Two cAMP-dependent pathways differentially regulate exocytosis of large dense-core and small vesicles in mouse beta-cells

Single-molecule analysis of intracellular insulin granule behavior and its application to analyzing cytoskeletal dependence and pathophysiological implications

Introduction: Mobilization of intracellular insulin granules to the plasma membrane plays a crucial role in regulating insulin secretion. However, the regulatory mechanisms of this mobilization process have been poorly understood due to technical limitations. In this study, we propose a convenient approach for assessing intracellular insulin granule behavior based on single-molecule analysis of insulin granule membrane proteins labeled with Quantum dot fluorescent nanocrystals. Methods: This approach allows us to analyze intracellular insulin granule movement with subpixel accuracy at 33 fps. We tracked two insulin granule membrane proteins, phogrin and zinc transporter 8, fused to HaloTag in rat insulinoma INS-1 cells and, by evaluating the tracks with mean-square displacement, demonstrated the characteristic behavior of insulin granules. Results and discussion: Pharmacological perturbations of microtubules and F-actin affected insulin granule behavior on distinct modalities. Specifically, microtubule dynamics and F-actin positively and negatively regulate insulin granule behavior, respectively, presumably by modulating each different behavioral mode. Furthermore, we observed impaired insulin granule behavior and cytoskeletal architecture under chronic treatment of high concentrations of glucose and palmitate. Our approach provides detailed information regarding intracellular insulin granule mobilization and its pathophysiological implications. This study sheds new light on the regulatory mechanisms of intracellular insulin granule mobilization and has important implications for understanding the pathogenesis of diabetes.

Utilizing the Off-Target Effects of T1R3 Antagonist Lactisole to Enhance Nitric Oxide Production in Basal Airway Epithelial Cells

Human airway sweet (T1R2 + T1R3), umami (T1R1 + T1R3), and bitter taste receptors (T2Rs) are critical components of the innate immune system, acting as sensors to monitor pathogenic growth. T2Rs detect bacterial products or bitter compounds to drive nitric oxide (NO) production in both healthy and diseased epithelial cell models. The NO enhances ciliary beating and also directly kills pathogens. Both sweet and umami receptors have been characterized to repress bitter taste receptor signaling in healthy and disease models. We hypothesized that the sweet/umami T1R3 antagonist lactisole may be used to alleviate bitter taste receptor repression in airway basal epithelial cells and enhance NO production. Here, we show that lactisole activates cAMP generation, though this occurs through a pathway independent of T1R3. This cAMP most likely signals through EPAC to increase ER Ca²⁺ efflux. Stimulation with denatonium benzoate, a bitter taste receptor agonist which activates largely nuclear and mitochondrial Ca²⁺ responses, resulted in a dramatically increased cytosolic Ca²⁺ response in cells treated with lactisole. This cytosolic Ca²⁺ signaling activated NO production in the presence of lactisole. Thus, lactisole may be useful coupled with bitter compounds as a therapeutic nasal rinse or spray to enhance beneficial antibacterial NO production in patients suffering from chronic inflammatory diseases such as chronic rhinosinusitis.

Savory Signaling: T1R Umami Receptor Modulates Endoplasmic Reticulum Calcium Store Content and Release Dynamics in Airway Epithelial Cells

T1Rs are expressed in solitary chemosensory cells of the upper airway where they detect apical glucose levels and repress bitter taste receptor Ca²⁺ signaling pathways. Microbial growth leads to a decrease in apical glucose levels. T1Rs detect this change and liberate bitter taste receptor signaling, initiating an innate immune response to both kill and expel pathogens through releasing antimicrobial peptides and increasing nitric oxide production and ciliary beat frequency. However, chronic inflammation due to disease, smoking, or viral infections causes a remodeling of the epithelial airway. The resulting squamous metaplasia causes a loss of multi-ciliated cells and solitary chemosensory cells, replaced by basal epithelial cells. To understand how T1R function is altered during disease, we used basal epithelial cells as a model to study the function of T1R3 on Ca²⁺ signaling dynamics. We found that both T1R1 and T1R3 detect amino acids and signal via cAMP, increasing the responsiveness of the cells to Ca²⁺ signaling stimuli. Either knocking down T1R1/3 or treating wild-type cells with MEM amino acids caused a reduction in ER Ca²⁺ content through a non-cAMP signaled pathway. Treatment with amino acids led to a reduction in downstream denatonium-induced Ca²⁺-signaled caspase activity. Thus, amino acids may be used to reduce unwanted apoptosis signaling in treatments containing bitter compounds.

Glycine Release Is Potentiated by cAMP via EPAC2 and Ca2+ Stores in a Retinal Interneuron

Neuromodulation via the intracellular second messenger cAMP is ubiquitous at presynaptic nerve terminals. This modulation of synaptic transmission allows exocytosis to adapt to stimulus levels and reliably encode information. The AII amacrine cell (AII-AC) is a central hub for signal processing in the mammalian retina. The main apical dendrite of the AII-AC is connected to several lobular appendages that release glycine onto OFF cone bipolar cells and ganglion cells. However, the influence of cAMP on glycine release is not well understood. Using membrane capacitance measurements from mouse AII-ACs to directly measure exocytosis, we observe that intracellular dialysis of 1 mm cAMP enhances exocytosis without affecting the L-type Ca²⁺ current. Responses to depolarizing pulses of various durations show that the size of the readily releasable pool of vesicles nearly doubles with cAMP, while paired-pulse depression experiments suggest that release probability does not change. Specific agonists and antagonists for exchange protein activated by cAMP 2 (EPAC2) revealed that the cAMP-induced enhancement of exocytosis requires EPAC2 activation. Furthermore, intact Ca²⁺ stores were also necessary for the cAMP potentiation of exocytosis. Postsynaptic recordings from OFF cone bipolar cells showed that increasing cAMP with forskolin potentiated the frequency of glycinergic spontaneous IPSCs. We propose that cAMP elevations in the AII-AC lead to a robust enhancement of glycine release through an EPAC2 and Ca²⁺ store signaling pathway. Our results thus contribute to a better understanding of how AII-AC crossover inhibitory circuits adapt to changes in ambient luminance.SIGNIFICANCE STATEMENT The mammalian retina operates over a wide dynamic range of light intensities and contrast levels. To optimize the signal-to-noise ratio of processed visual information, both excitatory and inhibitory synapses within the retina must modulate their gain in synaptic transmission to adapt to different levels of ambient light. Here we show that increases of cAMP concentration within AII amacrine cells produce enhanced exocytosis from these glycinergic interneurons. Therefore, we propose that light-sensitive neuromodulators may change the output of glycine release from AII amacrine cells. This novel mechanism may fine-tune the amount of tonic and phasic synaptic inhibition received by bipolar cell terminals and, consequently, the spiking patterns that ganglion cells send to the upstream visual areas of the brain.

Nano-analysis reveals high fraction of serotonin release during exocytosis from a gut epithelium model cell

Electrochemical methods were used to explore the exocytotic nature of serotonin (5‐HT) release in human carcinoid BON cells, an in vitro human enterochromaffin cell model, to understand the mechanisms operating the release of gut‐derived 5‐HT in the intestinal mucosal epithelium. We show that the fractional vesicular 5‐HT release in BON cells is 80 % compared to previous work in pancreatic beta cells (34 %). The fractional release increased from 80 % in control BON cells to 87 % with 5‐HT preincubation and nearly 100 % with the combination of 5‐HT and the 5‐HT₄ autoreceptor agonist, cisapride. Thus, partial release is the primary mechanism of exocytosis in BON cells, resulting in a variable amount of the vesicular content being released. Factors that control secretion of 5‐HT from enterochromaffin cells or BON cells are important as partial release provides a mechanism for development of effective therapeutic strategies to treat gastrointestinal diseases.

