Ubiquitination of a new form of alpha-synuclein by parkin from human brain: implications for Parkinson's disease

E2 conjugating enzymes: A silent but crucial player in ubiquitin biology

E2 conjugating enzymes serve as the linchpin of the Ubiquitin-Proteasome System (UPS), facilitating ubiquitin (Ub) transfer to substrate proteins and regulating diverse processes critical to cellular homeostasis. The interaction of E2s with E1 activating enzymes and E3 ligases singularly positions them as middlemen of the ubiquitin machinery that guides protein turnover. Structural determinants of E2 enzymes play a pivotal role in these interactions, enabling precise ubiquitin transfer and substrate specificity. Regulation of E2 enzymes is tightly controlled through mechanisms such as post-translational modifications (PTMs), allosteric control, and gene expression modulation. Specific residues that undergo PTMs highlight their impact on E2 function and their role in ubiquitin dynamics. E2 enzymes also cooperate with deubiquitinases (DUBs) to maintain proteostasis. Design of small molecule inhibitors to modulate E2 activity is emerging as promising avenue to restrict ubiquitination as a potential therapeutic intervention. Additionally, E2 enzymes have been implicated in the pathogenesis and progression of neurodegenerative disorders (NDDs), where their dysfunction contributes to disease mechanisms. In summary, examining E2 enzymes from structural and functional perspectives offers potential to advance our understanding of cellular processes and assist in discovery of new therapeutic targets.

TFE3-mediated neuroprotection: Clearance of aggregated α-synuclein and accumulated mitochondria in the AAV-α-synuclein model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by fibrillar neuronal inclusions containing aggregated α-synuclein (α-Syn). While the pathology of PD is multifaceted, the aggregation of α-Syn and mitochondrial dysfunction are well-established hallmarks in its pathogenesis. Recently, TFE3, a transcription factor, has emerged as a regulator of autophagy and metabolic processes. However, it remains unclear whether TFE3 can facilitate the degradation of α-Syn and regulate mitochondrial metabolism specifically in dopaminergic neurons. In this study, we demonstrate that TFE3 overexpression significantly mitigates the loss of dopaminergic neurons and reduces the decline in tyrosine hydroxylase-positive fiber density, thereby restoring motor function in an α-Syn overexpression model of PD. Mechanistically, TFE3 overexpression reversed α-Syn-mediated impairment of autophagy, leading to enhanced α-Syn degradation and reduced aggregation. Additionally, TFE3 overexpression inhibited α-Syn propagation. TFE3 overexpression also reversed the down-regulation of Parkin, promoting the clearance of accumulated mitochondria, and restored the expression of PGC1-α and TFAM, thereby enhancing mitochondrial biogenesis in the adeno-associated virus-α-Syn model. These findings further underscore the neuroprotective role of TFE3 in PD and provide insights into its underlying mechanisms, suggesting TFE3 as a potential therapeutic target for PD.

Interactions of VMAT2 with CDCrel-1 and Parkin in Methamphetamine Neurotoxicity

In recent years, methamphetamine (METH) misuse in the US has been rapidly increasing, and there is no FDA-approved pharmacotherapy for METH use disorder (MUD). In addition to being dependent on the drug, people with MUD develop a variety of neurological problems related to the toxicity of this drug. A variety of molecular mechanisms underlying METH neurotoxicity has been identified, including the dysfunction of the neuroprotective protein parkin. However, it is not known whether parkin loss of function within striatal dopaminergic (DAergic) terminals translates into decreased DA storage capacity. This study examined the relationship between parkin, its substrate cell division cycle related-1 (CDCrel-1) associated with synaptic vesicles, and vesicular monoamine transporter-2 (VMAT2) responsible for packaging DA in an in vivo model of METH neurotoxicity. To assess the individual differences in response to METH's neurotoxic effects, a large group of male Sprague Dawley rats were treated with binge METH or saline and sacrificed 1 h or 24 h later. This study is the first to show that CDCrel-1 interacts with VMAT2 in the rat striatum and that binge METH can alter this interaction as well as the levels and subcellular localization of CDCrel-1. The proteomic analysis of VMAT-2-associated proteins revealed the upregulation of several proteins involved in the exocytosis/endocytosis cycle and responses to stress. The results suggest that DAergic neurons are engaged in counteracting METH-induced toxic effects, including attempts to increase endocytosis and autophagy at 1 h after the METH binge, with the responses varying widely between individual rats. Studying CDCrel-1, VMAT2, and other proteins in large groups of outbred rats can help define individual genetic and molecular differences in responses to METH neurotoxicity, which, in turn, may aid treating humans suffering from MUD and its neurological consequences.

PACRG is Expressed on the Left Side of the Brain Vesicle in the Ascidian Halocynthia Larva

The ascidian larvae, which display a chordate ground body plan, are left-right asymmetric in several structures, including the brain vesicle. In ascidian larvae, the ocellus and otolith pigment cells, which are thought to detect light and gravity respectively, are located on the right side of the brain vesicle, while the coronet cells, which are presumed to be dopaminergic, are located on the left side. To study how left-right asymmetry of the brain vesicle in the ascidian Halocynthia roretzi larva is determined, I attempted to isolate a gene that is expressed in the brain vesicle. As a result, an ascidian Parkin co-regulated gene (PACRG) orthologue was cloned. Expression of PACRG begins weakly in the head region of the late tailbud embryos, and it thereafter is observed on the left side of the brain vesicle of the larvae just before hatching. The location of PACRG expression is estimated to overlap with the area stained by the coronet cell-specific antibody. Thus, it is suggested that PACRG might be involved in the formation of the left-side structures of the brain vesicle, including coronet cells, during ascidian embryogenesis.

Therapeutic potential of Parkin and its regulation in Parkinson's disease

Parkinson's disease (PD) is a debilitating neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the midbrain substantia nigra, resulting in motor and non-motor symptoms. While the exact etiology of PD remains elusive, a growing body of evidence suggests that dysfunction in the parkin protein plays a pivotal role in the pathogenesis of the disease. Parkin is an E3 ubiquitin ligase that ubiquitinates substrate proteins to control a number of crucial cellular processes including protein catabolism, immune response, and cellular apoptosis.While autosomal recessive mutations in the PARK2 gene, which codes for parkin, are linked to an inherited form of early-onset PD, heterozygous mutations in PARK2 have also been reported in the more commonly occurring sporadic PD cases. Impairment of parkin's E3 ligase activity is believed to play a pathogenic role in both familial and sporadic forms of PD.This article provides an overview of the current understanding of the mechanistic basis of parkin's E3 ligase activity, its major physiological role in controlling cellular functions, and how these are disrupted in familial and sporadic PD. The second half of the manuscript explores the currently available and potential therapeutic strategies targeting parkin structure and/or function in order to slow down or mitigate the progressive neurodegeneration in PD.

Alpha-Synuclein and Microglia in Parkinson's Disease: From Pathogenesis to Therapeutic Prospects

Parkinson's disease (PD) is a neurodegenerative disorder characterized by both motor symptoms and non-motor features. A hallmark of PD is the misfolding and accumulation of alpha-synuclein (α-syn), which triggers neuroinflammation and drives neurodegeneration. Microglia, brain cells that play a central role in neuroinflammatory responses and help clear various unnecessary molecules within the brain, thus maintaining the brain's internal environment, respond to α-syn through mechanisms involving inflammation, propagation, and clearance. This review delves into the complex interplay between α-syn and microglia, elucidating how these interactions drive PD pathogenesis. Furthermore, we discuss emerging therapeutic strategies targeting the α-syn-microglia axis, with a focus on modulating microglial functions to mitigate neuroinflammation, enhance clearance, and prevent α-syn propagation, emphasizing their potential to slow PD progression.

