Measurement of exocytosis by amperometry in adrenal chromaffin cells: effects of clostridial neurotoxins and activation of protein kinase C on fusion pore kinetics

Direct Acquisition of the Gap Height of Biological Tissue-Electronic Chemical Sensor Interfaces

Interfacing biological tissues with electronic sensors offers the exciting opportunity to accurately investigate multiple biological processes. Accurate signal collection and application are the foundation of these measurements, but a long-term issue is the signal distortion resulting from the interface gap. The height of the gap is the key characteristic needed to evaluate or model the distortion, but it is difficult to measure. By integrating a pair of nanopores at the electronic sensor plane and measuring the ion conductance between them, we developed a versatile and straightforward strategy to realize the direct cooperative evaluation of the gap height during exocytotic release from adrenal gland tissues. The signaling distortion of this gap has been theoretically evaluated and shows almost no influence on the amperometric recording of exocytosis in a classic "semi-artificial synapse" configuration. This strategy should benefit research concerning various bio/chemical/machine interfaces.

A two-subpopulation model that reflects heterogeneity of large dense core vesicles in exocytosis

Exocytosis of large dense core vesicles is responsible for hormone secretion in neuroendocrine cells. The population of primed vesicles ready to release upon cell excitation demonstrates large heterogeneity. However, there are currently no models that clearly reflect such heterogeneity. Here, we develop a novel model based on single vesicle release events from amperometry recordings of PC12 cells using carbon fiber microelectrode. In this model, releasable vesicles can be grouped into two subpopulations, namely, SP1 and SP2. SP1 vesicles replenish quickly, with kinetics of ~0.0368 s^-1, but likely undergo slow fusion pore expansion (amperometric signals rise at ~2.5 pA/ms), while SP2 vesicles demonstrate slow replenishment (kinetics of ~0.0048 s^-1) but prefer fast dilation of fusion pore, with an amperometric signal rising rate of ~9.1 pA/ms. Phorbol ester enlarges the size of SP2 partially via activation of protein kinase C and conveys SP1 vesicles into SP2. Inhibition of Rho GTPase-dependent actin rearrangement almost completely depletes SP2. We also propose that the phorbol ester-sensitive vesicle subpopulation (SP2) is analogous to the subset of superprimed synaptic vesicles in neurons. This model provides a meticulous description of the architecture of the readily releasable vesicle pool and elucidates the heterogeneity of the vesicle priming mechanism.

Anticancer Drug Tamoxifen Affects Catecholamine Transmitter Release and Storage from Single Cells

Electrochemical measurements of exocytosis combined with intracellular vesicle impact electrochemical cytometry have been used to evaluate the effect of an anticancer drug, tamoxifen, on catecholamine release at the single-cell level. Tamoxifen has been used for over 40 years to treat estrogen receptor-positive breast cancers during both early stages of the disease and in the adjuvant setting. Tamoxifen causes memory and cognitive dysfunction, but the reasons for the cognitive impairment and memory problems induced by this anticancer drug are not well-known. We show that tamoxifen, through a nongenomic mechanism, can modulate both exocytosis and vesicle catecholamine storage in a model cell line. The results indicate that exocytosis is inhibited at high concentrations of tamoxifen and is stimulated at low levels. Tamoxifen also elicits a significant concentration-dependent change in total catecholamine content of single vesicles, while sub-nanomolar concentrations of the drug have stimulatory activity on the catecholamine content of vesicles. In addition, it has profound effects on storage at higher concentrations. Tamoxifen also reduces the intracellular free Ca²⁺ but only at micromolar concentration, by acting on voltage-gated Ca²⁺ channels, which likely affects neurotransmitter secretion.

Using Single-Cell Amperometry and Intracellular Vesicle Impact Electrochemical Cytometry To Shed Light on the Biphasic Effects of Lidocaine on Exocytosis

Single cell amperometry and intracellular vesicle impact electrochemical cytometry were used to examine whether lidocaine can regulate neurotransmitter release or storage for PC12 cells to explain the biphasic effects whereby it can protect neurons and improve cognitive outcome at low concentration, but can cause neurotoxicity at high concentration. We show that lidocaine affects the behavior of PC12 cell exocytosis in a concentration dependent way, which exactly corresponds to its biphasic effects. At a relatively high concentration, it shows a much narrower pore size and a longer-duration fusion pore with less monoamine released than control cells. However, at a relatively low concentration, the fusion pore is open even longer than at high concentration, and with more monoamine released than control cells. Furthermore, intracellular vesicle impact electrochemical cytometry was used to confirm that lidocaine did not change the catecholamine content of the vesicles. These data provide a mechanism for the observed biphasic effects of the drug and suggest that lidocaine influences exocytosis through multiple mechanisms.

On the Action of General Anesthetics on Cellular Function: Barbiturate Alters the Exocytosis of Catecholamines in a Model Cell System

General anesthetics are essential in many areas, however, the cellular mechanisms of anesthetic-induced amnesia and unconsciousness are incompletely understood. Exocytosis is the main mechanism of signal transduction and neuronal communication through the release of chemical transmitters from vesicles to the extracellular environment. Here, we use disk electrodes placed on top of PC12 cells to show that treatment with barbiturate induces fewer molecules released during exocytosis and changes the event dynamics perhaps by inducing a less stable fusion pore that is prone to close faster during partial exocytosis. Larger events are essentially abolished. However, use of intracellular vesicle impact electrochemical cytometry using a nano-tip electrode inserted into a cell shows that the distribution of vesicle transmitter content does not change after barbiturate treatment. This indicates that barbiturate selectively alters the pore size of larger events or perhaps differentially between types of vesicles. Alteration of exocytosis in this manner could be linked to the effects of general anesthetics on memory loss.

Regulation of insulin exocytosis by calcium-dependent protein kinase C in beta cells

The control of insulin release from pancreatic beta cells helps ensure proper blood glucose level, which is critical for human health. Protein kinase C has been shown to be one key control mechanism for this process. After glucose stimulation, calcium influx into beta cells triggers exocytosis of insulin-containing dense-core granules and activates protein kinase C via calcium-dependent phospholipase C-mediated generation of diacylglycerol. Activated protein kinase C potentiates insulin release by enhancing the calcium sensitivity of exocytosis, likely by affecting two main pathways that could be linked: (1) the reorganization of the cortical actin network, and (2) the direct phosphorylation of critical exocytotic proteins such as munc18, SNAP25, and synaptotagmin. Here, we review what is currently known about the molecular mechanisms of protein kinase C action on each of these pathways and how these effects relate to the control of insulin release by exocytosis. We identify remaining challenges in the field and suggest how these challenges might be addressed to advance our understanding of the regulation of insulin release in health and disease.