Natural mechanisms and artificial PEG-induced mechanism that repair traumatic damage to the plasmalemma in eukaryotes

Eukaryotic tissues are composed of individual cells surrounded by a plasmalemma that consists of a phospholipid bilayer with hydrophobic heads that bind cell water. Bound-water creates a thermodynamic barrier that impedes the fusion of a plasmalemma with other membrane-bound intracellular structures or with the plasmalemma of adjacent cells. Plasmalemmal damage consisting of small or large holes or complete transections of a cell or axon results in calcium influx at the lesion site. Calcium activates fusogenic pathways that have been phylogenetically conserved and that lower thermodynamic barriers for fusion of membrane-bound structures. Calcium influx also activates phylogenetically conserved sealing mechanisms that mobilize the gradual accumulation and fusion of vesicles/membrane-bound structures that seal the damaged membrane. These naturally occurring sealing mechanisms for different cells vary based on the type of lesion, the type of cell, the proximity of intracellular membranous structures to the lesion and the relation to adjacent cells. The reliability of different measures to assess plasmalemmal sealing need be carefully considered for each cell type. Polyethylene glycol (PEG) bypasses calcium and naturally occurring fusogenic pathways to artificially fuse adjacent cells (PEG-fusion) or artificially seal transected axons (PEG-sealing). PEG-fusion techniques can also be used to rapidly rejoin the closely apposed, open ends of severed axons. PEG-fused axons do not (Wallerian) degenerate and PEG-fused nerve allografts are not immune-rejected, and enable behavioral recoveries not observed for any other clinical treatment. A better understanding of natural and artificial mechanisms that induce membrane fusion should provide better clinical treatment for many disorders involving plasmalemmal damage.

Strong stimulation triggers full fusion exocytosis and very slow endocytosis of the small dense core granules in carotid glomus cells

Chemosensory glomus cells of the carotid bodies release transmitters, including ATP and dopamine mainly via the exocytosis of small dense core granules (SDCGs, vesicular diameter of ∼100 nm). Using carbon-fiber amperometry, we showed previously that with a modest uniform elevation in cytosolic Ca²⁺ concentration ([Ca²⁺]_i of ∼0.5 µM), SDCGs of rat glomus cells predominantly underwent a "kiss-and-run" mode of exocytosis. Here, we examined whether a larger [Ca²⁺]_i rise influenced the mode of exocytosis. Activation of voltage-gated Ca²⁺ channels by a train of voltage-clamped depolarizations which elevated [Ca²⁺]_i to ∼1.6 μM increased the cell membrane capacitance by ∼2.5%. At 30 s after such a stimulus, only 5% of the added membrane was retrieved. Flash photolysis of caged-Ca²⁺ (which elevated [Ca²⁺]_i to ∼16 μM) increased cell membrane capacitance by ∼13%, and only ∼30% of the added membrane was retrieved at 30 s after the UV flash. When exocytosis and endocytosis were monitored using the two-photon excitation and extracellular polar tracer (TEP) imaging of FM1-43 fluorescence in conjunction with photolysis of caged Ca²⁺, almost uniform exocytosis was detected over the cell's entire surface and it was followed by slow endocytosis. Immunocytochemistry showed that the cytoplasmic densities of dynamin I, II and clathrin (key proteins that mediate endocytosis) in glomus cells were less than half of those in adrenal chromaffin cells, suggesting that a lower expression of endocytotic machinery may underlie the slow endocytosis in glomus cells. An analysis of the relative change in the signals from two fluorescent dyes that simultaneously monitored the addition of vesicular volume and plasma membrane surface area, suggested that with an intense stimulus, SDCGs of glomus cells underwent full fusion without any significant "compound" exocytosis. Therefore, during a severe hypoxic challenge, glomus granules undergo full fusion for a more complete release of transmitters.

Repair of traumatic plasmalemmal damage to neurons and other eukaryotic cells

The repair (sealing) of plasmalemmal damage, consisting of small holes to complete transections, is critical for cell survival, especially for neurons that rarely regenerate cell bodies. We first describe and evaluate different measures of cell sealing. Some measures, including morphological/ultra-structural observations, membrane potential, and input resistance, provide very ambiguous assessments of plasmalemmal sealing. In contrast, measures of ionic current flow and dye barriers can, if appropriately used, provide more accurate assessments. We describe the effects of various substances (calcium, calpains, cytoskeletal proteins, ESCRT proteins, mUNC-13, NSF, PEG) and biochemical pathways (PKA, PKC, PLC, Epac, cytosolic oxidation) on plasmalemmal sealing probability, and suggest that substances, pathways, and cellular events associated with plasmalemmal sealing have undergone a very conservative evolution. During sealing, calcium ion influx mobilizes vesicles and other membranous structures (lysosomes, mitochondria, etc.) in a continuous fashion to form a vesicular plug that gradually restricts diffusion of increasingly smaller molecules and ions over a period of seconds to minutes. Furthermore, we find no direct evidence that sealing occurs through the collapse and fusion of severed plasmalemmal leaflets, or in a single step involving the fusion of one large wound vesicle with the nearby, undamaged plasmalemma. We describe how increases in perikaryal calcium levels following axonal transection account for observations that cell body survival decreases the closer an axon is transected to the perikaryon. Finally, we speculate on relationships between plasmalemmal sealing, Wallerian degeneration, and the ability of polyethylene glycol (PEG) to seal cell membranes and rejoin severed axonal ends – an important consideration for the future treatment of trauma to peripheral nerves. A better knowledge of biochemical pathways and cytoplasmic structures involved in plasmalemmal sealing might provide insights to develop treatments for traumatic nerve injuries, stroke, muscular dystrophy, and other pathologies.

Type II PKAs are anchored to mature insulin secretory granules in INS-1 β-cells and required for cAMP-dependent potentiation of exocytosis

Specificity of the cAMP-dependent protein kinase (PKA) pathway relies on an extremely sophisticated compartmentalization mechanism of the kinase within a given cell, based on high-affinity binding of PKA tetramer pools to different A-Kinase Anchoring Proteins (AKAPs). We and others have previously shown that AKAPs-dependent PKA subcellular targeting is a requisite for optimal cAMP-dependent potentiation of insulin exocytosis. We thus hypothesized that a PKA pool may directly anchor to the secretory compartment to potentiate insulin exocytosis. Here, using immunofluorescence analyses combined to subcellular fractionations and purification of insulin secretory granules (ISGs), we identified discrete subpools of type II PKAs, RIIα and RIIβ PKAs, along with the catalytic subunit, physically associated with ISGs within pancreatic insulin-secreting β-cells. Ultrastructural analysis of native rodent β-cells confirmed in vivo the occurrence of PKA on dense-core ISGs. Isoform-selective disruption of binding of PKAs to AKAPs reinforced the requirement of type II PKA isoforms for cAMP potentiation of insulin exocytosis. This granular localization of PKA was of critical importance since siRNA-mediated depletion of either RIIα or RIIβ PKAs resulted in a significant reduction of cAMP-dependent potentiation of insulin release. The present work provides evidence for a previously unrecognized pool of type II PKAs physically anchored to the β-cell ISGs compartment and supports a non-redundant function for type II PKAs during cAMP potentiation of exocytosis.

Rp-cAMPS Prodrugs Reveal the cAMP Dependence of First-Phase Glucose-Stimulated Insulin Secretion

cAMP-elevating agents such as the incretin hormone glucagon-like peptide-1 potentiate glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells. However, a debate has existed since the 1970s concerning whether or not cAMP signaling is essential for glucose alone to stimulate insulin secretion. Here, we report that the first-phase kinetic component of GSIS is cAMP-dependent, as revealed through the use of a novel highly membrane permeable para-acetoxybenzyl (pAB) ester prodrug that is a bioactivatable derivative of the cAMP antagonist adenosine-3',5'-cyclic monophosphorothioate, Rp-isomer (Rp-cAMPS). In dynamic perifusion assays of human or rat islets, a step-wise increase of glucose concentration leads to biphasic insulin secretion, and under these conditions, 8-bromoadenosine-3',5'-cyclic monophosphorothioate, Rp-isomer, 4-acetoxybenzyl ester (Rp-8-Br-cAMPS-pAB) inhibits first-phase GSIS by up to 80%. Surprisingly, second-phase GSIS is inhibited to a much smaller extent (≤20%). Using luciferase, fluorescence resonance energy transfer, and bioluminescence resonance energy transfer assays performed in living cells, we validate that Rp-8-Br-cAMPS-pAB does in fact block cAMP-dependent protein kinase activation. Novel effects of Rp-8-Br-cAMPS-pAB to block the activation of cAMP-regulated guanine nucleotide exchange factors (Epac1, Epac2) are also validated using genetically encoded Epac biosensors, and are independently confirmed in an in vitro Rap1 activation assay using Rp-cAMPS and Rp-8-Br-cAMPS. Thus, in addition to revealing the cAMP dependence of first-phase GSIS from human and rat islets, these findings establish a pAB-based chemistry for the synthesis of highly membrane permeable prodrug derivatives of Rp-cAMPS that act with micromolar or even nanomolar potency to inhibit cAMP signaling in living cells.