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

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

Protein aggregation and its affecting mechanisms in neurodegenerative diseases

Protein aggregation serves as a critical pathological marker in a spectrum of neurodegenerative diseases (NDs), including the formation of amyloid β (Aβ) and Tau neurofibrillary tangles in Alzheimer's disease, as well as α-Synuclein (α-Syn) aggregates in Parkinson's disease, Parkinson's disease-related dementia (PDD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). A significant proportion of patients with amyotrophic lateral sclerosis (ALS) exhibit TDP-43 aggregates. Moreover, a confluence of brain protein pathologies, such as Aβ, Tau, α-Syn, and TDP-43, has been identified in individual NDs cases, highlighting the intricate interplay among these proteins that is garnering heightened scrutiny. Importantly, protein aggregation is modulated by an array of factors, with burgeoning evidence suggesting that it frequently results from perturbations in protein homeostasis, influenced by the cellular membrane milieu, metal ion concentrations, post-translational modifications, and genetic mutations. This review delves into the pathological underpinnings of protein aggregation across various NDs and elucidates the intercommunication among disparate proteins within the same disease context. Additionally, we examine the pathogenic mechanisms by which diverse factors impinge upon protein aggregation, offering fresh perspectives for the future therapeutic intervention of NDs.

Caspase-2 is a condensate-mediated deubiquitinase in protein quality control

Protein ubiquitination plays a critical role in protein quality control in response to cellular stress. The excessive accumulation of ubiquitinated conjugates can be detrimental to cells and is recognized as a hallmark of multiple neurodegenerative diseases. However, an in-depth understanding of how the excessive ubiquitin chains are removed to maintain ubiquitin homeostasis post stress remains largely unclear. Here we found that caspase-2 (CASP2) accumulates in a ubiquitin and proteasome-positive biomolecular condensate, which we named ubstressome, following stress and functions as a deubiquitinase to remove overloaded ubiquitin chains on proteins prone to misfolding. Mechanistically, CASP2 binds to the poly-ubiquitinated conjugates through its allosteric ubiquitin-interacting motif-like region and decreases overloaded ubiquitin chains in a protease-dependent manner to promote substrate degradation. CASP2 deficiency in mice results in excessive accumulation of poly-ubiquitinated TAR DNA-binding protein 43, leading to motor defects. Our findings uncover a stress-evoked deubiquitinating activity of CASP2 in the maintenance of cellular ubiquitin homeostasis, which differs from the well-known roles of caspase in apoptosis and inflammation. These data also reveal unrecognized protein quality control functions of condensates in the removal of stress-induced ubiquitin chains.

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

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

Serine phosphorylation mimics of Aβ form distinct, non-cross-seeding fibril morphs

The self-assembly of amyloid-β peptide (Aβ) into fibrils and oligomers is linked to Alzheimer's disease (AD). Fibrillar aggregates in AD patient's brains contain several post-translational modifications, including phosphorylation at positions 8 and 26. These play a key role in modifying the aggregation propensity of Aβ, yet how they affect the mechanism of aggregation is only poorly understood. Here we elucidate the aggregation mechanism of Aβ42 peptides with phosphomimic mutations at these positions, with glutamine mimicking the size, and glutamate mimicking both the size and charge effect. We find that all variants are less aggregation-prone than wild-type Aβ42 with the glutamate mutants showing the largest reduction. Secondary nucleation is the dominant nucleation route for all variants, as confirmed using seeding experiments; however, its rate is reduced by about an order of magnitude or more for all variants relative to wild-type. S26Q and S26E fibrils fail to catalyse nucleation of wild-type monomers and vice versa, while the S8 variants co-aggregate more readily with wild-type. Ultrastructural analyses by cryo-electron microscopy and small angle X-ray scattering reveal an altered structure with longer node-to-node distance and smaller cross-section dimensions of S26Q fibrils. These results imply that structural compatibility between fibrils and monomer is a key determinant in secondary nucleation, and that small modifications can alter the preferred fibril structure, and thus its potential to induce aggregation of other variants. Overall, our results indicate that phosphorylation could play a key role in controlling aggregation propensity and may lead to the formation of distinct, non-cross-seeding fibril populations.

The role of polo-like kinases 2 in the proteasomal and lysosomal degradation of alpha-synuclein in neurons

Parkinson's disease (PD) is a neurodegenerative disorder caused by the degeneration of dopaminergic neurons in the brain stem. PD is mostly sporadic, but familial PD (FPD) cases are recorded in different studies. The first gene mutation that is linked to FPD is α-synuclein (α-syn). It was then found that α-syn is also accumulated in Lewy body (LB), a classical pathological hallmark in PD patients. Different studies have shown that α-syn accumulation and aggregation can be a crucial factor contributing to the degeneration of dopaminergic neurons in PD. α-syn has been found to be degraded by the ubiquitin proteasomal system (UPS) and autophagy-lysosomal pathway (ALP). In this study, we initially explored how α-syn phosphorylation by GRK6, PLK2 and CK2α would facilitate its degradation in relation to the UPS or ALP. Unexpectedly, we found that the degradation of α-syn through PLK2 phosphorylation could be modulated by UPS and ALP in a novel mechanism. Specially, attenuation of UPS could increase the amount of PLK2 and then could facilitate the phosphorylation and degradation of α-syn through ALP. To test this further in vivo, we attenuate the proteasomal activity in a well-established A53T α-syn transgenic PD mouse model. We found that attenuation of proteasomal activity in the A53T α-syn transgenic mice could reduce the accumulation of α-syn in the striatum and midbrain. Based on our results, this study provides a new insight into how α-syn is degraded through the UPS and ALP.

Enhanced homeostatic sleep response and decreased neurodegenerative proteins in cereblon knock-out mice

Energy homeostasis and sleep have a bidirectional relationship. Cereblon (CRBN) regulates energy levels by ubiquitinating the AMP-activated protein kinase(AMPK), an energy sensor. However, whether CRBN participates in sleep is unclear. Here, we examine sleep-wake patterns in Crbn+/+ and Crbn-/- mice during 24-h baseline, 6-h sleep deprivation (SD), and following 6-h recovery sleep (RS). At baseline, overall sleep patterns are similar between genotypes. However, SD decreases CRBN expression in Crbn+/+ mice and increases phospho-Tau, phospho-α-synuclein, DNAJA1 (DJ2), and DNAJB1 (DJ1) in both genotypes, with Crbn-/- mice showing a lesser extent of increase in p-Tau and p-α-synuclein and a higher level of heat shock protein 70 (HSP70), DJ2, and DJ1. During RS, Crbn-/- mice show increased slow-wave activity in the low-delta range (0.5-2.5 Hz), suggesting higher homeostatic sleep propensity associated with AMPK hyperactivation. By illuminating the role of CRBN in regulating sleep-wake behaviors through AMPK, we suggest CRBN as a potential therapeutic target for managing sleep disorders and preventing neurodegeneration.

Pathological Involvement of Protein Phase Separation and Aggregation in Neurodegenerative Diseases

Neurodegenerative diseases are the leading cause of human disability and immensely reduce patients' life span and quality. The diseases are characterized by the functional loss of neuronal cells and share several common pathogenic mechanisms involving the malfunction, structural distortion, or aggregation of multiple key regulatory proteins. Cellular phase separation is the formation of biomolecular condensates that regulate numerous biological processes, including neuronal development and synaptic signaling transduction. Aberrant phase separation may cause protein aggregation that is a general phenomenon in the neuronal cells of patients suffering neurodegenerative diseases. In this review, we summarize the pathological causes of common neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and Huntington's disease, among others. We discuss the regulation of key amyloidogenic proteins with an emphasis of their aberrant phase separation and aggregation. We also introduce the approaches as potential therapeutic strategies to ameliorate neurodegenerative diseases through intervening protein aggregation. Overall, this review consolidates the research findings of phase separation and aggregation caused by misfolded proteins in a context of neurodegenerative diseases.

Molecular Signatures of Neurodegenerative Diseases Identified by Proteomic and Phosphoproteomic Analyses in Aging Mouse Brain

A central hallmark of neurodegenerative diseases is the irreversible accumulation of misfolded proteins in the brain by aberrant phosphorylation. Understanding the mechanisms underlying protein phosphorylation and its role in pathological protein aggregation within the context of aging is crucial for developing therapeutic strategies aimed at preventing or reversing such diseases. Here, we applied multi-protease digestion and quantitative mass spectrometry to compare and characterize dysregulated proteins and phosphosites in the mouse brain proteome using three different age groups: young-adult (3-4 months), middle-age (10 months), and old mice (19-21 months). Proteins associated with senescence, neurodegeneration, inflammation, cell cycle regulation, the p53 hallmark pathway, and cytokine signaling showed significant age-dependent changes in abundances and level of phosphorylation. Several proteins implicated in Alzheimer's disease (AD) and Parkinson's disease (PD) including tau (Mapt), Nefh, and Dpysl2 (also known as Crmp2) were hyperphosphorylated in old mice brain suggesting their susceptibility to the diseases. Cdk5 and Gsk3b, which are known to phosphorylate Dpysl2 at multiple specific sites, had also increased phosphorylation levels in old mice suggesting a potential crosstalk between them to contribute to AD. Hapln2, which promotes α-synuclein aggregation in patients with PD, was one of the proteins with highest abundance in old mice. CD9, which regulates senescence through the PI3K-AKT-mTOR-p53 signaling was upregulated in old mice and its regulation was correlated with the activation of phosphorylated AKT1. Overall, the findings identify a significant association between aging and the dysregulation of proteins involved in various pathways linked to neurodegenerative diseases with potential therapeutic implications.

