Punctate appearance of dopamine-beta-hydroxylase on the chromaffin cell surface reflects the fusion of individual chromaffin granules upon exocytosis

5 ns electric pulses induce Ca2+-dependent exocytotic release of catecholamine from adrenal chromaffin cells

Previously we reported that adrenal chromaffin cells exposed to a 5 ns, 5 MV/m pulse release the catecholamines norepinephrine (NE) and epinephrine (EPI) in a Ca²⁺-dependent manner. Here we determined that NE and EPI release increased with pulse number (one versus five and ten pulses at 1 Hz), established that release occurs by exocytosis, and characterized the exocytotic response in real-time. Evidence of an exocytotic mechanism was the appearance of dopamine-β-hydroxylase on the plasma membrane, and the demonstration by total internal reflection fluorescence microscopy studies that a train of five or ten pulses at 1 Hz triggered the release of the fluorescent dye acridine orange from secretory granules. Release events were Ca²⁺-dependent, longer-lived relative to those evoked by nicotinic receptor stimulation, and occurred with a delay of several seconds despite an immediate rise in Ca²⁺. In complementary studies, cells labeled with the plasma membrane fluorescent dye FM 1-43 and exposed to a train of ten pulses at 1 Hz underwent Ca²⁺-dependent increases in FM 1-43 fluorescence indicative of granule fusion with the plasma membrane due to exocytosis. These results demonstrate the effectiveness of ultrashort electric pulses for stimulating catecholamine release, signifying their promise as a novel electrostimulation modality for neurosecretion.

A membrane-bound dopamine β-hydroxylase highly expressed in granulocyte of Pacific oyster Crassostrea gigas

Dopamine β-hydroxylase (DBH) is one of key rate-limiting enzymes converting dopamine to norepinephrine. It locates not only in catecholaminergic neuron system, but also in immunocytes and plays roles in the immune response of vertebrates. However, the knowledge about the function of DBH in immune system is still very limited in invertebrates. In the present study, the DBH gene family with seven members was screened from Crassostrea gigas genome, and their mRNA expressions in various tissues were recorded. Among them, one DBH (designated CgDBH-1) with high expression level in oyster hemocytes was further characterized. The deduced amino acid sequence of CgDBH-1 was predicted to contain a transmembrane domain and shared 30.1% and 30.9% similarity with that in Mus musculus and Homo sapiens, respectively. CgDBH-1 was closely clustered with DBH from Aplysia californica in the phylogenetic tree. The recombinant protein of CgDBH-1 (rCgDBH-1) exhibited significant enzymatic activity (0.54 ± 0.019 pmol L^-1 min^-1) to synthesize norepinephrine. Importantly, the mRNA transcript of CgDBH-1 was highly expressed in oyster hemocytes, and the highest expression level was observed in granulocytes among the three types of hemocytes, which was 8.18-fold (p < 0.01) of that in agranulocytes. Moreover, the expression of CgDBH-1 in hemocytes was significantly increased at the late stage of immune response. The CgDBH-1 protein was mainly co-localized with the granules and endoplasmic reticulum (ER) of granulocytes. These results collectively suggested that CgDBH-1, as a novel molluscan norepinephrine synthesizing enzyme highly expressed in granulocytes, involved in the late-stage immune response of oysters, which provided vital insight to understand the crosstalk between neuroendocrine and immune systems in invertebrates.

Exocytosis and endocytosis in neuroendocrine cells: inseparable membranes!

Although much has been learned concerning the mechanisms of secretory vesicle formation and fusion at donor and acceptor membrane compartments, relatively little attention has been paid toward understanding how cells maintain a homeostatic membrane balance through vesicular trafficking. In neurons and neuroendocrine cells, release of neurotransmitters, neuropeptides, and hormones occurs through calcium-regulated exocytosis at the plasma membrane. To allow recycling of secretory vesicle components and to preserve organelles integrity, cells must initiate and regulate compensatory membrane uptake. This review relates the fate of secretory granule membranes after full fusion exocytosis in neuroendocrine cells. In particular, we focus on the potential role of lipids in preserving and sorting secretory granule membranes after exocytosis and we discuss the potential mechanisms of membrane retrieval.

