Complexins: cytosolic proteins that regulate SNAP receptor function

Identification of a minimal segment of complexin II essential for preferential distribution in axons

Complexin II (CPLX2) is a soluble pre-synaptic protein believed to regulate neurotransmitter release from pre-synaptic terminals. CPLX2 is localized in pre-synaptic terminals in mature brain, but the mechanism of selective localization remains unclear. Here we identified an essential segment of CPLX2 for preferential axonal distribution. Myc-tagged CPLX2 was expressed in cultured rat hippocampal neurons and its distribution between axons and dendrites was compared by immunocytochemistry and image analysis. Fluorescence signals were detected in both axons and dendrites; however, their respective distribution varied significantly. Despite the fact that signal intensity decreased almost linearly from the base to the tip of the dendrite, a substantial level was sustained along the axon, even at a position near the tip. Image analyses using a series of mutants indicated that the deletion of 19 amino acid residues, G71-P89, within the 'central core' for binding to soluble N-ethylmaleimide sensitive factor attachment protein receptor proteins resulted in the loss of preferential axonal distribution. The enhanced green fluorescent protein derivative fused with the G71-P89 fragment exhibited a similar localization to that of wild type CPLX2, indicating that the G71-P89 region of CPLX2 is essential and sufficient for preferential axonal distribution.

Direct interaction of SNARE complex binding protein synaphin/complexin with calcium sensor synaptotagmin 1

Although the binding of synaphin (also called complexin) to the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex is critical for synaptic vesicle exocytosis, the exact role of synaphin remains unclear. Here, we show that synaphin directly binds to synaptotagmin 1, a major Ca(2+) sensor for fast neurotransmitter release, in a 1:1 stoichiometry. Mapping of the synaphin site involved in synaptotagmin 1 binding revealed that the C-terminal region is essential for the interaction between these two proteins. Binding was sensitive to ionic strength, suggesting the involvement of charged residues in the C-terminus region. Mutation of the seven consecutive glutamic acid residues (residues 108-114) at the C-terminal region of synaphin to alanines or glutamines resulted in a dramatic reduction in synaptotagmin 1 binding activity. Furthermore, a peptide from the C-terminus of synaphin (residues 91-124) blocked the binding of synaptotagmin 1 to synaphin, an effect that was abolished by mutating the consecutive glutamic acid residues to alanine. Immunoprecipitation experiments with brain membrane extracts showed the presence of a complex consisting of synaphin, synaptotagmin 1, and SNAREs. We propose that synaphin recruits synaptotagmin 1 to the SNARE-based fusion complex and synergistically functions with synaptotagmin 1 in mediating fast synaptic vesicle exocytosis.

Understanding synapses: past, present, and future

Classical physiological work by Katz, Eccles, and others revealed the central importance of synapses in brain function, and characterized the mechanisms involved in synaptic transmission. Building on this work, major advances in the past two decades have elucidated how synapses work molecularly. In the present perspective, we provide a short description of our personal view of these advances, suggest a series of important future questions about synapses, and discuss ideas about how best to achieve further progress in the field.

Complexin II plays a positive role in Ca2+-triggered exocytosis by facilitating vesicle priming

SNARE-mediated exocytosis is a multistage process central to synaptic transmission and hormone release. Complexins (CPXs) are small proteins that bind very rapidly and with a high affinity to the SNARE core complex, where they have been proposed recently to inhibit exocytosis by clamping the complex and inhibiting membrane fusion. However, several other studies also suggest that CPXs are positive regulators of neurotransmitter release. Thus, whether CPXs are positive or negative regulators of exocytosis is not known, much less the stage in the vesicle life cycle at which they function. Here, we systematically dissect the vesicle stages leading up to exocytosis using a knockout-rescue strategy in a mammalian model system. We show that adrenal chromaffin cells from CPX II knockout mice exhibit markedly diminished releasable vesicle pools (comprising the readily and slowly releasable pools), while showing no change in the kinetics of fusion pore dilation or morphological vesicle docking. Overexpression of WT CPX II-but not of SNARE-binding-deficient mutants-restores the size of the the releasable pools in knockout cells, and in WT cells it markedly enlarges them. Our results show that CPXs regulate the size of the primed vesicle pools and have a positive role in Ca(2+)-triggered exocytosis.

Syntaxin 3b is a t-SNARE specific for ribbon synapses of the retina

Previous studies have demonstrated that ribbon synapses in the retina do not contain the t-SNARE (target-soluble N-ethylmaleimide-sensitive factor attachment protein receptor) syntaxin 1A that is found in conventional synapses of the nervous system. In contrast, ribbon synapses of the retina contain the related isoform syntaxin 3. In addition to its localization in ribbon synapses, syntaxin 3 is also found in nonneuronal cells, where it has been implicated in the trafficking of transport vesicles to the apical plasma membrane of polarized cells. The syntaxin 3 gene codes for four different splice forms, syntaxins 3A, 3B, 3C, and 3D. We demonstrate here by using analysis of EST databases, RT-PCR, in situ hybridization, and Northern blot analysis that cells in the mouse retina express only syntaxin 3B. In contrast, nonneuronal tissues, such as kidney, express only syntaxin 3A. The two major syntaxin isoforms (3A and 3B) have an identical N-terminal domain but differ in the C-terminal half of the SNARE domain and the C-terminal transmembrane domain. These two domains are thought to be directly involved in synaptic vesicle fusion. The interaction of syntaxin 1A and syntaxin 3B with other synaptic proteins was examined. We found that both proteins bind Munc18/N-sec1 with similar affinity. In contrast, syntaxin 3B had a much lower binding affinity for the t-SNARE SNAP25 compared with syntaxin 1A. By using an in vitro fusion assay, we could demonstrate that vesicles containing syntaxin 3B and SNAP25 could fuse with vesicles containing synaptobrevin2/VAMP2, demonstrating that syntaxin 3B can function as a t-SNARE.

S-nitrosylation of syntaxin 1 at Cys(145) is a regulatory switch controlling Munc18-1 binding

Exocytosis is regulated by NO in many cell types, including neurons. In the present study we show that syntaxin 1a is a substrate for S-nitrosylation and that NO disrupts the binding of Munc18-1 to the closed conformation of syntaxin 1a in vitro. In contrast, NO does not inhibit SNARE {SNAP [soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein] receptor} complex formation or binding of Munc18-1 to the SNARE complex. Cys(145) of syntaxin 1a is the target of NO, as a non-nitrosylatable C145S mutant is resistant to NO and novel nitrosomimetic Cys(145) mutants mimic the effect of NO on Munc18-1 binding in vitro. Furthermore, expression of nitrosomimetic syntaxin 1a in living cells affects Munc18-1 localization and alters exocytosis release kinetics and quantal size. Molecular dynamic simulations suggest that NO regulates the syntaxin-Munc18 interaction by local rearrangement of the syntaxin linker and H3c regions. Thus S-nitrosylation of Cys(145) may be a molecular switch to disrupt Munc18-1 binding to the closed conformation of syntaxin 1a, thereby facilitating its engagement with the membrane fusion machinery.

