Exocytotic exposure and retrieval of membrane antigens of chromaffin granules: quantitative evaluation of immunofluorescence on the surface of chromaffin cells

Basal and Stress-Induced Network Activity in the Adrenal Medulla In Vivo

The adrenal medulla plays a critical role in mammalian homeostasis and the stress response. It is populated by clustered chromaffin cells that secrete epinephrine or norepinephrine along with peptides into the bloodstream affecting distant target organs. Despite been heavily studied, the central control of adrenal medulla and in-situ spatiotemporal responsiveness remains poorly understood. For this work, we continuously monitored the electrical activity of individual adrenomedullary chromaffin cells in the living anesthetized rat using multielectrode arrays. We measured the chromaffin cell activity under basal and physiological stress conditions and characterized the functional micro-architecture of the adrenal medulla. Under basal conditions, chromaffin cells fired action potentials with frequencies between ~0.2 and 4 Hz. Activity was almost completely driven by sympathetic inputs coming through the splanchnic nerve. Chromaffin cells were organized into independent local networks in which cells fired in a specific order, with latencies from hundreds of microseconds to a few milliseconds. Electrical stimulation of the splanchnic nerve evoked almost exactly the same spatiotemporal firing patterns that occurred spontaneously. Hypoglycemic stress, induced by insulin administration resulted in increased activity of a subset of the chromaffin cells. In contrast, respiratory arrest induced by lethal anesthesia resulted in an increase in the activity of virtually all chromaffin cells before cessation of all activity. These results suggest a stressor-specific activation of adrenomedullary chromaffin cell networks and revealed a surprisingly complex electrical organization that likely reflects the dynamic nature of the adrenal medulla's neuroendocrine output during basal conditions and during different types of physiological stress.

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

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

The distribution of mitochondria and endoplasmic reticulum in relation with secretory sites in chromaffin cells

The distribution of mitochondria and ER in relation to exocytotic sites is relevant to understand the influence of these organelles in tuning calcium signals and secretion. Confocal images of probes tagged to mitochondria and F-actin cytoskeleton revealed the existence of two populations of mitochondria, one cortical and the other perinuclear. This mitochondrial distribution was also confirmed by using electron microscopy. In contrast, ER was sparse in the cortex and more abundant in deep cytoplasmic regions. The mitochondrial distribution may be due to organellar transport, which experiences increasing restrictions in the cell cortex. Further study of organelle distribution in relation to SNARE microdomains or the granule fusion sites revealed that 1/3 of the cortical mitochondria co-localized with exocytotic sites whereas another 1/3 located at a distance smaller than 2 vesicle diameters. ER structures were also present in the vicinity of secretory sites but at a lower density. Therefore, mitochondria and ER have a spatial distribution that suggests a specialized role in modulation of exocytosis and fits with cytosolic Ca2+ microdomains described before.

Exocytosis and endocytosis in neuroendocrine cells: inseparable membranes!

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

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

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

Tight coupling of the t-SNARE and calcium channel microdomains in adrenomedullary slices and not in cultured chromaffin cells

Regulated exocytosis involves calcium-dependent fusion of secretory vesicles with the plasma membrane with three SNARE proteins playing a central role: the vesicular synaptobrevin and the plasma membrane syntaxin1 and SNAP-25. Cultured bovine chromaffin cells possess defined plasma membrane microdomains that are specifically enriched in both syntaxin1 and SNAP-25. We now show that in both isolated cells and adrenal medulla slices these target SNARE (t-SNARE) patches quantitatively coincide with single vesicle secretory spots as detected by exposure of the intravesicular dopamine beta-hydroxylase onto the plasmalemma. During exocytosis, neither area nor density of the syntaxin1/SNAP-25 microdomains changes on the plasma membrane of both preparations confirming that preexisting clusters act as the sites for vesicle fusion. Our analysis reveals a high level of colocalization of L, N and P/Q type calcium channel clusters with SNAREs in adrenal slices; this close association is altered in individual cultured cells. Therefore, microdomains carrying syntaxin1/SNAP-25 and different types of calcium channels act as the sites for physiological granule fusion in "in situ" chromaffin cells. In the case of isolated cells, it is the t-SNAREs microdomains rather than calcium channels that define the sites of exocytosis.

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

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

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.

Transforming growth factor beta2 is released from PC12 cells via the regulated pathway of secretion

Transforming growth factor beta2 (TGF-beta2), a prototypic member of a large superfamily of multifunctional cytokines, is expressed by neurons and glial cells. Its subcellular compartmentalization and release from neurons, however, are largely unknown. Here we show that TGF-beta2 colocalizes with the trans-Golgi network marker TGN38 and a marker molecule for secretory granules, chromogranin B (CgB), in PC12 cells. Similarly, primary hippocampal neurons show colocalization of TGN38 and TGF-beta2. A substantial amount of endogenous as well as transfected TGF-beta2 in PC12 cells comigrates with CgB on an equilibrium gradient, suggesting costorage in secretory granules. TGF-beta biological activity peaks in identical fractions. Depolarization of PC12 cells with high potassium triggers colocalization of CgB and TGF-beta2 at the cell surface, suggesting their regulated corelease from secretory granules. High potassium also liberates biologically active TGF-beta from PC12 cells and primary neurons. Our results indicate that a substantial portion of TGF-beta2 is secreted by the regulated secretory pathway in PC12 cells and hippocampal neurons.

