The fusion pore

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

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

Src kinases regulate de novo actin polymerization during exocytosis in neuroendocrine chromaffin cells

The cortical actin network is dynamically rearranged during secretory processes. Nevertheless, it is unclear how de novo actin polymerization and the disruption of the preexisting actin network control transmitter release. Here we show that in bovine adrenal chromaffin cells, both formation of new actin filaments and disruption of the preexisting cortical actin network are induced by Ca²⁺ concentrations that trigger exocytosis. These two processes appear to regulate different stages of exocytosis; whereas the inhibition of actin polymerization with the N-WASP inhibitor wiskostatin restricts fusion pore expansion, thus limiting the release of transmitters, the disruption of the cortical actin network with cytochalasin D increases the amount of transmitter released per event. Further, the Src kinase inhibitor PP2, and cSrc SH2 and SH3 domains also suppress Ca²⁺-dependent actin polymerization, and slow down fusion pore expansion without disturbing the cortical F-actin organization. Finally, the isolated SH3 domain of c-Src prevents both the disruption of the actin network and the increase in the quantal release induced by cytochalasin D. These findings support a model where a rise in the cytosolic Ca²⁺ triggers actin polymerization through a mechanism that involves Src kinases. The newly formed actin filaments would speed up the expansion of the initial fusion pore, whereas the preexisting actin network might control a different step of the exocytosis process.

A comparison of coarse-grained and continuum models for membrane bending in lipid bilayer fusion pores

To establish the validity of continuum mechanics models quantitatively for the analysis of membrane remodeling processes, we compare the shape and energies of the membrane fusion pore predicted by coarse-grained (MARTINI) and continuum mechanics models. The results at these distinct levels of resolution give surprisingly consistent descriptions for the shape of the fusion pore, and the deviation between the continuum and coarse-grained models becomes notable only when the radius of curvature approaches the thickness of a monolayer. Although slow relaxation beyond microseconds is observed in different perturbative simulations, the key structural features (e.g., dimension and shape of the fusion pore near the pore center) are consistent among independent simulations. These observations provide solid support for the use of coarse-grained and continuum models in the analysis of membrane remodeling. The combined coarse-grained and continuum analysis confirms the recent prediction of continuum models that the fusion pore is a metastable structure and that its optimal shape is neither toroidal nor catenoidal. Moreover, our results help reveal a new, to our knowledge, bowing feature in which the bilayers close to the pore axis separate more from one another than those at greater distances from the pore axis; bowing helps reduce the curvature and therefore stabilizes the fusion pore structure. The spread of the bilayer deformations over distances of hundreds of nanometers and the substantial reduction in energy of fusion pore formation provided by this spread indicate that membrane fusion can be enhanced by allowing a larger area of membrane to participate and be deformed.

Dynamin-2 regulates fusion pore expansion and quantal release through a mechanism that involves actin dynamics in neuroendocrine chromaffin cells

Over the past years, dynamin has been implicated in tuning the amount and nature of transmitter released during exocytosis. However, the mechanism involved remains poorly understood. Here, using bovine adrenal chromaffin cells, we investigated whether this mechanism rely on dynamin’s ability to remodel actin cytoskeleton. According to this idea, inhibition of dynamin GTPase activity suppressed the calcium-dependent de novo cortical actin and altered the cortical actin network. Similarly, expression of a small interfering RNA directed against dynamin-2, an isoform highly expressed in chromaffin cells, changed the cortical actin network pattern. Disruption of dynamin-2 function, as well as the pharmacological inhibition of actin polymerization with cytochalasine-D, slowed down fusion pore expansion and increased the quantal size of individual exocytotic events. The effects of cytochalasine-D and dynamin-2 disruption were not additive indicating that dynamin-2 and F-actin regulate the late steps of exocytosis by a common mechanism. Together our data support a model in which dynamin-2 directs actin polymerization at the exocytosis site where both, in concert, adjust the hormone quantal release to efficiently respond to physiological demands.

Vesicles and vesicle fusion: coarse-grained simulations

Biological cells are highly dynamic, and continually move material around their own volume and between their interior and exterior. Much of this transport encapsulates the material inside phospholipid vesicles that shuttle to and from, fusing with, and budding from, other membranes. A feature of vesicles that is crucial for this transport is their ability to fuse to target membranes and release their contents to the distal side. In industry, some personal care products contain vesicles to help transport reagents across the skin, and research on drug formulation shows that packaging active compounds inside vesicles delays their clearance from the blood stream. In this chapter, we survey the biological role and physicochemical properties of phospholipids, and describe progress in coarse-grained simulations of vesicles and vesicle fusion. Because coarse-grained simulations retain only those molecular details that are thought to influence the large-scale processes of interest, they act as a model embodying our current understanding. Comparing the predictions of these models with experiments reveals the importance of the retained microscopic details and also the deficiencies that can suggest missing details, thereby furthering our understanding of the complex dynamic world of vesicles.

