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

Calcium imaging in intact mouse acinar cells in acute pancreas tissue slices

The physiology and pathophysiology of the exocrine pancreas are in close connection to changes in intra-cellular Ca²⁺ concentration. Most of our knowledge is based on in vitro experiments on acinar cells or acini enzymatically isolated from their surroundings, which can alter their structure, physiology, and limit our understanding. Due to these limitations, the acute pancreas tissue slice technique was introduced almost two decades ago as a complementary approach to assess the morphology and physiology of both the endocrine and exocrine pancreas in a more conserved in situ setting. In this study, we extend previous work to functional multicellular calcium imaging on acinar cells in tissue slices. The viability and morphological characteristics of acinar cells within the tissue slice were assessed using the LIVE/DEAD assay, transmission electron microscopy, and immunofluorescence imaging. The main aim of our study was to characterize the responses of acinar cells to stimulation with acetylcholine and compare them with responses to cerulein in pancreatic tissue slices, with special emphasis on inter-cellular and inter-acinar heterogeneity and coupling. To this end, calcium imaging was performed employing confocal microscopy during stimulation with a wide range of acetylcholine concentrations and selected concentrations of cerulein. We show that various calcium oscillation parameters depend monotonically on the stimulus concentration and that the activity is rather well synchronized within acini, but not between acini. The acute pancreas tissue slice represents a viable and reliable experimental approach for the evaluation of both intra- and inter-cellular signaling characteristics of acinar cell calcium dynamics. It can be utilized to assess many cells simultaneously with a high spatiotemporal resolution, thus providing an efficient and high-yield platform for future studies of normal acinar cell biology, pathophysiology, and screening pharmacological substances.

Measuring Exocytosis Rate Using Corrected Fluorescence Recovery After Photoconversion

Exocytosis plays crucial roles in regulating the distribution and function of plasma membrane (PM) and extracellular matrix proteins. However, measuring the exocytosis rate of a specific protein by conventional methods is very difficult because of exocytosis-independent trafficking such as endocytosis, which also affects membrane protein distribution. Here, we describe a novel method, corrected fluorescence recovery after photoconversion, in which exocytosis-dependent and -independent trafficking events are measured simultaneously to accurately determine exocytosis rate. In this method, the protein-of-interest is tagged with Dendra2, a green-to-red photoconvertible fluorescent protein. Following the photoconversion of PM-localized Dendra2, both the recovery of the green signal and the changes in the photoconverted red signal are measured, and the rate of exocytosis is calculated from the changing rates of these two signals.

Porosome in the Exocrine Pancreas: A Detailed EM Study suppressor

A major question in cell biology that accumulation of partially empty vesicles in cells following secretion is seen, while it is believed that secretion occurs via the complete merger of secretory vesicles with the cell plasma membrane. This important question was solved nearly two decades ago, with the discovery of the Porosome. Porosomes are cup-shaped lipoprotein structures found at the plasma membrane of all cells. Secretory vesicles dock and transiently fuse at the porosome base to form a continuous channel or fusion pore to release the pressurized vesicle contents to the outside. In a decade-long study by our group, we carefully examined using electron microscopy, the detailed structure of the porosome complex in acinar cells of the exocrine pancreas. Besides conformation of earlier findings, our study provides in much greater detail, the in situ morphology of the porosome complex in the exocrine pancreas. The discovery of the detailed morphology of the exocrine pancreas porosome complex in my laboratory is one of the major highlights of my academic career spanning nearly 50 years. Results from our EM studies, reveal for the first time the presence of tethers or cables, which are likely t-SNAREs, present at the porosome base. These EM studies further demonstrate for the first time the docking of a single secretory vesicle or zymogen granule at the base of more than one porosome complex. Detailed spoke-like elements lining the porosome cup were also observed for the first time in these studies, greatly advancing our understanding of the molecular architecture and physiology of the porosome in the exocrine pancreas.

Distinct contributions by ionotropic purinoceptor subtypes to ATP-evoked calcium signals in mouse parotid acinar cells

