shRNA targeting α-synuclein prevents neurodegeneration in a Parkinson's disease model

Epigenetic modifications in Parkinson's disease: A critical review

Parkinson's Disease (PD) is a progressive neurodegenerative disorder expected to increase by over 50% by 2030 due to increasing life expectancy. The disease's hallmarks include slow movement, tremors, and postural instability. Impaired protein processing is a major factor in the pathophysiology of PD, leading to the buildup of aberrant protein aggregates, particularly misfolded α-synuclein, also known as Lewy bodies. These Lewy bodies lead to inflammation and further death of dopaminergic neurons, leading to imbalances in excitatory and inhibitory neurotransmitters, causing excessive uncontrollable movements called dyskinesias. It was previously suggested that a complex interplay involving hereditary and environmental variables causes the specific death of neurons in PD; however, the exact mechanism of the association involving the two primary modifiers is yet unknown. An increasing amount of research points to the involvement of epigenetics in the onset and course of several neurological conditions, such as PD. DNA methylation, post-modifications of histones, and non-coding RNAs are the primary examples of epigenetic alterations, that is defined as alterations to the expression of genes and functioning without modifications in DNA sequence. Epigenetic modifications play a significant role in the development of PD, with genes such as Parkin, PTEN-induced kinase 1 (PINK1), DJ1, Leucine-Rich Repeat Kinase 2 (LRRK2), and alpha-synuclein associated with the disease. The aberrant epigenetic changes implicated in the pathophysiology of PD and their impact on the design of novel therapeutic approaches are the primary focus of this review.

Spatially Precise Genetic Engineering at the Electrode-Tissue Interface

The interface between electrodes and neural tissues plays a pivotal role in determining the efficacy and fidelity of neural activity recording and modulation. While considerable efforts have been made to improve the electrode-tissue interface, the majority of studies have primarily concentrated on the development of biocompatible neural electrodes through abiotic materials and structural engineering. In this study, an approach is presented that seamlessly integrates abiotic and biotic engineering principles into the electrode-tissue interface. Specifically, ultraflexible neural electrodes with short hairpin RNAs (shRNAs) designed to silence the expression of endogenous genes within neural tissues are combined. The system facilitates shRNA-mediated knockdown of phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and polypyrimidine tract-binding protein 1 (PTBP1), two essential genes associated in neural survival/growth and neurogenesis, within specific cell populations located at the electrode-tissue interface. Additionally, it is demonstrated that the downregulation of PTEN in neurons can result in an enlargement of neuronal cell bodies at the electrode-tissue interface. Furthermore, the system enables long-term monitoring of neuronal activities following PTEN knockdown in a mouse model of Parkinson's disease and traumatic brain injury. The system provides a versatile approach for genetically engineering the electrode-tissue interface with unparalleled precision, paving the way for the development of regenerative electronics and next-generation brain-machine interfaces.

ENGINEERED NANOBODIES WITH PROGRAMMABLE TARGET ANTIGEN PROTEOLYSIS (PTAP) FUSIONS REGULATE INTRACELLULAR ALPHA-SYNUCLEIN IN VITRO AND IN VIVO

Alpha-synuclein (αSyn) aggregation and the formation of Lewy pathology (LP) is a foundational pathophysiological phenomenon in synucleinopathies. Delivering therapeutic single-chain and single-domain antibodies that bind pathogenic targets can disrupt intracellular aggregation. The fusion of antibody fragments to a negatively-charged proteasomal targeting motif (PEST) creates bifunctional constructs that enhance both solubility and turnover. With sequence-specific point mutations of PEST sequences that modulate proteasomal degradation efficiency, we report the creation of Programmable Target Antigen Proteolysis (PTAP) technology that can provide graded control over the levels of target antigens. We have previously demonstrated our lead anti-αSyn intrabody, VH14-PEST, is capable of reducing the pathological burden of synucleinopathy in vitro and in vivo. Here, we report a family of fully humanized VH14-PTAP constructs for controllable, therapeutic targeting of intracellular α-Syn. In cells, we demonstrate successful target engagement and efficacy of VH14-hPEST intrabodies, and validate proof-of-principle in human cells using 3D human organoids derived from PD-patient induced pluripotent stem cells (iPSC). In two synuclein-based rat models, PTAP intrabodies attenuated nigral αSyn pathology, preserved nigrostriatal dopaminergic tone, and slowed the propagation of αSyn pathology. These data demonstrate the potency of intracellular αSyn targeting as a method to alleviate pathology and highlight the potential clinical utility of PTAP intrabodies.

Ultrasound-Assisted CRISPRi-Exosome for Epigenetic Modification of α-Synuclein Gene in a Mouse Model of Parkinson's Disease

Currently, there is a lack of effective treatment for Parkinson's disease (PD). In PD patients, aberrant methylation of SNCA (α-synuclein gene) has been reported and may be a potential therapeutic target. In this study, we established an epigenetic regulation platform based on an exosomal CRISPR intervention system. With the assist of focused ultrasound (FUS) opening the blood-brain barrier, engineered exosomes carrying RVG (rabies viral glycoprotein) targeting peptide, sgRNA (single guide RNA), and dCas9-DNMT3A (named RVG-CRISPRi-Exo) were efficiently delivered into the brain lesions and induced specific methylation of SNCA. In vivo, FUS combined with RVG-CRISPRi-Exo significantly improved motor performance, balance coordination, and neurosensitivity in PD mice, greatly down-regulated the elevation of α-synuclein (α-syn) caused by modeling, rescued cell apoptosis, and alleviated the progression of PD in mice. [18F]-FP-DTBZ imaging suggested that the synaptic function of the nigrostriatal pathway could be restored, which was conducive to the control of motor behavior in PD mice. Pyrosequencing results showed that RVG-CRISPRi-Exo could methylate CpG at specific sites of SNCA, and this fine-tuned editing achieved good therapeutic effects in PD model mice. In vitro, RVG-CRISPRi-Exo down-regulated SNCA transcripts and α-syn expression and relieved neuronal cell damage. Collectively, our findings provide a proof-of-principle for the development of targeted brain nanodelivery based on engineered exosomes and provide insights into epigenetic regulation of brain diseases.

Alpha-synuclein regulates the repair of genomic DNA double-strand breaks in a DNA-PKcs-dependent manner

α-synuclein (αSyn) is a presynaptic and nuclear protein that aggregates in important neurodegenerative diseases such as Parkinson's Disease (PD), Parkinson's Disease Dementia (PDD) and Lewy Body Dementia (LBD). Our past work suggests that nuclear αSyn may regulate forms of DNA double-strand break (DSB) repair in HAP1 cells after DNA damage induction with the chemotherapeutic agent bleomycin1. Here, we report that genetic deletion of αSyn specifically impairs the non-homologous end-joining (NHEJ) pathway of DSB repair using an extrachromosomal plasmid-based repair assay in HAP1 cells. Importantly, induction of a single DSB at a precise genomic location using a CRISPR/Cas9 lentiviral approach also showed the importance of αSyn in regulating NHEJ in HAP1 cells and primary mouse cortical neuron cultures. This modulation of DSB repair is dependent on the activity of the DNA damage response signaling kinase DNA-PKcs, since the effect of αSyn loss-of-function is reversed by DNA-PKcs inhibition. Using in vivo multiphoton imaging in mouse cortex after induction of αSyn pathology, we find an increase in longitudinal cell survival of inclusion-bearing neurons after Polo-like kinase (PLK) inhibition, which is associated with an increase in the amount of aggregated αSyn within inclusions. Together, these findings suggest that αSyn plays an important physiologic role in regulating DSB repair in both a transformed cell line and in primary cortical neurons. Loss of this nuclear function may contribute to the neuronal genomic instability detected in PD, PDD and DLB and points to DNA-PKcs and PLK as potential therapeutic targets.

Ameliorating Mitochondrial Dysfunction for the Therapy of Parkinson's Disease

Parkinson's disease (PD) is currently the second most incurable central neurodegenerative disease resulting from various pathogenesis. As the "energy factory" of cells, mitochondria play an extremely important role in supporting neuronal signal transmission and other physiological activities. Mitochondrial dysfunction can cause and accelerate the occurrence and progression of PD. How to effectively prevent and suppress mitochondrial disorders is a key strategy for the treatment of PD from the root. Therefore, the emerging mitochondria-targeted therapy has attracted considerable interest. Herein, the relationship between mitochondrial dysfunction and PD, the causes and results of mitochondrial dysfunction, and major strategies for ameliorating mitochondrial dysfunction to treat PD are systematically reviewed. The study also prospects the main challenges for the treatment of PD.

