Cooperative inhibition of SNARE-mediated vesicle fusion by α-synuclein monomers and oligomers

SynPull: An advanced method for studying neurodegeneration-related aggregates in synaptosomes using super-resolution microscopy

Synaptic dysfunction is a primary hallmark of both Alzheimer's and Parkinson's disease, leading to cognitive and behavioral decline. While alpha-synuclein, beta-amyloid, and tau are involved in the physiological functioning of synapses, their pathological aggregation has been linked to synaptopathology. The methodology for studying the small-soluble protein aggregates formed by these proteins is limited. Here we describe SynPull, a method combining single-molecule pull-down, super-resolution microscopy, and advanced computational analyses to characterize the protein aggregates in human and mouse synaptosomes. We show that AT8-positive tau aggregates are the predominant aggregate type in synaptosomes from postmortem Alzheimer's disease brain, although the aggregate size does not change in disease. Meanwhile, the relatively smaller amount of alpha-synuclein and beta-amyloid aggregates found in the synapses are larger than the extra-synaptic ones. Collectively, these results show the utility of SynPull to study pathological aggregates in neurodegeneration, elucidating the disease mechanisms causing synaptic dysfunction.

Direct evidence for ultrastructures of the α-synuclein-associated synaptic vesicle pool in presynaptic terminals

SNCA/PARK1 encodes α-synuclein, which is associated with familial Parkinson's disease. Despite its abundance in presynaptic terminals, the aggregation mechanism of α-synuclein and its relationship with Parkinson's disease have not yet been elucidated. Moreover, the ultrastructures of α-synuclein localization sites in neuronal presynaptic terminals remain unclear. Therefore, we herein generated transgenic mice expressing human α-synuclein tagged with mKate2 (hSNCA-mKate2 mice). These mice exhibited normal growth and fertility and had no motor dysfunction relative to their wild-type littermates, even at one year old. α-Synuclein-mKate2 accumulated in presynaptic terminals, particularly between Purkinje cells in the cerebellum and neurons in cerebellar nuclei. α-Synuclein-mKate2 was associated with the presynaptic marker, synaptophysin. In-resin CLEM and immunoelectron or electron microscopy revealed that α-synuclein-mKate2 localized on the surface of synaptic vesicles that were tightly arranged and assembled to form large synaptic pools in the cerebellum with negligible effects on the active zone. These results suggest that α-synuclein-associated ultrastructures in the presynaptic terminals of hSNCA-mKate2 mice reflect the structures of α-synuclein-assembled synaptic vesicle pools, and the size of vesicle pools increased. This transgenic mouse model will be a valuable tool for studying α-synuclein-associated synaptic vesicle pools.

Interplay of α-Synuclein Oligomers and Endoplasmic Reticulum Stress in Parkinson'S Disease: Insights into Cellular Dysfunctions

Oligomeric forms of α-synuclein (α-syn) are critical in the formation of α-synuclein fibrils, exhibiting neurotoxic properties that are pivotal in the pathogenesis of Parkinson's disease (PD). A salient feature of this pathology is the disruption of the protein folding capacity of the endoplasmic reticulum (ER), leading to a perturbation in the ER's protein quality control mechanisms. The accumulation of unfolded or misfolded proteins instigates ER stress. However, the onset of ER stress and the consequent activation of the Unfolded Protein Response (UPR) and Endoplasmic Reticulum-Associated Degradation (ERAD) pathways do not merely culminate in apoptosis when they fail to restore cellular homeostasis. More critically, this condition initiates a cascade of reactions involving ER-related structures and organelles, resulting in multifaceted cellular damage and, potentially, a feedback loop that precipitates neuroinflammation. In this review, we elucidate the interplay between UPR and ERAD, as well as the intricate crosstalk among the ER and other organelles such as mitochondria, lysosomes, and the Golgi apparatus, underscoring their roles in the neurodegenerative process.

The role of voltage-gated calcium channels in the pathogenesis of Parkinson's disease

Aim: Voltage-gated calcium (CaV) channels play an essential role in maintaining calcium homeostasis and regulating numerous physiological processes in neurons. Therefore, dysregulation of calcium signaling is relevant in many neurological disorders, including Parkinson's disease (PD). This review aims to introduce the role of CaV channels in PD and discuss some novel aspects of channel regulation and its impact on the molecular pathophysiology of the disease.