Perspectives on kiss-and-run: role in exocytosis, endocytosis, and neurotransmission

Regulated exocytosis and endocytosis are critical to the function of many intercellular networks, particularly the complex neural circuits underlying mammalian behavior. Kiss-and-run (KR) is an unconventional fusion between secretory vesicles and a target membrane that releases intravesicular content through a transient, nanometer-sized fusion pore. The fusing vesicle retains its gross shape, precluding full integration into the planar membrane, and enough molecular components for rapid retrieval, reacidification, and reuse. KR makes judicious use of finite presynaptic resources, and mounting evidence suggests that it influences synaptic information transfer. Here we detail emerging perspectives on KR and its role in neurotransmission. We additionally formulate a restraining force hypothesis as a plausible mechanistic basis for KR and its physiological modulation in small nerve terminals. Clarification of the mechanism and function of KR has bearing on understanding the kinetic transitions underlying SNARE-mediated fusion, interactions between vesicles and their local environment, and the influence of release dynamics on neural information processing.

Activity-dependent fusion pore expansion regulated by a calcineurin-dependent dynamin-syndapin pathway in mouse adrenal chromaffin cells

Neuroendocrine chromaffin cells selectively secrete a variety of transmitter molecules into the circulation as a function of sympathetic activation. Activity-dependent release of transmitter species is controlled through regulation of the secretory fusion pore. Under sympathetic tone, basal synaptic excitation drives chromaffin cells to selectively secrete modest levels of catecholamine through a restricted secretory fusion pore. In contrast, elevated sympathetic activity, experienced under stress, results in fusion pore expansion to evoke maximal catecholamine release and to facilitate release of copackaged peptide transmitters. Therefore, fusion pore expansion is a key control point for the activation of the sympatho-adrenal stress response. Despite the physiological importance of this process, the molecular mechanism by which it is regulated remains unclear. Here we employ fluorescence imaging with electrophysiological and electrochemical-based approaches to investigate the role of dynamin I in the regulation of activity-mediated fusion pore expansion in mouse adrenal chromaffin cells. We show that under elevated stimulation, dynamin I is dephosphorylated at Ser-774 by calcineurin. We also demonstrate that disruption of dynamin I-syndapin binding, an association regulated by calcineurin-dependent dynamin dephosphorylation, limits fusion pore expansion. Last, we show that perturbation of N-WASP function (a syndapin substrate) limits activity-mediated fusion pore expansion. Our results suggest that fusion pore expansion is regulated by a calcineurin-dependent dephosphorylation of dynamin I. Dephosphorylated dynamin I acts via a syndapin/N-WASP signaling cascade to mediate pore expansion.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Titration of synaptotagmin I expression differentially regulates release of norepinephrine and neuropeptide Y

Synaptotagmin (syt) I is a Ca(2+) sensor that has been thought to trigger all vesicle secretion with similar mechanisms. However, given the calcium and stimulation requirements of small clear, and large dense core vesicles, we hypothesized that syt I expression differentially regulates vesicle release. Therefore, in this study, we generated multiple stable cell lines of PC12 cells that each had a different and stable level of syt I expression. We determined the functional effects of titrated syt I expression on transmitter release from the two vesicle types, and showed that the transmitters, norepinephrine (NE) and neuropeptide Y (NPY), each have a threshold level of syt I expression required for their release that is different for the two transmitter types. We used carbon fiber amperometry to measure release of NE from single vesicles, and found that release ranged from 50% to 100% in the syt I-targeted cells compared to release from control cells. We used an immunoassay to measure NPY release and found that NPY release was abolished in cells that had abolished syt I expression, but cell lines that expressed 50-60% of control levels of syt I exhibited NPY release levels comparable to release of NPY from control cells. Furthermore, the vesicle fusion pore exhibited a reduced open duration when syt I was abolished, but a longer open duration time for 50% syt I expression than control cells. Therefore, vesicles have a threshold for syt I that is required to control opening of the fusion pore, expansion, and full fusion to release large dense core proteins, but not for full fusion of the small molecules like NE.

High resolution electrophysiological techniques for the study of calcium-activated exocytosis

BACKGROUND

Neurotransmitters, neuropeptides and hormones are released from secretory vesicles of nerve terminals and neuroendocrine cells by calcium-activated exocytosis. A key step in this process is the formation of a fusion pore between the vesicle membrane and the plasma membrane. Exocytotic fusion leads to an increase in plasma membrane area that can be measured as a proportional increase in plasma membrane capacitance.

SCOPE OF REVIEW

High resolution capacitance measurements in single cells, nerve terminals and small membrane patches have become possible with the development of the patch clamp technique. This review discusses the methods of whole cell patch clamp capacitance measurements and their use in conjunction with voltage clamp pulse stimulation and with stimulation by photorelease of caged calcium. It also discusses patch capacitance measurements for the study of single exocytotic events and fusion pore properties in neuroendocrine cells and nerve terminals.

MAJOR CONCLUSIONS

Capacitance measurements provide high resolution information on the extent and time course of fusion for the characterization of vesicle pools and the kinetics of exocytosis. They allow the characterization of the mode of fusion including distinction of single vesicle full fusion, transient kiss-and-run fusion or multivesicular compound exocytosis. Furthermore, measurement of fusion pore conductances and their dynamic behavior has enabled the characterization of fusion pore properties in a way that resembles the characterization of ion channel function through single channel recordings.

