Ca2+-mediated generation of inositol 1,4,5-triphosphate and inositol 1,3,4,5-tetrakisphosphate in pancreatic islets. Studies with K+, glucose, and carbamylcholine

Indicator-dependent differences in detection of local intracellular Ca2+ release events evoked by voltage-gated Ca2+ entry in pancreatic β-cells

Genetically encoded Ca²⁺ indicators have become widely used in cell signalling studies as they offer advantages over cell-loaded dye indicators in enabling specific cellular or subcellular targeting. Comparing responses from dye and protein-based indicators may provide information about indicator properties and cell physiology, but side-by-side recordings in cells are scarce. In this study, we compared cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) changes in insulin-secreting β-cells recorded with commonly used dyes and indicators based on circularly permuted fluorescent proteins. Total internal reflection fluorescence (TIRF) imaging of K⁺ depolarization-triggered submembrane [Ca²⁺]_i increases showed that the dyes Fluo-4 and Fluo-5F mainly reported stable [Ca²⁺]_i elevations, whereas the proteins R-GECO1 and GCaMP5G more often reported distinct [Ca²⁺]_i spikes from an elevated level. [Ca²⁺]_i spiking occurred also in glucose-stimulated cells. The spikes reflected Ca²⁺ release from the endoplasmic reticulum, triggered by autocrine activation of purinergic receptors after exocytotic release of ATP and/or ADP, and the spikes were consequently prevented by SERCA inhibition or P2Y₁-receptor antagonism. Widefield imaging, which monitors the entire cytoplasm, increased the spike detection by the Ca²⁺ dyes. The indicator-dependent response patterns were unrelated to Ca²⁺ binding affinity, buffering and mobility, and probably reflects the much slower dissociation kinetics of protein compared to dye indicators. Ca²⁺ dyes thus report signalling within the submembrane space excited by TIRF illumination, whereas the protein indicators also catch Ca²⁺ events originating outside this volume. The study highlights that voltage-dependent Ca²⁺ entry in β-cells is tightly linked to local intracellular Ca²⁺ release mediated via an autocrine route that may be more important than previously reported direct Ca²⁺ effects on phospholipase C or on intracellular channels mediating calcium-induced calcium release.

Noncanonical Regulation of cAMP-Dependent Insulin Secretion and Its Implications in Type 2 Diabetes

Impaired glucose tolerance (IGT) and β-cell dysfunction in insulin resistance associated with obesity lead to type 2 diabetes (T2D). Glucose-stimulated insulin secretion (GSIS) from β-cells occurs via a canonical pathway that involves glucose metabolism, ATP generation, inactivation of K ATP channels, plasma membrane depolarization, and increases in cytosolic concentrations of [Ca 2+ ] c . However, optimal insulin secretion requires amplification of GSIS by increases in cyclic adenosine monophosphate (cAMP) signaling. The cAMP effectors protein kinase A (PKA) and exchange factor activated by cyclic-AMP (Epac) regulate membrane depolarization, gene expression, and trafficking and fusion of insulin granules to the plasma membrane for amplifying GSIS. The widely recognized lipid signaling generated within β-cells by the β-isoform of Ca 2+ -independent phospholipase A 2 enzyme (iPLA 2 β) participates in cAMP-stimulated insulin secretion (cSIS). Recent work has identified the role of a G-protein coupled receptor (GPCR) activated signaling by the complement 1q like-3 (C1ql3) secreted protein in inhibiting cSIS. In the IGT state, cSIS is attenuated, and the β-cell function is reduced. Interestingly, while β-cell-specific deletion of iPLA 2 β reduces cAMP-mediated amplification of GSIS, the loss of iPLA 2 β in macrophages (MØ) confers protection against the development of glucose intolerance associated with diet-induced obesity (DIO). In this article, we discuss canonical (glucose and cAMP) and novel noncanonical (iPLA 2 β and C1ql3) pathways and how they may affect β-cell (dys)function in the context of impaired glucose intolerance associated with obesity and T2D. In conclusion, we provide a perspective that in IGT states, targeting noncanonical pathways along with canonical pathways could be a more comprehensive approach for restoring β-cell function in T2D. © 2023 American Physiological Society. Compr Physiol 13:5023-5049, 2023.

The regulation of insulin secretion via phosphoinositide-specific phospholipase Cβ signaling

Phospholipase Cβ (PLCβ) is a membrane-associated enzyme activated by membrane receptors, especially G-protein coupled receptors (GPCRs). It propagates intracellular signaling by mediating phospholipid metabolism and generating key second messengers, such as inositol triphosphate and diacylglycerol, leading to intracellular Ca²⁺ mobilization and activation of kinases, such as protein kinases C. In pancreatic β-cells, PLCβ-mediated signaling activated by various factors, such as free fatty acids and neuronal and hormonal ligands, has been confirmed as being involved in the regulation of insulin secretion, and PLCβs have been regarded as essential mediators for augmenting insulin secretion. In this review, we describe the physiological function of PLCβs in the regulation of glucose-stimulated insulin secretion and discuss emerging data on GPCR/PLCβ signaling that is being developed as a target for the treatment of diabetes mellitus.

Regulation of insulin exocytosis by calcium-dependent protein kinase C in beta cells

The control of insulin release from pancreatic beta cells helps ensure proper blood glucose level, which is critical for human health. Protein kinase C has been shown to be one key control mechanism for this process. After glucose stimulation, calcium influx into beta cells triggers exocytosis of insulin-containing dense-core granules and activates protein kinase C via calcium-dependent phospholipase C-mediated generation of diacylglycerol. Activated protein kinase C potentiates insulin release by enhancing the calcium sensitivity of exocytosis, likely by affecting two main pathways that could be linked: (1) the reorganization of the cortical actin network, and (2) the direct phosphorylation of critical exocytotic proteins such as munc18, SNAP25, and synaptotagmin. Here, we review what is currently known about the molecular mechanisms of protein kinase C action on each of these pathways and how these effects relate to the control of insulin release by exocytosis. We identify remaining challenges in the field and suggest how these challenges might be addressed to advance our understanding of the regulation of insulin release in health and disease.

Autocrine Signaling Underlies Fast Repetitive Plasma Membrane Translocation of Conventional and Novel Protein Kinase C Isoforms in β Cells

PKC signaling has been implicated in the regulation of many cell functions, including metabolism, cell death, proliferation, and secretion. Activation of conventional and novel PKC isoforms is associated with their Ca²⁺- and/or diacylglycerol (DAG)-dependent translocation to the plasma membrane. In β cells, exocytosis of insulin granules evokes brief (<10 s) local DAG elevations (“spiking”) at the plasma membrane because of autocrine activation of P2Y₁ purinoceptors by ATP co-released with insulin. Using total internal reflection microscopy, fluorescent protein-tagged PKCs, and signaling biosensors, we investigated whether DAG spiking causes membrane recruitment of PKCs and whether different classes of PKCs show characteristic responses. Glucose stimulation of MIN6 cells triggered DAG spiking with concomitant repetitive translocation of the novel isoforms PKCδ, PKCϵ, and PKCη. The conventional PKCα, PKCβI, and PKCβII isoforms showed a more complex pattern with both rapid and slow translocation. K⁺ depolarization-induced PKCϵ translocation entirely mirrored DAG spiking, whereas PKCβI translocation showed a sustained component, reflecting the subplasma membrane Ca²⁺ concentration ([Ca²⁺]_pm), with additional effect during DAG spikes. Interference with DAG spiking by purinoceptor inhibition prevented intermittent translocation of PKCs and reduced insulin secretion but did not affect [Ca²⁺]_pm elevation or sustained PKCβI translocation. The muscarinic agonist carbachol induced pronounced transient PKCβI translocation and sustained recruitment of PKCϵ. When rise of [Ca²⁺]_pm was prevented, the carbachol-induced DAG and PKCϵ responses were somewhat reduced, but PKCβI translocation was completely abolished. We conclude that exocytosis-induced DAG spikes efficiently recruit both conventional and novel PKCs to the β cell plasma membrane. PKC signaling is thus implicated in autocrine regulation of β cell function.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2β) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

