High calcium concentrations shift the mode of exocytosis to the kiss-and-run mechanism

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.

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.

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.

An inhibitory effect of extracellular Ca2+ on Ca2+-dependent exocytosis

Aim

Neurotransmitter release is elicited by an elevation of intracellular Ca²⁺ concentration ([Ca²⁺]_i). The action potential triggers Ca²⁺ influx through Ca²⁺ channels which causes local changes of [Ca²⁺]_i for vesicle release. However, any direct role of extracellular Ca²⁺ (besides Ca²⁺ influx) on Ca²⁺-dependent exocytosis remains elusive. Here we set out to investigate this possibility on rat dorsal root ganglion (DRG) neurons and chromaffin cells, widely used models for studying vesicle exocytosis.

Results

Using photolysis of caged Ca²⁺ and caffeine-induced release of stored Ca²⁺, we found that extracellular Ca²⁺ inhibited exocytosis following moderate [Ca²⁺]_i rises (2–3 µM). The IC₅₀ for extracellular Ca²⁺ inhibition of exocytosis (ECIE) was 1.38 mM and a physiological reduction (∼30%) of extracellular Ca²⁺ concentration ([Ca²⁺]_o) significantly increased the evoked exocytosis. At the single vesicle level, quantal size and release frequency were also altered by physiological [Ca²⁺]_o. The calcimimetics Mg²⁺, Cd²⁺, G418, and neomycin all inhibited exocytosis. The extracellular Ca²⁺-sensing receptor (CaSR) was not involved because specific drugs and knockdown of CaSR in DRG neurons did not affect ECIE.

Conclusion/Significance

As an extension of the classic Ca²⁺ hypothesis of synaptic release, physiological levels of extracellular Ca²⁺ play dual roles in evoked exocytosis by providing a source of Ca²⁺ influx, and by directly regulating quantal size and release probability in neuronal cells.

Temporal characteristics of vesicular fusion in astrocytes: examination of synaptobrevin 2-laden vesicles at single vesicle resolution

Astrocytes can release various gliotransmitters in response to stimuli that cause increases in intracellular Ca(2+) levels; this secretion occurs via a regulated exocytosis pathway. Indeed, astrocytes express protein components of the vesicular secretory apparatus. However, the detailed temporal characteristics of vesicular fusions in astrocytes are not well understood. In order to start addressing this issue, we used total internal reflection fluorescence microscopy (TIRFM) to visualize vesicular fusion events in astrocytes expressing the fluorescent synaptobrevin 2 derivative, synapto-pHluorin. Although our cultured astrocytes from visual cortex express synaptosome-associated protein of 23 kDa (SNAP23), but not of 25 kDa (SNAP25), these glial cells exhibited a slow burst of exocytosis under mechanical stimulation; the expression of SNAP25B did not affect bursting behaviour. The relative amount of two distinct types of events observed, transient and full fusions, depended on the applied stimulus. Expression of exogenous synaptotagmin 1 (Syt1) in astrocytes endogenously expressing Syt4, led to a greater proportion of transient fusions when astrocytes were stimulated with bradykinin, a stimulus otherwise resulting in more full fusions. Additionally, we studied the stability of the transient fusion pore by measuring its dwell time, relation to vesicular size, flickering and decay slope; all of these characteristics were secretagogue dependent. The expression of SNAP25B or Syt1 had complex effects on transient fusion pore stability in a stimulus-specific manner. SNAP25B obliterated the appearance of flickers and reduced the dwell time when astrocytes were mechanically stimulated, while astrocytes expressing SNAP25B and stimulated with bradykinin had a reduction in decay slope. Syt1 reduced the dwell time when astrocytes were stimulated either mechanically or with bradykinin. Our detailed study of temporal characteristics of astrocytic exocytosis will not only aid the general understanding of this process, but also the interpretation of the events at the tripartite synapse, both in health and disease.

Ca(v)2.3 Ca2+ channel interacts with the G1-subunit of V-ATPase

BACKGROUND

Calcium channels are essential in coupling action potential to signal transduction in cells. There are several types of calcium channels, which can be pharmacologically classified as L-, N-, P/Q-, R- and T-type. But molecular basis of R-type channels is less clearly understood compared the other channel types. Therefore the current study aims at understanding the molecular functions of R-type calcium channels by identifying interaction partners of the channel.

METHODS

In order to do so, a yeast two hybrid (Y2H) screen, with carboxy terminus of α1 subunit of the channel, as the bait, was performed. G1 subunit of v-ATPase was identified as a putative interaction partner of human Ca(v)2.3 by using the Y2H screening. The interaction was confirmed by immunoprecipitation. To study the functional importance of the interaction, bafilomycin A(1), a potent and specific inhibitor of v-ATPase was used in patch-clamp recordings in Ca(v)2.3 stably-transfected HEK-293 cells (2C6) as well as in electroretinography of the isolated bovine retina expressing R-type Ca(2+) channels.

RESULTS

G1 subunit of v-ATPase interacts with C-terminal tail of Ca(v)2.3 and bafilomycin A(1) reduces Ca(v)2.3 mediated calcium currents. Additionally peak I(Ca) is inhibited in retinal signal transduction when recorded as ERG b-wave.