There is emerging consensus that P2X₄ and P2X₇ ionotropic purinoceptors (P2X₄R and P2X₇R) are critical players in regulating [Ca²⁺]i dynamics and fluid secretion in the salivary gland. In contrast, details regarding their compartmentalization and selective activation, contributions to the spatiotemporal properties of intracellular signals and roles in regulating protein exocytosis and ion channel activity have remained largely undefined. To address these concerns, we profiled mouse parotid acinar cells using live-cell imaging to follow the spatial and temporal features of ATP-evoked Ca²⁺ dynamics and exocytotic activity. Selective activation of P2X7Rs revealed an apical-to-basal [Ca²⁺]i signal that initiated at the sub-luminal border and propagated with a wave speed estimated at 17.3 ± 4.3 μm s⁻¹ (n =6). The evoked Ca²⁺ spike consisted of Ca²⁺ influx and Ca²⁺-induced Ca²⁺ release from intracellular Ca²⁺ channels. In contrast, selective activation of P2X₄Rs induced a Ca²⁺ signal that initiated basally and propagated toward the lumen with a wave speed of 4.3 ± 0.2 μm s⁻¹ (n =8) that was largely independent of intracellular Ca²⁺ channel blockade. Consistent with these observations, P2X₇R expression was enriched in the sub-luminal regions of acinar cells while P2X₄R appeared localized to basal areas. In addition, we showed that P2X₄R and P2X₇R activation evokes exocytosis in parotid acinar cells. Our studies also demonstrate that the P2X₄R-mediated [Ca²⁺]i rise and subsequent protein exocytosis was enhanced by ivermectin (IVR). Thus, in addition to furthering our understanding of salivary gland physiology, this study identifies P2X₄R as a potential target for treatment of salivary hypofunction diseases.

Measuring calcium signals and exocytosis in tissues

BACKGROUND

Since the 1960s it has been clear that calcium is a key regulator of exocytosis. Early experiments directly showed that the secretory output was calcium dependent. But it has taken improvements in technology and clever experimentation to determine the relationships between the calcium signal and exocytosis. Today controversies still remain because of limitations in our ability to record both the calcium responses within the local domains that control secretion and in the methods used to record exocytosis.

SCOPE OF REVIEW

Here the techniques used to measure calcium and exocytosis are reviewed with a distinction being drawn between measurements in excitable cells versus measurements in non-excitable cells. The review has a focus on techniques that are relevant to in vitro studies of native tissues and recent in vivo recordings.

MAJOR CONCLUSIONS

There are a range of methods used to study the stimulus-secretion pathway. Each presents their own advantages and drawbacks. These are discussed with reference to the latest work determining the factors controlling exocytosis in tissues.

GENERAL SIGNIFICANCE

Stimulus-secretion coupling is the fundamental step in the control of neurotransmitter release, hormone secretion and protein secretion. Understanding secretory control is therefore important in understanding the physiological regulation of processes ranging from learning and memory to pancreatic secretion. Recent technological advances are now enabling us to study stimulus-secretion coupling within native tissues. This is helping us to understand the physiological complexities of secretory control. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Cholecystokinin-58 and cholecystokinin-8 exhibit similar actions on calcium signaling, zymogen secretion, and cell fate in murine pancreatic acinar cells

The gastrointestinal hormone CCK exists in various molecular forms, with differences in bioactivity between the well-characterized CCK-8 and larger CCK-58 previously reported. We have compared the effects of these peptides on cytosolic calcium concentration ([Ca(2+)](c)), mitochondrial metabolism, enzyme secretion, and cell fate in murine isolated pancreatic acinar cells using fluorescence confocal microscopy and patch-clamp electrophysiology. CCK-58 (1-10 pM) induced transient, oscillatory increases of [Ca(2+)](c), which showed apical to basolateral progression and were associated with a rise of mitochondrial NAD(P)H. CCK-58 (10 pM) induced zymogen exocytosis in isolated cells and amylase secretion from isolated cells and whole tissues. Hyperstimulation with supraphysiological CCK-58 (5 nM) induced a single large increase of [Ca(2+)](c) that declined to a plateau, which remained above the basal level 20 min after application and was dependent on external Ca(2+) entry. In cells dispersed from the same tissues, CCK-8 induced similar patterns of responses to those of CCK-58, with oscillatory increases of [Ca(2+)](c) at lower (pM) concentrations and sustained responses at 5 nM. CCK-58 and CCK-8 exhibited similar profiles of action on cell death, with increases in necrosis at high CCK-58 and CCK-8 (10 nM) that were not significantly different between peptides. The present experiments indicate that CCK-8 and CCK-58 have essentially identical actions on the acinar cell at high and low agonist concentrations, suggesting an action via the same receptor and that the differences observed in an intact rat model may result from indirect effects of the peptides. Our data strengthen the argument that CCK-58 is an important physiological form of this gastrointestinal hormone.

Duck pancreatic acinar cell as a unique model for independent cholinergic stimulation-secretion coupling

This paper investigated the role of acetylcholine (ACh) in physiological regulation of amylase secretion in avian exocrine pancreas. In the isolated duck pancreatic acini, ACh dose dependently stimulated amylase secretion, with a maximal effective concentration at 10 muM. The cAMP-mobilizing compounds forskolin, vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase activating peptide (PACAP) receptor (VPAC) agonists PACAP-38 and PACAP-27 had no effect on the dose-response curve. ACh dose dependently induced increases in cytosolic Ca(2+) concentration ([Ca(2+)]( c )), with increasing concentrations transforming oscillations into plateau increases. Forskolin (10 muM), PACAP-38 (1 nM), PACAP-27 (1 nM), or VIP (10 nM) alone did not stimulate [Ca(2+)]( c ) increase; neither did they modulate ACh-induced oscillations, nor made ACh low concentration effective. These data indicate that ACh-stimulated zymogen secretion in duck pancreatic acinar cells is not subject to modulation from the cAMP signaling pathway; whereas it has been widely reported in the rodents that ACh-stimulated exocrine pancreatic secretion is significantly enhanced by cAMP-mobilizing agents. This makes the duck exocrine pancreas unique in that cholinergic stimulus-secretion coupling is not subject to cAMP regulation.

