Synaptotagmin VII is targeted to secretory organelles in PC12 cells, where it functions as a high-affinity calcium sensor

A novel dual Ca2+ sensor system regulates Ca2+-dependent neurotransmitter release

Ca²⁺-dependent neurotransmitter release requires synaptotagmins as Ca²⁺ sensors to trigger synaptic vesicle (SV) exocytosis via binding of their tandem C2 domains—C2A and C2B—to Ca²⁺. We have previously demonstrated that SNT-1, a mouse synaptotagmin-1 (Syt1) homologue, functions as the fast Ca²⁺ sensor in Caenorhabditis elegans. Here, we report a new Ca²⁺ sensor, SNT-3, which triggers delayed Ca²⁺-dependent neurotransmitter release. snt-1;snt-3 double mutants abolish evoked synaptic transmission, demonstrating that C. elegans NMJs use a dual Ca²⁺ sensor system. SNT-3 possesses canonical aspartate residues in both C2 domains, but lacks an N-terminal transmembrane (TM) domain. Biochemical evidence demonstrates that SNT-3 binds both Ca²⁺ and the plasma membrane. Functional analysis shows that SNT-3 is activated when SNT-1 function is impaired, triggering SV release that is loosely coupled to Ca²⁺ entry. Compared with SNT-1, which is tethered to SVs, SNT-3 is not associated with SV. Eliminating the SV tethering of SNT-1 by removing the TM domain or the whole N terminus rescues fast release kinetics, demonstrating that cytoplasmic SNT-1 is still functional and triggers fast neurotransmitter release, but also exhibits decreased evoked amplitude and release probability. These results suggest that the fast and slow properties of SV release are determined by the intrinsically different C2 domains in SNT-1 and SNT-3, rather than their N-termini–mediated membrane tethering. Our findings therefore reveal a novel dual Ca²⁺ sensor system in C. elegans and provide significant insights into Ca²⁺-regulated exocytosis.

Synaptotagmin 7 is targeted to the axonal plasma membrane through γ-secretase processing to promote synaptic vesicle docking in mouse hippocampal neurons

Synaptotagmin 7 (SYT7) has emerged as a key regulator of presynaptic function, but its localization and precise role in the synaptic vesicle cycle remain the subject of debate. Here, we used iGluSnFR to optically interrogate glutamate release, at the single-bouton level, in SYT7KO-dissociated mouse hippocampal neurons. We analyzed asynchronous release, paired-pulse facilitation, and synaptic vesicle replenishment and found that SYT7 contributes to each of these processes to different degrees. 'Zap-and-freeze' electron microscopy revealed that a loss of SYT7 diminishes docking of synaptic vesicles after a stimulus and inhibits the recovery of depleted synaptic vesicles after a stimulus train. SYT7 supports these functions from the axonal plasma membrane, where its localization and stability require both γ-secretase-mediated cleavage and palmitoylation. In summary, SYT7 is a peripheral membrane protein that controls multiple modes of synaptic vesicle (SV) exocytosis and plasticity, in part, through enhancing activity-dependent docking of SVs.

Choreographing endo-lysosomal Ca2+ throughout the life of a phagosome

The emergence of endo-lysosomes as ubiquitous Ca²⁺ stores with their unique cohort of channels has resulted in their being implicated in a growing number of processes in an ever-increasing number of cell types. The architectural and regulatory constraints of these acidic Ca²⁺ stores distinguishes them from other larger Ca²⁺ sources such as the ER and influx across the plasma membrane. In view of recent advances in the understanding of the modes of operation, we discuss phagocytosis as a template for how endo-lysosomal Ca²⁺ signals (generated via TPC and TRPML channels) can be integrated in multiple sophisticated ways into biological processes. Phagocytosis illustrates how different endo-lysosomal Ca²⁺ signals drive different phases of a process, and how these can be altered by disease or infection.

Synaptotagmin-7 places dense-core vesicles at the cell membrane to promote Munc13-2- and Ca2+-dependent priming

Synaptotagmins confer calcium-dependence to the exocytosis of secretory vesicles, but how coexpressed synaptotagmins interact remains unclear. We find that synaptotagmin-1 and synaptotagmin-7 when present alone act as standalone fast and slow Ca²⁺-sensors for vesicle fusion in mouse chromaffin cells. When present together, synaptotagmin-1 and synaptotagmin-7 are found in largely non-overlapping clusters on dense-core vesicles. Synaptotagmin-7 stimulates Ca²⁺-dependent vesicle priming and inhibits depriming, and it promotes ubMunc13-2- and phorbolester-dependent priming, especially at low resting calcium concentrations. The priming effect of synaptotagmin-7 increases the number of vesicles fusing via synaptotagmin-1, while negatively affecting their fusion speed, indicating both synergistic and competitive interactions between synaptotagmins. Synaptotagmin-7 places vesicles in close membrane apposition (<6 nm); without it, vesicles accumulate out of reach of the fusion complex (20-40 nm). We suggest that a synaptotagmin-7-dependent movement toward the membrane is involved in Munc13-2/phorbolester/Ca²⁺-dependent priming as a prelude to fast and slow exocytosis triggering.

Function of Drosophila Synaptotagmins in membrane trafficking at synapses

The Synaptotagmin (SYT) family of proteins play key roles in regulating membrane trafficking at neuronal synapses. Using both Ca²⁺-dependent and Ca²⁺-independent interactions, several SYT isoforms participate in synchronous and asynchronous fusion of synaptic vesicles (SVs) while preventing spontaneous release that occurs in the absence of stimulation. Changes in the function or abundance of the SYT1 and SYT7 isoforms alter the number and route by which SVs fuse at nerve terminals. Several SYT family members also regulate trafficking of other subcellular organelles at synapses, including dense core vesicles (DCV), exosomes, and postsynaptic vesicles. Although SYTs are linked to trafficking of multiple classes of synaptic membrane compartments, how and when they interact with lipids, the SNARE machinery and other release effectors are still being elucidated. Given mutations in the SYT family cause disorders in both the central and peripheral nervous system in humans, ongoing efforts are defining how these proteins regulate vesicle trafficking within distinct neuronal compartments. Here, we review the Drosophila SYT family and examine their role in synaptic communication. Studies in this invertebrate model have revealed key similarities and several differences with the predicted activity of their mammalian counterparts. In addition, we highlight the remaining areas of uncertainty in the field and describe outstanding questions on how the SYT family regulates membrane trafficking at nerve terminals.