The Role of α-Synuclein in Etiology of Neurodegenerative Diseases

A presynaptic protein called α-synuclein plays a crucial role in synaptic function and neurotransmitter release. However, its misfolding and aggregation have been implicated in a variety of neurodegenerative diseases, particularly Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Emerging evidence suggests that α-synuclein interacts with various cellular pathways, including mitochondrial dysfunction, oxidative stress, and neuroinflammation, which contributes to neuronal cell death. Moreover, α-synuclein has been involved in the propagation of neurodegenerative processes through prion-like mechanisms, where misfolded proteins induce similar conformational changes in neighboring neurons. Understanding the multifaced roles of α-synuclein in neurodegeneration not only aids in acquiring more knowledge about the pathophysiology of these diseases but also highlights potential biomarkers and therapeutic targets for intervention in alpha-synucleinopathies. In this review, we provide a summary of the mechanisms by which α-synuclein contributes to neurodegenerative processes, focusing on its misfolding, oligomerization, and the formation of insoluble fibrils that form characteristic Lewy bodies. Furthermore, we compare the potential value of α-synuclein species in diagnosing and differentiating selected neurodegenerative diseases.

Neurotoxic Methamphetamine Doses Alter CDCel-1 Levels and Its Interaction with Vesicular Monoamine Transporter-2 in Rat Striatum

In recent years, methamphetamine METH misuse in the US has been rapidly increasing and there is no FDA-approved pharmacotherapy for METH use disorder (MUD). In addition to being dependent on the drug, people with MUD develop a variety of neurological problems related to the toxicity of this drug. A variety of molecular mechanisms underlying METH neurotoxicity has been identified, including dysfunction of the neuroprotective protein parkin. However, it is not known whether parkin loss of function within striatal dopaminergic (DAergic) terminals translates into a decrease in DA storage capacity. This study examined the relationship between parkin, its substrate cell division cycle related-1 (CDCrel-1), and vesicular monoamine transporter-2 (VMAT2) in METH neurotoxicity in male Sprague Dawley rats. To also assess individual differences in response to METH's neurotoxic effects, a large group of rats was treated with binge METH or saline and sacrificed 1h or 24h later. This study is the first to show that binge METH alters the levels and subcellular localization of CDCrel-1 and that CDCrel-1 interacts with VMAT2 and increases its levels at the plasma membrane. Furthermore, we found wide individual differences in the responses of measured indices to METH. Proteomic analysis of VMAT-2-associated proteins revealed upregulation of several proteins involved in the exocytosis/endocytosis cycle. The results suggest that at 1h after METH binge, DAergic neurons are engaged in counteracting METH-induced toxic effects, including oxidative stress- and hyperthermia-induced inhibition of synaptic vesicle cycling, with the responses varying between individual rats. Studying CDCrel-1, VMAT2, and other proteins in large groups of outbred rats can help define individual genetic and molecular differences in responses to METH neurotoxicity which, in turn, will aid treating humans suffering from METH use disorder and its neurological consequences.

Exploring the Role of Ubiquitin-Proteasome System in the Pathogenesis of Parkinson's Disease

Parkinson's disease (PD) is a prevalent neurodegenerative disorder among the elderly population. The pathogenesis of PD encompasses genetic alterations, environmental factors, and age-related neurodegenerative processes. Numerous studies have demonstrated that aberrant functioning of the ubiquitin-proteasome system (UPS) plays a crucial role in the initiation and progression of PD. Notably, E3 ubiquitin ligases serve as pivotal components determining substrate specificity within UPS and are intimately associated with the regulation of various proteins implicated in PD pathology. This review comprehensively summarizes the mechanisms by which E3 ubiquitin ligases and deubiquitinating enzymes modulate PD-associated proteins and signaling pathways, while exploring the intricate relationship between UPS dysfunctions and PD etiology. Furthermore, this article discusses recent research advancements regarding inhibitors targeting PD-related E3 ubiquitin ligases.

Pathological mechanisms and treatment of sporadic Parkinson's disease: past, present, and future

For a special issue, we review studies on the pathogenesis of nigral cell death and the treatment of sporadic Parkinson's disease (sPD) over the past few decades, with a focus on the studies performed by Prof. Mizuno and our group. Prof. Mizuno proposed the initial concept that mitochondrial function may be impaired in sPD. When working at Jichi Medical School, he found a decrease in complex I of the mitochondrial electron transfer complex in the substantia nigra of patients with Parkinson's disease (PD) and MPTP models. After moving to Juntendo University as a professor and chairman, he continued to study the mechanisms of cell death in the substantia nigra of patients with sPD. Under his supervision, I studied the relationships between PD and apoptosis, PD and iron involvement, mitochondrial dysfunction and apoptosis, and PD and neuroinflammation. Moving to Kitasato University, we focused on PD and the cytotoxicity of alpha synuclein (αSyn) as well as brain neuropathology. Eventually, I moved to Osaka University, where I continued working on PD and αSyn projects to promote therapeutic research. In this paper, we present the details of these studies in the following order: past, present, and future.

Catecholamines and Parkinson's disease: tyrosine hydroxylase (TH) over tetrahydrobiopterin (BH4) and GTP cyclohydrolase I (GCH1) to cytokines, neuromelanin, and gene therapy: a historical overview

The author identified the genes and proteins of human enzymes involved in the biosynthesis of catecholamines (dopamine, norepinephrine, epinephrine) and tetrahydrobiopterin (BH4): tyrosine hydroxylase (TH), aromatic L-amino acid decarboxylase (AADC), dopamine β-hydroxylase (DBH), phenylethanolamine N-methyltransferase (PNMT), and GTP cyclohydrolase I (GCH1). In Parkinson's disease (PD), the activities and levels of mRNA and protein of all catecholamine-synthesizing enzymes are decreased, especially in dopamine neurons in the substantia nigra. Hereditary GCH1 deficiency results in reductions in the levels of BH4 and the activities of TH, causing decreases in dopamine levels. Severe deficiencies in GCH1 or TH cause severe decreases in dopamine levels leading to severe neurological symptoms, whereas mild decreases in TH activity in mild GCH1 deficiency or in mild TH deficiency result in only modest reductions in dopamine levels and symptoms of DOPA-responsive dystonia (DRD, Segawa disease) or juvenile Parkinsonism. DRD is a treatable disease and small doses of L-DOPA can halt progression. The death of dopamine neurons in PD in the substantia nigra may be related to (i) inflammatory effect of extra neuronal neuromelanin, (ii) inflammatory cytokines which are produced by activated microglia, (iii) decreased levels of BDNF, and/or (iv) increased levels of apoptosis-related factors. This review also discusses progress in gene therapies for the treatment of PD, and of GCH1, TH and AADC deficiencies, by transfection of TH, AADC, and GCH1 via adeno-associated virus (AAV) vectors.

Proteostasis in neurodegenerative diseases

Proteostasis is essential for normal function of proteins and vital for cellular health and survival. Proteostasis encompasses all stages in the "life" of a protein, that is, from translation to functional performance and, ultimately, to degradation. Proteins need native conformations for function and in the presence of multiple types of stress, their misfolding and aggregation can occur. A coordinated network of proteins is at the core of proteostasis in cells. Among these, chaperones are required for maintaining the integrity of protein conformations by preventing misfolding and aggregation and guide those with abnormal conformation to degradation. The ubiquitin-proteasome system (UPS) and autophagy are major cellular pathways for degrading proteins. Although failure or decreased functioning of components of this network can lead to proteotoxicity and disease, like neuron degenerative diseases, underlying factors are not completely understood. Accumulating misfolded and aggregated proteins are considered major pathomechanisms of neurodegeneration. In this chapter, we have described the components of three major branches required for proteostasis-chaperones, UPS and autophagy, the mechanistic basis of their function, and their potential for protection against various neurodegenerative conditions, like Alzheimer's, Parkinson's, and Huntington's disease. The modulation of various proteostasis network proteins, like chaperones, E3 ubiquitin ligases, proteasome, and autophagy-associated proteins as therapeutic targets by small molecules as well as new and unconventional approaches, shows promise.

