Publications (53)286.04 Total impact
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Article: Congenital Hyperinsulinism Caused by Hexokinase I Expression or Glucokinase-Activating Mutation in a Subset of β-Cells.
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ABSTRACT: Congenital hyperinsulinism causes persistent hypoglycemia in neonates and infants. Most often, uncontrolled insulin secretion (IS) results from a lack of functional K(ATP) channels in all β-cells or only in β-cells within a resectable focal lesion. In more rare cases, without K(ATP) channel mutations, hyperfunctional islets are confined within few lobules, whereas hypofunctional islets are present throughout the pancreas. They also can be cured by selective partial pancreatectomy; however, unlike those with a K(ATP) focal lesion, they show clinical sensitivity to diazoxide. Here, we characterized in vitro IS by fragments of pathological and adjacent normal pancreas from six such cases. Responses of normal pancreas were unremarkable. In pathological region, IS was elevated at 1 mmol/L and was further increased by 15 mmol/L glucose. Diazoxide suppressed IS and tolbutamide antagonized the inhibition. The most conspicuous anomaly was a large stimulation of IS by 1 mmol/L glucose. In five of six cases, immunohistochemistry revealed undue presence of low-K(m) hexokinase-I in β-cells of hyperfunctional islets only. In one case, an activating mutation of glucokinase (I211F) was found in pathological islets only. Both abnormalities, attributed to somatic genetic events, may account for inappropriate IS at low glucose levels by a subset of β-cells. They represent a novel cause of focal congenital hyperinsulinism.Diabetes 12/2012; · 8.29 Impact Factor -
Article: Amplification of insulin secretion by acetylcholine or phorbol ester is independent of β-cell microfilaments and distinct from metabolic amplification.
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ABSTRACT: Insulin secretion (IS) triggered by β-cell [Ca(2+)](c) is amplified by metabolic and receptor-generated signals. Diacylglycerol largely mediates acetylcholine (ACh) effects through protein-kinase C and other effectors, which can be directly activated by phorbol-ester (PMA). Using mouse islets, we investigated the possible role of microfilaments in ACh/PMA-mediated amplification of IS. PMA had no steady-state impact on actin microfilaments. Although ACh slightly augmented and PMA diminished glucose- and tolbutamide-induced increases in β-cell [Ca(2+)](c), both amplified IS in control islets and after microfilament disruption (latrunculin) or stabilization (jasplakinolide). Both phases of IS were larger in response to glucose than tolbutamide, although [Ca(2+)](c) was lower. This difference in secretion, which reflects metabolic amplification, persisted in presence of ACh/PMA and was independent of microfilaments. Amplification of IS by ACh/PMA is thus distinct from metabolic amplification, but both pathways promote acquisition of release competence by insulin granules, which can access exocytotic sites without intervention of microfilaments.Molecular and Cellular Endocrinology 12/2012; · 4.19 Impact Factor -
Article: cAMP-Mediated and Metabolic Amplification of Insulin Secretion Are Distinct Pathways Sharing Independence of β-Cell Microfilaments.
