Sequential-replenishment mechanism of exocytosis in pancreatic acini

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.

Cross talk between metabotropic and ionotropic glutamate receptor-mediated signaling in parallel fiber-induced inositol 1,4,5-trisphosphate production in cerebellar Purkinje cells

In many excitatory glutamatergic synapses, both ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs) are closely distributed on the postsynaptic membrane. However, the functional significance of the close distribution of the two types of glutamate receptors has not been fully clarified. In this study, we examined the functional interaction between iGluR and mGluR at parallel fiber (PF)--> Purkinje cell synapses in the generation of inositol 1,4,5-trisphosphate (IP3), a key second messenger that regulates many important cellular functions. We visualized local IP3 dynamics in Purkinje cells using the green fluorescent protein-tagged pleckstrin homology domain (GFP-PHD) as a fluorescent IP3 probe. Purkinje cells were transduced with Sindbis virus encoding GFP-PHD and imaged with a two-photon laser scanning microscope. Translocation of GFP-PHD from the plasma membrane to the cytoplasm attributable to an increase in IP3 concentration was observed on PF stimulation in fine dendrites of Purkinje cells. Surprisingly, this PF-induced IP3 production was blocked not only by the group I mGluR antagonist but also by the AMPA receptor (AMPAR) antagonist. The PF-induced IP3 production was blocked by either the inhibition of G-protein activation by GDP-betaS or intracellular Ca2+ buffering by BAPTA. These results show that IP3 production is mediated cooperatively by group I mGluR and AMPAR through G-protein activation and Ca2+ influx at PF--> Purkinje cell synapses, identifying the robust cross talk between iGluR and mGluR for the generation of IP3 signals.

Stable Golgi-Mitochondria Complexes and Formation of Golgi Ca2+ Gradients in Pancreatic Acinar Cells

We have determined the localization of the Golgi with respect to other organelles in living pancreatic acinar cells and the importance of this localization to the establishment of Ca(2+) gradients over the Golgi. Using confocal microscopy and the Golgi-specific fluorescent probe 6-((N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoyl)sphingosine, we found Golgi structures localizing to the outer edge of the secretory granular region of individual acinar cells. We also assessed Golgi positioning in acinar cells located within intact pancreatic tissue using two-photon microscopy and found a similar localization. The mitochondria segregate the Golgi from lateral regions of the plasma membrane, the nucleus, and the basal part of the cytoplasm. The Golgi is therefore placed between the principal Ca(2+) release sites in the apical region of the cell and the important Ca(2+) sink formed by the peri-granular mitochondria. During acetylcholine-induced cytosolic Ca(2+) signals in the apical region, large Ca(2+) gradients form over the Golgi (decreasing from trans- to cis-Golgi). We further describe a novel, close interaction of the peri-granular mitochondria and the Golgi apparatus. The mitochondria and the Golgi structures form very close contacts, and these contacts remain stable over time. When the cell is forced to swell, the Golgi and mitochondria remain juxtaposed up to the point of cell lysis. The strategic position of the Golgi (closer to release sites than the bulk of the mitochondrial belt) makes this organelle receptive to local apical Ca(2+) transients. In addition the Golgi is ideally placed to be preferentially supplied by ATP from adjacent mitochondria.

Two phases of zymogen granule lifetime in mouse pancreas: ghost granules linger after exocytosis of contents

Different cell types show widely divergent mechanisms and kinetics of exocytosis. We investigated these processes in pancreatic acinar cells by using video-rate 2-photon microscopy to image entry of extracellular dye into individual zymogen granules undergoing exocytosis. Fluorescence signals display two distinct phases; an initial peak that then decays over several seconds to a prolonged plateau. Several observations suggest that the first component reflects the binding of dye to the granule contents and their subsequent release into the acinar duct. These observations include: the peak/plateau fluorescence ratio differs between different dyes; the initial fluorescence decay mirrors the loss of granule contents as monitored by differential interference contrast microscopy; and the fall in vesicular fluorescence is accompanied by a rise in fluorescence in the adjacent duct lumen. We thus propose the use of extracellular fluorescent probes as a convenient means to monitor the kinetics of loss of proteinaceous content from secretory granules. In pancreatic acinar cells the fusion pore remains open much longer than required to ensure secretion of the granule contents, and instead the persistent empty 'ghost-granule' may act as a conduit to which secondary granules can fuse and release their contents by compound exocytosis.