CONCLUSIONS

The results suggest that v-ATPase interacts physically and also functionally with Ca(v)2.3. This is the first demonstration of association of Ca(v)2.3 C-terminus with a protein complex which is involved in transmembrane signalling.

Role of zonadhesin during sperm-egg interaction: a species-specific acrosomal molecule with multiple functions

Sperm-zona adhesion is an essential event in mammalian fertilization, failure of which causes sterility. However, the molecular mechanisms involved in this process are still poorly understood. It has been suggested by few laboratories studying gamete interaction that acrosomal molecules are implicated in sperm-zona pellucida adhesion prior to the acrosome reaction (AR). Zonadhesin, a sperm-specific protein located in the acrosome is critically involved in zona binding. Here we describe the cellular and molecular interaction of zonadhesin during fertilization and also discuss its role in species-specific gamete interaction--an intriguing question in biology. We propose a model in which sperm could transiently expose acrosomal molecules that adhere to the zona independently of the AR in a 'kiss and run' mechanism. This could be a valuable framework for further investigations and a detailed understanding of the molecular events during gamete adhesion is likely to provide new approaches for the design of more effective male contraceptives and better diagnostic methods for sperm dysfunction.

Protein quantification at the single vesicle level reveals that a subset of synaptic vesicle proteins are trafficked with high precision

Protein sorting represents a potential point of regulation in neurotransmission because it dictates the protein composition of synaptic vesicles, the organelle that mediates transmitter release. Although the average number of most vesicle proteins has been estimated using bulk biochemical approaches (Takamori et al., 2006), no information exists on the intervesicle variability of protein number, and thus on the precision with which proteins are sorted to vesicles. To address this, we adapted a single molecule quantification approach (Mutch et al., 2007) and used it to quantify both the average number and variance of seven integral membrane proteins in brain synaptic vesicles. We report that four vesicle proteins, SV2, the proton ATPase, Vglut1, and synaptotagmin 1, showed little intervesicle variation in number, indicating they are sorted to vesicles with high precision. In contrast, the apparent number of VAMP2/synaptobrevin 2, synaptophysin, and synaptogyrin demonstrated significant intervesicle variability. These findings place constraints on models of protein function at the synapse and raise the possibility that changes in vesicle protein expression affect vesicle composition and functioning.

Modes of exocytotic and endocytotic events in tobacco BY-2 protoplasts

To analyze the kinetics and size of single exo- and endocytotic events in BY-2 protoplasts, we employed cell-attached membrane capacitance measurements. These measurements revealed different modes of fusion and fission of single vesicles. In about half of the observed exocytotic events, fusion occurred transiently, which facilitates rapid recycling of vesicles. In addition, transient sequential or multi-vesicular exocytosis observed in some recordings can contribute to an increase in efficiency of secretory product release. Microscopic analysis of the timescale of cellulose and pectin deposition in protoplasts demonstrates that rebuilding of the cell wall starts soon after isolation of protoplasts and that transient fusion events can fully account for secretion of the required soluble material. The capacitance measurements also allowed us to investigate formation of the fusion pore. We speculate that regulation of secretion may involve control of the length and/or size of fusion pore opening. Together, the different kinetic modes of exo- and endocytosis revealed by capacitance measurements underline the complexity of this process in plants and provide a basis for future research into the underlying mechanisms. The fact that similar fusion/fission kinetics are present in plant and animal cells suggests that many of these mechanisms are highly conserved among eukaryotes.

Frequency-dependent modes of synaptic vesicle endocytosis and exocytosis at adult mouse neuromuscular junctions

During locomotion, adult rodent lumbar motoneurons fire in high-frequency (80-100 Hz) 1-2 s bursts every several seconds, releasing between 10,000 and 20,000 vesicles per burst. The estimated total vesicle pool size indicates that all vesicles would be used within 30 s; thus, a mechanism for rapid endocytosis and vesicle recycling is necessary to maintain effective transmission and motor behavior. However, whether such rapid recycling exists at mouse neuromuscular junctions (NMJs) or how it is regulated has been unclear. Here, we show that much less FM1-43 dye is lost per stimulus with 100 Hz stimulation than with 10 Hz stimulation even when the same number of vesicles undergo exocytosis. Electrophysiological data using folimycin show this lesser amount of dye loss is caused in part by the rapid reuse of vesicles. We showed previously that a myosin light chain kinase (MLCK)-myosin II pathway was required for effective transmission at 100 Hz. Here, we confirm the activation of MLCK, based on increased nerve terminal phospho-MLC immunostaining, with 100 Hz but not with 10 Hz stimulation. We further demonstrate that activation of MLCK, by increased extracellular Ca(2+), by PKC (protein kinase C) activation, or by a MLCK agonist peptide, reduces the amount of dye lost even with 10 Hz stimulation. MLCK activation at 10 Hz also resulted in more vesicles being rapidly reused. Thus, MLCK activation by 100 Hz stimulation switches the mechanism of vesicle cycling to a rapid-reuse mode and is required to sustain effective transmission in adult mouse NMJs.

