Impaired intracellular trafficking defines early Parkinson's disease

Gastrointestinal dysmotility in rodent models of Parkinson's disease

Multiple studies describe prodromal, nonmotor dysfunctions that affect the quality of life of patients who subsequently develop Parkinson's disease (PD). These prodromal dysfunctions comprise a wide array of autonomic issues, including severe gastrointestinal (GI) motility disorders such as dysphagia, delayed gastric emptying, and chronic constipation. Indeed, strong evidence from studies in humans and animal models suggests that the GI tract and its neural, mainly vagal, connection to the central nervous system (CNS) could have a major role in the etiology of PD. In fact, misfolded α-synuclein aggregates that form Lewy bodies and neurites, i.e., the histological hallmarks of PD, are detected in the enteric nervous system (ENS) before clinical diagnosis of PD. The aim of the present review is to provide novel insights into the pathogenesis of GI dysmotility in PD, focusing our attention on functional, neurochemical, and molecular alterations in animal models.

Post-translational modification and mitochondrial function in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease with currently no cure. Most PD cases are sporadic, and about 5-10% of PD cases present a monogenic inheritance pattern. Mutations in more than 20 genes are associated with genetic forms of PD. Mitochondrial dysfunction is considered a prominent player in PD pathogenesis. Post-translational modifications (PTMs) allow rapid switching of protein functions and therefore impact various cellular functions including those related to mitochondria. Among the PD-associated genes, Parkin, PINK1, and LRRK2 encode enzymes that directly involved in catalyzing PTM modifications of target proteins, while others like α-synuclein, FBXO7, HTRA2, VPS35, CHCHD2, and DJ-1, undergo substantial PTM modification, subsequently altering mitochondrial functions. Here, we summarize recent findings on major PTMs associated with PD-related proteins, as enzymes or substrates, that are shown to regulate important mitochondrial functions and discuss their involvement in PD pathogenesis. We will further highlight the significance of PTM-regulated mitochondrial functions in understanding PD etiology. Furthermore, we emphasize the potential for developing important biomarkers for PD through extensive research into PTMs.

Linking acetylated α-Tubulin redistribution to α-Synuclein pathology in brain of Parkinson's disease patients

Highly specialized microtubules in neurons are crucial to both health and disease of the nervous system, and their properties are strictly regulated by different post-translational modifications, including α-Tubulin acetylation. An imbalance in the levels of acetylated α-Tubulin has been reported in experimental models of Parkinson's disease (PD) whereas pharmacological or genetic modulation that leads to increased acetylated α-Tubulin successfully rescues axonal transport defects and inhibits α-Synuclein aggregation. However, the role of acetylation of α-Tubulin in the human nervous system is largely unknown as most studies are based on in vitro evidence. To capture the complexity of the pathological processes in vivo, we analysed post-mortem human brain of PD patients and control subjects. In the brain of PD patients at Braak stage 6, we found a redistribution of acetylated α-Tubulin, which accumulates in the neuronal cell bodies in subcortical structures but not in the cerebral cortex, and decreases in the axonal compartment, both in putamen bundles of fibres and in sudomotor fibres. High-resolution and 3D reconstruction analysis linked acetylated α-Tubulin redistribution to α-Synuclein oligomerization and to phosphorylated Ser 129 α-Synuclein, leading us to propose a model for Lewy body (LB) formation. Finally, in post-mortem human brain, we observed threadlike structures, resembling tunnelling nanotubes that contain α-Synuclein oligomers and are associated with acetylated α-Tubulin enriched neurons. In conclusion, we support the role of acetylated α-Tubulin in PD pathogenesis and LB formation.

Neuronal Vulnerability to Degeneration in Parkinson's Disease and Therapeutic Approaches

Parkinson's disease is the second most common neurodegenerative disease affecting millions of people worldwide. Despite the crucial threat it poses, currently, no specific therapy exists that can completely reverse or halt the progression of the disease. Parkinson's disease pathology is driven by neurodegeneration caused by the intraneuronal accumulation of alpha-synuclein (α-syn) aggregates in Lewy bodies in the substantia nigra region of the brain. Parkinson's disease is a multiorgan disease affecting the central nervous system (CNS) as well as the autonomic nervous system. A bidirectional route of spreading α-syn from the gut to CNS through the vagus nerve and vice versa has also been reported. Despite our understanding of the molecular and pathophysiological aspects of Parkinson's disease, many questions remain unanswered regarding the selective vulnerability of neuronal populations, the neuromodulatory role of the locus coeruleus, and alpha-synuclein aggregation. This review article aims to describe the probable factors that contribute to selective neuronal vulnerability in Parkinson's disease, such as genetic predisposition, bioenergetics, and the physiology of neurons, as well as the interplay of environmental and exogenous modulators. This review also highlights various therapeutic strategies with cell transplants, through viral gene delivery, by targeting α-synuclein and aquaporin protein or epidermal growth factor receptors for the treatment of Parkinson's disease. The application of regenerative medicine and patient-specific personalized approaches have also been explored as promising strategies in the treatment of Parkinson's disease.

MAPT Locus in Parkinson's Disease Patients of Ashkenazi Origin: A Stratified Analysis

Introduction: MAPT locus is associated with Parkinson's disease (PD), which is located within a large inversion region of high linkage disequilibrium (LD). We aimed to determine whether the H2-haplotype protective effect and its effect size depends on the GBA1 or LRRK2 risk allele carrier status, and to further characterize genetic alterations that might contribute to its effect. Methods: LD analysis was performed using whole-genome sequencing data of 202 unrelated Ashkenazi Jewish (AJ) PDs. A haplotype-divergent variant was genotyped in a cohort of 1200 consecutively recruited AJ-PDs. The odd ratios were calculated using AJ-non-neuro cases from the gnomAD database as the controls in an un-stratified and a stratified manner according to the mutation carrier status, and the effect on the Age at Motor Symptom Onset (AMSO) was examined. Expression and splicing quantitative trait locus (eQTL and sQTL) analyses were carried out using brain tissues from a database. Results: The H2 haplotype exhibited significant association with PD protection, with a similar effect size in GBA1 carriers, LRRK2-G2019S carriers, and non-carriers (OR = 0.77, 0.69, and 0.82, respectively), and there was no effect on AMSO. The LD interval was narrowed to approximately 1.2 Mb. The H2 haplotype carried potential variants in candidate genes (MAPT and SPPL2C); structural deletions and segmental duplication (KANSL1); and variants affecting gene expression and intron excision ratio in brain tissues (LRRC37A/2). Conclusions: Our results demonstrate that H2 is associated with PD and its protective effect is not influenced by the GBA1/LRRK2 risk allele carrier status. This effect may be genetically complex, resulting from different levels of variations such as missense mutations in relevant genes, structural variations, epigenetic modifications, and RNA expression changes, which may operate independently or in synergy.

Membrane-stabilizing and protective effects of curcumin in a rotenone-induced rat model of Parkinson disease

Parkinson disease (PD) is a chronic progressive neurodegenerative disease characterized by both motor and non-motor features. Numerous risk factors (oxidative stress, free radical formation, and several environmental toxins) have been associated with PD. The experimental studies were carried out under in vivo conditions. Biochemical data analysis indicated that compared with the parameters of control (C) rats, rotenone-induced PD rats showed a significant decrease in the specific content of the total fraction of isoforms of O2--producing, heat-stable, NADPH-containing associates (NLP-Nox) from membrane formations of tissues (brain, liver, lung, and small intestine). Compared with the C group indices, in the PD and PD + curcumin (PD + CU) groups there is some change in the shape of the optical absorption spectra of isoforms associated with a change in the amount of Nox in the isoform composition of the total fraction of the NLP-Nox associate. Thus, daily administration of CU (200 mg/kg, i.p.) to PD rats for 63 days had a regulatory effect, bringing the specific content and O2--producing activity of the total fraction of NLP-Nox isoforms closer to the norm. CU has membrane-stabilizing effects in rotenone-induced PD.

