Docked granules, the exocytic burst, and the need for ATP hydrolysis in endocrine cells

High mobility of vesicles supports continuous exocytosis at a ribbon synapse

BACKGROUND

Most synapses release neurotransmitter as transient pulses, but ribbon synapses of sensory neurons support continuous exocytosis in response to maintained stimulation. We have investigated how the movement and retrieval of vesicles might contribute to continuous exocytosis at the ribbon synapse of retinal bipolar cells.

RESULTS

Using a combination of total internal reflection fluorescence microscopy and fluorescence recovery after photobleaching, we found that the great majority of vesicles within 50-120 nm of the plasma membrane move in a random fashion with an effective diffusion coefficient of approximately 1.5 x 10(-2) microm(2) s(-1). Using confocal microscopy, we found that vesicles are similarly mobile across the whole terminal and that this motion is not altered by calcium influx or the actin cytoskeleton. We calculated that the cytoplasmic reservoir of approximately 300,000 vesicles would generate about 900 vesicle collisions/s against ribbons and 28,000 collisions/s against the surface membrane. The efficient resupply of vesicles to ribbons was confirmed by electron microscopy. A 1 min depolarization, releasing 500-1000 vesicles/s, caused a 70% reduction in the number of vesicles docked at the active zone without reducing the number of vesicles attached to ribbons or remote areas of the plasma membrane. These sites were not repopulated by retrieved vesicles because 80-90% of the recycled membrane was taken up into cisternae that pinched off from the surface.

CONCLUSIONS

These results indicate that the random motion of cytoplasmic vesicles provides an efficient supply to the ribbon and plasma membrane and allows the maintenance of high rates of exocytosis without an equally rapid recycling of vesicles. The selective depletion of vesicles docked under ribbons suggests that the transfer of vesicles to the active zone limits the rate of exocytosis during maintained stimulation.

Real-Time Measurement of Exocytosis and Endocytosis Using Interference of Light

We describe a new approach for making real-time measurements of exocytosis and endocytosis in neurons and neuroendocrine cells. The method utilizes interference reflection microscopy (IRM) to image surface membrane in close contact with a glass coverslip (the "footprint"). At the synaptic terminal of retinal bipolar cells, the footprint expands during exocytosis and retracts during endocytosis, paralleling changes in total surface area measured by capacitance. In chromaffin cells, IRM detects the fusion of individual granules as the appearance of bright spots within the footprint with spatial and temporal resolution similar to total internal reflection fluorescence microscopy. Advantages of IRM over capacitance are that it can monitor changes in surface area while cells are electrically active and it can be applied to mammalian neurons with relatively small synaptic terminals. IRM reveals that vesicles at the synapse of bipolar cells rapidly collapse into the surface membrane while secretory granules in chromaffin cells do not.

Secretory vesicles membrane area is regulated in tandem with quantal size in chromaffin cells

The number of transmitter molecules released in a quantal event can be regulated, and recent studies suggest that the modulation of quantal size is associated with corresponding changes in vesicle volume (Colliver et al., 2000; Pothos et al., 2002). If so, this could occur either by distension of the vesicle membrane or by incorporation and removal of vesicle membrane. We performed simultaneous measurements of vesicle membrane area and catecholamine release in individual quantal events from chromaffin cells using cell-attached patch amperometry. Cells were treated with reserpine, a vesicular monoamine transport blocker that decreases quantal size, or l-dopa, a catecholamine precursor that increases quantal size. We show that decrease and increase in quantal size are associated with a respective decrease and increase in vesicle membrane area. These results point to a novel mechanism of vesicle membrane dynamics by which vesicles physically change their membrane area in response to changes in transmitter content such that the intravesicular concentration of transmitter is maintained.

Effects of nateglinide on the secretion of glycated insulin and glucose tolerance in type 2 diabetes

AIMS

Glycation of insulin has been demonstrated within pancreatic beta-cells and the resulting impaired bioactivity may contribute to insulin resistance in diabetes. We used a novel radioimmunoassay to evaluate the effect of nateglinide on plasma concentrations of glycated insulin and glucose tolerance in type 2 diabetes.

METHODS

Ten patients (5 M/5 F, age 57.8+/-1.9 years, HbA(1c) 7.6+/-0.5%, fasting plasma glucose 9.4+/-1.2 mmol/l, creatinine 81.6+/-4.5 microM/l) received oral nateglinide 120 mg or placebo, 10 min prior to 75 g oral glucose in a random, single blind, crossover design, 1 week apart. Blood samples were taken for glycated insulin, glucose, insulin and C-peptide over 225 min.

RESULTS

Plasma glucose and glycated insulin responses were reduced by 9% (P=0.005) and 38% (P=0.047), respectively, following nateglinide compared with placebo. Corresponding AUC measures for insulin and C-peptide were enhanced by 36% (P=0.005) and 25% (P=0.007) by nateglinide.

CONCLUSIONS

Glycated insulin in type 2 diabetes is reduced in response to the insulin secretagogue nateglinide, resulting in preferential release of native insulin. Since glycated insulin exhibits impaired biological activity, reduced glycated insulin release may contribute to the antihyperglycaemic action of nateglinide.

Tuning exocytosis for speed: fast and slow modes

Exocytic fusion reactions triggered by Ca(2+) are widespread in neural, endocrine, exocrine, hemapoetic and perhaps all cell types. These processes exhibit tremendous variation in latencies to fusion following a Ca(2+) rise and in rates of fusion. We review reported differences for synaptic vesicle (SV) and dense-core vesicle (DCV) exocytosis and attempt to identify key features in the molecular mechanisms of docking, priming and fusion of SVs and DCVs that may account for differences in speed.

Dense-core secretory vesicle docking and exocytotic membrane fusion in Paramecium cells

Work with Paramecium has contributed to the actual understanding of certain aspects of exocytosis regulation, including membrane fusion. The system is faster and more synchronous than any other dense-core vesicle system described and its highly regular design facilitates correlation of functional and ultrastructural (freeze-fracture) features. From early times on, several crucial aspects of exocytosis regulation have been found in Paramecium cells, e.g. genetically controlled microdomains (with distinct ultrastructure) for organelle docking and membrane fusion, involvement of calmodulin in establishing such microdomains, priming by ATP, occurrence of focal fusion with active participation of integral and peripheral proteins, decay of a population of integral proteins ("rosettes", mandatory for fusion capacity) into subunits and their lateral dispersal during fusion, etc. The size of rosette particles and their dispersal upon focal fusion would be directly compatible with proteolipid V(0) subunits of a V-ATPase, much better than the size predicted for oligomeric SNARE pins (SCAMPs are unknown from Paramecium at this time). However, there are some restrictions for a straightforward interpretation of ultrastructural results. The rather pointed, nipple-like tip of the trichocyst membrane could accommodate only one (or very few) potential V(0) counterpart(s), while the overlaying domain of the cell membrane contains numerous rosette particles. Particle size is compatible with V(0), but larger than that assumed for the SNARE complexes. When membrane fusion is induced in the presence of antibodies against cell surface components, focal fusion is seen to occur with dispersing rosette particles but without dispersal of their subunits and without pore expansion. Clearly, this is required for completing fusion and pore expansion. After cloning SNARE and V(0) components in Paramecium (with increasing details becoming rapidly available), we may soon be able to address the question more directly, whether any of these components or some new ones to be detected, serve exocytotic and/or any other membrane fusions in Paramecium.