How does synaptotagmin trigger neurotransmitter release?

Neurotransmitter release at synapses involves a highly specialized form of membrane fusion that is triggered by Ca(2+) ions and is optimized for speed. These observations were established decades ago, but only recently have the molecular mechanisms that underlie this process begun to come into view. Here, we summarize findings obtained from genetically modified neurons and neuroendocrine cells, as well as from reconstituted systems, which are beginning to reveal the molecular mechanism by which Ca(2+)-acting on the synaptic vesicle (SV) protein synaptotagmin I (syt)-triggers rapid exocytosis. This work sheds light not only on presynaptic aspects of synaptic transmission, but also on the fundamental problem of membrane fusion, which has remained a puzzle that has yet to be solved in any biological system.

Complexin-I-deficient sperm are subfertile due to a defect in zona pellucida penetration

Upon adhesion to the zona pellucida, sperm undergo regulated exocytosis of the acrosome. Although it is necessary for sperm to penetrate the zona pellucida and fertilize an egg, the acrosomal membrane fusion process is poorly understood. Complexins I and II are small, cytosolic proteins that bind to a complex of proteins termed the solubleN-ethylmaleimide-sensitive factor attachment protein receptor complex to regulate synaptic vesicle exocytosis. Complexin-II-deficient mice are fertile but the fertility of sperm from complexin-I-deficient male mice is unclear because the mice have ataxia and cannot mate. Here, we show that the genes encoding complexins I and II are expressed in primary spermatocytes and spermatids. Complexin proteins were found in/near the developing acrosome in spermatids and in or around the acrosome of mature sperm. Cell fractionation demonstrated that complexins I and II were predominantly found in the cytosolic fraction. Furthermore, sperm from complexin-I-deficient mice had normal morphology, number, and only small differences in motility, as assessed by computer-assisted semen analysis. Complexin-I-deficient sperm capacitated normally and bound to the zona pellucida. But when sperm from complexin-I-deficient mice were inseminated into females, a defect in fertility was observed, in concordance with previous data showing thatin vitrofertilization rate was also reduced. If the zona pellucida was removed prior toin vitrofertilization, fertility was normal, demonstrating that zona pellucida penetration was defective, a step requiring acrosomal exocytosis. Therefore, complexin-I-deficient sperm are subfertile due to faulty zona pellucida penetration.

Synaptic vesicle fusion

The core of the neurotransmitter release machinery is formed by SNARE complexes, which bring the vesicle and plasma membranes together and are key for fusion, and by Munc18-1, which controls SNARE-complex formation and may also have a direct role in fusion. In addition, SNARE complex assembly is likely orchestrated by Munc13s and RIMs, active-zone proteins that function in vesicle priming and diverse forms of presynaptic plasticity. Synaptotagmin-1 mediates triggering of release by Ca²⁺, probably through interactions with SNAREs and both membranes, as well as through a tight interplay with complexins. Elucidation of the release mechanism will require a full understanding of the network of interactions among all these proteins and the membranes.

Synaptotagmin arrests the SNARE complex before triggering fast, efficient membrane fusion in response to Ca2+

Neuronal communication is mediated by Ca(2+)-triggered fusion of transmitter-filled synaptic vesicles with the presynaptic plasma membrane. Synaptotagmin I functions as a Ca(2+) sensor that regulates exocytosis, whereas soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) proteins in the vesicle and target membrane assemble into complexes that directly catalyze bilayer fusion. Here we report that, before the Ca(2+) trigger, synaptotagmin interacts with SNARE proteins in the target membrane to halt SNARE complex assembly at a step after donor vesicles attach, or dock, to target membranes. This results in fusion complexes that, when subsequently triggered by Ca(2+), drive rapid, highly efficient lipid mixing. Ca(2+)-independent interactions with SNAREs also predispose synaptotagmin to selectively penetrate the target membrane in response to Ca(2+); we demonstrate that Ca(2+)-synaptotagmin must insert into the target membrane to accelerate SNARE-catalyzed fusion. These findings demonstrate that Ca(2+) converts synaptotagmin from a clamp to a trigger for exocytosis.

Complexin and Ca2+ stimulate SNARE-mediated membrane fusion

Ca²⁺-triggered, synchronized synaptic vesicle fusion underlies interneuronal communication. Complexin is a major binding partner of the SNARE complex, the core fusion machinery at the presynapse. The physiological data on complexin, however, have been at odds with each other, making delineation of its molecular function difficult. Here we report direct observation of two-faceted functions of complexin using the single-vesicle fluorescence fusion assay and EPR. We show that complexin I has two opposing effects on trans-SNARE assembly: inhibition of SNARE complex formation and stabilization of assembled SNARE complexes. Of note, SNARE-mediated fusion is markedly stimulated by complexin, and it is further accelerated by two orders of magnitude in response to an externally applied Ca²⁺ wave. We suggest that SNARE complexes, complexins and phospholipids collectively form a complex substrate for Ca²⁺ and Ca²⁺-sensing fusion effectors in neurotransmitter release.

Regulation of complexin 1 and complexin 2 in the developing human prefrontal cortex

Complexin 1 (CX1) and complexin 2 (CX2) are presynaptic proteins that modulate neurotransmitter release and are used as markers of inhibitory and excitatory synapses, respectively. The aim of this study was to gain insight into the development of inhibitory and excitatory synapses in human prefrontal cortex (PFC) by examining the expression of CX1 and CX2 in postmortem tissues. Relative complexin protein levels were measured by Western blotting in postmortem dorsolateral prefrontal cortex (DLPFC) of 42 subjects without neurological or psychiatric disease ranging in age from 18 gestational weeks to 25 years. Samples were batched a priori into fetal, 0-12 month, 1-5 years, 6-10 years, 11-15 years, 16-20 years, and 21-25 years age groups. CX1 and CX2 expression and CX2/CX1 demonstrated a significant effect of age group by ANOVA. Group CX1 level increased progressively across development and was lowest in the fetal group and highest in the young adult group, whereas group CX2 level increased between the fetal and the 6-10 years groups and then plateaued. Consistent with these divergent patterns, there was a significant effect of age group on CX2/CX1, which was higher in fetal and infant groups than in the young adult group. Furthermore, regression analysis demonstrated linear relationships of CX1 and CX2/CX1 with age, whereas CX2 was better described as having a curvilinear relationship with age. These data indicate that complexin expression increases during synaptic maturation in human DLPFC and that an increase in the influence of inhibitory synapses relative to that of excitatory synapses occurs during development in this cortical region.

