Rapid fluctuations in transmitter release from single vesicles in bovine adrenal chromaffin cells.

Single-vesicle release of catecholamines from chromaffin cells can be detected in real time as current spikes by the electrochemical method of amperometry. About 70% of spikes are preceded by a small "foot," the trickle of transmitter out of the early fusion pore. In addition, 20-50% of foot signals exhibit rapid fluctuations that we interpret as flickering of the fusion pore. There are also "stand-alone" foot signals, which may reflect transient fusions, in which the vesicles do not collapse completely into the plasma membrane. The number and frequency of the foot flickering are affected by intracellular Ca2+ concentration.     

Temporal separation of vesicle release from vesicle fusion during exocytosis

During exocytosis, vesicles in secretory cells fuse with the cellular membrane and release their contents in a Ca2+-dependent process. Release occurs initially through a fusion pore, and its rate is limited by the dissociation of the matrix-associated contents. To determine whether this dissociation is promoted by osmotic forces, we have examined the effects of elevated osmotic pressure on release and extrusion from vesicles at mast and chromaffin cells. The identity of the molecules released and the time course of extrusion were measured with fast scan cyclic voltammetry at carbon fiber microelectrodes. In external solutions of high osmolarity, release events following entry of divalent ions (Ba2+ or Ca2+) were less frequent. However, the vesicles appeared to be fused to the membrane without extruding their contents, since the maximal observed concentrations of events were less than 7% of those evoked in isotonic media. Such an isolated, intermediate fusion state, which we term "kiss-and-hold," was confirmed by immunohistochemistry at chromaffin cells. Transient exposure of cells in the kiss and hold state to isotonic solutions evoked massive release. These results demonstrate that an osmotic gradient across the fusion pore is an important driving force for exocytotic extrusion of granule contents from secretory cells following fusion pore formation.     

Aging differentially affects multiple aspects of vesicle fusion kinetics

How fusion pore formation during exocytosis affects the subsequent release of vesicle contents remains incompletely understood. It is unclear if the amount released per vesicle is dependent upon the nature of the developing fusion pore and whether full fusion and transient kiss and run exocytosis are regulated by similar mechanisms. We hypothesise that if consistent relationships exist between these aspects of exocytosis then they will remain constant across any age. Using amperometry in mouse chromaffin cells we measured catecholamine efflux during single exocytotic events at P0, 1 month and 6 months. At all ages we observed full fusion (amperometric spike only), full fusion preceded by fusion pore flickering (pre-spike foot (PSF) signal followed by a spike) and pure "kiss and run" exocytosis (represented by stand alone foot (SAF) signals). We observe age-associated increases in the size of all 3 modes of fusion but these increases occur at different ages. The release probability of PSF signals or full spikes alone doesn't alter across any age in comparison with an age-dependent increase in the incidence of "kiss and run" type events. However, the most striking changes we observe are age-associated changes in the relationship between vesicle size and the membrane bending energy required for exocytosis. Our data illustrates that vesicle size does not regulate release probability, as has been suggested, that membrane elasticity or flexural rigidity change with age and that the mechanisms controlling full fusion may differ from those controlling "kiss and run" fusion.     

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.     

Brefeldin A increases the quantal size and alters the kinetics of catecholamine release from rat adrenal chromaffin cells.

The fungal metabolite, brefeldin A (BFA), is known to inhibit guanine nucleotide exchange on the ADP-ribosylating factors that are involved in vesicle membrane trafficking. Here, we investigated the action of BFA on Ca2+-regulated exocytosis in single rat adrenal chromaffin cells. Incubation of chromaffin cells with BFA (1 or 10 microM) for 2 h effectively disrupted the Golgi membranes but did not affect the pattern of catecholamine release triggered by high extracellular K+, which was monitored with carbon fiber amperometry along with cytosolic Ca2+ measurement. The BFA treatment, however, increased the mean quantal size of catecholamine-containing vesicles and the occurrence of amperometric events with a "foot" or "stand alone" signal (which reflects sluggish or incomplete dilation of the fusion pore). To examine whether BFA altered the Ca2+-dependence of exocytosis, we employed the whole-cell recording technique in conjunction with the capacitance measurement to measure exocytosis evoked from the entire cell during voltage-gated Ca2+ entry. Our results suggested that BFA treatment did not alter either the initial rate of capacitance increase or the total amount of capacitance increase. Therefore, in chromaffin cells, BFA treatment affects Ca2+-regulated exocytosis predominantly by increasing the quantal size and by slowing the fusion kinetics of some vesicles.     

