Enhanced ubiquitin-dependent degradation by Nedd4 protects against α-synuclein accumulation and toxicity in animal models of Parkinson's disease

Tailored pharmacotherapy monitoring in Parkinson's disease and Schizophrenia using a rapid and sensitive α-Synuclein assay

BACKGROUND

While Parkinson's disease is a low dopamine neurodegenerative disorder, Schizophrenia is considered a high dopamine psychiatric disorder. Pharmacological interventions that are directed to normalize dopamine concentrations in the mid-brain for an extended duration lead to unintended consequences. Parkinson's disease patients experience psychosis, and Schizophrenia patients develop extra-pyramidal symptoms due to dopamine levels overshooting their physiological range. An objective monitoring technique is therefore required for better therapeutic efficacy in these two neurological diseases.

METHODS

A rapid and sensitive assay for α-Synuclein based on magnetic enrichment and enzymatic fluorescent signal generation was developed. This assay has been benchmarked with conventional ELISA and validated in 53 CSF and 36 serum samples.

RESULTS

Developed assay from an experimental perspective has a sensitivity of less than 10 pg/mL; requires a turnaround time of 45 mins; and uses 2 µL of CSF/serum fluid samples to quantify alpha synuclein. From a utility perspective, the assay showed (a) a two-fold linearity across clinical phenotypes of Parkinson's disease, neurological controls, and schizophrenia patients; (b) variation between the naïve and treated patients; (c) correlation with severity of the disease. From a diagnostic perspective, the serum-based assay had a 100 % specificity and a minimum of 67 % sensitivity in differentiating naïve patients from treated patients; the CSF/serum-based assays had a minimum of 91 % specificity and a minimum of 85 % sensitivity in differentiating patients from neurological controls.

CONCLUSIONS

The developed assay can be used to quantify alpha-synuclein in serum and CSF samples, thereby setting a translational platform for diagnosis, prognosis, and monitoring pharmacotherapy patients with Parkinson's disease and Schizophrenia.

The Role and Mechanisms of Ubiquitin-Proteasome System-Mediated Ferroptosis in Neurological Disorders

Ferroptosis is a form of cell death elicited by an imbalance in intracellular iron concentrations, leading to enhanced lipid peroxidation. In neurological disorders, both oxidative stress and mitochondrial damage can contribute to ferroptosis, resulting in nerve cell dysfunction and death. The ubiquitin-proteasome system (UPS) refers to a cellular pathway in which specific proteins are tagged with ubiquitin for recognition and degradation by the proteasome. In neurological conditions, the UPS plays a significant role in regulating ferroptosis. In this review, we outline how the UPS regulates iron metabolism, ferroptosis, and their interplay in neurological diseases. In addition, we discuss the future application of small-molecule inhibitors and identify potential drug targets. Further investigation into the mechanisms of UPS-mediated ferroptosis will provide novel insights and strategies for therapeutic interventions and clinical applications in neurological diseases.

Listerin promotes α-synuclein degradation to alleviate Parkinson's disease through the ESCRT pathway

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive accumulation of abnormal α-synuclein (α-syn) within dopaminergic neurons in the substantia nigra region of the brain. Despite excessive accumulation of α-syn being key to the pathogenesis of PD, the mechanisms governing its clearance remain elusive. In this study, we found that the endosomal sorting complex required for transport (ESCRT) system plays a crucial role in capturing and facilitating the degradation of ubiquitinated α-syn. The E3 ubiquitin ligase Listerin was found to promote K27-linked polyubiquitination of α-syn, directing it to the endosome for subsequent degradation. We showed that the deletion of the Listerin gene exacerbates the neurodegenerative progression in a mouse model of PD, whereas the overexpression of Listerin effectively mitigates disease progression in PD mice. Consequently, our study reveals a mechanism for α-syn degradation and identifies Listerin as a promising therapeutic target for the treatment of PD.

The E3 ubiquitin ligase Nedd4 fosters developmental myelination in the mouse central and peripheral nervous system

Ubiquitination is a major post-translational regulatory mechanism that tunes numerous aspects of ubiquitinated target proteins, including localization, stability, and function. During differentiation and myelination, Oligodendrocytes (OLs) in the central nervous system and Schwann cells (SCs) in the peripheral nervous system undergo major cellular changes, including the tightly controlled production of large and accurate amounts of proteins and lipids. Such processes have been implied to depend on regulatory aspects of ubiquitination, with E3 ubiquitin ligases being generally involved in the selection of specific ubiquitination substrates by ligating ubiquitin to targets and granting target selectivity. In this study, we have used multiple transgenic mouse lines to investigate the functional impact of the E3 ubiquitin ligase Nedd4 in the OL- and SC-lineages. Our findings in the developing spinal cord indicate that Nedd4 is required for the correct accumulation of differentiated OLs and ensures proper myelination, supporting and further expanding previously suggested conceptual models. In sciatic nerves, we found that Nedd4 is required for timely radial sorting of axons by SCs as a pre-requirement for correct onset of myelination. Moreover, Nedd4 ensures correct myelin thickness in both SCs and spinal cord OLs.

OTUD5 Protects Dopaminergic Neurons by Promoting the Degradation of α-Synuclein in Parkinson's Disease Model

Defective clearance and accumulation of α-synuclein (α-Syn) is the key pathogenic factor in Parkinson's disease (PD). Recent studies emphasize the importance of E3 ligases in regulating the degradation of α-Syn. However, the molecular mechanisms by which deubiquitinases regulate α-Syn degradation are scarcely studied. In this study, it is found that the protein levels of α-Syn are negatively regulated by ovarian tumor protease deubiquitinase 5 (OTUD5) which protects dopaminergic (DA) neurons in the PD model. Mechanistically, OTUD5 promotes K63-linked polyubiquitination of α-Syn independent of its deubiquitinating enzyme activity and mediates its endolysosomal degradation by recruiting the E3 ligase neural precursor cell expressed developmentally downregulated 4 (NEDD4). Furthermore, OTUD5 conditional knockout in DA neurons results in more severe α-Syn related pathology and dyskinesia after injection of α-Syn preformed fibrils (PFF). Overall, the data unveil a novel mechanism to regulate the degradation of α-Syn and provide a new therapeutic strategy to alleviate DA neurodegeneration.

Differentially Expressed Nedd4-binding Protein Ndfip1 Protects Neurons Against Methamphetamine-induced Neurotoxicity

To identify factors involved in methamphetamine (METH) neurotoxicity, we comprehensively searched for genes which were differentially expressed in mouse striatum after METH administration using differential display (DD) reverse transcription-PCR method and sequent single-strand conformation polymorphism analysis, and found two DD cDNA fragments later identified as mRNA of Nedd4 (neural precursor cell expressed developmentally downregulated 4) WW domain-binding protein 5 (N4WBP5), later named Nedd4 family-interacting protein 1 (Ndfip1). It is an adaptor protein for the binding between Nedd4 of ubiquitin ligase (E3) and target substrate protein for ubiquitination. Northern blot analysis confirmed drastic increases in Ndfip1 mRNA in the striatum after METH injections, and in situ hybridization histochemistry showed that the mRNA expression was increased in the hippocampus and cerebellum at 2 h-2 days, in the cerebral cortex and striatum at 18 h-2 days after single METH administration. The knockdown of Ndfip1 expression with Ndfip1 siRNA significantly aggravated METH-induced neurotoxicity in the cultured monoaminergic neuronal cells. These results suggest that drastic increases in Ndfip1 mRNA is compensatory reaction to protect neurons against METH-induced neurotoxicity.