Exocytotic insertion of calcium channels constrains compensatory endocytosis to sites of exocytosis

Proteins inserted into the cell surface by exocytosis are thought to be retrieved by compensatory endocytosis, suggesting that retrieval requires granule proteins. In sea urchin eggs, calcium influx through P-type calcium channels is required for retrieval, and the large size of sea urchin secretory granules permits the direct observation of retrieval. Here we demonstrate that retrieval is limited to sites of prior exocytosis. We tested whether channel distribution can account for the localization of retrieval at exocytotic sites. We find that P-channels reside on secretory granules before fertilization, and are translocated to the egg surface by exocytosis. Our study provides strong evidence that the transitory insertion of P-type calcium channels in the surface membrane plays an obligatory role in the mechanism coupling exocytosis and compensatory endocytosis.

Secretagogue-triggered transfer of membrane proteins from neuroendocrine secretory granules to synaptic-like microvesicles

The membrane proteins of all regulated secretory organelles (RSOs) recycle after exocytosis. However, the recycling of those membrane proteins that are targeted to both dense core granules (DCGs) and synaptic-like microvesicles (SLMVs) has not been addressed. Since neuroendocrine cells contain both RSOs, and the recycling routes that lead to either organelle overlap, transfer between the two pools of membrane proteins could occur during recycling. We have previously demonstrated that a chimeric protein containing the cytosolic and transmembrane domains of P-selectin coupled to horseradish peroxidase is targeted to both the DCG and the SLMV in PC12 cells. Using this chimera, we have characterized secretagogue-induced traffic in PC12 cells. After stimulation, this chimeric protein traffics from DCGs to the cell surface, internalizes into transferrin receptor (TFnR)-positive endosomes and thence to a population of secretagogue-responsive SLMVs. We therefore find a secretagogue-dependent rise in levels of HRP within SLMVs. In addition, the levels within SLMVs of the endogenous membrane protein, synaptotagmin, as well as a green fluorescent protein-tagged version of vesicle-associated membrane protein (VAMP)/synaptobrevin, also show a secretagogue-dependent increase.

Frog chromaffin and adrenocortical cell co-cultures: a model for the study of medullary control of corticosteroidogenesis

Phylogenetic, physiological and morphological evidence indicates that interactions between chromaffin and adrenocortical cells are involved in the differentiation and maintenance of function of both cell types. Chromaffin-adrenocortical interaction has become recognized as an important component of adrenocortical regulation; however, the mechanisms by which chromaffin cells modulate adrenocortical function are not well understood. To study directly chromaffin-adrenocortical cellular interactions, we developed primary frog (Rana pipiens) adrenal co-cultures. In these co-cultures, chromaffin cells extend processes that project towards or onto adrenocortical cells, mimicking their organization in vivo and indicating a potential for interaction between the two cell types. Cell survival and differentiation were optimized using a combination of NGF, FGF and histamine to enhance neurite outgrowth and fetal calf serum plus 10(-10) M ACTH to maintain steroidogenesis. Characterization of the cells by immunocytochemistry and histochemistry showed that chromaffin cells maintain expression of catecholamine biosynthetic enzymes and that adrenocortical cells maintain expression of steroidogenic enzymes. Furthermore, chromaffin cells release catecholamines upon stimulation with carbamylcholine or potassium while adrenocortical cells sustain a basal secretion rate of aldosterone and corticosterone that is augmented 10-40-fold by 0.1 nM to 10 nM ACTH. We therefore propose that these co-cultures serve as a useful model system to study the cellular and molecular mechanisms by which chromaffin cells modulate adrenocortical cell function.

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+.

Vesicle recycling revisited: rapid endocytosis may be the first step

Synaptic vesicle recycling is a critical feature of neuronal communication as it ensures a constant supply of releasable transmitter at the nerve terminal. Physiological studies predict that vesicle recycling is rapid and recent studies with fluorescent dyes have confirmed that the entire process may occur in less than a minute. Two competing hypotheses have been proposed for the first step in the process comprising endocytosis of vesicular membrane. The coated vesicle model proposes that vesicular membrane components merge with the plasma membrane and are subsequently recovered and possibly sorted in coated pits. These pinch off as coated vesicles that either fuse with a sorting endosome from which new vesicles emerge or uncoat to become synaptic vesicles directly. The alternative "kiss-and-run" model proposes that "empty" vesicles are retrieved intact from the plasma membrane after secretion occurs via a fusion pore; they are then immediately refilled with transmitter and re-enter the secretion-competent pool. This article summarizes the data for both models and focusses on new information that supports the kiss-and-run model. In particular, the phenomenon of rapid endocytosis, which may represent the key endocytotic step in recycling, is discussed. Rapid endocytosis has time-constants in the order of a few seconds, thus is temporally consistent with the rate of vesicle recycling. Moreover, rapid endocytosis appears to be clathrin-independent, thus does not involve the coated vesicle pathway. We present a model that accommodates both types of endocytosis, which appear to coexist in many secretory tissues including neurons. Rapid endocytosis may reflect the principal mechanism operative under normal physiological rates of stimulation while coated vesicles may come into play at higher rates of stimulation. These two processes may feed into different populations of vesicles corresponding to distinct pools defined by studies of the kinetics of transmitter release.

