Characterization of a novel protein regulated during the critical period for song learning in the zebra finch

The role of α-synuclein in exocytosis

The pathogenesis of degeneration in Parkinson's disease (PD) remains poorly understood but multiple lines of evidence have converged on the presynaptic protein α-synuclein (αsyn). αSyn has been shown to regulate several cellular processes, however, its normal function remains poorly understood. In this review, we will specifically focus on its role in exocytosis.

Relationship among α‑synuclein, aging and inflammation in Parkinson's disease (Review)

Parkinson's disease (PD) is a common neurodegenerative pathology whose major clinical symptoms are movement disorders. The main pathological characteristics of PD are the selective death of dopaminergic (DA) neurons in the pars compacta of the substantia nigra and the presence of Lewy bodies containing α-synuclein (α-Syn) within these neurons. PD is associated with numerous risk factors, including environmental factors, genetic mutations and aging. In many cases, the complex interplay of numerous risk factors leads to the onset of PD. The mutated α-Syn gene, which expresses pathologicalα-Syn protein, can cause PD. Another important feature of PD is neuroinflammation, which is conducive to neuronal death. α-Syn is able to interact with certain cell types in the brain, including through phagocytosis and degradation of α-Syn by glial cells, activation of inflammatory pathways by α-Syn in glial cells, transmission of α-Syn between glial cells and neurons, and interactions between peripheral immune cells and α-Syn. In addition to the aforementioned risk factors, PD may also be associated with aging, as the prevalence of PD increases with advancing age. The aging process impairs the cellular clearance mechanism, which leads to chronic inflammation and the accumulation of intracellular α-Syn, which results in DA neuronal death. In the present review, the age-associated α-Syn pathogenicity and the interactions between α-Syn and certain types of cells within the brain are discussed to facilitate understanding of the mechanisms of PD pathogenesis, which may potentially provide insight for the future clinical treatment of PD.

The C-terminus of α-Synuclein Regulates its Dynamic Cellular Internalization by Neurexin 1β

The aggregation of the disordered neuronal protein, α-Synuclein (αS), is the primary pathological feature of Parkinson's disease. Current hypotheses favor cell-to-cell spread of αS species as underlying disease progression, driving interest in identifying the molecular species and cellular processes involved in cellular internalization of αS. Prior work from our lab identified the chemically specific interaction between αS and the presynaptic adhesion protein neurexin-1β (N1β) to be capable of driving cellular internalization of both monomer and aggregated forms of αS. Here we explore the physical basis of N1β-driven internalization of αS. Specifically, we show that spontaneous internalization of αS by SH-SY5Y and HEK293 cells expressing N1β requires essentially all of the membrane-binding domain of αS; αS constructs truncated beyond residue 90 bind to N1β in the plasma membrane of HEK cells, but are not internalized. Interestingly, before internalization, αS and N1β codiffuse rapidly in the plasma membrane. αS constructs that are not internalized show very slow mobility themselves, as well as slow N1β diffusion. Finally, we find that truncated αS is capable of blocking internalization of full-length αS. Our results draw attention to the potential therapeutic value of blocking αS-N1β interactions.

The Bidirectional Interplay of α-Synuclein with Lipids in the Central Nervous System and Its Implications for the Pathogenesis of Parkinson's Disease

The alteration and aggregation of alpha-synuclein (α-syn) play a crucial role in neurodegenerative diseases collectively termed as synucleinopathies, including Parkinson's disease (PD). The bidirectional interaction of α-syn with lipids and biomembranes impacts not only α-syn aggregation but also lipid homeostasis. Indeed, lipid composition and metabolism are severely perturbed in PD. One explanation for lipid-associated alterations may involve structural changes in α-syn, caused, for example, by missense mutations in the lipid-binding region of α-syn as well as post-translational modifications such as phosphorylation, acetylation, nitration, ubiquitination, truncation, glycosylation, and glycation. Notably, different strategies targeting the α-syn-lipid interaction have been identified and are able to reduce α-syn pathology. These approaches include the modulation of post-translational modifications aiming to reduce the aggregation of α-syn and modify its binding properties to lipid membranes. Furthermore, targeting enzymes involved in various steps of lipid metabolism and exploring the neuroprotective potential of lipids themselves have emerged as novel therapeutic approaches. Taken together, this review focuses on the bidirectional crosstalk of α-syn and lipids and how alterations of this interaction affect PD and thereby open a window for therapeutic interventions.

Emergence of the Synucleins

This study establishes the origin and evolutionary history of the synuclein genes. A combination of phylogenetic analyses of the synucleins from twenty-two model species, characterization of local synteny similarities among humans, sharks and lampreys, and statistical comparisons among lamprey and human chromosomes, provides conclusive evidence for the current diversity of synuclein genes arising from the whole-genome duplications (WGDs) that occurred in vertebrates. An ancestral synuclein gene was duplicated in a first WGD, predating the diversification of all living vertebrates. The two resulting genes are still present in agnathan vertebrates. The second WGD, specific to the gnathostome lineage, led to the emergence of the three classical synuclein genes, SNCA, SNCB and SNCG, which are present in all jawed vertebrate lineages. Additional WGDs have added new genes in both agnathans and gnathostomes, while some gene losses have occurred in particular species. The emergence of synucleins through WGDs prevented these genes from experiencing dosage effects, thus avoiding the potential detrimental effects associated with individual duplications of genes that encode proteins prone to aggregation. Additional insights into the structural and functional features of synucleins are gained through the analysis of the highly divergent synuclein proteins present in chondrichthyans and agnathans.

Knocking out alpha-synuclein in melanoma cells downregulates L1CAM and decreases motility

The Parkinson's disease (PD) associated protein, alpha-synuclein (α-syn/SNCA), is highly expressed in aggressive melanomas. The goal of this study was to reveal possible mechanism(s) of α-syn involvement in melanoma pathogenesis. Herein, we asked whether α-syn modulates the expression of the pro-oncogenic adhesion molecules L1CAM and N-cadherin. We used two human melanoma cell lines (SK-MEL-28, SK-MEL-29), SNCA-knockout (KO) clones, and two human SH-SY5Y neuroblastoma cell lines. In the melanoma lines, loss of α-syn expression resulted in significant decreases in the expression of L1CAM and N-cadherin and concomitant significant decreases in motility. On average, there was a 75% reduction in motility in the four SNCA-KOs tested compared to control cells. Strikingly, comparing neuroblastoma SH-SY5Y cells that have no detectable α-syn to SH-SY5Y cells that stably express α-syn (SH/+αS), we found that expressing α-syn increased L1CAM and single-cell motility by 54% and 597%, respectively. The reduction in L1CAM level in SNCA-KO clones was not due to a transcriptional effect, rather we found that L1CAM is more efficiently degraded in the lysosome in SNCA-KO clones than in control cells. We propose that α-syn is pro-survival to melanoma (and possibly neuroblastoma) because it promotes the intracellular trafficking of L1CAM to the plasma membrane.

Enhancing the neuroprotective effect of squid outer skin astaxanthin against rotenone-induced neurotoxicity in in-vitro model for Parkinson's disease

Rotenone is a widely used organic pesticide that induces neurotoxicity via inhibition of mitochondrial complex I and oxidative stress actions for the most of dopaminergic neurons as that occurring in Parkinsonism disease (PD). Astaxanthin (ASX) is a natural pigment (carotenoids) and a potent therapeutic compound due to its antioxidant and anti-inflammatory properties. The commercially important cephalopod Doryteuthis singhalensis is widely distributed in tropical and subtropical waters in World Ocean. D. singhalensis is an important source of astaxanthin that contains valuable biological active compounds with many valuable pharmacological effects. The present study evaluated the effect of astaxanthin in preventing rotenone-induced toxicity of SK-N-SH human neuroblastoma cells in an in vitro model of experimental Parkinsonism. The results revealed the strongly significant antioxidant capability of extracted squid astaxanthin in 1,1- diphenyl- 2- picrylhydrazyl (DPPH) radical scavenging activity. In addition, astaxanthin treatment based on dose dependent manner significantly attenuated rotenone induced cytotoxicity, mitochondrial dysfunction and oxidative stress in SKN- SH cells. It is concluded that the marine squid derived astaxanthin could be used as a potential neuroprotector against rotenone induced toxicity due to its antioxidant, and anti-apoptotic properties. Consequently, it could be a supportive remedy for neurodegenerative diseases like Parkinson's disease.