Recalibrating the Why and Whom of Animal Models in Parkinson Disease: A Clinician's Perspective

Animal models have been used to gain pathophysiologic insights into Parkinson's disease (PD) and aid in the translational efforts of interventions with therapeutic potential in human clinical trials. However, no disease-modifying therapy for PD has successfully emerged from model predictions. These translational disappointments warrant a reappraisal of the types of preclinical questions asked of animal models. Besides the limitations of experimental designs, the one-size convergence and oversimplification yielded by a model cannot recapitulate the molecular diversity within and between PD patients. Here, we compare the strengths and pitfalls of different models, review the discrepancies between animal and human data on similar pathologic and molecular mechanisms, assess the potential of organoids as novel modeling tools, and evaluate the types of questions for which models can guide and misguide. We propose that animal models may be of greatest utility in the evaluation of molecular mechanisms, neural pathways, drug toxicity, and safety but can be unreliable or misleading when used to generate pathophysiologic hypotheses or predict therapeutic efficacy for compounds with potential neuroprotective effects in humans. To enhance the translational disease-modification potential, the modeling must reflect the biology not of a diseased population but of subtypes of diseased humans to distinguish What data are relevant and to Whom.

Advancements in Genetic and Biochemical Insights: Unraveling the Etiopathogenesis of Neurodegeneration in Parkinson's Disease

Parkinson's disease (PD) is the second most prevalent neurodegenerative movement disorder worldwide, which is primarily characterized by motor impairments. Even though multiple hypotheses have been proposed over the decades that explain the pathogenesis of PD, presently, there are no cures or promising preventive therapies for PD. This could be attributed to the intricate pathophysiology of PD and the poorly understood molecular mechanism. To address these challenges comprehensively, a thorough disease model is imperative for a nuanced understanding of PD's underlying pathogenic mechanisms. This review offers a detailed analysis of the current state of knowledge regarding the molecular mechanisms underlying the pathogenesis of PD, with a particular emphasis on the roles played by gene-based factors in the disease's development and progression. This study includes an extensive discussion of the proteins and mutations of primary genes that are linked to PD, including α-synuclein, GBA1, LRRK2, VPS35, PINK1, DJ-1, and Parkin. Further, this review explores plausible mechanisms for DAergic neural loss, non-motor and non-dopaminergic pathologies, and the risk factors associated with PD. The present study will encourage the related research fields to understand better and analyze the current status of the biochemical mechanisms of PD, which might contribute to the design and development of efficacious and safe treatment strategies for PD in future endeavors.

Inactive S. aureus Cas9 downregulates alpha-synuclein and reduces mtDNA damage and oxidative stress levels in human stem cell model of Parkinson's disease

Parkinson's disease (PD) is one of the most common neurodegenerative diseases, but no disease modifying therapies have been successful in clinical translation presenting a major unmet medical need. A promising target is alpha-synuclein or its aggregated form, which accumulates in the brain of PD patients as Lewy bodies. While it is not entirely clear which alpha-synuclein protein species is disease relevant, mere overexpression of alpha-synuclein in hereditary forms leads to neurodegeneration. To specifically address gene regulation of alpha-synuclein, we developed a CRISPR interference (CRISPRi) system based on the nuclease dead S. aureus Cas9 (SadCas9) fused with the transcriptional repressor domain Krueppel-associated box to controllably repress alpha-synuclein expression at the transcriptional level. We screened single guide (sg)RNAs across the SNCA promoter and identified several sgRNAs that mediate downregulation of alpha-synuclein at varying levels. CRISPRi downregulation of alpha-synuclein in iPSC-derived neuronal cultures from a patient with an SNCA genomic triplication showed functional recovery by reduction of oxidative stress and mitochondrial DNA damage. Our results are proof-of-concept in vitro for precision medicine by targeting the SNCA gene promoter. The SNCA CRISPRi approach presents a new model to understand safe levels of alpha-synuclein downregulation and a novel therapeutic strategy for PD and related alpha-synucleinopathies.

Combating Dopaminergic Neurodegeneration in Parkinson's Disease through Nanovesicle Technology

Parkinson's disease (PD) is characterized by dopaminergic neurodegeneration, resulting in dopamine depletion and motor behavior deficits. Since the discovery of L-DOPA, it has been the most prescribed drug for symptomatic relief in PD, whose prolonged use, however, causes undesirable motor fluctuations like dyskinesia and dystonia. Further, therapeutics targeting the pathological hallmarks of PD including α-synuclein aggregation, oxidative stress, neuroinflammation, and autophagy impairment have also been developed, yet PD treatment is a largely unmet success. The inception of the nanovesicle-based drug delivery approach over the past few decades brings add-on advantages to the therapeutic strategies for PD treatment in which nanovesicles (basically phospholipid-containing artificial structures) are used to load and deliver drugs to the target site of the body. The present review narrates the characteristic features of nanovesicles including their blood-brain barrier permeability and ability to reach dopaminergic neurons of the brain and finally discusses the current status of this technology in the treatment of PD. From the review, it becomes evident that with the assistance of nanovesicle technology, the therapeutic efficacy of anti-PD pharmaceuticals, phyto-compounds, as well as that of nucleic acids targeting α-synuclein aggregation gained a significant increment. Furthermore, owing to the multiple drug-carrying abilities of nanovesicles, combination therapy targeting multiple pathogenic events of PD has also found success in preclinical studies and will plausibly lead to effective treatment strategies in the near future.

Protective Effects of Polyphenol-Rich Extracts against Neurotoxicity Elicited by Paraquat or Rotenone in Cellular Models of Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder involving motor symptoms caused by a loss of dopaminergic neurons in the substantia nigra region of the brain. Epidemiological evidence suggests that anthocyanin (ANC) intake is associated with a low risk of PD. Previously, we reported that extracts enriched with ANC and proanthocyanidins (PAC) suppressed dopaminergic neuron death elicited by the PD-related toxin rotenone in a primary midbrain culture model. Here, we characterized botanical extracts enriched with a mixed profile of polyphenols, as well as a set of purified polyphenolic standards, in terms of their ability to mitigate dopaminergic cell death in midbrain cultures exposed to another PD-related toxicant, paraquat (PQ), and we examined underlying neuroprotective mechanisms. Extracts prepared from blueberries, black currants, grape seeds, grape skin, mulberries, and plums, as well as several ANC, were found to rescue dopaminergic neuron loss in PQ-treated cultures. Comparison of a subset of ANC-rich extracts for the ability to mitigate neurotoxicity elicited by PQ versus rotenone revealed that a hibiscus or plum extract was only neuroprotective in cultures exposed to rotenone or PQ, respectively. Several extracts or compounds with the ability to protect against PQ neurotoxicity increased the activity of the antioxidant transcription factor Nrf2 in cultured astrocytes, and PQ-induced dopaminergic cell death was attenuated in Nrf2-expressing midbrain cultures. In other studies, we found that extracts prepared from hibiscus, grape skin, or purple basil (but not plums) rescued defects in O₂ consumption in neuronal cells treated with rotenone. Collectively, these findings suggest that extracts enriched with certain combinations of ANC, PAC, stilbenes, and other polyphenols could potentially slow neurodegeneration in the brains of individuals exposed to PQ or rotenone by activating cellular antioxidant mechanisms and/or alleviating mitochondrial dysfunction.

The link between neuroinflammation and the neurovascular unit in synucleinopathies

The neurovascular unit (NVU) is composed of vascular cells, glial cells, and neurons. As a fundamental functional module in the central nervous system, the NVU maintains homeostasis in the microenvironment and the integrity of the blood-brain barrier. Disruption of the NVU and interactions among its components are involved in the pathophysiology of synucleinopathies, which are characterized by the pathological accumulation of α-synuclein. Neuroinflammation contributes to the pathophysiology of synucleinopathies, including Parkinson's disease, multiple system atrophy, and dementia with Lewy bodies. This review aims to summarize the neuroinflammatory response of glial cells and vascular cells in the NVU. We also review neuroinflammation in the context of the cross-talk between glial cells and vascular cells, between glial cells and pericytes, and between microglia and astroglia. Last, we discuss how α-synuclein affects neuroinflammation and how neuroinflammation influences the aggregation and spread of α-synuclein and analyze different properties of α-synuclein in synucleinopathies.