Methods: an exhaustive search of the literature in the field was carried out using the PubMed database of The National Center for Biotechnology Information. Systematic searches were performed from the initial date of publication to May 2022.

Results: Although α-synuclein aggregates are the main feature of PD, L-type calcium (CaV1) channels seem to play an essential role in the pathogenesis of PD. Changes in the functional expression of CaV1.3 channels alter Calcium homeostasis and contribute to the degeneration of dopaminergic neurons. Furthermore, recent studies suggest that CaV channel trafficking towards the cell membrane depends on the activity of the ubiquitin-proteasome system (UPS). In PD, there is an increase in the expression of L-type channels associated with a decrease in the expression of Parkin, an E3 enzyme of the UPS. Therefore, a link between Parkin and CaV channels could play a fundamental role in the pathogenesis of PD and, as such, could be a potentially attractive target for therapeutic intervention.

Conclusion: The study of alterations in the functional expression of CaV channels will provide a framework to understand better the neurodegenerative processes that occur in PD and a possible path toward identifying new therapeutic targets to treat this condition.

Cell biology of Parkinson's disease: Mechanisms of synaptic, lysosomal, and mitochondrial dysfunction

Parkinson's disease (PD) is a growing cause of disability worldwide and there is a critical need for the development of disease-modifying therapies to slow or stop disease progression. Recent advances in characterizing the genetics of PD have expanded our understanding of the cell biology of this disorder. Mitochondrial oxidative stress, defects in synaptic function, and impaired lysosomal activity have been shown to be linked in PD, resulting in a pathogenic feedback cycle involving the accumulation of toxic oxidized dopamine and alpha-synuclein. In this review, we will highlight recent data on a subset of PD-linked genes which have key roles in these pathways and the pathogenic cycle. We will furthermore discuss findings highlighting the importance of dynamic mitochondria-lysosome contact sites that mediate direct inter-organelle cross-talk in the pathogenesis of PD and related disorders.

Early Effects of Alpha-Synuclein Depletion by Pan-Neuronal Inactivation of Encoding Gene on Electroencephalogram Coherence between Different Brain Regions in Mice

Inactivation of the Snca gene in young mice by chronic injections of tamoxifen (TAM), a selective estrogen receptor modifier, has been shown to decrease the level of alpha-synuclein, a key peptide in the pathogenesis of Parkinson's disease. In young mice, different time courses of the effect were observed in different brain areas, meaning associated disturbances in the intracerebral relations, namely in brain function after TAM-induced synucleinopathy.

METHODS

We analyzed electroencephalogram (EEG) coherence ("functional connectivity") between the cortex (MC), putamen (Pt), and dopamine-producing brain regions (ventral tegmental area, VTA, and substantia nigra, SN) in two groups of two-month-old male mice. We compared EEG coherences in the conditional knockout Sncaflox/flox mice with those in their genetic background (C57Bl6J) one, two, and three months after chronic (for five days) intraperitoneal injections of TAM or the vehicle (corn oil). The EEG coherences in the TAM-treated group were compared with those in the alpha-synuclein knockout mice.

RESULTS

A significant suppression of EEG coherence in the TAM-treated mice versus the vehicle group was observed in all inter-structural relations, with the exception of MC-VTA at one and three months and VTA-SN at two months after the injections. Suppressive changes in EEG coherence were observed in the alpha-synuclein knockout mice as well; the changes were similar to those in TAM-treated mice three months after treatment.

CONCLUSION

our data demonstrate a combined time-dependent suppressive effect induced by TAM on intracerebral EEG coherence.

α-Synuclein oligomers and fibrils: partners in crime in synucleinopathies

The misfolding and aggregation of α-synuclein is the general hallmark of a group of devastating neurodegenerative pathologies referred to as synucleinopathies, such as Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. In such conditions, a range of different misfolded aggregates, including oligomers, protofibrils, and fibrils, are present both in neurons and glial cells. Growing experimental evidence supports the proposition that soluble oligomeric assemblies, formed during the early phases of the aggregation process, are the major culprits of neuronal toxicity; at the same time, fibrillar conformers appear to be the most efficient at propagating among interconnected neurons, thus contributing to the spreading of α-synuclein pathology. Moreover, α-synuclein fibrils have been recently reported to release soluble and highly toxic oligomeric species, responsible for an immediate dysfunction in the recipient neurons. In this review, we discuss the current knowledge about the plethora of mechanisms of cellular dysfunction caused by α-synuclein oligomers and fibrils, both contributing to neurodegeneration in synucleinopathies.