Acute effect of 3β-hidroxihop-22(29)ene on insulin secretion is mediated by GLP-1, potassium and calcium channels for the glucose homeostasis

The effect of 3β-hidroxihop-22(29)ene (3-BHO) on insulin and glucagon-like peptide 1 (GLP-1) secretion as well as the mechanism of action of the compound in pancreatic islet on glucose homeostasis was investigated. The data from in vivo treatment show that 3-BHO significantly reduces the hyperglycemia by increasing the insulin and GLP-1 secretion, as well as by accumulating hepatic glycogen in hyperglycemic rats. In rat pancreatic β-cell, 3-BHO stimulates the glucose uptake, insulin vesicles translocation to the plasma membrane and thus the insulin secretion through the involvement of potassium channels (ATP- and Ca(2+)-dependent K(+) channels) and calcium channels (L-type voltage-dependent calcium channels (L-VDCC)). Furthermore, this study also provides evidence for a crosstalk between intracellular high calcium concentration, PKA and PKC in the signal transduction of 3-BHO to stimulate insulin secretion. In conclusion, 3-BHO diminishes glycaemia, stimulates GLP-1 secretion and potentiates insulin secretion and increase hepatic glycogen content. Moreover, this triterpene modulates calcium influx characterizing ATP-K(+), Ca(2+)-K(+) and L-VDCC channels-dependent pathways as well as PKA and PKC activity in pancreatic islets underlying the signaling of 3-BHO for the secretory activity and contribution on glucose homeostasis.

The future of EPAC-targeted therapies: agonism versus antagonism

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Although tractable to drug development, targeting of cAMP signalling has side effects.

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Selectively targeting EPAC1 and EPAC2 cAMP sensor enzymes may limit some of these off-target effects.

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EPAC agonists could be used to treat vascular inflammation (EPAC1) or type 2 diabetes (EPAC2).

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EPAC1 and EPAC2 antagonists could be used to treat heart disease.

Pharmaceutical manipulation of cAMP levels exerts beneficial effects through the regulation of the exchange protein activated by cAMP (EPAC) and protein kinase A (PKA) signalling routes. Recent attention has turned to the specific regulation of EPAC isoforms (EPAC1 and EPAC2) as a more targeted approach to cAMP-based therapies. For example, EPAC2-selective agonists could promote insulin secretion from pancreatic β cells, whereas EPAC1-selective agonists may be useful in the treatment of vascular inflammation. By contrast, EPAC1 and EPAC2 antagonists could both be useful in the treatment of heart failure. Here we discuss whether the best way forward is to design EPAC-selective agonists or antagonists and the current strategies being used to develop isoform-selective, small-molecule regulators of EPAC1 and EPAC2 activity.

Somatostatin and insulin mediate glucose-inhibited glucagon secretion in the pancreatic α-cell by lowering cAMP

The dysregulation of glucose-inhibited glucagon secretion from the pancreatic islet α-cell is a critical component of diabetes pathology and metabolic disease. We show a previously uncharacterized [Ca(2+)]i-independent mechanism of glucagon suppression in human and murine pancreatic islets whereby cAMP and PKA signaling are decreased. This decrease is driven by the combination of somatostatin, which inhibits adenylyl cyclase production of cAMP via the Gαi subunit of the SSTR2, and insulin, which acts via its receptor to activate phosphodiesterase 3B and degrade cytosolic cAMP. Our data indicate that both somatostatin and insulin signaling are required to suppress cAMP/PKA and glucagon secretion from both human and murine α-cells, and the combination of these two signaling mechanisms is sufficient to reduce glucagon secretion from isolated α-cells as well as islets. Thus, we conclude that somatostatin and insulin together are critical paracrine mediators of glucose-inhibited glucagon secretion and function by lowering cAMP/PKA signaling with increasing glucose.

Molecular mechanisms underlying physiological and receptor pleiotropic effects mediated by GLP-1R activation

The incidence of type 2 diabetes in developed countries is increasing yearly with a significant negative impact on patient quality of life and an enormous burden on the healthcare system. Current biguanide and thiazolidinedione treatments for type 2 diabetes have a number of clinical limitations, the most serious long-term limitation being the eventual need for insulin replacement therapy (Table 1). Since 2007, drugs targeting the glucagon-like peptide-1 (GLP-1) receptor have been marketed for the treatment of type 2 diabetes. These drugs have enjoyed a great deal of success even though our underlying understanding of the mechanisms for their pleiotropic effects remain poorly characterized even while major pharmaceutical companies actively pursue small molecule alternatives. Coupling of the GLP-1 receptor to more than one signalling pathway (pleiotropic signalling) can result in ligand-dependent signalling bias and for a peptide receptor such as the GLP-1 receptor this can be exaggerated with the use of small molecule agonists. Better consideration of receptor signalling pleiotropy will be necessary for future drug development. This is particularly important given the recent failure of taspoglutide, the report of increased risk of pancreatitis associated with GLP-1 mimetics and the observed clinical differences between liraglutide, exenatide and the newly developed long-acting exenatide long acting release, albiglutide and dulaglutide.

Mechanisms of neuronal membrane sealing following mechanical trauma

Membrane integrity is crucial for maintaining the intricate signaling and chemically-isolated intracellular environment of neurons; disruption risks deleterious effects, such as unregulated ionic flux, neuronal apoptosis, and oxidative radical damage as observed in spinal cord injury and traumatic brain injury. This paper, in addition to a discussion of the current understanding of cellular tactics to seal membranes, describes two major factors involved in membrane repair. These are line tension, the hydrophobic attractive force between two lipid free-edges, and membrane tension, the rigidity of the lipid bilayer with respect to the tethered cortical cytoskeleton. Ca(2+), a major mechanistic trigger for repair processes, increases following flux through a membrane injury site, and activates phospholipase enzymes, calpain-mediated cortical cytoskeletal proteolysis, protein kinase cascades, and lipid bilayer microdomain modification. The membrane tension appears to be largely modulated through vesicle dynamics, cytoskeletal organization, membrane curvature, and phospholipase manipulation. Dehydration of the phospholipid gap edge and modification of membrane packaging, as in temperature variation, experimentally impact line tension. Due to the time-sensitive nature of axonal sealing, increasing the efficacy of axolemmal sealing through therapeutic modification would be of great clinical value, to deter secondary neurodegenerative effects. Better therapeutic enhancement of membrane sealing requires a complete understanding of its intricate underlying neuronal mechanism.

The Amino Acid Transporters of the Glutamate/GABA-Glutamine Cycle and Their Impact on Insulin and Glucagon Secretion

Intercellular communication is pivotal in optimizing and synchronizing cellular responses to keep homeostasis and to respond adequately to external stimuli. In the central nervous system (CNS), glutamatergic and GABAergic signals are postulated to be dependent on the glutamate/GABA-glutamine cycle for vesicular loading of neurotransmitters, for inactivating the signal and for the replenishment of the neurotransmitters. Islets of Langerhans release the hormones insulin and glucagon, but share similarities with CNS cells in for example transcriptional control of development and differentiation, and chromatin methylation. Interestingly, CNS proteins involved in secretion of the neurotransmitters and emitting their responses as well as the regulation of these processes, are also found in islet cells. Moreover, high levels of glutamate, GABA, and glutamine and their respective vesicular and plasma membrane transporters have been shown in the islet cells and there is emerging support for these amino acids and their transporters playing important roles in the maturation and secretion of insulin and glucagon. In this review, we will discuss the feasibility of recent data in the field in relation to the biophysical properties of the transporters (Slc1, Slc17, Slc32, and Slc38) and physiology of hormone secretion in islets of Langerhans.

