A complexin fusion clamp regulates spontaneous neurotransmitter release and synaptic growth

Complexin facilitates exocytosis and synchronizes vesicle release in two secretory model systems

Complexins (Cplxs) are small, SNARE-associated proteins believed to regulate fast, calcium-triggered exocytosis. However, studies have pointed to either an inhibitory and/or facilitatory role in exocytosis, and the role of Cplxs in synchronizing exocytosis is relatively unexplored. Here, we compare the function of two types of complexin, Cplx 1 and 2, in two model systems of calcium-dependent exocytosis. In mouse neuromuscular junctions (NMJs), we find that lack of Cplx 1 significantly reduces and desynchronizes calcium-triggered synaptic transmission; furthermore, high-frequency stimulation elicits synaptic facilitation, instead of normal synaptic depression, and the degree of facilitation is highly sensitive to the amount of cytoplasmic calcium buffering. In Cplx 2-null adrenal chromaffin cells, we also find decreased and desynchronized evoked release, and identify a significant reduction in the vesicle pool close to the calcium channels (immediately releasable pool, IRP). Viral transduction with either Cplx 1 or 2 rescues both the size of the evoked response and the synchronicity of release, and it restores the IRP size. Our findings in two model systems are mutually compatible and indicate a role of Cplx 1 and 2 in facilitating vesicle priming, and also lead to the new hypothesis that Cplxs may synchronize vesicle release by promoting coupling between secretory vesicles and calcium channels.

Development and plasticity of the Drosophila larval neuromuscular junction

The Drosophila larval neuromuscular system is relatively simple, containing only 32 motor neurons in each abdominal hemisegment, and its neuromuscular junctions (NMJs) have been studied extensively. NMJ synapses exhibit developmental and functional plasticity while displaying stereotyped connectivity. Drosophila Type I NMJ synapses are glutamatergic, while the vertebrate NMJ uses acetylcholine as its primary neurotransmitter. The larval NMJ synapses use ionotropic glutamate receptors (GluRs) that are homologous to AMPA-type GluRs in the mammalian brain, and they have postsynaptic scaffolds that resemble those found in mammalian postsynaptic densities. These features make the Drosophila neuromuscular system an excellent genetic model for the study of excitatory synapses in the mammalian central nervous system. The first section of the review presents an overview of NMJ development. The second section describes genes that regulate NMJ development, including: (1) genes that positively and negatively regulate growth of the NMJ, (2) genes required for maintenance of NMJ bouton structure, (3) genes that modulate neuronal activity and alter NMJ growth, (4) genes involved in transsynaptic signaling at the NMJ. The third section describes genes that regulate acute plasticity, focusing on translational regulatory mechanisms. As this review is intended for a developmental biology audience, it does not cover NMJ electrophysiology in detail, and does not review genes for which mutations produce only electrophysiological but no structural phenotypes.

Differential regulation of evoked and spontaneous neurotransmitter release by C-terminal modifications of complexin

Complexins are small α-helical proteins that modulate neurotransmitter release by binding to SNARE complexes during synaptic vesicle exocytosis. They have been found to function as fusion clamps to inhibit spontaneous synaptic vesicle fusion in the absence of Ca(2+), while also promoting evoked neurotransmitter release following an action potential. Complexins consist of an N-terminal domain and an accessory α-helix that regulates the activating and inhibitory properties of the protein, respectively, and a central α-helix that binds the SNARE complex and is essential for both functions. In addition, complexins contain a largely unstructured C-terminal domain whose role in synaptic vesicle cycling is poorly defined. Here, we demonstrate that the C-terminus of Drosophila complexin (DmCpx) regulates localization to synapses and that alternative splicing of the C-terminus can differentially regulate spontaneous and evoked neurotransmitter release. Characterization of the single DmCpx gene by mRNA analysis revealed expression of two alternatively expressed isoforms, DmCpx7A and DmCpx7B, which encode proteins with different C-termini that contain or lack a membrane tethering prenylation domain. The predominant isoform, DmCpx7A, is further modified by RNA editing within this C-terminal region. Functional analysis of the splice isoforms showed that both are similarly localized to synaptic boutons at larval neuromuscular junctions, but have differential effects on the regulation of evoked and spontaneous fusion. These data indicate that the C-terminus of Drosophila complexin regulates both spontaneous and evoked release through separate mechanisms and that alternative splicing generates isoforms with distinct effects on the two major modes of synaptic vesicle fusion at synapses.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Postsynaptic complexin controls AMPA receptor exocytosis during LTP

Long-term potentiation (LTP) is a compelling synaptic correlate of learning and memory. LTP induction requires NMDA receptor (NMDAR) activation, which triggers SNARE-dependent exocytosis of AMPA receptors (AMPARs). However, the molecular mechanisms mediating AMPAR exocytosis induced by NMDAR activation remain largely unknown. Here, we show that complexin, a protein that regulates neurotransmitter release via binding to SNARE complexes, is essential for AMPAR exocytosis during LTP but not for the constitutive AMPAR exocytosis that maintains basal synaptic strength. The regulated postsynaptic AMPAR exocytosis during LTP requires binding of complexin to SNARE complexes. In hippocampal neurons, presynaptic complexin acts together with synaptotagmin-1 to mediate neurotransmitter release. However, postsynaptic synaptotagmin-1 is not required for complexin-dependent AMPAR exocytosis during LTP. These results suggest a complexin-dependent molecular mechanism for regulating AMPAR delivery to synapses, a mechanism that is surprisingly similar to presynaptic exocytosis but controlled by regulators other than synaptotagmin-1.

Enlightening molecular mechanisms through study of protein interactions

The investigation of molecular mechanisms is a fascinating area of current biological research that unites efforts from scientists with very diverse expertise. This review provides a perspective on the characterization of protein interactions as a central aspect of this research. We discuss case studies on the neurotransmitter release machinery that illustrate a variety of principles and emphasize the power of combining nuclear magnetic resonance (NMR) spectroscopy with other biophysical techniques, particularly X-ray crystallography. These studies have shown that: (i) the soluble SNAP receptor (SNARE) proteins form a tight complex that brings the synaptic vesicle and plasma membranes together, which is key for membrane fusion; (ii) the SNARE syntaxin-1 adopts an autoinhibitory closed conformation; (iii) Munc18-1 plays crucial functions through interactions with closed syntaxin-1 and with the SNARE complex; (iv) Munc13s mediate the opening of syntaxin-1; (v) complexins play dual roles through distinct interactions with the SNARE complex; (vi) synaptotagmin-1 acts a Ca(2+) sensor, interacting simultaneously with the membranes and the SNAREs; and (vii) a Munc13 homodimer to Munc13-RIM heterodimer switch modulates neurotransmitter release. Overall, this research underlines the complexities involved in elucidating molecular mechanisms and how these mechanisms can depend critically on an interplay between strong and weak protein interactions.

