3D evaluation of the extracellular matrix of hypoxic pancreatic islets using light sheet fluorescence microscopy

Pancreatic islet transplantation is a promising treatment for type 1 diabetes, but the survival and function of transplanted islets are hindered by the loss of extracellular matrix (ECM) during islet isolation and by low oxygenation upon implantation. This study aimed to evaluate the impact of hypoxia on ECM using a cutting-edge imaging approach based on tissue clearing and 3D microscopy. Human and rat islets were cultured under normoxic (O2 21%) or hypoxic (O2 1%) conditions. Immunofluorescence staining targeting insulin, glucagon, CA9 (a hypoxia marker), ECM proteins (collagen 4, fibronectin, laminin), and E-cadherin (intercellular adhesion protein) was performed on fixed whole islets. The cleared islets were imaged using Light Sheet Fluorescence Microscopy (LSFM) and digitally analyzed. The volumetric analysis of target proteins did not show significant differences in abundance between the experimental groups. However, 3D projections revealed distinct morphological features that differentiated normoxic and hypoxic islets. Under normoxic conditions, ECM could be found throughout the islets. Hypoxic islets exhibited areas of scattered nuclei and central clusters of ECM proteins, indicating central necrosis. E-cadherin was absent in these areas. Our results, demonstrating a diminution of islets' functional mass in hypoxia, align with the functional decline observed in transplanted islets experiencing low oxygenation after grafting. This study provides a methodology combining tissue clearing, multiplex immunofluorescence, Light Sheet Fluorescence Microscopy, and digital image analysis to investigate pancreatic islet morphology. This 3D approach allowed us to highlight ECM organizational changes during hypoxia from a morphological perspective.

GLP-1 and GIP receptors signal through distinct β-arrestin 2-dependent pathways to regulate pancreatic β cell function

Glucagon-like peptide 1 (GLP-1R) and glucose-dependent insulinotropic polypeptide (GIPR) receptors are G-protein-coupled receptors involved in glucose homeostasis. Diabetogenic conditions decrease β-arrestin 2 (ARRB2) levels in human islets. In mouse β cells, ARRB2 dampens insulin secretion by partially uncoupling cyclic AMP (cAMP)/protein kinase A (PKA) signaling at physiological doses of GLP-1, whereas at pharmacological doses, the activation of extracellular signal-related kinase (ERK)/cAMP-responsive element-binding protein (CREB) requires ARRB2. In contrast, GIP-potentiated insulin secretion needs ARRB2 in mouse and human islets. The GIPR-ARRB2 axis is not involved in cAMP/PKA or ERK signaling but does mediate GIP-induced F-actin depolymerization. Finally, the dual GLP-1/GIP agonist tirzepatide does not require ARRB2 for the potentiation of insulin secretion. Thus, ARRB2 plays distinct roles in regulating GLP-1R and GIPR signaling, and we highlight (1) its role in the physiological context and the possible functional consequences of its decreased expression in pathological situations such as diabetes and (2) the importance of assessing the signaling pathways engaged by the agonists (biased/dual) for therapeutic purposes.

ER stress increases expression of intracellular calcium channel RyR1 to modify Ca2+ homeostasis in pancreatic beta cells

Pancreatic beta cells maintain glucose homeostasis by secreting pulses of insulin in response to a rise in plasma glucose. Pulsatile insulin secretion occurs as a result of glucose-induced oscillations in beta-cell cytosolic Ca2+. The endoplasmic reticulum (ER) helps regulate beta-cell cytosolic Ca2+, and ER stress can lead to ER Ca2+ reduction, beta-cell dysfunction, and an increased risk of type 2 diabetes. However, the mechanistic effects of ER stress on individual calcium channels are not well understood. To determine the effects of tunicamycin-induced ER stress on ER inositol 1,4,5-triphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) and their involvement in subsequent Ca2+ dysregulation, we treated INS-1 832/13 cells and primary mouse islets with ER stress inducer tunicamycin (TM). We showed TM treatment increased RyR1 mRNA without affecting RyR2 mRNA and decreased both IP3R1 and IP3R3 mRNA. Furthermore, we found stress reduced ER Ca2+ levels, triggered oscillations in cytosolic Ca2+ under subthreshold glucose conditions, and increased apoptosis and that these changes were prevented by cotreatment with the RyR1 inhibitor dantrolene. In addition, we demonstrated silencing RyR1-suppressed TM-induced subthreshold cytosolic Ca2+ oscillations, but silencing RyR2 did not affect these oscillations. In contrast, inhibiting IP3Rs with xestospongin-C failed to suppress the TM-induced cytosolic Ca2+ oscillations and did not protect beta cells from TM-induced apoptosis although xestospongin-C inclusion did prevent ER Ca2+ reduction. Taken together, these results show changes in RyR1 play a critical role in ER stress-induced Ca2+ dysfunction and beta-cell apoptosis.

GLP-1R agonists demonstrate potential to treat Wolfram syndrome in human preclinical models

AIMS/HYPOTHESIS

Wolfram syndrome is a rare autosomal recessive disorder caused by pathogenic variants in the WFS1 gene. It is characterised by insulin-dependent diabetes mellitus, optic nerve atrophy, diabetes insipidus, hearing loss and neurodegeneration. Considering the unmet treatment need for this orphan disease, this study aimed to evaluate the therapeutic potential of glucagon-like peptide 1 receptor (GLP-1R) agonists under wolframin (WFS1) deficiency with a particular focus on human beta cells and neurons.

METHODS

The effect of the GLP-1R agonists dulaglutide and exenatide was examined in Wfs1 knockout mice and in an array of human preclinical models of Wolfram syndrome, including WFS1-deficient human beta cells, human induced pluripotent stem cell (iPSC)-derived beta-like cells and neurons from control individuals and individuals affected by Wolfram syndrome, and humanised mice.

RESULTS

Our study shows that the long-lasting GLP-1R agonist dulaglutide reverses impaired glucose tolerance in WFS1-deficient mice, and that exenatide and dulaglutide improve beta cell function and prevent apoptosis in different human WFS1-deficient models including iPSC-derived beta cells from people with Wolfram syndrome. Exenatide improved mitochondrial function, reduced oxidative stress and prevented apoptosis in Wolfram syndrome iPSC-derived neural precursors and cerebellar neurons.

CONCLUSIONS/INTERPRETATION

Our study provides novel evidence for the beneficial effect of GLP-1R agonists on WFS1-deficient human pancreatic beta cells and neurons, suggesting that these drugs may be considered as a treatment for individuals with Wolfram syndrome.

Chemerin as an Inducer of β Cell Proliferation Mediates Mitochondrial Homeostasis and Promotes β Cell Mass Expansion

Loss of the β cell population is a crucial feature of type 2 diabetes. Restoring the β cell mass by stimulating β cell proliferation and preventing its apoptosis was proposed as a therapeutic approach to treating diabetes. Therefore, researchers have been increasingly interested in identifying exogenous factors that can stimulate β cell proliferation in situ and in vitro. Adipokine chemerin, which is secreted from adipose tissue and the liver, has been identified as a chemokine that plays a critical role in the regulation of metabolism. In this study, we demonstrate that chemerin as a circulating adipokine promotes β cell proliferation in vivo and in vitro. Chemerin serum levels and the expression of the main receptors within islets are highly regulated under a variety of challenging conditions, including obesity and type 2 diabetes. As compared to their littermates, mice overexpressing chemerin had a larger islet area and increased β cell mass with both a normal and high-fat diet. Moreover, in chemerin-overexpressed mice, we observed improved mitochondrial homeostasis and increased insulin synthesis. In summary, our findings confirm the potential role of chemerin as an inducer of β cell proliferation, and they provide novel insights into the helpful strategy to expand β cell population.

Prolonged culture of human pancreatic islets under glucotoxic conditions changes their acute beta cell calcium and insulin secretion glucose response curves from sigmoid to bell-shaped

AIMS/HYPOTHESIS

The rapid remission of type 2 diabetes by a diet very low in energy correlates with a marked improvement in glucose-stimulated insulin secretion (GSIS), emphasising the role of beta cell dysfunction in the early stages of the disease. In search of novel mechanisms of beta cell dysfunction after long-term exposure to mild to severe glucotoxic conditions, we extensively characterised the alterations in insulin secretion and upstream coupling events in human islets cultured for 1-3 weeks at ~5, 8, 10 or 20 mmol/l glucose and subsequently stimulated by an acute stepwise increase in glucose concentration.