Three distinct modes of exocytosis revealed by amperometry in neuroendocrine cells

Neurotransmission requires Ca(2+)-dependent release of secretory products through fusion pores that open and reclose (partial membrane distention) or open irreversibly (complete membrane distention). It has been challenging to distinguish between these release modes; however, in the work presented here, we were able to deduce different modes of depolarization-evoked exocytosis in neuroendocrine chromaffin and PC12 cells solely by analyzing amperometric recordings. After we determined the quantal size (Q), event half-width (t(50)), event amplitude (I(peak)), and event decay time constant (τ(decay)), we fitted scatter plots of log-transformed data with a mixture of one- and two-dimensional Gaussian distributions. Our analysis revealed three distinct and differently shaped clusters of secretory events, likely corresponding to different modes of exocytosis. Complete membrane distention, through fusion pores of widely varying conductances, accounted for 70% of the total amount of released catecholamine. Two different kinds of partial membrane distention (kiss-and-run and kiss-and-stay exocytosis), characterized by mode-specific fusion pores with unitary conductances, accounted for 20% and 10%, respectively. These results show that our novel one- and two-dimensional analysis of amperometric data reveals new release properties and enables one to distinguish at least three different modes of exocytosis solely by analyzing amperometric recordings.

Intravesicular factors controlling exocytosis in chromaffin cells

Chromaffin granules are similar organelles to the large dense core vesicles (LDCV) present in many secretory cell types including neurons. LDCV accumulate solutes at high concentrations (catecholamines, 0.5-1 M; ATP, 120-300 mM; or Ca(2+), 40 mM (Bulenda and Gratzl Biochemistry 24:7760-7765, 1985). Solutes seem to aggregate to a condensed matrix to elude osmotic lysis. The affinity of solutes for LDCV matrix is responsible for the delayed release of catecholamines during exocytosis. The aggregation of solutes occurs due to a specific H(+) pump denominated V-ATPase that maintains an inner acidic media (pH ≈5.5). This pH gradient against cytosol is also responsible for the vesicular accumulation of amines and Ca(2+). When this gradient is reduced by modulation of the V-ATPase activity, catecholamines and Ca(2+) are moved toward the cytosol. In addition, some drugs largely accumulate inside LDCV and not only impair the accumulation of natural solutes, but also act as false neurotransmitters when they are co-released with catecholamines. There is much experimental evidence to conclude that the physiological modulation of vesicle pH and the manipulation of intravesicular media with drugs affect the LDCV cargo and change the kinetics of exocytosis. Here, we will present some experimental data demonstrating the participation of drugs in the kinetics of exocytosis through changes in the composition of vesicular media. We also offer a model to explain the regulation of exocytosis by the intravesicular media that conciliate the experimentally obtained data.

Rapid endocytosis and vesicle recycling in neuroendocrine cells

Endocytosis is a crucial process for neuroendocrine cells that ensures membrane homeostasis, vesicle recycling, and hormone release reliability. Different endocytic mechanisms have been described in chromaffin cells, such as clathrin-dependent slow endocytosis and clathrin-independent rapid endocytosis. Rapid endocytosis, classically measured in terms of a fast decrease in membrane capacitance, exhibits two different forms, "rapid compensatory endocytosis" and "excess retrieval." While excess retrieval seems to be associated with formation of long-lasting endosomes, rapid compensatory endocytosis is well correlated with exocytotic activity, and it is regarded as a mechanism associated to rapid vesicle recycling during normal secretory activity. It has been suggested that rapid compensatory endocytosis may be related to the prevalence of a transient fusion mode of exo-endocytosis. In the latter mode, the fusion pore, a nanometric-sized channel formed at the onset of exocytosis, remains open for a few hundred milliseconds and later abruptly closes, releasing a small amount of transmitters. By this mechanism, endocrine cell selectively releases low molecular weight transmitters, and rapidly recycles the secretory vesicles. In this article, we discuss the cellular and molecular mechanisms that define the different forms of exocytosis and endocytosis and their impact on vesicle recycling pathways.

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.

Hormonal inhibition of endocytosis: novel roles for noradrenaline and G protein G(z)

The modulation of endocytosis following exocytosis by noradrenaline (NA), a physiological inhibitor of insulin secretion, was investigated in INS 832/13 cells using patch-clamp capacitance measurements. Endocytosis was inhibited by NA in a pertussis toxin-insensitive manner. Dialysing a synthetic peptide mimicking the C-terminus of the α-subunit of G(z) into the cells blocked the inhibition of endocytosis by NA. Cell-attached capacitance measurements indicated that inhibition by NA was due to a decreased number of endocytotic events without a change in vesicle size. Analysis of fission pore closure kinetics revealed two distinct fission modes, with NA selectively inhibiting the rapid fission pore closure events. Comparison of the actions of NA and deltamethrin, a calcineurin antagonist and potent inhibitor of endocytosis, demonstrated that they inhibit endocytosis by different mechanisms. These findings establish novel actions for NA and G(z) in insulin-secreting cells and possibly other cell types.