Could the Urease of the Gut Bacterium Proteus mirabilis Play a Role in the Altered Gut-Brain Talk Associated with Parkinson's Disease?

Intestinal dysbiosis seems to play a role in neurodegenerative pathologies. Parkinson's disease (PD) patients have an altered gut microbiota. Moreover, mice treated orally with the gut microbe Proteus mirabilis developed Parkinson's-like symptoms. Here, the possible involvement of P. mirabilis urease (PMU) and its B subunit (PmUreβ) in the pathogenesis of PD was assessed. Purified proteins were given to mice intraperitoneally (20 μg/animal/day) for one week. Behavioral tests were conducted, and brain homogenates of the treated animals were subjected to immunoassays. After treatment with PMU, the levels of TNF-α and IL-1β were measured in Caco2 cells and cellular permeability was assayed in Hek 293. The proteins were incubated in vitro with α-synuclein and examined via transmission electron microscopy. Our results showed that PMU treatment induced depressive-like behavior in mice. No motor deficits were observed. The brain homogenates had an increased content of caspase-9, while the levels of α-synuclein and tyrosine hydroxylase decreased. PMU increased the pro-inflammatory cytokines and altered the cellular permeability in cultured cells. The urease, but not the PmUreβ, altered the morphology of α-synuclein aggregates in vitro, forming fragmented aggregates. We concluded that PMU promotes pro-inflammatory effects in cultured cells. In vivo, PMU induces neuroinflammation and a depressive-like phenotype compatible with the first stages of PD development.

The Interplay between α-Synuclein and Microglia in α-Synucleinopathies

Synucleinopathies are a set of devastating neurodegenerative diseases that share a pathologic accumulation of the protein α-synuclein (α-syn). This accumulation causes neuronal death resulting in irreversible dementia, deteriorating motor symptoms, and devastating cognitive decline. While the etiology of these conditions remains largely unknown, microglia, the resident immune cells of the central nervous system (CNS), have been consistently implicated in the pathogenesis of synucleinopathies. Microglia are generally believed to be neuroprotective in the early stages of α-syn accumulation and contribute to further neurodegeneration in chronic disease states. While the molecular mechanisms by which microglia achieve this role are still being investigated, here we highlight the major findings to date. In this review, we describe how structural varieties of inherently disordered α-syn result in varied microglial receptor-mediated interactions. We also summarize which microglial receptors enable cellular recognition and uptake of α-syn. Lastly, we review the downstream effects of α-syn processing within microglia, including spread to other brain regions resulting in neuroinflammation and neurodegeneration in chronic disease states. Understanding the mechanism of microglial interactions with α-syn is vital to conceptualizing molecular targets for novel therapeutic interventions. In addition, given the significant diversity in the pathophysiology of synucleinopathies, such molecular interactions are vital in gauging all potential pathways of neurodegeneration in the disease state.

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.

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.

Sec22b is a critical and nonredundant regulator of plasma cell maintenance

Despite the essential role of plasma cells in health and disease, the cellular mechanisms controlling their survival and secretory capacity are still poorly understood. Here, we identified the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) Sec22b as a unique and critical regulator of plasma cell maintenance and function. In the absence of Sec22b, plasma cells were hardly detectable and serum antibody titers were dramatically reduced. Accordingly, Sec22b-deficient mice fail to mount a protective immune response. At the mechanistic level, we demonstrated that Sec22b contributes to efficient antibody secretion and is a central regulator of plasma cell maintenance through the regulation of their transcriptional identity and of the morphology of the endoplasmic reticulum and mitochondria. Altogether, our results unveil an essential and nonredundant role for Sec22b as a regulator of plasma cell fitness and of the humoral immune response.

α-Synuclein Disrupts Vesicle Fusion by Two Mutant-Specific Mechanisms

Synaptic accumulation of α-synuclein (α-Syn) oligomers and their interactions with VAMP2 have been reported to be the basis of synaptic dysfunction in Parkinson's disease (PD). α-Syn mutants associated with familial PD have also been known to be capable of interacting with VAMP2, but the exact mechanisms resulting from those interactions to eventual synaptic dysfunction are still unclear. Here, we investigate the effect of α-Syn mutant oligomers comprising A30P, E46K, and A53T on VAMP2-embedded vesicles. Specifically, A30P and A53T oligomers cluster vesicles in the presence of VAMP2, which is a shared mechanism with wild type α-Syn oligomers induced by dopamine. On the other hand, E46K oligomers reduce the membrane mobility of the planar bilayers, as revealed by single-particle tracking, and permeabilize the membranes in the presence of VAMP2. In the absence of VAMP2 interactions, E46K oligomers enlarge vesicles by fusing with one another. Our results clearly demonstrate that α-Syn mutant oligomers have aberrant effects on VAMP2-embedded vesicles and the disruption types are distinct depending on the mutant types. This work may provide one of the possible clues to explain the α-Syn mutant-type dependent pathological heterogeneity of familial PD.