Revisiting the role of SNAREs in exocytosis and membrane fusion

For over a decade SNARE hypotheses have been proposed to explain the mechanism of membrane fusion, yet the field still lacks sufficient evidence to conclusively identify the minimal components of native fusion. Consequently, debate concerning the postulated role(s) of SNAREs in membrane fusion continues. The focus of this review is to revisit original literature with a current perspective. Our analysis begins with the earliest studies of clostridial toxins, leading to various cellular and molecular approaches that have been used to test for the roles of SNAREs in exocytosis. We place much emphasis on distinguishing between specific effects on membrane fusion and effects on other critical steps in exocytosis. Although many systems can be used to study exocytosis, few permit selective access to specific steps in the pathway, such as membrane fusion. Thus, while SNARE proteins are essential to the physiology of exocytosis, assay limitations often prevent definitive conclusions concerning the molecular mechanism of membrane fusion. In all, the SNAREs are more likely to function upstream as modulators or priming factors of fusion.

Intracellular patch electrochemistry: regulation of cytosolic catecholamines in chromaffin cells

Alterations in the cytosolic pool directly affect neurotransmitter synthesis and release and are suggested to be key factors in various neurodegenerative disorders. Although this cytosolic pool is the most metabolically active, it is miniscule compared with the amount of vesicular transmitter and has never been quantified separately. Here, we introduce intracellular patch electrochemistry (IPE), a technique that for the first time provides direct measurements of cytosolic oxidizable molecules in single mammalian cells. In amperometric mode, IPE detects total catechols, whereas in cyclic voltammetric mode, it preferentially measures catecholamines. In cultured chromaffin cells, the total cytosolic catechol concentration was 50-500 microm, of which approximately 10% were catecholamines. Reserpine, a vesicular monoamine transporter inhibitor, had no effect on the catecholamine pool but increased total catechols by fourfold to fivefold. Combined with pargyline, a monoamine oxidase inhibitor, reserpine increased catecholamine levels in the cytosol by approximately sixfold. Amphetamine induced a transient approximately fivefold accumulation of cytosolic catecholamines and a slow increase of total catechols. In cells incubated with 3,4-dihydroxy-L-phenylalanine (L-DOPA), catecholamines increased by approximately 2.5-fold and total catechols increased by approximately fourfold. Cytosolic catecholamines returned to control levels <or=10 min after L-DOPA withdrawal, whereas total catechols remained approximately twofold elevated even after a 1.5 hr incubation in L-DOPA-free media. Our data indicate that cytosolic catecholamines are strictly maintained at a defined level, and drug-induced increases in their concentrations lead to the accumulation of other catecholamine derivatives, such as DOPAC and 3,4-dihydroxyphenylethyleneglycol. These derivatives reside in the cytosol for hours after treatment and may be an underlying cause of drug-related cytotoxicity.

Differential control of the releasable vesicle pools by SNAP-25 splice variants and SNAP-23

The SNARE complex, consisting of synaptobrevin, syntaxin, and SNAP-25, is essential for calcium-triggered exocytosis in neurosecretory cells. Little is known, however, about how developmentally regulated isoforms and other cognate SNARE components regulate vesicular fusion. To address this question, we examined neuroexocytosis from chromaffin cells of Snap25 null mice rescued by the two splice variants SNAP-25a and SNAP-25b and the ubiquitously expressed homolog SNAP-23. In the absence of SNAP-25, vesicle docking persisted, but primed vesicle pools were empty and fast calcium-triggered release abolished. Single vesicular fusion events showed normal characteristics, except for a shorter duration of the fusion pore. Overexpression of SNAP-25a, SNAP-25b, and SNAP-23 resulted in three distinct phenotypes; SNAP-25b induced larger primed vesicle pools than SNAP-25a, whereas SNAP-23 did not support a standing pool of primed vesicles. We conclude that three alternative SNARE components support exocytosis, but they differ in their ability to stabilize vesicles in the primed state.

Calcium-triggered membrane fusion proceeds independently of specific presynaptic proteins

Complexes of specific presynaptic proteins have been hypothesized to drive or catalyze the membrane fusion steps of exocytosis. Here we use a stage-specific preparation to test the roles of SNAREs, synaptotagmin, and SNARE-binding proteins in the mechanism of Ca2+-triggered membrane fusion. Excess exogenous proteins, sufficient to block SNARE interactions, did not inhibit either the Ca2+ sensitivity, extent, or kinetics of fusion. In contrast, despite a limited effect on SNARE and synaptotagmin densities, treatments with high doses of chymotrypsin markedly inhibited fusion. Conversely, low doses of chymotrypsin had no effect on the Ca2+ sensitivity or extent of fusion but did alter the kinetic profile, indicating a more direct involvement of other proteins in the triggered fusion pathway. SNAREs, synaptotagmin, and their immediate binding partners are critical to exocytosis at a stage other than membrane fusion, although they may still influence the triggered steps.

Exocytosis at the ribbon synapse of retinal bipolar cells studied in patches of presynaptic membrane

The distribution of exocytic sites and ion channels in the synaptic terminal of retinal bipolar cells was investigated by measuring capacitance and conductance changes in cell-attached patches of presynaptic membrane. Patch depolarization evoked capacitance and conductance increases that were inhibited by blocking Ca(2+) influx or loading the terminal with EGTA. The increase in capacitance declined as the depolarization approached the reversal potential for Ca(2+), indicating that it was a result of Ca(2+)-dependent exocytosis. The conductance increase was caused by K(Ca) channels that were also activated by Ca(2+) influx. Two observations indicated that sites of exocytosis and endocytosis colocalized with clusters of Ca(2+) channels and K(Ca) channels; the initial rate of exocytosis was correlated with the activation of K(Ca) channels, and exocytosis did not occur in the 41% of patches lacking this conductance. Electron microscopy demonstrated that there were approximately 16 vesicles docked to the plasma membrane at each active zone marked by a ribbon, but vesicles were also attached to the rest of the membrane at a density of 1.5/microm(2). The density of ribbons was 0.10 +/- 0.02/microm(2), predicting that approximately 43% of cell-attached patches would lack an active zone. The density of Ca(2+) channel clusters assayed by capacitance and conductance responses was therefore similar to the density of ribbons. These results are consistent with the idea that Ca(2+) channel clusters were colocalized with ribbons but do not exclude the possibility that calcium channels also occurred at other sites. The wide distribution of vesicles docked to the plasma membrane suggests that exocytosis might also be triggered by the spread of Ca(2+) from Ca(2+) channel clusters.

Regulated secretion: SNARE density, vesicle fusion and calcium dependence

SNAREs such as VAMP, SNAP-25 and syntaxin are essential for intracellular trafficking, but what are their exact molecular roles and how are their interactions with other proteins manifest? Capitalizing on the differential sensitivity of SNAREs to exogenous proteases, we quantified the selective removal of identified SNAREs from native secretory vesicles without loss of fusion competence. Using previously established fusion assays and a high sensitivity immunoblotting protocol, we analyzed the relationship between these SNARE proteins and Ca2+-triggered membrane fusion. Neither the extent of fusion nor the number of intermembrane fusion complexes per vesicle were correlated with the measured density of identified egg cortical vesicle (CV) SNAREs. Without syntaxin, CVs remained fusion competent. Surprisingly, for one (but not another) protease the Ca2+ dependence of fusion was correlated with CV SNARE density, suggesting a native protein complex that associates with SNAREs, the architecture of which ensures high Ca2+ sensitivity. As SNAREs may function during CV docking in vivo, and as further proteolysis after SNARE removal eventually ablates fusion, we hypothesize that the triggered steps of regulated fusion (Ca2+ sensitivity and the catalysis and execution of fusion) require additional proteins that function downstream of SNAREs.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

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

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

Myosins II and V in chromaffin cells: myosin V is a chromaffin vesicle molecular motor involved in secretion

The presence of myosin II and V in chromaffin cells and their subcellular distribution is described. Myosin II and V distribution in sucrose density gradients showed only a strong correlation between the distribution of myosin V and secretory vesicle markers. Confocal microscopy images demonstrated colocalization of myosin V with dopamine beta-hydroxylase, a chromaffin vesicle marker, whereas myosin II was present mainly in the cell cortex. Cell depolarization induced, in a Ca2+ and time-dependent manner, the dissociation of myosin V from chromaffin vesicles suggesting that this association was not permanent but determined by secretory cycle requirements. Myosin II was also found in the crude granule fraction, however, its distribution was not affected by cell depolarization. Myosin V head antibodies were able to inhibit secretion whereas myosin II antibodies had no inhibitory effect. The pattern of inhibition indicated that these treatments interfered with the transport of vesicles from the reserve to the release-ready compartment, suggesting the involvement of myosin V and not myosin II in this transport process. The results described here suggest that myosin V is a molecular motor involved in chromaffin vesicle secretion. However, these results do not discard an indirect role for myosin II in secretion through its interaction with F-actin networks.

