Synaptotagmins Syt1, Syt2, Syt7, and Syt9 act as Ca2+-sensors for synaptic and neuroendocrine exocytosis, but the function of other synaptotagmins remains unknown. Here, we show that olfactory bulb neurons secrete IGF-1 by an activity-dependent pathway of exocytosis, and that Syt10 functions as the Ca2+-sensor that triggers IGF-1 exocytosis in these neurons. Deletion of Syt10 impaired activity-dependent IGF-1 secretion in olfactory bulb neurons, resulting in smaller neurons and an overall decrease in synapse numbers. Exogenous IGF-1 completely reversed the Syt10 knockout phenotype. Syt10 co-localized with IGF-1 in somatodendritic vesicles of olfactory bulb neurons, and Ca2+-binding to Syt10 caused these vesicles to undergo exocytosis, thereby secreting IGF-1. Thus, Syt10 controls a previously unrecognized pathway of Ca2+-dependent exocytosis that is spatially and temporally distinct from Ca2+-dependent synaptic vesicle exocytosis controlled by Syt1 in the same neurons, and two different synaptotagmins regulate distinct Ca2+-dependent membrane fusion reactions during exocytosis in the same neuron.
Fibroblast growth factor (FGF)1 is released from cells as a constituent of a complex that contains the small calcium binding protein S100A13, and the p40 kDa form of synaptotagmin (Syt)1, through an ER-Golgi-independent stress-induced pathway. FGF1 and the other components of its secretory complex are signal peptide-less proteins. We examined their capability to interact with lipid bilayers by studying protein-induced carboxyfluorescein release from liposomes of different phospholipid (pL) compositions. FGF1, p40 Syt1, and S100A13 induced destabilization of liposomes composed of acidic but not of zwitterionic pL. We produced mutants of FGF1 and p40 Syt1, in which specific basic amino acid residues in the regions that bind acidic pL were substituted. The ability of these mutants to induce liposomes destabilization was strongly attenuated, and they exhibited drastically diminished spontaneous and stress-induced release. Apparently, the non-classical release of FGF1 and p40 Syt1 involves destabilization of membranes containing acidic pL.
FGF1; synaptotagmin 1; S100A13; non-classical release; membrane; phospholipid; 5,6 carboxyfluorescein (CF); fibroblast growth factor (FGF); molten globule (MG); phospholipid (pL); phosphatidylinositol (pI); phosphatidylserine (pS); phosphatidylglycerol (pG); phosphatidylcholine (pC); synaptotagmin (Syt); wild type (WT)
The Ca2+ sensor synaptotagmin (Syt) VII regulates the exocytosis of conventional lysosomes in several cell types. In CTLs, the Ca2+-regulated exocytosis of lytic granules/secretory lysosomes is responsible for the perforin/granzyme-mediated lysis of target cells. To investigate the role of Syt VII in CTL effector function, the expression and function of Syt VII were examined in wild-type and Syt VII-deficient mice. In comparison with Syt VII+/+ controls, Syt VII−/− animals were impaired in their ability to clear an infection with the intracellular pathogen Listeria monocytogenes. When isolated CTLs were examined, we found that Syt VII is expressed upon CTL activation and localizes to granzyme A-containing lytic granules. Syt VII-deficient CTLs have no defects in proliferation and cytokine production, and their lytic granules contain normal amounts of perforin and granzyme A and polarize normally at the immunological synapse. However, despite normal conjugate formation with target cells, CTLs from Syt VII−/− mice exhibit reduced effector activity, when compared with controls. Treatment of Syt VII+/+ or Syt VII−/− CTLs with an inhibitor of the perforin-mediated lytic pathway resulted in comparable levels of cytotoxic activity, suggesting that Syt VII regulates perforin-mediated cytolytic CTL responses.