Nuclease-dead S. aureus Cas9 downregulates alpha-synuclein and reduces mtDNA damage and oxidative stress levels in patient-derived stem cell model of Parkinson's disease

Parkinson's disease (PD) is one of the most common neurodegenerative diseases, but no disease modifying therapies have been successful in clinical translation presenting a major unmet medical need. A promising target is alpha-synuclein or its aggregated form, which accumulates in the brain of PD patients as Lewy bodies. While it is not entirely clear which alpha-synuclein protein species is disease relevant, mere overexpression of alpha-synuclein in hereditary forms leads to neurodegeneration. To specifically address gene regulation of alpha-synuclein, we developed a CRISPR interference (CRISPRi) system based on the nuclease dead S. aureus Cas9 (SadCas9) fused with the transcriptional repressor domain Krueppel-associated box to controllably repress alpha-synuclein expression at the transcriptional level. We screened single guide (sg)RNAs across the SNCA promoter and identified several sgRNAs that mediate downregulation of alpha-synuclein at varying levels. CRISPRi downregulation of alpha-synuclein in iPSC-derived neuronal cultures from a patient with an SNCA genomic triplication showed functional recovery by reduction of oxidative stress and mitochondrial DNA damage. Our results are proof-of-concept in vitro for precision medicine by targeting the SNCA gene promoter. The SNCA CRISPRi approach presents a new model to understand safe levels of alpha-synuclein downregulation and a novel therapeutic strategy for PD and related alpha-synucleinopathies.

A Mouse Model to Test Novel Therapeutics for Parkinson's Disease: an Update on the Thy1-aSyn ("line 61") Mice

Development of neuroprotective therapeutics for Parkinson's disease (PD) is facing a lack of translation from pre-clinical to clinical trials. One strategy for improvement is to increase predictive validity of pre-clinical studies by using extensively characterized animal models with a comprehensive set of validated pharmacodynamic readouts. Mice over-expressing full-length, human, wild-type alpha-synuclein under the Thy-1 promoter (Thy1-aSyn line 61) reproduce key features of sporadic PD, such as progressive loss of striatal dopamine, alpha-synuclein pathology, deficits in motor and non-motor functions, and elevation of inflammatory markers. Extensive work with this model by multiple laboratories over the past decade further increased confidence in its robustness and validity, especially for analyzing pathomechanisms of alpha-synuclein pathology and down-stream pathways, and for pre-clinical drug testing. Interestingly, while postnatal transgene expression is widespread in central and peripheral neurons, the extent and progression of down-stream pathology differs between brain regions, thereby replicating the characteristic selective vulnerability of neurodegenerative diseases. In-depth characterization of these readouts in conjunction with behavioral deficits has led to more informative endpoints for pre-clinical trials. Each drug tested in Thy1-aSyn line 61 enhances knowledge on how molecular targets, pathology, and functional behavioral readouts are interconnected, thereby further optimizing the platform towards predictive validity for clinical trials. Here, we present the current state of the art using Thy1-aSyn line 61 for drug target discovery, validation, and pre-clinical testing.

A Conceptual Study on the Peripheral Clearance of Brain-Derived α-Synuclein in Humans

BACKGROUND

Abnormal intracellular expression and aggregation of α-synuclein (α-syn) is the histopathological hallmark of several neurodegenerative diseases especially Parkinson's disease. However, safe and efficient approaches to clear α-syn remain unavailable.

OBJECTIVE

This study aimed to investigate the process of peripheral catabolism of brain-derived α-syn.

METHODS

Thirty patients with atrioventricular reentrant tachycardia (AVRT) (left accessory pathways) who underwent radiofrequency catheter ablation (RFCA) were enrolled in this study. Blood was collected via catheters from superior vena cava (SVC), inferior vena cava (IVC) proximal to the hepatic vein (HV), the right femoral vein (FV), and femoral artery (FA) simultaneously during RFCA. Plasma α-syn levels of AVRT patients and soluble α-syn levels of the brain samples were measured using enzyme-linked immunosorbent assay kits.

RESULTS

The α-syn concentrations in different locations of veins were divided by time-matched arterial α-syn concentrations to generate the venous/arterial (V/A) ratio. The V/A ratio of α-syn from the SVC was 1.204 (1.069-1.339, 95% CI), while the V/A ratio of α-syn from IVC was 0.831 (0.734-0.928, 95% CI), suggesting that brain-derived α-syn in the arterial blood was physiologically cleared while going through the peripheral organs and tissues. And it was estimated that about half of brain soluble α-syn could efflux and be cleared in the periphery. Moreover, the glomerular filtration rate was found correlated with V-A difference (FA-ICV) (p = 0.0272).

CONCLUSION

Under physiological conditions, brain-derived α-syn could efflux into and be catabolized by the peripheral system. The kidney may play a potential role in the clearance of α-syn.

Intronic enhancers of the human SNCA gene predominantly regulate its expression in brain in vivo

Evidence from patients with Parkinson's disease (PD) and our previously reported α-synuclein (SNCA) transgenic rat model support the idea that increased SNCA protein is a substantial risk factor of PD pathogenesis. However, little is known about the transcription control of the human SNCA gene in the brain in vivo. Here, we identified that the DYT6 gene product THAP1 (THAP domain-containing apoptosis-associated protein 1) and its interaction partner CTCF (CCCTC-binding factor) act as transcription regulators of SNCA. THAP1 controls SNCA intronic enhancers' activities, while CTCF regulates its enhancer-promoter loop formation. The SNCA intronic enhancers present neurodevelopment-dependent activities and form enhancer clusters similar to "super-enhancers" in the brain, in which the PD-associated single-nucleotide polymorphisms are enriched. Deletion of the SNCA intronic enhancer clusters prevents the release of paused RNA polymerase II from its promoter and subsequently reduces its expression drastically in the brain, which may provide new therapeutic approaches to prevent its accumulation and thus related neurodegenerative diseases defined as synucleinopathies.

Transfer of neuron-derived α-synuclein to astrocytes induces neuroinflammation and blood-brain barrier damage after methamphetamine exposure: Involving the regulation of nuclear receptor-associated protein 1

The α-synuclein (α-syn) is involved in methamphetamine (METH)-induced neurotoxicity. Neurons can transfer excessive α-syn to neighboring neurons and glial cells. The effects of α-syn aggregation in astrocytes after METH exposure on the blood-brain barrier (BBB) remains unclear. Our previous study demonstrated that nuclear receptor-related protein 1 (Nurr1), a member of the nuclear receptor family widely expressed in the brain, was involved in the process of METH-induced α-syn accumulated in astrocytes to activate neuroinflammation. The role Nurr1 plays in astrocyte-mediated neuroinflammation, which results in BBB injury induced by METH, remains uncertain. This study found that METH up-regulated α-syn expression in neurons extended to astrocytes, thereby eliciting astrocyte activation, increasing and decreasing IL-1β, IL-6, TNF-α, and GDNF levels by down-regulating Nurr1 expression, and ultimately damaging the BBB. Specifically, the permeability of BBB to Evans blue and sodium fluorescein (NaF) increased; IgG deposits in the brain parenchyma increased; the Claudin5, Occludin, and PDGFRβ levels decreased. Several ultrastructural pathological changes occurred in the BBB, such as abnormal cerebral microvascular diameter, astrocyte end-foot swelling, decreased pericyte coverage, and loss of tight junctions. However, knockout or inhibition of α-syn or astrocyte-specific overexpression of Nurr1 partially alleviated these symptoms and BBB injury. Moreover, the in vitro experiments confirmed that METH increased α-syn level in the primary cultured neurons, which could be further transferred to primary cultured astrocytes, resulting in decreased Nurr1 levels. The decreased Nurr1 levels mediated the increase of IL-1β, IL-6, and TNF-α, and the decrease of GDNF, thereby changing the permeability to NaF, transendothelial electrical resistance, and Claudin5 and Occludin levels of primary cultured brain microvascular endothelial cells. Based on our findings, we proposed a new mechanism to elucidate METH-induced BBB injury and presented α-syn and Nurr1 as promising drug intervention targets to reduce BBB injury and resulting neurotoxicity in METH abusers.

