Exocytosis, dependent on Ca2+ release from Ca2+ stores, is regulated by Ca2+ microdomains

Vitamin C Modes of Action in Calcium-Involved Signaling in the Brain

Vitamin C (ascorbic acid) is well known for its potent antioxidant properties, as it can neutralize ROS and free radicals, thereby protecting cellular elements from oxidative stress. It predominantly exists as an ascorbate anion and after oxidation to dehydroascorbic acid and further breakdown, is removed from the cells. In nervous tissue, a progressive decrease in vitamin C level or its prolonged deficiency have been associated with an increased risk of disturbances in neurotransmission, leading to dysregulation in brain function. Therefore, understanding the regulatory function of vitamin C in antioxidant defence and identification of its molecular targets deserves more attention. One of the key signalling ions is calcium and a transient rise in its concentration is crucial for all neuronal processes. Extracellular Ca²⁺ influx (through specific ion channels) or Ca²⁺ release from intracellular stores (endoplasmic reticulum, mitochondria) are precisely controlled. Ca²⁺ regulates the functioning of the CNS, including growth, development, myelin formation, synthesis of catecholamines, modulation of neurotransmission and antioxidant protection. A growing body of evidence indicates a unique role for vitamin C in these processes. In this short review, we focus on vitamin C in the regulation of calcium-involved pathways under physiological and stress conditions in the brain.

Mitochondria-Endoplasmic Reticulum Interplay Regulates Exo-Cytosis in Human Neuroblastoma Cells

Mitochondria–endoplasmic reticulum (ER) contact sites (MERCS) have been emerging as a multifaceted subcellular region of the cell which affects several physiological and pathological mechanisms. A thus far underexplored aspect of MERCS is their contribution to exocytosis. Here, we set out to understand the role of these contacts in exocytosis and find potential mechanisms linking these structures to vesicle release in human neuroblastoma SH-SY5Y cells. We show that increased mitochondria to ER juxtaposition through Mitofusin 2 (Mfn2) knock-down resulted in a substantial upregulation of the number of MERCS, confirming the role of Mfn2 as a negative regulator of these structures. Furthermore, we report that both vesicle numbers and vesicle protein levels were decreased, while a considerable upregulation in exocytotic events upon cellular depolarization was detected. Interestingly, in Mfn2 knock-down cells, the inhibition of the inositol 1,4,5-trisphosphate receptor (IP3R) and the mitochondrial calcium (Ca²⁺) uniporter (MCU) restored vesicle protein content and attenuated exocytosis. We thus suggest that MERCS could be targeted to prevent increased exocytosis in conditions in which ER to mitochondria proximity is upregulated.

Case Report: Hypoglycemia Due to a Novel Activating Glucokinase Variant in an Adult - a Molecular Approach

We present a case of an obese 22-year-old man with activating GCK variant who had neonatal hypoglycemia, re-emerging with hypoglycemia later in life. We investigated him for asymptomatic hypoglycemia with a family history of hypoglycemia. Genetic testing yielded a novel GCK missense class 3 variant that was subsequently found in his mother, sister and nephew and reclassified as a class 4 likely pathogenic variant. Glucokinase enables phosphorylation of glucose, the rate-limiting step of glycolysis in the liver and pancreatic β cells. It plays a crucial role in the regulation of insulin secretion. Inactivating variants in GCK cause hyperglycemia and activating variants cause hypoglycemia. Spleen-preserving distal pancreatectomy revealed diffuse hyperplastic islets, nuclear pleomorphism and periductular islets. Glucose stimulated insulin secretion revealed increased insulin secretion in response to glucose. Cytoplasmic calcium, which triggers exocytosis of insulin-containing granules, revealed normal basal but increased glucose-stimulated level. Unbiased gene expression analysis using 10X single cell sequencing revealed upregulated INS and CKB genes and downregulated DLK1 and NPY genes in β-cells. Further studies are required to see if alteration in expression of these genes plays a role in the metabolic and histological phenotype associated with glucokinase pathogenic variant. There were more large islets in the patient's pancreas than in control subjects but there was no difference in the proportion of β cells in the islets. His hypoglycemia was persistent after pancreatectomy, was refractory to diazoxide and improved with pasireotide. This case highlights the variable phenotype of GCK mutations. In-depth molecular analyses in the islets have revealed possible mechanisms for hyperplastic islets and insulin hypersecretion.