Stable isotope labeling and ultra-high-resolution NanoSIMS imaging reveal alpha-synuclein-induced changes in neuronal metabolism in vivo

In Parkinson's disease, pathogenic factors such as the intraneuronal accumulation of the protein α-synuclein affect key metabolic processes. New approaches are required to understand how metabolic dysregulations cause degeneration of vulnerable subtypes of neurons in the brain. Here, we apply correlative electron microscopy and NanoSIMS isotopic imaging to map and quantify 13C enrichments in dopaminergic neurons at the subcellular level after pulse-chase administration of 13C-labeled glucose. To model a condition leading to neurodegeneration in Parkinson's disease, human α-synuclein was unilaterally overexpressed in the substantia nigra of one brain hemisphere in rats. When comparing neurons overexpressing α-synuclein to those located in the control hemisphere, the carbon anabolism and turnover rates revealed metabolic anomalies in specific neuronal compartments and organelles. Overexpression of α-synuclein enhanced the overall carbon turnover in nigral neurons, despite a lower relative incorporation of carbon inside the nucleus. Furthermore, mitochondria and Golgi apparatus showed metabolic defects consistent with the effects of α-synuclein on inter-organellar communication. By revealing changes in the kinetics of carbon anabolism and turnover at the subcellular level, this approach can be used to explore how neurodegeneration unfolds in specific subpopulations of neurons.

Myeloid-specific blockade of notch signaling alleviates dopaminergic neurodegeneration in Parkinson's disease by dominantly regulating resident microglia activation through NF-κB signaling

Yolk sac-derived microglia and peripheral monocyte-derived macrophages play a key role during Parkinson's disease (PD) progression. However, the regulatory mechanism of microglia/macrophage activation and function in PD pathogenesis remains unclear. Recombination signal-binding protein Jκ (RBP-J)-mediated Notch signaling regulates macrophage development and activation. In this study, with an 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) hydrochloride-induced acute murine PD model, we found that Notch signaling was activated in amoeboid microglia accompanied by a decrease in tyrosine hydroxylase (TH)-positive neurons. Furthermore, using myeloid-specific RBP-J knockout (RBP-JcKO) mice combined with a PD model, our results showed that myeloid-specific disruption of RBP-J alleviated dopaminergic neurodegeneration and improved locomotor activity. Fluorescence-activated cell sorting (FACS) analysis showed that the number of infiltrated inflammatory macrophages and activated major histocompatibility complex (MHC) II+ microglia decreased in RBP-JcKO mice compared with control mice. Moreover, to block monocyte recruitment by using chemokine (C-C motif) receptor 2 (CCR2) knockout mice, the effect of RBP-J deficiency on dopaminergic neurodegeneration was not affected, indicating that Notch signaling might regulate neuroinflammation independent of CCR2+ monocyte infiltration. Notably, when microglia were depleted with the PLX5622 formulated diet, we found that myeloid-specific RBP-J knockout resulted in more TH+ neurons and fewer activated microglia. Ex vitro experiments demonstrated that RBP-J deficiency in microglia might reduce inflammatory factor secretion, TH+ neuron apoptosis, and p65 nuclear translocation. Collectively, our study first revealed that RBP-J-mediated Notch signaling might participate in PD progression by mainly regulating microglia activation through nuclear factor kappa-B (NF-κB) signaling.

Dystonia and Parkinson's disease: Do they have a shared biology?

Parkinsonism and dystonia co-occur across many movement disorders and are most encountered in the setting of Parkinson's disease. Here we aim to explore the shared neurobiological underpinnings of dystonia and parkinsonism through the clinical lens of the conditions in which these movement disorders can be seen together. Foregrounding the discussion, we briefly review the circuits of the motor system and the neuroanatomical and neurophysiological aspects of motor control and highlight their relevance to the proposed pathophysiology of parkinsonism and dystonia. Insight into shared biology is then sought from dystonia occurring in PD and other forms of parkinsonism including those disorders in which both can be co-expressed simultaneously. We organize these within a biological schema along with important questions to be addressed in this space.

Neuropathology of incidental Lewy body & prodromal Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is a progressive neurodegenerative disorder associated with a loss of dopaminergic (DA) neurons. Despite symptomatic therapies, there is currently no disease-modifying treatment to halt neuronal loss in PD. A major hurdle for developing and testing such curative therapies results from the fact that most DA neurons are already lost at the time of the clinical diagnosis, rendering them inaccessible to therapy. Understanding the early pathological changes that precede Lewy body pathology (LBP) and cell loss in PD will likely support the identification of novel diagnostic and therapeutic strategies and help to differentiate LBP-dependent and -independent alterations. Several previous studies identified such specific molecular and cellular changes that occur prior to the appearance of Lewy bodies (LBs) in DA neurons, but a concise map of such early disease events is currently missing.

METHODS

Here, we conducted a literature review to identify and discuss the results of previous studies that investigated cases with incidental Lewy body disease (iLBD), a presumed pathological precursor of PD.

RESULTS

Collectively, our review demonstrates numerous cellular and molecular neuropathological changes occurring prior to the appearance of LBs in DA neurons.

CONCLUSIONS

Our review provides the reader with a summary of early pathological events in PD that may support the identification of novel therapeutic and diagnostic targets and aid to the development of disease-modifying strategies in PD.

Mitochondrial Dysfunction and Parkinson's Disease: Pathogenesis and Therapeutic Strategies

Parkinson's disease (PD) is a common age-related neurodegenerative disorder whose pathogenesis is not completely understood. Mitochondrial dysfunction and increased oxidative stress have been considered as major causes and central events responsible for the progressive degeneration of dopaminergic (DA) neurons in PD. Therefore, investigating mitochondrial disorders plays a role in understanding the pathogenesis of PD and can be an important therapeutic target for this disease. This study discusses the effect of environmental, genetic and biological factors on mitochondrial dysfunction and also focuses on the mitochondrial molecular mechanisms underlying neurodegeneration, and its possible therapeutic targets in PD, including reactive oxygen species generation, calcium overload, inflammasome activation, apoptosis, mitophagy, mitochondrial biogenesis, and mitochondrial dynamics. Other potential therapeutic strategies such as mitochondrial transfer/transplantation, targeting microRNAs, using stem cells, photobiomodulation, diet, and exercise were also discussed in this review, which may provide valuable insights into clinical aspects. A better understanding of the roles of mitochondria in the pathophysiology of PD may provide a rationale for designing novel therapeutic interventions in our fight against PD.

Super-resolution fluorescence microscopic imaging in pathogenesis and drug treatment of neurological disease

Since super-resolution fluorescence microscopic technology breaks the diffraction limit that has existed for a long time in optical imaging, it can observe the process of synapses formed between nerve cells and the protein aggregation related to neurological disease. Thus, super-resolution fluorescence microscopic imaging has significantly impacted several industries, including drug development and pathogenesis research, and it is anticipated that it will significantly alter the future of life science research. Here, we focus on several typical super-resolution fluorescence microscopic technologies, introducing their benefits and drawbacks, as well as applications in several common neurological diseases, in the hope that their services will be expanded and improved in the pathogenesis and drug treatment of neurological diseases.