METHODS

Human islets from 49 non-diabetic donors (ND-islets) and six type 2 diabetic donors (T2D-islets) were obtained from five isolation centres. After shipment, the islets were precultured for 3-7 days in RPMI medium containing ~5 mmol/l glucose and 10% (vol/vol) heat-inactivated FBS with selective islet picking at each medium renewal. Islets were then cultured for 1-3 weeks in RPMI containing ~5, 8, 10 or 20 mmol/l glucose before measurement of insulin secretion during culture, islet insulin and DNA content, beta cell apoptosis and cytosolic and mitochondrial glutathione redox state, and assessment of dynamic insulin secretion and upstream coupling events during acute stepwise stimulation with glucose [NAD(P)H autofluorescence, ATP/(ATP+ADP) ratio, electrical activity, cytosolic Ca²⁺ concentration ([Ca²⁺]_c)].

RESULTS

Culture of ND-islets for 1-3 weeks at 8, 10 or 20 vs 5 mmol/l glucose did not significantly increase beta cell apoptosis or oxidative stress but decreased insulin content in a concentration-dependent manner and increased beta cell sensitivity to subsequent acute stimulation with glucose. Islet glucose responsiveness was higher after culture at 8 or 10 vs 5 mmol/l glucose and markedly reduced after culture at 20 vs 5 mmol/l glucose. In addition, the [Ca²⁺]_c and insulin secretion responses to acute stepwise stimulation with glucose were no longer sigmoid but bell-shaped, with maximal stimulation at 5 or 10 mmol/l glucose and rapid sustained inhibition above that concentration. Such paradoxical inhibition was, however, no longer observed when islets were acutely depolarised by 30 mmol/l extracellular K⁺. The glucotoxic alterations of beta cell function were fully reversible after culture at 5 mmol/l glucose and were mimicked by pharmacological activation of glucokinase during culture at 5 mmol/l glucose. Similar results to those seen in ND-islets were obtained in T2D-islets, except that their rate of insulin secretion during culture at 8 and 20 mmol/l glucose was lower, their cytosolic glutathione oxidation increased after culture at 8 and 20 mmol/l glucose, and the alterations in GSIS and upstream coupling events were greater after culture at 8 mmol/l glucose.

CONCLUSIONS/INTERPRETATION

Prolonged culture of human islets under moderate to severe glucotoxic conditions markedly increased their glucose sensitivity and revealed a bell-shaped acute glucose response curve for changes in [Ca²⁺]_c and insulin secretion, with maximal stimulation at 5 or 10 mmol/l glucose and rapid inhibition above that concentration. This novel glucotoxic alteration may contribute to beta cell dysfunction in type 2 diabetes independently from a detectable increase in beta cell apoptosis.

In depth functional characterization of human induced pluripotent stem cell-derived beta cells in vitro and in vivo

In vitro differentiation of human induced pluripotent stem cells (iPSCs) into beta cells represents an important cell source for diabetes research. Here, we fully characterized iPSC-derived beta cell function in vitro and in vivo in humanized mice. Using a 7-stage protocol, human iPSCs were differentiated into islet-like aggregates with a yield of insulin-positive beta cells comparable to that of human islets. The last three stages of differentiation were conducted with two different 3D culture systems, rotating suspension or static microwells. In the latter, homogeneously small-sized islet-like aggregates were obtained, while in rotating suspension size was heterogeneous and aggregates often clumped. In vitro function was assessed by glucose-stimulated insulin secretion, NAD(P)H and calcium fluctuations. Stage 7 aggregates slightly increased insulin release in response to glucose in vitro. Aggregates were transplanted under the kidney capsule of NOD-SCID mice to allow for further in vivo beta cell maturation. In transplanted mice, grafts showed glucose-responsiveness and maintained normoglycemia after streptozotocin injection. In situ kidney perfusion assays showed modulation of human insulin secretion in response to different secretagogues. In conclusion, iPSCs differentiated with equal efficiency into beta cells in microwells compared to rotating suspension, but the former had a higher experimental success rate. In vitro differentiation generated aggregates lacking fully mature beta cell function. In vivo, beta cells acquired the functional characteristics typical of human islets. With this technology an unlimited supply of islet-like organoids can be generated from human iPSCs that will be instrumental to study beta cell biology and dysfunction in diabetes.

Glucose inhibits glucagon secretion by decreasing [Ca2+]c and by reducing the efficacy of Ca2+ on exocytosis via somatostatin-dependent and independent mechanisms

Objective

Methods

Results

Conclusions

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Glucose modulates [Ca²⁺]_c in α-cells within islets but not in dispersed α-cells.

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In α-cells within islets, it decreases [Ca²⁺]_c independently of their K_ATP channels.

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It decreases α-cell [Ca²⁺]_c partly via somatostatin.

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All glucose-induced [Ca²⁺]_c changes trigger parallel changes in glucagon release.

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Glucose also decreases the efficacy of Ca²⁺ on exocytosis (attenuating pathway).

The endoplasmic reticulum-plasma membrane tethering protein TMEM24 is a regulator of cellular Ca2+ homeostasis

Endoplasmic reticulum (ER)–plasma membrane (PM) contacts are sites of lipid exchange and Ca²⁺ transport, and both lipid transport proteins and Ca²⁺ channels specifically accumulate at these locations. In pancreatic β-cells, both lipid and Ca²⁺ signaling are essential for insulin secretion. The recently characterized lipid transfer protein TMEM24 (also known as C2CD2L) dynamically localizes to ER–PM contact sites and provides phosphatidylinositol, a precursor of phosphatidylinositol-4-phosphate [PI(4)P] and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂], to the PM. β-cells lacking TMEM24 exhibit markedly suppressed glucose-induced Ca²⁺ oscillations and insulin secretion, but the underlying mechanism is not known. We now show that TMEM24 only weakly interacts with the PM, and dissociates in response to both diacylglycerol and nanomolar elevations of cytosolic Ca²⁺. Loss of TMEM24 results in hyper-accumulation of Ca²⁺ in the ER and in excess Ca²⁺ entry into mitochondria, with resulting impairment in glucose-stimulated ATP production.

Summary: TMEM24 reversibly localizes to ER–PM contact sites and participates in the regulation of both ER and mitochondrial Ca²⁺ homeostasis and in glucose-dependent ATP production in insulin-secreting cells.

LDHA is enriched in human islet alpha cells and upregulated in type 2 diabetes

The lactate dehydrogenase isoform A (LDHA) is a key metabolic enzyme that preferentially catalyzes the conversion of pyruvate to lactate. Whereas LDHA is highly expressed in many tissues, its expression is turned off in the differentiated adult β-cell within the pancreatic islets. The repression of LDHA under normal physiological condition and its inappropriate upregulation under a diabetogenic environment is well-documented in rodent islets/β-cells but little is known about LDHA expression in human islet cells and whether its abundance is altered under diabetic conditions. Analysis of public single-cell RNA-seq (sc-RNA seq) data as well as cell type-specific immunolabeling of human pancreatic islets showed that LDHA was mainly localized in human α-cells while it is expressed at a very low level in β-cells. Furthermore, LDHA, both at mRNA and protein, as well as lactate production is upregulated in human pancreatic islets exposed to chronic high glucose treatment. Microscopic analysis of stressed human islets and autopsy pancreases from individuals with type 2 diabetes (T2D) showed LDHA upregulation mainly in human α-cells. Pharmacological inhibition of LDHA in isolated human islets enhanced insulin secretion under physiological conditions but did not significantly correct the deregulated secretion of insulin or glucagon under diabetic conditions.

Inhibition of aquaporin-1 prevents myocardial remodeling by blocking the transmembrane transport of hydrogen peroxide

Pathological remodeling of the myocardium has long been known to involve oxidant signaling, but strategies using systemic antioxidants have generally failed to prevent it. We sought to identify key regulators of oxidant-mediated cardiac hypertrophy amenable to targeted pharmacological therapy. Specific isoforms of the aquaporin water channels have been implicated in oxidant sensing, but their role in heart muscle is unknown. RNA sequencing from human cardiac myocytes revealed that the archetypal AQP1 is a major isoform. AQP1 expression correlates with the severity of hypertrophic remodeling in patients with aortic stenosis. The AQP1 channel was detected at the plasma membrane of human and mouse cardiac myocytes from hypertrophic hearts, where it colocalized with NADPH oxidase-2 and caveolin-3. We show that hydrogen peroxide (H₂O₂), produced extracellularly, is necessary for the hypertrophic response of isolated cardiac myocytes and that AQP1 facilitates the transmembrane transport of H₂O₂ through its water pore, resulting in activation of oxidant-sensitive kinases in cardiac myocytes. Structural analysis of the amino acid residues lining the water pore of AQP1 supports its permeation by H₂O₂ Deletion of Aqp1 or selective blockade of the AQP1 intrasubunit pore inhibited H₂O₂ transport in mouse and human cells and rescued the myocyte hypertrophy in human induced pluripotent stem cell-derived engineered heart muscle. Treatment of mice with a clinically approved AQP1 inhibitor, Bacopaside, attenuated cardiac hypertrophy. We conclude that cardiac hypertrophy is mediated by the transmembrane transport of H₂O₂ by the water channel AQP1 and that inhibitors of AQP1 represent new possibilities for treating hypertrophic cardiomyopathies.