α-Synuclein ubiquitination - functions in proteostasis and development of Lewy bodies

Synucleinopathies are neurodegenerative disorders characterized by the accumulation of α-synuclein containing Lewy bodies. Ubiquitination, a key post-translational modification, has been recognized as a pivotal regulator of α-synuclein's cellular dynamics, influencing its degradation, aggregation, and associated neurotoxicity. This review examines comprehensively the current understanding of α-synuclein ubiquitination and its role in the pathogenesis of synucleinopathies, particularly in the context of Parkinson's disease. We explore the molecular mechanisms responsible for α-synuclein ubiquitination, with a focus on the roles of E3 ligases and deubiquitinases implicated in the degradation process which occurs primarily through the endosomal lysosomal pathway. The review further discusses how the dysregulation of these mechanisms contributes to α-synuclein aggregation and LB formation and offers suggestions for future investigations into the role of α-synuclein ubiquitination. Understanding these processes may shed light on potential therapeutic avenues that can modulate α-synuclein ubiquitination to alleviate its pathological impact in synucleinopathies.

Posttranslational Modifications of -Synuclein, Their Therapeutic Potential, and Crosstalk in Health and Neurodegenerative Diseases

α-Synuclein (α-Syn) aggregation in Lewy bodies and Lewy neurites has emerged as a key pathogenetic feature in Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Various factors, including posttranslational modifications (PTMs), can influence the propensity of α-Syn to misfold and aggregate. PTMs are biochemical modifications of a protein that occur during or after translation and are typically mediated by enzymes. PTMs modulate several characteristics of proteins including their structure, activity, localization, and stability. α-Syn undergoes various posttranslational modifications, including phosphorylation, ubiquitination, SUMOylation, acetylation, glycation, O-GlcNAcylation, nitration, oxidation, polyamination, arginylation, and truncation. Different PTMs of a protein can physically interact with one another or work together to influence a particular physiological or pathological feature in a process known as PTMs crosstalk. The development of detection techniques for the cooccurrence of PTMs in recent years has uncovered previously unappreciated mechanisms of their crosstalk. This has led to the emergence of evidence supporting an association between α-Syn PTMs crosstalk and synucleinopathies. In this review, we provide a comprehensive evaluation of α-Syn PTMs, their impact on misfolding and pathogenicity, the pharmacological means of targeting them, and their potential as biomarkers of disease. We also highlight the importance of the crosstalk between these PTMs in α-Syn function and aggregation. Insight into these PTMS and the complexities of their crosstalk can improve our understanding of the pathogenesis of synucleinopathies and identify novel targets of therapeutic potential. SIGNIFICANCE STATEMENT: α-Synuclein is a key pathogenic protein in Parkinson's disease and other synucleinopathies, making it a leading therapeutic target for disease modification. Multiple posttranslational modifications occur at various sites in α-Synuclein and alter its biophysical and pathological properties, some interacting with one another to add to the complexity of the pathogenicity of this protein. This review details these modifications, their implications in disease, and potential therapeutic opportunities.

Therapeutic Implications and Regulations of Protein Post-translational Modifications in Parkinsons Disease

Parkinsons disease (PD) is a neurodegenerative disorder characterized by dopaminergic neuron loss and alpha-synuclein aggregation. This comprehensive review examines the intricate role of post-translational modifications (PTMs) in PD pathogenesis, focusing on DNA methylation, histone modifications, phosphorylation, SUMOylation, and ubiquitination. Targeted PTM modulation, particularly in key proteins like Parkin, DJ1, and PINK1, emerges as a promising therapeutic strategy for mitigating dopaminergic degeneration in PD. Dysregulated PTMs significantly contribute to the accumulation of toxic protein aggregates and dopaminergic neuronal dysfunction observed in PD. Targeting PTMs, including epigenetic strategies, addressing aberrant phosphorylation events, and modulating SUMOylation processes, provides potential avenues for intervention. The ubiquitin-proteasome system, governed by enzymes like Parkin and Nedd4, offers potential targets for clearing misfolded proteins and developing disease-modifying interventions. Compounds like ginkgolic acid, SUMO E1 enzyme inhibitors, and natural compounds like Indole-3-carbinol illustrate the feasibility of modulating PTMs for therapeutic purposes in PD. This review underscores the therapeutic potential of PTM-targeted interventions in modulating PD-related pathways, emphasizing the need for further research in this promising area of Parkinsons disease therapeutics.

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.

Are Therapies That Target α-Synuclein Effective at Halting Parkinson's Disease Progression? A Systematic Review

There are currently no pharmacological treatments available that completely halt or reverse the progression of Parkinson's Disease (PD). Hence, there is an unmet need for neuroprotective therapies. Lewy bodies are a neuropathological hallmark of PD and contain aggregated α-synuclein (α-syn) which is thought to be neurotoxic and therefore a suitable target for therapeutic interventions. To investigate this further, a systematic review was undertaken to evaluate whether anti-α-syn therapies are effective at preventing PD progression in preclinical in vivo models of PD and via current human clinical trials. An electronic literature search was performed using MEDLINE and EMBASE (Ovid), PubMed, the Web of Science Core Collection, and Cochrane databases to collate clinical evidence that investigated the targeting of α-syn. Novel preclinical anti-α-syn therapeutics provided a significant reduction of α-syn aggregations. Biochemical and immunohistochemical analysis of rodent brain tissue demonstrated that treatments reduced α-syn-associated pathology and rescued dopaminergic neuronal loss. Some of the clinical studies did not provide endpoints since they had not yet been completed or were terminated before completion. Completed clinical trials displayed significant tolerability and efficacy at reducing α-syn in patients with PD with minimal adverse effects. Collectively, this review highlights the capacity of anti-α-syn therapies to reduce the accumulation of α-syn in both preclinical and clinical trials. Hence, there is potential and optimism to target α-syn with further clinical trials to restrict dopaminergic neuronal loss and PD progression and/or provide prophylactic protection to avoid the onset of α-syn-induced PD.

