Complexins: cytosolic proteins that regulate SNAP receptor function

Molecular underpinnings of synaptic vesicle pool heterogeneity

Neuronal communication relies on chemical synaptic transmission for information transfer and processing. Chemical neurotransmission is initiated by synaptic vesicle fusion with the presynaptic active zone resulting in release of neurotransmitters. Classical models have assumed that all synaptic vesicles within a synapse have the same potential to fuse under different functional contexts. In this model, functional differences among synaptic vesicle populations are ascribed to their spatial distribution in the synapse with respect to the active zone. Emerging evidence suggests, however, that synaptic vesicles are not a homogenous population of organelles, and they possess intrinsic molecular differences and differential interaction partners. Recent studies have reported a diverse array of synaptic molecules that selectively regulate synaptic vesicles' ability to fuse synchronously and asynchronously in response to action potentials or spontaneously irrespective of action potentials. Here we discuss these molecular mediators of vesicle pool heterogeneity that are found on the synaptic vesicle membrane, on the presynaptic plasma membrane, or within the cytosol and consider some of the functional consequences of this diversity. This emerging molecular framework presents novel avenues to probe synaptic function and uncover how synaptic vesicle pools impact neuronal signaling.

Dysregulation of Norepinephrine Release in the Absence of Functional Synaptotagmin 7

Synaptotagmin 7 (Syt7) is expressed in cardiac sympathetic nerve terminals where norepinephrine (NE) is released in both Ca(2+)-dependent exocytosis and Ca(2+)-independent norepinephrine transporter (NET)-mediated overflow. The role of Syt7 in the regulation of NE release from cardiac sympathetic nerve terminals is tested by employing a Syt7 knock-in mouse line that expresses a non-functional mutant form of Syt7. In cardiac sympathetic nerve terminals prepared from these Syt7 knock-in mice, the Ca(2+)-dependent component of NE release was diminished. However, these terminals displayed upregulated function of NET (∼130% of controls) and a significant increase in Ca(2+)-independent NE overflow (∼140% of controls), which is greater than the Ca(2+)-dependent component of NE exocytosis occurring in wild-type controls. Consistent with a significant increase in NE overflow, the Syt7 knock-in mice showed significantly higher blood pressures compared to those of littermate wild-type and heterozygous mice. Our results indicate that the lack of functional Syt7 dysregulates NE release from cardiac sympathetic nerve terminals.

Transmission, Development, and Plasticity of Synapses

Chemical synapses are sites of contact and information transfer between a neuron and its partner cell. Each synapse is a specialized junction, where the presynaptic cell assembles machinery for the release of neurotransmitter, and the postsynaptic cell assembles components to receive and integrate this signal. Synapses also exhibit plasticity, during which synaptic function and/or structure are modified in response to activity. With a robust panel of genetic, imaging, and electrophysiology approaches, and strong evolutionary conservation of molecular components, Drosophila has emerged as an essential model system for investigating the mechanisms underlying synaptic assembly, function, and plasticity. We will discuss techniques for studying synapses in Drosophila, with a focus on the larval neuromuscular junction (NMJ), a well-established model glutamatergic synapse. Vesicle fusion, which underlies synaptic release of neurotransmitters, has been well characterized at this synapse. In addition, studies of synaptic assembly and organization of active zones and postsynaptic densities have revealed pathways that coordinate those events across the synaptic cleft. We will also review modes of synaptic growth and plasticity at the fly NMJ, and discuss how pre- and postsynaptic cells communicate to regulate plasticity in response to activity.

Evolutionary conservation of complexins: from choanoflagellates to mice

Complexins are synaptic SNARE complex-binding proteins that cooperate with synaptotagmins in activating Ca(2+)-stimulated, synaptotagmin-dependent synaptic vesicle exocytosis and in clamping spontaneous, synaptotagmin-independent synaptic vesicle exocytosis. Here, we show that complexin sequences are conserved in some non-metazoan unicellular organisms and in all metazoans, suggesting that complexins are a universal feature of metazoans that predate metazoan evolution. We show that complexin from Nematostella vectensis, a cnidarian sea anemone far separated from mammals in metazoan evolution, functionally replaces mouse complexins in activating Ca(2+)-triggered exocytosis, but is unable to clamp spontaneous exocytosis. Thus, the activating function of complexins is likely conserved throughout metazoan evolution.

Complexin stabilizes newly primed synaptic vesicles and prevents their premature fusion at the mouse calyx of held synapse

Complexins (Cplxs) are small synaptic proteins that cooperate with SNARE-complexes in the control of synaptic vesicle (SV) fusion. Studies involving genetic mutation, knock-down, or knock-out indicated two key functions of Cplx that are not mutually exclusive but cannot easily be reconciled, one in facilitating SV fusion, and one in "clamping" SVs to prevent premature fusion. Most studies on the role of Cplxs in mammalian synapse function have relied on cultured neurons, heterologous expression systems, or membrane fusion assays in vitro, whereas little is known about the function of Cplxs in native synapses. We therefore studied consequences of genetic ablation of Cplx1 in the mouse calyx of Held synapse, and discovered a developmentally exacerbating phenotype of reduced spontaneous and evoked transmission but excessive asynchronous release after stimulation, compatible with combined facilitating and clamping functions of Cplx1. Because action potential waveforms, Ca(2+) influx, readily releasable SV pool size, and quantal size were unaltered, the reduced synaptic strength in the absence of Cplx1 is most likely a consequence of a decreased release probability, which is caused, in part, by less tight coupling between Ca(2+) channels and docked SV. We found further that the excessive asynchronous release in Cplx1-deficient calyces triggered aberrant action potentials in their target neurons, and slowed-down the recovery of EPSCs after depleting stimuli. The augmented asynchronous release had a delayed onset and lasted hundreds of milliseconds, indicating that it predominantly represents fusion of newly recruited SVs, which remain unstable and prone to premature fusion in the absence of Cplx1.

Complexins: small but capable

Despite intensive research, it is still unclear how an immediate and profound acceleration of exocytosis is triggered by appropriate Ca(2+)-stimuli in presynaptic terminals. This is due to the fact that the molecular mechanisms of "docking" and "priming" reactions, which set up secretory vesicles to fuse at millisecond time scale, are extremely hard to study. Yet, driven by a fruitful combination of in vitro and in vivo analyses, our mechanistic understanding of Ca(2+)-triggered vesicle fusion has certainly advanced in the past few years. In this review, we aim to highlight recent progress and emerging views on the molecular mechanisms, by which constitutively forming SNAREpins are organized in functional, tightly regulated units for synchronized release. In particular, we will focus on the role of the small regulatory factor complexin whose function in Ca(2+)-dependent exocytosis has been controversially discussed for more than a decade. Special emphasis will also be laid on the functional relationship of complexin and synaptotagmin, as both proteins possibly act as allies and/or antagonists to govern SNARE-mediated exocytosis.