Only a Fraction of Quantal Content is Released During Exocytosis as Revealed by Electrochemical Cytometry of Secretory Vesicles

The primary method for neuronal communication involves the release of chemical messengers that are packaged intracellularly in vesicles. Although experiments measuring release at single cells have classically been thought to assess the entire content of vesicles, there is evidence in the literature that suggests that the total transmitter stored in vesicles is not expelled during exocytosis. In this work, we introduce a novel technology using a microfluidic-based platform to electrochemically probe individual PC12 cell vesicles isolated from the cell environment. We measure the total vesicular content using methodology that circumvents the biophysical processes of the cell associated with exocytosis. Direct comparisons of amperometric data from release experiments at single PC12 cells versus our cell-free model reveal that on average vesicles release only 40% of their total transmitter load. The data support the intriguing hypothesis that the average vesicle does not open all the way during the normal exocytosis process, resulting in incomplete distention of the vesicular contents. In addition, we have shown that vesicular catecholamine levels can be altered with pharmacological manipulation and variances observed from these treatments can be resolved at the single vesicle level in a high-throughput manner, a process that we have termed electrochemical cytometry. Upon establishing that release in exocytotic processes proceeds in an incomplete manner, electrochemical data quantified from both single cell release experiments and electrochemical cytometry of vesicles were related to vesicular volume from electron microscopy measurements to investigate the location of intravesicular catecholamine stores retained post-fusion.

Suppression of Ca2+ syntillas increases spontaneous exocytosis in mouse adrenal chromaffin cells

A central concept in the physiology of neurosecretion is that a rise in cytosolic [Ca²⁺] in the vicinity of plasmalemmal Ca²⁺ channels due to Ca²⁺ influx elicits exocytosis. Here, we examine the effect on spontaneous exocytosis of a rise in focal cytosolic [Ca²⁺] in the vicinity of ryanodine receptors (RYRs) due to release from internal stores in the form of Ca²⁺ syntillas. Ca²⁺ syntillas are focal cytosolic transients mediated by RYRs, which we first found in hypothalamic magnocellular neuronal terminals. (scintilla, Latin for spark; found in nerve terminals, normally synaptic structures.) We have also observed Ca²⁺ syntillas in mouse adrenal chromaffin cells. Here, we examine the effect of Ca²⁺ syntillas on exocytosis in chromaffin cells. In such a study on elicited exocytosis, there are two sources of Ca²⁺: one due to influx from the cell exterior through voltage-gated Ca²⁺ channels, and that due to release from intracellular stores. To eliminate complications arising from Ca²⁺ influx, we have examined spontaneous exocytosis where influx is not activated. We report here that decreasing syntillas leads to an increase in spontaneous exocytosis measured amperometrically. Two independent lines of experimentation each lead to this conclusion. In one case, release from stores was blocked by ryanodine; in another, stores were partially emptied using thapsigargin plus caffeine, after which syntillas were decreased. We conclude that Ca²⁺ syntillas act to inhibit spontaneous exocytosis, and we propose a simple model to account quantitatively for this action of syntillas.

The molecular physiology of activity-dependent bulk endocytosis of synaptic vesicles

Central nerve terminals release neurotransmitter in response to a wide variety of stimuli. Because maintenance of neurotransmitter release is dependent on the continual supply of synaptic vesicles (SVs), nerve terminals possess an array of endocytosis modes to retrieve and recycle SV membrane and proteins. During mild stimulation conditions, single SV retrieval modes such as clathrin-mediated endocytosis predominate. However, during increased neuronal activity, additional SV retrieval capacity is required, which is provided by activity-dependent bulk endocytosis (ADBE). ADBE is the dominant SV retrieval mechanism during elevated neuronal activity. It is a high capacity SV retrieval mode that is immediately triggered during such stimulation conditions. This review will summarize the current knowledge regarding the molecular mechanism of ADBE, including molecules required for its triggering and subsequent steps, including SV budding from bulk endosomes. The molecular relationship between ADBE and the SV reserve pool will also be discussed. It is becoming clear that an understanding of the molecular physiology of ADBE will be of critical importance in attempts to modulate both normal and abnormal synaptic function during intense neuronal activity.

Biophysical characterization of styryl dye-membrane interactions

Styryl dyes (also referred to as FM dyes) become highly fluorescent upon binding to membranes and are often used to study synaptic vesicle recycling in neurons. To date, however, no direct comparisons of the fluorescent properties, or time-resolved (millisecond) measurements of dye-membrane binding and unbinding reactions, for all members of this family of probes have been reported. Here, we compare the fluorescence intensities of each member of the FM dye family when bound to membranes. This analysis included SGC5, a new lipophilic fluorescent dye with a unique structure. Fluorescence intensities depended on the length of the lipophilic tail of each dye, with a rank order as follows: SGC5 > FM1-84 > FM1-43 > SynaptoGreen C3 > FM2-10/FM4-64/FM5-95. Stopped-flow measurements revealed that dye hydrophobicity determined the affinity and departitioning rates for dye-membrane interactions. All of the dyes dissociated from membranes on the millisecond timescale, which is orders of magnitude faster than the overall destaining rate (timescale of seconds) of these dyes from presynaptic boutons. Departitioning kinetics were faster at higher temperatures, but were unaffected by pH or cholesterol. The data reported here aid interpretation of dye-release kinetics from single synaptic vesicles, and indicate that these probes dissociate from membranes on more rapid timescales than previously appreciated.

Calcium dependence of exo- and endocytotic coupling at a glutamatergic synapse

The mechanism coupling exocytosis and endocytosis remains to be elucidated at central synapses. Here, we show that the mechanism linking these two processes is dependent on microdomain-[Ca2+](i) similar to that which triggers exocytosis, as well as the exocytotic protein synaptobrevin/VAMP. Furthermore, block of endocytosis has a limited, retrograde action on exocytosis, delaying recruitment of release-ready vesicles and enhancing short-term depression. This effect sets in so rapidly that it cannot be explained by the nonavailability of recycled vesicles. Rather, we postulate that perturbation of a step linking exocytosis and endocytosis temporarily prevents new vesicles from docking at specialized sites for exocytosis.