Synaptotagmin 7 switches short-term synaptic plasticity from depression to facilitation by suppressing synaptic transmission

Short-term synaptic plasticity is a fast and robust modification in neuronal presynaptic output that can enhance release strength to drive facilitation or diminish it to promote depression. The mechanisms that determine whether neurons display short-term facilitation or depression are still unclear. Here we show that the Ca²⁺-binding protein Synaptotagmin 7 (Syt7) determines the sign of short-term synaptic plasticity by controlling the initial probability of synaptic vesicle (SV) fusion. Electrophysiological analysis of Syt7 null mutants at Drosophila embryonic neuromuscular junctions demonstrate loss of the protein converts the normally observed synaptic facilitation response during repetitive stimulation into synaptic depression. In contrast, overexpression of Syt7 dramatically enhanced the magnitude of short-term facilitation. These changes in short-term plasticity were mirrored by corresponding alterations in the initial evoked response, with SV release probability enhanced in Syt7 mutants and suppressed following Syt7 overexpression. Indeed, Syt7 mutants were able to display facilitation in lower [Ca²⁺] where release was reduced. These data suggest Syt7 does not act by directly sensing residual Ca²⁺ and argues for the existence of a distinct Ca²⁺ sensor beyond Syt7 that mediates facilitation. Instead, Syt7 normally suppresses synaptic transmission to maintain an output range where facilitation is available to the neuron.

Diverse roles of Synaptotagmin-7 in regulating vesicle fusion

Synaptotagmin 7 (Syt7) is a multifunctional calcium sensor expressed throughout the body. Its high calcium affinity makes it well suited to act in processes triggered by modest calcium increases within cells. In synaptic transmission, Syt7 has been shown to mediate asynchronous neurotransmitter release, facilitation, and vesicle replenishment. In this review we provide an update on recent developments, and the newly emerging roles of Syt7 in frequency invariant synaptic transmission and in suppressing spontaneous release. Additionally, we discuss Syt7's regulation of membrane fusion in non-neuronal cells, and its involvement in disease. How such diversity of functions is regulated remains an open question. We discuss several potential factors including temperature, presynaptic calcium signals, the localization of Syt7, and its interaction with other Syt isoforms.

Phosphatidylinositol 4,5-bisphosphate drives Ca2+-independent membrane penetration by the tandem C2 domain proteins synaptotagmin-1 and Doc2β

Exocytosis mediates the release of neurotransmitters and hormones from neurons and neuroendocrine cells. Tandem C2 domain proteins in the synaptotagmin (syt) and double C2 domain (Doc2) families regulate exocytotic membrane fusion via direct interactions with Ca²⁺ and phospholipid bilayers. Syt1 is a fast-acting, low-affinity Ca²⁺ sensor that penetrates membranes upon binding Ca²⁺ to trigger synchronous vesicle fusion. The closely related Doc2β is a slow-acting, high-affinity Ca²⁺ sensor that triggers spontaneous and asynchronous vesicle fusion, but whether it also penetrates membranes is unknown. Both syt1 and Doc2β bind the dynamically regulated plasma membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP₂), but it is unclear whether PIP₂ serves only as a membrane contact or enables specialized membrane-binding modes by these Ca²⁺ sensors. Furthermore, it has been shown that PIP₂ uncaging can trigger rapid, syt1-dependent exocytosis in the absence of Ca²⁺ influx, suggesting that current models for the action of these Ca²⁺ sensors are incomplete. Here, using a series of steady-state and time-resolved fluorescence measurements, we show that Doc2β, like syt1, penetrates membranes in a Ca²⁺-dependent manner. Unexpectedly, we observed that PIP₂ can drive membrane penetration by both syt1 and Doc2β in the absence of Ca²⁺, providing a plausible mechanism for Ca²⁺-independent, PIP₂-dependent exocytosis. Quantitative measurements of penetration depth revealed that, in the presence of Ca²⁺, PIP₂ drives Doc2β, but not syt1, substantially deeper into the membrane, defining a biophysical regulatory mechanism specific to this high-affinity Ca²⁺ sensor. Our results provide evidence of a novel role for PIP₂ in regulating, and under some circumstances triggering, exocytosis.

Synaptotagmin-7 controls the size of the reserve and resting pools of synaptic vesicles in hippocampal neurons

Continuous neurotransmitter release is subjected to synaptic vesicle availability, which in turn depends on vesicle recycling and the traffic of vesicles between pools. We studied the role of Synaptotagmin-7 (Syt-7) in synaptic vesicle accessibility for release in hippocampal neurons in culture. Synaptic boutons from Syt-7 knockout (KO) mice displayed normal basal secretion with no alteration in the RRP size or the probability of release. However, stronger stimuli revealed an increase in the size of the reserve and resting vesicle pools in Syt-7 KO boutons compared with WT. These data suggest that Syt-7 plays a significant role in the vesicle pool homeostasis and, consequently, in the availability of vesicles for synaptic transmission during strong stimulation, probably, by facilitating advancing synaptic vesicles to the readily releasable pool.

Distinct patterns of exocytosis elicited by Ca2+, Sr2+ and Ba2+ in bovine chromaffin cells

Three divalent cations can elicit secretory responses in most neuroendocrine cells, including chromaffin cells. The extent to which secretion is elicited by the cations in intact depolarized cells was Ba²⁺ > Sr²⁺ ≥ Ca²⁺, contrasting with that elicited by these cations in permeabilized cells (Ca²⁺ > Sr²⁺ > Ba²⁺). Current-clamp recordings show that extracellular Sr²⁺ and Ba²⁺ cause membrane depolarization and action potentials, which are not blocked by Cd²⁺ but that can be mimicked by tetra-ethyl-ammonium. When applied intracellularly, only Ba²⁺ provokes action potentials. Voltage-clamp monitoring of Ca²⁺-activated K⁺ channels (K_Ca) shows that Ba²⁺ reduces outward currents, which were enhanced by Sr²⁺. Extracellular Ba²⁺ increases cytosolic Ca²⁺ concentrations in Fura-2-loaded intact cells, and it induces long-lasting catecholamine release. Conversely, amperometric recordings of permeabilized cells show that Ca²⁺ promotes the longest lasting secretion, as Ba²⁺ only provokes secretion while it is present and Sr²⁺ induces intermediate-lasting secretion. Intracellular Ba²⁺ dialysis provokes exocytosis at concentrations 100-fold higher than those of Ca²⁺, whereas Sr²⁺ exhibits an intermediate sensitivity. These results are compatible with the following sequence of events: Ba²⁺ blocks K_Ca channels from both the outside and inside of the cell, causing membrane depolarization that, in turn, opens voltage-sensitive Ca²⁺ channels and favors the entry of Ca²⁺ and Ba²⁺. Although Ca²⁺ is less permeable through its own channels, it is more efficient in triggering exocytosis. Strontium possesses both an intermediate permeability and an intermediate ability to induce secretion.

The high-affinity calcium sensor synaptotagmin-7 serves multiple roles in regulated exocytosis

MacDougall et al. review the structure and function of the calcium sensor synaptotagmin-7 in exocytosis.