Using Single-Cell Amperometry To Reveal How Cisplatin Treatment Modulates the Release of Catecholamine Transmitters during Exocytosis

The pretreatment of cultured pheochromocytoma (PC12) cells with cis-diamminedichloroplatinum (cisplatin), an anti-cancer drug, influences the exocytotic ability of the cells in a dose-dependent manner. Low concentrations of cisplatin stimulate catecholamine release whereas high concentrations inhibit it. Single-cell amperometry reflects that 2 μm cisplatin treatment increases the frequency of exocytotic events and reduces their duration, whereas 100 μm cisplatin treatment decreases the frequency of exocytotic events and increases their duration. Furthermore, the stability of the initial fusion pore that is formed in the lipid membrane during exocytosis is also regulated differentially by different cisplatin concentrations. This study thus suggests that cisplatin influences exocytosis by multiple mechanisms.

Heterogeneous distribution of exocytotic microdomains in adrenal chromaffin cells resolved by high-density diamond ultra-microelectrode arrays

Here we describe the ability of a high-density diamond microelectrode array targeted to resolve multi-site detection of fast exocytotic events from single cells. The array consists of nine boron-doped nanocrystalline diamond ultra-microelectrodes (9-Ch NCD-UMEA) radially distributed within a circular area of the dimensions of a single cell. The device can be operated in voltammetric or chronoamperometric configuration. Sensitivity to catecholamines, tested by dose-response calibrations, set the lowest detectable concentration of adrenaline to ∼5 μm. Catecholamine release from bovine or mouse chromaffin cells could be triggered by electrical stimulation or external KCl-enriched solutions. Spikes detected from the cell apex using carbon fibre microelectrodes showed an excellent correspondence with events measured at the bottom of the cell by the 9-Ch NCD-UMEA, confirming the ability of the array to resolve single quantal secretory events. Subcellular localization of exocytosis was provided by assigning each quantal event to one of the nine channels based on its location. The resulting mapping highlights the heterogeneous distribution of secretory activity in cell microdomains of 12-27 μm2. In bovine chromaffin cells, secretion was highly heterogeneous with zones of high and medium activity in 54% of the cell surface and zones of low or no activity in the remainder. The 'non-active' ('silent') zones covered 24% of the total and persisted for 6-8 min, indicating stable location. The 9-Ch NCD-UMEA therefore appears suitable for investigating the microdomain organization of neurosecretion with high spatial resolution.

Efficient transfection of dissociated mouse chromaffin cells using small-volume electroporation

We have developed an improved procedure for isolating and transfecting a chromaffin cell-enriched population of primary cells from adult mouse adrenal glands. Significantly, the parameters of a novel electroporation transfection technique were optimized to achieve an average transfection efficiency of 45 % on the small number of cells derived from the mouse glands. Such transfection efficiency was previously unachievable with the electroporation protocols conventionally used with bovine chromaffin cells, even with use of large cell numbers. Our small scale technique now makes feasible the use of genetically homogenous inbred mouse models for investigations on the exocytotic pathway without the time, expense, and cellular changes associated with viral approaches. High fidelity co-expression of multiple plasmids in individual cells is a further advantage of the procedure. To assess whether the biophysical characteristics of mouse adrenal chromaffin cells were altered by this process, we examined structural integrity using immunocytochemistry and functional response to stimuli using calcium imaging, amperometry, and whole-cell capacitance and current clamp recordings. We conclude these parameters are minimally affected. Finally, we demonstrate that high transfection efficiency makes possible the use of primary mouse adrenal chromaffin cells, rather than a cell line, in human growth hormone secretion assays for high throughput evaluation of secretion.

Amperometric measurements at cells support a role for dynamin in the dilation of the fusion pore during exocytosis

Dynamin is a GTPase mechanochemical enzyme involved in the late steps of endocytosis, where it separates the endocytotic vesicle from the cell membrane. However, recent reports have emphasized its role in exocytosis. In this case, dynamin may contribute to the control of the exocytotic pore, thus suggesting a direct control on the efflux of neurotransmitters. Dynasore, a selective inhibitor of the GTPase activity of dynamin, was used to investigate the role of dynamin in exocytosis. Exocytosis was analyzed by amperometry, thus revealing that dynasore inhibits exocytosis in a dose-dependent manner. Analysis of the exocytotic peaks shows that the inhibition of the GTPase activity of dynamin leads to shorter, smaller events. This observation, together with the rapid effect of dynasore, suggests that the blocking of the GTPase induces the formation of a more narrow and short-lived fusion pore. These results suggest that the GTPase properties of dynamin are involved in the duration and kinetics of exocytotic release. Interestingly, and in strong contrast with its role in endocytosis, the mechanochemical properties of dynamin appear to contribute to the dilation and stability of the pore during exocytosis.

Gabapentin inhibits catecholamine release from adrenal chromaffin cells

BACKGROUND

Gabapentin is most commonly prescribed for chronic pain, but acute perioperative effects, including preemptive analgesia and hemodynamic stabilization, have been reported. Adrenal chromaffin cells are a widely used model to investigate neurosecretion, and adrenal catecholamines play important physiologic roles and contribute to the acute stress response. However, the effects of gabapentin on adrenal catecholamine release have never been tested.

METHODS

Primary cultures of bovine adrenal chromaffin cells were treated with gabapentin or vehicle for 18-24 h. The authors quantified catecholamine secretion from dishes of cells using high-performance liquid chromatography and resolved exocytosis of individual secretory vesicles from single cells using carbon fiber amperometry. Voltage-gated calcium channel currents were recorded using patch clamp electrophysiology and intracellular [Ca2+] using fluorescent imaging.

RESULTS

Gabapentin produced statistically significant reductions in catecholamine secretion evoked by cholinergic agonists (24 ± 3%, n = 12) or KCl (16 ± 4%, n = 8) (mean ± SEM) but did not inhibit Ca2+ entry or calcium channel currents. Amperometry (n = 51 cells) revealed that gabapentin inhibited the number of vesicles released upon stimulation, with no change in quantal size or kinetics of these unitary events.