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ABSTRACT: Insulin secretion is triggered by an increase in the cytosolic calcium concentration ([Ca(2+)](c)) in β-cells. Ca(2+)-induced exocytosis of insulin granules can be augmented by metabolic amplification (unknown signals generated through glucose metabolism) or neurohormonal amplification (in particular cAMP mediated). Functional actin microfilaments are not required for metabolic amplification, but their possible role in cAMP-mediated amplification is unknown. It is also uncertain whether cAMP (generated in response to glucose) is implicated in metabolic amplification. These questions were addressed using isolated mouse islets. cAMP levels were increased by phosphodiesterase inhibition (with isobutylmethylxanthine) and adenylate-cyclase stimulation (with forskolin or glucagon-like peptide-1, 7-36 amide). Raising cAMP levels had no steady-state impact on actin polymerization in control islets. Neither disruption (depolymerization by latrunculin) nor stabilization (polymerization by jasplakinolide) of actin microfilaments was counteracted by cAMP. Both changes increased both phases of glucose- or tolbutamide-induced insulin secretion but did not prevent further amplification by cAMP. These large changes in secretion were not caused by changes in [Ca(2+)](c), which was only slightly increased by cAMP. Both phases of insulin secretion were larger in response to glucose than tolbutamide, although [Ca(2+)](c) was lower. This difference in secretion, which reflects metabolic amplification, was independent of microfilaments, was not attributable to differences in cAMP, and persisted in presence of dibutyryl-cAMP or when cAMP levels were variably raised by isobutylmethylxanthine + forskolin or glucagon-like peptide-1, 7-36 amide. We conclude that metabolic and cAMP-mediated amplification of insulin secretion are distinct pathways that accelerate acquisition of release competence by insulin granules that can access exocytotic sites without intervention of microfilaments.Endocrinology 09/2012; 153(10):4644-54. · 4.46 Impact Factor -
Article: Developmental defects and rescue from glucose intolerance of a catalytically-inactive novel Ship2 mutant mouse.
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ABSTRACT: The function of the phosphoinositide 5-phosphatase Ship2 was investigated in a new mouse model expressing a germline catalytically-inactive Ship2(∆/∆) mutant protein. Ship2(∆/∆) mice were viable with defects in somatic growth and in development of muscle, adipose tissue and female genital tract. Lipid metabolism and insulin secretion were also affected in these mice, but glucose tolerance, insulin sensitivity and insulin-induced PKB phosphorylation were not. We expected that the expression of the catalytically inactive Ship2 protein in PI 3'-kinase-defective p110α(D933A/+) mice would counterbalance the phenotypes of parental mice by restoring normal PKB signaling but, for most of the parameters tested, this was not the case. Indeed, often, the Ship2(∆/∆) phenotype had a dominant effect over the p110α(D933A/+) phenotype and, sometimes, there was a surprising additive effect of both mutations. p110α(D933A/+)Ship2(∆/∆) mice still displayed a reduced PKB phosphorylation in response to insulin, compared to wild type mice yet had a normal glucose tolerance and insulin sensitivity, like the Ship2(∆/∆) mice. Together, our results suggest that the Ship2(∆/∆) phenotype is not dependent on an overstimulated class I PI 3-kinase-PKB signaling pathway and thus, indirectly, that it may be more dependent on the lack of Ship2-produced phosphatidylinositol 3,4-bisphosphate and derived phosphoinositides.Cellular signalling 06/2012; 24(11):1971-80. · 4.09 Impact Factor -
Article: Disruption and stabilization of β-cell actin microfilaments differently influence insulin secretion triggered by intracellular Ca2+ mobilization or store-operated Ca2+ entry.
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ABSTRACT: Latrunculin depolymerizes and jasplakinolide polymerizes β-cell actin microfilaments. Both increase insulin secretion when Ca(2+) enters β-cells during depolarization by glucose, sulfonylureas or potassium. Mouse islets were held hyperpolarized with diazoxide, and stimulated with acetylcholine to test the role of microfilaments in insulin secretion triggered by intracellular Ca(2+) mobilization and store-operated Ca(2+) entry (SOCE). Jasplakinolide slightly attenuated Ca(2+) mobilization and did not affect SOCE, but consistently inhibited the attending insulin secretion. Latrunculin did not affect Ca(2+) changes induced by acetylcholine, but consistently increased insulin secretion, its effect being larger in response to Ca(2+) entry than to Ca(2+) mobilization. Microfilaments have thus a distinct impact on exocytosis of insulin granules depending on the source of triggering Ca(2+).FEBS letters 12/2011; 586(1):89-95. · 3.54 Impact Factor -
Article: In vitro insulin secretion by pancreatic tissue from infants with diazoxide-resistant congenital hyperinsulinism deviates from model predictions.