Overexpression of alpha synuclein disrupts APP and Endolysosomal axonal trafficking in a mouse model of synucleinopathy

Mutations or triplication of the alpha synuclein (ASYN) gene contribute to synucleinopathies including Parkinson's disease (PD), Dementia with Lewy bodies (DLB) and multiple system atrophy (MSA). Recent evidence suggests that ASYN also plays an important role in amyloid-induced neurotoxicity, although the mechanism(s) remains unknown. One hypothesis is that accumulation of ASYN alters endolysosomal pathways to impact axonal trafficking and processing of the amyloid precursor protein (APP). To define an axonal function for ASYN, we used a transgenic mouse model of synucleinopathy that expresses a GFP-human ASYN (GFP-hASYN) transgene and an ASYN knockout (ASYN-/-) mouse model. Our results demonstrate that expression of GFP-hASYN in primary neurons derived from a transgenic mouse impaired axonal trafficking and processing of APP. In addition, axonal transport of BACE1, Rab5, Rab7, lysosomes and mitochondria were also reduced in these neurons. Interestingly, axonal transport of these organelles was also affected in ASYN-/- neurons, suggesting that ASYN plays an important role in maintaining normal axonal transport function. Therefore, selective impairment of trafficking and processing of APP by ASYN may act as a potential mechanism to induce pathological features of Alzheimer's disease (AD) in PD patients.

α-Synuclein modulates fibronectin expression in the trabecular meshwork independent of TGFβ2

α-Synuclein (α-Syn) is implicated in Parkinson's disease (PD), a neuromotor disorder with prominent visual symptoms. The underlying cause of motor dysfunction has been studied extensively, and is attributed to the death of dopaminergic neurons mediated in part by intracellular aggregation of α-Syn. The cause of visual symptoms, however, is less clear. Neuroretinal degeneration due to the presence of aggregated α-Syn has been reported, but the evidence is controversial. Other symptoms including those arising from primary open angle glaucoma (POAG) are believed to be the side-effects of medications prescribed for PD. Here, we explored the alternative hypothesis that dysfunction of α-Syn in the anterior eye alters the interaction between the actin cytoskeleton of trabecular meshwork (TM) cells with the extracellular matrix (ECM), impairing their ability to respond to physiological changes in intraocular pressure (IOP). A similar dysfunction in neurons is responsible for impaired neuritogenesis, a characteristic feature of PD. Using cadaveric human and bovine TM tissue and primary human TM cells as models, we report two main observations: 1) α-Syn is expressed in human and bovine TM cells, and significant amounts of monomeric and oligomeric α-Syn are present in the AH, and 2) primary human TM cells and human and bovine TM tissue endocytose extracellular recombinant monomeric and oligomeric α-Syn via the prion protein (PrP^C), and upregulate fibronectin (FN) and α-smooth muscle actin (α-SMA), fibrogenic proteins implicated in POAG. Transforming growth factor β2 (TGFβ2), a fibrogenic cytokine implicated in ∼50% cases of POAG, is also increased, and so is RhoA-associated coiled-coil-containing protein kinase 1 (ROCK-1). However, silencing of α-Syn in primary human TM cells reduces FN, α-SMA, and ROCK-1 in the absence or presence of over-expressed active TGFβ2, suggesting modulation of FN and ROCK-1 independent of, or upstream of TGFβ2. These observations suggest that extracellular α-Syn modulates ECM proteins in the TM independently or via PrP^C by activating the RhoA-ROCK pathway. These observations reveal a novel function of α-Syn in the anterior eye, and offer new therapeutic options.

Lysosomal Pathogenesis of Parkinson's Disease: Insights From LRRK2 and GBA1 Rodent Models

The discovery of mutations in LRRK2 and GBA1 that are linked to Parkinson's disease provided further evidence that autophagy and lysosome pathways are likely implicated in the pathogenic process. Their protein products are important regulators of lysosome function. LRRK2 has kinase-dependent effects on lysosome activity, autophagic efficacy and lysosomal Ca2+ signaling. Glucocerebrosidase (encoded by GBA1) is a hydrolytic enzyme contained in the lysosomes and contributes to the degradation of alpha-synuclein. PD-related mutations in LRRK2 and GBA1 slow the degradation of alpha-synuclein, thus directly implicating the dysfunction of the process in the neuropathology of Parkinson's disease. The development of genetic rodent models of LRRK2 and GBA1 provided hopes of obtaining reliable preclinical models in which to study pathogenic processes and perform drug validation studies. Here, I will review the extensive characterization of these models, their impact on understanding lysosome alterations in the course of Parkinson's disease and what novel insights have been obtained. In addition, I will discuss how these models fare with respect to the features of a "gold standard" animal models and what could be attempted in future studies to exploit LRRK2 and GBA1 rodent models in the fight against Parkinson's disease.

The Treatment of Parkinson's Disease with Sodium Oxybate

Sodiun Oxybate (SO) has a number of attributes that may mitigate the metabolic stress on the substantia nigra pars compacta (SNpc) dopaminergic (DA) neurons in Parkinson's disease (PD). These neurons function at the borderline of energy sufficiency. SO is metabolized to succinate and supplies energy to the cell by generating ATP. SO is a GABAB agonist and, as such, also arrests the high energy requiring calcium pace-making activity of these neurons. In addition, blocking calcium entry impedes the synaptic release and subsequent neurotransmission of aggregated synuclein species. As DA neurons degenerate, a homeostatic failure exposes these neurons to glutamate excitotoxicity, which in turn accelerates the damage. SO inhibits the neuronal release of glutamate and blocks its agonistic actions. Most important, SO generates NADPH, the cell's major antioxidant cofactor. Excessive free radical production within DA neurons and even more so within activated microglia are early and key features of the degenerative process that are present long before the onset of motor symptoms. NADPH maintains cell glutathione levels and alleviates oxidative stress and its toxic consequences. SO, a histone deacetylase inhibitor also suppresses the expression of microglial NADPH oxidase, the major source of free radicals in Parkinson brain. The acute clinical use of SO at night has been shown to reduce daytime sleepiness and fatigue in patients with PD. With long-term use, its capacity to supply energy to DA neurons, impede synuclein transmission, block excitotoxicity and maintain an anti-oxidative redox environment throughout the night may delay the onset of PD and slow its progress.

Methylphenidate Analogues as a New Class of Potential Disease-Modifying Agents for Parkinson's Disease: Evidence from Cell Models and Alpha-Synuclein Transgenic Mice

Parkinson's disease (PD) is characterized by dopaminergic nigrostriatal neurons degeneration and Lewy body pathology, mainly composed of α-synuclein (αSyn) fibrillary aggregates. We recently described that the neuronal phosphoprotein Synapsin III (Syn III) participates in αSyn pathology in PD brains and is a permissive factor for αSyn aggregation. Moreover, we reported that the gene silencing of Syn III in a human αSyn transgenic (tg) mouse model of PD at a pathological stage, manifesting marked insoluble αSyn deposits and dopaminergic striatal synaptic dysfunction, could reduce αSyn aggregates, restore synaptic functions and motor activities and exert neuroprotective effects. Interestingly, we also described that the monoamine reuptake inhibitor methylphenidate (MPH) can recover the motor activity of human αSyn tg mice through a dopamine (DA) transporter-independent mechanism, which relies on the re-establishment of the functional interaction between Syn III and α-helical αSyn. These findings support that the pathological αSyn/Syn III interaction may constitute a therapeutic target for PD. Here, we studied MPH and some of its analogues as modulators of the pathological αSyn/Syn III interaction. We identified 4-methyl derivative I-threo as a lead candidate modulating αSyn/Syn III interaction and having the ability to reduce αSyn aggregation in vitro and to restore the motility of αSyn tg mice in vivo more efficiently than MPH. Our results support that MPH derivatives may represent a novel class of αSyn clearing agents for PD therapy.

