The association of vesicular contents and its effects on release

Vesicular release dynamics are altered by the interaction between the chemical cargo and vesicle membrane lipids

The release of the cargo from soft vesicles, an essential process for chemical delivery, is mediated by multiple factors. Among them, the regulation by the interaction between the chemical cargo species and the vesicular membrane, widely existing in all vesicles, has not been investigated to date. Yet, these interactions hold the potential to complicate the release process. We used liposomes loaded with different monoamines, dopamine (DA) and serotonin (5-HT), to simulate vesicular release and to monitor the dynamics of chemical release from isolated vesicles during vesicle impact electrochemical cytometry (VIEC). The release of DA from liposomes presents a longer release time compared to 5-HT. Modelling the release time showed that DA filled vesicles had a higher percentage of events where the time for the peak fall was better fit to a double exponential (DblExp) decay function, suggesting multiple kinetic steps in the release. By fitting to a desorption-release model, where the transmitters adsorbed to the vesicle membrane, the dissociation rates of DA and 5-HT from the liposome membrane were estimated. DA has a lower desorption rate constant, which leads to slower DA release than that observed for 5-HT, whereas there is little difference in pore size. The alteration of vesicular release dynamics due to the interaction between the chemical cargo and vesicle membrane lipids provides an important mechanism to regulate vesicular release in chemical and physiological processes. It is highly possible that this introduces a fundamental chemical regulation difference between transmitters during exocytosis.

Two Japanese pepper (Zanthoxylum piperitum) fruit-derived compounds attenuate IgE-mediated allergic response in vitro and in vivo via inhibition of mast cell degranulation

Zanthoxylum piperitum (ZP, 'Japanese pepper') is a traditional medicine and pepper used in Asian countries such as Japan. Hydroxy-α-sanshool, a pungent-tasting substance contained within ZP, has been reported to slightly suppress immunoglobulin E (IgE)-mediated mast cell degranulation. The current study aims to newly identify anti-allergic compounds derived from ZP. We examine the inhibitory mechanisms behind IgE-mediated mast cell degranulation. By inhibitory effect-guided isolation, we identified degranulation inhibitory compounds derived from ZP fruit: 1-acetoxy-7-hydroxy-3, 7-dimethylocta-2E, 5E-diene (ZP1) and 8-hydroxygeranyl acetate (ZP2). ZP1 and ZP2 inhibited IgE-mediated degranulation and A23187-mediated degranulation in RBL-2H3 mast cells. Our findings suggest the inhibition of degranulation by ZP1 and ZP2 was by inhibition of Lyn phosphorylation, followed by inhibition of intracellular Ca²⁺ mobilization, protein kinase C alpha phosphorylation, membrane ruffling, and granule-to-plasma membrane fusion. Oral administration of ZP1 or ZP2 attenuated an IgE-mediated passive cutaneous anaphylactic reaction in mice. Histological observation suggests that this effect occurred via inhibition of mast cell degranulation. These findings indicate that ZP1 and ZP2 attenuate allergic reaction via inhibition of IgE-mediated mast cell degranulation.

Intestinal and peritoneal mast cells differ in kinetics of quantal release

5-hydroxytriptamine (5-HT, serotonin) storage and release in mast cell (MC) secretory granules (SG) are dependent on serglycin proteoglycans (PG). This notion is based on the studies of MC of the connective tissue subtype that predominantly contain PG of the heparin type, whereas intestinal mucosal MC, which contain predominantly chondroitin sulfate, have been poorly explored. In the present study, we addressed the possibility that PG contents may differently affect the storage and release of preformed mediators in these two MC subclasses and explain in part their different functional properties. Rat peritoneal (PMC) and intestinal mast cells (IMC) were isolated and purified using a percoll gradient, and the efflux of 5-HT from each SG was measured by amperometric detection. IMC exhibited a ∼34% reduction in the release of 5-HT compared with PMC because of a lower number of exocytotic events, rather than a lower secretion per single exocytotic event. Amperometric spikes from IMC exhibited a slower decay phase and increased half-width but a similar ascending phase and foot parameters, indicating that the fusion pore kinetics are comparable in both MC subclasses. We conclude that both PG subtypes are equally efficient systems, directly involved in serotonin accumulation, and play a crucial role in regulating the kinetics of exocytosis from SG, providing specific secretory properties for the two cellular subtypes.

