The Neuroprotective Role of Protein Quality Control in Halting the Development of Alpha-Synuclein Pathology

Enhanced mTORC1 signaling and protein synthesis in pathologic α-synuclein cellular and animal models of Parkinson's disease

Pathologic α-synuclein plays an important role in the pathogenesis of α-synucleinopathies such as Parkinson's disease (PD). Disruption of proteostasis is thought to be central to pathologic α-synuclein toxicity; however, the molecular mechanism of this deregulation is poorly understood. Complementary proteomic approaches in cellular and animal models of PD were used to identify and characterize the pathologic α-synuclein interactome. We report that the highest biological processes that interacted with pathologic α-synuclein in mice included RNA processing and translation initiation. Regulation of catabolic processes that include autophagy were also identified. Pathologic α-synuclein was found to bind with the tuberous sclerosis protein 2 (TSC2) and to trigger the activation of the mammalian target of rapamycin (mTOR) complex 1 (mTORC1), which augmented mRNA translation and protein synthesis, leading to neurodegeneration. Genetic and pharmacologic inhibition of mTOR and protein synthesis rescued the dopamine neuron loss, behavioral deficits, and aberrant biochemical signaling in the α-synuclein preformed fibril mouse model and Drosophila transgenic models of pathologic α-synuclein-induced degeneration. Pathologic α-synuclein furthermore led to a destabilization of the TSC1-TSC2 complex, which plays an important role in mTORC1 activity. Constitutive overexpression of TSC2 rescued motor deficits and neuropathology in α-synuclein flies. Biochemical examination of PD postmortem brain tissues also suggested deregulated mTORC1 signaling. These findings establish a connection between mRNA translation deregulation and mTORC1 pathway activation that is induced by pathologic α-synuclein in cellular and animal models of PD.

Significant Role of Long Non-coding RNAs in Parkinson's Disease

BACKGROUND

Parkinson's disease (PD) is the second most common neurodegenerative disease in the world, with clinical manifestations of resting tremor, akinesia (or bradykinesia), rigidity, and postural instability. However, the molecular pathogenesis of PD is still unclear, and its effective treatments are limited. Substantial evidence demonstrates that long non-coding RNAs (lncRNAs) have important functions in various human diseases, such as cancer, cardiovascular disease, and neurodegenerative diseases. Therefore, the main purpose of this study is to review the role of lncRNAs in the pathogenesis of PD.

METHODS

The role of lncRNAs in the pathogenesis of PD is summarized by reviewing Pubmed.

RESULTS

Thirty different lncRNAs are aberrantly expressed in PD and promote or inhibit PD by mediating ubiquitin-proteasome system, autophagy-lysosomal pathway, dopamine (DA) neuronal apoptosis, mitochondrial function, oxidative stress, and neuroinflammation.

CONCLUSION

In this direction, lncRNA may contribute to the treatment of PD as a diagnostic and therapeutic target for PD.

Opicapone, a Novel Catechol-O-methyl Transferase Inhibitor, for Treatment of Parkinson's Disease "Off" Episodes

Parkinson's Disease (PD) is a common neurodegenerative disorder and the leading cause of disability. It causes significant morbidity and disability through a plethora of symptoms, including movement disorders, sleep disturbances, and cognitive and psychiatric symptoms. The traditional pathogenesis theory of PD involves the loss of dopaminergic neurons in the substantia nigra (SN). Classically, treatment is pursued with an assortment of medications that are directed at overcoming this deficiency with levodopa being central to most treatment plans. Patients taking levodopa tend to experience "off episodes" with decreasing medication levels, causing large fluctuations in their symptoms. These off episodes are disturbing and a source of morbidity for these patients. Opicapone is a novel, peripherally acting Catechol-O-methyl transferase (COMT) inhibitor that is used as adjunctive therapy to carbidopa/levodopa for treatment and prevention of "off episodes." It has been approved for use as an adjunct to levodopa since 2016 in Europe and has recently (April 2020) gained FDA approval for use in the USA. By inhibiting COMT, opicapone slows levodopa metabolism and increases its availability. Several clinical studies demonstrated significant improvement in treatment efficacy and reduction in duration of "off episodes." The main side effect demonstrated was dyskinesia, mostly with the 100mg dose, which is higher than the approved, effective dose of 50mg. Post-marketing surveillance and analysis are required to further elucidate its safety profile and contribute to patient selection. This paper reviews the seminal and latest evidence in the treatment of PD "off episodes" with the novel drug Opicapone, including efficacy, safety, and clinical indications.

