Fast exocytosis and endocytosis triggered by depolarisation in single adrenal chromaffin cells before rapid Ca2+ current run-down

Ca2+ Dependence of Synaptic Vesicle Endocytosis

Ca(2+)-dependent synaptic vesicle recycling is essential for structural homeostasis of synapses and maintenance of neurotransmission. Although, the executive role of intrasynaptic Ca(2+) transients in synaptic vesicle exocytosis is well established, identifying the exact role of Ca(2+) in endocytosis has been difficult. In some studies, Ca(2+) has been suggested as an essential trigger required to initiate synaptic vesicle retrieval, whereas others manipulating synaptic Ca(2+) concentrations reported a modulatory role for Ca(2+) leading to inhibition or acceleration of endocytosis. Molecular studies of synaptic vesicle endocytosis, on the other hand, have consistently focused on the roles of Ca(2+)-calmodulin dependent phosphatase calcineurin and synaptic vesicle protein synaptotagmin as potential Ca(2+) sensors for endocytosis. Most studies probing the role of Ca(2+) in endocytosis have relied on measurements of synaptic vesicle retrieval after strong stimulation. Strong stimulation paradigms elicit fusion and retrieval of multiple synaptic vesicles and therefore can be affected by several factors besides the kinetics and duration of Ca(2+) signals that include the number of exocytosed vesicles and accumulation of released neurotransmitters thus altering fusion and retrieval processes indirectly via retrograde signaling. Studies monitoring single synaptic vesicle endocytosis may help resolve this conundrum as in these settings the impact of Ca(2+) on synaptic fusion probability can be uncoupled from its putative role on synaptic vesicle retrieval. Future experiments using these single vesicle approaches will help dissect the specific role(s) of Ca(2+) and its sensors in synaptic vesicle endocytosis.

Syndapin 3 modulates fusion pore expansion in mouse neuroendocrine chromaffin cells

Adrenal neuroendocrine chromaffin cells receive excitatory synaptic input from the sympathetic nervous system and secrete hormones into the peripheral circulation. Under basal sympathetic tone, modest amounts of freely soluble catecholamine are selectively released through a restricted fusion pore formed between the secretory granule and the plasma membrane. Upon activation of the sympathoadrenal stress reflex, elevated stimulation drives fusion pore expansion, resulting in increased catecholamine secretion and facilitating release of copackaged peptide hormones. Thus regulated expansion of the secretory fusion pore is a control point for differential hormone release of the sympathoadrenal stress response. Previous work has shown that syndapin 1 deletion alters transmitter release and that the dynamin 1-syndapin 1 interaction is necessary for coupled endocytosis in neurons. Dynamin has also been shown to be involved in regulation of fusion pore expansion in neuroendocrine chromaffin cells through an activity-dependent association with syndapin. However, it is not known which syndapin isoform(s) contributes to pore dynamics in neuroendocrine cells. Nor is it known at what stage of the secretion process dynamin and syndapin associate to modulate pore expansion. Here we investigate the expression and localization of syndapin isoforms and determine which are involved in mediating fusion pore expansion. We show that all syndapin isoforms are expressed in the adrenal medulla. Mutation of the SH3 dynamin-binding domain of all syndapin isoforms shows that fusion pore expansion and catecholamine release are limited specifically by mutation of syndapin 3. The mutation also disrupts targeting of syndapin 3 to the cell periphery. Syndapin 3 exists in a persistent colocalized state with dynamin 1.

Complexin facilitates exocytosis and synchronizes vesicle release in two secretory model systems

Complexins (Cplxs) are small, SNARE-associated proteins believed to regulate fast, calcium-triggered exocytosis. However, studies have pointed to either an inhibitory and/or facilitatory role in exocytosis, and the role of Cplxs in synchronizing exocytosis is relatively unexplored. Here, we compare the function of two types of complexin, Cplx 1 and 2, in two model systems of calcium-dependent exocytosis. In mouse neuromuscular junctions (NMJs), we find that lack of Cplx 1 significantly reduces and desynchronizes calcium-triggered synaptic transmission; furthermore, high-frequency stimulation elicits synaptic facilitation, instead of normal synaptic depression, and the degree of facilitation is highly sensitive to the amount of cytoplasmic calcium buffering. In Cplx 2-null adrenal chromaffin cells, we also find decreased and desynchronized evoked release, and identify a significant reduction in the vesicle pool close to the calcium channels (immediately releasable pool, IRP). Viral transduction with either Cplx 1 or 2 rescues both the size of the evoked response and the synchronicity of release, and it restores the IRP size. Our findings in two model systems are mutually compatible and indicate a role of Cplx 1 and 2 in facilitating vesicle priming, and also lead to the new hypothesis that Cplxs may synchronize vesicle release by promoting coupling between secretory vesicles and calcium channels.

Cav1.3 and Cav1.2 channels of adrenal chromaffin cells: emerging views on cAMP/cGMP-mediated phosphorylation and role in pacemaking

Voltage-gated Ca²⁺ channels (VGCCs) are voltage sensors that convert membrane depolarizations into Ca²⁺ signals. In the chromaffin cells of the adrenal medulla, the Ca²⁺ signals driven by VGCCs regulate catecholamine secretion, vesicle retrievals, action potential shape and firing frequency. Among the VGCC-types expressed in these cells (N-, L-, P/Q-, R- and T-types), the two L-type isoforms, Ca(v)1.2 and Ca(v)1.3, control key activities due to their particular activation-inactivation gating and high-density of expression in rodents and humans. The two isoforms are also effectively modulated by G protein-coupled receptor pathways delimited in membrane micro-domains and by the cAMP/PKA and NO/cGMP/PKG phosphorylation pathways which induce prominent Ca²⁺ current changes if opposingly regulated. The two L-type isoforms shape the action potential and directly participate to vesicle exocytosis and endocytosis. The low-threshold of activation and slow rate of inactivation of Ca(v)1.3 confer to this channel the unique property of carrying sufficient inward current at subthreshold potentials able to activate BK and SK channels which set the resting potential, the action potential shape, the cell firing mode and the degree of spike frequency adaptation during spontaneous firing or sustained depolarizations. These properties help chromaffin cells to optimally adapt when switching from normal to stress-mimicking conditions. Here, we will review past and recent findings on cAMP- and cGMP-mediated modulations of Ca(v)1.2 and Ca(v)1.3 and the role that these channels play in the control of chromaffin cell firing. This article is part of a Special Issue entitled: Calcium channels.

Activity-dependent fusion pore expansion regulated by a calcineurin-dependent dynamin-syndapin pathway in mouse adrenal chromaffin cells

Neuroendocrine chromaffin cells selectively secrete a variety of transmitter molecules into the circulation as a function of sympathetic activation. Activity-dependent release of transmitter species is controlled through regulation of the secretory fusion pore. Under sympathetic tone, basal synaptic excitation drives chromaffin cells to selectively secrete modest levels of catecholamine through a restricted secretory fusion pore. In contrast, elevated sympathetic activity, experienced under stress, results in fusion pore expansion to evoke maximal catecholamine release and to facilitate release of copackaged peptide transmitters. Therefore, fusion pore expansion is a key control point for the activation of the sympatho-adrenal stress response. Despite the physiological importance of this process, the molecular mechanism by which it is regulated remains unclear. Here we employ fluorescence imaging with electrophysiological and electrochemical-based approaches to investigate the role of dynamin I in the regulation of activity-mediated fusion pore expansion in mouse adrenal chromaffin cells. We show that under elevated stimulation, dynamin I is dephosphorylated at Ser-774 by calcineurin. We also demonstrate that disruption of dynamin I-syndapin binding, an association regulated by calcineurin-dependent dynamin dephosphorylation, limits fusion pore expansion. Last, we show that perturbation of N-WASP function (a syndapin substrate) limits activity-mediated fusion pore expansion. Our results suggest that fusion pore expansion is regulated by a calcineurin-dependent dephosphorylation of dynamin I. Dephosphorylated dynamin I acts via a syndapin/N-WASP signaling cascade to mediate pore expansion.