Redox Imbalance in Neurological Disorders in Adults and Children

Oxygen is a central molecule for numerous metabolic and cytophysiological processes, and, indeed, its imbalance can lead to numerous pathological consequences. In the human body, the brain is an aerobic organ and for this reason, it is very sensitive to oxygen equilibrium. The consequences of oxygen imbalance are especially devastating when occurring in this organ. Indeed, oxygen imbalance can lead to hypoxia, hyperoxia, protein misfolding, mitochondria dysfunction, alterations in heme metabolism and neuroinflammation. Consequently, these dysfunctions can cause numerous neurological alterations, both in the pediatric life and in the adult ages. These disorders share numerous common pathways, most of which are consequent to redox imbalance. In this review, we will focus on the dysfunctions present in neurodegenerative disorders (specifically Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis) and pediatric neurological disorders (X-adrenoleukodystrophies, spinal muscular atrophy, mucopolysaccharidoses and Pelizaeus-Merzbacher Disease), highlighting their underlining dysfunction in redox and identifying potential therapeutic strategies.

Rotenone-Induced Model of Parkinson's Disease: Beyond Mitochondrial Complex I Inhibition

Parkinson's disease (PD) is usually diagnosed through motor symptoms that make the patient incapable of carrying out daily activities; however, numerous non-motor symptoms include olfactory disturbances, constipation, depression, excessive daytime sleepiness, and rapid eye movement at sleep; they begin years before motor symptoms. Therefore, several experimental models have been studied to reproduce several PD functional and neurochemical characteristics; however, no model mimics all the PD motor and non-motor symptoms to date, which becomes a limitation for PD study. It has become increasingly relevant to find ways to study the disease from its slowly progressive nature. The experimental models most frequently used to reproduce PD are based on administering toxic chemical compounds, which aim to imitate dopamine deficiency. The most used toxic compounds to model PD have been 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA), which inhibit the complex I of the electron transport chain but have some limitations. Another toxic compound that has drawn attention recently is rotenone, the classical inhibitor of mitochondrial complex I. Rotenone triggers the progressive death of dopaminergic neurons and α-synuclein inclusions formation in rats; also, rotenone induces microtubule destabilization. This review presents information about the experimental model of PD induced by rotenone, emphasizing its molecular characteristics beyond the inhibition of mitochondrial complex I.

The effects of KTKEGV repeat motif and intervening ATVA sequence on α-synuclein solubility and assembly

Alpha-synuclein (αS), the key protein in Parkinson's disease, is typically described as an intrinsically disordered protein. Consistent with this notion, several context-dependent folding states may coexist in neurons. Unfolded soluble monomers, helical monomers at membranes and helical multimers (soluble or at membranes) have all been reported and may be in an equilibrium with each other. We previously found that αS can be stabilized in its membrane-associated monomeric form by genetically increasing the hydrophobicity of the membrane-embedded half of the αS helix. αS amphipathic helix formation at membranes is governed by up to nine 11-amino acid repeats with the core motif KTKEGV. However, this repeat is only imperfectly conserved; for example, it consists of KAKEGV in repeat #1, KTKEQV in repeat #5, and AVVTGV in the poorly conserved repeat #6. Here we explored the effect of perfecting the αS core repeat to nine times KTKEGV ("9KV") and found by sequential protein extraction that this engineered mutant accumulates in the cytosolic phase of neural cells. Intact-cell cross-linking trapped a part of the cytosolic portion at multimeric positions (30, 60, 80, 100 kDa). Thus, compared to wild-type αS, αS 9KV seems less prone to populating the membrane-associated monomeric form. Removing the "ATVA" intervening amino-acid sequence between repeats 4 and 5 slightly increased cytosolic localization while adding "ATVA" in between all repeats 1-8 caused αS to be trapped as a monomer in membrane fractions. Our results contribute to an ongoing debate on the dynamic structure of αS, highlighting that wild-type αS is unlikely to be fully multimeric/monomeric or fully cytosolic/membrane-associated in cells, but protein engineering can create αS variants that preferentially adopt a certain state. Overall, the imperfect nature of the KTKEGV repeat motifs and the presence of ATVA in between repeats 4 and 5 seem to prevent a strong cytosolic localization of αS and thus play a major role in the protein's ability to dynamically populate cytosolic vs. membrane-associated and monomeric vs. multimeric states.

α-Synuclein in synaptic function and dysfunction

α-Synuclein is a neuronal protein that is enriched in presynaptic terminals. Under physiological conditions, it binds to synaptic vesicle membranes and functions in neurotransmitter release, although the molecular details remain unclear, and it is controversial whether α-synuclein inhibits or facilitates neurotransmitter release. Pathologically, in synucleinopathies including Parkinson's disease (PD), α-synuclein forms aggregates that recruit monomeric α-synuclein and spread throughout the brain, which triggers neuronal dysfunction at molecular, cellular, and organ levels. Here, we present an overview of the effects of α-synuclein on SNARE-complex assembly, neurotransmitter release, and synaptic vesicle pool homeostasis, and discuss how the observed divergent effects of α-synuclein on neurotransmitter release can be reconciled. We also discuss how gain-of-function versus loss-of-function of α-synuclein may contribute to pathogenesis in synucleinopathies.

Mitochondrial Toxicant-Induced Neuronal Apoptosis in Parkinson's Disease: What We Know so Far

Parkinson's disease (PD) is one of the most common progressive neurodegenerative diseases caused by the loss of dopamine-producing neuronal cells in the region of substantia nigra pars compacta of the brain. During biological aging, neuronal cells slowly undergo degeneration, but the rate of cell death increases tremendously under some pathological conditions, leading to irreversible neurodegenerative diseases. By the time symptoms of PD usually appear, more than 50 to 60% of neuronal cells have already been destroyed. PD symptoms often start with tremors, followed by slow movement, stiffness, and postural imbalance. The etiology of PD is still unknown; however, besides genetics, several factors contribute to neurodegenerative disease, including exposure to pesticides, environmental chemicals, solvents, and heavy metals. Postmortem brain tissues of patients with PD show mitochondrial abnormalities, including dysfunction of the electron transport chain. Most chemicals present in our environment have been shown to target the mitochondria; remarkably, patients with PD show a mild deficiency in NADH dehydrogenase activity, signifying a possible link between PD and mitochondrial dysfunction. Inhibition of electron transport complexes generates free radicals that further attack the macromolecules leading to neuropathological conditions. Apart from that, oxidative stress also causes neuroinflammation-mediated neurodegeneration due to the activation of microglial cells. However, the mechanism that causes mitochondrial dysfunction, especially the electron transport chain, in the pathogenesis of PD remains unclear. This review discusses the recent updates and explains the possible mechanisms of mitochondrial toxicant-induced neuroinflammation and neurodegeneration in PD.