Genetic modifiers of synucleinopathies-lessons from experimental models

α-Synuclein is a pleiotropic protein underlying a group of progressive neurodegenerative diseases, including Parkinson's disease and dementia with Lewy bodies. Together, these are known as synucleinopathies. Like all neurological diseases, understanding of disease mechanisms is hampered by the lack of access to biopsy tissues, precluding a real-time view of disease progression in the human body. This has driven researchers to devise various experimental models ranging from yeast to flies to human brain organoids, aiming to recapitulate aspects of synucleinopathies. Studies of these models have uncovered numerous genetic modifiers of α-synuclein, most of which are evolutionarily conserved. This review discusses what we have learned about disease mechanisms from these modifiers, and ways in which the study of modifiers have supported ongoing efforts to engineer disease-modifying interventions for synucleinopathies.

Extracellular vesicle-based therapeutics: Extracellular vesicles as therapeutic targets and agents

Extracellular vesicles (EVs) are cell-derived membrane vesicles composed of a lipid bilayer. EVs contain biological molecules, such as nucleic acids, lipids, and proteins. As these molecules are transferred to cells that receive EVs, EVs function as intercellular communication tools. EV-mediated intercellular communication is involved in various diseases, such as cancer and neurodegenerative diseases, and biological events, such as immune reactions and inflammation. Therefore, EVs are suggested to be useful as therapeutic targets for various diseases. However, an EV-based drug delivery system (DDS) that utilizes its therapeutic properties has not yet been reported. The biological activities of EVs are derived from their endogenous components; hence, they can be directly applied as drugs. In this review, the basic aspects of EVs, such as their types, methods of isolation, and in vivo behavior, are briefly summarized. Moreover, the potential of using therapeutics targeting EVs has been discussed in cancer and neurodegenerative diseases. Various therapeutics using EVs, including DDSs, are listed and their associated advantages and challenges are discussed.

α-synuclein build-up is alleviated via ESCRT-dependent endosomal degradation brought about by p38MAPK inhibition in cells expressing p25α

α-synucleinopathy is driven by an imbalance of synthesis and degradation of α-synuclein (αSyn), causing a build up of αSyn aggregates and post-translationally modified species, which not only interfere with normal cellular metabolism but also by their secretion propagates the disease. Therefore, a better understanding of αSyn degradation pathways is needed to address α-synucleinopathy. Here, we used the nerve growth factor–differentiated catecholaminergic PC12 neuronal cell line, which was conferred α-synucleinopathy by inducible expression of αSyn and tubulin polymerization-promoting protein p25α. p25α aggregates αSyn, and imposes a partial autophagosome–lysosome block to mimic aspects of lysosomal deficiency common in neurodegenerative disease. Under basal conditions, αSyn was degraded by multiple pathways but most prominently by macroautophagy and Nedd4/Ndfip1-mediated degradation. We found that expression of p25α induced strong p38MAPK activity. Remarkably, when opposed by inhibitor SB203580 or p38MAPK shRNA knockdown, endolysosomal localization and degradation of αSyn increased, and αSyn secretion and cytotoxicity decreased. This effect was specifically dependent on Hsc70 and the endosomal sorting complex required for transport machinery, but different from classical microautophagy, as the αSyn Hsc70 binding motif was unnecessary. Furthermore, in a primary neuronal (h)-αSyn seeding model, p38MAPK inhibition decreased pathological accumulation of phosphorylated serine-129-αSyn and cytotoxicity. In conclusion, p38MAPK inhibition shifts αSyn degradation from various forms of autophagy to an endosomal sorting complex required for transport–dependent uptake mechanism, resulting in increased αSyn turnover and cell viability in p25α-expressing cells. More generally, our results suggest that under conditions of autophagolysosomal malfunction, the uninterrupted endosomal pathway offers a possibility to achieve disease-associated protein degradation.

Role of post-translational modifications on the alpha-synuclein aggregation-related pathogenesis of Parkinson’s disease

Together with neuronal loss, the existence of insoluble inclusions of alpha-synuclein (α-syn) in the brain is widely accepted as a hallmark of synucleinopathies including Parkinson’s disease (PD), multiple system atrophy, and dementia with Lewy body. Because the α-syn aggregates are deeply involved in the pathogenesis, there have been many attempts to demonstrate the mechanism of the aggregation and its potential causative factors including post-translational modifications (PTMs). Although no concrete conclusions have been made based on the previous study results, growing evidence suggests that modifications such as phosphorylation and ubiquitination can alter α-syn characteristics to have certain effects on the aggregation process in PD; either facilitating or inhibiting fibrillization. In the present work, we reviewed studies showing the significant impacts of PTMs on α-syn aggregation. Furthermore, the PTMs modulating α-syn aggregation-induced cell death have been discussed. 

Inhibition of Ubiquitin-Specific Protease-13 Improves Behavioral Performance in Alpha-Synuclein Expressing Mice

Ubiquitin-Specific Protease-13 (USP13) promotes protein de-ubiquitination. USP13 levels are upregulated in post-mortem Parkinson's disease, whereas USP13 knockdown via shRNA reduces alpha-synuclein levels in animal models. We studied the role of USP13 in knockout mice expressing lentiviral human alpha-synuclein and investigated the impact of a small molecule inhibitor of USP13, BK50118-C, on alpha-synuclein pathology and animal behavior. Alpha-synuclein was expressed unilaterally in substantia nigra (SN) of USP13 deficient mice that were treated with a daily intraperitoneal injection of 100 mg/kg BK50118-C or DMSO for four consecutive weeks, and behavioral and functional assays were performed. Wild-type USP13^+/+ mice expressing lentiviral human alpha-synuclein showed motor and behavioral defects that were not seen in partially (USP13^+/-) or completely (USP13^-/-) deficient USP13 mice. BK50118-C displayed a wide and favorable therapeutic dose range in vivo. Treatment with BK50118-C significantly reduced ubiquitinated alpha-synuclein, increased dopamine levels, and improved motor and behavioral symptoms in wild-type (USP13^+/+), but not USP13 deficient, mice. These data suggest that USP13 is critical to the neuropathology of alpha-synuclein, whereas a novel small molecule inhibitor of USP13 is a potential therapeutic agent of alpha-synucleinopathies.

Role of post-translational modifications on the alpha-synuclein aggregation-related pathogenesis of Parkinson's disease

Together with neuronal loss, the existence of insoluble inclusions of alpha-synuclein (α-syn) in the brain is widely accepted as a hallmark of synucleinopathies including Parkinson's disease (PD), multiple system atrophy, and dementia with Lewy body. Because the α-syn aggregates are deeply involved in the pathogenesis, there have been many attempts to demonstrate the mechanism of the aggregation and its potential causative factors including post-translational modifications (PTMs). Although no concrete conclusions have been made based on the previous study results, growing evidence suggests that modifications such as phosphorylation and ubiquitination can alter α-syn characteristics to have certain effects on the aggregation process in PD; either facilitating or inhibiting fibrillization. In the present work, we reviewed studies showing the significant impacts of PTMs on α-syn aggregation. Furthermore, the PTMs modulating α-syn aggregation-induced cell death have been discussed. [BMB Reports 2022; 55(7): 323-335].