PepFect14 Signaling and Transfection

PepFect14 is a cell-penetrating peptide (CPP) derived from stearylated transportan-10 (strearil-TP10) with which it shares the stearic acid residue on C' terminus and the amino acid sequence except for lysines that in PepFect14 are substituted with ornithines. Being non-proteinogenic amino acids, ornithines make PepFect14 less sensitive to serum proteases and due to its positive charges the CPP can form complexes with negatively charged cargos, such as splice correcting oligonucleotides (SCOs), plasmid DNA (pDNA), and proteins. It has been reported that PepFect14/SCO complexes enter the cells mainly through endocytosis, in particular: macopinocitosys and caveolae-mediated endocytosis through the interaction with two receptors of the scavenger receptors class A family (SCARAs). PepFect14 and its complexes trigger the chaperone-mediated autophagy response involving the heat shock protein family (HSP70) whose inhibition leads to an increase of PepFect14 transfection efficacy. Exploiting the interaction between HSP70 and PepFect14 and their ability to form nanoparticle. HSP70 has been delivered in Bomirsky Hamster Melanoma cells (BHM) using PepFect14 of which a protocol is described at the end of this chapter.

Targeted Drug Delivery for Cardiovascular Disease: Modeling of Modulated Extracellular Vesicle Release Rates

Invasive and medical therapy has led to major improvements in cardiovascular disease management, but important challenges remain open. The discovery of a nano-sized system of extracellular vesicles (EVs) is opening new possibilities for reprogramming malfunctioning cells and indicates that EVs can be employed in therapeutic biomedical applications as engineered drug vehicles. Molecular communication (MC) has applications for treating cells with directed drug delivery, employing special targeting transmembrane proteins. In this paper, we propose a novel drug delivery system for cardiovascular diseases using an EV-mediated MC platform and exemplify the potential use in hypertrophic cardiomyopathy. We utilize intracellular calcium signaling as a natural mediator of EVs released from synthetic cells and model the release rate. We propose to use the cells as a therapeutic release system with a control signal input which modulates the EVs release rate as the output signal. We also study the frequency domain of the proposed model and estimate the transfer function of the therapeutic release system model numerically where the root-mean-square error for two separate estimated output signals are 0.0353 and 0.0124. The proposed EV-mediated targeted drug delivery system can make breakthroughs in future healthcare, in cardiovascular and other diseases where targeting is required.

Induction of Ca2+-Dependent Exocytotic Processes by Laser Ablation of Endothelial Cells

Ca²⁺ regulates a variety of cellular processes that are essential to maintain cell integrity and function. Different methods have been used to study these processes by increasing intracellular Ca²⁺ levels. Here, we describe a protocol to initiate Ca²⁺-dependent membrane-related events, using laser ablation by near-infrared irradiation. This creates a rupture in the plasma membrane that allows the extracellular Ca²⁺ to enter the cell and thereby induce a receptor-independent Ca²⁺ increase. We report laser ablation protocols to study two different Ca²⁺-induced processes in human endothelial cells-membrane resealing and exocytosis of secretory granules called Weibel-Palade bodies (WPBs). Thus, laser ablation represents a technique that permits the analysis of different Ca²⁺-regulated processes at high spatiotemporal resolution in a controlled manner.