Complexins facilitate neurotransmitter release at excitatory and inhibitory synapses in mammalian central nervous system

Complexins (Cplxs) are key regulators of synaptic exocytosis, but whether they act as facilitators or inhibitors is currently being disputed controversially. We show that genetic deletion of all Cplxs expressed in the mouse brain causes a reduction in Ca(2+)-triggered and spontaneous neurotransmitter release at both excitatory and inhibitory synapses. Our results demonstrate that at mammalian central nervous system synapses, Cplxs facilitate neurotransmitter release and do not simply act as inhibitory clamps of the synaptic vesicle fusion machinery.

For better or for worse: complexins regulate SNARE function and vesicle fusion

In contrast to constitutive secretion, SNARE-mediated synaptic vesicle fusion is controlled by multiple regulatory proteins, which determine the Ca(2+) sensitivity of the vesicle fusion process and the speed of excitation-secretion coupling. Complexins are among the best characterized SNARE regulators known to date. They operate by binding to trimeric SNARE complexes consisting of the vesicle protein synaptobrevin and the plasma membrane proteins syntaxin and SNAP-25. The question as to whether complexins facilitate or inhibit SNARE-mediated fusion processes is currently a matter of significant controversy. This is mainly because of the fact that biochemical experiments in vitro and studies on vertebrate complexins in vivo have yielded apparently contradictory results. In this review, I provide a summary of available data on the role of complexins in SNARE-mediated vesicle fusion and attempt to define a model of complexin function that incorporates evidence for both facilitatory and inhibitory roles of complexins in SNARE-mediated fusion.

Diagnosis of a terminal deletion of 4p with duplication of Xp22.31 in a patient with findings of Opitz G/BBB syndrome and Wolf-Hirschhorn syndrome

Opitz G/BBB syndrome (OS) is a congenital midline malformation syndrome characterized by hypertelorism, hypospadias, cleft lip/palate, laryngotracheoesophageal abnormalities, imperforate anus, developmental delay and cardiac defects. The X-linked form is caused by mutations in the MID1 gene, while no gene has yet been identified for the autosomal dominant form. Here, we report on a 15-year-old boy who was referred for MID1 mutation analysis with findings typical of OS, including apparent hypertelorism, hypospadias, a history of feeding difficulties, dysphagia secondary to esophageal arteria lusoria, growth retardation and developmental delay. No MID1 mutation was found, but subsequent sub-megabase resolution array CGH unexpectedly documented a 2.34 Mb terminal 4p deletion, suggesting a diagnosis of WHS, and a duplication in Xp22.31. Wolf-Hirschhorn syndrome (WHS) is a contiguous gene deletion syndrome involving terminal chromosome 4p deletions, in particular 4p16.3. WHS is characterized by typical facial appearance ("Greek helmet facies"), mental retardation, congenital hypotonia, and growth retardation. While the severity of developmental delay in this patient supports the diagnosis of WHS rather than OS, this case illustrates the striking similarities of clinical findings in seemingly unrelated syndromes, suggesting common or interacting pathways at the molecular and pathogenetic level. This is the first report of arteria lusoria (esophageal vascular ring) in a patient with WHS.

Unsolved mysteries in membrane traffic

Remarkable strides have been made over the past 20 years in elucidating the molecular basis of membrane trafficking. Indeed, a combination of biochemical and genetic approaches have determined the identity and function of many of the core constituents needed for protein secretion and endocytosis. But much remains to be learned. This review highlights underlying themes in membrane traffic to help us refocus and solve many remaining and newly emerging issues that are fundamental to mammalian cell biology and human physiology.

A complexin fusion clamp regulates spontaneous neurotransmitter release and synaptic growth

Neuronal signaling occurs through both action potential-triggered synaptic vesicle fusion and spontaneous release, although the fusion clamp machinery that prevents premature exocytosis of synaptic vesicles in the absence of calcium is unknown. Here we demonstrate that spontaneous release at synapses is regulated by complexin, a SNARE complex-binding protein. Analysis of Drosophila melanogaster complexin null mutants showed a marked increase in spontaneous fusion and a profound overgrowth of synapses, suggesting that complexin functions as the fusion clamp in vivo and may modulate structural remodeling of neuronal connections by controlling the rate of spontaneous release.

Distinct domains of complexin I differentially regulate neurotransmitter release

Complexins constitute a family of four synaptic high-affinity SNARE complex binding proteins. They positively regulate a late, post-priming step in Ca²⁺-triggered synchronous neurotransmitter release, but the underlying molecular mechanisms are unclear. We show here that SNARE complex binding of Complexin I via its central α-helix is necessary but unexpectedly not sufficient for its key function in promoting neurotransmitter release. An accessory α-helix N-terminal of the SNARE complex binding region plays an inhibitory role in fast synaptic exocytosis, while its N-terminally adjacent sequences facilitate Ca²⁺-triggered release even in the absence of the Ca²⁺ sensor Synaptotagmin 1. Our results indicate that distinct functional domains of Complexins differentially regulate synaptic exocytosis, and that via the interplay between these domains Complexins play a crucial role in fine-tuning Ca²⁺-triggered fast neurotransmitter release.

Complexin I is required for mammalian sperm acrosomal exocytosis

Regulated exocytosis in many cells is controlled by the SNARE complex, whose core includes three proteins that promote membrane fusion. Complexins I and II are highly related cytosolic proteins that bind tightly to the assembled SNARE complex and regulate neuronal exocytosis. Like somatic cells, sperm undergo regulated exocytosis; however, sperm release a single large vesicle, the acrosome, whose release has different characteristics than neuronal exocytosis. Acrosomal release is triggered upon sperm adhesion to the mammalian egg extracellular matrix (zona pellucida) to allow penetration of the egg coat. Membrane fusion occurs at multiple points within the acrosome but how fusion is activated and the formation and progression of fusion points is synchronized is unclear. We show that complexins I and II are found in acrosome-intact mature sperm, bind to SNARE complex proteins, and are not detected in sperm after acrosomal exocytosis (acrosome reaction). Although complexin-I-deficient sperm acrosome-react in response to calcium ionophore, they do not acrosome-react in response to egg zona pellucida proteins and have reduced fertilizing ability, in vitro. Complexin II is present in the complexin-I-deficient sperm and its expression is increased in complexin-I-deficient testes. Therefore, complexin I functions in exocytosis in two related but morphologically distinct secretory processes. Sperm are unusual because they express both complexins I and II but have a unique and specific requirement for complexin I.