Plasma membrane domains enriched in cortical endoplasmic reticulum function as membrane protein trafficking hubs

In mammalian cells, the cortical endoplasmic reticulum (cER) is a network of tubules and cisterns that lie in close apposition to the plasma membrane (PM). We provide evidence that PM domains enriched in underlying cER function as trafficking hubs for insertion and removal of PM proteins in HEK 293 cells. By simultaneously visualizing cER and various transmembrane protein cargoes with total internal reflectance fluorescence microscopy, we demonstrate that the majority of exocytotic delivery events for a recycled membrane protein or for a membrane protein being delivered to the PM for the first time occur at regions enriched in cER. Likewise, we observed recurring clathrin clusters and functional endocytosis of PM proteins preferentially at the cER-enriched regions. Thus the cER network serves to organize the molecular machinery for both insertion and removal of cell surface proteins, highlighting a novel role for these unique cellular microdomains in membrane trafficking.

A nibbling mechanism for clathrin-mediated retrieval of secretory granule membrane after exocytosis

Clathrin-mediated endocytosis is the major pathway for recycling of granule membrane components after strong stimulation and high exocytotic rates. It resembles "classical" receptor-mediated endocytosis but has a trigger that is unique to secretion, the sudden appearance of the secretory granule membrane in the plasma membrane. The spatial localization, the relationship to individual fusion events, the nature of the cargo, and the timing and nature of the nucleation events are unknown. Furthermore, a size mismatch between chromaffin granules (∼300-nm diameter) and typical clathrin-coated vesicles (∼90 nm) makes it unlikely that clathrin-mediated endocytosis internalizes as a unit the entire fused granule membrane. We have used a combination of total internal reflection fluorescence microscopy of transiently expressed proteins and time-resolved quantitative confocal imaging of endogenous proteins along with a fluid-phase marker to address these issues. We demonstrate that the fused granule membrane remains a distinct entity and serves as a nucleation site for clathrin- and dynamin-mediated endocytosis that internalizes granule membrane components in small increments.

Kv2.1 cell surface clusters are insertion platforms for ion channel delivery to the plasma membrane

Voltage-gated K+ (Kv) channels regulate membrane potential in many cell types. Although the channel surface density and location must be well controlled, little is known about Kv channel delivery and retrieval on the cell surface. The Kv2.1 channel localizes to micron-sized clusters in neurons and transfected human embryonic kidney (HEK) cells, where it is nonconducting. Because Kv2.1 is postulated to be involved in soluble N-ethylmaleimide–sensitive factor attachment protein receptor–mediated membrane fusion, we examined the hypothesis that these surface clusters are specialized platforms involved in membrane protein trafficking. Total internal reflection–based fluorescence recovery after photobleaching studies and quantum dot imaging of single Kv2.1 channels revealed that Kv2.1-containing vesicles deliver cargo at the Kv2.1 surface clusters in both transfected HEK cells and hippocampal neurons. More than 85% of cytoplasmic and recycling Kv2.1 channels was delivered to the cell surface at the cluster perimeter in both cell types. At least 85% of recycling Kv1.4, which, unlike Kv2.1, has a homogeneous surface distribution, is also delivered here. Actin depolymerization resulted in Kv2.1 exocytosis at cluster-free surface membrane. These results indicate that one nonconducting function of Kv2.1 is to form microdomains involved in membrane protein trafficking. This study is the first to identify stable cell surface platforms involved in ion channel trafficking.

Localized topological changes of the plasma membrane upon exocytosis visualized by polarized TIRFM

Imaging of individual secretory granules reveals how exocytosis curves the membrane.

Total internal reflection fluorescence microscopy (TIRFM) images the plasma membrane–cytosol interface and has allowed insights into the behavior of individual secretory granules before and during exocytosis. Much less is known about the dynamics of the other partner in exocytosis, the plasma membrane. In this study, we report the implementation of a TIRFM-based polarization technique to detect rapid submicrometer changes in plasma membrane topology as a result of exocytosis. A theoretical analysis of the technique is presented together with image simulations of predicted topologies of the postfusion granule membrane–plasma membrane complex. Experiments on diI-stained bovine adrenal chromaffin cells using polarized TIRFM demonstrate rapid and varied submicrometer changes in plasma membrane topology at sites of exocytosis that occur immediately upon fusion. We provide direct evidence for a persistent curvature in the exocytotic region that is altered by inhibition of dynamin guanosine triphosphatase activity and is temporally distinct from endocytosis measured by VMAT2-pHluorin.