Interplay between α-synuclein and parkin genes: Insights of Parkinson's disease

Parkinson's disease (PD) is a complex and debilitating neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra. The pathogenesis of PD is intimately linked to the roles of two key molecular players, α-synuclein (α-syn) and Parkin. Understanding the intricate interplay between α-syn and Parkin is essential for unravelling the molecular underpinnings of PD. Their roles in synaptic function and protein quality control underscore their significance in neuronal health. Dysregulation of these processes, as seen in PD, highlights the potential for targeted therapeutic strategies aimed at restoring normal protein homeostasis and mitigating neurodegeneration. Investigating the connections between α-syn, Parkin, and various pathological mechanisms provides insights into the complex web of factors contributing to PD pathogenesis and offers hope for the development of more effective treatments for this devastating neurological disorder. The present compilation provides an overview of their structures, regional and cellular locations, associations, physiological functions, and pathological roles in the context of PD.

Abnormal protein post-translational modifications induces aggregation and abnormal deposition of protein, mediating neurodegenerative diseases

Protein post-translational modifications (PPTMs) refer to a series of chemical modifications that occur after the synthesis of protein. Proteins undergo different modifications such as phosphorylation, acetylation, ubiquitination, and so on. These modifications can alter the protein's structure, function, and interaction, thereby regulating its biological activity. In neurodegenerative diseases, several proteins undergo abnormal post-translational modifications, which leads to aggregation and abnormal deposition of protein, thus resulting in neuronal death and related diseases. For example, the main pathological features of Alzheimer's disease are the aggregation of beta-amyloid protein and abnormal phosphorylation of tau protein. The abnormal ubiquitination and loss of α-synuclein are related to the onset of Parkinson's disease. Other neurodegenerative diseases such as Huntington's disease, amyotrophic lateral sclerosis, and so on are also connected with abnormal PPTMs. Therefore, studying the abnormal PPTMs in neurodegenerative diseases is critical for understanding the mechanism of these diseases and the development of significant therapeutic strategies. This work reviews the implications of PPTMs in neurodegenerative diseases and discusses the relevant therapeutic strategies.

Cell-to-cell transmitted alpha-synuclein recapitulates experimental Parkinson's disease

Parkinson's disease is characterized by a progressive accumulation of alpha-Synuclein (αSyn) neuronal inclusions called Lewy bodies in the nervous system. Lewy bodies can arise from the cell-to-cell propagation of αSyn, which can occur via sequential steps of secretion and uptake. Here, by fusing a removable short signal peptide to the N-terminus of αSyn, we developed a novel mouse model with enhanced αSyn secretion and cell-to-cell transmission. Expression of the secreted αSyn in the mouse brain was under the control of a novel hybrid promoter in combination with adeno-associated virus serotype 9 (AAV9). This combination of promoter and viral vector induced a robust expression in neurons but not in the glia of injected mice. Biochemical characterization of the secreted αSyn revealed that, in cultured cells, this protein is released to the extracellular milieu via conventional secretion. The released αSyn is then internalized and processed by acceptor cells via the endosome-lysosome pathway indicating that the secreted αSyn is cell-to-cell transmitted. The secreted αSyn is aggregation-prone and amyloidogenic, and when expressed in the brain of wild-type non-transgenic mice, it induces a Parkinson's disease-like phenotype that includes a robust αSyn pathology in the substantia nigra, neuronal loss, neuroinflammation, and motor deficits, all the key features of experimental animal models of Parkinson's disease. In summary, a novel animal model of Parkinson's disease based on enhanced cell-to-cell transmission of αSyn was developed. The neuron-produced cell-to-cell transmitted αSyn triggers all phenotypic features of experimental Parkinson's disease in mice.

Alpha-Synuclein Contribution to Neuronal and Glial Damage in Parkinson's Disease

Parkinson's disease (PD) is a complex neurodegenerative disease characterized by the progressive loss of dopaminergic neurons in the substantia nigra and the widespread accumulation of alpha-synuclein (αSyn) protein aggregates. αSyn aggregation disrupts critical cellular processes, including synaptic function, mitochondrial integrity, and proteostasis, which culminate in neuronal cell death. Importantly, αSyn pathology extends beyond neurons-it also encompasses spreading throughout the neuronal environment and internalization by microglia and astrocytes. Once internalized, glia can act as neuroprotective scavengers, which limit the spread of αSyn. However, they can also become reactive, thereby contributing to neuroinflammation and the progression of PD. Recent advances in αSyn research have enabled the molecular diagnosis of PD and accelerated the development of targeted therapies. Nevertheless, despite more than two decades of research, the cellular function, aggregation mechanisms, and induction of cellular damage by αSyn remain incompletely understood. Unraveling the interplay between αSyn, neurons, and glia may provide insights into disease initiation and progression, which may bring us closer to exploring new effective therapeutic strategies. Herein, we provide an overview of recent studies emphasizing the multifaceted nature of αSyn and its impact on both neuron and glial cell damage.

Saccharomyces cerevisiae as a Model for Studying Human Neurodegenerative Disorders: Viral Capsid Protein Expression

In this article, we briefly describe human neurodegenerative diseases (NDs) and the experimental models used to study them. The main focus is the yeast Saccharomyces cerevisiae as an experimental model used to study neurodegenerative processes. We review recent experimental data on the aggregation of human neurodegenerative disease-related proteins in yeast cells. In addition, we describe the results of studies that were designed to investigate the molecular mechanisms that underlie the aggregation of reporter proteins. The advantages and disadvantages of the experimental approaches that are currently used to study the formation of protein aggregates are described. Special attention is given to the similarity between aggregates that form as a result of protein misfolding and viral factories-special structural formations in which viral particles are formed inside virus-infected cells. A separate part of the review is devoted to our previously published study on the formation of aggregates upon expression of the insect densovirus capsid protein in yeast cells. Based on the reviewed results of studies on NDs and related protein aggregation, as well as viral protein aggregation, a new experimental model system for the study of human NDs is proposed. The core of the proposed system is a comparative transcriptomic analysis of changes in signaling pathways during the expression of viral capsid proteins in yeast cells.

Linking Heat Shock Protein 70 and Parkin in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disease that affects millions of elderly people worldwide and is characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). The precise mechanisms underlying the pathogenesis of PD are still not fully understood, but it is well accepted that the misfolding, aggregation, and abnormal degradation of proteins are the key causative factors of PD. Heat shock protein 70 (Hsp70) is a molecular chaperone that participates in the degradation of misfolded and aggregated proteins in living cells and organisms. Parkin, an E3 ubiquitin ligase, participates in the degradation of proteins via the proteasome pathway. Recent studies have indicated that both Hsp70 and Parkin play pivotal roles in PD pathogenesis. In this review, we focus on discussing how dysregulation of Hsp70 and Parkin leads to PD pathogenesis, the interaction between Hsp70 and Parkin in the context of PD and their therapeutic applications in PD.

The Role of Ubiquitin-Proteasome System and Mitophagy in the Pathogenesis of Parkinson's Disease

Parkinson's disease (PD) is a common neurodegenerative disease that is mainly in middle-aged people and elderly people, and the pathogenesis of PD is complex and diverse. The ubiquitin-proteasome system (UPS) is a master regulator of neural development and the maintenance of brain structure and function. Dysfunction of components and substrates of this UPS has been linked to neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. Moreover, UPS can regulate α-synuclein misfolding and aggregation, mitophagy, neuroinflammation and oxidative stress to affect the development of PD. In the present study, we review the role of several related E3 ubiquitin ligases and deubiquitinating enzymes (DUBs) on the pathogenesis of PD such as Parkin, CHIP, USP8, etc. On this basis, we summarize the connections and differences of different E3 ubiquitin ligases in the pathogenesis, and elaborate on the regulatory progress of different DUBs on the pathogenesis of PD. Therefore, we can better understand their relationships and provide feasible and valuable therapeutic clues for UPS-related PD treatment research.

