Targeting intracellular and extracellular alpha-synuclein as a therapeutic strategy in Parkinson's disease and other synucleinopathies

Sustained effect of prasinezumab on Parkinson's disease motor progression in the open-label extension of the PASADENA trial

The Phase II trial of Anti-alpha-Synuclein Antibody in Early Parkinson's Disease (PASADENA) is an ongoing double-blind, placebo-controlled trial evaluating the safety and efficacy of prasinezumab in early-stage Parkinson's disease (PD). During the double-blind period, prasinezumab-treated individuals showed less progression of motor signs (Movement Disorders Society-sponsored revision of the Unified Parkinson's Disease Rating Scale (MDS-UPDRS) Part III) than placebo-treated individuals. We evaluated whether the effect of prasinezumab on motor progression, assessed as a change in MDS-UPDRS Part III score in the OFF and ON states, and MDS-UPDRS Part II score, was sustained for 4 years from the start of the trial. We compared participants enrolled in the PASADENA open-label extension study with those enrolled in an external comparator arm derived from the Parkinson's Progression Markers Initiative observational study. The PASADENA delayed-start (n = 94) and early-start (n = 177) groups showed a slower decline (a smaller increase in score) in MDS-UPDRS Part III scores in the OFF state (delayed start, -51%; early start, -65%), ON state (delayed start, -94%; early start, -118%) and MDS-UPDRS Part II (delayed start, -48%; early start, -40%) than did the Parkinson's Progression Markers Initiative external comparator (n = 303). This exploratory analysis, which requires confirmation in future studies, suggested that the effect of prasinezumab in slowing motor progression in PD may be sustained long term. PASADENA ClinicalTrials.gov no. NCT03100149 .

Long noncoding RNAs in ubiquitination, protein degradation, and human diseases

Protein stability and turnover is critical in normal cellular and physiological process and their misregulation may contribute to accumulation of unwanted proteins causing cellular malfunction, neurodegeneration, mitochondrial malfunction, and disrupted metabolism. Signaling mechanism associated with protein degradation is complex and is extensively studied. Many protein and enzyme machineries have been implicated in regulation of protein degradation. Despite these insights, our understanding of protein degradation mechanisms remains limited. Emerging studies suggest that long non-coding RNAs (lncRNAs) play critical roles in various cellular and physiological processes including metabolism, cellular homeostasis, and protein turnover. LncRNAs, being large nucleic acids (>200 nt long) can interact with various proteins and other nucleic acids and modulate protein structure and function leading to regulation of cell signaling processes. LncRNAs are widely distributed across cell types and may exhibit tissue specific expression. They are detected in body fluids including blood and urine. Their expressions are also altered in various human diseases including cancer, neurological disorders, immune disorder, and others. LncRNAs are being recognized as novel biomarkers and therapeutic targets. This review article focuses on the emerging role of noncoding RNAs (ncRNAs), particularly long noncoding RNAs (lncRNAs), in the regulation of protein polyubiquitination and proteasomal degradation.

Navigating the Neurobiology of Parkinson's: The Impact and Potential of α-Synuclein

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease worldwide; therefore, since its initial description, significant progress has been made, yet a mystery remains regarding its pathogenesis and elusive root cause. The widespread distribution of pathological α-synuclein (αSyn) aggregates throughout the body raises inquiries regarding the etiology, which has prompted several hypotheses, with the most prominent one being αSyn-associated proteinopathy. The identification of αSyn protein within Lewy bodies, coupled with genetic evidence linking αSyn locus duplication, triplication, as well as point mutations to familial Parkinson's disease, has underscored the significance of αSyn in initiating and propagating Lewy body pathology throughout the brain. In monogenic and sporadic PD, the presence of early inflammation and synaptic dysfunction leads to αSyn aggregation and neuronal death through mitochondrial, lysosomal, and endosomal functional impairment. However, much remains to be understood about αSyn pathogenesis, which is heavily grounded in biomarkers and treatment strategies. In this review, we provide emerging new evidence on the current knowledge about αSyn's pathophysiological impact on PD, and its presumable role as a specific disease biomarker or main target of disease-modifying therapies, highlighting that this understanding today offers the best potential of disease-modifying therapy in the near future.

Neuroprotection and Non-Invasive Brain Stimulation: Facts or Fiction?

Non-Invasive Brain Stimulation (NIBS) techniques, such as transcranial Direct Current Stimulation (tDCS) and repetitive Magnetic Transcranial Stimulation (rTMS), are well-known non-pharmacological approaches to improve both motor and non-motor symptoms in patients with neurodegenerative disorders. Their use is of particular interest especially for the treatment of cognitive impairment in Alzheimer's Disease (AD), as well as axial disturbances in Parkinson's (PD), where conventional pharmacological therapies show very mild and short-lasting effects. However, their ability to interfere with disease progression over time is not well understood; recent evidence suggests that NIBS may have a neuroprotective effect, thus slowing disease progression and modulating the aggregation state of pathological proteins. In this narrative review, we gather current knowledge about neuroprotection and NIBS in neurodegenerative diseases (i.e., PD and AD), just mentioning the few results related to stroke. As further matter of debate, we discuss similarities and differences with Deep Brain Stimulation (DBS)-induced neuroprotective effects, and highlight possible future directions for ongoing clinical studies.

Kisspeptin-10 Rescues Cholinergic Differentiated SHSY-5Y Cells from α-Synuclein-Induced Toxicity In Vitro

The neuropathological substrate of dementia with Lewy bodies (DLB) is defined by the inextricable cross-seeding accretion of amyloid-β (Aβ) and α-synuclein (α-syn)-laden deposits in cholinergic neurons. The recent revelation that neuropeptide kisspeptin-10 (KP-10) is able to mitigate Aβ toxicity via an extracellular binding mechanism may provide a new horizon for innovative drug design endeavors. Considering the sequence similarities between α-syn’s non-amyloid-β component (NAC) and Aβ’s C-terminus, we hypothesized that KP-10 would enhance cholinergic neuronal resistance against α-syn’s deleterious consequences through preferential binding. Here, human cholinergic SH-SY5Y cells were transiently transformed to upsurge the mRNA expression of α-syn while α-syn-mediated cholinergic toxicity was quantified utilizing a standardized viability-based assay. Remarkably, the E46K mutant α-syn displayed elevated α-syn mRNA levels, which subsequently induced more cellular toxicity compared with the wild-type α-syn in choline acetyltransferase (ChAT)-positive cholinergic neurons. Treatment with a high concentration of KP-10 (10 µM) further decreased cholinergic cell viability, while low concentrations of KP-10 (0.01–1 µM) substantially suppressed wild-type and E46K mutant α-syn-mediated toxicity. Correlating with the in vitro observations are approximations from in silico algorithms, which inferred that KP-10 binds favorably to the C-terminal residues of wild-type and E46K mutant α-syn with CDOCKER energy scores of −118.049 kcal/mol and −114.869 kcal/mol, respectively. Over the course of 50 ns simulation time, explicit-solvent molecular dynamics conjointly revealed that the docked complexes were relatively stable despite small-scale fluctuations upon assembly. Taken together, our findings insinuate that KP-10 may serve as a novel therapeutic scaffold with far-reaching implications for the conceptualization of α-syn-based treatments.