Linking α-synuclein-induced synaptopathy and neural network dysfunction in early Parkinson's disease

The prodromal phase of Parkinson's disease is characterized by aggregation of the misfolded pathogenic protein α-synuclein in select neural centres, co-occurring with non-motor symptoms including sensory and cognitive loss, and emotional disturbances. It is unclear whether neuronal loss is significant during the prodrome. Underlying these symptoms are synaptic impairments and aberrant neural network activity. However, the relationships between synaptic defects and network-level perturbations are not established. In experimental models, pathological α-synuclein not only impacts neurotransmission at the synaptic level, but also leads to changes in brain network-level oscillatory dynamics-both of which likely contribute to non-motor deficits observed in Parkinson's disease. Here we draw upon research from both human subjects and experimental models to propose a 'synapse to network prodrome cascade' wherein before overt cell death, pathological α-synuclein induces synaptic loss and contributes to aberrant network activity, which then gives rise to prodromal symptomology. As the disease progresses, abnormal patterns of neural activity ultimately lead to neuronal loss and clinical progression of disease. Finally, we outline goals and research needed to unravel the basis of functional impairments in Parkinson's disease and other α-synucleinopathies.

Inter-organellar Communication in Parkinson's and Alzheimer's Disease: Looking Beyond Endoplasmic Reticulum-Mitochondria Contact Sites

Neurodegenerative diseases (NDs) are generally considered proteinopathies but whereas this may initiate disease in familial cases, onset in sporadic diseases may originate from a gradually disrupted organellar homeostasis. Herein, endolysosomal abnormalities, mitochondrial dysfunction, endoplasmic reticulum (ER) stress, and altered lipid metabolism are commonly observed in early preclinical stages of major NDs, including Parkinson's disease (PD) and Alzheimer's disease (AD). Among the multitude of underlying defective molecular mechanisms that have been suggested in the past decades, dysregulation of inter-organellar communication through the so-called membrane contact sites (MCSs) is becoming increasingly apparent. Although MCSs exist between almost every other type of subcellular organelle, to date, most focus has been put on defective communication between the ER and mitochondria in NDs, given these compartments are critical in neuronal survival. Contributions of other MCSs, notably those with endolysosomes and lipid droplets are emerging, supported as well by genetic studies, identifying genes functionally involved in lysosomal homeostasis. In this review, we summarize the molecular identity of the organelle interactome in yeast and mammalian cells, and critically evaluate the evidence supporting the contribution of disturbed MCSs to the general disrupted inter-organellar homeostasis in NDs, taking PD and AD as major examples.

The Parkinson's disease protein alpha-synuclein is a modulator of processing bodies and mRNA stability

Alpha-synuclein (αS) is a conformationally plastic protein that reversibly binds to cellular membranes. It aggregates and is genetically linked to Parkinson's disease (PD). Here, we show that αS directly modulates processing bodies (P-bodies), membraneless organelles that function in mRNA turnover and storage. The N terminus of αS, but not other synucleins, dictates mutually exclusive binding either to cellular membranes or to P-bodies in the cytosol. αS associates with multiple decapping proteins in close proximity on the Edc4 scaffold. As αS pathologically accumulates, aberrant interaction with Edc4 occurs at the expense of physiologic decapping-module interactions. mRNA decay kinetics within PD-relevant pathways are correspondingly disrupted in PD patient neurons and brain. Genetic modulation of P-body components alters αS toxicity, and human genetic analysis lends support to the disease-relevance of these interactions. Beyond revealing an unexpected aspect of αS function and pathology, our data highlight the versatility of conformationally plastic proteins with high intrinsic disorder.

Functional Screening of Parkinson's Disease Susceptibility Genes to Identify Novel Modulators of α-Synuclein Neurotoxicity in Caenorhabditis elegans

Idiopathic Parkinson's disease (PD) is characterized by progressive loss of dopaminergic (DA) neurons during aging. The pathological hallmark of PD is the Lewy body detected in postmortem brain tissue, which is mainly composed of aggregated α-Synuclein (αSyn). However, it is estimated that 90% of PD cases have unknown pathogenetic triggers. Here, we generated a new transgenic Caenorhabditis elegans PD model eraIs1 expressing green fluorescent protein- (GFP-) based reporter of human αSyn in DA neurons, and exhibited a nice readout of the developed αSyn inclusions in DA neurons, leading to their degeneration during aging. Using these animals in a preliminary reverse genetic screening of >100-PD genome-wide association study- (GWAS-) based susceptibility genes, we identified 28 orthologs of C. elegans and their inactivation altered the phenotype of eraIs1; 10 knockdowns exhibited reduced penetrance of αSyn:Venus inclusions formed in the axons of cephalic (CEP) DA neurons, 18 knockdowns exhibited increased penetrance of disrupted CEP dendrite integrity among which nine knockdowns also exhibited disrupted neuronal morphology independent of the expressed αSyn reporter. Loss-of-function alleles of the five identified genes, such as sac-2, rig-6 or lfe-2, unc-43, and nsf-1, modulated the corresponding eraIs1 phenotype, respectively, and supported the RNA interference (RNAi) data. The Western blot analysis showed that the levels of insoluble αSyn:Venus were not correlated with the observed phenotypes in these mutants. However, RNAi of 12 identified modulators reduced the formation of pro-aggregating polyglutamine Q40:YFP foci in muscle cells, suggesting the possible role of these genes in cellular proteotoxicity. Therefore, modulators identified by their associated biological pathways, such as calcium signaling or vesicular trafficking, represent new potential therapeutic targets for neurodegenerative proteopathies and other diseases associated with aging.

Ubiquitin Proteasome System and Microtubules Are Master Regulators of Central and Peripheral Nervous System Axon Degeneration

Axonal degeneration is an active process that differs from neuronal death, and it is the hallmark of many disorders affecting the central and peripheral nervous system. Starting from the analyses of Wallerian degeneration, the simplest experimental model, here we describe how the long projecting neuronal populations affected in Parkinson’s disease and chemotherapy-induced peripheral neuropathies share commonalities in the mechanisms and molecular players driving the earliest phase of axon degeneration. Indeed, both dopaminergic and sensory neurons are particularly susceptible to alterations of microtubules and axonal transport as well as to dysfunctions of the ubiquitin proteasome system and protein quality control. Finally, we report an updated review on current knowledge of key molecules able to modulate these targets, blocking the on-going axonal degeneration and inducing neuronal regeneration. These molecules might represent good candidates for disease-modifying treatment, which might expand the window of intervention improving patients’ quality of life.