KATP channel blockers control glucagon secretion by distinct mechanisms: A direct stimulation of α-cells involving a [Ca2+]c rise and an indirect inhibition mediated by somatostatin

Objective

Glucagon is secreted by pancreatic α-cells in response to hypoglycemia and its hyperglycemic effect helps to restore normal blood glucose. Insulin and somatostatin (SST) secretions from β- and δ-cells, respectively, are stimulated by glucose by mechanisms involving an inhibition of their ATP-sensitive K⁺ (K_ATP) channels, leading to an increase in [Ca²⁺]_c that triggers exocytosis. Drugs that close K_ATP channels, such as sulfonylureas, are used to stimulate insulin release in type 2 diabetic patients. α-cells also express K_ATP channels. However, the mechanisms by which sulfonylureas control glucagon secretion are still largely debated and were addressed in the present study. In particular, we studied the effects of K_ATP channel blockers on α-cell [Ca²⁺]_c and glucagon secretion in the presence of a low (1 mM) or a high (15 mM) glucose concentration and evaluated the role of SST in these effects.

Methods

Using a transgenic mouse model expressing the Ca²⁺-sensitive fluorescent protein, GCaMP6f, specifically in α-cells, we measured [Ca²⁺]_c in α-cells either dispersed or within whole islets (by confocal microscopy). By measuring [Ca²⁺]_c in α-cells within islets and glucagon secretion using the same perifusion protocols, we tested whether glucagon secretion correlated with changes in [Ca²⁺]_c in response to sulfonylureas. We studied the role of SST in the effects of sulfonylureas using multiple approaches including genetic ablation of SST, or application of SST-14 and SST receptor antagonists.

Results

Application of the sulfonylureas, tolbutamide, or gliclazide, to a medium containing 1 mM or 15 mM glucose increased [Ca²⁺]_c in α-cells by a direct effect as in β-cells. At low glucose, sulfonylureas inhibited glucagon secretion of islets despite the rise in α-cell [Ca²⁺]_c that they triggered. This glucagonostatic effect was indirect and attributed to SST because, in the islets of SST-knockout mice, sulfonylureas induced a stimulation of glucagon secretion which correlated with an increase in α-cell [Ca²⁺]_c. Experiments with exogenous SST-14 and SST receptor antagonists indicated that the glucagonostatic effect of sulfonylureas mainly resulted from an inhibition of the efficacy of cytosolic Ca²⁺ on exocytosis. Although SST-14 was also able to inhibit glucagon secretion by decreasing α-cell [Ca²⁺]_c, no decrease in [Ca²⁺]_c occurred during sulfonylurea application because it was largely counterbalanced by the direct stimulatory effect of these drugs on α-cell [Ca²⁺]_c. At high glucose, i.e., in conditions where glucagon release was already low, sulfonylureas stimulated glucagon secretion because their direct stimulatory effect on α-cells exceeded the indirect effect by SST. Our results also indicated that, unexpectedly, SST-14 poorly decreased the efficacy of Ca²⁺ on exocytosis in β-cells.

Conclusions

Sulfonylureas exert two opposite actions on α-cells: a direct stimulation as in β-cells and an indirect inhibition by SST. This suggests that any alteration of SST paracrine influence, as described in diabetes, will modify the effect of sulfonylureas on glucagon release. In addition, we suggest that δ-cells inhibit α-cells more efficiently than β-cells.

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K_ATP channel blockers control glucagon secretion by two mechanisms.

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The first one is the direct stimulation of α-cell by a [Ca²⁺]_c rise, as in β-cells.

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The second one is an indirect inhibition mediated by δ-cells releasing somatostatin.

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Somatostatin mainly reduces the efficacy of Ca²⁺ on exocytosis in α-cells.

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Somatostatin more potently inhibits glucagon than insulin secretion.

SGLT2 is not expressed in pancreatic α- and β-cells, and its inhibition does not directly affect glucagon and insulin secretion in rodents and humans

Objective

Sodium-glucose cotransporter 2 (SGLT2) inhibitors (SGLT2i), or gliflozins, are anti-diabetic drugs that lower glycemia by promoting glucosuria, but they also stimulate endogenous glucose and ketone body production. The likely causes of these metabolic responses are increased blood glucagon levels, and decreased blood insulin levels, but the mechanisms involved are hotly debated. This study verified whether or not SGLT2i affect glucagon and insulin secretion by a direct action on islet cells in three species, using multiple approaches.

Methods

We tested the in vivo effects of two selective SGLT2i (dapagliflozin, empagliflozin) and a SGLT1/2i (sotagliflozin) on various biological parameters (glucosuria, glycemia, glucagonemia, insulinemia) in mice. mRNA expression of SGLT2 and other glucose transporters was assessed in rat, mouse, and human FACS-purified α- and β-cells, and by analysis of two human islet cell transcriptomic datasets. Immunodetection of SGLT2 in pancreatic tissues was performed with a validated antibody. The effects of dapagliflozin, empagliflozin, and sotagliflozin on glucagon and insulin secretion were assessed using isolated rat, mouse and human islets and the in situ perfused mouse pancreas. Finally, we tested the long-term effect of SGLT2i on glucagon gene expression.

Results

SGLT2 inhibition in mice increased the plasma glucagon/insulin ratio in the fasted state, an effect correlated with a decline in glycemia. Gene expression analyses and immunodetections showed no SGLT2 mRNA or protein expression in rodent and human islet cells, but moderate SGLT1 mRNA expression in human α-cells. However, functional experiments on rat, mouse, and human (29 donors) islets and the in situ perfused mouse pancreas did not identify any direct effect of dapagliflozin, empagliflozin or sotagliflozin on glucagon and insulin secretion. SGLT2i did not affect glucagon gene expression in rat and human islets.

Conclusions

The data indicate that the SGLT2i-induced increase of the plasma glucagon/insulin ratio in vivo does not result from a direct action of the gliflozins on islet cells.

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Gliflozins (SGLT2 and SGLT1/2 inhibitors) increase plasma glucagon/insulin ratio.

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SGLT2 is not expressed in rodent and human pancreatic α- and β-cells.

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SGLT1 is however expressed in human α-cells.

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SGLT2 and SGLT1/2 inhibitors do not directly affect glucagon and insulin secretion.

γ-Hydroxybutyrate does not mediate glucose inhibition of glucagon secretion

Hypersecretion of glucagon from pancreatic α-cells strongly contributes to diabetic hyperglycemia. Moreover, failure of α-cells to increase glucagon secretion in response to falling blood glucose concentrations compromises the defense against hypoglycemia, a common complication in diabetes therapy. However, the mechanisms underlying glucose regulation of glucagon secretion are poorly understood and likely involve both α-cell-intrinsic and intraislet paracrine signaling. Among paracrine factors, glucose-stimulated release of the GABA metabolite γ-hydroxybutyric acid (GHB) from pancreatic β-cells might mediate glucose suppression of glucagon release via GHB receptors on α-cells. However, the direct effects of GHB on α-cell signaling and glucagon release have not been investigated. Here, we found that GHB (4-10 μm) lacked effects on the cytoplasmic concentrations of the secretion-regulating messengers Ca2+ and cAMP in mouse α-cells. Glucagon secretion from perifused mouse islets was also unaffected by GHB at both 1 and 7 mm glucose. The GHB receptor agonist 3-chloropropanoic acid and the antagonist NCS-382 had no effects on glucagon secretion and did not affect stimulation of secretion induced by a drop in glucose from 7 to 1 mm Inhibition of endogenous GHB formation with the GABA transaminase inhibitor vigabatrin also failed to influence glucagon secretion at 1 mm glucose and did not prevent the suppressive effect of 7 mm glucose. In human islets, GHB tended to stimulate glucagon secretion at 1 mm glucose, an effect mimicked by 3-chloropropanoic acid. We conclude that GHB does not mediate the inhibitory effect of glucose on glucagon secretion.