Synaptotagmin (Syt) proteins comprise a 17-member family, many of which trigger exocytosis in response to calcium. Historically, most studies have focused on the isoform Syt-1, which serves as the primary calcium sensor in synchronous neurotransmitter release. Recently, Syt-7 has become a topic of broad interest because of its extreme calcium sensitivity and diversity of roles in a wide range of cell types. Here, we review the known and emerging roles of Syt-7 in various contexts and stress the importance of its actions. Unique functions of Syt-7 are discussed in light of recent imaging, electrophysiological, and computational studies. Particular emphasis is placed on Syt-7–dependent regulation of synaptic transmission and neuroendocrine cell secretion. Finally, based on biochemical and structural data, we propose a mechanism to link Syt-7’s role in membrane fusion with its role in subsequent fusion pore expansion via strong calcium-dependent phospholipid binding.

Synaptotagmin isoforms confer distinct activation kinetics and dynamics to chromaffin cell granules

Chromaffin cells release transmitters from populations of granules to which synaptotagmin-1 and synaptotagmin-7 are selectively sorted. Rao et al. characterize the functional properties of these granules and show that synaptotagmin-7 confers fast kinetics and high efficacy to the exocytotic event.

Adrenomedullary chromaffin cells respond to sympathetic nervous system activation by secreting a cocktail of potent neuropeptides and hormones into the circulation. The distinct phases of the chromaffin cell secretory response have been attributed to the progressive fusion of distinct populations of dense core granules with different activation kinetics. However, it has been difficult to define what distinguishes these populations at the molecular level. Functional segregation of granule pools may depend on selective sorting of synaptotagmin-1 (Syt-1) and synaptotagmin-7 (Syt-7), which our previous work showed are rarely cosorted to the same granule. Here we assess the consequences of selective sorting of Syt isoforms in chromaffin cells, particularly with respect to granule dynamics and activation kinetics. Upon depolarization of cells expressing fluorescent Syt isoforms using elevated K⁺, we find that Syt-7 granules fuse with faster kinetics than Syt-1 granules, irrespective of stimulation strength. Pharmacological blockade of Ca²⁺ channels reveals differential dependence of Syt-1 versus Syt-7 granule exocytosis on Ca²⁺ channel subtypes. Syt-7 granules also show a greater tendency to fuse in clusters than Syt-1 granules, and granules harboring Syt-1 travel a greater distance before fusion than those with Syt-7, suggesting that there is spatial and fusion-site heterogeneity among the two granule populations. However, the greatest functional difference between granule populations is their responsiveness to Ca²⁺. Upon introduction of Ca²⁺ into permeabilized cells, Syt-7 granules fuse with fast kinetics and high efficacy, even at low Ca²⁺ levels (e.g., when cells are weakly stimulated). Conversely, Syt-1 granules require a comparatively larger increase in intracellular Ca²⁺ for activation. At Ca²⁺ concentrations above 30 µM, activation kinetics are faster for Syt-1 granules than for Syt-7 granules. Our study provides evidence for functional specialization of chromaffin cell granules via selective expression of Syt isoforms with different Ca²⁺ sensitivities.

GABA Signaling and Neuroactive Steroids in Adrenal Medullary Chromaffin Cells

Gamma-aminobutyric acid (GABA) is produced not only in the brain, but also in endocrine cells by the two isoforms of glutamic acid decarboxylase (GAD), GAD65 and GAD67. In rat adrenal medullary chromaffin cells only GAD67 is expressed, and GABA is stored in large dense core vesicles (LDCVs), but not synaptic-like microvesicles (SLMVs). The α3β2/3γ2 complex represents the majority of GABAA receptors expressed in rat and guinea pig chromaffin cells, whereas PC12 cells, an immortalized rat chromaffin cell line, express the α1 subunit as well as the α3. The expression of α3, but not α1, in PC12 cells is enhanced by glucocorticoid activity, which may be mediated by both the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). GABA has two actions mediated by GABAA receptors in chromaffin cells: it induces catecholamine secretion by itself and produces an inhibition of synaptically evoked secretion by a shunt effect. Allopregnanolone, a neuroactive steroid which is secreted from the adrenal cortex, produces a marked facilitation of GABAA receptor channel activity. Since there are no GABAergic nerve fibers in the adrenal medulla, GABA may function as a para/autocrine factor in the chromaffin cells. This function of GABA may be facilitated by expression of the immature isoforms of GAD and GABAA receptors and the lack of expression of plasma membrane GABA transporters (GATs). In this review, we will consider how the para/autocrine function of GABA is achieved, focusing on the structural and molecular mechanisms for GABA signaling.

Molecular underpinnings of synaptic vesicle pool heterogeneity

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

Exocytosis and synaptic vesicle function

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

Distinct fusion properties of synaptotagmin-1 and synaptotagmin-7 bearing dense core granules

Adrenal chromaffin cells express two synaptotagmin isoforms, Syt-1 and Syt-7. Isoforms are usually sorted to separate secretory granules, are differentially activated by depolarizing stimuli, and favor discrete modes of exocytosis. It is proposed that stimulus/Ca⁺-dependent secretion in the chromaffin cell relies on selective Syt isoform activation.

Adrenal chromaffin cells release hormones and neuropeptides that are essential for physiological homeostasis. During this process, secretory granules fuse with the plasma membrane and deliver their cargo to the extracellular space. It was once believed that fusion was the final regulated step in exocytosis, resulting in uniform and total release of granule cargo. Recent evidence argues for nonuniform outcomes after fusion, in which cargo is released with variable kinetics and selectivity. The goal of this study was to identify factors that contribute to the different outcomes, with a focus on the Ca²⁺-sensing synaptotagmin (Syt) proteins. Two Syt isoforms are expressed in chromaffin cells: Syt-1 and Syt-7. We find that overexpressed and endogenous Syt isoforms are usually sorted to separate secretory granules and are differentially activated by depolarizing stimuli. In addition, overexpressed Syt-1 and Syt-7 impose distinct effects on fusion pore expansion and granule cargo release. Syt-7 pores usually fail to expand (or reseal), slowing the dispersal of lumenal cargo proteins and granule membrane proteins. On the other hand, Syt-1 diffuses from fusion sites and promotes the release of lumenal cargo proteins. These findings suggest one way in which chromaffin cells may regulate cargo release is via differential activation of synaptotagmin isoforms.