The Role of Axonal Transport in Glaucoma

Glaucoma is a neurodegenerative disease that affects the retinal ganglion cells (RGCs) and leads to progressive vision loss. The first pathological signs can be seen at the optic nerve head (ONH), the structure where RGC axons leave the retina to compose the optic nerve. Besides damage of the axonal cytoskeleton, axonal transport deficits at the ONH have been described as an important feature of glaucoma. Axonal transport is essential for proper neuronal function, including transport of organelles, synaptic components, vesicles, and neurotrophic factors. Impairment of axonal transport has been related to several neurodegenerative conditions. Studies on axonal transport in glaucoma include analysis in different animal models and in humans, and indicate that its failure happens mainly in the ONH and early in disease progression, preceding axonal and somal degeneration. Thus, a better understanding of the role of axonal transport in glaucoma is not only pivotal to decipher disease mechanisms but could also enable early therapies that might prevent irreversible neuronal damage at an early time point. In this review we present the current evidence of axonal transport impairment in glaucomatous neurodegeneration and summarize the methods employed to evaluate transport in this disease.

Rescue of α-synuclein aggregation in Parkinson's patient neurons by synergistic enhancement of ER proteostasis and protein trafficking

Neurodegenerative disorders are characterized by a collapse in proteostasis, as shown by the accumulation of insoluble protein aggregates in the brain. Proteostasis involves a balance of protein synthesis, folding, trafficking, and degradation, but how aggregates perturb these pathways is unknown. Using Parkinson's disease (PD) patient midbrain cultures, we find that aggregated α-synuclein induces endoplasmic reticulum (ER) fragmentation and compromises ER protein folding capacity, leading to misfolding and aggregation of immature lysosomal β-glucocerebrosidase. Despite this, PD neurons fail to initiate the unfolded protein response, indicating perturbations in sensing or transducing protein misfolding signals in the ER. Small molecule enhancement of ER proteostasis machinery promotes β-glucocerebrosidase solubility, while simultaneous enhancement of trafficking improves ER morphology, lysosomal function, and reduces α-synuclein. Our studies suggest that aggregated α-synuclein perturbs the ability of neurons to respond to misfolded proteins in the ER, and that synergistic enhancement of multiple proteostasis branches may provide therapeutic benefit in PD.

Can the lack of fibrillar form of alpha-synuclein in Lewy bodies be explained by its catalytic activity?

Finding the causative pathophysiological mechanisms for Parkinson's disease (PD) is important for developing therapeutic interventions. Until recently, it was believed that Lewy bodies (LBs), the hallmark of PD, are mostly composed of alpha-synuclein (α-syn) fibrils. Recent results (Shahmoradian et al. (2019)) demonstrated that the fibrillar form of α-syn is lacking from LBs. Here we propose that this surprising observation can be explained by the catalytic activity of the fibrillar form of α-syn. We assumed that α-syn fibrils catalyze the formation of LBs, but do not become part of them. We developed a mathematical model based on this hypothesis. By using the developed model, we investigated the consequences of this hypothesis. In particular, the model suggests that the long incubation time of PD can be explained by a two-step aggregation process that leads to its development: (i) aggregation of monomeric α-syn into α-syn oligomers and fibrils and (ii) clustering of membrane-bound organelles, which may cause disruption of axonal trafficking and lead to neuron starvation and death. The model shows that decreasing the rate of destruction of α-syn aggregates in somatic lysosomes accelerates the formation of LBs. Another consequence of the model is the prediction that removing α-syn aggregates from the brain after the aggregation of membrane-bound organelles into LBs has started may not stop the progression of PD because LB formation is an autocatalytic process; hence, the formation of LBs will be catalyzed by aggregates of membrane-bound organelles even in the absence of α-syn aggregates. The performed sensitivity study made it possible to establish the hierarchy of model parameters with respect to their effect on the formation of vesicle aggregates in the soma.

The increase of α-synuclein and alterations of dynein in A53T transgenic and aging mouse

The dynein protein plays a key role in the degradation pathway by attaching to targeted molecules and transporting the autophagosome to the centrosome. Aging plays an important role in the pathogenesis of Parkinson's disease (PD), but its effect on dynein is not clear. In this study we analyzed behavioral characteristics using the rod endurance test and climbing rod time test in different aged mice (3 months, 12 months, 20 months), and measured protein expression of dynein, α-synuclein, Tctex-1, and LC3 in the substantianigra of the mice by Western blot. The mRNA levels of dynein, α-synuclein, LC3 and Tctex-1 were measured by quantitative real time reverse transcription PCR, and detecting expression of dynein and α-synuclein by immunofluorescence. We found the motor functions of A53T mutant mice, in 12 months and 20 months, decreased more significantly compared with normal mice (p < 0.05). In addition, the expression of dynein, LC3-Ⅱ and Tctex-1 proteins in the substantia nigra of the two groups decreased with age. However, α-synuclein protein increased gradually with age, with significantly higher levels in the PD groups compared with age matched controls (p < 0.05). These results were confirmed by immunofluorescence. Our data demonstrates that dynein and other autophagy proteins change with age, and this is associated with increased α-synuclein. Therefore, therapeutics that prevent dynein dysfunction may offer novel treatments for PD and other autophagy related diseases.

The Role of GM130 in Nervous System Diseases

Golgi matrix protein 130 (GM130) is a Golgi-shaping protein located on the cis surface of the Golgi apparatus (GA). It is one of the most studied Golgin proteins so far. Its biological functions are involved in many aspects of life processes, including mitosis, autophagy, apoptosis, cell polarity, and directed migration at the cellular level, as well as intracellular lipid and protein transport, microtubule formation and assembly, lysosome function maintenance, and glycosylation modification. Mutation inactivation or loss of expression of GM130 has been detected in patients with different diseases. GM130 plays an important role in the development of the nervous system, but the studies on it are limited. This article reviewed the current research progress of GM130 in nervous system diseases. It summarized the physiological functions of GM130 in the occurrence and development of Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), microcephaly (MCPH), sepsis associated encephalopathy (SAE), and Ataxia, aiming to provide ideas for the further study of GM130 in nervous system disease detection and treatment.

Alpha-Synuclein Aggregation in Parkinson's Disease

Parkinson's disease (PD), a neurodegenerative disorder characterized by distinct aging-independent loss of dopaminergic neurons in substantia nigra pars compacta (SNpc) region urging toward neuronal loss. Over the decade, various key findings from clinical perspective to molecular pathogenesis have aided in understanding the genetics with assorted genes related with PD. Subsequently, several pathways have been incriminated in the pathogenesis of PD, involving mitochondrial dysfunction, protein aggregation, and misfolding. On the other hand, the sporadic form of PD cases is found with no genetic linkage, which still remain an unanswered question? The exertion in ascertaining vulnerability factors in PD considering the genetic factors are to be further dissevered in the forthcoming decades with advancement in research studies. One of the major proponents behind the prognosis of PD is the pathogenic transmutation of aberrant alpha-synuclein protein into amyloid fibrillar structures, which actuates neurodegeneration. Alpha-synuclein, transcribed by SNCA gene is a neuroprotein found predominantly in brain. It is implicated in the modulation of synaptic vesicle transport and eventual release of neurotransmitters. Due to genetic mutations and other elusive factors, the alpha-synuclein misfolds into its amyloid form. Therefore, this review aims in briefing the molecular understanding of the alpha-synuclein associated with PD.

