Jacqueline Burré

α-Synuclein Promotes SNARE-Complex Assembly in Vivo and in Vitro

α-Synuclein and Aging Transgenic α-synuclein can reverse the otherwise lethal neurodegeneration of cysteine string protein-α knockout mice via changes in SNARE proteins, which mediate synaptic vesicle release. Using experiments with purified recombinant proteins, triple αβγ-synuclein knockout mice, and studies of mouse aging, Burré et al. (p. 1663, published online 26 August) now demonstrate that α-synuclein directly interacts with the SNARE protein synaptobrevin and functions as a catalyst for SNARE-complex assembly. The role of synucleins is fully dispensable in young animals, but becomes essential late in life, which suggests that α-synuclein maintains normal synaptic function during aging. A protein implicated in neurodegeneration promotes the assembly of membrane fusion complexes. Presynaptic nerve terminals release neurotransmitters repeatedly, often at high frequency, and in relative isolation from neuronal cell bodies. Repeated release requires cycles of soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE)–complex assembly and disassembly, with continuous generation of reactive SNARE-protein intermediates. Although many forms of neurodegeneration initiate presynaptically, only few pathogenic mechanisms are known, and the functions of presynaptic proteins linked to neurodegeneration, such as α-synuclein, remain unclear. Here, we show that maintenance of continuous presynaptic SNARE-complex assembly required a nonclassical chaperone activity mediated by synucleins. Specifically, α-synuclein directly bound to the SNARE-protein synaptobrevin-2/vesicle-associated membrane protein 2 (VAMP2) and promoted SNARE-complex assembly. Moreover, triple-knockout mice lacking synucleins developed age-dependent neurological impairments, exhibited decreased SNARE-complex assembly, and died prematurely. Thus, synucleins may function to sustain normal SNARE-complex assembly in a presynaptic terminal during aging.

CSPα promotes SNARE-complex assembly by chaperoning SNAP-25 during synaptic activity

A neuron forms thousands of presynaptic nerve terminals on its axons, far removed from the cell body. The protein CSPα resides in presynaptic terminals, where it forms a chaperone complex with Hsc70 and SGT. Deletion of CSPα results in massive neurodegeneration that impairs survival in mice and flies. In CSPα-knockout mice, levels of presynaptic SNARE complexes and the SNARE protein SNAP-25 are reduced, suggesting that CSPα may chaperone SNARE proteins, which catalyse synaptic vesicle fusion. Here, we show that the CSPα–Hsc70–SGT complex binds directly to monomeric SNAP-25 to prevent its aggregation, enabling SNARE-complex formation. Deletion of CSPα produces an abnormal SNAP-25 conformer that inhibits SNARE-complex formation, and is subject to ubiquitylation and proteasomal degradation. Even in wild-type mouse terminals, SNAP-25 degradation is regulated by synaptic activity; this degradation is decreased by CSPα overexpression, and enhanced by CSPα deletion. Thus, SNAP-25 function is maintained during rapid SNARE cycles by equilibrium between CSPα-dependent chaperoning and ubiquitin-dependent degradation, revealing unique protein quality-control machinery within the presynaptic compartment.

α-Synuclein assembles into higher-order multimers upon membrane binding to promote SNARE complex formation