Chaperon-mediated autophagy can regulate diquat-induced apoptosis by inhibiting α-synuclein accumulation cooperatively with macroautophagy

α-Synuclein, which is associated with Parkinson's disease, is cleared by the ubiquitin-proteasome system and autophagy lysosome system. Chaperon-mediated autophagy (CMA) and macroautophagy are major subtypes of autophagy and play a critical role in pesticide-induced α-synucleinopathy. In this study, we explored the role of CMA in diquat (DQ)-induced α-synucleinopathy and characterized the relationship between CMA and macroautophagy in the clearance of pathologic α-synuclein for the prevention of DQ neurotoxicity. DQ was cytotoxic to SH-SY5Y cells in a concentration-dependent manner, as shown by decreased cell viability and increased cytotoxicity. DQ treatment was also found to induce autophagy such as CMA and macroautophagy by monitoring the expression of Lamp2A and microtubule-associated protein 1A/1B light chain 3B (LC3-II) respectively. Following DQ treatment, SH-SY5Y cells were found to have induced phosphorylated and detergent-insoluble α-synuclein deposits, and MG132, a proteasome inhibitor, effectively potentiated both CMA and macroautophagy for preventing α-synuclein aggregation. Interestingly, CMA impairment by Lamp2A-knock down decreased the LC3II expression compared to in DQ-treated cells transfected with control siRNA. In Lamp2-knock down cells, pathologic α-synuclein was increased 12 h after DQ treatment, but there was no change observed at 24 h. In DQ-treated cells, macroautophagy by 3-methyladenine and bafilomycin inhibition increased Lamp2A expression, indicating an increase in CMA activity. In addition, CMA modulation affected apoptosis, and inhibiting lysosome activity by NH₄Cl increased apoptosis in DQ-treated cells. An increase in autophagy was confirmed to compensate for the decrease in lysosome activity. Pretreatment with z-VAD-fmk, a pan-caspase inhibitor, significantly enhanced the macroautophagy response of DQ-exposed cells without alterations in Lamp2A expression. Our results suggest that CMA can regulate DQ-induced α-synucleinopathy cooperatively with macroautophagy, and crosstalk between macroautophagy and CMA plays an important role in DQ-induced cytotoxicity. Taken together, autophagy modulation may be a useful treatment strategy in pesticide-induced neurodegenerative disorders through preventing α-synucleinopathy.

Cellular Prion Protein Mediates α-Synuclein Uptake, Localization, and Toxicity In Vitro and In Vivo

BACKGROUND

The cellular prion protein (PrP^C ) is a membrane-bound, multifunctional protein mainly expressed in neuronal tissues. Recent studies indicate that the native trafficking of PrP^C can be misused to internalize misfolded amyloid beta and α-synuclein (aSyn) oligomers.

OBJECTIVES

We define PrP^C 's role in internalizing misfolded aSyn in α-synucleinopathies and identify further involved proteins.

METHODS

We performed comprehensive behavioral studies on four transgenic mouse models (ThySyn and ThySynPrP00, TgM83 and TgMPrP00) at different ages. We developed PrP^C -(over)-expressing cell models (cell line and primary cortical neurons), used confocal laser microscopy to perform colocalization studies, applied mass spectrometry to identify interactomes, and determined disassociation constants using surface plasmon resonance (SPR) spectroscopy.

RESULTS

Behavioral deficits (memory, anxiety, locomotion, etc.), reduced lifespans, and higher oligomeric aSyn levels were observed in PrP^C -expressing mice (ThySyn and TgM83), but not in homologous Prnp ablated mice (ThySynPrP00 and TgMPrP00). PrP^C colocalized with and facilitated aSyn (oligomeric and monomeric) internalization in our cell-based models. Glimepiride treatment of PrP^C -overexpressing cells reduced aSyn internalization in a dose-dependent manner. SPR analysis showed that the binding affinity of PrP^C to monomeric aSyn was lower than to oligomeric aSyn. Mass spectrometry-based proteomic studies identified clathrin in the immunoprecipitates of PrP^C and aSyn. SPR was used to show that clathrin binds to recombinant PrP, but not aSyn. Experimental disruption of clathrin-coated vesicles significantly decreased aSyn internalization.

CONCLUSION

PrP^C 's native trafficking can be misused to internalize misfolded aSyn through a clathrin-based mechanism, which may facilitate the spreading of pathological aSyn. Disruption of aSyn-PrP^C binding is, therefore, an appealing therapeutic target in α-synucleinopathies. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

X1INH, an improved next-generation affinity-optimized hydrazonic ligand, attenuates abnormal copper(I)/copper(II)-α-Syn interactions and affects protein aggregation in a cellular model of synucleinopathy

Although normal aging presents an accumulation of copper and iron in the brain, this becomes more relevant in neurodegeneration. α-Synuclein (α-Syn) misfolding has long been linked with the development of Parkinson's disease (PD). Copper binding promotes aggregation of α-Syn, as well as generalized oxidative stress. In this sense, the use of therapies that target metal dyshomeostasis has been in focus in the past years. Metal-Protein Attenuating Compounds (MPACs) are moderate chelators that aim at disrupting specific, abnormal metal-protein interactions. Our research group has now established that N-acylhydrazones compose a set of truly encouraging MPACs for the bioinorganic management of metal-enhanced aggregopathies. In the present work, a novel ligand, namely 1-methyl-1H-imidazole-2-carboxaldehyde isonicotinoyl hydrazone (X1INH), is reported. We describe solution studies on the interaction and affinity of this compound for copper(ii) ions showing that a fine tuning of metal-affinity was achieved. A series of in vitro biophysical NMR experiments were performed in order to assess the X1INH ability to compete with α-Syn monomers for the binding of both copper(i) and copper(ii) ions, which are central in PD pathology. A preference for copper(i) has been observed. X1INH is less toxic to human neuroglioma (H4) cells in comparison to structure-related compounds. Finally, we show that treatment with X1INH results in a higher number of smaller, less compact inclusions in a well-established model of α-Syn aggregation. Thus, X1INH constitutes a promising MPAC for the treatment of Parkinson's disease.

Innovative treatment targeting gangliosides aimed at blocking the formation of neurotoxic α-synuclein oligomers in Parkinson's disease

Parkinson's disease (PD) is a major neurodegenerative disorder which exhibits many of the characteristics of a pandemic. Current therapeutic strategies are centered on the dopaminergic system, with limited efficacy, so that a treatment that has a direct impact on the underlying disease pathogenesis is urgently needed. Although α-synuclein is a privileged target for such therapies, this protein has been in the past wrongly considered as exclusively intracellular, so that the impact of paracrine neurotoxicity mechanisms in PD have been largely ignored. In this article we review the data showing that lipid rafts act as plasma membrane machineries for the formation of α-synuclein pore-like oligomers which trigger an increase of intracellular Ca²⁺. This Ca²⁺ influx is responsible for a self-sustained cascade of neurotoxic events, including mitochondrial oxidative stress, tau phosphorylation, Ca²⁺ release from the endoplasmic reticulum, Lewy body formation, and extracellular release of α-synuclein in exosomes. The first step of this cascade is the binding of α-synuclein to lipid raft gangliosides, suggesting that PD should be considered as both a proteinopathy and a ganglioside membrane disorder lipidopathy. Accordingly, blocking α-synuclein-ganglioside interactions should annihilate the whole neurotoxic cascade and stop disease progression. A pipeline of anti-oligomer molecules is under development, among which an in-silico designed synthetic peptide AmyP53 which is the first drug targeting gangliosides and thus able to prevent the formation of α-synuclein oligomers and all downstream neurotoxicity. These new therapeutic avenues challenge the current symptomatic approaches by finally targeting the root cause of PD through a long-awaited paradigm shift.