GENERAL SIGNIFICANCE

The combination of patch clamp capacitance measurements with pharmacological and molecular manipulations of exocytosis is emerging as a powerful approach to investigate the molecular mechanisms of calcium-activated exocytotic fusion pore formation. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

VISUALIZING SOFT MATTER: MESOSCOPIC SIMULATIONS OF MEMBRANES, VESICLES AND NANOPARTICLES

Biological membranes have properties and behavior that emerge from the propagation of the molecular characteristics of their components across many scales. Artificial smart materials, such as drug delivery vehicles and nanoparticles, often rely on modifying naturally-occurring soft matter, such as polymers and lipid vesicles, so that they possess useful behavior. Mesoscopic simulations allow in silico experiments to be easily and cheaply performed on complex, soft materials requiring as input only the molecular structure of the constituents at a coarse-grained level. They can therefore act as a guide to experimenters prior to performing costly assays. Additionally, mesoscopic simulations provide the only currently feasible window on the length and time scales relevant to important biophysical processes such as vesicle fusion. We describe here recent work using Dissipative Particle Dynamics simulations to explore the structure and behavior of amphiphilic membranes, the fusion of vesicles, and the interactions between rigid nanoparticles and soft surfaces.

Microwell device for targeting single cells to electrochemical microelectrodes for high-throughput amperometric detection of quantal exocytosis

Electrochemical microelectrodes are commonly used to detect spikes of amperometric current that correspond to exocytosis of oxidizable transmitter from individual vesicles, i.e., quantal exocytosis. We are developing transparent multielectrochemical electrode arrays on microchips in order to automate measurement of quantal exocytosis. Here, we report development of an improved device to target individual cells to each microelectrode in an array. Efficient targeting (~75%) is achieved using cell-sized microwell traps fabricated in SU-8 photoresist together with patterning of poly(l-lysine) in register with electrodes to promote cell adhesion. The surface between electrodes is made resistant to cell adhesion using poly(ethylene glycol) in order to facilitate movement of cells to electrode "docking sites". We demonstrate the activity of the electrodes using the test analyte ferricyanide and perform recordings of quantal exocytosis from bovine adrenal chromaffin cells on the device. Multiple cell recordings on a single device demonstrate the consistency of spike measurements, and multiple recordings from the same electrodes demonstrate that the device can be cleaned and reused without degradation of performance. The new device will enable high-throughput studies of quantal exocytosis and may also find application in rapidly screening drugs or toxins for effects on exocytosis.

Assessing the efficacy of vesicle fusion with planar membrane arrays using a mitochondrial porin as reporter

Reconstitution of functionally active membrane protein into artificially made lipid bilayers is a challenge that must be overcome to create a membrane-based biomimetic sensor and separation device. In this study we address the efficacy of proteoliposome fusion with planar membrane arrays. We establish a protein incorporation efficacy assay using the major non-specific porin of Fusobacterium nucleatum (FomA) as reporter. We use electrical conductance measurements and fluorescence microscopy to characterize proteoliposome fusion with an array of planar membranes. We show that protein reconstitution in biomimetic membrane arrays may be quantified using the developed FomA assay. Specifically, we show that FomA vesicles are inherently fusigenic. Optimal FomA incorporation is obtained with a proteoliposome lipid-to-protein molar ratio (LPR)=50 more than 10(5) FomA proteins could be incorporated in a bilayer array with a total membrane area of 2mm(2) within 20 min. This novel assay for quantifying protein delivery into lipid bilayers may be a useful tool in developing biomimetic membrane applications.

From spike to graph--a complete automated single-spike analysis

Amperometry is a commonly used technique for detecting the kinetics of single-vesicle exocytosis with excellent temporal and spatial resolution. However, different methods of analyzing the amperometric signals can produce conflicting conclusions. We developed an efficient automated method for kinetics analysis of single spikes that does not filter the data and therefore prevents distortion of the results. The algorithm assesses the signal-to-noise ratios (SNRs) and accordingly, separates the signals using an adjustable two-threshold calculation. This enables comparing data with different SNRs from different setups. The software also includes a complete statistical analysis, with an automated selection of the most appropriate statistical tests and a graphical representation. The algorithms can be used for any other experimental results requiring the separation of signals from noise, making this method useful for many applications.

The association of dynamin with synaptophysin regulates quantal size and duration of exocytotic events in chromaffin cells

Although synaptophysin is one of the most abundant integral proteins of synaptic vesicle membranes, its contribution to neurotransmitter release remains unclear. One possibility is that through its association with dynamin it controls the fine tuning of transmitter release. To test this hypothesis, we took advantage of amperometric measurements of quantal catecholamine release from chromaffin cells. First, we showed that synaptophysin and dynamin interact in chromaffin granule-rich fractions and that this interaction relies on the C terminal of synaptophysin. Experimental maneuvers that are predicted to disrupt the association between these two proteins, such as injection of antibodies against dynamin or synaptophysin, or peptides homologous to the C terminal of synaptophysin, increased the quantal size and duration of amperometric spikes. In contrast, the amperometric current that precedes the spike remained unchanged, indicating that synaptophysin/dynamin association does not regulate the initial fusion pore, but it appears to target a later step of exocytosis to control the amount of catecholamines released during a single vesicle fusion event.

Automated targeting of cells to electrochemical electrodes using a surface chemistry approach for the measurement of quantal exocytosis

Here we describe a method to fabricate a multi-channel high-throughput microchip device for measurement of quantal transmitter release from individual cells. Instead of bringing carbon-fiber electrodes to cells, the device uses a surface chemistry approach to bring cells to an array of electrochemical microelectrodes. The microelectrodes are small and "cytophilic" in order to promote adhesion of a single cell whereas all other areas of the chip are covered with a thin "cytophobic" film to block cell attachement and facilitate movement of cells to electrodes. This cytophobic film also insulates unused areas of the conductive film, thus the alignment of cell docking sites to working electrodes is automatic. Amperometric spikes resulting from single-granule fusion events were recorded on the device and had amplitudes and kinetics similar to those measured using carbon-fiber microelectrodes. Use of this device will increase the pace of basic neuroscience research and may also find applications in drug discovery or validation.