Cell-Permeable Stimuli-Responsive Ubiquitin Probe for Time-Resolved Monitoring of Substrate Ubiquitination in Live Cells

Dynamic monitoring of intracellular ubiquitin (Ub) conjugates is instrumental to understanding the Ub regulatory machinery. Although many biochemical approaches have been developed to characterize protein ubiquitination, chemical tools capable of temporal resolution probing of ubiquitination events remain to be developed. Here, we report the development of the first cell-permeable and stimuli-responsive Ub probe and its application for the temporal resolution profiling of ubiquitinated substrates in live cells. The probe carrying the photolabile group N-(2-nitrobenzyl)-Gly (Nbg) on the amide bond between Ub Gly75 and Gly76 is readily prepared through chemical synthesis and can be delivered to live cells by conjugation via a disulfide bond with the cyclic cell-penetrating peptide cR10D (i.e., 4-((4-(dimethylamino)phenyl)-azo)-benzoic acid-modified cyclic deca-arginine). Both in vitro and in vivo experiments showed that Ub-modifying enzymes (E1, E2s, and E3s) could not install the Ub probe onto substrate proteins prior to removal of the nitrobenzyl group, which was easily accomplished via photoirradiation. The utility and practicality of this probe were exemplified by the time-resolved biochemical and proteomic investigation of ubiquitination events in live cells during a H2O2-mediated oxidative stress response. This work shows a conceptually new family of chemical Ub tools for the time-resolved studies on dynamic protein ubiquitination in different biological processes and highlights the utility of modern chemical protein synthesis in obtaining custom-designed tools for biological studies.

Fluorescent reporter of Caenorhabditis elegans Parkin: Regulators of its abundance and role in autophagy-lysosomal dynamics

Background: Parkin, which when mutated leads to early-onset Parkinson's disease, acts as an E3 ubiquitin ligase. How Parkin is regulated for selective protein and organelle targeting is not well understood. Here, we used protein interactor and genetic screens in Caenorhabditis elegans ( C. elegans) to identify new regulators of Parkin abundance and showed their impact on autophagy-lysosomal dynamics and alpha-Synuclein processing. Methods: We generated a transgene encoding mCherry-tagged C. elegans Parkin - Parkinson's Disease Related 1 (PDR-1). We performed protein interactor screen using Co-immunoprecipitation followed by mass spectrometry analysis to identify putative interacting partners of PDR-1. Ribonucleic acid interference (RNAi) screen and an unbiased mutagenesis screen were used to identify genes regulating PDR-1 abundance. Confocal microscopy was used for the identification of the subcellular localization of PDR-1 and alpha-Synuclein processing. Results: We show that the mCherry::pdr-1 transgene rescues the mitochondrial phenotype of pdr-1 mutants and that the expressed PDR-1 reporter is localized in the cytosol with enriched compartmentalization in the autophagy-lysosomal system. We determined that the transgenic overexpression of the PDR-1 reporter, due to inactivated small interfering RNA (siRNA) generation pathway, disrupts autophagy-lysosomal dynamics. From the RNAi screen of putative PDR-1 interactors we found that the inactivated Adenine Nucleotide Translocator ant-1.1/hANT, or hybrid ubiquitin genes ubq-2/h UBA52 and ubl-1/h RPS27A encoding a single copy of ubiquitin fused to the ribosomal proteins L40 and S27a, respectively, induced PDR-1 abundance and affected lysosomal dynamics. In addition, we demonstrate that the abundant PDR-1 plays a role in alpha-Synuclein processing. Conclusions: These data show that the abundant reporter of  C. elegans Parkin affects the autophagy-lysosomal system together with alpha-Synuclein processing which can help in understanding the pathology in Parkin-related diseases.

Dessert or Poison? The Roles of Glycosylation in Alzheimer's, Parkinson's, Huntington's Disease, and Amyotrophic Lateral Sclerosis

Ministry of Education and Key Laboratory of Neurons and glial cells of the central nervous system (CNS) are modified by glycosylation and rely on glycosylation to achieve normal neural function. Neurodegenerative disease is a common disease of the elderly, affecting their healthy life span and quality of life, and no effective treatment is currently available. Recent research implies that various glycosylation traits are altered during neurodegenerative diseases, suggesting a potential implication of glycosylation in disease pathology. Herein, we summarized the current knowledge about glycosylation associated with Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and Amyotrophic lateral sclerosis (ALS) pathogenesis, focusing on their promising functional avenues. Moreover, we collected research aimed at highlighting the need for such studies to provide a wealth of disease-related glycosylation information that will help us better understand the pathophysiological mechanisms and hopefully specific glycosylation information to provide further diagnostic and therapeutic directions for neurodegenerative diseases.

Integrated analysis of plasma proteome and cortex single-cell transcriptome reveals the novel biomarkers during cortical aging

Background

With the increase of age, multiple physiological functions of people begin gradually degenerating. Regardless of natural aging or pathological aging, the decline in cognitive function is one of the most obvious features in the process of brain aging. Brain aging is a key factor for several neuropsychiatric disorders and for most neurodegenerative diseases characterized by onset typically occurring late in life and with worsening of symptoms over time. Therefore, the early prevention and intervention of aging progression are particularly important. Since there is no unified conclusion about the plasma diagnostic biomarkers of brain aging, this paper innovatively employed the combined multi-omics analysis to delineate the plasma markers of brain aging.

Methods

In order to search for specific aging markers in plasma during cerebral cortex aging, we used multi-omics analysis to screen out differential genes/proteins by integrating two prefrontal cortex (PFC) single-nucleus transcriptome sequencing (snRNA-seq) datasets and one plasma proteome sequencing datasets. Then plasma samples were collected from 20 young people and 20 elder people to verify the selected differential genes/proteins with ELISA assay.

Results

We first integrated snRNA-seq data of the post-mortem human PFC and generated profiles of 65,064 nuclei from 14 subjects across adult (44-58 years), early-aging (69-79 years), and late-aging (85-94 years) stages. Seven major cell types were classified based on established markers, including oligodendrocyte, excitatory neurons, oligodendrocyte progenitor cells, astrocytes, microglia, inhibitory neurons, and endotheliocytes. A total of 93 cell-specific genes were identified to be significantly associated with age. Afterward, plasma proteomics data from 2,925 plasma proteins across 4,263 young adults to nonagenarians (18-95 years old) were combined with the outcomes from snRNA-seq data to obtain 12 differential genes/proteins (GPC5, CA10, DGKB, ST6GALNAC5, DSCAM, IL1RAPL2, TMEM132C, VCAN, APOE, PYH1R, CNTN2, SPOCK3). Finally, we verified the 12 differential genes by ELISA and found that the expression trends of five biomarkers (DSCAM, CNTN2, IL1RAPL2, CA10, GPC5) were correlated with brain aging.

Conclusion

Five differentially expressed proteins (DSCAM, CNTN2, IL1RAPL2, CA10, GPC5) can be considered as one of the screening indicators of brain aging, and provide a scientific basis for clinical diagnosis and intervention.

Neuroprotective effects of the PPARβ/δ antagonist GSK0660 in in vitro and in vivo Parkinson's disease models

BACKGROUND

The underlying mechanism of Parkinson's disease are still unidentified, but excitotoxicity, oxidative stress, and neuroinflammation are considered key actors. Proliferator activated receptors (PPARs) are transcription factors involved in the control of numerous pathways. Specifically, PPARβ/δ is recognized as an oxidative stress sensor, and we have previously reported that it plays a detrimental role in neurodegeneration.

METHODS

Basing on this concept, in this work, we tested the potential effects of a specific PPARβ/δ antagonist (GSK0660) in an in vitro model of Parkinson's disease. Specifically, live-cell imaging, gene expression, Western blot, proteasome analyses, mitochondrial and bioenergetic studies were performed. Since we obtained promising results, we tested this antagonist in a 6-hydroxydopamine hemilesioned mouse model. In the animal model, behavioral tests, histological analysis, immunofluorescence and western blot of substantia nigra and striatum upon GSK0660 were assayed.