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.

Phosphatidylinositol 3-kinase regulates Ca2+ signaling in pancreatic acinar cells through inhibition of sarco(endo)plasmic reticulum Ca2+-ATPase

Calcium is a key mediator of hormone-induced enzyme secretion in pancreatic acinar cells. At the same time, abnormal Ca(2+) responses are associated with pancreatitis. We have recently shown that inhibition of phosphatidylinositol 3-kinase (PI3-kinase) by LY-294002 and wortmannin, as well as genetic deletion of PI3-kinase-gamma, regulates Ca(2+) responses and the Ca(2+)-sensitive trypsinogen activation in pancreatic acinar cells. The present study sought to determine the mechanisms of PI3-kinase involvement in Ca(2+) responses induced in these cells by CCK and carbachol. The PI3-kinase inhibitors inhibited both Ca(2+) influx and mobilization from intracellular stores induced by stimulation of acini with physiological and pathological concentrations of CCK, as well as with carbachol. PI3-kinase inhibition facilitated the decay of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) oscillations observed in individual acinar cells. The PI3-kinase inhibitors decreased neither CCK-induced inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] production nor Ins(1,4,5)P(3)-induced Ca(2+) mobilization, suggesting that the effect of PI3-kinase inhibition is not through Ins(1,4,5)P(3) or Ins(1,4,5)P(3) receptors. PI3-kinase inhibition did not affect Ca(2+) mobilization induced by thapsigargin, a specific inhibitor of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA). Moreover, SERCA blockade with thapsigargin abolished the effects of pharmacological and genetic PI3-kinase inhibition on [Ca(2+)](i) signals, suggesting SERCA as a downstream target of PI3-kinase. Both pharmacological PI3-kinase inhibition and genetic deletion of PI3-kinase-gamma increased the amount of Ca(2+) in intracellular stores during CCK stimulation. Finally, addition of the PI3-kinase product phosphatidylinositol 3,4,5-trisphosphate to permeabilized acini significantly attenuated Ca(2+) reloading into the endoplasmic reticulum. The results indicate that PI3-kinase regulates Ca(2+) signaling in pancreatic acinar cells through its inhibitory effect on SERCA.

Sequential exocytosis of insulin granules is associated with redistribution of SNAP25

We have investigated sequential exocytosis in beta cells of intact pancreatic islets with the use of two-photon excitation imaging of a polar fluorescent tracer, sulforhodamine B, and a fusion protein comprising enhanced cyan fluorescent protein (ECFP) and the SNARE protein SNAP25 (synaptosome-associated protein of 25 kD) transfected with an adenoviral vector. Sequential exocytosis was found to account for <10% of exocytic events in beta cells stimulated either with glucose under various conditions or by photolysis of a caged-Ca2+ compound. Multigranular exocytosis, in which granule-to-granule fusion occurs before exocytosis, was rarely found. We detected redistribution of ECFP-SNAP25 from the plasma membrane into the membrane of the fused granule occurred in a large proportion (54%) of sequential exocytic events but in only a small fraction (5%) of solitary fusion events. Removal of cholesterol in the plasma membrane by methyl-beta-cyclodextrin facilitated both redistribution of ECFP-SNAP25 and sequential exocytosis by threefold. These observations support the hypothesis that SNAP25 is a plasma membrane factor that is responsible for sequential exocytosis.

Stabilization of Exocytosis by Dynamic F-actin Coating of Zymogen Granules in Pancreatic Acini

Reorganization of F-actin in the apical region of mouse pancreatic acinar cells during Ca(2+)-dependent exocytosis of zymogen granules was investigated by two-photon excitation microscopy with intact acini. Granules were rapidly coated with F-actin in response to either agonist stimulation or photolysis of a caged-Ca(2+) compound. Such F-actin coating occurred exclusively at the surface of granules undergoing exocytosis and was prevented either by latrunculin-A, which inhibits actin polymerization, or by Clostridium botulinum exoenzyme C3, which inhibits the small GTPase Rho. Latrunculin-A or exoenzyme C3 also triggered the formation of vacuoles in acinar cells, a characteristic of acute pancreatitis. Stimulation of acini with high concentrations of cholecystokinin, which cause acute pancreatitis in mice, also impaired the F-actin coating of granules and induced vacuole formation. Latrunculin-A reduced the latency to exocytosis but did not affect the total number of exocytic events, suggesting that F-actin slows and further stabilizes exocytosis by facilitating F-actin coating. Rho-dependent F-actin coating of granule membranes thus stabilizes exocytic structures and is necessary for physiological progression of sequetial compound exocytosis in the exocrine pancreas and for prevention of acute pancreatitis.