Synaptotagmins (Syts) I and II are believed to act as Ca2+ sensors in the control of neurotransmission. Here we demonstrate that mast cells express Syt II in their lysosomal fraction. We further show that activation of mast cells by either aggregation of FcεRI or by Ca2+ ionophores results in exocytosis of lysosomes, in addition to the well documented exocytosis of their secretory granules. Syt II directly regulates lysosomal exocytosis, whereby overexpression of Syt II inhibited Ca2+-triggered release of the lysosomal processed form of cathepsin D, whereas suppression of Syt II expression markedly potentiated this release. These findings provide evidence for a novel function of Syt II in negatively regulating Ca2+-triggered exocytosis of lysosomes, and suggest that Syt II–regulated secretion from lysosomes may play an important role in mast cell biology.
mast cells; lysosomes; calcium binding proteins; exocytosis; immunoglobulin E
The intracellular protozoan parasite Trypanosoma cruzi causes Chagas' disease, which affects millions of people in Latin America. T. cruzi enters a large number of cell types by an unusual mechanism that involves Ca2+-triggered fusion of lysosomes with the plasma membrane. Here we show that synaptotagmin VII (Syt VII), a ubiquitously expressed synaptotagmin isoform that regulates exocytosis of lysosomes, is localized on the membranes of intracellular vacuoles containing T. cruzi. Antibodies against the C2A domain of Syt VII or recombinant peptides including this domain inhibit cell entry by T. cruzi, but not by Toxoplasma gondii or Salmonella typhimurium. The C2A domains of other ubiquitously expressed synaptotagmin isoforms have no effect on T. cruzi invasion, and mutation of critical residues on Syt VII C2A abolish its inhibitory activity. These findings indicate that T. cruzi exploits the Syt VII–dependent, Ca2+-regulated lysosomal exocytic pathway for invading host cells.
trypanosome; parasite; intracellular; secretion; calcium
Acidic fibroblast growth factor (aFGF) is a signal peptide-less protein that is secreted into the extracellular compartment as part of a multiprotein release complex, consisting of aFGF, S100A13 (a calcium binding protein), and a 40 kDa (p40) form of synaptotagmin (Syt1), a protein that participates in the docking of a variety of secretory vesicles. p40 Syt1, and specifically its C2A domain, is believed to play a major role in the non-classical secretion of the aFGF release complex mediated by the interaction of aFGF and p40 Syt1with the phospholipids of the cell membrane inner leaflet. In the present study, we investigate the structural characteristics of aFGF and the C2A domain of p40 Syt1 under acidic conditions, using a variety of biophysical techniques including multidimensional NMR spectroscopy. Urea-induced equilibrium unfolding (at pH 3.4) of both aFGF and the C2A domain are non-cooperative and proceed with the accumulation of stable intermediate states. 1-Anilino-8-napthalene sulfonate (ANS) binding and size-exclusion chromatography results suggest that both aFGF and the C2A domain exist as partially structured states under acidic conditions (pH 3.4). Limited trypsin digestion analysis and 1H-15N chemical shift perturbation data reveal that the flexibility of certain portions of the protein backbone is increased in the partially structured state(s) of aFGF. The residues that are perturbed in the partially structured state(s) in aFGF are mostly located at the N- and C-terminal ends of the protein. In marked contrast, most of the interactions stabilizing the native secondary structure are preserved in the partially structured state of the C2A domain. Isothermal titration calorimetry data indicate that the binding affinity between aFGF and the C2A domain is significantly enhanced at pH 3.4. In addition, both aFGF and the C2A domain exhibit much higher lipid binding affinity in their partially structured states. The translocation of the multiprotein FGF release complex across the membrane appears to be facilitated by the formation of partially structured states of aFGF and the C2A domain of p40 Syt1.
Synaptotagmin-1 regulates synaptic vesicle fusion, but little is known about the remaining syt isoforms. Syt-pHluorin reporters are used to show that only two syts are on synaptic vesicles, whereas other isoforms are selectively targeted to dendrites, axons, or both axons and dendrites, where they undergo exocytosis and endocytosis with distinct kinetics.