In Search of Effective Treatments Targeting α-Synuclein Toxicity in Synucleinopathies: Pros and Cons

Parkinson's disease (PD), multiple system atrophy (MSA) and Dementia with Lewy bodies (DLB) represent pathologically similar, progressive neurodegenerative disorders characterized by the pathological aggregation of the neuronal protein α-synuclein. PD and DLB are characterized by the abnormal accumulation and aggregation of α-synuclein in proteinaceous inclusions within neurons named Lewy bodies (LBs) and Lewy neurites (LNs), whereas in MSA α-synuclein inclusions are mainly detected within oligodendrocytes named glial cytoplasmic inclusions (GCIs). The presence of pathologically aggregated α-synuclein along with components of the protein degradation machinery, such as ubiquitin and p62, in LBs and GCIs is considered to underlie the pathogenic cascade that eventually leads to the severe neurodegeneration and neuroinflammation that characterizes these diseases. Importantly, α-synuclein is proposed to undergo pathogenic misfolding and oligomerization into higher-order structures, revealing self-templating conformations, and to exert the ability of "prion-like" spreading between cells. Therefore, the manner in which the protein is produced, is modified within neural cells and is degraded, represents a major focus of current research efforts in the field. Given that α-synuclein protein load is critical to disease pathogenesis, the identification of means to limit intracellular protein burden and halt α-synuclein propagation represents an obvious therapeutic approach in synucleinopathies. However, up to date the development of effective therapeutic strategies to prevent degeneration in synucleinopathies is limited, due to the lack of knowledge regarding the precise mechanisms underlying the observed pathology. This review critically summarizes the recent developed strategies to counteract α-synuclein toxicity, including those aimed to increase protein degradation, to prevent protein aggregation and cell-to-cell propagation, or to engage antibodies against α-synuclein and discuss open questions and unknowns for future therapeutic approaches.

Synergy between plasminogen activator inhibitor-1, α-synuclein, and neuroinflammation in Parkinson's disease

Parkinson's disease (PD) is a progressive degenerative nervous system disorder and is the second most common neurodegenerative disorder in the elderly population. The disease originates from the loss of dopamine-producing neurons in the substantia nigra in the brain, resulting in unregulated activity of the basal ganglia. Αlpha-synuclein (α-syn) is a protein found to aggregate in the substantia nigra region of patients with PD, forming Lewy Body inclusions; its aggregation may contribute to neuronal cell death in PD. This work hypothesizes about the synergistic relationship between α-syn aggregation and neuroinflammation to up-regulate expression of the serine protease inhibitor (serpin) plasminogen activator inhibitor-1 (PAI-1). The protease, plasmin, has been shown to cleave extracellular α-syn (including its monomeric, oligomeric, and fibrillary forms), resulting in less aggregation and Lewy Body formation. The zymogen plasminogen is converted to its active serine protease form, plasmin, either by tissue plasminogen activator (tPA) or by urokinase plasminogen activator (uPA) bound to urokinase receptor (uPAR). Both tPA and uPA/uPAR are inhibited by PAI-1. Thus, when PAI-1 levels increase, less plasmin is generated, which would lead to reduced proteolysis of α-syn. Expression of PAI-1 is increased both in inflammatory environments and in the presence of extracellular α-syn aggregates. This scenario suggests a pathological amplification loop: increased extracellular α-syn aggregation activates an inflammatory response from microglia and astrocytes, increasing PAI-1 levels, and decreasing the generation of plasmin. With reduced plasmin, less α-syn can be cleaved, and aggregation continues, sustaining the pathological process. Understanding this putative pathogenic loop could provide insight into the means by which neurodegeneration progresses in PD, and it may offer possible novel therapeutic strategies.

GDNF-expressing macrophages restore motor functions at a severe late-stage, and produce long-term neuroprotective effects at an early-stage of Parkinson's disease in transgenic Parkin Q311X(A) mice

There is an unmet medical need in the area of Parkinson's disease (PD) to develop novel therapeutic approaches that can stop and reverse the underlying mechanisms responsible for the neuronal death. We previously demonstrated that systemically administered autologous macrophages transfected ex vivo to produce glial cell line-derived neurotrophic factor (GDNF) readily migrate to the mouse brain with acute toxin-induced neuroinflammation and ameliorate neurodegeneration in PD mouse models. We hypothesized that the high level of cytokines due to inflammatory process attracted GDNF-expressing macrophages and ensured targeted drug delivery to the PD brain. Herein, we validated a therapeutic potential of GDNF-transfected macrophages in a transgenic Parkin Q311X(A) mice with slow progression and mild brain inflammation. Systemic administration of GDNF-macrophages at a severe late stage of the disease leaded to a near complete restoration of motor functions in Parkin Q311X(A) mice and improved brain tissue integrity with healthy neuronal morphology. Furthermore, intravenous injections of GDNF-macrophages at an early stage of disease resulted in potent sustained therapeutic effects in PD mice for more than a year after the treatment. Importantly, multiple lines of evidence for therapeutic efficacy were observed including: diminished neuroinflammation and α-synuclein aggregation, increased survival of dopaminergic neurons, and improved locomotor functions. In summary, GDNF-transfected macrophages represent a promising therapeutic strategy for PD at both late- and early-stages of the disease.

A cullin-RING ubiquitin ligase targets exogenous α-synuclein and inhibits Lewy body-like pathology

Parkinson's disease (PD) is a neurological disorder characterized by the progressive accumulation of neuronal α-synuclein (αSyn) inclusions called Lewy bodies. It is believed that Lewy bodies spread throughout the nervous system due to the cell-to-cell propagation of αSyn via cycles of secretion and uptake. Here, we investigated the internalization and intracellular accumulation of exogenous αSyn, two key steps of Lewy body pathogenesis, amplification and spreading. We found that stable αSyn fibrils substantially accumulate in different cell lines upon internalization, whereas αSyn monomers, oligomers, and dissociable fibrils do not. Our data indicate that the uptake-mediated accumulation of αSyn in a human-derived neuroblastoma cell line triggered an adaptive response that involved proteins linked to ubiquitin ligases of the S-phase kinase-associated protein 1 (SKP1), cullin-1 (Cul1), and F-box domain-containing protein (SCF) family. We found that SKP1, Cul1, and the F-box/LRR repeat protein 5 (FBXL5) colocalized and physically interacted with internalized αSyn in cultured cells. Moreover, the SCF containing the F-box protein FBXL5 (SCFFBXL5) catalyzed αSyn ubiquitination in reconstitution experiments in vitro using recombinant proteins and in cultured cells. In the human brain, SKP1 and Cul1 were recruited into Lewy bodies from brainstem and neocortex of patients with PD and related neurological disorders. In both transgenic and nontransgenic mice, intracerebral administration of exogenous αSyn fibrils triggered a Lewy body-like pathology, which was amplified by SKP1 or FBXL5 loss of function. Our data thus indicate that SCFFXBL5 regulates αSyn in vivo and that SCF ligases may constitute targets for the treatment of PD and other α-synucleinopathies.

Anodal transcranial direct current stimulation prevents methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurotoxicity by modulating autophagy in an in vivo mouse model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the accumulation of protein inclusions and the loss of dopaminergic neurons. Transcranial direct current stimulation (tDCS) is a non-invasive brain-stimulating technique that has demonstrated promising results in clinical studies of PD. Despite accumulating evidence indicating that tDCS exerts a protective effect, the mechanism underlying its activity remains unknown. In the present study, we first investigated the neuroprotective effect of tDCS in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mouse model and then evaluated the effect of tDCS on the autophagy pathway. tDCS improved behavioral alterations, increased tyrosine hydroxylase protein levels and suppressed α-synuclein protein levels in MPTP-treated mice. MPTP-treated mice subjected to tDCS also had lower levels of autophagy-related proteins, such as microtubule-associated protein 1 light chain 3 and AMP-activated protein kinase, and higher levels of mechanistic target of rapamycin and p62. In addition, the protein levels of phosphoinositide 3-kinase and brain-derived neurotrophic factor were higher, and the levels of unc-51-like kinase 1 were lower in MPTP-treated mice subjected to tDCS. Our findings suggest that tDCS protected against MPTP-induced PD in a mouse model by modulating autophagy.