How could SNARE proteins open a fusion pore?

The SNARE (Soluble NSF Attachment protein REceptor) complex, which in mammalian neurosecretory cells is composed of the proteins synaptobrevin 2 (also called VAMP2), syntaxin, and SNAP-25, plays a key role in vesicle fusion. In this review, we discuss the hypothesis that, in neurosecretory cells, fusion pore formation is directly accomplished by a conformational change in the SNARE complex via movement of the transmembrane domains.

Re-visiting the trans insertion model for complexin clamping

We have previously proposed that complexin cross-links multiple pre-fusion SNARE complexes via a trans interaction to function as a clamp on SNARE-mediated neurotransmitter release. A recent NMR study was unable to detect the trans clamping interaction of complexin and therefore questioned the previous interpretation of the fluorescence resonance energy transfer and isothermal titration calorimetry data on which the trans clamping model was originally based. Here we present new biochemical data that underscore the validity of our previous interpretation and the continued relevancy of the trans insertion model for complexin clamping.

DOI:http://dx.doi.org/10.7554/eLife.04463.001

Molecules called neurotransmitters are used to carry signals between neurons. The neurotransmitters in the first neuron are stored in small bubble-like structures called synaptic vesicles. When this neuron is ready to send a signal to a second neuron, the membrane that encloses the synaptic vesicle fuses with the cell membrane that surrounds the neuron. This involves SNARE proteins in the vesicle membrane interacting with similar proteins in the cell membrane to form a SNARE complex, which then proceeds to ‘zip’ the two membranes together.

Other proteins are involved in the fusion process and the release of the neurotransmitters. For example, complexins bind to SNARE proteins during the formation of the SNARE complex in order to temporarily halt the fusion process. This ‘clamping’ interaction ensures that the neurotransmitters are released at the appropriate time.

Researchers have proposed two different models of the clamping interaction. In the trans clamping model a region in the complexins called the accessory helix extends forward and clamps SNARE proteins that are present on the two membranes. An alternative model explains clamping in terms of electrostatic interactions between the accessory helix and the two membranes. These interactions are repulsive because the accessory helix and the membranes are all negatively charged.

Now Krishnakumar, Li et al.—including some of the researchers who first proposed the trans clamping model—have used a variety of biochemical techniques to re-examine the clamping interaction. These experiments support the idea that the accessory helix binds to and clamps a SNARE protein, as suggested by the trans clamping model. The results of recent in vivo experiments on fruit flies have also provided support for the trans clamping model, although further work is need to compare the models in both in vitro and in vivo systems.

DOI:http://dx.doi.org/10.7554/eLife.04463.002

Retinoic Acid and LTP Recruit Postsynaptic AMPA Receptors Using Distinct SNARE-Dependent Mechanisms

Retinoic acid (RA)-dependent homeostatic plasticity and NMDA receptor-dependent long-term potentiation (LTP), a form of Hebbian plasticity, both enhance synaptic strength by increasing the abundance of postsynaptic AMPA receptors (AMPARs). However, it is unclear whether the molecular mechanisms mediating AMPAR trafficking during homeostatic and Hebbian plasticity differ, and it is unknown how RA signaling impacts Hebbian plasticity. Here, we show that RA increases postsynaptic AMPAR abundance using an activity-dependent mechanism that requires a unique SNARE (soluble NSF-attachment protein receptor)-dependent fusion machinery different from that mediating LTP. Specifically, RA-induced AMPAR trafficking did not involve complexin, which activates SNARE complexes containing syntaxin-1 or -3, but not complexes containing syntaxin-4, whereas LTP required complexin. Moreover, RA-induced AMPAR trafficking utilized the Q-SNARE syntaxin-4, whereas LTP utilized syntaxin-3; both additionally required the Q-SNARE SNAP-47 and the R-SNARE synatobrevin-2. Finally, acute RA treatment blocked subsequent LTP expression, probably by increasing AMPAR trafficking. Thus, RA-induced homeostatic plasticity involves a novel, activity-dependent postsynaptic AMPAR-trafficking pathway mediated by a unique SNARE-dependent fusion machinery.

Expression of the synaptic exocytosis-regulating molecule complexin 2 in taste buds and its participation in peripheral taste transduction

Taste information from type III taste cells to gustatory neurons is thought to be transmitted via synapses. However, the molecular mechanisms underlying taste transduction through this pathway have not been fully elucidated. In this study, to identify molecules that participate in synaptic taste transduction, we investigated whether complexins (Cplxs), which play roles in regulating membrane fusion in synaptic vesicle exocytosis, were expressed in taste bud cells. Among four Cplx isoforms, strong expression of Cplx2 mRNA was detected in type III taste cells. To investigate the function of CPLX2 in taste transduction, we observed taste responses in CPLX2‐knockout mice. When assessed with electrophysiological and behavioral assays, taste responses to some sour stimuli in CPLX2‐knockout mice were significantly lower than those in wild‐type mice. These results suggested that CPLX2 participated in synaptic taste transduction from type III taste cells to gustatory neurons. A part of taste information is thought to be transmitted via synapses. However, the molecular mechanisms have not been fully elucidated. To identify molecules that participate in synaptic taste transduction, we investigated complexins (Cplxs) expression in taste bud cells. Strong expression of Cplx2 mRNA was detected in taste bud cells. Furthermore, taste responses to some sour stimuli in CPLX2‐ knockout mice were significantly lower than those in wild‐type mice. These suggested that CPLX2 participated in synaptic taste transduction.

Synaptosome-related (SNARE) genes and their interactions contribute to the susceptibility and working memory of attention-deficit/hyperactivity disorder in males

BACKGROUNDS

N-ethylmaleimide-sensitive attachment protein receptor (SNARE) complex involved in neurotransmission via exocytosis was implicated in attention-deficit/hyperactivity disorder (ADHD). The present study investigated the influence of SNARE related genes and their interaction on ADHD susceptibility and their cognitive functions.

METHODS

We genotyped eight single nucleotide polymorphisms (SNP) of Syntaxin 1A (STX1A), vesicle-associated membrane protein 2 (VAMP2) and synaptosomal-associated protein 25 kDa (SNAP25) and conducted case-control studies in 1404 male ADHD and 617 male controls. Quantitative analyses were performed for genotypes and performance on the Rey-Osterrieth complex figure test (RCFT), digit span test and Stroop test in 383 ADHD males. In addition, we explored gene-gene interactions by generalized multifactor dimensionality reduction (GMDR) followed with logistic regression and analyses of covariance for verifying.