Function suggests nano-structure: towards a unified theory for secretion rate, a statistical mechanics approach

The inventory of secretory granules along the plasma membrane can be viewed as maintained in two restricted compartments. The release-ready pool represents docked granules available for an initial stage of fast, immediate secretion, followed by a second stage of granule set-aside secretion pool, with significantly slower rate. Transmission electron microscopy ultra-structural investigations correlated with electrophysiological techniques and mathematical modelling have allowed the categorization of these secretory vesicle compartments, in which vesicles can be in various states of secretory competence. Using the above-mentioned approaches, the kinetics of single vesicle exocytosis can be worked out. The ultra-fast kinetics, explored in this study, represents the immediately available release-ready pool, in which granules bound to the plasma membrane are exocytosed upon Ca(2+) influx at the SNARE rosette at the base of porosomes. Formalizing Dodge and Rahamimoff findings on the effect of calcium concentration and incorporating the effect of SNARE transient rosette size, we postulate that secretion rate (rate), the number (X) of intracellular calcium ions available for fusion, calcium capacity (0 ≤ M ≤ 5) and the fusion nano-machine size (as measured by the SNARE rosette size K) satisfy the parsimonious M-K relation rate ≈ C × [Ca(2+)](min(X,M))e(-K/2).

Metabolic signaling in fuel-induced insulin secretion

The pancreatic islet β cell senses circulating levels of calorigenic nutrients to secrete insulin according to the needs of the organism. Altered insulin secretion is linked to various disorders such as diabetes, hypoglycemic states, and cardiometabolic diseases. Fuel stimuli, including glucose, free fatty acids, and amino acids, promote insulin granule exocytosis primarily via their metabolism in β cells and the production of key signaling metabolites. This paper reviews our current knowledge of the pathways involved in both positive and negative metabolic signaling for insulin secretion and assesses the role of established and candidate metabolic coupling factors, keeping recent developments in focus.

Cyclic AMP dynamics in the pancreatic β-cell

Insulin secretion from pancreatic β-cells is tightly regulated by glucose and other nutrients, hormones, and neural factors. The exocytosis of insulin granules is triggered by an elevation of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) and is further amplified by cyclic AMP (cAMP). Cyclic AMP is formed primarily in response to glucoincretin hormones and other G(s)-coupled receptor agonists, but generation of the nucleotide is critical also for an optimal insulin secretory response to glucose. Nutrient and receptor stimuli trigger oscillations of the cAMP concentration in β-cells. The oscillations arise from variations in adenylyl cyclase-mediated cAMP production and phosphodiesterase-mediated degradation, processes controlled by factors like cell metabolism and [Ca(2+)](i). Protein kinase A and the guanine nucleotide exchange factor Epac2 mediate the actions of cAMP in β-cells and operate at multiple levels to promote exocytosis and pulsatile insulin secretion. The cAMP signaling system contains important targets for pharmacological improvement of insulin secretion in type 2 diabetes.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Multivesicular exocytosis in rat pancreatic beta cells

AIMS/HYPOTHESIS

To establish the occurrence, modulation and functional significance of compound exocytosis in insulin-secreting beta cells.

METHODS

Exocytosis was monitored in rat beta cells by electrophysiological, biochemical and optical methods. The functional assays were complemented by three-dimensional reconstruction of confocal imaging, transmission and block face scanning electron microscopy to obtain ultrastructural evidence of compound exocytosis.

RESULTS

Compound exocytosis contributed marginally (<5% of events) to exocytosis elicited by glucose/membrane depolarisation alone. However, in beta cells stimulated by a combination of glucose and the muscarinic agonist carbachol, 15-20% of the release events were due to multivesicular exocytosis, but the frequency of exocytosis was not affected. The optical measurements suggest that carbachol should stimulate insulin secretion by ∼40%, similar to the observed enhancement of glucose-induced insulin secretion. The effects of carbachol were mimicked by elevating [Ca(2+)](i) from 0.2 to 2 μmol/l Ca(2+). Two-photon sulforhodamine imaging revealed exocytotic events about fivefold larger than single vesicles and that these structures, once formed, could persist for tens of seconds. Cells exposed to carbachol for 30 s contained long (1-2 μm) serpentine-like membrane structures adjacent to the plasma membrane. Three-dimensional electron microscopy confirmed the existence of fused multigranular aggregates within the beta cell, the frequency of which increased about fourfold in response to stimulation with carbachol.

CONCLUSIONS/INTERPRETATION

Although contributing marginally to glucose-induced insulin secretion, compound exocytosis becomes quantitatively significant under conditions associated with global elevation of cytoplasmic calcium. These findings suggest that compound exocytosis is a major contributor to the augmentation of glucose-induced insulin secretion by muscarinic receptor activation.

Optimized amplification and single-cell analysis identify GnRH-mediated activation of Rap1b in primary rat gonadotropes

Identifying the early gene program induced by GnRH would help understand how GnRH-activated signaling pathways modulate gonadotrope secretory response. We previously analyzed GnRH-induced early genes in LβT2 cells, however these lack GnRH self-potentiation, a physiological attribute of gonadotropes. To minimize cellular heterogeneity, rat primary pituitary cultures were enriched for gonadotropes by 40-60% using a sedimentation gradient. Given the limited number of gonadotropes, RNA was amplified prior to microarray analysis. Thirty-three genes were up-regulated 40 min after GnRH stimulation. Real-time PCR confirmed regulation of several transcripts including fosB, c-fos, egr-2 and rap1b, a small GTPase and member of the Ras family. GnRH stimulated rap1b gene expression in gonadotropes, measured by a sensitive single cell assay. Immunocytochemistry revealed increased Rap1 protein in GnRH-stimulated gonadotropes. These data establish rap1b as a novel gene rapidly induced by GnRH and a candidate to modulate gonadotropin secretion in rat gonadotropes.

Coupling of metabolic, second messenger pathways and insulin granule dynamics in pancreatic beta-cells: a computational analysis

Insulin secretory responses to nutrient stimuli and hormonal modulators in pancreatic beta-cells are controlled by a variety of secondary messengers. We have analyzed numerous mechanisms responsible for regulated exocytosis in these cells and present an integrated mathematical model of cytosolic Ca²⁺, cAMP and granule dynamics. The insulin-containing granules in the beta-cell were divided into four classes: a large "reserve" granule pool, a smaller pool of the morphologically docked granules that is chemically 'primed' for release or the "readily releasable pool", and a pool of "restless newcomer granules" that undergoes preferential exocytosis. The model incorporates glucose and other aspects of metabolism, the cAMP amplifying pathway, insulin granule dynamics and the exocyst concept for granule binding. The values of most of the model parameters were inferred from available experimental data. The model can generate both the fast first phase and slow biphasic insulin secretion found experimentally in response to a step increase of membrane potential or of glucose. The numerical simulations have also reproduced a variety of experimental conditions, such as periodic stimulation by high K⁺ and the potentiation induced in islets by pre-incubation with cAMP pathway activators. The explicit incorporation of Ca²⁺ channels, Ca²⁺ and cAMP dynamics allows the model to be further connected to current models for calcium and metabolic dynamics and provides an interpretation of the roles of the triggering and amplifying pathways of glucose-stimulated insulin secretion. The model may be important in the identification of pharmacological targets for improving insulin secretion in type 2 diabetes.

Calcium dependencies of regulated exocytosis in different endocrine cells

Exocytotic machinery in neuronal and endocrine tissues is sensitive to changes in intracellular Ca(2+) concentration. Endocrine cell models, that are most frequently used to study the mechanisms of regulated exocytosis, are pancreatic beta cells, adrenal chromaffin cells and pituitary cells. To reliably study the Ca(2+) sensitivity in endocrine cells, accurate and fast determination of Ca(2+) dependence in each tested cell is required. With slow photo-release it is possible to induce ramp-like increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) that leads to a robust exocytotic activity. Slow increases in the [Ca(2+)](i) revealed exocytotic phases with different Ca(2+) sensitivities that have been largely masked in step-like flash photo-release experiments. Strikingly, in the cells of the three described model endocrine tissues (beta, chromaffin and melanotroph cells), distinct Ca(2+) sensitivity 'classes' of secretory vesicles have been observed: a highly Ca(2+)-sensitive, a medium Ca(2+)-sensitive and a low Ca(2+)-sensitive kinetic phase of secretory vesicle exocytosis. We discuss that a physiological modulation of a cellular activity, e.g. by activating cAMP/PKA transduction pathway, can switch the secretory vesicles between Ca(2+) sensitivity classes. This significantly alters late steps in the secretory release of hormones even without utilization of an additional Ca(2+) sensor protein.