Complexin synchronizes primed vesicle exocytosis and regulates fusion pore dynamics

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

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

Synaptotagmin 7 functions as a Ca2+-sensor for synaptic vesicle replenishment

Synaptotagmin (syt) 7 is one of three syt isoforms found in all metazoans; it is ubiquitously expressed, yet its function in neurons remains obscure. Here, we resolved Ca(2+)-dependent and Ca(2+)-independent synaptic vesicle (SV) replenishment pathways, and found that syt 7 plays a selective and critical role in the Ca(2+)-dependent pathway. Mutations that disrupt Ca(2+)-binding to syt 7 abolish this function, suggesting that syt 7 functions as a Ca(2+)-sensor for replenishment. The Ca(2+)-binding protein calmodulin (CaM) has also been implicated in SV replenishment, and we found that loss of syt 7 was phenocopied by a CaM antagonist. Moreover, we discovered that syt 7 binds to CaM in a highly specific and Ca(2+)-dependent manner; this interaction requires intact Ca(2+)-binding sites within syt 7. Together, these data indicate that a complex of two conserved Ca(2+)-binding proteins, syt 7 and CaM, serve as a key regulator of SV replenishment in presynaptic nerve terminals. DOI: http://dx.doi.org/10.7554/eLife.01524.001.

Hydrophobic contributions to the membrane docking of synaptotagmin 7 C2A domain: mechanistic contrast between isoforms 1 and 7

Synaptotagmin (Syt) triggers Ca(2+)-dependent membrane fusion via its tandem C2 domains, C2A and C2B. The 17 known human isoforms are active in different secretory cell types, including neurons (Syt1 and others) and pancreatic β cells (Syt7 and others). Here, quantitative fluorescence measurements reveal notable differences in the membrane docking mechanisms of Syt1 C2A and Syt7 C2A to vesicles comprised of physiological lipid mixtures. In agreement with previous studies, the Ca(2+) sensitivity of membrane binding is much higher for Syt7 C2A. We report here for the first time that this increased sensitivity is due to the slower target membrane dissociation of Syt7 C2A. Association and dissociation rate constants for Syt7 C2A are found to be ~2-fold and ~60-fold slower than Syt1 C2A, respectively. Furthermore, the membrane dissociation of Syt7 C2A but not Syt1 C2A is slowed by Na(2)SO(4) and trehalose, solutes that enhance the hydrophobic effect. Overall, the simplest model consistent with these findings proposes that Syt7 C2A first docks electrostatically to the target membrane surface and then inserts into the bilayer via a slow hydrophobic mechanism. In contrast, the membrane docking of Syt1 C2A is known to be predominantly electrostatic. Thus, these two highly homologous domains exhibit distinct mechanisms of membrane binding correlated with their known differences in function.

Membrane trafficking in osteoblasts and osteoclasts: new avenues for understanding and treating skeletal diseases

The endocytic and exocytic/secretory pathways are two major intracellular membrane trafficking routes that regulate numerous cellular functions in a variety of cell types. Osteoblasts and osteoclasts, two major bone cells responsible for bone remodeling and homeostasis, are no exceptions. During the past few years, emerging evidence has pinpointed a critical role for endocytic and secretory pathways in osteoblast and osteoclast differentiation and function. The endosomal membrane provides a platform to integrate bone tropic signals of hormones and growth factors in osteoblasts. In osteoclasts, endocytosis, followed by transcytosis, of degraded bone matrix promotes bone resorption. Secretory pathways, especially lysosome secretion, not only participate in bone matrix deposition by osteoblasts and degradation of mineralized bone matrix by osteoclasts; they may also be involved in the coupling of bone resorption and bone formation during bone remodeling. More importantly, mutations in genes encoding regulatory factors within the endocytic and secretory pathways have been identified as causes for bone diseases. Identification of the molecular mechanisms of these genes in bone cells may provide new therapeutic targets for skeletal disorders.

Axonal and dendritic synaptotagmin isoforms revealed by a pHluorin-syt functional screen

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.

Evidence for lysosomal exocytosis and release of aggrecan-degrading hydrolases from hypertrophic chondrocytes, in vitro and in vivo

The abundant proteoglycan, aggrecan, is resorbed from growth plate cartilage during endochondral bone ossification, yet mice with genetically-ablated aggrecan-degrading activity have no defects in bone formation. To account for this apparent anomaly, we propose that lysosomal hydrolases degrade extracellular, hyaluronan-bound aggrecan aggregates in growth plate cartilage, and that lysosomal hydrolases are released from hypertrophic chondrocytes into growth plate cartilage via Ca²⁺-dependent lysosomal exocytosis. In this study we confirm that hypertrophic chondrocytes release hydrolases via lysosomal exocytosis in vitro and we show in vivo evidence for lysosomal exocytosis in hypertrophic chondrocytes during skeletal development. We show that lysosome-associated membrane protein 1 (LAMP1) is detected at the cell surface following in vitro treatment of epiphyseal chondrocytes with the calcium ionophore, ionomycin. Furthermore, we show that in addition to the lysosomal exocytosis markers, cathepsin D and β-hexosaminidase, ionomycin induces release of aggrecan- and hyaluronan-degrading activity from cultured epiphyseal chondrocytes. We identify VAMP-8 and VAMP7 as v-SNARE proteins with potential roles in lysosomal exocytosis in hypertrophic chondrocytes, based on their colocalisation with LAMP1 at the cell surface in secondary ossification centers in mouse tibiae. We propose that resorbing growth plate cartilage involves release of destructive hydrolases from hypertrophic chondrocytes, via lysosomal exocytosis.

Doc2 is a Ca2+ sensor required for asynchronous neurotransmitter release

Synaptic transmission involves a fast synchronous phase and a slower asynchronous phase of neurotransmitter release that are regulated by distinct Ca(2+) sensors. Though the Ca(2+) sensor for rapid exocytosis, synaptotagmin I, has been studied in depth, the sensor for asynchronous release remains unknown. In a screen for neuronal Ca(2+) sensors that respond to changes in [Ca(2+)] with markedly slower kinetics than synaptotagmin I, we observed that Doc2--another Ca(2+), SNARE, and lipid-binding protein--operates on timescales consistent with asynchronous release. Moreover, up- and downregulation of Doc2 expression levels in hippocampal neurons increased or decreased, respectively, the slow phase of synaptic transmission. Synchronous release, when triggered by single action potentials, was unaffected by manipulation of Doc2 but was enhanced during repetitive stimulation in Doc2 knockdown neurons, potentially due to greater vesicle availability. In summary, we propose that Doc2 is a Ca(2+) sensor that is kinetically tuned to regulate asynchronous neurotransmitter release.

Multiple Ca2+ sensors in secretion: teammates, competitors or autocrats?

Regulated neurotransmitter secretion depends on Ca(2+) sensors, C2 domain proteins that associate with phospholipids and soluble N-ethylmaleimide-sensitive fusion attachment protein receptor (SNARE) complexes to trigger release upon Ca(2+) binding. Ca(2+) sensors are thought to prevent spontaneous fusion at rest (clamping) and to promote fusion upon Ca(2+) activation. At least eight, often coexpressed, Ca(2+) sensors have been identified in mammals. Accumulating evidence suggests that multiple Ca(2+) sensors interact, rather than work autonomously, to produce the complex secretory response observed in neurons and secretory cells. In this review, we present several working models to describe how different sensors might be arranged to mediate synchronous, asynchronous and spontaneous neurotransmitter release. We discuss the scenario that different Ca(2+) sensors typically act on one shared vesicle pool and compete for binding the multiple SNARE complexes that are likely to assemble at single vesicles, to exert both clamping and fusion-promoting functions.