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

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

Secretagogue-induced translocation of CRHSP-28 within an early apical endosomal compartment in acinar cells

Ca(2+)-regulated heat-stable protein (CRHSP-28) is a member of the TPD52 protein family that has been shown to regulate Ca(2+)-dependent secretory activity in pancreatic acinar cells. Immunofluorescence microscopy of isolated lobules demonstrated that CRHSP-28 is localized to a supranuclear apical compartment in acini and accumulates immediately below the apical membrane within 2 min of CCK octapeptide (CCK-8) stimulation. Dual-immunofluorescence microscopy demonstrated an endosomal localization of CRHSP-28 that strongly overlapped with early endosomal antigen-1 (EEA-1) on vesicular structures throughout the apical cytoplasm but showed only minimal overlap with the transferrin receptor, which is present in basolaterally derived endosomes. Significant overlapping of CRHSP-28 with the trans-Golgi network marker-38 was also noted in supranuclear regions of acini. Interestingly, treatment of lobules with brefeldin A reversibly disrupted the vesicular localization of CRHSP-28 and EEA-1 within the apical cytoplasm. The CCK-8-induced accumulation of CRHSP-28 in subapical regions of acini was not altered by inhibition of apical endocytosis with the actin filament-disrupting agent latrunculin B. Immunoelectron microscopy confirmed that CRHSP-28 is associated with the limiting membrane of irregularly shaped vesicular structures of low electron density in the apical cytoplasm that are positive for EEA-1 staining. Sparse, but significant, CRHSP-28 immunoreactivity was also observed along the limiting membrane of zymogen granules. Consistent with immunofluorescence data, CRHSP-28 was found to accumulate in clusters on endosomes and positioned between zymogen granules below the cell apex on CCK-8 stimulation. These data indicate that CRHSP-28 is present within endocytic and exocytic compartments of acinar cells and is acutely regulated by secretagogue stimulation.

Noc2 is essential in normal regulation of exocytosis in endocrine and exocrine cells

Rab3 is a subfamily of the small GTP-binding protein Rab family and plays an important role in exocytosis. Several potential effectors of Rab3, including rabphilin3 and Rims (Rim1 and Rim2), have been isolated and characterized. Noc2 was identified originally in endocrine pancreas as a molecule homologous to rabphilin3, but its role in exocytosis is unclear. To clarify the physiological function of Noc2 directly, we have generated Noc2 knockout (Noc2(-/-)) mice. Glucose intolerance with impaired insulin secretion was induced in vivo by acute stress in Noc2(-/-) mice, but not in wild-type (Noc2(+/+)) mice. Ca(2+)-triggered insulin secretion from pancreatic isles of Noc2(-/-) mice was markedly impaired, but was completely restored by treatment with pertussis toxin, which inhibits inhibitory G protein Gi/o signaling. In addition, the inhibitory effect of clonidine, an alpha(2)-adrenoreceptor agonist, on insulin secretion was significantly greater in Noc2(-/-) islets than in Noc2(+/+) islets. Impaired Ca(2+)-triggered insulin secretion was rescued by adenovirus gene transfer of wild-type Noc2 but not by that of mutant Noc2, which does not bind to Rab3. Accordingly, Noc2 positively regulates insulin secretion from endocrine pancreas by inhibiting Gi/o signaling, and the interaction of Noc2 and Rab3 is required for the effect. Interestingly, we also found a marked accumulation of secretory granules in various exocrine cells of Noc2(-/-) mice, especially in exocrine pancreas with no amylase response to stimuli. Thus, Noc2, a critical effector of Rab3, is essential in normal regulation of exocytosis in both endocrine and exocrine cells.

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.