SNARE Modulators and SNARE Mimetic Peptides

The soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE) proteins play a central role in most forms of intracellular membrane trafficking, a key process that allows for membrane and biocargo shuffling between multiple compartments within the cell and extracellular environment. The structural organization of SNARE proteins is relatively simple, with several intrinsically disordered and folded elements (e.g., SNARE motif, N-terminal domain, transmembrane region) that interact with other SNAREs, SNARE-regulating proteins and biological membranes. In this review, we discuss recent advances in the development of functional peptides that can modify SNARE-binding interfaces and modulate SNARE function. The ability of the relatively short SNARE motif to assemble spontaneously into stable coiled coil tetrahelical bundles has inspired the development of reduced SNARE-mimetic systems that use peptides for biological membrane fusion and for making large supramolecular protein complexes. We evaluate two such systems, based on peptide-nucleic acids (PNAs) and coiled coil peptides. We also review how the self-assembly of SNARE motifs can be exploited to drive on-demand assembly of complex re-engineered polypeptides.

The Effect of Aggregated Alpha Synuclein on Synaptic and Axonal Proteins in Parkinson’s Disease—A Systematic Review

α-synuclein is a core component of Lewy bodies, one of the pathological hallmarks of Parkinson’s disease. Aggregated α-synuclein can impair both synaptic functioning and axonal transport. However, understanding the pathological role that α-synuclein plays at a cellular level is complicated as existing findings are multifaceted and dependent on the mutation, the species, and the quantity of the protein that is involved. This systematic review aims to stratify the research findings to develop a more comprehensive understanding of the role of aggregated α-synuclein on synaptic and axonal proteins in Parkinson’s disease models. A literature search of the PubMed, Scopus, and Web of Science databases was conducted and a total of 39 studies were included for analysis. The review provides evidence for the dysregulation or redistribution of synaptic and axonal proteins due to α-synuclein toxicity. However, due to the high quantity of variables that were used in the research investigations, it was challenging to ascertain exactly what effect α-synuclein has on the expression of the proteins. A more standardized experimental approach regarding the variables that are employed in future studies is crucial so that existing literature can be consolidated. New research involving aggregated α-synuclein at the synapse and regarding axonal transport could be advantageous in guiding new treatment solutions.

Alpha-Synuclein and Its Role in Melanocytes

Pigmentation is an important process in skin physiology and skin diseases and presumably also plays a role in Parkinson’s disease (PD). In PD, alpha-Synuclein (aSyn) has been shown to be involved in the pigmentation of neurons. The presynaptic protein is intensively investigated for its pathological role in PD, but its physiological function remains unknown. We hypothesized that aSyn is both involved in melanocytic differentiation and melanosome trafficking processes. We detected a strong expression of aSyn in human epidermal melanocytes (NHEMs) and observed its regulation in melanocytic differentiation via the microphthalmia-associated transcription factor (MITF), a central regulator of differentiation. Moreover, we investigated its role in pigmentation by performing siRNA experiments but found no effect on the total melanin content. We discovered a localization of aSyn to melanosomes, and further analysis of aSyn knockdown revealed an important role in melanocytic morphology and a reduction in melanosome release. Additionally, we found a reduction of transferred melanosomes in co-culture experiments of melanocytes and keratinocytes but no complete inhibition of melanosome transmission. In summary, this study highlights a novel physiological role of aSyn in melanocytic morphology and its so far unknown function in the pigment secretion in melanocytes.