RESULTS

Genotypic distribution of rs875342 of STX1A was significantly different between ADHD and controls. The SNPs, rs363039 of SNAP25 and rs1150 of VAMP2, were significantly associated with RCFT scores, while rs875342 of STX1A with digit span. We found genetic interaction models between these three genes and ADHD susceptibility as well as working memory function evaluated by RCFT.

CONCLUSION

SNARE complex genes and their interactions may play a significant role in susceptibility and working memory of ADHD.

Otoferlin deficiency in zebrafish results in defects in balance and hearing: rescue of the balance and hearing phenotype with full-length and truncated forms of mouse otoferlin

Sensory hair cells convert mechanical motion into chemical signals. Otoferlin, a six-C2 domain transmembrane protein linked to deafness in humans, is hypothesized to play a role in exocytosis at hair cell ribbon synapses. To date, however, otoferlin has been studied almost exclusively in mouse models, and no rescue experiments have been reported. Here we describe the phenotype associated with morpholino-induced otoferlin knockdown in zebrafish and report the results of rescue experiments conducted with full-length and truncated forms of otoferlin. We found that expression of otoferlin occurs early in development and is restricted to hair cells and the midbrain. Immunofluorescence microscopy revealed localization to both apical and basolateral regions of hair cells. Knockdown of otoferlin resulted in hearing and balance defects, as well as locomotion deficiencies. Further, otoferlin morphants had uninflated swim bladders. Rescue experiments conducted with mouse otoferlin restored hearing, balance, and inflation of the swim bladder. Remarkably, truncated forms of otoferlin retaining the C-terminal C2F domain also rescued the otoferlin knockdown phenotype, while the individual N-terminal C2A domain did not. We conclude that otoferlin plays an evolutionarily conserved role in vertebrate hearing and that truncated forms of otoferlin can rescue hearing and balance.

The dendritic SNARE fusion machinery involved in AMPARs insertion during long-term potentiation

Sorting endosomes carry α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) from their maturation sites to their final destination at the dendritic plasma membrane through both constitutive and regulated exocytosis. Insertion of functional AMPARs into the postsynaptic membrane is essential for maintaining fast excitatory synaptic transmission and plasticity. Despite this crucial role in neuronal function, the machinery mediating the fusion of AMPAR-containing endosomes in dendrites has been largely understudied in comparison to presynaptic vesicle exocytosis. Increasing evidence suggests that similarly to neurotransmitter release, AMPARs insertion relies on the formation of a SNARE complex (soluble NSF-attachment protein receptor), whose composition in dendrites has just begun to be elucidated. This review analyzes recent findings of the fusion machinery involved in regulated AMPARs insertion and discusses how dendritic exocytosis and AMPARs lateral diffusion may work together to support synaptic plasticity.

Exocytosis and synaptic vesicle function

Synaptic vesicles release their vesicular contents to the extracellular space by Ca(2+)-triggered exocytosis. The Ca(2+)-triggered exocytotic process is regulated by synaptotagmin (Syt), a vesicular Ca(2+)-binding C2 domain protein. Synaptotagmin 1 (Syt1), the most studied major isoform among 16 Syt isoforms, mediates Ca(2+)-triggered synaptic vesicle exocytosis by interacting with the target membranes and SNARE/complexin complex. In synapses of the central nervous system, synaptobrevin 2, a major vesicular SNARE protein, forms a ternary SNARE complex with the plasma membrane SNARE proteins, syntaxin 1 and SNAP25. The affinities of Ca(2+)-dependent interactions between Syt1 and its targets (i.e., SNARE complexes and membranes) are well correlated with the efficacies of the corresponding exocytotic processes. Therefore, different SNARE protein isoforms and membrane lipids, which interact with Syt1 with various affinities, are capable of regulating the efficacy of Syt1-mediated exocytosis. Otoferlin, another type of vesicular C2 domain protein that binds to the membrane in a Ca(2+)-dependent manner, is also involved in the Ca(2+)-triggered synaptic vesicle exocytosis in auditory hair cells. However, the functions of otoferlin in the exocytotic process are not well understood. In addition, at least five different types of synaptic vesicle proteins such as synaptic vesicle protein 2, cysteine string protein α, rab3, synapsin, and a group of proteins containing four transmembrane regions, which includes synaptophysin, synaptogyrin, and secretory carrier membrane protein, are involved in modulating the exocytotic process by regulating the formation and trafficking of synaptic vesicles.

The principle of membrane fusion in the cell (Nobel lecture)

Cells contain small membrane-enclosed vesicles which transport many kinds of cargo between the compartments of the cell. The result is a choreographed program of secretory, biosynthetic, and endocytic protein traffic that serves the cell's internal physiologic needs.

The molecular machinery of neurotransmitter release (Nobel lecture)

The most important property of synaptic transmission is its speed, which is crucial for the overall workings of the brain. In his Nobel Lecture, T. C. Südhof explains how the synaptic vesicle and the plasma membrane undergo rapid fusion during neurotransmitter release and how this process is spatially organized, such that opening of Ca(2+) -channels allows rapid translation of the entering Ca(2+) signal into a fusion event.

Thiamine deficiency: an update of pathophysiologic mechanisms and future therapeutic considerations

Thiamine is an essential vitamin that is necessary to maintain the functional integrity of cells in the brain. Its deficiency is the underlying cause of Wernicke's encephalopathy (WE), a disorder primarily associated with, but not limited to, chronic alcoholism. Thiamine deficiency leads to the development of impaired energy metabolism due to mitochondrial dysfunction in focal regions of the brain resulting in cerebral vulnerability. The consequences of this include oxidative stress, excitotoxicity, inflammatory responses, decreased neurogenesis, blood-brain barrier disruption, lactic acidosis and a reduction in astrocyte functional integrity involving a loss of glutamate transporters and other astrocyte-specific proteins which together contribute in a major way to the resulting neurodegeneration. Exactly how these factors acting in concert lead to the demise of neurons is unclear. In this review we reassess their relative importance in the light of more recent findings and discuss therapeutic possibilities that may provide hope for the future for individuals with WE.

Vesicular transport earns a Nobel

The field of intracellular traffic is celebrating the awarding of the 2013 Nobel Prize in Physiology or Medicine to three of its most distinguished scientists--James Rothman, Randy Schekman, and Thomas Südhof--for their discoveries on the molecular mechanisms of vesicular transport. Their outstanding achievements serve as inspiration for the next generation of scientists to continue the task of uncovering new principles of intracellular compartmentalization and dynamics.

