the MPTP/p mouse model by targeting α-synuclein and regulating α-synuclein-related inflammatory responses, including BBB damage, glial activation, NF-κB signaling, and the NLRP3 inflammasome pathway.

Membrane Binding of Parkinson's Protein α-Synuclein: Effect of Phosphorylation at Positions 87 and 129 by the S to D Mutation Approach

Human α-synuclein, a protein relevant in the brain with so-far unknown function, plays an important role in Parkinson's disease. The phosphorylation state of αS was related to the disease, prompting interest in this process. The presumed physiological function and the disease action of αS involves membrane interaction. Here, we study the effect of phosphorylation at positions 87 and 129, mimicked by the mutations S87A, S129A (nonphosphorylated) and S87D, S129D (phosphorylated) on membrane binding. Local binding is detected by spin-label continuous-wave electron paramagnetic resonance. For S87A/D, six positions (27, 56, 63, 69, 76, and 90) are probed; and for S129A/D, three (27, 56, and 69). Binding to large unilamellar vesicles of 100 nm diameter of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine in a 1 : 1 composition is not affected by the phosphorylation state of S129. For phosphorylation at S87, local unbinding of αS from the membrane is observed. We speculate that modulating the local membrane affinity by phosphorylation could tune the way αS interacts with different membranes; for example, tuning its membrane fusion activity.