A microfluidic cell trap device for automated measurement of quantal catecholamine release from cells

Neurons and endocrine cells secrete neurotransmitter and hormones in discrete packets in a process called quantal exocytosis. Electrochemical microelectrodes can detect spikes in current resulting from the oxidation of individual quanta of transmitter only if the electrodes are small and directly adjacent to release sites on the cell. Here we report development of a microchip device that uses microfluidic traps to automatically target individual or small groups of cells to small electrochemical electrodes. Microfluidic channels and traps were fabricated by multi-step wet etch of a silicon wafer whereas Pt electrodes were patterned in register with the trap sites. We demonstrate high-resolution amperometric measurement of quantal exocytosis of catecholamines from chromaffin cells on the device. This reusable device is a step towards developing high-throughput lab-on-a-chip instruments for recording quantal exocytosis to increase the pace of basic neuroscience research and to enable screening of drugs that target exocytosis.

Minimum membrane bending energies of fusion pores

Membranes fuse by forming highly curved intermediates, culminating in structures described as fusion pores. These hourglass-like figures that join two fusing membranes have high bending energies, which can be estimated using continuum elasticity models. Fusion pore bending energies depend strongly on shape, and the present study developed a method for determining the shape that minimizes bending energy. This was first applied to a fusion pore modeled as a single surface and then extended to a more realistic model treating a bilayer as two monolayers. For the two-monolayer model, fusion pores were found to have metastable states with energy minima at particular values of the pore diameter and bilayer separation. Fusion pore energies were relatively insensitive to membrane thickness but highly sensitive to spontaneous curvature and membrane asymmetry. With symmetrical bilayers and monolayer spontaneous curvatures of -0.1 nm(-1) (a typical value) separated by 6 nm (closest distance determined by repulsive hydration forces), fusion pore formation required 43-65 kT. The pore radius of approximately 2.25 nm fell within the range estimated from conductance measurements. With bilayer separation >6 nm, fusion pore formation required less energy, suggesting that protein scaffolds can promote fusion by bending membranes toward one another. With nonzero spontaneous monolayer curvature, the shape that minimized the energy change during fusion pore formation differed from the shape that minimized its energy after it formed. Thus, a nascent fusion pore will relax spontaneously to a new shape, consistent with the experimentally observed expansion of nascent fusion pores during viral fusion.

Compound vesicle fusion increases quantal size and potentiates synaptic transmission

Exocytosis at synapses involves fusion between vesicles and the plasma membrane. Although compound fusion between vesicles was proposed to occur at ribbon-type synapses, whether it exists, how it is mediated, and what role it plays at conventional synapses remain unclear. Here we report the existence of compound fusion, its underlying mechanism, and its role at a nerve terminal containing conventional active zones in rats and mice. We found that high potassium application and high frequency firing induced giant capacitance up-steps, reflecting exocytosis of vesicles larger than regular ones, followed by giant down-steps, reflecting bulk endocytosis. These intense stimuli also induced giant vesicle-like structures, as observed with electron microscopy, and giant miniature excitatory postsynaptic currents (mEPSCs), reflecting more transmitter release. Calcium and its sensor for vesicle fusion, synaptotagmin, were required for these giant events. After high frequency firing, calcium/synaptotagmin-dependent mEPSC size increase was paralleled by calcium/synaptotagmin-dependent post-tetanic potentiation. These results suggest a new route of exocytosis and endocytosis composed of three steps. First, calcium/synaptotagmin mediates compound fusion between vesicles. Second, exocytosis of compound vesicles increases quantal size, which increases synaptic strength and contributes to the generation of post-tetanic potentiation. Third, exocytosed compound vesicles are retrieved via bulk endocytosis. We suggest that this vesicle cycling route be included in models of synapses in which only vesicle fusion with the plasma membrane is considered.

Cortical F-actin, the exocytic mode, and neuropeptide release in mouse chromaffin cells is regulated by myristoylated alanine-rich C-kinase substrate and myosin II

Adrenal medullary chromaffin cells are innervated by the sympathetic splanchnic nerve and translate graded sympathetic firing into a differential hormonal exocytosis. Basal sympathetic firing elicits a transient kiss-and-run mode of exocytosis and modest catecholamine release, whereas elevated firing under the sympathetic stress response results in full granule collapse to release catecholamine and peptide transmitters into the circulation. Previous studies have shown that rearrangement of the cell actin cortex regulates the mode of exocytosis. An intact cortex favors kiss-and-run exocytosis, whereas disrupting the cortex favors the full granule collapse mode. Here, we investigate the specific roles of two actin-associated proteins, myosin II and myristoylated alanine-rich C-kinase substrate (MARCKS) in this process. Our data demonstrate that MARCKS phosphorylation under elevated cell firing is required for cortical actin disruption but is not sufficient to elicit peptide transmitter exocytosis. Our data also demonstrate that myosin II is phospho-activated under high stimulation conditions. Inhibiting myosin II activity prevented disruption of the actin cortex, full granule collapse, and peptide transmitter release. These results suggest that phosphorylation of both MARCKS and myosin II lead to disruption of the actin cortex. However, myosin II, but not MARCKS, is required for the activity-dependent exocytosis of the peptide transmitters.