Complexin inhibits spontaneous release and synchronizes Ca2+-triggered synaptic vesicle fusion by distinct mechanisms

Previously we showed that fast Ca²⁺-triggered vesicle fusion with reconstituted neuronal SNAREs and synaptotagmin-1 begins from an initial hemifusion-free membrane point contact, rather than a hemifusion diaphragm, using a single vesicle–vesicle lipid/content mixing assay (Diao et al., 2012). When complexin-1 was included, a more pronounced Ca²⁺-triggered fusion burst was observed, effectively synchronizing the process. Here we show that complexin-1 also reduces spontaneous fusion in the same assay. Moreover, distinct effects of several complexin-1 truncation mutants on spontaneous and Ca²⁺-triggered fusion closely mimic those observed in neuronal cultures. The very N-terminal domain is essential for synchronization of Ca²⁺-triggered fusion, but not for suppression of spontaneous fusion, whereas the opposite is true for the C-terminal domain. By systematically varying the complexin-1 concentration, we observed differences in titration behavior for spontaneous and Ca²⁺-triggered fusion. Taken together, complexin-1 utilizes distinct mechanisms for synchronization of Ca²⁺-triggered fusion and inhibition of spontaneous fusion.

DOI:http://dx.doi.org/10.7554/eLife.03756.001

Fast, Ca2+-dependent exocytosis at nerve terminals: shortcomings of SNARE-based models

Investigations over the last two decades have made major inroads in clarifying the cellular and molecular events that underlie the fast, synchronous release of neurotransmitter at nerve endings. Thus, appreciable progress has been made in establishing the structural features and biophysical properties of the calcium (Ca2+) channels that mediate the entry into nerve endings of the Ca2+ ions that trigger neurotransmitter release. It is now clear that presynaptic Ca2+ channels are regulated at many levels and the interplay of these regulatory mechanisms is just beginning to be understood. At the same time, many lines of research have converged on the conclusion that members of the synaptotagmin family serve as the primary Ca2+ sensors for the action potential-dependent release of neurotransmitter. This identification of synaptotagmins as the proteins which bind Ca2+ and initiate the exocytotic fusion of synaptic vesicles with the plasma membrane has spurred widespread efforts to reveal molecular details of synaptotagmin's action. Currently, most models propose that synaptotagmin interfaces directly or indirectly with SNARE (soluble, N-ethylmaleimide sensitive factor attachment receptors) proteins to trigger membrane fusion. However, in spite of intensive efforts, the field has not achieved consensus on the mechanism by which synaptotagmins act. Concurrently, the precise sequence of steps underlying SNARE-dependent membrane fusion remains controversial. This review considers the pros and cons of the different models of SNARE-mediated membrane fusion and concludes by discussing a novel proposal in which synaptotagmins might directly elicit membrane fusion without the intervention of SNARE proteins in this final fusion step.

Genetic analysis of the Complexin trans-clamping model for cross-linking SNARE complexes in vivo

Complexin (Cpx) is a SNARE-binding protein that regulates neurotransmission by clamping spontaneous synaptic vesicle fusion in the absence of Ca(2+) influx while promoting evoked release in response to an action potential. Previous studies indicated Cpx may cross-link multiple SNARE complexes via a trans interaction to function as a fusion clamp. During Ca(2+) influx, Cpx is predicted to undergo a conformational switch and collapse onto a single SNARE complex in a cis-binding mode to activate vesicle release. To test this model in vivo, we performed structure-function studies of the Cpx protein in Drosophila. Using genetic rescue approaches with cpx mutants that disrupt SNARE cross-linking, we find that manipulations that are predicted to block formation of the trans SNARE array disrupt the clamping function of Cpx. Unexpectedly, these same mutants rescue action potential-triggered release, indicating trans-SNARE cross-linking by Cpx is not a prerequisite for triggering evoked fusion. In contrast, mutations that impair Cpx-mediated cis-SNARE interactions that are necessary for transition from an open to closed conformation fail to rescue evoked release defects in cpx mutants, although they clamp spontaneous release normally. Our in vivo genetic manipulations support several predictions made by the Cpx cross-linking model, but unexpected results suggest additional mechanisms are likely to exist that regulate Cpx's effects on SNARE-mediated fusion. Our findings also indicate that the inhibitory and activating functions of Cpx are genetically separable, and can be mapped to distinct molecular mechanisms that differentially regulate the SNARE fusion machinery.

Complexin synchronizes primed vesicle exocytosis and regulates fusion pore dynamics

ComplexinII and SynaptotagminI coordinately transform the constitutively active SNARE-mediated fusion mechanism into a highly synchronized, Ca²⁺-triggered release apparatus.

ComplexinII (CpxII) and SynaptotagminI (SytI) have been implicated in regulating the function of SNARE proteins in exocytosis, but their precise mode of action and potential interplay have remained unknown. In this paper, we show that CpxII increases Ca²⁺-triggered vesicle exocytosis and accelerates its secretory rates, providing two independent, but synergistic, functions to enhance synchronous secretion. Specifically, we demonstrate that the C-terminal domain of CpxII increases the pool of primed vesicles by hindering premature exocytosis at submicromolar Ca²⁺ concentrations, whereas the N-terminal domain shortens the secretory delay and accelerates the kinetics of Ca²⁺-triggered exocytosis by increasing the Ca²⁺ affinity of synchronous secretion. With its C terminus, CpxII attenuates fluctuations of the early fusion pore and slows its expansion but is functionally antagonized by SytI, enabling rapid transmitter discharge from single vesicles. Thus, our results illustrate how key features of CpxII, SytI, and their interplay transform the constitutively active SNARE-mediated fusion mechanism into a highly synchronized, Ca²⁺-triggered release apparatus.

Re-examining how complexin inhibits neurotransmitter release

Complexins play activating and inhibitory functions in neurotransmitter release. The complexin accessory helix inhibits release and was proposed to insert into SNARE complexes to prevent their full assembly. This model was supported by ‘superclamp’ and ‘poor-clamp’ mutations that enhanced or decreased the complexin-I inhibitory activity in cell–cell fusion assays, and by the crystal structure of a superclamp mutant bound to a synaptobrevin-truncated SNARE complex. NMR studies now show that the complexin-I accessory helix does not insert into synaptobrevin-truncated SNARE complexes in solution, and electrophysiological data reveal that superclamp mutants have slightly stimulatory or no effects on neurotransmitter release, whereas a poor-clamp mutant inhibits release. Importantly, increasing or decreasing the negative charge of the complexin-I accessory helix inhibits or stimulates release, respectively. These results suggest a new model whereby the complexin accessory helix inhibits release through electrostatic (and perhaps steric) repulsion enabled by its location between the vesicle and plasma membranes.