Functional and spatial segregation of secretory vesicle pools according to vesicle age

Synaptic terminals and neuroendocrine cells are packed with secretory vesicles, only a few of which are docked at the plasma membrane and readily releasable. The remainder are thought to constitute a large cytoplasmic reserve pool awaiting recruitment into the readily releasable pool (RRP) for exocytosis. How vesicles are prioritized in recruitment is still unknown: the choice could be random, or else the oldest or the newest ones might be favoured. Here we show, using a fluorescent cargo protein that changes colour with time, that vesicles in bovine adrenal chromaffin cells segregate into distinct populations, based on age. Newly assembled vesicles are immobile (morphologically docked) at the plasma membrane shortly after biogenesis, whereas older vesicles are mobile and located deeper in the cell. Different secretagogues selectively release vesicles from the RRP or, surprisingly, selectively from the deeper cytoplasmic pool. Thus, far from being equal, vesicles are segregated functionally and spatially according to age.

Inhibition and facilitation by pimobendan, a calcium sensitizer, of catecholamine secretion from bovine adrenal chromaffin cells

The effects of pimobendan, a Ca(2+) sensitizer with inhibitory action against cyclic-GMP-inhibited phosphodiesterase (PDE-III), on catecholamine (CA) secretion were studied in bovine adrenal chromaffin cells. In intact cells, pimobendan (10 - 100 microM) inhibited CA secretion stimulated by acetylcholine (10 and 30 microM) and 1,1-dimethyl-4-phenyl-piperazinium (DMPP) (3 and 10 microM), but facilitated CA secretion stimulated by high K(+) (30 mM), histamine (3 microM), and angiotensin-II (3 microM). Histamine and angiotensin-II had no effect on CA secretion in Ca(2+)-free medium. The inhibition or facilitation by pimobendan of the stimulation-evoked CA secretion was not affected by H-89 (1 microM) and H-8 (30 microM), inhibitors of cyclic-AMP-dependent protein kinase. Milrinone (10 and 30 microM) and amrinone (100 and 300 microM), inhibitors of PDE-III, did not affect the stimulation-evoked CA secretion. In beta-escin-permeabilized cells, pimobendan (10 - 100 microM) did not affect CA secretion stimulated by Ca(2+) (0.1 - 10 microM) in the presence and absence of MgATP (2 mM). These results indicate that pimobendan has dual effects, inhibition and facilitation, on CA secretion. The inhibition may be due to an inhibitory action on nicotinic receptors and the facilitation may be due to a facilitatory action on stimulation-induced Ca(2+) influx. Neither Ca(2+) sensitizing nor PDE-III inhibiting actions seem to be related to these effects.

Involvement of actin polymerization in vesicle recruitment at the calyx of Held synapse

Depletion and replenishment of pools of synaptic vesicles are important determinants of short-term synaptic plasticity, but the underlying molecular mechanisms are not yet clear. As a first step toward understanding the process of vesicle recruitment, we have applied various specific agents directly to the presynaptic terminal of the calyx of Held synapse. Here we show that the nonhydrolyzable ATP analog ATP-gammaS retards the recovery from vesicle pool depletion, as does latrunculin A. Phalloidin has no effects on recovery, suggesting that dynamic actin reorganization is not necessary. Unexpectedly, neither N-ethylmaleimide nor staurosporine affected the recovery, calling into question the role of N-ethylmaleimide-sensitive factor and protein kinases. The results suggest that intact actin polymerization is involved in vesicle recruitment.

Mechanisms of exocytosis in insulin-secreting B-cells and glucagon-secreting A-cells

In pancreatic B- and A-cells, metabolic stimuli regulate biochemical and electrical processes that culminate in Ca2+-influx and release of insulin or glucagon, respectively. Like in other (neuro)endocrine cells, Ca2+-influx triggers the rapid exocytosis of hormone-containing secretory granules. Only a small fraction of granules (<1% in insulin-secreting B-cells) can be released immediately, while the remainder requires translocation to the plasma membrane and further "priming" for release by several ATP- and Ca2+-dependent reactions. Such functional organization may account for systemic features such as the biphasic time course of glucose-stimulated insulin secretion. Since this release pattern is altered in type-2 diabetes mellitus, it is conceivable that disturbances in the exocytotic machinery underlie the disease. Here I will review recent data from our laboratory relevant for the understanding of these processes in insulin-secreting B-cells and glucagon-secreting A-cells and for the identification of novel targets for antidiabetic drug action. Two aspects are discussed in detail: 1) The importance of a tight interaction between L-type Ca2+-channels and the exocytotic machinery for efficient secretion; and 2) the role of intragranular acidification for the priming of secretory granules and its regulation by a granular 65-kDa sulfonylurea-binding protein.

Quantal size and variation determined by vesicle size in normal and mutant Drosophila glutamatergic synapses

Quantal size and variation at chemical synapses could be determined presynaptically by the amount of neurotransmitter released from synaptic vesicles or postsynaptically by the number of receptors available for activation. We investigated these possibilities at Drosophila glutamatergic neuromuscular synapses formed by two separate motor neurons innervating the same muscle cell. At wild-type synapses of the two neurons we found a difference in quantal size corresponding to a difference in mean synaptic vesicle volume. The same finding applied to two mutants (dlg and lap) in which synaptic vesicle size was altered. Quantal variances at wild-type and mutant synapses were similar and could be accounted for by variation in vesicular volume. The linear relationship between quantal size and vesicular volume for several different genotypes indicates that glutamate is regulated homeostatically to the same intravesicular concentration in all cases. Thus functional differences in synaptic strength among glutamatergic neurons of Drosophila result in part from intrinsic differences in vesicle size.

Protein kinase C-dependent phosphorylation of synaptosome-associated protein of 25 kDa at Ser187 potentiates vesicle recruitment

Activation of protein kinase C (PKC) constitutes a key event in the upregulation of secretory strength in neurons and neurosecretory cells during extensive stimulation, presumably by speeding up vesicle supply. However, the molecular targets and their mode of action remain elusive. We studied the only PKC-dependent phosphorylation site in the neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, Ser(187), in synaptosome-associated protein of 25 kDa (SNAP-25). This phosphorylation site is located within the negatively charged C-terminal end of SNAP-25, which has been shown to be of critical importance in calcium-triggered exocytosis. We combined mutational studies that used overexpression in chromaffin cells with capacitance measurements and flash photolysis of caged calcium, allowing for high time resolution during both the stimulation and measurement of exocytosis. Overexpression of mutants simulating the phosphorylated form of Ser(187) accelerated vesicle recruitment after the emptying of the releasable vesicle pools. Overexpression of mutants simulating the nonphosphorylated form, or block of PKC, impaired the refilling of the vesicle pools to similar extents. Biochemical studies verified the phosphorylation of a subpopulation of SNAP-25 after elevation of intracellular calcium concentrations. Some of the mutations led to a moderately decreased fast exocytotic burst component, which did not seem to be associated with the phosphorylation state of SNAP-25. Thus the C terminus of SNAP-25 plays a role for both fast exocytosis triggering and vesicle recruitment, and the latter process is regulated by PKC-dependent phosphorylation.