Quantitative investigations of amperometric spike feet suggest different controlling factors of the fusion pore in exocytosis at chromaffin cells

Around 30% of exocytosis events recorded by amperometry at carbon fiber microelectrodes exhibit a pre-spike feature (PSF) termed a "foot". This wave is associated with the release of the neurotransmitters via a transitory fusion pore, whilst the large, main exocytotic spike is due to complete release. The amperometric data reported herein were obtained using bovine chromaffin cells stimulated with either potassium or barium ions, two commonly-employed elicitors of exocytosis. Identical trends are observed with both activators: (i) they induce the same ratio (close to 30%) of events with a foot in the population of amperometric spikes, and (ii) spikes with a foot can be divided into two primary categories, depending on the temporal variation of the current wave (viz. as a ramp, or a ramp followed by a plateau). Correlations between the characteristics of the whole current spike, and of its observed foot, have been sought; such analyses demonstrate that the maximum current of both foot and spike signals are highly correlated, but, in contrast, the integrated charges of both are poorly correlated. Moreover, the temporal duration of the PSF is fully uncorrelated with any parameter pertaining to the main current spike. On the basis of these reproducible observations, it is hypothesized that the characteristics (dimensions and topology, at least) of each secretory vesicle determine the probability of formation of the fusion pore and its maximum size, whilst molecular factors of the cell membrane control its duration, and, consequently, the amount delivered prior to the massive exocytosis of catecholamines observed as a spike in amperometry.

The fusion of membranes and vesicles: pathway and energy barriers from dissipative particle dynamics

The fusion of lipid bilayers is studied with dissipative particle dynamics simulations. First, to achieve control over membrane properties, the effects of individual simulation parameters are studied and optimized. Then, a large number of fusion events for a vesicle and a planar bilayer are simulated using the optimized parameter set. In the observed fusion pathway, configurations of individual lipids play an important role. Fusion starts with individual lipids assuming a splayed tail configuration with one tail inserted in each membrane. To determine the corresponding energy barrier, we measure the average work for interbilayer flips of a lipid tail, i.e., the average work to displace one lipid tail from one bilayer to the other. This energy barrier is found to depend strongly on a certain dissipative particle dynamics parameter, and, thus, can be adjusted in the simulations. Overall, three subprocesses have been identified in the fusion pathway. Their energy barriers are estimated to lie in the range 8-15 k(B)T. The fusion probability is found to possess a maximum at intermediate tension values. As one decreases the tension, the fusion probability seems to vanish before the tensionless membrane state is attained. This would imply that the tension has to exceed a certain threshold value to induce fusion.

Enhancement of the endosomal endocytic pathway increases quantal size

We combined recordings of spontaneous quantal events with electron microscopy analysis of synaptic ultrastructure to demonstrate that the size of a neurosecretory quantum increases following an activation of the endosomal endocytic pathway. We reversibly activated the endosomal endocytic pathway in Drosophila motor boutons by application of high K+ solution. This treatment produced the formation of numerous cisternae, vacuoles and enlarged vesicles. Spontaneous quantal events recorded immediately after the cessation of high K+ application were significantly enlarged, and this increase in quantal size was reversed after a 10 minute resting period. Actin depolymerization produced by latrunculin B pretreatment inhibited both the formation of endosome-like structures and the increase in quantal size. Loading the preparations with the dye FM1-43 followed by photoconversion of the dye combined with electron microscopy analysis revealed that the observed cisternae are likely to be the product of both bulk membrane retrieval and vesicle fusion.

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.

Tethering forces of secretory granules measured with optical tweezers

Fusion of a vesicle with its target membrane is preceded by tethering or docking. However, the physical mechanism of vesicle-tethering is unknown. To study this mechanism, we used eosinophil secretory granules, which undergo stimulated homotypic fusion events inside the cell during degranulation. Using a dual optical trap system, we observed tether formation between isolated eosinophil secretory granules. The results show that secretory granules interact stochastically with a target membrane forming physical tethers linking the vesicle and target membrane, rather than via interactions with the cytoskeleton. The necessary components are membrane-associated, and the addition of cytosolic components is not required. Tether-lifetime measurements as a function of applied mechanical force revealed at least three kinetically distinct tethered states. The tethered-state lifetimes of isolated eosinophil granules match the residence times of chromaffin granules at the plasma membrane in intact cells, suggesting that the tethering mechanisms reported here may represent the physiological mechanisms of vesicle-tethering in the cell.

The mouth of a dense-core vesicle opens and closes in a concerted action regulated by calcium and amphiphysin

Secretion of hormones and peptides by neuroendocrine cells occurs through fast and slow modes of vesicle fusion but the mechanics of these processes are not understood. We used interference reflection microscopy to monitor deformations of the membrane surface and found that both modes of fusion involve the tightly coupled dilation and constriction of the vesicle. The rate of opening is calcium dependent and occurs rapidly at concentrations <5 muM [corrected] The fast mode of fusion is blocked selectively by a truncation mutant of amphiphysin. Vesicles do not collapse when fusion is triggered by strontium, rather they remain locked open and membrane scission is blocked. In contrast, constriction of the vesicle opening continues when endocytosis is blocked by inhibiting the function of dynamin. Thus, fast and slow modes of fusion involve similar membrane deformations and vesicle closure can be uncoupled from membrane scission. Regulation of these processes by calcium and amphiphysin may provide a mechanism for controlling the release of vesicle contents.