Membrane cycling after the excess retrieval mode of rapid endocytosis in mouse chromaffin cells

AIM

After exocytosis, neuroendocrine cells and neurones keep constant the plasma membrane and the releasable vesicle pools by performing endocytosis and vesicular cycling. Patch-clamp capacitance measurements on chromaffin cells showed that strong Ca(+2) entry activates excess retrieval: a rapid endocytosis process that retrieves more membrane than the one fused by preceding exocytosis. The main purpose of the present experiments was to study the recycling pathway that follows excess retrieval, which is unknown.

METHODS

Membrane recycling after exocytosis-endocytosis can be studied by fluorescence imaging assays with FM1-43 (Perez Bay et al. Am J Physiol Cell Physiol 2007; 293, C1509). In this work, we used this assay in combination with fluorescent dextrans and specific organelle-targeted antibodies to study the membrane recycling after excess retrieval in mouse chromaffin cells.

RESULTS

Excess retrieval was observed after the application of high-K(+) or cholinergic agonists during 15 or 30 s in the presence of FM1-43. We found that the excess retrieval membrane pool (defined as endocytosis-exocytosis) was associated with the generation of a non-releasable fraction of membrane (up to 30% of plasma membrane surface) colocalizing with the lysosomal compartment. The excess retrieval membrane pool followed a saturable cytosolic Ca(2+) dependency, and it was suppressed by inhibitors of L-type Ca(2+) channels, endoplasmic reticulum Ca(2+) release and PKC.

CONCLUSION

Excess retrieval is not associated with the cycling of releasable vesicles, but it is related to the formation of non-releasable endosomes. This process is activated by a concerted contribution of Ca(2+) entry through L-channels and Ca(2+) release from endoplasmic reticulum.

Pituitary adenylate cyclase-activating peptide (PACAP) recruits low voltage-activated T-type calcium influx under acute sympathetic stimulation in mouse adrenal chromaffin cells

Low voltage-activated T-type Ca(v)3.2 calcium channels are expressed in neurosecretory chromaffin cells of the adrenal medulla. Previous studies have shown that naïve adrenal chromaffin cells express a nominal Ca(v)3.2-dependent conductance. However, Ca(v)3.2 conductance is up-regulated following chronic hypoxia or long term exposure to cAMP analogs. Thus, although a link between chronic stressors and up-regulation of Ca(v)3.2 exists, there are no reports testing the specific role of Ca(v)3.2 channels in the acute sympathoadrenal stress response. In this study, we examined the effects of acute sympathetic stress on T-type Ca(v)3.2 calcium influx in mouse chromaffin cells in situ. Pituitary adenylate cyclase-activating peptide (PACAP) is an excitatory neuroactive peptide transmitter released by the splanchnic nerve under elevated sympathetic activity to stimulate the adrenal medulla. PACAP stimulation did not evoke action potential firing in chromaffin cells but did cause a persistent subthreshold membrane depolarization that resulted in an immediate and robust Ca(2+)-dependent catecholamine secretion. Moreover, PACAP-evoked secretion was sensitive to block by nickel chloride and was acutely inhibited by protein kinase C blockers. We utilized perforated patch electrophysiological recordings conducted in adrenal tissue slices to investigate the mechanism of PACAP-evoked calcium entry. We provide evidence that stimulation with exogenous PACAP and native neuronal stress stimulation both lead to a protein kinase C-mediated phosphodependent recruitment of a T-type Ca(v)3.2 Ca(2+) influx. This in turn evokes catecholamine release during the acute sympathetic stress response.

Rapid endocytosis and vesicle recycling in neuroendocrine cells

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

Permissive role of sphingosine on calcium-dependent endocytosis in chromaffin cells

Sphingosine has been shown to modulate neurotransmitter release. Because membrane fusion and fission involve lipid metabolism, we asked here whether sphingosine had a role in regulating endocytosis. To explore this hypothesis, we monitored changes of membrane capacitance (Cm) to study the effects of intracellular sphingosine on membrane retrieval after chromaffin cell stimulation with depolarising pulses (DPs). We found that: (1) sphingosine dialysis through the patch-clamp pipette (SpD) using the whole-cell configuration of the patch-clamp technique (WCC) favours the appearance of a pronounced endocytotic response; (2) SpD-elicited endocytosis was Ca(2+)-dependent but Ba(2+) did not substitute Ca(2+); (3) under WCC, such endocytotic response disappeared with repetitive DPs; (4) in cells preincubated with sphingomyelinase to augment endogenous sphingosine synthesis, and then voltage-clamped under the perforated-patch configuration of the patch-clamp technique (PPC), endocytosis decayed little with repeated stimulation; (5) sphingosine-1-phosphate (S1P), a metabolite of sphingosine, had a meagre effect on endocytosis; and (6) neither dynamin inhibitor dynasore nor calmodulin blocker calmidazolium affected the sphingosine elicited endocytosis. We believe this is the first report showing that sphingosine plays a permissive role in activating Ca(2+)-dependent endocytosis during cell depolarisation. This effect requires high subplasmalemmal cytosolic Ca(2+) concentrations and a cytosolic factor(s) that is dialysed with the pipette solution. Independence of dynamin and calmodulin suggests that sphingosine-dependent endocytosis could be a novel, more direct pathway for vesicle recycling under mild depolarisation stimuli.

Differential variations in Ca2+ entry, cytosolic Ca2+ and membrane capacitance upon steady or action potential depolarizing stimulation of bovine chromaffin cells

AIMS

This study looks into the physiology of the exocytosis of catecholamines released by adrenal medullary chromaffin cells. We have comparatively explored the exocytotic responses elicited by two different patterns of depolarizing stimulation: the widely employed square depolarizing pulses (DPs) and trains of acetylcholine-like action potentials (APs), likely the physiological mode of stimulation in the intact innervated adrenal medulla. APs were applied at 30 Hz, a frequency similar to that produced in a stressful situation.

METHODS

Patch-clamp, cell membrane capacitance, single cell amperometry and fluorescence were the techniques used. The variations of calcium entry measured as the integral of the calcium current, cytosolic calcium (measured with the calcium-sensitive fluorescent probe fluo-4) and exo-endocytosis (membrane capacitance variations) were the parameters measured.

RESULTS

Trains of AP depolarizations produced distinct responses compared to those of square depolarizations: (1) Calcium current amplitude decreased to a lesser extent along the AP train; (2) calcium entry and capacitance increments raised linearly with stimulation time whereas they deviated from linearity when square depolarizations were used; (3) slower activation and faster delayed decay phase of cytosolic calcium transients; (4) capacitance increments varied linearly with calcium entry with APs and deviated from linearity with longer depolarizations; (5) little endocytosis after stimulation with longer trains of APs and pronounced endocytosis with longer square depolarizations.