Differential regulation of exocytotic fusion and granule-granule fusion in eosinophils by Ca2+ and GTP analogs

Dynamics of degranulation was studied in horse eosinophils by patch clamp capacitance measurements. Degranulation was stimulated by intracellular application of calcium, and GTPgammaS or guanosine 5'-(beta,gamma-imido)triphosphate at different concentrations via the patch pipette. Degranulation was quantified by measuring the delay time between the beginning of intracellular perfusion and the first exocytotic event, determining the distribution of time intervals between fusion events and the capacitance step size distributions under the different conditions. The degranulation dynamics could be well reproduced using a computer model assuming three independent rate constants for granule-plasma membrane fusion, granule fusion with already exocytosed granules, and intracellular granule-granule fusion. The rate of granule-plasma membrane fusion is sensitive to both, the GTP analog and [Ca2+]i. The rate of granule-exocytosed granule fusion is sensitive to [Ca2+]i but insensitive to the GTP analogs, and the rate of granule-to-granule fusion is sensitive to the GTP analog but insensitive to [Ca2+]i. Granule fusions with the three different target compartments thus involve different regulatory mechanisms.

Compound exocytosis and cumulative fusion in eosinophils

Focal release of cytotoxic proteins by eosinophils onto the target surface plays an important role in parasite killing. Degranulation was stimulated by intracellular application of calcium and guanosine 5'-3-O-(thio)triphosphate via the recording patch pipette or via streptolysin-O permeabilization. Exocytotic fusion was monitored by capacitance measurements, whereas release of fluorescent weak bases, which accumulate selectively within eosinophil granules, was followed by fluorescence imaging. Several distinct types of granule fusion events were directly observed by simultaneous capacitance and fluorescence measurements. These are fusion of a single granule with the plasma membrane, intracellular granule-granule fusion, fusion of large compounds of pre-fused granules with the plasma membrane (compound exocytosis), and sequential fusion of granules to granules previously fused to the plasma membrane. Extensive granule-granule fusion was also observed by electron microscopy of permeabilized cells. All these fusion mechanisms contribute to focal release. The coexistence of distinct modes of exocytosis suggests that their regulation may modulate effector functions of eosinophils during helminth infection and allergic response.

NAADP mobilizes Ca2+ from a thapsigargin-sensitive store in the nuclear envelope by activating ryanodine receptors

Ca2+ release from the envelope of isolated pancreatic acinar nuclei could be activated by nicotinic acid adenine dinucleotide phosphate (NAADP) as well as by inositol 1,4,5-trisphosphate (IP3) and cyclic ADP-ribose (cADPR). Each of these agents reduced the Ca2+ concentration inside the nuclear envelope, and this was associated with a transient rise in the nucleoplasmic Ca2+ concentration. NAADP released Ca2+ from the same thapsigargin-sensitive pool as IP3. The NAADP action was specific because, for example, nicotineamide adenine dinucleotide phosphate was ineffective. The Ca2+ release was unaffected by procedures interfering with acidic organelles (bafilomycin, brefeldin, and nigericin). Ryanodine blocked the Ca2+-releasing effects of NAADP, cADPR, and caffeine, but not IP3. Ruthenium red also blocked the NAADP-elicited Ca2+ release. IP3 receptor blockade did not inhibit the Ca2+ release elicited by NAADP or cADPR. The nuclear envelope contains ryanodine and IP3 receptors that can be activated separately and independently; the ryanodine receptors by either NAADP or cADPR, and the IP3 receptors by IP3.

Whole-field fluorescence microscope with digital micromirror device: imaging of biological samples

We have developed a whole-field fluorescence microscope equipped with a Digital Micromirror Device to acquire optically sectioned images by using the fringe-projection technique and the phase-shift method. This system allows free control of optical sectioning strength through computer-controlled alteration of the fringe period projected onto a sample. We have employed this system to image viable cells expressing fluorescent proteins and discussed its biological applications.

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.

Modulation of IP(3)-sensitive Ca(2+) release by 2,3-butanedione monoxime

We describe the actions of 2,3-butanedione monoxime (BDM) on calcium responses in secretory cells. Our studies were prompted by the widespread use of BDM as a myosin-ATPase inhibitor. Application of 10 mM BDM almost completely inhibited agonist-evoked amylase secretion from mouse pancreatic acinar cells. This action might be interpreted as indicating a role for myosin in secretion. However, BDM alone elicited a calcium response in single cells and this calcium signal was sufficient to activate calcium-dependent chloride currents. Furthermore, in some cases, BDM potentiated agonist-evoked calcium signals but almost always blocked agonist-evoked calcium oscillations. These effects of BDM were not due to an action on calcium influx pathways but rather to direct effects on IP(3)-sensitive stores. We conclude that BDM cannot be used for unequivocal identification of the involvement of myosin motors in a cellular response. Further, our evidence suggests that BDM can act directly to modify the opening of IP(3) receptors.