Secretory control: evidence for agonist regulation of post-fusion vesicle behaviour

1. Here, we review recent work on vesicular secretion, with a focus on the control of post-fusion events as a means of regulating secretory output. 2. In the classical model of secretion, each fused vesicle releases the entirety of its content in an all-or-none manner. In this way, the secretory output of a cell is controlled by regulating the numbers of fused vesicles. The realisation that post-fusion events can control secretory output leads to a distinct model of partial release of vesicle content. 3. Recent work shows that post-fusion events are under cellular control. Further, new data from our laboratory demonstrates agonist-dependent regulation of fusion pore behaviour. 4. We conclude that post-fusion events are not epiphenomena, but are likely an important mechanism of secretory control.

Loose coupling between calcium channels and sites of exocytosis in chromaffin cells

Calcium microdomains generated by tight clusters of calcium channels regulate fusion of small vesicles at the synaptic terminal and have also been suggested to trigger exocytosis of large dense-core vesicles from neuroendocrine cells. To test this idea, we have compared sites of exocytosis and the spatial distribution of calcium channels in chromaffin cells. Fusion of individual vesicles was visualized using interference reflection microscopy and the submembranous calcium signal was assessed using total internal reflection fluorescence microscopy. Depolarization triggered a burst of exocytosis from up to seven sites in a membrane area of 11 microm(2), but these sites did not colocalize with calcium microdomains. Instead, calcium influx occurred in large patches (averaging 34 microm(2)) containing a mixture of P/Q- and N-type channels. About 20% of fusion events occurred outside calcium channel patches. Further, the delay between the onset of stimulation and a burst of exocytosis was prolonged for several seconds by increasing the concentration of the slow calcium chelator EGTA from 1.5 to 5 mM. These results demonstrate that while calcium channels and release sites tend to congregate in specialized regions of the surface membrane, these have dimensions of several micrometres. The dominant calcium signal regulating release in chromaffin cells is generated by the cooperative action of many channels operating over distances of many micrometres rather than discrete clusters of calcium channels generating localized microdomains.

Differences in the quantal release of catecholamines in chromaffin cells of rat embryos and their mothers

Studies on the bulk catecholamine release from fetal and neonatal rat adrenals, adrenal slices, or isolated chromaffin cells stimulated with high K+, hypoxia, hypercapnia, or acidosis are available. However, a study analyzing the kinetics of quantal secretion is lacking. We report here such a study in which we compare the quantal release of catecholamines from immature rat embryo chromaffin cells (ECCs) and their mothers' (MCCs). Cell challenging with a strong depolarizing stimulus (75 mM K+) caused spike bursts having the following characteristics. ECCs released more multispike events and wave envelopes than MCCs. This, together with narrower single-spike events, a faster decay, and a threefold smaller quantal size suggest a faster secretory machinery in ECCs. Furthermore, with a milder stimulus (25 mM K+) enhanced Ca2+ entry by L-type Ca2+ channel activator BAY K 8644 did not change the kinetic parameters of single spikes in ECCs; in contrast, augmentation of Ca2+ entry increased spike amplitude and width, quantal size, and decay time in MCCs. This suggests that in mature MCCs, the last exocytotic steps are more tightly regulated than in immature ECCs. Finally, we found that quantal secretion was fully controlled by L-type voltage-dependent Ca2+ channels (VDCCs) in ECCs, whereas both L- and non-L VDCCs (N and PQ) contributed equally to secretion control in MCCs. Our results have the following physiological, pharmacological, and clinical relevance: 1) they may help to better understand the regulation of adrenal catecholamine release in response to stress during fetal life and delivery; 2) if clinically used, L-type Ca2+ channel blockers may augment the incidence of sudden infant death syndrome (SIDS); and 3) so-called Ca2+ promotors or activators of Ca2+ entry through L-type VDCCs may be useful to secure a healthy catecholamine surge upon violent stress during fetal life, at birth, or to prevent the SIDS in neonates at risk.

Exocytosis of post-Golgi vesicles is regulated by components of the endocytic machinery

Post-Golgi vesicles target and deliver most biosynthetic cargoes to the cell surface. However, the molecules and mechanisms involved in fusion of these vesicles are not well understood. We have employed a system to simultaneously monitor release of luminal and membrane biosynthetic cargoes from individual post-Golgi vesicles. Exocytosis of these vesicles is not calcium triggered. Release of luminal cargo can be accompanied by complete, partial, or no release of membrane cargo. Partial and no release of membrane cargo of a fusing vesicle are fates associated with kiss-and-run exocytosis, and we find that these are the predominant mode of post-Golgi vesicle exocytosis. Partial cargo release by post-Golgi vesicles occurs because of premature closure of the fusion pore and is modulated by the activity of clathrin, actin, and dynamin. Our results demonstrate that these components of the endocytic machinery modulate the nature and extent of biosynthetic cargo delivery by post-Golgi vesicles at the cell membrane.