P2Y₁ receptor-dependent diacylglycerol signaling microdomains in β cells promote insulin secretion

Diacylglycerol (DAG) controls numerous cell functions by regulating the localization of C1-domain-containing proteins, including protein kinase C (PKC), but little is known about the spatiotemporal dynamics of the lipid. Here, we explored plasma membrane DAG dynamics in pancreatic β cells and determined whether DAG signaling is involved in secretagogue-induced pulsatile release of insulin. Single MIN6 cells, primary mouse β cells, and human β cells within intact islets were transfected with translocation biosensors for DAG, PKC activity, or insulin secretion and imaged with total internal reflection fluorescence microscopy. Muscarinic receptor stimulation triggered stable, homogenous DAG elevations, whereas glucose induced short-lived (7.1 ± 0.4 s) but high-amplitude elevations (up to 109 ± 10% fluorescence increase) in spatially confined membrane regions. The spiking was mimicked by membrane depolarization and suppressed after inhibition of exocytosis or of purinergic P2Y₁, but not P2X receptors, reflecting involvement of autocrine purinoceptor activation after exocytotic release of ATP. Each DAG spike caused local PKC activation with resulting dissociation of its substrate protein MARCKS from the plasma membrane. Inhibition of spiking reduced glucose-induced pulsatile insulin secretion. Thus, stimulus-specific DAG signaling patterns appear in the plasma membrane, including distinct microdomains, which have implications for the kinetic control of exocytosis and other membrane-associated processes.

Bursting and calcium oscillations in pancreatic beta-cells: specific pacemakers for specific mechanisms

Oscillatory phenomenon in electrical activity and cytoplasmic calcium concentration in response to glucose are intimately connected to multiple key aspects of pancreatic β-cell physiology. However, there is no single model for oscillatory mechanisms in these cells. We set out to identify possible pacemaker candidates for burst activity and cytoplasmic Ca(2+) oscillations in these cells by analyzing published hypotheses, their corresponding mathematical models, and relevant experimental data. We found that although no single pacemaker can account for the variety of oscillatory phenomena in β-cells, at least several separate mechanisms can underlie specific kinds of oscillations. According to our analysis, slowly activating Ca(2+)-sensitive K(+) channels can be responsible for very fast Ca(2+) oscillations; changes in the ATP/ADP ratio and in the endoplasmic reticulum calcium concentration can be pacemakers for both fast bursts and cytoplasmic calcium oscillations, and cyclical cytoplasmic Na(+) changes may underlie patterning of slow calcium oscillations. However, these mechanisms still lack direct confirmation, and their potential interactions raises new issues. Further studies supported by improved mathematical models are necessary to understand oscillatory phenomena in β-cell physiology.

The Gq/G11-mediated signaling pathway is critical for autocrine potentiation of insulin secretion in mice

A variety of neurotransmitters, gastrointestinal hormones, and metabolic signals are known to potentiate insulin secretion through GPCRs. We show here that beta cell-specific inactivation of the genes encoding the G protein alpha-subunits Galphaq and Galpha11 resulted in impaired glucose tolerance and insulin secretion in mice. Interestingly, the defects observed in Galphaq/Galpha11-deficient beta cells were not restricted to loss of muscarinic or metabolic potentiation of insulin release; the response to glucose per se was also diminished. Electrophysiological recordings revealed that glucose-induced depolarization of isolated beta cells was impaired in the absence of Galphaq/Galpha11, and closure of KATP channels was inhibited. We provide evidence that this reduced excitability was due to a loss of beta cell-autonomous potentiation of insulin secretion through factors cosecreted with insulin. We identified as autocrine mediators involved in this process extracellular nucleotides such as uridine diphosphate acting through the Gq/G11-coupled P2Y6 receptor and extracellular calcium acting through the calcium-sensing receptor. Thus, the Gq/G11-mediated signaling pathway potentiates insulin secretion in response to glucose by integrating systemic as well as autocrine/paracrine mediators.

Distinct plasma-membrane PtdIns(4)P and PtdIns(4,5)P2 dynamics in secretagogue-stimulated beta-cells

Phosphoinositides regulate numerous processes in various subcellular compartments. Whereas many stimuli trigger changes in the plasma-membrane PtdIns(4,5)P(2) concentration, little is known about its precursor, PtdIns(4)P, in particular whether there are stimulus-induced alterations independent of those of PtdIns(4,5)P(2). We investigated plasma-membrane PtdIns(4)P and PtdIns(4,5)P(2) dynamics in insulin-secreting MIN6 cells using fluorescent translocation biosensors and total internal reflection microscopy. Loss of PtdIns(4,5)P(2) induced by phospholipase C (PLC)-activating receptor agonists or stimulatory glucose concentrations was paralleled by increased PtdIns(4)P levels. In addition, glucose-stimulated cells regularly showed anti-synchronous oscillations of the two lipids. Whereas glucose-induced PtdIns(4)P elevation required voltage-gated Ca(2+) entry and was mimicked by membrane-depolarizing stimuli, the receptor-induced response was Ca(2+) independent, but sensitive to protein kinase C (PKC) inhibition and mimicked by phorbol ester stimulation. We conclude that glucose and PLC-activating receptor stimuli trigger Ca(2+)- and PKC-dependent changes in the plasma-membrane PtdIns(4)P concentration that are independent of the effects on PtdIns(4,5)P(2). These findings indicate that enhanced formation of PtdIns(4)P, apart from ensuring efficient replenishment of the PtdIns(4,5)P(2) pool, might serve an independent signalling function by regulating the association of PtdIns(4)P-binding proteins with the plasma membrane.

Glucocorticoids in vivo induce both insulin hypersecretion and enhanced glucose sensitivity of stimulus-secretion coupling in isolated rat islets

Although glucocorticoids are widely used as antiinflammatory agents in clinical therapies, they may cause serious side effects that include insulin resistance and hyperinsulinemia. To study the potential functional adaptations of the islet of Langerhans to in vivo glucocorticoid treatment, adult Wistar rats received dexamethasone (DEX) for 5 consecutive days, whereas controls (CTL) received only saline. The analysis of insulin release in freshly isolated islets showed an enhanced secretion in response to glucose in DEX-treated rats. The study of Ca(2+) signals by fluorescence microscopy also demonstrated a higher response to glucose in islets from DEX-treated animals. However, no differences in Ca(2+) signals were found between both groups with tolbutamide or KCl, indicating that the alterations were probably related to metabolism. Thus, mitochondrial function was explored by monitoring oxidation of nicotinamide dinucleotide phosphate autofluorescence and mitochondrial membrane potential. Both parameters revealed a higher response to glucose in islets from DEX-treated rats. The mRNA and protein content of glucose transporter-2, glucokinase, and pyruvate kinase was similar in both groups, indicating that changes in these proteins were probably not involved in the increased mitochondrial function. Additionally, we explored the status of Ca(2+)-dependent signaling kinases. Unlike calmodulin kinase II, we found an augmented phosphorylation level of protein kinase C alpha as well as an increased response of the phospholipase C/inositol 1,4,5-triphosphate pathway in DEX-treated rats. Finally, an increased number of docked secretory granules were observed in the beta-cells of DEX animals using transmission electron microscopy. Thus, these results demonstrate that islets from glucocorticoid-treated rats develop several adaptations that lead to an enhanced stimulus-secretion coupling and secretory capacity.