CONCLUSIONS

Stimulation of chromaffin cells with trains of APs produced patterns of cytosolic calcium transients, exocytotic and endocytotic responses quite different from those elicited by the widely employed DPs. Our study is relevant from the methodological and physiological points of view.

Dynamin I plays dual roles in the activity-dependent shift in exocytic mode in mouse adrenal chromaffin cells

Under low stimulation, adrenal chromaffin cells release freely soluble catecholamines through a restricted granule fusion pore while retaining the large neuropeptide-containing proteinacious granule core. Elevated activity causes dilation of the pore and release of all granule contents. Thus, physiological differential transmitter release is achieved through regulation of fusion pore dilation. We examined the mechanism for pore dilation utilizing a combined approach of peptide transfection, electrophysiology, electrochemistry and quantitative imaging techniques. We report that disruption of dynamin I function alters both fusion modes. Under low stimulation, interference with dynamin I does not affect granule fusion but blocks its re-internalization. In full collapse mode, disruption of dynamin I limits fusion pore dilation, but does not block membrane re-internalization. These data suggest that dynamin I is involved in both modes of exocytosis by regulating contraction or dilation of the fusion pore and thus contributes to activity-dependent differential transmitter release from the adrenal medulla.

Compensatory endocytosis in chromaffin cells

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

A physiological view of the central and peripheral mechanisms that regulate the release of catecholamines at the adrenal medulla

Here we review the tight neural control of the differential secretion into the circulation, of the adrenal medullary hormones adrenaline and noradrenaline. One or the other catecholamines are differentially released on various stress conditions. This is specifically controlled by central nervous system nuclei at the cortex, hypothalamus and spinal cord. Different firing patterns of splanchnic nerves and nicotinic or muscarinic receptors cause the selective release of noradrenaline or adrenaline, to adapt the body to the 'fight or flight' reaction, or during severe hypoglycaemia, haemorrhage, cold, acute myocardial infarction or other severe stressful conflicts. Endogenously acetylcholine (ACh) released at the splanchnic nerve-chromaffin cell synapse, acting on muscarinic and nicotinic receptors, causes membrane depolarization and action potentials (AP) in chromaffin cells. These changes vary with the animal species, the cell preparation (intact bisected adrenal, adrenal slices, or isolated fresh or cultured cells) or the recording technique (intracellular microelectrodes, patch-clamp, perforated-patch, cell-attached). Conflicting results leave many open questions concerning the actions of ACh on chromaffin cell excitability. The use of adrenal slices and field electrical stimulation will surely provide new insights into these mechanisms. Chromaffin cells have been thoroughly used as models to study the relationship between Ca2+ entry, cytosolic Ca2+ signals, exocytosis and endocytosis, using patch-clamp and amperometric techniques. Cells have been stimulated with single depolarizing pulses (DPs), DP trains and with simulated AP waveforms. These approaches have provided useful information but we have no data on APs generated by pulsatile secretory quanta of ACh, trying to mimic the intermittent and repetitive splanchnic nerve discharge of the neurotransmitter. We present some recent experiments using ultrashort ACh pulses (25 ms), that cause non-desensitizing repetitive APs with each ACh pulse, at low ACh concentrations (30 microM). Ultrashort pulses of a high ACh concentration (1000 microM) causes a single AP followed by a prolonged depolarization. It could be interesting trying to correlate these 'patterns of splanchnic nerve discharge' with Ca2+ signals and exocytosis. This, together with the use of adrenal slices and transmural electrical stimulation of splanchnic nerves will provide new physiologically sound data on the regulation of adrenal medullary secretion.

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.

Calcium-regulated fusion of yolk granules is important for yolk degradation during early embryogenesis of Rhodnius prolixusStahl

This study examined the process of membrane fusion of yolk granules (YGs)during early embryogenesis of Rhodnius prolixus. We show that eggs collected at days 0 and 3 after oviposition contain different populations of YGs, for example day-3 eggs are enriched in large YGs (LYGs). Day-3 eggs also contain the highest free [Ca2+] during early embryogenesis of this insect. In vitro incubations of day-0 YGs with [Ca2+]similar to those found in day-3 eggs resulted in the formation of LYGs, as observed in vivo. Fractionation of LYGs and small YGs (SYGs) and their subsequent incubation with the fluorescent membrane marker PKH67 showed a calcium-dependent transference of fluorescence from SYGs to LYGs, possibly as the result of membrane fusion. Acid phosphatase and H+-PPase activities were remarkably increased in day-3 LYGs and in calcium-treated day-0 LYGs. Both fractions were found to contain vitellins as major components, and incubation of YGs with calcium induced yolk proteolysis in vitro. Altogether, our results suggest that calcium-induced membrane fusion events take part in yolk degradation, leading to the assembly of the yolk mobilization machinery.

An activity-dependent increased role for L-type calcium channels in exocytosis is regulated by adrenergic signaling in chromaffin cells

Chromaffin cells of the adrenal medulla represent a primary output of the sympathetic nervous system. Their electrical stimulation evokes the fusion of large dense core granules with the cell membrane and the exocytic release of multiple transmitter molecules into the circulation. There the transmitters contribute to the regulation of basic metabolism of the organism. Under physiological activity, granule fusion and transmitter release are limited by activity-dependent Ca(2+) influx, entering through multiple isoforms of voltage-gated calcium channels. In this study we utilize perforated-patch voltage-clamp recordings and depolarize mouse chromaffin cells in situ with action potential-like waveforms to mimic physiological firing. We measure calcium influx through specific isoforms and measure cell capacitance as an index of granule fusion. Combining these approaches we calculate specific stimulus-secretion efficiencies for L-type, N-type, P/Q-type and R-type calcium channels under varied physiological activity levels. Current influx through all channel subtypes exhibited an activity-dependent depression. As expected P/Q-type channels, while responsible for modest Ca(2+) influx, are tightly coupled to catecholamine secretion under all conditions. We further find that stimulation designed to match sympathetic input under the acute stress response recruits L-type channels to a state of enhanced stimulus-secretion efficiency. N- and R-type channels do not undergo activity-dependent recruitment and remain loosely coupled to the secretion. Thus, only L-type channels exhibit activity-dependent changes in their stimulus-secretion function under physiological stimulation. Lastly, we show that treatment with the beta-adrenergic agonist, isoproterenol, specifically blocks the increase in the stimulus-secretion function of L-type channels. Thus, increased cell firing specifically enhances stimulus-secretion coupling of L-type Ca(2+) channels in chromaffin cells in situ. This mechanism is regulated by an adrenergic signaling pathway.

Calcium-regulated fusion of yolk granules during early embryogenesis ofPeriplaneta americana

This work reported membrane fusion of yolk granules (YGs) during early embryogenesis of the insect Periplaneta americana (P. americana). We showed that eggs from Day 5 of embryogenesis possess a greater amount of enlarged YGs in comparison with Day 1. Day 5 is also the period when the largest amount of free calcium is found (approximately 17 mM) within the oothecae from early embryogenesis. Treatment of Day 1-YGs fraction with 17 mM Ca2+ resulted in a YG size pattern very similar to the one observed in Day 5 eggs, where enlarged YGs were formed. YG membrane fusion was observed by fluorescent membrane dye transfer from previously labeled small YGs to larger ones and was also visualized by electron microscopy. We also showed that the small "in fusion" YGs seemed to be acidic, suggesting that acidification is correlated with YG membrane fusion. Hence, it was shown that YGs are capable of membrane fusion in a calcium-dependent manner and this process probably occurs in vivo during early embryogenesis of P. americana.