Inhibition of N and PQ calcium channels by calcium entry through L channels in chromaffin cells

Why adrenal chromaffin cells express various subtypes of voltage-dependent Ca(2+) channels and whether a given channel is specialized to perform a specific function are puzzling and unanswered questions. In this study, we have used the L Ca(2+) channel activator FPL64176 (FPL) to test the hypothesis that enhanced Ca(2+) entry through this channel favors the inhibition of N and PQ channels in voltage-clamped bovine adrenal chromaffin cells. Using 2 mM Ca(2+) as charge carrier and under the perforated-patch configuration (PPC) of the patch-clamp technique, FPL caused a paradoxical inhibition of the whole-cell inward Ca(2+) current (I (Ca)). Such inhibition turned on into an augmentation upon cell loading with EGTA-AM. Also, under the whole-cell configuration (WCC) of the patch-clamp technique, FPL decreased I (Ca) in the absence of EGTA from the pipette solution and increased the current in its presence. Using 2 mM Ba(2+) as charge carrier, FPL augmented the Ba(2+) current under both recording conditions, WCC and PPC. FPL augmented the residual current remaining after blockade of N and PQ channels with omega-conotoxin MVIIC or by holding the membrane potential at -50 mV. The data support the view that Ca(2+) entering the cell through the lesser inactivating L channels serves to modulate the more inactivating N and PQ channels. They also suggest a close colocalization of L and N/PQ Ca(2+) channels. This kind of L channel specialization may be relevant to cell excitability, exocytosis, and cell survival mechanisms.

Trafficking and fusion of neuropeptide Y-containing dense-core granules in astrocytes

It is becoming clear that astrocytes are active participants in synaptic functioning and exhibit properties, such as the secretion of classical transmitters, previously thought to be exclusively neuronal. Whether these similarities extend to the release of neuropeptides, the other major class of transmitters, is less clear. Here we show that cortical astrocytes can synthesize both native and foreign neuropeptides and can secrete them in a stimulation-dependent manner. Reverse transcription-PCR and mass spectrometry indicate that cortical astrocytes contain neuropeptide Y (NPY), a widespread neuronal transmitter. Immunocytochemical studies reveal NPY-immunoreactive (IR) puncta that colocalize with markers of the regulated secretory pathway. These NPY-IR puncta are distinct from the synaptic-like vesicles that contain classical transmitters, and the two types of organelles are differentially distributed. After activation of metabotropic glutamate receptors and the release of calcium from intracellular stores, the NPY-IR puncta fuse with the cell membrane, and the peptide-containing dense cores are displayed. To determine whether peptide secretion subsequently occurred, exocytosis was monitored from astrocytes expressing NPY-red fluorescent protein (RFP). In live cells, after activation of glutamate receptors, the intensity of the NPY-RFP-labeled puncta declined in a step-like manner indicating a regulated release of the granular contents. Because NPY is a widespread and potent regulator of synaptic transmission, these results suggest that astrocytes could play a role in the peptidergic modulation of synaptic signaling in the CNS.

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.

Real-time bioluminescence imaging of a protein secretory pathway in living mammalian cells using Gaussia luciferase

Using photon counting and charge-coupled device (CCD) cameras, we have applied the method of real-time bioluminescence imaging to investigate protein trafficking in mammalian cells. In the living cells of Chinese hamster ovary and PC12D cells, exocytotic secretion of protein and protein targeting on the cell surface were visualized using the secreted Gaussia luciferase (GLase) as a reporter protein in a minute. After incubation of the cells with luciferin (coelenterazine) for 10min, luciferin was imported into the cells and the vesicle transport network in the cells could be shown by luminescence images of GLase activity. Further, we demonstrate that GLase with a heterologous signal peptide sequence is targeted to the cell surface in neuronally differentiated PC12D cells and luminescence signals could be detected in a few seconds.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

FM1-43 measurements of local exocytotic events in rat melanotrophs

We have explored the existence of fusion- and secretion-competent sites on the plasma membrane of peptide secreting rat pituitary melanotrophs at rest, and following stimulation with glutamate. We monitored changes in fluorescence of FM1-43, a styryl dye which labels plasma membrane. The results show spontaneous local increases in FM1-43 reporting changes in membrane surface area due to cumulative exocytosis. Addition of glutamate, further increased the occurrence of these events. Statistical analysis of local FM1-43 fluorescence changes suggests that this is due to the recruitment of inactive exocytotic domains and due to the stimulation of already active exocytotic domains.