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

We have used carbon-fibre amperometry to examine the kinetics of individual secretory granule fusion/release events in bovine adrenal chromaffin cells. Transfection with plasmids encoding the light chains of botulinum neurotoxins (BoNTs) was used to investigate the effects of cleavage of syntaxin or SNAP-25 on exocytosis. Expression of BoNT/C1 or BoNT/E inhibited the extent of exocytosis that was evoked by application of digitonin/Ca(2+) to permeabilise and stimulate single chromaffin cells. Following neurotoxin expression, the residual release events were no different from those of control cells in their magnitude and kinetics from analysis of the amperometric spikes. In contrast, activation of protein kinase C (PKC) resulted in a modification of the kinetics of single granule release events. Following phorbol ester treatment, the amperometric spikes showed a significant decrease in their total charge due to a decrease in their mean half-width with increases in the rate of the initial rise and also the fall to baseline of the spikes. These changes were prevented by pre-treatment with the PKC inhibitor bisindolylmaleimide. These results suggest that PKC regulates the rate of fusion pore expansion and also subsequent pore closure or granule retrieval. A PKC-mediated regulation of kiss-and-run fusion may, therefore, control the extent of catecholamine release from single secretory granules. The experimental approach used here may provide further information on the protein constituents and regulation of the fusion pore machinery.     

Myosin II contributes to fusion pore expansion during exocytosis

During exocytosis, the fusion pore expands to allow release of neurotransmitters and hormones to the extracellular space. To understand the process of synaptic transmission, it is of outstanding importance to know the properties of the fusion pore and how these properties affect the release process. Many proteins have been implicated in vesicle fusion; however, there is little evidence for proteins involved in fusion pore expansion. Myosin II has been shown to participate in the transport of vesicles and, surprisingly, in the final phases of exocytosis, affecting the kinetics of catecholamine release in adrenal chromaffin cells as measured by amperometry. Here, we have studied single vesicle exocytosis in chromaffin cells overexpressing an unphosphorylatable form (T18AS19A RLC-GFP) of myosin II that produces an inactive protein by patch amperometry. This method allows direct determination of fusion pore expansion by measuring its conductance, whereas the release of catecholamines is recorded simultaneously by amperometry. Here we demonstrated that the fusion pore is of critical importance to control the release of catecholamines during single vesicle secretion in chromaffin cells. We proved that myosin II acts as a molecular motor on the fusion pore expansion by hindering its dilation when it lacks the phosphorylation sites.     

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.     

cis-Unsaturated fatty acids induce the fusion of chromaffin granules aggregated by synexin

When isolated chromaffin granules were aggregated by synexin (a Ca2+-binding protein present in chromaffin and other secretory tissues) and then exposed to cis-unsaturated fatty acids at 37 degrees C, they fused together to form large vesicles. The fusion was monitored by phase and electron microscopy and by turbidity measurements on the granule suspension. Arachidonic acid was the most effective fusogen, whereas trans-unsaturated fatty acids, saturated fatty acids, detergents or lysolecithin were inactive. During fusion some of the epinephrine of the granules was released but the soluble core proteins remained trapped in the resulting vesicles. These vesicles swelled to enclose the maximum volume. Although this swelling could be inhibited by increasing the osmotic strength of the medium, it did not appear to depend on the chemiosmotic properties of the granule membranes as it was not influenced by ATP, a proton ionophore, or an anion transport inhibitor. The regulators of this in vitro fusion--Ca2+, synexin, and free, cis-unsaturated fatty acids--may be present in the cytoplasm of the chromaffin cell when it is stimulated to release epinephrine and granule proteins by exocytosis. Therefore, this fusion event may be the same that occurs between chromaffin granules undergoing compound exocytosis.     