Ultrastructural characterization of endoplasmic reticulum--Golgi transport containers (EGTC)

Recent observations made in live cells expressing green fluorescent protein (GFP)-tagged cargo markers have demonstrated the existence of large, mobile transport intermediates linking peripheral ER exit sites (ERES) to the perinuclear Golgi. Using a procedure of rapid ethane freezing, we examined ultrastructurally the intermediates involved in ER-Golgi transport of the vesicular stomatitis virus (VSV) G protein. When released at the permissive temperature of 32 degrees C, VSVG is first found to be concentrated in pleiomorphic, membrane-bound structures (of about 0.4 to 1 microm in diameter) with extensive budding profiles. These structures are devoid of COPII components and Golgi markers, but are enriched in COPI, the retrograde cargo ERGIC53, and the tethering protein p115. The structures appear to be able to undergo fusion with the Golgi stack and are tentatively referred to as ER-Golgi transport containers, or EGTCs. VSVG protein exiting the ERES at 15 degrees C is first found in clusters or strings of COPII-containing small vesicles, and morphological analysis indicates that these clusters and strings of COPII vesicles may coalesce by homotypic fusion to form the EGTCs. Together with the large transport containers mediating transport from the trans-Golgi network to the plasma membrane, EGTCs represents an emerging class of large membranous structures mediating anterograde transport between the major stations of the exocytic pathway.

Inducible system in human hepatoma cell lines for hepatitis C virus production

We cloned the complete complementary DNA of an isolate of the hepatitis C virus, HCV-S1, into a tetracycline-inducible expression vector and stably transfected it into two human hepatoma cell lines, Huh7 and HepG2. Twenty-six Huh7 and two HepG2-positive clones were obtained after preliminary screening. Two Huh7 (SH-7 and -9) and one HepG2 (G-19) clones were chosen for further characterisation. Expression of HCV proteins in these cells accumulated from 6 h to 4 days posttreatment. Full-length viral plus-strand RNA was detected by Northern analyses. Using RT-PCR and ribonuclease protection assay, we also detected the synthesis of minus-strand HCV RNA. Plus- and minus-strand viral RNA was still detected after treatment with actinomycin D. Indirect immunofluorescence staining with anti-E2, NS4B, and NS5A revealed that these proteins were mostly localised to the endoplasmic reticulum (ER). Culture media from tet-induced SH-9 cells was separated on sucrose density gradients and analysed for the presence of HCV RNA. Viral RNA levels peaked at two separate ranges, one with a buoyant density of 1.08 g/ml and another from 1.17 to 1.39 g/ml. Electron microscopy demonstrated the presence of subviral-like particles (approximately 20-25 nm in diameter) in the cytoplasm of SH-9 and G-19 cells, which were positively labelled by anti-HCV core antibodies. Anti-E2 antibodies strongly labelled cytoplasmic vesicular structures and some viral-like particles. Complete viral particles of about 50 nm which reacted with anti-E2 antibodies were observed in the culture media of tet-induced SH-9 cells following negative staining. Supernatant from tet-treated SH-9 cells was found to infect nai;ve Huh7 and stable Huh7-human CD81 cells.

Pathways that control cortical F-actin dynamics during secretion

Chromaffin cells possess a mesh of filamentous actin underneath the plasma membrane which acts as a barrier to the chromaffin vesicles access to exocytotic sites. Disassembly of cortical F-actin in response to stimulation allows the movement of vesicles from the reserve pool to the release-ready vesicle pool and, therefore, to exocytotic sites. The dynamics of cortical F-actin is controlled by two mechanisms: a) stimulation-induced Ca2+ entry and scinderin activation and b) protein kinase C (PKC) activation and MARCKS phosphorylation as demonstrated here by experiments with recombinant proteins, antisense olygodeoxynucleotides and vector mediated transient expressions. Under physiological conditions (i.e., cholinergic receptor stimulation followed by Ca2+ entry), mechanism (a) is the most important for the control of cortical F-actin network whereas when Ca2+ is released from intracellular stores (i.e., histamine stimulation) cortical F-actin is regulated mainly by mechanism b.

Molecular motors involved in chromaffin cell secretion

Neurosecretory cells, including chromaffin cells, possess a mesh of filamentous actin underneath the plasma membrane. It has been proposed that filamentous actin network separates the secretory vesicles into two compartments: the reserve pool and the release-ready vesicle pool. Disassembly of chromaffin cell cortical filamentous actin in response to stimulation allows the movement of vesicles from the reserve pool into the release-ready vesicle pool. Electron microscopy of cytoskeletons revealed the presence of polygonal areas almost devoid of actin filaments in stimulated cells. The percentage of stimulated cells showing disrupted cytoskeleton correlates well with the increase in secretion in these cells. Fine filaments also remain in these areas of disassembly, and these reacted with actin antibodies, as demonstrated by immunogold staining. In addition, the movement of vesicles between pools requires Ca(2+) and ATP, a condition for activation of a molecular motor. Confocal microscopy images demonstrated colocalization of myosin Va with dopamine-beta-hydroxylase. Cell depolarization induced the dissociation of myosin Va from chromaffin vesicles. 2,3-Butadione-2-monoxime (BDM), an inhibitor of myosin ATPase, inhibited secretion, suggesting a blockage for chromaffin vesicle transport between the reserve pool and the release-ready vesicle pool. On the other hand, myosin II subcellular distribution was not affected by cell depolarization. Confocal microscopy images show myosin II to be localized in the cell cortex and in some perinuclear structures. Chromaffin vesicles were not stained by myosin II antibody.

Localization of phosphatidylinositol 4,5-P(2) important in exocytosis and a quantitative analysis of chromaffin granule motion adjacent to the plasma membrane

A slow ATP-dependent priming step precedes a rapid, Ca(2+)-dependent triggering step in exocytosis in chromaffin cells and in most, if not all, differentiated secretory cells. A major component of ATP-dependent secretion in permeabilized cells reflects the maintenance of the polyphosphoinositides, especially PtdIns-4,5-P2. Here we summarize recent experiments with PH-GFP (binds to PtdIns-4,5-P2) that indicate that PtdIns-4,5-P2 is localized primarily on the plasma membrane in chromaffin cells, and that it is this pool that plays a role in exocytosis. It is demonstrated that transiently expressed PH-GFP inhibits secretion in subsequently permeabilized cells. Recent studies using total internal reflection fluorescent microscopy (TIRFM) to measure chromaffin granule motion adjacent to the plasma membrane are also summarized. The quantitative analysis indicates that chromaffin granule motion is highly restricted and suggests that chromaffin granules are caged or tethered immediately adjacent to the plasma membrane.