Mouse chromaffin cells have two populations of dense core vesicles

The quantal hypothesis states that neurotransmitter is released in discrete packages, quanta, thought to represent the neurotransmitter content of individual vesicles. If true, then vesicle size should influence quantal size. Although chromaffin cells are generally thought to have a single population of secretory vesicles, our electron microscopy analysis suggested two populations as the size distribution was best described as the sum of two Gaussians. The average volume difference was fivefold. To test whether this difference in volume affected quantal size, neurotransmitter release from permeabilized cells exposed to 100 microM Ca2+ was measured with amperometry. Quantal content was bimodally distributed with both large and small events; the distribution of vesicle sizes predicted by amperometry was extremely similar to those measured with electron microscopy. In addition, each population of events exhibited distinct release kinetics. These results suggest that chromaffin cells have two populations of dense core vesicles (DCV) with unique secretory properties and which may represent two distinct synthetic pathways for DCV biogenesis or alternatively they may represent different stages of biosynthesis.

Recapture after exocytosis causes differential retention of protein in granules of bovine chromaffin cells

After exocytosis, chromaffin granules release essentially all their catecholamines in small fractions of a second, but it is unknown how fast they release stored peptides and proteins. Here we compare the exocytic release of fluorescently labelled neuropeptide Y (NPY) and tissue plasminogen activator from single granules. Exocytosis was tracked by measuring the membrane capacitance, and single granules in live cells were imaged by evanescent field microscopy. Neuropeptide Y left most granules in small fractions of a second, while tissue plasminogen activator remained in open granules for minutes. Taking advantage of the dependence on pH of the fluorescence of green fluorescent protein, we used rhythmic external acidification to determine whether and when granules re-sealed. One-third of them re-sealed within 100 s and retained significant levels of tissue plasminogen activator. Re-sealing accounts for only a fraction of the endocytosis monitored in capacitance measurements. When external [Ca2+] was raised, even neuropeptide Y remained in open granules until they re-sealed. It is concluded that a significant fraction of chromaffin granules re-seal after exocytosis, and retain those proteins that leave granules slowly. We suggest that granules vary the stoichiometry of release by varying both granule re-sealing and the association of proteins with the granule matrix.

Visualization of regulated exocytosis with a granule-membrane probe using total internal reflection microscopy

Secretory granules labeled with Vamp-green fluorescent protein (GFP) showed distinct signatures upon exocytosis when viewed by total internal reflection fluorescence microscopy. In approximately 90% of fusion events, we observed a large increase in fluorescence intensity coupled with a transition from a small punctate appearance to a larger, spreading cloud with free diffusion of the Vamp-GFP into the plasma membrane. Quantitation suggests that these events reflect the progression of an initially fused and spherical granule flattening into the plane of the plasma membrane as the Vamp-GFP simultaneously diffuses through the fusion junction. Approximately 10% of the events showed a transition from puncta to ring-like structures coupled with little or no spreading. The ring-like images correspond quantitatively to granules fusing and retaining concavity (recess of approximately 200 nm). A majority of fusion events involved granules that were present in the evanescent field for at least 12 s. However, approximately 20% of the events involved granules that were present in the evanescent field for no more than 0.3 s, indicating that the interaction of the granule with the plasma membrane that leads to exocytosis can occur within that time. In addition, approximately 10% of the exocytotic sites were much more likely to occur within a granule diameter of a previous event than can be accounted for by chance, suggestive of sequential (piggy-back) exocytosis that has been observed in other cells. Overall granule behavior before and during fusion is strikingly similar to exocytosis previously described in the constitutive secretory pathway.

Recycling of intact dense core vesicles in neurites of NGF-treated PC12 cells

Exocytic fusion in neuroendocrine cells does not always result in complete release of the peptide contents from dense core vesicles (DCVs). In this study, we use fluorescence imaging and immunoelectron microscopy to examine the retention, endocytosis and recycling of chromogranin B in DCVs of NGF-treated PC12 cells. Our results indicate that DCVs retained and retrieved an intact core that was available for subsequent exocytic release. The endocytic process was inhibited by cyclosporine A or by substitution of extracellular Ca(2+) with Ba(2+) and the total recycling time was less than 5 min.