Tubulin and Tau: Possible targets for diagnosis of Parkinson's and Alzheimer's diseases

Neurodegenerative diseases including Alzheimer’s disease (AD) and Parkinson’s disease (PD) are characterized by progressive neuronal loss and pathological accumulation of some proteins. Developing new biomarkers for both diseases is highly important for the early diagnosis and possible development of neuro-protective strategies. Serum antibodies (AIAs) against neuronal proteins are potential biomarkers for AD and PD that may be formed in response to their release into systemic circulation after brain damage. In the present study, two AIAs (tubulin and tau) were measured in sera of patients of PD and AD, compared to healthy controls. Results showed that both antibodies were elevated in patients with PD and AD compared to match controls. Curiously, the profile of elevation of antibodies was different in both diseases. In PD cases, tubulin and tau AIAs levels were similar. On the other hand, AD patients showed more elevation of tau AIAs compared to tubulin. Our current results suggested that AIAs panel could be able to identify cases with neuro-degeneration when compared with healthy subjects. More interestingly, it is possible to differentiate between PD and AD cases through identifying specific AIAs profile for each neurodegenerative states.

Abundant fish protein inhibits α-synuclein amyloid formation

The most common allergen in fish, the highly-abundant protein β-parvalbumin, forms amyloid structures as a way to avoid gastrointestinal degradation and transit to the blood. In humans, the same amyloid structures are mostly associated with neurodegenerative disorders such as Alzheimer’s and Parkinson’s. We here assessed a putative connection between these amyloids using recombinant Atlantic cod β-parvalbumin and the key amyloidogenic protein in Parkinson’s disease, α-synuclein. Using a set of in vitro biophysical methods, we discovered that β-parvalbumin readily inhibits amyloid formation of α-synuclein. The underlying mechanism was found to involve α-synuclein binding to the surface of β-parvalbumin amyloid fibers. In addition to being a new amyloid inhibition mechanism, the data suggest that health benefits of fish may be explained in part by cross-reaction of β-parvalbumin with human amyloidogenic proteins.

Conformational Ensembles of α-Synuclein Derived Peptide with Different Osmolytes from Temperature Replica Exchange Sampling

Intrinsically disordered proteins (IDP) are a class of proteins that do not have a stable three-dimensional structure and can adopt a range of conformations playing various vital functional role. Alpha-synuclein is one such IDP which can aggregate into toxic protofibrils and has been associated largely with Parkinson's disease (PD) along with other neurodegenerative diseases. Osmolytes are small organic compounds that can alter the environment around the proteins by acting as denaturants or protectants for the proteins. In the present study, we have conducted a series of replica exchange molecular dynamics simulations to explore the role of osmolytes, urea which is a denaturant and TMAO (trimethylamine N-oxide), a protecting osmolyte, in aggregation and conformations of the synuclein peptide. We observed that both the osmolytes have significantly distinct impacts on the peptide and led to transitions of the conformations of the peptide from one state to other. Our findings highlighted that urea attenuated peptide aggregation and resulted in the formation of extended peptide structures whereas TMAO led to compact and folded forms of the peptide.

A Molecular Tweezer Ameliorates Motor Deficits in Mice Overexpressing α-Synuclein

Aberrant accumulation and self-assembly of α-synuclein are tightly linked to several neurodegenerative diseases called synucleinopathies, including idiopathic Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Deposition of fibrillar α-synuclein as insoluble inclusions in affected brain cells is a pathological hallmark of synucleinopathies. However, water-soluble α-synuclein oligomers may be the actual culprits causing neuronal dysfunction and degeneration in synucleinopathies. Accordingly, therapeutic approaches targeting the toxic α-synuclein assemblies are attractive for these incurable disorders. The "molecular tweezer" CLR01 selectively remodels abnormal protein self-assembly through reversible binding to Lys residues. Here, we treated young male mice overexpressing human wild-type α-synuclein under control of the Thy-1 promoter (Thy1-aSyn mice) with CLR01 and examined motor behavior and α-synuclein in the brain. Intracerebroventricular administration of CLR01 for 28 days to the mice improved motor dysfunction in the challenging beam test and caused a significant decrease of buffer-soluble α-synuclein in the striatum. Proteinase-K-resistant, insoluble α-synuclein deposits remained unchanged in the substantia nigra, whereas levels of diffuse cytoplasmic α-synuclein in dopaminergic neurons increased in mice receiving CLR01 compared with vehicle. More moderate improvement of motor deficits was also achieved by subcutaneous administration of CLR01, in 2/5 trials of the challenging beam test and in the pole test, which requires balance and coordination. The data support further development of molecular tweezers as therapeutic agents for synucleinopathies.

α-Synuclein aggregation modulation: an emerging approach for the treatment of Parkinson's disease

Parkinson's disease (PD) is a multifactorial progressive neurological disorder. Pathological hallmarks of PD are characterized by the presence of α-synuclein (αSyn) aggregates known as Lewy bodies. αSyn aggregation is one of the leading causes for the neuronal dysfunction and death in PD. It is also associated with neurotransmitter and calcium release. Current therapies of PD are limited to only symptomatic relief without addressing the underlying pathogenic factors of the disease process such as aggregation of αSyn. Consequently, the progression of the disease continues with the current therapies. Therefore, the modulation of αSyn aggregation is an emerging approach as a novel therapeutic target to treat PD. There are two major aspects that might be targeted therapeutically: first, protein is prone to aggregation, therefore, anti-aggregative or compounds that can break the pre-existing aggregates should be helpful. Second, there are number of molecular events that may be targeted to combat the disease.

Anti-MAG autoantibodies are increased in Parkinson's disease but not in atypical parkinsonism

There is emerging evidence that glial cells are involved in the neuropathological process in Parkinson's disease (PD) in addition to degeneration of neuronal structures. Recently, we confirmed the presence of an adaptive immune response against different glial-derived antigens in PD, with a possible role of anti-MAG, anti-MBP and anti-PLP antibodies in the disease progression. The aim of the present study was to assess humoral response against myelin-associated glycoprotein (MAG) in patients with parkinsonism (both idiopathic and atypical) to check whether these antibodies could serve as biomarkers of PD, its severity and progression. Anti-MAG autoantibodies were measured by an ELISA system in 99 PD patients, 33 atypical parkinsonism patients, and 36 control subjects. In PD patients, anti-MAG IgM autoantibodies were significantly higher in comparison to healthy control subjects (p = 0.038). IgM anti-MAG autoantibodies titers were also significantly higher in the whole group of patients with parkinsonism (either idiopathic or atypical) in comparison to healthy control subjects (1.88 ± 0.84 vs 1.70 ± 1.19, p = 0.017). This difference was mainly driven by the PD group, as the atypical parkinsonism group did not differ significantly from the control group in anti-MAG antibody levels (p = 0.51). A negative correlation between anti-MAG levels and disease duration was found in PD patients. Our study provides evidence for an increased production of autoantibodies against a protein of glial origin in PD. The negative correlation between anti-MAG antibodies and disease duration may suggest possible involvement of the immune system in disease progression. Increasing evidence that glia are involved in the neurodegenerative process to a greater extent than previously thought may turn out be useful in the search for biomarkers of the neurodegenerative process in PD.

Low-Intensity Ultrasound Decreases α-Synuclein Aggregation via Attenuation of Mitochondrial Reactive Oxygen Species in MPP(+)-Treated PC12 Cells

Many studies have shown that mitochondrial dysfunction and the subsequent oxidative stress caused by excessive reactive oxygen species (ROS) generation play a central role in the pathogenesis of Parkinson's disease (PD). We have previously shown that low-intensity ultrasound (LIUS) could reduce ROS generation by L-buthionine-(S,R)-sulfoximine (BSO) in retinal pigment epithelial cells. In this study, we studied the effects of LIUS stimulation on the ROS-dependent α-synuclein aggregation in 1-methyl-4-phenylpyridinium ion (MPP⁺)-treated PC12 cells. We found that LIUS stimulation suppressed the MPP⁺-induced ROS generation and inhibition of mitochondrial complex I activity in PC12 cells in an intensity-dependent manner at 30, 50, and 100 mW/cm². Furthermore, LIUS stimulation at 100 mW/cm² suppressed inhibition of mitochondrial complex activity by MPP⁺ and actually resulted in a decrease of α-synuclein phosphorylation and aggregation induced by MMP⁺ treatment in PC12 cells. LIUS stimulation also inhibited expression of casein kinase 2 (CK2) that appears to mediate ROS-dependent α-synuclein aggregation. Finally, LIUS stimulation alleviated the death of PC12 cells by MPP⁺ treatment in an intensity-dependent manner. We, hence, suggest that LIUS stimulation inhibits ROS generation by MPP⁺ treatment, thereby suppressing α-synuclein aggregation in PC12 cells.

