Anne Wuttke

Glucose-induced cyclic AMP oscillations regulate pulsatile insulin secretion

Cyclic AMP (cAMP) and Ca(2+) are key regulators of exocytosis in many cells, including insulin-secreting beta cells. Glucose-stimulated insulin secretion from beta cells is pulsatile and involves oscillations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), but little is known about the detailed kinetics of cAMP signaling. Using evanescent-wave fluorescence imaging we found that glucose induces pronounced oscillations of cAMP in the submembrane space of single MIN6 cells and primary mouse beta cells. These oscillations were preceded and enhanced by elevations of [Ca(2+)](i). However, conditions raising cytoplasmic ATP could trigger cAMP elevations without accompanying [Ca(2+)](i) rise, indicating that adenylyl cyclase activity may be controlled also by the substrate concentration. The cAMP oscillations correlated with pulsatile insulin release. Whereas elevation of cAMP enhanced secretion, inhibition of adenylyl cyclases suppressed both cAMP oscillations and pulsatile insulin release. We conclude that cell metabolism directly controls cAMP and that glucose-induced cAMP oscillations regulate the magnitude and kinetics of insulin exocytosis.

Rapid Turnover of Phosphatidylinositol-4,5-Bisphosphate in Insulin-Secreting Cells Mediated by Ca2+ and the ATP-to-ADP Ratio

Phosphatidylinositol-4,5-bisphosphate (PIP2) 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 β-cells. Here, we applied evanescent wave microscopy and the PIP2-binding pleckstrin homology domain from phospholipase C (PLC)-δ fused to the green fluorescent protein to characterize the regulation of plasma membrane PIP2 in individual insulin-secreting MIN6 β-cells. Elevation of the glucose concentration from 3 to 11 mmol/l evoked antisynchronous oscillations of [PIP2] and cytoplasmic Ca2+concentration, consistent with PLC being periodically activated by the voltage-dependent Ca2+ influx. The effect of adenine nucleotides on [PIP2] was studied in cells permeabilized with α-toxin. ATP dose- dependently stimulated PIP2 synthesis with half-maximal effect at 300 μmol/l. Omission of the nucleotide resulted in rapid loss of PIP2 with t1/2 < 40 s. ADP also stimulated PIP2 formation, but this effect reflected local ATP formation and was prevented by the adenylate kinase inhibitor diadenosine-pentaphosphate. The ATP-induced PIP2 synthesis was counteracted by the ADP analog adenosine-5′-O-2-thiodiphosphate. We conclude that plasma membrane PIP2 is dynamically regulated by intracellular Ca2+ and the ATP-to-ADP ratio in insulin-secreting cells. The rapid turnover allows maintenance of PIP2 levels while generating second messengers of critical importance for insulin secretion.

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.—Wuttke, A., Idevall‐Hagren, O., Tengholm, A. P2Y1 receptor‐dependent diacylglycerol signaling microdomains in β cells promote insulin secretion. FASEB J. 27, 1610–1620 (2013). www.fasebj.org

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

Phosphoinositides regulate numerous processes in various subcellular compartments. Whereas many stimuli trigger changes in the plasma-membrane PtdIns(4,5)P2 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)P2. We investigated plasma-membrane PtdIns(4)P and PtdIns(4,5)P2 dynamics in insulin-secreting MIN6 cells using fluorescent translocation biosensors and total internal reflection microscopy. Loss of PtdIns(4,5)P2 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 Ca2+ entry and was mimicked by membrane-depolarizing stimuli, the receptor-induced response was Ca2+ 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 Ca2+- and PKC-dependent changes in the plasma-membrane PtdIns(4)P concentration that are independent of the effects on PtdIns(4,5)P2. These findings indicate that enhanced formation of PtdIns(4)P, apart from ensuring efficient replenishment of the PtdIns(4,5)P2 pool, might serve an independent signalling function by regulating the association of PtdIns(4)P-binding proteins with the plasma membrane.