DOI:http://dx.doi.org/10.7554/eLife.02391.001

The instructions sent to, from and within the brain are rapidly transmitted along neurons in the form of electrical signals. These signals cannot pass across the small gaps—called synapses—that separate neighboring neurons. Instead, neurons release chemicals called neurotransmitters into the synapses, and these relay the signal to the next neuron.

The neurotransmitters are stored inside neurons in small bubbles called vesicles. To release these neurotransmitters into the synapse, the membrane that encloses the vesicle fuses with the membrane that surrounds the neuron. To fuse the membranes, proteins embedded in the vesicle membrane interact with similar proteins in the neuron membrane to form a structure called a SNARE complex. Additional proteins control membrane fusion to ensure that the signal is passed to the other neuron at the right time and with the appropriate efficiency.

Among these proteins are the complexins, which are often found attached to SNARE complexes. Although different parts of complexins can both help and hinder membrane fusion, a part known as an accessory helix is thought to have only one role—to stop the membranes from fusing together. Several models have been suggested for how the accessory helix interferes with fusion. However, after performing a range of analyses by diverse biophysical techniques, Trimbuch, Xu et al. suggest these models are unlikely to describe the process accurately.

Instead, Trimbuch, Xu et al. propose a new model based on the electrostatic properties of two molecules that are both negatively charged. An accessory helix taken from a fruit fly complexin was more negatively charged than a mammalian version, and experiments showed it was also better at preventing the release of neurotransmitters. It is thought that the negative charges on the helix hold the membranes apart because the helix is located between the membranes, which are also negatively charged. Consistent with this model, Trimbuch, Xu et al. showed that the membranes fused more easily when some of the negative charges on the accessory helix were replaced with positive charges. The next challenges are to test the model further with additional studies, and to explain how other proteins work with complexins to control neurotransmitter release.

DOI:http://dx.doi.org/10.7554/eLife.02391.002

All roads lead to neuroscience: the 2013 Nobel Prize in Physiology or Medicine

Numerous metaphors have been employed to describe the achievements of the 2013 Nobel Laureates in Physiology or Medicine, James E. Rothman, Randy W. Schekman, and Thomas C. Südhof, who were honored for "their discoveries of machinery regulating vesicle traffic, a major transport system in our cells." Most of these metaphors referred to the mundane issue of business logistics, and there is probably no other cell type in which the logistics problem is more pressing than in neurons.

Tocotrienols enhance melanosome degradation through endosome docking/fusion proteins in B16F10 melanoma cells

Vitamin E inhibits tyrosinase activity and acts as a melanogenesis inhibitor in epidermal melanocytes in vitro. However, there is no direct evidence indicating that melanosomes are degraded in lysosomes in the presence of vitamin E. To determine whether vitamin E-induced melanosome disintegration is related to the expression of endosome docking/fusion proteins in B16F10 melanoma cells, electron microscopy, reverse transcription-polymerase chain reaction (RT-PCR), and real-time PCR were used to observe the effects of tocomin (α-tocopherols and α,γ,δ-tocotrienols in palm oil) on B16F10 melanoma cells. Melanosomal integrity was lost in lysosomes of B16F10 melanoma cells when treated with tocomin, indicating that tocomin caused the degradation of melanosomes in the lysosomal compartment. RT-PCR and real-time PCR analysis demonstrated mRNA expression of tyrosinase and the endosome docking/fusion proteins (syntaxin7, Rab7, Vps11, Vps16, Vps33, Vps39, and Vps41). Expression of syntaxin7, Vps16, Vps33, and Vps41 mRNA increased significantly in cells treated with tocomin compared with that in controls. These results indicate that the tocomin-induced degradation of melanosomes in the lysosomal compartment occurs with an increase in endosome docking/fusion proteins (syntaxin7, Vps16, Vps33, and Vps41) in cultured B16F10 melanoma cells.

Milk secretion: The role of SNARE proteins

During lactation, polarized mammary epithelial secretory cells (MESCs) secrete huge quantities of the nutrient molecules that make up milk, i.e. proteins, fat globules and soluble components such as lactose and minerals. Some of these nutrients are only produced by the MESCs themselves, while others are to a great extent transferred from the blood. MESCs can thus be seen as a crossroads for both the uptake and the secretion with cross-talks between intracellular compartments that enable spatial and temporal coordination of the secretion of the milk constituents. Although the physiology of lactation is well understood, the molecular mechanisms underlying the secretion of milk components remain incompletely characterized. Major milk proteins, namely caseins, are secreted by exocytosis, while the milk fat globules are released by budding, being enwrapped by the apical plasma membrane. Prolactin, which stimulates the transcription of casein genes, also induces the production of arachidonic acid, leading to accelerated casein transport and/or secretion. Because of their ability to form complexes that bridge two membranes and promote their fusion, SNARE (Soluble N-ethylmaleimide-Sensitive Factor Attachment Protein Receptor) proteins are involved in almost all intracellular trafficking steps and exocytosis. As SNAREs can bind arachidonic acid, they could be the effectors of the secretagogue effect of prolactin in MESCs. Indeed, some SNAREs have been observed between secretory vesicles and lipid droplets suggesting that these proteins could not only orchestrate the intracellular trafficking of milk components but also act as key regulators for both the coupling and coordination of milk product secretion in response to hormones.

Voltage-gated calcium channels function as Ca2+-activated signaling receptors

Voltage-gated calcium channels (VGCCs) are transmembrane cell surface proteins responsible for multifunctional signals. In response to voltage, VGCCs trigger synaptic transmission, drive muscle contraction, and regulate gene expression. Voltage perturbations open VGCCs enabling Ca(2+) binding to the low affinity Ca(2+) binding site of the channel pore. Subsequent to permeation, Ca(2+) targets selective proteins to activate diverse signaling pathways. It is becoming apparent that the Ca(2+)-bound channel triggers secretion in excitable cells and drives contraction in cardiomyocytes prior to Ca(2+) permeation. Here, I highlight recent data implicating receptor-like function of the Ca(2+)-bound channel in converting external Ca(2+) into an intracellular signal. The two sequential mechanistic perspectives of VGCC function are discussed in the context of the prevailing and long-standing current models of depolarization-evoked secretion and cardiac contraction.