The Role of α-Synuclein in SNARE-mediated Synaptic Vesicle Fusion

Neuronal communication depends on exquisitely regulated membrane fusion between synaptic vesicles and presynaptic neurons, which results in neurotransmitter release in precisely timed patterns. Presynaptic dysfunctions are known to occur prior to the onset of neurodegenerative diseases, including Parkinson's disease. Synaptic accumulation of α-synuclein (α-Syn) oligomers has been implicated in the pathway leading to such outcomes. α-Syn oligomers exert aberrant effects on presynaptic fusion machinery through their interactions with synaptic vesicles and proteins. Here, we summarize in vitro bulk and single-vesicle assays for investigating the functions of α-Syn monomers and oligomers in synaptic vesicle fusion and then discuss the current understanding of the roles of α-Syn monomers and oligomers in synaptic vesicle fusion. Finally, we suggest a new therapeutic avenue specifically targeting the mechanisms of α-Syn oligomer toxicity rather than the oligomer itself.

Liquid-liquid Phase Separation of α-Synuclein: A New Mechanistic Insight for α-Synuclein Aggregation Associated with Parkinson's Disease Pathogenesis

Aberrant aggregation of the misfolded presynaptic protein, α-Synuclein (α-Syn) into Lewy body (LB) and Lewy neuritis (LN) is a major pathological hallmark of Parkinson's disease (PD) and other synucleinopathies. Numerous studies have suggested that prefibrillar and fibrillar species of the misfolded α-Syn aggregates are responsible for cell death in PD pathogenesis. However, the precise molecular events during α-Syn aggregation, especially in the early stages, remain elusive. Emerging evidence has demonstrated that liquid-liquid phase separation (LLPS) of α-Syn occurs in the nucleation step of α-Syn aggregation, which offers an alternate non-canonical aggregation pathway in the crowded microenvironment. The liquid-like α-Syn droplets gradually undergo an irreversible liquid-to-solid phase transition into amyloid-like hydrogel entrapping oligomers and fibrils. This new mechanism of α-Syn LLPS and gel formation might represent the molecular basis of cellular toxicity associated with PD. This review aims to demonstrate the recent development of α-Syn LLPS, the underlying mechanism along with the microscopic events of aberrant phase transition. This review further discusses how several intrinsic and extrinsic factors regulate the thermodynamics and kinetics of α-Syn LLPS and co-LLPS with other proteins, which might explain the pathophysiology of α-Syn in various neurodegenerative diseases.

Functional and Pathological Effects of α-Synuclein on Synaptic SNARE Complexes

α-Synuclein is an abundant protein at the neuronal synapse that has been implicated in Parkinson's disease for over 25 years and characterizes the hallmark pathology of a group of neurodegenerative diseases now known as the synucleinopathies. Physiologically, α-synuclein exists in an equilibrium between a synaptic vesicle membrane-bound α-helical multimer and a cytosolic largely unstructured monomer. Through its membrane-bound state, α-synuclein functions in neurotransmitter release by modulating several steps in the synaptic vesicle cycle, including synaptic vesicle clustering and docking, SNARE complex assembly, and homeostasis of synaptic vesicle pools. These functions have been ascribed to α-synuclein's interactions with the synaptic vesicle SNARE protein VAMP2/synaptobrevin-2, the synaptic vesicle-attached synapsins, and the synaptic vesicle membrane itself. How α-synuclein affects these processes, and whether disease is due to loss-of-function or gain-of-toxic-function of α-synuclein remains unclear. In this review, we provide an in-depth summary of the existing literature, discuss possible reasons for the discrepancies in the field, and propose a working model that reconciles the findings in the literature.

Cross-talk between α-synuclein and the microtubule cytoskeleton in neurodegeneration

Looking at the puzzle that depicts the molecular determinants in neurodegeneration, many pieces are lacking and multiple interconnections among key proteins and intracellular pathways still remain unclear. Here we focus on the concerted action of α-synuclein and the microtubule cytoskeleton, whose interplay, indeed, is emerging but remains largely unexplored in both its physiology and pathology. α-Synuclein is a key protein involved in neurodegeneration, underlying those diseases termed synucleinopathies. Its propensity to interact with other proteins and structures renders the identification of neuronal death trigger extremely difficult. Conversely, the unbalance of microtubule cytoskeleton in terms of structure, dynamics and function is emerging as a point of convergence in neurodegeneration. Interestingly, α-synuclein and microtubules have been shown to interact and mediate cross-talks with other intracellular structures. This is supported by an increasing amount of evidence ranging from their direct interaction to the engagement of in-common partners and culminating with their respective impact on microtubule-dependent neuronal functions. Last, but not least, it is becoming even more clear that α-synuclein and tubulin work synergically towards pathological aggregation, ultimately resulting in neurodegeneration. In this respect, we supply a novel perspective towards the understanding of α-synuclein biology and, most importantly, of the link between α-synuclein with microtubule cytoskeleton and its impact for neurodegeneration and future development of novel therapeutic strategies.

An Insight into the Molecular Mechanism of Mitochondrial Toxicant-induced Neuronal Apoptosis in Parkinson's Disease

Parkinson's disease (PD) is one of the most common progressive neurodegenerative disorders affecting approximately 1% of the world's population at the age of 50 and above. Majority of PD cases are sporadic and show symptoms after the age of 60 and above. At that time, most of the dopaminergic neurons in the region of substantia nigra pars compacta have been degenerated. Although in past decades, discoveries of genetic mutations linked to PD have significantly impacted our current understanding of the pathogenesis of this devastating disorder, it is likely that the environment also plays a critical role in the etiology of sporadic PD. Recent epidemiological and experimental studies indicate that exposure to environmental agents, including a number of agricultural and industrial chemicals, may contribute to the pathogenesis of several neurodegenerative disorders, including PD. Furthermore, there is a strong correlation between mitochondrial dysfunction and several forms of neurodegenerative disorders, including Alzheimer's disease (AD), Huntington's disease (HD), Amyotrophic lateral sclerosis (ALS) and PD. Interestingly, substantia nigra of patients with PD has been shown to have a mild deficiency in mitochondrial respiratory electron transport chain NADH dehydrogenase (Complex I) activity. This review discusses the role of mitochondrial toxicants in the selective degeneration of dopaminergic neurons targeting the electron transport system that leads to Parkinsonism.

The Role of Membrane Affinity and Binding Modes in Alpha-Synuclein Regulation of Vesicle Release and Trafficking

Alpha-synuclein is a presynaptic protein linked to Parkinson's disease with a poorly characterized physiological role in regulating the synaptic vesicle cycle. Using RBL-2H3 cells as a model system, we earlier reported that wild-type alpha-synuclein can act as both an inhibitor and a potentiator of stimulated exocytosis in a concentration-dependent manner. The inhibitory function is constitutive and depends on membrane binding by the helix-2 region of the lipid-binding domain, while potentiation becomes apparent only at high concentrations. Using structural and functional characterization of conformationally selective mutants via a combination of spectroscopic and cellular assays, we show here that binding affinity for isolated vesicles similar in size to synaptic vesicles is a primary determinant of alpha-synuclein-mediated potentiation of vesicle release. Inhibition of release is sensitive to changes in the region linking the helix-1 and helix-2 regions of the N-terminal lipid-binding domain and may require some degree of coupling between these regions. Potentiation of release likely occurs as a result of alpha-synuclein interactions with undocked vesicles isolated away from the active zone in internal pools. Consistent with this, we observe that alpha-synuclein can disperse vesicles from in vitro clusters organized by condensates of the presynaptic protein synapsin-1.

A penetratin-derived peptide reduces the membrane permeabilization and cell toxicity of α-synuclein oligomers

Parkinson's disease is a neurodegenerative movement disorder associated with the intracellular aggregation of α-synuclein (α-syn). Cytotoxicity is mainly associated with the oligomeric species (αSOs) formed at early stages in α-syn aggregation. Consequently, there is an intense focus on the discovery of novel inhibitors such as peptides to inhibit oligomer formation and toxicity. Here, using peptide arrays, we identified nine peptides with high specificity and affinity for αSOs. Of these, peptides p194, p235, and p249 diverted α-syn aggregation from fibrils to amorphous aggregates with reduced β-structures and increased random coil content. However, they did not reduce αSO cytotoxicity and permeabilization of large anionic unilamellar vesicles. In parallel, we identified a non-self-aggregating peptide (p216), derived from the cell-penetrating peptide penetratin, which showed 12-fold higher binding affinity to αSOs than to α-syn monomers (K_d^app 2.7 and 31.2 μM, respectively). p216 reduced αSOs-induced large anionic unilamellar vesicle membrane permeability at 10^-1 to 10^-3 mg/ml by almost 100%, was not toxic to SH-SY5Y cells, and reduced αSOs cytotoxicity by about 20%. We conclude that p216 is a promising starting point from which to develop peptides targeting toxic αSOs in Parkinson's disease.