Complexin arrests a pool of docked vesicles for fast Ca2+-dependent release

Regulated exocytosis requires that the assembly of the basic membrane fusion machinery is temporarily arrested. Synchronized membrane fusion is then caused by a specific trigger--a local rise of the Ca(2+) concentration. Using reconstituted giant unilamellar vesicles (GUVs), we have analysed the role of complexin and membrane-anchored synaptotagmin 1 in arresting and synchronizing fusion by lipid-mixing and cryo-electron microscopy. We find that they mediate the formation and consumption of docked small unilamellar vesicles (SUVs) via the following sequence of events: Synaptotagmin 1 mediates v-SNARE-SUV docking to t-SNARE-GUVs in a Ca(2+)-independent manner. Complexin blocks vesicle consumption, causing accumulation of docked vesicles. Together with synaptotagmin 1, complexin synchronizes and stimulates rapid fusion of accumulated docked vesicles in response to physiological Ca(2+) concentrations. Thus, the reconstituted assay resolves both the stimulatory and inhibitory function of complexin and mimics key aspects of synaptic vesicle fusion.

The absence of Complexin 3 and Complexin 4 differentially impacts the ON and OFF pathways in mouse retina

Complexins (Cplxs) regulate the speed and Ca(2+)-sensitivity of synaptic vesicle fusion. It has been shown that all four known Cplxs are present at mouse retinal synapses--at conventional amacrine cell synapses (Cplx 1 to Cplx 3) and at photoreceptor and bipolar cell ribbon synapses (Cplx 3 and Cplx 4) [K. Reim et al. (2005) J. Cell Biol., 169, 669-680]. Electroretinographic recordings in Cplx 3/Cplx 4 double-knockout (DKO) mice showed perturbed transmission in the outer plexiform layer, and possible changes in the inner plexiform layer [K. Reim et al. (2009) J. Cell Sci., 122, 1352-1361]. In the present study, we examined the effects of the absence of Cplx 3 and Cplx 4 on ganglion cell responses. We report that the lack of Cplx 3 and Cplx 4 differentially impacts the ON and OFF pathways. Under photopic conditions, the responses in the cone OFF pathway are largely unaffected, whereas the responses in the cone ON pathway are diminished in Cplx 3/Cplx 4 DKO mice. Under scotopic conditions, both ON and OFF response rates are reduced and high-sensitivity OFF responses are missing in Cplx 3/Cplx 4 DKO mice. The electrophysiological findings are corroborated by new immunocytochemical findings. We now show that rod spherules contain only Cplx 4. However, both Cplx 3 and Cplx 4 co-localize in cone pedicles. In the inner plexiform layer, Cplx 3 is present in rod bipolar cell terminals and in amacrine cell processes. Most importantly, Cplx 3 is localized in the lobular appendages of AII amacrine cells, the sites of signal transmission from the primary rod pathway into the OFF pathway in the inner plexiform layer.

C-terminal complexin sequence is selectively required for clamping and priming but not for Ca2+ triggering of synaptic exocytosis

Complexins are small soluble proteins that bind to assembling SNARE complexes during synaptic vesicle exocytosis, which in turn mediates neurotransmitter release. Complexins are required for clamping of spontaneous "mini " release and for the priming and synaptotagmin-dependent Ca(2+) triggering of evoked release. Mammalian genomes encode four complexins that are composed of an N-terminal unstructured sequence that activates synaptic exocytosis, an accessory α-helix that clamps exocytosis, an essential central α-helix that binds to assembling SNARE complexes and is required for all of its functions, and a long, apparently unstructured C-terminal sequence whose function remains unclear. Here, we used cultured mouse neurons to show that the C-terminal sequence of complexin-1 is not required for its synaptotagmin-activating function but is essential for its priming and clamping functions. Wild-type complexin-3 did not clamp exocytosis but nevertheless fully primed and activated exocytosis. Strikingly, exchanging the complexin-1 C terminus for the complexin-3 C terminus abrogated clamping, whereas exchanging the complexin-3 C terminus for the complexin-1 C terminus enabled clamping. Analysis of point mutations in the complexin-1 C terminus identified two single amino-acid substitutions that impaired clamping without altering the activation function of complexin-1. Examination of release induced by stimulus trains revealed that clamping-deficient C-terminal complexin mutants produced a modest relative increase in delayed release. Overall, our results show that the relatively large C-terminal complexin-1 sequence acts in priming and clamping synaptic exocytosis and demonstrate that the clamping function is not conserved in complexin-3, presumably because of its distinct C-terminal sequences.

Molecular machines in the synapse: overlapping protein sets control distinct steps in neurosecretion

Activity regulated neurotransmission shapes the computational properties of a neuron and involves the concerted action of many proteins. Classical, intuitive working models often assign specific proteins to specific steps in such complex cellular processes, whereas modern systems theories emphasize more integrated functions of proteins. To test how often synaptic proteins participate in multiple steps in neurotransmission we present a novel probabilistic method to analyze complex functional data from genetic perturbation studies on neuronal secretion. Our method uses a mixture of probabilistic principal component analyzers to cluster genetic perturbations on two distinct steps in synaptic secretion, vesicle priming and fusion, and accounts for the poor standardization between different studies. Clustering data from 121 perturbations revealed that different perturbations of a given protein are often assigned to different steps in the release process. Furthermore, vesicle priming and fusion are inversely correlated for most of those perturbations where a specific protein domain was mutated to create a gain-of-function variant. Finally, two different modes of vesicle release, spontaneous and action potential evoked release, were affected similarly by most perturbations. This data suggests that the presynaptic protein network has evolved as a highly integrated supramolecular machine, which is responsible for both spontaneous and activity induced release, with a group of core proteins using different domains to act on multiple steps in the release process.

Synapses, which are small structures where information is transmitted between neurons, are the functional units of computation in the brain implicated in information processing, learning and memory. In the last few years several genes that are expressed in synapses have been linked to brain disorders such as autism, depression and schizophrenia. However, good understanding of the molecular basis of these diseases requires more insight in the functional organization of the protein network. In the classic view proteins are involved in a specific step of a cellular process, whereas modern system theories suggest a more integrated and complex network where proteins can be involved in several steps. We have developed a method that allows comparison of high-end but low-throughput functional data from different genetic perturbation studies despite the poor standardization between studies. Our analysis shows that synaptic proteins are not restricted to a single function but that they are part of a highly integrated supramolecular machine, with overlapping protein sets being involved in distinct steps in neurotransmitter release. Further characterization of these complex protein network relations will be important for new drug design strategies.