CONCLUSIONS

The authors show Ca2+ entry was not inhibited by gabapentin but was less effective at triggering vesicle fusion. The work also demonstrates that chromaffin cells are a useful model for additional investigation of the cellular mechanism(s) by which gabapentin controls neurosecretion. In addition, it identifies altered adrenal catecholamine release as a potential contributor to some of the beneficial perioperative effects of gabapentin.

Structure-function study of mammalian Munc18-1 and C. elegans UNC-18 implicates domain 3b in the regulation of exocytosis

Munc18-1 is an essential synaptic protein functioning during multiple stages of the exocytotic process including vesicle recruitment, docking and fusion. These functions require a number of distinct syntaxin-dependent interactions; however, Munc18-1 also regulates vesicle fusion via syntaxin-independent interactions with other exocytotic proteins. Although the structural regions of the Munc18-1 protein involved in closed-conformation syntaxin binding have been thoroughly examined, regions of the protein involved in other interactions are poorly characterised. To investigate this we performed a random transposon mutagenesis, identifying domain 3b of Munc18-1 as a functionally important region of the protein. Transposon insertion in an exposed loop within this domain specifically disrupted Mint1 binding despite leaving affinity for closed conformation syntaxin and binding to the SNARE complex unaffected. The insertion mutation significantly reduced total amounts of exocytosis as measured by carbon fiber amperometry in chromaffin cells. Introduction of the equivalent mutation in UNC-18 in Caenorhabditis elegans also reduced neurotransmitter release as assessed by aldicarb sensitivity. Correlation between the two experimental methods for recording changes in the number of exocytotic events was verified using a previously identified gain of function Munc18-1 mutation E466K (increased exocytosis in chromaffin cells and aldicarb hypersensitivity of C. elegans). These data implicate a novel role for an exposed loop in domain 3b of Munc18-1 in transducing regulation of vesicle fusion independent of closed-conformation syntaxin binding.

Monitoring of vesicular exocytosis from single cells using micrometer and nanometer-sized electrochemical sensors

Communication between cells by release of specific chemical messengers via exocytosis plays crucial roles in biological process. Electrochemical detection based on ultramicroelectrodes (UMEs) has become one of the most powerful techniques in real-time monitoring of an extremely small number of released molecules during very short time scales, owing to its intrinsic advantages such as fast response, excellent sensitivity, and high spatiotemporal resolution. Great successes have been achieved in the use of UME methods to obtain quantitative and kinetic information about released chemical messengers and to reveal the molecular mechanism in vesicular exocytosis. In this paper, we review recent developments in monitoring exocytosis by use of UMEs-electrochemical-based techniques including electrochemical detection using micrometer and nanometer-sized sensors, scanning electrochemical microscopy (SECM), and UMEs implemented in lab-on-a-chip (LOC) microsystems. These advances are of great significance in obtaining a better understanding of vesicular exocytosis and chemical communications between cells, and will facilitate developments in many fields, including analytical chemistry, biological science, and medicine. Furthermore, future developments in electrochemical probing of exocytosis are also proposed.

The functions of Munc18-1 in regulated exocytosis

The activation of regulated exocytosis occurs by a rise in cytosolic Ca(2+) concentration. Synaptotagmins act as the Ca(2+) sensors, whereas the machinery that allows fusion of secretory vesicles with the plasma membrane consists of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, including syntaxin 1, SNAP-25, and VAMP. Within the pathway leading to exocytosis, there is an essential requirement for a member of the conserved Sec1/Munc18 (SM) protein family, which in neurotransmitter and neurohormone release in mammalian cells is Munc18-1. The exact role of Munc18-1 and the steps within exocytosis in which it acts have been intensively investigated. Current evidence suggests that Munc18-1 acts via distinct modes of interactions with syntaxin 1 and the other SNARE proteins and influences all of the steps leading to exocytosis, including vesicle recruitment, tethering, docking, priming, and membrane fusion.

UNC-18 modulates ethanol sensitivity in Caenorhabditis elegans

Acute ethanol exposure affects the nervous system as a stimulant at low concentrations and as a depressant at higher concentrations, eventually resulting in motor dysfunction and uncoordination. A recent genetic study of two mouse strains with varying ethanol preference indicated a correlation with a polymorphism (D216N) in the synaptic protein Munc18-1. Munc18-1 functions in exocytosis via a number of discrete interactions with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein syntaxin-1. We report that the mutation affects binding to syntaxin but not through either a closed conformation mode of interaction or through binding to the syntaxin N terminus. The D216N mutant instead has a specific impairment in binding the assembled SNARE complex. Furthermore, the mutation broadens the duration of single exocytotic events. Expression of the orthologous mutation (D214N) in the Caenorhabditis elegans UNC-18 null background generated transgenic rescues with phenotypically similar locomotion to worms rescued with the wild-type protein. Strikingly, D214N worms were strongly resistant to both stimulatory and sedative effects of acute ethanol. Analysis of an alternative Munc18-1 mutation (I133V) supported the link between reduced SNARE complex binding and ethanol resistance. We conclude that ethanol acts, at least partially, at the level of vesicle fusion and that its acute effects are ameliorated by point mutations in UNC-18.

Specific effects of KChIP3/calsenilin/DREAM, but not KChIPs 1, 2 and 4, on calcium signalling and regulated secretion in PC12 cells

The KChIPs (K⁺ channel-interacting proteins) are members of the NCS (neuronal calcium sensor) protein family of Ca²⁺-binding proteins. It is unclear to what extent the KChIPs have distinct functions although they all interact with Kv4 K⁺ channels. KChIP3 has also been shown to repress transcription of specific genes via binding to DRE (downstream regulatory element) motifs and all KChIPs may share this function. In the present study, we have compared the function of isoforms of the four KChIPs. KChIPs 1–4 were found to stimulate the traffic of Kv4.2 channels to the plasma membrane. KChIP3 expression in PC12 cells resulted in an increase in exocytosis evoked by activation of purinergic receptors. In contrast, KChIPs 1, 2 and 4, although expressed to the same extent, had no effect on secretion. In addition, KChIP3 but not KChIPs 1, 2 and 4 modified the ATP-induced Ca²⁺ signal resulting in a delay in recovery after the peak Ca²⁺ elevation and also specifically resulted in down-regulation of the Na⁺/Ca²⁺ exchanger NCX3, which could explain the effects on the Ca²⁺ signal and secretion. Regulation of NCX3 by KChIP3 has been shown to occur via its DREAM (DRE antagonist modulator) function [Gomez-Villafuertes, Torres, Barrio, Savignac, Gabellini, Rizzato, Pintado, Gutierrez-Adan, Mellstrom, Carafoli and Naranjo (2005) J. Neurosci. 25, 10822–10830] suggesting that this activity might depend on the cellular context of expression of the various KChIPs. These results reveal a new role for KChIP3 in the regulation of Ca²⁺-regulated secretion and also suggest that the functions of each of the KChIPs may be more specialized than previously appreciated.