Expression, localization, and functional role for synaptotagmins in pancreatic acinar cells

Secretagogue-induced changes in intracellular Ca(2+) play a pivotal role in secretion in pancreatic acini yet the molecules that respond to Ca(2+) are uncertain. Zymogen granule (ZG) exocytosis is regulated by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes. In nerve and endocrine cells, Ca(2+)-stimulated exocytosis is regulated by the SNARE-associated family of proteins termed synaptotagmins. This study examined a potential role for synaptotagmins in acinar secretion. RT-PCR revealed that synaptotagmin isoforms 1, 3, 6, and 7 are present in isolated acini. Immunoblotting and immunofluorescence using three different antibodies demonstrated synaptotagmin 1 immunoreactivity in apical cytoplasm and ZG fractions of acini, where it colocalized with vesicle-associated membrane protein 2. Synaptotagmin 3 immunoreactivity was detected in membrane fractions and colocalized with an endolysosomal marker. A potential functional role for synaptotagmin 1 in secretion was indicated by results that introduction of synaptotagmin 1 C2AB domain into permeabilized acini inhibited Ca(2+)-dependent exocytosis by 35%. In contrast, constructs of synaptotagmin 3 had no effect. Confirmation of these findings was achieved by incubating intact acini with an antibody specific to the intraluminal domain of synaptotagmin 1, which is externalized following exocytosis. Externalized synaptotagmin 1 was detected exclusively along the apical membrane. Treatment with CCK-8 (100 pM, 5 min) enhanced immunoreactivity by fourfold, demonstrating that synaptotagmin is inserted into the apical membrane during ZG fusion. Collectively, these data indicate that acini express synaptotagmin 1 and support that it plays a functional role in secretion whereas synaptotagmin 3 has an alternative role in endolysosomal membrane trafficking.

Activity-dependent IGF-1 exocytosis is controlled by the Ca(2+)-sensor synaptotagmin-10

Synaptotagmins Syt1, Syt2, Syt7, and Syt9 act as Ca(2+)-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 Ca(2+)-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 colocalized with IGF-1 in somatodendritic vesicles of olfactory bulb neurons, and Ca(2+)-binding to Syt10 caused these vesicles to undergo exocytosis, thereby secreting IGF-1. Thus, Syt10 controls a previously unrecognized pathway of Ca(2+)-dependent exocytosis that is spatially and temporally distinct from Ca(2+)-dependent synaptic vesicle exocytosis controlled by Syt1. Our findings thereby reveal that two different synaptotagmins can regulate functionally distinct Ca(2+)-dependent membrane fusion reactions in the same neuron.

Release mode of large and small dense-core vesicles specified by different synaptotagmin isoforms in PC12 cells

Different synaptotagmin isoforms (syt I, VII, and IX) sort to populations of dense-core vesicles with different sizes. These isoforms differ in their sensitivities to divalent cations and trigger different modes of exocytosis. Exocytosis triggered by these isoforms also differs in its sensitivity to inhibition by another isoform, syt IV.

Many cells release multiple substances in different proportions according to the specific character of a stimulus. PC12 cells, a model neuroendocrine cell line, express multiple isoforms of the exocytotic Ca²⁺ sensor synaptotagmin. We show that these isoforms sort to populations of dense-core vesicles that differ in size. These synaptotagmins differ in their Ca²⁺ sensitivities, their preference for full fusion or kiss-and-run, and their sensitivity to inhibition by synaptotagmin IV. In PC12 cells, vesicles that harbor these different synaptotagmin isoforms can be preferentially triggered to fuse by different forms of stimulation. The mode of fusion is specified by the synaptotagmin isoform activated, and because kiss-and-run exocytosis can filter small molecules through a size-limiting fusion pore, the activation of isoforms that favor kiss-and-run will select smaller molecules over larger molecules packaged in the same vesicle. Thus synaptotagmin isoforms can provide multiple levels of control in the release of different molecules from the same cell.

Differential distribution of synaptotagmin-1, -4, -7, and -9 in rat adrenal chromaffin cells

Neurons and certain kinds of endocrine cells, such as adrenal chromaffin cells, have large dense-core vesicles (LDCVs) and synaptic vesicles or synaptic-like microvesicles (SLMVs). These secretory vesicles exhibit differences in Ca(2+) sensitivity and contain diverse signaling substances. The present work was undertaken to identify the synaptotagmin (Syt) isoforms present in secretory vesicles. Fractionation analysis of lysates of the bovine adrenal medulla and immunocytochemistry in rat chromaffin cells indicated that Syt 1 was localized in LDCVs and SLMVs, whereas Syt 7 was the predominant isoform present in LDCVs. In contrast to PC12 cells and the pancreatic β cell line INS-1, Syt 9 was not immunodetected in LDCVs in rat chromaffin cells. Double-staining revealed that Syt 9-like immunoreactivity was nearly identical with fluorescent thapsigargin binding, suggesting the presence of Syt 9 in the endoplasmic reticulum (ER).The exogenous expression of Syt 1-GFP in INS-1 cells, which had a negligible level of endogenous Syt 1, resulted in an increase in the amount of Syt 9 in the ER, suggesting that Syt 9 competes with Syt 1 for trafficking from the ER to the Golgi complex. We conclude that LDCVs mainly contain Syt 7, whereas SLMVs contain Syt 1, but not Syt 7, in rat and bovine chromaffin cells.

Push-and-pull regulation of the fusion pore by synaptotagmin-7

In chromaffin cells, Ca(2+) binding to synaptotagmin-1 and -7 triggers exocytosis by promoting fusion pore opening and fusion pore expansion. Synaptotagmins contain two C2 domains that both bind Ca(2+) and contribute to exocytosis; however, it remains unknown whether the C2 domains act similarly or differentially to promote opening and expansion of fusion pores. Here, we use patch amperometry measurements in WT and synaptotagmin-7-mutant chromaffin cells to analyze the role of Ca(2+) binding to the two synaptotagmin-7 C2 domains in exocytosis. We show that, surprisingly, Ca(2+) binding to the C2A domain suffices to trigger fusion pore opening but that the resulting fusion pores are unstable and collapse, causing a dramatic increase in kiss-and-run fusion events. Thus, synaptotagmin-7 controls fusion pore dynamics during exocytosis via a push-and-pull mechanism in which Ca(2+) binding to both C2 domains promotes fusion pore opening, but the C2B domain is selectively essential for continuous expansion of an otherwise unstable fusion pore.