Bone Marrow Stem Cell-Exo-Derived TSG-6 Attenuates 1-Methyl-4-Phenylpyridinium+-Induced Neurotoxicity via the STAT3/miR-7/NEDD4/LRRK2 Axis

Bone marrow mesenchymal stem cell-derived exosome (BMSCs-Exo)-derived TNF-stimulated gene-6 (TSG-6) has anti-inflammatory and antioxidative stress-related properties that may be beneficial in the treatment of Parkinson disease (PD) patients. To elucidate the mechanisms involved, we analyzed the effects of BMSCs-Exo-derived TSG-6 on in vitro models of PD induced with 1-methyl-4-phenylpyridinium (MPP+). TSG-6 was abundant in BMSCs-Exo and it attenuated MPP+-induced neurotoxicity. Moreover, BMSCs-Exo reversed the MPP+-induced toxicity accelerated by neural precursor cells expressed developmentally downregulated 4 (NEDD4) knockdown or miR-7 mimics. Further analysis indicated that NEDD4 combined with leucine-rich repeat kinase 2 (LRRK2) to accelerate ubiquitin degradation of LRRK2. Signal transducer and activator of transcription 3 (STAT3) bound to the miR-7 promoter and miR-7 targeted NEDD4. These data indicate that BMSCs-Exo-derived TSG-6 attenuated neurotoxicity via the STAT3-miR-7-NEDD4 axis. Our results define the specific mechanisms for BMSCs-Exo-derived TSG-6 regulation of MPP+-induced neurotoxicity that are relevant to understanding PD pathogenesis and developing therapies for PD patients.

p21-activated kinase 4 controls the aggregation of α-synuclein by reducing the monomeric and aggregated forms of α-synuclein: involvement of the E3 ubiquitin ligase NEDD4-1

Aggregation of misfolded alpha-synuclein (α-synuclein) is a central player in the pathogenesis of neurodegenerative diseases. Therefore, the regulatory mechanism underlying α-synuclein aggregation has been intensively studied in Parkinson's disease (PD) but remains poorly understood. Here, we report p21-activated kinase 4 (PAK4) as a key regulator of α-synuclein aggregation. Immunohistochemical analysis of human PD brain tissues revealed an inverse correlation between PAK4 activity and α-synuclein aggregation. To investigate their causal relationship, we performed loss-of-function and gain-of-function studies using conditional PAK4 depletion in nigral dopaminergic neurons and the introduction of lentivirus expressing a constitutively active form of PAK4 (caPAK4; PAK4^S445N/S474E), respectively. For therapeutic relevance in the latter setup, we injected lentivirus into the striatum following the development of motor impairment and analyzed the effects 6 weeks later. In the loss-of-function study, Cre-driven PAK4 depletion in dopaminergic neurons enhanced α-synuclein aggregation, intracytoplasmic Lewy body-like inclusions and Lewy-like neurites, and reduced dopamine levels in PAK4^DAT-CreER mice compared to controls. Conversely, caPAK4 reduced α-synuclein aggregation, as assessed by a marked decrease in both proteinase K-resistant and Triton X100-insoluble forms of α-synuclein in the AAV-α-synuclein-induced PD model. Mechanistically, PAK4 specifically interacted with the NEDD4-1 E3 ligase, whose pharmacological inhibition and knockdown suppressed the PAK4-mediated downregulation of α-synuclein. Collectively, these results provide new insights into the pathogenesis of PD and suggest PAK4-based gene therapy as a potential disease-modifying therapy in PD.

Novel molecular targets and mechanisms for neuroprotective modulation in neurodegenerative disorders

BACKGROUND

Neuronal death underlies the symptoms of several human neurological disorders, including Alzheimer's, Parkinson's and Huntington's diseases, and amyotrophic lateral sclerosis that their precise pathophysiology have not yet been elucidated. According to various studies the prohibition is the best therapy with neuroprotective approaches which are advanced and safe methods.

METHODS

This review summarizes some of the already-known and newly emerged neuroprotective targets and strategies that their experimental effects have been reported. Accordingly, literature was studied from 2000 to 2021 and appropriate articles were searched in Google Scholar and Scopus with the keywords given in the Keywords section of the current review.

RESULTS

Lewy bodies are the histopathologic characteristics of neurodegenerative disorders and are protein-rich intracellular deposits in which Alpha-Synuclein is its major protein. Alpha-Synuclein's toxic potential provides a compelling rationale for therapeutic strategies aimed at decreasing its burden in neuronal cells through numerous pathways including ubiquitin-proteasome system and autophagy-lysosome Pathway, proteolytic breakdown via cathepsin D, kallikrein-6 (neurosin), calpain-1 or MMP9, heat shock proteins, and proteolysis targeting chimera which consists of a target protein ligand and an E3 ubiquitin ligase (E3) followed by target protein ubiquitination (PROTACs). Other targets that have been noticed recently are the mutant huntingtin, tau proteins and glycogen synthase kinase 3β that their accumulation proceeds extensive neuronal damage and up to the minute approach such as Proteolysis Targeting Chimera promotes its degradation in cells. As various studies demonstrated that Mendelian gene mutations can result into the neurodegenerative diseases, additional target that has gained much interest is epigenetics such as mutation, phosphodiesterase, RNA binding proteins and Nuclear respiratory factor 1.

CONCLUSION

The novel molecular targets and new strategies compiled and introduced here can be used by scientists to design and discover more efficient small molecule drugs against the neurodegenerative diseases. And also the genes in which their mutations can lead to the α-synuclein aggregation or accumulation are discussed and considered a valuable information of epigenetics in dementia.

Ubiquitin and Ubiquitin-like Proteins in Cancer, Neurodegenerative Disorders, and Heart Diseases

Post-translational modification (PTM) is an essential mechanism for enhancing the functional diversity of proteins and adjusting their signaling networks. The reversible conjugation of ubiquitin (Ub) and ubiquitin-like proteins (Ubls) to cellular proteins is among the most prevalent PTM, which modulates various cellular and physiological processes by altering the activity, stability, localization, trafficking, or interaction networks of its target molecules. The Ub/Ubl modification is tightly regulated as a multi-step enzymatic process by enzymes specific to this family. There is growing evidence that the dysregulation of Ub/Ubl modifications is associated with various diseases, providing new targets for drug development. In this review, we summarize the recent progress in understanding the roles and therapeutic targets of the Ub and Ubl systems in the onset and progression of human diseases, including cancer, neurodegenerative disorders, and heart diseases.

The Roles of NEDD4 Subfamily of HECT E3 Ubiquitin Ligases in Neurodevelopment and Neurodegeneration

The ubiquitin pathway regulates the function of many proteins and controls cellular protein homeostasis. In recent years, it has attracted great interest in neurodevelopmental and neurodegenerative diseases. Here, we have presented the first review on the roles of the 9 proteins of the HECT E3 ligase NEDD4 subfamily in the development and function of neurons in the central nervous system (CNS). We discussed their regulation and their direct or indirect involvement in neurodevelopmental diseases, such as intellectual disability, and neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease or Amyotrophic Lateral Sclerosis. Further studies on the roles of these proteins, their regulation and their targets in neurons will certainly contribute to a better understanding of neuronal function and dysfunction, and will also provide interesting information for the development of therapeutics targeting them.