Effects of phorbol ester on vesicle dynamics as revealed by total internal reflection fluorescence microscopy

Exocytosis of neurotransmitter or hormone-filled vesicles is a highly dynamic process regulated by various proteins and lipids. As mainly revealed indirectly by the electrophysiological methods, exocytosis is believed to involve multiple kinetic steps in which vesicles transit from one state to another. Using total internal reflection fluorescence microscopy which enables direct visualization of individual vesicles, we developed an analytical framework to track and analyze vesicle dynamics. We demonstrated that all subplasmalemmal vesicles generally undergo constant and caged Brownian motion. And they can be classified into three populations that differ in their motion characteristics and fusion competence. Furthermore, we showed that these vesicle pools are differentially modulated by phorbol-12-myristate-13-acetate, a phorbol ester analog to endogenous diacylglycerol, through both protein-kinase-C-dependent and -independent pathways.

A gain-of-function mutant of Munc18-1 stimulates secretory granule recruitment and exocytosis and reveals a direct interaction of Munc18-1 with Rab3

Munc18-1 plays a crucial role in regulated exocytosis in neurons and neuroendocrine cells through modulation of vesicle docking and membrane fusion. The molecular basis for Munc18 function is still unclear, as are the links with Rabs and SNARE [SNAP (soluble N-ethylmaleimide-sensitive factor-attachment protein) receptor] proteins that are also required. Munc18-1 can bind to SNAREs through at least three modes of interaction, including binding to the closed conformation of syntaxin 1. Using a gain-of-function mutant of Munc18-1 (E466K), which is based on a mutation in the related yeast protein Sly1p, we have identified a direct interaction of Munc18-1 with Rab3A, which is increased by the mutation. Expression of Munc18-1 with the E466K mutation increased exocytosis in adrenal chromaffin cells and PC12 cells (pheochromocytoma cells) and was found to increase the density of secretory granules at the periphery of PC12 cells, suggesting a stimulatory effect on granule recruitment through docking or tethering. Both the increase in exocytosis and changes in granule distribution appear to require Munc18-1 E466K binding to the closed form of syntaxin 1, suggesting a role for this interaction in bridging Rab- and SNARE-mediated events in exocytosis.

Measuring secretion in chromaffin cells using electrophysiological and electrochemical methods

Our present understanding of exocytosis of catecholamines has benefited tremendously from the arrival of single-cell electrochemical methods (amperometry and voltammetry), electrophysiological techniques (whole-cell and patch capacitance) and from the combination of both techniques (patch amperometry). In this brief review, we will outline the strengths and limitations of amperometric and electrophysiological methods and highlight the major contribution obtained with the use of these techniques in chromaffin cells.

DSCR1/RCAN1 regulates vesicle exocytosis and fusion pore kinetics: implications for Down syndrome and Alzheimer's disease

Genes located on chromosome 21, over-expressed in Down syndrome (DS) and Alzheimer's disease (AD) and which regulate vesicle trafficking, are strong candidates for involvement in AD neuropathology. Regulator of calcineurin activity 1 (RCAN1) is one such gene. We have generated mutant mice in which RCAN1 is either over-expressed (RCAN1(ox)) or ablated (Rcan1-/-) and examined whether exocytosis from chromaffin cells, a classic cellular model of neuronal exocytosis, is altered using carbon fibre amperometry. We find that Rcan1 regulates the number of vesicles undergoing exocytosis and the speed at which the vesicle fusion pore opens and closes. Cells from both Rcan1-/- and RCAN1(ox) mice display reduced levels of exocytosis. Changes in single-vesicle fusion kinetics are also evident resulting in the less catecholamine released per vesicle with increasing Rcan1 expression. Acute calcineurin inhibition did not replicate the effect of RCAN1 overexpression. These changes are not due to alterations in Ca2+ entry or the readily releasable vesicle pool size. Thus, we illustrate a novel regulator of vesicle exocytosis, Rcan1, which influences both exocytotic rate and vesicle fusion kinetics. If Rcan1 functions similarly in neurons then overexpression of this protein, as occurs in DS and AD brains, will reduce both the number of synaptic vesicles undergoing exocytosis and the amount of neurotransmitter released per fusion event. This has direct implications for the pathogenesis of these diseases as sufficient levels of neurotransmission are required for synaptic maintenance and the prevention of neurodegeneration and vesicle trafficking defects are the earliest hallmark of AD neuropathology.

A 20-nm step toward the cell membrane preceding exocytosis may correspond to docking of tethered granules

In endocrine cells, plasma membrane (PM)-bound secretory granules must undergo a number of maturation stages (i.e., priming) to become fusion-competent. Despite identification of several molecules involved in binding granules to the PM and priming them, the exact nature of events occurring at the PM still largely remains a mystery. In stimulated BON cells, we used evanescent wave microscopy to study trajectories of granules shortly before their exocytoses, which provided a physical description of vesicle-PM interactions at an unprecedented level of detail, and directly lead to an original mechanistic model. In these cells, tethered (T), nonfusogenic, vesicles are prevented from converting to fusogenic, docked (D) ones in resting conditions. Upon elevation of calcium, T-vesicles perform a 21-nm step toward the PM to become D, and fuse approximately 3 s thereafter. Our ability to directly visualize different modes of PM-attachment paves the way for clarifying the exact role of various molecules implicated in attachment and priming of granules in future studies.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Controlled on-chip stimulation of quantal catecholamine release from chromaffin cells using photolysis of caged Ca2+ on transparent indium-tin-oxide microchip electrodes