ATP13A2 Declines Zinc-Induced Accumulation of α-Synuclein in a Parkinson’s Disease Model

Parkinson’s disease (PD) is characterized by the presence of Lewy bodies caused by α-synuclein. The imbalance of zinc homeostasis is a major cause of PD, promoting α-synuclein accumulation. ATP13A2, a transporter found in acidic vesicles, plays an important role in Zn²⁺ homeostasis and is highly expressed in Lewy bodies in PD-surviving neurons. ATP13A2 is involved in the transport of zinc ions in lysosomes and exosomes and inhibits the aggregation of α-synuclein. However, the potential mechanism underlying the regulation of zinc homeostasis and α-synuclein accumulation by ATP13A2 remains unexplored. We used α-synuclein-GFP transgenic mice and HEK293 α-synuclein-DsRed cell line as models. The spatial exploration behavior of mice was significantly reduced, and phosphorylation levels of α-synuclein increased upon high Zn²⁺ treatment. High Zn²⁺ also inhibited the autophagy pathway by reducing LAMP2a levels and changing the expression of LC3 and P62, by reducing mitochondrial membrane potential and increasing the expression of cytochrom C, and by activating the ERK/P38 apoptosis signaling pathway, ultimately leading to increased caspase 3 levels. These protein changes were reversed after ATP13A2 overexpression, whereas ATP13A2 knockout exacerbated α-synuclein phosphorylation levels. These results suggest that ATP13A2 may have a protective effect on Zn²⁺-induced abnormal aggregation of α-synuclein, lysosomal dysfunction, and apoptosis.

Molecular and Functional Interactions of Alpha-Synuclein with Rab3a

Alpha-synuclein (a-Syn) is a presynaptic protein, the misfolding of which is associated with Parkinson’s disease. Rab GTPases are small guanine nucleotide binding proteins that play key roles in vesicle trafficking and have been associated with a-Syn function and dysfunction. a-Syn is enriched on synaptic vesicles, where it has been reported to interact with GTP-bound Rab3a, a master regulator of synaptic vesicle trafficking. a-Syn is known to bind weakly to Rab8a in solution via a positively charged patch, but the physiological implications of such interactions have not been explored. Here, we investigate direct interactions between a-Syn and Rab3a in solution and on lipid membranes using NMR spectroscopy. We find that the C terminus of a-Syn interacts with Rab3a in a manner similar to its previously reported interaction with Rab8a. While weak in solution, we demonstrate that this interaction becomes stronger when the proteins are bound to a membrane surface. The Rab3a binding site for a-Syn is similar to the surface that contacts the Rab3a effector rabphilin-3A, which modulates the enzymatic activity of Rab3a. Accordingly, we show that a-Syn inhibits GTP hydrolysis by Rab3a and that inhibition is more potent on the membrane surface, suggesting that their interaction may be functionally relevant. Finally, we show that phosphorylation of a-Syn residue Ser 129, a modification associated with Parkinson’s disease pathology, enhances its interactions with Rab3a and increases its ability to inhibit Rab3a GTP hydrolysis. These results represent the first observation of a functional role for synuclein-Rab interactions and for a-Syn Ser 129 phosphorylation.

Pathological characterization of a novel mouse model expressing the PD-linked CHCHD2-T61I mutation

Coiled-coil-helix-coiled-coil-helix domain containing 2 (CHCHD2) is a mitochondrial protein that plays important roles in cristae structure, oxidative phosphorylation and apoptosis. Multiple mutations in CHCHD2 have been associated with Lewy body disorders (LBDs), such as Parkinson's disease (PD) and dementia with Lewy bodies, with the CHCHD2-T61I mutation being the most widely studied. However, at present, only CHCHD2 knockout or CHCHD2/CHCHD10 double knockout mouse models have been investigated. They do not recapitulate the pathology seen in patients with CHCHD2 mutations. We generated the first transgenic mouse model expressing the human PD-linked CHCHD2-T61I mutation driven by the mPrP promoter. We show that CHCHD2-T61I Tg mice exhibit perinuclear mitochondrial aggregates, neuroinflammation, and have impaired long-term synaptic plasticity associated with synaptic dysfunction. Dopaminergic neurodegeneration, a hallmark of PD, is also observed along with α-synuclein pathology. Significant motor dysfunction is seen with no changes in learning and memory at 1 year of age. A minor proportion of the CHCHD2-T61I Tg mice (~10%) show a severe motor phenotype consistent with human Pisa Syndrome, an atypical PD phenotype. Unbiased proteomics analysis reveals surprising increases in many insoluble proteins predominantly originating from mitochondria and perturbing multiple canonical biological pathways as assessed by ingenuity pathway analysis, including neurodegenerative disease-associated proteins such as tau, cofilin, SOD1 and DJ-1. Overall, CHCHD2-T61I Tg mice exhibit pathological and motor changes associated with LBDs, indicating that this model successfully captures phenotypes seen in human LBD patients with CHCHD2 mutations and demonstrates changes in neurodegenerative disease-associated proteins, which delineates relevant pathological pathways for further investigation.

NMDA and AMPA Receptors at Synapses: Novel Targets for Tau and α-Synuclein Proteinopathies

A prominent feature of neurodegenerative diseases is synaptic dysfunction and spine loss as early signs of neurodegeneration. In this context, accumulation of misfolded proteins has been identified as one of the most common causes driving synaptic toxicity at excitatory glutamatergic synapses. In particular, a great effort has been placed on dissecting the interplay between the toxic deposition of misfolded proteins and synaptic defects, looking for a possible causal relationship between them. Several studies have demonstrated that misfolded proteins could directly exert negative effects on synaptic compartments, altering either the function or the composition of pre- and post-synaptic receptors. In this review, we focused on the physiopathological role of tau and α-synuclein at the level of postsynaptic glutamate receptors. Tau is a microtubule-associated protein mainly expressed by central nervous system neurons where it exerts several physiological functions. In some cases, it undergoes aberrant post-translational modifications, including hyperphosphorylation, leading to loss of function and toxic aggregate formation. Similarly, aggregated species of the presynaptic protein α-synuclein play a key role in synucleinopathies, a group of neurological conditions that includes Parkinson’s disease. Here, we discussed how tau and α-synuclein target the postsynaptic compartment of excitatory synapses and, specifically, AMPA- and NMDA-type glutamate receptors. Notably, recent studies have reported their direct functional interactions with these receptors, which in turn could contribute to the impaired glutamatergic transmission observed in many neurodegenerative diseases.

The role of NURR1 in metabolic abnormalities of Parkinson's disease

A constant metabolism and energy supply are crucial to all organs, particularly the brain. Age-dependent neurodegenerative diseases, such as Parkinson’s disease (PD), are associated with alterations in cellular metabolism. These changes have been recognized as a novel hot topic that may provide new insights to help identify risk in the pre-symptomatic phase of the disease, understand disease pathogenesis, track disease progression, and determine critical endpoints. Nuclear receptor-related factor 1 (NURR1), an orphan member of the nuclear receptor superfamily of transcription factors, is a major risk factor in the pathogenesis of PD, and changes in NURR1 expression can have a detrimental effect on cellular metabolism. In this review, we discuss recent evidence that suggests a vital role of NURR1 in dopaminergic (DAergic) neuron development and the pathogenesis of PD. The association between NURR1 and cellular metabolic abnormalities and its implications for PD therapy have been further highlighted.