The Role of α-Cells in Islet Function and Glucose Homeostasis in Health and Type 2 Diabetes

Pancreatic α-cells are the major source of glucagon, a hormone that counteracts the hypoglycemic action of insulin and strongly contributes to the correction of acute hypoglycemia. The mechanisms by which glucose controls glucagon secretion are hotly debated, and it is still unclear to what extent this control results from a direct action of glucose on α-cells or is indirectly mediated by β- and/or δ-cells. Besides its hyperglycemic action, glucagon has many other effects, in particular on lipid and amino acid metabolism. Counterintuitively, glucagon seems also required for an optimal insulin secretion in response to glucose by acting on its cognate receptor and, even more importantly, on GLP-1 receptors. Patients with diabetes mellitus display two main alterations of glucagon secretion: a relative hyperglucagonemia that aggravates hyperglycemia, and an impaired glucagon response to hypoglycemia. Under metabolic stress states, such as diabetes, pancreatic α-cells also secrete GLP-1, a glucose-lowering hormone, whereas the gut can produce glucagon. The contribution of extrapancreatic glucagon to the abnormal glucose homeostasis is unclear. Here, I review the possible mechanisms of control of glucagon secretion and the role of α-cells on islet function in healthy state. I discuss the possible causes of the abnormal glucagonemia in diabetes, with particular emphasis on type 2 diabetes, and I briefly comment the current antidiabetic therapies affecting α-cells.

Metallothionein 1 negatively regulates glucose-stimulated insulin secretion and is differentially expressed in conditions of beta cell compensation and failure in mice and humans

AIMS/HYPOTHESIS

The mechanisms responsible for beta cell compensation in obesity and for beta cell failure in type 2 diabetes are poorly defined. The mRNA levels of several metallothionein (MT) genes are upregulated in islets from individuals with type 2 diabetes, but their role in beta cells is not clear. Here we examined: (1) the temporal changes of islet Mt1 and Mt2 gene expression in mouse models of beta cell compensation and failure; and (2) the role of Mt1 and Mt2 in beta cell function and glucose homeostasis in mice.

METHODS

Mt1 and Mt2 expression was assessed in islets from: (1) control lean (chow diet-fed) and diet-induced obese (high-fat diet-fed for 6 weeks) mice; (2) mouse models of diabetes (db/db mice) at 6 weeks old (prediabetes) and 16 weeks old (after diabetes onset) and age-matched db/+ (control) mice; and (3) obese non-diabetic ob/ob mice (16-week-old) and age-matched ob/+ (control) mice. MT1E, MT1X and MT2A expression was assessed in islets from humans with and without type 2 diabetes. Mt1-Mt2 double-knockout (KO) mice, transgenic mice overexpressing Mt1 under the control of its natural promoter (Tg-Mt1) and corresponding control mice were also studied. In MIN6 cells, MT1 and MT2 were inhibited by small interfering RNAs. mRNA levels were assessed by real-time RT-PCR, plasma insulin and islet MT levels by ELISA, glucose tolerance by i.p. glucose tolerance tests and overnight fasting-1 h refeeding tests, insulin tolerance by i.p. insulin tolerance tests, insulin secretion by RIA, cytosolic free Ca²⁺ concentration with Fura-2 leakage resistant (Fura-2 LR), cytosolic free Zn²⁺ concentration with Fluozin-3, and NAD(P)H by autofluorescence.

RESULTS

Mt1 and Mt2 mRNA levels were reduced in islets of murine models of beta cell compensation, whereas they were increased in diabetic db/db mice. In humans, MT1X mRNA levels were significantly upregulated in islets from individuals with type 2 diabetes in comparison with non-diabetic donors, while MT1E and MT2A mRNA levels were unchanged. Ex vivo, islet Mt1 and Mt2 mRNA and MT1 and MT2 protein levels were downregulated after culture with glucose at 10-30 mmol/l vs 2-5 mmol/l, in association with increased insulin secretion. In human islets, mRNA levels of MT1E, MT1X and MT2A were downregulated by stimulation with physiological and supraphysiological levels of glucose. In comparison with wild-type (WT) mice, Mt1-Mt2 double-KO mice displayed improved glucose tolerance in association with increased insulin levels and enhanced insulin release from isolated islets. In contrast, isolated islets from Tg-Mt1 mice displayed impaired glucose-stimulated insulin secretion (GSIS). In both Mt1-Mt2 double-KO and Tg-Mt1 models, the changes in GSIS occurred despite similar islet insulin content, rises in cytosolic free Ca²⁺ concentration and NAD(P)H levels, or intracellular Zn²⁺ concentration vs WT mice. In MIN6 cells, knockdown of MT1 but not MT2 potentiated GSIS, suggesting that Mt1 rather than Mt2 affects beta cell function.

CONCLUSIONS/INTERPRETATION

These findings implicate Mt1 as a negative regulator of insulin secretion. The downregulation of Mt1 is associated with beta cell compensation in obesity, whereas increased Mt1 accompanies beta cell failure and type 2 diabetes.

Glucose Acutely Reduces Cytosolic and Mitochondrial H2O2 in Rat Pancreatic Beta Cells

Aims: Whether H₂O₂ contributes to the glucose-dependent stimulation of insulin secretion (GSIS) by pancreatic β cells is highly controversial. We used two H₂O₂-sensitive probes, roGFP2-Orp1 (reduction/oxidation-sensitive enhanced green fluorescent protein fused to oxidant receptor peroxidase 1) and HyPer (hydrogen peroxide sensor) with its pH-control SypHer, to test the acute effects of glucose, monomethyl succinate, leucine with glutamine, and α-ketoisocaproate on β cell cytosolic and mitochondrial H₂O₂ concentrations. We then tested the effects of low H₂O₂ and menadione concentrations on insulin secretion. Results: RoGFP2-Orp1 was more sensitive than HyPer to H₂O₂ (response at 2-5 vs. 10 μM) and less pH-sensitive. Under control conditions, stimulation with glucose reduced mitochondrial roGFP2-Orp1 oxidation without affecting cytosolic roGFP2-Orp1 and HyPer fluorescence ratios, except for the pH-dependent effects on HyPer. However, stimulation with glucose decreased the oxidation of both cytosolic probes by 15 μM exogenous H₂O₂. The glucose effects were not affected by overexpression of catalase, mitochondrial catalase, or superoxide dismutase 1 and 2. They followed the increase in NAD(P)H autofluorescence, were maximal at 5 mM glucose in the cytosol and 10 mM glucose in the mitochondria, and were partly mimicked by the other nutrients. Exogenous H₂O₂ (1-15 μM) did not affect insulin secretion. By contrast, menadione (1-5 μM) did not increase basal insulin secretion but reduced the stimulation of insulin secretion by 20 mM glucose. Innovation: Subcellular changes in β cell H₂O₂ levels are better monitored with roGFP2-Orp1 than HyPer/SypHer. Nutrients acutely lower mitochondrial H₂O₂ levels in β cells and promote degradation of exogenously supplied H₂O₂ in both cytosolic and mitochondrial compartments. Conclusion: The GSIS occurs independently of a detectable increase in β cell cytosolic or mitochondrial H₂O₂ levels.

Impaired Store-Operated Calcium Entry and STIM1 Loss Lead to Reduced Insulin Secretion and Increased Endoplasmic Reticulum Stress in the Diabetic β-Cell

Store-operated Ca²⁺ entry (SOCE) is a dynamic process that leads to refilling of endoplasmic reticulum (ER) Ca²⁺ stores through reversible gating of plasma membrane Ca²⁺ channels by the ER Ca²⁺ sensor Stromal Interaction Molecule 1 (STIM1). Pathogenic reductions in β-cell ER Ca²⁺ have been observed in diabetes. However, a role for impaired SOCE in this phenotype has not been tested. We measured the expression of SOCE molecular components in human and rodent models of diabetes and found a specific reduction in STIM1 mRNA and protein levels in human islets from donors with type 2 diabetes (T2D), islets from hyperglycemic streptozotocin-treated mice, and INS-1 cells (rat insulinoma cells) treated with proinflammatory cytokines and palmitate. Pharmacologic SOCE inhibitors led to impaired islet Ca²⁺ oscillations and insulin secretion, and these effects were phenocopied by β-cell STIM1 deletion. STIM1 deletion also led to reduced ER Ca²⁺ storage and increased ER stress, whereas STIM1 gain of function rescued β-cell survival under proinflammatory conditions and improved insulin secretion in human islets from donors with T2D. Taken together, these data suggest that the loss of STIM1 and impaired SOCE contribute to ER Ca²⁺ dyshomeostasis under diabetic conditions, whereas efforts to restore SOCE-mediated Ca²⁺ transients may have the potential to improve β-cell health and function.