Pancreatic calcium signaling: role in health and disease

In order to control cell functions, extracellular agents, such as hormones or neurotransmitters among others, generate a diversity of calcium (Ca(2+)) signals in target cells. Here, we review the components involved in Ca(2+) handling and effectors, both members of the known calcium signaling pathways. In the pancreas, Ca(2+) signal appears as local increases, global elevations or Ca(2+) oscillations. Ca(2+) plays a key role in the pancreatic cells, regulating secretion in exocrine cells, a widely used model for studying the coupling between Ca(2+) signaling and secretion, and the release of insulin, glucagon and somatostatin in the exocrine pancreas. Interestingly, Ca(2+) deregulations have been related to pancreatitis and aging of the pancreas, and treatment with melatonin has shown beneficial effects suggesting that melatonin could be an adequate therapeutic approach.

Oscillatory control of insulin secretion

Pancreatic beta-cells possess an inherent ability to generate oscillatory signals that trigger insulin release. Coordination of the secretory activity among beta-cells results in pulsatile insulin secretion from the pancreas, which is considered important for the action of the hormone in the target tissues. This review focuses on the mechanisms underlying oscillatory control of insulin secretion at the level of the individual beta-cell. Recent studies have demonstrated that oscillations of the cytoplasmic Ca(2+) concentration are synchronized with oscillations in beta-cell metabolism, intracellular cAMP concentration, phospholipase C activity and plasma membrane phosphoinositide lipid concentrations. There are complex interdependencies between the different messengers and signalling pathways that contribute to amplitude regulation and shaping of the insulin secretory response to nutrient stimuli and neurohormonal modulators. Several of these pathways may be important pharmacological targets for improving pulsatile insulin secretion in type 2 diabetes.

The diverse roles of protein kinase C in pancreatic beta-cell function

Members of the serine/threonine PKC (protein kinase C) family perform diverse functions in multiple cell types. All members of the family are activated in signalling cascades triggered by occupation of cell surface receptors, but the cPKC (conventional PKC) and nPKC (novel PKC) isoforms are also responsive to fatty acid metabolites. PKC isoforms are involved in various aspects of pancreatic beta-cell function, including cell proliferation, differentiation and death, as well as regulation of secretion in response to glucose and muscarinic receptor agonists. Recently, the nPKC isoform, PKCepsilon, has also been implicated in the loss of insulin secretory responsiveness that underpins the development of Type 2 diabetes.

Long lasting synchronization of calcium oscillations by cholinergic stimulation in isolated pancreatic islets

Individual mouse pancreatic islets exhibit oscillations in [Ca(2+)](i) and insulin secretion in response to glucose in vitro, but how the oscillations of a million islets are coordinated within the human pancreas in vivo is unclear. Islet to islet synchronization is necessary, however, for the pancreas to produce regular pulses of insulin. To determine whether neurohormone release within the pancreas might play a role in coordinating islet activity, [Ca(2+)](i) changes in 4-6 isolated mouse islets were simultaneously monitored before and after a transient pulse of a putative synchronizing agent. The degree of synchronicity was quantified using a novel analytical approach that yields a parameter that we call the "Synchronization Index". Individual islets exhibited [Ca(2+)](i) oscillations with periods of 3-6 min, but were not synchronized under control conditions. However, raising islet [Ca(2+)](i) with a brief application of the cholinergic agonist carbachol (25 microM) or elevated KCl in glucose-containing saline rapidly synchronized islet [Ca(2+)](i) oscillations for >/=30 min, long after the synchronizing agent was removed. In contrast, the adrenergic agonists clonidine or norepinephrine, and the K(ATP) channel inhibitor tolbutamide, failed to synchronize islets. Partial synchronization was observed, however, with the K(ATP) channel opener diazoxide. The synchronizing action of carbachol depended on the glucose concentration used, suggesting that glucose metabolism was necessary for synchronization to occur. To understand how transiently perturbing islet [Ca(2+)](i) produced sustained synchronization, we used a mathematical model of islet oscillations in which complex oscillatory behavior results from the interaction between a fast electrical subsystem and a slower metabolic oscillator. Transient synchronization simulated by the model was mediated by resetting of the islet oscillators to a similar initial phase followed by transient "ringing" behavior, during which the model islets oscillated with a similar frequency. These results suggest that neurohormone release from intrapancreatic neurons could help synchronize islets in situ. Defects in this coordinating mechanism could contribute to the disrupted insulin secretion observed in Type 2 diabetes.

Rapid turnover of phosphatidylinositol-4,5-bisphosphate in insulin-secreting cells mediated by Ca2+ and the ATP-to-ADP ratio

Phosphatidylinositol-4,5-bisphosphate (PIP(2)) is important for a variety of cellular processes as a precursor for second messengers and by regulating ion channels, the cytoskeleton, and vesicle traffic in many types of cells, including insulin-secreting beta-cells. Here, we applied evanescent wave microscopy and the PIP(2)-binding pleckstrin homology domain from phospholipase C (PLC)-delta fused to the green fluorescent protein to characterize the regulation of plasma membrane PIP(2) in individual insulin-secreting MIN6 beta-cells. Elevation of the glucose concentration from 3 to 11 mmol/l evoked antisynchronous oscillations of [PIP(2)] and cytoplasmic Ca(2+)concentration, consistent with PLC being periodically activated by the voltage-dependent Ca(2+) influx. The effect of adenine nucleotides on [PIP(2)] was studied in cells permeabilized with alpha-toxin. ATP dose- dependently stimulated PIP(2) synthesis with half-maximal effect at 300 mumol/l. Omission of the nucleotide resulted in rapid loss of PIP(2) with t(1/2) < 40 s. ADP also stimulated PIP(2) formation, but this effect reflected local ATP formation and was prevented by the adenylate kinase inhibitor diadenosine-pentaphosphate. The ATP-induced PIP(2) synthesis was counteracted by the ADP analog adenosine-5'-O-2-thiodiphosphate. We conclude that plasma membrane PIP(2) is dynamically regulated by intracellular Ca(2+) and the ATP-to-ADP ratio in insulin-secreting cells. The rapid turnover allows maintenance of PIP(2) levels while generating second messengers of critical importance for insulin secretion.

Desensitization of the pancreatic beta-cell: effects of sustained physiological hyperglycemia and potassium

The impact of modest but prolonged (3 h) exposure to high physiological glucose concentrations and hyperkalemia on the insulin secretory and phospholipase C (PLC) responses of rat pancreatic islets was determined. In acute studies, glucose (5-20 mM) caused a dose-dependent increase in secretion with maximal release rates 25-fold above basal secretion. When measured after 3 h of exposure to 5-10 mM glucose, subsequent stimulation of islets with 10-20 mM glucose during a dynamic perifusion resulted in dose-dependent decrements in secretion and PLC activation. Acute hyperkalemia (15-30 mM) stimulated calcium-dependent increases in both insulin secretion and PLC activation; however, prolonged hyperkalemia resulted in a biochemical and secretory lesion similar to that induced by sustained modest hyperglycemia. Glucose- (8 mM) desensitized islets retained significant sensitivity to stimulation by either carbachol or glucagon-like peptide-1. These findings emphasize the vulnerability of the beta-cell to even moderate sustained hyperglycemia and provide a biochemical rationale for achieving tight glucose control in diabetic patients. They also suggest that PLC activation plays a critically important role in the physiological regulation of glucose-induced secretion and in the desensitization of release that follows chronic hyperglycemia or hyperkalemia.