Bicarbonate permeation through anion channels: its role in health and disease

Many anion channels, frequently referred as Cl^- channels, are permeable to different anions in addition to Cl^-. As the second-most abundant anion in the human body, HCO₃^- permeation via anion channels has many important physiological roles. In addition to its classical role as an intracellular pH regulator, HCO₃^- also controls the activity and stability of dissolved proteins in bodily fluids such as saliva, pancreatic juice, intestinal fluid, and airway surface liquid. Moreover, HCO₃^- permeation through these channels affects membrane potentials that are the driving forces for transmembrane transport of solutes and water in epithelia and affect neuronal excitability in nervous tissue. Consequently, aberrant HCO₃^- transport via anion channels causes a number of human diseases in respiratory, gastrointestinal, genitourinary, and neuronal systems. Notably, recent studies have shown that the HCO₃^- permeabilities of several anion channels are not fixed and can be altered by cellular stimuli, findings which may have both physiological and pathophysiological significance. In this review, we summarize recent progress in understanding the molecular mechanisms and the physiological roles of HCO₃^- permeation through anion channels. We hope that the present discussions can stimulate further research into this very important topic, which will provide the basis for human disorders associated with aberrant HCO₃^- transport.

Modeling of Modulated Exosome Release From Differentiated Induced Neural Stem Cells for Targeted Drug Delivery

A novel implantable and externally controllable stem-cell-based platform for the treatment of Glioblastoma brain cancer has been proposed to bring hope to patients who suffer from this devastating cancer type. Induced Neural Stem Cells (iNSCs), known to have potent therapeutic effects through exosomes-based molecular communication, play a pivotal role in this platform. Transplanted iNSCs demonstrate long-term survival and differentiation into neurons and glia which then fully functionally integrate with the existing neural network. Recent studies have shown that specific types of calcium channels in differentiated neurons and astrocytes are inhibited or activated upon cell depolarization leading to the increased intracellular calcium concentration levels which, in turn, interact with mobilization of multivesicular bodies and exosomal release. In order to provide a platform towards treating brain cancer with the optimum therapy dosage, we propose mathematical models to compute the therapeutic exosomal release rate that is modulated by cell stimulation patterns applied from the external wearable device. This study serves as an initial and required step in the evaluation of controlled exosomal secretion and release via induced stimulation with electromagnetic, optical and/or ultrasonic waves.

The TRPV4 Agonist GSK1016790A Regulates the Membrane Expression of TRPV4 Channels

TRPV4 is a non-selective cation channel that tunes the function of different tissues including the vascular endothelium, lung, chondrocytes, and neurons. GSK1016790A is the selective and potent agonist of TRPV4 and a pharmacological tool that is used to study the TRPV4 physiological function in vitro and in vivo. It remains unknown how the sensitivity of TRPV4 to this agonist is regulated. The spatial and temporal dynamics of receptors are the major determinants of cellular responses to stimuli. Membrane translocation has been shown to control the response of several members of the transient receptor potential (TRP) family of ion channels to different stimuli. Here, we show that TRPV4 stimulation with GSK1016790A caused an increase in [Ca²⁺]_i that is stable for a few minutes. Single molecule analysis of TRPV4 channels showed that the density of TRPV4 at the plasma membrane is controlled through two modes of membrane trafficking, complete, and partial vesicular fusion. Further, we show that the density of TRPV4 at the plasma membrane decreased within 20 min, as they translocate to the recycling endosomes and that the surface density is dependent on the release of calcium from the intracellular stores and is controlled via a PI3K, PKC, and RhoA signaling pathway.