The synaptotagmins (syts) are a family of molecules that regulate membrane fusion. There are 17 mammalian syt isoforms, most of which are expressed in the brain. However, little is known regarding the subcellular location and function of the majority of these syts in neurons, largely due to a lack of isoform-specific antibodies. Here we generated pHluorin-syt constructs harboring a luminal domain pH sensor, which reports localization, pH of organelles to which syts are targeted, and the kinetics and sites of exocytosis and endocytosis. Of interest, only syt-1 and 2 are targeted to synaptic vesicles, whereas other isoforms selectively recycle in dendrites (syt-3 and 11), axons (syt-5, 7, 10, and 17), or both axons and dendrites (syt-4, 6, 9, and 12), where they undergo exocytosis and endocytosis with distinctive kinetics. Hence most syt isoforms localize to distinct secretory organelles in both axons and dendrites and may regulate neuropeptide/neurotrophin release to modulate neuronal function.
Bacteriophage Mu has one of the best studied, most efficient and largest transposition machineries of the prokaryotic world. To harness this attractive integration machinery for use in mammalian cells, we cloned the coding sequences of the phage factors MuA and MuB in a eukaryotic expression cassette and fused them to a FLAG epitope and a SV40-derived nuclear localization signal. We demonstrate that these N-terminal extensions were sufficient to target the Mu proteins to the nucleus, while their function in Escherichia coli was not impeded. In vivo transposition in mammalian cells was analysed by co-transfection of the MuA and MuB expression vectors with a donor construct, which contained a miniMu transposon carrying a Hygromycin-resistance marker (HygR). In all co-transfections, a significant but moderate (up to 2.7-fold) increase in HygR colonies was obtained if compared with control experiments in which the MuA vector was omitted. To study whether the increased efficiency was the result of bona fide Mu transposition, integrated vector copies were cloned from 43 monoclonal and one polyclonal cell lines. However, in none of these clones, the junction between the vector and the chromosomal DNA was localized precisely at the border of the Att sites. From our data we conclude that expression of MuA and MuB increases the integration of miniMu vectors in mammalian cells, but that this increase is not the result of bona fide Mu-induced transposition.
Insulin regulates blood glucose by promoting uptake by fat and muscle, and inhibiting production by liver. Insulin-stimulated glucose uptake is mediated by GLUT4, which translocates from an intracellular compartment to the plasma membrane. GLUT4 traffic and insulin secretion both rely on calcium-dependent, regulated exocytosis. Deletion of the voltage-gated potassium channel Kv1.3 results in constitutive expression of GLUT4 at the plasma membrane. Inhibition of channel activity stimulated GLUT4 translocation through a calcium dependent mechanism. The synaptotagmins (Syt) are calcium sensors for vesicular traffic, and Syt VII mediates lysosomal and secretory granule exocytosis. We asked if Syt VII regulates insulin secretion by pancreatic β cells, and GLUT4 translocation in insulin-sensitive tissues mouse model. Syt VII deletion (Syt VII −/−) results in glucose intolerance and a marked decrease in glucose-stimulated insulin secretion in vivo. Pancreatic islet cells isolated from Syt VII −/− cells secreted significantly less insulin than islets of littermate controls. Syt VII deletion disrupted GLUT4 traffic as evidenced by constitutive expression of GLUT4 present at the plasma membrane of fat and skeletal muscle cells and unresponsiveness to insulin. These data document a key role for Syt VII in peripheral glucose homeostasis through its action on both insulin secretion and GLUT4 traffic.