Ca2+ microdomains and the control of insulin secretion

Nutrient-induced increases in intracellular free Ca(2+) concentrations are the key trigger for insulin release from pancreatic islet beta-cells. These Ca(2+) changes are tightly regulated temporally, occurring as Ca(2+) influx-dependent baseline oscillations. We explore here the concept that locally high [Ca(2+)] concentrations (i.e. Ca(2+) microdomains) may control exocytosis via the recruitment of key effector proteins to sites of exocytosis. Importantly, recent advances in the development of organelle- and membrane-targeted green fluorescent protein (GFP-) or aequorin-based Ca(2+) indicators, as well as in rapid imaging techniques, are providing new insights into the potential role of these Ca(2+) microdomains in beta-cells. We summarise here some of the evidence indicating that Ca(2+) microdomains beneath the plasma membrane and at the surface of large dense core vesicles may be important in the normal regulation of insulin secretion, and may conceivably contribute to "ATP-sensitive K(+)-channel independent" effects of glucose. We also discuss evidence that, in contrast to certain non-excitable cells, direct transfer of Ca(2+) from the ER to mitochondria via localised physical contacts between these organelles is relatively less important for efficient mitochondrial Ca(2+) uptake in beta-cells. Finally, we discuss evidence from single cell imaging that increases in cytosolic Ca(2+) are not required for the upstroke of oscillations in mitochondrial redox state, but may underlie the reoxidation process.

Synaptotagmin 8 is expressed both as a calcium-insensitive soluble and membrane protein in neurons, neuroendocrine and endocrine cells

Synaptotagmins (syt) form a large family of transmembrane proteins and some of its isoforms are known to regulate calcium-induced membrane fusion during vesicular traffic. In view of the reported implication of the isoform syt8 in exocytosis we investigated the expression, localisation and calcium-sensitivity of syt8 in secretory cells. An immunopurified antipeptide antibody was generated which is directed against a C-terminal sequence and devoid of crossreactivity towards syt1 to 12. Subcellular fractionation and immunocytochemistry revealed two forms of synaptotagmin 8 (50 and 40 kDa). Whereas the 40-kDa was present in the cytosol in brain, in PC12 and in clonal beta-cells, the 50-kDa form was localised in very typical clusters and partially colocalised with the SNARE protein Vti1a. Moreover, in primary hippocampal neurons syt8 was only found within the soma. Amplification of syt8 by RT-PCR indicated that the observed protein variants were not generated by alternative splicing of the 6th exon and are most likely linked to variations in the N-terminal region. In contrast to the established calcium sensor syt2, endogenous cytosolic syt8 and transiently expressed syt8-C2AB-eGFP did not translocate upon a raise in cytosolic calcium in living cells. Syt8 is therefore not a calcium sensor in exocytotic membrane fusion in endocrine cells.

Inositol (1,4,5)-trisphosphate dynamics and intracellular calcium oscillations in pancreatic beta-cells

Glucose-stimulated insulin secretion is associated with transients of intracellular calcium concentration ([Ca2+]i) in the pancreatic beta-cell. We tested the hypothesis that inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] [Ca2+]i release is incorporated in glucose-induced [Ca2+]i oscillations in mouse islets and MIN6 cells. We found that depletion of intracellular Ca2+ stores with thapsigargin increased the oscillation frequency by twofold and inhibited the slow recovery phase of [Ca2+]i oscillations. We employed a pleckstrin homology domain-containing fluorescent biosensor, phospholipase C partial differential pleckstrin homology domain-enhanced green fluorescent protein, to visualize Ins(1,4,5)P3 dynamics in insulin-secreting MIN6 cells and mouse islets in real time using a video-rate confocal system. In both types of cells, stimulation with carbamoylcholine (CCh) and depolarization with KCl results in an increase in Ins(1,4,5)P3 accumulation in the cytoplasm. When stimulated with glucose, the Ins(1,4,5)P3 concentration in the cytoplasm oscillates in parallel with oscillations of [Ca2+]i. Maximal accumulation of Ins(1,4,5)P3 in these oscillations coincides with the peak of [Ca2+]i and tracks changes in frequencies induced by the voltage-gated K+ channel blockade. We show that Ins(1,4,5)P3 release in insulin-secreting cells can be stimulated by depolarization-induced Ca2+ flux. We conclude that Ins(1,4,5)P3 concentration oscillates in parallel with [Ca2+]i in response to glucose stimulation, but it is not the driving force for [Ca2+]i oscillations.

Cortically restricted production of IP3 leads to propagation of the fertilization Ca2+ wave along the cell surface in a model of the Xenopus egg

The fertilization Ca2+ wave in Xenopus laevis is a single, large wave of elevated free cytosolic Ca2+ concentration that emanates from the point of sperm-egg fusion and traverses the entire diameter of the egg. This phenomenon appears to involve an increase in inositol-1,4,5-trisphosphate (IP3) resulting from interaction of the sperm and egg, which then results in the activation of the endoplasmic reticulum Ca2+ release machinery. We have proposed models based on a static elevated distribution of IP3, and dynamic [IP3], however, these models have suggested that the fertilization wave passes through the center of the egg. Complementing these earlier models, we propose a more detailed model of the fertilization Ca2+ wave in Xenopus eggs to explore the hypothesis that IP3 is produced only at or near the plasma membrane. In this case, we find that the wave propagates primarily through the cortex of the egg, and that Ca2+ -induced production of IP3 at the plasma membrane allows IP3 to propagate in advance of the wave. Our model includes Ca2+ -dependent production of IP3 at the plasma membrane and IP3 degradation. Simulations in 1 dimension and axi-symmetric 3 dimensions illustrate the basic features of the wave.

Removal of Ca2+ channel beta3 subunit enhances Ca2+ oscillation frequency and insulin exocytosis

An oscillatory increase in pancreatic beta cell cytoplasmic free Ca2+ concentration, [Ca2+]i, is a key feature in glucose-induced insulin release. The role of the voltage-gated Ca2+ channel beta3 subunit in the molecular regulation of these [Ca2+]i oscillations has now been clarified by using beta3 subunit-deficient beta cells. beta3 knockout mice showed a more efficient glucose homeostasis compared to wild-type mice due to increased glucose-stimulated insulin secretion. This resulted from an increased glucose-induced [Ca2+]i oscillation frequency in beta cells lacking the beta3 subunit, an effect accounted for by enhanced formation of inositol 1,4,5-trisphosphate (InsP3) and increased Ca2+ mobilization from intracellular stores. Hence, the beta3 subunit negatively modulated InsP3-induced Ca2+ release, which is not paralleled by any effect on the voltage-gated L type Ca2+ channel. Since the increase in insulin release was manifested only at high glucose concentrations, blocking the beta3 subunit in the beta cell may constitute the basis for a novel diabetes therapy.

Atypical Ca2+-induced Ca2+ release from a sarco-endoplasmic reticulum Ca2+-ATPase 3-dependent Ca2+ pool in mouse pancreatic beta-cells

The contribution of Ca(2+) release from intracellular stores to the rise in the free cytosolic Ca(2+) concentration ([Ca(2+)](c)) triggered by Ca(2+) influx was investigated in mouse pancreatic beta-cells. Depolarization of beta-cells by 45 mm K(+) (in the presence of 15 mm glucose and 0.1 mm diazoxide) evoked two types of [Ca(2+)](c) responses: a monotonic and sustained elevation; or a sustained elevation superimposed by a transient [Ca(2+)](c) peak (TCP) (40-120 s after the onset of depolarization). Simultaneous measurements of [Ca(2+)](c) and voltage-dependent Ca(2+) current established that the TCP did not result from a larger Ca(2+) current. Abolition of the TCP by thapsigargin and its absence in sarco-endoplasmic reticulum Ca(2+)-ATPase 3 (SERCA3) knockout mice show that it is caused by Ca(2+) mobilization from the endoplasmic reticulum. A TCP could not be evoked by the sole depolarization of beta-cells but required a rise in [Ca(2+)](c) pointing to a Ca(2+)-induced Ca(2+) release (CICR). This CICR did not involve inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) because it was resistant to heparin. Nor did it involve ryanodine receptors (RyRs) because it persisted after blockade of RyRs with ryanodine, and was not mimicked by caffeine, a RyR agonist. Moreover, RyR1 and RyR2 mRNA were not found and RyR3 mRNA was only slightly expressed in purified beta-cells. A CICR could also be detected in a limited number of cells in response to glucose. Our data demonstrate, for the first time in living cells, the existence of an atypical CICR that is independent from the IP(3)R and the RyR. This CICR is prominent in response to a supraphysiological stimulation with high K(+), but plays little role in response to glucose in non-obese mouse pancreatic beta-cells.