Staurosporine restores GTPgammaS induced block of rapid endocytosis in chromaffin cells

Fast capacitance measurements demonstrated that chromaffin cells retrieve membrane by several kinetically different pathways. Here, we show that rapid endocytosis is blocked and slow endocytosis reduced by intracellular application of GTPgammaS, an activator of G-proteins, but not by the competitive blocker GDPbetaS. The inhibition of rapid endocytosis by GTPgammaS can be restored with GDPbetaS or staurosporine completely. But only staurosporine partially abolishes the reduction of slow endocytosis by GTPgammaS. Besides triggering exocytosis, GTPgammaS elicits large exo- and endocytotic vesicles that contributed significantly to the total membrane traffic, indicating a third pathway of membrane shuttle.

Adrenal medulla calcium channel population is not conserved in bovine chromaffin cells in culture

During the stress response adrenal medullary chromaffin cells release catecholamines to the bloodstream. Voltage-activated calcium channels present in the cell membrane play a crucial role in this process. Although the electrophysiological and pharmacological properties of chromaffin cell calcium channels have been studied in detail, the molecular composition of these channels has not been defined yet. Another aspect that needs to be explored is the extent to which chromaffin cells in culture reflect the adrenal medulla calcium channel characteristics. In this sense, it has been described that catecholamine release in the intact adrenal gland recruits different calcium channels than those recruited during secretion from cultured chromaffin cells. Additionally, recent electrophysiological studies show that chromaffin cells in culture differ from those located in the intact adrenal medulla in the contribution of several calcium channel types to the whole cell current. However there is not yet any study that compares the population of calcium channels in chromaffin cells with that one present in the adrenal medulla. In order to gain some insight into the roles that calcium channels might play in the adrenal medullary cells we have analyzed the alpha1 subunit mRNA expression profile. We demonstrate that the expression pattern of voltage-dependent calcium channels in cultured bovine chromaffin cells markedly differs from that found in the native adrenal medulla and that glucocorticoids are only partially involved in those differences. Additionally, we show, for the first time, that the cardiac isoform of L-type calcium channel is present in both bovine adrenal medulla and cultured chromaffin cells and that its levels of expression do not vary during culture.

Low frequency stimulation of mouse adrenal slices reveals a clathrin-independent, protein kinase C-mediated endocytic mechanism

Evidence suggests that chromaffin cells employ separate mechanisms for evoked endocytosis and granule recycling when stimulated at basal (approximately 0.5 Hz) and stress-activated (approximately 15 Hz) rates. Previous studies have focused mainly on elucidating the cellular mechanisms responsible for membrane recycling under conditions similar to the stress-activated state and indicate a clathrin/dephosphin-mediated retrieval via coated pits. However, the mechanism for membrane internalisation at basal stimulus intensity remains largely unexplored. We electrically stimulated chromaffin cells in adrenal tissue slices at the sympathetic basal firing rate and measured cell capacitance in the perforated voltage clamp configuration. A new method for the separation of non-secretory from secretory cell capacitance signals is presented. Simultaneous catecholamine release was measured electrochemically to isolate the exocytic from endocytic components of the capacitance responses. Using this approach we demonstrate that firing patterns that mimic basal sympathetic input results in rapid and graded membrane retrieval. We show that block of the calcium-mediated protein phosphatase 2B, a common step in clathrin-mediated processes, did not alter endocytosis elicited at basal firing levels. We further blocked clathrin-mediated retrieval with a clathrin/dephosphin-disrupting peptide (PP-19) and found endocytosis to be blocked at 15 Hz stimulation but complete and indistinguishable from control cells at 0.5 Hz stimulation. Lastly, pharmacological treatments show that conventional isoforms of protein kinase C (cPKC) are required for the 0.5 Hz-evoked retrieval mechanism. From these data we conclude that unlike endocytosis evoked under stress conditions, basal firing activity results in a clathrin-independent rapid membrane retrieval mediated through conventional isoforms of PKC.

Calcium and calmodulin in membrane fusion

Regulated exocytosis was the first intracellular membrane fusion step that was suggested to involve both Ca(2+) and calmodulin. In recent years, it has become clear that calmodulin is not an essential Ca(2+) sensor for exocytosis but that it is likely to have a more regulatory role. A requirement for cytosolic Ca(2+) in other vesicle fusion events within cells has become apparent and in certain cases, such as homotypic fusion of early endosomes and yeast vacuoles, calmodulin may be the primary Ca(2+) sensor. A number of distinct targets for calmodulin have been identified including SNARE proteins and subunits of the vacuolar ATPase. The extent to which calmodulin regulates different intracellular fusion events through conserved SNARE-dependent or other mechanisms remains to be resolved.

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 in bovine chromaffin cells: studies with patch-clamp capacitance and FM1-43 fluorescence

In response to physiological stimuli, neuroendocrine cells secrete neurotransmitters through a Ca(2+)-dependent fusion of secretory granules with the plasma membrane. We studied insertion of granules in bovine chromaffin cells using capacitance as a measure of plasma membrane area and fluorescence of a membrane marker FM1-43 as a measure of exocytosis. Intracellular dialysis with [Ca(2+)] (1.5-100 microM) evoked massive exocytosis that was sufficient to double plasma membrane area but did not swell cells. In principle, in the absence of endocytosis, the addition of granule membrane would be anticipated to produce similar increases in the capacitance and FM1-43 fluorescence responses. However, when endocytosis was minimal, the changes in capacitance were markedly larger than the corresponding changes in FM1-43 fluorescence. Moreover, the apparent differences between capacitance and FM1-43 fluorescence changes increased with larger exocytic responses, as more granules fused with the plasma membrane. In experiments in which exocytosis was suppressed, increasing membrane tension by osmotically induced cell swelling increased FM1-43 fluorescence, suggesting that FM1-43 fluorescence is sensitive to changes in the membrane tension. Thus, increasing membrane area through exocytosis does not swell chromaffin cells but may decrease membrane tension.

Dynamin-dependent and dynamin-independent processes contribute to the regulation of single vesicle release kinetics and quantal size

Accumulating evidence suggests that the kinetics of release from single secretory vesicles can be regulated and that quantal size can be modified during fast kiss-and-run fusion. Multiple pathways for vesicle retrieval have been identified involving clathrin and dynamin. It has been unclear whether dynamin could participate in a fast kiss-and-run process to reclose a transient fusion pore and thereby limit vesicle release. We have disrupted dynamin function in adrenal chromaffin cells by expression of the amphiphysin Src-homology domain 3 (SH3) or by application of guanosine 5'-[gamma-thio]triphosphate (GTP gamma S), and have monitored single vesicle release events, evoked by digitonin and Ca(2+), by using carbon-fiber amperometry. Under both conditions, there was an increase in mean quantal size accompanying an increase in the half-width of amperometric spikes and a slowing of the fall time. These data suggest the existence of a dynamin-dependent process that can terminate vesicle release under basal conditions. Protein kinase C activation changed release kinetics and decreased quantal size by shortening the release period. The effects of phorbol ester treatment were not prevented by expression of the amphiphysin SH3 domain or by GTP gamma S suggesting the existence of alternative dynamin-independent process underlying fast kiss-and-run exocytosis.