Recent developments in nucleic acid-based therapies for Parkinson’s disease: Current status, clinical potential, and future strategies

Parkinson’s disease is the second most common progressive neurodegenerative disease diagnosed mainly based on clinical symptoms caused by loss of nigrostriatal dopaminergic neurons. Although currently available pharmacological therapies provide symptomatic relief, however, the disease continues to progress eventually leading to severe motor and cognitive decline and reduced quality of life. The hallmark pathology of Parkinson’s disease includes intraneuronal inclusions known as Lewy bodies and Lewy neurites, including fibrillar α-synuclein aggregates. These aggregates can progressively spread across synaptically connected brain regions leading to emergence of disease symptoms with time. The α-synuclein level is considered important in its fibrillization and aggregation. Nucleic acid therapeutics have recently been shown to be effective in treating various neurological diseases, raising the possibility of developing innovative molecular therapies for Parkinson’s disease. In this review, we have described the advancements in genetic dysregulations in Parkinson’s disease along with the disease-modifying strategies involved in genetic regulation with particular focus on downregulation of α-synuclein gene using various novel technologies, notably antisense oligonucleotides, microRNA, short interfering RNA, short hairpin RNAs, DNA aptamers, and gene therapy of vector-assisted delivery system-based therapeutics. In addition, the current status of preclinical and clinical development for nucleic acid-based therapies for Parkinson’s disease have also been discussed along with their limitations and opportunities.

RNA interference (RNAi)-based therapeutics for treatment of rare neurologic diseases

Advances in genome sequencing have greatly facilitated the identification of genomic variants underlying rare neurodevelopmental and neurodegenerative disorders. Understanding the fundamental causes of rare monogenic disorders has made gene therapy a possible treatment approach for these conditions. RNA interference (RNAi) technologies such as small interfering RNA (siRNA), microRNA (miRNA), and short hairpin RNA (shRNA), and other oligonucleotide-based modalities such as antisense oligonucleotides (ASOs) are being developed as potential therapeutic approaches for manipulating expression of the genes that cause a variety of neurological diseases. Here, we offer a brief review of the mechanism of action of these RNAi approaches; provide deeper discussion of the advantages, challenges, and specific considerations related to the development of RNAi therapeutics for neurological disease; and highlight examples of rare neurological diseases for which RNAi therapeutics hold great promise.

Delivering the Promise of Gene Therapy with Nanomedicines in Treating Central Nervous System Diseases

Central Nervous System (CNS) diseases, such as Alzheimer's diseases (AD), Parkinson's Diseases (PD), brain tumors, Huntington's disease (HD), and stroke, still remain difficult to treat by the conventional molecular drugs. In recent years, various gene therapies have come into the spotlight as versatile therapeutics providing the potential to prevent and treat these diseases. Despite the significant progress that has undoubtedly been achieved in terms of the design and modification of genetic modulators with desired potency and minimized unwanted immune responses, the efficient and safe in vivo delivery of gene therapies still poses major translational challenges. Various non-viral nanomedicines have been recently explored to circumvent this limitation. In this review, an overview of gene therapies for CNS diseases is provided and describes recent advances in the development of nanomedicines, including their unique characteristics, chemical modifications, bioconjugations, and the specific applications that those nanomedicines are harnessed to deliver gene therapies.

Oxidative stress and synaptic dysfunction in rodent models of Parkinson's disease

Parkinson's disease (PD) is a multifactorial disorder involving a complex interplay between a variety of genetic and environmental factors. In this scenario, mitochondrial impairment and oxidative stress are widely accepted as crucial neuropathogenic mechanisms, as also evidenced by the identification of PD-associated genes that are directly involved in mitochondrial function. The concept of mitochondrial dysfunction is closely linked to that of synaptic dysfunction. Indeed, compelling evidence supports the role of mitochondria in synaptic transmission and plasticity, although many aspects have not yet been fully elucidated. Here, we will provide a brief overview of the most relevant evidence obtained in different neurotoxin-based and genetic rodent models of PD, focusing on mitochondrial impairment and synaptopathy, an early central event preceding overt nigrostriatal neurodegeneration. The identification of early deficits occurring in PD pathogenesis is crucial in view of the development of potential disease-modifying therapeutic strategies.

Gene-Based Therapeutics for Parkinson’s Disease

Parkinson’s disease (PD) is a complex multifactorial disorder that is not yet fully surmised, and it is only when such a disease is tackled on multiple levels simultaneously that we should expect to see fruitful results. Gene therapy is a modern medical practice that theoretically and, so far, practically, has demonstrated its capability in joining the battle against PD and other complex disorders on most if not all fronts. This review discusses how gene therapy can efficiently replace current forms of therapy such as drugs, personalized medicine or invasive surgery. Furthermore, we discuss the importance of enhancing delivery techniques to increase the level of transduction and control of gene expression or tissue specificity. Importantly, the results of current trials establish the safety, efficacy and applicability of gene therapy for PD. Gene therapy’s variety of potential in interfering with PD’s pathology by improving basal ganglial circuitry, enhancing dopamine synthesis, delivering neuroprotection or preventing neurodegeneration may one day achieve symptomatic benefit, disease modification and eradication.

Alpha-Synuclein Aggregation Pathway in Parkinson's Disease: Current Status and Novel Therapeutic Approaches

Following Alzheimer’s, Parkinson’s disease (PD) is the second-most common neurodegenerative disorder, sharing an unclear pathophysiology, a multifactorial profile, and massive social costs worldwide. Despite this, no disease-modifying therapy is available. PD is tightly associated with α-synuclein (α-Syn) deposits, which become organised into insoluble, amyloid fibrils. As a typical intrinsically disordered protein, α-Syn adopts a monomeric, random coil conformation in an aqueous solution, while its interaction with lipid membranes drives the transition of the molecule part into an α-helical structure. The central unstructured region of α-Syn is involved in fibril formation by converting to well-defined, β-sheet rich secondary structures. Presently, most therapeutic strategies against PD are focused on designing small molecules, peptides, and peptidomimetics that can directly target α-Syn and its aggregation pathway. Other approaches include gene silencing, cell transplantation, stimulation of intracellular clearance with autophagy promoters, and degradation pathways based on immunotherapy of amyloid fibrils. In the present review, we sum marise the current advances related to α-Syn aggregation/neurotoxicity. These findings present a valuable arsenal for the further development of efficient, nontoxic, and non-invasive therapeutic protocols for disease-modifying therapy that tackles disease onset and progression in the future.

NADPH oxidase 2 activity in Parkinson's disease

Mitochondrial dysfunction and oxidative stress are strongly implicated in Parkinson’s disease (PD) pathogenesis and there is evidence that mitochondrially-generated superoxide can activate NADPH oxidase 2 (NOX2). Although NOX2 has been examined in the context of PD, most attention has focused on glial NOX2, and the role of neuronal NOX2 in PD remains to be defined. Additionally, pharmacological NOX2 inhibitors have typically lacked specificity. Here we devised and validated a proximity ligation assay for NOX2 activity and demonstrated that in human PD and two animal models thereof, both neuronal and microglial NOX2 are highly active in substantia nigra under chronic conditions. However, in acute and sub-acute PD models, we observed neuronal, but not microglial NOX2 activation, suggesting that neuronal NOX2 may play a primary role in the early stages of the disease. Aberrant NOX2 activity is responsible for the formation of oxidative stress-related post-translational modifications of α-synuclein, and impaired mitochondrial protein import in vitro in primary ventral midbrain neuronal cultures and in vivo in nigrostriatal neurons in rats. In a rat model, administration of a brain-penetrant, highly specific NOX2 inhibitor prevented NOX2 activation in nigrostriatal neurons and its downstream effects in vivo, such as activation of leucine-rich repeat kinase 2 (LRRK2). We conclude that NOX2 is an important enzyme that contributes to progressive oxidative damage which in turn can lead to α-synuclein accumulation, mitochondrial protein import impairment, and LRRK2 activation. In this context, NOX2 inhibitors hold potential as a disease-modifying therapy in PD.