The Role of AlphαSynuclein in Mouse Models of Acute, Inflammatory and Neuropathic Pain

(1) AlphαSynuclein (αSyn) is a synaptic protein which is expressed in the nervous system and has been linked to neurodegenerative diseases, in particular Parkinson’s disease (PD). Symptoms of PD are mainly due to overexpression and aggregation of αSyn and include pain. However, the interconnection of αSyn and pain has not been clarified so far. (2) We investigated the potential effects of a αSyn knock-out on the nociceptive behaviour in mouse models of acute, inflammatory and neuropathic pain. Furthermore, we assessed the impact of αSyn deletion on pain-related cellular and molecular mechanisms in the spinal cord in these models. (3) Our results showed a reduction of acute cold nociception in αSyn knock-out mice while responses to acute heat and mechanical noxious stimulation were similar in wild type and knock-out mice. Inflammatory nociception was not affected by αSyn knock-out which is also mirrored by unaltered inflammatory gene expression. In contrast, in the SNI model of neuropathic pain, αSyn knock-out mice showed decreased mechanical allodynia as compared to wild type mice. This effect was associated with reduced proinflammatory mechanisms and suppressed activation of MAP kinase signalling in the spinal cord while endogenous antinociceptive mechanisms are not inhibited. (4) Our data indicate that αSyn plays a role in neuropathy and its inhibition might be useful to ameliorate pain symptoms after nerve injury.

Unique seeding profiles and prion-like propagation of synucleinopathies are highly dependent on the host in human α-synuclein transgenic mice

α-synuclein (αSyn) is an intrinsically disordered protein which can undergo structural transformations, resulting in the formation of stable, insoluble fibrils. αSyn amyloid-type nucleation can be induced by misfolded 'seeds' serving as a conformational template, tantamount to the prion-like mechanism. Accumulation of αSyn inclusions is a key feature of dementia with Lewy bodies (DLB) and multiple system atrophy (MSA), and are found as additional pathology in Alzheimer's disease (AD) such as AD with amygdala predominant Lewy bodies (AD/ALB). While these disorders accumulate the same pathological protein, they exhibit heterogeneity in clinical and histological features; however, the mechanism(s) underlying this variability remains elusive. Accruing data from human autopsy studies, animal inoculation modeling, and in vitro characterization experiments, have lent credence to the hypothesis that conformational polymorphism of the αSyn amyloid-type fibril structure results in distinct "strains" with categorical infectivity traits. Herein, we directly compare the seeding abilities and outcome of human brain lysates from these diseases, as well as recombinant preformed human αSyn fibrils by the intracerebral inoculation of transgenic mice overexpressing either human wild-type αSyn or human αSyn with the familial A53T mutation. Our study has revealed that the initiating inoculum heavily dictates the phenotypic and pathological course of disease. Interestingly, we have also established relevant host-dependent distinctions between propagation profiles, including burden and spread of inclusion pathology throughout the neuroaxis, as well as severity of neurological symptoms. These findings provide compelling evidence supporting the hypothesis that diverse prion-type conformers may explain the variability seen in synucleinopathies.

Structures of tau and α-synuclein filaments from brains of patients with neurodegenerative diseases

Intracellular accumulations and aggregates of abnormal protein, consisting of amyloid-like fibrils, are common neuropathological features of many neurodegenerative diseases. The distributions and spreading of these pathological proteins are closely correlated with clinical symptoms and progression. Recent evidence supports the idea that template-mediated amplification of amyloid-like fibrils and intracellular propagation of fibril seeds are the main mechanisms by which pathological features spread along the neural circuits in the brain. Here, we review recent developments in the structural analysis of amyloid-like fibrils from brains of patients with various types of tauopathy and alpha-synucleinopathy, focusing on cryo-electron microscopy and mass analysis, and we discuss their relevance to the mechanisms of template-mediated amplification and intracellular propagation.

Vocal changes in a zebra finch model of Parkinson's disease characterized by alpha-synuclein overexpression in the song-dedicated anterior forebrain pathway

Deterioration in the quality of a person's voice and speech is an early marker of Parkinson's disease (PD). In humans, the neural circuit that supports vocal motor control consists of a cortico-basal ganglia-thalamo-cortico loop. The basal ganglia regions, striatum and globus pallidus, in this loop play a role in modulating the acoustic features of vocal behavior such as loudness, pitch, and articulatory rate. In PD, this area is implicated in pathogenesis. In animal models of PD, the accumulation of toxic aggregates containing the neuronal protein alpha-synuclein (αsyn) in the midbrain and striatum result in limb and vocal motor impairments. It has been challenging to study vocal impairments given the lack of well-defined cortico-basal ganglia circuitry for vocalization in rodent models. Furthermore, whether deterioration of voice quality early in PD is a direct result of αsyn-induced neuropathology is not yet known. Here, we take advantage of the well-characterized vocal circuits of the adult male zebra finch songbird to experimentally target a song-dedicated pathway, the anterior forebrain pathway, using an adeno-associated virus expressing the human wild-type αsyn gene, SNCA. We found that overexpression of αsyn in this pathway coincides with higher levels of insoluble, monomeric αsyn compared to control finches. Impairments in song production were also detected along with shorter and poorer quality syllables, which are the most basic unit of song. These vocal changes are similar to the vocal abnormalities observed in individuals with PD.

Neuropathology of α-synuclein in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by progressive movement disability accompanied by non-motor symptoms. The neuropathology hallmark of PD is the loss of dopaminergic neurons predominantly in the substantia nigra pars compacta and the presence of intracellular inclusions termed Lewy bodies (LBs), which are mainly composed of α-synuclein (αSyn). Detailed staging based on the distribution and progression pattern of αSyn pathology in the postmortem brains of PD patients revealed correlation with the clinical phenotypes but not invariably. Cumulative evidence from cell and animal studies has implied that αSyn propagation contributes to the anatomical spread of αSyn pathology in the brain. Here, we recount the studies over the past two centuries on the anatomopathological foundations of PD documented. We also review studies on the structural analysis of αSyn and LBs, Braak staging of αSyn pathology, the cell-to-cell propagation of αSyn as well as αSyn fibril polymorphisms, which underlie the phenotypic differences in synucleinopathies.

Fatty acid-binding protein 7 triggers α-synuclein oligomerization in glial cells and oligodendrocytes associated with oxidative stress

We previously show that fatty acid-binding protein 3 (FABP3) triggers α-synuclein (Syn) accumulation and induces dopamine neuronal cell death in Parkinson disease mouse model. But the role of fatty acid-binding protein 7 (FABP7) in the brain remains unclear. In this study we investigated whether FABP7 was involved in synucleinopathies. We showed that FABP7 was co-localized and formed a complex with Syn in Syn-transfected U251 human glioblastoma cells, and treatment with arachidonic acid (100 M) significantly promoted FABP7-induced Syn aggregation, which was associated with cell death. We demonstrated that synthetic FABP7 ligand 6 displayed a high affinity against FABP7 with K_d value of 209 nM assessed in 8-anilinonaphthalene-1-sulfonic acid (ANS) assay; ligand 6 improved U251 cell survival via disrupting the FABP7-Syn interaction. We showed that activation of phospholipase A2 (PLA2) by psychosine (10 M) triggered oligomerization of endogenous Syn and FABP7, and induced cell death in both KG-1C human oligodendroglia cells and oligodendrocyte precursor cells (OPCs). FABP7 ligand 6 (1 M) significantly decreased Syn oligomerization and aggregation thereby prevented KG-1C and OPC cell death. This study demonstrates that FABP7 triggers α-synuclein oligomerization through oxidative stress, while FABP7 ligand 6 can inhibit FABP7-induced Syn oligomerization and aggregation, thereby rescuing glial cells and oligodendrocytes from cell death.