Mechanical forces impeding exocytotic surfactant release revealed by optical tweezers

The release of surfactant from alveolar type II cells is essential to lower the surface tension in the lung and to facilitate inspiration. However, the factors controlling dispersal and diffusion of this hydrophobic material are still poorly understood. Here we report that release of surfactant from the fused vesicle, termed lamellar body (LB), resisted mechanical forces applied by optical tweezers: At constant trapping force, the probability to expand LB contents, i.e., to "pull" surfactant into the extracellular fluid, increased with time after LB fusion with the plasma membrane, consistent with slow fusion pore expansion in these cells. Elevations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) had a similar effect. Inasmuch as surfactant did not disintegrate in the extracellular space, this method permitted for the first time the determination of elastic and recoil properties of the macromolecular complex, yielding a spring constant of approximately 12.5 pN/ micro m. This is the first functional evidence that release of hydrophobic material is mechanically impeded and occurs in an "all-or-none" fashion. This mode of release is most probably the result of cohesive forces of surfactant, combined with adhesive forces and/or retaining forces exerted by a constrictive fusion pore acting as a regulated mechanical barrier, withstanding forces up to 160 pN. In independent experiments equiaxial strain was exerted on cells without optical tweezers. Strain facilitated surfactant release from preexisting fused vesicles, consistent with the view of mechanical impediments during the release process, which can be overcome by cell strain.

Effects of selective endocrine or exocrine induction of AR42J on SNARE and Munc18 protein expression

INTRODUCTION AND AIM

We used the amphicrine AR42J as an excellent model to study the differentiation of the secretory machinery of pancreatic endocrine and exocrine cells. Dexamethasone treatment induced the AR42J to differentiate towards the exocrine phenotype capable of secreting amylase in response to cholecystokinin. In contrast, activin A plus hepatocyte growth factor treatment of a subclone of AR42J, AR42J-B13, induced this cell to differentiate morphologically and functionally toward an insulin-containing and insulin-secreting endocrine phenotype. We took advantage of these unique properties of selective exocrine and endocrine induction of the AR42J to reveal which distinct combinations of exocytic SNARE complex proteins (syntaxin, SNAP-25 and VAMP) and associated Munc18 proteins were preferentially expressed to play a role in enzyme and insulin secretion.

RESULTS AND CONCLUSION

To our surprise, both endocrine and exocrine induction of AR42J and AR42J-B13 caused very similar upregulation in the expression of the exocytic member isoforms of the syntaxin, SNAP-25, VAMP, and Munc18 families. We conclude that whereas the differentiation of the proximal components of the secretory machinery of the exocrine acinar and endocrine islet beta-cells is distinct, the differentiation of the distal components of exocytosis between these two cell types is very similar.

Switch to anaerobic glucose metabolism with NADH accumulation in the beta-cell model of mitochondrial diabetes. Characteristics of betaHC9 cells deficient in mitochondrial DNA transcription