Photorelease of caged Ca(2+) is a uniquely powerful tool to study the dynamics of Ca(2+)-triggered exocytosis from individual cells. Using photolithography and other microfabrication techniques, we have developed transparent microchip devices to enable photorelease of caged Ca(2+), together with electrochemical detection of quantal catecholamine secretion from individual cells or cell arrays as a step towards developing high-throughput experimental devices. A 100 nm thick transparent indium-tin-oxide (ITO) film was sputter-deposited onto glass coverslips, which were then patterned into 24 cell-sized working electrodes (approximately 20 microm by 20 microm). We loaded bovine chromaffin cells with acetoxymethyl (AM) ester derivatives of the Ca(2+) cage NP-EGTA and Ca(2+) indicator dye fura-4F, then transferred these cells onto the working ITO electrodes for amperometric recordings. Upon flash photorelease of caged Ca(2+), a uniform rise of [Ca(2+)](i) within the target cell leads to quantal release of oxidizable catecholamines measured amperometrically by the underlying ITO electrode. We observed a burst of amperometric spikes upon rapid elevation of [Ca(2+)](i) and a "priming" effect of sub-stimulatory [Ca(2+)](i) on the response of cells to subsequent [Ca(2+)](i) elevation, similar to previous reports using different techniques. We conclude that UV photolysis of caged Ca(2+) is a suitable stimulation technique for higher-throughput studies of Ca(2+)-dependent exocytosis on transparent electrochemical microelectrode arrays.

Specificity, promiscuity and localization of ARF protein interactions with NCS-1 and phosphatidylinositol-4 kinase-III beta

ADP-ribosylation factor (ARF) proteins are involved in multiple intracellular vesicular transport pathways. Most studies have focused on the functions of ARF1 or ARF6 and little is known about the remaining ARF isoforms. Although the mammalian ARF proteins share a high degree of sequence identity, recent evidence has indicated that they may control distinct trafficking steps within cells. A unanswered issue is the degree of specificity of ARF family members for different interacting proteins. To investigate potential functional differences between the human ARF proteins, we have examined the localization of all human ARF isoforms and their interactions with two ARF1 binding proteins, neuronal calcium sensor-1 (NCS-1) and phosphatidylinositol-4 kinase-IIIbeta (PI4Kbeta). Use of a fluorescent protein fragment complementation method showed direct interactions between ARFs 1, 3, 5 and 6 with NCS-1 but at different intracellular locations in live HeLa cells. Photobleaching experiments indicated that complementation did not detect dynamic changes in protein interactions over short-time scales. A more specific interaction between ARFs 1/3 and PI4Kbeta was observed. Consistent with these latter findings ARF1 but not ARF5 or 6 enhanced the stimulatory effect of PI4Kbeta on regulated exocytosis, suggesting a specific role for class-I ARFs in the regulation of PI4Kbeta.

Differential dynamics of Rab3A and Rab27A on secretory granules

We have assessed the dynamics of the association of Rab3A and Rab27A with secretory granules at various stages of their life in PC12 cells. Endogenous Rab3A colocalised with the secretory granule marker secretogranin II (SGII) and expressed EGFP-Rab3A and ECFP-Rab27A colocalised with one another. The extent of colocalisation between EGFP-Rab3A or EGFP-Rab27 and SGII increased after longer times post transfection suggesting that these Rab proteins are preferentially recruited to newly synthesised granules. Following the release of immature secretory granules from the trans-Golgi network, Rab3A and Rab27A became associated with the immature granules after a lag period of around 20 minutes. Rab dynamics on granules were analysed in fluorescence recovery after photobleaching (FRAP) experiments. The recovery profile of EGFP-Rab27A was comparable to that of ppANF-EGFP, whereas the recovery profile of EGFP-Rab3A was significantly faster, indicating that Rab3A but not Rab27A might be rapidly exchanged between granules and cytosol. Inhibition of heat-shock protein 90 with 10 muM geldanamycin did not affect the exchange process or regulated exocytosis. Rab dynamics during stimulation with 300 muM ATP were analysed in live cells. Loss of granular ppANF-EGFP fluorescence was seen at the cell periphery after stimulation but only limited changes in EGFP-Rab3A and EGFP-Rab27A fluorescence was observed, indicating that the Rab proteins do not immediately dissociate or disperse on stimulation. The data suggest potentially distinct roles for Rab3A and Rab27A and we suggest that the finding that young secretory granules have a higher capacity for binding Rab3A and Rab27A is functionally important for preferential exocytosis from these granules.

Physiological stimulation regulates the exocytic mode through calcium activation of protein kinase C in mouse chromaffin cells

Adrenal medullary chromaffin cells release catecholamines and neuropeptides in an activity-dependent manner controlled by the sympathetic nervous system. Under basal sympathetic tone, catecholamines are preferentially secreted. During acute stress, increased sympathetic firing evokes release of both catecholamines as well as neuropeptides. Both signalling molecules are co-packaged in the same large dense core granules, thus release of neuropeptide transmitters must be regulated after granule fusion with the cell surface. Previous work has indicated this may be achieved through a size-exclusion mechanism whereby, under basal sympathetic firing, the catecholamines are selectively released through a restricted fusion pore, while less-soluble neuropeptides are left behind in the dense core. Only under the elevated firing experienced during the sympathetic stress response do the granules fully collapse to expel catecholamines and neuropeptides. However, mechanistic description and physiological regulation of this process remain to be determined. We employ electrochemical amperometry, fluid-phase dye uptake and electrophysiological capacitance noise analysis to probe the fusion intermediate in mouse chromaffin cells under physiological electrical stimulation. We show that basal firing rates result in the selective release of catecholamines through an Omega-form 'kiss and run' fusion event characterized by a narrow fusion pore. Increased firing raises calcium levels and activates protein kinase C, which then promotes fusion pore dilation until full granule collapse occurs. Our results demonstrate that the transition between 'kiss and run' and 'full collapse' exocytosis serves a vital physiological regulation in neuroendocrine chromaffin cells and help effect a proper acute stress response.