Somatostatin Is Only Partly Required for the Glucagonostatic Effect of Glucose but Is Necessary for the Glucagonostatic Effect of KATP Channel Blockers

The mechanisms of control of glucagon secretion are largely debated. In particular, the paracrine role of somatostatin (SST) is unclear. We studied its role in the control of glucagon secretion by glucose and K_ATP channel blockers, using perifused islets and the in situ perfused pancreas. The involvement of SST was evaluated by comparing glucagon release of control tissue or tissue without paracrine influence of SST (pertussis toxin-treated islets, or islets or pancreas from Sst^-/- mice). We show that removal of the paracrine influence of SST suppresses the ability of K_ATP channel blockers or K_ATP channel ablation to inhibit glucagon release, suggesting that in control islets, the glucagonostatic effect of K_ATP channel blockers/ablation is fully mediated by SST. By contrast, the glucagonostatic effect of glucose in control islets is mainly independent of SST for low glucose concentrations (0-7 mmol/L) but starts to involve SST for high concentrations of the sugar (15-30 mmol/L). This demonstrates that the glucagonostatic effect of glucose only partially depends on SST. Real-time quantitative PCR and pharmacological experiments indicate that the glucagonostatic effect of SST is mediated by two types of SST receptors, SSTR2 and SSTR3. These results suggest that alterations of the paracrine influence of SST will affect glucagon release.

TALK-1 channels control β cell endoplasmic reticulum Ca2+ homeostasis

Ca²⁺ handling by the endoplasmic reticulum (ER) serves critical roles in controlling pancreatic β cell function and becomes perturbed during the pathogenesis of diabetes. ER Ca²⁺ homeostasis is determined by ion movements across the ER membrane, including K⁺ flux through K⁺ channels. We demonstrated that K⁺ flux through ER-localized TALK-1 channels facilitated Ca²⁺ release from the ER in mouse and human β cells. We found that β cells from mice lacking TALK-1 exhibited reduced basal cytosolic Ca²⁺ and increased ER Ca²⁺ concentrations, suggesting reduced ER Ca²⁺ leak. These changes in Ca²⁺ homeostasis were presumably due to TALK-1-mediated ER K⁺ flux, because we recorded K⁺ currents mediated by functional TALK-1 channels on the nuclear membrane, which is continuous with the ER. Moreover, overexpression of K⁺-impermeable TALK-1 channels in HEK293 cells did not reduce ER Ca²⁺ stores. Reduced ER Ca²⁺ content in β cells is associated with ER stress and islet dysfunction in diabetes, and islets from TALK-1-deficient mice fed a high-fat diet showed reduced signs of ER stress, suggesting that TALK-1 activity exacerbated ER stress. Our data establish TALK-1 channels as key regulators of β cell ER Ca²⁺ and suggest that TALK-1 may be a therapeutic target to reduce ER Ca²⁺ handling defects in β cells during the pathogenesis of diabetes.

Sodium-myoinositol cotransporter-1, SMIT1, mediates the production of reactive oxygen species induced by hyperglycemia in the heart

Hyperglycemia (HG) stimulates the production of reactive oxygen species in the heart through activation of NADPH oxidase 2 (NOX2). This production is independent of glucose metabolism but requires sodium/glucose cotransporters (SGLT). Seven SGLT isoforms (SGLT1 to 6 and sodium-myoinositol cotransporter-1, SMIT1) are known, although their expression and function in the heart remain elusive. We investigated these 7 isoforms and found that only SGLT1 and SMIT1 were expressed in mouse, rat and human hearts. In cardiomyocytes, galactose (transported through SGLT1) did not activate NOX2. Accordingly, SGLT1 deficiency did not prevent HG-induced NOX2 activation, ruling it out in the cellular response to HG. In contrast, myo-inositol (transported through SMIT1) reproduced the toxic effects of HG. SMIT1 overexpression exacerbated glucotoxicity and sensitized cardiomyocytes to HG, whereas its deletion prevented HG-induced NOX2 activation. In conclusion, our results show that heart SMIT1 senses HG and triggers NOX2 activation. This could participate in the redox signaling in hyperglycemic heart and contribute to the pathophysiology of diabetic cardiomyopathy.

SERCA2 Deficiency Impairs Pancreatic β-Cell Function in Response to Diet-Induced Obesity

The sarcoendoplasmic reticulum (ER) Ca(2+) ATPase 2 (SERCA2) pump is a P-type ATPase tasked with the maintenance of ER Ca(2+) stores. Whereas β-cell SERCA2 expression is reduced in diabetes, the role of SERCA2 in the regulation of whole-body glucose homeostasis has remained uncharacterized. To this end, SERCA2 heterozygous mice (S2HET) were challenged with a high-fat diet (HFD) containing 45% of kilocalories from fat. After 16 weeks of the HFD, S2HET mice were hyperglycemic and glucose intolerant, but adiposity and insulin sensitivity were not different between HFD-fed S2HET mice and HFD-fed wild-type controls. Consistent with a defect in β-cell function, insulin secretion, glucose-induced cytosolic Ca(2+) mobilization, and the onset of steady-state glucose-induced Ca(2+) oscillations were impaired in HFD-fed S2HET islets. Moreover, HFD-fed S2HET mice exhibited reduced β-cell mass and proliferation, altered insulin production and proinsulin processing, and increased islet ER stress and death. In contrast, SERCA2 activation with a small molecule allosteric activator increased ER Ca(2+) storage and rescued tunicamycin-induced β-cell death. In aggregate, these data suggest a critical role for SERCA2 and the regulation of ER Ca(2+) homeostasis in the β-cell compensatory response to diet-induced obesity.

Cocaine- and amphetamine-regulated transcript: a novel regulator of energy homeostasis expressed in a subpopulation of pancreatic islet cells

Type 2 diabetes is characterised by chronic hyperglycaemia and its incidence is highly increased by exaggerated food consumption. It results from a lack of insulin action/production, but growing evidence suggests that it might also involve hyperglucagonaemia and impaired control of glucose homeostasis by the brain. In recent years, the cocaine and amphetamine-regulated transcript (CART) peptides have generated a lot of interest in the battle against obesity because, via the brain, they exert anorexic effects and they increase energy expenditure. They are also localised, outside the brain, in discrete regions of the body and play a hormonal role in controlling various functions. In this issue of Diabetologia, the Wierup group (doi: 10.1007/s00125-016-4020-6 ) shows that CART peptides are expressed heterogeneously in islet cells of various species, including humans, and that their expression is upregulated in diabetes. The authors also shine a spotlight on some interesting effects of CART peptides on islet function, including stimulation of insulin secretion and inhibition of glucagon release. CART peptides would thus be at the centre of a cooperation between the brain and the endocrine pancreas to control glucose homeostasis. Although the mechanisms of action of CART peptides remain enigmatic because no specific receptor for these peptides has so far been discovered, their potential therapeutic use is evident and represents a new challenge for future research.

Paracrine nitric oxide induces expression of cardiac sarcomeric proteins in adult progenitor cells through soluble guanylyl cyclase/cyclic-guanosine monophosphate and Wnt/β-catenin inhibition

AIM

Cardiac progenitor cells (CPC) from adult hearts can differentiate to several cell types composing the myocardium but the underlying molecular pathways are poorly characterized. We examined the role of paracrine nitric oxide (NO) in the specification of CPC to the cardiac lineage, particularly through its inhibition of the canonical Wnt/β-catenin pathway, a critical step preceding cardiac differentiation.

METHODS AND RESULTS

Sca1 + CPC from adult mouse hearts were isolated by magnetic-activated cell sorting and clonally expanded. Pharmacologic NO donors increased their expression of cardiac myocyte-specific sarcomeric proteins in a concentration and time-dependent manner. The optimal time window for NO efficacy coincided with up-regulation of CPC expression of Gucy1a3 (coding the alpha1 subunit of guanylyl cyclase). The effect of paracrine NO was reproduced in vitro upon co-culture of CPC with cardiac myocytes expressing a transgenic NOS3 (endothelial nitric oxide synthase) and in vivo upon injection of CPC in infarcted hearts from cardiac-specific NOS3 transgenic mice. In mono- and co-cultures, this effect was abrogated upon inhibition of soluble guanylyl cyclase or nitric oxide synthase, and was lost in CPC genetically deficient in Gucy1a3. Mechanistically, NO inhibits the constitutive activity of the canonical Wnt/β-catenin in CPC and in cell reporter assays in a guanylyl cyclase-dependent fashion. This was paralleled with decreased expression of β-catenin and down-regulation of Wnt target genes in CPC and abrogated in CPC with a stabilized, non-inhibitable β-catenin.