Distinct roles for two synaptotagmin isoforms in synchronous and asynchronous transmitter release at zebrafish neuromuscular junction

An obligatory role for the calcium sensor synaptotagmins in stimulus-coupled release of neurotransmitter is well established, but a role for synaptotagmin isoform involvement in asynchronous release remains conjecture. We show, at the zebrafish neuromuscular synapse, that two separate synaptotagmins underlie these processes. Specifically, knockdown of synaptotagmin 2 (syt2) reduces synchronous release, whereas knockdown of synaptotagmin 7 (syt7) reduces the asynchronous component of release. The zebrafish neuromuscular junction is unique in having a very small quantal content and a high release probability under conditions of either low-frequency stimulation or high-frequency augmentation. Through these features, we further determined that during the height of shared synchronous and asynchronous transmission these two modes compete for the same release sites.

Synaptotagmins in neurodegeneration

Synaptotagmins (Syts) are transmembrane proteins involved in the regulation of membrane trafficking. Here, we summarize literature data that provide growing evidence that several Syts are involved in the pathophysiological mechanisms of temporal lobe epilepsy and Parkinson's disease, as well as few reports related to brain ischemia and Alzheimer's disease (AD). We also report new data from our laboratories, showing changes of the expression of several Syts in Tg2576 mouse model of AD that may be related to neuroinflammation surrounding the beta-amyloid plaques. Furthermore, we demonstrate N-methyl-D-aspartate receptor-mediated upregulation of Syt 4 mRNA in a model of excitotoxic striatal lesion induced by unilateral striatal injection of quinolinic acid, associating the upregulation of Syt 4 with mechanisms of excitotoxicity. We propose that pharmacological manipulation of Syt expression in animal models of neurodegeneration should be further explored, as it may help to clarify the role of individual Syt isoforms in the regulation of membrane trafficking in neurodegeneration.

Silence of Synaptotagmin VII inhibits release of dense core vesicles in PC12 cells

Synaptotagmin VII (Syt VII), which has a higher Ca(2+) affinity and slower disassembly kinetics with lipid than Syt I and Syt IX, was regarded as being uninvolved in synaptic vesicle (SV) exocytosis but instead possibly as a calcium sensor for the slower kinetic phase of dense core vesicles (DCVs) release. By using high temporal resolution capacitance and amperometry measurements, it was demonstrated that the knockdown of endogenous Syt VII attenuated the fusion of DCV with the plasma membrane, reduced the amplitude of the exocytotic burst of the Ca(2+)-triggered DCV release without affecting the slope of the sustained component, and blocked the fusion pore expansion. This suggests that Syt VII is the Ca(2+) sensor of DCV fusion machinery and is an essential factor for the establishment and maintenance of the pool size of releasable DCVs in PC12 cells.

Synaptotagmin IV: a multifunctional regulator of peptidergic nerve terminals

Many members of the synaptotagmin (Syt) protein family bind Ca(2+) and trigger exocytosis, but some Syt proteins appear to have no Ca(2+)-dependent actions and their biological functions remain obscure. Syt IV is an activity-induced brain protein with no known Ca(2+)-dependent interactions and its subcellular localization and biological functions have sparked considerable controversy. We found Syt IV on both micro- and dense-core vesicles in posterior pituitary nerve terminals in mice. In terminals from Syt IV knockout mice compared with those from wild types, low Ca(2+) entry triggered more exocytosis, high Ca(2+) entry triggered less exocytosis and endocytosis was accelerated. In Syt IV knockouts, dense-core and microvesicle fusion was enhanced in cell-attached patches and dense-core vesicle fusion pores had conductances that were half as large as those in wild types. Given the neuroendocrine functions of the posterior pituitary, changes in Syt IV levels could be involved in endocrine transitions involving alterations in the release of the neuropeptides oxytocin and vasopressin.

Synaptotagmin-1 utilizes membrane bending and SNARE binding to drive fusion pore expansion

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 Ca(2+) is mediated by specific isoforms of synaptotagmin (Syt), which employ both SNARE complex and membrane binding. Ca(2+) also promotes fusion pore expansion and Syts have been implicated in this process but the mechanisms involved are unclear. We determined the role of Ca(2+)-dependent Syt-effector interactions in fusion pore expansion by expressing Syt-1 mutants selectively altered in Ca(2+)-dependent SNARE binding or in Ca(2+)-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 Ca(2+)-dependent SNARE binding exhibited reduced fusion pore opening probabilities and reduced fusion pore expansion. Cells with gain-of-function Syt-1 mutants for Ca(2+)-dependent membrane insertion exhibited normal fusion pore opening probabilities but the fusion pores dilated extensively. The results indicate that Syt-1 uses both Ca(2+)-dependent membrane insertion and SNARE binding to drive fusion pore expansion.

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.

Functional significance of repressor element 1 silencing transcription factor (REST) target genes in pancreatic beta cells

AIMS/HYPOTHESIS

The expression of several neuronal genes in pancreatic beta cells is due to the absence of the transcription factor repressor element 1 (RE-1) silencing transcription factor (REST). The identification of these traits and their functional significance in beta cells has only been partly elucidated. Herein, we investigated the biological consequences of a repression of REST target genes by expressing REST in beta cells.

METHODS

The effect of REST expression on glucose homeostasis, insulin content and release, and beta cell mass was analysed in transgenic mice selectively expressing REST in beta cells. Relevant target genes were identified in INS-1E and primary beta cells expressing REST.

RESULTS

Transgenic mice featuring a beta cell-targeted expression of REST exhibited glucose intolerance and reduced beta cell mass. In primary beta cells, REST repressed several proteins of the exocytotic machinery, including synaptosomal-associated protein (SNAP) 25, synaptotagmin (SYT) IV, SYT VII, SYT IX and complexin II; it impaired first and second phases of insulin secretion. Using RNA interference in INS-1E cells, we showed that SYT IV and SYT VII were implicated in the control of insulin release.

CONCLUSIONS/INTERPRETATION

The data document the critical role of REST target genes in pancreatic beta cells. Specifically, we provide evidence that the downregulation of these genes is detrimental for the exocytosis of large dense core vesicles, thus contributing to beta cell dysfunction and impaired glucose homeostasis.