Small molecule inhibitors of α-synuclein oligomers identified by targeting early dopamine-mediated motor impairment in C. elegans

BACKGROUND

Parkinson's disease is a disabling neurodegenerative movement disorder characterized by dopaminergic neuron loss induced by α-synuclein oligomers. There is an urgent need for disease-modifying therapies for Parkinson's disease, but drug discovery is challenged by lack of in vivo models that recapitulate early stages of neurodegeneration. Invertebrate organisms, such as the nematode worm Caenorhabditis elegans, provide in vivo models of human disease processes that can be instrumental for initial pharmacological studies.

METHODS

To identify early motor impairment of animals expressing α-synuclein in dopaminergic neurons, we first used a custom-built tracking microscope that captures locomotion of single C. elegans with high spatial and temporal resolution. Next, we devised a method for semi-automated and blinded quantification of motor impairment for a population of simultaneously recorded animals with multi-worm tracking and custom image processing. We then used genetic and pharmacological methods to define the features of early motor dysfunction of α-synuclein-expressing C. elegans. Finally, we applied the C. elegans model to a drug repurposing screen by combining it with an artificial intelligence platform and cell culture system to identify small molecules that inhibit α-synuclein oligomers. Screen hits were validated using in vitro and in vivo mammalian models.

RESULTS

We found a previously undescribed motor phenotype in transgenic α-synuclein C. elegans that correlates with mutant or wild-type α-synuclein protein levels and results from dopaminergic neuron dysfunction, but precedes neuronal loss. Together with artificial intelligence-driven in silico and in vitro screening, this C. elegans model identified five compounds that reduced motor dysfunction induced by α-synuclein. Three of these compounds also decreased α-synuclein oligomers in mammalian neurons, including rifabutin which has not been previously investigated for Parkinson's disease. We found that treatment with rifabutin reduced nigrostriatal dopaminergic neurodegeneration due to α-synuclein in a rat model.

CONCLUSIONS

We identified a C. elegans locomotor abnormality due to dopaminergic neuron dysfunction that models early α-synuclein-mediated neurodegeneration. Our innovative approach applying this in vivo model to a multi-step drug repurposing screen, with artificial intelligence-driven in silico and in vitro methods, resulted in the discovery of at least one drug that may be repurposed as a disease-modifying therapy for Parkinson's disease.

T-Type Ca2+ Enhancer SAK3 Activates CaMKII and Proteasome Activities in Lewy Body Dementia Mice Model

Lewy bodies are pathological characteristics of Lewy body dementia (LBD) and are composed of α-synuclein (α-Syn), which is mostly degraded via the ubiquitin–proteasome system. More importantly, 26S proteasomal activity decreases in the brain of LBD patients. We recently introduced a T-type calcium channel enhancer SAK3 (ethyl-8-methyl-2,4-dioxo-2-(piperidin-1-yl)- 2H-spiro[cyclopentane-1,3-imidazo [1,2-a]pyridin]-2-ene-3-carboxylate) for Alzheimer’s disease therapeutics. SAK3 enhanced the proteasome activity via CaMKII activation in amyloid precursor protein knock-in mice, promoting the degradation of amyloid-β plaques to improve cognition. At this point, we addressed whether SAK3 promotes the degradation of misfolded α-Syn and the aggregates in α-Syn preformed fibril (PFF)-injected mice. The mice were injected with α-Syn PFF in the dorsal striatum, and SAK3 (0.5 or 1.0 mg/kg) was administered orally for three months, either immediately or during the last month after injection. SAK3 significantly inhibited the accumulation of fibrilized phosphorylated-α-Syn in the substantia nigra. Accordingly, SAK3 significantly recovered mesencephalic dopamine neurons from cell death. Decreased α-Syn accumulation was closely associated with increased proteasome activity. Elevated CaMKII/Rpt-6 signaling possibly mediates the enhanced proteasome activity after SAK3 administration in the cortex and hippocampus. CaMKII/Rpt-6 activation also accounted for improved memory and cognition in α-Syn PFF-injected mice. These findings indicate that CaMKII/Rpt-6-dependent proteasomal activation by SAK3 recovers from α-Syn pathology in LBD.