Significance Physiologically, α-synuclein promotes soluble NSF attachment protein receptor (SNARE) complex assembly during synaptic exocytosis. Pathologically, however, α-synuclein forms neurotoxic aggregates that promote neurodegeneration and represent hallmarks of Parkinsons disease and other synucleinopathies. α-Synuclein exists in a monomeric unfolded state in solution and in an α-helical folded state upon binding to membranes. Yet the relation between these conformational states and their physiological and pathological roles remain unknown. Here, we demonstrate that α-synuclein multimerizes during membrane binding and that the membrane-bound, multimeric form of α-synuclein mediates SNARE complex assembly in presynaptic terminals. Our data delineate a folding pathway for α-synuclein that ranges from a monomeric unfolded form in cytosol to a physiologically functional multimeric form that is membrane bound and chaperones SNARE complex assembly, and that may protect against neurodegeneration. Physiologically, α-synuclein chaperones soluble NSF attachment protein receptor (SNARE) complex assembly and may also perform other functions; pathologically, in contrast, α-synuclein misfolds into neurotoxic aggregates that mediate neurodegeneration and propagate between neurons. In neurons, α-synuclein exists in an equilibrium between cytosolic and membrane-bound states. Cytosolic α-synuclein appears to be natively unfolded, whereas membrane-bound α-synuclein adopts an α-helical conformation. Although the majority of studies showed that cytosolic α-synuclein is monomeric, it is unknown whether membrane-bound α-synuclein is also monomeric, and whether chaperoning of SNARE complex assembly by α-synuclein involves its cytosolic or membrane-bound state. Here, we show using chemical cross-linking and fluorescence resonance energy transfer (FRET) that α-synuclein multimerizes into large homomeric complexes upon membrane binding. The FRET experiments indicated that the multimers of membrane-bound α-synuclein exhibit defined intermolecular contacts, suggesting an ordered array. Moreover, we demonstrate that α-synuclein promotes SNARE complex assembly at the presynaptic plasma membrane in its multimeric membrane-bound state, but not in its monomeric cytosolic state. Our data delineate a folding pathway for α-synuclein that ranges from a monomeric, natively unfolded form in cytosol to a physiologically functional, multimeric form upon membrane binding, and show that only the latter but not the former acts as a SNARE complex chaperone at the presynaptic terminal, and may protect against neurodegeneration.

Immunoisolation of two synaptic vesicle pools from synaptosomes: a proteomics analysis.

The nerve terminal proteome governs neurotransmitter release as well as the structural and functional dynamics of the presynaptic compartment. In order to further define specific presynaptic subproteomes we used subcellular fractionation and a monoclonal antibody against the synaptic vesicle protein SV2 for immunoaffinity purification of two major synaptosome‐derived synaptic vesicle‐containing fractions: one sedimenting at lower and one sedimenting at higher sucrose density. The less dense fraction contains free synaptic vesicles, the denser fraction synaptic vesicles as well as components of the presynaptic membrane compartment. These immunoisolated fractions were analyzed using the cationic benzyldimethyl‐n‐hexadecylammonium chloride (BAC) polyacrylamide gel system in the first and sodium dodecyl sulfate–polyacrylamide gel electrophoresis in the second dimension. Protein spots were subjected to analysis by matrix‐assisted laser desorption ionization time of flight mass spectrometry (MALDI TOF MS). We identified 72 proteins in the free vesicle fraction and 81 proteins in the plasma membrane‐containing denser fraction. Synaptic vesicles contain a considerably larger number of protein constituents than previously anticipated. The plasma membrane‐containing fraction contains synaptic vesicle proteins, components of the presynaptic fusion and retrieval machinery and numerous other proteins potentially involved in regulating the functional and structural dynamics of the nerve terminal.

Native α-synuclein induces clustering of synaptic-vesicle mimics via binding to phospholipids and synaptobrevin-2/VAMP2

α-Synuclein is a presynaptic protein that is implicated in Parkinsons and other neurodegenerative diseases. Physiologically, native α-synuclein promotes presynaptic SNARE-complex assembly, but its molecular mechanism of action remains unknown. Here, we found that native α-synuclein promotes clustering of synaptic-vesicle mimics, using a single-vesicle optical microscopy system. This vesicle-clustering activity was observed for both recombinant and native α-synuclein purified from mouse brain. Clustering was dependent on specific interactions of native α-synuclein with both synaptobrevin-2/VAMP2 and anionic lipids. Out of the three familial Parkinsons disease-related point mutants of α-synuclein, only the lipid-binding deficient mutation A30P disrupted clustering, hinting at a possible loss of function phenotype for this mutant. α-Synuclein had little effect on Ca2+-triggered fusion in our reconstituted single-vesicle system, consistent with in vivo data. α-Synuclein may therefore lead to accumulation of synaptic vesicles at the active zone, providing a ‘buffer’ of synaptic vesicles, without affecting neurotransmitter release itself. DOI: http://dx.doi.org/10.7554/eLife.00592.001