Synaptotagmin VII regulates bone remodeling by modulating osteoclast and osteoblast secretion

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 SNAP-25 linker as an adaptation toward fast exocytosis

The assembly of four soluble N-ethylmaleimide-sensitive factor attachment protein receptor domains into a complex is essential for membrane fusion. In most cases, the four SNARE-domains are encoded by separate membrane-targeted proteins. However, in the exocytotic pathway, two SNARE-domains are present in one protein, connected by a flexible linker. The significance of this arrangement is unknown. We characterized the role of the linker in SNAP-25, a neuronal SNARE, by using overexpression techniques in synaptosomal-associated protein of 25 kDa (SNAP-25) null mouse chromaffin cells and fast electrophysiological techniques. We confirm that the palmitoylated linker-cysteines are important for membrane association. A SNAP-25 mutant without cysteines supported exocytosis, but the fusion rate was slowed down and the fusion pore duration prolonged. Using chimeric proteins between SNAP-25 and its ubiquitous homologue SNAP-23, we show that the cysteine-containing part of the linkers is interchangeable. However, a stretch of 10 hydrophobic and charged amino acids in the C-terminal half of the SNAP-25 linker is required for fast exocytosis and in its absence the calcium dependence of exocytosis is shifted toward higher concentrations. The SNAP-25 linker therefore might have evolved as an adaptation toward calcium triggering and a high rate of execution of the fusion process, those features that distinguish exocytosis from other membrane fusion pathways.

Ca-dependent nonsecretory vesicle fusion in a secretory cell

We have compared Ca-dependent exocytosis in excised giant membrane patches and in whole-cell patch clamp with emphasis on the rat secretory cell line, RBL. Stable patches of 2–4 pF are easily excised from RBL cells after partially disrupting actin cytoskeleton with latrunculin A. Membrane fusion is triggered by switching the patch to a cytoplasmic solution containing 100–200 μM free Ca. Capacitance and amperometric recording show that large secretory granules (SGs) containing serotonin are mostly lost from patches. Small vesicles that are retained (non-SGs) do not release serotonin or other substances detected by amperometry, although their fusion is reduced by tetanus toxin light chain. Non-SG fusion is unaffected by N-ethylmaleimide, phosphatidylinositol-4,5-bis-phosphate (PI(4,5)P₂) ligands, such as neomycin, a PI-transfer protein that can remove PI from membranes, the PI(3)-kinase inhibitor LY294002 and PI(4,5)P₂, PI(3)P, and PI(4)P antibodies. In patch recordings, but not whole-cell recordings, fusion can be strongly reduced by ATP removal and by the nonspecific PI-kinase inhibitors wortmannin and adenosine. In whole-cell recording, non-SG fusion is strongly reduced by osmotically induced cell swelling, and subsequent recovery after shrinkage is then inhibited by wortmannin. Thus, membrane stretch that occurs during patch formation may be a major cause of differences between excised patch and whole-cell fusion responses. Regarding Ca sensors for non-SG fusion, fusion remains robust in synaptotagmin (Syt) VII−/− mouse embryonic fibroblasts (MEFs), as well as in PLCδ1, PLC δ1/δ4, and PLCγ1−/− MEFs. Thus, Syt VII and several PLCs are not required. Furthermore, the Ca dependence of non-SG fusion reflects a lower Ca affinity (K_D ∼71 μM) than expected for these C2 domain–containing proteins. In summary, we find that non-SG membrane fusion behaves and is regulated substantially differently from SG fusion, and we have identified an ATP-dependent process that restores non-SG fusion capability after it is perturbed by membrane stretch or cell dilation.

Increased plasma membrane localization of O-glycosylation-deficient mutant of synaptotagmin I in PC12 cells

Synaptotagmin I (Syt I) is a Ca2+-binding protein on synaptic vesicles and presumably functions as a Ca2+ sensor for neurotransmitter release. Native Syt I protein in neuroendocrine PC12 cells undergoes several posttranslational modifications, such as O-glycosylation, N-glycosylation, and fatty acylation, and the latter two modifications have been shown to be required for the proper function of murine Syt I in PC12 cells. However, nothing is known about the physiological significance of the O-glycosylation of Syt I in dense-core vesicle exocytosis in PC12 cells. In this study, we created an O-glycosylation-deficient mutant (named TA = T15A/T16A) and an N-glycosylation-deficient mutant of Syt I (named T26A) and investigated their subcellular distribution in Syt I-deficient PC12 cells, where other Syt isoforms (e.g., IV and IX) and other membrane trafficking proteins (e.g., Rab27A, SNAP-25, syntaxin-1, and VAMP-2) are normally expressed. We found that some cells expressing high level of recombinant wild-type (WT) Syt I protein show mistargeting of Syt I(WT) protein to the plasma membrane, whereas most of the cells show normal dense-core vesicle localization of Syt I(WT) protein. Similar mistargeting was also observed in cells expressing high levels of the Syt I(T26A) and Syt I(TA) mutants, but the mistargeting of the Syt I(TA) mutant to the plasma membrane was much more evident than with the Syt I(WT) or (T26A) mutant. The results indicate that O-glycosylation, not N-glycosylation, is partially involved in efficient targeting of Syt I protein to dense-core vesicles in PC12 cells.

Mechanisms of neuromodulation as dissected using Sr2+ at motor nerve endings

The use of binomial analysis as a tool for determining the sites of action of neuromodulators may be complicated by the nonuniformity of release probability. One of the potential sources for nonuniformity of release probability is the presence of multiple forms of synaptotagmins, the Ca2+ sensors responsible for triggering vesicular exocytosis. In this study we have used Sr2+, an ion whose actions may be restricted to a subpopulation of synaptotagmins, in an attempt to obtain meaningful estimates of the binomial parameters p (the probability of evoked acetylcholine [Ach] release) and n (the immediate available store of ACh quanta, whereby m = np). In contrast to results in Ca2+ solutions, binomial analysis of Sr2+-dependent release reveals a dramatically reduced dependence of n on extracellular Sr2+ concentrations. In Sr2+ solutions, blockade of potassium channels with 3,4-diaminopyridine increased m by an exclusive increase in p, whereas treatment with phorbol ester increased m solely by effects on n. The cyclic adenosine monophosphate (cAMP) analogue CPT-cAMP increased m by increasing both n and p. The effect of CPT-cAMP on p but not on n was blocked by protein kinase A (PKA) inhibitors, whereas the effect on n was mimicked by 8-CPT-2'-O-Me-cAMP, a selective agonist for exchange protein directly activated by cAMP, otherwise known as the cAMP-sensitive guanine nucleotide-exchange protein. The results demonstrate both the utility of the binomial distribution in Sr2+ solutions and the dual effects of cyclic AMP on both PKA-dependent and PKA-independent processes at the amphibian neuromuscular junction.