The effect of copy number on α-synuclein's toxicity and its protective role in Bax-induced apoptosis, in yeast

Apoptosis is a form of programmed cell death which is essential for the growth of dividing human cells whereas, in contrast, it is deleterious for post-mitotic cells such as neurons. Bax and α-synuclein are two human proteins which play a role in the induction of neuronal apoptosis in neurodegenerative diseases like Alzheimer's and Parkinson's. Human Bax and α-synuclein also induce cell death when expressed in baker's yeast, Saccharomyces cerevisiae. Quite unexpectedly, the human α-synuclein gene had been identified as an inhibitor of pro-apoptotic Bax using a yeast-based screen of a human hippocampal cDNA library. Plasmids were constructed with different promoters, which allow expression of wildtype and Parkinson's disease (PD)-related mutant α-synuclein genes, from (i) multi-copy 2µ (episomal) plasmids and (ii) integrative plasmids that compel expression of genes from chromosomal sites in varying copy numbers (1-3). All α-synuclein-containing plasmids were introduced, through transformation, into a yeast strain which already contained a chromosomally integrated copy of Bax. It is for the first time that it was observed that, depending on gene dosage, only wildtype α-synuclein is anti-apoptotic while mutant α-synuclein is not. The results also indicate that wildtype α-synuclein has a remarkable ability to manifest two contrasting effects depending on its level of expression: (i) normally, it would negate apoptosis but (ii) when overexpressed, it tends to induce apoptosis which is probably what happens in PD.

Distinct alpha-Synuclein species induced by seeding are selectively cleared by the Lysosome or the Proteasome in neuronally differentiated SH-SY5Y cells

A major pathological feature of Parkinson's disease (PD) is the aberrant accumulation of misfolded assemblies of alpha-synuclein (α-Syn). Protein clearance appears as a regulator of the 'α-Syn burden' underlying PD pathogenesis. The picture emerging is that a combination of pathways with complementary roles, including the Proteasome System and the Autophagy-Lysosome Pathway, contributes to the intracellular degradation of α-Syn. This study addresses the mechanisms governing the degradation of α-Syn species seeded by exogenous fibrils in neuronally differentiated SH-SY5Y neuroblastoma cells with inducible expression of α-Syn. Using human α-Syn recombinant fibrils (pre-formed fibrils, PFFs), seeding and aggregation of endogenous Proteinase K (PK)-resistant α-Syn species occurs within a time frame of 6 days, and is still prominent after 12 days of PFF addition. Clearance of α-Syn assemblies in this inducible model was enhanced after switching off α-Syn expression with doxycycline. Lysosomal inhibition led to accumulation of SDS-soluble α-Syn aggregates 6 days after PFF-addition or when switching off α-Syn expression. Additionally, the autophagic enhancer, rapamycin, induced the clearance of α-Syn aggregates 13 days post-PFF addition, indicating that autophagy is the major pathway for aggregated α-Syn clearance. SDS-soluble phosphorylated α-Syn at S129 was only apparent at 7 days of incubation with a higher amount of PFFs. Proteasomal inhibition resulted in further accumulation of SDS-soluble phosphorylated α-Syn at S129, with limited PK resistance. Our data suggest that in this inducible model autophagy is mainly responsible for the degradation of fibrillar α-Syn, whereas the proteasome system is responsible, at least in part, for the selective clearance of phosphorylated α-Syn oligomers.