Adhesion energy can regulate vesicle fusion and stabilize partially fused states

Release of neurotransmitters from nerve terminals occurs by fusion of synaptic vesicles with the plasma membrane, and this process is highly regulated. Although major molecular components that control docking and fusion of vesicles to the synaptic membrane have been identified, the detailed mechanics of this process is not yet understood. We have developed a mathematical model that predicts how adhesion forces imposed by docking and fusion molecular machinery would affect the fusion process. We have computed the membrane stress that is produced by adhesion-driven vesicle bending and find that it is compressive. Further, our computations of the membrane curvature predict that strong adhesion can create a metastable state with a partially opened pore that would correspond to the 'kiss and run' release mode. Our model predicts that the larger the vesicle size, the more likely the metastable state with a transiently opened pore. These results contribute to understanding the mechanics of the fusion process, including possible clamping of the fusion by increasing molecular adhesion, and a balance between 'kiss and run' and full collapse fusion modes.

v-SNARE composition distinguishes synaptic vesicle pools

Synaptic vesicles belong to two distinct pools, a recycling pool responsible for the evoked release of neurotransmitter and a resting pool unresponsive to stimulation. The uniform appearance of synaptic vesicles has suggested that differences in location or cytoskeletal association account for these differences in function. We now find that the v-SNARE tetanus toxin-insensitive vesicle-associated membrane protein (VAMP7) differs from other synaptic vesicle proteins in its distribution to the two pools, providing evidence that they differ in molecular composition. We also find that both resting and recycling pools undergo spontaneous release, and when activated by deletion of the longin domain, VAMP7 influences the properties of release. Further, the endocytosis that follows evoked and spontaneous release differs in mechanism, and specific sequences confer targeting to the different vesicle pools. The results suggest that different endocytic mechanisms generate synaptic vesicles with different proteins that can endow the vesicles with distinct properties.

Synapsin regulation of vesicle organization and functional pools

Synaptic vesicles are organized in clusters, and synapsin maintains vesicle organization and abundance in nerve terminals. At the functional level, vesicles can be subdivided into three pools: the releasable pool, the recycling pool, and the reserve pool, and synapsin mediates transitions between these pools. Synapsin directs vesicles into the reserve pool, and synapsin II isoform has a primary role in this function. In addition, synapsin actively delivers vesicles to active zones. Finally, synapsin I isoform mediates coupling release events to action potentials at the latest stages of exocytosis. Thus, synapsin is involved in multiple stages of the vesicle cycle, including vesicle clustering, maintaining the reserve pool, vesicle delivery to active zones, and synchronizing release events. These processes are regulated via a dynamic synapsin phosphorylation/dephosphorylation cycle which involves multiple phosphorylation sites and several pathways. Different synapsin isoforms have unique and non-redundant roles in the multifaceted synapsin function.

Complexin activates and clamps SNAREpins by a common mechanism involving an intermediate energetic state

The core mechanism of intracellular vesicle fusion consists of SNAREpin zippering between vesicular and target membranes. Recent studies indicate that the same SNARE-binding protein, complexin (CPX), can act either as a facilitator or as an inhibitor of membrane fusion, constituting a controversial dilemma. Here we take energetic measurements with the surface force apparatus that reveal that CPX acts sequentially on assembling SNAREpins, first facilitating zippering by nearly doubling the distance at which v- and t-SNAREs can engage and then clamping them into a half-zippered fusion-incompetent state. Specifically, we find that the central helix of CPX allows SNAREs to form this intermediate energetic state at 9-15 nm but not when the bilayers are closer than 9 nm. Stabilizing the activated-clamped state at separations of less than 9 nm requires the accessory helix of CPX, which prevents membrane-proximal assembly of SNAREpins.

Complexin cross-links prefusion SNAREs into a zigzag array

Complexin prevents SNAREs from releasing neurotransmitters until an action potential arrives at the synapse. To understand the mechanism for this inhibition, we determined the structure of complexin bound to a mimetic of a prefusion SNAREpin lacking the portion of the v-SNARE that zippers last to trigger fusion. The 'central helix' of complexin is anchored to one SNARE complex, while its 'accessory helix' extends away at ~45° and bridges to a second complex, occupying the vacant v-SNARE binding site to inhibit fusion. We expected the accessory helix to compete with the v-SNARE for t-SNARE binding but found instead that the interaction occurs intermolecularly. Thus, complexin organizes the SNAREs into a zigzag topology that, when interposed between the vesicle and plasma membranes, is incompatible with fusion.

A presynaptic endosomal trafficking pathway controls synaptic growth signaling

Association of Nwk with SNX16 promotes down-regulation of synaptic growth signaling at the interface between early and recycling endosomes.

Structural remodeling of synapses in response to growth signals leads to long-lasting alterations in neuronal function in many systems. Synaptic growth factor receptors alter their signaling properties during transit through the endocytic pathway, but the mechanisms controlling cargo traffic between endocytic compartments remain unclear. Nwk (Nervous Wreck) is a presynaptic F-BAR/SH3 protein that regulates synaptic growth signaling in Drosophila melanogaster. In this paper, we show that Nwk acts through a physical interaction with sorting nexin 16 (SNX16). SNX16 promotes synaptic growth signaling by activated bone morphogenic protein receptors, and live imaging in neurons reveals that SNX16-positive early endosomes undergo transient interactions with Nwk-containing recycling endosomes. We identify an alternative signal termination pathway in the absence of Snx16 that is controlled by endosomal sorting complex required for transport (ESCRT)–mediated internalization of receptors into the endosomal lumen. Our results define a presynaptic trafficking pathway mediated by SNX16, NWK, and the ESCRT complex that functions to control synaptic growth signaling at the interface between endosomal compartments.

Regulation of transmitter release by Ca(2+) and synaptotagmin: insights from a large CNS synapse

Transmitter release at synapses is driven by elevated intracellular Ca(2+) concentration ([Ca(2+)](i)) near the sites of vesicle fusion. [Ca(2+)](i) signals of profoundly different amplitude and kinetics drive the phasic release component during a presynaptic action potential, and asynchronous release at later times. Studies using direct control of [Ca(2+)](i) at a large glutamatergic terminal, the calyx of Held, have provided significant insight into how intracellular Ca(2+) regulates transmitter release over a wide concentration range. Synaptotagmin-2 (Syt2), the major isoform of the Syt1/2 Ca(2+) sensors at these synapses, triggers highly Ca(2+)-cooperative release above 1μM [Ca(2+)](i), but suppresses release at low [Ca(2+)](i). Thus, neurons utilize a highly sophisticated release apparatus to maximize the dynamic range of Ca(2+)-evoked versus spontaneous release.