Direct activation of cytosolic Ca2+ signaling and enzyme secretion by cholecystokinin in human pancreatic acinar cells

BACKGROUND & AIMS

Cholecystokinin (CCK) has been thought to act only indirectly on human pancreatic acinar cells via vagal nerve stimulation, rather than by direct CCK receptor activation as on rodent pancreatic acinar cells. We tested whether CCK (CCK-8 and human CCK-58) can act directly on human pancreatic acinar cells.

METHODS

Human acinar cells were freshly isolated from pancreatic transection line samples, loaded with Fluo4-AM or quinacrine, and examined for Ca(2+), metabolic and secretory responses to CCK-8, human CCK-58, or acetylcholine with confocal microscopy.

RESULTS

CCK-8 and human CCK-58 at physiologic concentrations (1-20 pmol/L) elicited rapid, robust, oscillatory increases of the cytosolic Ca(2+) ion concentration, showing apical to basal progression, in acinar cells from 14 patients with unobstructed pancreata. The cytosolic Ca(2+) ion concentration increases were followed by increases in mitochondrial adenosine triphosphate production and secretion. CCK-elicited Ca(2+) signals and exocytosis were not inhibited by atropine (1 mumol/L) or tetrodotoxin (100 nmol/L), showing that CCK was unlikely to have acted via neurotransmitter release. CCK-elicited Ca(2+) signals were inhibited reversibly by caffeine (5-20 mmol/L), indicating involvement of intracellular inositol trisphosphate receptor Ca(2+) release channels. Acetylcholine (50 nmol/L) elicited similar Ca(2+) signals.

CONCLUSIONS

CCK at physiologic concentrations in the presence of atropine and tetrodotoxin elicits cytosolic Ca(2+) signaling, activates mitochondrial function, and stimulates enzyme secretion in isolated human pancreatic acinar cells. We conclude that CCK acts directly on acinar cells in the human pancreas.

Identification of SNAREs that mediate zymogen granule exocytosis

A secretagogue-stimulated pancreatic acinar cell releases digestive enzymes from its apical pole. We attempted to identify the SNAREs involved in zymogen granule exocytosis. Antibodies against syntaxins 2 and 3, SNAP-23 and VAMP 8, and the corresponding recombinant SNAREs, inhibited amylase secretion from streptolysin O-permeabilised acini; other anti-SNARE antibodies and SNAREs had no effect. Botulinum neurotoxin C, which cleaved syntaxin 2 and (to a lesser extent) syntaxin 3, but not syntaxins 4, 7 or 8, also inhibited exocytosis. We propose that syntaxin 2, SNAP-23 and VAMP 8 mediate primary granule-plasma membrane fusion. Syntaxin 3 may be involved in secondary granule-granule fusion.

Two-photon excitation imaging of exocytosis and endocytosis and determination of their spatial organization

Two-photon excitation imaging is the least invasive optical approach to study living tissues. We have established two-photon extracellular polar-tracer (TEP) imaging with which it is possible to visualize and quantify all exocytic events in the plane of focus within secretory tissues. This technology also enables estimate of the precise diameters of vesicles independently of the spatial resolution of the optical microscope, and determination of the fusion pore dynamics at nanometer resolution using TEP-imaging based quantification (TEPIQ). TEP imaging has been applied to representative secretory glands, e.g., exocrine pancreas, endocrine pancreas, adrenal medulla and a pheochromocytoma cell line (PC12), and has revealed unexpected diversity in the spatial organization of exocytosis and endocytosis crucial for the physiology and pathology of secretory tissues and neurons. TEP imaging and TEPIQ analysis are powerful tools for elucidating the molecular and cellular mechanisms of exocytosis and certain related diseases, such as diabetes mellitus, and the development of new therapeutic agents and diagnostic tools.

