Live pancreatic acinar imaging of exocytosis using syncollin-pHluorin

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.

Pancreas-specific SNAP23 depletion prevents pancreatitis by attenuating pathological basolateral exocytosis and formation of trypsin-activating autolysosomes

Intrapancreatic trypsin activation by dysregulated macroautophagy/autophagy and pathological exocytosis of zymogen granules (ZGs), along with activation of inhibitor of NFKB/NF-κB kinase (IKK) are necessary early cellular events in pancreatitis. How these three pancreatitis events are linked is unclear. We investigated how SNAP23 orchestrates these events leading to pancreatic acinar injury. SNAP23 depletion was by knockdown (SNAP23-KD) effected by adenovirus-shRNA (Ad-SNAP23-shRNA/mCherry) treatment of rodent and human pancreatic slices and in vivo by infusion into rat pancreatic duct. In vitro pancreatitis induction by supraphysiological cholecystokinin (CCK) or ethanol plus low-dose CCK were used to assess SNAP23-KD effects on exocytosis and autophagy. Pancreatitis stimuli resulted in SNAP23 translocation from its native location at the plasma membrane to autophagosomes, where SNAP23 would bind and regulate STX17 (syntaxin17) SNARE complex-mediated autophagosome-lysosome fusion. This SNAP23 relocation was attributed to IKBKB/IKKβ-mediated SNAP23 phosphorylation at Ser95 Ser120 in rat and Ser120 in human, which was blocked by IKBKB/IKKβ inhibitors, and confirmed by the inability of IKBKB/IKKβ phosphorylation-disabled SNAP23 mutant (Ser95A Ser120A) to bind STX17 SNARE complex. SNAP23-KD impaired the assembly of STX4-driven basolateral exocytotic SNARE complex and STX17-driven SNARE complex, causing respective reduction of basolateral exocytosis of ZGs and autolysosome formation, with consequent reduction in trypsinogen activation in both compartments. Consequently, pancreatic SNAP23-KD rats were protected from caerulein and alcoholic pancreatitis. This study revealed the roles of SNAP23 in mediating pathological basolateral exocytosis and IKBKB/IKKβ's involvement in autolysosome formation, both where trypsinogen activation would occur to cause pancreatitis. SNAP23 is a strong candidate to target for pancreatitis therapy.

Mechanism and effects of pulsatile GABA secretion from cytosolic pools in the human beta cell

Pancreatic beta cells synthesize and secrete the neurotransmitter γ-aminobutyric acid (GABA) as a paracrine and autocrine signal to help regulate hormone secretion and islet homeostasis. Islet GABA release has classically been described as a secretory vesicle-mediated event. Yet, a limitation of the hypothesized vesicular GABA release from islets is the lack of expression of a vesicular GABA transporter in beta cells. Consequentially, GABA accumulates in the cytosol. Here we provide evidence that the human beta cell effluxes GABA from a cytosolic pool in a pulsatile manner, imposing a synchronizing rhythm on pulsatile insulin secretion. The volume regulatory anion channel (VRAC), functionally encoded by LRRC8A or Swell1, is critical for pulsatile GABA secretion. GABA content in beta cells is depleted and secretion is disrupted in islets from type 1 and type 2 diabetic patients, suggesting that loss of GABA as a synchronizing signal for hormone output may correlate with diabetes pathogenesis.

Adult human pancreatic acinar cells dedifferentiate into an embryonic progenitor-like state in 3D suspension culture