Novel functions for the anaphase-promoting complex in neurobiology

In recent years, diverse and unexpected neurobiological functions have been uncovered for the major cell cycle-regulated ubiquitin ligase, the anaphase-promoting complex (APC). Functions of the APC in the nervous system range from orchestrating neuronal morphogenesis and synapse development to the regulation of neuronal differentiation, survival, and metabolism. The APC acts together with the coactivating proteins Cdh1 and Cdc20 in neural cells to target specific substrates for ubiquitination and consequent degradation by the proteasome. As we continue to unravel APC functions and mechanisms in neurobiology, these studies should advance our understanding of the molecular mechanisms of neuronal connectivity, with important implications for the study of brain development and disease.

Altered gene regulation and synaptic morphology in Drosophila learning and memory mutants

Genetic studies in Drosophila have revealed two separable long-term memory pathways defined as anesthesia-resistant memory (ARM) and long-lasting long-term memory (LLTM). ARM is disrupted in radish (rsh) mutants, whereas LLTM requires CREB-dependent protein synthesis. Although the downstream effectors of ARM and LLTM are distinct, pathways leading to these forms of memory may share the cAMP cascade critical for associative learning. Dunce, which encodes a cAMP-specific phosphodiesterase, and rutabaga, which encodes an adenylyl cyclase, both disrupt short-term memory. Amnesiac encodes a pituitary adenylyl cyclase-activating peptide homolog and is required for middle-term memory. Here, we demonstrate that the Radish protein localizes to the cytoplasm and nucleus and is a PKA phosphorylation target in vitro. To characterize how these plasticity pathways may manifest at the synaptic level, we assayed synaptic connectivity and performed an expression analysis to detect altered transcriptional networks in rutabaga, dunce, amnesiac, and radish mutants. All four mutants disrupt specific aspects of synaptic connectivity at larval neuromuscular junctions (NMJs). Genome-wide DNA microarray analysis revealed ∼375 transcripts that are altered in these mutants, suggesting defects in multiple neuronal signaling pathways. In particular, the transcriptional target Lapsyn, which encodes a leucine-rich repeat cell adhesion protein, localizes to synapses and regulates synaptic growth. This analysis provides insights into the Radish-dependent ARM pathway and novel transcriptional targets that may contribute to memory processing in Drosophila.

Complexin has opposite effects on two modes of synaptic vesicle fusion

BACKGROUND

Synaptic transmission can occur in a binary or graded fashion, depending on whether transmitter release is triggered by action potentials or by gradual changes in membrane potential. Molecular differences of these two types of fusion events and their differential regulation in a physiological context have yet to be addressed. Complexin is a conserved SNARE-binding protein that has been proposed to regulate both spontaneous and stimulus-evoked synaptic vesicle (SV) fusion.

RESULTS

Here we examine complexin function at a graded synapse in C. elegans. Null complexin (cpx-1) mutants are viable, although nervous system function is significantly impaired. Loss of CPX-1 results in a 3-fold increase in the rate of tonic synaptic transmission at the neuromuscular junction, whereas stimulus-evoked SV fusion is decreased 10-fold. A truncated CPX-1 missing its C-terminal domain can rescue stimulus-evoked synaptic vesicle exocytosis but fails to suppress tonic activity, demonstrating that these two modes of exocytosis can be distinguished at the molecular level. A CPX-1 variant with impaired SNARE binding also rescues evoked, but not tonic, neurotransmitter release. Finally, tonic, but not evoked, release can be rescued in a syntaxin point mutant by removing CPX-1. Rescue of either form of exocytosis partially restores locomotory behavior, indicating that both types of synaptic transmission are relevant.

CONCLUSION

These observations suggest a dual role for CPX-1: suppressing SV exocytosis, driven by low levels of endogenous neural activity, while promoting synchronous fusion of SVs driven by a depolarizing stimulus. Thus, patterns of synaptic activity regulate complexin's inhibitory and permissive roles at a graded synapse.

Complexin clamps asynchronous release by blocking a secondary Ca(2+) sensor via its accessory α helix

Complexin activates and clamps neurotransmitter release; impairing complexin function decreases synchronous, but increases spontaneous and asynchronous synaptic vesicle exocytosis. Here, we show that complexin-different from the Ca(2+) sensor synaptotagmin-1-activates synchronous exocytosis by promoting synaptic vesicle priming, but clamps spontaneous and asynchronous exocytosis-similar to synaptotagmin-1-by blocking a secondary Ca(2+) sensor. Activation and clamping functions of complexin depend on distinct, autonomously acting sequences, namely its N-terminal region and accessory α helix, respectively. Mutations designed to test whether the accessory α helix of complexin clamps exocytosis by inserting into SNARE-complexes support this hypothesis, suggesting that the accessory α helix blocks completion of trans-SNARE-complex assembly until Ca(2+) binding to synaptotagmin relieves this block. Moreover, a juxtamembranous mutation in the SNARE-protein synaptobrevin-2, which presumably impairs force transfer from nascent trans-SNARE complexes onto fusing membranes, also unclamps spontaneous fusion by disinhibiting a secondary Ca(2+) sensor. Thus, complexin performs mechanistically distinct activation and clamping functions that operate in conjunction with synaptotagmin-1 by controlling trans-SNARE-complex assembly.

Complexin maintains vesicles in the primed state in C. elegans

BACKGROUND

Complexin binds the SNARE complex at synapses and regulates exocytosis, but genetic studies indicate contradictory roles: in flies it predominantly inhibits synaptic vesicle fusion, whereas in mice it promotes evoked responses.

RESULTS

Here we characterize the complexin mutant in the nematode Caenorhabditis elegans and reveal bipolar functions in neurotransmission: complexin inhibits spontaneous fusion of synaptic vesicles but is also essential for evoked responses. Complexin mutants exhibit a doubling of vesicle fusion in the absence of extracellular calcium. Even more profoundly, mutants exhibit an almost complete loss of evoked responses, and current amplitudes are reduced by 94%. One possible interpretation is that complexin is required for the stabilization of docked vesicles and that, in its absence, vesicles may fuse or undock from the plasma membrane. Consistent with this hypothesis, docked synaptic vesicles are reduced by 70% in complexin-1 mutants.

CONCLUSION

These data suggest that the main function of complexin is to maintain the docked state both by inhibiting fusion and by promoting priming.