Neurotransmitter release: the last millisecond in the life of a synaptic vesicle

During an action potential, Ca(2+) entering a presynaptic terminal triggers synaptic vesicle exocytosis and neurotransmitter release in less than a millisecond. How does Ca(2+) stimulate release so rapidly and precisely? Work over the last decades revealed that Ca(2+) binding to synaptotagmin triggers release by stimulating synaptotagmin binding to a core fusion machinery composed of SNARE and SM proteins that mediates membrane fusion during exocytosis. Complexin adaptor proteins assist synaptotagmin by activating and clamping this core fusion machinery. Synaptic vesicles containing synaptotagmin are positioned at the active zone, the site of vesicle fusion, by a protein complex containing RIM proteins. RIM proteins activate docking and priming of synaptic vesicles and simultaneously recruit Ca(2+) channels to active zones, thereby connecting in a single complex primed synaptic vesicles to Ca(2+) channels. This architecture allows direct flow of Ca(2+) ions from Ca(2+) channels to synaptotagmin, which then triggers fusion, thus mediating tight millisecond coupling of an action potential to neurotransmitter release.

Microarray and FISH-based genotype-phenotype analysis of 22 Japanese patients with Wolf-Hirschhorn syndrome

Wolf-Hirschhorn syndrome (WHS) is a contiguous gene deletion syndrome of the distal 4p chromosome, characterized by craniofacial features, growth impairment, intellectual disability, and seizures. Although genotype-phenotype correlation studies have previously been published, several important issues remain to be elucidated including seizure severity. We present detailed clinical and molecular-cytogenetic findings from a microarray and fluorescence in situ hybridization (FISH)-based genotype-phenotype analysis of 22 Japanese WHS patients, the first large non-Western series. 4p deletions were terminal in 20 patients and interstitial in two, with deletion sizes ranging from 2.06 to 29.42 Mb. The new Wolf-Hirschhorn syndrome critical region (WHSCR2) was deleted in all cases, and duplication of other chromosomal regions occurred in four. Complex mosaicism was identified in two cases: two different 4p terminal deletions; a simple 4p terminal deletion and an unbalanced translocation with the same 4p breakpoint. Seizures began in infancy in 33% (2/6) of cases with small (<6 Mb) deletions and in 86% (12/14) of cases with larger deletions (>6 Mb). Status epilepticus occurred in 17% (1/6) with small deletions and in 87% (13/15) with larger deletions. Renal hypoplasia or dysplasia and structural ocular anomalies were more prevalent in those with larger deletions. A new susceptible region for seizure occurrence is suggested between 0.76 and 1.3 Mb from 4 pter, encompassing CTBP1 and CPLX1, and distal to the previously-supposed candidate gene LETM1. The usefulness of bromide therapy for seizures and additional clinical features including hypercholesterolemia are also described.

Deconstructing complexin function in activating and clamping Ca2+-triggered exocytosis by comparing knockout and knockdown phenotypes

Complexin, a presynaptic protein that avidly binds to assembled SNARE complexes, is widely acknowledged to activate Ca(2+)-triggered exocytosis. In addition, studies of invertebrate complexin mutants and of mouse neurons with a double knockdown (DKD) of complexin-1 and -2 suggested that complexin maintains the readily releasable pool (RRP) of vesicles and clamps spontaneous exocytosis. In contrast, studies of mouse neurons with a double knockout (DKO) of complexin-1 and -2, largely carried out in hippocampal autapses, did not detect changes in the RRP size or in spontaneous exocytosis. To clarify complexin function, we here directly compared in two different preparations, cultured cortical and olfactory bulb neurons, the phenotypes of complexin DKD and DKO neurons. We find that complexin-deficient DKD and DKO neurons invariably exhibit a ~50% decrease in vesicle priming. Moreover, the DKD consistently increased spontaneous exocytosis, but the DKO did so in cortical but not olfactory bulb neurons. Furthermore, the complexin DKD but not the complexin DKO caused a compensatory increase in complexin-3 and -4 mRNA levels; overexpression of complexin-3 but not complexin-1 increased spontaneous exocytosis. Complexin-3 but not complexin-1 contains a C-terminal lipid anchor attaching it to the plasma membrane; addition of a similar lipid anchor to complexin-1 converted complexin-1 from a clamp into an activator of spontaneous exocytosis. Viewed together, our data suggest that complexin generally functions in priming and Ca(2+) triggering of exocytosis, and additionally contributes to the control of spontaneous exocytosis dependent on the developmental history of a neuron and on the subcellular localization of the complexin.

AMPAR trafficking in synapse maturation and plasticity

Glutamate ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) mediate most fast excitatory synaptic transmission in the central nervous system. The content and composition of AMPARs in postsynaptic membranes (which determine synaptic strength) are dependent on the regulated trafficking of AMPAR subunits in and out of the membranes. AMPAR trafficking is a key mechanism that drives nascent synapse development, and is the main determinant of both Hebbian and homeostatic plasticity in mature synapses. Hebbian plasticity seems to be the biological substrate of at least some forms of learning and memory; while homeostatic plasticity (also known as synaptic scaling) keeps neuronal circuits stable by maintaining changes within a physiological range. In this review, we examine recent findings that provide further understanding of the role of AMPAR trafficking in synapse maturation, Hebbian plasticity, and homeostatic plasticity.

NOX2 deficiency ameliorates cerebral injury through reduction of complexin II-mediated glutamate excitotoxicity in experimental stroke

Although NADPH oxidase (NOX)-mediated oxidative stress is considered one of the major mechanisms triggering the pathogenic actions of ischemic stroke and very recent studies have indicated that NADPH oxidase is a major source of reactive oxygen species (ROS) production controlling glutamate release, how neuronal NADPH oxidase activation is coupled to glutamate release is not well understood. Therefore, in this study, we used an in vivo transient middle cerebral artery occlusion model and in vitro primary cell cultures to test whether complexins, the regulators of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes necessary for vesicle fusion, are associated with NOX2-derived ROS and contribute to glutamate-mediated excitotoxicity in ischemic stroke. In this study, we first identified the upregulation of complexin II in the ischemic brain and evaluated its potential role in ischemic stroke showing that gene silencing of complexin II ameliorated cerebral injury as evidenced by reduced infarction volume, neurological deficit, and neuron necrosis accompanied by decreased glutamate levels, consistent with the results from NOX2(-/-) mice with ischemic stroke. We further demonstrated that complexin II expression was mediated by NOX2 in primary cultured neurons subjected to oxygen-glucose deprivation (OGD) and contributed to OGD-induced glutamate release and neuron necrosis via SNARE signaling. Taken together, these findings for the first time provide evidence that complexin II is a central target molecule that links NADPH oxidase-derived ROS to glutamate-mediated neuronal excitotoxicity in ischemic stroke.