The Role of NEDD4 E3 Ubiquitin–Protein Ligases in Parkinson’s Disease

Parkinson’s disease (PD) is a debilitating neurodegenerative disease that causes a great clinical burden. However, its exact molecular pathologies are not fully understood. Whilst there are a number of avenues for research into slowing, halting, or reversing PD, one central idea is to enhance the clearance of the proposed aetiological protein, oligomeric α-synuclein. Oligomeric α-synuclein is the main constituent protein in Lewy bodies and neurites and is considered neurotoxic. Multiple E3 ubiquitin-protein ligases, including the NEDD4 (neural precursor cell expressed developmentally downregulated protein 4) family, parkin, SIAH (mammalian homologues of Drosophila seven in absentia), CHIP (carboxy-terminus of Hsc70 interacting protein), and SCFFXBL5 SCF ubiquitin ligase assembled by the S-phase kinase-associated protein (SKP1), cullin-1 (Cul1), a zinc-binding RING finger protein, and the F-box domain/Leucine-rich repeat protein 5-containing protein FBXL5), have been shown to be able to ubiquitinate α-synuclein, influencing its subsequent degradation via the proteasome or lysosome. Here, we explore the link between NEDD4 ligases and PD, which is not only via α-synuclein but further strengthened by several additional substrates and interaction partners. Some members of the NEDD4 family of ligases are thought to crosstalk even with PD-related genes and proteins found to be mutated in familial forms of PD. Mutations in NEDD4 family genes have not been observed in PD patients, most likely because of their essential survival function during development. Following further in vivo studies, it has been thought that NEDD4 ligases may be viable therapeutic targets in PD. NEDD4 family members could clear toxic proteins, enhancing cell survival and slowing disease progression, or might diminish beneficial proteins, reducing cell survival and accelerating disease progression. Here, we review studies to date on the expression and function of NEDD4 ubiquitin ligases in the brain and their possible impact on PD pathology.

Small but Mighty-Exosomes, Novel Intercellular Messengers in Neurodegeneration

Simple Summary

Exosomes are biological nanoparticles recently recognized as intercellular messengers. They contain a cargo of lipids, proteins, and RNA. They can transfer their content to not only cells in the vicinity but also to cells at a distance. This unique ability empowers them to modulate the physiology of recipient cells. In brain, exosomes play a role in neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease and amyotrophic lateral sclerosis.

Abstract

Exosomes of endosomal origin are one class of extracellular vesicles that are important in intercellular communication. Exosomes are released by all cells in our body and their cargo consisting of lipids, proteins and nucleic acids has a footprint reflective of their parental origin. The exosomal cargo has the power to modulate the physiology of recipient cells in the vicinity of the releasing cells or cells at a distance. Harnessing the potential of exosomes relies upon the purity of exosome preparation. Hence, many methods for isolation have been developed and we provide a succinct summary of several methods. In spite of the seclusion imposed by the blood–brain barrier, cells in the CNS are not immune from exosomal intrusive influences. Both neurons and glia release exosomes, often in an activity-dependent manner. A brief description of exosomes released by different cells in the brain and their role in maintaining CNS homeostasis is provided. The hallmark of several neurodegenerative diseases is the accumulation of protein aggregates. Recent studies implicate exosomes’ intercellular communicator role in the spread of misfolded proteins aiding the propagation of pathology. In this review, we discuss the potential contributions made by exosomes in progression of Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Understanding contributions made by exosomes in pathogenesis of neurodegeneration opens the field for employing exosomes as therapeutic agents for drug delivery to brain since exosomes do cross the blood–brain barrier.

Ubiquitination-Proteasome System (UPS) and Autophagy Two Main Protein Degradation Machineries in Response to Cell Stress

In response to environmental stimuli, cells make a series of adaptive changes to combat the injury, repair the damage, and increase the tolerance to the stress. However, once the damage is too serious to repair, the cells will undergo apoptosis to protect the overall cells through suicidal behavior. Upon external stimulation, some intracellular proteins turn into unfolded or misfolded protein, exposing their hydrophobic regions to form protein aggregation, which may ultimately produce serious damage to the cells. Ubiquitin plays an important role in the degradation of these unnatural proteins by tagging with ubiquitin chains in the ubiquitin-proteasome or autophagy system. If the two processes fail to eliminate the abnormal protein aggregates, the cells will move to apoptosis and death. Dysregulation of ubiquitin-proteasome system (UPS) and autophagy may result in the development of numerous diseases. This review focuses on the molecular mechanisms of UPS and autophagy in clearance of intracellular protein aggregates, and the relationship between dysregulation of ubiquitin network and diseases.

Lentiviral Vectors Expressing Chimeric NEDD4 Ubiquitin Ligases: An Innovative Approach for Interfering with Alpha-Synuclein Accumulation

One of the main pathological features of Parkinson’s disease (PD) is a diffuse accumulation of alpha-synuclein (aS) aggregates in neurons. The NEDD4 E3 Ub ligase promotes aS degradation by the endosomal–lysosomal route. Interestingly, NEDD4, as well as being a small molecule able to trigger its functions, is protective against human aS toxicity in evolutionary distant models. While pharmacological activation of E3 enzymes is not easy to achieve, their flexibility and the lack of “consensus” motifs for Ub-conjugation allow the development of engineered Ub-ligases, able to target proteins of interest. We developed lentiviral vectors, encoding well-characterized anti-human aS scFvs fused in frame to the NEDD4 catalytic domain (ubiquibodies), in order to target ubiquitinate aS. We demonstrate that, while all generated ubiquibodies bind to and ubiquitinate aS, the one directed against the non-amyloid component (NAC) of aS (Nac32HECT) affects aS’s intracellular levels. Furthermore, Nac32HECT expression partially rescues aS’s overexpression or mutation toxicity in neural stem cells. Overall, our data suggest that ubiquibodies, and Nac32HECT in particular, represent a valid platform for interfering with the effects of aS’s accumulation and aggregation in neurons.

Small Molecule Improvement of Trafficking Defects in Models of Neurodegeneration

Disrupted cellular trafficking and transport processes are hallmarks of many neurodegenerative disorders (NDs). Recently, efforts have been made toward developing and implementing experimental platforms to identify small molecules that may help restore normative trafficking functions. There have been a number of successes in targeting endomembrane trafficking with the identification of compounds that restore cell viability through rescue of protein transport and trafficking. Here, we describe some of the experimental platforms implemented for small molecule screening efforts for rescue of trafficking defects in neurodegeneration. A survey of phenotypically active small molecules identified to date is provided, including a summary of medicinal chemistry efforts and insights into putative targets and mechanisms of action. In particular, emphasis is put on ligands that demonstrate activity in more than one model of neurodegeneration as retention of phenotypic activity across ND models suggests conservation of biological targets across NDs.