Direct effect of acute metabolic and respiratory acidosis on parathyroid hormone secretion in the dog

Because both metabolic (Met Acid) and respiratory acidosis (Resp Acid) have diverse effects on mineral metabolism, it has been difficult to establish whether acidosis directly affects parathyroid hormone (PTH) secretion. Our goal was to determine whether acute Met Acid and Resp Acid directly affected PTH secretion. Three groups of dogs were studied: control, acute Met Acid induced by HCl infusion, and acute Resp Acid induced by hypoventilation. EDTA was infused to prevent acidosis-induced increases in ionized calcium, but more EDTA was needed in Met Acid than in Resp Acid. The PTH response to EDTA-induced hypocalcemia was evaluated also. Magnesium needed to be infused in groups receiving EDTA to prevent hypomagnesemia. The half-life of intact PTH (iPTH) was determined during hypocalcemia when PTH was measured after parathyroidectomy. During normocalcemia, PTH values were greater (p < 0.05) in Met Acid (92 +/- 19 pg/ml) and Resp Acid (77 +/- 22 pg/ml) than in controls (27 +/- 5 pg/ml); the respective pH values were 7.23 +/- 0.01, 7.24 +/- 0.01, and 7.39 +/- 0.02. The maximal PTH response to hypocalcemia was greater (p < 0.05) in Met Acid (443 +/- 54 pg/ml) than in Resp Acid (267 +/- 37 pg/ml) and controls (262 +/- 48 pg/ml). The half-life of PTH was greater (p < 0.05) in Met Acid than in controls, but the PTH secretion rate also was greater (p < 0.05) in Met Acid than in the other two groups. In conclusion, (1) both acute Met Acid and Resp Acid increase PTH secretion when the ionized calcium concentration is normal; (2) acute Met Acid may increase the bone efflux of calcium more than Resp Acid; (3) acute Met Acid acts as a secretogogue for PTH secretion because it enhances the maximal PTH response to hypocalcemia.

Rab3D is not required for exocrine exocytosis but for maintenance of normally sized secretory granules

Rab3D, a member of the Rab3 subfamily of the Rab/ypt GTPases, is expressed on zymogen granules in the pancreas as well as on secretory vesicles in mast cells and in the parotid gland. To shed light on the function of Rab3D, we have generated Rab3D-deficient mice. These mice are viable and have no obvious phenotypic changes. Secretion of mast cells is normal as revealed by capacitance patch clamping. Furthermore, enzyme content and overall morphology are unchanged in pancreatic and parotid acinar cells of knockout mice. Both the exocrine pancreas and the parotid gland show normal release kinetics in response to secretagogue stimulation, suggesting that Rab3D is not involved in exocytosis. However, the size of secretory granules in both the exocrine pancreas and the parotid gland is significantly increased, with the volume being doubled. We conclude that Rab3D exerts its function during granule maturation, possibly by preventing homotypic fusion of secretory granules.

Perturbation of synaptic vesicle delivery during neurotransmitter release triggered independently of calcium influx

Although much evidence suggests that calcium (Ca(2+)) usually triggers synaptic vesicle exocytosis and neurotransmitter release, the role of Ca(2+) in vesicle endocytosis and in the delivery of fusion-competent vesicles (i.e. mobilisation and/or priming) in nerve terminals remains unclear. To address this issue, we have studied synaptic vesicle dynamics in cultured rat neurones under conditions where neurotransmitter release is triggered independently of Ca(2+) using the secretagogue Ruthenium Red (RR). Using a prolonged stimulation protocol, we find that RR causes a rapid increase in neurotransmitter release followed by a gradually decrementing response. In contrast, when release is triggered by moderate membrane depolarisation caused by saline containing 18 mM K(+), release is sustained. These observations suggest that when release is triggered independently of a rise in Ca(2+), endocytosis or vesicle mobilisation/priming are perturbed. Using FM2-10, a fluorescent indicator of synaptic vesicle cycling, we find that neurotransmitter release triggered by RR is accompanied by both uptake and release of this dye, thereby suggesting that vesicle endocytosis is not blocked. To evaluate whether synaptic vesicle mobilisation/priming is perturbed in the absence of a rise in Ca(2+), we compared the kinetics of FM2-10 loss during prolonged stimulation. While 18 mM K(+) induced gradual and continuous dye loss, RR only induced substantial dye loss during the first minute of stimulation. In the presence of low concentrations of the Ca(2+) ionophore ionomycin, release caused by RR was prolonged. Taken together, these results provide evidence suggesting that, although a rise in intraterminal Ca(2+) is not required for endocytosis, it is essential for the continuous delivery of fusion-competent vesicles and to maintain neurotransmitter release during prolonged stimulation.

Granule-specific ATP requirements for Ca2+-induced exocytosis in human neutrophils. Evidence for substantial ATP-independent release

Ca2+-induced exocytosis in neuronal and neuroendocrine cells involves ATP-dependent steps believed to `prime' vesicles for exocytosis. Primed, docked vesicles are released in response to Ca2+ influx through voltage-gated Ca2+ channels. Neutrophils, however, do not possess voltage-gated Ca2+ channels and appear to have no docked vesicles. Furthermore, neutrophils have several types of granules with markedly different Ca2+ requirements for exocytosis. These differential Ca2+ dependencies were used as a tool to investigate the ATP dependence of different granule populations. Here we demonstrate distinct ATP requirements for release of neutrophil granule populations, with respect to rate as well as amplitude. Intracellular ATP was depleted to various levels, and exocytosis was stimulated with different Ca2+concentrations and measured with the patch-clamp capacitance technique or by detecting enzyme release. Primary granule exocytosis displayed a distinct ATP dependence with an apparent Km of ∼80 μM ATP and no ATP-independent exocytosis. Release of secondary and tertiary granules displayed a more shallow ATP dependence (Km∼330 μM), and more than 50% of secondary and tertiary granules appeared not to need ATP at all for their release. Individual granules in human neutrophils have distinct ATP requirements for exocytosis, suggesting that the ATP-sensitive elements are localised to the granules. Primary granule exocytosis has a very low affinity for ATP. Furthermore, substantial ATP-independent exocytosis of secondary and tertiary granule occurs despite the absence of docked granules. These characteristics should help neutrophils to fulfil their bactericidal functions at poorly irrigated sites of infection with low glucose supply.

Roles of ATP in depletion and replenishment of the releasable pool of synaptic vesicles

Synaptic terminals of retinal bipolar neurons contain a pool of readily releasable synaptic vesicles that undergo rapid calcium-dependent release. ATP hydrolysis is required for the functional refilling of this vesicle pool. However, it was unclear which steps required ATP hydrolysis: delivery of vesicles to their anatomical release sites or preparation of synaptic vesicles and/or the secretory apparatus for fusion. To address this, we dialyzed single synaptic terminals with ATP or the poorly hydrolyzable analogue ATP-gammaS and examined the size of the releasable pool, refilling of the releasable pool, and the number of vesicles at anatomical active zones. After minutes of dialysis with ATP-gammaS, vesicles already in the releasable pool could still be discharged. This pool was not functionally refilled despite the fact that its anatomical correlate, the number of synaptic vesicles tethered to active zone synaptic ribbons, was completely normal. We conclude 1) because the existing releasable pool is stable during prolonged inhibition of ATP hydrolysis, whereas entry into the functional pool is blocked, a vesicle on entering the pool will tend to remain there until it fuses; 2) because the anatomical pool is unaffected by inhibition of ATP hydrolysis, failure to refill the functional pool is not caused by failure of vesicle movement; 3) local vesicle movements important for pool refilling and fusion are independent of conventional ATP-dependent motor proteins; and 4) ATP hydrolysis is required for the biochemical transition of vesicles and/or release sites to fusion-competent status.