Human pancreatic exocrine cells were cultured in 3D suspension and formed pancreatospheres composed of acinar-derived and duct-like cells. We investigated, up to 6 days, the fate of human pancreatic acinar cells using fluorescein-conjugated Ulex Europaeus Agglutinin 1 lectin, a previously published acinar-specific non-genetic lineage tracing strategy. At day 4, fluorescence-activated cell sort for the intracellularly incorporated FITC-conjugated UEA1 lectin and the duct-specific CA19.9 surface marker, distinguished acinar-derived cells (UEA1⁺CA19.9^-) from duct-like cells (UEA1^-CA19.9⁺) and acinar-to-duct-like transdifferentiated cells (UEA1⁺CA19.9⁺). mRNA expression analysis of the acinar-derived (UEA1⁺CA19.9^-) and duct-like (UEA1^-CA19.9⁺) cell fractions with concomitant immunocytochemical analysis of the pancreatospheres revealed acquisition of an embryonic signature in the UEA1⁺CA19.9^- acinar-derived cells characterized by de novo expression of SOX9 and CD142, robust expression of PDX1 and surface expression of GP2. The colocalisation of CD142, a multipotent pancreatic progenitor surface marker, PDX1, SOX9 and GP2 is reminiscent of a cellular state present during human embryonic development. Addition of TGF-beta signalling inhibitor Alk5iII, induced a 28-fold increased KI67-labeling in pancreatospheres, more pronounced in the CD142⁺GP2⁺ acinar-derived cells. These findings with human cells underscore the remarkable plasticity of pancreatic exocrine acinar cells, previously described in rodents, and could find applications in the field of regenerative medicine.

THE M-RNA, EXPRESSION OF SERCA2 AND NCX1 IN THE PROCESS OF PHARMACOLOGICAL CELL PROTECTION IN EXPERIMENTAL ACUTE PANCREATITIS INDUCED BY TAUROCHOLATE

Background:

Intracellular calcium overload is known to be a precipitating factor of pancreatic cell injury in acute pancreatitis (AP). Intracellular calcium homeostasis depends of Plasmatic Membrane Calcium ATPase (PMCA), Sarcoplasmic Endothelial Reticulum Calcium ATPase 2 (SERCA 2) and the Sodium Calcium Exchanger (NCX1). The antioxidant melatonin (Mel) and Trisulfate Disaccharide (TD) that accelerates NCX1 action could reduce the cell damage determined by the AP.

Aim:

To evaluate m-RNA expressions of SERCA2 and NCX1 in acute pancreatitis induced by sodium taurocholate in Wistar rats pre-treated with melatonin and/or TD.

Methods:

Wistar rats were divided in groups: 1) without AP; 2) AP without pre-treatment; 3) AP and Melatonin; 4) AP and TD; 5) AP and Melatonin associated to TD. Pancreatic tissue samples were collected for detection of SERCA2 and NCX1 m-R NA levels by polymerase chain reaction (PCR).

Results:

Increased m-RNA expression of SERCA2 in the melatonin treated group, without increase of m-RNA expression of the NCX1. The TD did not affect levels of SERCA2 and NCX1 m-RNA expressions. The combined melatonin and TD treatment reduced the m-RNA expression of SERCA2.

Conclusions:

The effect of melatonin is restricted to increased m-RNA expression of SERCA2. Although TD does not affect gene expression, its action in accelerating calcium exchanger function can explain the slightest expression of SERCA2 m-RNA when associated with Melatonin, perhaps by a joint action of drugs with different and but possibly complementary mechanisms.

Pancreatitis-Induced Depletion of Syntaxin 2 Promotes Autophagy and Increases Basolateral Exocytosis

BACKGROUND & AIMS

Pancreatic acinar cells are polarized epithelial cells that store enzymes required for digestion as inactive zymogens, tightly packed at the cell apex. Stimulation of acinar cells causes the zymogen granules to fuse with the apical membrane, and the cells undergo exocytosis to release proteases into the intestinal lumen. Autophagy maintains homeostasis of pancreatic acini. Syntaxin 2 (STX2), an abundant soluble N-ethyl maleimide sensitive factor attachment protein receptor in pancreatic acini, has been reported to mediate apical exocytosis. Using human pancreatic tissues and STX2-knockout (KO) mice, we investigated the functions of STX2 in zymogen granule-mediated exocytosis and autophagy.

METHODS

We obtained pancreatic tissues from 5 patients undergoing surgery for pancreatic cancer and prepared 80-μm slices; tissues were exposed to supramaximal cholecystokinin octapeptide (CCK-8) or ethanol and a low concentration of CCK-8 and analyzed by immunoblot and immunofluorescence analyses. STX2-KO mice and syntaxin 2+/+ C57BL6 mice (controls) were given intraperitoneal injections of supramaximal caerulein (a CCK-8 analogue) or fed ethanol and then given a low dose of caerulein to induce acute pancreatitis, or saline (controls); pancreata were isolated and analyzed by histology and immunohistochemistry. Acini were isolated from mice, incubated with CCK-8, and analyzed by immunofluorescence microscopy or used in immunoprecipitation experiments. Exocytosis was quantified using live-cell exocytosis and Ca2+ imaging analyses and based on formation of exocytotic soluble N-ethyl maleimide sensitive factor attachment protein receptor complexes. Dysregulations in autophagy were identified using markers, electron and immunofluorescence microscopy, and protease activation assays.