RESULTS

Our findings suggested that PPARβ/δ antagonist has neuroprotective potential due to neurotrophic support, anti-apoptotic and anti-oxidative effects paralleled to an amelioration of mitochondria and proteasome activity. These findings are strongly supported also by the siRNA results demonstrating that by silencing PPARβ/δ a significative rescue of the dopaminergic neurons was obtained, thus indicating an involvement of PPARβ/δ in PD's pathogenesis. Interestingly, in the animal model, GSK0660 treatment confirmed neuroprotective effects observed in the in vitro studies. Neuroprotective effects were highlighted by the behavioural performance and apomorphine rotation tests amelioration and the reduction of dopaminergic neuronal loss. These data were also confirmed by imaging and western blotting, indeed, the tested compound decreased astrogliosis and activated microglia, concomitant with an upregulation of neuroprotective pathways.

CONCLUSIONS

In summary, PPARβ/δ antagonist displayed neuroprotective activities against 6-hydroxydopamine detrimental effects both in vitro and in vivo models of Parkinson's disease, suggesting that it may represent a novel therapeutic approach for this disorder.

Structural mechanisms of mitochondrial quality control mediated by PINK1 and parkin

Parkinson's disease (PD) is the second most common neurodegenerative disease and represents a looming public health crisis as the global population ages. While the etiology of the more common, idiopathic form of the disease remains unknown, the last ten years have seen a breakthrough in our understanding of the genetic forms related to two proteins that regulate a quality control system for the removal of damaged or non-functional mitochondria. Here, we review the structure of these proteins, PINK1, a protein kinase, and parkin, a ubiquitin ligase with an emphasis on the molecular mechanisms responsible for their recognition of dysfunctional mitochondria and control of the subsequent ubiquitination cascade. Recent atomic structures have revealed the basis of PINK1 substrate specificity and the conformational changes responsible for activation of PINK1 and parkin catalytic activity. Progress in understanding the molecular basis of mitochondrial quality control promises to open new avenues for therapeutic interventions in PD.

Autophagy and neurodegeneration: Unraveling the role of C9ORF72 in the regulation of autophagy and its relationship to ALS-FTD pathology

The proper functioning of the cell clearance machinery is critical for neuronal health within the central nervous system (CNS). In normal physiological conditions, the cell clearance machinery is actively involved in the elimination of misfolded and toxic proteins throughout the lifetime of an organism. The highly conserved and regulated pathway of autophagy is one of the important processes involved in preventing and neutralizing pathogenic buildup of toxic proteins that could eventually lead to the development of neurodegenerative diseases (NDs) such as Alzheimer’s disease or Amyotrophic lateral sclerosis (ALS). The most common genetic cause of ALS and frontotemporal dementia (FTD) is a hexanucleotide expansion consisting of GGGGCC (G4C2) repeats in the chromosome 9 open reading frame 72 gene (C9ORF72). These abnormally expanded repeats have been implicated in leading to three main modes of disease pathology: loss of function of the C9ORF72 protein, the generation of RNA foci, and the production of dipeptide repeat proteins (DPRs). In this review, we discuss the normal physiological role of C9ORF72 in the autophagy-lysosome pathway (ALP), and present recent research deciphering how dysfunction of the ALP synergizes with C9ORF72 haploinsufficiency, which together with the gain of toxic mechanisms involving hexanucleotide repeat expansions and DPRs, drive the disease process. This review delves further into the interactions of C9ORF72 with RAB proteins involved in endosomal/lysosomal trafficking, and their role in regulating various steps in autophagy and lysosomal pathways. Lastly, the review aims to provide a framework for further investigations of neuronal autophagy in C9ORF72-linked ALS-FTD as well as other neurodegenerative diseases.

Human mini-brains for reconstituting central nervous system disorders

Neurological disorders in the central nervous system (CNS) are progressive and irreversible diseases leading to devastating impacts on patients' life as they cause cognitive impairment, dementia, and even loss of essential body functions. The development of effective medicines curing CNS disorders is, however, one of the most ambitious challenges due to the extremely complex functions and structures of the human brain. In this regard, there are unmet needs to develop simplified but physiopathologically-relevant brain models. Recent advances in the microfluidic techniques allow multicellular culture forming miniaturized 3D human brains by aligning parts of brain regions with specific cells serving suitable functions. In this review, we overview designs and strategies of microfluidics-based human mini-brains for reconstituting CNS disorders, particularly Alzheimer's disease (AD), Parkinson's disease (PD), traumatic brain injury (TBI), vascular dementia (VD), and environmental risk factor-driven dementia (ERFD). Afterward, the applications of the mini-brains in the area of medical science are introduced in terms of the clarification of pathogenic mechanisms and identification of promising biomarkers. We also present expanded model systems ranging from the CNS to CNS-connecting organ axes to study the entry pathways of pathological risk factors into the brain. Lastly, the advantages and potential challenges of current model systems are addressed with future perspectives.

Parkin and mitochondrial signalling

Aging, toxic chemicals and changes to the cellular environment are sources of oxidative damage to mitochondria which contribute to neurodegenerative conditions including Parkinson's disease. To counteract this, cells have developed signalling mechanisms to identify and remove select proteins and unhealthy mitochondria to maintain homeostasis. Two important proteins that work in concert to control mitochondrial damage are the protein kinase PINK1 and the E3 ligase parkin. In response to oxidative stress, PINK1 phosphorylates ubiquitin present on proteins at the mitochondrial surface. This signals the translocation of parkin, accelerates further phosphorylation, and stimulates ubiquitination of outer mitochondrial membrane proteins such as Miro1/2 and Mfn1/2. The ubiquitination of these proteins is the key step needed to target them for degradation via the 26S proteasomal machinery or eliminate the entire organelle through mitophagy. This review highlights the signalling mechanisms used by PINK1 and parkin and presents several outstanding questions yet to be resolved.

Hallmarks of neurodegenerative diseases

Decades of research have identified genetic factors and biochemical pathways involved in neurodegenerative diseases (NDDs). We present evidence for the following eight hallmarks of NDD: pathological protein aggregation, synaptic and neuronal network dysfunction, aberrant proteostasis, cytoskeletal abnormalities, altered energy homeostasis, DNA and RNA defects, inflammation, and neuronal cell death. We describe the hallmarks, their biomarkers, and their interactions as a framework to study NDDs using a holistic approach. The framework can serve as a basis for defining pathogenic mechanisms, categorizing different NDDs based on their primary hallmarks, stratifying patients within a specific NDD, and designing multi-targeted, personalized therapies to effectively halt NDDs.

Translational molecular imaging and drug development in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder that primarily affects elderly people and constitutes a major source of disability worldwide. Notably, the neuropathological hallmarks of PD include nigrostriatal loss and the formation of intracellular inclusion bodies containing misfolded α-synuclein protein aggregates. Cardinal motor symptoms, which include tremor, rigidity and bradykinesia, can effectively be managed with dopaminergic therapy for years following symptom onset. Nonetheless, patients ultimately develop symptoms that no longer fully respond to dopaminergic treatment. Attempts to discover disease-modifying agents have increasingly been supported by translational molecular imaging concepts, targeting the most prominent pathological hallmark of PD, α-synuclein accumulation, as well as other molecular pathways that contribute to the pathophysiology of PD. Indeed, molecular imaging modalities such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) can be leveraged to study parkinsonism not only in animal models but also in living patients. For instance, mitochondrial dysfunction can be assessed with probes that target the mitochondrial complex I (MC-I), while nigrostriatal degeneration is typically evaluated with probes designed to non-invasively quantify dopaminergic nerve loss. In addition to dopaminergic imaging, serotonin transporter and N-methyl-D-aspartate (NMDA) receptor probes are increasingly used as research tools to better understand the complexity of neurotransmitter dysregulation in PD. Non-invasive quantification of neuroinflammatory processes is mainly conducted by targeting the translocator protein 18 kDa (TSPO) on activated microglia using established imaging agents. Despite the overwhelming involvement of the brain and brainstem, the pathophysiology of PD is not restricted to the central nervous system (CNS). In fact, PD also affects various peripheral organs such as the heart and gastrointestinal tract - primarily via autonomic dysfunction. As such, research into peripheral biomarkers has taken advantage of cardiac autonomic denervation in PD, allowing the differential diagnosis between PD and multiple system atrophy with probes that visualize sympathetic nerve terminals in the myocardium. Further, α-synuclein has recently gained attention as a potential peripheral biomarker in PD. This review discusses breakthrough discoveries that have led to the contemporary molecular concepts of PD pathophysiology and how they can be harnessed to develop effective imaging probes and therapeutic agents. Further, we will shed light on potential future trends, thereby focusing on potential novel diagnostic tracers and disease-modifying therapeutic interventions.