Identification of complexin II in astrocytes: a possible regulator of glutamate release in these cells

Complexins are a family of SNARE complex-binding proteins which regulate neurotransmitter release by playing a crucial role in triggering fast exocytosis at the synapse. Current evidence indicates astrocytes can release glutamate via a vesicular mechanism similar to that at nerve terminals and thereby modulate synaptic activity. In addition, components of the biochemical machinery associated with synaptic release have been identified in these cells. However, whether complexins are also present in astrocytes and may therefore participate in the vesicular release of glutamate is a key issue that is yet to be determined. In the present study we therefore examined if astrocytes express complexin I (Cpx I) and/or complexin II (Cpx II). Our results indicate these cells contain Cpx II but not Cpx I in primary culture. In addition, serum deprivation for 24 h led to a 2.6-fold increase in Cpx II, suggesting this protein is responsive to insults. These findings point to Cpx II being a likely key modulator of synaptic activity at the level of these glial cells. Given the considered involvement of complexins in neurologic and psychiatric illness, astrocytic Cpx II represents a potentially important therapeutic target for the future treatment of such maladies.

Comparative analysis of Drosophila and mammalian complexins as fusion clamps and facilitators of neurotransmitter release

The SNARE-binding protein complexin (Cpx) has been demonstrated to regulate synaptic vesicle fusion. Previous studies are consistent with Cpx functioning either as a synaptic vesicle fusion clamp to prevent premature exocytosis, or as a facilitator to directly stimulate release. Here we examined conserved roles of invertebrate and mammalian Cpx isoforms in the regulation of neurotransmitter release using the Drosophila neuromuscular junction as a model synapse. We find that SNARE binding by Cpx is required for its role as a fusion clamp. All four mammalian Cpx proteins (mCpx), which have been demonstrated to facilitate release, also function as fusion clamps when expressed in Drosophila cpx null mutants, though their clamping abilities vary between isoforms. Moreover, expression of mCpx I, II or III isoforms dramatically enhance evoked release compared to mCpx IV or Drosophila Cpx. Differences in the clamping and facilitating properties of complexin isoforms can be partially attributed to differences in the C-terminal membrane tethering domain. Our findings indicate that the function of complexins as fusion clamps and facilitators of fusion are conserved across evolution, and that these roles are genetically separable within an isoform and across different isoforms.

Clorgyline-mediated reversal of neurological deficits in a Complexin 2 knockout mouse

Complexin 2 is a protein modulator of neurotransmitter release that is downregulated in humans suffering from depression, animal models of depression and neurological disorders such as Huntington's disease in which depression is a major symptom. Although complexin 2 knockout (Cplx2-/-) mice are overtly normal, they show significant abnormalities in cognitive function and synaptic plasticity. Here we show that Cplx2-/- mice also have disturbances in emotional behaviours that include abnormal social interactions and depressive-like behaviour. Since neurotransmitter deficiencies are thought to underlie depression, we examined neurotransmitter levels in Cplx2-/- mice and found a significant decrease in levels of noradrenaline and the serotonin metabolite 5-hydroxyindoleacetic acid in the hippocampus. Chronic treatment with clorgyline, an irreversible inhibitor of monoamine oxidase A, restored hippocampal noradrenaline to normal levels (from 60 to 97% of vehicle-treated Cplx2+/+ mice, P<0.001), and reversed the behavioural deficits seen in Cplx2-/- mice. For example, clorgyline-treated Cplx2-/- mice spent significantly more time interacting with a novel visitor mouse compared with vehicle-treated Cplx2-/- mice in the social recognition test (34 compared with 13%, P<0.01). We were also able to reverse the selective deficit seen in mossy fibre-long-term potentiation (MF-LTP) in Cplx2-/- mice using the noradrenergic agonist isoprenaline. Pre-treatment with isoprenaline in vitro increased MF-LTP by 125% (P<0.001), thus restoring it to control levels. Our data strongly support the idea that complexin 2 is a key player in normal neurological function, and that downregulation of complexin 2 could lead to changes in neurotransmitter release sufficient to cause significant behavioural abnormalities such as depression.

Opposing functions of two sub-domains of the SNARE-complex in neurotransmission

The SNARE-complex consisting of synaptobrevin-2/VAMP-2, SNAP-25 and syntaxin-1 is essential for evoked neurotransmission and also involved in spontaneous release. Here, we used cultured autaptic hippocampal neurons from Snap-25 null mice rescued with mutants challenging the C-terminal, N-terminal and middle domains of the SNARE-bundle to dissect out the involvement of these domains in neurotransmission. We report that the stabilities of two different sub-domains of the SNARE-bundle have opposing functions in setting the probability for both spontaneous and evoked neurotransmission. Destabilizing the C-terminal end of the SNARE-bundle abolishes spontaneous neurotransmitter release and reduces evoked release probability, indicating that the C-terminal end promotes both modes of release. In contrast, destabilizing the middle or deleting the N-terminal end of the SNARE-bundle increases both spontaneous and evoked release probabilities. In both cases, spontaneous release was affected more than evoked neurotransmission. In addition, the N-terminal deletion delays vesicle priming after a high-frequency train. We propose that the stability of N-terminal two-thirds of the SNARE-bundle has a function for vesicle priming and limiting spontaneous release.

Binding of the complexin N terminus to the SNARE complex potentiates synaptic-vesicle fusogenicity

Complexins facilitate and inhibit neurotransmitter release through distinct domains, and their function was proposed to be coupled to the Ca(2+) sensor synaptotagmin-1 (Syt1). However, the mechanisms underlying complexin function remain unclear. We now uncover an interaction between the complexin N terminus and the SNARE complex C terminus, and we show that disrupting this interaction abolishes the facilitatory function of complexins in mouse neurons. Analyses of hypertonically induced exocytosis show that complexins enhance synaptic-vesicle fusogenicity. Genetic experiments crossing complexin- and Syt1-null mice indicate a functional interaction between these proteins but also show that complexins can promote Ca(2+)-triggered release in the absence of Syt1. We propose that the complexin N terminus stabilizes the SNARE complex C terminus and/or helps release the inhibitory function of complexins, thereby activating the fusion machinery in a manner that may cooperate with Syt1 but does not require Syt1.

Real-time visualization of complexin during single exocytic events

Understanding the fundamental role of soluble NSF attachment protein receptor (SNARE) complexes in membrane fusion requires knowledge of the spatiotemporal dynamics of their assembly. We visualized complexin (cplx), a cytosolic protein that binds assembled SNARE complexes, during single exocytic events in live cells. We found that cplx appeared briefly during full fusion. However, a truncated version of cplx containing only the SNARE-complex binding region persisted at fusion sites for seconds and caused fusion to be transient. Resealing pores with the mutant cplx only partially released transmitter and lipid probes, indicating that the pores are narrow and not purely lipidic in structure. Depletion of cplx similarly caused secretory cargo to be retained. These data suggest that cplx is recruited at a late step in exocytosis and modulates fusion pores composed of SNARE complexes.