Slowing Parkinson's Disease Progression with Vaccination and Other Immunotherapies

Parkinson's disease (PD) is the second most common neurodegenerative disorder. There are several recognized pathways leading up to dopaminergic neuron loss in the substantia nigra pars compacta and other cells in the brain as a result of age-related, genetic, environmental, and other processes. Of these, the most prominent is the role played by the protein α-synuclein, which aggregates and is the primary component of Lewy bodies, the histopathological hallmark of PD. The latest disease-modifying treatment options being investigated in PD are active and passive immunization against α-synuclein. There are currently five different monoclonal antibodies investigated as passive immunization and three drugs being studied as active immunization modalities in PD. These work through different mechanisms but with a common goal-to minimize or prevent α-synuclein-driven neurotoxicity by reducing α-synuclein synthesis, increasing α-synuclein degradation, and preventing aggregation and propagation from cell to cell. These promising strategies, along with other potential therapies, may favorably alter disease progression in PD.

Autophagy system as a potential therapeutic target for neurodegenerative diseases

Autophagy is an evolutionally conserved process by which cytoplasmic contents including protein aggregates and damaged organelles such as mitochondria and lysosomes, are sequestered by double-membrane structure, autophagosomes, and delivered to the lysosomes for degradation. Recently, considerable efforts have been made to reveal the role of autophagy in neurodegenerative diseases like Alzheimer's disease, Parkinson's disease and Huntington's disease. Impairment of autophagy aggravates the accumulation of misfolded protein and damaged organelles in neurons, while sufficient autophagic activity reduces such accumulation in nervous system and ameliorates the pathology. Here we summarize recent progress regarding the role of autophagy in several neurodegenerative diseases and the potential autophagy-associated therapies for them.

Exosomes in Parkinson: Revisiting Their Pathologic Role and Potential Applications

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by bradykinesia, rigidity, and tremor. Considerable progress has been made to understand the exact mechanism leading to this disease. Most of what is known comes from the evidence of PD brains' autopsies showing a deposition of Lewy bodies-containing a protein called α-synuclein (α-syn)-as the pathological determinant of PD. α-syn predisposes neurons to neurotoxicity and cell death, while the other associated mechanisms are mitochondrial dysfunction and oxidative stress, which are underlying precursors to the death of dopaminergic neurons at the substantia nigra pars compacta leading to disease progression. Several mechanisms have been proposed to unravel the pathological cascade of these diseases; most of them share a particular similarity: cell-to-cell communication through exosomes (EXOs). EXOs are intracellular membrane-based vesicles with diverse compositions involved in biological and pathological processes, which their secretion is driven by the NLR family pyrin domain-containing three proteins (NLRP3) inflammasome. Toxic biological fibrils are transferred to recipient cells, and the disposal of damaged organelles through generating mitochondrial-derived vesicles are suggested mechanisms for developing PD. EXOs carry various biomarkers; thus, they are promising to diagnose different neurological disorders, including neurodegenerative diseases (NDDs). As nanovesicles, the applications of EXOs are not only restricted as diagnostics but also expanded to treat NDDs as therapeutic carriers and nano-scavengers. Herein, the aim is to highlight the potential incrimination of EXOs in the pathological cascade and progression of PD and their role as biomarkers and therapeutic carriers for diagnosing and treating this neuro-debilitating disorder.

Therapeutics in the Pipeline Targeting α-Synuclein for Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder and the fastest growing neurologic disease in the world, yet no disease-modifying therapy is available for this disabling condition. Multiple lines of evidence implicate the protein α-synuclein (α-Syn) in the pathogenesis of PD, and as such, there is intense interest in targeting α-Syn for potential disease modification. α-Syn is also a key pathogenic protein in other synucleionpathies, most commonly dementia with Lewy bodies. Thus, therapeutics targeting this protein will have utility in these disorders as well. Here we discuss the various approaches that are being investigated to prevent and mitigate α-Syn toxicity in PD, including clearing its pathologic aggregates from the brain using immunization strategies, inhibiting its misfolding and aggregation, reducing its expression level, enhancing cellular clearance mechanisms, preventing its cell-to-cell transmission within the brain and perhaps from the periphery, and targeting other proteins associated with or implicated in PD that contribute to α-Syn toxicity. We also discuss the therapeutics in the pipeline that harness these strategies. Finally, we discuss the challenges and opportunities for the field in the discovery and development of therapeutics for disease modification in PD. SIGNIFICANCE STATEMENT: PD is the second most common neurodegenerative disorder, for which disease-modifying therapies remain a major unmet need. A large body of evidence points to α-synuclein as a key pathogenic protein in this disease as well as in dementia with Lewy bodies, making it of leading therapeutic interest. This review discusses the various approaches being investigated and progress made to date toward discovering and developing therapeutics that would slow and stop progression of these disabling diseases.

Ghrelin alleviates 6-hydroxydopamine-induced neurotoxicity in SH-SY5Y cells

Ghrelin is a neuropeptide that has various physiological functions and has been demonstrated to be neuroprotective in a number of neurological disease models. However, the underlying mechanisms of ghrelin in Parkinson’s disease remain largely unexplored. The current study aimed to study the effects of ghrelin in a 6-hydroxydopamine (6-OHDA)-induced Parkinson’s disease model and evaluate the potential underlying mechanisms. In the present study, we treated an SH-SY5Y cell model with 6-OHDA, and observed that pretreatment with different concentrations of ghrelin (1, 10, and 100 nM) for 30 minutes relieved the neurotoxic effects of 6-OHDA, as revealed by Cell Counting Kit-8 and Annexin V/propidium iodide (PI) apoptosis assays. Reverse transcription quantitative polymerase chain reaction and western blot assay results demonstrated that 6-OHDA treatment upregulated α-synuclein and lincRNA-p21 and downregulated TG-interacting factor 1 (TGIF1), which was predicted as a potential transcription regulator of the gene encoding α-synuclein (SNCA). Ghrelin pretreatment was able to reverse the trends caused by 6-OHDA. The Annexin V/PI apoptosis assay results revealed that inhibiting either α-synuclein or lincRNA-p21 expression with small interfering RNA (siRNA) relieved 6-OHDA-induced cell apoptosis. Furthermore, inhibiting lincRNA-p21 also partially upregulated TGIF1. By retrieving information from a bioinformatics database and performing both double luciferase and RNA immunoprecipitation assays, we found that lincRNA-p21 and TGIF1 were able to form a double-stranded RNA-binding protein Staufen homolog 1 (STAU1) binding site and further activate the STAU1-mediated mRNA decay pathway. In addition, TGIF1 was able to transcriptionally regulate α-synuclein expression by binding to the promoter of SNCA. The Annexin V/PI apoptosis assay results showed that either knockdown of TGIF1 or overexpression of lincRNA-p21 notably abolished the neuroprotective effects of ghrelin against 6-OHDA-induced neurotoxicity. Collectively, these findings suggest that ghrelin exerts neuroprotective effects against 6-OHDA-induced neurotoxicity via the lincRNA-p21/TGIF1/α-synuclein pathway.

Toward a Disease-Modifying Therapy of Alpha-Synucleinopathies: New Molecules and New Approaches Came into the Limelight

The accumulation of the various products of alpha-synuclein aggregation has been associated with the etiology and pathogenesis of several neurodegenerative conditions, including both familial and sporadic forms of Parkinson’s disease (PD) and dementia with Lewy bodies (DLB). It is now well established that the aggregation and spread of alpha-synuclein aggregation pathology activate numerous pathogenic mechanisms that contribute to neurodegeneration and, ultimately, to disease progression. Therefore, the development of a safe and effective disease-modifying therapy that limits or prevents the accumulation of the toxic intermediate products of alpha-synuclein aggregation and the spread of alpha-synuclein aggregation pathology could provide significant positive clinical outcomes in PD/DLB cohorts. It has been suggested that this goal can be achieved by reducing the intracellular and/or extracellular levels of monomeric and already aggregated alpha-synuclein. The principal aim of this review is to critically evaluate the potential of therapeutic strategies that target the post-transcriptional steps of alpha-synuclein production and immunotherapy-based approaches to alpha-synuclein degradation in PD/DLB patients. Strategies aimed at the downregulation of alpha-synuclein production are at an early preclinical stage of drug development and, although they have shown promise in animal models of alpha-synuclein aggregation, many limitations need to be resolved before in-human clinical trials can be seriously considered. In contrast, many strategies aimed at the degradation of alpha-synuclein using immunotherapeutic approaches are at a more advanced stage of development, with some in-human Phase II clinical trials currently in progress. Translational barriers for both strategies include the limitations of alpha-synuclein aggregation models, poor understanding of the therapeutic window for the alpha-synuclein knockdown, and variability in alpha-synuclein pathology across patient cohorts. Overcoming such barriers should be the main focus of further studies. However, it is already clear that these strategies do have the potential to achieve a disease-modifying effect in PD and DLB.