Comparative Analysis of Antigenic Strength and In vivo Serum Antibodies Concentration of Tetanus Toxoid Vaccine Adsorbed in Pakistan

Clostridium tetani produce tetanospasmin, a potent exotoxin; that causes tetanus or lockjaw disease. Scientists developed an anti-tetanus toxoid to protect the body from the spasm's neurotoxic effect. In Pakistan recently, 478 cases of neonatal tetanus were reported. The study was carried out at The National Control Laboratory for Biologicals Islamabad, aiming to decipher the effectiveness of the most frequently used tetanus toxoid vaccine adsorbed in Pakistan in comparison to standard reference vaccine having earlier known consistent values. The vaccines included domestic public sector, domestic private sector, imported private sector I, and imported private sector II. The triplicate experiments on purebred Swiss albino mice were performed by immunizing with Tetanus toxoid and then tested parallel with standard reference vaccine. Various analytical tests were performed on the test organism that included flocculation test/identity test, antibody quantification using enzyme-linked immunosorbent assay (ELISA), potency test, abnormal toxicity test, osmolality, pH test, liquid sub-visible particle test, and sterility test. Results of all the vaccines were compared in comparison with the standard reference vaccine. Absorbances of test vaccines were recorded at the lowest dilution by ELISA. The domestic private sector, imported private sector I, imported private sector II and standard reference vaccine were flocculated at mean dilution (Mean: 0.24, 95% CI: 0.1903–0.2897), and the domestic public sector was flocculated at mean dilution (Mean: 0.23, 95% CI: 0.2052–0.2548). All the products were found within the normal ranges where it was concluded that the maximum average titer of 2.81 was observed at dilution 10^−1.6, indicating that these vaccines were adequate/suitable for the prevention of tetanus.

Phosphorylation of endogenous α-synuclein induced by extracellular seeds initiates at the pre-synaptic region and spreads to the cell body

Accumulation of phosphorylated α-synuclein aggregates has been implicated in several diseases, such as Parkinson's disease (PD) and dementia with Lewy bodies (DLB), and is thought to spread in a prion-like manner. Elucidating the mechanisms of prion-like transmission of α-synuclein is important for the development of therapies for these diseases, but little is known about the details. Here, we injected α-synuclein fibrils into the brains of wild-type mice and examined the early phase of the induction of phosphorylated α-synuclein accumulation. We found that phosphorylated α-synuclein appeared within a few days after the intracerebral injection. It was observed initially in presynaptic regions and subsequently extended its localization to axons and cell bodies.　These results suggest that extracellular α-synuclein fibrils are taken up into the presynaptic region and seed-dependently convert the endogenous normal α-synuclein that is abundant there to an abnormal phosphorylated form, which is then transported through the axon to the cell body.

Vitamin D Inhibits the Early Aggregation of α-Synuclein and Modulates Exocytosis Revealed by Electrochemical Measurements

Alpha-synuclein (α-Syn) localizes at presynaptic terminal and modulates synaptic functions. Increasing evidence demonstrate that α-Syn oligomers, forming at the early of aggregation, are cytotoxic and is thus related to brain neurodegenerative diseases. Herein, we find that vitamin D (VD) can reduce neurocytotoxicity. The reduced neurocytotoxicity might be attributed to the less amount of large-sized α-Syn oligomers inhibited by VD, measured by electrochemical collision at single particle level, which are not observable with traditionally ensembled method. Single-cell amperometry (SCA) results show that VD can recover the amount of neurotransmitter release during exocytosis induced by α-Syn oligomers, further verifying the neuroprotection of VD. Our study reveals the neuroprotective role of VD through inhibiting α-Syn aggregation, which is envisioned to be of great importance in treatment and prevention of the neurodegenerative diseases.

Alpha-Synuclein and Lipids: The Elephant in the Room?

Since the initial identification of alpha-synuclein (α-syn) at the synapse, numerous studies demonstrated that α-syn is a key player in the etiology of Parkinson's disease (PD) and other synucleinopathies. Recent advances underline interactions between α-syn and lipids that also participate in α-syn misfolding and aggregation. In addition, increasing evidence demonstrates that α-syn plays a major role in different steps of synaptic exocytosis. Thus, we reviewed literature showing (1) the interplay among α-syn, lipids, and lipid membranes; (2) advances of α-syn synaptic functions in exocytosis. These data underscore a fundamental role of α-syn/lipid interplay that also contributes to synaptic defects in PD. The importance of lipids in PD is further highlighted by data showing the impact of α-syn on lipid metabolism, modulation of α-syn levels by lipids, as well as the identification of genetic determinants involved in lipid homeostasis associated with α-syn pathologies. While questions still remain, these recent developments open the way to new therapeutic strategies for PD and related disorders including some based on modulating synaptic functions.