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

Beyond canonical PROTAC: biological targeted protein degradation (bioTPD)

Targeted protein degradation (TPD) is an emerging therapeutic strategy with the potential to modulate disease-associated proteins that have previously been considered undruggable, by employing the host destruction machinery. The exploration and discovery of cellular degradation pathways, including but not limited to proteasomes and lysosome pathways as well as their degraders, is an area of active research. Since the concept of proteolysis-targeting chimeras (PROTACs) was introduced in 2001, the paradigm of TPD has been greatly expanded and moved from academia to industry for clinical translation, with small-molecule TPD being particularly represented. As an indispensable part of TPD, biological TPD (bioTPD) technologies including peptide-, fusion protein-, antibody-, nucleic acid-based bioTPD and others have also emerged and undergone significant advancement in recent years, demonstrating unique and promising activities beyond those of conventional small-molecule TPD. In this review, we provide an overview of recent advances in bioTPD technologies, summarize their compositional features and potential applications, and briefly discuss their drawbacks. Moreover, we present some strategies to improve the delivery efficacy of bioTPD, addressing their challenges in further clinical development.

Development of non-viral vectors for neuronal-targeted delivery of CRISPR-Cas9 RNA-proteins as a therapeutic strategy for neurological disorders

The aging population contributes to an increase in the prevalence of neurodegenerative diseases, such as Parkinson's disease (PD). Due to the progressive nature of these diseases and an incomplete understanding of their pathophysiology, current drugs are inefficient, with a limited efficacy and major side effects. In this study, CRISPR-Cas9 RNA-proteins (RNP) composed of a Cas9 nuclease and single-guide RNA were delivered with a non-viral targeted delivery system to rescue the PD-associated phenotype in neuronal cells. Here, we fused the cell-penetrating amphipathic peptide, PepFect14 (PF14), with a short fragment of the rabies virus glycoprotein (C2) previously shown to have an affinity towards nicotinic acetylcholine receptors expressed on neuronal cells and on the blood-brain barrier. The resultant peptide, C2-PF14, was used to complex with and deliver RNPs to neuronal cells. We observed that RNP/C2-PF14 complexes formed nanosized, monodispersed, and nontoxic nanoparticles that led to a specific delivery into neuronal cells. α-Synuclein (α-syn) plays a major role in the pathology of PD and is considered to be a target for therapy. We demonstrated that CRISPR/Cas9 RNP delivered by C2-PF14 achieved α-syn gene (SNCA) editing in neuronal cells as determined by T7EI assay and western blotting. Furthermore, RNP/C2-PF14 relieved PD-associated toxicity in neuronal cells in vitro. This is a proof-of-concept towards simple and safe targeted genome-editing for treating PD and other neurological disorders.

α-Synuclein in Parkinson's Disease: Does a Prion-Like Mechanism of Propagation from Periphery to the Brain Play a Role?

Parkinson's disease (PD) is one of the most common neurodegenerative diseases, defined as motor and non-motor symptoms associated with the loss of dopaminergic neurons and a decreased release of dopamine (DA). Currently, PD patients are believed to have a neuropathological basis denoted by the presence of Lewy bodies (LBs) or Lewy neurites (LNs), which mostly comprise α-synuclein (α-syn) inclusions. Remarkably, there is a growing body of evidence indicating that the inclusions undergo template-directed aggregation and propagation via template-directed among the brain and peripheral organs, mainly in a prion-like manner. Interestingly, some studies reported that an integral loop was reminiscent of the mechanism of Parkinson's disease, denoting that α-syn as prionoid was transmitted from the periphery to the brain via specific pathways. Also the systematic life cycle of α-syn in the cellular level is illustrated. In this review, we critically assess landmark evidence in the field of Parkinson's disease with a focus on the genesis and prion-like propagation of the α-syn pathology. The anatomical and cell-to-cell evidences are discussed to depict the theory behind the propagation and transferred pathways. Furthermore, we highlight effective therapeutic perspectives and clinical trials targeting prion-like mechanisms. Major controversies surrounding this topic are also discussed.

Pharmacological Targeting of Microglial Activation: New Therapeutic Approach

Mounting evidence suggests that neuroinflammation is not just a consequence but a vital contributor to the development and progression of Parkinson’s disease (PD). Microglia in particular, may contribute to the induction and modulation of inflammation in PD. Upon stimulation, microglia convert into activated phenotypes, which exist along a dynamic continuum and bear different immune properties depending on the disease stage and severity. Activated microglia release various factors involved in neuroinflammation, such as cytokines, chemokines, growth factors, reactive oxygen species (ROS), reactive nitrogen species (RNS), and prostaglandins (PGs). Further, activated microglia interact with other cell types (e.g., neurons, astrocytes and mast cells) and are closely associated with α-synuclein (α-syn) pathophysiology and iron homeostasis disturbance. Taken together, microglial activation and microglia-mediated inflammatory responses play essential roles in the pathogenesis of PD and elucidation of the complexity and imbalance of microglial activation may shed light on novel therapeutic approaches for PD.