Properties of native brain α-synuclein

Arising from T. Bartels, J. G. Choi & D. J. Selkoe. 477, 107–110 (2011).10.1038/nature10324α-Synuclein is an abundant presynaptic protein that binds to negatively charged phospholipids, functions as a SNARE-complex chaperone and contributes to Parkinson’s disease pathogenesis. Recombinant α-synuclein in solution is largely unfolded and devoid of tertiary structure, but Bartels et al. have proposed that native α-synuclein purified from human erythrocytes forms a stably folded, soluble tetramer that resists aggregation. By contrast, we show here that native α-synuclein purified from mouse brain consists of a largely unstructured monomer, exhibits no stable tetramer formation, and is prone to aggregation. The native state of α-synuclein is important for understanding its pathological effects as a stably folded protein would be much less prone to aggregation than a conformationally labile protein. There is a Reply to this Brief Communication Arising by Bartels, T. & Selkoe, D. J. Nature 498, http://dx.doi.org/10.1038/nature12126 (2013).

Systematic Mutagenesis of α-Synuclein Reveals Distinct Sequence Requirements for Physiological and Pathological Activities

α-Synuclein is an abundant presynaptic protein that binds to phospholipids and synaptic vesicles. Physiologically, α-synuclein functions as a SNARE-protein chaperone that promotes SNARE-complex assembly for neurotransmitter release. Pathologically, α-synuclein mutations and α-synuclein overexpression cause Parkinsons disease, and aggregates of α-synuclein are found as Lewy bodies in multiple neurodegenerative disorders (“synucleinopathies”). The relation of the physiological functions to the pathological effects of α-synuclein remains unclear. As an initial avenue of addressing this question, we here systematically examined the effect of α-synuclein mutations on its physiological and pathological activities. We generated 26 α-synuclein mutants spanning the entire molecule, and analyzed them compared with wild-type α-synuclein in seven assays that range from biochemical studies with purified α-synuclein, to analyses of α-synuclein expression in cultured neurons, to examinations of the effects of virally expressed α-synuclein introduced into the mouse substantia nigra by stereotactic injections. We found that both the N-terminal and C-terminal sequences of α-synuclein were required for its physiological function as SNARE-complex chaperone, but that these sequences were not essential for its neuropathological effects. In contrast, point mutations in the central region of α-synuclein, referred to as nonamyloid β component (residues 61–95), as well as point mutations linked to Parkinsons disease (A30P, E46K, and A53T) increased the neurotoxicity of α-synuclein but did not affect its physiological function in SNARE-complex assembly. Thus, our data show that the physiological function of α-synuclein, although protective of neurodegeneration in some contexts, is fundamentally distinct from its neuropathological effects, thereby dissociating the two activities of α-synuclein.

CSPα knockout causes neurodegeneration by impairing SNAP-25 function.

At a synapse, the synaptic vesicle protein cysteine‐string protein‐α (CSPα) functions as a co‐chaperone for the SNARE protein SNAP‐25. Knockout (KO) of CSPα causes fulminant neurodegeneration that is rescued by α‐synuclein overexpression. The CSPα KO decreases SNAP‐25 levels and impairs SNARE‐complex assembly; only the latter but not the former is reversed by α‐synuclein. Thus, the question arises whether the CSPα KO phenotype is due to decreased SNAP‐25 function that then causes neurodegeneration, or due to the dysfunction of multiple as‐yet uncharacterized CSPα targets. Here, we demonstrate that decreasing SNAP‐25 levels in CSPα KO mice by either KO or knockdown of SNAP‐25 aggravated their phenotype. Conversely, increasing SNAP‐25 levels by overexpression rescued their phenotype. Inactive SNAP‐25 mutants were unable to rescue, showing that the rescue was specific. Under all conditions, the neurodegenerative phenotype precisely correlated with SNARE‐complex assembly, indicating that impaired SNARE‐complex assembly due to decreased SNAP‐25 levels is the ultimate correlate of neurodegeneration. Our findings suggest that the neurodegeneration in CSPα KO mice is primarily produced by defective SNAP‐25 function, which causes neurodegeneration by impairing SNARE‐complex assembly.