Oscillations of phospholipase C activity triggered by depolarization and Ca2+ influx in insulin-secreting cells

Phospholipase C (PLC) is a ubiquitous enzyme involved in the regulation of a variety of cellular processes. Its dependence on Ca2+ is well recognized, but it is not known how PLC activity is affected by physiological variations of the cytoplasmic Ca2+ concentration ([Ca2+](i)). Here, we applied evanescent wave microscopy to monitor PLC activity in parallel with [Ca2+](i) in individual insulin-secreting INS-1 cells using the phosphatidylinositol 4,5-bisphosphate- and inositol 1,4,5-trisphosphate-binding pleckstrin homology domain from PLCdelta(1) fused to green fluorescent protein (PH(PLCdelta1)-GFP) and the Ca2+ indicator fura red. In resting cells, PH(PLCdelta1)-GFP was located predominantly at the plasma membrane. Activation of PLC by muscarinic or purinergic receptor stimulation resulted in PH(PLCdelta1)-GFP translocation from the plasma membrane to the cytoplasm, detected as a decrease in evanescent wave-excited PH(PLCdelta1)-GFP fluorescence. Using this translocation as a measure of PLC activity, we found that depolarization by raising extracellular [K+] triggered activation of the enzyme. This effect could be attributed both to a rise of [Ca2+](i) and to depolarization per se, because some translocation persisted during depolarization in a Ca2+-deficient medium containing the Ca2+ chelator EGTA. Moreover, oscillations of [Ca2+](i) resulting from depolarization with Ca2+ influx evoked concentration-dependent periodic activation of PLC. We conclude that PLC activity is under tight dynamic control of [Ca2+](i). In insulin-secreting beta-cells, this mechanism provides a link between Ca2+ influx and release from intracellular stores that may be important in the regulation of insulin secretion.

PKC alpha is activated but not required during glucose-induced insulin secretion from rat pancreatic islets

The role of protein kinase C (PKC) in glucose-stimulated insulin secretion (GSIS) is controversial. Using recombinant adenoviruses for overexpression of PKC alpha and PKC delta, in both wild-type (WT) and kinase-dead (KD) forms, we here demonstrate that activation of these two PKCs is neither necessary nor sufficient for GSIS from batch-incubated, rat pancreatic islets. In contrast, responses to the pharmacologic activator 12-O-tetradecanoylphorbol-13-acetate (TPA) were reciprocally modulated by overexpression of the PKC alpha WT or PKC alpha KD but not the corresponding PKC delta adenoviruses. The kinetics of the secretory response to glucose (monitored by perifusion) were not altered in either cultured islets overexpressing PKC alpha KD or freshly isolated islets stimulated in the presence of the conventional PKC (cPKC) inhibitor Go6976. However, the latter did inhibit the secretory response to TPA. Using phosphorylation state-specific antisera for consensus PKC phosphorylation sites, we also showed that (compared with TPA) glucose causes only a modest and transient functional activation of PKC (maximal at 2-5 min). However, glucose did promote a prolonged (15 min) phosphorylation of PKC substrates in the presence of the phosphatase inhibitor okadaic acid. Overall, the results demonstrate that glucose does stimulate PKC alpha in pancreatic islets but that this makes little overall contribution to GSIS.

NAADP: a new second messenger for glucose-induced Ca2+ responses in clonal pancreatic beta cells

Important questions remain concerning how elevated blood glucose levels are coupled to insulin secretion from pancreatic beta cells and how this process is impaired in type 2 diabetes. Glucose uptake and metabolism in beta cells cause the intracellular Ca(2+) concentration ([Ca(2+)](i)) to increase to a degree necessary and sufficient for triggering insulin release. Although both Ca(2+) influx and Ca(2+) release from internal stores are critical, the roles of inositol 1,4,5-trisphosphate (IP(3)) and cyclic adenosine dinucleotide phosphate ribose (cADPR) in regulating the latter have proven equivocal. Here we show that glucose also increases [Ca(2+)](i) via the novel Ca(2+)-mobilizing agent nicotinic acid adenine dinucleotide phosphate (NAADP) in the insulin-secreting beta-cell line MIN6. NAADP binds to specific, high-affinity membrane binding sites and at low concentrations elicits robust Ca(2+) responses in intact cells. Higher concentrations of NAADP inactivate NAADP receptors and attenuate the glucose-induced Ca(2+) increases. Importantly, glucose stimulation increases endogenous NAADP levels, providing strong evidence for recruitment of this pathway. In conclusion, our results support a model in which NAADP mediates glucose-induced Ca(2+) signaling in pancreatic beta cells and are the first demonstration in mammalian cells of the presence of endogenous NAADP levels that can be regulated by a physiological stimulus.

Phosphorylated inositol compounds in beta -cell stimulus-response coupling

Pancreatic beta-cell function is essential for the regulation of glucose homeostasis in humans, and its impairment leads to the development of type 2 diabetes. Inputs from glucose and cell surface receptors act together to initiate the beta-cell stimulus-response coupling that ultimately leads to the release of insulin. Phosphorylated inositol compounds have recently emerged as key players at all levels of the stimulus-secretion coupling process. In this current review, we seek to highlight recent advances in beta-cell phosphoinositide research by dividing our examination into two sections. The first involves the events that lead to insulin secretion. This includes both new roles for inositol polyphosphates, particularly inositol hexakisphosphate, and both conventional and 3-phosphorylated inositol lipids. In the second section, we deal with the more novel concept of the autocrine role of insulin. Here, released insulin initiates signal transduction cascades, principally through the activity of phosphatidylinositol 3-kinase. This new round of signal transduction has been established to activate key beta-cell genes, particularly the insulin gene itself. More controversially, this insulin feedback has also been suggested to either terminate or enhance insulin secretion events.

Inositol 3,4,5,6-tetrakisphosphate inhibits insulin granule acidification and fusogenic potential

ClC Cl(-) channels in endosomes, synaptosomes, lysosomes, and beta-cell insulin granules provide charge neutralization support for the functionally indispensable acidification of the luminal interior by electrogenic H(+)-ATPases (Jentsch, T. J., Stein, V., Weinreich, F., and Zdebik, A. A. (2002) Physiol. Rev. 82, 503-568). Regulation of ClC activity is, therefore, of widespread biological significance (Forgac, M. (1999) J. Biol. Chem. 274, 12951-12954). We now ascribe just such a regulatory function to the increases in cellular levels of inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P(4)) that inevitably accompany activation of the ubiquitous Ins(1,4,5)P(3) signaling pathway. We used confocal imaging to record insulin granule acidification in single mouse pancreatic beta-cells. Granule acidification was reduced by perfusion of single cells with 10 microm Ins(3,4,5,6)P(4) (the concentration following receptor activation), whereas at 1 microm ("resting" levels), Ins(3,4,5,6)P(4) was ineffective. This response to Ins(3,4,5,6)P(4) was not mimicked by 100 microm Ins(1,4,5,6)P(4) or by 100 microm Ins(1,3,4,5,6)P(5). Ins(3,4,5,6)P(4) did not affect granular H(+)-ATPase activity or H(+) leak, indicating that Ins(3,4,5,6)P(4) instead inhibited charge neutralization by ClC. The Ins(3,4,5,6)P(4)-mediated inhibition of vesicle acidification reduced exocytic release of insulin as determined by whole-cell capacitance recordings. This may impinge upon type 2 diabetes etiology. Regulatory control over vesicle acidification by this negative signaling pathway in other cell types should be considered.