Physiological stimuli evoke two forms of endocytosis in bovine chromaffin cells

1. Exocytosis and endocytosis were measured following single, or trains of, simulated action potentials (sAP) in bovine adrenal chromaffin cells. Catecholamine secretion was measured by oxidative amperometry and cell membrane turnover was measured by voltage clamp cell capacitance measurements. 2. The sAPs evoked inward Na(+) and Ca(2+) currents that were statistically identical to those evoked by native action potential waveforms. On average, a single secretory granule underwent fusion following sAP stimulation. An equivalent amount of membrane was then quickly internalised (tau = 560 ms). 3. Stimulation with sAP trains revealed a biphasic relationship between cell firing rate and endocytic activity. At basal stimulus frequencies (single to 0.5 Hz) cells exhibited a robust membrane internalisation that then diminished as firing increased to intermediate levels (1.9 and 6 Hz). However at the higher stimulation rates (10 and 16 Hz) endocytic activity rebounded and was again able to effectively maintain cell surface near pre-stimulus levels. 4. Treatment with cyclosporin A and FK506, inhibitors of the phosphatase calcineurin, left endocytosis characteristics unaltered at the lower basal stimulus levels, but blocked the resurgence in endocytosis seen in control cells at higher sAP frequencies. 5. Based on these findings we propose that, under physiological electrical stimulation, chromaffin cells internalise membrane via two distinct pathways that are separable. One is prevalent at basal stimulus frequencies, is lessened with increased firing, and is insensitive to cyclosporin A and FK506. A second endocytic form is activated by increased firing frequencies, and is selectively blocked by cyclosporin A and FK506.

Sustained stimulation of exocytosis triggers continuous membrane retrieval in rat pituitary somatotrophs

We studied the relationship between exocytosis and endocytosis in rat pituitary somatotrophs using patch-clamp capacitance, FM1-43 fluorescence imaging and amperometry. Stimulation of exocytosis through voltage-dependent Ca2+ channels by depolarizations (1-5 s) increased the capacitance by 4.3 +/- 0.9 % and the fluorescence by 6.6 +/- 1.1 % (10 cells). The correlation between the capacitance and fluorescence changes indicated that the cell membrane and granule membrane added via exocytosis were stained with the membrane-bound fluorescent dye FM1-43 in a quantitatively similar manner. Intracellular dialysis (0.5-4.5 min) with elevated Ca2+ (1.5-100 microM) evoked continuous exocytosis that was detected with a carbon fibre electrode from dopamine-loaded cells (10 cells) or as an increase in FM1-43 fluorescence (56 +/- 10 %; 21 cells). Interestingly during Ca2+ dialysis the capacitance did not significantly change (2 +/- 1 %; 31 cells), indicating that endocytosis efficiently retrieved increased cell membrane. Sustained endocytosis was not blocked when the intracellular GTP (300 microM) was replaced with GTP[gamma]S. Replacing intracellular Ca2+ (100 microM) with Ba2+ (300 microM) or Sr2+ (200 microM), or reducing the pH of the intracellular solution from 7.2 to 6.2 did not block sustained endocytosis. Our results suggest that pituitary somatotrophs have the ability to undergo continuous exocytosis and membrane retrieval that persist in whole-cell recordings.

Real-time studies of zymogen granule exocytosis in intact rat pancreatic acinar cells

An adequate understanding of secretion requires the measurement of exocytosis on the same time scale as that used for second messenger dynamics. To investigate the kinetics of ACh-evoked secretion in pancreatic acinar cells, exocytosis of zymogen granules was quantified by continuous, time-differential analysis of digital images. The validity of this method was confirmed by simultaneous fluorescence imaging of quinacrine-loaded zymogen granules. Basal rates of exocytosis were low (0.2 events min(-1)). ACh stimulated a biphasic increase in secretory activity, maximal rates exceeding 20 events min(-1) after 10 s of ACh application (10 microM). Over the next 15 s the rate of exocytosis fell to less than 4 events min(-1); then began a second phase of secretion that peaked 15 s later at approximately 11 events min(-1), but subsequently declined in the continued presence of agonist. Measurements of fura-2 fluorescence demonstrated a biphasic increase in intracellular [Ca2+] ([Ca2+]i). Comparison of the [Ca2+]i records and time-differential analysis revealed that the fall in exocytotic rate following the initial burst occurred despite the fact that [Ca2+]i remained high. The second phase of secretion depended on both [Ca2+]i and [ACh]. At 10 microM ACh there was a decrease in the steepness of the relationship between [Ca2+]i and exocytosis that led to an enhancement of the slow secretory phase. We propose that acinar cells contain two pools of secretory vesicles: a small pool of granules that is exocytosed rapidly, but is quickly depleted; and a reserve pool of granules that can be recruited by ACh in a process that is modulated by second messengers other than calcium.

Munc13-1 acts as a priming factor for large dense-core vesicles in bovine chromaffin cells

In chromaffin cells the number of large dense-core vesicles (LDCVs) which can be released by brief, intense stimuli represents only a small fraction of the 'morphologically docked' vesicles at the plasma membrane. Recently, it was shown that Munc13-1 is essential for a post-docking step of synaptic vesicle fusion. To investigate the role of Munc13-1 in LDCV exocytosis, we overexpressed Munc13-1 in chromaffin cells and stimulated secretion by flash photolysis of caged calcium. Both components of the exocytotic burst, which represent the fusion of release-competent vesicles, were increased by a factor of three. The sustained component, which represents vesicle maturation and subsequent fusion, was increased by the same factor. The response to a second flash, however, was greatly reduced, indicating a depletion of release-competent vesicles. Since there was no apparent change in the number of docked vesicles, we conclude that Munc13-1 acts as a priming factor by accelerating the rate constant of vesicle transfer from a pool of docked, but unprimed vesicles to a pool of release-competent, primed vesicles.

Hrs-2 regulates receptor-mediated endocytosis via interactions with Eps15

Hrs-2, via interactions with SNAP-25, plays a regulatory role on the exocytic machinery. We now show that Hrs-2 physically interacts with Eps15, a protein required for receptor-mediated endocytosis. The Hrs-2/Eps15 interaction is calcium dependent, inhibited by SNAP-25 and alpha-adaptin, and results in the inhibition of receptor-mediated endocytosis. Immunoelectron microscopy reveals Hrs-2 localization on the limiting membrane of multivesicular bodies, organelles in the endosomal pathway. These data show that Hrs-2 regulates endocytosis, delineate a biochemical pathway (Hrs-2-Eps15-AP2) in which Hrs-2 functions, and suggest that Hrs-2 acts to provide communication between endo- and exocytic processes.

Mechanisms of synaptic vesicle recycling illuminated by fluorescent dyes

The recycling of synaptic vesicles in nerve terminals involves multiple steps, underlies all aspects of synaptic transmission, and is a key to understanding the basis of synaptic plasticity. The development of styryl dyes as fluorescent molecules that label recycling synaptic vesicles has revolutionized the way in which synaptic vesicle recycling can be investigated, by allowing an examination of processes in neurons that have long been inaccessible. In this review, we evaluate the major aspects of synaptic vesicle recycling that have been revealed and advanced by studies with styryl dyes, focussing upon synaptic vesicle fusion, retrieval, and trafficking. The greatest impact of styryl dyes has been to allow the routine visualization of endocytosis in central nerve terminals for the first time. This has revealed the kinetics of endocytosis, its underlying sequential steps, and its regulation by Ca2+. In studies of exocytosis, styryl dyes have helped distinguish between different modes of vesicle fusion, provided direct support for the quantal nature of exocytosis and endocytosis, and revealed how the probability of exocytosis varies enormously from one nerve terminal to another. Synaptic vesicle labelling with styryl dyes has helped our understanding of vesicle trafficking by allowing better understanding of different synaptic vesicle pools within the nerve terminal, vesicle intermixing, and vesicle clustering at release sites. Finally, the dyes are now being used in innovative ways to reveal further insights into synaptic plasticity.