RESULTS

Human pancreatic tissues and dispersed pancreatic acini from control mice exposed to CCK-8 or ethanol plus CCK-8 were depleted of STX2. STX2-KO developed more severe pancreatitis after administration of supramaximal caerulein or a 6-week ethanol diet compared with control. Acini from STX2-KO mice had increased apical exocytosis after exposure to CCK-8, as well as increased basolateral exocytosis, which led to ectopic release of proteases. These increases in apical and basolateral exocytosis required increased formation of fusogenic soluble N-ethyl maleimide sensitive factor attachment protein receptor complexes, mediated by STX3 and STX4. STX2 bound ATG16L1 and prevented it from binding clathrin. Deletion of STX2 from acini increased binding of AT16L1 to clathrin, increasing formation of pre-autophagosomes and inducing autophagy. Induction of autophagy promoted the CCK-8-induced increase in autolysosome formation and the activation of trypsinogen.

CONCLUSIONS

In studies of human pancreatic tissues and pancreata from STX2-KO and control mice, we found STX2 to block STX3- and STX4-mediated fusion of zymogen granules with the plasma membrane and exocytosis and prevent binding of ATG16L1 to clathrin, which contributes to induction of autophagy. Exposure of pancreatic tissues to CCK-8 or ethanol depletes acinar cells of STX2, increasing basolateral exocytosis and promoting autophagy induction, leading to activation of trypsinogen.

Depletion of the membrane-fusion regulator Munc18c attenuates caerulein hyperstimulation-induced pancreatitis

Epithelial pancreatic acinar cells perform crucial functions in food digestion, and acinar cell homeostasis required for secretion of digestive enzymes relies on SNARE-mediated exocytosis. The ubiquitously expressed Sec1/Munc18 protein mammalian uncoordinated-18c (Munc18c) regulates membrane fusion by activating syntaxin-4 (STX-4) to bind cognate SNARE proteins to form a SNARE complex that mediates exocytosis in many cell types. However, in the acinar cell, Munc18c's functions in exocytosis and homeostasis remain inconclusive. Here, we found that pancreatic acini from Munc18c-depleted mice (Munc18c^+/-) and human pancreas (lenti-Munc18c-shRNA-treated) exhibit normal apical exocytosis of zymogen granules (ZGs) in response to physiologic stimulation with the intestinal hormone cholecystokinin (CCK-8). However, when stimulated with supraphysiologic CCK-8 levels to mimic pancreatitis, Munc18c-depleted (Munc18c^+/-) mouse acini exhibited a reduction in pathological basolateral exocytosis of ZGs resulting from a decrease in fusogenic STX-4 SNARE complexes. This reduced basolateral exocytosis in part explained the less severe pancreatitis observed in Munc18c^+/- mice after hyperstimulation with the CCK-8 analog caerulein. Likely as a result of this secretory blockade, Munc18c-depleted acini unexpectedly activated a component of the endoplasmic reticulum (ER) stress response that contributed to autophagy induction, resulting in downstream accumulation of autophagic vacuoles and autolysosomes. We conclude that Munc18c's role in mediating ectopic basolateral membrane fusion of ZGs contributes to the initiation of CCK-induced pancreatic injury, and that blockade of this secretory process could increase autophagy induction.

Probes for monitoring regulated exocytosis

Regulated secretion is a fundamental cellular process that serves diverse functions in neurobiology, endocrinology, immunology, and numerous other aspects of animal physiology. In response to environmental or biological cues, cells release contents of secretory granules into an extracellular medium to communicate with or impact neighboring or distant cells through paracrine or endocrine signaling. To investigate mechanisms governing stimulus-secretion coupling, to better understand how cells maintain or regulate their secretory activity, and to characterize secretion defects in human diseases, probes for tracking various exocytotic events at the cellular or sub-cellular level have been developed over the years. This review summarizes different strategies and recent progress in developing optical probes for monitoring regulated secretion in mammalian cells.