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ABSTRACT: Congenital hyperinsulinism (CHI) is the major cause of persistent neonatal hypoglycemia. CHI most often occurs due to mutations in the ABCC8 (which encodes sulfonylurea receptor 1) or KCNJ11 (which encodes the potassium channel Kir6.2) gene, which result in a lack of functional KATP channels in pancreatic β cells. Diffuse forms of CHI (DiCHI), in which all β cells are abnormal, often require subtotal pancreatectomy, whereas focal forms (FoCHI), which are characterized by localized hyperplasia of abnormal β cells, can be cured by resection of the lesion. Here, we characterized the in vitro kinetics of insulin secretion by pancreatic fragments from 6 DiCHI patients and by focal lesion and normal adjacent pancreas from 18 FoCHI patients. Responses of normal pancreas were similar to those reported for islets from adult organ donors. Compared with normal pancreas, basal insulin secretion was elevated in both FoCHI and DiCHI tissue. Affected tissues were heterogeneous in their secretory responses, with increased glucose levels often producing a rapid increase in insulin secretion that could be followed by a paradoxical decrease below prestimulatory levels. The KATP channel blocker tolbutamide was consistently ineffective in stimulating insulin secretion; conversely, the KATP channel activator diazoxide often caused an unanticipated increase in insulin secretion. These observed alterations in secretory behavior were similar in focal lesion and DiCHI tissue, and independent of the specific mutation in ABCC8 or KCNJ11. They cannot be explained by classic models of β cell function. Our results provide insight into the excessive and sometimes paradoxical changes in insulin secretion observed in CHI patients with inactivating mutations of KATP channels.The Journal of clinical investigation 09/2011; 121(10):3932-42. · 15.39 Impact Factor -
Article: The dual control of insulin secretion by glucose involves triggering and amplifying pathways in β-cells.
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ABSTRACT: This review outlines the two pathways that interact in β-cells to ensure temporal and amplitude control of insulin secretion by glucose. The most well known triggering pathway involves the following steps: acceleration of glucose metabolism, closure of ATP-sensitive potassium channels, depolarization, influx of Ca(2+) through voltage-gated calcium channels, and a rise in the concentration of cytosolic ionized Ca(2+) that triggers exocytosis of insulin-containing granules. This classic sequence is, however, incomplete. Additional mechanisms, involving other channels, are necessarily implicated in the production of the triggering Ca(2+) signal. It is also clear that the effect of glucose on insulin secretion would be poor if Ca(2+)-induced exocytosis was not markedly augmented (approximately doubled) through a metabolic amplifying pathway, mechanistically distinct from neurohormonal amplifying pathways. This metabolic amplifying pathway is physiologically relevant for both phases of glucose-induced insulin secretion and for the potentiation, by glucose, of insulin secretion triggered by non-metabolized secretagogues (e.g. arginine). Three important challenges for future studies will be to identify the additional targets mediating control of the triggering Ca(2+) signal by glucose, to elucidate the cellular mechanisms of the metabolic amplifying pathway and to determine the contribution of each pathway in the alterations of insulin secretion in type 2 diabetic patients.Diabetes research and clinical practice 08/2011; 93 Suppl 1:S27-31. · 2.16 Impact Factor -
Article: Metabolic amplification of insulin secretion by glucose is independent of β-cell microtubules.