Novel therapeutic interventions for combating Parkinson's disease and prospects of Nose-to-Brain drug delivery

Parkinson disease (PD) is a progressive neurodegenerative disorder prevalent mainly in geriatric population. While, L-DOPA remains one of the major choices for the therapeutic management of PD, various motor and non-motor manifestations complicate the management of PD. In the last two decades, exhaustive research has been carried out to explore novel therapeutic approaches for mitigating motor and non-motor symptoms of PD. These approaches majorly include receptor-based, anti-inflammatory, stem-cell and nucleic acid based. The major limitations of existing therapeutic interventions (of commonly oral route) are low efficacy due to low brain bioavailability and associated side effects. Nanotechnology has been exploited and has gained wide attention in the recent years as an approach for enhancement of bioavailability of various small molecule drugs in the brain. To address the challenges associated with PD therapy, nose-to-brain delivery utilizing nanomedicine-based approaches has been found to be encouraging in published evidence. Therefore, the present work summarises the major challenges and limitations with antiparkinsonian drugs, novel therapeutic interventions, and scope of nanomedicine-based nose-to-brain delivery in addressing the current challenges of antiparkinsonian therapy. The manuscript tries to sensitize the researchers for designing brain-targeted nanomedicine loaded with natural/synthetic scaffolds, biosimilars, and nucleic acids that can bypass the first-pass effect for the effective management of PD.

Viral alpha-synuclein knockdown prevents spreading synucleinopathy

The accumulation of aggregated alpha-synuclein (α-syn) in Parkinson's disease, dementia with Lewy bodies and multiple system atrophy is thought to involve a common prion-like mechanism, whereby misfolded α-syn provides a conformational template for further accumulation of pathological α-syn. We tested whether silencing α-syn gene expression could reduce native non-aggregated α-syn substrate and thereby disrupt the propagation of pathological α-syn initiated by seeding with synucleinopathy-affected mouse brain homogenates. Unilateral intracerebral injections of adeno-associated virus serotype-1 encoding microRNA targeting the α-syn gene reduced the extent and severity of both the α-syn pathology and motor deficits. Importantly, a moderate 50% reduction in α-syn was sufficient to prevent the spread of α-syn pathology to distal brain regions. Our study combines behavioural, immunohistochemical and biochemical data that strongly support α-syn knockdown gene therapy for synucleinopathies.

MicroRNA-7 Protects Against Neurodegeneration Induced by α-Synuclein Preformed Fibrils in the Mouse Brain

α-Synuclein is a key protein in the pathogenesis of Parkinson's disease as it accumulates in fibrillar form in affected brain regions. Misfolded α-synuclein seeds recruit monomeric α-synuclein to form aggregates, which can spread to anatomically connected brain regions, a phenomenon that correlates with clinical disease progression. Thus, downregulating α-synuclein levels could reduce seeding and inhibit aggregate formation and propagation. We previously reported that microRNA-7 (miR-7) protects neuronal cells by downregulating α-synuclein expression through its effect on the 3'-untranslated region of SNCA mRNA; however, whether miR-7 blocks α-synuclein seeding and propagation in vivo remains unknown. Here, we induced miR-7 overexpression in the mouse striatum unilaterally by infusing adeno-associated virus 1 (AAV-miR-7) followed by inoculation with recombinant α-synuclein preformed fibrils (PFF) a month later. Compared with control mice injected with non-targeting AAV-miR-NT followed by PFF, AAV-miR-7 pre-injected mice exhibited lower levels of monomeric and high-molecular-weight α-synuclein species in the striatum, and reduced amount of phosphorylated α-synuclein in the striatum and in nigral dopamine neurons. Accordingly, AAV-miR-7-injected mice had less pronounced degeneration of the nigrostriatal pathway and better behavioral performance. The neuroinflammatory reaction to α-synuclein PFF inoculation was also significantly attenuated. These data suggest that miR-7 inhibits the formation and propagation of pathological α-synuclein and protects against neurodegeneration induced by PFF. Collectively, these findings support the potential of miR-7 as a disease modifying biologic agent for Parkinson's disease and related α-synucleinopathies.

Precision Medicine on the Fly: Using Drosophila to Decipher Gene-Environment Interactions in Parkinson's Disease

Big data approaches have profoundly influenced state-of-the-art in many fields of research, with toxicology being no exception. Here, we use Parkinson's disease as a window through which to explore the challenges of a dual explosion of metabolomic data addressing the myriad environmental exposures individuals experience and genetic analyses implicating many different loci as risk factors for disease. We argue that new experimental approaches are needed to convert the growing body of omics data into molecular mechanisms of disease that can be therapeutically targeted in specific patients. We outline one attractive strategy, which capitalizes on the rapid generation time and advanced molecular tools available in the fruit fly, Drosophila, to provide a platform for mechanistic dissection and drug discovery.

Roles for α-Synuclein in Gene Expression

α-Synuclein (α-Syn) is a small cytosolic protein associated with a range of cellular compartments, including synaptic vesicles, the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes. In addition to its physiological role in regulating presynaptic function, the protein plays a central role in both sporadic and familial Parkinson’s disease (PD) via a gain-of-function mechanism. Because of this, several recent strategies propose to decrease α-Syn levels in PD patients. While these therapies may offer breakthroughs in PD management, the normal functions of α-Syn and potential side effects of its depletion require careful evaluation. Here, we review recent evidence on physiological and pathological roles of α-Syn in regulating activity-dependent signal transduction and gene expression pathways that play fundamental role in synaptic plasticity.

RNA pull-down confocal nanoscanning (RP-CONA) detects quercetin as pri-miR-7/HuR interaction inhibitor that decreases α-synuclein levels

RNA-protein interactions are central to all gene expression processes and contribute to a variety of human diseases. Therapeutic approaches targeting RNA-protein interactions have shown promising effects on some diseases that are previously regarded as 'incurable'. Here, we developed a fluorescent on-bead screening platform, RNA Pull-Down COnfocal NAnoscanning (RP-CONA), to identify RNA-protein interaction modulators in eukaryotic cell extracts. Using RP-CONA, we identified small molecules that disrupt the interaction between HuR, an inhibitor of brain-enriched miR-7 biogenesis, and the conserved terminal loop of pri-miR-7-1. Importantly, miR-7's primary target is an mRNA of α-synuclein, which contributes to the aetiology of Parkinson's disease. Our method identified a natural product quercetin as a molecule able to upregulate cellular miR-7 levels and downregulate the expression of α-synuclein. This opens up new therapeutic avenues towards treatment of Parkinson's disease as well as provides a novel methodology to search for modulators of RNA-protein interaction.