Water channels and zymogen granules in salivary glands

Salivary secretion occurs in response to stimulation by neurotransmitters released from autonomic nerve endings. The molecular mechanisms underlying the secretion of water, a main component of saliva, from salivary glands are not known; the plasma membrane is a major barrier to water transport. A 28-kDa integral membrane protein, distributed in highly water-permeable tissues, was identified as a water channel protein, aquaporin (AQP). Thirteen AQPs (AQP0 - AQP12) have been identified in mammals. AQP5 is localized in lipid rafts under unstimulated conditions and translocates to the apical plasma membrane in rat parotid glands upon stimulation by muscarinic agonists. The importance of increases in intracellular calcium concentration [Ca(2+)](i) and the nitric oxide synthase and protein kinase G signaling pathway in the translocation of AQP5 is reviewed in section I. Signals generated by the activation of Ca(2+) mobilizing receptors simultaneously trigger and regulate exocytosis. Zymogen granule exocytosis occurs under the control of essential process, stimulus-secretion coupling, in salivary glands. Ca(2+) signaling is a principal signal in both protein and water secretion from salivary glands induced by cholinergic stimulation. On the other hand, the cyclic adenosine monophosphate (cAMP)/cAMP-dependent protein kinase system has a major role in zymogen granule exocytosis without significant increases in [Ca(2+)](i). In section II, the mechanisms underlying the control of salivary protein secretion and its dysfunction are reviewed.

cGMP modulation of ACh-stimulated exocytosis in guinea pig antral mucous cells

In guinea pig antral mucous cells, ACh stimulates the Ca(2+)-regulated exocytosis, which has a characteristics feature: an initial transient phase followed by a sustained phase. The effects of cGMP on ACh-stimulated exocytosis were studied in guinea pig antral mucous cells using video microscopy. cGMP enhanced the frequency of ACh-stimulated exocytotic events, whereas cGMP alone did not induce any exocytotic events under the ACh-unstimulated condition. cGMP did not stimulate either Ca(2+) mobilization or cAMP accumulation. The Ca(2+) dose-response studies demonstrated that cGMP shifted the dose-response curve upward with no shift to the lower concentration. This indicates that cGMP increased maximal responsiveness of the Ca(2+)-regulated exocytosis, but not the Ca(2+) sensitivity. Moreover, under a condition of ATP depletion by dinitrophenol (DNP) or anoxia (N(2) bubbling), ACh evoked only a sustained phase in exocytotic events with no initial transient phase. However, ACh evoked an initial transient phase followed by a sustained phase with addition of cGMP before ATP depletion, whereas only a sustained phase was evoked in a case of cGMP addition after ATP depletion. Thus cGMP-induced enhancement in ACh-stimulated exocytotic events requires ATP, suggesting that cGMP modulates ATP-dependent priming of Ca(2+)-regulated exocytosis in antral mucous cells. In conclusion, cGMP increases the number of primed granules via acceleration of the ATP-dependent priming, which enhances the Ca(2+)-regulated exocytosis stimulated by ACh.

Enhancement of Ca2+-regulated exocytosis by indomethacin in guinea-pig antral mucous cells: arachidonic acid accumulation

Ca2+-regulated exocytosis is enhanced by an autocrine mechanism via the PGE2-cAMP pathway in antral mucous cells of guinea-pigs. The inhibition of the PGE2-cAMP pathway by H-89 (an inhibitor of protein kinase A, PKA) or aspirin (ASA, an inhibitor of cyclo-oxygenase, COX) decreased the frequency of ACh-stimulated exocytotic events by 60%. Indomethacin (IDM, an inhibitor of COX), however, decreased the frequency of ACh-stimulated exocytotic events only by 30%. Moreover, IDM increased the frequency of ACh-stimulated exocytotic events by 50% in H-89-treated or ASA-treated cells. IDM inhibits the synthesis of Prostaglandin (PGG/H) and (15R)-15-hydroxy-5,8,11 cis-13-trans-eicosatetraenoic acid (15R-HPETE), while ASA inhibits only the synthesis of PGG/H. Thus, IDM may accumulate arachidonic acid (AA). AACOCF3 or N-(p-amylcinnamoyl) anthranilic acid (ACA; both inhibitors of phospholipase A2, PLA2), which inhibits AA synthesis, decreased the frequency of ACh-stimulated exocytotic events by 60%. IDM, however, did not increase the frequency in AACOCF3-treated cells. AA increased the frequency of ACh-stimulated exocytotic events in AACOCF3- or ASA-treated cells, similar to IDM in ASA- and H-89-treated cells. Moreover, in the presence of AA, IDM did not increase the frequency of ACh-stimulated exocytotic events in ASA-treated cells. The PGE2 release from antral mucosa indicates that inhibition of PLA2 by ACA inhibits the AA accumulation in unstimulated and ACh-stimulated antral mucosa. The dose-response study of AA and IDM demonstrated that the concentration of intracellular AA accumulated by IDM is less than 100 nm. In conclusion, IDM modulates the ACh-stimulated exocytosis via AA accumulation in antral mucous cells.