Imatinib mesilate-induced phosphatidylinositol 3-kinase signalling and improved survival in insulin-producing cells: role of Src homology 2-containing inositol 5′-phosphatase interaction with c-Abl

Aims/hypothesisIt is not clear how small tyrosine kinase inhibitors, such as imatinib mesilate, protect against diabetes and beta cell death. The aim of this study was to determine whether imatinib, as compared with the non-cAbl-inhibitor sunitinib, affects pro-survival signalling events in the phosphatidylinositol 3-kinase (PI3K) pathway.MethodsHuman EndoC-βH1 cells, murine beta TC-6 cells and human pancreatic islets were used for immunoblot analysis of insulin receptor substrate (IRS)-1, Akt and extracellular signal-regulated kinase (ERK) phosphorylation. Phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] plasma membrane concentrations were assessed in EndoC-βH1 and MIN6 cells using evanescent wave microscopy. Src homology 2-containing inositol 5′-phosphatase 2 (SHIP2) tyrosine phosphorylation and phosphatase and tensin homologue deleted on chromosome 10 (PTEN) serine phosphorylation, as well as c-Abl co-localisation with SHIP2, were studied in HEK293 and EndoC-βH1 cells by immunoprecipitation and immunoblot analysis. Gene expression was assessed using RT-PCR. Cell viability was measured using vital staining.ResultsImatinib stimulated ERK(thr202/tyr204) phosphorylation in a c-Abl-dependent manner. Imatinib, but not sunitinib, also stimulated IRS-1(tyr612), Akt(ser473) and Akt(thr308) phosphorylation. This effect was paralleled by oscillatory bursts in plasma membrane PI(3,4,5)P3 levels. Wortmannin induced a decrease in PI(3,4,5)P3 levels, which was slower in imatinib-treated cells than in control cells, indicating an effect on PI(3,4,5)P3-degrading enzymes. In line with this, imatinib decreased the phosphorylation of SHIP2 but not of PTEN. c-Abl co-immunoprecipitated with SHIP2 and its binding to SHIP2 was largely reduced by imatinib but not by sunitinib. Imatinib increased total β-catenin levels and cell viability, whereas sunitinib exerted negative effects on cell viability.Conclusions/interpretationImatinib inhibition of c-Abl in beta cells decreases SHIP2 activity, which results in enhanced signalling downstream of PI3 kinase.

A case study on stress preconditioning of a Lactobacillus strain prior to freeze-drying

Freeze-drying of bacterial cells with retained viability and activity after storage requires appropriate formulation, i.e. mixing of physiologically adapted cell populations with suitable protective agents, and control of the freeze-drying process. Product manufacturing may alter the clinical effects of probiotics and it is essential to identify and understand possible factor co-dependencies during manufacturing. The physical solid-state behavior of the formulation and the freeze-drying parameters are critical for bacterial survival and thus process optimization is important, independent of strain. However, the maximum yield achievable is also strain-specific and strain survival is governed by e.g. medium, cell type, physiological state, excipients used, and process. The use of preferred compatible solutes for cross-protection of Lactobacilli during industrial manufacturing may be a natural step to introduce robustness, but knowledge is lacking on how compatible solutes, such as betaine, influence formulation properties and cell survival. This study characterized betaine formulations, with and without sucrose, and tested these with the model lactic acid bacteria Lactobacillus coryniformis Si3. Betaine alone did not act as a lyo-protectant and thus betaine import prior to freeze-drying should be avoided. Differences in protective agents were analyzed by calorimetry, which proved to be a suitable tool for evaluating the characteristics of the freeze-dried end products.

Imaging phosphoinositide dynamics in living cells

To improve our understanding of the important roles played by inositol lipid derivatives in signalling and other cellular processes, it is crucial to measure phosphoinositide concentration changes in individual cells with high spatial and temporal resolution. A number of protein domains that interact with inositol lipids in a specific manner have been identified. Tagged with the green fluorescent protein or its colour variants, these protein modules can be used as probes to visualize various phosphoinositide species in different sub-cellular compartments. Here, we present protocols for fluorescence imaging of phosphoinositide dynamics in single living cells. Total internal reflection fluorescence microscopy is particularly powerful for time-lapse recordings of phosphoinositides in the plasma membrane. We demonstrate how this technique can be used to record phospholipase C- and PI3-kinase-induced changes in inositol lipids in insulin-secreting cells. These procedures should be applicable to studies of the spatio-temporal regulation of phosphoinositide metabolism in many types of cells.