Positively charged amino acids at the SNAP-25 C terminus determine fusion rates, fusion pore properties, and energetics of tight SNARE complex zippering

SNAP-25 is a Q-SNARE protein mediating exocytosis of neurosecretory vesicles including chromaffin granules. Previous results with a SNAP-25 construct lacking the nine C terminal residues (SNAP-25Δ9) showed changed fusion pore properties (Fang et al., 2008), suggesting a model for fusion pore mechanics that couple C terminal zipping of the SNARE complex to the opening of the fusion pore. The deleted fragment contains the positively charged residues R198 and K201, adjacent to layers 7 and 8 of the SNARE complex. To determine how fusion pore conductance and dynamics depend on these residues, single exocytotic events in bovine chromaffin cells expressing R198Q, R198E, K201Q, or K201E mutants were investigated by carbon fiber amperometry and cell-attached patch capacitance measurements. Coarse grain molecular dynamics simulations revealed spontaneous transitions between a loose and tightly zippered state at the SNARE complex C terminus. The SNAP-25 K201Q mutant showed no changes compared with SNAP-25 wild-type. However, K201E, R198Q, and R198E displayed reduced release frequencies, slower release kinetics, and prolonged fusion pore duration that were correlated with reduced probability to engage in the tightly zippered state. The results show that the positively charged amino acids at the SNAP-25 C terminus promote tight SNARE complex zippering and are required for high release frequency and rapid release in individual fusion events.

Emodin attenuates A23187-induced mast cell degranulation and tumor necrosis factor-α secretion through protein kinase C and IκB kinase 2 signaling

Mast cells are known to play a pivotal role in allergic diseases. Cross-linking of the high-affinity IgE receptor (FcεRI) is known to be one of the major causes that lead to degranulation and allergic inflammation. An increase in intracellular calcium (Ca(2+)) concentration also triggers degranulation, bypassing receptor activation. Emodin (1,3,8-trihydroxy-6-methylanthraquinone) is known to exhibit a variety of pharmacological activities including anti-allergic effects. However, the detailed molecular mechanisms involved in exhibiting anti-allergic effects by emodin were remained to be clarified. In the present investigation we report the regulatory function of emodin on the allergic signal mediators through Ca(2+) ionophore activation in mast cells. Emodin significantly inhibited A23187-induced tumor necrosis factor-α production and degranulation through the attenuation of protein kinase C, IκB kinase 2, and soluble N-ethylmaleimide-sensitive fusion factor attachment protein receptor complex formation, bypassing FcεRI activation. Data from our study indicated that emodin acts by regulating multiple signaling pathways in inhibiting the allergic reactions in mast cells.

Quantal regulation and exocytosis of platelet dense-body granules

This study reports how quantal size, or the quantity of chemical messengers within a storage granule, is regulated in platelet dense-body granules via dynamic adaption of granule size according to changing levels of granule contents. Mechanistic studies using carbon-fiber microelectrode fast-scan cyclic voltammetry and amperometry methods correlated with transmission electron microscopy analysis reveal the impact of granule structural changes on granular content secretion kinetics and highlight the dynamic interplay between soluble granule contents and membrane components in exocytosis. Despite the distinct chemical profile of platelet dense-body granules, these secretory granules act according to general biochemical/biophysical phenomena using charge-charge interactions to sequester chemical messengers and employ known conserved exocytotic machinery to deliver them; therefore, the mechanistic information obtained herein further advances the general understanding of exocytosis while revealing fundamental details about blood platelets.