Melatonin attenuates MPTP-induced neurotoxicity via preventing CDK5-mediated autophagy and SNCA/α-synuclein aggregation

Autophagy is involved in the pathogenesis of neurodegenerative diseases including Parkinson disease (PD). However, little is known about the regulation of autophagy in neurodegenerative process. In this study, we characterized aberrant activation of autophagy induced by neurotoxin 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) and demonstrated that melatonin has a protective effect on neurotoxicity. We found an excessive activation of autophagy in monkey brain tissues and C6 cells, induced by MPTP, which is mediated by CDK5 (cyclin-dependent kinase 5). MPTP treatment significantly reduced total dendritic length and dendritic complexity of cultured primary cortical neurons and melatonin could reverse this effect. Decreased TH (tyrosine hydroxylase)-positive cells and dendrites of dopaminergic neurons in the substantia nigra pars compacta (SNc) were observed in MPTP-treated monkeys and mice. Along with decreased TH protein level, we observed an upregulation of CDK5 and enhanced autophagic activity in the striatum of mice with MPTP injection. These changes could be salvaged by melatonin treatment or knockdown of CDK5. Importantly, melatonin or knockdown of CDK5 reduced MPTP-induced SNCA/α-synuclein aggregation in mice, which is widely thought to trigger the pathogenesis of PD. Finally, melatonin or knockdown of CDK5 counteracted the PD phenotype in mice induced by MPTP. Our findings uncover a potent role of CDK5-mediated autophagy in the pathogenesis of PD, and suggest that control of autophagic pathways may provide an important clue for exploring potential target for novel therapeutics of PD.

Impairment of chaperone-mediated autophagy induces dopaminergic neurodegeneration in rats

Chaperone-mediated autophagy (CMA) involves the selective lysosomal degradation of cytosolic proteins such as SNCA (synuclein α), a protein strongly implicated in Parkinson disease (PD) pathogenesis. However, the physiological role of CMA and the consequences of CMA failure in the living brain remain elusive. Here we show that CMA inhibition in the adult rat substantia nigra via adeno-associated virus-mediated delivery of short hairpin RNAs targeting the LAMP2A receptor, involved in CMA's rate limiting step, was accompanied by intracellular accumulation of SNCA-positive puncta, which were also positive for UBIQUITIN, and in accumulation of autophagic vacuoles within LAMP2A-deficient nigral neurons. Strikingly, LAMP2A downregulation resulted in progressive loss of nigral dopaminergic neurons, severe reduction in striatal dopamine levels/terminals, increased astro- and microgliosis and relevant motor deficits. Thus, this study highlights for the first time the importance of the CMA pathway in the dopaminergic system and suggests that CMA impairment may underlie PD pathogenesis.

What can trigger the onset of Parkinson's disease - A modeling study based on a compartmental model of α-synuclein transport and aggregation in neurons

The aim of this paper is to develop a minimal model describing events leading to the onset of Parkinson's disease (PD). The model accounts for α-synuclein (α-syn) production in the soma, transport toward the synapse, misfolding, and aggregation. The production and aggregation of polymeric α-syn is simulated using a minimalistic 2-step Finke-Watzky model. We utilized the developed model to analyze what changes in a healthy neuron are likely to lead to the onset of α-syn aggregation. We checked the effects of interruption of α-syn transport toward the synapse, entry of misfolded (infectious) α-syn into the somatic and synaptic compartments, increasing the rate of α-syn synthesis in the soma, and failure of α-syn degradation machinery. Our model suggests that failure of α-syn degradation machinery is probably the most likely cause for the onset of α-syn aggregation leading to PD.

Constructing Structure Ensembles of Intrinsically Disordered Proteins from Chemical Shift Data

Modeling the structural ensemble of intrinsically disordered proteins (IDPs), which lack fixed structures, is essential in understanding their cellular functions and revealing their regulation mechanisms in signaling pathways of related diseases (e.g., cancers and neurodegenerative disorders). Though the ensemble concept has been widely believed to be the most accurate way to depict 3D structures of IDPs, few of the traditional ensemble-based approaches effectively address the degeneracy problem that occurs when multiple solutions are consistent with experimental data and is the main challenge in the IDP ensemble construction task. In this article, based on a predefined conformational library, we formalize the structure ensemble construction problem into a least squares framework, which provides the optimal solution when the data constraints outnumber unknown variables. To deal with the degeneracy problem, we further propose a regularized regression approach based on the elastic net technique with the assumption that the weights to be estimated for individual structures in the ensemble are sparse. We have validated our methods through a reference ensemble approach as well as by testing the real biological data of three proteins, including alpha-synuclein, the translocation domain of Colocin N, and the K18 domain of Tau protein.

Autophagy and Alpha-Synuclein: Relevance to Parkinson's Disease and Related Synucleopathies

Evidence from human postmortem material, transgenic mice, and cellular/animal models of PD link alpha-synuclein accumulation to alterations in the autophagy lysosomal pathway. Conversely, alpha-synuclein mutations related to PD pathogenesis, as well as post-translational modifications of the wild-type protein, result in the generation of aberrant species that may impair further the function of the autophagy lysosomal pathway, thus generating a vicious cycle leading to neuronal death. Moreover, PD-linked mutations in lysosomal-related genes, such as glucocerebrosidase, have been also shown to contribute to alpha-synuclein accumulation and related toxicity, indicating that lysosomal dysfunction may, in part, account for the neurodegeneration observed in synucleinopathies. In the current review, we summarize findings related to the inter-relationship between alpha-synuclein and lysosomal proteolytic pathways, focusing especially on recent experimental strategies based on the manipulation of the autophagy lysosomal pathway to counteract alpha-synuclein-mediated neurotoxicity in vivo. Pinpointing the factors that regulate alpha-synuclein association to the lysosome may represent potential targets for therapeutic interventions in PD and related synucleinopathies.

Altered Expression Patterns of Inflammation-Associated and Trophic Molecules in Substantia Nigra and Striatum Brain Samples from Parkinson's Disease, Incidental Lewy Body Disease and Normal Control Cases

Evidence of inflammation has been consistently associated with pathology in Parkinson's disease (PD)-affected brains, and has been suggested as a causative factor. Dopaminergic neurons in the substantia nigra (SN) pars compacta, whose loss results in the clinical symptoms associated with PD, are particularly susceptible to inflammatory damage and oxidative stress. Inflammation in the striatum, where SN dopaminergic neurons project, is also a feature of PD brains. It is not known whether inflammatory changes occur first in striatum or SN. Many animal models of PD have implicated certain inflammatory molecules with dopaminergic cell neuronal loss; however, there have been few studies to validate these findings by measuring the levels of these and other inflammatory factors in human PD brain samples. This study also included samples from incidental Lewy body disease (ILBD) cases, since ILBD is considered a non-symptomatic precursor to PD, with subjects having significant loss of tyrosine hydroxylase-producing neurons. We hypothesized that there may be a progressive change in key inflammatory factors in ILBD samples intermediate between neurologically normal and PD. To address this, we used a quantitative antibody-array platform (Raybiotech-Quantibody arrays) to measure the levels of 160 different inflammation-associated cytokines, chemokines, growth factors, and related molecules in extracts of SN and striatum from clinically and neuropathologically characterized PD, ILBD, and normal control cases. Patterns of changes in inflammation and related molecules were distinctly different between SN and striatum. Our results showed significantly different levels of interleukin (IL)-5, IL-15, monokine induced by gamma interferon, and IL-6 soluble receptor in SN between disease groups. A different panel of 13 proteins with significant changes in striatum, with IL-15 as the common feature, was identified. Although the ability to detect some proteins was limited by sensitivity, patterns of expression indicated involvement of certain T-cell cytokines, vascular changes, and loss of certain growth factors, with disease progression. The results demonstrate the feasibility of profiling inflammatory molecules using diseased human brain samples, and have provided additional targets to validate in relation to PD pathology.