Synaptotagmin-1 and -7 are functionally overlapping Ca2+ sensors for exocytosis in adrenal chromaffin cells

Synaptotagmin-1, the canonical isoform of the synaptotagmin family, is a Ca(2+) sensor for fast synchronous neurotransmitter release in forebrain neurons and chromaffin cells. Even though deletion of synaptotagmin-1 abolishes fast exocytosis in chromaffin cells, it reduces overall secretion by only 20% because of the persistence of slow exocytosis. Therefore, another Ca(2+) sensor dominates release in these cells. Synaptotagmin-7 has a higher Ca(2+) affinity and slower binding kinetics than synaptotagmin-1, matching the proposed properties for the second, slower Ca(2+) sensor. Here, we examined Ca(2+)-triggered exocytosis in chromaffin cells from KO mice lacking synaptotagmin-7, and from knockin mice containing normal levels of a mutant synaptotagmin-7 whose C(2)B domain does not bind Ca(2+). In both types of mutant chromaffin cells, Ca(2+)-triggered exocytosis was decreased dramatically. Moreover, in chromaffin cells lacking both synaptotagmin-1 and -7, only a very slow release component, accounting for approximately 30% of WT exocytosis, persisted. These data establish synaptotagmin-7 as a major Ca(2+) sensor for exocytosis in chromaffin cells, which, together with synaptotagmin-1, mediates almost all of the Ca(2+) triggering of exocytosis in these cells, a surprising result, considering the lack of a role of synaptotagmin-7 in synaptic vesicle exocytosis.

Genetic analysis of synaptotagmin-7 function in synaptic vesicle exocytosis

Synaptotagmin-7 is a candidate Ca(2+) sensor for exocytosis that is at least partly localized to synapses. Similar to synaptotagmin-1, which functions as a Ca(2+) sensor for fast synaptic vesicle (SV) exocytosis, synaptotagmin-7 contains C(2)A and C(2)B domains that exhibit Ca(2+)-dependent phospholipid binding. However, synaptotagmin-7 cannot replace synaptotagmin-1 as a Ca(2+) sensor for fast SV exocytosis, raising questions about the physiological significance of its Ca(2+)-binding properties. Here, we examine how synaptotagmin-7 binds Ca(2+) and test whether this Ca(2+) binding regulates Ca(2+)-triggered SV exocytosis. We show that the synaptotagmin-7 C(2)A domain exhibits a Ca(2+)-binding mode similar to that of the synaptotagmin-1 C(2)A domain, suggesting that the synaptotagmin-1 and -7 C(2) domains generally employ comparable Ca(2+)-binding mechanisms. We then generated mutant mice that lack synaptotagmin-7 or contain point mutations inactivating Ca(2+) binding either to both C(2) domains of synaptotagmin-7 or only to its C(2)B domain. Synaptotagmin-7-mutant mice were viable and fertile. Inactivation of Ca(2+) binding to both C(2) domains caused an approximately 70% reduction in synaptotagmin-7 levels, whereas inactivation of Ca(2+) binding to only the C(2)B domain did not alter synaptotagmin-7 levels. The synaptotagmin-7 deletion did not change fast synchronous release, slow asynchronous release, or short-term synaptic plasticity of release of neurotransmitters. Thus, our results show that Ca(2+) binding to the synaptotagmin-7 C(2) domains is physiologically important for stabilizing synaptotagmin-7, but that Ca(2+) binding by synaptotagmin-7 likely does not regulate SV exocytosis, consistent with a role for synaptotagmin-7 in other forms of Ca(2+)-dependent synaptic exocytosis.

Stable RNA interference of synaptotagmin I in PC12 cells results in differential regulation of transmitter release

In sympathetic neurons, it is well-established that the neurotransmitters, norepinephrine (NE), neuropeptide Y (NPY), and ATP are differentially coreleased from the same neurons. In this study, we determined whether synaptotagmin (syt) I, the primary Ca(2+) sensor for regulated release, could function as the protein that differentially regulates release of these neurotransmitters. Plasmid-based RNA interference was used to specifically and stably silence expression of syt I in a model secretory cell line. Whereas stimulated release of NPY and purines was abolished, stimulated catecholamine (CA) release was only reduced by approximately 50%. Although expression levels of tyrosine hydroxylase, the rate-limiting enzyme in the dopamine synthesis pathway, was unaffected, expression of the vesicular monoamine transporter 1 was reduced by 50%. To evaluate whether NPY and CAs are found within the same vesicles and whether syt I is found localized to each of these NPY- and CA-containing vesicles, we used immunocytochemistry to determine that syt I colocalized with large dense core vesicles, with NPY, and with CAs. Furthermore, both CAs and NPY colocalized with one another and with large dense core vesicles. Electron micrographs show that large dense core vesicles are synthesized and available for release in cells that lack syt I. These results are consistent with syt I regulating differential release of transmitters.

Synaptotagmin I and IX function redundantly in controlling fusion pore of large dense core vesicles

Synaptotagmins (Syts) constitute a large family of at least 16 members and individual Syt isoforms exhibit distinct Ca(2+)-binding properties and subcellular localization. It remains to be demonstrated whether multiple Syt isoforms can function independently or cooperatively on certain type of vesicle. In the current study, we have developed NPY-pHluorin to specifically assess exocytosis of large dense core vesicles (LDCVs) and studied the requirement of Syt I and Syt IX for LDCV exocytosis in PC12 cells. We found that down-regulation of both Syt I and Syt IX resulted in a significant loss of Ca(2+)-dependent LDCV exocytosis. Moreover, our results suggest Syt I and Syt IX play redundant role in controlling the choice of fusion modes. Down-regulation of both Syt I and Syt IX renders more fusion in the kiss-and-run mode. We conclude that Syt I and Syt IX function redundantly in Ca(2+)-sensing and fusion pore dilation on LDCVs in PC12 cells.

Distinct v-SNAREs regulate direct and indirect apical delivery in polarized epithelial cells

SNARE [soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor] proteins control the membrane-fusion events of eukaryotic membrane-trafficking pathways. Specific vesicular and target SNAREs operate in specific trafficking routes, but the degree of specificity of SNARE functions is still elusive. Apical fusion requires the polarized distribution at the apical surface of the t-SNARE syntaxin 3, and several v-SNAREs including TI-VAMP and VAMP8 operate at the apical plasma membrane in polarized epithelial cells. It is not known, however, whether specific v-SNAREs are involved in direct and indirect routes to the apical surface. Here, we used RNAi to assess the role of two tetanus-neurotoxin-insensitive v-SNAREs, TI-VAMP/VAMP7 and VAMP8, in the sorting of raft- and non-raft-associated apical markers that follow either a direct or a transcytotic delivery, respectively, in FRT or Caco2 cells. We show that TI-VAMP mediates the direct apical delivery of both raft- and non-raft-associated proteins. By contrast, sorting by means of the transcytotic pathway is not affected by TI-VAMP knockdown but does appear to be regulated by VAMP8. Together with the specific role of VAMP3 in basolateral transport, our results demonstrate a high degree of specificity in v-SNARE function in polarized cells.

Regulation of insulin secretion and GLUT4 trafficking by the calcium sensor synaptotagmin VII

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 beta 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.