Retention and stimulus-dependent recycling of dense core vesicle content in neuroendocrine cells

We have used fluorescence imaging of individual exocytic events in combination with immunogold electron microscopy and FM1-43 photoconversion to study the stimulus-dependent recycling of dense core vesicle content in isolated rat pituitary lactotrophs. Secretory stimulation with high external [K+] resulted in 100 exocytic sites per cell that were labeled by extracellular antibodies against the peptide hormone prolactin. Morphological analysis demonstrated that the prolactin was retained and internalized in intact dense cores. Vesicles containing non-secreted, internalized prolactin did not colocalize with DiI-LDL that had been chased into lysosomes but did transiently colocalize with internalized transferrin. The recycling vesicles also trafficked through a syntaxin 6-positive compartment but not the TGN38-positive trans-Golgi. Recycling vesicles, which returned to the cell surface in a slow basal manner, could also be stimulated to undergo exocytosis with a high release probability during subsequent exocytic stimulation with external K+. These studies suggest a functional role for recycling vesicles that retain prolactin.

Calcium regulates exocytosis at the level of single vesicles

Ca2+ microdomains that form during the opening of voltage-gated Ca2+ channels have been implicated in regulating the kinetics of hormone and transmitter release. Direct assessment of the interaction between a single Ca2+ microdomain and a single secretory vesicle has been impossible because of technical limitations. Using evanescent field imaging of near-membrane micromolar Ca2+ concentration ([Ca2+]) and fluorescently labeled vesicles, we have observed exocytosis of individual chromaffin dense-core vesicles that was triggered by Ca2+ microdomains. Ca2+ microdomains selectively triggered the release of vesicles that were docked within 300 nm. Not all vesicles exposed to a Ca2+ microdomain were released, indicating that some vesicles are docked but are not ready for release. In addition to its established role as a trigger for release, elevated near-membrane [Ca2+] reduced the distance between docked vesicles and Ca2+ entry sites. Our results suggest a new mechanism for stimulation-dependent facilitation of exocytosis, whereby vesicles are moved closer to Ca2+ entry sites, thereby increasing a Ca2+ microdomain's efficacy to trigger vesicle fusion.

Secretory granules are recaptured largely intact after stimulated exocytosis in cultured endocrine cells

Classical cell biology teaches that exocytosis causes the membrane of exocytic vesicles to disperse into the cell surface and that a cell must later retrieve by molecular sorting whatever membrane components it wishes to keep inside. We have tested whether this view applies to secretory granules in intact PC-12 cells. Three granule proteins were labeled with fluorescent proteins in different colors, and two-color evanescent-field microscopy was used to view single granules during and after exocytosis. Whereas neuro-peptide Y was lost from granules in seconds, tissue plasminogen activator (tPA) and the membrane protein phogrin remained at the granule site for over 1 min, thus providing markers for postexocytic granules. When tPA was imaged simultaneously with cyan fluorescent protein (CFP) as a cytosolic marker, the volume occupied by the granule appeared as a dark spot where it excluded CFP. The spot remained even after tPA reported exocytosis, indicating that granules failed to flatten into the cell surface. Phogrin was labeled with GFP at its luminal end and used to sense the pH in granules. When exocytosis caused the acidic granule interior to neutralize, GFP-phogrin at first brightened and later dimmed again as the interior separated from the extracellular space and reacidified. Reacidification and dimming could be reversed by application of NH(4)Cl. We conclude that most granules reseal in <10 s after releasing cargo, and that these empty or partially empty granules are recaptured otherwise intact.

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.