Spatiotemporal detection and analysis of exocytosis reveal fusion "hotspots" organized by the cytoskeleton in endocrine cells

Total internal reflection fluorescence microscope has often been used to study the molecular mechanisms underlying vesicle exocytosis. However, the spatial occurrence of the fusion events within a single cell is not frequently explored due to the lack of sensitive and accurate computer-assisted programs to analyze large image data sets. Here, we have developed an image analysis platform for the nonbiased identification of different types of vesicle fusion events with high accuracy in different cell types. By performing spatiotemporal analysis of stimulus-evoked exocytosis in insulin-secreting INS-1 cells, we statistically prove that individual vesicle fusion events are clustered at hotspots. This spatial pattern disappears upon the disruption of either the actin or the microtubule network; this disruption also severely inhibits evoked exocytosis. By demonstrating that newcomer vesicles are delivered from the cell interior to the surface membrane for exocytosis, we highlight a previously unappreciated mechanism in which the cytoskeleton-dependent transportation of secretory vesicles organizes exocytosis hotspots in endocrine cells.

Sorting of a HaloTag protein that has only a signal peptide sequence into exocrine secretory granules without protein aggregation

The mechanism involved in the sorting and accumulation of secretory cargo proteins, such as amylase, into secretory granules of exocrine cells remains to be solved. To clarify that sorting mechanism, we expressed a reporter protein HaloTag fused with partial sequences of salivary amylase protein in primary cultured parotid acinar cells. We found that a HaloTag protein fused with only the signal peptide sequence (Met(1)-Ala(25)) of amylase, termed SS25H, colocalized well with endogenous amylase, which was confirmed by immunofluorescence microscopy. Percoll-density gradient centrifugation of secretory granule fractions shows that the distributions of amylase and SS25H were similar. These results suggest that SS25H is transported to secretory granules and is not discriminated from endogenous amylase by the machinery that functions to remove proteins other than granule cargo from immature granules. Another reporter protein, DsRed2, that has the same signal peptide sequence also colocalized with amylase, suggesting that the sorting to secretory granules is not dependent on a characteristic of the HaloTag protein. Whereas Blue Native PAGE demonstrates that endogenous amylase forms a high-molecular-weight complex, SS25H does not participate in the complex and does not form self-aggregates. Nevertheless, SS25H was released from cells by the addition of a β-adrenergic agonist, isoproterenol, which also induces amylase secretion. These results indicate that addition of the signal peptide sequence, which is necessary for the translocation in the endoplasmic reticulum, is sufficient for the transportation and storage of cargo proteins in secretory granules of exocrine cells.

Species- and Dose-Specific Pancreatic Responses and Progression in Single- and Repeat-Dose Studies with GI181771X

Compound-induced pancreatic injury is a serious liability in preclinical toxicity studies. However, its relevance to humans should be cautiously evaluated because of interspecies variations. To highlight such variations, we evaluated the species- and dose-specific pancreatic responses and progression caused by GI181771X, a novel cholecystokinin 1 receptor agonist investigated by GlaxoSmithKline for the treatment of obesity. Acute (up to 2,000 mg/kg GI181771X, as single dose) and repeat-dose studies in mice and/or rats (0.25–250 mg/kg/day for 7 days to 26 weeks) showed wide-ranging morphological changes in the pancreas that were dose and duration dependent, including necrotizing pancreatitis, acinar cell hypertrophy/atrophy, zymogen degranulation, focal acinar cell hyperplasia, and interstitial inflammation. In contrast to rodents, pancreatic changes were not observed in cynomolgus monkeys given GI181771X (1–500 mg/kg/day with higher systemic exposure than rats) for up to 52 weeks. Similarly, no GI181771X treatment-associated abnormalities in pancreatic structure were noted in a 24-week clinical trial with obese patients (body mass index >30 or >27 kg/m2) as assessed by abdominal ultrasound or by magnetic resonance imaging. Mechanisms for interspecies variations in the pancreatic response to CCK among rodents, monkeys, and humans and their relevance to human risk are discussed.