Development of Passive Immunotherapies for Synucleinopathies

Immunotherapy using antibodies targeting alpha-synuclein has proven to be an effective strategy for ameliorating pathological and behavioral deficits induced by excess pathogenic alpha-synuclein in various animal and/or cellular models. However, the process of selecting the anti-alpha-synuclein antibody with the best potential to treat synucleinopathies in humans is not trivial. Critical to this process is a better understanding of the pathological processes involved in the synucleinopathies and how antibodies are able to influence these. We will give an overview of the first proof-of-concept studies in rodent disease models and discuss challenges associated with developing antibodies against alpha-synuclein resulting from the distribution and structural characteristics of the protein. We will also provide a status on the passive immunization approaches targeting alpha-synuclein that have entered, or are expected to enter, clinical evaluation.

Targeting α-synuclein as a therapeutic strategy for Parkinson's disease

INTRODUCTION

α-Synuclein, a neuronal protein, plays a central role in the pathophysiology of Parkinson's disease (PD), the second most prevalent neurodegenerative disorder. Cases of PD have increased tremendously over the past decade necessitating the identification of new therapeutic targets to reduce patient morbidity and to improve PD patients' quality of life.

AREAS COVERED

The purpose of this article is to provide an update on the role of α-synuclein in fibrils formation and review its role as an effective immunotherapeutic target for PD. The rapidly expanding evidence for the contribution of α-synuclein to the pathogenesis of PD led to the development of antibodies against the C terminus of α-synuclein and other molecules involved in the inflammatory signaling pathways that were found to contribute significantly to initiation and progression of the disease.

EXPERT OPINION

The readers will obtain new insights on the mechanisms by which α-synuclein can trigger the development of PD and other related degenerative disorders along with the potential role of active and passive antibodies targeted against specific form of α-synuclein aggregates to clear neurotoxicity, stop the propagation of the prion-like behavior of these oligomers and reverse neuronal degeneration associated with PD.

The degree of astrocyte activation in multiple system atrophy is inversely proportional to the distance to α-synuclein inclusions

Multiple system atrophy (MSA) exhibits widespread astrogliosis together with α-synuclein (α-syn) glial cytoplasmic inclusions (GCIs) in mature oligodendrocytes. We quantified astrocyte activation by morphometric analysis of MSA cases, and investigated the correlation to GCI proximity. Using Imaris software, we obtained "skinned" three-dimensional models of GFAP-positive astrocytes in MSA and control tissue (n=75) from confocal z-stacks and measured the astrocyte process length and thickness and radial distance to the GCI. Astrocytes proximal to GCI-containing oligodendrocytes (r<25μm) had significantly (p, 0.05) longer and thicker processes characteristic of activation than distal astrocytes (r>25μm), with a reciprocal linear correlation (m, 90μm(2)) between mean process length and radial distance to the nearest GCI (R(2), 0.7). In primary cell culture studies, α-syn addition caused ERK-dependent activation of rat astrocytes and perinuclear α-syn inclusions in mature (MOSP-positive) rat oligodendrocytes. Activated astrocytes were also observed in close proximity to α-syn deposits in a unilateral rotenone-lesion mouse model. Moreover, unilateral injection of MSA tissue-derived α-syn into the mouse medial forebrain bundle resulted in widespread neuroinflammation in the α-syn-injected, but not sham-injected hemisphere. Taken together, our data suggests that the action of localized concentrations of α-syn may underlie both astrocyte and oligodendrocyte MSA pathological features.

Chaperone mediated autophagy to the rescue: A new-fangled target for the treatment of neurodegenerative diseases

One of the main pathways of lysosomal proteolysis is chaperone-mediated autophagy (CMA), which represents a selective mechanism for the degradation of specific soluble proteins within lysosomes. Along with the other two lysosomal pathways, macro- and micro-autophagy, CMA contributes to cellular quality control through the removal of damaged or malfunctioning proteins. The two intrinsic characteristics of CMA are the selective targeting and the direct translocation of substrate proteins into the lysosomal lumen, in a fine-tuned manner through the orchestrated action of a chaperone/co-chaperone complex localized both at the cytosol and the lysosomes. Even though CMA was originally identified as a stress-induced pathway, basal CMA activity is detectable in most cell types analyzed so far, including neurons. Additionally, CMA activity declines with age and this may become a major aggravating factor contributing to neurodegeneration. More specifically, it has been suggested that CMA impairment may underlie the accumulation of misfolded/aggregated proteins, such as alpha-synuclein or LRRK2, whose levels or conformations are critical to Parkinson's disease pathogenesis. On the other hand, CMA induction might accelerate clearance of pathogenic proteins and promote cell survival, suggesting that CMA represents a viable therapeutic target for the treatment of various proteinopathies. In the current review, we provide an overview of the current state of knowledge regarding the role of CMA under physiological and pathological conditions of the nervous system and discuss the implications of these findings for therapeutic interventions for Parkinson's disease and other neurodegenerative disorders. This article is part of Special Issue entitled "Neuronal Protein".

Anti-human α-synuclein N-terminal peptide antibody protects against dopaminergic cell death and ameliorates behavioral deficits in an AAV-α-synuclein rat model of Parkinson's disease

The protein α-synuclein (α-Syn) has a central role in the pathogenesis of Parkinson’s disease (PD) and immunotherapeutic approaches targeting this molecule have shown promising results. In this study, novel antibodies were generated against specific peptides from full length human α-Syn and evaluated for effectiveness in ameliorating α-Syn-induced cell death and behavioral deficits in an AAV-α-Syn expressing rat model of PD. Fisher 344 rats were injected with rAAV vector into the right substantia nigra (SN), while control rats received an AAV vector expressing green fluorescent protein (GFP). Beginning one week after injection of the AAV-α-Syn vectors, rats were treated intraperitoneally with either control IgG or antibodies against the N-terminal (AB1), or central region (AB2) of α-Syn. An unbiased stereological estimation of TH+, NeuN+, and OX6 (MHC-II) immunostaining revealed that the α-Syn peptide antibodies (AB1 and AB2) significantly inhibited α-Syn-induced dopaminergic cell (DA) and NeuN+ cell loss (one-way ANOVA (F (3, 30) = 5.8, p = 0.002 and (F (3, 29) = 7.92, p = 0.002 respectively), as well as decreasing the number of activated microglia in the ipsilateral SN (one-way ANOVA F = 14.09; p = 0.0003). Antibody treated animals also had lower levels of α-Syn in the ipsilateral SN (one-way ANOVA F (7, 37) = 9.786; p = 0.0001) and demonstrated a partial intermediate improvement of the behavioral deficits. Our data suggest that, in particular, an α-Syn peptide antibody against the N-terminal region of the protein can protect against DA neuron loss and, to some extent behavioral deficits. As such, these results may be a potential therapeutic strategy for halting the progression of PD.