Mitochondria play a critical role in shaping the exocytotic response of rat pancreatic acinar cells

We have previously demonstrated [M. Campos-Toimil, T. Bagrij, J.M. Edwardson, P. Thomas, Two modes of secretion in pancreatic acinar cells: involvement of phosphatidylinositol 3-kinase and regulation by capacitative Ca(2+) entry, Curr. Biol. 12 (2002) 211-215] that in rat pancreatic acinar cells, Gd(3+)-sensitive Ca(2+) entry is instrumental in governing which second messenger pathways control secretory activity. However, in those studies, we were unable to demonstrate a significant increase in cytoplasmic [Ca(2+)] during agonist application as a result of this entry pathway. In the present study, we combined pharmacology with ratiometric imaging of fura-2 fluorescence to resolve this issue. We found that 2 microM Gd(3+) significantly inhibits store-mediated Ca(2+) entry. Furthermore, both the protonophore, CCCP (5 microM) and the mitochondrial Ca(2+)-uptake blocker, RU360 (10 microM), led to an enhancement of the plateau phase of the biphasic Ca(2+) response induced by acetylcholine (1 microM). This enhancement was completely abolished by Gd(3+); and as has been previously shown for Gd(3+), RU360 led to a switch to a wortmannin-sensitive form of exocytosis. Using MitoTracker Red staining we found a close association of mitochondria with the lateral plasma membrane. We propose that in rat pancreatic acinar cells, capacitative Ca(2+) entry is targeted directly to mitochondria; and that as a result of Ca(2+) uptake, these mitochondria release "third" messengers which both enhance exocytosis and suppress phosphatidylinositol 3-kinase-dependent secretion.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

Regulation of zymogen granule exocytosis by Ca2+, cAMP, and PKC in pancreatic acinar cells

The effect of cAMP and PKC on zymogen granule exocytosis was investigated by simultaneously measuring cytosolic Ca2+ concentration ([Ca2+]c) and individual zymogen granule exocytosis in isolated mouse pancreatic acini. When acinar cells were stimulated with acetylcholine (ACh, 10 microM), exocytic events were detected through granule-attached apical membranes with [Ca2+]c rise. Application of secretin, forskolin (an adenylate cyclase activator), or PMA (a PKC activator) alone did not elicit any [Ca2+]c rise or zymogen granule exocytosis, but co-stimulation with ACh led to exocytosis in that the total number of secreted granules increased markedly without a significant difference in [Ca2+]c rises. When we evoked exocytosis by [Ca2+]c ramps, pretreatment with forskolin or PMA elicited exocytosis at lower [Ca2+]c levels. These results indicate that PKC or cAMP alone could not directly elicit zymogen granule exocytosis, but that they increase the total releasable pool by rendering zymogen granules more sensitive to Ca2+.

Syncollin is required for efficient zymogen granule exocytosis

Syncollin is a 13 kDa protein that is present in the exocrine pancreas, where the majority of the protein is tightly attached to the luminal surface of the zymogen granule membrane. We have addressed the physiological role of syncollin by studying the phenotype of syncollin KO (knockout) mice. These mice show pancreatic hypertrophy and elevated pancreatic amylase levels. Further, secretagogue-stimulated amylase release from pancreatic lobules of syncollin KO mice was found to be reduced by about 45% compared with wild-type lobules, and the delivery of newly synthesized protein to zymogen granules was delayed, indicating that the mice have a pancreatic secretory defect. As determined by two-photon imaging, the number of secretagogue-stimulated exocytotic events in acini from syncollin KO mice was reduced by 50%. This reduction was accounted for predominantly by a loss of later, 'secondary' fusion events between zymogen granules and other granules that had already fused with the plasma membrane. We conclude that syncollin is required for efficient exocytosis in the pancreatic acinar cell, and that it plays a particularly important role in compound exocytosis.

Spatiotemporal analysis of exocytosis in mouse parotid acinar cells

Exocrine cells of the digestive system are specialized to secrete protein and fluid in response to neuronal and/or hormonal input. Although morphologically similar, parotid and pancreatic acinar cells exhibit important functional divergence in Ca(2+) signaling properties. To address whether there are fundamental differences in exocytotic release of digestive enzyme from exocrine cells of salivary gland versus pancreas, we applied electrophysiological and optical methods to investigate spatial and temporal characteristics of zymogen-containing secretory granule fusion at the single-acinar cell level by direct or agonist-induced Ca(2+) and cAMP elevation. Temporally resolved membrane capacitance measurements revealed that two apparent phases of exocytosis were induced by Ca(2+) elevation: a rapidly activated initial phase that could not be resolved as individual fusion events and a second phase that was activated after a delay, increased in a staircaselike fashion, was augmented by cAMP elevation, and likely reflected both sequential compound and multivesicular fusion of zymogen-containing granules. Optical measurements of exocytosis with time-differential imaging analysis revealed that zymogen granule fusion was induced after a minimum delay of approximately 200 ms, occurred initially at apical and basolateral borders of acinar cells, and under strong stimulation proceeded from apical pole to deeper regions of the cell interior. Zymogen granule fusions appeared to coordinate subsequent fusions and produced persistent structures that generally lasted several minutes. In addition, parotid gland slices were used to assess secretory dynamics in a more physiological context. Parotid acinar cells were shown to exhibit both similar and divergent properties compared with the better-studied pancreatic acinar cell regarding spatial organization and kinetics of exocytotic fusion of zymogen granules.