The role of the C terminus of the SNARE protein SNAP-25 in fusion pore opening and a model for fusion pore mechanics

Formation of a fusion pore between a vesicle and its target membrane is thought to involve the so-called SNARE protein complex. However, there is no mechanistic model explaining how the fusion pore is opened by conformational changes in the SNARE complex. It has been suggested that C-terminal zipping triggers fusion pore opening. A SNAP-25 mutant named SNAP-25Delta9 (lacking the last nine C-terminal residues) should lead to a less-tight C-terminal zipping. Single exocytotic events in chromaffin cells expressing this mutant were characterized by carbon fiber amperometry and cell-attached patch capacitance measurements. Cells expressing SNAP-25Delta9 displayed smaller amperometric "foot-current" currents, reduced fusion pore conductances, and lower fusion pore expansion rates. We propose that SNARE/lipid complexes form proteolipid fusion pores. Fusion pores involving the SNAP-25Delta9 mutant will be less tightly zipped and may lead to a longer fusion pore structure, consistent with the observed decrease of fusion pore conductance.

Kiss-and-run exocytosis and fusion pores of secretory vesicles in human beta-cells

Exocytosis of secretory vesicles results in the release of insulin from pancreatic beta-cells, although little is known about this process in humans. We examined the exocytosis of single secretory vesicles and their associated fusion pores in human beta-cells by cell-attached capacitance and conductance measurement. Unitary capacitance steps were observed, consistent with the exocytosis of single secretory vesicles. These were often coincident with increases in patch conductance representing the presence of a stable fusion pore. In some events, the fusion pore closed, mediating kiss-and-run, which contributed 20% of the exocytotic events. The cAMP-raising agent forskolin (5 microM) doubled the relative contribution of kiss-and-run. This effect was confirmed visually in MIN6 cells expressing a fluorescent granule probe. Thus, we demonstrate the unitary capacitance steps and fusion pores during single vesicle exocytosis in human beta-cells. Furthermore, these secretory vesicles can undergo rapid recycling by kiss-and-run, and this process is up-regulated by cAMP.

Dynamin I plays dual roles in the activity-dependent shift in exocytic mode in mouse adrenal chromaffin cells

Under low stimulation, adrenal chromaffin cells release freely soluble catecholamines through a restricted granule fusion pore while retaining the large neuropeptide-containing proteinacious granule core. Elevated activity causes dilation of the pore and release of all granule contents. Thus, physiological differential transmitter release is achieved through regulation of fusion pore dilation. We examined the mechanism for pore dilation utilizing a combined approach of peptide transfection, electrophysiology, electrochemistry and quantitative imaging techniques. We report that disruption of dynamin I function alters both fusion modes. Under low stimulation, interference with dynamin I does not affect granule fusion but blocks its re-internalization. In full collapse mode, disruption of dynamin I limits fusion pore dilation, but does not block membrane re-internalization. These data suggest that dynamin I is involved in both modes of exocytosis by regulating contraction or dilation of the fusion pore and thus contributes to activity-dependent differential transmitter release from the adrenal medulla.

Massive Ca-induced membrane fusion and phospholipid changes triggered by reverse Na/Ca exchange in BHK fibroblasts

Baby hamster kidney (BHK) fibroblasts increase their cell capacitance by 25-100% within 5 s upon activating maximal Ca influx via constitutively expressed cardiac Na/Ca exchangers (NCX1). Free Ca, measured with fluo-5N, transiently exceeds 0.2 mM with total Ca influx amounting to approximately 5 mmol/liter cell volume. Capacitance responses are half-maximal when NCX1 promotes a free cytoplasmic Ca of 0.12 mM (Hill coefficient approximately 2). Capacitance can return to baseline in 1-3 min, and responses can be repeated several times. The membrane tracer, FM 4-64, is taken up during recovery and can be released at a subsequent Ca influx episode. Given recent interest in signaling lipids in membrane fusion, we used green fluorescent protein (GFP) fusions with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and diacylglycerol (DAG) binding domains to analyze phospholipid changes in relation to these responses. PI(4,5)P(2) is rapidly cleaved upon activating Ca influx and recovers within 2 min. However, PI(4,5)P(2) depletion by activation of overexpressed hM1 muscarinic receptors causes only little membrane fusion, and subsequent fusion in response to Ca influx remains massive. Two results suggest that DAG may be generated from sources other than PI(4,5)P in these protocols. First, acylglycerols are generated in response to elevated Ca, even when PI(4,5)P(2) is metabolically depleted. Second, DAG-binding C1A-GFP domains, which are brought to the cell surface by exogenous ligands, translocate rapidly back to the cytoplasm in response to Ca influx. Nevertheless, inhibitors of PLCs and cPLA2, PI(4,5)P(2)-binding peptides, and PLD modification by butanol do not block membrane fusion. The cationic agents, FM 4-64 and heptalysine, bind profusely to the extracellular cell surface during membrane fusion. While this binding might reflect phosphatidylserine (PS) "scrambling" between monolayers, it is unaffected by a PS-binding protein, lactadherin, and by polylysine from the cytoplasmic side. Furthermore, the PS indicator, annexin-V, binds only slowly after fusion. Therefore, we suggest that the luminal surfaces of membrane vesicles that fuse to the plasmalemma may be rather anionic. In summary, our results provide no support for any regulatory or modulatory role of phospholipids in Ca-induced membrane fusion in fibroblasts.