Parkin coregulates glutathione metabolism in adult mammalian brain

We recently discovered that the expression of PRKN, a young-onset Parkinson disease-linked gene, confers redox homeostasis. To further examine the protective effects of parkin in an oxidative stress model, we first combined the loss of prkn with Sod2 haploinsufficiency in mice. Although adult prkn-/-//Sod2± animals did not develop dopamine cell loss in the S. nigra, they had more reactive oxidative species and a higher concentration of carbonylated proteins in the brain; bi-genic mice also showed a trend for more nitrotyrosinated proteins. Because these redox changes were seen in the cytosol rather than mitochondria, we next explored the thiol network in the context of PRKN expression. We detected a parkin deficiency-associated increase in the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) in murine brain, PRKN-linked human cortex and several cell models. This shift resulted from enhanced recycling of GSSG back to GSH via upregulated glutathione reductase activity; it also correlated with altered activities of redox-sensitive enzymes in mitochondria isolated from mouse brain (e.g., aconitase-2; creatine kinase). Intriguingly, human parkin itself showed glutathione-recycling activity in vitro and in cells: For each GSSG dipeptide encountered, parkin regenerated one GSH molecule and was S-glutathionylated by the other (GSSG + P-SH [Formula: see text] GSH + P-S-SG), including at cysteines 59, 95 and 377. Moreover, parkin's S-glutathionylation was reversible by glutaredoxin activity. In summary, we found that PRKN gene expression contributes to the network of available thiols in the cell, including by parkin's participation in glutathione recycling, which involves a reversible, posttranslational modification at select cysteines. Further, parkin's impact on redox homeostasis in the cytosol can affect enzyme activities elsewhere, such as in mitochondria. We posit that antioxidant functions of parkin may explain many of its previously described, protective effects in vertebrates and invertebrates that are unrelated to E3 ligase activity.

Concomitant protein pathogenesis in Parkinson's disease and perspective mechanisms

Comorbidity is a common phenotype in Parkinson's disease (PD). Patients with PD not only have motor deficit symptoms, but also have heterogeneous non-motor symptoms, including cognitive impairment and emotional changes, which are the featured symptoms observed in patients with Alzheimer's disease (AD), frontotemporal dementia (FTD) and cerebrovascular disease. Moreover, autopsy studies have also confirmed the concomitant protein pathogenesis, such as the co-existences of α-synuclein, amyloid-β and tau pathologies in PD and AD patients' brains. Here, we briefly summarize the recent reports regarding the comorbidity issues in PD from both clinical observations and neuropathological evidences. Furthermore, we provide some discussion about the perspective potential mechanisms underlying such comorbidity phenomenon, with a focus on PD and related neurodegenerative diseases.

The Role of Voltage-Gated Calcium Channels in Basal Ganglia Neurodegenerative Disorders

Calcium (Ca2+) plays a central role in regulating many cellular processes and influences cell survival. Several mechanisms can disrupt Ca2+ homeostasis to trigger cell death, including oxidative stress, mitochondrial damage, excitotoxicity, neuroinflammation, autophagy, and apoptosis. Voltage-gated Ca2+ channels (VGCCs) act as the main source of Ca2+ entry into electrically excitable cells, such as neurons, and they are also expressed in glial cells such as astrocytes and oligodendrocytes. The dysregulation of VGCC activity has been reported in both Parkinson's disease (PD) and Huntington's (HD). PD and HD are progressive neurodegenerative disorders (NDs) of the basal ganglia characterized by motor impairment as well as cognitive and psychiatric dysfunctions. This review will examine the putative role of neuronal VGCCs in the pathogenesis and treatment of central movement disorders, focusing on PD and HD. The link between basal ganglia disorders and VGCC physiology will provide a framework for understanding the neurodegenerative processes that occur in PD and HD, as well as a possible path towards identifying new therapeutic targets for the treatment of these debilitating disorders.

Genetic Study of Early Onset Parkinson's Disease in Cyprus

Parkinson's Disease (PD) is a multifactorial neurodegenerative disease characterized by motor and non-motor symptoms. The etiology of PD remains unclear. However, several studies have demonstrated the interplay of genetic, epigenetic, and environmental factors in PD. Early-onset PD (EOPD) is a subgroup of PD diagnosed between the ages of 21 and 50. Population genetic studies have demonstrated great genetic variability amongst EOPD patients. Hence, this study aimed to obtain a genetic landscape of EOPD in the Cypriot population. Greek-Cypriot EOPD patients (n = 48) were screened for variants in the six most common EOPD-associated genes (PINK1, PRKN, FBXO7, SNCA, PLA2G6, and DJ-1). This included DNA sequencing and Multiplex ligation-dependent probe amplification (MLPA). One previously described frameshift variant in PINK1 (NM_032409.3:c.889del) was detected in five patients (10.4%)-the largest number to be detected to date. Copy number variations in the PRKN gene were identified in one homozygous and 3 compound heterozygous patients (8.3%). To date, the pathogenic variants identified in this study have explained the PD phenotype for 18.8% of the EOPD cases. The results of this study may contribute to the genetic screening of EOPD in Cyprus.

Ingenuity pathway analysis of α-synuclein predicts potential signaling pathways, network molecules, biological functions, and its role in neurological diseases

Despite the knowledge that mutation, multiplication, and anomalous function of α-synuclein cause progressive transformation of α-synuclein monomers into toxic amyloid fibrils in neurodegenerative diseases, the understanding of canonical signaling, interaction network molecules, biological functions, and role of α-synuclein remains ambiguous. The evolution of artificial intelligence and Bioinformatics tools have enabled us to analyze a vast pool of data to draw meaningful conclusions about the events occurring in complex biological systems. We have taken the advantage of such a Bioinformatics tool, ingenuity pathway analysis (IPA) to decipher the signaling pathways, interactome, biological functions, and role of α-synuclein. IPA of the α-synuclein NCBI gene dataset revealed neuroinflammation, Huntington’s disease, TREM1, phagosome maturation, and sirtuin signaling as the key canonical signaling pathways. IPA further revealed Parkinson’s disease (PD), sumoylation, and SNARE signaling pathways specific to the toxicity of α-synuclein. A frequency distribution analysis of α-synuclein-associated genes from the NCBI dataset that appeared in the predicted canonical pathways revealed that NFKB1 was the most populated gene across the predicted pathways followed by FOS, PRKCD, TNF, GSK3B, CDC42, IL6, MTOR, PLCB1, and IL1B. Overlapping of the predicted top-five canonical signaling pathways and the α-synuclein NCBI gene dataset divulged that neuroinflammation signaling was the most overlapped pathway, while NFKB1, TNF, and CASP1 were the most shared molecules among the pathways. The major diseases associated with α-synuclein were predicted to be neurological diseases, organismal injury and abnormalities, skeletal and muscular disorders, psychological disorders, and hereditary disorders. The molecule activity predictor (MAP) analysis of the principal interaction network of α-synuclein gene SNCA revealed that SNCA directly interacts with APP, CLU, and NEDD4, whereas it indirectly communicates with CALCA and SOD1. Besides, IPA also predicted amyloid plaque forming APP, cytokines/inflammatory mediators IL1B, TNF, MIF, PTGS2, TP53, and CCL2, and kinases of MAPK family Mek, ERK, and P38 MAPK as the top upstream regulators of α-synuclein signaling cascades. Taken together, the first IPA analysis of α-synuclein predicted PD as the key toxicity pathway, neurodegeneration as the major pathological outcome, and inflammatory mediators as the critical interacting partners of α-synuclein.