Targeting α-Synuclein in Parkinson's Disease: Progress Towards the Development of Disease-Modifying Therapeutics

Parkinson's disease (PD), the second most common neurodegenerative movement disorder, is characterized by progressive motor and non-motor symptoms [1]. Despite treatment with pharmacologic and surgical therapies, the disease will continue to relentlessly advance. Hence, there is a great deal of interest in potential disease-modifying therapies with the hope that the neurodegenerative process can be slowed or halted. The purpose of this review is to highlight the role toxic α-synuclein (α-syn) plays in PD pathogenesis and critically review the relevant literature about therapeutic modalities targeting α-syn. Toxic α-syn plays a key role in PD pathogenesis, disrupting important cellular functions, and, thus, targeting α-syn is a reasonable disease-modifying strategy. Current approaches under investigation include decreasing α-syn production with RNA interference (RNAi), inhibiting α-syn aggregation, promoting intracellular degradation of α-syn aggregates (via enhancing autophagy and enhancing lysosomal degradation), and promoting extracellular degradation of α-syn via active and passive immunization.

Prion-Like Mechanisms in Parkinson's Disease

Formation and aggregation of misfolded proteins in the central nervous system (CNS) is a key hallmark of several age-related neurodegenerative diseases, including Parkinson’s disease (PD), Alzheimer’s disease (AD), and amyotrophic lateral sclerosis (ALS). These diseases share key biophysical and biochemical characteristics with prion diseases. It is believed that PD is characterized by abnormal protein aggregation, mainly that of α-synuclein (α-syn). Of particular importance, there is growing evidence indicating that abnormal α-syn can spread to neighboring brain regions and cause aggregation of endogenous α-syn in these regions as seeds, in a “prion-like” manner. Abundant studies in vitro and in vivo have shown that α-syn goes through a templated conformational change, propagates from the original region to neighboring regions, and eventually cause neuron degeneration in the substantia nigra and striatum. The objective of this review is to summarize the mechanisms involved in the aggregation of abnormal intracellular α-syn and its subsequent cell-to-cell transmission. According to these findings, we look forward to effective therapeutic perspectives that can block the progression of neurodegenerative diseases.

Interferon-γ Potentiates α-Synuclein-induced Neurotoxicity Linked to Toll-like Receptors 2 and 3 and Tumor Necrosis Factor-α in Murine Astrocytes

α-Synuclein (α-syn), a metabolite of neurons, induces glial activation and neuroinflammation and participates in pathogenesis of neurodegenerative diseases. This inflammatory response involves activation of toll-like receptors (TLRs) and its neurotoxic outcomes such as cytokine expression and release. However, regulatory role of cytokines on α-syn-induced neurotoxicity is still unclear. In this study, we used interferon (IFN)-γ to costimulate primary astrocytes with wild-type or A53T mutant α-syn, and evaluated inflammatory pathway activation. Four α-syn concentrations (0.5, 2, 8 and 20 μg/mL, 24 h) and four α-syn time-points (3, 12, 24 and 48 h, 2 μg/mL) were chosen to coincubate with one IFN-γ concentration (2 ng/mL). IFN-γ alone upregulated expressions of TLR3 and tumor necrosis factor (TNF)-α (mRNA level), and A53T mutant or wild-type α-syn alone activated the pathway components including TLR2, TLR3, nuclear factor-κB, TNF-α and interleukin (IL)-1β. Additive application of IFN-γ amplified this activation effect except for IL-1β at mRNA and protein levels or TNF-α release, displaying a synergistic effect of α-syn and IFN-γ. Blocking TLR2 other than TLR4 suppressed TLR3, TLR2 and TNF-α expressions induced by α-syn or plus IFN-γ, reflecting an interaction of TLR2 and TLR3 in TNF-α expression. These data collectively showed that IFN-γ potentiated α-syn stimulation and inflammatory outcomes via TLR2, TLR3 and TNF-α other than IL-1β in astrocytes, suggesting that involvement of IFN-γ in α-syn-induced innate immunity may be required for initiation and maintenance of glial activation, a novel neurotoxic mechanism underlying pathogenesis of neurodegenerative diseases. Graphical Abstract IFN-γ potentiates α-synuclein (A53T or wild-type)-induced innate immunity, involving expressions of TLR2, TLR3, NF-κB, and TNF-α, other than IL-1β. This effect is suppressed by blockage of TLR2 other than TLR4, reflecting an interaction of TLR2 and TLR3 in TNF-α expression. Thus, involvement of IFN-γ in α-syn-induced neurotoxicity may be required for initiation and maintenance of glial activation, a novel neurotoxic mechanism underlying pathogenesis of neurodegenerative diseases.