Robust Sequence Determinants of α-Synuclein Toxicity in Yeast Implicate Membrane Binding

Protein conformations are shaped by cellular environments, but how environmental changes alter the conformational landscapes of specific proteins in vivo remains largely uncharacterized, in part due to the challenge of probing protein structures in living cells. Here, we use deep mutational scanning to investigate how a toxic conformation of α-synuclein, a dynamic protein linked to Parkinson's disease, responds to perturbations of cellular proteostasis. In the context of a course for graduate students in the UCSF Integrative Program in Quantitative Biology, we screened a comprehensive library of α-synuclein missense mutants in yeast cells treated with a variety of small molecules that perturb cellular processes linked to α-synuclein biology and pathobiology. We found that the conformation of α-synuclein previously shown to drive yeast toxicity-an extended, membrane-bound helix-is largely unaffected by these chemical perturbations, underscoring the importance of this conformational state as a driver of cellular toxicity. On the other hand, the chemical perturbations have a significant effect on the ability of mutations to suppress α-synuclein toxicity. Moreover, we find that sequence determinants of α-synuclein toxicity are well described by a simple structural model of the membrane-bound helix. This model predicts that α-synuclein penetrates the membrane to constant depth across its length but that membrane affinity decreases toward the C terminus, which is consistent with orthogonal biophysical measurements. Finally, we discuss how parallelized chemical genetics experiments can provide a robust framework for inquiry-based graduate coursework.

Progress in the molecular pathogenesis and nucleic acid therapeutics for Parkinson's disease in the precision medicine era

Parkinson's disease (PD) is one of the most common neurodegenerative disorders that manifest various motor and nonmotor symptoms. Although currently available therapies can alleviate some of the symptoms, the disease continues to progress, leading eventually to severe motor and cognitive decline and reduced life expectancy. The past two decades have witnessed rapid progress in our understanding of the molecular and genetic pathogenesis of the disease, paving the way for the development of new therapeutic approaches to arrest or delay the neurodegenerative process. As a result of these advances, biomarker‐driven subtyping is making it possible to stratify PD patients into more homogeneous subgroups that may better respond to potential genetic‐molecular pathway targeted disease‐modifying therapies. Therapeutic nucleic acid oligomers can bind to target gene sequences with very high specificity in a base‐pairing manner and precisely modulate downstream molecular events. Recently, nucleic acid therapeutics have proven effective in the treatment of a number of severe neurological and neuromuscular disorders, drawing increasing attention to the possibility of developing novel molecular therapies for PD. In this review, we update the molecular pathogenesis of PD and discuss progress in the use of antisense oligonucleotides, small interfering RNAs, short hairpin RNAs, aptamers, and microRNA‐based therapeutics to target critical elements in the pathogenesis of PD that could have the potential to modify disease progression. In addition, recent advances in the delivery of nucleic acid compounds across the blood–brain barrier and challenges facing PD clinical trials are also reviewed.

Dementia associated with disorders of the basal ganglia

Dementia is now the leading cause of death in the United Kingdom, accounting for over 12% of all deaths and is the fifth most common cause of death worldwide. As treatments for heart disease and cancers improve and the population ages, the number of sufferers will only increase, with the chance of developing dementia doubling every 5 years after the age of 65. Finding an effective treatment is ever more critical to avert this pandemic health (and economic) crisis. To date, most dementia-related research has focused on the cortex and the hippocampus; however, with dementia becoming more fully recognized as aspects of diseases historically categorized as motor disorders (e.g., Parkinson's and Huntington's diseases), the role of the basal ganglia in dementia is coming to the fore. Conversely, it is highly likely that neuronal pathways in these structures traditionally considered as spared in Alzheimer's disease are also affected, particularly in later stages of the disease. In this review, we examine some of the limited evidence linking the basal ganglia to dementia.

Autophagy activation promotes clearance of α-synuclein inclusions in fibril-seeded human neural cells

There is much interest in delineating the mechanisms by which the α-synuclein protein accumulates in brains of individuals with Parkinson's disease (PD). Preclinical studies with rodent and primate models have indicated that fibrillar forms of α-synuclein can initiate the propagation of endogenous α-synuclein pathology. However, the underlying mechanisms by which α-synuclein fibrils seed pathology remain unclear. To investigate this further, we have used exogenous fibrillar α-synuclein to seed endogenous α-synuclein pathology in human neuronal cell lines, including primary human neurons differentiated from induced pluripotent stem cells. Fluorescence microscopy and immunoblot analyses were used to monitor levels of α-synuclein and key autophagy/lysosomal proteins over time in the exogenous α-synuclein fibril-treated neurons. We observed that temporal changes in the accumulation of cytoplasmic α-synuclein inclusions were associated with changes in the key autophagy/lysosomal markers. Of note, chloroquine-mediated blockade of autophagy increased accumulation of α-synuclein inclusions, and rapamycin-induced activation of autophagy, or use of 5'-AMP-activated protein kinase (AMPK) agonists, promoted the clearance of fibril-mediated α-synuclein pathology. These results suggest a key role for autophagy in clearing fibrillar α-synuclein pathologies in human neuronal cells. We propose that our findings may help inform the development of human neural cell models for screening of potential therapeutic compounds for PD or for providing insight into the mechanisms of α-synuclein propagation. Our results further add to existing evidence that AMPK activation may be a therapeutic option for managing PD.