Exocytosis through the Lens

Exocytosis, the process in which material is transported from the cell interior to the extracellular space, proceeds through a complex mechanism. Defects in this process are linked to a number of serious illnesses including diabetes, cancer, and a range of neuropathologies. In neuroendocrine cells, exocytosis involves the fusion of secretory vesicles, carrying signaling molecules, with the plasma membrane through the coordinated interplay of proteins, lipids, and small molecules. This process is highly regulated and occurs in a complex three-dimensional environment within the cell precisely coupled to the stimulus. The study of exocytosis poses significant challenges, involving rapidly changing, nano-scale, protein-protein, and protein-lipid interactions, at specialized sites in the cell. Over the last decade our understanding of neuroendocrine exocytosis has been greatly enhanced by developments in fluorescence microscopy. Modern microscopy encompasses a toolbox of advanced techniques, pushing the limits of sensitivity and resolution, to probe different properties of exocytosis. In more recent years, the development of super-resolution microscopy techniques, side-stepping the limits of optical resolution imposed by the physical properties of light, have started to provide an unparalleled view of exocytosis. In this review we will discuss how advances in fluorescence microscopy are shedding light on the spatial and temporal organization of the exocytotic machinery.

Exocyst sec5 regulates exocytosis of newcomer insulin granules underlying biphasic insulin secretion

The exocyst complex subunit Sec5 is a downstream effector of RalA-GTPase which promotes RalA-exocyst interactions and exocyst assembly, serving to tether secretory granules to docking sites on the plasma membrane. We recently reported that RalA regulates biphasic insulin secretion in pancreatic islet β cells in part by tethering insulin secretory granules to Ca²⁺ channels to assist excitosome assembly. Here, we assessed β cell exocytosis by patch clamp membrane capacitance measurement and total internal reflection fluorescence microscopy to investigate the role of Sec5 in regulating insulin secretion. Sec5 is present in human and rodent islet β cells, localized to insulin granules. Sec5 protein depletion in rat INS-1 cells inhibited depolarization-induced release of primed insulin granules from both readily-releasable pool and mobilization from the reserve pool. This reduction in insulin exocytosis was attributed mainly to reduction in recruitment and exocytosis of newcomer insulin granules that undergo minimal docking time at the plasma membrane, but which encompassed a larger portion of biphasic glucose stimulated insulin secretion. Sec5 protein knockdown had little effect on predocked granules, unless vigorously stimulated by KCl depolarization. Taken together, newcomer insulin granules in β cells are more sensitive than predocked granules to Sec5 regulation.

Fluorescent probes for monitoring regulated secretion

Numerous secretory cells use the regulated secretory pathway to release signaling molecules. Regulated secretion is an essential component of the intercellular communication network of a multicellular organism and serves diverse functions in neurobiology, endocrinology, and many other aspects of animal physiology. Probes that can monitor a specific exocytotic event with high temporal and spatial resolution would be invaluable tools for studying the molecular and cellular mechanisms underlying stimulus-secretion coupling, and for characterizing secretion defects that are found in different human diseases. This review summarizes different strategies and recent progress in developing fluorescent sensors for imaging regulated cell secretion.

Syntaxin-3 regulates newcomer insulin granule exocytosis and compound fusion in pancreatic beta cells

AIMS/HYPOTHESIS

The molecular basis of the exocytosis of secretory insulin-containing granules (SGs) during biphasic glucose-stimulated insulin secretion (GSIS) from pancreatic beta cells remains unclear. Syntaxin (SYN)-1A and SYN-4 have been shown to mediate insulin exocytosis. The insulin-secretory function of SYN-3, which is particularly abundant in SGs, is unclear.

METHODS

Mouse pancreatic islets and INS-1 cells were treated with adenovirus carrying Syn-3 (also known as Stx3) or small interfering RNA targeting Syn-3 in order to examine insulin secretion by radioimmunoassay. The localisation and distribution of insulin granules were examined by confocal and electron microscopy. Dynamic single-granule fusion events were assessed using total internal reflection fluorescence microscopy (TIRFM).