Cellular and Molecular Aspects of Parkinson Treatment: Future Therapeutic Perspectives

Parkinson's disease is a neurodegenerative disorder accompanied by depletion of dopamine and loss of dopaminergic neurons in the brain that is believed to be responsible for the motor and non-motor symptoms in this disease. The main drug prescribed for Parkinsonian patients is L-dopa, which can be converted to dopamine by passing through the blood-brain barrier. Although L-dopa is able to improve motor function and improve the quality of life in the patients, there is inter-individual variability and some patients do not achieve the therapeutic effect. Variations in treatment response and side effects of current drugs have convinced scientists to think of treating Parkinson's disease at the cellular and molecular level. Molecular and cellular therapy for Parkinson's disease include (i) cell transplantation therapy with human embryonic stem (ES) cells, human induced pluripotent stem (iPS) cells and human fetal mesencephalic tissue, (ii) immunological and inflammatory therapy which is done using antibodies, and (iii) gene therapy with AADC-TH-GCH gene therapy, viral vector-mediated gene delivery, RNA interference-based therapy, CRISPR-Cas9 gene editing system, and alternative methods such as optogenetics and chemogenetics. Although these methods currently have a series of challenges, they seem to be promising techniques for Parkinson's treatment in future. In this study, these prospective therapeutic approaches are reviewed.

Genomic DNA levels of mutant alpha-synuclein correlate with non-motor symptoms in an A53T Parkinson's disease mouse model

Alpha-synuclein plays a key role in the pathogenesis of Parkinson's disease (PD). A robust transgenic mouse model has been generated that overexpresses the mutant human A53T alpha-synuclein under the mouse prion protein gene promoter; these mice develop age-dependent motor deficits. Recently, compared to wild-type (WT) littermates, A53T alpha-synuclein mice were reported to display non-motor symptom deficits, e.g., anxiety-like and depressive-like behaviors, odor discrimination and detection impairments, and gastrointestinal dysfunction, at 6 months of age or older. However, the differences between heterozygous and homozygous mice in terms of non-motor symptoms and whether the genomic DNA levels of alpha-synuclein correlate with the symptoms have not yet been elucidated. In the present work, we used littermate WT and heterozygous and homozygous A53T mice that were characterized by a modified genotyping protocol and observed a unilateral decline in the dopamine transporter (DAT) distribution from 3 months to 12 months of age in homozygous mice. We evaluated non-motor symptoms by measuring colon motility, anxiety-like and depressive-like behaviors, and motor coordination. The results showed that homozygous A53T mice exhibited earlier abnormal non-motor symptoms compared to their heterozygous littermates. The severity of impaired colon motility as well as anxiety-like and depressive-like behaviors were correlated with the genomic DNA levels of A53T mutant alpha-synuclein. More noticeable, motor coordination aberrances were also observed in homozygous A53T mice. This study provides direct evidence that the genomic DNA levels of mutant alpha-synuclein correlate with non-motor symptoms in an A53T mouse model, indicating that the genomic DNA levels of mutant alpha-synuclein should be tightly manipulated in PD model studies.

Emerging preclinical pharmacological targets for Parkinson's disease

Parkinson's disease (PD) is a progressive neurological condition caused by the degeneration of dopaminergic neurons in the basal ganglia. It is the most prevalent form of Parkinsonism, categorized by cardinal features such as bradykinesia, rigidity, tremors, and postural instability. Due to the multicentric pathology of PD involving inflammation, oxidative stress, excitotoxicity, apoptosis, and protein aggregation, it has become difficult to pin-point a single therapeutic target and evaluate its potential application. Currently available drugs for treating PD provide only symptomatic relief and do not decrease or avert disease progression resulting in poor patient satisfaction and compliance. Significant amount of understanding concerning the pathophysiology of PD has offered a range of potential targets for PD. Several emerging targets including AAV-hAADC gene therapy, phosphodiesterase-4, potassium channels, myeloperoxidase, acetylcholinesterase, MAO-B, dopamine, A2A, mGlu5, and 5-HT-1A/1B receptors are in different stages of clinical development. Additionally, alternative interventions such as deep brain stimulation, thalamotomy, transcranial magnetic stimulation, and gamma knife surgery, are also being developed for patients with advanced PD. As much as these therapeutic targets hold potential to delay the onset and reverse the disease, more targets and alternative interventions need to be examined in different stages of PD. In this review, we discuss various emerging preclinical pharmacological targets that may serve as a new promising neuroprotective strategy that could actually help alleviate PD and its symptoms.