CONCLUSIONS

Exogenous or paracrine sources of NO promote the specification towards the myocyte lineage and expression of cardiac sarcomeric proteins of adult CPC. This is contingent upon the expression and activity of the alpha1 subunit of guanylyl cyclase in CPC that is necessary for NO-mediated inhibition of the canonical Wnt/β-catenin pathway.

Inter-domain tagging implicates caveolin-1 in insulin receptor trafficking and Erk signaling bias in pancreatic beta-cells

OBJECTIVE

The role and mechanisms of insulin receptor internalization remain incompletely understood. Previous trafficking studies of insulin receptors involved fluorescent protein tagging at their termini, manipulations that may be expected to result in dysfunctional receptors. Our objective was to determine the trafficking route and molecular mechanisms of functional tagged insulin receptors and endogenous insulin receptors in pancreatic beta-cells.

METHODS

We generated functional insulin receptors tagged with pH-resistant fluorescent proteins between domains. Confocal, TIRF and STED imaging revealed a trafficking pattern of inter-domain tagged insulin receptors and endogenous insulin receptors detected with antibodies.

RESULTS

Surprisingly, interdomain-tagged and endogenous insulin receptors in beta-cells bypassed classical Rab5a- or Rab7-mediated endocytic routes. Instead, we found that removal of insulin receptors from the plasma membrane involved tyrosine-phosphorylated caveolin-1, prior to trafficking within flotillin-1-positive structures to lysosomes. Multiple methods of inhibiting caveolin-1 significantly reduced Erk activation in vitro or in vivo, while leaving Akt signaling mostly intact.

CONCLUSIONS

We conclude that phosphorylated caveolin-1 plays a role in insulin receptor internalization towards lysosomes through flotillin-1-positive structures and that caveolin-1 helps bias physiological beta-cell insulin signaling towards Erk activation.

Involvement of nitric oxide in iodine deficiency-induced microvascular remodeling in the thyroid gland: role of nitric oxide synthase 3 and ryanodine receptors

Iodine deficiency (ID) induces microvascular changes in the thyroid gland via a TSH-independent reactive oxygen species-hypoxia inducible factor (HIF)-1α-vascular endothelial growth factor (VEGF) pathway. The involvement of nitric oxide (NO) in this pathway and the role of calcium (Ca(2+)) and of ryanodine receptors (RYRs) in NO synthase 3 (NOS3) activation were investigated in a murine model of goitrogenesis and in 3 in vitro models of ID, including primary cultures of human thyrocytes. ID activated NOS3 and the production of NO in thyrocytes in vitro and increased the thyroid blood flow in vivo. Using bevacizumab (a blocking antibody against VEGF-A) in mice, it appeared that NOS3 is activated upstream of VEGF-A. L-nitroarginine methyl ester (a NOS inhibitor) blocked the ID-induced increase in thyroid blood flow in vivo and NO production in vitro, as well as ID-induced VEGF-A mRNA and HIF-1α expression in vitro, whereas S-nitroso-acetyl-penicillamine (a NO donor) did the opposite. Ca(2+) is involved in this pathway as intracellular Ca(2+) flux increased after ID, and thapsigargin activated NOS3 and increased VEGF-A mRNA expression. Two of the 3 known mammalian RYR isoforms (RYR1 and RYR2) were shown to be expressed in thyrocytes. RYR inhibition using ryanodine at 10μM decreased ID-induced NOS3 activation, HIF-1α, and VEGF-A expression, whereas RYR activation with ryanodine at 1nM increased NOS3 activation and VEGF-A mRNA expression. In conclusion, during the early phase of TSH-independent ID-induced microvascular activation, ID sequentially activates RYRs and NOS3, thereby supporting ID-induced activation of the NO/HIF-1α/VEGF-A pathway in thyrocytes.

AMPK activation by glucagon-like peptide-1 prevents NADPH oxidase activation induced by hyperglycemia in adult cardiomyocytes

Exposure of cardiomyocytes to high glucose concentrations (HG) stimulates reactive oxygen species (ROS) production by NADPH oxidase (NOX2). NOX2 activation is triggered by enhanced glucose transport through a sodium-glucose cotransporter (SGLT) but not by a stimulation of glucose metabolism. The aim of this work was to identify potential therapeutic approaches to counteract this glucotoxicity. In cultured adult rat cardiomyocytes incubated with 21 mM glucose (HG), AMP-activated protein kinase (AMPK) activation by A769662 or phenformin nearly suppressed ROS production. Interestingly, glucagon-like peptide 1 (GLP-1), a new antidiabetic drug, concomitantly induced AMPK activation and prevented the HG-mediated ROS production (maximal effect at 100 nM). α2-AMPK, the major isoform expressed in cardiomyocytes (but not α1-AMPK), was activated in response to GLP-1. Anti-ROS properties of AMPK activators were not related to changes in glucose uptake or glycolysis. Using in situ proximity ligation assay, we demonstrated that AMPK activation prevented the HG-induced p47phox translocation to caveolae, whatever the AMPK activators used. NOX2 activation by either α-methyl-d-glucopyranoside, a glucose analog transported through SGLT, or angiotensin II was also counteracted by GLP-1. The crucial role of AMPK in limiting HG-mediated NOX2 activation was demonstrated by overexpressing a constitutively active form of α2-AMPK using adenoviral infection. This overexpression prevented NOX2 activation in response to HG, whereas GLP-1 lost its protective action in α2-AMPK-deficient mouse cardiomyocytes. Under HG, the GLP-1/AMPK pathway inhibited PKC-β2 phosphorylation, a key element mediating p47phox translocation. In conclusion, GLP-1 induces α2-AMPK activation and blocks HG-induced p47phox translocation to the plasma membrane, thereby preventing glucotoxicity.

Pancreatic and duodenal homeobox protein 1 (Pdx-1) maintains endoplasmic reticulum calcium levels through transcriptional regulation of sarco-endoplasmic reticulum calcium ATPase 2b (SERCA2b) in the islet β cell

Although the pancreatic duodenal homeobox 1 (Pdx-1) transcription factor is known to play an indispensable role in β cell development and secretory function, recent data also implicate Pdx-1 in the maintenance of endoplasmic reticulum (ER) health. The sarco-endoplasmic reticulum Ca(2+) ATPase 2b (SERCA2b) pump maintains a steep Ca(2+) gradient between the cytosol and ER lumen. In models of diabetes, our data demonstrated loss of β cell Pdx-1 that occurs in parallel with altered SERCA2b expression, whereas in silico analysis of the SERCA2b promoter revealed multiple putative Pdx-1 binding sites. We hypothesized that Pdx-1 loss under inflammatory and diabetic conditions leads to decreased SERCA2b levels and activity with concomitant alterations in ER health. To test this, siRNA-mediated knockdown of Pdx-1 was performed in INS-1 cells. The results revealed reduced SERCA2b expression and decreased ER Ca(2+), which was measured using fluorescence lifetime imaging microscopy. Cotransfection of human Pdx-1 with a reporter fused to the human SERCA2 promoter increased luciferase activity 3- to 4-fold relative to an empty vector control, and direct binding of Pdx-1 to the proximal SERCA2 promoter was confirmed by chromatin immunoprecipitation. To determine whether restoration of SERCA2b could rescue ER stress induced by Pdx-1 loss, Pdx1(+/-) mice were fed a high-fat diet. Isolated islets demonstrated an increased spliced-to-total Xbp1 ratio, whereas SERCA2b overexpression reduced the Xbp1 ratio to that of wild-type controls. Together, these results identify SERCA2b as a novel transcriptional target of Pdx-1 and define a role for altered ER Ca(2+) regulation in Pdx-1-deficient states.

Glucose regulation of glucagon secretion

Glucagon secreted by pancreatic α-cells is the major hyperglycemic hormone correcting acute hypoglycaemia (glucose counterregulation). In diabetes the glucagon response to hypoglycaemia becomes compromised and chronic hyperglucagonemia appears. There is increasing awareness that glucagon excess may underlie important manifestations of diabetes. However opinions differ widely how glucose controls glucagon secretion. The autonomous nervous system plays an important role in the glucagon response to hypoglycaemia. But it is clear that glucose controls glucagon secretion also by mechanisms involving direct effects on α-cells or indirect effects via paracrine factors released from non-α-cells within the pancreatic islets. The present review discusses these mechanisms and argues that different regulatory processes are involved in a glucose concentration-dependent manner. Direct glucose effects on the α-cell and autocrine mechanisms are probably most significant for the glucagon response to hypoglycaemia. During hyperglycaemia, when secretion from β- and δ-cells is stimulated, paracrine inhibitory factors generate pulsatile glucagon release in opposite phase to pulsatile release of insulin and somatostatin. High concentrations of glucose have also stimulatory effects on glucagon secretion that tend to balance and even exceed the inhibitory influence. The latter actions might underlie the paradoxical hyperglucagonemia that aggravates hyperglycaemia in persons with diabetes.