Molecular physiology of glucagon-like peptide-1 insulin secretagogue action in pancreatic β cells

Insulin secretion from pancreatic β cells is stimulated by glucagon-like peptide-1 (GLP-1), a blood glucose-lowering hormone that is released from enteroendocrine L cells of the distal intestine after the ingestion of a meal. GLP-1 mimetics (e.g., Byetta) and GLP-1 analogs (e.g., Victoza) activate the β cell GLP-1 receptor (GLP-1R), and these compounds stimulate insulin secretion while also lowering levels of blood glucose in patients diagnosed with type 2 diabetes mellitus (T2DM). An additional option for the treatment of T2DM involves the administration of dipeptidyl peptidase-IV (DPP-IV) inhibitors (e.g., Januvia, Galvus). These compounds slow metabolic degradation of intestinally released GLP-1, thereby raising post-prandial levels of circulating GLP-1 substantially. Investigational compounds that stimulate GLP-1 secretion also exist, and in this regard a noteworthy advance is the demonstration that small molecule GPR119 agonists (e.g., AR231453) stimulate L cell GLP-1 secretion while also directly stimulating β cell insulin release. In this review, we summarize what is currently known concerning the signal transduction properties of the β cell GLP-1R as they relate to insulin secretion. Emphasized are the cyclic AMP, protein kinase A, and Epac2-mediated actions of GLP-1 to regulate ATP-sensitive K⁺ channels, voltage-dependent K⁺ channels, TRPM2 cation channels, intracellular Ca⁺ release channels, and Ca⁺-dependent exocytosis. We also discuss new evidence that provides a conceptual framework with which to understand why GLP-1R agonists are less likely to induce hypoglycemia when they are administered for the treatment of T2DM.

Phospholipase C-ε links Epac2 activation to the potentiation of glucose-stimulated insulin secretion from mouse islets of Langerhans

Glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells is potentiated by cAMP-elevating agents, such as the incretin hormone glucagon-like peptide-1 (GLP-1), and cAMP exerts its insulin secretagogue action by activating both protein kinase A (PKA) and the cAMP-regulated guanine nucleotide exchange factor designated as Epac2. Although prior studies of mouse islets demonstrated that Epac2 acts via Rap1 GTPase to potentiate GSIS, it is not understood which downstream targets of Rap1 promote the exocytosis of insulin. Here, we measured insulin secretion stimulated by a cAMP analog that is a selective activator of Epac proteins in order to demonstrate that a Rap1-regulated phospholipase C-epsilon (PLC-ε) links Epac2 activation to the potentiation of GSIS. Our analysis demonstrates that the Epac activator 8-pCPT-2'-O-Me-cAMP-AM potentiates GSIS from the islets of wild-type (WT) mice, whereas it has a greatly reduced insulin secretagogue action in the islets of Epac2 (-/-) and PLC-ε (-/-) knockout (KO) mice. Importantly, the insulin secretagogue action of 8-pCPT-2'-O-Me-cAMP-AM in WT mouse islets cannot be explained by an unexpected action of this cAMP analog to activate PKA, as verified through the use of a FRET-based A-kinase activity reporter (AKAR3) that reports PKA activation. Since the KO of PLC-ε disrupts the ability of 8-pCPT-2'-O-Me-cAMP-AM to potentiate GSIS, while also disrupting its ability to stimulate an increase of β-cell [Ca2+]i, the available evidence indicates that it is a Rap1-regulated PLC-ε that links Epac2 activation to Ca2+-dependent exocytosis of insulin.

Epac2-dependent rap1 activation and the control of islet insulin secretion by glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) binds its Class II G protein-coupled receptor to stimulate cyclic adenosine monophosphate (cAMP) production and to potentiate the glucose metabolism-dependent secretion of insulin from pancreatic β cells located within the islets of Langerhans. Prior clinical studies demonstrate that this cAMP-mediated action of GLP-1 to potentiate glucose-stimulated insulin secretion (GSIS) is of major therapeutic importance when evaluating the abilities of GLP-1 receptor (GLP-1R) agonists to lower levels of blood glucose in type 2 diabetic subjects. Surprisingly, recent in vitro studies of human or rodent islets of Langerhans provide evidence for the existence of a noncanonical mechanism of β cell cAMP signal transduction, one that may explain how GLP-1R agonists potentiate GSIS. What these studies demonstrate is that a cAMP-regulated guanine nucleotide exchange factor designated as Epac2 couples β cell cAMP production to the protein kinase A-independent stimulation of insulin exocytosis. Provided here is an overview of the Epac2 signal transduction system in β cells, with special emphasis on Rap1, a Ras-related GTPase that is an established target of Epac2.

cAMP increases the sensitivity of exocytosis to Ca²+ primarily through protein kinase A in mouse pancreatic beta cells

Cyclic AMP regulates the late step of Ca²+-dependent exocytosis in many secretory cells through two major mechanisms: a protein kinase A-dependent and a cAMP-GEF/Epac-dependent pathway. We designed a protocol to characterize the role of these two cAMP-dependent pathways on the Ca²+ sensitivity and kinetics of regulated exocytosis in mouse pancreatic beta cells, using a whole-cell patch-clamp based capacitance measurements. A train of depolarizing pulses or slow photo-release of caged Ca²+ were stimuli for the exocytotic activity. In controls, due to exocytosis after slow photo-release, the C(m) change had typically two phases. We observed that the Ca²+-dependency of the rate of the first C(m) change follows saturation kinetics with high cooperativity and half-maximal rate at 2.9±0.2 μM. The intracellular depletion of cAMP did not change amp1, while rate1 and amp2 were strongly reduced. This manipulation pushed the Ca²+-dependency of the exocytotic burst to significantly lower [Ca²+](i). To address the question of which of the cAMP-dependent mechanisms regulates the observed shifts in Ca²+ dependency we included regulators of PKA and Epac2 activity in the pipette solution. PKA activation with 100 μM 6-Phe-cAMP or inhibition with 500 μM Rp-cAMPs in beta cells significantly shifted the EC(50) in the opposite directions. Specific activation of Epac2 did not change Ca²+ sensitivity. Our findings suggest that cAMP modulates Ca²+-dependent exocytosis in mouse beta cells mainly through a PKA-dependent mechanism by sensitizing the insulin releasing machinery to [Ca²+](i); Epac2 may contribute to enhance the rates of secretory vesicle fusion.

A model for sealing plasmalemmal damage in neurons and other eukaryotic cells

Plasmalemmal repair is necessary for survival of damaged eukaryotic cells. Ca(2+) influx through plasmalemmal disruptions activates calpain, vesicle accumulation at lesion sites, and membrane fusion proteins; Ca(2+) influx also initiates competing apoptotic pathways. Using the formation of a dye barrier (seal) to assess plasmalemmal repair, we now report that B104 hippocampal cells with neurites transected nearer (<50 μm) to the soma seal at a lower frequency and slower rate compared to cells with neurites transected farther (>50 μm) from the soma. Analogs of cAMP, including protein kinase A (PKA)-specific and Epac-specific cAMP, each increase the frequency and rate of sealing and can even initiate sealing in the absence of Ca(2+) influx at both transection distances. Furthermore, Epac activates a cAMP-dependent, PKA-independent, pathway involved in plasmalemmal sealing. The frequency and rate of plasmalemmal sealing are decreased by a small molecule inhibitor of PKA targeted to its catalytic subunit (KT5720), a peptide inhibitor targeted to its regulatory subunits (PKI), an inhibitor of a novel PKC (an nPKCη pseudosubstrate fragment), and an antioxidant (melatonin). Given these and other data, we propose a model for redundant parallel pathways of Ca(2+)-dependent plasmalemmal sealing of injured neurons mediated in part by nPKCs, cytosolic oxidation, and cAMP activation of PKA and Epac. We also propose that the evolutionary origin of these pathways and substances was to repair plasmalemmal damage in eukaryotic cells. Greater understanding of vesicle interactions, proteins, and pathways involved in plasmalemmal sealing should suggest novel neuroprotective treatments for traumatic nerve injuries and neurodegenerative disorders.