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

A secretion from cultured bovine chromaffin cells was stimulated to examine the pattern of exocytotic fusion on the plasma membrane. Confocal microscopy revealed that dopamine-beta-hydroxylase immunofluorescence in intact cells stimulated for 20s with the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium was almost entirely punctate and evenly distributed on the cell surface. The basis for the fine, punctate appearance of dopamine-beta-hydroxylase was investigated. Dopamine-beta-hydroxylase presentation on the surface of permeabilized cells stimulated with 1-30 microM Ca2+ was punctate and similar to that on the plasma membrane of intact cells. The fluorescence intensities of both surface dopamine-beta-hydroxylase sites and internal chromaffin granules were estimated by computerized digital image analysis. The surface area of punctate surface dopamine-beta-hydroxylase (0.218 +/- 0.013 microm2, mean +/- S.E.M.) is similar to the surface area of a 0.28 microm diameter chromaffin granule (0.25 microm2). The average fluorescence intensity integrated over the area of the surface spots was 25-30% of the average chromaffin granule intensity, a fraction that is similar to the published values of 40-50% of the dopamine-beta-hydroxylase in the chromaffin granule being membrane bound. The surface density of the spots is consistent with the number of granules undergoing exocytosis. The spots do not tend to be clumped. The key conclusions from this work are that each individual punctate site of dopamine-beta-hydroxylase represents the fusion of a single chromaffin granule and that the distribution of dopamine-beta-hydroxylase spots over the cell surface is extensive and random, suggesting that each individual granule associates with its own release site.

Dynamic responses of presynaptic terminal membrane pools following KCl and sucrose stimulation

The cholinergic presynaptic terminals of Torpedo electric organ have been examined morphometrically following stimulation by KCI and sucrose. The objective was to confirm correlations predicted by the vesicle hypothesis between miniature end-plate potentials (MEPPs) and morphometric changes in terminal ultrastructure. Both secretegogues generated high frequencies of MEPPs and also distinctive though differing ultrastructural changes. The synaptic vesicles show classes of 68 and 90 nm diameters and both store acetylcholine (ACh). KCl stimulation depleted the 90 nm class first whereas sucrose reversed the order of depletion. Very few instances of actual vesicle fusion were seen. Dose-response correlations between vesicle density and secretegogue strength (mM) and duration were higher with sucrose. Both secretegogues produced declines in vesicle numbers and densities and yielded multimodal distributions of large vesicles with an average 160 nm mean diameter. No meaningful correlations were detected between numbers of MEPPs and vesicles and little evidence was found to indicate that vesicles were fusing to terminal plasma membrane in numbers approximating MEPP release. Linear regression analysis was used to quantitatively examine relationships between the vesicle membrane pool and other pools of the putative exo/endocytotic pathway. Correlation coefficients between vesicle and terminal plasma membrane pools were non-significant and of positive sign, indicating independent, similar responses. Non-significant, negative coefficients were obtained when vacuole and 160 nm vesicle membrane values were included. These tests further argue against claims that vesicles are actively fusing with the plasma membrane. These conflicting findings for both secretegogues preclude meaningful correlations between vesicle changes and numbers of MEPPs generated and again emphasize the difficulty of validating the vesicle hypothesis by ultrastructural means. On the other hand, the study shows that vesicular, vacuolar and terminal membrane pools are dynamically changing during transmitter release, presumably interacting with cytosolic membrane constituents. A dynamical release process therefore has been proposed to account for the two classes of MEPPs, the rapid changes in class ratio and the mutable characteristics of the bell-MEPP that presently challenge the quantal-vesicular claims of prepackaged, immutable, exocytotically released packets of transmitter. This model features a state for each MEPP class with class and size determined at moment of release. For example, a single flicker of a channel would generate the sub-MEPP (defined subunit of an MEPP) and 7-20 flickering channels would generate the bell-MEPP.

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

The kinetics of exocytosis and membrane retrieval (endocytosis) were examined in bovine chromaffin cells using membrane capacitance measurement during whole-cell recording. At early times after breakthrough to the whole-cell recording mode, depolarisation for 1 s resulted in a fast (600 vesicles per s) exocytotic response and efficient membrane retrieval with a time constant of 25 s. The ability to activate fast exocytosis and retrieval was lost during intracellular dialysis, with a time constant of 40 s. At later times, a slow exocytotic response could be elicited with no membrane retrieval following single depolarisations. The wash-out of the responses appeared to be due to a rapid loss of a portion of the Ca2+ current. Trains of depolarisation at late times after breakthrough could elicit a fast (time constant 4 s) retrieval. These data show that in addition to a previously studied slow Ca(2+)-independent retrieval mechanism, chromaffin cells also possess an efficient and rapid retrieval pathway coupled to exocytosis that can be activated following depolarisation. The fast endocytosis appears to have a higher threshold for activation than exocytosis, probably due to a higher Ca2+ requirement. Rapid membrane retrieval appears to occur via a clathrin-independent pathway in chromaffin cells.

Exocytosis in adrenal chromaffin cells

Recent advances have led to an increased understanding of the Ca(2+)-signalling pathway leading to exocytosis in bovine adrenal chromaffin cells. Video-imaging studies have allowed the temporal and spatial aspects of the Ca2+ signal to be investigated in detail. Ca2+ entry at the plasma membrane appears to be crucial for the activation of exocytosis. Ca2+ can enter through the nicotinic channel or characterised voltage-activated channels, or through other poorly defined pathways due to a variety of agonists. Emptying of internal Ca2+ stores is sufficient to activate a Ca2+ entry pathway. The elevation of cytosolic Ca2+ concentration leads to a reorganisation of the cortical actin network and to the triggering of exocytosis. Studies on permeabilised chromaffin cells have resulted in the identification of some of the proteins that control Ca(2+)-dependent exocytosis. These include the peripheral plasma membrane protein annexin II and the cytosolic proteins, protein kinase C and 14-3-3 proteins (Exo1).