Attenuation of the early events of α-synuclein aggregation: a fluorescence correlation spectroscopy and laser scanning microscopy study in the presence of surface-coated Fe3O4 nanoparticles

The aggregation of α-synuclein (A-syn) has been implicated in the pathogenesis of Parkinson's disease (PD). Although the early events of aggregation and not the matured amyloid fibrils are believed to be responsible for the toxicity, it has been difficult to probe the formation of early oligomers experimentally. We studied the effect of Fe3O4 nanoparticle (NP) in the early stage of aggregation of A-syn using fluorescence correlation spectroscopy (FCS) and laser scanning microscopy. The binding between the monomeric protein and NPs was also studied using FCS at single-molecule resolution. Our data showed that the addition of bare Fe3O4 NPs accelerated the rate of early aggregation, and it did not bind the monomeric A-syn. In contrast, L-lysine (Lys)-coated Fe3O4 NPs showed strong binding with the monomeric A-syn, inhibiting the early events of aggregation. Lys-coated Fe3O4 NPs showed significantly less cell toxicity compared with bare Fe3O4 NPs and can be explored as a possible strategy to develop therapeutic application against PD. To the best of our knowledge, this report is the first example of using a small molecule to attenuate the early (and arguably the most relevant in terms of PD pathogenesis) events of A-syn aggregation.

Reduced Risk of Parkinson's Disease in Users of Calcium Channel Blockers: A Meta-Analysis

Aim. To pool the data currently available to determine the association between calcium channel blockers (CCBs) and risk of Parkinson's disease (PD). Methods. Literature search in PubMed, EBSCO, and Cochrane library was undertaken through March 2014, looking for observational studies evaluating the association between CCBs use and PD. Pooled relative risk (RR) estimates and 95% confidence intervals (CIs) were calculated using random-effects model. Subgroup analyses, sensitivity Analysis, and cumulative meta-analysis were also performed. Results. Six studies were included in our meta-analysis according to the selection criteria, including three cohort studies and three case-control studies involving 27,67,990 subjects including 11,941 PD cases. We found CCBs use was associated with significant decreased risk of PD, compared with not using CCBs (random effects model pooled RR, 0.81 (95% CI, 0.69–0.95)); a significant heterogeneity was found between studies (P = 0.031; I² 54.6%). Both the classes of CCB, that is, dihydropyridine calcium channel blockers (DiCCB) (0.80 (95% CI, 0.65–0.98) P = 0.032) and non-DiCCB (0.70 (95% CI, 0.53–0.92) P = 0.013), were found to be reducing the risk of PD. Conclusion. In our analysis, we found that CCBs use was associated with a Significantly decreased risk of PD compared with non-CCB use.

Is Parkinson's disease truly a prion-like disorder? An appraisal of current evidence

Parkinson's disease (PD) is the world's second most common neurodegenerative disease and most common movement disorder. Characterised by a loss of dopaminergic neurons and the development of intraneuronal inclusions known as Lewy bodies, it has classically been thought of as a cell-autonomous disease. However, in 2008, two groups reported the startling observation of Lewy bodies within embryonic neuronal grafts transplanted into PD patients little more than a decade previously, suggesting that PD pathology can be propagated to neighbouring cells and calling basic assumptions of our understanding of the disease into question. Subsequent research has largely served to confirm this interpretation, pointing towards a prion-like intercellular transfer of misfolded α-synuclein, the main component of Lewy bodies, as central to PD. This shift in thinking offers a revolutionary approach to PD treatment, potentially enabling a transition from purely symptomatic therapy to direct targeting of the pathology that drives disease progression. In this short review, we appraise current experimental support for PD as a prion-like disease, whilst highlighting areas of controversy or inconsistency which must be resolved. We also offer a brief discussion of the therapeutic implications of these discoveries.

Protein Transmission, Seeding and Degradation: Key Steps for α-Synuclein Prion-Like Propagation

Converging lines of evidence suggest that cell-to-cell transmission and the self-propagation of pathogenic amyloidogenic proteins play a central role in the initiation and the progression of several neurodegenerative disorders. This "prion-like" hypothesis has been recently reported for α-synuclein, a presynaptic protein implicated in the pathogenesis of Parkinson's disease (PD) and related disorders. This review summarizes recent findings on α-synuclein prion-like propagation, focusing on its transmission, seeding and degradation and discusses some key questions that remain to be explored. Understanding how α-synuclein exits cells and propagates from one brain region to another will lead to the development of new therapeutic strategies for the treatment of PD, aiming at slowing or stopping the disease progression.

Alpha-synuclein biology in Lewy body diseases

α-Synuclein is an abundantly expressed neuronal protein that is at the center of focus in understanding a group of neurodegenerative disorders called α-synucleinopathies, which are characterized by the presence of aggregated α-synuclein intracellularly. Primary α-synucleinopathies include Parkinson’s disease (PD), dementia with Lewy bodies and multiple system atrophy, with α-synuclein also found secondarily in a number of other diseases, including Alzheimer’s disease. Understanding how α-synuclein aggregates form in these different disorders is important for the understanding of its pathogenesis in Lewy body diseases. PD is the most prevalent of the α-synucleinopathies and much of the initial research on α-synuclein Lewy body pathology was based on PD but is also relevant to Lewy bodies in other diseases (dementia with Lewy bodies and Alzheimer’s disease). Polymorphism and mutation studies of SNCA, the gene that encodes α-synuclein, provide much evidence for a causal link between α-synuclein and PD. Among the primary α-synucleinopathies, multiple system atrophy is unique in that α-synuclein deposition occurs in oligodendrocytes rather than neurons. It is unclear whether α-synuclein originates from oligodendrocytes or whether it is transmitted somehow from neurons. α-Synuclein exists as a natively unfolded monomer in the cytosol, but in the presence of lipid membranes it is thought to undergo a conformational change to a folded α-helical secondary structure that is prone to forming dimers and oligomers. Posttranslational modification of α-synuclein, such as phosphorylation, ubiquitination and nitration, has been widely implicated in α-synuclein aggregation process and neurotoxicity. Recent studies using animal and cell models, as well as autopsy studies of patients with neuron transplants, provided compelling evidence for prion-like propagation of α-synuclein. This observation has implications for therapeutic strategies, and much recent effort is focused on developing antibodies that target extracellular α-synuclein.

Multifunctional D2/D3 agonist D-520 with high in vivo efficacy: modulator of toxicity of alpha-synuclein aggregates

We have developed a series of dihydroxy compounds and related analogues based on our hybrid D2/D3 agonist molecular template to develop multifunctional drugs for symptomatic and neuroprotective treatment for Parkinson's disease (PD). The lead compound (-)-24b (D-520) exhibited high agonist potency at D2/D3 receptors and produced efficacious activity in the animal models for PD. The data from thioflavin T (ThT) assay and from transmission electron microscopy (TEM) analysis demonstrate that D-520 is able to modulate aggregation of alpha-synuclein (αSN). Additionally, coincubation of D-520 with αSN is able to reduce toxicity of preformed aggregates of αSN compared to control αSN alone. Finally, in a neuroprotection study with dopaminergic MN9D cells, D-520 clearly demonstrated the effect of neuroprotection from toxicity of 6-hydroxydopamine. Thus, compound D-520 possesses properties characteristic of multifunctionality conducive to symptomatic and neuroprotective treatment of PD.

A30P α-Synuclein interferes with the stable integration of adult-born neurons into the olfactory network

Impaired olfaction is an early symptom in Parkinson disease (PD), although the exact cause is as yet unknown. Here, we investigated the link between PD-related mutant α-Synuclein (α-SYN) pathology and olfactory deficit, by examining the integration of adult-born neurons in the olfactory bulb (OB) of A30P α-SYN overexpressing mice. To this end, we chose to label one well-known vulnerable subpopulation of adult-born cells, the dopaminergic neurons. Using in vivo two-photon imaging, we followed the dynamic process of neuronal turnover in transgenic A30P α-SYN and wild-type mice over a period of 2.5 months. Our results reveal no difference in the number of cells that reach, and possibly integrate into, the glomerular layer in the OB. However, in mutant transgenic mice these new neurons have a significantly shortened survival, resulting in an overall reduction in the addition of neurons to the glomerular layer over time. We therefore propose unstable integration and impaired homeostasis of functional new neurons as a likely contributor to odour discrimination deficits in mutant α-SYN mice.