Imaging secretory vesicles by fluorescent protein insertion in propeptide rather than mature secreted peptide

We combined confocal and live-cell imaging with a novel molecular strategy aimed at revealing mechanisms underlying glucose-regulated insulin vesicle secretion. The 'Ins-C-GFP' reporter monitors secretory peptide targeting, trafficking, and exocytosis without directly tagging the mature secreted peptide. We trapped a green fluorescent protein (GFP) reporter in equimolar quantity within the secretory vesicle by fusing it within the C peptide of proinsulin which only after nascent vesicle sealing and acidification is cleaved from the mature secreted A and B chains of insulin. Ins-C-GFP expression in mouse islets without fail exhibited punctate distribution of green fluorescence by confocal microscopy. Ins-C-GFP colocalized GFP with insulin at vesicle dense cores by immuno-electron microscopy. Glucose stimulation decreased vesicle fluorescence coordinately with enhanced secretion from islets of C-GFP detected by anti-GFP Western blots, and of insulin detected by anti-insulin radioimmunoassay. An insulin secretagogue with a red fluorescent label, glibenclamide BODIPY TR, was applied to islets expressing Ins-C-GFP. The stimulus response was imaged as a rise in red secretagogue leading to marked loss in green granules. Since neuropeptides as well as peptide hormones are processed from propeptides after sealing of secretory granules, vesicle trapping likely is widely applicable for studies on targeting, trafficking, and regulated release of secretory peptides.

Trapping forces, force constants, and potential depths for dielectric spheres in the presence of spherical aberrations

We present and verify a theoretical model that predicts trapping forces (escape forces), force constants (trap stiffnesses), and trapping potential depths for dielectric spheres with diameters smaller than or equal to the wavelength of the trapping light. Optical forces can be calculated for arbitrary incident light distributions with a two-component approach that determines the gradient and the scattering force separately. We investigate the influence of spherical aberrations that are due to refractive-index mismatch on the maximum trapping force, the force constant, and the potential depth of a trap, which are important for optical tweezer applications. The relationships between the three parameters are explained and studied for different degrees of spherical aberration and various spheres (refractive indices n(s) = 1.39-1.57, radii a = 0.1-0.5 microm, lambda(0) = 1.064 microm). We find that all three parameters decrease when the distance to the coverslip increases. Effects that could make the interpretation of experimental results ambiguous are simulated and explained. Computational results are compared with the experimental data found in the literature. A good coincidence can be established.

The SNARE protein SNAP-25 is linked to fast calcium triggering of exocytosis

Synchronous neurotransmission depends on the tight coupling between Ca(2+) influx and fusion of neurotransmitter-filled vesicles with the plasma membrane. The vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein synaptobrevin 2 and the plasma membrane SNAREs syntaxin 1 and synaptosomal protein of 25 kDa (SNAP-25) are essential for calcium-triggered exocytosis. However, the link between calcium triggering and SNARE function remains elusive. Here we describe mutations in two sites on the surface of the SNARE complex formed by acidic and hydrophilic residues of SNAP-25 and synaptobrevin 2, which were found to coordinate divalent cations in the neuronal SNARE complex crystal structure. By reducing the net charge of the site in SNAP-25 we identify a mutation that interferes with calcium triggering of exocytosis when overexpressed in chromaffin cells. Exocytosis was elicited by photorelease of calcium from a calcium cage and evaluated by using patch-clamp capacitance measurements at millisecond time resolution. We present a method for monitoring the dependence of exocytotic rate upon calcium concentration at the release site and demonstrate that the mutation decreased the steepness of this relationship, indicating that the number of sequential calcium-binding steps preceding exocytosis is reduced by one. We conclude that the SNARE complex is linked directly to calcium triggering of exocytosis, most likely in a complex with auxiliary proteins.

Differential regulation of exocytosis by alpha- and beta-SNAPs

We examined the role of SNAPs, soluble proteins that attach N-ethylmaleimide-sensitive factor (NSF), in regulating exocytosis in single rat adrenal chromaffin cells. Whole-cell dialysis of Ca2+-buffered solution or photolysis of caged-Ca2+ was used to manipulate cytosolic Ca2+ concentration ([Ca2+]i), whereas exocytosis was measured via carbon fiber amperometry or membrane capacitance. Buffering [Ca2+]i to approximately 170 nm produced a mean rate of exocytosis of approximately one amperometric event per minute. Including alpha-SNAP (60 or 500 nm) in the intracellular solution dramatically increased the mean rate of exocytosis. The stimulatory action of alpha-SNAP requires ATP hydrolysis mediated via NSF, because this action was blocked by intracellular dialysis of ATP-gamma-S (2 mm) and could not be mimicked by a mutant alpha-SNAP that does not stimulate the ATPase activity of NSF. This action of alpha-SNAP was significant only at [Ca2+]i between 100 and 300 nm and was not shared by beta-SNAP (500 nm), suggesting that alpha-SNAP enhanced a component of exocytosis that is regulated by a high-affinity Ca2+ sensor. In cells dialyzed with both alpha- and beta-SNAP, the rate of exocytosis was smaller than that produced by alpha-SNAP alone, suggesting that alpha- and beta-SNAP interact competitively. Although only alpha-SNAP stimulated exocytosis at [Ca2+]i between 100 and 300 nm, both alpha- and beta-SNAP isoforms equally slowed the time-dependent rundown of the exocytic response. Our results indicate that alpha- and beta-SNAP have different actions in exocytosis. Thus, the ratio of different isoforms of SNAPs can determine release probability at the levels of [Ca2+]i that are involved in regulation of exocytosis.

A subset of 50 secretory granules in close contact with L-type Ca2+ channels accounts for first-phase insulin secretion in mouse beta-cells

Capacitance measurements were applied to mouse pancreatic beta-cells to elucidate the cellular mechanisms underlying biphasic insulin secretion. We report here that only <50 of the beta-cell's >10,000 granules are immediately available for release. The releasable granules tightly associate with the voltage-gated alpha(1C) Ca(2+) channels, and it is proposed that the release of these granules accounts for first-phase insulin secretion. Subsequent replenishment of the releasable pool by priming of previously nonreleasable granules is required for second-phase insulin secretion. The latter reaction depends on intragranular acidification due to the concerted action of granular bafilomycin-sensitive v-type H(+)-ATPase and 4,4-diisothiocyanostilbene-2,2-disulfonate--blockable ClC-3 Cl(-) channels. Lowering the cytoplasmic ATP/ADP ratio prevents granule acidification, granule priming, and refilling of the releasable pool. The latter finding provides an explanation to the transient nature of insulin secretion elicited by, for example, high extracellular K(+) in the absence of metabolizable fuels.

Fast and cAMP-sensitive mode of Ca(2+)-dependent exocytosis in pancreatic beta-cells

The fast component (mode 1) of Ca(2+)-dependent exocytosis in pancreatic beta-cells, unlike that in adrenal chromaffin cells, is regulated by cytosolic ATP in a concentration-dependent manner. This action of ATP is apparent within 3 min and does not require ATP hydrolysis; rather, it requires the production of cAMP by adenylate cyclase. Moreover, the effect of cAMP is ATP dependent, as revealed by the observation that the fast component of exocytosis is facilitated by ATP, even in the presence of a saturating concentration of cAMP (200 micromol/l). Thus, the amplitude of mode-1 exocytosis depends quadratically on the cytosolic ATP concentration and is facilitated by ATP, even in the absence of an increase in the concentration of cAMP. Given that high glucose concentrations increase the cytosolic ATP concentration, glucose-induced insulin secretion likely involves this action of ATP on mode-1 exocytosis, together with its effect on ATP-dependent K(+) channels. In contrast to the fast component of exocytosis, the slow component (mode 2) of this process is independent of cAMP and ATP and can account for the slow component of insulin secretion, which does not require these nucleotides.