The Plasma Membrane Q-SNARE Syntaxin 2 Enters the Zymogen Granule Membrane during Exocytosis in the Pancreatic Acinar Cell

During exocytosis in the pancreatic acinar cell, zymogen granules fuse directly with the apical plasma membrane and also with granules that have themselves fused with the plasma membrane. Together, these primary and secondary fusion events constitute the process of compound exocytosis. It has been suggested that the sequential nature of primary and secondary fusion is a consequence of the requirement for plasma membrane soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors, such as syntaxin 2, to enter the membrane of the primary fused granule. We have tested this possibility by determining the location of syntaxin 2 in unstimulated and stimulated pancreatic acini. Syntaxin 2 was imaged by confocal immunofluorescence microscopy. Fused granules were detected both through their filling with the aqueous dye lysine-fixable Texas Red-dextran and through the decoration of their cytoplasmic surfaces with filamentous actin. In unstimulated cells, syntaxin 2 was exclusively present on the apical plasma membrane. In contrast, after stimulation, syntaxin 2 had moved into the membranes of fused granules, as judged by its location around dye-filled structures of 1-mum diameter that were coated with filamentous actin. At long times of stimulation (5 min), the majority (85%) of dye-filled granules were also positive for syntaxin 2. In contrast, at shorter times (1 min), more dye-filled granules (29%) were syntaxin 2-negative. We conclude that syntaxin 2 enters the membrane of a fused zymogen granule after the opening of the fusion pore, and we suggest that this movement might permit the onset of secondary fusion.

Inhibition of ACh-stimulated exocytosis by NSAIDs in guinea pig antral mucous cells: autocrine regulation of mucin secretion by PGE2

The effects of indomethacin (IDM) and aspirin (ASA) on ACh (10 microM) -stimulated exocytotic events were studied in guinea pig antral mucous cells by using video optical microscopy. IDM or ASA, which inhibits cyclooxygenase (COX), decreased the frequency of ACh-stimulated exocytotic events by 30% or 60%, respectively. The extent of inhibition induced by ASA (60%) decreased by 30% when IDM or arachidonic acid (AA, the substrate of COX) was added. IDM, unlike ASA, appears to induce the accumulation of AA, which enhances the frequency of ACh-stimulated exocytotic events in ASA-treated cells. ONO-8713 (100 microM; an inhibitor of the EP1-EP4 prostaglandin receptors) and N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, HCl (H-89, 20 microM; an inhibitor of PKA) also decreased the frequency of ACh-stimulated exocytotic events by 60%. However, the supplementation of PGE(2) (1 microM) prevented the IDM-induced decrease in the frequency of ACh-stimulated exocytotic events. SC-560 (an inhibitor of COX-1) decreased the frequency of ACh-stimulated exocytotic events by 30%, but NS-398 (an inhibitor of COX-2) did not. Moreover, IDM decreased the frequency of exocytotic events stimulated by ionomycin, suggesting that COX-1 activity is stimulated by an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). ACh and ionomycin increased PGE(2) release in antral mucosal cells. In conclusion, in ACh-stimulated antral mucous cells, an increase in [Ca(2+)](i) activates Ca(2+)-regulated exocytotic events and PGE(2) release mediated by COX-1. The released PGE(2) induces the accumulation of cAMP, which enhances the Ca(2+)-regulated exocytosis. The autocrine mechanism mediated by PGE(2) maintains the high-level mucin release from antral mucous cells during ACh stimulation.

Zymogen granule exocytosis is characterized by long fusion pore openings and preservation of vesicle lipid identity

The dynamics of the fusion pore that forms between a secretory vesicle and the plasma membrane are important in the regulation of both exocytosis and endocytosis. Here, we describe characteristics of fusion during zymogen granule exocytosis in exocrine pancreatic acinar cells. By using fluorescence recovery after photobleaching techniques, we show that the fusion pore remains open to allow free aqueous exchange with the vesicle lumen. There is no lipid interchange between the plasma and granule membranes during this time, and at the end of its life, the intact granule shrinks in situ, probably by a gradual pinching off of membrane patches. We propose that the protracted fusion pore lifetime is adapted to permit compound exocytosis, whereby the lingering primary granule acts as a conduit through which the contents of a secondary granule can be released. The lack of lipid intermixing may then facilitate selective recycling of granule membrane and preservation of apical membrane integrity.