Extracellular α--synuclein-a novel and crucial factor in Lewy body diseases

Misfolding and intracellular aggregation of α-synuclein are thought to be crucial factors in the pathogenesis of Lewy body diseases (LBDs), such as Parkinson disease. However, the pathogenic modifications of this protein and the mechanisms underlying its activity have not been fully characterized. Recent studies suggest that small amounts of α-synuclein are released from neuronal cells by unconventional exocytosis, and that this extracellular α-synuclein contributes to the major pathological features of LBD, such as neurodegeneration, progressive spreading of α-synuclein pathology, and neuroinflammation. In this article, we review a rapidly growing body of literature on possible mechanisms by which extracellular α-synuclein contributes to LBD pathology, and discuss therapeutic approaches to target this form of α-synuclein to halt disease progression.

Valproic Acid Regulates α-Synuclein Expression through JNK Pathway in Rat Primary Astrocytes

Although the role of α-synuclein aggregation on Parkinson's disease is relatively well known, the physiological role and the regulatory mechanism governing the expression of α-synuclein are unclear yet. We recently reported that α-synuclein is expressed and secreted from cultured astrocytes. In this study, we investigated the effect of valproic acid (VPA), which has been suggested to provide neuroprotection by increasing α-synuclein in neuron, on α-synuclein expression in rat primary astrocytes. VPA concentrationdependently increased the protein expression level of α-synuclein in cultured rat primary astrocytes with concomitant increase in mRNA expression level. Likewise, the level of secreted α-synuclein was also increased by VPA. VPA increased the phosphorylation of Erk1/2 and JNK and pretreatment of a JNK inhibitor SP600125 prevented the VPA-induced increase in α-synuclein. Whether the increased α-synuclein in astrocytes is involved in the reported neuroprotective effects of VPA awaits further investigation.

Over-expression of an inactive mutant cathepsin D increases endogenous alpha-synuclein and cathepsin B activity in SH-SY5Y cells

Parkinson's disease is a neurodegenerative movement disorder. The histopathology of Parkinson's disease comprises proteinaceous inclusions known as Lewy bodies, which contains aggregated α-synuclein. Cathepsin D (CD) is a lysosomal protease previously demonstrated to cleave α-synuclein and decrease its toxicity in both cell lines and mouse brains in vivo. Here, we show that pharmacological inhibition of CD, or introduction of catalytically inactive mutant CD, resulted in decreased CD activity and increased cathepsin B activity, suggesting a possible compensatory response to inhibition of CD activity. However, this increased cathepsin B activity was not sufficient to maintain α-synuclein degradation, as evidenced by the accumulation of endogenous α-synuclein. Interestingly, the levels of LC3, LAMP1, and LAMP2, proteins involved in autophagy-lysosomal activities, as well as total lysosomal mass as assessed by LysoTracker flow cytometry, were unchanged. Neither autophagic flux nor proteasomal activities differs between cells over-expressing wild-type versus mutant CD. These observations point to a critical regulatory role for that endogenous CD activity in dopaminergic cells in α-synuclein homeostasis which cannot be compensated for by increased Cathepsin B. These data support the potential need to enhance CD function in order to attenuate α-synuclein accumulation as a therapeutic strategy against development of synucleinopathy.

Parkinson disease: from pathology to molecular disease mechanisms

Parkinson disease (PD) is a complex neurodegenerative disorder with both motor and nonmotor symptoms owing to a spreading process of neuronal loss in the brain. At present, only symptomatic treatment exists and nothing can be done to halt the degenerative process, as its cause remains unclear. Risk factors such as aging, genetic susceptibility, and environmental factors all play a role in the onset of the pathogenic process but how these interlink to cause neuronal loss is not known. There have been major advances in the understanding of mechanisms that contribute to nigral dopaminergic cell death, including mitochondrial dysfunction, oxidative stress, altered protein handling, and inflammation. However, it is not known if the same processes are responsible for neuronal loss in nondopaminergic brain regions. Many of the known mechanisms of cell death are mirrored in toxin-based models of PD, but neuronal loss is rapid and not progressive and limited to dopaminergic cells, and drugs that protect against toxin-induced cell death have not translated into neuroprotective therapies in humans. Gene mutations identified in rare familial forms of PD encode proteins whose functions overlap widely with the known molecular pathways in sporadic disease and these have again expanded our knowledge of the neurodegenerative process but again have so far failed to yield effective models of sporadic disease when translated into animals. We seem to be missing some key parts of the jigsaw, the trigger event starting many years earlier in the disease process, and what we are looking at now is merely part of a downstream process that is the end stage of neuronal death.

Extracellular alpha-synuclein induces calpain-dependent overactivation of cyclin-dependent kinase 5 in vitro

Extracellular alpha-synuclein (ASN) could be involved in the pathomechanism of Parkinson's disease (PD) via disturbances of calcium homeostasis, activation of nitric oxide synthase and oxidative/nitrosative stress. In this study we analyzed the role of cyclin-dependent kinase 5 (Cdk5) in the molecular mechanism(s) of ASN toxicity. We found that exposure of PC12 cells to ASN increases Cdk5 activity via calpain-dependent p25 formation and by enhancement of Cdk5 phosphorylation at Tyr15. Cdk5 and calpain inhibitors prevented ASN-evoked cell death. Our findings, indicating the participation of Cdk5 in ASN toxicity, provide new insight into how extracellular ASN may trigger dopaminergic cell dysfunction in PD.

Deregulation of calcium homeostasis mediates secreted α-synuclein-induced neurotoxicity

α-Synuclein (AS) plays a crucial role in Parkinson's disease pathogenesis. AS is normally secreted from neuronal cells and can thus exert paracrine effects. We have previously demonstrated that naturally secreted AS species, derived from SH-SY5Y cells inducibly overexpressing human wild type AS, can be toxic to recipient neuronal cells. In the current study, we show that application of secreted AS alters membrane fluidity and increases calcium (Ca2+) entry. This influx is reduced on pharmacological inhibition of voltage-operated Ca2+ channels. Although no change in free cytosolic Ca2+ levels is observed, a significantly increased mitochondrial Ca2+ sequestration is found in recipient cells. Application of voltage-operated Ca2+ channel blockers or Ca2+ chelators abolishes AS-mediated toxicity. AS-treated cells exhibit increased calpain activation, and calpain inhibition greatly alleviates the observed toxicity. Collectively, our data suggest that secreted AS exerts toxicity through engagement, at least in part, of the Ca2+ homeostatic machinery. Therefore, manipulating Ca2+ signaling pathways might represent a potential therapeutic strategy for Parkinson's disease.

The neurobiology of LRRK2 and its role in the pathogenesis of Parkinson's disease

Leucine-rich repeat kinase 2 (LRRK2) is a large, widely expressed protein of largely unknown function. Mutations in the gene encoding LRRK2 have been linked to multiple diseases, including a prominent association with familial and sporadic Parkinson's disease (PD), as well as inflammatory bowel disorders such as Crohn's disease. The LRRK2 protein possesses both kinase and GTPase signaling domains, as well as multiple protein interaction domains. Experimental studies in both cellular and in vivo models of mutant LRRK2-induced neurodegeneration have given clues to potential function(s) of LRRK2, yet much remains unknown. For example, while it is known that intact kinase and GTPase activity are required for mutant forms of the protein to trigger cell death, the specific targets of these enzymatic activities that mediate the death of neurons are not known. In this review, we discuss the evidence linking LRRK2 to various cellular/neuronal activities such as extrinsic death and inflammatory signaling, lysosomal protein degradation, the cytoskeletal system and neurite outgrowth, vesicle trafficking, mitochondrial dysfunction, as well as multiple points of interaction with several other genes linked to the pathogenesis of PD. In order for more effective therapeutic strategies to be envisioned and implemented, the mechanisms underlying LRRK2-mediated neurodegeneration need to be better characterized. Furthermore, insights into LRRK2-associated PD pathogenesis can potentially advance our understanding of the more common sporadic forms of PD.