Small-molecule PET Tracers for Imaging Proteinopathies

In this chapter, we provide a review of the challenges and advances in developing successful PET imaging agents for 3 major types of aggregated amyloid proteins: amyloid-beta (Aβ), tau, and alpha-synuclein (α-syn). These 3 amyloids are involved in the pathogenesis of a variety of neurodegenerative diseases, referred to as proteinopathies or proteopathies, that include Alzheimer disease, Lewy body dementias, multiple system atrophy, and frontotemporal dementias, among others. In the Introduction section, we briefly discuss the history of amyloid in neurodegenerative diseases and describe why progress in developing effective imaging agents has been hampered by the failure of crystallography to provide definitive ligand-protein interactions for rational radioligand design efforts. Instead, the field has relied on largely serendipitous, trial-and-error methods to achieve useful and specific PET amyloid imaging tracers for Aβ, tau, and α-syn deposits. Because many of the proteopathies involve more than 1 amyloid protein, it is important to develop selective PET tracers for the different amyloids to help assess the relative contribution of each to total amyloid burden. We use Pittsburgh compound B to illustrate some of the critical steps in developing a potent and selective Aβ PET imaging agent. Other selective Aβ and tau PET imaging compounds have followed similar pathways in their developmental processes. Success for selective α-syn PET imaging agents has not been realized yet, but work is ongoing in multiple laboratories throughout the world. In the tau sections, we provide background regarding 3-repeat (3R) and 4-repeat (4R) tau proteins and how they can affect the binding of tau radioligands in different tauopathies. We review the ongoing efforts to assess the properties of tau ligands, which are useful in 3R, 4R, or combined 3R-4R tauopathies. Finally, we describe in the α-syn sections recent attempts to develop selective tracers to image α-synucleinopathies.

Current Pharmaceutical Treatments and Alternative Therapies of Parkinson's Disease

Over the decades, pharmaceutical treatments, particularly dopaminergic (DAergic) drugs have been considered as the main therapy against motor symptoms of Parkinson's disease (PD). It is proposed that DAergic drugs in combination with other medications, such as monoamine oxidase type B inhibitors, catechol-O-methyl transferase inhibitors, anticholinergics and other newly developed non-DAergic drugs can make a better control of motor symptoms or alleviate levodopa-induced motor complications. Moreover, non-motor symptoms of PD, such as cognitive, neuropsychiatric, sleep, autonomic and sensory disturbances caused by intrinsic PD pathology or drug-induced side effects, are gaining increasing attention and urgently need to be taken care of due to their impact on quality of life. Currently, neuroprotective therapies have been investigated extensively in pre-clinical studies, and some of them have been subjected to clinical trials. Furthermore, non-pharmaceutical treatments, including deep brain stimulation (DBS), gene therapy, cell replacement therapy and some complementary managements, such as Tai chi, Yoga, traditional herbs and molecular targeted therapies have also been considered as effective alternative therapies to classical pharmaceutics. This review will provide us updated information regarding the current drugs and non-drugs therapies for PD.

Novel multifunctional dopamine D2/D3 receptors agonists with potential neuroprotection and anti-alpha synuclein protein aggregation properties

Our ongoing drug development endeavor to design compounds for symptomatic and neuroprotective treatment of Parkinson's disease (PD) led us to carry out a structure activity relationship study based on dopamine agonists pramipexole and 5-OHDPAT. Our goal was to incorporate structural elements in these agonists in a way to preserve their agonist activity while producing inhibitory activity against aggregation of α-synuclein protein. In our design we appended various catechol and related phenol derivatives to the parent agonists via different linker lengths. Structural optimization led to development of several potent agonists among which (-)-8a, (-)-14 and (-)-20 exhibited potent neuroprotective properties in a cellular PD model involving neurotoxin 6-OHDA. The lead compounds (-)-8a and (-)-14 were able to modulate aggregation of α-synuclein protein efficiently. Finally, in an in vivo PD animal model, compound (-)-8a exhibited efficacious anti-parkinsonian effect.

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

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