The adrenal chromaffin granule: a model for large dense core vesicles of endocrine and nervous tissue

More than 25 years have elapsed since R. E. Coupland made his classic observations on the ultrastructure of chromaffin granules, on the histochemical differentiation of noradrenaline and adrenaline storage granules and on their release by exocytosis. This essay attempts to demonstrate that subsequent studies on the biochemistry of chromaffin granules have yielded analytical and functional data relevant for all large dense core vesicles of endocrine and nervous tissue.

Glycoprotein III (clusterin, sulfated glycoprotein 2) in endocrine, nervous, and other tissues: immunochemical characterization, subcellular localization, and regulation of biosynthesis

Specific antisera were raised against the A and B chains of glycoprotein III. Immunoblotting revealed that in adrenal medulla both chains migrate very closely together in two-dimensional electrophoresis. Both chains with slightly differing molecular sizes are found in several endocrine tissues and in brain, kidney, liver, and serum. The mRNA has an analogous widespread distribution. In primary cultures of chromaffin cells the level of message becomes significantly increased by treatment with histamine or 12-O-tetradecanoylphorbol 13-acetate/forskolin. However, the increase is small when compared with that of secretogranin II. The subcellular localization of glycoprotein III in endocrine organs and in the posterior pituitary was investigated by subcellular fractionation and immunoelectron microscopy. Glycoprotein III was found to be confined to the large dense-core vesicles of these organs. For a discussion of the function of glycoprotein III, its localization in these organelles has to be taken into account.

Histamine stimulates exocytosis in a sub-population of bovine adrenal chromaffin cells

Both nicotinic stimulation and histamine are able to raise cytosolic free calcium concentration in the majority of cells in a population of bovine adrenal medullary chromaffin cells to comparable levels. Nevertheless, histamine induces much less catecholamine secretion than does nicotine. In order to test whether this is due to heterogeneity in the responses of chromaffin cells to histamine we examined exocytosis in response to nicotine and histamine using an immunofluorescence method based on staining with anti-DBH to detect inserted secretory vesicle membrane. The results show that while up to 98% of the chromaffin cells in culture undergo exocytosis in response to nicotine, histamine stimulates exocytosis in only a sub-population of cells.

Membrane retrieval following exocytosis in isolated neurosecretory nerve endings

In the neurosecretory nerve endings of the neurohypophysis depolarization-induced exocytosis is followed by endocytosis of vacuole-like structures with diameter similar to that of neurosecretory granules. However, it remains unknown whether the membrane of the endocytotic vacuoles is comprised primarily of retrieved secretory granule membrane, plasma membrane or of a mixture of the granule and plasma membrane. In the present paper membrane retrieval following depolarization-induced exocytosis has been studied in isolated neurosecretory nerve endings from the rat neurohypophysis. The origin of the retrieved membrane was assessed by pre-labeling the plasma membrane with an antibody against neural cell adhesion molecule, a plasma membrane specific protein. Horseradish peroxidase was used as an index of fluid endocytosis and secretion of vasopressin was measured by radioimmunoassay. Following potassium-induced depolarization, endocytotic vacuoles showed labeling with the fluid phase marker horseradish peroxidase but never showed significant neural cell adhesion molecule labeling. The time-course of endocytosis following closely that of exocytosis as endocytotic vacuoles labeled with horseradish peroxidase were only observed when the fluid phase marker was present in the extracellular medium during the period of evoked exocytosis. Our results are consistent with a model in which in neurosecretory nerve endings, after transient exocytotic fusion of the granule membrane with the plasma membrane, the granule membrane is rapidly and selectively retrieved into the nerve endings in the form of vacuoles similar in size to that of the neurosecretory granules.

Calcium requirements for secretion in bovine chromaffin cells

1. Measurements of membrane capacitance and intracellular Ca2+ concentration, [Ca2+]i, were used to examine the Ca2+ dependence of secretion in single adrenal chromaffin cells. 2. Intracellular dialysis of Ca2+, through a patch pipette, promoted secretion; the rate of secretion increased monotonically as [Ca2+]i was elevated, while the total amount of secretion reached a maximum at 1.5 microM-Ca2+ and declined at high [Ca2+]i. 3. Release of Ca2+ from internal stores, using bradykinin or ionomycin, transiently elevated [Ca2+]i and the rate of secretion. 4. Considering responses to both Ca2+ dialysis and release from internal stores, it appears that the rate of secretion increases over a range of [Ca2+]i levels above 0.2 microM and saturates at concentrations greater than 10 microM, if at all. Secretion appears to have a Hill coefficient for Ca2+ of about 2. At [Ca2+]i greater than 1-2 microM, prolonged elevation of [Ca2+]i, via dialysis, produced lower rates of secretion than transient elevation of [Ca2+]i caused by release from internal stores. This may have been caused by a depletion of readily releasable chromaffin granules during prolonged elevation of [Ca2+]i. 5. Brief depolarizing pulses produced transient rises in both [Ca2+]i and the rate of secretion. The ability of these pulses to evoke secretion 'washed out' during prolonged intracellular dialysis, due to both reduced Ca2+ influx and a diminished ability of the cell to secrete in response to a given Ca2+ load. 6. The kinetics of the secretory response depended upon the size of the depolarization-induced Ca2+ load; small rises in [Ca2+]i increased membrane capacitance only during the depolarization, while larger rises in [Ca2+]i produced increases both during and following the depolarization. The secretory responses that outlasted the depolarization appeared to be due to persistent elevation of [Ca2+]i. Secretory responses were sometimes followed by a slower decline in membrane capacitance, probably due to endocytosis of membrane. 7. Comparison of the rates of secretion measured during depolarization to those produced by Ca2+ dialysis or release from internal stores suggests that [Ca2+]i at secretory sites can exceed 10 microM during depolarization. The spatially averaged measurements of [Ca2+]i indicate much smaller levels of [Ca2+]i; thus, there must be pronounced spatial gradients of [Ca2+]i during depolarization.