Microglia-specific overexpression of α-synuclein leads to severe dopaminergic neurodegeneration by phagocytic exhaustion and oxidative toxicity

Recent findings in human samples and animal models support the involvement of inflammation in the development of Parkinson's disease. Nevertheless, it is currently unknown whether microglial activation constitutes a primary event in neurodegeneration. We generated a new mouse model by lentiviral-mediated selective α-synuclein (αSYN) accumulation in microglial cells. Surprisingly, these mice developed progressive degeneration of dopaminergic (DA) neurons without endogenous αSYN aggregation. Transcriptomics and functional assessment revealed that αSYN-accumulating microglial cells developed a strong reactive state with phagocytic exhaustion and excessive production of oxidative and proinflammatory molecules. This inflammatory state created a molecular feed-forward vicious cycle between microglia and IFNγ-secreting immune cells infiltrating the brain parenchyma. Pharmacological inhibition of oxidative and nitrosative molecule production was sufficient to attenuate neurodegeneration. These results suggest that αSYN accumulation in microglia induces selective DA neuronal degeneration by promoting phagocytic exhaustion, an excessively toxic environment and the selective recruitment of peripheral immune cells.

Real-time three-dimensional tracking of single vesicles reveals abnormal motion and pools of synaptic vesicles in neurons of Huntington's disease mice

Although defective synaptic transmission was suggested to play a role in neurodegenerative diseases, the dynamics and vesicle pools of synaptic vesicles during neurodegeneration remain elusive. Here, we performed real-time three-dimensional tracking of single synaptic vesicles in cortical neurons from a mouse model of Huntington's disease (HD). Vesicles in HD neurons had a larger net displacement and radius of gyration compared with wild-type neurons. Vesicles with high release probability (P_r) were interspersed with low-P_r vesicles in HD neurons, whereas high-P_r vesicles were closer to fusion sites than low-P_r in wild-type neurons. Non-releasing vesicles in HD neurons had an abnormally high prevalence of irregular oscillatory motion. These abnormal dynamics and vesicle pools were rescued by overexpressing Rab11, and the abnormal irregular oscillatory motion was rescued by jasplakinolide. Our studies reveal the abnormal dynamics and pools of synaptic vesicles in the early stages of HD, suggesting a possible pathogenic mechanism of neurodegenerative diseases.

Nanoparticles With Affinity for α-Synuclein Sequester α-Synuclein to Form Toxic Aggregates in Neurons With Endolysosomal Impairment

Parkinson's disease (PD) is one of the most common neurodegenerative diseases. It is characterized pathologically by the aggregation of α-synuclein (αS) in the form of Lewy bodies and Lewy neurites. A major challenge in PD therapy is poor efficiency of drug delivery to the brain due to the blood-brain barrier (BBB). For this reason, nanomaterials, with significant advantages in drug delivery, have gained attention. On the other hand, recent studies have shown that nanoparticles can promote αS aggregation in salt solution. Therefore, we tested if nanoparticles could have the same effect in cell models. We found that nanoparticle can induce cells to form αS inclusions as shown in immunocytochemistry, and detergent-resistant αS aggregates as shown in biochemical analysis; and nanoparticles of smaller size can induce more αS inclusions. Moreover, the induction of αS inclusions is in part dependent on endolysosomal impairment and the affinity of αS to nanoparticles. More importantly, we found that the abnormally high level of endogenous lysosomotropic biomolecules (e.g., sphingosine), due to impairing the integrity of endolysosomes could be a determinant factor for the susceptibility of cells to nanoparticle-induced αS aggregation; and deletion of GBA1 gene to increase the level of intracellular sphingosine can render cultured cells more susceptible to the formation of αS inclusions in response to nanoparticle treatment. Ultrastructural examination of nanoparticle-treated cells revealed that the induced inclusions contained αS-immunopositive membranous structures, which were also observed in inclusions seeded by αS fibrils. These results suggest caution in the use of nanoparticles in PD therapy. Moreover, this study further supports the role of endolysosomal impairment in PD pathogenesis and suggests a possible mechanism underlying the formation of membrane-associated αS pathology.

Neuroaspis PLP10™, a nutritional formula rich in omega-3 and omega-6 fatty acids with antioxidant vitamins including gamma-tocopherol in early Parkinson's disease: A randomized, double-blind, placebo-controlled trial

In the present study, we investigated whether Neuroaspis PLP10™, a well-designed intervention, rich in omega-3 (n-3) and omega-6 (n-6) polyunsaturated fatty acids (PUFAs) with specific antioxidant vitamins, may exert positive action in the improvement of Parkinson's disease symptoms and perhaps delay the progression of the disease when used as an adjuvant to the conventional treatment. Forty patients were randomized 1:1 to receive either 20 ml dose, once daily, of control (pure virgin olive oil) or Neuroaspis PLP 10™, a formula containing a mixture of omega-3 (810 mg Eicosapentaenoic acid and 4140 mg Docosahexaenoic acid) and omega-6 fatty acids (1800 mg gamma-Linolenic acid and 3150 mg Linoleic acid) (1:1 w/w), with 0.6 mg vitamin A, vitamin E (22 mg) plus pure gamma (γ)-tocopherol (760 mg), for a total of 30 months in a randomized double-blind, placebo-controlled trial. Participants completed assessments based on the Hoehn and Yahr Staging Scale of Parkinson's Disease (HY scale) and the Unified Parkinson's Disease Rating Scale (UPDRS) III. Overall, for this small sample size clinical trial, Neuroaspis PLP10™ supplementation as an adjuvant treatment for 30 months in PD patients significantly delayed disease progression according to UPDRS (p ≤ 0.05) Vs placebo.

Therapeutic Potential of Natural Products in Treating Neurodegenerative Disorders and Their Future Prospects and Challenges

Natural products derived from plants, as well as their bioactive compounds, have been extensively studied in recent years for their therapeutic potential in a variety of neurodegenerative diseases (NDs), including Alzheimer's (AD), Huntington's (HD), and Parkinson's (PD) disease. These diseases are characterized by progressive dysfunction and loss of neuronal structure and function. There has been little progress in designing efficient treatments, despite impressive breakthroughs in our understanding of NDs. In the prevention and therapy of NDs, the use of natural products may provide great potential opportunities; however, many clinical issues have emerged regarding their use, primarily based on the lack of scientific support or proof of their effectiveness and patient safety. Since neurodegeneration is associated with a myriad of pathological processes, targeting multi-mechanisms of action and neuroprotection approaches that include preventing cell death and restoring the function of damaged neurons should be employed. In the treatment of NDs, including AD and PD, natural products have emerged as potential neuroprotective agents. This current review will highlight the therapeutic potential of numerous natural products and their bioactive compounds thatexert neuroprotective effects on the pathologies of NDs.

Promising drug targets and associated therapeutic interventions in Parkinson's disease

Parkinson's disease (PD) is one of the most debilitating brain diseases. Despite the availability of symptomatic treatments, response towards the health of PD patients remains scarce. To fulfil the medical needs of the PD patients, an efficacious and etiological treatment is required. In this review, we have compiled the information covering limitations of current therapeutic options in PD, novel drug targets for PD, and finally, the role of some critical beneficial natural products to control the progression of PD.

Chaperoning SNARE Folding and Assembly

SNARE proteins and Sec1/Munc18 (SM) proteins constitute the core molecular engine that drives nearly all intracellular membrane fusion and exocytosis. While SNAREs are known to couple their folding and assembly to membrane fusion, the physiological pathways of SNARE assembly and the mechanistic roles of SM proteins have long been enigmatic. Here, we review recent advances in understanding the SNARE-SM fusion machinery with an emphasis on biochemical and biophysical studies of proteins that mediate synaptic vesicle fusion. We begin by discussing the energetics, pathways, and kinetics of SNARE folding and assembly in vitro. Then, we describe diverse interactions between SM and SNARE proteins and their potential impact on SNARE assembly in vivo. Recent work provides strong support for the idea that SM proteins function as chaperones, their essential role being to enable fast, accurate SNARE assembly. Finally, we review the evidence that SM proteins collaborate with other SNARE chaperones, especially Munc13-1, and briefly discuss some roles of SNARE and SM protein deficiencies in human disease.