Release of small transmitters through kiss-and-run fusion pores in rat pancreatic beta cells

Exocytosis of secretory vesicles begins with a fusion pore connecting the vesicle lumen to the extracellular space. This pore may then expand or it may close to recapture the vesicle intact. The contribution of the latter, termed kiss-and-run, to exocytosis of pancreatic beta cell large dense-core vesicles (LDCVs) is controversial. Examination of single vesicle fusion pores demonstrated that rat beta cell LDCVs can undergo exocytosis by rapid pore expansion, by the formation of stable pores, or via small transient kiss-and-run fusion pores. Elevation of cAMP shifted LDCV fusion pore openings to the transient mode. Under this condition, the small fusion pores were sufficient for release of ATP, stored within LDCVs together with insulin. Individual ATP release events occurred coincident with amperometric "stand alone feet" representing kiss-and-run. Therefore, the LDCV kiss-and-run fusion pores allow small transmitter release but likely retain the larger insulin peptide. This may represent a mechanism for selective intraislet signaling.     

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.     

Exocytosis of single chromaffin granules in cell-free inside-out membrane patches

In chromaffin cells, exocytosis of single granules and properties of the fusion pore--the first connection between vesicular lumen and extracellular space --can be studied by cell-attached patch amperometry, which couples patch-clamp capacitance measurements with simultaneous amperometric recordings of transmitter release. Here we have studied exocytosis of single chromaffin granules and endocytosis of single vesicles in cell-free inside-out membrane patches by patch capacitance measurements and patch amperometry. We excised patches from chromaffin cells by using methods developed for studying properties of single ion channels. With low calcium concentrations in the pipette and bath, the patches showed no spontaneous exocytosis, but exocytosis could be induced in some patches by applying calcium to the cytoplasmic side of the patch. Exocytosis was also stimulated by calcium entry through the patch membrane. Initial conductances of the fusion pore were undistinguishable in cell-attached and excised patch recordings, but the subsequent pore expansion was slower in excised patches. The properties of exocytotic fusion pores in chromaffin cells are very similar to those observed in mast cells and granulocytes. Excised patches provide a tool with which to study the mechanisms of fusion pore formation and endocytosis in vitro.     

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.     

Exocytosis of catecholamine (CA)-containing and CA-free granules in chromaffin cells

Recent evidence suggests that endocytosis in neuroendocrine cells and neurons can be tightly coupled to exocytosis, allowing rapid retrieval from the plasma membrane of fused vesicles for future use. This can be a much faster mechanism for membrane recycling than classical clathrin-mediated endocytosis. During a fast exo-endocytotic cycle, the vesicle membrane does not fully collapse into the plasma membrane; nevertheless, it releases the vesicular contents through the fusion pore. Once the vesicle is depleted of transmitter, its membrane is recovered without renouncing its identity. In this report, we show that chromaffin cells contain catecholamine-free granules that retain their ability to fuse with the plasma membrane. These catecholamine-free granules represent 7% of the total population of fused vesicles, but they contributed to 47% of the fusion events when the cells were treated with reserpine for several hours. We propose that rat chromaffin granules that transiently fuse with the plasma membrane preserve their exocytotic machinery, allowing another round of exocytosis.     

Relationship between amperometric pre-spike feet and secretion granule composition in chromaffin cells: an overview

Amperometry is a simple and powerful technique to study exocytosis at the single cell level. By positioning and polarizing (at an appropriate potential at which the molecules released by the cell can be oxidized) a carbon fiber microelectrode at the top of the cell, each exocytotic event is detected as an amperometric spike. More particularly, a portion of these spikes has previously been shown to present a foot, i.e. a small pedestal of current that precedes the spike itself. Among the important number of works dealing with the monitoring of exocytosis by amperometry under different conditions, only a few studies focus on amperometric spikes with a foot. In this work, by coupling our previous and recent experiments on chromaffin cells (that release catecholamines after stimulation) with literature data, we bring more light on what an amperometric foot and particularly its features, represents.     

Neurotransmitter release: fusion or 'kiss-and-run'?

The clear synaptic vesicles of neurons release their contents at the presynaptic membrane and are then quickly retrieved. However, it is unclear whether a complete cycle of exocytosis and endocytosis is always involved or whether neurotransmitter can be released by a transient interaction. Recent findings in chromaffin and mast cells suggest that exocytosis is preceded by the formation of a pore that has similar conductance properties to ion channels. The content of the secretory organelle partially escapes at this early step, but the pore can close before the vesicle fuses fully. This article looks at the evidence that quantal release of neurotransmitter from clear synaptic vesicles may occur by a similar 'kiss-and-run' mechanism.     