Class IA phosphatidylinositol 3-kinase in pancreatic β cells controls insulin secretion by multiple mechanisms

Type 2 diabetes is characterized by insulin resistance and pancreatic β cell dysfunction, the latter possibly caused by a defect in insulin signaling in β cells. Inhibition of class IA phosphatidylinositol 3-kinase (PI3K), using a mouse model lacking the pik3r1 gene specifically in β cells and the pik3r2 gene systemically (βDKO mouse), results in glucose intolerance and reduced insulin secretion in response to glucose. β cells of βDKO mice had defective exocytosis machinery due to decreased expression of soluble N-ethylmaleimide attachment protein receptor (SNARE) complex proteins and loss of cell-cell synchronization in terms of Ca(2+) influx. These defects were normalized by expression of a constitutively active form of Akt in the islets of βDKO mice, preserving insulin secretion in response to glucose. The class IA PI3K pathway in β cells in vivo is important in the regulation of insulin secretion and may be a therapeutic target for type 2 diabetes.

Unperturbed islet α-cell function examined in mouse pancreas tissue slices

Critical investigation into α-cell biology in health and diabetes has been sparse and at times inconsistent because of the technical difficulties with employing conventional strategies of isolated islets and dispersed single cells. An acute pancreas slice preparation was developed to overcome the enzymatic and mechanical perturbations inherent in conventional islet cell isolation procedures. This preparation preserves intra-islet cellular communication and islet architecture in their in situ native state. α-Cells within tissue slices were directly assessed by patch pipette and electrophysiologically characterized. The identity of the patched cells was confirmed by biocytin dye labelling and immunocytochemistry. α-Cells in mouse pancreas slices exhibited well-described features of I(Na) (excitable at physiological membrane potential), I(KATP), small cell size, low resting membrane conductance, and inducible low and high voltage-activated I(Ca), the latter correlating with exocytosis determined by capacitance measurements. In contrast to previous reports, our large unbiased sampling of α-cells revealed a wide range distribution of all of these parameters, including the amount of K(ATP) conductance, Na+ and Ca2+ current amplitudes, and capacitance changes induced by a train of depolarization pulses. The proposed pancreas slice preparation in combination with standard patch-clamping technique allowed large sampling and rapid assessment of α-cells, which revealed a wide distribution in α-cell ion channel properties. This specific feature explains the apparent inconsistency of previous reports on these α-cell ion channel properties. Our innovative approach will enable future studies into elucidating islet α-cell dysregulation occurring during diabetes.

Epac2-dependent mobilization of intracellular Ca²+ by glucagon-like peptide-1 receptor agonist exendin-4 is disrupted in β-cells of phospholipase C-ε knockout mice

Calcium can be mobilized in pancreatic β-cells via a mechanism of Ca(2+)-induced Ca(2+) release (CICR), and cAMP-elevating agents such as exendin-4 facilitate CICR in β-cells by activating both protein kinase A and Epac2. Here we provide the first report that a novel phosphoinositide-specific phospholipase C- (PLC-) is expressed in the islets of Langerhans, and that the knockout (KO) of PLC- gene expression in mice disrupts the action of exendin-4 to facilitate CICR in the β-cells of these mice. Thus, in the present study, in which wild-type (WT) C57BL/6 mouse β-cells were loaded with the photolabile Ca(2+) chelator NP-EGTA, the UV flash photolysis-catalysed uncaging of Ca(2+) generated CICR in only 9% of the β-cells tested, whereas CICR was generated in 82% of the β-cells pretreated with exendin-4. This action of exendin-4 to facilitate CICR was reproduced by cAMP analogues that activate protein kinase A (6-Bnz-cAMP-AM) or Epac2 (8-pCPT-2'-O-Me-cAMP-AM) selectively. However, in β-cells of PLC- KO mice, and also Epac2 KO mice, these test substances exhibited differential efficacies in the CICR assay such that exendin-4 was partly effective, 6-Bnz-cAMP-AM was fully effective, and 8-pCPT-2'-O-Me-cAMP-AM was without significant effect. Importantly, transduction of PLC- KO β-cells with recombinant PLC- rescued the action of 8-pCPT-2'-O-Me-cAMP-AM to facilitate CICR, whereas a K2150E PLC- with a mutated Ras association (RA) domain, or a H1640L PLC- that is catalytically dead, were both ineffective. Since 8-pCPT-2'-O-Me-cAMP-AM failed to facilitate CICR in WT β-cells transduced with a GTPase activating protein (RapGAP) that downregulates Rap activity, the available evidence indicates that a signal transduction 'module' comprised of Epac2, Rap and PLC- exists in β-cells, and that the activities of Epac2 and PLC- are key determinants of CICR in this cell type.

How pancreatic beta-cells discriminate long and short timescale cAMP signals

The translocation of catalytic protein kinase A (cPKA) in response to cyclic-adenosine mono-phosphate (cAMP) depends on the pattern of stimulus applied to the cell. Experiments with IBMX have shown that 1), sustained cAMP elevation is more effective than oscillations of cAMP at getting cPKA into the nucleus; and 2), cPKA enters the nucleus by diffusion. We constructed mathematical models of cAMP activation of cPKA and their diffusion in order to study nuclear translocation of cPKA, and conclude that hindered diffusion of cPKA through the nuclear membrane by a rapid-binding process, but not globally reduced diffusion, can explain the experimental data. Perturbation analysis suggests that normal physiological oscillations of glucose would not result in nuclear translocation, but chronically high glucose that produces extended calcium plateaus and/or chronic glucagonlike peptide-1 stimulation could result in elevated levels of nuclear cPKA.

SNARE conformational changes that prepare vesicles for exocytosis

When cells release hormones and neurotransmitters through exocytosis, cytosolic Ca(2+) triggers the fusion of secretory vesicles with the plasma membrane. It is well known that this fusion requires assembly of a SNARE protein complex. However, the timing of SNARE assembly relative to vesicle fusion--essential for understanding exocytosis--has not been demonstrated. To investigate this timing, we constructed a probe that detects the assembly of two plasma membrane SNAREs, SNAP25 and syntaxin-1A, through fluorescence resonance energy transfer (FRET). With two-photon imaging, we simultaneously measured FRET signals and insulin exocytosis in beta cells from the pancreatic islet of Langerhans. In some regions of the cell, we found that the SNARE complex was preassembled, which enabled rapid exocytosis. In other regions, SNARE assembly followed Ca(2+) influx, and exocytosis was slower. Thus, SNARE proteins exist in multiple stable preparatory configurations, from which Ca(2+) may trigger exocytosis through distinct mechanisms and with distinct kinetics.