Cooperative inhibition of SNARE-mediated vesicle fusion by α-synuclein monomers and oligomers

The primary hallmark of Parkinson's disease (PD) is the generation of Lewy bodies of which major component is α-synuclein (α-Syn). Because of increasing evidence of the fundamental roles of α-Syn oligomers in disease progression, α-Syn oligomers have become potential targets for therapeutic interventions for PD. One of the potential toxicities of α-Syn oligomers is their inhibition of SNARE-mediated vesicle fusion by specifically interacting with vesicle-SNARE protein synaptobrevin-2 (Syb2), which hampers dopamine release. Here, we show that α-Syn monomers and oligomers cooperatively inhibit neuronal SNARE-mediated vesicle fusion. α-Syn monomers at submicromolar concentrations increase the fusion inhibition by α-Syn oligomers. This cooperative pathological effect stems from the synergically enhanced vesicle clustering. Based on this cooperative inhibition mechanism, we reverse the fusion inhibitory effect of α-Syn oligomers using small peptide fragments. The small peptide fragments, derivatives of α-Syn, block the binding of α-Syn oligomers to Syb2 and dramatically reverse the toxicity of α-Syn oligomers in vesicle fusion. Our findings demonstrate a new strategy for therapeutic intervention in PD and related diseases based on this specific interaction of α-Syn.

Microtubule acetylation: A reading key to neural physiology and degeneration

Neurons are the perfect example of cells where microtubules are essential to achieve an extraordinary degree of morphological and functional complexity. Different tubulin isoforms and associated post-translational modifications are the basis to establish the diversity in biochemical and biophysical properties of microtubules including their stability and the control of intracellular transport. Acetylation is one of the key tubulin modifications and it can influence important structural, mechanical and biological traits of the microtubule network. Here, we present the emerging evidence for the essential role of microtubule acetylation in the control of neuronal and glial function in healthy and degenerative conditions. In particular, we discuss the pathogenic role of tubulin acetylation in neurodegenerative disorders and focus on Parkinson's disease. We also provide a critical analysis about the possibility to target tubulin acetylation as a novel therapeutic intervention for neuroprotective strategies.

PGC-1s in the Spotlight with Parkinson's Disease

Parkinson's disease is one of the most common neurodegenerative disorders worldwide, characterized by a progressive loss of dopaminergic neurons mainly localized in the substantia nigra pars compacta. In recent years, the detailed analyses of both genetic and idiopathic forms of the disease have led to a better understanding of the molecular and cellular pathways involved in PD, pointing to the centrality of mitochondrial dysfunctions in the pathogenic process. Failure of mitochondrial quality control is now considered a hallmark of the disease. The peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1) family acts as a master regulator of mitochondrial biogenesis. Therefore, keeping PGC-1 level in a proper range is fundamental to guarantee functional neurons. Here we review the major findings that tightly bond PD and PGC-1s, raising important points that might lead to future investigations.

The cell biology of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder resulting from the death of dopamine neurons in the substantia nigra pars compacta. Our understanding of PD biology has been enriched by the identification of genes involved in its rare, inheritable forms, termed PARK genes. These genes encode proteins including α-syn, LRRK2, VPS35, parkin, PINK1, and DJ1, which can cause monogenetic PD when mutated. Investigating the cellular functions of these proteins has been instrumental in identifying signaling pathways that mediate pathology in PD and neuroprotective mechanisms active during homeostatic and pathological conditions. It is now evident that many PD-associated proteins perform multiple functions in PD-associated signaling pathways in neurons. Furthermore, several PARK proteins contribute to non-cell-autonomous mechanisms of neuron death, such as neuroinflammation. A comprehensive understanding of cell-autonomous and non-cell-autonomous pathways involved in PD is essential for developing therapeutics that may slow or halt its progression.

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

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

Sigma-2 receptor antagonists rescue neuronal dysfunction induced by Parkinson's patient brain-derived α-synuclein

α‐Synuclein oligomers are thought to have a pivotal role in sporadic and familial Parkinson's disease (PD) and related α‐synucleinopathies, causing dysregulation of protein trafficking, autophagy/lysosomal function, and protein clearance, as well as synaptic function impairment underlying motor and cognitive symptoms of PD. Moreover, trans‐synaptic spread of α‐synuclein oligomers is hypothesized to mediate disease progression. Therapeutic approaches that effectively block α‐synuclein oligomer‐induced pathogenesis are urgently needed. Here, we show for the first time that α‐synuclein species isolated from human PD patient brain and recombinant α‐synuclein oligomers caused similar deficits in lipid vesicle trafficking rates in cultured rat neurons and glia, while α‐synuclein species isolated from non‐PD human control brain samples did not. Recombinant α‐synuclein oligomers also increased neuronal expression of lysosomal‐associated membrane protein‐2A (LAMP‐2A), the lysosomal receptor that has a critical role in chaperone‐mediated autophagy. Unbiased screening of several small molecule libraries (including the NIH Clinical Collection) identified sigma‐2 receptor antagonists as the most effective at blocking α‐synuclein oligomer‐induced trafficking deficits and LAMP‐2A upregulation in a dose‐dependent manner. These results indicate that antagonists of the sigma‐2 receptor complex may alleviate α‐synuclein oligomer‐induced neurotoxicity and are a novel therapeutic approach for disease modification in PD and related α‐synucleinopathies.

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

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

Dysfunctional Autophagy and Endolysosomal System in Neurodegenerative Diseases: Relevance and Therapeutic Options

Autophagy and endolysosomal trafficking are crucial in neuronal development, function and survival. These processes ensure efficient removal of misfolded aggregation-prone proteins and damaged organelles, such as dysfunctional mitochondria, thus allowing the maintenance of proper cellular homeostasis. Beside this, emerging evidence has pointed to their involvement in the regulation of the synaptic proteome needed to guarantee an efficient neurotransmitter release and synaptic plasticity. Along this line, an intimate interplay between the molecular machinery regulating synaptic vesicle endocytosis and synaptic autophagy is emerging, suggesting that synaptic quality control mechanisms need to be tightly coupled to neurosecretion to secure release accuracy. Defects in autophagy and endolysosomal pathway have been associated with neuronal dysfunction and extensively reported in Alzheimer's, Parkinson's, Huntington's and amyotrophic lateral sclerosis among other neurodegenerative diseases, with common features and emerging genetic bases. In this review, we focus on the multiple roles of autophagy and endolysosomal system in neuronal homeostasis and highlight how their defects probably contribute to synaptic default and neurodegeneration in the above-mentioned diseases, discussing the most recent options explored for therapeutic interventions.