To elucidate the mechanism underlying diabetes caused by mitochondrial gene mutations, we created a model by applying 0.4 microg/ml ethidium bromide (EtBr) to the murine pancreatic beta cell line betaHC9; in this model, transcription of mitochondrial DNA, but not that of nuclear DNA, was suppressed in association with impairment of glucose-stimulated insulin release (Hayakawa, T., Noda, M., Yasuda, K., Yorifuji, H., Taniguchi, S., Miwa, I., Sakura, H., Terauchi, Y., Hayashi, J.-I., Sharp, G. W. G., Kanazawa, Y., Akanuma, Y., Yazaki, Y., and Kadowaki, T. (1998) J. Biol. Chem. 273, 20300-20307). To elucidate fully the metabolism-secretion coupling in these cells, we measured glucose oxidation, utilization, and lactate production. We also evaluated NADH autofluorescence in betaHC9 cells using two-photon excitation laser microscopy. In addition, we recorded the membrane potential and determined the ATP and ADP contents of the cells. The results indicated 22.2 mm glucose oxidation to be severely decreased by EtBr treatment compared with control cells (by 63% on day 4 and by 78% on day 6; both p < 0.01). By contrast, glucose utilization was only marginally decreased. Lactate production under 22.2 mm glucose was increased by 2.9- and 3.5-fold by EtBr treatment on days 4 and 6, respectively (both p < 0.01). Cellular NADH at 2.8 mm glucose was increased by 35 and 43% by EtBr on days 4 and 6 (both p < 0.01). These data suggest that reduced expression of the mitochondrial electron transport system causes NADH accumulation in beta cells, thereby halting the tricarboxylic acid cycle on one hand, and on the other hand facilitating anaerobic glucose metabolism. Glucose-induced insulin secretion was lost rapidly along with the EtBr treatment with concomitant losses of membrane potential depolarization and the [Ca(2+)](i) increase, whereas glibenclamide-induced changes persisted. This is the first report to demonstrate the connection between metabolic alteration of electron transport system and that of tricarboxylic acid cycle and its impact on insulin secretion.

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.

Fusion pore dynamics and insulin granule exocytosis in the pancreatic islet

Insulin secretion from intact mouse pancreatic islets was investigated with two-photon excitation imaging. Insulin granule exocytosis occurred mainly toward the interstitial space, away from blood vessels. The fusion pore was unusually stable with a lifetime of 1.8 seconds. Opening of the 1.4-nanometer-diameter pore was preceded by unrestricted lateral diffusion of lipids along the inner wall of the pore, supporting the idea that this structure is composed of membrane lipids. When the pore dilated to 12 nanometers, the granules rapidly flattened and discharged their contents. Thus, our methodology reveals fusion pore dynamics in intact tissues at nanometer resolution.

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.

Visualization of sequential exocytosis in rat pancreatic islet beta cells

The examination of insulin exocytosis at the single cell level by conventional electrophysiologic and amperometric methods possesses inherent limitations, and may not accurately reflect the morphologic events of exocytosis of the insulin granule. To overcome some of these limitations, we show by epifluorescent microscopy of a fluorescent dye, FM1-43, its incorporation into the plasma membrane and oncoming insulin granules undergoing exocytosis, and their core proteins. Using this method, we tracked exocytosis in real-time in insulinoma INS-1 and single rat islet beta cells in response to KCl and glucose. We observed both single transient and multi-stepwise increases in membrane FM1-43 fluorescence, suggesting single granule exocytosis as well as sequential and compound exocytosis, respectively. Confocal microscopy of nonpermeabilized cells shows that some of the exocytosed insulin granules labeled by the FM1-43 dye could also be labeled with insulin antibodies, suggesting prolonged openings of the fusion pores and slow dissolution of the granule core proteins on the membrane surface.

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.

Transformation of local Ca2+ spikes to global Ca2+ transients: the combinatorial roles of multiple Ca2+ releasing messengers

In pancreatic acinar cells, low, threshold concentrations of acetylcholine (ACh) or cholecystokinin (CCK) induce repetitive local cytosolic Ca2+ spikes in the apical pole, while higher concentrations elicit global signals. We have investigated the process that transforms local Ca2+ spikes to global Ca2+ transients, focusing on the interactions of multiple intracellular messengers. ACh-elicited local Ca2+ spikes were transformed into a global sustained Ca2+ response by cyclic ADP-ribose (cADPR) or nicotinic acid adenine dinucleotide phosphate (NAADP), whereas inositol 1,4,5-trisphosphate (IP3) had a much weaker effect. In contrast, the response elicited by a low CCK concentration was strongly potentiated by IP3, whereas cADPR and NAADP had little effect. Experiments with messenger mixtures revealed a local interaction between IP3 and NAADP and a stronger global potentiating interaction between cADPR and NAADP. NAADP strongly amplified the local Ca2+ release evoked by a cADPR/IP3 mixture eliciting a vigorous global Ca2+ response. Different combinations of Ca2+ releasing messengers can shape the spatio-temporal patterns of cytosolic Ca2+ signals. NAADP and cADPR are emerging as key messengers in the globalization of Ca2+ signals.

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.