Selective release of molecules from Weibel-Palade bodies during a lingering kiss

Exocytosis of specialized endothelial cell secretory organelles, Weibel-Palade bodies (WPBs), is thought to play an important role in regulating hemostasis and intravascular inflammation. The major WPB core proteins are Von Willebrand factor (VWF) and its propolypeptide (Proregion), constituting more than 95% of the content. Although the composition of the WPBs can be fine-tuned to include cytokines and chemokines (eg, interleukin-8 [IL-8] and eotaxin-3), it is generally assumed that WPB exocytosis is inextricably associated with secretion of VWF. Here we show that WPBs can undergo a form of exocytosis during which VWF and Proregion are retained while smaller molecules, such as IL-8, are released. Imaging individual WPBs containing fluorescent cargo molecules revealed that during weak stimulation approximately 25% of fusion events result in a failure to release VWF or Proregion. The WPB membrane protein P-selectin was also retained; however, the membrane tetraspannin CD63 was released. Accumulation or exclusion of extracellular fluorescent dextran molecules ranging from 3 kDa to 2 mDa show that these events arise due to the formation of a fusion pore approximately 12 nm in diameter. The pore behaves as a molecular filter, allowing selective release of WPB core and membrane proteins. WPB exocytosis is not inextricably associated with secretion of VWF.

Myosin II activation and actin reorganization regulate the mode of quantal exocytosis in mouse adrenal chromaffin cells

Chromaffin cells of the adrenal medulla are innervated by the sympathetic nervous system. Stimulation causes chromaffin cells to fire action potentials, leading to the exocytosis of various classes of transmitters into the circulation. Low-frequency electrical stimulation (action potentials delivered at 0.5 Hz) causes adrenal chromaffin cells to selectively release catecholamines through a kiss-and-run fusion event. Elevated electrical stimulation (action potentials at 15 Hz) evokes fusion pore dilation, full granule collapse, and additional release of the neuropeptide-containing proteinaceous granule core. Here we apply single-cell electrophysiological, electrochemical, and fluorescence measurements to investigate the cellular mechanism for this shift in exocytic behavior. We show that at low-frequency stimulation, a filamentous-actin cell cortex plays a key role in stabilizing the kiss-and-run fusion event. Increased stimulation disrupts the actin cortex, driving full granule collapse. We show that pharmacological perturbation of the actin cortex supersedes stimulus frequency in controlling exocytic mode. Finally, we show that nonmuscle myosin II activation contributes to the cytoskeleton-dependent control of the fusion event. Inhibition of myosin II or myosin light chain kinase under elevated stimulation frequencies inhibits fusion pore dilation and maintains the granule in a kiss-and-run mode of exocytosis. These results demonstrate an essential role for activity-evoked cytoskeletal rearrangement and the action of myosin II in the regulation of catecholamine and neuropeptide exocytosis and represent an essential element of the sympathetic stress response.

Matching native electrical stimulation by graded chemical stimulation in isolated mouse adrenal chromaffin cells

Adrenal chromaffin cells release multiple transmitters in response to sympathetic stimulation. Modest cell firing, matching sympathetic tone, releases small freely soluble catecholamines. Elevated electrical firing rates matching input under sympathetic stress results in release of catecholamines as well as semi-soluble vaso- and neuro-active peptides packaged within the dense core of the secretory granule. This activity-dependent differential transmitter release has been shown to rely on a mechanistic shift in the mode of exocytosis through the regulated dilation of the secretory fusion pore between granule and cell surface membranes. However, biochemical description of the mechanism regulating fusion pore dilation remains elusive. In the experimental setting, electrical stimulation designed to mimic sympathetic input, is achieved through single-cell voltage-clamp. While precise, this approach is incompatible with biochemical and proteomic analysis, both of which require large sample sizes. We address this limitation in the current study. We describe a bulk chemical stimulation paradigm calibrated to match defined electrical activity. We utilize calcium and single-cell amperometric measurements to match extracellular potassium concentrations to physiological electrical stimulation under sympathetic tone as well as acute stress conditions. This approach provides larger samples of uniformly stimulated cells for determining molecular players in activity-dependent differential transmitter release from adrenal chromaffin cells.

Calcium channel subtypes differentially regulate fusion pore stability and expansion

Various studies have focused in the relative contribution of different voltage-activated Ca(2+) channels (VACC) to total transmitter release. However, how Ca(2+) entry through a given VACC subtype defines the pattern of individual exocytotic events remains unknown. To address this question, we have used amperometry in bovine chromaffin cells. L, N, and P/Q channels were individually or jointly blocked with furnidipine, omega-conotoxin GVIA, omega-agatoxin IVA, or omega-conotoxin MVIIC. The three channel types contributed similarly to cytosolic Ca(2+) signals induced by 70 mmol/L K(+). However, they exhibited different contributions to the frequency of exocytotic events and they were shown to differently regulate the final steps of the exocytosis. When compared with the other VACC subtypes, Ca(2+) entry through P/Q channels effectively induced exocytosis, it decreased fusion pore stability and accelerated its expansion. Conversely, Ca(2+) entry through N channels was less efficient in inducing exocytotic events, also slowing fusion pore expansion. Finally, Ca(2+) entry through L channels inefficiently induced exocytosis, and the individual blockade of this channel significantly modified fusion pore dynamics. The distance between a given VACC subtype and the release sites could account for the differential effects of the distinct VACC on the fusion pore dynamics.