Complexin/synaptotagmin interplay controls acrosomal exocytosis

Regulated secretion is a fundamental process underlying the function of many cell types. In particular, acrosomal exocytosis in mammalian sperm is essential for egg fertilization. Regulated secretion requires SNARE proteins and, in neurons, also synaptotagmin I and complexin. Recent reports suggest that complexin imposes a fusion block that is released by Ca(2+) and synaptotagmin I. However, no direct evidence for this model in secreting cells has been provided and whether this complexin/synaptotagmin interplay functions in other types of secretion is unknown. In this report, we show that the C2B domain of synaptotagmin VI and an anti-complexin antibody blocked the formation of trans SNARE complexes in permeabilized human sperm, and that this effect was reversed by adding complexin. In contrast, an excess of complexin stopped exocytosis at a later step, when SNAREs were assembled in loose trans complexes. Interestingly, this blockage was released by the addition of the synaptotagmin VI C2B domain in the presence of Ca(2+). We have previously demonstrated that the activity of this domain is regulated by protein kinase C-mediated phosphorylation. Here, we show that a phosphomimetic mutation in the polybasic region of the C2B domain strongly affects its Ca(2+) and phospholipids binding properties. Importantly, this mutation completely abrogates its ability to rescue the complexin block. Our results show that the functional interplay between complexin and synaptotagmin has a central role in a physiological secretion event, and that this interplay can be modulated by phosphorylation of the C2B domain.

Dynamin 2 associates with complexins and is found in the acrosomal region of mammalian sperm

Previous data showed that complexin I, a SNARE regulatory protein, is localized in and/or around the acrosome and is necessary for the acrosome reaction in sperm. To understand how complexin I regulates the acrosome reaction, we used complexin-GST pulldown assays to identify interacting proteins. We showed that both complexins I and II bound mouse sperm dynamin 2. Dynamin 2 is a 100 kDa GTPase essential to many aspects of endocytosis but its potential role in exocytosis is unknown. Dynamin 2 is expressed in rat testis and widely expressed in other tissues; however, the function of dynamin 2 in germ cells is uncertain. Dynamin 2 protein was detected in mouse testis and was most abundant in or around the developing acrosome of spermatids. In addition, dynamin 2 was co-localized with complexin I in the acrosomal region of mammalian sperm. Its co-localization and interaction with complexin I suggest that dynamin 2 may play a role during acrosome formation and/or acrosomal exocytosis.

Kinetics of complexin binding to the SNARE complex: correcting single molecule FRET measurements for hidden events

Virtually all measurements of biochemical kinetics have been derived from macroscopic measurements. Single-molecule methods can reveal the kinetic behavior of individual molecular complexes and thus have the potential to determine heterogeneous behaviors. Here we have used single-molecule fluorescence resonance energy transfer to determine the kinetics of binding of SNARE (soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptor) complexes to complexin and to a peptide derived from the central SNARE binding region of complexin. A Markov model was developed to account for the presence of unlabeled competitor in such measurements. We find that complexin associates rapidly with SNARE complexes anchored in lipid bilayers with a rate constant of 7.0 x 10(6) M(-1) s(-1) and dissociates slowly with a rate constant of 0.3 s(-1). The complexin peptide associates with SNARE complexes at a rate slower than that of full-length complexin (1.2 x 10(6) M(-1) s(-1)), and dissociates much more rapidly (rate constant >67 s(-1)). Comparison of single-molecule fluorescence resonance energy transfer measurements made using several dye attachment sites illustrates that dye labeling of complexin can modify its rate of unbinding from SNAREs. These rate constants provide a quantitative framework for modeling of the cascade of reactions underlying exocytosis. In addition, our theoretical correction establishes a general approach for improving single-molecule measurements of intermolecular binding kinetics.

Trans-SNARE complex assembly and yeast vacuole membrane fusion

cis-SNARE complexes (anchored in one membrane) are disassembled by Sec17p (alpha-SNAP) and Sec18p (NSF), permitting the unpaired SNAREs to assemble in trans. We now report a direct assay of trans-SNARE complex formation during yeast vacuole docking. SNARE complex assembly and fusion is promoted by high concentrations of the SNARE Vam7p or Nyv1p or by addition of HOPS (homotypic fusion and vacuole protein sorting), a Ypt7p (Rab)-effector complex with a Sec1/Munc18-family subunit. Inhibitors that target Ypt7p, HOPS, or key regulatory lipids prevent trans-SNARE complex assembly and ensuing fusion. Strikingly, the lipid ligand MED (myristoylated alanine-rich C kinase substrate effector domain) or elevated concentrations of Sec17p, which can displace HOPS from SNARE complexes, permit full trans-SNARE pairing but block fusion. These findings suggest that efficient fusion requires trans-SNARE complex associations with factors such as HOPS and subsequent regulated lipid rearrangements.

SNAP-23 and syntaxin-2 localize to the extracellular surface of the platelet plasma membrane

SNARE proteins direct membrane fusion events required for platelet granule secretion. These proteins are oriented in cell membranes such that most of the protein resides in a cytosolic compartment. Evaluation of SNARE protein localization in activated platelets using immunonanogold staining and electron microscopy, however, demonstrated expression of SNAP-23 and syntaxin-2 on the extracellular surface of the platelet plasma membrane. Flow cytometry of intact platelets confirmed trypsin-sensitive SNAP-23 and syntaxin-2 localization to the extracellular surface of the plasma membrane. Acyl-protein thioesterase 1 and botulinum toxin C light chain released SNAP-23 and syntaxin-2, respectively, from the surface of intact platelets. When resting platelets were incubated with both acyl-protein thioesterase 1 and botulinum toxin C light chain, a complex that included both SNAP-23 and syntaxin-2 was detected in supernatants, indicating that extracellular SNARE proteins retain their ability to bind one another. These observations represent the first description of SNARE proteins on the extracellular surface of a cell.