Calcium dependence of exocytosis in lacrimal gland acinar cells

Simultaneous measurements of membrane capacitance and intracellular calcium concentration were used to examine the calcium dependence of exocytosis in single acinar cells from mouse lacrimal gland and to establish the quantitative relation between calcium concentration and rate of exocytosis. Application of adrenergic or muscarinic agonists elevated intracellular calcium and evoked exocytosis, as indicated by an increase in membrane capacitance of single cells. The capacitance response to agonist stimulation was eliminated by internal dialysis with the calcium buffer EGTA, which demonstrated that the increase in intracellular calcium was necessary for agonist-evoked exocytosis. When internal calcium was elevated by application of the calcium ionophore ionomycin, exocytosis was evoked in the absence of agonist stimulation. Thus an increase in intracellular calcium was necessary and sufficient for exocytosis in single acinar cells. The rate of change of membrane capacitance increased as approximately the third power of the calcium concentration, which is similar to the dependence of exocytosis rate on calcium concentration in other secretory cells.

Two-photon excitation imaging of pancreatic islets with various fluorescent probes

Various fluorescent probes were assessed for investigating intact islets of Langerhans using two-photon excitation imaging. Polar fluorescent tracers applied on the outside rapidly (within 3 min) penetrated deep into the islets via microvessels. Likewise, an adenovirus carrying a Ca(2+)-sensitive green fluorescent protein mutant gene, yellow cameleon 2.1, was successfully transfected and enabled ratiometric cytosolic Ca(2+) measurement of cells in the deep layers of the islets. Interestingly, FM1-43, which is lipophilic and does not permeate the plasma membrane, also rapidly reached deep cell layers of the islets. In contrast, lipophilic fluorescent probes that permeate the plasma membrane (for example, fura-2-acetoxymethyl and BODIPY-forskolin) accumulated in the superficial cell layers of the islets, even 30 min after application. Thus, two-photon excitation imaging of pancreatic islets is a promising method for clarifying signaling mechanisms of islet cells, particularly when it is combined with membrane-impermeable probes. In addition, our data suggest that membrane-permeable antagonists may affect only the superficial cell layers of islets, and so their negative effects should be interpreted with caution.

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

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

Supramaximal cholecystokinin displaces Munc18c from the pancreatic acinar basal surface, redirecting apical exocytosis to the basal membrane

Exocytosis at the apical surface of pancreatic acinar cells occurs in the presence of physiological concentrations of cholecystokinin (CCK) but is inhibited at high concentrations. Here we show that Munc18c is localized predominantly to the basal membranes of acinar cells. Supramaximal but not submaximal CCK stimulation caused Munc18c to dissociate from the plasma membrane, and this displacement was blocked by protein kinase C (PKC) inhibitors. Conversely, whereas the CCK analog CCK-OPE alone failed to displace Munc18c from the membrane, this agent caused Munc18c displacement following minimal PKC activation. To determine the physiological significance of this displacement, we used the fluorescent dye FM1-43 to visualize individual exocytosis events in real-time from rat acinar cells in culture. We showed that supramaximal CCK inhibition of secretion resulted from impaired apical secretion and a redirection of exocytic events to restricted basal membrane sites. In contrast, CCK-OPE evoked apical exocytosis and could only induce basolateral exocytosis following activation of PKC. Infusion of supraphysiological concentrations of CCK in rats, a treatment that induced tissue changes reminiscent of mild acute pancreatitis, likewise resulted in rapid displacement of Munc18c from the basal membrane in vivo.

Applying multiphoton imaging to the study of membrane dynamics in living cells

The endomembrane system of a cell is a highly dynamic, ephemeral structure that is difficult to visualize. Reconstructions from sections of fixed material can provide high-resolution information on intercellular membrane architecture, but such techniques are fraught with artifacts and are of little help in understanding the dynamics of intracellular membrane traffic. Recently, the availability of fluorescent membrane probes and the development of techniques for optically sectioning intact specimens have allowed glimpses of membrane dynamics to be visualized in living tissue. In this review we discuss the potential of a new optical sectioning technique, multiphoton imaging, for visualizing membrane dynamics in living cells. Multiphoton microscopy offers an unparalleled ability to obtain images from deep within specimens while minimizing the effects of phototoxicity.