Delay between fusion pore opening and peptide release from large dense-core vesicles in neuroendocrine cells

Peptidergic neurotransmission is slow compared to that mediated by classical neurotransmitters. We have studied exocytotic membrane fusion and cargo release by simultaneous capacitance measurements and confocal imaging of single secretory vesicles in neuroendocrine cells. Depletion of the readily releasable pool (RRP) correlated with exocytosis of 10%-20% of the docked vesicles. Some remaining vesicles became releasable after recovery of RRP. Expansion of the fusion pore, seen as an increase in luminal pH, occurred after approximately 0.3 s, and peptide release was delayed by another 1-10 s. We conclude that (1) RRP refilling involves chemical modification of vesicles already in place, (2) the release of large neuropeptides via the fusion pore is negligible and only proceeds after complete fusion, and (3) sluggish peptidergic transmission reflects the time course of vesicle emptying.

Role of Ca(2+) in the synchronization of transmitter release at calyceal synapses in the auditory system of rat

The synchronization of transmitter release in the synapse of the medial nucleus of the trapezoid body (MNTB) is achieved during early postnatal development as a consequence of elimination of delayed asynchronous releases and appears to reflect changes in the dynamics of Ca(2+) entry and clearance. To examine the role of Ca(2+) in regulating synchronization of transmitter release in the mature synapse (after postnatal day 9, P9), we perturbed Ca(2+) dynamics systematically. Replacement of external Ca(2+) (2 mM) with Sr(2+) induced delayed asynchronous release following the major EPSC. We tried to reproduce asynchronous releases without using Sr(2+) and instead by manipulating the time course and the size of Ca(2+) transient in the presynaptic terminal, under the assumption that replacement of external Na(+) with Li(+) or application of eosin-Y would prolong the lifetime of Ca(2+) transient by reducing the rate of Ca(2+) extrusion from the terminal. With application of Li(+), Ca(2+) transient in the terminal was prolonged, the EPSC decay time course was prolonged, and the EPSC amplitude increased. However, these EPSCs were not followed by delayed asynchronous release. When Ca(2+) influx was reduced, either by partial Ca(2+) channel blockade with a low concentration of Cd(2+) or omega-agatoxin IVA, a marked asynchronous release resulted. This was further enhanced by the combined application of Li(+) or eosin-Y. These results suggest that cooperative increases of both Ca(2+) influx and Ca(2+) clearance capacities leading to a sharper Ca(2+) spike in the presynaptic terminal underlie synchronized transmitter release in the presynaptic terminal of the MNTB.

A common molecular machinery for exocytosis and the ‘kiss-and-run’ mechanism in chromaffin cells is controlled by phosphorylation

Exocytosis and ‘kiss-and-run’ secretion coexist in chromaffin cells. Our findings suggest that these mechanisms are closely related, based on their common molecular machinery. Here we present a model that describes how chromaffin cells regulate catecholamine release by switching the mode of secretion between the two pathways, a process controlled by phosphorylation. Stimulation-dependent vesicle-plasma membrane interactions in chromaffin cells were analysed by simultaneous ‘on-cell’ capacitance and conductance measurements, a technique that allows the monitoring of single vesicles. Capacitance steps represent fusions of large dense-core vesicles with the plasma membrane, whereas capacitance flickers correspond to transient connections of the vesicle lumen with the extracellular space. All these events require the presence of extracellular calcium in millimolar concentrations. ‘Kiss-and-run’ type of release is enhanced by the kinase inhibitor staurosporine, which suggests that this secretion mode is regulated by protein phosphorylation. We also observed capacitance bursts, which most probably represent ‘hot spots’ of secretion and we found that ‘kiss-and-run’ is the prevalent mechanism during these episodes. The significance of ‘kiss-and run’ for neurohormone release is even higher at physiological temperature, because up to half of all secretion events are mediated by this mechanism.

Nutrient-secretion coupling in the pancreatic islet beta-cell: recent advances

Insulin secretion from pancreatic islet beta-cells is a tightly regulated process, under the close control of blood glucose concentrations, and several hormones and neurotransmitters. Defects in glucose-triggered insulin secretion are ultimately responsible for the development of type II diabetes, a condition in which the total beta-cell mass is essentially unaltered, but beta-cells become progressively "glucose blind" and unable to meet the enhanced demand for insulin resulting for peripheral insulin resistance. At present, the mechanisms by which glucose (and other nutrients including certain amino acids) trigger insulin secretion in healthy individuals are understood only in part. It is clear, however, that the metabolism of nutrients, and the generation of intracellular signalling molecules including the products of mitochondrial metabolism, probably play a central role. Closure of ATP-sensitive K+(K(ATP)) channels in the plasma membrane, cell depolarisation, and influx of intracellular Ca2+, then prompt the "first phase" on insulin release. However, recent data indicate that glucose also enhances insulin secretion through mechanisms which do not involve a change in K(ATP) channel activity, and seem likely to underlie the second, sustained phase of glucose-stimulated insulin secretion. In this review, I will discuss recent advances in our understanding of each of these signalling processes.

The function of Ca(2+) channel subtypes in exocytotic secretion: new perspectives from synaptic and non-synaptic release

By mediating the Ca(2+) influx that triggers exocytotic fusion, Ca(2+) channels play a central role in a wide range of secretory processes. Ca(2+) channels consist of a complex of protein subunits, including an alpha(1) subunit that constitutes the voltage-dependent Ca(2+)-selective membrane pore, and a group of auxiliary subunits, including beta, gamma, and alpha(2)-delta subunits, which modulate channel properties such as inactivation and channel targeting. Subtypes of Ca(2+) channels are constituted by different combinations of alpha(1) subunits (of which 10 have been identified) and auxiliary subunits, particularly beta (of which 4 have been identified). Activity-secretion coupling is determined not only by the biophysical properties of the channels involved, but also by the relationship between channels and the exocytotic apparatus, which may differ between fast and slow types of secretion. Colocalization of Ca(2+) channels at sites of fast release may depend on biochemical interactions between channels and exocytotic proteins. The aim of this article is to review recent work on Ca(2+) channel structure and function in exocytotic secretion. We discuss Ca(2+) channel involvement in selected types of secretion, including central neurotransmission, endocrine and neuroendocrine secretion, and transmission at graded potential synapses. Several different Ca(2+) channel subtypes are involved in these types of secretion, and their function is likely to involve a variety of relationships with the exocytotic apparatus. Elucidating the relationship between Ca(2+) channel structure and function is central to our understanding of the fundamental process of exocytotic secretion.

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

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

Direct observation of membrane retrieval in chromaffin cells by capacitance measurements

This study was focussed on the identification of the endocytic organelles in chromaffin cells which retrieve large, dense core vesicle (LDCV)-membrane components from the plasma membrane. For this purpose, 'on-cell' capacitance measurements and electron microscopy were employed. We found capacitance steps and capacitance flickers, corresponding to single exo- and endocytic events. The analysis revealed that the total membrane surface of completely fused LDCVs is recycled by large endocytic vesicles and smaller, most likely clathrin-coated vesicles, at approximately the same ratio. These results were confirmed by rapid-freeze immuno-electron microscopy, where an extracellular marker was rapidly internalized into endocytic vesicles that morphologically resembled LDCVs.