Synaptobrevin N-terminally bound to syntaxin-SNAP-25 defines the primed vesicle state in regulated exocytosis

Time-resolved measurements of exocytosis identify a domain of the SNARE complex required to keep vesicles readily releasable.

Rapid neurotransmitter release depends on the ability to arrest the SNAP receptor (SNARE)–dependent exocytosis pathway at an intermediate “cocked” state, from which fusion can be triggered by Ca²⁺. It is not clear whether this state includes assembly of synaptobrevin (the vesicle membrane SNARE) to the syntaxin–SNAP-25 (target membrane SNAREs) acceptor complex or whether the reaction is arrested upstream of that step. In this study, by a combination of in vitro biophysical measurements and time-resolved exocytosis measurements in adrenal chromaffin cells, we find that mutations of the N-terminal interaction layers of the SNARE bundle inhibit assembly in vitro and vesicle priming in vivo without detectable changes in triggering speed or fusion pore properties. In contrast, mutations in the last C-terminal layer decrease triggering speed and fusion pore duration. Between the two domains, we identify a region exquisitely sensitive to mutation, possibly constituting a switch. Our data are consistent with a model in which the N terminus of the SNARE complex assembles during vesicle priming, followed by Ca²⁺-triggered C-terminal assembly and membrane fusion.

The dynamic ubiquitin ligase duo: Cdh1-APC and Cdc20-APC regulate neuronal morphogenesis and connectivity

The proper development and patterning of axons, dendrites, and synapses is essential for the establishment of accurate neuronal circuits in the brain. A major goal in neurobiology is to identify the mechanisms and principles that govern these fundamental developmental events of neuronal circuit formation. In recent years, exciting new studies have suggested that ubiquitin signaling pathways may play crucial roles in the control of neuronal connectivity. Among E3 ubiquitin ligases, Cdh1-anaphase promoting complex (Cdh1-APC) and Cdc20-APC have emerged as key regulators of diverse aspects of neuronal connectivity, from axon and dendrite morphogenesis to synapse differentiation and remodeling.

Accessory alpha-helix of complexin I can displace VAMP2 locally in the complexin-SNARE quaternary complex

The calcium-triggered neurotransmitter release requires three SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins: synaptobrevin 2 (or vesicle-associated membrane protein 2) on the synaptic vesicle and syntaxin 1 and SNAP-25 (synaptosome-associated protein of 25 kDa) at the presynaptic plasma membrane. This minimal fusion machinery is believed to drive fusion of the vesicle to the presynaptic membrane. Complexin, also known as synaphin, is a neuronal cytosolic protein that acts as a major regulator of synaptic vesicle exocytosis. Stimulatory and inhibitory effects of complexin have both been reported, suggesting the duality of its function. To shed light on the molecular basis of the complexin's dual function, we have performed an EPR investigation of the complexin-SNARE quaternary complex. We found that the accessory alpha-helix (amino acids 27-48) by itself has the capacity to replace the C-terminus of the SNARE motif of vesicle-associated membrane protein 2 in the four-helix bundle and makes the SNARE complex weaker when the N-terminal region of complexin I (amino acids 1-26) is removed. However, the accessory alpha-helix remains detached from the SNARE core when the N-terminal region of complexin I is present. Thus, our data show the possibility that the balance between the activities of the accessory alpha-helix and the N-terminal domain might determine the final outcome of the complexin function, either stimulatory or inhibitory.

Complexins living up to their name--new light on their role in exocytosis

Ca(2+)-dependent exocytosis of synaptic vesicles is mediated by the SNARE proteins synaptobrevin/VAMP, SNAP-25, and syntaxin. SNARE function is controlled by conserved regulatory proteins, including the complexins. In a study by Xue et al. in this issue of Neuron, contradictory data from Drosophila and mouse complexin mutants have been resolved, revealing a complex pattern of facilitatory and inhibitory domains.

Tilting the balance between facilitatory and inhibitory functions of mammalian and Drosophila Complexins orchestrates synaptic vesicle exocytosis

SNARE-mediated synaptic exocytosis is orchestrated by facilitatory and inhibitory mechanisms. Genetic ablations of Complexins, a family of SNARE-complex-binding proteins, in mice and Drosophila cause apparently opposite effects on neurotransmitter release, leading to contradictory hypotheses of Complexin function. Reconstitution experiments with different fusion assays and Complexins also yield conflicting results. We therefore performed cross-species rescue experiments to compare the functions of murine and Drosophila Complexins in both mouse and fly synapses. We found that murine and Drosophila Complexins employ conserved mechanisms to regulate exocytosis despite their strikingly different overall effects on neurotransmitter release. Both Complexins contain distinct domains that facilitate or inhibit synaptic vesicle fusion, and the strength of each facilitatory or inhibitory function differs significantly between murine and Drosophila Complexins. Our results show that a relative shift in the balance of facilitatory and inhibitory functions results in differential regulation of neurotransmitter release by murine and Drosophila Complexins in vivo, reconciling previous incompatible findings.

A Cdc20-APC ubiquitin signaling pathway regulates presynaptic differentiation

Presynaptic axonal differentiation is essential for synapse formation and the establishment of neuronal circuits. However, the mechanisms that coordinate presynaptic development in the brain are largely unknown. We found that the major mitotic E3 ubiquitin ligase Cdc20-anaphase promoting complex (Cdc20-APC) regulates presynaptic differentiation in primary postmitotic mammalian neurons and in the rat cerebellar cortex. Cdc20-APC triggered the degradation of the transcription factor NeuroD2 and thereby promoted presynaptic differentiation. The NeuroD2 target gene encoding Complexin II, which acts locally at presynaptic sites, mediated the ability of NeuroD2 to suppress presynaptic differentiation. Thus, our findings define a Cdc20-APC ubiquitin signaling pathway that governs presynaptic development, which holds important implications for neuronal connectivity and plasticity in the brain.

Proteomic analysis reveals differentially expressed proteins in the rat frontal cortex after methamphetamine treatment

Methamphetamine (MA) is an addictive psycho-stimulant and the illicit use of the drug is escalating. In the present study, we examined protein expression profiles in the rat frontal cortex exposed to a total of eight MA injections (1 mg/kg, intraperitoneal) using 2-DE based proteomics. We investigated protein changes occurring in both the cytosolic fraction and the membrane fraction. 2-DE analysis resulted in 62 cytosolic and 44 membrane protein spots that were differentially regulated in the frontal cortex of rats exposed to MA when compared to control animals. Of these spots, 47 cytosolic and 42 membrane proteins were identified respectively, using ESI-Quad-TOF, which included ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCH-L1), beta-synuclein, 78 kDa glucose-regulated protein (GRP 78), gamma-enolase, dihydropyrimidase-related protein 2 (DRP 2), complexin 2 and synapsin II. These proteins are associated with protein degradation, redox regulation, energy metabolism, cellular growth, cytoskeletal modifications and synaptic function. Proteomic research may be useful in exploring the complex underlying molecular mechanisms of MA dependence.