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ABSTRACT: Glucose-induced insulin secretion (IS) by β-cells is controlled by two pathways. The triggering pathway involves ATP-sensitive potassium (K(ATP)) channel-dependent depolarization, Ca(2+) influx, and rise in the cytosolic Ca(2+) concentration ([Ca(2+)](c)), which triggers exocytosis of insulin granules. The metabolic amplifying pathway augments IS without further increasing [Ca(2+)](c). After exclusion of the contribution of actin microfilaments, we here tested whether amplification implicates microtubule-dependent granule mobilization. Mouse islets were treated with nocodazole or taxol, which completely depolymerized and polymerized tubulin. They were then perifused to measure [Ca(2+)](c) and IS. Metabolic amplification was studied during imposed steady elevation of [Ca(2+)](c) by tolbutamide or KCl or by comparing [Ca(2+)](c) and IS responses to glucose and tolbutamide. Nocodazole did not alter [Ca(2+)](c) or IS changes induced by the three secretagogues, whereas taxol caused a small inhibition of IS that is partly ascribed to a decrease in [Ca(2+)](c). When [Ca(2+)](c) was elevated and controlled by KCl or tolbutamide, the amplifying action of glucose was unaffected by microtubule disruption or stabilization. Both phases of IS were larger in response to glucose than tolbutamide, although triggering [Ca(2+)](c) was lower. This difference, due to amplification, persisted in nocodazole- or taxol-treated islets, even when IS was augmented fourfold by microfilament disruption with cytochalasin B or latrunculin B. In conclusion, metabolic amplification rapidly augments first and second phases of IS independently of insulin granule translocation along microtubules. We therefore extend our previous proposal that it does not implicate the cytoskeleton but corresponds to acceleration of the priming process conferring release competence to insulin granules.AJP Cell Physiology 12/2010; 300(3):C697-706. · 3.54 Impact Factor -
Article: Metabolic amplifying pathway increases both phases of insulin secretion independently of beta-cell actin microfilaments.
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ABSTRACT: Two pathways control glucose-induced insulin secretion (IS) by beta-cells. The triggering pathway involves ATP-sensitive potassium (K(ATP)) channel-dependent depolarization, Ca(2+) influx, and a rise in the cytosolic Ca(2+) concentration ([Ca(2+)](c)), which triggers exocytosis of insulin granules. The metabolic amplifying pathway augments IS without further increasing [Ca(2+)](c). The underlying mechanisms are unknown. Here, we tested the hypothesis that amplification implicates actin microfilaments. Mouse islets were treated with latrunculin B and cytochalasin B to depolymerize actin or jasplakinolide to polymerize actin. They were then perifused to measure [Ca(2+)](c) and IS. Metabolic amplification was studied during imposed steady elevation of [Ca(2+)](c) by tolbutamide or KCl or by comparing the magnitude of [Ca(2+)](c) and IS changes produced by glucose and tolbutamide. Both actin polymerization and depolymerization augmented IS triggered by all stimuli without increasing (sometimes decreasing) [Ca(2+)](c), which indicates a predominantly inhibitory function of microfilaments in exocytosis at a step distal to [Ca(2+)](c) increase. When [Ca(2+)](c) was elevated and controlled by KCl or tolbutamide, the amplifying action of glucose was facilitated by actin depolymerization and unaffected by polymerization. Both phases of IS were larger in response to high-glucose than to tolbutamide in low-glucose, although triggering [Ca(2+)](c) was lower. This difference in IS, due to amplification, persisted when the IS rate was doubled by actin depolymerization or polymerization. In conclusion, metabolic amplification is rapid and influences the first as well as the second phase of IS. It is a late step of stimulus-secretion coupling, which does not require functional actin microfilaments and could correspond to acceleration of the priming process conferring release competence to insulin granules.AJP Cell Physiology 08/2010; 299(2):C389-98. · 3.54 Impact Factor -
Article: β-Cell research - A decade of rapid growth.