Visualization of IP(3) dynamics reveals a novel AMPA receptor-triggered IP(3) production pathway mediated by voltage-dependent Ca(2+) influx in Purkinje cells

IP(3) signaling in Purkinje cells is involved in the regulation of cell functions including LTD. We have used a GFP-tagged pleckstrin homology domain to visualize IP(3) dynamics in Purkinje cells. Surprisingly, IP(3) production was observed in response not only to mGluR activation, but also to AMPA receptor activation in Purkinje cells in culture. AMPA-induced IP(3) production was mediated by depolarization-induced Ca(2+) influx because it was mimicked by depolarization and was blocked by inhibition of the P-type Ca(2+) channel. Furthermore, trains of complex spikes, elicited by climbing fiber stimulation (1 Hz), induced IP(3) production in Purkinje cells in cerebellar slices. These results revealed a novel IP(3) signaling pathway in Purkinje cells that can be elicited by synaptic inputs from climbing fibers.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Effects of triphenyltin on cytosolic Na(+) and Ca(2+) response to glucose and acetylcholine in pancreatic beta-cells from hamster

Oral administration of triphenyltin chloride (TPT) (60 mg/kg body wt) inhibits insulin secretion by perturbing the cytoplasmic Ca(2+) concentration ([Ca(2+)]i) in pancreatic beta-cells of the hamster. To test the possibility that the abnormal levels of [Ca(2+)]i induced by TPT administration could be due to a defect in the cytoplasmic Na(+) concentration ([Na(+)]i) in the beta-cells, we investigated the effects of TPT administration on the changes of [Na(+)]i and [Ca(2+)]i induced by glucose or acetylcholine (ACh) and on the [Na(+)]i induced by ouabain, a potent inhibitor of Na(+),K(+)-ATPase. The changes of [Na(+)]i and [Ca(2+)]i were measured in islet cells loaded with sodium-binding benzofuran isophthalate and fura 2, respectively. TPT administration strongly reduced the rise in [Ca(2+)]i induced by 15 mM glucose with and without extracellular 135 mM Na(+). TPT administration also significantly reduced the rise of [Ca(2+)]i by 100 microM ACh in the presence of 5.5 or 15 mM glucose but not the amplitude of [Ca(2+)]i by 100 microM ACh in Na(+)-free medium. TPT administration attenuated the rise in [Na(+)]i induced by 100 microM ACh in the presence of either 3 or 5.5 mM glucose. However, TPT administration did not impair the [Na(+)]i in the presence of glucose (3, 5.5, and 15 mM) or of 100 microM ouabain with 3 mM glucose. TPT administration significantly suppressed the insulin secretion induced by 15 and 27.8 mM glucose or 100 microM ACh in the presence of 5.5 mM glucose. Our study suggests that triphenyltin has inhibitory effects on the cellular Ca(2+) response due to a reduction of [Ca(2+)]i after Na(+)-dependent and Na(+)-independent depolarization in islet cells of the hamster.

Phospholipase C-gamma mediates the hydrolysis of phosphatidylinositol, but not of phosphatidylinositol 4,5-bisphoshate, in carbamylcholine-stimulated islets of langerhans

In pancreatic islets the activation of phospholipase C (PLC) by the muscarinic receptor agonist carbamyolcholine (carbachol) results in the hydrolysis of both phosphatidylinositol 4,5-bisphosphate (PtdInsP(2)) and phosphatidylinositol (PtdIns). Here we tested the hypothesis that PtdIns hydrolysis is mediated by PLCgamma1, which is known to be regulated by activation of tyrosine kinases and PtdIns 3-kinase. PtdIns breakdown was more sensitive than that of PtdInsP(2) to the tyrosine kinase inhibitor, genistein. Conversely, the tyrosine phosphatase inhibitor, vanadate, alone promoted PtdIns hydrolysis and acted non-additively with carbachol. Vanadate did not stimulate PtdInsP(2) breakdown. Carbachol also stimulated a rapid (maximal at 1-2 min) tyrosine phosphorylation of several islet proteins, although not of PLCgamma1 itself. Two structurally unrelated inhibitors of PtdIns 3-kinase, wortmannin and LY294002, more effectively attenuated the hyrolysis of PtdIns compared with PtdInsP(2). Adenovirally mediated overexpression of PLCgamma1 significantly increased carbachol-stimulated PtdIns hydrolysis without affecting that of PtdInsP(2). Conversely overexpression of PLCbeta1 up-regulated the PtdInsP(2), but not PtdIns, response. These results indicate that the hydrolysis of PtdIns and PtdInsP(2) are independently regulated in pancreatic islets and that PLCgamma1 selectively mediates the breakdown of PtdIns. The activation mechanism of PLCgamma involves tyrosine phosphorylation (but not of PLCgamma directly) and PtdIns 3-kinase. Our findings point to a novel bifurcation of signaling pathways downstream of muscarinic receptors and suggest that hydrolysis of PtdIns and PtdInsP(2) might serve different physiological ends.

Fuel and hormone regulation of phospholipase C beta 1 and delta 1 overexpressed in RINm5F pancreatic beta cells

The mechanism by which glucose and other fuels stimulate phosphoinositide-specific phospholipase C (PLC) in pancreatic islet beta cells is not known. Previous studies have suggested that glucose may couple to PLC beta 1 and PLC delta 1. To determine directly if fuels activate these PLC isozymes, clones stably overexpressing PLC beta 1 or PLC delta 1 were generated in the fuel-sensitive beta cell line RINm5F, and secretagogue regulation of these PLC isoforms was determined. Overexpression of PLC beta 1 or PLC delta 1 significantly increased PLC activity in isolated cell fractions, consistent with overexpression of active PLC isoforms in these clones. In paired experiments, stimulation of inositol phosphate (IP) accumulation by the fuel glyceraldehyde was enhanced in clones overexpressing PLC beta 1, in parallel with the G-protein alpha subunit activator, AlF(4)(-), suggesting a coupling between glyceraldehyde and this PLC isoform. In contrast, overexpression of PLC delta 1 had no effect on glyceraldehyde- or AlF(4)(-)-stimulated IP accumulation. Similarly, IP accumulation stimulated by ionomycin was enhanced in PLC beta 1, but not PLC delta 1 clones, indicating that increases in intracellular free calcium [Ca(2+)](i) can regulate PLC beta 1 but not PLC delta 1 overexpressed in this cell line. Interestingly, [Arg(8)] vasopressin-stimulated, but not carbachol-stimulated, IP accumulation was significantly increased in clones overexpressing either PLC beta 1 or PLC delta 1. These studies illustrate unique pathways coupling diverse secretagogues to specific PLC isoforms in islet beta cells, and demonstrate that glyceraldehyde can activate PLC beta 1 but not PLC delta 1; whereas, vasopressin, but not carbachol, can stimulate either isoform.

Ca2+-induced Ca2+ release from the endoplasmic reticulum amplifies the Ca2+ signal mediated by activation of voltage-gated L-type Ca2+ channels in pancreatic beta-cells

Stimulus-secretion coupling in pancreatic beta-cells involves membrane depolarization and Ca(2+) entry through voltage-gated L-type Ca(2+) channels, which is one determinant of increases in the cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)). We investigated how the endoplasmic reticulum (ER)-associated Ca(2+) apparatus further modifies this Ca(2+) signal. When fura-2-loaded mouse beta-cells were depolarized by KCl in the presence of 3 mm glucose, [Ca(2+)](i) increased to a peak in two phases. The second phase of the [Ca(2+)](i) increase was abolished when ER Ca(2+) stores were depleted by thapsigargin. The steady-state [Ca(2+)](i) measured at 300 s of depolarization was higher in control cells compared with cells in which the ER Ca(2+) pools were depleted. The amount of Ca(2+) presented to the cytoplasm during depolarization as estimated from the integral of the increment in [Ca(2+)](i) over time (integralDelta[Ca(2+)](i).dt) was approximately 30% higher compared with that in the Ca(2+) pool-depleted cells. neo-thapsigargin, an inactive analog, did not affect [Ca(2+)](i) response. Using Sr(2+) in the extracellular medium and exploiting the differences in the fluorescence properties of Ca(2+)- and Sr(2+)-bound fluo-3, we found that the incoming Sr(2+) triggered Ca(2+) release from the ER. Depolarization-induced [Ca(2+)](i) response was not altered by, an inhibitor of phosphatidylinositol-specific phospholipase C, suggesting that stimulation of the enzyme by Ca(2+) is not essential for amplification of Ca(2+) signaling. [Ca(2+)](i) response was enhanced when cells were depolarized in the presence of 3 mm glucose, forskolin, and caffeine, suggesting involvement of ryanodine receptors in the amplification process. Pretreatment with ryanodine (100 microm) diminished the second phase of the depolarization-induced increase in [Ca(2+)](i). We conclude that Ca(2+) entry through L-type voltage-gated Ca(2+) channels triggers Ca(2+) release from the ER and that such a process amplifies depolarization-induced Ca(2+) signaling in beta-cells.