Activity-dependent bulk synaptic vesicle endocytosis--a fast, high capacity membrane retrieval mechanism

Central nerve terminals are placed under considerable stress during intense stimulation due to large numbers of synaptic vesicles (SVs) fusing with the plasma membrane. Classical clathrin-dependent SV endocytosis cannot correct for the large increase in nerve terminal surface area in the short term, due to its slow kinetics and low capacity. During such intense stimulation, an additional SV retrieval pathway is recruited called bulk endocytosis. Recent studies have shown that bulk endocytosis fulfils all of the physiological requirements to remedy the acute changes in nerve terminal surface area to allow the nerve terminal to continue to function. This review will summarise the recent developments in the field that characterise the physiology of bulk endocytosis which show that it is a fast, activity-dependent and high capacity mechanism that is essential for the function of central nerve terminals.

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.

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

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

Short-term plasticity of small synaptic vesicle (SSV) and large dense-core vesicle (LDCV) exocytosis

Synaptic plasticity results from changes in the strength of synaptic transmission upon repetitive stimulation. The amount of neurotransmitter released from presynaptic terminals can regulate short-term plasticity that lasts for a few minutes. This review focuses on short-term plasticity of small synaptic vesicle (SSV) and large dense-core vesicle (LDCV) exocytosis. Whereas SSVs contain classical neurotransmitters and activate ion channels, LDCVs contain neuropeptides and hormones which primarily activate G protein-coupled receptors (GPCRs). Thus, LDCV exocytosis is mainly associated with modulation of synaptic activity and cannot induce synaptic activity by itself. As in SSV exocytosis, repetitive stimulation leads to short-term enhancement of LDCV exocytosis: i.e., activity-dependent potentiation (ADP) which represents potentiation of neurotransmitter release. Short-term plasticity of SSV exocytosis results from Ca2+ accumulation, but ADP of LDCV exocytosis does not. Here, we review the signaling mechanisms and differences of short-term plasticity in exocytotic processes of SSV and LDCV.

Myosin 2 maintains an open exocytic fusion pore in secretory epithelial cells

Many studies have implicated F-actin and myosin 2 in the control of regulated secretion. Most recently, evidence suggests a role for the microfilament network in regulating the postfusion events of vesicle dynamics. This is of potential importance as postfusion behavior can influence the loss of vesicle content and may provide a new target for drug therapy. We have investigated the role of myosin 2 in regulating exocytosis in secretory epithelial cells by using novel assays to determine the behavior of the fusion pore in individual granules. We immunolocalize myosin 2A to the apical region of pancreatic acinar cells, suggesting it is this isoform that plays a role in granule exocytosis. We further show myosin 2 phosphorylation increased on cell stimulation, consistent with a regulatory role in secretion. Importantly, in a single-cell, single-granule secretion assay, neither the myosin 2 inhibitor (-)-blebbistatin nor the myosin light chain kinase inhibitor ML-9 had any effect on the numbers of granules stimulated to fuse after cell stimulation. These data indicate that myosin 2, if it has any action on secretion, must be targeting postfusion granule behavior. This interpretation is supported by direct study of fusion pore opening in which we show that (-)-blebbistatin and ML-9 promote fusion pore closure and decrease fusion pore lifetimes. Our work now adds to a growing body of evidence showing that myosin 2 is an essential regulator of postfusion granule behavior. In particular, in the case of the secretory epithelial cells, myosin 2 activity is necessary to maintain fusion pore opening.

Matrix-dependent local retention of secretory vesicle cargo in cortical neurons

Neurons secrete many diffusible signals from synaptic and other secretory vesicles. We characterized secretion of guidance cues, neuropeptides, neurotrophins, and proteases from single secretory vesicles using pHluorin-tagged cargo in cortical neurons. Stimulation triggered transient and persistent fusion events. Transient events represented full release followed by cargo diffusion or incomplete release followed by vesicle retrieval, as previously observed in neuroendocrine cells. Unexpectedly, we also observed that certain cargo, such as Semaphorin 3A (Sema3A), was delivered at the cell surface as stable deposits. Stable deposits and transient events were observed for single cargo and both were SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) and calcium dependent. The ratio between stable and transient events did not depend on cargo size, subcellular localization (synaptic vs extrasynaptic secretion), or the presence of the extracellular matrix. Instead, the ratio is cargo specific and depends on an interaction with the vesicle matrix through a basic domain in the cargo protein. Inhibition of this interaction through deletion of the basic domain in Sema3A abolished stable deposits and rendered all events transient. Strikingly, cargo favoring transient release was stably deposited after corelease with cargo favoring stable deposit. These data argue against cargo diffusion after exocytosis as a general principle. Instead, the vesicle matrix retains secreted signals, probably for focal signaling at the cell surface.