Postsynaptic regulation of synaptic plasticity by synaptotagmin 4 requires both C2 domains

Analogous to synaptotagmin 1, a calcium-sensitive regulator of presynaptic vesicle fusion, synaptotagmin 4 needs both of its calcium-binding sites to regulate synaptic plasticity via postsynaptic retrograde signaling.

Ca²⁺ influx into synaptic compartments during activity is a key mediator of neuronal plasticity. Although the role of presynaptic Ca²⁺ in triggering vesicle fusion though the Ca²⁺ sensor synaptotagmin 1 (Syt 1) is established, molecular mechanisms that underlie responses to postsynaptic Ca²⁺ influx remain unclear. In this study, we demonstrate that fusion-competent Syt 4 vesicles localize postsynaptically at both neuromuscular junctions (NMJs) and central nervous system synapses in Drosophila melanogaster. Syt 4 messenger RNA and protein expression are strongly regulated by neuronal activity, whereas altered levels of postsynaptic Syt 4 modify synaptic growth and presynaptic release properties. Syt 4 is required for known forms of activity-dependent structural plasticity at NMJs. Synaptic proliferation and retrograde signaling mediated by Syt 4 requires functional C2A and C2B Ca²⁺–binding sites, as well as serine 284, an evolutionarily conserved substitution for a key Ca²⁺-binding aspartic acid found in other synaptotagmins. These data suggest that Syt 4 regulates activity-dependent release of postsynaptic retrograde signals that promote synaptic plasticity, similar to the role of Syt 1 as a Ca²⁺ sensor for presynaptic vesicle fusion.

Aberrant function and structure of retinal ribbon synapses in the absence of complexin 3 and complexin 4

Complexins regulate the speed and Ca(2+) sensitivity of SNARE-mediated synaptic vesicle fusion at conventional synapses. Two of the vertebrate complexins, Cplx3 and Cplx4, are specifically localized to retinal ribbon synapses. To test whether Cplx3 and Cplx4 contribute to the highly efficient transmitter release at ribbon synapses, we studied retina function and structure in Cplx3 and Cplx4 single- and double-knockout mice. Electroretinographic recordings from single and double mutants revealed a cooperative perturbing effect of Cplx3 and Cplx4 deletion on the b-wave amplitude, whereas most other detected effects in both plexiform synaptic layers were additive. Light and electron microscopic analyses uncovered a disorganized outer plexiform layer in the retinae of mice lacking Cplx3 and Cplx4, with a significant proportion of photoreceptor terminals containing spherical free-floating ribbons. These structural and functional aberrations were accompanied by behavioural deficits indicative of a vision deficit. Our results show that Cplx3 and Cplx4 are essential regulators of transmitter release at retinal ribbon synapses. Their loss leads to aberrant adjustment and fine-tuning of transmitter release at the photoreceptor ribbon synapse, alterations in transmission at bipolar cell terminals, changes in the temporal structure of synaptic processing in the inner plexiform layer of the retina and perturbed vision.

Complexin-I is required for high-fidelity transmission at the endbulb of Held auditory synapse

Complexins (CPXs I-IV) presumably act as regulators of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex, but their function in the intact mammalian nervous system is not well established. Here, we explored the role of CPXs in the mouse auditory system. Hearing was impaired in CPX I knock-out mice but normal in knock-out mice for CPXs II, III, IV, and III/IV as measured by auditory brainstem responses. Complexins were not detectable in cochlear hair cells but CPX I was expressed in spiral ganglion neurons (SGNs) that give rise to the auditory nerve. Ca(2+)-dependent exocytosis of inner hair cells and sound encoding by SGNs were unaffected in CPX I knock-out mice. In the absence of CPX I, the resting release probability in the endbulb of Held synapses of the auditory nerve fibers with bushy cells in the cochlear nucleus was reduced. As predicted by computational modeling, bushy cells had decreased spike rates at sound onset as well as longer and more variable first spike latencies explaining the abnormal auditory brainstem responses. In addition, we found synaptic transmission to outlast the stimulus at many endbulb of Held synapses in vitro and in vivo, suggesting impaired synchronization of release to stimulus offset. Although sound encoding in the cochlea proceeds in the absence of complexins, CPX I is required for faithful processing of sound onset and offset in the cochlear nucleus.

Investigation of SNARE-Mediated Membrane Fusion Mechanism Using Atomic Force Microscopy

Membrane fusion is driven by specialized proteins that reduce the free energy penalty for the fusion process. In neurons and secretory cells, soluble N-ethylmaleimide-sensitive factor-attachment protein (SNAP) receptors (SNAREs) mediate vesicle fusion with the plasma membrane during vesicular content release. Although, SNAREs have been widely accepted as the minimal machinery for membrane fusion, the specific mechanism for SNARE-mediated membrane fusion remains an active area of research. Here, we summarize recent findings based on force measurements acquired in a novel experimental system that uses atomic force microscope (AFM) force spectroscopy to investigate the mechanism(s) of membrane fusion and the role of SNAREs in facilitating membrane hemifusion during SNARE-mediated fusion. In this system, protein-free and SNARE-reconstituted lipid bilayers are formed on opposite (trans) substrates and the forces required to induce membrane hemifusion and fusion or to unbind single v-/t-SNARE complexes are measured. The obtained results provide evidence for a mechanism by which the pulling force generated by interacting trans-SNAREs provides critical proximity between the membranes and destabilizes the bilayers at fusion sites by broadening the hemifusion energy barrier and consequently making the membranes more prone to fusion.

A role of complexin-lipid interactions in membrane fusion

Complexins (Cpxs) and synaptotagmins regulate calcium-dependent exocytosis. A central helix in Cpx confers specific binding to the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) fusion machinery. An accessory helix in the amino-terminal region inhibits membrane fusion by blocking SNAREpin zippering. We now show that an amphipathic helix in the carboxy-terminal region of CpxI binds lipid bilayers and affects SNARE-mediated lipid mixing in a liposome fusion assay. The substitution of a hydrophobic amino acid within the helix by a charged residue abolishes the lipid interaction and the stimulatory effect of CpxI in liposome fusion. In contrast, the introduction of the bulky hydrophobic amino acid tryptophan stimulates lipid binding and liposome fusion. This data shows that local Cpx-lipid interactions can play a role in membrane fusion.