Hyperactivity and cortical disinhibition in mice with restricted expression of mutant huntingtin to parvalbumin-positive cells

Recent evidence suggests that interneurons are involved in the pathophysiology of Huntington Disease (HD). Abnormalities in the function of interneurons expressing the calcium buffer parvalbumin (PV) have been observed in multiple mouse models of HD, although it is not clear how PV-positive interneuron dysfunction contributes to behavioral and synaptic deficits. Here, we use the cre-lox system to drive expression of mutant huntingtin (mthtt) in parvalbumin (PV)-positive neurons and find that mutant mice exhibit diffuse mthtt immunoreactivity in PV-rich areas at 10months of age and mthtt aggregates in PV-positive processes at 24months of age. At midlife, mutant mice are hyperactive and display impaired GABA release in the motor cortex, characterized by reduced miniature inhibitory events and severely blunted responses to gamma frequency stimulation, without a loss of PV-positive interneurons. In contrast, 24month-old mutant mice show normalized behavior and responses to gamma frequency stimulation, possibly due to compensatory changes in pyramidal neurons or the formation of inclusions with age. These data indicate that mthtt expression in PV-positive neurons is sufficient to drive a hyperactive phenotype and suggest that mthtt-mediated dysfunction in PV-positive neuronal populations could be a key factor in the hyperkinetic behavior observed in HD. Further clarification of the roles for specific PV-positive populations in this phenotype is warranted to definitively identify cellular targets for intervention.

Rapid release revealed: honoring the synapse

This year, the Albert Lasker Basic Medical Research Award will be shared by Richard Scheller and Thomas Südhof for their elucidation of the molecular mechanisms underlying neurotransmitter release. Their discoveries provided insight into the molecular basis of synaptic transmission and enhanced our understanding of how synaptic dysfunction may cause neuropsychiatric disorders.

Subtle Interplay between synaptotagmin and complexin binding to the SNARE complex

Ca²⁺-triggered neurotransmitter release depends on the formation of SNARE complexes that bring the synaptic vesicle and plasma membranes together, on the Ca²⁺ sensor synaptotagmin-1 and on complexins, which play active and inhibitory roles. Release of the complexin inhibitory activity by binding of synaptotagmin-1 to the SNARE complex, causing complexin displacement, was proposed to trigger exocytosis. However, the validity of this model was questioned based on the observation of simultaneous binding of complexin-I and a fragment containing the synaptotagmin-1 C2 domains (C2AB) to membrane-anchored SNARE complex. Using diverse biophysical techniques, here we show that C2AB and complexin-I do not bind to each other but can indeed bind simultaneously to the SNARE complex in solution. Hence, the SNARE complex contains separate binding sites for both proteins. However, total internal reflection fluorescence microscopy experiments show that C2AB can displace a complexin-I fragment containing its central SNARE-binding helix and an inhibitory helix (Cpx26-83) from membrane-anchored SNARE complex under equilibrium conditions. Interestingly, full-length complexin-I binds more tightly to membrane-anchored SNARE complex than Cpx26-83, and it is not displaced by C2AB. These results show that interactions of N- and/or C-terminal sequences of complexin-I with the SNARE complex and/or phospholipids increase the affinity of complexin-I for the SNARE complex, hindering dissociation induced by C2AB. We propose a model whereby binding of synaptotagmin-1 to the SNARE complex directly or indirectly causes a rearrangement of the complexin-I inhibitory helix without inducing complexin-I dissociation, thus relieving the inhibitory activity and enabling cooperation between synaptotagmin-1 and complexin-I in triggering release.

Ultrahigh-resolution imaging reveals formation of neuronal SNARE/Munc18 complexes in situ

Membrane fusion is mediated by complexes formed by SNAP-receptor (SNARE) and Secretory 1 (Sec1)/mammalian uncoordinated-18 (Munc18)-like (SM) proteins, but it is unclear when and how these complexes assemble. Here we describe an improved two-color fluorescence nanoscopy technique that can achieve effective resolutions of up to 7.5-nm full width at half maximum (3.2-nm localization precision), limited only by stochastic photon emission from single molecules. We use this technique to dissect the spatial relationships between the neuronal SM protein Munc18-1 and SNARE proteins syntaxin-1 and SNAP-25 (25 kDa synaptosome-associated protein). Strikingly, we observed nanoscale clusters consisting of syntaxin-1 and SNAP-25 that contained associated Munc18-1. Rescue experiments with syntaxin-1 mutants revealed that Munc18-1 recruitment to the plasma membrane depends on the Munc18-1 binding to the N-terminal peptide of syntaxin-1. Our results suggest that in a primary neuron, SNARE/SM protein complexes containing syntaxin-1, SNAP-25, and Munc18-1 are preassembled in microdomains on the presynaptic plasma membrane. Our superresolution imaging method provides a framework for investigating interactions between the synaptic vesicle fusion machinery and other subcellular systems in situ.

Molecular mechanisms of COMPLEXIN fusion clamp function in synaptic exocytosis revealed in a new Drosophila mutant

The COMPLEXIN (CPX) proteins play a critical role in synaptic vesicle fusion and neurotransmitter release. Previous studies demonstrated that CPX functions in both activation of evoked neurotransmitter release and inhibition/clamping of spontaneous synaptic vesicle fusion. Here we report a new cpx mutant in Drosophila melanogaster, cpx(1257), revealing spatially defined and separable pools of CPX which make distinct contributions to the activation and clamping functions. In cpx(1257), lack of only the last C-terminal amino acid of CPX is predicted to disrupt prenylation and membrane targeting of CPX. Immunocytochemical analysis established localization of wild-type CPX to active zone (AZ) regions containing neurotransmitter release sites as well as broader presynaptic membrane compartments including synaptic vesicles. Parallel biochemical studies confirmed CPX membrane association and demonstrated robust binding interactions of CPX with all three SNAREs. This is in contrast to the cpx(1257) mutant, in which AZ localization of CPX persists but general membrane localization and, surprisingly, the bulk of CPX-SNARE protein interactions are abolished. Furthermore, electrophysiological analysis of neuromuscular synapses revealed interesting differences between cpx(1257) and a cpx null mutant. The cpx null exhibited a marked decrease in the EPSC amplitude, slowed EPSC rise and decay times and an increased mEPSC frequency with respect to wild-type. In contrast, cpx(1257) exhibited a wild-type EPSC with an increased mEPSC frequency and thus a selective failure to clamp spontaneous release. These results indicate that spatially distinct and separable interactions of CPX with presynaptic membranes and SNARE proteins mediate separable activation and clamping functions of CPX in neurotransmitter release.