Convenient Monitoring System of Intracellular microRNA Expression during Adipogenesis via Mechanical Stimulus-Induced Exocytosis of Lipovesicular miRNA Beacon

Noninvasive investigation of microRNAs (miRNAs) expression, which is deeply related to biological phenomena such as stem cell differentiation, in culture soup is particularly useful for monitoring of stem cell differentiation without phototoxicity of living cells, especially when cell morphologies remain unchanged during differentiation. However, real-time detection of miRNA in culture soup is not recommended because of insufficient miRNA amounts in culture soup. In this study, a convenient method is introduced for real-time assessing intracellular miRNA in culture soup by using lipovesicular miRNA beacon (Lipo-mB) and mechanical stimulus-mediated exocytosis. Pipetting-harvest of culture soup induces exocytosis-secretion of fluorescence signal of Lipo-mB from cytoplasm into culture soup. To demonstrate this method, Lipo-mB is applied for monitoring of adipogenesis by analyzing the expression levels of various intracellular miRNAs, which are related to adipogenesis regulators. The fluorescence intensity profile of the culture soup is correlated with the quantitative reverse-transcription-polymerase chain reaction data and absorbance of Oil Red O staining. These results demonstrate that Lipo-mB can successfully monitor stem cell differentiation by sensing changes in miRNA expression from culture soup of living cells. Lipo-mB can be further developed as an accurate sensing system for analyzing subtle differences in genotype, even when changes in phenotype cannot be observed.

Ex vivo human pancreatic slice preparations offer a valuable model for studying pancreatic exocrine biology

A genuine understanding of human exocrine pancreas biology and pathobiology has been hampered by a lack of suitable preparations and reliance on rodent models employing dispersed acini preparations. We have developed an organotypic slice preparation of the normal portions of human pancreas obtained from cancer resections. The preparation was assessed for physiologic and pathologic responses to the cholinergic agonist carbachol (Cch) and cholecystokinin (CCK-8), including 1) amylase secretion, 2) exocytosis, 3) intracellular Ca²⁺ responses, 4) cytoplasmic autophagic vacuole formation, and 5) protease activation. Cch and CCK-8 both dose-dependently stimulated secretory responses from human pancreas slices similar to those previously observed in dispersed rodent acini. Confocal microscopy imaging showed that these responses were accounted for by efficient apical exocytosis at physiologic doses of both agonists and by apical blockade and redirection of exocytosis to the basolateral plasma membrane at supramaximal doses. The secretory responses and exocytotic events evoked by CCK-8 were mediated by CCK-A and not CCK-B receptors. Physiologic agonist doses evoked oscillatory Ca²⁺ increases across the acini. Supraphysiologic doses induced formation of cytoplasmic autophagic vacuoles and activation of proteases (trypsin, chymotrypsin). Maximal atropine pretreatment that completely blocked all the Cch-evoked responses did not affect any of the CCK-8-evoked responses, indicating that rather than acting on the nerves within the pancreas slice, CCK cellular actions directly affected human acinar cells. Human pancreas slices represent excellent preparations to examine pancreatic cell biology and pathobiology and could help screen for potential treatments for human pancreatitis.

Role of phosphatidylinositol-4,5-bisphosphate 3-kinase signaling in vesicular trafficking

Phosphatidylinositol-4,5-bisphosphate 3-kinases (PI3Ks) are regulatory enzymes involved in the generation of lipid species that modulate cellular signaling pathways through downstream effectors to influence a variety of cellular functions. Years of intensive study of PI3Ks have produced a significant body of literature in many areas, including that PI3K can mediate intracellular vesicular trafficking and through these actions contribute to a number of important physiological functions. This review focuses on the crucial roles that PI3K and AKT, a major downstream partner of PI3K, play in the regulation of vesicle trafficking during various forms of vesicular endocytosis and exocytosis.

HCO3- Transport through Anoctamin/Transmembrane Protein ANO1/TMEM16A in Pancreatic Acinar Cells Regulates Luminal pH

The identification of ANO1/TMEM16A as the likely calcium-dependent chloride channel of exocrine glands has led to a more detailed understanding of its biophysical properties. This includes a calcium-dependent change in channel selectivity and evidence that HCO3 (-) permeability can be significant. Here we use freshly isolated pancreatic acini that preserve the luminal structure to measure intraluminal pH and test the idea that ANO1/TMEM16A contributes to luminal pH balance. Our data show that, under physiologically relevant stimulation with 10 pm cholesystokinin, the luminal acid load that results from the exocytic fusion of zymogen granules is significantly blunted by HCO3 (-) buffer in comparison with HEPES, and that this is blocked by the specific TMEM16A inhibitor T16inh-A01. Furthermore, in a model of acute pancreatitis, we observed substantive luminal acidification and provide evidence that ANO1/TMEM16A acts to attenuate this pH shift. We conclude that ANO1/TMEM16A is a significant pathway in pancreatic acinar cells for HCO3 (-) secretion into the lumen.