Recent Insights into the Interplay of Alpha-Synuclein and Sphingolipid Signaling in Parkinson's Disease

Molecular studies have provided increasing evidence that Parkinson’s disease (PD) is a protein conformational disease, where the spread of alpha-synuclein (ASN) pathology along the neuraxis correlates with clinical disease outcome. Pathogenic forms of ASN evoke oxidative stress (OS), neuroinflammation, and protein alterations in neighboring cells, thereby intensifying ASN toxicity, neurodegeneration, and neuronal death. A number of evidence suggest that homeostasis between bioactive sphingolipids with opposing function—e.g., sphingosine-1-phosphate (S1P) and ceramide—is essential in pro-survival signaling and cell defense against OS. In contrast, imbalance of the “sphingolipid biostat” favoring pro-oxidative/pro-apoptotic ceramide-mediated changes have been indicated in PD and other neurodegenerative disorders. Therefore, we focused on the role of sphingolipid alterations in ASN burden, as well as in a vast range of its neurotoxic effects. Sphingolipid homeostasis is principally directed by sphingosine kinases (SphKs), which synthesize S1P—a potent lipid mediator regulating cell fate and inflammatory response—making SphK/S1P signaling an essential pharmacological target. A growing number of studies have shown that S1P receptor modulators, and agonists are promising protectants in several neurological diseases. This review demonstrates the relationship between ASN toxicity and alteration of SphK-dependent S1P signaling in OS, neuroinflammation, and neuronal death. Moreover, we discuss the S1P receptor-mediated pathways as a novel promising therapeutic approach in PD.

Gene Therapy to Modulate Alpha-Synuclein in Synucleinopathies

The protein alpha-Synuclein (α-Syn) is a key contributor to the etiology of Parkinson’s disease (PD) with aggregation, trans-neuronal spread, and/or depletion of α-Syn being viewed as crucial events in the molecular processes that result in neurodegeneration. The exact succession of pathological occurrences that lead to neuronal death are still largely unknown and are likely to be multifactorial in nature. Despite this unknown, α-Syn dose and stability, autophagy-lysosomal dysfunction, and inflammation, amongst other cellular impairments, have all been described as participatory events in the neurodegenerative process. To that end, in this review we discuss the logical points for gene therapy to intervene in α-Syn-mediated disease and review the preclinical body of work where gene therapy has been used, or could conceptually be used, to ameliorate α-Syn induced neurotoxicity. We discuss gene therapy in the traditional sense of modulating gene expression, as well as the use of viral vectors and nanoparticles as methods to deliver other therapeutic modalities.

"Janus-Faced" α-Synuclein: Role in Parkinson's Disease

Parkinson's disease (PD) is a pathological condition characterized by the aggregation and the resultant presence of intraneuronal inclusions termed Lewy bodies (LBs) and Lewy neurites which are mainly composed of fibrillar α-synuclein (α-syn) protein. Pathogenic aggregation of α-syn is identified as the major cause of LBs deposition. Several mutations in α-syn showing varied aggregation kinetics in comparison to the wild type (WT) α-syn are reported in PD (A30P, E46K, H 50Q, G51D, A53E, and A53T). Also, the cell-to-cell spread of pathological α-syn plays a significant role in PD development. Interestingly, it has also been suggested that the pathology of PD may begin in the gastrointestinal tract and spread via the vagus nerve (VN) to brain proposing the gut-brain axis of α-syn pathology in PD. Despite multiple efforts, the behavior and functions of this protein in normal and pathological states (specifically in PD) is far from understood. Furthermore, the etiological factors responsible for triggering aggregation of this protein remain elusive. This review is an attempt to collate and present latest information on α-syn in relation to its structure, biochemistry and biophysics of aggregation in PD. Current advances in therapeutic efforts toward clearing the pathogenic α-syn via autophagy/lysosomal flux are also reviewed and reported.

Mechanism of Pacemaker Activity in Zebrafish DC2/4 Dopaminergic Neurons

Zebrafish models are used increasingly to study the molecular pathogenesis of Parkinson's disease (PD), owing to the extensive array of techniques available for their experimental manipulation and analysis. The ascending dopaminergic projection from the posterior tuberculum (TPp; diencephalic populations DC2 and DC4) to the subpallium is considered the zebrafish correlate of the mammalian nigrostriatal projection, but little is known about the neurophysiology of zebrafish DC2/4 neurons. This is an important knowledge gap, because autonomous activity in mammalian substantia nigra (SNc) dopaminergic neurons contributes to their vulnerability in PD models. Using a new transgenic zebrafish line to label living dopaminergic neurons, and a novel brain slice preparation, we conducted whole-cell patch clamp recordings of DC2/4 neurons from adult zebrafish of both sexes. Zebrafish DC2/4 neurons share many physiological properties with mammalian dopaminergic neurons, including the cell-autonomous generation of action potentials. However, in contrast to mammalian dopaminergic neurons, the pacemaker driving intrinsic rhythmic activity in zebrafish DC2/4 neurons does not involve calcium conductances, hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, or sodium leak currents. Instead, voltage clamp recordings and computational models show that interactions between three components - a small, predominantly potassium, leak conductance, voltage-gated sodium channels, and voltage-gated potassium channels - are sufficient for pacemaker activity in zebrafish DC2/4 neurons. These results contribute to understanding the comparative physiology of the dopaminergic system and provide a conceptual basis for interpreting data derived from zebrafish PD models. The findings further suggest new experimental opportunities to address the role of dopaminergic pacemaker activity in the pathogenesis of PD.SIGNIFICANCE STATEMENT Posterior tuberculum (TPp) DC2/4 dopaminergic neurons are considered the zebrafish correlate of mammalian substantia nigra (SNc) neurons, whose degeneration causes the motor signs of Parkinson's disease (PD). Our study shows that DC2/4 and SNc neurons share a number of electrophysiological properties, including depolarized membrane potential, high input resistance, and continual, cell-autonomous pacemaker activity, that strengthen the basis for the increasing use of zebrafish models to study the molecular pathogenesis of PD. The mechanisms driving pacemaker activity differ between DC2/4 and SNc neurons, providing: (1) experimental opportunities to dissociate the contributions of intrinsic activity and underlying pacemaker currents to pathogenesis; and (2) essential information for the design and interpretation of studies using zebrafish PD models.

α-Synuclein antisense oligonucleotides as a disease-modifying therapy for Parkinson's disease

Parkinson's disease (PD) is a prevalent neurodegenerative disease with no approved disease-modifying therapies. Multiplications, mutations, and single nucleotide polymorphisms in the SNCA gene, encoding α-synuclein (aSyn) protein, either cause or increase risk for PD. Intracellular accumulations of aSyn are pathological hallmarks of PD. Taken together, reduction of aSyn production may provide a disease-modifying therapy for PD. We show that antisense oligonucleotides (ASOs) reduce production of aSyn in rodent preformed fibril (PFF) models of PD. Reduced aSyn production leads to prevention and removal of established aSyn pathology and prevents dopaminergic cell dysfunction. In addition, we address the translational potential of the approach through characterization of human SNCA-targeting ASOs that efficiently suppress the human SNCA transcript in vivo. We demonstrate broad activity and distribution of the human SNCA ASOs throughout the nonhuman primate brain and a corresponding decrease in aSyn cerebral spinal fluid (CSF) levels. Taken together, these data suggest that, by inhibiting production of aSyn, it may be possible to reverse established pathology; thus, these data support the development of SNCA ASOs as a potential disease-modifying therapy for PD and related synucleinopathies.

Gene-based therapies for neurodegenerative diseases

Gene therapy is making a comeback. With its twin promise of targeting disease etiology and 'long-term correction', gene-based therapies (defined here as all forms of genome manipulation) are particularly appealing for neurodegenerative diseases, for which conventional pharmacologic approaches have been largely disappointing. The recent success of a viral-vector-based gene therapy in spinal muscular atrophy-promoting survival and motor function with a single intravenous injection-offers a paradigm for such therapeutic intervention and a platform to build on. Although challenges remain, the newfound optimism largely stems from advances in the development of viral vectors that can diffusely deliver genes throughout the CNS, as well as genome-engineering tools that can manipulate disease pathways in ways that were previously impossible. Surely spinal muscular atrophy cannot be the only neurodegenerative disease amenable to gene therapy, and one can imagine a future in which the toolkit of a clinician will include gene-based therapeutics. The goal of this Review is to highlight advances in the development and application of gene-based therapies for neurodegenerative diseases and offer a prospective look into this emerging arena.