RESULTS

Depletion of endogenous SYN-3 inhibited insulin release. TIRFM showed no change in the number or fusion competence of previously docked SGs but, instead, a marked reduction in the recruitment of newcomer SGs and their subsequent exocytotic fusion during biphasic GSIS. Conversely, overexpression of Syn-3 enhanced both phases of GSIS, owing to the increase in newcomer SGs and, remarkably, to increased SG-SG fusion, which was confirmed by electron microscopy.

CONCLUSIONS/INTERPRETATION

In insulin secretion, SYN-3 plays a role in the mediation of newcomer SG exocytosis and SG-SG fusion that contributes to biphasic GSIS.

Network for Pancreatic Organ Donors with Diabetes (nPOD): developing a tissue biobank for type 1 diabetes

BACKGROUND

The Network for Pancreatic Organ Donors with Diabetes (nPOD) was established to recover and characterize pancreata and related organs from cadaveric organ donors with various risk levels for type 1 diabetes (T1D). These biospecimens are available to investigators for collaborative studies aimed at addressing questions related to T1D natural history and pathogenesis.

RESEARCH DESIGN AND METHODS

Organ donors included T1D patients (new onset to long term), non-diabetic autoantibody-positive subjects, non-diabetic controls and individuals with disorders relevant to β-cell function. Pancreas recovery and transport met transplant-grade criteria. Additional samples recovered included serum, whole blood, spleen and pancreatic and non-pancreatic lymph nodes. Biospecimens were processed for cryopreserved cells, fixed paraffin and fresh frozen blocks and snap frozen samples. T1D autoantibodies, C-peptide levels and high-resolution HLA genotyping for risk alleles were also determined.

RESULTS

Over 160 donors have been enrolled (ages of 1 day to >90 years). Standard operating procedures were established along with a quality management system. Donor demographics, laboratory assays and histopathological characterizations were shared through an open online informatics system. Biospecimens were distributed to more than 60 investigators.

CONCLUSIONS

The nPOD programme provides access to high quality biospecimens without cost to investigators. Collaborations and open data sharing are emphasized to maximize research potential of each donor. On the basis of initial successes, the nPOD programme is expanding to recover additional organs relevant to T1D pathogenesis and complications from European countries (PanFin network).

Effects of ethanol metabolites on exocytosis of pancreatic acinar cells in rats

BACKGROUND & AIMS

During development of alcoholic pancreatitis, oxidative (acetaldehyde) and nonoxidative metabolites (ethyl palmitate, ethyl oleate), rather than ethanol itself, mediate toxic injury. Exposure of pancreatic acini to ethanol blocks cholecystokinin (CCK)-8-stimulated apical exocytosis and redirects exocytosis to the basolateral plasma membrane, causing interstitial pancreatitis. We examined how each ethanol metabolite contributes to these changes in exocytosis.

METHODS

Rat pancreatic acini were incubated with concentrations of ethanol associated with alcoholic pancreatitis (20-50 mmol/L) or ethanol metabolites (1-3 mmol/L) and then stimulated with CCK-8. We performed single zymogen granule (ZG) exocytosis assays, Ca(2+) imaging studies, ultrastructural analyses (with electron microscopy), and confocal microscopy to assess the actin cytoskeleton and track the movement of vesicle-associated membrane protein (VAMP)-8-containing ZGs. Coimmunoprecipitation assays were used to identify complexes that contain the distinct combinations of Munc18 and the soluble N-ethylmaleimide sensitive factor attachment protein receptor proteins, which mediate apical (ZG-apical plasma membrane) and basolateral exocytosis and fusion between ZGs (ZG-ZG).