Bovine chromaffin granule membranes undergo Ca(2+)-regulated exocytosis in frog oocytes

We have devised a new method that permits the investigation of exogenous secretory vesicle function using frog oocytes and bovine chromaffin granules, the secretory vesicles from adrenal chromaffin cells. Highly purified chromaffin granule membranes were injected into Xenopus laevis oocytes. Exocytosis was detected by the appearance of dopamine-beta-hydroxylase of the chromaffin granule membrane in the oocyte plasma membrane. The appearance of dopamine-beta-hydroxylase on the oocyte surface was strongly Ca(2+)-dependent and was stimulated by coinjection of the chromaffin granule membranes with InsP3 or Ca2+/EGTA buffer (18 microM free Ca2+) or by incubation of the injected oocytes in medium containing the Ca2+ ionophore ionomycin. Similar experiments were performed with a subcellular fraction from cultured chromaffin cells enriched with [3H]norepinephrine-containing chromaffin granules. Because the release of [3H]norepinephrine was strongly correlated with the appearance of dopamine-beta-hydroxylase on the oocyte surface, it is likely that intact chromaffin granules and chromaffin granule membranes undergo exocytosis in the oocyte. Thus, the secretory vesicle membrane without normal vesicle contents is competent to undergo the sequence of events leading to exocytosis. Furthermore, the interchangeability of mammalian and amphibian components suggests substantial biochemical conservation of the regulated exocytotic pathway during the evolutionary progression from amphibians to mammals.

Lateral diffusion of luminal membrane components during secretion in parotid acinar cells of the rat. Immunocytochemical and freeze-fracture studies

The movements of the molecular components of the luminal plasma membrane during exocytotic secretion in parotid acinar cells were examined. For immunocytochemical study, we used an antiserum of dipeptidyl peptidase IV as a marker for the components of the luminal plasma membrane of acinar cells. In unstimulated acinar cells, dipeptidyl peptidase IV immunoreactivity is restricted to the luminal plasma membrane. However, after secretion was stimulated with a beta-adrenergic agonist, isoproterenol, immunostaining became detectable on the membrane of discharged granules. Freeze-fracture images showed that the density of intra-membrane particles on the P-fracture leaflets of discharged granule membranes is much higher than that of undischarged granule membranes during secretion. These results suggest that in parotid acinar cells of the rat, the components of the luminal plasma membrane move laterally, during secretion, to the membranes of discharged granules.

Redistribution of synaptophysin and synapsin I during alpha-latrotoxin-induced release of neurotransmitter at the neuromuscular junction

The distribution of two synaptic vesicle-specific phosphoproteins, synaptophysin and synapsin I, during intense quantal secretion was studied by applying an immunogold labeling technique to ultrathin frozen sections. In nerve-muscle preparations treated for 1 h with a low dose of alpha-latrotoxin in the absence of extracellular Ca2+ (a condition under which nerve terminals are depleted of both quanta of neurotransmitter and synaptic vesicles), the immunolabeling for both proteins was distributed along the axolemma. These findings indicate that, in the presence of a block of endocytosis, exocytosis leads to the permanent incorporation of the synaptic vesicle membrane into the axolemma and suggest that, under this condition, at least some of the synapsin I molecules remain associated with the vesicle membrane after fusion. When the same dose of alpha-latrotoxin was applied in the presence of extracellular Ca2+, the immunoreactivity patterns resembled those obtained in resting preparations: immunogold particles were selectively associated with the membrane of synaptic vesicles, whereas the axolemma was virtually unlabeled. Under this condition an active recycling of both quanta of neurotransmitter and vesicles operates. These findings indicate that the retrieval of components of the synaptic vesicle membrane is an efficient process that does not involve extensive intermixing between molecular components of the vesicle and plasma membrane, and show that synaptic vesicles that are rapidly recycling still have the bulk of synapsin I associated with their membrane.

Inhibition of exocytosis by intracellularly applied antibodies against a chromaffin granule-binding protein

Exocytotic secretion requires the interaction and fusion of secretory vesicles with the plasma membrane. This process could be mediated by specific recognition molecules acting as intracellular, membrane-bound receptors and ligands. One possible component of such a recognition site on the plasma membrane is a protein of relative molecular mass (Mr) 51,000 (51K) that has been isolated from bovine adrenal chromaffin cells. This protein binds strongly to chromaffin granules, the secretory vesicles of these cells. To determine the function of this membrane-anchored chromaffin granule-binding protein in exocytosis, we tested the effect of intracellularly injected antibodies on secretion. Here we show, by two independent techniques in two different cell types, that antibodies against this protein inhibit exocytosis. In rat pheochromocytoma cell cultures, monospecific antibodies, applied by erythrocyte ghost fusion, impair the release of 3H-noradrenaline. The same antibodies, introduced into individual chromaffin cells through a patch pipette, block exocytosis, as revealed by the measurement of membrane capacitance. These results demonstrate the functional involvement in exocytosis of a plasma membrane protein with high affinity for secretory vesicles.