G protein betagamma-subunits activated by serotonin mediate presynaptic inhibition by regulating vesicle fusion properties

Neurotransmitters are thought to be released as quanta, where synaptic vesicles deliver packets of neurotransmitter to the synaptic cleft by fusion with the plasma membrane. However, synaptic vesicles may undergo incomplete fusion. We provide evidence that G protein-coupled receptors inhibit release by causing such incomplete fusion. 5-hydroxytryptamine (5-HT) receptor signaling potently inhibits excitatory postsynaptic currents (EPSCs) between lamprey reticulospinal axons and their postsynaptic targets by a direct action on the vesicle fusion machinery. We show that 5-HT receptor-mediated presynaptic inhibition, at this synapse, involves a reduction in EPSC quantal size. Quantal size was measured directly by comparing unitary quantal amplitudes of paired EPSCs before and during 5-HT application and indirectly by determining the effect of 5-HT on the relationship between mean-evoked EPSC amplitude and variance. Results from FM dye-labeling experiments indicate that 5-HT prevents full fusion of vesicles. 5-HT reduces FM1-43 staining of vesicles with a similar efficacy to its effect on the EPSC. However, destaining of FM1-43-labeled vesicles is abolished by lower concentrations of 5-HT that leave a substantial EPSC. The use of a water-soluble membrane impermeant quenching agent in the extracellular space reduced FM1-43 fluorescence during stimulation in 5-HT. Thus vesicles contact the extracellular space during inhibition of synaptic transmission by 5-HT. We conclude that 5-HT, via free Gbetagamma, prevents the collapse of synaptic vesicles into the presynaptic membrane.

Regulation of exocytosis by protein kinase C

PKC (protein kinase C) has been known for many years to modulate regulated exocytosis in a wide variety of cell types. In neurons and neuroendocrine cells, PKC regulates several different stages of the exocytotic process, suggesting that these multiple actions of PKC are mediated by phosphorylation of distinct protein targets. In recent years, a variety of exocytotic proteins have been identified as PKC substrates, the best characterized of which are SNAP-25 (25 kDa synaptosome-associated protein) and Munc18. In the present study, we review recent evidence suggesting that site-specific phosphorylation of SNAP-25 and Munc18 by PKC regulates distinct stages of exocytosis.

Protein kinase B/Akt is a novel cysteine string protein kinase that regulates exocytosis release kinetics and quantal size

Protein kinase B/Akt has been implicated in the insulin-dependent exocytosis of GLUT4-containing vesicles, and, more recently, insulin secretion. To determine if Akt also regulates insulin-independent exocytosis, we used adrenal chromaffin cells, a popular neuronal model. Akt1 was the predominant isoform expressed in chromaffin cells, although lower levels of Akt2 and Akt3 were also found. Secretory stimuli in both intact and permeabilized cells induced Akt phosphorylation on serine 473, and the time course of Ca2+-induced Akt phosphorylation was similar to that of exocytosis in permeabilized cells. To determine if Akt modulated exocytosis, we transfected chromaffin cells with Akt constructs and monitored catecholamine release by amperometry. Wild-type Akt had no effect on the overall number of exocytotic events, but slowed the kinetics of catecholamine release from individual vesicles, resulting in an increased quantal size. This effect was due to phosphorylation by Akt, because it was not seen in cells transfected with kinase-dead mutant Akt. As overexpression of cysteine string protein (CSP) results in a similar alteration in release kinetics and quantal size, we determined if CSP was an Akt substrate. In vitro 32P-phosphorylation studies revealed that Akt phosphorylates CSP on serine 10. Using phospho-Ser10-specific antisera, we found that both transfected and endogenous cellular CSP is phosphorylated by Akt on this residue. Taken together, these findings reveal a novel role for Akt phosphorylation in regulating the late stages of exocytosis and suggest that this is achieved via the phosphorylation of CSP on serine 10.

Calcium-dependent regulation of exocytosis

A rapid increase in intracellular calcium directly triggers regulated exocytosis. In addition, changes in intracellular calcium concentration can adjust the extent of exocytosis (quantal content) or the magnitude of individual release events (quantal size) in both the short- and long-term. It is generally agreed that calcium achieves this regulation via an interaction with a number of different molecular targets located at or near to the site of membrane fusion. We review here the synaptic proteins with defined calcium-binding domains and protein kinases activated by calcium, summarize what is known about their function in membrane fusion and the experimental evidence in support of their involvement in synaptic plasticity.

Activation of GPCRs modulates quantal size in chromaffin cells through G(betagamma) and PKC

Exocytosis proceeds by either full fusion or 'kiss-and-run' between vesicle and plasma membrane. Switching between these two modes permits the cell to regulate the kinetics and amount of secretion. Here we show that ATP receptor activation reduces secretion downstream from cytosolic Ca2+ elevation in rat adrenal chromaffin cells. This reduction is mediated by activation of a pertussis toxin-sensitive G(i/o) protein, leading to activation of G(betagamma) subunits, which promote the 'kiss-and-run' mode by reducing the total open time of the fusion pore during a vesicle fusion event. Furthermore, parallel activation of the muscarinic acetylcholine receptor removes the inhibitory effects of ATP on secretion. This is mediated by a G(q) pathway through protein kinase C activation. The inhibitory effects of ATP and its reversal by protein kinase C activation are also shared by opioids and somatostatin. Thus, a variety of G protein pathways exist to modulate Ca2+-evoked secretion at specific steps in fusion pore formation.

Amisyn regulates exocytosis and fusion pore stability by both syntaxin-dependent and syntaxin-independent mechanisms

Amisyn and tomosyn are related by the possession of a C-terminal vesicle-associated membrane protein-like domain that allows them to bind to syntaxin 1 and assemble into SNARE complexes. The formation of inactive complexes may sequester syntaxin and allow tomosyn and amisyn to act as inhibitors of exocytosis. We aimed to use adrenal chromaffin and PC12 cells to probe this possible mode of action of amisyn and tomosyn in dense core granule exocytosis. Although tomosyn is expressed by adrenal chromaffin and PC12 cells, amisyn expression could not be detected allowing examination of the effect of introduction of amisyn expression onto a neuronal-like background. Overexpression of m-tomosyn1 and expression of amisyn both inhibited Ca2+-induced exocytosis in transfected PC12 cells. Surprisingly, this inhibition was not removed when amisyn and tomosyn constructs were used in which key residues required for efficient binding to syntaxin1 were mutated. The effect of amisyn was further characterized using carbon fiber amperometry in chromaffin cells. Expression of amisyn had no effect on the basic characteristics of the amperometric spikes but reduced the number of spikes elicited. This inhibitory action on the extent of exocytosis was also seen with the amisyn mutant deficient in syntaxin1 binding. In addition, expression of amisyn resulted in an increase in the lifetime of the prespike foot, and this effect was abolished by the mutations. These results show that tomosyn and amisyn can negatively regulate exocytosis independently of syntaxin and also that amisyn can regulate the stability of the fusion pore.