RESULTS

The ethanol metabolites acetaldehyde, ethyl palmitate, and ethyl oleate reduced CCK-8-stimulated apical exocytosis and formation of apical exocytotic complexes (between Munc18b and Syntaxin-2, synaptosomal-associated protein of 23 kilodaltons [SNAP23], and VAMP2) in rat pancreatic acini. Acetaldehyde and ethyl oleate redirected CCK-8-stimulated exocytosis to the basal and lateral plasma membranes and translocation of VAMP8-containing ZGs toward the basolateral plasma membrane. This process was mediated primarily via formation of basolateral exocytotic complexes (between Munc18c and Syntaxin-4, SNAP23, and VAMP8). Exposure of the acini to acetaldehyde and ethyl oleate followed by CCK-8 stimulation mildly perturbed the actin cytoskeleton and Ca(2+) signaling; exposure to ethyl palmitate severely affected Ca(2+) signaling. Acetaldehyde, like ethanol, promoted fusion between ZGs by the formation of ZG-ZG exocytotic complexes (between Munc18b and Syntaxin-3, SNAP23, and VAMP8), whereas ethyl palmitate and ethyl oleate reduced ZG-ZG fusion and formation of these complexes.

CONCLUSIONS

The ethanol metabolites acetaldehyde, ethyl palmitate, and ethyl oleate perturb exocytosis processes in cultured rat pancreatic acini (apical blockade, basolateral exocytosis, and fusion between ZGs). Acetaldehyde and, to a lesser degree, ethyl oleate produce many of the same pathologic effects of ethanol on CCK-8-stimulated exocytosis in pancreatic acini.

pHlash: a new genetically encoded and ratiometric luminescence sensor of intracellular pH

We report the development of a genetically encodable and ratiometic pH probe named “pHlash” that utilizes Bioluminescence Resonance Energy Transfer (BRET) rather than fluorescence excitation. The pHlash sensor–composed of a donor luciferase that is genetically fused to a Venus fluorophore–exhibits pH dependence of its spectral emission in vitro. When expressed in either yeast or mammalian cells, pHlash reports basal pH and cytosolic acidification in vivo. Its spectral ratio response is H⁺ specific; neither Ca⁺⁺, Mg⁺⁺, Na⁺, nor K⁺ changes the spectral form of its luminescence emission. Moreover, it can be used to image pH in single cells. This is the first BRET-based sensor of H⁺ ions, and it should allow the approximation of pH in cytosolic and organellar compartments in applications where current pH probes are inadequate.

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.

Regulated exocytosis: novel insights from intravital microscopy

Regulated exocytosis is a fundamental process that every secretory cell uses to deliver molecules to the cell surface and the extracellular space by virtue of membranous carriers. This process has been extensively studied using various approaches such as biochemistry, electrophysiology and electron microscopy. However, recent developments in time-lapse light microscopy have made possible imaging individual exocytic events, hence, advancing our understanding of this process at a molecular level. In this review, we focus on intravital microscopy (IVM), a light microscopy-based approach that enables imaging subcellular structures in live animals, and discuss its recent application to study regulated exocytosis. IVM has revealed differences in regulation and modality of regulated exocytosis between in vitro and in vivo model systems, unraveled novel aspects of this process that can be appreciated only in in vivo settings and provided valuable and novel information on its molecular machinery. In conclusion, we make the case for IVM being a mature technique that can be used to investigate the molecular machinery of several intracellular events under physiological conditions.

Calcium and reactive oxygen species in acute pancreatitis: friend or foe?

SIGNIFICANCE

Acute pancreatitis (AP) is a debilitating and, at times, lethal inflammatory disease, the causes and progression of which are incompletely understood. Disruption of Ca(2+) homeostasis in response to precipitants of AP leads to loss of mitochondrial integrity and cellular necrosis.

RECENT ADVANCES

While oxidative stress has been implicated as a major player in the pathogenesis of this disease, its precise roles remain to be defined. Recent developments are challenging the perception of reactive oxygen species (ROS) as nonspecific cytotoxic agents, suggesting that ROS promote apoptosis that may play a vital protective role in cellular stress since necrosis is avoided.

CRITICAL ISSUES

Fresh clinical findings have indicated that antioxidant treatment does not ameliorate AP and may actually worsen the outcome. This review explores the complex links between cellular Ca(2+) signaling and the intracellular redox environment, with particular relevance to AP.

FUTURE DIRECTIONS

Recent publications have underlined the importance of both Ca(2+) and ROS within the pathogenesis of AP, particularly in the determination of cell fate. Future research should elucidate the subtle interplay between Ca(2+) and redox mechanisms that operate to modulate mitochondrial function, with a view to devising strategies for the preservation of organellar function.