Reduced expression of complexins I and II in rats bred for learned helplessness

Disturbed synaptic transmission contributes to the pathophysiology of mood disorders. Post mortem studies reported reduced expression of the synaptic vesicle protein (SVP) complexins I and II in depression. Antidepressants were found to induce the expression of these genes. Since animals with congenital susceptibility to learned helplessness provide a valid animal model of depression, we investigated the expression of different SVPs in this system by semiquantitative in situ hybridization. Rats bred for congenital learned helpless behavior (cLH, N=6) failed to interrupt foot shock currents by lever pressing (mean 12.3 failures out of 15 trials). These animals showed significantly lower expression of complexins I and II mRNA in hippocampal, limbic and cortical brain areas compared to not helpless animals (cNLH, N=6) with a mean failure rate of 0.83 out of 15 trials. Expression levels of complexins I and II significantly correlated with the failure rate in the test paradigm. In contrast, the expressions of synaptotagmin I and synaptophysin were found unchanged. This investigation provides a further validation of the LH model of depression. The experimental data fit well into current pathogenetic concepts of mood disorders and support the hypothesis, that complexins are pivotal players in the pathophysiology of depression and tentative targets of antidepressants.

Overexpression of complexin in PC12 cells inhibits exocytosis by preventing SNARE complex recycling

Complexin is an important protein that functions during Ca2+-dependent neurotransmitter release. Substantial evidence supports that complexin performs its role through rapid interaction with SNARE complex with high affinity. However, alpha-SNAP/NSF, which can disassemble the cis-SNARE complex in the presence of MgATP, competes with complexin to bind to SNARE complex. In addition, injection of alpha-SNAP into chromaffin cells enhances the size of the readily releasable pool, and mutation disrupting the ATPase activity of NSF results in the accumulation of SNARE complex. Thus, whether high concentrations of complexin could result in a reverse result is unclear. In this paper, we demonstrate that when stably overexpressed in PC12 cells, high levels of complexin result in the accumulation of SNARE complex. This in turn leads to a reduction in the size of the readily releasable pool of large dense core vesicles. These results suggest that high levels of complexin seem to prevent SNARE complex recycling, presumably by displacing NSF and alpha-SNAP from SNARE complex.

Depletion of Complexin II does not affect disease progression in a mouse model of Huntington's disease (HD); support for role for complexin II in behavioural pathology in a mouse model of HD

Huntington's disease (HD) is a progressive, inherited, neurological disorder with a complicated phenotype that is characterised by movement abnormalities, cognitive impairments and psychiatric symptoms. Although HD is a neurodegenerative disease, recent evidence indicates that neurological dysfunction, rather than frank neurodegeneration contributes to, and may even cause early symptoms in the absence of neurodegeneration. One protein that may contribute to neurological dysfunction in HD is complexin II. Complexins are presynaptic proteins that are believed to modulate neurotransmitter release. Complexin II levels are reduced in human HD striatum and cortex, and a progressive depletion of complexin II mRNA and protein has also been shown in the R6/2 mouse model of HD. Interestingly, complexin II knockout mice share behavioural deficits in reversal learning in common with R6/2 mice. Further, the two strains both show abnormalities in long-term potentiation. This evidence led us to wonder whether or not loss of complexin II underlies some of the behavioural deficits seen in R6/2 mice. To investigate this, we crossbred complexin II knockout mice with R6/2 mice to generate a double mutant mouse. The behavioural phenotype of R6/2 mice on a null complexin II background was characterised and was compared to that seen in control mice. Complete knockout of complexin II did not significantly affect the phenotype of R6/2 mice. This indicates that loss of complexin II is part of the mechanism underlying the R6/2 phenotype. Whether it is causal or compensatory remains to be determined.

Putting the clamps on membrane fusion: How complexin sets the stage for calcium-mediated exocytosis

Three recent papers have addressed a long-standing question in exocytosis: how does a sudden calcium influx trigger a coordinated synchronous release in regulated exocytosis [Giraudo, C.G., Eng, W.S., Melia, T.J. and Rothman, J.E. (2006) A clamping mechanism involved in SNARE-dependent exocytosis. Science 313, 676-680; Schaub, J.R., Lu, X., Doneske, B., Shin, Y.K. and McNew, J.A. (2006) Hemifusion arrest by complexin is relieved by Ca(2+)-synaptotagmin I. Nat. Struct. Mol. Biol. 13, 748-750; Tang, J., Maximov, A., Shin, O.H., Dai, H., Rizo, J. and Sudhof, T.C. (2006) A complexin/synaptotagmin 1 switch controls fast synaptic vesicle exocytosis. Cell 126, 1175-1187]? Using diverse approaches that include cell-free reconstitution of the membrane fusion machinery and in vivo manipulation of fusogenic proteins, these groups have established that the complexin proteins are fusion clamps. By arresting vesicle secretion just prior to fusion, complexin primes select vesicles for a fast, synchronous response to calcium.

A search for genes that may confer divergent morphology and function in the carotid body between two strains of mice

The carotid body (CB) is the primary hypoxic chemosensory organ. Its hypoxic response appears to be genetically controlled. We have hypothesized that: 1) genes related to CB function are expressed less in the A/J mice (low responder to hypoxia) compared with DBA/2J mice (high responder to hypoxia); and 2) gene expression levels of morphogenic and trophic factors of the CB are significantly lower in the A/J mice than DBA/2J mice. This study utilizes microarray analysis to test these hypotheses. Three sets of CBs were harvested from both strains. RNA was isolated and used for global gene expression profiling (Affymetrix Mouse 430 v2.0 array). Statistically significant gene expression was determined as a minimum six counts of nine pairwise comparisons, a minimum 1.5-fold change, and P ≤ 0.05. Our results demonstrated that 793 genes were expressed less and that 568 genes were expressed more in the A/J strain vs. the DBA/2J strain. Analysis of individual genes indicates that genes encoding ion channels are differentially expressed between the two strains. Genes related to neurotransmitter metabolism, synaptic vesicles, and the development of neural crest-derived cells are expressed less in the A/J CB vs. the DBA/2J CB. Through pathway analysis, we have constructed a model that shows gene interactions and offers a roadmap to investigate CB development and hypoxic chemosensing/chemotransduction processes. Particularly, Gdnf, Bmp2, Kcnmb2, Tph1, Hif1a, and Arnt2 may contribute to the functional differences in the CB between the two strains. Bmp2, Phox2b, Dlx2, and Msx2 may be important for the morphological differences.