Diabetes Obesity and Metabolism 11/2009; 11 Suppl 4:iv-ix. · 3.38 Impact Factor -
Article: Alterations of insulin secretion from mouse islets treated with sulphonylureas: perturbations of Ca2+ regulation prevail over changes in insulin content
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ABSTRACT: To determine how pretreatment with sulphonylureas alters the β cell function, mouse islets were cultured (18–20 h) without (controls) or with (test) 0.01 μM glibenclamide. Acute responses to glucose were then determined in the absence of glibenclamide.Test islets were insensitive to drugs (sulphonylureas and diazoxide) acting on K+-ATP channels, and their [Ca2+]i was already elevated in the absence of stimulation.Insulin secretion was increased in the absence of glucose, and mainly stimulated between 0–10 instead of 7–20 mM glucose in controls. The maximum response was halved, but this difference disappeared after correction for the 45% decrease in the islet insulin content.The first phase of glucose-induced insulin secretion was abrogated because of a paradoxical decrease of the high basal [Ca2+]i in β cells. The second phase was preserved but occurred with little rise of [Ca2+]i. These abnormalities did not result from alterations of glucose metabolism (NADPH fluorescence).In islets cultured with 50 μM tolbutamide, glucose induced biphasic increases in [Ca2+]i and insulin secretion. The decrease in the secretory response was matched by the decrease in insulin content (45%) except at maximal glucose concentrations. Islets pretreated with tolbutamide, however, behaved like those cultured with glibenclamide if tolbutamide was also present during the acute functional tests.In conclusion, treatment with a low glibenclamide concentration causes long-lasting blockade of K+-ATP channels and rise of [Ca2+]i in β cells. Glucose-induced insulin secretion occurs at lower concentrations, is delayed and is largely mediated by a modulation of Ca2+ action on exocytosis. It is suggested that glucose regulation of insulin secretion mainly depends on a K+-ATP channel-independent pathway during in vivo sulphonylurea treatment.British Journal of Pharmacology (1999) 127, 1883–1891; doi:10.1038/sj.bjp.0702731British Journal of Pharmacology 01/2009; 127(8):1883 - 1891. · 4.41 Impact Factor -
Article: Glucose and pharmacological modulators of ATP-sensitive K+ channels control [Ca2+]c by different mechanisms in isolated mouse alpha-cells.
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ABSTRACT: We studied how glucose and ATP-sensitive K(+) (K(ATP)) channel modulators affect alpha-cell [Ca(2+)](c). GYY mice (expressing enhanced yellow fluorescent protein in alpha-cells) and NMRI mice were used. [Ca(2+)](c), the K(ATP) current (I(KATP), perforated mode) and cell metabolism [NAD(P)H fluorescence] were monitored in single alpha-cells and, for comparison, in single beta-cells. In 0.5 mmol/l glucose, [Ca(2+)](c) oscillated in some alpha-cells and was basal in the others. Increasing glucose to 15 mmol/l decreased [Ca(2+)](c) by approximately 30% in oscillating cells and was ineffective in the others. alpha-Cell I(KATP) was inhibited by tolbutamide and activated by diazoxide or the mitochondrial poison azide, as in beta-cells. Tolbutamide increased alpha-cell [Ca(2+)](c), whereas diazoxide and azide abolished [Ca(2+)](c) oscillations. Increasing glucose from 0.5 to 15 mmol/l did not change I(KATP) and NAD(P)H fluorescence in alpha-cells in contrast to beta-cells. The use of nimodipine showed that L-type Ca(2+) channels are the main conduits for Ca(2+) influx in alpha-cells. gamma-Aminobutyric acid and zinc did not decrease alpha-cell [Ca(2+)](c), and insulin, although lowering [Ca(2+)](c) very modestly, did not affect glucagon secretion. alpha-Cells display similarities with beta-cells: K(ATP) channels control Ca(2+) influx mainly through L-type Ca(2+) channels. However, alpha-cells have distinct features from beta-cells: Most K(ATP) channels are already closed at low glucose, glucose does not affect cell metabolism and I(KATP), and it slightly decreases [Ca(2+)](c). Hence, glucose and K(ATP) channel modulators exert distinct effects on alpha-cell [Ca(2+)](c). The direct small glucose-induced drop in alpha-cell [Ca(2+)](c) contributes likely only partly to the strong glucose-induced inhibition of glucagon secretion in islets.Diabetes 12/2008; 58(2):412-21. · 8.29 Impact Factor -
Article: Pancreatic beta-cell mass or beta-cell function? That is the question!