PARkinson's: From cellular mechanisms to potential therapeutics

Our understanding of the progression and mechanisms underlying the onset of Parkinson's disease (PD) has grown enormously in the past few decades. There is growing evidence suggesting that poly (ADP-ribose) polymerase 1 (PARP-1) hyperactivation is involved in various neurodegenerative disorders, including PD, and that poly (ADP-ribose) (PAR)-dependent cell death is responsible for neuronal loss. In this review, we discuss the contribution of PARP-1 and PAR in the pathological process of PD. We describe the potential pathways regulated by the enzyme, review clinically relevant PARP-1 inhibitors as potential disease-modifying therapeutics for PD, and outline important factors that need to be considered for repurposing PARP-1 inhibitors for use in PD.

[Role of fatty acid-binding protein 7 and novel therapeutic approach in synucleinopathies]

The synucleinopathies are neurodegenerative disease caused by abnormal accumulation of the 140-amino acid-containing protein α-synuclein (αSyn), including Parkinson's disease (PD), diffuse Lewy body dementia (DLBD), and multiple system atrophy (MSA). In patients with PD and DLBD, αSyn is misfolded in neurons, and its aggregation forms Lewy bodies (LB) and Lewy neurites (LN). On the other hand, in patients with MSA, αSyn accumulates primarily in oligodendrocytes (OLGs) and forms glial inclusion bodies (GCIs), a typical pathological feature of MSA. We recently demonstrated a making complex between αSyn and fatty acid-binding proteins (FABPs) in synucleinopathies and received wide attention. Fatty acid-binding protein 3 (FABP3) in dopamine nerves, and fatty acid-binding protein 7 (FABP7) in glial cells promoted αSyn accumulation and aggregation, respectively and caused cell death. Here, we introduced the current studies about the role of αSyn and FABP7 in MSA and novel therapeutic approach targeting for FABP7.

Brain regions susceptible to alpha-synuclein spreading

The spreading of misfolded alpha-synuclein (α-syn) protein has been observed in animal models of Parkinson's disease (PD) and other α-synucleinopathies that mimic human PD pathologies. In animal models, the spreading of α-syn has been associated with motor dysfunction and neuronal death. However, variability in both susceptible brain regions and cellular populations limits our understanding of the consequences of α-syn spreading and the development of associated therapies. Here, we have reviewed the physiological and pathological functions of α-syn and summarized the susceptible brain regions and cell types identified from human postmortem studies and exogenous α-syn injection-based animal models. We have reviewed the methods for inducing α-syn aggregation, the specific hosts, the inoculation sites, the routes of propagation, and other experimental settings that may affect the spreading pattern of α-syn, as reported in current studies. Understanding the spread of α-syn to produce a consistent PD animal model is vital for future drug discovery.

Phosphorylation of the aggregate-forming protein alpha-synuclein on serine-129 inhibits its DNA-bending properties

Alpha-synuclein (aSyn) is a vertebrate protein, normally found within the presynaptic nerve terminal and nucleus, which is known to form somatic and neuritic aggregates in certain neurodegenerative diseases. Disease-associated aggregates of aSyn are heavily phosphorylated at serine-129 (pSyn), while normal aSyn protein is not. Within the nucleus, aSyn can directly bind DNA, but the mechanism of binding and the potential modulatory roles of phosphorylation are poorly understood. Here we demonstrate using a combination of electrophoretic mobility shift assay and atomic force microscopy approaches that both aSyn and pSyn can bind DNA within the major groove, in a DNA length-dependent manner and with little specificity for DNA sequence. Our data are consistent with a model in which multiple aSyn molecules bind a single 300 base pair (bp) DNA molecule in such a way that stabilizes the DNA in a bent conformation. We propose that serine-129 phosphorylation decreases the ability of aSyn to both bind and bend DNA, as aSyn binds 304 bp circular DNA forced into a bent shape, but pSyn does not. Two aSyn paralogs, beta- and gamma-synuclein, also interact with DNA differently than aSyn, and do not stabilize similar DNA conformations. Our work suggests that reductions in aSyn's ability to bind and bend DNA induced by serine-129 phosphorylation may be important for modulating aSyn's known roles in DNA metabolism, including the regulation of transcription and DNA repair.

Lipotoxicity Downstream of α-Synuclein Imbalance: A Relevant Pathomechanism in Synucleinopathies?

Neuronal loss in Parkinson's disease and related brain diseases has been firmly linked to the abundant neuronal protein α-synuclein (αS). However, we have gained surprisingly little insight into how exactly αS exerts toxicity in these diseases. Hypotheses of proteotoxicity, disturbed vesicle trafficking, mitochondrial dysfunction and other toxicity mechanisms have been proposed, and it seems possible that a combination of different mechanisms may drive pathology. A toxicity mechanism that has caught increased attention in the recent years is αS-related lipotoxicity. Lipotoxicity typically occurs in a cell when fatty acids exceed the metabolic needs, triggering a flux into harmful pathways of non-oxidative metabolism. Genetic and experimental approaches have revealed a significant overlap between lipid storage disorders, most notably Gaucher's disease, and synucleinopathies. There is accumulating evidence for lipid aberrations causing synuclein misfolding as well as for αS excess and misfolding causing lipid aberration. Does that mean the key problem in synucleinopathies is lipotoxicity, the accumulation of harmful lipid species or alteration in lipid equilibrium? Here, we review the existing literature in an attempt to get closer to an answer.

Spiers Memorial Lecture: Analysis and de novo design of membrane-interactive peptides

Membrane-peptide interactions play critical roles in many cellular and organismic functions, including protection from infection, remodeling of membranes, signaling, and ion transport. Peptides interact with membranes in a variety of ways: some associate with membrane surfaces in either intrinsically disordered conformations or well-defined secondary structures. Peptides with sufficient hydrophobicity can also insert vertically as transmembrane monomers, and many associate further into membrane-spanning helical bundles. Indeed, some peptides progress through each of these stages in the process of forming oligomeric bundles. In each case, the structure of the peptide and the membrane represent a delicate balance between peptide-membrane and peptide-peptide interactions. We will review this literature from the perspective of several biologically important systems, including antimicrobial peptides and their mimics, α-synuclein, receptor tyrosine kinases, and ion channels. We also discuss the use of de novo design to construct models to test our understanding of the underlying principles and to provide useful leads for pharmaceutical intervention of diseases.

Friend or Foe? The Varied Faces of Homeostatic Synaptic Plasticity in Neurodegenerative Disease

Homeostatic synaptic plasticity (HSP) regulates synaptic strength both pre- and postsynaptically to ensure stability and efficient information transfer in neural networks. A number of neurological diseases have been associated with deficits in HSP, particularly diseases characterised by episodic network instability such as migraine and epilepsy. Recently, it has become apparent that HSP also plays a role in many neurodegenerative diseases. In this mini review, we present an overview of the evidence linking HSP to each of the major neurodegenerative diseases, finding that HSP changes in each disease appear to belong to one of three broad functional categories: (1) deficits in HSP at degenerating synapses that contribute to pathogenesis or progression; (2) HSP induced in a heterosynaptic or cell non-autonomous manner to support the function of networks of which the degenerating synapses or cells are part; and (3) induction of HSP within the degenerating population of synapses to preserve function and to resist the impact of synapse loss. Understanding the varied manifestations of HSP in neurodegeneration will not only aid understanding mechanisms of disease but could also inspire much-needed novel approaches to therapy.