An efficient method for infection of adrenal chromaffin cells using the Semliki Forest virus gene expression system

We have expanded the use of the Semliki Forest virus (SFV) by infecting chromaffin cells with synaptic proteins at high efficiency. Using the SFV gene expression system, up to 40% of cultured bovine chromaffin cells express the protein of interest within 12-48 h after infection. In order to learn about the basic physiological properties of infected cells, we performed membrane capacitance measurements using the whole-cell patch-clamp technique and monitored catecholamine release with amperometry. We found that chromaffin cells infected with green fluorescent protein (GFP) were comparable to control cells in intracellular calcium concentrations ([Ca2+]i), leak currents and cell sizes. In response to depolarization, calcium currents were elicited and the cells secreted catecholamine. Comparison of the calcium current amplitude and the size of the readily releasable pool of vesicles revealed a small decrease in these parameters compared to control cells. The refilling kinetics after pool depletion, however, were not altered. Overexpressed munc13-1 translocates to the plasma membrane in response to phorbol esters, an effect that is also observed in fibroblasts transfected with conventional methods. Thus, the use of the SFV gene expression system to infect chromaffin cells represents a major improvement in infection efficiency compared to other methods. It opens up new opportunities to introduce synaptic proteins into chromaffin cells and study their role in secretion.

Properties of fast endocytosis at hippocampal synapses

Regulation of synaptic transmission is a widespread means for dynamic alterations in nervous system function. In several cases, this regulation targets vesicular recycling in presynaptic terminals and may result in substantial changes in efficiency of synaptic transmission. Traditionally, experimental accessibility of the synaptic vesicle cycle in central neuronal synapses has been largely limited to the exocytotic side, which can be monitored with electrophysiological responses to neurotransmitter release. Recently, physiological measurements on the endocytotic portion of the cycle have been made possible by the introduction of styryl dyes such as FM1-43 as fluorescent markers for recycling synaptic vesicles. Here we demonstrate the existence of fast endocytosis in hippocampal nerve terminals and derive its kinetics from fluorescence measurements using dyes with varying rates of membrane departitioning. The rapid mode of vesicular retrieval was greatly speeded by exposure to staurosporine or elevated extracellular calcium. The effective time-constant for retrieval can be < 2 seconds under appropriate conditions. Thus, hippocampal synapses capitalize on efficient mechanisms for endocytosis and their vesicular retrieval is subject to modulatory control.

Rapid exocytosis in single chromaffin cells recorded from mouse adrenal slices

We report here that brief depolarizations such as action potentials trigger exocytosis in thin mouse adrenal slices. The secretory rates obtained in membrane capacitance recordings from chromaffin cells in slices are faster than those observed in isolated cells. Fast exocytosis in slices is attributable to the rapid release of a small pool of vesicles. The pool recovers from depletion with a time constant of 10 sec. Recruitment of the rapidly released vesicles is strongly hindered by the fast Ca2+ chelator BAPTA and much less by the slower chelator EGTA. We suggest that these vesicles are located in close proximity to Ca2+ channels. Spatial coupling of Ca2+ entry and exocytosis may be sensitive to cell isolation and culture.

A persistent activity-dependent facilitation in chromaffin cells is caused by Ca2+ activation of protein kinase C

Activity-dependent facilitation was studied in bovine adrenal chromaffin cells. Stimulation with a train of depolarizations caused subsequent triggered exocytotic activity to be significantly enhanced. After the facilitating stimulus train, the readily releasable vesicle pool (RRP) size was estimated from capacitance jumps in response to paired depolarizations and found to be elevated for a period of at least 10 min. The time dependency of onset and degree of facilitation could be well fitted assuming protein kinase C (PKC)-dependent and independent Ca2+-mediated processes. Both processes increase the recruitment of vesicles from the reserve pool to the RRP, resulting in an greater number of releasable vesicles. The data suggest that cell activity can act as a trigger to increase cytosolic Ca2+ to a level sufficient to cause an increase in the number of readily releasable secretory vesicles, with the more persistent component of the evoked facilitation being mediated through activity-dependent activation of PKC.

Effect of Ca2+ and cAMP on capacitance-measured hormone secretion in human GH-secreting adenoma cells

Membrane capacitance (Cm) was measured as an index of exocytosis in human growth hormone-secreting adenoma cells using the perforated whole cell, patch-clamp technique; the effects of membrane depolarization, growth hormone-releasing hormone, and 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) were examined. Cm was increased by membrane depolarization to potentials beyond the threshold necessary to open voltage-gated Ca2+ channels. These voltage-dependent changes in Cm varied as a function of both depolarization amplitude and duration and were blocked in the presence of the Ca2+ channel antagonist nitrendipine (10(-6) M). When membrane potential was clamped at the holding potential (-78 mV), voltage-gated Ca2+ channels were closed, and neither application of growth hormone-releasing hormone nor 8-BrcAMP affected Cm. However, when these agents were applied to depolarized cells, where the voltage-gated Ca2+ channels were open, the increases in Cm were augmented. From these data, it was concluded that elevation of intracellular cAMP, per se, did not stimulate exocytosis. Rather, Ca2+ influx through voltage-gated channels was a prerequisite for cAMP-induced exocytosis.

Kinetics and regulation of fast endocytosis at hippocampal synapses

Presynaptic nerve terminals often contain as few as a hundred vesicles and so must recycle them soon after exocytosis to preserve synaptic transmission and presynaptic morphology during repetitive firing. The kinetics and mechanisms of vesicular endocytosis and repriming have therefore been studied. Vesicles in hippocampal nerve terminals can become available to release their contents within approximately 40 s of the previous round of exocytosis. Studies using the styryl dye FM1-43 have estimated the time constant for endocytosis as approximately 20-30 s at least half of the total recycling time, which is much slower than endocytosis in other secretory systems. It seems paradoxical that the neurosecretory terminals that could benefit the most from rapid endocytosis do not use such a mechanism. Here we demonstrate the existence of fast endocytosis in hippocampal nerve terminals and derive its kinetics from fluorescence measurements using dyes with varying rates of membrane departitioning. The rapid mode of vesicular retrieval was much faster after exposure to staurosporine or elevated extracellular calcium. Thus hippocampal synapses take advantage of efficient mechanisms for endocytosis, and their vesicular retrieval is subject to modulatory control.

The synaptic vesicle cycle

The ins and outs of the synaptic vesicle cycle are being examined in increasing detail with diverse investigative tools in a variety of cell types, particularly those with large granules. The cycle begins with the opening of a fusion pore that connects the vesicle lumen to the extracellular fluid. Sensitive electrophysiological techniques reveal the often-stuttering behavior of single pores in non-neuronal cells, through which small molecules trickle until the fusion pore expands and the remaining contents erupt from the vesicle. The granule membranes are then retrieved by multiple processes that appear to act in parallel and that are distinguished from each other kinetically and ultrastructurally. Following endocytosis, synaptic vesicles are then shuttled back into the vesicle pool, where they briefly mix with other vesicles, become immobilized, and remain gelled with their neighbors, even while moving en masse again to the presynaptic membrane as a prelude for another round of exocytosis.