The synaptic vesicle proteome

Synaptic vesicles are key organelles in neurotransmission. Vesicle integral or membrane‐associated proteins mediate the various functions the organelle fulfills during its life cycle. These include organelle transport, interaction with the nerve terminal cytoskeleton, uptake and storage of low molecular weight constituents, and the regulated interaction with the pre‐synaptic plasma membrane during exo‐ and endocytosis. Within the past two decades, converging work from several laboratories resulted in the molecular and functional characterization of the proteinaceous inventory of the synaptic vesicle compartment. However, up until recently and due to technical difficulties, it was impossible to screen the entire organelle thoroughly. Recent advances in membrane protein identification and mass spectrometry (MS) have dramatically promoted this field. A comparison of different techniques for elucidating the proteinaceous composition of synaptic vesicles revealed numerous overlaps but also remarkable differences in the protein constituents of the synaptic vesicle compartment, indicating that several protein separation techniques in combination with differing MS approaches are required to identify and characterize the synaptic vesicle proteome. This review highlights the power of various gel separation techniques and MS analyses for the characterization of the proteome of highly purified synaptic vesicles. Furthermore, the newly detected protein assignments to synaptic vesicles, especially those proteins which are new to the inventory of the synaptic vesicle proteome, are critically discussed.

Proteasome Inhibition Alleviates SNARE-Dependent Neurodegeneration

Neurodegeneration in a mouse model is abrogated by proteasome inhibitors that reverse impaired SNARE-complex assembly. Ensnaring Neurodegeneration Many believe that a decrease in proteasome activity contributes to the pathogenesis of neurodegenerative disorders, such as Alzheimer’s and Parkinson’s disease. Thus, activation of proteasome activity has been considered a promising therapeutic strategy for treating neurodegenerative diseases. In a new study, Sharma et al. tested proteasome inhibitors in mice with neurodegeneration caused by deletion of cysteine string protein-α. Mice lacking cysteine string protein-α die early because of loss of synapses and neuronal death, which results from loss of the SNARE protein SNAP-25 (for which cysteine string protein-α is a chaperone), and a decrease in the assembly of SNARE complexes. Surprisingly, the authors found the opposite of what they expected: Instead of accelerating neurodegeneration, proteasome inhibitors alleviated neurodegeneration. This unexpected result demonstrates that at least for this form of neurodegeneration, proteasome inhibition does not represent a pathogenic mechanism, but instead can be used as a therapeutic strategy. The researchers showed that the proteasome inhibitors alleviated neurodegeneration by increasing SNAP-25 concentrations and enhancing SNARE-complex assembly. They then demonstrated that SNARE-complex assembly is impaired in human brain tissue from patients with neurodegenerative diseases. The impact of this study may go beyond neurodegeneration because systemic administration of proteasome inhibitors is currently being tested as a cancer treatment. Proteasome inhibitors also may help in the treatment of other diseases such as cystic fibrosis and nephrogenic diabetes insipidus, which are caused by proteasomal degradation of functionally important proteins. Activation of the proteasomal degradation of misfolded proteins has been proposed as a therapeutic strategy for treating neurodegenerative diseases, but it is unclear whether proteasome dysfunction contributes to neurodegeneration. We tested the role of proteasome activity in neurodegeneration developed by mice lacking cysteine string protein–α (CSPα). Unexpectedly, we found that proteasome inhibitors alleviated neurodegeneration in CSPα-deficient mice, reversing impairment of SNARE (soluble N-ethylmaleimide–sensitive factor attachment protein receptor)–complex assembly and extending life span. We tested whether dysfunctional SNARE-complex assembly could contribute to neurodegeneration in Alzheimer’s and Parkinson’s disease by analyzing postmortem brain tissue from these patients; we found reduced SNARE-complex assembly in the brain tissue samples. Our results suggest that proteasomal activation may not always be beneficial for alleviating neurodegeneration and that blocking the proteasome may represent a potential therapeutic avenue for treating some forms of neurodegenerative disease.