Therapeutic Potential of α-Synuclein Evolvability for Autosomal Recessive Parkinson's Disease

The majority of Parkinson's disease (PD) is sporadic in elderly and is characterized by α-synuclein (αS) aggregation and other alterations involving mitochondria, ubiquitin-proteasome, and autophagy. The remaining are familial PD associated with gene mutations of either autosomal dominant or recessive inheritances. However, the former ones are similar to sporadic PD, and the latter ones are accompanied by impaired mitophagy during the reproductive stage. Since no radical therapies are available for PD, the objective of this paper is to discuss a mechanistic role for amyloidogenic evolvability, a putative physiological function of αS, among PD subtypes, and the potential relevance to therapy. Presumably, αS evolvability might benefit familial PD due to autosomal dominant genes and also sporadic PD during reproduction, which may manifest as neurodegenerative diseases through antagonistic pleiotropy mechanism in aging. Indeed, there are some reports describing that αS prevents apoptosis and mitochondrial alteration under the oxidative stress conditions, notwithstanding myriads of papers on the neuropathology of αS. Importantly, β-synuclein (βS), the nonamyloidogenic homologue of αS, might buffer against evolvability of αS protofibrils associated with neurotoxicity. Finally, it is intriguing to predict that increased αS evolvability through suppression of βS expression might protect against autosomal recessive PD. Collectively, further studies are warranted to better understand αS evolvability in PD pathogenesis, leading to rational therapy development.

α-Synuclein-mediated neurodegeneration in Dementia with Lewy bodies: the pathobiology of a paradox

Dementia with Lewy bodies (DLB) is epitomized by the pathognomonic manifestation of α-synuclein-laden Lewy bodies within selectively vulnerable neurons in the brain. By virtue of prion-like inheritance, the α-synuclein protein inexorably undergoes extensive conformational metamorphoses and culminate in the form of fibrillar polymorphs, instigating calamitous damage to the brain's neuropsychological networks. This epiphenomenon is nebulous, however, by lingering uncertainty over the quasi "pathogenic" behavior of α-synuclein conformers in DLB pathobiology. Despite numerous attempts, a monolithic "α-synuclein" paradigm that is able to untangle the enigma enshrouding the clinicopathological spectrum of DLB has failed to emanate. In this article, we review conceptual frameworks of α-synuclein dependent cell-autonomous and non-autonomous mechanisms that are likely to facilitate the transneuronal spread of degeneration through the neuraxis. In particular, we describe how the progressive demise of susceptible neurons may evolve from cellular derangements perpetrated by α-synuclein misfolding and aggregation. Where pertinent, we show how these bona fide mechanisms may mutually accentuate α-synuclein-mediated neurodegeneration in the DLB brain.

An Emerging Role for Phosphoinositides in the Pathophysiology of Parkinson’s Disease

Recent data support an involvement of defects in homeostasis of phosphoinositides (PIPs) in the pathophysiology of Parkinson’s disease (PD). Genetic mutations have been identified in genes encoding for PIP-regulating and PIP-interacting proteins, that are associated with familial and sporadic PD. Many of these proteins are implicated in vesicular membrane trafficking, mechanisms that were recently highlighted for their close associations with PD. PIPs are phosphorylated forms of the membrane phospholipid, phosphatidylinositol. Their composition in the vesicle’s membrane of origin, as well as membrane of destination, controls vesicular membrane trafficking. We review the converging evidence that points to the involvement of PIPs in PD. The review describes PD- and PIP-associated proteins implicated in clathrin-mediated endocytosis and autophagy, and highlights the involvement of α-synuclein in these mechanisms.

Astrocytes in Neurodegenerative Diseases: A Perspective from Tauopathy and α-Synucleinopathy

Neurodegenerative diseases are aging-associated chronic pathological conditions affecting primarily neurons in humans. Inclusion bodies containing misfolded proteins have emerged as a common pathologic feature for these diseases. In many cases, misfolded proteins produced by a neuron can be transmitted to another neuron or a non-neuronal cell, leading to the propagation of disease-associated pathology. While undergoing intercellular transmission, misfolded proteins released from donor cells can often change the physiological state of recipient cells. Accumulating evidence suggests that astrocytes are highly sensitive to neuron-originated proteotoxic insults, which convert them into an active inflammatory state. Conversely, activated astrocytes can release a plethora of factors to impact neuronal functions. This review summarizes our current understanding of the complex molecular interplays between astrocyte and neuron, emphasizing on Tau and α-synuclein (α-syn), the disease-driving proteins for Alzheimer's and Parkinson's diseases, respectively.

The subcellular arrangement of alpha-synuclein proteoforms in the Parkinson's disease brain as revealed by multicolor STED microscopy

Various post-translationally modified (PTM) proteoforms of alpha-synuclein (aSyn)-including C-terminally truncated (CTT) and Serine 129 phosphorylated (Ser129-p) aSyn-accumulate in Lewy bodies (LBs) in different regions of the Parkinson's disease (PD) brain. Insight into the distribution of these proteoforms within LBs and subcellular compartments may aid in understanding the orchestration of Lewy pathology in PD. We applied epitope-specific antibodies against CTT and Ser129-p aSyn proteoforms and different aSyn domains in immunohistochemical multiple labelings on post-mortem brain tissue from PD patients and non-neurological, aged controls, which were scanned using high-resolution 3D multicolor confocal and stimulated emission depletion (STED) microscopy. Our multiple labeling setup highlighted a consistent onion skin-type 3D architecture in mature nigral LBs in which an intricate and structured-appearing framework of Ser129-p aSyn and cytoskeletal elements encapsulates a core enriched in CTT aSyn species. By label-free CARS microscopy we found that enrichments of proteins and lipids were mainly localized to the central portion of nigral aSyn-immunopositive (aSyn+) inclusions. Outside LBs, we observed that 122CTT aSyn+ punctae localized at mitochondrial membranes in the cytoplasm of neurons in PD and control brains, suggesting a physiological role for 122CTT aSyn outside of LBs. In contrast, very limited to no Ser129-p aSyn immunoreactivity was observed in brains of non-neurological controls, while the alignment of Ser129-p aSyn in a neuronal cytoplasmic network was characteristic for brains with (incidental) LB disease. Interestingly, Ser129-p aSyn+ network profiles were not only observed in neurons containing LBs but also in neurons without LBs particularly in donors at early disease stage, pointing towards a possible subcellular pathological phenotype preceding LB formation. Together, our high-resolution and 3D multicolor microscopy observations in the post-mortem human brain provide insights into potential mechanisms underlying a regulated LB morphogenesis.

Disease-, region- and cell type specific diversity of α-synuclein carboxy terminal truncations in synucleinopathies

Synucleinopathies, including Parkinson's disease (PD), Lewy body dementia (LBD), Alzheimer's disease with amygdala restricted Lewy bodies (AD/ALB), and multiple system atrophy (MSA) comprise a spectrum of neurodegenerative disorders characterized by the presence of distinct pathological α-synuclein (αSyn) inclusions. Experimental and pathological studies support the notion that αSyn aggregates contribute to cellular demise and dysfunction with disease progression associated with a prion-like spread of αSyn aggregates via conformational templating. The initiating event(s) and factors that contribute to diverse forms of synucleinopathies remain poorly understood. A major post-translational modification of αSyn associated with pathological inclusions is a diverse array of specific truncations within the carboxy terminal region. While these modifications have been shown experimentally to induce and promote αSyn aggregation, little is known about their disease-, region- and cell type specific distribution. To this end, we generated a series of monoclonal antibodies specific to neo-epitopes in αSyn truncated after residues 103, 115, 119, 122, 125, and 129. Immunocytochemical investigations using these new tools revealed striking differences in the αSyn truncation pattern between different synucleinopathies, brain regions and specific cellular populations. In LBD, neuronal inclusions in the substantia nigra and amygdala were positive for αSyn cleaved after residues 103, 119, 122, and 125, but not 115. In contrast, in the same patients' brain αSyn cleaved at residue 115, as well as 103, 119 and 122 were abundant in the dorsal motor nucleus of the vagus. In patients with AD/ALB, these modifications were only weakly or not detected in amygdala αSyn inclusions. αSyn truncated at residues 103, 115, 119, and 125 was readily present in MSA glial cytoplasmic inclusions, but 122 cleaved αSyn was only weakly or not present. Conversely, MSA neuronal pathology in the pontine nuclei was strongly reactive to the αSyn x-122 neo-epitope but did not display any reactivity for αSyn 103 cleavage. These studies demonstrate significant disease-, region- and cell type specific differences in carboxy terminal αSyn processing associated with pathological inclusions that likely contributes to their distinct strain-like prion properties and promotes the diversity displayed in the degrees of these insidious diseases.