Imaging of evoked dense-core-vesicle exocytosis in hippocampal neurons reveals long latencies and kiss-and-run fusion events

Evoked neuropeptide secretion in the central nervous system occurs slowly, but the basis for slow release is not fully understood. Whereas exocytosis of single synaptic vesicles in neurons and of dense-core vesicles (DCVs) in endocrine cells have been directly visualized, single DCV exocytic events in neurons of the central nervous system have not been previously studied. We imaged DCV exocytosis in primary cultured hippocampal neurons using fluorescent propeptide cargo and total internal reflectance fluorescence microscopy. The majority of Ca(2+)-triggered exocytic events occurred from immobile plasma-membrane-proximal DCVs in the cell soma, whereas there were few events in the neurites. Strikingly, DCVs in the cell soma exhibited 50-fold greater release probabilities than those in neurites. Latencies to depolarization-evoked fusion for DCVs were surprisingly long, occurring with an average time constant (tau) of 16 seconds for DCVs in the soma and even longer for DCVs in neurites. All of the single DCV release events exhibited rapid fusion-pore openings and closures, the kinetics of which were highly dependent upon Ca(2+) levels. These ;kiss-and-run' events were associated with limited cargo secretion. Thus, the slow evoked release of neuropeptides could be attributed to very prolonged latencies from stimulation to fusion and transient fusion-pore openings that might limit cargo secretion.

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.

How does synaptotagmin trigger neurotransmitter release?

Neurotransmitter release at synapses involves a highly specialized form of membrane fusion that is triggered by Ca(2+) ions and is optimized for speed. These observations were established decades ago, but only recently have the molecular mechanisms that underlie this process begun to come into view. Here, we summarize findings obtained from genetically modified neurons and neuroendocrine cells, as well as from reconstituted systems, which are beginning to reveal the molecular mechanism by which Ca(2+)-acting on the synaptic vesicle (SV) protein synaptotagmin I (syt)-triggers rapid exocytosis. This work sheds light not only on presynaptic aspects of synaptic transmission, but also on the fundamental problem of membrane fusion, which has remained a puzzle that has yet to be solved in any biological system.

The fusion pores of Ca2+ -triggered exocytosis

The aqueous compartment inside a vesicle makes its first connection with the extracellular fluid through an intermediate structure termed the exocytotic fusion pore. Progress in exocytosis can be measured in terms of the formation and growth of the fusion pore. The fusion pore has become a major focus of research in exocytosis; sensitive biophysical measurements have provided various glimpses of what it looks like and how it behaves. Some of the principal questions about the molecular mechanism of exocytosis can be cast explicitly in terms of properties and transitions of fusion pores. This Review will present current knowledge about fusion pores in Ca(2+)-triggered exocytosis, highlight recent advances and relate questions about fusion pores to broader issues concerning how cells regulate exocytosis and how nerve terminals release neurotransmitter.

Regulation of quantal shape by Rab3A: evidence for a fusion pore-dependent mechanism

The function of Rab3A, a small GTPase located on synaptic vesicles, is not well understood. Studies in the Rab3A(-/-) mouse support a role in activity-dependent plasticity, but have not reported any effects on spontaneously occurring miniature synaptic currents, except that there is a decrease in resting frequency at the neuromuscular junction. Therefore we were surprised to find an increase in the occurrence of mEPCs with abnormally long half-widths at the neuromuscular junctions of Rab3A(-/-) mice. The abnormal miniature endplate currents (mEPCs), which have significantly greater charge than the average mEPCs for the same fibres, could arise from larger vesicles. However, the type of mEPC most increased in Rab3A(-/-) mice has a slow rise, which suggests it is not the result of full collapse fusion. To test if the slow mEPCs increased after loss of Rab3A could be due to malfunctioning fusion pores, we used carbon fibre amperometry to record pre-spike feet, which have been shown to correspond to the initial opening of a narrow fusion pore, in adrenal chromaffin cells of wild-type and Rab3A(-/-) mice. We found that small amplitude pre-spike feet with abnormally long durations were increased in Rab3A(-/-) cells. The correspondence between mEPC and amperometric data supports our interpretation that slow rising, long half-width mEPCs are caused by reduced diameter fusion pores that remain open longer. These data could be explained by a direct action of Rab3A on the fusion pore, or by Rab3A-dependent control of vesicles with unusual fusion pore characteristics.

Low-noise recording of single-vesicle capacitance steps in cell-attached patches

Capacitance recording provides a readout of membrane area that can be used to monitor exo- and endocytosis in neurons and secretory cells in real time. By interfacing a lock-in amplifier to a patch-clamp amplifier, the capacitance of cell-attached membrane patches can be measured with sufficient sensitivity to reveal the fusion and retrieval of single vesicles as unitary stepwise changes in capacitance. The small size of many secretory vesicles, especially of synaptic vesicles, places a premium on the reduction of noise in a capacitance recording. With care, the capacitance noise in cell-attached patches can be reduced to below 10 aF root-mean-square (rms), thus bringing into view steps resulting from the fusion of vesicles as small as about 18 nm in diameter. Thus, the lowest achievable noise level enables the resolution of changes in capacitance associated with the smallest secretory vesicles. This chapter presents the method of capacitance recording from cell-attached patches with an emphasis on noise reduction. It also addresses the closely related issue of extracting fusion pore properties from these recordings.

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.

Measuring secretion in chromaffin cells using electrophysiological and electrochemical methods

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

Histogenesis and morphofunctional characteristics of chromaffin cells

This article reviews the current status of research about the histogenesis and morphofunctional characteristics of chromaffin cells in the adrenal medulla. First, this study reports the selective migration, transcription and activation factors, and the morphological events of the chromaffin cell precursors during adrenal medulla development. Subsequently, the morphofunctional characteristics of adrenergic and non-adrenergic cells are considered, with particular reference to the characteristics of chromaffin granules and their biological steps, including their formation, traffic (storage, targeting and docking), exocytosis in the strict sense and recapture. Moreover, the relationship of chromaffin cells with other tissue components of the adrenal medulla is also revised, comprising the ganglion cells, sustentacular cells, nerves and connective-vascular tissue.