CaM kinase II-dependent mobilization of secretory granules underlies acetylcholine-induced stimulation of exocytosis in mouse pancreatic B-cells

1. Measurements of cell capacitance were used to investigate the mechanisms by which acetylcholine (ACh) stimulates Ca2+-induced exocytosis in single insulin-secreting mouse pancreatic B-cells. 2. ACh (250 microM) increased exocytotic responses elicited by voltage-clamp depolarizations 2.3-fold. This effect was mediated by activation of muscarinic receptors and dependent on elevation of the cytoplasmic Ca2+ concentration ([Ca2+]i) attributable to mobilization of Ca2+ from intracellular stores. The latter action involved interference with the buffering of [Ca2+]i and the time constant (tau) for the recovery of [Ca2+]i following a voltage-clamp depolarization increased 5-fold. As a result, Ca2+ was present at concentrations sufficient to promote the replenishment of the readily releasable pool of granules (RRP; > 0.2 microM) for much longer periods in the presence than in the absence of the agonist. 3. The effect of Ca2+ on exocytosis was mediated by activation of CaM kinase II, but not protein kinase C, and involved both an increased size of the RRP from 40 to 140 granules and a decrease in tau for the refilling of the RRP from 31 to 19 s. 4. Collectively, the effects of ACh on the RRP and tau result in a > 10-fold stimulation of the rate at which granules are supplied for release.

Uptake and release of Ca2+ by the endoplasmic reticulum contribute to the oscillations of the cytosolic Ca2+ concentration triggered by Ca2+ influx in the electrically excitable pancreatic B-cell

The role of intracellular Ca2+ pools in oscillations of the cytosolic Ca2+ concentration ([Ca2+]c) triggered by Ca2+ influx was investigated in mouse pancreatic B-cells. [Ca2+]c oscillations occurring spontaneously during glucose stimulation or repetitively induced by pulses of high K+ (in the presence of diazoxide) were characterized by a descending phase in two components. A rapid decrease in [Ca2+]c coincided with closure of voltage-dependent Ca2+ channels and was followed by a slower phase independent of Ca2+ influx. Blocking the SERCA pump with thapsigargin or cyclopiazonic acid accelerated the rising phase of [Ca2+]c oscillations and increased their amplitude, which suggests that the endoplasmic reticulum (ER) rapidly takes up Ca2+. It also suppressed the slow [Ca2+]c recovery phase, which indicates that this phase corresponds to the slow release of Ca2+ that was taken up by the ER during the upstroke of the [Ca2+]c transient. Glucose promoted the buffering capacity of the ER and amplified the slow [Ca2+]c recovery phase. The slow phase induced by high K+ pulses was not affected by modulators of Ca2+- or inositol 1,4,5-trisphosphate-induced Ca2+ release, did not involve a depolarization-induced Ca2+ release, and was also observed at the end of a rapid rise in [Ca2+]c triggered from caged Ca2+. It is attributed to passive leakage of Ca2+ from the ER. We suggest that the ER displays oscillations of the Ca2+ concentration ([Ca2+]ER) concomitant and parallel to [Ca2+]c. The observation that thapsigargin depolarizes the membrane of B-cells supports the proposal that the degree of Ca2+ filling of the ER modulates the membrane potential. Therefore, [Ca2+]ER oscillations occurring during glucose stimulation are likely to influence the bursting behavior of B-cells and eventually [Ca2+]c oscillations.

Metabolism of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate by the oocytes of Xenopus laevis

The pathway and kinetics of inositol 1,4,5-trisphosphate (IP3) metabolism were measured in Xenopus laevis oocytes and cytoplasmic extracts of oocytes. Degradation of microinjected IP3 in intact oocytes was similar to that in the extracts containing comparable concentrations of IP3 ([IP3]). The rate and route of metabolism of IP3 depended on the [IP3] and the intracellular free Ca2+ concentration ([Ca2+]). At low [IP3] (100 nM) and high [Ca2+] (>/=1 microM), IP3 was metabolized predominantly by inositol 1,4, 5-trisphosphate 3-kinase (3-kinase) with a half-life of 60 s. As the [IP3] was increased, inositol polyphosphate 5-phosphatase (5-phosphatase) degraded progressively more IP3. At a [IP3] of 8 microM or greater, the dephosphorylation of IP3 was the dominant mode of IP3 removal irrespective of the [Ca2+]. At low [IP3] and low [Ca2+] (both </=400 nM), the activities of the 5-phosphatase and 3-kinase were comparable. The calculated range of action of IP3 in the oocyte was approximately 300 micron suggesting that IP3 acts as a global messenger in oocytes. In contrast to IP3, inositol 1,3,4, 5-tetrakisphosphate (IP4) was metabolized very slowly. The half-life of IP4 (100 nM) was 30 min and independent of the [Ca2+]. IP4 may act to sustain Ca2+ signals initiated by IP3. The half-life of both IP3 and IP4 in Xenopus oocytes was an order of magnitude or greater than that in small mammalian cells.

Impaired cytosolic Ca2+ response to glucose and gastric inhibitory polypeptide in pancreatic beta-cells from triphenyltin-induced diabetic hamster

Oral administration of a single dose of triphenyltin compounds induces diabetes with decreased insulin secretion in rabbits and hamsters after 2-3 days without any morphological changes in pancreatic islets. In the present study, to test the possibility that the impaired insulin secretion induced by triphenyltin compounds could result from an impaired Ca2+ response in pancreatic beta-cells, we investigated the effect of triphenyltin-chloride (TPTCl) administration on the changes in the cytoplasmic Ca2+ concentration ([Ca2+]i) induced by secretagogues, such as glucose, high K+, gastric inhibitory polypeptide (GIP), and acetylcholine (ACh) in hamster pancreatic beta-cells. TPTCl administration caused partial suppression in 10 mM K+-induced rise in [Ca2+]i without suppressing the increase in [Ca2+]i evoked by 20-50 mM K+. Administration of TPTCl strongly inhibited the rises in [Ca2+]i induced by 27.8 mM glucose, 100 microM ACh in the presence of 5.5 mM glucose, and by 100 nM GIP in the presence of 5.5 mM glucose. In the ACh-induced response, TPTCl administration strongly suppressed the late sustained phase, while weakly suppressing the initial rise in [Ca2+]i. TPTCl administration significantly suppressed the rise of cAMP content in islet cells induced by 100 nM GIP with 1 mM 3-isobutyl-1-methylxanthine in the presence of 5.5 mM glucose (P < 0.01, N = 5-11). TPTCl administration also impaired the insulin secretion in islet cells induced by 27.8 mM glucose, 100 nM GIP in the presence of 5.5 mM glucose, and 100 microM ACh in the presence of 5.5 mM glucose (P < 0.05, N = 9-16). We conclude that the pathology of triphenyltin-induced diabetes in hamsters involves a defect in cellular Ca2+ response due to a reduced Ca2+-influx through voltage-gated Ca2+ channels.