A quaternary SNARE-synaptotagmin-Ca2+-phospholipid complex in neurotransmitter release

The function of synaptotagmin as a Ca(2+) sensor in neurotransmitter release involves Ca(2+)-dependent phospholipid binding to its two C(2) domains, but this activity alone does not explain why Ca(2+) binding to the C(2)B domain is more critical for release than Ca(2+) binding to the C(2)A domain. Synaptotagmin also binds to SNARE complexes, which are central components of the membrane fusion machinery, and displaces complexins from the SNAREs. However, it is unclear how phospholipid binding to synaptotagmin is coupled to SNARE binding and complexin displacement. Using supported lipid bilayers deposited within microfluidic channels, we now show that Ca(2+) induces simultaneous binding of synaptotagmin to phospholipid membranes and SNARE complexes, resulting in an intimate quaternary complex that we name SSCAP complex. Mutagenesis experiments show that Ca(2+) binding to the C(2)B domain is critical for SSCAP complex formation and displacement of complexin, providing a clear rationale for the preponderant role of the C(2)B domain in release. This and other correlations between the effects of mutations on SSCAP complex formation and their functional effects in vivo suggest a key role for this complex in release. We propose a model whereby the highly positive electrostatic potential at the tip of the SSCAP complex helps to induce membrane fusion during release.

Phosphorylated synaphin/complexin found in the brain exhibits enhanced SNARE complex binding

The cytosolic protein synaphin/complexin critically regulates fast neurotransmitter release at the synapse by binding to SNARE complex. However, the exact mechanism of its action remains unclear, and very little is known about how it is physiologically regulated. Here we show that synaphins (Syps) 1 and 2 can be phosphorylated in vitro by protein kinase CK2 (CK2). The only phosphorylation site by CK2 was serine-115 (Ser-115) of Syps 1 and 2. Syps 1 and 2 exhibited higher affinities to native and recombinant SNARE complexes when phosphorylated at Ser-115. We found Ser-115-phosphorylated Syp 1 (pS115-Syp 1) in the cytosolic fraction of the rat brain using polyclonal antibody specific to pS115-Syps 1 and 2. These results suggest that the activity of Syp is regulated by CK2 phosphorylation of its Ser-115 in vivo. The phosphorylation may provide a new route for modulating fast neurotransmitter release.

Hippocampal abnormalities and memory deficits: new evidence of a strong pathophysiological link in schizophrenia

The central goals of this manuscript are (1) to better characterize what appears to be the most parsimonious account of schizophrenic long-term memory impairment in the neuropsychological literature: a contextual binding deficit rooted in the medial temporal lobes; (2) to link this deficit to concrete abnormalities at the level of the hippocampus; and (3) to suggest that this deficit could lead to the functional impairment experienced by schizophrenia patients in their daily lives. As far as long-term memory is concerned in schizophrenia, there seems to be a general agreement to conclude that explicit mechanisms are disturbed compared to relatively spared implicit mechanisms. More precisely, both subsystems of explicit memory (i.e., episodic and semantic) appear to be dysfunctional in this patient population. Errors during the encoding processes could be responsible for this dysfunction even if retrieval per se is not totally spared. Recently, a number of studies have suggested that impairments in conscious recollection and contextual binding are closely linked to episodic memory deficit. Since the hippocampal formation is considered to be the central element in the neural support for contextual binding and episodic memory, we have conducted an extensive review of the literature concerning the hippocampal formation in schizophrenia. Emerging evidence from varying disciplines confirm the coherence of the different anomalies reported concurrently at the neuroanatomical, neurodevelopmental, biochemical, and genetic levels. It seems highly probable that the synaptic disorganization in the hippocampus concerns the regions crucial for encoding and contextual binding memory processes. The consequences of these deficits could result in schizophrenia patients experiencing major difficulties when facing usual events which have not been encoded with their proper context.

Arachidonic acid potentiates exocytosis and allows neuronal SNARE complex to interact with Munc18a

Neuronal communication relies on the fusion of neurotransmitter-containing vesicles with the neuronal plasma membrane. Recent genetic studies have highlighted the critical role played by polyunsaturated fatty acids in neurotransmission, however, there is little information available about which fatty acids act on exocytosis and, more importantly, by what mechanism. We have used permeabilized chromaffin cells to screen various fatty acids of the n-3 and n-6 series for their acute effects on exocytosis. We have demonstrated that an n-6 series polyunsaturated fatty acid, arachidonic acid, potentiates secretion from intact neurosecretory cells regardless of the secretagogue used. We have shown that arachidonic acid dose dependently increases soluble NSF attachment protein receptor complex formation in chromaffin cells and bovine cortical brain extracts and that a non-hydrolysable analogue of arachidonic acid causes a similar increase in SNARE complex formation. This prompted us to examine the effect of arachidonic acid on SNARE protein interactions with Munc18a, a protein known to prevent Syntaxin1a engagement into the SNARE complex in vitro. In the presence of arachidonic acid, we show that Munc18a can interact with the neuronal SNARE complex in a dose-dependent manner. We further demonstrate that arachidonic acid directly interacts with Syntaxin1a.

Early motor development is abnormal in complexin 1 knockout mice

Complexin I expression is dysregulated in a number of neurological diseases including schizophrenia and depression. Adult complexin 1 knockout (Cplx1(-/-)) mice are severely ataxic and show deficits in exploration and emotional reactivity. Here, we evaluated early behavioural development of Cplx1(-/-) mice. Cplx1(-/-) mice showed marked abnormalities. They develop ataxia by post-natal day 7 (P7), and by P21 show marked deficits in tasks requiring postural skills and complex movement. These deficits are consistent with abnormalities in sensory and motor development found in infants that develop schizophrenia in later life. A role for complexin I depletion should be considered in diseases where deficits in early sensory and motor development exist, such as autism and schizophrenia.

A gain-of-function mutation in synaptotagmin-1 reveals a critical role of Ca2+-dependent soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex binding in synaptic exocytosis

Synaptotagmin-1, the Ca2+ sensor for fast neurotransmitter release, was proposed to function by Ca2+-dependent phospholipid binding and/or by Ca2+-dependent soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex binding. Extensive in vivo data support the first hypothesis, but testing the second hypothesis has been difficult because no synaptotagmin-1 mutation is known that selectively interferes with SNARE complex binding. Using knock-in mice that carry aspartate-to-asparagine substitutions in a Ca2+-binding site of synaptotagmin-1 (the D232N or D238N substitutions), we now show that the D232N mutation dramatically increases Ca2+-dependent SNARE complex binding by native synaptotagmin-1, but leaves phospholipid binding unchanged. In contrast, the adjacent D238N mutation does not significantly affect SNARE complex binding, but decreases phospholipid binding. Electrophysiological recordings revealed that the D232N mutation increased Ca2+-triggered release, whereas the D238N mutation decreased release. These data establish that fast vesicle exocytosis is driven by a dual Ca2+-dependent activity of synaptotagmin-1, namely Ca2+-dependent binding both to SNARE complexes and to phospholipids.