Predicting PY motif-mediated protein-protein interactions in the Nedd4 family of ubiquitin ligases

The Nedd4 family contains several structurally related but functionally distinct HECT-type ubiquitin ligases. The members of the Nedd4 family are known to recognize substrates through their multiple WW domains, which recognize PY motifs (PPxY, LPxY) or phospho-threonine or phospho-serine residues. To better understand protein interactor recognition mechanisms across the Nedd4 family, we report the development and implementation of a python-based tool, PxYFinder, to identify PY motifs in the primary sequences of previously identified interactors of Nedd4 and related ligases. Using PxYFinder, we find that, on average, half of Nedd4 family interactions are likely PY-motif mediated. Further, we find that PPxY motifs are more prevalent than LPxY motifs and are more likely to occur in proline-rich regions and that PPxY regions are more disordered on average relative to LPxY-containing regions. Informed by consensus sequences for PY motifs across the Nedd4 interactome, we rationally designed a focused peptide library and employed a computational screen, revealing sequence- and biomolecular interaction-dependent determinants of WW-domain/PY-motif interactions. Cumulatively, our efforts provide a new bioinformatic tool and expand our understanding of sequence and structural factors that contribute to PY-motif mediated interactor recognition across the Nedd4 family.

A novel yeast-based screening system for potential compounds that can alleviate human α-synuclein toxicity

Aims

This study aimed to establish a yeast‐based screening system for potential compounds that can alleviate the toxicity of α‐synuclein (α‐syn), a neuropathological hallmark of Parkinson’s disease, either inhibition of α‐syn aggregation or promotion of ubiquitin‐mediated degradation of α‐syn.

Methods and Results

A powerful yeast‐based screening assay using the rsp5^A401E‐mutant strain, which is hypersensitive to α‐syn aggregation, was established by two‐step gene replacement and further overexpressed the GFP‐fused α‐syn in the drug‐sensitive yeast strain with a galactose‐inducible multicopy plasmid. The rsp5^A401E‐mutant strain treated with baicalein, a known α‐syn aggregation inhibitor, showed better α‐syn toxicity alleviation than the same background wild type strain as accessed by comparison on the reduction kinetics of viable dye resazurin fluorometrically (λ_ex540/λ_em590 nm). The rsp5^A401E‐mutant yeast‐based assay system showed high sensitivity as it could detect as low as 3.13 µmol l⁻¹ baicalein, the concentration that lower than previously report detected by the in vitro assay.

Conclusions

Our yeast‐based system has been effective for screening potential compounds that can alleviate α‐syn toxicity with high sensitivity and specificity.

Significance and Impact of the Study

Yeast‐based assay system can be used to discover novel neuroprotective drug candidates which may be either efficiently suppress‐α‐syn aggregation or enhance ubiquitin‐dependent degradation.

Oligomerization of Selective Autophagy Receptors for the Targeting and Degradation of Protein Aggregates

The selective targeting and disposal of solid protein aggregates are essential for cells to maintain protein homoeostasis. Autophagy receptors including p62, NBR1, Cue5/TOLLIP (CUET), and Tax1-binding protein 1 (TAX1BP1) proteins function in selective autophagy by targeting ubiquitinated aggregates through ubiquitin-binding domains. Here, we summarize previous beliefs and recent findings on selective receptors in aggregate autophagy. Since there are many reviews on selective autophagy receptors, we focus on their oligomerization, which enables receptors to function as pathway determinants and promotes phase separation.

Understanding amphisomes

Amphisomes are intermediate/hybrid organelles produced through the fusion of endosomes with autophagosomes within cells. Amphisome formation is an essential step during a sequential maturation process of autophagosomes before their ultimate fusion with lysosomes for cargo degradation. This process is highly regulated with multiple protein machineries, such as SNAREs, Rab GTPases, tethering complexes, and ESCRTs, are involved to facilitate autophagic flux to proceed. In neurons, autophagosomes are robustly generated in axonal terminals and then rapidly fuse with late endosomes to form amphisomes. This fusion event allows newly generated autophagosomes to gain retrograde transport motility and move toward the soma, where proteolytically active lysosomes are predominantly located. Amphisomes are not only the products of autophagosome maturation but also the intersection of the autophagy and endo-lysosomal pathways. Importantly, amphisomes can also participate in non-canonical functions, such as retrograde neurotrophic signaling or autophagy-based unconventional secretion by fusion with the plasma membrane. In this review, we provide an updated overview of the recent discoveries and advancements on the molecular and cellular mechanisms underlying amphisome biogenesis and the emerging roles of amphisomes. We discuss recent developments towards the understanding of amphisome regulation as well as the implications in the context of major neurodegenerative diseases, with a comparative focus on Alzheimer's disease and Parkinson's disease.

Deletion of ubiquitin ligase Nedd4l exacerbates ischemic brain damage

Ubiquitination by Nedd4 (neuronally expressed developmentally downregulated 4) family of HECT type E3 ligases plays a key role in degrading misfolded and damaged proteins, and its disruption leads to neurodegeneration. Parkinson's disease-causing protein α-Synuclein (α-Syn) is ubiquitinated by the Nedd4 family and degraded by endosomes. Nedd4l is the only Nedd4 homolog that showed upregulation in post-stroke surviving cortical neurons where it correlated with neuroprotection. We tested the role of Nedd4l after stroke by subjecting the Nedd4l^-/- mice to transient middle cerebral artery occlusion. Focal ischemia significantly increased Nedd4l expression and poly-ubiquitinated α-Syn levels, and knockout of Nedd4l reduced post-ischemic poly-ubiquitinated α-Syn that is majorly located in the peri-infarct neurons. Co-immunoprecipitation further shows that focal ischemia enhances the α-Syn-Nedd4l interaction resulting in increased ubiquitination of α-Syn. Nedd4l knockout mice (n = 7 mice/group) showed exacerbated post-ischemic motor dysfunction manifested by decreased time on the rotarod and increased number of foot faults, and significantly increased ischemic brain damage. This suggests that Nedd4l might be a potential therapeutic target to minimize α-Syn-mediated toxicity after cerebral ischemia.

Evolution of mammalian longevity: age-related increase in autophagy in bats compared to other mammals

Autophagy maintains cellular homeostasis and its dysfunction has been implicated in aging. Bats are the longest-lived mammals for their size, but the molecular mechanisms underlying their extended healthspan are not well understood. Here, drawing on >8 years of mark-recapture field studies, we report the first longitudinal analysis of autophagy regulation in bats. Mining of published population level aging blood transcriptomes (M. myotis, mouse and human) highlighted a unique increase of autophagy related transcripts with age in bats, but not in other mammals. This bat-specific increase in autophagy transcripts was recapitulated by the western blot determination of the autophagy marker, LC3II/I ratio, in skin primary fibroblasts (Myotis myotis,Pipistrellus kuhlii, mouse), that also showed an increase with age in both bat species. Further phylogenomic selection pressure analyses across eutherian mammals (n=70 taxa; 274 genes) uncovered 10 autophagy-associated genes under selective pressure in bat lineages. These molecular adaptations potentially mediate the exceptional age-related increase of autophagy signalling in bats, which may contribute to their longer healthspans.