Network topologies and convergent aetiologies arising from deletions and duplications observed in individuals with autism

Autism Spectrum Disorders (ASD) are highly heritable and characterised by impairments in social interaction and communication, and restricted and repetitive behaviours. Considering four sets of de novo copy number variants (CNVs) identified in 181 individuals with autism and exploiting mouse functional genomics and known protein-protein interactions, we identified a large and significantly interconnected interaction network. This network contains 187 genes affected by CNVs drawn from 45% of the patients we considered and 22 genes previously implicated in ASD, of which 192 form a single interconnected cluster. On average, those patients with copy number changed genes from this network possess changes in 3 network genes, suggesting that epistasis mediated through the network is extensive. Correspondingly, genes that are highly connected within the network, and thus whose copy number change is predicted by the network to be more phenotypically consequential, are significantly enriched among patients that possess only a single ASD-associated network copy number changed gene (p = 0.002). Strikingly, deleted or disrupted genes from the network are significantly enriched in GO-annotated positive regulators (2.3-fold enrichment, corrected p = 2×10⁻⁵), whereas duplicated genes are significantly enriched in GO-annotated negative regulators (2.2-fold enrichment, corrected p = 0.005). The direction of copy change is highly informative in the context of the network, providing the means through which perturbations arising from distinct deletions or duplications can yield a common outcome. These findings reveal an extensive ASD-associated molecular network, whose topology indicates ASD-relevant mutational deleteriousness and that mechanistically details how convergent aetiologies can result extensively from CNVs affecting pathways causally implicated in ASD.

Autism Spectrum Disorders (ASD) are characterised by impairments in social interaction and communication, and restricted and repetitive behaviours. ASD are highly heritable and many different stretches of DNA have been found to be duplicated or deleted in individuals with ASD. We found that an unusually high number of genes affected by these DNA deletions/duplications are associated with the functioning of synaptic transmission between nerve cells. The proteins made by many of these genes are known to interact with each other and, together with proteins from other deleted/duplicated genes, form a large interlinked biological network. This network was affected by almost 50% of the deletions/duplications in the ASD patients considered. Many individual ASD patients had deletions or duplications of multiple genes within this network, but for those patients with just a single gene from the network changed, that single gene appeared to play an important role. Furthermore, the network predicts that the effects arising from the genes in the deletions are similar to the effects arising from the genes in the duplications. Thus, the way that this ASD-associated network is wired together contributes to the understanding of the impact of these DNA deletions and duplications.

LTP requires a unique postsynaptic SNARE fusion machinery

Membrane fusion during exocytosis is mediated by assemblies of SNARE (soluble NSF-attachment protein receptor) and SM (Sec1/Munc18-like) proteins. The SNARE/SM proteins involved in vesicle fusion during neurotransmitter release are well understood, whereas little is known about the protein machinery that mediates activity-dependent AMPA receptor (AMPAR) exocytosis during long-term potentiation (LTP). Using direct measurements of LTP in acute hippocampal slices and an in vitro LTP model of stimulated AMPAR exocytosis, we demonstrate that the Q-SNARE proteins syntaxin-3 and SNAP-47 are required for regulated AMPAR exocytosis during LTP but not for constitutive basal AMPAR exocytosis. In contrast, the R-SNARE protein synaptobrevin-2/VAMP2 contributes to both regulated and constitutive AMPAR exocytosis. Both the central complexin-binding and the N-terminal Munc18-binding sites of syntaxin-3 are essential for its postsynaptic role in LTP. Thus, postsynaptic exocytosis of AMPARs during LTP is mediated by a unique fusion machinery that is distinct from that used during presynaptic neurotransmitter release.

Complexin activates exocytosis of distinct secretory vesicles controlled by different synaptotagmins

Complexins are SNARE-complex binding proteins essential for the Ca(2+)-triggered exocytosis mediated by synaptotagmin-1, -2, -7, or -9, but the possible role of complexins in other types of exocytosis controlled by other synaptotagmin isoforms remains unclear. Here we show that, in mouse olfactory bulb neurons, synaptotagmin-1 localizes to synaptic vesicles and to large dense-core secretory vesicles as reported previously, whereas synaptotagmin-10 localizes to a distinct class of peptidergic secretory vesicles containing IGF-1. Both synaptotagmin-1-dependent synaptic vesicle exocytosis and synaptotagmin-10-dependent IGF-1 exocytosis were severely impaired by knockdown of complexins, demonstrating that complexin acts as a cofactor for both synaptotagmin-1 and synaptotagmin-10 despite the functional differences between these synaptotagmins. Rescue experiments revealed that only the activating but not the clamping function of complexins was required for IGF-1 exocytosis controlled by synaptotagmin-10. Thus, our data indicate that complexins are essential for activation of multiple types of Ca(2+)-induced exocytosis that are regulated by different synaptotagmin isoforms. These results suggest that different types of regulated exocytosis are mediated by similar synaptotagmin-dependent fusion mechanisms, that particular synaptotagmin isoforms confer specificity onto different types of regulated exocytosis, and that complexins serve as universal synaptotagmin adaptors for all of these types of exocytosis independent of which synaptotagmin isoform is involved.

Complexin controls spontaneous and evoked neurotransmitter release by regulating the timing and properties of synaptotagmin activity

Neurotransmitter release following synaptic vesicle (SV) fusion is the fundamental mechanism for neuronal communication. Synaptic exocytosis is a specialized form of intercellular communication that shares a common SNARE-mediated fusion mechanism with other membrane trafficking pathways. The regulation of synaptic vesicle fusion kinetics and short-term plasticity is critical for rapid encoding and transmission of signals across synapses. Several families of SNARE-binding proteins have evolved to regulate synaptic exocytosis, including Synaptotagmin (SYT) and Complexin (CPX). Here, we demonstrate that Drosophila CPX controls evoked fusion occurring via the synchronous and asynchronous pathways. cpx(-/-) mutants show increased asynchronous release, while CPX overexpression largely eliminates the asynchronous component of fusion. We also find that SYT and CPX coregulate the kinetics and Ca(2+) co-operativity of neurotransmitter release. CPX functions as a positive regulator of release in part by coupling the Ca(2+) sensor SYT to the fusion machinery and synchronizing its activity to speed fusion. In contrast, syt(-/-); cpx(-/-) double mutants completely abolish the enhanced spontaneous release observe in cpx(-/-) mutants alone, indicating CPX acts as a fusion clamp to block premature exocytosis in part by preventing inappropriate activation of the SNARE machinery by SYT. CPX levels also control the size of synaptic vesicle pools, including the immediate releasable pool and the ready releasable pool-key elements of short-term plasticity that define the ability of synapses to sustain responses during burst firing. These observations indicate CPX regulates both spontaneous and evoked fusion by modulating the timing and properties of SYT activation during the synaptic vesicle cycle.

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.