Exocytosis in non-neuronal cells

Exocytosis is the process by which stored neurotransmitters and hormones are released via the fusion of secretory vesicles with the plasma membrane. It is a dynamic, rapid and spatially restricted process involving multiple steps including vesicle trafficking, tethering, docking, priming and fusion. For many years great steps have been undertaken in our understanding of how exocytosis occurs in different cell types, with significant focus being placed on synaptic release and neurotransmission. However, this process of exocytosis is an essential component of cell signalling throughout the body and underpins a diverse array of essential physiological pathways. Many similarities exist between different cell types with regard to key aspects of the exocytosis pathway, such as the need for Ca(2+) to trigger it or the involvement of members of the N-ethyl maleimide-sensitive fusion protein attachment protein receptor protein families. However, it is also equally clear that non-neuronal cells have acquired highly specialized mechanisms to control the release of their own unique chemical messengers. This review will focus on several important non-neuronal cell types and discuss what we know about the mechanisms they use to control exocytosis and how their specialized output is relevant to the physiological role of each individual cell type. These include enteroendocrine cells, pancreatic β cells, astrocytes, lactotrophs and cytotoxic T lymphocytes. Non-neuronal cells have acquired highly specialized mechanisms to control the release of unique chemical messengers, such as polarised fusion of insulin granules in pancreatic β cells targeted towards the vasculature (top). This review discusses mechanisms used in several important non-neuronal cell types to control exocytosis, and the relevance of intermediate vesicle fusion pore states (bottom) and their specialized output to the physiological role of each cell type. These include enteroendocrine cells, pancreatic β cells, astrocytes, lactotrophs and cytotoxic T lymphocytes. This article is part of a mini review series on Chromaffin cells (ISCCB Meeting, 2015).

Microtubule-Associated Protein EB3 Regulates IP3 Receptor Clustering and Ca(2+) Signaling in Endothelial Cells

The mechanisms by which the microtubule cytoskeleton regulates the permeability of endothelial barrier are not well understood. Here, we demonstrate that microtubule-associated end-binding protein 3 (EB3), a core component of the microtubule plus-end protein complex, binds to inositol 1,4,5-trisphosphate receptors (IP3Rs) through an S/TxIP EB-binding motif. In endothelial cells, α-thrombin, a pro-inflammatory mediator that stimulates phospholipase Cβ, increases the cytosolic Ca(2+) concentration and elicits clustering of IP3R3s. These responses, and the resulting Ca(2+)-dependent phosphorylation of myosin light chain, are prevented by depletion of either EB3 or mutation of the TxIP motif of IP3R3 responsible for mediating its binding to EB3. We also show that selective EB3 gene deletion in endothelial cells of mice abrogates α-thrombin-induced increase in endothelial permeability. We conclude that the EB3-mediated interaction of IP3Rs with microtubules controls the assembly of IP3Rs into effective Ca(2+) signaling clusters, which thereby regulate microtubule-dependent endothelial permeability.

Modelling the transition from simple to complex Ca²⁺ oscillations in pancreatic acinar cells

A mathematical model is proposed which systematically investigates complex calcium oscillations in pancreatic acinar cells. This model is based on calcium-induced calcium release via inositol trisphosphate receptors (IPR) and ryanodine receptors (RyR) and includes calcium modulation of inositol (1,4,5) trisphosphate (IP3) levels through feedback regulation of degradation and production. In our model, the apical and the basal regions are separated by a region containing mitochondria, which is capable of restricting Ca2+ responses to the apical region. We were able to reproduce the observed oscillatory patterns, from baseline spikes to sinusoidal oscillations. The model predicts that calcium-dependent production and degradation of IP3 is a key mechanism for complex calcium oscillations in pancreatic acinar cells. A partial bifurcation analysis is performed which explores the dynamic behaviour of the model in both apical and basal regions.