Dynamics of immature secretory granules: role of cytoskeletal elements during transport, cortical restriction, and F-actin-dependent tethering

Secretory granules store neuropeptides and hormones and exhibit regulated exocytosis upon appropriate cellular stimulation. They are generated in the trans-Golgi network as immature secretory granules, short-lived vesicular intermediates, which undergo a complex and poorly understood maturation process. Due to their short half-life and low abundance, real-time studies of immature secretory granules have not been previously possible. We describe here a pulse/chase-like system based on the expression of a human chromogranin B-GFP fusion protein in neuroendocrine PC12 cells, which permits direct visualization of the budding of immature secretory granules and their dynamics during maturation. Live cell imaging revealed that newly formed immature secretory granules are transported in a direct and microtubule-dependent manner within a few seconds to the cell periphery. Our data suggest that the cooperative action of microtubules and actin filaments restricts immature secretory granules to the F-actin-rich cell cortex, where they move randomly and mature completely within a few hours. During this maturation period, secretory granules segregate into pools of different motility. In a late phase of maturation, 60% of secretory granules were found to be immobile and about half of these underwent F-actin-dependent tethering.

Quantal size is dependent on stimulation frequency and calcium entry in calf chromaffin cells

To what extent the quantal hypothesis of transmitter release applies to dense-core vesicle (DCV) secretion is unknown. We determined the characteristics of individual secretory events in calf chromaffin cells using catecholamine amperometry combined with different patterns of stimulation. Raising the frequency of action potential trains from 0.25-10 Hz in 2 mM [Ca(2+)]o or [Ca(2+)]o from 0.25-7 mM at 7 Hz elevated the amount released per event (quantal size). With increased stimulation, quantal size rose continuously, not abruptly, suggesting that release efficiency from a single population of DCVs rather than recruitment of different-sized vesicles contributed to the effect. These results suggest that catecholamine secretion does not conform to the quantal model. Inhibition of rapid endocytosis damped secretion in successive episodes, implying an essential role for this process in the recycling of vesicles needed for continuous secretion.

Somatostatin inhibits exocytosis in rat pancreatic alpha-cells by G(i2)-dependent activation of calcineurin and depriming of secretory granules

1. Measurements of cell capacitance were used to investigate the molecular mechanisms by which somatostatin inhibits Ca(2+)-induced exocytosis in single rat glucagon-secreting pancreatic alpha-cells. 2. Somatostatin decreased the exocytotic responses elicited by voltage-clamp depolarisations by 80 % in the presence of cyclic AMP-elevating agents such as isoprenaline and forskolin. Inhibition was time dependent and half-maximal within 22 s. 3. The inhibitory action of somatostatin was concentration dependent with an IC(50) of 68 nM and prevented by pretreatment of the cells with pertussis toxin. The latter effect was mimicked by intracellular dialysis with specific antibodies to G(i1/2) and by antisense oligonucleotides against G proteins of the subtype G(i2). 4. Somatostatin lacked inhibitory action when applied in the absence of forskolin or in the presence of the L-type Ca(2+) channel blocker nifedipine. The size of the omega-conotoxin-sensitive and forskolin-independent component of exocytosis was limited to 60 fF. By contrast, somatostatin abolished L-type Ca(2+) channel-dependent exocytosis in alpha-cells exposed to forskolin. The magnitude of the latter pool amounted to 230 fF. 5. The inhibitory effect of somatostatin on exocytosis was mediated by activation of the serine/threonine protein phosphatase calcineurin and was prevented by pretreatment with cyclosporin A and deltamethrin or intracellularly applied calcineurin autoinhibitory peptide. Experiments using the stable ATP analogue AMP-PCP indicate that somatostatin acts by depriming of granules. 6. We propose that somatostatin receptors associate with L-type Ca(2+) channels and couple to G(i2) proteins leading to a localised activation of calcineurin and depriming of secretory granules situated close to the L-type Ca(2+) channels.

A kinetic analysis of calcium-triggered exocytosis

Although the relationship between exocytosis and calcium is fundamental both to synaptic and nonneuronal secretory function, analysis is problematic because of the temporal and spatial properties of calcium, and the fact that vesicle transport, priming, retrieval, and recycling are coupled. By analyzing the kinetics of sea urchin egg secretory vesicle exocytosis in vitro, the final steps of exocytosis are resolved. These steps are modeled as a three-state system: activated, committed, and fused, where interstate transitions are given by the probabilities that an active fusion complex commits (alpha) and that a committed fusion complex results in fusion, p. The number of committed complexes per vesicle docking site is Poisson distributed with mean n. Experimentally, p and n increase with increasing calcium, whereas alpha and the pn ratio remain constant, reducing the kinetic description to only one calcium-dependent, controlling variable, n. On average, the calcium dependence of the maximum rate (R(max)) and the time to reach R(max) (T(peak)) are described by the calcium dependence of n. Thus, the nonlinear relationship between the free calcium concentration and the rate of exocytosis can be explained solely by the calcium dependence of the distribution of fusion complexes at vesicle docking sites.

Molecular chaperones and the regulation of neurotransmitter exocytosis11Abbreviations: SNARE, soluble NSF attachment protein (SNAP) receptor; NSF, N-ethylmaleimide-sensitive factor; Hsc70, 70-kDa heat-shock cognate protein; CSP, cysteine-string protein; VAMP vesicle-associated membrane protein; SNAP-25, synaptosome-associated protein 25 kDa; NEM, N-ethylmaleimide; AAA ATPases, ATPases Associated to a variety of Activities; and Hsp70, Hsp90, and Hsp60, 70-kDa, 90-kDa, and 60-kDa heat-shock protein, respectively

Regulated neurotransmitter release depends on a precise sequence of events that lead to repeated cycles of exocytosis and endocytosis. These events are mediated by a series of molecular interactions among vesicular, plasma membrane, and cytosolic proteins. An emerging theme has been that molecular chaperones may guide the sequential restructuring of stable or transient protein complexes to promote a temporal and spatial regulation of the endo- and exocytotic machinery and to ensure a vectorial passage through the vesicle cycle. Chaperones, specialized for a few substrates, are ideally suited to participate in regulatory processes that require some molecular dexterity to rearrange conformational or oligomeric protein structures. This article emphasizes the significance of three molecular chaperone systems in regulated neurotransmitter release: the regulation of soluble NSF attachment protein receptor (SNARE) complexes by N-ethylmaleimide-sensitive factor (NSF) and the soluble NSF attachment protein (SNAP), the uncoating of clathrin-coated vesicles by the 70 kDa heat-shock cognate protein (Hsc70), and the regulation of SNARE complex-associated protein interactions by cysteine-string protein and Hsc70.

Secretagogue-dependent phosphorylation of the insulin granule membrane protein phogrin is mediated by cAMP-dependent protein kinase

Phogrin, a 60/64-kDa integral membrane protein of dense-core granules in neuroendocrine cells, is phosphorylated in a Ca(2+)-sensitive manner in response to secretagogue stimulation of pancreatic beta-cells. Phosphorylation of the phogrin cytosolic domain by beta-cell homogenates was Ca(2+)-independent but stimulated by cAMP. Recombinant protein kinase A (PKA) could phosphorylate phogrin directly. High performance liquid chromatography analysis of tryptic phosphopeptides, combined with site-directed mutagenesis of candidate sites, revealed the presence of two phosphorylation sites at Ser-680 and Thr-699, located in the juxtamembrane region between the transmembrane span and the protein-tyrosine phosphatase homology domain of phogrin. Full-length wild-type phogrin, as well as mutant versions where Ser-680 and Thr-699 had been replaced either by alanines or by aspartic acid residues, were targeted to secretory granules in transfected AtT20 neuroendocrine cells. Stimulation of these cells with a range of secretagogues, including K(+), BaCl(2), and forskolin, demonstrated that the in vivo phosphorylation sites are the same as those identified in vitro. In MIN6 beta-cells, the PKA inhibitor H-89 prevented Ca(2+)-dependent phogrin phosphorylation in response to glucose, suggesting that Ca(2+) exerts its effect on phogrin phosphorylation through regulating the activity of PKA.