Development of an α-synuclein knockdown peptide and evaluation of its efficacy in Parkinson's disease models

Convincing evidence supports the premise that reducing α-synuclein levels may be an effective therapy for Parkinson's disease (PD); however, there has been lack of a clinically applicable α-synuclein reducing therapeutic strategy. This study was undertaken to develop a blood-brain barrier and plasma membrane-permeable α-synuclein knockdown peptide, Tat-βsyn-degron, that may have therapeutic potential. The peptide effectively reduced the level of α-synuclein via proteasomal degradation both in cell cultures and in animals. Tat-βsyn-degron decreased α-synuclein aggregates and microglial activation in an α-synuclein pre-formed fibril model of spreading synucleinopathy in transgenic mice overexpressing human A53T α-synuclein. Moreover, Tat-βsyn-degron reduced α-synuclein levels and significantly decreased the parkinsonian toxin-induced neuronal damage and motor impairment in a mouse toxicity model of PD. These results show the promising efficacy of Tat-βsyn-degron in two different animal models of PD and suggest its potential use as an effective PD therapeutic that directly targets the disease-causing process.

Targeted attenuation of elevated histone marks at SNCA alleviates α-synuclein in Parkinson's disease

Epigenetic deregulation of α-synuclein plays a key role in Parkinson's disease (PD). Analysis of the SNCA promoter using the ENCODE database revealed the presence of important histone post-translational modifications (PTMs) including transcription-promoting marks, H3K4me3 and H3K27ac, and repressive mark, H3K27me3. We investigated these histone marks in post-mortem brains of controls and PD patients and observed that only H3K4me3 was significantly elevated at the SNCA promoter of the substantia nigra (SN) of PD patients both in punch biopsy and in NeuN-positive neuronal nuclei samples. To understand the importance of H3K4me3 in regulation of α-synuclein, we developed CRISPR/dCas9-based locus-specific H3K4me3 demethylating system where the catalytic domain of JARID1A was recruited to the SNCA promoter. This CRISPR/dCas9 SunTag-JARID1A significantly reduced H3K4me3 at SNCA promoter and concomitantly decreased α-synuclein both in the neuronal cell line SH-SY5Y and idiopathic PD-iPSC derived dopaminergic neurons. In sum, this study indicates that α-synuclein expression in PD is controlled by SNCA's histone PTMs and modulation of the histone landscape of SNCA can reduce α-synuclein expression.

Current Status and Challenges Associated with CNS-Targeted Gene Delivery across the BBB

The era of the aging society has arrived, and this is accompanied by an increase in the absolute numbers of patients with neurological disorders, such as Alzheimer’s disease (AD) and Parkinson’s disease (PD). Such neurological disorders are serious costly diseases that have a significant impact on society, both globally and socially. Gene therapy has great promise for the treatment of neurological disorders, but only a few gene therapy drugs are currently available. Delivery to the brain is the biggest hurdle in developing new drugs for the central nervous system (CNS) diseases and this is especially true in the case of gene delivery. Nanotechnologies such as viral and non-viral vectors allow efficient brain-targeted gene delivery systems to be created. The purpose of this review is to provide a comprehensive review of the current status of the development of successful drug delivery to the CNS for the treatment of CNS-related disorders especially by gene therapy. We mainly address three aspects of this situation: (1) blood-brain barrier (BBB) functions; (2) adeno-associated viral (AAV) vectors, currently the most advanced gene delivery vector; (3) non-viral brain targeting by non-invasive methods.

Protection from α-Synuclein induced dopaminergic neurodegeneration by overexpression of the mitochondrial import receptor TOM20

Dopaminergic neurons of the substantia nigra are selectively vulnerable to mitochondrial dysfunction, which is hypothesized to be an early and fundamental pathogenic mechanism in Parkinson's disease (PD). Mitochondrial function depends on the successful import of nuclear-encoded proteins, many of which are transported through the TOM20-TOM22 outer mitochondrial membrane import receptor machinery. Recent data suggests that post-translational modifications of α-synuclein promote its interaction with TOM20 at the outer mitochondrial membrane and thereby inhibit normal protein import, leading to dysfunction, and death of dopaminergic neurons. As such, preservation of mitochondrial import in the face of α-synuclein accumulation might be a strategy to prevent dopaminergic neurodegeneration, however, this is difficult to assess using current in vivo models of PD. To this end, we established an exogenous co-expression system, utilizing AAV2 vectors to overexpress human α-synuclein and TOM20, individually or together, in the adult Lewis rat substantia nigra to assess whether TOM20 overexpression attenuates α-synuclein-induced dopaminergic neurodegeneration. Twelve weeks after viral injection, we observed that AAV2-TOM20 expression was sufficient to prevent loss of nigral dopaminergic neurons caused by AAV2-αSyn overexpression. The observed TOM20-mediated dopaminergic neuron preservation appeared to be due, in part, to the rescued expression (and presumed import) of nuclear-encoded mitochondrial electron transport chain proteins that were inhibited by α-synuclein overexpression. In addition, TOM20 overexpression rescued the expression of the chaperone protein GRP75/mtHSP70/mortalin, a stress-response protein involved in α-synuclein-induced injury. Collectively, these data indicate that TOM20 expression prevents α-synuclein-induced mitochondrial dysfunction, which is sufficient to rescue dopaminergic neurons in the adult rat brain.

Acquired dysregulation of dopamine homeostasis reproduces features of Parkinson's disease

The catecholamine neurotransmitter dopamine has the potential to act as an endogenous neurotoxin when its vesicular sequestration is dysregulated. Despite postmortem analyses from patients with Parkinson’s disease that demonstrate decreased vesicular sequestration of dopamine with a corresponding increase in dopamine metabolism, dopamine’s contribution to nigrostriatal dopaminergic degeneration in Parkinson’s disease has been debated. Here, we present a new in vivo model demonstrating the induction of Parkinson’s disease-associated pathogenic mechanisms of degeneration resulting from acquired dysregulation of dopamine sequestration in nigrostriatal dopaminergic neurons in adult rats. Utilizing adeno-associated virus (serotype 2), viral-mediated small-hairpin RNA interference of endogenous vesicular monoamine transporter 2 (VMAT2) expression resulted in a loss of VMAT2 protein expression in transduced dopaminergic cell bodies in the substantia nigra with a corresponding loss of VMAT2 protein within the striatal terminals. The loss of VMAT2 resulted in an accumulation of cytosolic dopamine and subsequent increased dopamine metabolism, deficits in dopamine-mediated behaviors, and degeneration of nigrostriatal dopaminergic neurons that was rescued through reintroduction of exogenous VMAT2, demonstrating that the toxicity was specific to the loss of VMAT2. Analysis of parkinsonian pathogenic mechanisms of degeneration identified oxidative damage, activation of Parkinson’s disease-associated kinase LRRK2, and the formation of aberrant α-synuclein. This model demonstrates that a progressive acquired loss of VMAT2 expression in adulthood is sufficient to induce Parkinson’s disease-associated pathogenic mechanisms of degeneration and provides a new model to further investigate the consequences of cytosolic dopamine.

Stereological investigation of 5-HT3 receptors in the substantia nigra and dorsal raphe nucleus in the rat

Serotonin (5-HT) is a common neurotransmitter in mammals, playing a central role in the regulation of various processes such as sleep, perception, cognitive and autonomic functions in the nervous system. Previous studies have demonstrated that 5-HT type 3 (5-HT₃) receptors are expressed in either or both the substantia nigra (SN) and the dorsal raphe nucleus (DRN) in humans, marmosets, rats and Syrian hamsters. Here, we quantify the distribution of 5-HT₃ receptors across these regions in the adult rat. Fluorescent immunohistochemistry was performed on sections of rat brain covering the entire rostro-caudal extent of the SN and DRN with antibodies specific to the 5-HT_3A receptor subunit, as well as others targeting the monoaminergic markers tyrosine hydroxylase (TH) and the 5-HT transporter (SERT). The number of 5-HT_3A receptor-positive, TH-positive (n = 28,428 ± 888, Gundersen's m = 1 coefficient of error [CE] = 0.05) and SERT-positive (n = 12,852 ± 462, CE = 0.06) cells were estimated in both the SN and the DRN using stereology. We found that 5-HT_3A receptor-positive cells are present in the SNr (n = 1250 ± 64, CE = 0.24), but they did not co-localise with TH-positive cells, nor were they present in the SNc. In contrast, no 5-HT_3A receptor-positive cells were found in the DRN. These results support the presence of 5-HT₃ receptors in the SN, but not in the DRN, and do not support their expression on monoaminergic cells within these two brain areas.

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

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