Diabetes Obesity and Metabolism 11/2008; 10 Suppl 4:1-4. · 3.38 Impact Factor -
Article: Glucose controls cytosolic Ca2+ and insulin secretion in mouse islets lacking adenosine triphosphate-sensitive K+ channels owing to a knockout of the pore-forming subunit Kir6.2.
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ABSTRACT: Glucose-induced insulin secretion is classically attributed to the cooperation of an ATP-sensitive potassium (K ATP) channel-dependent Ca2+ influx with a subsequent increase of the cytosolic free Ca2+ concentration ([Ca2+]c) (triggering pathway) and a K ATP channel-independent augmentation of secretion without further increase of [Ca2+]c (amplifying pathway). Here, we characterized the effects of glucose in beta-cells lacking K ATP channels because of a knockout (KO) of the pore-forming subunit Kir6.2. Islets from 1-yr and 2-wk-old Kir6.2KO mice were used freshly after isolation and after 18 h culture to measure glucose effects on [Ca2+]c and insulin secretion. Kir6.2KO islets were insensitive to diazoxide and tolbutamide. In fresh adult Kir6.2KO islets, basal [Ca2+]c and insulin secretion were marginally elevated, and high glucose increased [Ca2+]c only transiently, so that the secretory response was minimal (10% of controls) despite a functioning amplifying pathway (evidenced in 30 mm KCl). Culture in 10 mm glucose increased basal secretion and considerably improved glucose-induced insulin secretion (200% of controls), unexpectedly because of an increase in [Ca2+]c with modulation of [Ca2+]c oscillations. Similar results were obtained in 2-wk-old Kir6.2KO islets. Under selected conditions, high glucose evoked biphasic increases in [Ca2+]c and insulin secretion, by inducing K ATP channel-independent depolarization and Ca2+ influx via voltage-dependent Ca2+ channels. In conclusion, Kir6.2KO beta-cells down-regulate insulin secretion by maintaining low [Ca2+]c, but culture reveals a glucose-responsive phenotype mainly by increasing [Ca2+]c. The results support models implicating a K ATP channel-independent amplifying pathway in glucose-induced insulin secretion, and show that K ATP channels are not the only possible transducers of metabolic effects on the triggering Ca2+ signal.Endocrinology 10/2008; 150(1):33-45. · 4.46 Impact Factor -
Article: Insulin secretion in islets from mice with a double knockout for the dense core vesicle proteins islet antigen-2 (IA-2) and IA-2beta.
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ABSTRACT: Islet antigen-2 (IA-2 or ICA 512) and IA-2beta (or phogrin) are major autoantigens in type 1 diabetes. They are located in dense core secretory vesicles including insulin granules, but their role in beta-cell function is unclear. Targeted disruption of either IA-2 or IA-2beta, or both, impaired glucose tolerance, an effect attributed to diminution of insulin secretion. In this study, we therefore characterized the dynamic changes in cytosolic Ca2+([Ca2+](c)) and insulin secretion in islets from IA-2/IA-2beta double knockout (KO) mice. High glucose (15 mM) induced biphasic insulin secretion in IA-2/IA-2beta KO islets, with a similar first phase and smaller second phase compared with controls. Since the insulin content of IA-2/IA-2beta KO islets was approximately 45% less than that of controls, fractional insulin secretion (relative to content) was thus increased during first phase and unaffected during second phase. This peculiar response occurred in spite of a slightly smaller rise in [Ca2+](c), could not be attributed to an alteration of glucose metabolism (NADPH fluorescence) and also was observed with tolbutamide. The dual control of insulin secretion via the K(ATP) channel-dependent triggering pathway and K(ATP) channel-independent amplifying pathway was unaltered in IA-2/IA-2beta KO islets, and so were the potentiations by acetylcholine or cAMP (forskolin). Intriguingly, amino acids, in particular the cationic arginine and lysine, induced larger fractional insulin secretion in IA-2/IA-2beta KO than control islets. In conclusion, IA-2 and IA-2beta are dispensable for exocytosis of insulin granules, but are probably more important for cargo loading and/or stability of dense core vesicles.Journal of Endocrinology 04/2008; 196(3):573-81. · 3.55 Impact Factor -
Article: Of mighty mice and diabetic men: animal models of islet dysfunction.