Lipid signalling dynamics at the β-cell plasma membrane.

Pancreatic β-cells are clustered in islets of Langerhans and secrete insulin in response to increased concentrations of circulating glucose. Insulin in turn acts on liver, muscle and fat tissue to store energy and normalize the blood glucose level. Inappropriate insulin release may lead to impaired glucose tolerance and diabetes. In addition to glucose, other nutrients, neural stimuli and hormonal stimuli control insulin secretion. Many of these signals are perceived at the plasma membrane, which is also the site where insulin granules undergo exocytosis. Therefore, it is not surprising that membrane lipids play an important role in the regulation of insulin secretion. β-cells release insulin in a pulsatile fashion. Signalling lipids integrate the nutrient and neurohormonal inputs to fine-tune, shape and co-ordinate the pulsatility. An important group of signalling lipids are phosphoinositides and their downstream messengers. This MiniReview will discuss new insights into lipid signalling dynamics in β-cells obtained from live-cell imaging experiments with fluorescent translocation biosensors. The plasma membrane concentration of several phosphoinositides and of their downstream messengers changes rapidly upon nutrient or neurohormonal stimulation. Glucose induces the most complex spatio-temporal patterns, typically involving oscillations of messenger concentrations, which sometimes are locally restricted. The tightly controlled levels of lipid messengers can mediate specific binding of downstream effectors to the plasma membrane, contributing to the appropriate regulation of insulin secretion.

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.

Exocytotic release of ATP triggers diacylglycerol spiking in insulin-secreting cells

Background and aims: TCF7L2 is both an activator and an inhibitor of transcription and the most highly associated type 2 diabetes gene known to date. It influences beta cell survival and function, i.e. incretin hormonal effects, insulin processing and secretion. However, its target genes in pancreatic islets are not fully described and the molecular mechanism whereby it propagates its effects on islet function is not known. The aim of this study is to identify the molecular mechanisms through which TCF7L2 influence beta cell survival and function. Materials and methods: Wister rat primary islets and INS-1 (832/13) cells were incubated with siRNA against Tcf7l2, both Tcf7l2 and TP53INP1 or both TCF7L2 and TP53 in 5.5 mM and 14.3 mM glucose. TCF7L2 activity, p53 activity and target gene expression (using qPCR) were measured after siRNA treatment. INS-1 cell apoptosis was measured by DNA degradation levels, caspase-3/7 levels and by using antibodies against Annexin V, and 7-AAD, visualized using confocal microscopy. Rat islet viability was estimated measuring metabolic rate. Rat islet apoptosis was estimated by measuring Caspase-3/7 level. Results: The type 2 diabetes associated genes TP53INP1, FTO, GIPR and ADAMTS9 were identified as TCF7L2 potential target gene using chromatin immunoprecipitation on microarrays. In INS-1 cells, siRNA mediated Tcf7l2 knock down (69.5 %) resulted in decreased TCF7L2 activity (91%) and differential expression of the target genes: Tp53 (14.5% increase), TP53INP1 (65.9% increase) and ADAMTS9 (82.8% decrease). TCF7L2 knockdown also lead to reduced cell viability (65%) and increased apoptosis (113%). The TCF7L2 induced cell death was replicated in rat primary islets. When restoring (decreasing) the Tp53inp1 expression level in TCF7L2 depleted islets, the decrease in cell viability and increase in apoptosis were prevented, suggesting that the Tcf7l2 effect is mediated via Tp53inp1. Furthermore, p53 depletion prohibited TCF7L2 down regulation induced cell death and elevation of Tp53inp1 expression in both INS-1 cells and rat primary islets. Conclusion: The type 2 diabetes associated genes TP53INP1 and ADAMTS9 are target genes of TCF7L2 in pancreatic islets. TCF7L2 induced apoptosis and decreased cell viability are mediated through activation of p53 and increased p53INP1 expression.