The recycling of a secretory granule membrane protein

We have used N-hydroxysuccinimido-d-biotin as a reagent for labeling proteins exposed at the surface of cultured bovine adrenal chromaffin cells during Ba2+-stimulated secretion. A specific secretory granule membrane constituent, dopamine-beta-hydroxylase (DBH), has been investigated using immunoprecipitation followed by electrophoresis. Within 30 min of stimulation, exposed DBH had been cleared from the cell surface. Nevertheless, quantitation of labeled DBH using [125I] streptavidin suggested that it remained undegraded over a period of 24 h, a time during which secretory granule stores of catecholamines were being replenished. Subcellular fractionation of the cultured cells suggested that, after 3 or 4 h, the biotinylated DBH, which was still membrane-bound, was located in particulate material that also contained cytochrome b561, another major secretory granule membrane component.

Actions of somatostatin on perfused bovine adrenal glands and cultured bovine adrenal medullary cells

The effects of somatostatin on catecholamine secretion and inositol phosphate accumulation have been studied using isolated perfused bovine adrenal glands and cultured bovine adrenal medullary cells. Somatostatin had no effect on basal adrenaline or noradrenaline secretion from either preparation. At concentrations above 1 microM, somatostatin inhibited the secretion of both catecholamines induced by 5 microM nicotine from cultured chromaffin cells. In contrast, over the concentration range 0.1 nM-10 microM, somatostatin had no effect on the secretory responses produced by 10 nM angiotensin II or 1 microM histamine. Inositol phosphate accumulation in cultured bovine adrenal medullary cells was unaffected by 0.1 nM-0.1 microM somatostatin, however at 1 and 10 microM somatostatin it was significantly increased, by 23% and 103% respectively. The effects of somatostatin (0.1 nM-10 microM) and of 50 microM muscarine on inositol phosphate accumulation were simply additive. Similarly, somatostatin at 0.1 nM and 10 nM together with 10 nM angiotensin II or 1 microM histamine produced additive inositol phosphate responses. In contrast, 1 microM somatostatin gave significantly more-than-additive (synergistic) inositol phosphate responses with angiotensin II and histamine. The results suggest that some adrenal medullary cells possess several types of receptors, and that these receptors may interact to produce non-additive responses.

Synaptophysin (p38) at the frog neuromuscular junction: its incorporation into the axolemma and recycling after intense quantal secretion

Recycling of synaptophysin (p38), a synaptic vesicle integral membrane protein, was studied by the use of antisera raised against the protein purified from frog brain. When frog cutaneous pectoris muscles were fixed at rest, a bright, specific immunofluorescent signal was observed in nerve-terminal regions only if their plasma membranes had been previously permeabilized. When muscles were fixed after they had been treated for 1 h with a low dose of alpha-latrotoxin in Ca2+-free medium, an equally intense fluorescence could be observed without previous permeabilization. Under this condition, alpha-latrotoxin depletes nerve terminals of their quantal store of acetylcholine and of synaptic vesicles. These results indicate that fusion of synaptic vesicles leads to the exposure of intravesicular antigenic determinants of synaptophysin on the outer surface of the axolemma, and provide direct support for the vesicle hypothesis of neurotransmitter release. After 1 h treatment with the same dose of alpha-latrotoxin in the presence of 1.8 mM extracellular Ca2+, immunofluorescent images were obtained only after permeabilization with detergents. Under this condition, the vesicle population was maintained by an active process of recycling and more than two times the initial store of quanta were secreted. Thus, despite the active turnover of synaptic vesicles and of quanta of neurotransmitter, no extensive intermixing occurs between components of the vesicle and presynaptic plasma membrane.

Digitonin-permeabilized cells are exocytosis competent

Release of norepinephrine from PC12 cells can be stimulated by free Ca2+ in micromolar concentrations after permeabilization with 10 micrograms/ml of digitonin. This release is time and temperature dependent, half-maximal at 0.3 microM Ca2+, and, after washing out of endogenous ATP, half-maximal at about 0.5 mM MgATP when exogenously added. Similar results were obtained with bovine adrenal chromaffin cells using the same protocol. Support for the idea that the mechanism of release from both permeabilized cell types is still exocytosis is demonstrated at the electron microscopic level by immunolabeling chromaffin granule membrane antigens that were introduced into the plasma membrane following stimulation. Electron micrographs furthermore demonstrate that chromaffin granules retain typical dense cores after permeabilization, indicating that leakiness of catecholamines from the granules was not a major factor. Pores, formed by digitonin in the plasma membranes, were utilized to introduce antibodies into such exocytosis-competent cells. Anti-actin and anti-chromaffin granule membrane antibodies show a staining pattern similar to conventionally fixed and stained preparations. Our results demonstrate that pores formed by digitonin do not impair the process of exocytosis although they are big enough to allow macromolecules to pass in both directions. The digitonin-permeabilized cell is therefore an ideal in vitro system with which to study the fusion process between chromaffin granules and the plasma membrane.