Insulin; Glucose; Diabetes; Insulin resistance
Many members of the synaptotagmin (Syt) protein family bind Ca2+ and trigger exocytosis, but some Syts appear to have no Ca2+-dependent actions and their biological functions remain obscure. Syt IV is an activity-induced brain protein with no known Ca2+-dependent interactions; its sub-cellular localization and biological functions have sparked considerable controversy. This study reports the presence of Syt IV on both dense-core and microvesicles in posterior pituitary nerve terminals. In terminals from Syt IV knock-out mice compared to wild-type, low Ca2+ entry triggered more exocytosis, high Ca2+ entry triggered less exocytosis, and endocytosis was accelerated. Dense-core and microvesicle fusion was enhanced in cell-attached patches, and dense-core vesicle fusion pores had conductances half as large as in wild-type. Given the neuroendocrine functions of the posterior pituitary, changes in Syt IV levels could play roles in endocrine transitions involving alterations in release of the neuropeptides oxytocin and vasopressin.
Synaptotagmin (syt) I is thought to act as a Ca2+ sensor that regulates neuronal exocytosis. Fifteen additional isoforms of syt have been identified, but their functions are less well understood. Here, we used PC12 cells to test the idea that different isoforms of syt impart cells with distinct metal (i.e., Ca2+, Ba2+, and Sr2+) requirements for secretion. These cells express syt's I and IX (syt IX sometimes referred to as syt V), which have low apparent metal affinities, at much higher levels than syt VII, which we show has a relatively high apparent affinity for metals. We found that syt I and VII partially colocalize on large dense core vesicles and that upregulation of syt VII produces a concomitant increase in the divalent cation sensitivity of catecholamine release from PC12 cells. Furthermore, RNA interference-mediated knockdown of endogenous syt VII reduced the metal sensitivity of release. These data support the hypothesis that the complement of syt's expressed by a cell, in conjunction with their metal affinity, determines the divalent cation sensitivity of exocytosis.
In neuroendocrine PC12 cells, immature secretory granules (ISGs) mature through homotypic fusion and membrane remodeling. We present evidence that the ISG-localized synaptotagmin IV (Syt IV) is involved in ISG maturation. Using an in vitro homotypic fusion assay, we show that the cytoplasmic domain (CD) of Syt IV, but not of Syt I, VII, or IX, inhibits ISG homotypic fusion. Moreover, Syt IV CD binds specifically to ISGs and not to mature secretory granules (MSGs), and Syt IV binds to syntaxin 6, a SNARE protein that is involved in ISG maturation. ISG homotypic fusion was inhibited in vivo by small interfering RNA–mediated depletion of Syt IV. Furthermore, the Syt IV CD, as well as Syt IV depletion, reduces secretogranin II (SgII) processing by prohormone convertase 2 (PC2). PC2 is found mostly in the proform, suggesting that activation of PC2 is also inhibited. Granule formation, and the sorting of SgII and PC2 from the trans-Golgi network into ISGs and MSGs, however, is not affected. We conclude that Syt IV is an essential component for secretory granule maturation.
Synaptotagmin-IV (syt-IV) is a membrane trafficking protein that influences learning and memory, but its localization and role in synaptic function remain unclear. Here we discovered that syt-IV localizes to BDNF-containing vesicles in hippocampal neurons. Syt-IV/BDNF-harboring vesicles undergo exocytosis in both axons and dendrites, and syt-IV inhibits BDNF release at both sites. Knockout of syt-IV increases, and over-expression decreases, the rate of FM dye destaining from presynaptic terminals indirectly via changes in post-synaptic release of BDNF. Hence, post-synaptic syt-IV regulates the trans-synaptic action of BDNF to control presynaptic vesicle dynamics. Furthermore, selective loss of presynaptic syt-IV increased spontaneous quantal release, while loss of post-synaptic syt-IV increased quantal amplitude. Finally, syt-IV knockout mice exhibit enhanced LTP, which depends entirely on disinhibition of BDNF release. Thus, regulation of BDNF secretion by syt-IV emerges as a mechanism to maintain synaptic strength within a useful range during long-term potentiation.