The debate on the kiss-and-run fusion at synapses

It has long been proposed that following vesicle fusion, a small pore might open and close rapidly without full dilation. Such 'kiss-and-run' vesicle fusion can in principle result in rapid vesicle recycling and influence the size and the kinetics of the resulting synaptic current. However, the existence of kiss-and-run remains highly controversial, as revealed by recent imaging and electrophysiological studies at several synapses, including hippocampal synapses, neuromuscular junctions and retinal bipolar synapses. Only a minor fraction of fusion events has been shown to be kiss-and-run, as determined using cell-attached capacitance recordings in endocrine cells, pituitary nerve terminals and calyx-type synapses. Further work is needed to determine whether kiss-and-run is a major mode of fusion and has a major role in controlling synaptic strength at synapses.

Exocytotic catecholamine release is not associated with cation flux through channels in the vesicle membrane but Na+ influx through the fusion pore

Release of charged neurotransmitter molecules through a narrow fusion pore requires charge compensation by other ions. It has been proposed that this may occur by ion flow from the cytosol through channels in the vesicle membrane, which would generate a net outward current. This hypothesis was tested in chromaffin cells using cell-attached patch amperometry that simultaneously measured catecholamine release from single vesicles and ionic current across the patch membrane. No detectable current was associated with catecholamine release indicating that <2% of cations, if any, enter the vesicle through its membrane. Instead, we show that flux of catecholamines through the fusion pore, measured as an amperometric foot signal, decreases when the extracellular cation concentration is reduced. The results reveal that the rate of transmitter release through the fusion pore is coupled to net Na+ influx through the fusion pore, as predicted by electrodiffusion theory applied to fusion-pore permeation, and suggest a prefusion rather than postfusion role for vesicular cation channels.

Giant vesicles in electric fields

This review is dedicated to electric field effects on giant unilamellar vesicles, a cell-size membrane system. We summarize various types of behavior observed when vesicles are subjected either to weak AC fields at various frequency, or to strong DC pulses. Different processes such as electro-deformation, -poration and -fusion of giant vesicles are considered. We describe some recent developments, which allowed us to detect the dynamics of the vesicle response with a resolution below milliseconds for all of these processes. Novel aspects on electric field effects on vesicles in the gel phase are introduced.

Presynaptic G-protein-coupled receptors regulate synaptic cleft glutamate via transient vesicle fusion

When synaptic vesicles fuse with the plasma membrane, they may completely collapse or fuse transiently. Transiently fusing vesicles remain structurally intact and therefore have been proposed to represent a form of rapid vesicle recycling. However, the impact of a transient synaptic vesicle fusion event on neurotransmitter release, and therefore on synaptic transmission, has yet to be determined. Recently, the molecular mechanism by which a serotonergic presynaptic G-protein-coupled receptor (GPCR) regulates synaptic vesicle fusion and inhibits synaptic transmission was identified. By making paired electrophysiological recordings in the presence and absence of low-affinity antagonists, we now demonstrate that activation of this presynaptic GPCR lowers the peak synaptic cleft glutamate concentration independently of the probability of vesicle fusion. Furthermore, this change in cleft glutamate concentration differentially inhibits synaptic NMDA and AMPA receptor-mediated currents. We conclude that a presynaptic GPCR regulates the profile of glutamate in the synaptic cleft through altering the mechanism of vesicle fusion leading to qualitative as well as quantitative changes in neural signaling.

Temperature-dependent differences between readily releasable and reserve pool vesicles in chromaffin cells

Statistical differences between amperometric traces recorded from chromaffin cells using K(+) and Ba(2+) secretagogues support the assertion that readily releasable pool (RRP) and reserve pool (RP) vesicles can be probed with pool-specific secretagogues. Release from the RRP was evoked by K(+) while release from the RP was evoked by Ba(2+). Similar temperature-dependent changes in spike area and half-width for both pools suggest that the content of RRP and RP vesicles is similar and packaged in the same way. Differences between the vesicle pools were revealed in the temperature dependence of spike frequency. While the burst spike frequency of the RRP, which is comprised of pre-docked and primed vesicles, increased 2.8% per degrees C, the RP spike frequency increased 12% per degrees C. This difference is attributed to a temperature-dependent mobilization of the RP. Furthermore, the RP exhibited more foot events at room temperature than the RRP but this difference was not apparent at 37 degrees C. This trend suggests that RP vesicle membranes have a compromised surface tension compared to RRP vesicles. Collectively, the changes of release characteristics with temperature reveal distinctions between the RRP and the RP.

Pathway of membrane fusion with two tension-dependent energy barriers

Fusion of bilayer membranes is studied via dissipative particle dynamics (DPD) simulations. A new set of DPD parameters is introduced which leads to an energy barrier for flips of lipid molecules between adhering membranes. A large number of fusion events is monitored for a vesicle in contact with a planar membrane. Several time scales of the fusion process are found to depend exponentially on the membrane tension. This implies an energy barrier of about 10k(B)T for intermembrane flips and a second size-dependent barrier for the nucleation of a small hemifused membrane patch.