Role of calcium signaling in epithelial bicarbonate secretion

Transepithelial bicarbonate secretion plays a key role in the maintenance of fluid and protein secretion from epithelial cells and the protection of the epithelial cell surface from various pathogens. Epithelial bicarbonate secretion is mainly under the control of cAMP and calcium signaling. While the physiological roles and molecular mechanisms of cAMP-induced bicarbonate secretion are relatively well defined, those induced by calcium signaling remain poorly understood in most epithelia. The present review summarizes the current status of knowledge on the role of calcium signaling in epithelial bicarbonate secretion. Specifically, this review introduces how cytosolic calcium signaling can increase bicarbonate secretion by regulating membrane transport proteins and how it synergizes with cAMP-induced mechanisms in epithelial cells. In addition, tissue-specific variations in the pancreas, salivary glands, intestines, bile ducts, and airways are discussed. We hope that the present report will stimulate further research into this important topic. These studies will provide the basis for future medicines for a wide spectrum of epithelial disorders including cystic fibrosis, Sjögren's syndrome, and chronic pancreatitis.

Dopamine Release Suppression Dependent on an Increase of Intracellular Ca(2+) Contributed to Rotenone-induced Neurotoxicity in PC12 Cells

Rotenone is an inhibitor of mitochondrial complex I that produces a model of Parkinson's disease (PD), in which neurons undergo dopamine release dysfunction and other features. In neurons, exocytosis is one of the processes associated with dopamine release and is dependent on Ca(2+) dynamic changes of the cell. In the present study, we have investigated the exocytosis of dopamine and the involvement of Ca(2+) in dopamine release in PC12 cells administrated with rotenone. Results demonstrated that rotenone led to an elevation of intracellular Ca(2+) through Ca(2+) influx by opening of the voltage-gated Ca(2+) channel and influenced the soluble N-ethylmaleimide attachment protein receptor (SNARE) proteins expression (including syntaxin, vesicle-associated membrane protein 2 (VAMP2) and synaptosome-associated protein 25 (SNAP-25)); pretreatment with a blocker of L-type voltage-activated Ca(2+) channels (nifedipine) decreased the intracellular dopamine levels and ROS formation, increased the cell viability and enhanced the neurite outgrowth and exocytosis of synaptic vesicles. These results indicated that the involvement of intracellular Ca(2+) was one of the factors resulting in suppression of dopamine release suppression in PC12 cells intoxicated with rotenone, which was associated with the rotenone-induced dopamine neurotoxicity.

Dynamic modulation of ANO1/TMEM16A HCO3(-) permeability by Ca2+/calmodulin

Anoctamin 1 (ANO1)/transmembrane protein 16A (TMEM16A) is a calcium-activated anion channel that may play a role in HCO(3)(-) secretion in epithelial cells. Here, we report that the anion selectivity of ANO1 is dynamically regulated by the Ca(2+)/calmodulin complex. Whole-cell current measurements in HEK 293T cells indicated that ANO1 becomes highly permeable to HCO(3)(-) at high [Ca(2+)](i). Interestingly, this result was not observed in excised patches, indicating the involvement of cytosolic factors in this process. Further studies revealed that the direct association between ANO1 and calmodulin at high [Ca(2+)](i) is responsible for changes in anion permeability. Calmodulin physically interacted with ANO1 in a [Ca(2+)](i)-dependent manner, and addition of recombinant calmodulin to the cytosolic side of excised patches reversibly increased P(HCO3)/P(Cl). In addition, the high [Ca(2+)](i)-induced increase in HCO(3)(-) permeability was reproduced in mouse submandibular gland acinar cells, in which ANO1 plays a critical role in fluid secretion. These results indicate that the HCO(3)(-) permeability of ANO1 can be dynamically modulated and that ANO1 may play an important role in cellular HCO(3)(-) transport, especially in transepithelial HCO(3)(-) secretion.