Barium triggers rapid endocytosis in calf adrenal chromaffin cells

1. Changes in cell capacitance were monitored in whole-cell patch-clamp recordings from calf adrenal chromaffin cells using a software-based phase-tracking technique. Rapid endocytosis and exocytosis were observed in extracellular solutions containing either Ca2+ or Ba2+. 2. There was no significant difference in the magnitude or the time course of rapid endocytosis of cells stimulated in Ca2+ as compared to Ba2+. When cells were pretreated with caffeine and thapsigargin in order to deplete intracellular Ca2+ stores, rapid endocytosis in Ba2+ was not affected. This indicates that Ba2+ itself is capable of supporting rapid endocytosis. 3. The application of the calmodulin inhibitor calmidazolium via the intracellular pipette solution did not inhibit rapid endocytosis. Although our findings are inconsistent with an immediate requirement for calmodulin in rapid endocytosis, they do not rule out an involvement on a longer time scale. 4. While rapid endocytosis was not affected by the substitution of Ca2+ with Ba2+, the maximum rate of exocytosis was higher in cells stimulated in Ca2+ than in Ba2+. Since Ba2+ currents were much larger than Ca2+ currents during depolarizations to +10 mV (the test potential used in these experiments), Ba2+ appears to be less efficient at promoting exocytosis than Ca2+.

Compensatory and excess retrieval: two types of endocytosis following single step depolarizations in bovine adrenal chromaffin cells

1. Endocytosis following exocytosis evoked by single step depolarizations was examined in bovine adrenal chromaffin cells using high resolution capacitance measurements in perforated-patch voltage clamp recordings. 2. Endocytosis was detected as a smooth exponential decline in membrane capacitance to either the pre-stimulus level ('compensatory retrieval') or far below the pre-stimulus level ('excess retrieval'). During excess retrieval, > 10% of the cell surface could be internalized in under 5 s. 3. Compensatory retrieval was equal in magnitude to stimulus-evoked exocytosis for membrane additions > 100 fF (about fifty large dense-cored vesicles). In contrast, excess retrieval surpassed both the stimulus-evoked exocytosis, and the initial capacitance level recorded at the onset of phase-tracking measurements. Cell capacitance was not maintained at the level achieved by excess retrieval but slowly returned to pre-stimulus levels, even in the absence of stimulation. 4. A large percentage of capacitance increases < 100 fF, usually evoked by 40 ms depolarizations, were not accompanied by membrane retrieval. 5. Compensatory retrieval could occur with any amount of Ca2+ entry, but excess retrieval was never triggered below a threshold Ca2+ current integral of 70 pC. 6. The kinetics of compensatory and excess retrieval differed by an order of magnitude. Compensatory retrieval was usually fitted with a single exponential function that had a median time constant of 5.7 s. Excess retrieval usually occurred with double exponential kinetics that had an extremely fast first time constant (median, 670 ms) and a second time constant indistinguishable from that of compensatory retrieval. 7. The speed of compensatory retrieval was Ca2+ dependent: the largest mono-exponential time constants occurred for the smallest amounts of Ca2+ entry and decreased with increasing Ca2+ entry. The Ca2+ dependence of mono-exponential time constants was disrupted by cyclosporin A (CsA), an inhibitor of the Ca(2+)- and calmodulin-dependent phosphatase calcineurin. 8. CsA also reduced the proportion of responses with excess retrieval, but this action was caused by a shift in Ca2+ entry values below the threshold for activation. The lower total Ca2+ entry in the presence of CsA was due to an increase in the rate of Ca2+ current inactivation rather than a reduction in peak amplitude. 9. Our data suggest that compensatory and excess retrieval represent two independent, Ca(2+)-regulated mechanisms of rapid membrane internalization in bovine adrenal chromaffin cells. Alternatively, there is a single membrane internalization mechanism that can switch between two distinct modes of behaviour.

Spontaneous calcium oscillations in embryonic stem cell-derived primitive endodermal cells

In vitro differentiation of mouse embryonic stem cells within three-dimensional cell aggregates called embryoid bodies parallels the development of postimplantation embryos at the egg cylinder stage, where visceral and parietal endoderm diverge from the primitive endoderm. We have investigated spontaneous [Ca2+]i oscillations by means of confocal laser-scanning microscopy in primitive endodermal cell layers of embryoid bodies during their differentiation to parietal and visceral endoderm. The frequency of [Ca2+]i oscillations increased from day 4 to day 19 of development, whereas their duration decreased from day 3 to days 16-17. Oscillations depended on both extracellular Ca2+ and Ca2+ release from intracellular stores as they were abolished in Ca(2+)-free solution and in the prescence of Ni2+ and thapsigargin. Signal transduction operated via the phospholipase C (PLC)-mediated inositol 1,4,5-triphosphate (InsP3) pathway with a negative feedback loop via protein kinase C (PKC) as U73,122, a blocker of PLC; bisindolylmaleimide 1, staurosporine, and H-7, blockers of PKC; and 10 mM caffeine totally inhibited [Ca2+]i spiking. Thimerosal, which hypersensitizes the InsP3 receptor, as well as vasopressin and bradykinin, which act via the InsP3 pathway, increased the frequency of [Ca2+]i spikes. In the prescence of brefeldin A (50 microM) or monensin (20 microM), which both inhibit endo/exocytotic vesicle pathways, an immediate transient increase in spiking activity was followed by a decline within 1 to 2 h. In the presence of brefeldin A or thapsigargin or in the absence of extracellular Ca2+, endocytotic vesicles were absent, suggesting that oscillating [Ca2+]i transients are involved in the exo/endocytotic vesicle shuttle.

Short-term changes in the Ca2+-exocytosis relationship during repetitive pulse protocols in bovine adrenal chromaffin cells

Stimulus-secretion coupling was monitored with capacitance detection in bovine chromaffin cells recorded in perforated patch mode and stimulated with trains of depolarizing pulses. A subset of stimulus trains evoked a response with a Ca2+-exocytosis relationship identical to that obtained for single depolarizing pulses (Engisch and Nowycky, 1996). Other trains evoked responses with enhanced or diminished Ca2+ efficacy relative to this input-output function. The probability of obtaining a particular Ca2+-exocytosis relationship was correlated with the amount of Ca2+ entry per pulse, such that shorter pulses or smaller currents were associated with the greatest efficacy, and longer pulses and larger currents with the lowest efficacy. Apparent enhancements in Ca2+ efficacy were not caused by residual Ca2+ summing between pulses, because decreasing the interval between pulses usually reduced efficacy in the same cell; conversely, increasing the interval between pulses did not prevent an enhanced Ca2+-exocytosis relationship. Apparent decreases in Ca2+ efficacy were not caused by depletion of an available pool of release-ready vesicles, because an equivalent amount of total Ca2+ entry during a single long depolarizing pulse usually evoked a much larger secretory response in the same cell. Finally, there were no striking differences in global Ca2+ levels monitored with the fluorescent indicator Fura Red that could account for apparent changes in Ca2+ efficacy during repetitive stimulus protocols. It appears that in chromaffin cells, the Ca2+-exocytosis relationship is subject to activity-dependent changes during a stimulus train and can be modulated up or down from a basal state accessed by single pulse stimulations.