Characterization of exocytotic events from single PC12 cells: amperometric studies in native PC12h, DA-loaded PC12h and bovine adrenal chromaffin cells

Exocytotic events from rat pheochromocytoma (PC12) cells were characterized by amperometric analysis. For single-cell amperometric recordings, PC12h cells cultured onto poly-L-lysine corted glass-base dish were incubated with 1 mM dopamine (DA) for 60 min. Amperometric recordings, with a carbon fiber microelectrode (5 mum diameter), of catecholamine release from the individual cells were conducted under an inverted microscope at 25 degrees C. To characterize a single exocytotic event that is detected as a single spike current, the spike number, spike parameters (rise time, middle width and area) and spike shape were analyzed. Exposure of DA-loaded PC12h cells to 60 mM KCl (1000 hps) for 5 min and for 4 s evoked a train of events with the event number of 114+/-19 (spikes/response for 5 min) and 12+/-3 (spikes/response for 15 s), respectively. We observed distinctive kinetics in the events (rise time=0.83+/-0.19 ms, middle width=2.89+/-0.62 ms, area=62+/-7.6 fC and the spikes with a "foot"=15.4+/-2.7% of total spikes). The number and mean height of the events were 3- to 4-fold higher than that in DA-unloaded cells, and the values of rise time and middle width in DA-loaded PC12h cells were approx. 5- and 10-fold less than those observed in cultured adrenal chromaffin cells. The successful application of amperometry to monitor DA released from secretory vesicles in DA-loaded PC12h cell suggest that this technique is applicable to characterize exocytotic events in neurons.

A molecular basis underlying differences in the toxicity of botulinum serotypes A and E

Botulinum neurotoxins (BoNTs) block neurotransmitter release through their specific proteolysis of the proteins responsible for vesicle exocytosis. Paradoxically, two serotypes of BoNTs, A and E, cleave the same molecule, synaptosome-associated protein with relative molecular mass 25K (SNAP-25), and yet they cause synaptic blockade with very different properties. Here we compared the action of BoNTs A and E on the plasma membrane fusion machinery composed of syntaxin and SNAP-25. We now show that the BoNT/A-cleaved SNAP-25 maintains its association with two syntaxin isoforms in vitro, which is mirrored by retention of SNAP-25 on the plasma membrane in vivo. In contrast, BoNT/E severely compromises the ability of SNAP-25 to bind the plasma membrane syntaxin isoforms, leading to dissociation of SNAP-25. The distinct properties of botulinum intoxication, therefore, can result from the ability of shortened SNAP-25 to maintain its association with syntaxins-in the case of BoNT/A poisoning resulting in unproductive syntaxin/SNAP-25 complexes that impede vesicle exocytosis.

Regulation of exocytosis in neurons and neuroendocrine cells

Neurons communicate with one another through the release of molecules from synaptic vesicles and large dense core granules through the process of exocytosis. During exocytosis, molecules are released to the extracellular space through a fusion pore, which can either dilate, resulting in full fusion, or close, resulting in incomplete exocytosis, often referred to as 'kiss and run' exocytosis. Recently, there has been much interest in the regulation of this process in both neurons and neuroendocrine cells. There has been much recent work that addresses the existence of incomplete exocytosis in neurons and neuroendocrine cells, as well as recent work probing the molecular components and modulation of the fusion pore.

Lipid raft association of SNARE proteins regulates exocytosis in PC12 cells

SNAP25 and SNAP23 are plasma membrane SNARE proteins essential for regulated exocytosis in diverse cell types. Several recent studies have shown that these proteins are partly localized in lipid rafts, domains of the plasma membrane enriched in sphingolipids, and cholesterol. Here, we have employed cysteine mutants of SNAP25/SNAP23, which have modified affinities for raft domains, to examine whether raft association of these proteins is important for the regulation of exocytosis. PC12 cells were engineered that express the light chain of botulinum neurotoxin; in these cells all of the SNAP25 was cleaved to a lower molecular weight form, and regulated exocytosis was essentially absent. Exocytosis was rescued by expressing toxin-resistant SNAP25 or wild-type SNAP23, which is naturally toxin-resistant. Remarkably, a mutant SNAP25 protein with an increased affinity for rafts displayed a reduced ability to support exocytosis, whereas SNAP23 mutants with a decreased affinity for rafts displayed an enhancement of exocytosis when compared with wild-type SNAP23. The effects of the mutant proteins on exocytosis were dependent upon the integrity of the plasma membrane and lipid rafts. These results provide the first direct evidence that rafts regulate SNARE function and exocytosis and identify the central cysteine-rich region of SNAP25/23 as an important regulatory domain.

Interaction of neuronal calcium sensor-1 and ADP-ribosylation factor 1 allows bidirectional control of phosphatidylinositol 4-kinase beta and trans-Golgi network-plasma membrane traffic

We have identified a novel Ca(2+)-dependent interaction between neuronal calcium sensor-1 (NCS-1) and the GTPase ARF1. Both of these proteins are localized to the Golgi complex, and both regulate phosphatidylinositol 4-kinase IIIbeta (PI(4)Kbeta). Spatial and temporal control of phosphatidylinositol 4-phosphate levels through activation of PI(4)Kbeta is important for the recruitment of trafficking complexes to the trans-Golgi network (TGN) and vesicular traffic from this organelle. The NCS-1-ARF1 interaction and its specificity have been demonstrated through in vitro binding assays, in vitro enzyme assay, and through functional cellular assays. We show that NCS-1 can exert bidirectional effects to activate PI(4)Kbeta on its own or inhibit the activation by ARF1. NCS-1 was shown to modulate the effects of expression of ARF mutants that disrupt Golgi morphology and to recruit GDP-loaded ARF to the Golgi complex in a Ca(2+)-dependent manner. We demonstrate antagonist effects of NCS-1 and ARF on constitutive and regulated exocytosis. The NCS-1-ARF1 interaction provides evidence for functional cross-talk between Ca(2+)-dependent and ARF-dependent pathways in TGN to plasma membrane traffic.