Cyclic AMP signaling in pancreatic islets

Cyclic 3'5'AMP (cAMP) is an important physiological amplifier of glucose-induced insulin secretion by the pancreatic islet beta-cell, where it is formed by the activity of adenylyl cyclases, which are stimulated by glucose, through elevation in intracellular calcium concentrations, and by the incretin hormones (GLP-1 and GIP). cAMP is rapidly degraded in the pancreatic islet beta-cell by various cyclic nucleotide phosphodiesterase (PDE) enzymes. Many steps involved in glucose-induced insulin secretion are modulated by cAMP, which is also important in regulating pancreatic islet beta-cell differentiation, growth and survival. This chapter discusses the formation, destruction and actions of cAMP in the islets with particular emphasis on the beta-cell.

cAMP mediators of pulsatile insulin secretion from glucose-stimulated single beta-cells

Pulsatile insulin release from glucose-stimulated beta-cells is driven by oscillations of the Ca(2+) and cAMP concentrations in the subplasma membrane space ([Ca(2+)](pm) and [cAMP](pm)). To clarify mechanisms by which cAMP regulates insulin secretion, we performed parallel evanescent wave fluorescence imaging of [cAMP](pm), [Ca(2+)](pm), and phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) in the plasma membrane. This lipid is formed by autocrine insulin receptor activation and was used to monitor insulin release kinetics from single MIN6 beta-cells. Elevation of the glucose concentration from 3 to 11 mm induced, after a 2.7-min delay, coordinated oscillations of [Ca(2+)](pm), [cAMP](pm), and PIP(3). Inhibitors of protein kinase A (PKA) markedly diminished the PIP(3) response when applied before glucose stimulation, but did not affect already manifested PIP(3) oscillations. The reduced PIP(3) response could be attributed to accelerated depolarization causing early rise of [Ca(2+)](pm) that preceded the elevation of [cAMP](pm). However, the amplitude of the PIP(3) response after PKA inhibition was restored by a specific agonist to the cAMP-dependent guanine nucleotide exchange factor Epac. Suppression of cAMP formation with adenylyl cyclase inhibitors reduced already established PIP(3) oscillations in glucose-stimulated cells, and this effect was almost completely counteracted by the Epac agonist. In cells treated with small interfering RNA targeting Epac2, the amplitudes of the glucose-induced PIP(3) oscillations were reduced, and the Epac agonist was without effect. The data indicate that temporal coordination of the triggering [Ca(2+)](pm) and amplifying [cAMP](pm) signals is important for glucose-induced pulsatile insulin release. Although both PKA and Epac2 partake in initiating insulin secretion, the cAMP dependence of established pulsatility is mediated by Epac2.

PKA-dependent potentiation of glucose-stimulated insulin secretion by Epac activator 8-pCPT-2'-O-Me-cAMP-AM in human islets of Langerhans

Potential insulin secretagogue properties of an acetoxymethyl ester of a cAMP analog (8-pCPT-2'-O-Me-cAMP-AM) that activates the guanine nucleotide exchange factors Epac1 and Epac2 were assessed using isolated human islets of Langerhans. RT-QPCR demonstrated that the predominant variant of Epac expressed in human islets was Epac2, although Epac1 was detectable. Under conditions of islet perifusion, 8-pCPT-2'-O-Me-cAMP-AM (10 microM) potentiated first- and second-phase 10 mM glucose-stimulated insulin secretion (GSIS) while failing to influence insulin secretion measured in the presence of 3 mM glucose. The insulin secretagogue action of 8-pCPT-2'-O-Me-cAMP-AM was associated with depolarization and an increase of [Ca(2+)](i) that reflected both Ca(2+) influx and intracellular Ca(2+) mobilization in islet beta-cells. As expected for an Epac-selective cAMP analog, 8-pCPT-2'-O-Me-cAMP-AM (10 microM) failed to stimulate phosphorylation of PKA substrates CREB and Kemptide in human islets. Furthermore, 8-pCPT-2'-O-Me-cAMP-AM (10 microM) had no significant ability to activate AKAR3, a PKA-regulated biosensor expressed in human islet cells by viral transduction. Unexpectedly, treatment of human islets with an inhibitor of PKA activity (H-89) or treatment with a cAMP antagonist that blocks PKA activation (Rp-8-CPT-cAMPS) nearly abolished the action of 8-pCPT-2'-O-Me-cAMP-AM to potentiate GSIS. It is concluded that there exists a permissive role for PKA activity in support of human islet insulin secretion that is both glucose dependent and Epac regulated. This permissive action of PKA may be operative at the insulin secretory granule recruitment, priming, and/or postpriming steps of Ca(2+)-dependent exocytosis.

Exocytosis in islet beta-cells

The development of technologies that allow for live optical imaging of exocytosis from beta-cells has greatly improved our understanding of insulin secretion. Two-photon imaging, in particular, has enabled researchers to visualize the exocytosis of large dense-core vesicles (LDCVs) containing insulin from beta-cells in intact islets of Langerhans. These studies have revealed that high glucose levels induce two phases of insulin secretion and that this release is dependent upon cytosolic Ca(2+) and cAMP. This technology has also made it possible to examine the spatial profile of insulin exocytosis in these tissues and compare that profile with those of other secretory glands. Such studies have led to the discovery of the massive exocytosis of synaptic-like microvesicles (SLMVs) in beta-cells. These imaging studies have also helped clarify facets of insulin exocytosis that cannot be properly addressed using the currently available electrophysiological techniques. This chapter provides a concise introduction to the field of optical imaging for those researchers who wish to characterize exocytosis from beta-cells in the islets of Langerhans.

Glucose-dependent potentiation of mouse islet insulin secretion by Epac activator 8-pCPT-2'-O-Me-cAMP-AM

Epac2 is a cAMP-regulated guanine nucleotide exchange factor (cAMP-GEF) that is proposed to mediate stimulatory actions of the second messenger cAMP on mouse islet insulin secretion. Here we have used methods of islet perifusion to demonstrate that the acetoxymethyl ester (AM-ester) of an Epac-selective cAMP analog (ESCA) penetrates into mouse islets and is capable of potentiating both first and second phases of glucose-stimulated insulin secretion (GSIS). When used at low concentrations (1-10 μM), 8-pCPT-2'-O-Me-cAMP-AM activates Rap1 GTPase but exhibits little or no ability to activate protein kinase A (PKA), as validated in assays of in vitro PKA activity (phosphorylation of Kemptide), Ser (133) CREB phosphorylation status, RIP1-CRE-Luc reporter gene activity, and PKA-dependent AKAR3 biosensor activation. Since quantitative PCR demonstrates Epac2 mRNA to be expressed at levels ca. 5.3-fold greater than that of Epac1, available evidence indicates that Epac2 does in fact mediate stimulatory actions of cAMP on mouse islet GSIS.

Epac activates the small G proteins Rap1 and Rab3A to achieve exocytosis

Exocytosis of the acrosome (the acrosome reaction) relies on cAMP production, assembly of a proteinaceous fusion machinery, calcium influx from the extracellular medium, and mobilization from inositol 1,4,5-trisphosphate-sensitive intracellular stores. Addition of cAMP to human sperm suspensions bypasses some of these requirements and elicits exocytosis in a protein kinase A- and extracellular calcium-independent manner. The relevant cAMP target is Epac, a guanine nucleotide exchange factor for the small GTPase Rap. We show here that a soluble adenylyl cyclase synthesizes the cAMP required for the acrosome reaction. Epac stimulates the exchange of GDP for GTP on Rap1, upstream of a phospholipase C. The Epac-selective cAMP analogue 8-pCPT-2'-O-Me-cAMP induces a phospholipase C-dependent calcium mobilization in human sperm suspensions. In addition, our studies identify a novel connection between cAMP and Rab3A, a secretory granule-associated protein, revealing that the latter functions downstream of soluble adenylyl cyclase/cAMP/Epac but not of Rap1. Challenging sperm with calcium or 8-pCPT-2'-O-Me-cAMP boosts the exchange of GDP for GTP on Rab3A. Recombinant Epac does not release GDP from Rab3A in vitro, suggesting that the Rab3A-GEF activation by cAMP/Epac in vivo is indirect. We propose that Epac sits at a critical point during the exocytotic cascade after which the pathway splits into two limbs, one that assembles the fusion machinery into place and another that elicits intracellular calcium release.

PACAP regulates immediate catecholamine release from adrenal chromaffin cells in an activity-dependent manner through a protein kinase C-dependent pathway

Adrenal medullary chromaffin cells are a major peripheral output of the sympathetic nervous system. Catecholamine release from these cells is driven by synaptic excitation from the innervating splanchnic nerve. Acetylcholine has long been shown to be the primary transmitter at the splanchnic-chromaffin synapse, acting through ionotropic nicotinic acetylcholine receptors to elicit action potential-dependent secretion from the chromaffin cells. This cholinergic stimulation has been shown to desensitize under sustained stimulation, yet catecholamine release persists under this same condition. Recent evidence supports synaptic chromaffin cell stimulation through alternate transmitters. One candidate is pituitary adenylate cyclase activating peptide (PACAP), a peptide transmitter present in the adrenal medulla shown to have an excitatory effect on chromaffin cell secretion. In this study we utilize native neuronal stimulation of adrenal chromaffin cells in situ and amperometric catecholamine detection to demonstrate that PACAP specifically elicits catecholamine release under elevated splanchnic firing. Further data reveal that the immediate PACAP-evoked stimulation involves a phospholipase C and protein kinase C-dependent pathway to facilitate calcium influx through a Ni2+ and mibefradil-sensitive calcium conductance that results in catecholamine release. These data demonstrate that PACAP acts as a primary secretagogue at the sympatho-adrenal synapse under the stress response.