Anxiety induced by prenatal stress is associated with suppression of hippocampal genes involved in synaptic function

Exposure of pregnant women or animals to stress during a critical period of foetal brain development increases the likelihood of anxiety, depression and learning deficits that are associated with structural alterations in the offspring hippocampus. In this study, we report the effect of gestational stress in rats on anxiogenic behaviour and hippocampal gene expression of their 23-day-old female offspring. As the rat brain continues to develop after birth, we also used the procedure of handling (H) during the first 10 days of life to reverse the anxiogenic behaviour of prenatally stressed (PS) rats. By means of micro-array analysis on hippocampal extracts, we found that the expression of about 6.1% of 9505 valid genes was significantly altered by prenatal stress (p<0.05). Of these, 48% were over-expressed and 52% under-expressed. The latter included approximately 300 genes that participate in axonal growth, regulation of ion channels and transporters, trafficking of synaptic vesicles and neurotransmitter release. About 30% of the genes that were down-regulated in PS rats were restored to control levels by H. These include genes that play a role in pre-synaptic organization and function. Our results provide a possible relationship between hippocampal gene expression and changes in behaviour resulting from prenatal stress.

A complexin/synaptotagmin 1 switch controls fast synaptic vesicle exocytosis

Ca(2+) binding to synaptotagmin 1 triggers fast exocytosis of synaptic vesicles that have been primed for release by SNARE-complex assembly. Besides synaptotagmin 1, fast Ca(2+)-triggered exocytosis requires complexins. Synaptotagmin 1 and complexins both bind to assembled SNARE complexes, but it is unclear how their functions are coupled. Here we propose that complexin binding activates SNARE complexes into a metastable state and that Ca(2+) binding to synaptotagmin 1 triggers fast exocytosis by displacing complexin from metastable SNARE complexes. Specifically, we demonstrate that, biochemically, synaptotagmin 1 competes with complexin for SNARE-complex binding, thereby dislodging complexin from SNARE complexes in a Ca(2+)-dependent manner. Physiologically, increasing the local concentration of complexin selectively impairs fast Ca(2+)-triggered exocytosis but retains other forms of SNARE-dependent fusion. The hypothesis that Ca(2+)-induced displacement of complexins from SNARE complexes triggers fast exocytosis accounts for the loss-of-function and gain-of-function phenotypes of complexins and provides a molecular explanation for the high speed and synchronicity of fast Ca(2+)-triggered neurotransmitter release.

Fusion pores and fusion machines in Ca2+-triggered exocytosis

Exocytosis is initiated within a highly localized region of contact between two biological membranes. Small areas of these membranes draw close, molecules on the two surfaces interact, and structural transformations take place. Membrane fusion requires the action of proteins specialized for this task, and these proteins act as a fusion machine. At a critical point in this process, a fusion pore forms within the membrane contact site and then expands as the spherical vesicle merges with the flat target membrane. Hence, the operation of a fusion machine must be realized through the formation and expansion of a fusion pore. Delineating the relation between the fusion machine and the fusion pore thus emerges as a central goal in elucidating the mechanisms of membrane fusion. We summarize present knowledge of fusion machines and fusion pores studied in vitro, in neurons, and in neuroendocrine cells, and synthesize this knowledge into some specific and detailed hypotheses for exocytosis.

Hemifusion arrest by complexin is relieved by Ca2+-synaptotagmin I

Synaptic transmission relies on an exquisitely orchestrated series of protein-protein interactions. Here we show that fusion driven by neuronal SNAREs is inhibited by the regulatory protein complexin. Furthermore, inner-leaflet mixing is strongly impaired relative to total lipid mixing, indicating that inhibition by complexin arrests fusion at hemifusion. When the calcium sensor synaptotagmin is added in the presence of calcium, inhibition by complexin is relieved and full fusion rapidly proceeds.

Vesicle pools, docking, priming, and release

The release of neurotransmitter from synaptic vesicles represents the final event by which presynapses send their chemical signal to the receiving postsynapses. Prior to fusion, synaptic vesicles undergo a series of maturation events, most notably the membrane-delimited docking and priming steps. Physiological and optical experiments with high-time resolution have allowed the distinction of vesicles in different maturation states with respect to fusion, the so-called vesicle pools. In this review, we define the various vesicle pools and discuss pathways leading into and out of these pools. We also provide an overview of an array of proteins that have been identified or are speculated to play a role in the transition between the various vesicle pools.

Complexin is able to bind to SNARE core complexes in different assembled states with distinct affinity

The formation of the functional SNARE complex in vivo is central to the fast neurotransmitter release at the neuronal terminal. Numerous studies revealed that this process involves progressive assembly of an alpha-helical bundle and is dynamically reversible. So far many proteins directly or indirectly take part in this process. Complexin, one of such factors, has revealed rapid association with the SNARE complex, however, whether or not complexin can interact with partially assembled SNARE complex is critical and yet unknown. Here, we present evidence that complexin is able to bind to various mutant versions of the SNARE complex mimicking its quaternary structure at different assembly stages. In addition, the affinity of complexin for the SNARE complex is correlated with the extent to which the SNARE complex is assembled. These results suggest that complexin is able to bind to SNARE complex before its complete formation.

A clamping mechanism involved in SNARE-dependent exocytosis

During neurotransmitter release at the synapse, influx of calcium ions stimulates the release of neurotransmitter. However, the mechanism by which synaptic vesicle fusion is coupled to calcium has been unclear, despite the identification of both the core fusion machinery [soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)] and the principal calcium sensor (synaptotagmin). Here, we describe what may represent a basic principle of the coupling mechanism: a reversible clamping protein (complexin) that can freeze the SNAREpin, an assembled fusion-competent intermediate en route to fusion. When calcium binds to the calcium sensor synaptotagmin, the clamp would then be released. SNARE proteins, and key regulators like synaptotagmin and complexin, can be ectopically expressed on the cell surface. Cells expressing such "flipped" synaptic SNAREs fuse constitutively, but when we coexpressed complexin, fusion was blocked. Adding back calcium triggered fusion from this intermediate in the presence of synaptotagmin.