Morphological and functional changes of dissociated single pancreatic acinar cells: testing the suitability of the single cell as a model for exocytosis and calcium signaling

Isolated single pancreatic acinar cells have long been used as a model for studying many kinds of signaling processes due to their structural and functional polarities, but without significant validation. In this study, we examined the morphological and functional changes of dissociated single pancreatic acinar cells. Acutely isolated single cells showed a collapsed membrane potential and a much reduced secretion of zymogen granules in response to acetylcholine (ACh) stimulation, whereas clustered cells showed a much more negative membrane potential and potent exocytotic secretion. The isolated single cells became vertically flattened due to the loss of supporting adhesions with nearby cells, and the granule-attached luminal membrane was severely reduced versus that of clustered cells. However, polarized Ca(2+) signals and mitochondrial localizations were relatively well preserved in the isolated single cells, in that Ca(2+) release by ACh commenced at the indented luminal membrane. In clusters, the Ca(2+) release site was closest to the lumen where more than three cells met or at the tips of conical regions of the luminal membrane. These findings suggest that the dissociated single pancreatic acinar cells preserve an intact Ca(2+) signaling machinery but alter in shape and have impaired exocytotic functions and resting membrane potentials.

Receptor biology and intracellular regulatory mechanisms in pancreatic acinar cells

Continuing progress is being made in understanding the regulation of pancreatic acinar cell function by receptor-activated intracellular signaling mechanisms. Knowledge of how ligands interact at the molecular level with their receptors and activate heterotrimeric G proteins is increasing. In addition to inositol trisphosphate, intracellular messengers include cyclic ADP ribose, nicotinic acid adenine dinucleotide phosphate, arachidonic acid, and diacylglycerol. Ca signaling involves the interaction of inositol trisphosphate, cyclic ADP ribose, and nicotinic acid adenine dinucleotide phosphate with distinct subcellular Ca stores. Ca signals ultimately induce exocytosis of zymogen granules and identification of the proteins involved on the granule and plasma membrane, and understanding of their roles is continuing. Other receptor-activated signaling pathways primarily regulate nonsecretory events. Considerable progress has been made in understanding how the mammalian target of rapamycin pathway regulates protein synthesis through translation factors and ribosomal proteins. Other pathways in acinar cells include the mitogen-activated protein kinases, the tyrosine kinases, and the transforming growth factor-beta-Smad pathways.

The minor regulated pathway, a rapid component of salivary secretion, may provide docking/fusion sites for granule exocytosis at the apical surface of acinar cells

Recently, we reported that the minor regulated and constitutive-like pathways are the main source of resting secretion by parotid acinar cells. Using tissue lobules biosynthetically labeled with [35S]amino acids, we now show that discharge of the minor regulated pathway precedes granule exocytosis stimulated by isoproterenol (≥1 μM) or carbachol (2μM). Stimulation of the minor regulated pathway by 40 nM carbachol as well as altering its trafficking, either by adding brefeldin A or by incubating in K+-free medium, cause potentiation of amylase secretion stimulated by isoproterenol, suggesting that the minor regulated pathway contributes to the mechanism of potentiation. Both exocytosis of the minor regulated pathway and the potentiation-inducing treatments induce relocation of immunostained subapical puncta of the SNARE protein syntaxin 3 into the apical plasma membrane. Rab11 and possibly VAMP2 may be concentrated in the same relocating foci. These results suggest that the minor regulated pathway and granule exocytosis are functionally linked and that the minor regulated pathway has a second role beyond contributing to resting secretion — providing surface docking/fusion sites for granule exocytosis. In the current model of salivary protein export, discharge of the minor regulated pathway by eitherβ-adrenergic or cholinergic stimulation is an obligatory first step. Ensuing granule exocytosis is controlled mainly by β-adrenergic stimulation whereas cholinergic stimulation mainly regulates the number of surface sites where release occurs.

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

cAMP modulation of Ca(2+)-regulated exocytosis in ACh-stimulated antral mucous cells of guinea pig

Effects of cAMP accumulation on ATP-dependent priming and Ca(2+)-dependent fusion in Ca(2+)-regulated exocytosis were examined in antral mucous cells of guinea pigs by using video-enhanced contrast microscopy. The Ca(2+)-regulated exocytosis activated by 1 microM ACh consisted of two phases, an initial transient phase followed by a sustained phase, which were potentiated by cAMP accumulation. Depletion of ATP by 100 microM dinitrophenol (uncoupler of oxidative phosphorylation) or anoxia induced the sustained phase without the initial transient phase in Ca(2+)-regulated exocytosis. However, accumulation of cAMP before depletion of ATP induced and potentiated an initial transient phase followed by a sustained phase in Ca(2+)-regulated exocytosis. This suggests that the initial transient phase of Ca(2+)-regulated exocytosis is induced by fusion of all primed granules maintained by ATP and that accumulation of cAMP accelerates ATP-dependent priming of the exocytotic cycle. Moreover, ACh and Ca(2+) dose-response studies showed that accumulation of cAMP shifted the dose-response curves to the low concentration side, suggesting that it increases Ca(2+) sensitivity in the fusion of the exocytotic cycle. In conclusion, cAMP accumulation increases the number of primed granules and Ca(2+) sensitivity of the fusion, which potentiates Ca(2+)-regulated exocytosis in antral mucous cells.