F-actin and myosin II accelerate catecholamine release from chromaffin granules

The roles of nonmuscle myosin II and cortical actin filaments in chromaffin granule exocytosis were studied by confocal fluorescence microscopy, amperometry, and cell-attached capacitance measurements. Fluorescence imaging indicated decreased mobility of granules near the plasma membrane following inhibition of myosin II function with blebbistatin. Slower fusion pore expansion rates and longer fusion pore lifetimes were observed after inhibition of actin polymerization using cytochalasin D. Amperometric recordings revealed increased amperometric spike half-widths without change in quantal size after either myosin II inhibition or actin disruption. These results suggest that actin and myosin II facilitate release from individual chromaffin granules by accelerating dissociation of catecholamines from the intragranular matrix possibly through generation of mechanical forces.

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.

Exocytosis of norepinephrine at axon varicosities and neuronal cell bodies in the rat brain

Norepinephrine secretion from central neurons was widely assumed to occur by exocytosis, but the essential characteristics of this process remained unknown. We developed an approach to study it directly by amperometry using carbon fiber microelectrodes in organotypic rat brainstem slice cultures. Noradrenergic neurons from areas A1 and A2 were fluorescently labeled by an adenoviral vector with noradrenergic-specific promoter. Quantal events, consistent with exocytotic release of norepinephrine, were registered at noradrenergic axonal varicosities as well as at cell bodies. According to their charge integrals, events were grouped into two populations. The majority (approximately 40 fC) were compatible with full exocytotic fusion of small clear and dense core vesicles shown in previous morphometric studies. The quantal size distribution was modulated by treatment with reserpine and amitriptyline. In addition, much larger quantal events (>1 pC) occurred at predominantly axonal release sites. The time course of signals was severalfold faster than in adrenal chromaffin cells, suggesting profound differences in the release machinery between these cell types. Tetrodotoxin eliminated the majority of events, indicating that release was partially, but not entirely, action potential driven. In conclusion, central norepinephrine release has unique characteristics, distinguishing it from those of other monoaminergic cells in periphery and brain.

Activity-dependent potentiation of large dense-core vesicle release modulated by mitogen-activated protein kinase/extracellularly regulated kinase signaling

Large dense-core vesicles (LDCVs), containing neuropeptides, hormones, and amines, play a crucial role in the activation of the sympathetic nervous system and synaptic modulation. In some secretory cells, LDCVs show activity-dependent potentiation (ADP), which represents enhancement of subsequent exocytosis, compared with the previous one. Here we report the signaling mechanism involved in ADP of LDCV release. First, ADP of LDCV release, induced by repetitive stimulation of nicotinic acetylcholine receptors (nAChRs), was augmented by increasing calcium influx, showing calcium dependence of ADP. Second, translocation of vesicles was involved in ADP. Electron microscope analysis revealed that nAChR stimulation resulted in LDCV translocation to the plasma membrane and increase of fused LDCVs in response to repetitive stimulation was observed by amperometry. Third, we provide evidence for involvement of MAPK signaling in ADP. MAPK signaling was activated by nAChR-induced calcium influx, and ADP as well as vesicle translocation was suppressed by inhibition of MAPK signaling with MAPK kinase blockers, such as PD 098059 and U0126. Fourth, PD 098059 inhibited nAChR stimulation-induced F-actin disassembly, which has been reported to control vesicle translocation. Taken together, we suggest that ADP of LDCV release is modulated by calcium-dependent activation of MAPK signaling via regulating F-actin disassembly.

Analysis of exocytotic events recorded by amperometry

Amperometry is widely used to study exocytosis of neurotransmitters and hormones in various cell types. Analysis of the shape of the amperometric spikes that originate from the oxidation of monoamine molecules released during the fusion of individual secretory vesicles provides information about molecular steps involved in stimulation-dependent transmitter release. Here we present an overview of the methodology of amperometric signal processing, including (i) amperometric signal acquisition and filtering, (ii) detection of exocytotic events and determining spike shape characteristics, and (iii) data manipulation and statistical analysis. The purpose of this review is to provide practical guidelines for performing amperometric recordings of exocytotic activity and interpreting the results based on shape characteristics of individual release events.