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.

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.

Vesicle-associated membrane protein 8 (VAMP8) is a SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) selectively required for sequential granule-to-granule fusion

Compound exocytosis is found in many cell types and is the major form of regulated secretion in acinar and mast cells. Its key characteristic is the homotypic fusion of secretory granules. These then secrete their combined output through a single fusion pore to the outside. The control of compound exocytosis remains poorly understood. Although soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) such as syntaxin 2, SNAP23 (synaptosome-associated protein of 23 kDa), and SNAP25 have been suggested to play a role, none has been proven. Vesicle-associated membrane protein 8 (VAMP8) is a SNARE first associated with endocytic processes but more recently has been suggested as an R-SNARE in regulated exocytosis. Secretion in acinar cells is reduced when VAMP8 function is inhibited and is less in VAMP8 knock-out mice. Based on electron microscopy experiments, it was suggested that VAMP8 may be involved in compound exocytosis. Here we have tested the hypothesis that VAMP8 controls homotypic granule-to-granule fusion during sequential compound exocytosis. We use a new assay to distinguish primary fusion events (fusion with the cell membrane) from secondary fusion events (granule-granule fusion). Our data show the pancreatic acinar cells from VAMP8 knock-out animals have a specific reduction in secondary granule fusion but that primary granule fusion is unaffected. Furthermore, immunoprecipitation experiments show syntaxin 2 association with VAMP2, whereas syntaxin 3 associates with VAMP8. Taken together our data indicate that granule-to-granule fusion is regulated by VAMP8 containing SNARE complexes distinct from those that regulate primary granule fusion.

Exploring lung physiology in health and disease with lung slices

The development of therapeutic approaches to treat lung disease requires an understanding of both the normal and disease physiology of the lung. Although traditional experimental approaches only address either organ or cellular physiology, the use of lung slice preparations provides a unique approach to investigate integrated physiology that links the cellular and organ responses. Living lung slices are robust and can be prepared from a variety of species, including humans, and they retain many aspects of the cellular and structural organization of the lung. Functional portions of intrapulmonary airways, arterioles and veins are present within the alveoli parenchyma. The dynamics of macroscopic changes of contraction and relaxation associated with the airways and vessels are readily observed with conventional low-magnification microscopy. The microscopic changes associated with cellular events, that determine the macroscopic responses, can be observed with confocal or two-photon microscopy. To investigate disease processes, lung slices can either be prepared from animal models of disease or animals exposed to disease invoking conditions. Alternatively, the lung slices themselves can be experimentally manipulated. Because of the ability to observe changes in cell physiology and how these responses manifest themselves at the level of the organ, lung slices have become a standard tool for the investigation of lung disease.

Pancreatic acinar cell: new insights into the control of secretion

Pancreatic acinar cells secrete fluid and digestive enzymes. Both types of secretion are activated by a rise in intracellular calcium but how the stimulus-secretion cascade actually regulates secretory output is not well understood. It has long been known that the calcium response of acinar cells to physiological stimulation is complex. Dependent on the type and concentration of agonist, it consists of either local or global calcium increases as well as spreading waves of calcium across the cell. In the past it has been speculated that these different calcium signals drive different secretory responses. Now, recent employment of two-photon microscopy has enabled the simultaneous recording of both enzyme secretion and calcium signals and is beginning to resolve this issue. The data shows that local calcium responses exclusively drive fluid secretion. Where-as, global calcium responses drive both fluid and enzyme secretion. This differential control of secretory output is likely central to controlling the physiological responses of pancreatic acinar cells.