Diabetes Obesity and Metabolism 12/2007; 9 Suppl 2:1-4. · 3.38 Impact Factor -
Article: The GluCre-ROSA26EYFP mouse: a new model for easy identification of living pancreatic alpha-cells.
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ABSTRACT: The control of glucagon secretion by pancreatic alpha-cells is poorly understood, largely because of the difficulty to recognize living alpha-cells. We describe a new mouse model, referred to as GluCre-ROSA26EYFP (or GYY), allowing easy alpha-cell identification because of specific expression of EYFP. GYY mice displayed normal glycemic control during a fasting/refeeding test or intraperitoneal insulin injection. Glucagon secretion by isolated islets was normally inhibited by glucose and stimulated by adrenaline. [Ca(2+)](c) responses to arginine, adrenaline, diazoxide and tolbutamide, were similar in GYY and control mice. Hence, this new mouse model is a reliable and powerful tool to specifically study alpha-cells.FEBS Letters 10/2007; 581(22):4235-40. · 3.54 Impact Factor -
Article: Glucose-induced cytosolic pH changes in beta-cells and insulin secretion are not causally related: studies in islets lacking the Na+/H+ exchangeR NHE1.
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ABSTRACT: The contribution of Na(+)/H(+) exchange (achieved by NHE proteins) to the regulation of beta-cell cytosolic pH(c), and the role of pH(c) changes in glucose-induced insulin secretion are disputed and were examined here. Using real-time PCR, we identified plasmalemmal NHE1 and intracellular NHE7 as the two most abundant NHE isoforms in mouse islets. We, therefore, compared insulin secretion, cytosolic free Ca(2+) ([Ca(2+)](c)) and pH(c) in islets from normal mice and mice bearing an inactivating mutation of NHE1 (Slc9A1-swe/swe). The experiments were performed in HCO(-)(3)/CO(2) or HEPES/NaOH buffers. PCR and functional approaches showed that NHE1 mutant islets do not express compensatory pH-regulating mechanisms. NHE1 played a greater role than HCO(-)(3)-dependent mechanisms in the correction of an acidification imposed by a pulse of NH(4)Cl. In contrast, basal pH(c) (in low glucose) and the alkalinization produced by high glucose were independent of NHE1. Dimethylamiloride, a classic blocker of Na(+)/H(+) exchange, did not affect pH(c) but increased insulin secretion in NHE1 mutant islets, indicating unspecific effects. In control islets, glucose similarly increased [Ca(2+)](c) and insulin secretion in HCO(-)(3) and HEPES buffer, although pH(c) changed in opposite directions. The amplification of insulin secretion that glucose produces when [Ca(2+)](c) is clamped at an elevated level by KCl was also unrelated to pH(c) and pH(c) changes. All effects of glucose on [Ca(2+)](c) and insulin secretion proved independent of NHE1. In conclusion, NHE1 protects beta-cells against strong acidification, but has no role in stimulus-secretion coupling. The changes in pH(c) produced by glucose involve HCO(-)(3)-dependent mechanisms. Variations in beta-cell pH(c) are not causally related to changes in insulin secretion.Journal of Biological Chemistry 09/2007; 282(34):24538-46. · 4.77 Impact Factor -
Article: Glucose-induced Cytosolic pH Changes in β-Cells and Insulin Secretion Are Not Causally Related
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ABSTRACT: The contribution of Na+/H+ exchange (achieved by NHE proteins) to the regulation of β-cell cytosolic pHc, and the role of pHc changes in glucose-induced insulin secretion are disputed and were examined here. Using real-time PCR, we identified plasmalemmal NHE1 and intracellular NHE7 as the two most abundant NHE isoforms in mouse islets. We, therefore, compared insulin secretion, cytosolic free Ca2+ ([Ca2+]c) and pHc in islets from normal mice and mice bearing an inactivating mutation of NHE1 (Slc9A1-swe/swe). The experiments were performed in HCO–3/CO2 or HEPES/NaOH buffers. PCR and functional approaches showed that NHE1 mutant islets do not express compensatory pH-regulating mechanisms. NHE1 played a greater role