Multiple forms of endocytosis in bovine adrenal chromaffin cells

We studied endocytosis in chromaffin cells with both perforated patch and whole cell configurations of the patch clamp technique using cell capacitance measurements in combination with amperometric catecholamine detection. We found that chromaffin cells exhibit two relatively rapid, kinetically distinct forms of stimulus-coupled endocytosis. A more prevalent "compensatory" retrieval occurs reproducibly after stimulation, recovering an approximately equivalent amount of membrane as added through the immediately preceding exocytosis. Membrane is retrieved through compensatory endocytosis at an initial rate of approximately 6 fF/s. Compensatory endocytotic activity vanishes within a few minutes in the whole cell configuration. A second form of triggered membrane retrieval, termed "excess" retrieval, occurs only above a certain stimulus threshold and proceeds at a faster initial rate of approximately 248 fF/s. It typically undershoots the capacitance value preceding the stimulus, and its magnitude has no clear relationship to the amount of membrane added through the immediately preceding exocytotic event. Excess endocytotic activity persists in the whole cell configuration. Thus, two kinetically distinct forms of endocytosis coexist in intact cells during perforated patch recording. Both are fast enough to retrieve membrane after exocytosis within a few seconds. We argue that the slower one, termed compensatory endocytosis, exhibits properties that make it the most likely mechanism for membrane recycling during normal secretory activity.

Purinergic agonists, but not cAMP, stimulate coupled granule fusion and Cl- conductance in HT29-Cl.16E

Cl- conductance and capacitance were simultaneously measured in the mucin-secreting cell line, HT29-Cl.16E (Cl.16E), and its sister cell line, HT29-Cl.19A (Cl.19A), which lacks mucin granules. Purinergic stimulation by extracellular ATP transiently increased Cl- conductance in both cell lines with similar peak increases of 0.92 and 1.00 nS/pF in Cl.16E and Cl.19A cells, respectively (baseline of 0.08 nS/pF). Cell capacitance increased only in Cl.16E cells (17% above baseline of 22 pF in Cl.16E and 1% above baseline of 18 pF in Cl.19A cells). Wortmannin inhibited the purinergically activated Cl- conductance and capacitance increases in Cl.16E by 50 and 80%, respectively, but had no effects in Cl.19A cells. In Cl.16E cells, adenosine 3',5'-cyclic monophosphate (cAMP) signaling increased Cl- conductance from 0.08 to 0.52 nS/pF without changing capacitance. Cl- secretion in Cl.16E monolayers was additive in response to supramaximal stimulation of purinergic receptors and adenylyl cyclase, even though granule fusion is nine times greater with purinergic than adenylyl cyclase stimulation. In conclusion, 1) wortmannin does not directly inhibit activation of Cl- conductance, 2) at least 50% of purinergically activated Cl- conductance in Cl.16E is associated with granule fusion, and 3) cAMP-activated Cl- conductance is not associated with granules.

Estimation of mean exocytic vesicle capacitance in mouse adrenal chromaffin cells

Whole-cell membrane capacitance measurements are frequently used to monitor neuronal and nonneuronal secretory activity. However, unless individual fusion events can be resolved, the type of the fusing vesicles cannot be identified in these experiments. Here we apply statistical analysis of trial-to-trial variations between depolarization-induced capacitance increases of mouse adrenal chromaffin cells and obtain estimates for the capacitance contribution of individual exocytic vesicles between 0.6 and 2 fF. For comparison, measurements of membrane capacitance were combined with amperometric recordings of catecholamine release during intracellular perfusion of chromaffin cells with high [Ca2+]. Crosscorrelation of both signals yielded a mean capacitance contribution of individual catecholaminergic vesicles of 1.3 fF. We suggest that depolarization-induced capacitance increases in mouse adrenal chromaffin cells mainly represent fusion of chromaffin granules.

Intracellular acidification reversibly reduces endocytosis at the neuromuscular junction

The close spatial and temporal coupling of endocytosis and exocytosis in nerve terminals has made it difficult to elucidate the mechanisms and the regulation of endocytosis per se. Despite significant advances in our knowledge of the molecules involved in endocytosis, it has not yet been possible to selectively manipulate endocytosis in nerve terminals. We report that the substitution of propionate for chloride in the saline bathing a lizard neuromuscular junction reduces internal pH and reversibly blocks activity-dependent endocytosis. When intraterminal pH is reduced by approximately 0.7 pH units, the uptake of FM1-43 in nerve terminals, but not activity-dependent destaining, is reduced. Normalization of intracellular pH by removing the propionate, raising extracellular pH, or adding ammonium chloride immediately restores FM1-43 uptake. Electron microscopy indicates that intracellular acidification reversibly reduces activity-dependent endocytosis in nerve terminals, because depolarization in propionate saline leads to a depletion of vesicles and the appearance of large intramembraneous infoldings.

Depletion and replenishment of vesicle pools at a ribbon-type synaptic terminal

Synaptic depression was studied using capacitance measurements in synaptic terminals of retinal bipolar neurons. Single 250 msec depolarizations evoked saturating capacitance responses averaging approximately 150 fF, whereas trains of 250 msec depolarizations produced plateau capacitance increases of approximately 300 fF. Both types of stimuli were followed by pronounced synaptic depression, which recovered with a time constant of approximately 8 sec after single pulses but required >20 sec for full recovery after pulse trains. Inactivation of presynaptic calcium current could not account for depression, which is attributed instead to depletion of releasable and reserve vesicle pools that are recruited and replenished at different rates. Recovery from depression was normal in the absence of fast endocytosis, suggesting that replenishment was from a reserve pool of preformed vesicles rather than from preferential recycling of recently fused vesicles. Given the in vivo light response of the class of bipolar neuron studied here, it is likely that, under at least some illumination conditions, the cells produce a fast and phasic bout of exocytosis rather than tonic release.

Nerve activity but not intracellular calcium determines the time course of endocytosis at the frog neuromuscular junction

We used FM1-43 imaging and intracellular recordings of synaptic potentials to measure the time course of endocytosis in frog motor nerve terminals following tetanic nerve stimulation, and we used fura-2 imaging of intraterminal Ca2+ concentration to compare endocytic rate and [Ca2+]i. Following a 30 Hz tetanus, endocytosis declined exponentially with a time constant that depended on the duration of stimulation. The level of [Ca2+]i rose from a resting value of about 100 nM to more than 500 nM during 30 Hz stimulation, and rapidly declined to 200-250 nM after stimulation. [Ca2+]i returned to resting level with a time course that, like endocytosis, depended on the duration of tetanic stimulation. However, the rate of [Ca2+]i recovery was much slower than the rate of endocytosis, leading to the conclusion that endocytic rate is not determined solely by the instantaneous level of [Ca2+]i.

Patch-clamp capacitance analysis of the effects of alpha-SNAP on exocytosis in adrenal chromaffin cells

We have examined the effect of alpha-SNAP on exocytosis in adrenal chromaffin cells by direct assay of exocytosis using patch-clamp capacitance analysis. Cells were recorded using the whole cell patch-clamp configuration and the cells dialysed with control pipette solution or with a pipette solution containing alpha-SNAP or the deletion mutant alpha-SNAP(41–295). The deletion mutant was found to be unable to bind to syntaxin allowing a test of the requirement for syntaxin-binding for any effect of alpha-SNAP on exocytosis. Following cell dialysis for 10 minutes, cells were depolarised five times at 2 minute intervals. At each depolarisation step cells dialysed with alpha-SNAP showed a significant increase in both the initial rate and extent of exocytosis which was seen as a rise in membrane capacitance. This increase in exocytosis was not observed with alpha-SNAP(41–295) which instead produced some inhibition of the extent but had no effect on the initial rate of exocytosis. These results show directly that alpha-SNAP has a specific and marked stimulatory effect on exocytosis in chromaffin cells.