Tolbutamide controls glucagon release from mouse islets differently than glucose: involvement of K(ATP) channels from both α-cells and δ-cells

We evaluated the role of ATP-sensitive K⁺ (K(ATP)) channels, somatostatin, and Zn²⁺ in the control of glucagon secretion from mouse islets. Switching from 1 to 7 mmol/L glucose inhibited glucagon release. Diazoxide did not reverse the glucagonostatic effect of glucose. Tolbutamide decreased glucagon secretion at 1 mmol/L glucose (G1) but stimulated it at 7 mmol/L glucose (G7). The reduced glucagon secretion produced by high concentrations of tolbutamide or diazoxide, or disruption of K(ATP) channels (Sur1(-/-) mice) at G1 could be inhibited further by G7. Removal of the somatostatin paracrine influence (Sst(-/-) mice or pretreatement with pertussis toxin) strongly increased glucagon release, did not prevent the glucagonostatic effect of G7, and unmasked a marked glucagonotropic effect of tolbutamide. Glucose inhibited glucagon release in the absence of functional K(ATP) channels and somatostatin signaling. Knockout of the Zn²⁺ transporter ZnT8 (ZnT8(-/-) mice) did not prevent the glucagonostatic effect of glucose. In conclusion, glucose can inhibit glucagon release independently of Zn²⁺, K(ATP) channels, and somatostatin. Closure of K(ATP) channels controls glucagon secretion by two mechanisms, a direct stimulation of α-cells and an indirect inhibition via somatostatin released from δ-cells. The net effect on glucagon release results from a balance between both effects.

UCP2 regulates the glucagon response to fasting and starvation

Glucagon is important for maintaining euglycemia during fasting/starvation, and abnormal glucagon secretion is associated with type 1 and type 2 diabetes; however, the mechanisms of hypoglycemia-induced glucagon secretion are poorly understood. We previously demonstrated that global deletion of mitochondrial uncoupling protein 2 (UCP2(-/-)) in mice impaired glucagon secretion from isolated islets. Therefore, UCP2 may contribute to the regulation of hypoglycemia-induced glucagon secretion, which is supported by our current finding that UCP2 expression is increased in nutrient-deprived murine and human islets. Further to this, we created α-cell-specific UCP2 knockout (UCP2AKO) mice, which we used to demonstrate that blood glucose recovery in response to hypoglycemia is impaired owing to attenuated glucagon secretion. UCP2-deleted α-cells have higher levels of intracellular reactive oxygen species (ROS) due to enhanced mitochondrial coupling, which translated into defective stimulus/secretion coupling. The effects of UCP2 deletion were mimicked by the UCP2 inhibitor genipin on both murine and human islets and also by application of exogenous ROS, confirming that changes in oxidative status and electrical activity directly reduce glucagon secretion. Therefore, α-cell UCP2 deletion perturbs the fasting/hypoglycemic glucagon response and shows that UCP2 is necessary for normal α-cell glucose sensing and the maintenance of euglycemia.

In situ electrophysiological examination of pancreatic α cells in the streptozotocin-induced diabetes model, revealing the cellular basis of glucagon hypersecretion

Early-stage type 1 diabetes (T1D) exhibits hyperglucagonemia by undefined cellular mechanisms. Here we characterized α-cell voltage-gated ion channels in a streptozotocin (STZ)-induced diabetes model that lead to increased glucagon secretion mimicking T1D. GYY mice expressing enhanced yellow fluorescence protein in α cells were used to identify α cells within pancreas slices. Mice treated with low-dose STZ exhibited hyperglucagonemia, hyperglycemia, and glucose intolerance, with 71% reduction of β-cell mass. Although α-cell mass of STZ-treated mice remained unchanged, total pancreatic glucagon content was elevated, coinciding with increase in size of glucagon granules. Pancreas tissue slices enabled in situ examination of α-cell electrophysiology. α cells of STZ-treated mice exhibited the following: 1) increased exocytosis (serial depolarization-induced capacitance), 2) enhanced voltage-gated Na(+) current density, 3) reduced voltage-gated K(+) current density, and 4) increased action potential (AP) amplitude and firing frequency. Hyperglucagonemia in STZ-induced diabetes is thus likely due to increased glucagon content arising from enlarged glucagon granules and increased AP firing frequency and amplitude coinciding with enhanced Na(+) and reduced K(+) currents. These alterations may prime α cells in STZ-treated mice for more glucagon release per cell in response to low glucose stimulation. Thus, our study provides the first insight that STZ treatment sensitizes release mechanisms of α cells.

Frequency-dependent mitochondrial Ca(2+) accumulation regulates ATP synthesis in pancreatic β cells

Pancreatic β cells respond to increases in glucose concentration with enhanced metabolism, the closure of ATP-sensitive K⁺ channels and electrical spiking. The latter results in oscillatory Ca²⁺ influx through voltage-gated Ca²⁺ channels and the activation of insulin release. The relationship between changes in cytosolic and mitochondrial free calcium concentration ([Ca²⁺]_cyt and [Ca²⁺]_mit, respectively) during these cycles is poorly understood. Importantly, the activation of Ca²⁺-sensitive intramitochondrial dehydrogenases, occurring alongside the stimulation of ATP consumption required for Ca²⁺ pumping and other processes, may exert complex effects on cytosolic ATP/ADP ratios and hence insulin secretion. To explore the relationship between these parameters in single primary β cells, we have deployed cytosolic (Fura red, Indo1) or green fluorescent protein-based recombinant-targeted (Pericam, 2mt8RP for mitochondria; D4ER for the ER) probes for Ca²⁺ and cytosolic ATP/ADP (Perceval) alongside patch-clamp electrophysiology. We demonstrate that: (1) blockade of mitochondrial Ca²⁺ uptake by shRNA-mediated silencing of the uniporter MCU attenuates glucose- and essentially blocks tolbutamide-stimulated, insulin secretion; (2) during electrical stimulation, mitochondria decode cytosolic Ca²⁺ oscillation frequency as stable increases in [Ca²⁺]_mit and cytosolic ATP/ADP; (3) mitochondrial Ca²⁺ uptake rates remained constant between individual spikes, arguing against activity-dependent regulation (“plasticity”) and (4) the relationship between [Ca²⁺]_cyt and [Ca²⁺]_mit is essentially unaffected by changes in endoplasmic reticulum Ca²⁺ ([Ca²⁺]_ER). Our findings thus highlight new aspects of Ca²⁺ signalling in β cells of relevance to the actions of both glucose and sulphonylureas.

The mitochondrial Ca2+ uniporter MCU is essential for glucose-induced ATP increases in pancreatic β-cells

Glucose induces insulin release from pancreatic β-cells by stimulating ATP synthesis, membrane depolarisation and Ca²⁺ influx. As well as activating ATP-consuming processes, cytosolic Ca²⁺ increases may also potentiate mitochondrial ATP synthesis. Until recently, the ability to study the role of mitochondrial Ca²⁺ transport in glucose-stimulated insulin secretion has been hindered by the absence of suitable approaches either to suppress Ca²⁺ uptake into these organelles, or to examine the impact on β-cell excitability. Here, we have combined patch-clamp electrophysiology with simultaneous real-time imaging of compartmentalised changes in Ca²⁺ and ATP/ADP ratio in single primary mouse β-cells, using recombinant targeted (Pericam or Perceval, respectively) as well as entrapped intracellular (Fura-Red), probes. Through shRNA-mediated silencing we show that the recently-identified mitochondrial Ca²⁺ uniporter, MCU, is required for depolarisation-induced mitochondrial Ca²⁺ increases, and for a sustained increase in cytosolic ATP/ADP ratio. By contrast, silencing of the mitochondrial Na⁺-Ca²⁺ exchanger NCLX affected the kinetics of glucose-induced changes in, but not steady state values of, cytosolic ATP/ADP. Exposure to gluco-lipotoxic conditions delayed both mitochondrial Ca²⁺ uptake and cytosolic ATP/ADP ratio increases without affecting the expression of either gene. Mitochondrial Ca²⁺ accumulation, mediated by MCU and modulated by NCLX, is thus required for normal glucose sensing by pancreatic β-cells, and becomes defective in conditions mimicking the diabetic milieu.