than HCO–3-dependent mechanisms in the correction of an acidification imposed by a pulse of NH4Cl. In contrast, basal pHc (in low glucose) and the alkalinization produced by high glucose were independent of NHE1. Dimethylamiloride, a classic blocker of Na+/H+ exchange, did not affect pHc but increased insulin secretion in NHE1 mutant islets, indicating unspecific effects. In control islets, glucose similarly increased [Ca2+]c and insulin secretion in HCO–3 and HEPES buffer, although pHc changed in opposite directions. The amplification of insulin secretion that glucose produces when [Ca2+]c is clamped at an elevated level by KCl was also unrelated to pHc and pHc changes. All effects of glucose on [Ca2+]c and insulin secretion proved independent of NHE1. In conclusion, NHE1 protects β-cells against strong acidification, but has no role in stimulus-secretion coupling. The changes in pHc produced by glucose involve HCO–3-dependent mechanisms. Variations in β-cell pHc are not causally related to changes in insulin secretion.Journal of Biological Chemistry 08/2007; 282(34):24538-24546. · 4.77 Impact Factor -
Article: Overnight culture unmasks glucose-induced insulin secretion in mouse islets lacking ATP-sensitive K+ channels by improving the triggering Ca2+ signal.
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ABSTRACT: A current model ascribes glucose-induced insulin secretion to the interaction of a triggering pathway (K(ATP) channel-dependent Ca(2+) influx and rise in cytosolic [Ca(2+)](c)) and an amplifying pathway (K(ATP) channel-independent augmentation of secretion without further increase of [Ca(2+)](c)). However, several studies of sulfonylurea receptor 1 null mice (Sur1KO) failed to measure significant effects of glucose in their islets lacking K(ATP) channels. We addressed this issue that challenges the model. Compared with controls, fresh Sur1KO islets showed slightly elevated basal [Ca(2+)](c) and insulin secretion. In 15 mm glucose, the absolute rate of secretion was approximately 3-fold lower in Sur1KO than control islets, with only poor increase above base line. Overnight culture of Sur1KO islets in 10 mm glucose (not in 5 mm) augmented basal insulin secretion and considerably improved the response to 15 mm glucose, which reached higher values than in control islets, in which culture had little impact. Glucose stimulation during KCl depolarization showed that the amplifying pathway is functional in fresh and cultured Sur1KO islets. The differences in insulin secretion between fresh and cultured Sur1KO islets and between Sur1KO and control islets were not attributable to differences in insulin content, glucose oxidation rate, or synchronization of [Ca(2+)](c) oscillations. The unmasking of glucose-induced insulin secretion in beta-cells lacking K(ATP) channels is paradoxically due to improvement in the production of a triggering signal (elevated [Ca(2+)](c)). The results show that K(ATP) channels are not the only transducer of glucose effects on [Ca(2+)](c) in beta-cells. They explain controversies in the literature and refute arguments raised against the model implicating an amplifying pathway in glucose-induced insulin secretion.Journal of Biological Chemistry 05/2007; 282(20):14768-76. · 4.77 Impact Factor
Top co-authors
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Institutions
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1997–2012
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Université Catholique de Louvain
- Institute of Experimental and Clinical Research (IREC)
Louvain-la-Neuve, WAL, Belgium
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2003–2011
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Vrije Universiteit Brussel
- • Department of Metabolism and Endocrinology
- • Diabetes Research Center
Brussels, BRU, Belgium
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2002–2004
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University Hospital Brussels
Brussels, BRU, Belgium
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