Co-localization of vesicles and P/Q Ca2+-channels explains the preferential distribution of exocytotic active zones in neurites emitted by bovine chromaffin cells

We have taken advantage of the differences between the preferential localization of secretion in the terminals of neurite-emitting bovine chromaffin cells in contrast with the random distribution secretion in spherical cells to study the possible molecular factors determining such localization by using immunofluorescence and confocal microscopy techniques. By analyzing the distribution of dopamine beta-hydroxylase present in the membrane of chromaffin granules, we found that vesicles migrate and accumulate in dense packages in the terminals of neurite processes. Neither members of the fusion core complex such as SNAP-25, nor nicotinic receptors are preferentially located in the terminals as would be expected from elements defining sites of release, thereby suggesting the presence of additional factors. Interestingly, we observed a preferential distribution of the P/Q subtype of Ca2+ channels in these neurite terminals and co-localization with vesicles present in these structures, in sharp contrast with the overall distribution of the L subtype channels. Using the same immunofluorescence techniques we were unable to detect N-type calcium channels. In addition, omega-agatoxin IVA was able to block 70% of the exocytotic release occurring into the neurites, whereas L-type blockers had a weak effect. Taken together our results strongly indicate that the co-localization of vesicles and clusters of P/Q Ca2+ channels may explain the precise localization of exocytotic sites in the terminals of neurite-emitting chromaffin cells, whereas the distribution of secretory sites in round cells may arise from the random presence of these factors as indicated by their partial co-localization.

Restriction of secretory granule motion near the plasma membrane of chromaffin cells

We used total internal reflection fluorescence microscopy to study quantitatively the motion and distribution of secretory granules near the plasma membrane (PM) of living bovine chromaffin cells. Within the approximately 300-nm region measurably illuminated by the evanescent field resulting from total internal reflection, granules are preferentially concentrated close to the PM. Granule motion normal to the substrate (the z direction) is much slower than would be expected from free Brownian motion, is strongly restricted over tens of nanometer distances, and tends to reverse directions within 0.5 s. The z-direction diffusion coefficients of granules decrease continuously by two orders of magnitude within less than a granule diameter of the PM as granules approach the PM. These analyses suggest that a system of tethers or a heterogeneous matrix severely limits granule motion in the immediate vicinity of the PM. Transient expression of the light chains of tetanus toxin and botulinum toxin A did not disrupt the restricted motion of granules near the PM, indicating that SNARE proteins SNAP-25 and VAMP are not necessary for the decreased mobility. However, the lack of functional SNAREs on the plasma or granule membranes in such cells reduces the time that some granules spend immediately adjacent to the PM.

A preferential pole for exocytosis in cultured chromaffin cells revealed by confocal microscopy

Histological studies suggest that adrenal medulla chromaffin cells in situ are polarised, but functional evidence is lacking. We present here the first demonstration for polarisation of exocytosis in isolated, spherical, bovine chromaffin cells. Cells were stimulated with 70 mM K(+) to cause a marked enhancement of catecholamine release, monitored amperometrically. FM1-43 and dopamine beta-hydroxylase antibodies provided fluorescence confocal pictures that were 2-3-fold more intense in the bottom of the cells, as compared to equatorial or apex planes. This suggests that the solid phase to which the cell attaches serves as a 'trophic' signal for the polarisation of its secretory apparatus.

A current activated on depletion of intracellular Ca2+ stores can regulate exocytosis in adrenal chromaffin cells

Exocytosis in excitable cells is strongly coupled to Ca2+ entry through voltage-gated channels but can be evoked by activation of membrane receptors that release Ca2+ from inositol 1,4, 5-trisphosphate-sensitive internal stores. In many cell types, depletion of Ca2+ stores activates Ca2+ influx across the plasma membrane, a process known as capacitative or store-operated Ca2+ entry. This influx is mediated by a number of voltage-independent, Ca2+-selective currents. In addition to replenishing Ca2+ stores, these currents are hypothesized to play an important role in agonist-evoked secretion in nonexcitable cells, although this has not been confirmed experimentally. The existence and physiological function of such currents in excitable cells is not known. Using the capacitance detection technique to monitor exocytosis, we provide direct experimental evidence that a similar mechanism exists in bovine adrenal chromaffin cells. Depletion of intracellular Ca2+ stores with thapsigargin, a SERCA pump inhibitor, or with BAPTA, an exogenous Ca2+ chelator, activates a small-amplitude, voltage-independent current that is carried by Ca2+ and Na+ ions. Ca2+ entry through this pathway is sufficient to stimulate exocytosis at negative membrane potentials. In addition, depolarization-evoked exocytosis is markedly facilitated on activation of the current. These data suggest that excitable cells possess a store-operated Ca2+ influx mechanism that may both directly trigger exocytosis and modulate excitation-secretion coupling.