Synaptotagmins (syts) are abundant, evolutionarily conserved integral membrane proteins that play essential roles in regulated exocytosis in nervous and endocrine systems. There are at least 17 syt isoforms in mammals, all with tandem C-terminal C2 domains with highly variable capacities for Ca2+ binding. Many syts play roles in neurotransmitter release or hormone secretion or both, and a growing body of work supports a role for some syts as Ca2+ sensors of exocytosis. Work in many types of endocrine cells has documented the presence of a number of syt isoforms on dense-core vesicles containing various hormones. Syts can influence the kinetics of exocytotic fusion pores and the choice of release mode between kiss-and-run and full-fusion. Vesicles harboring different syt isoforms can preferentially undergo distinct modes of exocytosis with different forms of stimulation. The diverse properties of syt isoforms enable these proteins to shape Ca2+ sensing in endocrine cells, thus contributing to the regulation of hormone release and the organization of complex endocrine functions.
exocytosis; neuropeptides; dense-core vesicle; norepinephrine; insulin; calcium; fusion pores; kiss-and-run
Synaptotagmins are known to mediate diverse forms of Ca2+-triggered exocytosis through their C2 domains, but the principles underlying functional differentiation among them are unclear. Synaptotagmin-1 functions as a Ca2+ sensor in neurotransmitter release at central nervous system synapses, but synaptotagmin-7 does not, and yet both isoforms act as Ca2+ sensors in chromaffin cells. To shed light into this apparent paradox, we have performed rescue experiments in neurons from synaptotagmin-1 knockout mice using a chimera that contains the synaptotagmin-1 sequence with its C2B domain replaced by the synaptotagmin-7 C2B domain (Syt1/7). Rescue was not achieved either with the WT Syt1/7 chimera or with nine mutants where residues that are distinct in synaptotagmin-7 were restored to those present in synaptotagmin-1. To investigate whether these results arise because of unique conformational features of the synaptotagmin-7 C2B domain, we determined its crystal structure at 1.44 Å resolution. The synaptotagmin-7 C2B domain structure is very similar to that of the synaptotagmin-1 C2B domain and contains three Ca2+-binding sites. Two of the Ca2+-binding sites of the synaptotagmin-7 C2B domain are also present in the synaptotagmin-1 C2B domain and have analogous ligands to those determined for the latter by NMR spectroscopy, suggesting that a discrepancy observed in a crystal structure of the synaptotagmin-1 C2B domain arose from crystal contacts. Overall, our results suggest that functional differentiation in synaptotagmins arises in part from subtle sequence changes that yield dramatic functional differences.
The release of signal peptideless proteins occurs through nonclassical export pathways and the release of fibroblast growth factor (FGF)1 in response to cellular stress is well documented. Although biochemical evidence suggests that the formation of a multiprotein complex containing S100A13 and Synaptotagmin (Syt)1 is important for the release of FGF1, it is unclear where this intracellular complex is assembled. As a result, we employed real-time analysis using confocal fluorescence microscopy to study the spatio-temporal aspects of this nonclassical export pathway and demonstrate that heat shock stimulates the redistribution of FGF1 from a diffuse cytosolic pattern to a locale near the inner surface of the plasma membrane where it colocalized with S100A13 and Syt1. In addition, coexpression of dominant-negative mutant forms of S100A13 and Syt1, which both repress the release of FGF1, failed to inhibit the stress-induced peripheral redistribution of intracellular FGF1. However, amlexanox, a compound that is known to attenuate actin stress fiber formation and FGF1 release, was able to repress this process. These data suggest that the assembly of the intracellular complex involved in the release of FGF1 occurs near the inner surface of the plasma membrane and is dependent on the F-actin cytoskeleton.