Real-time measurements of noradrenaline release in periphery and central nervous system

Noradrenaline (NA) plays important hormonal and neurotransmitter roles in the periphery and central nervous system, respectively. The cells that produce and release NA, namely, adrenal chromaffin cells (ACCs), sympathetic postganglionic neurones and central neurones, show both commonalities as well as profound differences in morphology, physiological function and characteristics of NA secretion. In order to address disorders which have been associated with the dysregulation of NA release, such as essential hypertension, a better understanding of the molecular mechanisms governing and modulating NA release in neurones is urgently required. Due to profound technical challenges, the molecular basis of NA release has been investigated much more thoroughly in ACCs than in neurones. This review discusses suitable approaches for detecting NA secretion in periphery as well as brain tissues. Membrane capacitance and high-resolution electrochemical measurements have proven particularly useful when combined with fluorescence microscopy. ACCs and peripheral and central NAergic neurones are compared regarding their vesicle morphologies, as well as possible locations of release sites, and the trajectory of secreted NA. Further, current views on the properties of single vesicle release events, including proposed release probabilities in these cell types, are presented.

Imaging of Zn2+ release from pancreatic beta-cells at the level of single exocytotic events

Regulated secretion of Zn2+ from isolated pancreatic beta-cells was imaged using laser-scanning confocal microscopy. In the method, beta-cells were incubated in a solution containing the novel fluorescent Zn2+ indicator FluoZin-3. Zn2+ released from the cells reacted with the dye to form a fluorescent product, which was detected by the confocal microscope. The new dye is much brighter than Zinquin, previously used for this application, allowing detection limits of 10-40 nM and temporal resolution of 16 ms/image. The high temporal resolution allowed imaging of isolated fluorescent transients that occurred at the edge of the cells following stimulation with 20 mM glucose or 40 mM K+. Fluorescent transients took 16-50 ms to reach a peak from the initial rise and returned to baseline after 170 +/- 50 ms (n = 78 transients from 15 cells). It was concluded that the transients correspond to detection of exocytotic release of Zn2+. Analysis of the temporal and spatial dispersion of the transients indicates that the release of Zn2+ is not diffusion limited but is instead kinetically controlled in agreement with previous observations of insulin release detected by amperometry.

Ceramide kinase is a mediator of calcium-dependent degranulation in mast cells

Ceramide kinase (CERK) catalyzes the conversion of ceramide to ceramide 1-phosphate (C1P) and is known to be activated by calcium. Although several groups have examined the functions of CERK and its product C1P, the functions of C1P and CERK are not understood. We studied the RBL-2H3 cell line, a widely used model for mast cells, and found that CERK and C1P are required for activation of the degranulation process in mast cells. We found that C1P formation was enhanced during activation induced by IgE/antigen or by Ca(2+) ionophore A23187. The formation of C1P required the intracellular elevation of Ca(2+). We generated RBL-2H3 cells that stably express CERK, and when these cells were treated with A23187, a concomitant C1P formation was observed and degranulation increased 4-fold, compared with mock transfectants. The cell-permeable N-acetylsphingosine (C(2)-ceramide), a poor substrate of CERK, inhibited both the formation of C1P and degranulation, indicating that C1P formation was necessary for degranulation. Exogenous introduction of CERK into permeabilized RBL-2H3 cells caused degranulation. We identified a cytosolic localization of CERK that provides exposure to cytosolic Ca(2+). Taken together, these results indicate that C1P formation is a necessary step in the degranulation pathway in RBL-2H3 cells.

The fusion pore

The secretory process requires many different steps and stages. Vesicles must be formed and transported to the target membrane. They must be tethered or docked at the appropriate sites and must be prepared for fusion (priming). As the last step, a fusion pore is formed and the contents are released. Release of neurotransmitter is an extremely rapid event leading to rise times of the postsynaptic response of less than 100 micro s. The release thus occurs during the initial formation of the exocytotic fusion pore. To understand the process of synaptic transmission, it is thus of outstanding importance to understand the molecular structure of the fusion pore, what are the properties of the initial fusion pore, how these properties affect the release process and what other factors may be limiting the kinetics of release. Here we review the techniques currently employed in fusion pore studies and discuss recent data and opinions on exocytotic fusion pore properties.