Impaired phosphoinositide metabolism in glucose-incompetent islets of neonatally streptozotocin-diabetic rats

The effects of nutrient and neurotransmitter stimuli on insulin release, loss of phosphoinositides (PI), and production of inositol phosphates (InsP) were investigated in islets from neonatally streptozotocin-injected (nSTZ) rats. In islets from nSTZ rats, insulin secretory responses to 16.7 mM D-glucose and 10.0 mM D-glyceraldehyde were reduced compared with controls. Contents in phosphatidylinositol 4-monophosphate [PtdIns(4)P] and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], but not in phosphatidylinositol, were diminished. Glucose effects on breakdown of PtdIns(4)P and PtdIns(4,5)P2 and on total InsP accumulation were both reduced. D-Glucose was unable to increase the levels of both inositol trisphosphate isomers, Ins(1,3,4)P3 and Ins(1,4,5)P3. Glyceraldehyde also failed to promote InsP formation. By contrast, the ability of 1.0 mM carbachol or 300 nM cholecystokinin to stimulate insulin secretion and InsP generation was still observed. Thus a disturbed coupling between nutrient recognition and activation of phospholipase C, possibly together with a shortage of available polyphosphoinositides, could be responsible for the altered islet PI turnover in the nSTZ rats. It is proposed that such defects may contribute to the impairment of glucose-stimulated insulin secretion in this model of non-insulin-dependent diabetes mellitus.

Signal transduction of PACAP and GLP-1 in pancreatic beta cells

PACAP and GLP-1 depolarize pancreatic beta cells and stimulate insulin secretion in the presence of glucose. Depolarization occurs through at least two distinct mechanisms: (1) closure of ATP-sensitive K+ channels, and (2) activation of nonselective cation channels (NSCCs). Under physiological conditions the NSCCs carry a predominantly Na(+)-dependent current. The current may also have a Ca2+ component, but this remains to be determined. Acting together, these two signaling systems reinforce each other and serve to promote membrane depolarization, a rise of [Ca2+]i, and exocytosis of insulin-containing secretory granules. The NSCCs in beta cells are dually regulated by intracellular cAMP and [Ca2+]i. In view of this dual regulation, it appears likely that NSCC channel activation results from signaling events occurring not only at the plasma membrane (gating of channels by cAMP; protein kinase A-mediated phosphorylation of channels) but also at intracellular sites (mobilization of calcium stores by an as yet to be determined process). It is noteworthy that activation of NSCCs has also been reported following stimulation of beta-cells with maitotoxin, or after depletion of intracellular Ca2+ stores. Therefore, the possibility arises that PACAP, GLP-1, and maitotoxin all act on the same types of ion channels in these cells, and that these channels are sensitive to alterations in the content of intracellular calcium. FIGURE 6 summarizes our current knowledge concerning the properties of the PACAP and GLP-1 signaling systems as they pertain to the regulation of NSCCs and intracellular calcium homeostasis in the beta cell. Given that PACAP and GLP-1 are proven to be exceptionally potent insulin secretagogues, it is of considerable interest to determine their usefulness as blood glucose-lowering agents. Initial evaluations of the therapeutic effectiveness of GLP-1 indicate a role for this peptide in the treatment of NIDDM, and also possibly insulin-dependent diabetes mellitus (IDDM). A very attractive feature of such a strategy is the demonstrated lack of hypoglycemic side effects attendant to administration of GLP-1 to diabetic subjects. These observations reinforce the notion that peptides of the PACAP/glucagon/VIP family represent important pharmacological tools for use in experimental therapeutics.

Membrane potential and cytosolic free calcium levels modulate acetylcholine-induced inositol phosphate production in insulin-secreting BTC3 cells

Effects of membrane potential and cytosolic free Ca2+ concentrations ([Ca2+]i) on acetycholine (ACh)-induced inositol phosphate production were investigated in insulin secreting betaTC3 cells. ACh (10 microM) caused a rapid inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) production and increase in [Ca2+]i reaching a maximum within 5 s. The rise in Ins(1,4,5)P3 production was reduced by 79 +/- 5% when [Ca2+]i was kept low in cells loaded with the Ca2+ chelator BAPTA. The ACh-evoked Ins(1,4,5)P3 production also depended on the membrane potential as it was reduced by 31 +/- 6% in cells hyperpolarized by diazoxide, an opener of ATP-sensitive K+ channels. The Ca2+ ionophore ionomycin caused a rapid increase in [Ca2+]i and in the cellular Ins(1,4,5)P3 content. We conclude that stimulation-induced changes in membrane potential and [Ca2+]i play an important role in controlling Ins(1,4,5)P3 production in insulin-secreting betaTC3 cells.

Effects of short-term culturing on islet phosphoinositide and insulin secretory responses to glucose and carbachol

The ability of glucose and carbachol, alone or in combination, to stimulate islet cell phosphoinositide (PI) hydrolysis and insulin secretory responses in freshly isolated or in 20-24 h cultured rat islets was assessed. In freshly isolated, 3H-inositol-prelabeled islets, 20 mM glucose alone or 1 mM carbachol alone stimulated significant increments in 3H-inositol efflux and inositol phosphate (IP) accumulation. When stimulated with both agonists, a dramatic and synergistic effect on IP accumulation was noted. Carbachol (1 mM) alone had no sustained stimulatory effect on insulin secretion. Glucose (20 mM) alone induced a biphasic insulin secretory response. When compared to prestimulatory secretory rates of 18 +/- 4 pg/islet/min, peak first and second phase responses now averaged 422 +/- 61 and 1016 +/- 156 pg/islet/min, respectively. In contrast to freshly studied islets, culturing islets for 20-24 h in CMRL-1066 medium attenuated all measured responses. The increases in 3H-inositol efflux rates in response to glucose, carbachol, or their combination were significantly less than those observed with fresh islets. The IP responses were also attenuated. Second phase insulin secretory responses to 20 mM glucose alone 68 +/- 9 pg/islet/min) or the combination of 20 mM glucose plus 1 mM carbachol (358 +/- 85 pg/islet/min) were also significantly decreased when compared with fresh islets. We conclude from these studies that the process of culturing islets for one day in CMRL-1066 significantly decreases islet cell PI hydrolysis and insulin secretory responsiveness. These observations may help to explain the discordant conclusions reached concerning the involvement of PI hydrolysis and protein kinase C activation in the regulation of insulin release from freshly isolated versus cultured islets.

Synergistic interaction of glucose and neurohumoral agonists to stimulate islet phosphoinositide hydrolysis

The interaction between neurohumoral agonists and glucose to stimulate phosphoinositide (PI)-specific phospholipase C (PLC) and insulin release was examined. In freshly isolated rat islets, maximal glucose (40 mM), cholecystokinin (CCK; 300 nM), or carbachol (CCh; 1 mM) stimulated PI hydrolysis 6.5-, 9.8-, and 5.7-fold, respectively, above basal. The combination of glucose and CCK or of glucose and CCh, but not of CCK and CCh, synergistically increased PI hydrolysis 23.2- and 21.6-fold, respectively, indicating that these secretagogues activate PLC by distinct pathways and that there is an interaction between them. This synergy was maximal at physiological concentrations of stimulatory glucose (8-10 mM) and was paralleled by a marked synergistic stimulation of insulin secretion. The enhanced PI response was partially Ca2+ dependent and may involve the activation of distinct isozymes of PLC, which we identify in islets. These studies demonstrate for the first time a unique and highly sensitive synergistic interaction between glucose and neurohumoral agonists to stimulate PI hydrolysis, and they suggest that enhanced PI hydrolysis is important in the potentiation of glucose- and neurohumoral-stimulated insulin secretion.