A resting pool of vesicles is responsible for spontaneous vesicle fusion at the synapse

Synapses relay information through the release of neurotransmitters stored in presynaptic vesicles. The identity, kinetics and location of the vesicle pools that are mobilized by neuronal activity have been studied using a variety of techniques. We created a genetically encoded probe, biosyn, which consists of a biotinylated VAMP2 expressed at presynaptic terminals. We exploited the high-affinity interaction between streptavidin and biotin to label biosyn with fluorescent streptavidin during vesicle fusion. This approach allowed us to tag vesicles sequentially to visualize and establish the identity of presynaptic pools. Using this technique, we were able to distinguish between two different pools of vesicles in rat hippocampal neurons: one that was released in response to presynaptic activity and another, distinct vesicle pool that spontaneously fused with the plasma membrane. We found that the spontaneous vesicles belonged to a 'resting pool' that is normally not mobilized by neuronal activity and whose function was previously unknown.

Pulling force generated by interacting SNAREs facilitates membrane hemifusion

In biological systems, membrane fusion is mediated by specialized proteins. Although soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptors (SNAREs) provide the minimal molecular machinery required to drive membrane fusion, the precise mechanism for SNARE-mediated fusion remains to be established. Here, we used atomic force microscope (AFM) spectroscopy to determine whether the pulling force generated by interacting SNAREs is directly coupled to membrane fusion. The mechanical strength of the SNARE binding interaction was determined by single molecule force measurements. It was revealed that the forced unbinding of the SNARE complex formed between opposing (trans) bilayers involves two activation barriers; where the steep inner barrier governs the transition from the bound to an intermediate state and the outer barrier governs the transition between the intermediate and the unbound state. Moreover, truncation of either SNAP-25 or VAMP 2 reduced the slope of the inner barrier significantly and, consequently, reduced the pulling strength of the SNARE complex; thus, suggesting that the inner barrier determines the binding strength of the SNARE complex. In parallel, AFM compression force measurements revealed that truncated SNAREs were less efficient than native SNAREs in facilitating hemifusion of the apposed bilayers. Together, these findings reveal a mechanism by which a pulling force generated by interacting trans-SNAREs reduces the slope of the hemifusion barrier and, subsequently, facilitates hemifusion and makes the membranes more prone to fusion.

Concurrent binding of complexin and synaptotagmin to liposome-embedded SNARE complexes

Synaptotagmin and complexin regulate SNARE-mediated synaptic vesicle exocytosis. It has been proposed that complexin clamps membrane fusion and that Ca²⁺-synaptotagmin displaces complexin from SNARE complexes to relieve this clamping activity. Using a reconstituted system, we demonstrate that complexin and synaptotagmin simultaneously bind to neuronal SNARE complexes and that both apo-synaptotagmin and complexin inhibit SNARE-mediated membrane fusion. Moreover, the clamping ability of apo-synaptotagmin occluded the clamping activity of complexin until the arrival of a Ca²⁺ trigger, at which point synaptotagmin accelerated fusion while high concentrations of complexin inhibited fusion. Thus, the inhibitory patterns of synaptotagmin and complexin are different, suggesting that SNAREs assemble into distinct states along the fusion pathway. These data also suggest that during synaptotagmin-regulated vesicle−vesicle fusion, complexin does not function as a fusion clamp that is relieved by Ca²⁺-synaptotagmin.

Snapin facilitates the synchronization of synaptic vesicle fusion

Synaptic vesicle (SV) fusion is a fine-tuned process requiring a concert of fusion machineries. Using cortical neurons from snapin-deficient mice, we reveal a role for Snapin in facilitating synchronous release. In addition to reduced frequency of miniature excitatory postsynaptic currents (mini-EPSCs) and smaller release-ready vesicle pool (RRP) size, snapin deficiency results in EPSCs with multiple peaks and increased rise and decay times, reflecting "desynchronized" SV fusion. These defects impair both synaptic precision and efficacy during sustained neurotransmission. Transient expression of Snapin not only rescues the slowed kinetics of EPSCs, but also further accelerates the rate found in wild-type neurons. Furthermore, expression of Snapin-C66A, a dimerization-defective mutant with impaired interactions with SNAP-25 and Synaptotagmin, reduces the RRP size but exhibits less effect on synchronized fusion. Our studies provide mechanistic insights into a dual role of Snapin in enhancing the efficacy of SV priming and in fine-tuning synchronous SV fusion.

Complexin controls the force transfer from SNARE complexes to membranes in fusion

Trans-SNAP receptor (SNARE, where SNAP is defined as soluble NSF attachment protein, and NSF is defined as N-ethylmaleimide-sensitive factor) complexes catalyze synaptic vesicle fusion and bind complexin, but the function of complexin binding to SNARE complexes remains unclear. Here we show that in neuronal synapses, complexin simultaneously suppressed spontaneous fusion and activated fast calcium ion-evoked fusion. The dual function of complexin required SNARE binding and also involved distinct amino-terminal sequences of complexin that localize to the point where trans-SNARE complexes insert into the fusing membranes, suggesting that complexin controls the force that trans-SNARE complexes apply onto the fusing membranes. Consistent with this hypothesis, a mutation in the membrane insertion sequence of the v-SNARE synaptobrevin/vesicle-associated membrane protein (VAMP) phenocopied the complexin loss-of-function state without impairing complexin binding to SNARE complexes. Thus, complexin probably activates and clamps the force transfer from assembled trans-SNARE complexes onto fusing membranes.

The carboxy-terminal domain of complexin I stimulates liposome fusion

Regulated exocytosis requires tight coupling of the membrane fusion machinery to a triggering signal and a fast response time. Complexins are part of this regulation and, together with synaptotagmins, control calcium-dependent exocytosis. Stimulatory and inhibitory functions have been reported for complexins. To test if complexins directly affect membrane fusion, we analyzed the 4 known mammalian complexin isoforms in a reconstituted fusion assay. In contrast to complexin III (CpxIII) and CpxIV, CpxI and CpxII stimulated soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-pin assembly and membrane fusion. This stimulatory effect required a preincubation at low temperature and was specific for neuronal t-SNAREs. Stimulation of membrane fusion was lost when the carboxy-terminal domain of CpxI was deleted or serine 115, a putative phosphorylation site, was mutated. Transfer of the carboxy-terminal domain of CpxI to CpxIII resulted in a stimulatory CpxIII-I chimera. Thus, the carboxy-terminal domains of CpxI and CpxII promote the fusion of high-curvature liposomes.