Dendritic spine geometry is critical for AMPA receptor expression in hippocampal CA1 pyramidal neurons

Dendritic spines serve as preferential sites of excitatory synaptic connections and are pleomorphic. To address the structure-function relationship of the dendritic spines, we used two-photon uncaging of glutamate to allow mapping of functional glutamate receptors at the level of the single synapse. Our analyses of the spines of CA1 pyramidal neurons reveal that AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)-type glutamate receptors are abundant (up to 150/spine) in mushroom spines but sparsely distributed in thin spines and filopodia. The latter may be serving as the structural substrates of the silent synapses that have been proposed to play roles in development and plasticity of synaptic transmission. Our data indicate that distribution of functional AMPA receptors is tightly correlated with spine geometry and that receptor activity is independently regulated at the level of single spines.

Fusion pore expansion is a slow, discontinuous, and Ca2+-dependent process regulating secretion from alveolar type II cells

In alveolar type II cells, the release of surfactant is considerably delayed after the formation of exocytotic fusion pores, suggesting that content dispersal may be limited by fusion pore diameter and subject to regulation at a postfusion level. To address this issue, we used confocal FRAP and N-(3-triethylammoniumpropyl)-4-(4-[dibutylamino]styryl) pyridinium dibromide (FM 1-43), a dye yielding intense localized fluorescence of surfactant when entering the vesicle lumen through the fusion pore (Haller, T., J. Ortmayr, F. Friedrich, H. Volkl, and P. Dietl. 1998. Proc. Natl. Acad. Sci. USA. 95:1579-1584). Thus, we have been able to monitor the dynamics of individual fusion pores up to hours in intact cells, and to calculate pore diameters using a diffusion model derived from Fick's law. After formation, fusion pores were arrested in a state impeding the release of vesicle contents, and expanded at irregular times thereafter. The expansion rate of initial pores and the probability of late expansions were increased by elevation of the cytoplasmic Ca2+ concentration. Consistently, content release correlated with the occurrence of Ca2+ oscillations in ATP-treated cells, and expanded fusion pores were detectable by EM. This study supports a new concept in exocytosis, implicating fusion pores in the regulation of content release for extended periods after initial formation.

Multiphoton microscopy in biological research

From its conception a decade ago, multiphoton microscopy has evolved from a photonic novelty to an indispensable tool for gleaning information from subcellular events within organized tissue environments. Its relatively deep optical penetration has recently been exploited for subcellularly resolved investigations of disease models in living transgenic mice. Its enhanced spectral accessibility enables aberration-free imaging of fluorescent molecules absorbing in deep-UV energy regimes with simultaneous imaging of species having extremely diverse emission spectra. Although excited fluorescence is the primary signal for multiphoton microscopy, harmonic generation by multiphoton scattering processes are also valuable for imaging species with large anharmonic modes, such as collagen structures and membrane potential sensing dyes.

Local uncaging of caged Ca(2+) reveals distribution of Ca(2+)-activated Cl(-) channels in pancreatic acinar cells

In exocrine acinar cells, Ca(2+)-activated Cl(-) channels in the apical membrane are essential for fluid secretion, but it is unclear whether such channels are important for Cl(-) uptake at the base. Whole-cell current recording, combined with local uncaging of caged Ca(2+), was used to reveal the Cl(-) channel distribution in mouse pancreatic acinar cells, where approximately 90% of the current activated by Ca(2+) in response to acetylcholine was carried by Cl(-). When caged Ca(2+) in the cytosol was uncaged locally in the apical pole, the Cl(-) current was activated, whereas local Ca(2+) uncaging in the basal or lateral areas of the cell had no effect. Even when Ca(2+) was uncaged along the whole inner surface of the basolateral membrane, no Cl(-) current was elicited. There was little current deactivation at a high cytosolic Ca(2+) concentration ([Ca(2+)](c)), but at a low [Ca(2+)](c) there was clear voltage-dependent deactivation, which increased with hyperpolarization. Functional Ca(2+)-activated Cl(-) channels are expressed exclusively in the apical membrane and channel opening is strictly regulated by [Ca(2+)](c) and membrane potential. Ca(2+)-activated Cl(-) channels do not mediate Cl(-) uptake at the base, but acetylcholine-elicited local [Ca(2+)](c) spiking in the apical pole can regulate fluid secretion by controlling the opening of these channels in the apical membrane.