Munc18-1 regulates early and late stages of exocytosis via syntaxin-independent protein interactions

Sec1/Munc18 (SM) proteins are involved in various intracellular membrane trafficking steps. Many SM proteins bind to appropriate syntaxin homologues involved in these steps, suggesting that SM proteins function as syntaxin chaperones. Organisms with mutations in SM genes, however, exhibit defects in either early (docking) or late (fusion) stages of exocytosis, implying that SM proteins may have multiple functions. To gain insight into the role of SM proteins, we introduced mutations modeled on those identified in Caenorhabditis elegans, Drosophila melanogaster, and Saccharomyces cerevisiae into mammalian Munc18-1. As expected, several mutants exhibited reduced binding to syntaxin1A. However, three mutants displayed wild-type syntaxin binding affinities, indicating syntaxin-independent defects. Expression of these mutants in chromaffin cells either increased the rate and extent of exocytosis or altered the kinetics of individual release events. This latter effect was associated with a reduced Mint binding affinity in one mutant, implying a potential mechanism for the observed alteration in release kinetics. Furthermore, this phenotype persisted when the mutation was combined with a second mutation that greatly reduced syntaxin binding affinity. These results clarify the data on the function of SM proteins in mutant organisms and indicate that Munc18-1 controls multiple stages of exocytosis via both syntaxin-dependent and -independent protein interactions.

Regulation of the fusion pore conductance during exocytosis by cyclin-dependent kinase 5

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase involved in synaptogenesis and brain development, and its enzymatic activity is essential for slow forms of synaptic vesicle endocytosis. Recent work also has implicated Cdk5 in exocytosis and synaptic plasticity. Pharmacological inhibition of Cdk5 modifies secretion in neuroendocrine cells, synaptosomes, and brain slices; however, the specific mechanisms involved remain unclear. Here we demonstrate that dominant-negative inhibition of Cdk5 increases quantal size and broadens the kinetics of individual exocytotic events measured by amperometry in adrenal chromaffin cells. Conversely, Cdk5 overexpression narrows the kinetics of fusion, consistent with an increase in the extent of kiss-and-run exocytosis. Cdk5 inhibition also increases the total charge and current of catecholamine released during the amperometric foot, representing a modification of the conductance of the initial fusion pore connecting the granule and plasma membrane. We suggest that these effects are not attributable to an alteration in catecholamine content of secretory granules and therefore represent an effect on the fusion mechanism itself. Finally, mutational silencing of the Cdk5 phosphorylation site in Munc18, an essential protein of the late stages of vesicle fusion, has identical effects on amperometric spikes as dominant-negative Cdk5 but does not affect the amperometric feet. Cells expressing Munc18 T574A have increased quantal size and broader kinetics of fusion. These results suggest that Cdk5 could, in part, control the kinetics of exocytosis through phosphorylation of Munc18, but Cdk5 also must have Munc18-independent effects that modify fusion pore conductance, which may underlie a role of Cdk5 in synaptic plasticity.

Syntaxin/Munc18 interactions in the late events during vesicle fusion and release in exocytosis

The SNARE proteins, syntaxin, SNAP-25, and VAMP, form part of the core machinery for membrane fusion during regulated exocytosis. Additional proteins are required to account for the speed, spatial restriction, and tight control of exocytosis and a key role is played by members of the Sec1/Munc18 family of proteins that have been implicated either in vesicle docking or fusion itself through their interactions with the corresponding syntaxin. Using amperometry to assay the kinetics of single vesicle fusion/release events in adrenal chromaffin cells, the effect of expression of syntaxin 1A mutants was examined. Overexpression of wild-type syntaxin or its cytoplasmic domain had no effect on the kinetics of release during single exocytotic events although the cytoplasmic domain reduced the frequency of exocytosis. In contrast, expression of either an open syntaxin 1A or the I233A mutant resulted in increased quantal size and a slowing of the kinetics of release. The wild-type and mutant syntaxins were overexpressed to a similar extent and the only common defect shown by the syntaxin 1A mutants was reduced binding to Munc18-1. These results are consistent with a role for Munc18-1 in controlling the late stages of exocytosis by binding to and limiting the availability of syntaxin in its open conformation. Modification of the Munc18-1/syntaxin 1A interaction would therefore be a key mechanism for the regulation of quantal size.

Dopamine neurons release transmitter via a flickering fusion pore

A key question in understanding mechanisms of neurotransmitter release is whether the fusion pore of a synaptic vesicle regulates the amount of transmitter released during exocytosis. We measured dopamine release from small synaptic vesicles of rat cultured ventral midbrain neurons using carbon fiber amperometry. Our data indicate that small synaptic vesicle fusion pores flicker either once or multiple times in rapid succession, with each flicker releasing approximately 25-30% of vesicular dopamine. The incidence of events with multiple flickers was reciprocally regulated by phorbol esters and staurosporine. Thus, dopamine neurons regulate the amount of neurotransmitter released by small synaptic vesicles by controlling the number of fusion pore flickers per exocytotic event. This mode of exocytosis is a potential mechanism whereby neurons can rapidly reuse vesicles without undergoing the comparatively slow process of recycling.

Physiological regulation of Munc18/nSec1 phosphorylation on serine-313

Increased protein phosphorylation enhances exocytosis in most secretory cell types, including neurones. However, the molecular mechanisms by which this occurs and the specific protein targets remain unclear. Munc18-1/nSec1 is essential for exocytosis in neurones, and is known to be phosphorylated by protein kinase C (PKC) in vitro at Ser-313. This phosphorylation has been shown to decrease its affinity for syntaxin, and to alter the kinetics of exocytosis in chromaffin cells. However, there are no data on the physiological regulation of Ser-313 phosphorylation. Using phospho-Ser-313-specific antisera, we demonstrate here that Ser-313 is phosphorylated in intact and permeabilized chromaffin cells in response to histamine and Ca2+ respectively. Furthermore, Ser-313 is rapidly and transiently phosphorylated in intact synaptosomes in response to depolarization by KCl treatment or by 4-aminopyridine, and by the metabotropic glutamate receptor agonist dihydroxyphenylglycine. PKC was identified as the kinase, and PP1 and PP2B as the phosphatases responsible for regulating Ser-313 phosphorylation. As phosphorylation of nSec1 on Ser-313 affects the rate of transmitter release in chromaffin cells, the demonstration here that this phosphorylation event occurs in neurones suggests that synaptic neurotransmitter release may be similarly regulated by nSec1 phosphorylation. Furthermore, such changes in release kinetics are associated with long-term potentiation and depression, thus implicating nSec1 phosphorylation as a potential regulatory mechanism underlying presynaptic plasticity.