fibroblast growth factor; heat shock; S100A13; Synaptotagmin1; confocal microscopy
Newly synthesized secretory cargo molecules pass through the Golgi apparatus while resident Golgi proteins remain in the organelle. However, the pathways of membrane traffic within the Golgi are still uncertain. Most of the available data can be accommodated by the cisternal maturation model, which postulates that Golgi cisternae form de novo, carry the secretory cargoes forward, and ultimately disappear. The entry face of the Golgi receives material that has been exported from transitional ER sites, and the exit face of the Golgi is intimately connected with endocytic compartments. These conserved features are enhanced by cell type-specific elaborations such as tubular connections between mammalian Golgi cisternae. Key questions remain about how Golgi cisternae form and then mature, how resident Golgi proteins recycle, how Golgi compartments achieve their identities, how the unique architecture of the Golgi is established, and how different structural elements contribute to Golgi function.
cisternal maturation; COPI; transitional ER; recycling endosomes; Golgi matrix; Golgi tubules
In vitro reconstitution fusion assays incorporating full-length membrane-anchored synaptotagmin I clarify its role in several steps in the secretory pathway.
The synaptic vesicle protein synaptotagmin I (syt) promotes exocytosis via its ability to penetrate membranes in response to binding Ca2+ and through direct interactions with SNARE proteins. However, studies using full-length (FL) membrane-embedded syt in reconstituted fusion assays have yielded conflicting results, including a lack of effect, or even inhibition of fusion, by Ca2+. In this paper, we show that reconstituted FL syt promoted rapid docking of vesicles (<1 min) followed by a priming step (3–9 min) that was required for subsequent Ca2+-triggered fusion between v- and t-SNARE liposomes. Moreover, fusion occurred only when phosphatidylinositol 4,5-bisphosphate was included in the target membrane. This system also recapitulates some of the effects of syt mutations that alter synaptic transmission in neurons. Finally, we demonstrate that the cytoplasmic domain of syt exhibited mixed agonist/antagonist activity during regulated membrane fusion in vitro and in cells. Together, these findings reveal further convergence of reconstituted and cell-based systems.
Protein trafficking between the endoplasmic reticulum (ER) and Golgi apparatus is central to cellular homeostasis. ER export signals are utilized by a subset of proteins to rapidly exit the ER by direct uptake into COPII vesicles for transport to the Golgi. Norwalk virus nonstructural protein p22 contains a YXΦESDG motif that mimics a di-acidic ER export signal in both sequence and function. However, unlike normal ER export signals, the ER export signal mimic of p22 is necessary for apparent inhibition of normal COPII vesicle trafficking, which leads to Golgi disassembly and antagonism of Golgi-dependent cellular protein secretion. This is the first reported function for p22. Disassembly of the Golgi apparatus was also observed in cells replicating Norwalk virus, which may contribute to pathogenesis by interfering with cellular processes that are dependent on an intact secretory pathway. These results indicate that the ER export signal mimic is critical to the antagonistic function of p22, shown herein to be a novel antagonist of ER/Golgi trafficking. This unique and well-conserved human norovirus motif is therefore an appealing target for antiviral drug development.
Maintenance of bone mass and integrity requires a tight balance between resorption by osteoclasts and formation by osteoblasts. Exocytosis of functional proteins is a prerequisite for the activity of both cells. In the present study, we show that synaptotagmin VII, a calcium sensor protein that regulates exocytosis, is associated with lysosomes in osteoclasts and bone matrix protein-containing vesicles in osteoblasts. Absence of synaptotagmin VII inhibits cathepsin K secretion and formation of the ruffled border in osteoclasts and bone matrix protein deposition in osteoblasts, without affecting the differentiation of either cell. Reflecting these in vitro findings, synaptotagmin VII-deficient mice are osteopenic due to impaired bone resorption and formation. Therefore, synaptotagmin VII plays an important role in bone remodeling and homeostasis by modulating secretory pathways functionally important in osteoclasts and osteoblasts.