Regulation of exocytosis by protein kinases and Ca(2+) in pancreatic duct epithelial cells

We asked if the mechanisms of exocytosis and its regulation in epithelial cells share features with those in excitable cells. Cultured dog pancreatic duct epithelial cells were loaded with an oxidizable neurotransmitter, dopamine or serotonin, and the subsequent release of these exogenous molecules during exocytosis was detected by carbon-fiber amperometry. Loaded cells displayed spontaneous exocytosis that may represent constitutive membrane transport. The quantal amperometric events induced by fusion of single vesicles had a rapid onset and decay, resembling those in adrenal chromaffin cells and serotonin-secreting leech neurons. Quantal events were frequently preceded by a "foot," assumed to be leak of transmitters through a transient fusion pore, suggesting that those cell types share a common fusion mechanism. As in neurons and endocrine cells, exocytosis in the epithelial cells could be evoked by elevating cytoplasmic Ca(2+) using ionomycin. Unlike in neurons, hyperosmotic solutions decreased exocytosis in the epithelial cells, and giant amperometric events composed of many concurrent quantal events were observed occasionally. Agents known to increase intracellular cAMP in the cells, such as forskolin, epinephrine, vasoactive intestinal peptide, or 8-Br-cAMP, increased the rate of exocytosis. The forskolin effect was inhibited by the Rp-isomer of cAMPS, a specific antagonist of protein kinase A, whereas the Sp-isomer, a specific agonist of PKA, evoked exocytosis. Thus, PKA is a downstream effector of cAMP. Finally, activation of protein kinase C by phorbol-12-myristate-13-acetate also increased exocytosis. The PMA effect was not mimicked by the inactive analogue, 4alpha-phorbol-12,13-didecanoate, and it was blocked by the PKC antagonist, bisindolylmaleimide I. Elevation of intracellular Ca(2+) was not needed for the actions of forskolin or PMA. In summary, exocytosis in epithelial cells can be stimulated directly by Ca(2+), PKA, or PKC, and is mediated by physical mechanisms similar to those in neurons and endocrine cells.     

A rapid exocytosis mode in chromaffin cells with a neuronal phenotype

We have used astrocyte-conditioned medium (ACM) to promote the transdifferentiation of bovine chromaffin cells and study modifications in the exocytotic process when these cells acquire a neuronal phenotype. In the ACM-promoted neuronal phenotype, secretory vesicles and intracellular Ca2+ rise were preferentially distributed in the neurite terminals. Using amperometry, we observed that the exocytotic events also occurred mainly in the neurite terminals, wherein the individual exocytotic events had smaller quantal size than in undifferentiated cells. Additionally, duration of pre-spike current was significantly shorter, suggesting that ACM also modifies the fusion pore stability. After long exposure (7-9 days) to ACM, the kinetics of catecholamine release from individual vesicles was markedly accelerated. The morphometric analysis of vesicle diameters suggests that the rapid exocytotic events observed in neurites of ACM-treated cells correspond to the exocytosis of large dense-core vesicles (LDCV). On the other hand, experiments performed in EGTA-loaded cells suggest that ACM treatment promotes a better coupling between voltage-gated calcium channels (VGCC) and LDCV. Thus, our findings reveal that ACM promotes a neuronal phenotype in chromaffin cells, wherein the exocytotic kinetics is accelerated. Such rapid exocytosis mode could be caused at least in part by a better coupling between secretory vesicles and VGCC.     

Synapsin II negatively regulates catecholamine release

We have assessed the role of synapsins in catecholamine release by comparing the properties of exocytosis in adrenal chromaffin cells from wild-type and synapsin triple knock-out (TKO) mice. Brief depolarizations led to a greater amount of catecholamine release in chromaffin cells from TKO mice in comparison to chromaffin cells from wild-type mice. This increase in catecholamine release was due to an increased number of exocytotic events, while the properties of individual quanta of released catecholamine were unchanged. Barium ions produced similar amounts of catecholamine release from TKO and wild-type chromaffin cells, suggesting that the reserve pool of chromaffin granules is unchanged following loss of synapsins. Because expression of synapsin IIa in TKO chromaffin cells rescued the defect in depolarization-induced exocytosis, the TKO phenotype apparently results from loss of synapsin IIa. We conclude that synapsin IIa serves as a negative regulator of catecholamine release and that this protein influences exocytosis from a readily releasable pool of chromaffin granules. Further, because these defects in catecholamine release are different from those observed for glutamate and GABA release in TKO mice, we conclude that the functions of synapsins differ for vesicles containing different types of neurotransmitters.