Rabphilin regulates SNARE-dependent re-priming of synaptic vesicles for fusion

Synaptic vesicle fusion is catalyzed by assembly of synaptic SNARE complexes, and is regulated by the synaptic vesicle GTP-binding protein Rab3 that binds to RIM and to rabphilin. RIM is a known physiological regulator of fusion, but the role of rabphilin remains obscure. We now show that rabphilin regulates recovery of synaptic vesicles from use-dependent depression, probably by a direct interaction with the SNARE protein SNAP-25. Deletion of rabphilin dramatically accelerates recovery of depressed synaptic responses; this phenotype is rescued by viral expression of wild-type rabphilin, but not of mutant rabphilin lacking the second rabphilin C2 domain that binds to SNAP-25. Moreover, deletion of rabphilin also increases the size of synaptic responses in synapses lacking the vesicular SNARE protein synaptobrevin in which synaptic responses are severely depressed. Our data suggest that binding of rabphilin to SNAP-25 regulates exocytosis of synaptic vesicles after the readily releasable pool has either been physiologically exhausted by use-dependent depression, or has been artificially depleted by deletion of synaptobrevin.

Genetic analysis of synaptotagmin 2 in spontaneous and Ca2+-triggered neurotransmitter release

Synaptotagmin 2 resembles synaptotagmin 1, the Ca2+ sensor for fast neurotransmitter release in forebrain synapses, but little is known about synaptotagmin 2 function. Here, we describe a severely ataxic mouse strain that harbors a single, destabilizing amino-acid substitution (I377N) in synaptotagmin 2. In Calyx of Held synapses, this mutation causes a delay and a decrease in Ca2+-induced but not in hypertonic sucrose-induced release, suggesting that synaptotagmin 2 mediates Ca2+ triggering of evoked release in brainstem synapses. Unexpectedly, we additionally observed in synaptotagmin 2 mutant synapses a dramatic increase in spontaneous release. Synaptotagmin 1-deficient excitatory and inhibitory cortical synapses also displayed a large increase in spontaneous release, demonstrating that this effect was shared among synaptotagmins 1 and 2. Our data suggest that synaptotagmin 1 and 2 perform equivalent functions in the Ca2+ triggering of action potential-induced release and in the restriction of spontaneous release, consistent with a general role of synaptotagmins in controlling 'release slots' for synaptic vesicles at the active zone.

Dopamine release is severely compromised in the R6/2 mouse model of Huntington's disease

Recently, alterations in dopamine signaling have been implicated in Huntington's disease. In this work, dopamine release and uptake was measured in striatal slices from the R6/2 transgenic mouse model of Huntington's disease using fast-scan cyclic voltammetry at carbon-fiber microelectrodes. Dopamine release in brain slices from 6-week-old R6/2 mice is substantially reduced (53% of wild type), while dopamine uptake is unaffected. In agreement with this, R6/2 mice injected with the dopamine uptake inhibitor cocaine exhibited a blunted motor activity response (54% of wild type). At 10 weeks of age, an even more dramatic motor activity decrease in response to cocaine injection (21% of wild type) was observed. Moreover, the pre-drug activity of 10-week-old R6/2 mice was significantly reduced (by 37%) compared with 6-week-old R6/2 mice. Striatal dopamine release decreased with age, indicating that progressive alterations in dopaminergic pathways may affect motor activity. The inhibition constants of cocaine and methamphetamine (METH) determined in brain slices differed little between genotype or age group, suggesting that the decreased responses to cocaine and METH arise from compromised dopamine release rather than differences in uptake or drug action. Collectively, these data demonstrate (i) a reduction in the ability of dopamine terminals to release dopamine and (ii) the importance of this attenuation of release on the motor symptoms of Huntington's disease.

Promiscuous interaction of SNAP-25 with all plasma membrane syntaxins in a neuroendocrine cell

SNAP-25 (25 kDa synaptosome-associated protein) is found in cells that release neurotransmitters and hormones, and plays a central role in the fusion of secretory vesicles with the plasma membrane. SNAP-25 has been shown to interact specifically with syntaxin 1, a 35 kDa membrane protein, to mediate the fusion process. Here, we investigated whether other known syntaxin isoforms found at the plasma membrane can serve as binding partners for SNAP-25 in vivo. In our analysis, we employed rat phaeochromocytoma PC12 cells that are often used as a model of neuronal functions. We now show that these cells contain large amounts of SNAP-25, which interacts not only with syntaxin 1, but also with ubiquitous syntaxins 2, 3 and 4. The plasma membrane syntaxins appear to occupy complementary domains at the plasma membrane. In defined reactions, the ubiquitous plasma membrane syntaxin isoforms, when in binary complexes with SNAP-25, readily bound vesicular synaptobrevin to form SDS-resistant SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) complexes implicated in membrane fusion. However, vesicular synaptotagmin and cytosolic complexin, both implicated in the fusion process, exhibited differential ability to interact with the SNARE complexes formed by syntaxins 1-4, suggesting that the plasma membrane syntaxins may mediate vesicle fusion events with different properties.

Early, Transient Increase in Complexin I and Complexin II in the Cerebral Cortex following Traumatic Brain Injury Is Attenuated by N-Acetylcysteine

Alteration of excitatory neurotransmission is a key feature of traumatic brain injury (TBI) in which extracellular glutamate levels rise. Although increased synaptic release of glutamate occurs at the injury site, the precise mechanism is unclear. Complexin I and complexin II constitute a family of cytosolic proteins involved in the regulation of neurotransmitter release, competing with the chaperone protein alpha-SNAP (soluble N-ethylmaleimide-sensitive factor-attachment protein) for binding to the synaptic vesicle protein synaptobrevin as well as the synaptic membrane proteins SNAP-25 and syntaxin, which together form the SNAP receptor (SNARE) complex. Complexin I is predominantly a marker of axosomatic (inhibitory) synapses, whereas complexin II mainly labels axodendritic and axospinous synapses, the majority of which are excitatory. In order to examine the role of these proteins in TBI, we have studied levels of both complexins in the injured hemisphere by immunoblotting over a time period ranging from 6 h to 7 days following lateral fluid-percussion brain injury in the rat. Transient increases in the levels of complexin I and complexin II proteins were detected in the injured cerebral cortex 6 h following TBI. This increase was followed by a decrease of complexin I in the injured cortex and hippocampus, and a decrease in both complexins in the injured thalamus region at day 3 and day 7 post-injury. The early, transient increase in the injured cortex was completely blocked by N-acetylcysteine (NAC) administered 5 min following trauma, suggesting an involvement of oxidative stress. Neuronal loss was also reduced in the injured hemisphere with post-TBI NAC treatment. Our findings suggest a dysregulation of both inhibitory and excitatory neurotransmission following traumatic injury that is responsive to antioxidant treatment. These alterations in complexin levels may also play an important role in neuronal cell loss following TBI, and thus contribute to the pathophysiology of cerebral damage following brain injury.