Quantal release of free radicals during exocytosis of phagosomes

Secretion of lysosomes and related organelles is important for immune system function. High-resolution membrane capacitance techniques were used to track changes in membrane area in single phagocytes during opsonized polystyrene bead uptake and release. Secretagogue stimulation of cells preloaded with beads resulted in immediate vesicle discharge, visualized as step increases in capacitance. The size of the increases were consistent with phagosome size. This hypothesis was confirmed by direct observation of dye release from bead-containing phagosomes after secretagogue stimulation. Capacitance recordings of exocytosis were correlated with quantal free radical release, as determined by amperometry. Thus, phagosomes undergo regulated secretion in macrophages, one function of which may be to deliver sequestered free radicals to the extracellular space.

The regulation of exocytosis in the pancreatic acinar cell

The pancreatic acinar cell synthesises a variety of digestive enzymes. In transit through the secretory pathway, these enzymes are separated from constitutively secreted proteins and packaged into zymogen granules, which are localised in the apical pole of the cell. Stimulation of the cell by secretagogues such as acetylcholine and cholecystokinin, acting at receptors on the basolateral plasma membrane, causes the generation of an intracellular Ca(2+) signal. This signal, in turn, triggers the fusion of the zymogen granules with the apical plasma membrane, leading to the polarised secretion of the enzymes. This review describes recent advances in our understanding of the control of secretion in the acinar cell. In particular, we discuss the mechanisms underlying the sorting of digestive enzymes into the zymogen granules, the molecular components of the exocytotic "membrane fusion machine," the generation and propagation of the Ca(2+ signal and the development of new techniques for the visualisation of single granule fusion events.

Acetylcholine-induced zymogen granule exocytosis: comparison between acini and single pancreatic acinar cells

INTRODUCTION

Numerous studies have been carried out on the agonist-evoked calcium responses of single pancreatic acinar cells; however, several reports have shown that dissociation of the exocrine pancreas into predominantly single cells has an adverse effect on agonist-evoked amylase secretion.

AIMS AND METHODOLOGY

To determine whether single acinar cells behave in an anomalous manner compared with cells within an intact acinus, we measured exocytosis in both single acinar cells and acini (2-5 cells) present in the same preparation. Exocytosis of individual zymogen granules was quantified in real-time by using the technique of continuous time-differential analysis of brightfield digital images.

RESULTS

Basal rates of exocytosis were low in both single cells and intact acini. Application of 10 microM acetylcholine for 6 minutes stimulated a biphasic secretory response in acinar cells. Additionally, we found that exocytotic events occur repetitively in specific locations within the apical domain; i.e., there are exocytotic "hot spots." There were no statistically significant differences between the exocytotic rates, nor were there any differences in the characteristics of the exocytotic hot spots of single cells compared with those of acini.

CONCLUSION

We conclude that time-differential analysis of brightfield images appears to be a useful tool for the investigation of the role of gap junctions in zymogen granule exocytosis and that single acinar cells provide a reasonable model for studies of acinar cell signaling and secretion.

Two modes of secretion in pancreatic acinar cells: involvement of phosphatidylinositol 3-kinase and regulation by capacitative Ca(2+) entry

In pancreatic acinar cells, muscarinic agonists stimulate both the release of Ca(2+) from intracellular stores and the influx of extracellular Ca(2+). The part played by Ca(2+) released from intracellular stores in the regulation of secretion is well established; however, the role of Ca(2+) influx in exocytosis is unclear. Recently, we observed that supramaximal concentrations of acetylcholine (>or=10 microM) elicited an additional component of exocytosis despite reducing Ca(2+) influx. In the present study, we found that supramaximal exocytosis was substantially inhibited (approximately 70%) by wortmannin (100 nM), an inhibitor of phosphatidylinositol 3-kinase. In contrast, exocytosis evoked by a lower concentration of acetylcholine (1 microM) was potentiated (approximately 45%) by wortmannin. Exocytosis stimulated by 1 microM acetylcholine in the absence of extracellular Ca(2+) was, like supramaximal exocytosis, inhibited by wortmannin. The switch to a wortmannin-inhibitable form of exocytosis depended upon a reduction in Ca(2+) entry through store-operated Ca(2+) channels, as the switch in exocytotic mode could also be brought about by the selective blockade of these channels by Gd(3+) (2 microM), but not by inhibition of noncapacitative Ca(2+) entry by SB203580 (10 microM). We conclude that supramaximal doses of acetylcholine lead to a switch in the mode of zymogen granule exocytosis by inhibiting store-dependent Ca(2+) influx.