Regulatory Role of Ubiquitin Specific Protease-13 (USP13) in Misfolded Protein Clearance in Neurodegenerative Diseases

Ubiquitin Specific Protease (USP)-13 is a de-ubiquitinase member of the cysteine-dependent protease superfamily that cleaves ubiquitin off protein substrates to reverse ubiquitin-mediated protein degradation. Several findings implicate USPs in neurodegeneration. Ubiquitin targets proteins to major degradation pathways, including the proteasome and the lysosome. In melanoma cells, USP13 regulates the degradation of several proteins primarily via ubiquitination and de-ubiquitination. However, the significance of USP13 in regulating protein clearance in neurodegeneration is largely unknown. This mini-review summarizes the most recent evidence pertaining to the role of USP13 in protein clearance via autophagy and the proteasome in neurodegenerative diseases.

Evidence for an Interaction Between NEDD4 and Childhood Trauma on Clinical Characters of Schizophrenia With Family History of Psychosis

Background: Neural precursor cell-expressed developmentally downregulated 4 (NEDD4) polymorphisms and childhood trauma (CT) are associated with schizophrenia. However, whether NEDD4 interacts with CT on symptoms of schizophrenia remains unknown. This study aimed to investigate the gene-environment interaction effect. Methods: We recruited 289 schizophrenia patients and 487 controls and genotyped rs2303579, rs3088077, rs7162435, rs11550869, and rs62043855 in their NEDD4 gene. Results: We found significant differences in the rs2303579 and rs3088077 between the two groups. Patients with the rs2303579 CC genotype had higher scores compared with other genotype (P = 0.026) in the test of positive schizophrenia syndrome scores, whereas patients with the rs3088077 TT (P = 0.037) and rs7162435 CC genotypes (P = 0.009) had higher scores compared with the other genotypes in the test of excitement factor. Patients with a family history of psychosis (FH+) reported higher negative scores (P = 0.012) than those without. Patients exposed to physical abuse (PA) reported a lower language learning and memory score (P = 0.017) and working memory score (P = 0.047) than those not. Patients exposed to sexual abuse (SA) reported a lower reasoning and problem-solving skills score (P = 0.025); those exposed to emotional neglect (EN) reported a lower social cognition score (P = 0.044); and those exposed to physical neglect reported a lower social cognition score (P = 0.036) but higher visual learning and memory score (P = 0.032). Rs3088077 could interact with EN to increase risk for schizophrenia. Optimal model rs62043855 × EA, rs3088077 × rs7162435 × rs11550869 × SA × EN and rs2303579 × rs7162435 × rs11550869 × rs62043855 × EA × PA could explain positive symptom, excitement symptom and working memory, respectively, in FH+ group. Conclusion: The study highlighted that the combined interaction of NEDD4 and CT may be associated with symptoms of schizophrenia especially for those with FH+.

Ropinirole silver nanocomposite attenuates neurodegeneration in the transgenic Drosophila melanogaster model of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease due to the degeneration of dopaminergic neurons in substantia nigra pars compacta of the mid brain. The present study investigates the neuro-protective role of synthesized ropinirole silver nanocomposite (RPAgNC) in Drosophila model of PD. α-synuclein accumulation in the brain of flies (PD flies) leads to the damage of dopaminergic neurons, dopamine depletion, impaired muscular coordination, memory decline and increase in oxidative stress. Ingestion of the RPAgNC by Drosophila significantly prevented the neuronal degeneration compared to only ropinirole. The results confirm that the RPAgNC exerts more neuro-protective effect compared to dopamine agonist i.e. ropinirole as such drug in experimental PD flies. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Targeting α-synuclein for PD Therapeutics: A Pursuit on All Fronts

Parkinson's Disease (PD) is characterized both by the loss of dopaminergic neurons in the substantia nigra and the presence of cytoplasmic inclusions called Lewy Bodies. These Lewy Bodies contain the aggregated α-synuclein (α-syn) protein, which has been shown to be able to propagate from cell to cell and throughout different regions in the brain. Due to its central role in the pathology and the lack of a curative treatment for PD, an increasing number of studies have aimed at targeting this protein for therapeutics. Here, we reviewed and discussed the many different approaches that have been studied to inhibit α-syn accumulation via direct and indirect targeting. These analyses have led to the generation of multiple clinical trials that are either completed or currently active. These clinical trials and the current preclinical studies must still face obstacles ahead, but give hope of finding a therapy for PD with time.

Pathways of protein synthesis and degradation in PD pathogenesis

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

Autophagic and endo-lysosomal dysfunction in neurodegenerative disease

Due to their post-mitotic state, metabolic demands and often large polarised morphology, the function and survival of neurons is dependent on an efficient cellular waste clearance system both for generation of materials for metabolic processes and removal of toxic components. It is not surprising therefore that deficits in protein clearance can tip the balance between neuronal health and death. Here we discuss how autophagy and lysosome-mediated degradation pathways are disrupted in several neurological disorders. Both genetic and cell biological evidence show the diversity and complexity of vesicular clearance dysregulation in cells, and together may ultimately suggest a unified mechanism for neuronal demise in degenerative conditions. Causative and risk-associated mutations in Alzheimer's disease, Frontotemporal Dementia, Amyotrophic Lateral Sclerosis, Parkinson's disease, Huntington's disease and others have given the field a unique mechanistic insight into protein clearance processes in neurons. Through their broad implication in neurodegenerative diseases, molecules involved in these genetic pathways, in particular those involved in autophagy, are emerging as appealing therapeutic targets for intervention in neurodegeneration.

Lipid and immune abnormalities causing age-dependent neurodegeneration and Parkinson's disease

This article describes pathogenic concepts and factors, in particular glycolipid abnormalities, that create cell dysfunction and synaptic loss in neurodegenerative diseases. By phenocopying lysosomal storage disorders, such as Gaucher disease and related disorders, age- and dose-dependent changes in glycolipid cell metabolism can lead to Parkinson's disease and related dementias. Recent results show that perturbation of sphingolipid metabolism can precede or is a part of abnormal protein handling in both genetic and idiopathic Parkinson's disease and Lewy body dementia. In aging and genetic predisposition with lipid disturbance, α-synuclein's normal vesicular and synaptic role may be detrimentally shifted toward accommodating and binding such lipids. Specific neuronal glycolipid, protein, and vesicular interactions create potential pathophysiology that is amplified by astroglial and microglial immune mechanisms resulting in neurodegeneration. This perspective provides a new logic for therapeutic interventions that do not focus on protein aggregation, but rather provides a guide to the complex biology and the common sequence of events that lead to age-dependent neurodegenerative disorders.