Enhanced Rap1 activation and insulin secretagogue properties of an acetoxymethyl ester of an Epac-selective cyclic AMP analog in rat INS-1 cells: studies with 8-pCPT-2'-O-Me-cAMP-AM

To ascertain the identities of cyclic nucleotide-binding proteins that mediate the insulin secretagogue action of cAMP, the possible contributions of the exchange protein directly activated by cAMP (Epac) and protein kinase A (PKA) were evaluated in a pancreatic beta cell line (rat INS-1 cells). Assays of Rap1 activation, CREB phosphorylation, and PKA-dependent gene expression were performed in combination with live cell imaging and high throughput screening of a fluorescence resonance energy transfer-based cAMP sensor (Epac1-camps) to validate the selectivity with which acetoxymethyl esters (AM-esters) of cAMP analogs preferentially activate Epac or PKA. Selective activation of Epac or PKA was achieved following exposure of INS-1 cells to 8-pCPT-2'-O-Me-cAMP-AM or Bt(2)cAMP-AM, respectively. Both cAMP analogs exerted dose-dependent and glucose metabolism-dependent actions to stimulate insulin secretion, and when each was co-administered with the other, a supra-additive effect was observed. Because 2.4-fold more insulin was secreted in response to a saturating concentration (10 microm) of Bt(2)cAMP-AM as compared with 8-pCPT-2'-O-Me-cAMP-AM, and because the action of Bt(2)cAMP-AM but not 8-pCPT-2'-O-Me-cAMP-AM was nearly abrogated by treatment with 3 microm of the PKA inhibitor H-89, it is concluded that for INS-1 cells, it is PKA that acts as the dominant cAMP-binding protein in support of insulin secretion. Unexpectedly, 10-100 microm of the non-AM-ester of 8-pCPT-2'-O-Me-cAMP failed to stimulate insulin secretion and was a weak activator of Rap1 in INS-1 cells. Moreover, 10 microm of the AM-ester of 8-pCPT-2'-O-Me-cAMP stimulated insulin secretion from mouse islets, whereas the non-AM-ester did not. Thus, the membrane permeability of 8-pCPT-2'-O-Me-cAMP in insulin-secreting cells is so low as to limit its biological activity. It is concluded that prior reports documenting the failure of 8-pCPT-2'-O-Me-cAMP to act in beta cells, or other cell types, need to be re-evaluated through the use of the AM-ester of this cAMP analog.

Phosphatidylinositol-4,5-bisphosphate-dependent facilitation of the ATP-dependent secretory activity in mouse pituitary cells

Phosphatidylinositol-4, 5-bisphosphate [PI(4,5)P(2)] has been implicated in the priming of large dense-core vesicles in many secretory cells; however, its role in the Ca(2+)-dependent secretory activity in pituitary cells remains elusive. We assessed the effect of elevated intracellular PI(4,5)P(2) on the kinetics of Ca(2+)-dependent exocytosis, using a whole-cell patch-clamp technique in wild-type mouse melanotrophs from fresh pituitary tissue slices. We found that 1 micromol/L PI(4,5)P(2) significantly increased Ca(2+)-dependent exocytosis of vesicles that need to go through ATP-dependent reactions; however, the exocytosis of release-ready vesicles (ATP-independent release) and voltage-activated Ca(2+) currents remained unaffected. We suggest that PI(4,5)P(2) increases the size of the readily releasable vesicle pool by regulating the effectiveness of vesicular mobilization and fusion in an ATP-dependent manner.

Rescuing the subprime meltdown in insulin exocytosis in diabetes

Neuroendocrine pancreatic islet beta-cells secrete the hormone insulin in response to glucose stimulation and adapt efficiently to increased demand by peripheral tissues to maintain glucose homeostasis. Insulin is packed within dense-core granules, which traffic and dock onto the plasma membrane whereby a Ca(2+) stimulus evokes exocytosis by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE), complex-mediated, membrane fusion. Recent studies have unveiled postdocking steps mediated by "priming" factors that influence SNARE complex assembly to confer fusion readiness to the docked granules. This review will summarize recent insights into the priming role for Munc13 in the exocytosis of insulin granules. We present evidence for the interaction of Munc13-1 with exocytotic substrates involved in cAMP-mediated potentiation of insulin release, the latter we show to mediate enhanced granule-to-granule fusion events underlying compound exocytosis. We thus also further review the current understanding of granule-to-granule fusion. As agents acting on cAMP signaling are clinically used to augment insulin release in diabetes, this better understanding of priming steps may reveal additional novel therapeutic strategies to increase the capacity for insulin release to improve the treatment of diabetes.

EPAC proteins transduce diverse cellular actions of cAMP

It has now been over 10 years since efforts to completely understand the signalling actions of cAMP (3'-5'-cyclic adenosine monophosphate) led to the discovery of exchange protein directly activated by cAMP (EPAC) proteins. In the current review we will highlight important advances in the understanding of EPAC structure and function and demonstrate that EPAC proteins mediate multiple actions of cAMP in cells, revealing future targets for pharmaceutical intervention. It has been known for some time that drugs that elevate intracellular cAMP levels have proven therapeutic benefit for diseases ranging from depression to inflammation. The challenge now is to determine which of these positive actions of cAMP involve activation of EPAC-regulated signal transduction pathways. EPACs are specific guanine nucleotide exchange factors for the Ras GTPase homologues, Rap1 and Rap2, which they activate independently of the classical routes for cAMP signalling, cyclic nucleotide-gated ion channels and protein kinase A. Rather, EPAC activation is triggered by internal conformational changes induced by direct interaction with cAMP. Leading from this has been the development of EPAC-specific agonists, which has helped to delineate numerous cellular actions of cAMP that rely on subsequent activation of EPAC. These include regulation of exocytosis and the control of cell adhesion, growth, division and differentiation. Recent work also implicates EPAC in the regulation of anti-inflammatory signalling in the vascular endothelium, namely negative regulation of pro-inflammatory cytokine signalling and positive support of barrier function. Further elucidation of these important signalling mechanisms will no doubt support the development of the next generation of anti-inflammatory drugs.

Quantal ATP release in rat beta-cells by exocytosis of insulin-containing LDCVs

Quantal release of adenosine triphosphate (ATP) was monitored in rat pancreatic beta-cells expressing P2X(2) receptors. Stimulation of exocytosis evoked rapidly activating and deactivating ATP-dependent transient inward currents (TICs). The unitary charge (q) of the events recorded at 0.2 microM [Ca(2+)](i) averaged 4.3 pC. The distribution of the 3 square root q of these events could be described by a single Gaussian. The rise times averaged approximately 5 ms over a wide range of TIC amplitudes. In beta-cells preloaded with 5-hydroxytryptamine (5-HT; accumulating in insulin granules), ATP was coreleased with 5-HT during >90% of the release events. Following step elevation of [Ca(2+)](i) to approximately 5 microM by photo release of caged Ca(2+), an increase in membrane capacitance was observed after 33 ms, whereas ATP release first became detectable after 43 ms. The step increase in [Ca(2+)](i) produced an initial large TIC followed by a series of smaller events that echoed the changes in membrane capacitance (DeltaC(m)). Mathematical modeling suggests that the large initial TIC reflects the superimposition of many unitary events. Exocytosis, measured as DeltaC(m) or TICs, was complete within 2 s after elevation of [Ca(2+)](i) with no sign of endocytosis masking the capacitance increase. The relationship between total charge (Q) and DeltaC(m) was linear with a slope of approximately 1.2 pC/fF. The latter value predicts a capacitance increase of 3.6 fF for the observed mean value of q, close to that expected for exocytosis of individual insulin granules. Our results indicate that measurements of ATP release and DeltaC(m) principally (> or =85-95%) report exocytosis of insulin granules.