Alpha-Synuclein PET Tracer Development-An Overview about Current Efforts

Neurodegenerative diseases such as Parkinson’s disease (PD) are manifested by inclusion bodies of alpha-synuclein (α-syn) also called α-synucleinopathies. Detection of these inclusions is thus far only possible by histological examination of postmortem brain tissue. The possibility of non-invasively detecting α-syn will therefore provide valuable insights into the disease progression of α-synucleinopathies. In particular, α-syn imaging can quantify changes in monomeric, oligomeric, and fibrillic α-syn over time and improve early diagnosis of various α-synucleinopathies or monitor treatment progress. Positron emission tomography (PET) is a non-invasive in vivo imaging technique that can quantify target expression and drug occupancies when a suitable tracer exists. As such, novel α-syn PET tracers are highly sought after. The development of an α-syn PET tracer faces several challenges. For example, the low abundance of α-syn within the brain necessitates the development of a high-affinity ligand. Moreover, α-syn depositions are, in contrast to amyloid proteins, predominantly localized intracellularly, limiting their accessibility. Furthermore, another challenge is the ligand selectivity over structurally similar amyloids such as amyloid-beta or tau, which are often co-localized with α-syn pathology. The lack of a defined crystal structure of α-syn has also hindered rational drug and tracer design efforts. Our objective for this review is to provide a comprehensive overview of current efforts in the development of selective α-syn PET tracers.

Soluble endogenous oligomeric α-synuclein species in neurodegenerative diseases: Expression, spreading, and cross-talk

There is growing recognition in the field of neurodegenerative diseases that mixed proteinopathies are occurring at greater frequency than originally thought. This is particularly true for three amyloid proteins defining most of these neurological disorders, amyloid-beta (Aβ), tau, and alpha-synuclein (αSyn). The co-existence and often co-localization of aggregated forms of these proteins has led to the emergence of concepts positing molecular interactions and cross-seeding between Aβ, tau, and αSyn aggregates. Amongst this trio, αSyn has received particular attention in this context during recent years due to its ability to modulate Aβ and tau aggregation in vivo, to interact at a molecular level with Aβ and tau in vivo and to cross-seed tau in mice. Here we provide a comprehensive, critical, and accessible review about the expression, role and nature of endogenous soluble αSyn oligomers because of recent developments in the understanding of αSyn multimerization, misfolding, aggregation, cross-talk, spreading and cross-seeding in neurodegenerative disorders, including Parkinson's disease, dementia with Lewy bodies, multiple system atrophy, Alzheimer's disease, and Huntington's disease. We will also discuss our current understanding about the relative toxicity of endogenous αSyn oligomers in vivo and in vitro, and introduce potential opportunities to counter their deleterious effects.

Excess membrane binding of monomeric alpha-, beta-, and gamma-synuclein is invariably associated with inclusion formation and toxicity

α-Synuclein (αS) has been well-documented to play a role in human synucleinopathies such as Parkinson’s disease (PD) and dementia with Lewy bodies (DLB). First, the lesions found in PD/DLB brains—Lewy bodies and Lewy neurites—are rich in aggregated αS. Second, genetic evidence links missense mutations and increased αS expression to familial forms of PD/DLB. Third, toxicity and cellular stress can be caused by αS under certain experimental conditions. In contrast, the homologs β-synuclein (βS) and γ-synuclein (γS) are not typically found in Lewy bodies/neurites, have not been clearly linked to brain diseases and have been largely non-toxic in experimental settings. In αS, the so-called non-amyloid-β component of plaques (NAC) domain, constituting amino acids 61–95, has been identified to be critical for aggregation in vitro. This domain is partially absent in βS and only incompletely conserved in γS, which could explain why both homologs do not cause disease. However, αS in vitro aggregation and cellular toxicity have not been firmly linked experimentally, and it has been proposed that excess αS membrane binding is sufficient to induce neurotoxicity. Indeed, recent characterizations of Lewy bodies have highlighted the accumulation of lipids and membranous organelles, raising the possibility that βS and γS could also become neurotoxic if they were more prone to membrane/lipid binding. Here, we increased βS and γS membrane affinity by strategic point mutations and demonstrate that these proteins behave like membrane-associated monomers, are cytotoxic and form round cytoplasmic inclusions that can be prevented by inhibiting stearoyl-CoA desaturase.

Altered conformation of α-synuclein drives dysfunction of synaptic vesicles in a synaptosomal model of Parkinson's disease

While misfolding of alpha-synuclein (αSyn) is central to the pathogenesis of Parkinson's disease (PD), fundamental questions about its structure and function at the synapse remain unanswered. We examine synaptosomes from non-transgenic and transgenic mice expressing wild-type human αSyn, the E46K fPD-causing mutation, or an amplified form of E46K ("3K"). Synaptosomes from mice expressing the 3K mutant show reduced Ca²⁺-dependent vesicle exocytosis, altered synaptic vesicle ultrastructure, decreased SNARE complexes, and abnormal levels of certain synaptic proteins. With our intra-synaptosomal nuclear magnetic resonance (NMR) method, we reveal that WT αSyn participates in heterogeneous interactions with synaptic components dependent on endogenous αSyn and synaptosomal integrity. The 3K mutation markedly alters these interactions. The synaptic microenvironment is necessary for αSyn to reach its native conformations and establish a physiological interaction network. Its inability to populate diverse conformational ensembles likely represents an early step in αSyn dysfunction that contributes to the synaptotoxicity observed in synucleinopathies.

APOE genotype regulates pathology and disease progression in synucleinopathy

Apolipoprotein E (APOE) ε4 genotype is associated with increased risk of dementia in Parkinson's disease (PD), but the mechanism is not clear, because patients often have a mixture of α-synuclein (αSyn), amyloid-β (Aβ), and tau pathologies. APOE ε4 exacerbates brain Aβ pathology, as well as tau pathology, but it is not clear whether APOE genotype independently regulates αSyn pathology. In this study, we generated A53T αSyn transgenic mice (A53T) on Apoe knockout (A53T/EKO) or human APOE knockin backgrounds (A53T/E2, E3, and E4). At 12 months of age, A53T/E4 mice accumulated higher amounts of brainstem detergent-insoluble phosphorylated αSyn compared to A53T/EKO and A53T/E3; detergent-insoluble αSyn in A53T/E2 mice was undetectable. By immunohistochemistry, A53T/E4 mice displayed a higher burden of phosphorylated αSyn and reactive gliosis compared to A53T/E2 mice. A53T/E2 mice exhibited increased survival and improved motor performance compared to other APOE genotypes. In a complementary model of αSyn spreading, striatal injection of αSyn preformed fibrils induced greater accumulation of αSyn pathology in the substantia nigra of A53T/E4 mice compared to A53T/E2 and A53T/EKO mice. In two separate cohorts of human patients with PD, APOE ε4/ε4 individuals showed the fastest rate of cognitive decline over time. Our results demonstrate that APOE genotype directly regulates αSyn pathology independent of its established effects on Aβ and tau, corroborate the finding that APOE ε4 exacerbates pathology, and suggest that APOE ε2 may protect against αSyn aggregation and neurodegeneration in synucleinopathies.