α-Synuclein Trafficking in Parkinson's Disease: Insights From Fly and Mouse Models

Protein aggregation and accumulation are common pathological hallmarks in neurodegenerative diseases. To efficiently clear and eliminate such aggregation becomes an important cellular strategy for cell survival. Lewy bodies inclusion and aggregation of α-Synuclein (α-Syn) during the pathogenesis of Parkinson's disease (PD) serve as a good example and are potentially linked to other pathological PD features such as progressive dopaminergic neuron cell death, behavioral defects, and nonmotor symptoms like anosmia, cognitive impairment, and depression. Years of research have revealed a variety of mechanisms underlying α-Syn aggregation, clearance, and spread. Particularly, vesicular routes associated with the trafficking of α-Syn, leading to its aggregation and accumulation, have been shown to play vital roles in PD pathogenesis. How α-Syn proteins propagate among cells in a prion-like manner, either from or to neurons and glia, via means of uptake or secretion, are questions under active investigation and have been of central interest in the field of PD study. This review covers components and pathways of possible vesicular routes involved in α-Syn trafficking. Events including but not limited to exocytosis and endocytosis will be discussed within the context of an overall cellular trafficking theme. Recent advances on α-Syn trafficking mechanisms and their significance in mediating PD pathogenesis will be thoroughly reviewed, ending with a discussion on the advantages and limitations of different animal PD models.

How is alpha-synuclein cleared from the cell?

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

The Roles of Post-translational Modifications on α-Synuclein in the Pathogenesis of Parkinson's Diseases

Parkinson’s disease is the second most common neurodegenerative disorder. Although the pathogenesis of Parkinson’s disease is not entirely clear, the aberrant aggregation of α-synuclein has long been considered as an important risk factor. Elucidating the mechanisms that influence the aggregation of α-synuclein is essential for developing an effective diagnostic, preventative and therapeutic strategy to treat this devastating disease. The aggregation of α-synuclein is influenced by several post-translational modifications. Here, we summarized the major post-translational modifications (phosphorylation, ubiquitination, truncation, nitration, O-GlcNAcylation) of α-synuclein and the effect of these modifications on α-synuclein aggregation, which may provide potential targets for future therapeutics.

Monitoring α-Synuclein Proteotoxicity in Drosophila Models

Parkinson's disease is the second most common neurodegenerative disease without cure. It is characterized by α-synuclein accumulation and aggregation in dopaminergic and other types of neurons. Because α-synuclein accumulation leads to a toxic gain of function, its ectopic expression in Drosophila has been a useful in vivo model for testing modifiers of its toxicity. This chapter describes four assays: the rapid iterative negative geotaxis, rough eye phenotype, quantification of dopaminergic neuronal loss, and measurements of circadian effects.

Ubiquitin specific protease-13 independently regulates parkin ubiquitination and alpha-synuclein clearance in alpha-synucleinopathies

Ubiquitin specific proteases (USPs) are de-ubiquitinases (DUBs) that control protein ubiquitination cycle. The role of DUBs is poorly understood in neurodegenerative diseases. We found that USP13 is overexpressed in post-mortem Parkinson's disease (PD) brains. We investigated whether changes in USP13 levels can affect two molecules, parkin and alpha-synuclein, that are implicated in PD pathogenesis. Parkin is an E3 ubiquitin ligase that is regulated by ubiquitination and targets certain proteins for degradation, and alpha-synuclein may be ubiquitinated and recycled in the normal brain. We found that USP13 independently regulates parkin and alpha-synuclein ubiquitination in models of alpha-synucleinopathies. USP13 shRNA knockdown increases alpha-synuclein ubiquitination and clearance, in a parkin-independent manner. Furthermore, USP13 overexpression counteracts the effects of a tyrosine kinase inhibitor, Nilotinib, while USP13 knockdown facilitates Nilotinib effects on alpha-synculein clearance, suggesting that alpha-synuclein ubiquitnation is important for its clearance. These studies provide novel evidence of USP13 effects on parkin and alpha-synuclein metabolism and suggest that USP13 is a potential therapeutic target in the alpha-synucleinopathies.

From Yeast to Humans: Leveraging New Approaches in Yeast to Accelerate Discovery of Therapeutic Targets for Synucleinopathies

Neurodegenerative diseases (ND) represent a growing, global health crisis, one that lacks any disease-modifying therapeutic strategy. This critical need for new therapies must be met with an exhaustive approach to exploit all tools available. A yeast (Saccharomyces cerevisiae) model of α-synuclein toxicity-the protein causally linked to Parkinson's disease and other synucleinopathies-offers a powerful approach that takes advantage of the unique offerings of this system: tractable genetics, robust high-throughput screening strategies, unparalleled data repositories, powerful computational tools, and extensive evolutionary conservation of fundamental biological pathways. These attributes have enabled genetic and small molecule screens that have revealed toxic phenotypes and drug targets that translate directly to patient-derived iPSC neurons. Extending these insights, recent advances in genetic network analyses have generated the first "humanized" α-synuclein network, which has identified druggable proteins and led to validation of the toxic phenotypes in patient-derived cells. Unbiased phenotypic small molecule screens can identify compounds targeting critical proteins within α-synuclein networks. While identification of direct drug targets for phenotypic screen hits represents a bottleneck, high-throughput chemical genetic methods provide a means to uncover cellular targets and pathways for large numbers of compounds in parallel. Taken together, the yeast α-synuclein model and associated tools can reveal insights into underlying cellular pathologies, lead molecules and their cognate targets, and strategies to translate mechanisms of toxicity and cytoprotection into complex neuronal systems.

Ubiquitin Specific Protease 13 Regulates Tau Accumulation and Clearance in Models of Alzheimer's Disease

Ubiquitin Specific Protease-13 (USP13) is a de-ubiquinating enzyme that regulates protein ubiquitination and clearance. The role of USP13 is largely unknown in neurodegeneration. In this study we aim to demonstrate whether tau accumulation and/or clearance depends on ubiquitination/de-ubiquitination via USP-13. We used transgenic animal models of human amyloid precursor protein (APP) or P301L tau mutations and genetically knocked-down USP13 expression via shRNA to determine USP13 effects on tau ubiquitination and levels. We found a two-fold increase of USP13 levels in postmortem Alzheimer's disease (AD) brains. USP13 knockdown significantly increased the activity of the 20S proteasome and reduced the levels of hyper-phosphorylated tau (p-tau) in primary cortical neurons. USP13 knockdown also reduced the levels of amyloid and increased p-tau ubiquitination and clearance in transgenic animal models that overexpress murine tau as a result of the expression of familial APP mutations (TgAPP) and the human mutant P301L tau (rTg4510), respectively. Clearance of p-tau appears to be mediated by autophagy in these animal models. Taken together, these data suggest that USP13 knockdown reduces p-tau accumulation via regulation of ubiquitination/de-ubiquitination and mediates its clearance via autophagy and/or the proteasome. These results suggest that USP13 inhibition may be a therapeutic strategy to reduce accumulation of plaques and toxic p-tau in AD and human tauopathies.