Membrane recycling after exocytosis: an ultrastructural study of cultured chromaffin cells

When exocytosis of granule contents is induced by nicotine stimulation, glycoprotein III (a chromaffin granule membrane constituent) is exposed on the surface of cultured chromaffin cells, where it may be labeled with an immunocytochemical tracer. The subsequent fate of this glycoprotein after endocytosis was followed at the ultrastructural level using immunogold methods and was analyzed by morphometry. After stimulation exocytosis membranes newly inserted into the plasma membrane labeled with gold particles for glycoprotein III were found to be endocytosed via coated vesicles and finally found in organelles devoid of chromogranin A, the major secretory granule protein. At intervals between 30 min and 24 h after cell stimulation and immunolabeling, most labeled structures were identified by two different morphological approaches as prelysosomes and lysosomes. In contrast with results obtained on freshly isolated chromaffin cells, it is thus concluded that in cultured cells granule membrane recycling into new granules does not occur. It is suggested that the fate of granule membrane endocytosed after cell stimulation may be influenced by the external conditions to which cells are previously exposed.

Kinetic analysis of the triggered exocytosis/endocytosis secretory cycle in cultured bovine adrenal medullary cells

Cultured bovine adrenal medullary cells are an excellent preparation for quantitative analysis of the secretory exocytosis/endocytosis cycle. In this paper we examine the kinetics of endocytosis after stimulation of secretion. Membrane retrieval was monitored by uptake of the fluid phase marker horseradish peroxidase. Horseradish peroxidase was found to be suitable because it can be washed off completely, assayed quantitatively, and its uptake increases linearly with concentration. If this marker is present during stimulation, the rate of uptake is initially slower than catecholamine secretion but faster at a later time, suggesting that the formation of endocytotic vesicles follows exocytosis. To monitor the time-dependent concentration of secretory vesicle-plasma membrane fusion product (omega-profiles), secretion was halted at various time intervals after stimulation and the excess membrane allowed to transform into endocytotic vesicles in the presence of horseradish peroxidase. By adding horseradish peroxidase at various times after inhibition of secretion, the time course of membrane retrieval could be measured directly. All our results are consistent with a two-step kinetic model in which exocytosis and membrane retrieval are consecutive events. The estimated volumes of the compartments involved are roughly equal. The rate of endocytosis is strongly temperature-dependent but unaffected by extracellular calcium in the range of 10(-8)-2.5 X 10(-3) M, suggesting that calcium is not required at the site of endocytotic membrane fusion. Membrane retrieval is also unaffected by Lanthanum (1 mM) but is slowed by hypertonic media.

Limited and selective transfer of plasma membrane glycoproteins to membrane of secondary lysosomes

Radioactive galactose, covalently bound to cell surface glycoconjugates on mouse macrophage cells, P388D1, was used as a membrane marker to study the composition, and the kinetics of exchange, of plasma membrane-derived constituents in the membrane of secondary lysosomes. Secondary lysosomes were separated from endosomes and plasma membrane on self-forming Percoll density gradients. Horseradish peroxidase, taken up by fluid-phase pinocytosis, served as a vesicle contents marker to monitor transfer of endosomal contents into secondary lysosomes. Concurrently, the fraction of plasma membrane-derived label in secondary lysosomes increased by first order kinetics (k = [56 min]-1) from less than 0.1% (background level) to a steady-state level of approximately 2.5% of the total label. As analyzed by NaDodSO4 PAGE, labeled molecules of Mr 160-190 kD were depleted and of Mr 100-120 kD were enriched in lysosome membrane compared with the relative composition of label on the cell surface. No corresponding selectivity was observed for the degradation of label, with all Mr classes being affected to the same relative extent. The results indicate that endocytosis-derived transfer of plasma membrane constituents to secondary lysosomes is a limited and selective process, and that only approximately 1% of internalized membrane is recycled via a membrane pool of secondary lysosomes.

Platelet surface glycoproteins. Studies on resting and activated platelets and platelet membrane microparticles in normal subjects, and observations in patients during adult respiratory distress syndrome and cardiac surgery

The accurate definition of surface glycoprotein abnormalities in circulating platelets may provide better understanding of bleeding and thrombotic disorders. Platelet surface glycoproteins were measured on intact platelets in whole blood and platelet membrane microparticles were assayed in cell-free plasma using 125I-monoclonal antibodies. The glycoproteins (GP) studied were: GP Ib and GP IIb-IIIa, two of the major intrinsic plasma membrane glycoproteins; GMP-140, an alpha-granule membrane glycoprotein that becomes exposed on the platelet surface following secretion; and thrombospondin (TSP), an alpha-granule secreted glycoprotein that rebinds to the platelet surface. Thrombin-induced secretion in normal platelets caused the appearance of GMP-140 and TSP on the platelet surface, increased exposure of GP IIb-IIIa, and decreased antibody binding to GP Ib. Patients with adult respiratory distress syndrome had an increased concentration of GMP-140 and TSP on the surface of their platelets, demonstrating in vivo platelet secretion, but had no increase of platelet microparticles in their plasma. In contrast, patients after cardiac surgery with cardiopulmonary bypass demonstrated changes consistent with membrane fragmentation without secretion: a decreased platelet surface concentration of GP Ib and GP IIb with no increase of GMP-140 and TSP, and an increased plasma concentration of platelet membrane microparticles. These methods will help to define acquired abnormalities of platelet surface glycoproteins.