Brain aging: A Ianus-faced player between health and neurodegeneration

Neurodegenerative diseases are incurable debilitating disorders characterized by structural and functional neuronal loss. Approximately 30 million people are affected worldwide, and this number is predicted to reach more than 150 million by 2050. Neurodegenerative disorders include Alzheimer's, Parkinson's, and prion diseases among others. These disorders are characterized by the accumulation of aggregating proteins forming amyloid, responsible for the disease-associated pathological lesions. The aggregation of amyloidogenic proteins can result either in gaining of toxic functions, derived from the damage provoked by these deposits in affected tissue, or in a loss of functions, due to the sequestration and the consequent inability of the aggregating protein to ensure its physiological role. While it is widely accepted that aging represents the main risk factor for neurodegeneration, there is still no clear cut-off line between the two conditions. Indeed, many of the pathways that are commonly altered in neurodegeneration-misfolded protein accumulation, chronic inflammation, mitochondrial dysfunction, impaired iron homeostasis, epigenetic modifications-have been often correlated also with healthy aging. This overlap could be explained by the fact that the continuous accumulation of cellular damages, together with a progressive decline in metabolic efficiency during aging, makes the neurons more vulnerable to toxic injuries. When a given threshold is exceeded, all these alterations might give rise to pathological phenotypes that ultimately lead to neurodegeneration.

Formation of Heterotypic Amyloids: α-Synuclein in Co-Aggregation

Protein misfolding resulting in the formation of ordered amyloid aggregates is associated with a number of devastating human diseases. Intrinsically disordered proteins (IDPs) do not autonomously fold up into a unique stable conformation and remain as an ensemble of rapidly fluctuating conformers. Many IDPs are prone to convert into the β-rich amyloid state. One such amyloidogenic IDP is α-synuclein that is involved in Parkinson's disease. Recent studies have indicated that other neuronal proteins, especially IDPs, can co-aggregate with α-synuclein in many pathological ailments. This article describes several such observations highlighting the role of heterotypic protein-protein interactions in the formation of hetero-amyloids. It is believed that the characterizations of molecular cross talks between amyloidogenic proteins as well as the mechanistic studies of heterotypic protein aggregation will allow us to decipher the role of the interacting proteins in amyloid proteomics.

MicroRNAs to differentiate Parkinsonian disorders: Advances in biomarkers and therapeutics

Parkinsonian disorders are a set of progressive neurodegenerative movement disorders characterized by rigidity, tremor, bradykinesia, postural instability and their distinction has significant implications in terms of management and prognosis. Parkinson's disease (PD) is the most common among them. Its clinical diagnosis is challenging and, it can be misdiagnosed in the early stages. Multiple system atrophy and progressive supranuclear palsy are the close mimickers in early stages, due to overlapping clinical features. MicroRNAs are a class of stable non-coding small RNA molecules implicated in post-transcriptional gene regulation. Current studies propose that miRNAs play an essential role in the pathobiology of multiple neurodegenerative disorders including Parkinsonism, and they seem to be one of the reasonably available methods to aid in the differential diagnosis between PD and related disorders. MicroRNA-based diagnostic biomarkers and therapeutics are a powerful tool to understand and explore the function of the pathogenic gene/s, their mechanism in the disease pathobiology, and to validate drug targets. In this review, we emphasize on the recent developments in the usage of miRNAs as diagnostic biomarkers to identify PD and to differentiate it from atypical parkinsonian conditions, their role in disease pathogenesis, and their possible utility in the therapy of these disorders.