Short-Chain Fatty Acid-Producing Gut Microbiota Is Decreased in Parkinson's Disease but Not in Rapid-Eye-Movement Sleep Behavior Disorder

Gut dysbiosis has been repeatedly reported in Parkinson's disease (PD) but only once in idiopathic rapid-eye-movement sleep behavior disorder (iRBD) from Germany. Abnormal aggregation of α-synuclein fibrils causing PD possibly starts from the intestine, although this is still currently under debate. iRBD patients frequently develop PD. Early-stage gut dysbiosis that is causally associated with PD is thus expected to be observed in iRBD. We analyzed gut microbiota in 26 iRBD patients and 137 controls by 16S rRNA sequencing (16S rRNA-seq). Our iRBD data set was meta-analyzed with the German iRBD data set and was compared with gut microbiota in 223 PD patients. Unsupervised clustering of gut microbiota by LIGER, a topic model-based tool for single-cell RNA sequencing (RNA-seq) analysis, revealed four enterotypes in controls, iRBD, and PD. Short-chain fatty acid (SCFA)-producing bacteria were conserved in an enterotype observed in controls and iRBD, whereas they were less conserved in enterotypes observed in PD. Genus Akkermansia and family Akkermansiaceae were consistently increased in both iRBD in two countries and PD in five countries. Short-chain fatty acid (SCFA)-producing bacteria were not significantly decreased in iRBD in two countries. In contrast, we previously reported that recognized or putative SCFA-producing genera Faecalibacterium, Roseburia, and Lachnospiraceae ND3007 group were consistently decreased in PD in five countries. In α-synucleinopathy, increase of mucin-layer-degrading genus Akkermansia is observed at the stage of iRBD, whereas decrease of SCFA-producing genera becomes obvious with development of PD.IMPORTANCE Twenty studies on gut microbiota in PD have been reported, whereas only one study has been reported on iRBD from Germany. iRBD has the highest likelihood ratio to develop PD. Our meta-analysis of iRBD in Japan and Germany revealed increased mucin-layer-degrading genus Akkermansia in iRBD. Genus Akkermansia may increase the intestinal permeability, as we previously observed in PD patients, and may make the intestinal neural plexus exposed to oxidative stress, which can lead to abnormal aggregation of prion-like α-synuclein fibrils in the intestine. In contrast to PD, SCFA-producing bacteria were not decreased in iRBD. As SCFA induces regulatory T (Treg) cells, a decrease of SCFA-producing bacteria may be a prerequisite for the development of PD. We propose that prebiotic and/or probiotic therapeutic strategies to increase the intestinal mucin layer and to increase intestinal SCFA potentially retard the development of iRBD and PD.

Novel PET Biomarkers to Disentangle Molecular Pathways across Age-Related Neurodegenerative Diseases

There is a need to disentangle the etiological puzzle of age-related neurodegenerative diseases, whose clinical phenotypes arise from known, and as yet unknown, pathways that can act distinctly or in concert. Enhanced sub-phenotyping and the identification of in vivo biomarker-driven signature profiles could improve the stratification of patients into clinical trials and, potentially, help to drive the treatment landscape towards the precision medicine paradigm. The rapidly growing field of neuroimaging offers valuable tools to investigate disease pathophysiology and molecular pathways in humans, with the potential to capture the whole disease course starting from preclinical stages. Positron emission tomography (PET) combines the advantages of a versatile imaging technique with the ability to quantify, to nanomolar sensitivity, molecular targets in vivo. This review will discuss current research and available imaging biomarkers evaluating dysregulation of the main molecular pathways across age-related neurodegenerative diseases. The molecular pathways focused on in this review involve mitochondrial dysfunction and energy dysregulation; neuroinflammation; protein misfolding; aggregation and the concepts of pathobiology, synaptic dysfunction, neurotransmitter dysregulation and dysfunction of the glymphatic system. The use of PET imaging to dissect these molecular pathways and the potential to aid sub-phenotyping will be discussed, with a focus on novel PET biomarkers.

Autophagy and Redox Homeostasis in Parkinson's: A Crucial Balancing Act

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated primarily from endogenous biochemical reactions in mitochondria, endoplasmic reticulum (ER), and peroxisomes. Typically, ROS/RNS correlate with oxidative damage and cell death; however, free radicals are also crucial for normal cellular functions, including supporting neuronal homeostasis. ROS/RNS levels influence and are influenced by antioxidant systems, including the catabolic autophagy pathways. Autophagy is an intracellular lysosomal degradation process by which invasive, damaged, or redundant cytoplasmic components, including microorganisms and defunct organelles, are removed to maintain cellular homeostasis. This process is particularly important in neurons that are required to cope with prolonged and sustained operational stress. Consequently, autophagy is a primary line of protection against neurodegenerative diseases. Parkinson's is caused by the loss of midbrain dopaminergic neurons (mDANs), resulting in progressive disruption of the nigrostriatal pathway, leading to motor, behavioural, and cognitive impairments. Mitochondrial dysfunction, with associated increases in oxidative stress, and declining proteostasis control, are key contributors during mDAN demise in Parkinson's. In this review, we analyse the crosstalk between autophagy and redoxtasis, including the molecular mechanisms involved and the detrimental effect of an imbalance in the pathogenesis of Parkinson's.

From inflammasome to Parkinson's disease: Does the NLRP3 inflammasome facilitate exosome secretion and exosomal alpha-synuclein transmission in Parkinson's disease?

A pivotal neuropathological manifestation of synucleinopathies, like Parkinson's disease (PD), is the aggregation of α-synuclein. In a recent cell-to-cell transmission model of α-synuclein, α-synuclein propagation was demonstrated to resemble that of prion proteins in the central nervous system. Furthermore, exosomes, as biomolecule carriers, have been shown to transmit α-synuclein from neuron to neuron. However, the mechanisms underlying exosomal α-synuclein transmission have not been well understood. The NLR family pyrin domain containing 3 protein (NLRP3) inflammasome activation in microglia, and the subsequent release of proinflammatory cytokines, are two crucial pathological events involved in neuroinflammation and PD progression. Research has revealed that the NLRP3 inflammasome may facilitate the secretion of extracellular vesicles, as well as exosomal transmission of proteins like aggregated α-synuclein. However, only a few reports have evaluated these pathogenic mechanisms. Herein we evaluate for the first time the current evidence for the involvement of the NLRP3 inflammasome in microvesicle generation by microglial cells, and the various mechanisms regarding the production, shedding, and content of exosomes in relation to α-synuclein transmission from neuron to neuron. Furthermore, we propose a model of microglial NLRP3 inflammasome-dependent exosome secretion and exosomal α-synuclein transmission in PD. This knowledge may lead to the identification of novel potential targets for drug development and stimulate further research in PD.

Parkinson disease and the gut: new insights into pathogenesis and clinical relevance

The classic view portrays Parkinson disease (PD) as a motor disorder resulting from loss of substantia nigra pars compacta dopaminergic neurons. Multiple studies, however, describe prodromal, non-motor dysfunctions that affect the quality of life of patients who subsequently develop PD. These prodromal dysfunctions comprise a wide array of gastrointestinal motility disorders including dysphagia, delayed gastric emptying and chronic constipation. The histological hallmark of PD - misfolded α-synuclein aggregates that form Lewy bodies and neurites - is detected in the enteric nervous system prior to clinical diagnosis, suggesting that the gastrointestinal tract and its neural (vagal) connection to the central nervous system could have a major role in disease aetiology. This Review provides novel insights on the pathogenesis of PD, including gut-to-brain trafficking of α-synuclein as well as the newly discovered nigro-vagal pathway, and highlights how vagal connections from the gut could be the conduit by which ingested environmental pathogens enter the central nervous system and ultimately induce, or accelerate, PD progression. The pathogenic potential of various environmental neurotoxicants and the suitability and translational potential of experimental animal models of PD will be highlighted and appraised. Finally, the clinical manifestations of gastrointestinal involvement in PD and medications will be discussed briefly.