Study of the transit of an integral membrane protein from secretory granules through the plasma membrane of secreting rat basophilic leukemia cells using a specific monoclonal antibody

The monoclonal antibody 5G10 reacted specifically with an 80-kD integral membrane protein in rat basophilic leukemia (RBL) cells. Immunofluorescence microscopy studies of RBL cells, fixed and permeabilized, revealed that the 80-kD protein was located in the membrane of cytoplasmic vesicles. The vesicles were identified as secretory granules by their content in immunoreactive serotonin. Expression of the 5G10 antigen on the surface of unstimulated RBL cells was low. However, RBL cells stimulated to secrete with anti-dinitrophenyl IgE followed by dinitrophenyl-bovine serum albumin or with the Ca2+ ionophore A-23187 displayed an increased expression of the antigen on their surface. Surface exposure of the 5G10 antigen was maximal at 5 min after stimulation of secretion. Removal of dinitrophenyl-bovine serum albumin from the incubation medium resulted in internalization of 50% of the antigen within 10 min.

Exocytotic exposure and recycling of membrane antigens of chromaffin granules: ultrastructural evaluation after immunolabeling

The exocytotic exposure and retrieval of an antigen of chromaffin granule membranes were studied with chromaffin cells isolated from bovine adrenal medulla. Cells were incubated with an antiserum against glycoprotein III followed by fluorescein- or gold-labeled anti-IgG. Immunofluorescence on the cell surface was present in a patchy distribution irrespective of whether bivalent antibodies or Fab fragments were used. During subsequent incubation these fluorescent membrane patches were internalized within 45 min. At the ultrastructural level immunogold-labeled patches were present on the surface of stimulated cells. During incubation (5 min to 6 h) these immunolabeled membrane patches became coated, giving rise to coated vesicles and finally to smooth vesicles. These latter vesicles were found spread throughout the cytoplasm including the Golgi region, but Golgi stacks did not become labeled. Part of the immunolabel was transferred to multivesicular bodies, which probably represent a lysosomal pathway. 30 min after incubation immunolabel was also found in electron-dense vesicles apparently representing newly formed chromaffin granules. After 6 h of incubation immunolabel was found in vesicles indistinguishable from mature chromaffin granules. These results provide direct evidence that after exocytosis membranes of chromaffin granules are selectively retrieved from the plasma membrane and are partly recycled to newly formed chromaffin granules, providing a shuttle service from the Golgi region to the plasma membrane.

Biogenesis of chromaffin granules: incorporation of sulfate into chromogranin B and into a proteoglycan

The incorporation of [35S]sulfate into the soluble proteins of chromaffin granules was studied. Isolated bovine chromaffin cells were pulse-labeled with [35S]sulfate. The radioactively labeled products were characterized by one- and two-dimensional electrophoresis. Three proteins of chromaffin granules were preferentially labeled. One was identified by immunoprecipitation as chromogranin B (Mr 100,000). This result explains why during cellular synthesis the chromogranin B precursor is converted into a significantly more acidic protein. During chase periods, the newly synthesized chromogranin B was progressively degraded by endogenous proteases. A second labeled protein, much less labeled than chromogranin B, was identified as chromogranin A. The largest portion of the radioactive label was found in a heterogeneous component (Mr 86,000-100,000; pI 4.3-5.0). Digestion experiments with chondroitinase ABC demonstrated that this labeled component and a comigrating Coomassie Blue-stained spot were selectively degraded by this enzyme. This establishes that this component is a proteoglycan.

A platelet alpha-granule membrane protein (GMP-140) is expressed on the plasma membrane after activation

We have previously characterized a monoclonal antibody, S12, that binds only to activated platelets (McEver, R.P., and M.N. Martin, 1984, J. Biol. Chem., 259:9799-9804). It identifies a platelet membrane protein of Mr 140,000, which we have designated as GMP-140. Using immunocytochemical techniques we have now localized this protein in unstimulated and thrombin-stimulated platelets. Polyclonal antibodies to purified GMP-140 were used to enhance the sensitivity of detection. Nonpermeabilized, unstimulated platelets, incubated with anti-GMP-140 antibodies, and then with IgG-gold probes, showed very little label for GMP-140 along their plasma membranes. In contrast, thrombin-stimulated platelets exhibited at least a 50-fold increase in the amount of label along the plasma membrane. On frozen thin sections of unstimulated platelets we observed immunogold label along the alpha-granule membranes. We also employed the more sensitive technique of permeabilizing with saponin unstimulated platelets in suspension, and then incubating the cells with polyclonal anti-GMP-140 antibodies and Fab-peroxidase conjugate. Alpha-granule membranes showed heavy reaction product, but no other intracellular organelles were specifically labeled. These results demonstrate that GMP-140 is an alpha-granule membrane protein that is expressed on the platelet plasma membrane during degranulation.

Cell-free and cellular synthesis of chromogranin A and B of bovine adrenal medulla

We have studied the cell-free and cellular synthesis of chromogranins A and B, two immunologically distinct protein families of adrenal chromaffin granules. Two cell-free systems (wheat germ and reticulocyte lysate) were used for translating messenger RNA isolated from bovine adrenal medulla. Two primary translation products could be immunoprecipitated in case of chromogranin A. In the presence of microsomes the two chromogranin A precursors (pre-chromogranins A) were converted into a single protein, apparently by the removal of different signal peptides. For chromogranin B only one precursor (pre-chromogranin B) was translated. In isolated chromaffin cells only one chromogranin A protein was synthesized which corresponded to the processed cell-free translation product. During prolonged incubation this protein became slightly larger and more acidic, probably due to glycosylation in the Golgi region. Chromogranin B is post-translationally converted to a significantly more acidic protein. It is concluded that proteolytic breakdown of newly synthesized chromogranin A and B in chromaffin granules is a slow process comparable to that of the enkephalin precursors. It is not yet known what function these chromogranins have and whether breakdown to smaller subunits is necessary for any function to evolve.