The Golgi receives the entire output of newly synthesized cargo from the endoplasmic reticulum (ER), processes it in the stack largely through modification of bound oligosaccharides, and sorts it in the trans-Golgi network (TGN). GRASP65 and GRASP55, two proteins localized to the Golgi stack and early secretory pathway, mediate processes including Golgi stacking, Golgi ribbon linking, and unconventional secretion. Previously we have shown that GRASP depletion in cells disrupts Golgi stack formation. Here we report that knockdown of the GRASP proteins, alone or combined, accelerates protein trafficking through the Golgi membranes but also has striking negative effects on protein glycosylation and sorting. These effects are not caused by Golgi ribbon unlinking, unconventional secretion, or ER stress. We propose that GRASP55/65 are negative regulators of exocytic transport and that this slowdown helps to ensure more complete protein glycosylation in the Golgi stack and proper sorting at the TGN.
Synaptotagmin isoforms and mutants altered fusion event frequency and fusion pore transitions. These effects showed a strong correlation with PS binding, but not with SNARE binding. Synaptotagmin-PS interaction thus function in two distinct kinetic steps in Ca2+ triggered exocytosis, and stabilize open fusion pores.
Synaptotagmin (syt) serves as a Ca2+ sensor in the release of neurotransmitters and hormones. This function depends on the ability of syt to interact with other molecules. Syt binds to phosphatidylserine (PS)-containing lipid bilayers as well as to soluble N-ethylmaleimide sensitive factor receptors (SNAREs) and promotes SNARE assembly. All these interactions are regulated by Ca2+, but their specific roles in distinct kinetic steps of exocytosis are not well understood. To explore these questions we used amperometry recording from PC12 cells to investigate the kinetics of exocytosis. Syt isoforms and syt I mutants were overexpressed to perturb syt-PS and syt-SNARE interactions to varying degrees and evaluate the effects on fusion event frequency and the rates of fusion pore transitions. Syt I produced more rapid dilation of fusion pores than syt VII or syt IX, consistent with its role in synchronous synaptic release. Stronger syt-PS interactions were accompanied by a higher frequency of fusion events and more stable fusion pores. By contrast, syt-SNARE interactions and syt-induced SNARE assembly were uncorrelated with rates of exocytosis. This associates the syt-PS interaction with two distinct kinetic steps in Ca2+ triggered exocytosis and supports a role for the syt-PS interaction in stabilizing open fusion pores.
Complexins are SNARE-complex binding proteins essential for the Ca2+-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 co-factor 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 Ca2+-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.
In regulated vesicle exocytosis, SNARE protein complexes drive membrane fusion to connect the vesicle lumen with the extracellular space. The triggering of fusion pore formation by Ca2+ is mediated by specific isoforms of synaptotagmin (Syt), which employ both SNARE complex and membrane binding. Ca2+ also promotes fusion pore expansion and Syts have been implicated in this process but the mechanisms involved are unclear. We determined the role of Ca2+-dependent Syt-effector interactions in fusion pore expansion by expressing Syt-1 mutants selectively altered in Ca2+-dependent SNARE binding or in Ca2+-dependent membrane insertion in PC12 cells that lack vesicle Syts. The release of different-sized fluorescent peptide-EGFP vesicle cargo or the vesicle capture of different-sized external fluorescent probes was used to assess the extent of fusion pore dilation. We found that PC12 cells expressing partial loss-of-function Syt-1 mutants impaired in Ca2+-dependent SNARE binding exhibited reduced fusion pore opening probabilities and reduced fusion pore expansion. Cells with gain-of-function Syt-1 mutants for Ca2+-dependent membrane insertion exhibited normal fusion pore opening probabilities but the fusion pores dilated extensively. The results indicate that Syt-1 uses both Ca2+-dependent membrane insertion and SNARE binding to drive fusion pore expansion.
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 co-regulate the kinetics and Ca2+ cooperativity of neurotransmitter release. CPX functions as a positive regulator of release in part by coupling the Ca2+ 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.