Compensatory endocytosis in chromaffin cells

Exocytosis occurs via fusion of secretory granules with the cell membrane, whereupon the granule content is at least partially released and the granule membrane is temporarily added to the plasma membrane. Exocytosis is balanced by compensatory endocytosis to achieve net equilibrium of the cell surface area and to recycle and redistribute components of the exocytosis machinery. The underlying molecular mechanisms remain a matter of debate. In this review, we summarize and discuss recent progress in the understanding of compensatory endocytosis, with the focus on chromaffin cells as a useful model for studying mechanisms of regulated secretion.

Temperature dependence of fusion kinetics and fusion pores in Ca2+-triggered exocytosis from PC12 cells

The temperature dependence of Ca²⁺-triggered exocytosis was studied using carbon fiber amperometry to record the release of norepinephrine from PC12 cells. Single-vesicle fusion events were examined at temperatures varying from 12 to 28°C, and with release elicited by depolarization. Measurements were made of the initial and maximum frequencies of exocytotic events, of fusion pore lifetime, flux through the open fusion pore, kiss-and-run versus full-fusion probability, and parameters associated with the shapes of amperometric spikes. The fusion pore open-state flux, and all parameters associated with spike shape, including area, rise time, and decay time, had weak temperature dependences and activation energies in the range expected for bulk diffusion in an aqueous solution. Kiss-and-run events also varied with temperature, with lower temperatures increasing the relative probability of kiss-and-run events by ∼50%. By contrast, kinetic parameters relating to the frequency of exocytotic events and fusion pore transitions depended much more strongly on temperature, suggesting that these processes entail structural rearrangements of proteins or lipids or both. The weak temperature dependence of spike shape suggests that after the fusion pore has started to expand, structural transitions of membrane components are no longer kinetically limiting. This indicates that the content of a vesicle is expelled completely after fusion pore expansion.

Ca2+-dependent mobility of vesicles capturing anti-VGLUT1 antibodies

Several aspects of secretory vesicle cycle have been studied in the past, but vesicle trafficking in relation to the fusion site is less well understood. In particular, the mobility of recaptured vesicles that traffic back toward the central cytoplasm is still poorly defined. We exposed astrocytes to antibodies against the vesicular glutamate transporter 1 (VGLUT1), a marker of glutamatergic vesicles, to fluorescently label vesicles undergoing Ca(2+)-dependent exocytosis and examined their number, fluorescence intensity, and mobility by confocal microscopy. In nonstimulated cells, immunolabeling revealed discrete fluorescent puncta, indicating that VGLUT1 vesicles, which are approximately 50 nm in diameter, cycle slowly between the plasma membrane and the cytoplasm. When the cytosolic Ca(2+) level was raised with ionomycin, the number and fluorescence intensity of the puncta increased, likely because the VGLUT1 epitopes were more accessible to the extracellularly applied antibodies following Ca(2+)-triggered exocytosis. In nonstimulated cells, the mobility of labeled vesicles was limited. In stimulated cells, many vesicles exhibited directional mobility that was abolished by cytoskeleton-disrupting agents, indicating dependence on intact cytoskeleton. Our findings show that postfusion vesicle mobility is regulated and may likely play a role in synaptic vesicle cycle, and also more generally in the genesis and removal of endocytic vesicles.

Rapid recovery of releasable vesicles and formation of nonreleasable endosomes follow intense exocytosis in chromaffin cells

Neurons and neuroendocrine cells must retrieve plasma membrane excess and refill vesicle pools depleted by exocytosis. To perform these tasks cells can use different endocytosis/recycling mechanisms whose selection will impact on vesicle recycling time and secretion performance. We used FM1-43 to evaluate in the same experiment exocytosis, endocytosis, and recovery of releasable vesicles on mouse chromaffin cells. Various exocytosis levels were induced by a variety of stimuli, and we discriminated the resultant endocytosis-recycling responses according to their ability to rapidly generate releasable vesicles. Exocytosis of ≤20% of plasma membrane (provoked by nicotine/acetylcholine) was followed by total recovery of releasable vesicles. If a stronger stimulus (50 mM K+and 2 mM Ca2+) provoking intense exocytosis (51 ± 7%) was applied, endocytosis still retrieved all the fused membrane, but only a fraction (19 ± 2%) was releasable by a second stimulus. Using ADVASEP-7 or bromophenol blue to quickly eliminate fluorescence from noninternalized FM1-43, we determined that this fraction became releasable in <2 min. The remaining nonreleasable fraction was distributed mainly as fluorescent spots (∼0.7 μm) selectively labeled by 40- to 70-kDa dextrans and was suppressed by a phosphatidylinositol-3-phosphate kinase inhibitor, suggesting that it had been formed by a bulk retrieval mechanism. We concluded that chromaffin cells can rapidly recycle significant fractions of their total vesicle population, and that this pathway prevails when cholinergic agonists are used as secretagogues. When exocytosis exceeded ∼20% of plasma membrane, an additional mechanism was activated, which was unable to produce secretory vesicles in our experimental time frame but appeared crucial to maintaining membrane surface homeostasis under extreme conditions.