Pathogenesis of α-Synuclein in Parkinson's Disease: From a Neuron-Glia Crosstalk Perspective

Parkinson's disease (PD) is a progressive neurodegenerative disorder. The classical behavioral defects of PD patients involve motor symptoms such as bradykinesia, tremor, and rigidity, as well as non-motor symptoms such as anosmia, depression, and cognitive impairment. Pathologically, the progressive loss of dopaminergic (DA) neurons in the substantia nigra (SN) and the accumulation of α-synuclein (α-syn)-composed Lewy bodies (LBs) and Lewy neurites (LNs) are key hallmarks. Glia are more than mere bystanders that simply support neurons, they actively contribute to almost every aspect of neuronal development and function; glial dysregulation has been implicated in a series of neurodegenerative diseases including PD. Importantly, amounting evidence has added glial activation and neuroinflammation as new features of PD onset and progression. Thus, gaining a better understanding of glia, especially neuron-glia crosstalk, will not only provide insight into brain physiology events but also advance our knowledge of PD pathologies. This review addresses the current understanding of α-syn pathogenesis in PD, with a focus on neuron-glia crosstalk. Particularly, the transmission of α-syn between neurons and glia, α-syn-induced glial activation, and feedbacks of glial activation on DA neuron degeneration are thoroughly discussed. In addition, α-syn aggregation, iron deposition, and glial activation in regulating DA neuron ferroptosis in PD are covered. Lastly, we summarize the preclinical and clinical therapies, especially targeting glia, in PD treatments.

MS-based glycomics: An analytical tool to assess nervous system diseases

Neurological diseases affect millions of peopleochemistryorldwide and are continuously increasing due to the globe's aging population. Such diseases affect the nervous system and are characterized by a progressive decline in brain function and progressive cognitive impairment, decreasing the quality of life for those with the disease as well as for their families and loved ones. The increased burden of nervous system diseases demands a deeper insight into the biomolecular mechanisms at work during disease development in order to improve clinical diagnosis and drug design. Recently, evidence has related glycosylation to nervous system diseases. Glycosylation is a vital post-translational modification that mediates many biological functions, and aberrant glycosylation has been associated with a variety of diseases. Thus, the investigation of glycosylation in neurological diseases could provide novel biomarkers and information for disease pathology. During the last decades, many techniques have been developed for facilitation of reliable and efficient glycomic analysis. Among these, mass spectrometry (MS) is considered the most powerful tool for glycan analysis due to its high resolution, high sensitivity, and the ability to acquire adequate structural information for glycan identification. Along with MS, a variety of approaches and strategies are employed to enhance the MS-based identification and quantitation of glycans in neurological samples. Here, we review the advanced glycomic tools used in nervous system disease studies, including separation techniques prior to MS, fragmentation techniques in MS, and corresponding strategies. The glycan markers in common clinical nervous system diseases discovered by utilizing such MS-based glycomic tools are also summarized and discussed.

Human Induced Pluripotent Stem Cell Phenotyping and Preclinical Modeling of Familial Parkinson's Disease

Parkinson's disease (PD) is primarily idiopathic and a highly heterogenous neurodegenerative disease with patients experiencing a wide array of motor and non-motor symptoms. A major challenge for understanding susceptibility to PD is to determine the genetic and environmental factors that influence the mechanisms underlying the variations in disease-associated traits. The pathological hallmark of PD is the degeneration of dopaminergic neurons in the substantia nigra pars compacta region of the brain and post-mortem Lewy pathology, which leads to the loss of projecting axons innervating the striatum and to impaired motor and cognitive functions. While the cause of PD is still largely unknown, genome-wide association studies provide evidence that numerous polymorphic variants in various genes contribute to sporadic PD, and 10 to 15% of all cases are linked to some form of hereditary mutations, either autosomal dominant or recessive. Among the most common mutations observed in PD patients are in the genes LRRK2, SNCA, GBA1, PINK1, PRKN, and PARK7/DJ-1. In this review, we cover these PD-related mutations, the use of induced pluripotent stem cells as a disease in a dish model, and genetic animal models to better understand the diversity in the pathogenesis and long-term outcomes seen in PD patients.

The RING finger protein family in health and disease

Ubiquitination is a highly conserved and fundamental posttranslational modification (PTM) in all eukaryotes regulating thousands of proteins. The RING (really interesting new gene) finger (RNF) protein, containing the RING domain, exerts E3 ubiquitin ligase that mediates the covalent attachment of ubiquitin (Ub) to target proteins. Multiple reviews have summarized the critical roles of the tripartite-motif (TRIM) protein family, a subgroup of RNF proteins, in various diseases, including cancer, inflammatory, infectious, and neuropsychiatric disorders. Except for TRIMs, since numerous studies over the past decades have delineated that other RNF proteins also exert widespread involvement in several diseases, their importance should not be underestimated. This review summarizes the potential contribution of dysregulated RNF proteins, except for TRIMs, to the pathogenesis of some diseases, including cancer, autoimmune diseases, and neurodegenerative disorder. Since viral infection is broadly involved in the induction and development of those diseases, this manuscript also highlights the regulatory roles of RNF proteins, excluding TRIMs, in the antiviral immune responses. In addition, we further discuss the potential intervention strategies targeting other RNF proteins for the prevention and therapeutics of those human diseases.

Distribution and Localization of Mahogunin Ring Finger 1 in the Mouse Central Nervous System

Mahogunin ring finger 1 (MGRN1), an E3 ubiquitin, is involved in several physiological and neuropathological processes. Although mgrn1 mRNA is widely distributed in the central nervous system (CNS), detailed information on its cellular and subcellular localization is lacking and its physiological role remains unclear. In this study, we aimed to determine the distribution of MGRN1 in the mouse CNS using a newly produced antibody against MGRN1. We found that the MGRN1 protein was expressed in most neuronal cell bodies. An intense MGRN1 expression was also observed in the neuropil of the gray matter in different regions of the CNS, including the main olfactory bulb, cerebral cortex, caudate, putamen, thalamic nuclei, hypothalamic nuclei, medial eminence, superior colliculus, hippocampus, dentate gyrus, and spinal cord. Contrastingly, no MGRN1 expression was observed in glial cells. Double fluorescence and immunoelectron microscopic analyses revealed the intracellular distribution of MGRN1 in pre-synapses and near the outer membrane of the mitochondria in neurons. These findings indicate that MGRN1 is more widely expressed throughout the CNS; additionally, the intracellular expression of MGRN1 suggests that it may play an important role in synaptic and mitochondrial functions.

α-mangostin derivative 4e as a PDE4 inhibitor promote proteasomal degradation of alpha-synuclein in Parkinson's disease models through PKA activation

BACKGROUND

Parkinson's disease (PD) is a multi-factorial neurodegenerative disease affecting motor function of patients. The hall markers of PD are dopaminergic neuron loss in the midbrain and the presence of intra-neuronal inclusion bodies mainly composed of aggregation-prone protein alpha-synuclein (α-syn). Ubiquitin-proteasome system (UPS) is a multi-step reaction process responsible for more than 80% intracellular protein degradation. Impairment of UPS function has been observed in the brain tissue of PD patients. PDE4 inhibitors have been shown to activate cAMP-PKA pathway and promote UPS activity in Alzheimer's disease model. α-mangostin is a natural xanthonoid with broad biological activities, such as antioxidant, antimicrobial and antitumour activities. Structure-based optimizations based on α-mangostin produced a potent PDE4 inhibitor, 4e. Herein, we studied whether 4e could promote proteasomal degradation of α-syn in Parkinson's disease models through PKA activation.

METHODS

cAMP Assay was conducted to quantify cAMP levels in samples. Model UPS substrates (Ub-G76V-GFP and Ub-R-GFP) were used to monitor UPS-dependent activity. Proteasome activity was investigated by short peptide substrate, Suc-LLVY-AMC, cleavage of which by the proteasome increases fluorescence sensitivity. Tet-on WT, A30P, and A53T α-syn-inducible PC12 cells and primary mouse cortical neurons from A53T transgenic mice were used to evaluate the effect of 4e against α-syn in vitro. Heterozygous A53T transgenic mice were employed to assess the effect of 4e on the clearance of α-syn in vivo, and further validations were applied by western blotting and immunohistochemistry.

RESULTS

Taken together, α-mangostin derivative 4e, a PDE4 inhibitor, efficiently activated the cAMP/PKA pathway in neuronal cells, and promoted UPS activity as evidenced by enhanced degradation of UPS substrate Ub-G76V-GFP and Ub-R-GFP, as well as elevated proteasomal enzyme activity. Interestingly, 4e dramatically accelerated degradation of inducibly-expressed WT and mutant α-syn in PC12 cells, in a UPS dependent manner. Besides, 4e consistently activated PKA in primary neuron and A53T mice brain, restored UPS inhibition and alleviated α-syn accumulation in the A53T mice brain.

CONCLUSIONS

4e is a natural compound derived highly potent PDE4 inhibitor. We revealed its potential effect in promoting UPS activity to degrade pathogenic proteins associated with PD.