Mechanisms of control of the free Ca2+ concentration in the endoplasmic reticulum of mouse pancreatic β-cells: interplay with cell metabolism and [Ca2+]c and role of SERCA2b and SERCA3

OBJECTIVE

Sarco-endoplasmic reticulum Ca(2+)-ATPase 2b (SERCA2b) and SERCA3 pump Ca(2+) in the endoplasmic reticulum (ER) of pancreatic β-cells. We studied their role in the control of the free ER Ca(2+) concentration ([Ca(2+)](ER)) and the role of SERCA3 in the control of insulin secretion and ER stress.

RESEARCH DESIGN AND METHODS

β-Cell [Ca(2+)](ER) of SERCA3(+/+) and SERCA3(-/-) mice was monitored with an adenovirus encoding the low Ca(2+)-affinity sensor D4 addressed to the ER (D4ER) under the control of the insulin promoter. Free cytosolic Ca(2+) concentration ([Ca(2+)](c)) and [Ca(2+)](ER) were simultaneously recorded. Insulin secretion and mRNA levels of ER stress genes were studied.

RESULTS

Glucose elicited synchronized [Ca(2+)](ER) and [Ca(2+)](c) oscillations. [Ca(2+)](ER) oscillations were smaller in SERCA3(-/-) than in SERCA3(+/+) β-cells. Stimulating cell metabolism with various [glucose] in the presence of diazoxide induced a similar dose-dependent [Ca(2+)](ER) rise in SERCA3(+/+) and SERCA3(-/-) β-cells. In a Ca(2+)-free medium, glucose moderately raised [Ca(2+)](ER) from a highly buffered cytosolic Ca(2+) pool. Increasing [Ca(2+)](c) with high [K] elicited a [Ca(2+)](ER) rise that was larger but more transient in SERCA3(+/+) than SERCA3(-/-) β-cells because of the activation of a Ca(2+) release from the ER in SERCA3(+/+) β-cells. Glucose-induced insulin release was larger in SERCA3(-/-) than SERCA3(+/+) islets. SERCA3 ablation did not induce ER stress.

CONCLUSIONS

[Ca(2+)](c) and [Ca(2+)](ER) oscillate in phase in response to glucose. Upon [Ca(2+)](c) increase, Ca(2+) is taken up by SERCA2b and SERCA3. Strong Ca(2+) influx triggers a Ca(2+) release from the ER that depends on SERCA3. SERCA3 deficiency neither impairs Ca(2+) uptake by the ER upon cell metabolism acceleration and insulin release nor induces ER stress.

Placental lactogens induce serotonin biosynthesis in a subset of mouse beta cells during pregnancy

AIMS/HYPOTHESIS

Upregulation of the functional beta cell mass is required to match the physiological demands of mother and fetus during pregnancy. This increase is dependent on placental lactogens (PLs) and prolactin receptors, but the mechanisms underlying these events are only partially understood. We studied the mRNA expression profile of mouse islets during pregnancy to gain a better insight into these changes.

METHODS

RNA expression was measured ex vivo via microarrays and quantitative RT-PCR. In vivo observations were extended by in vitro models in which ovine PL was added to cultured mouse islets and MIN6 cells.

RESULTS

mRNA encoding both isoforms of the rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase (TPH), i.e. Tph1 and Tph2, were strongly induced (fold change 25- to 200-fold) during pregnancy. This induction was mimicked by exposing islets or MIN6 cells to ovine PLs for 24 h and was dependent on janus kinase 2 and signal transducer and activator of transcription 5. Parallel to Tph1 mRNA and protein induction, islet serotonin content increased to a peak level that was 200-fold higher than basal. Interestingly, only a subpopulation of the beta cells was serotonin-positive in vitro and in vivo. The stored serotonin pool in pregnant islets and PL-treated MIN6 cells was rapidly released (turnover once every 2 h).

CONCLUSIONS/INTERPRETATION

A very strong lactogen-dependent upregulation of serotonin biosynthesis occurs in a subpopulation of mouse islet beta cells during pregnancy. Since the newly formed serotonin is rapidly released, this lactogen-induced beta cell function may serve local or endocrine tasks, the nature of which remains to be identified.

Tissue-specific disallowance of housekeeping genes: the other face of cell differentiation

We report on a hitherto poorly characterized class of genes that are expressed in all tissues, except in one. Often, these genes have been classified as housekeeping genes, based on their nearly ubiquitous expression. However, the specific repression in one tissue defines a special class of "disallowed genes." In this paper, we used the intersection-union test to screen for such genes in a multi-tissue panel of genome-wide mRNA expression data. We propose that disallowed genes need to be repressed in the specific target tissue to ensure correct tissue function. We provide mechanistic data of repression with two metabolic examples, exercise-induced inappropriate insulin release and interference with ketogenesis in liver. Developmentally, this repression is established during tissue maturation in the early postnatal period involving epigenetic changes in histone methylation. In addition, tissue-specific expression of microRNAs can further diminish these repressed mRNAs. Together, we provide a systematic analysis of tissue-specific repression of housekeeping genes, a phenomenon that has not been studied so far on a genome-wide basis and, when perturbed, can lead to human disease.

Subplasmalemmal Ca(2+) measurements in mouse pancreatic beta cells support the existence of an amplifying effect of glucose on insulin secretion

AIMS/HYPOTHESIS

Glucose-induced insulin secretion is attributed to a rise of beta cell cytosolic free [Ca(2+)] ([Ca(2+)](c)) (triggering pathway) and amplification of the action of Ca(2+). This concept of amplification rests on observations that glucose can increase Ca(2+)-induced insulin secretion without further elevating an imposed already high [Ca(2+)](c). However, it remains possible that this amplification results from an increase in [Ca(2+)] just under the plasma membrane ([Ca(2+)](SM)), which escaped detection by previous measurements of global [Ca(2+)](c). This was the hypothesis that we tested here by measuring [Ca(2+)](SM).

METHODS

The genetically encoded Ca(2+) indicators D3-cpv (untargeted) and LynD3-cpv (targeted to plasma membrane) were expressed in clusters of mouse beta cells. LynD3-cpv was also expressed in beta cells within intact islets. [Ca(2+)](SM) changes were monitored using total internal reflection fluorescence microscopy. Insulin secretion was measured in parallel.

RESULTS

Beta cells expressing D3cpv or LynD3cpv displayed normal [Ca(2+)] changes and insulin secretion in response to glucose. Distinct [Ca(2+)](SM) fluctuations were detected during repetitive variations of KCl between 30 and 32-35 mmol/l, attesting to the adequate sensitivity of our system. When the amplifying pathway was evaluated (high KCl + diazoxide), increasing glucose from 3 to 15 mmol/l consistently lowered [Ca(2+)](SM) while stimulating insulin secretion approximately two fold. Blocking Ca(2+) uptake by the endoplasmic reticulum largely attenuated the [Ca(2+)](SM) decrease produced by high glucose but did not unmask localised [Ca(2+)](SM) increases.

CONCLUSIONS/INTERPRETATION

Glucose can increase Ca(2+)-induced insulin secretion without causing further elevation of beta cell [Ca(2+)](SM). The phenomenon is therefore a true amplification of the triggering action of Ca(2+).

Effects of fructosamine-3-kinase deficiency on function and survival of mouse pancreatic islets after prolonged culture in high glucose or ribose concentrations

Due to their high glucose permeability, insulin-secreting pancreatic beta-cells likely undergo strong intracellular protein glycation at high glucose concentrations. They may, however, be partly protected from the glucotoxic alterations of their survival and function by fructosamine-3-kinase (FN3K), a ubiquitous enzyme that initiates deglycation of intracellular proteins. To test that hypothesis, we cultured pancreatic islets from Fn3k-knockout (Fn3k(-/-)) mice and their wild-type (WT) littermates for 1-3 wk in the presence of 10 or 30 mmol/l glucose (G10 or G30, respectively) and measured protein glycation, apoptosis, preproinsulin gene expression, and Ca(2+) and insulin secretory responses to acute glucose stimulation. The more potent glycating agent d-ribose (25 mmol/l) was used as positive control for protein glycation. In WT islets, a 1-wk culture in G30 significantly increased the amount of soluble intracellular protein-bound fructose-epsilon-lysines and the glucose sensitivity of beta-cells for changes in Ca(2+) and insulin secretion, whereas it decreased the islet insulin content. After 3 wk, culture in G30 also strongly decreased beta-cell glucose responsiveness and preproinsulin mRNA levels, whereas it increased islet cell apoptosis. Although protein-bound fructose-epsilon-lysines were more abundant in Fn3k(-/-) vs. WT islets, islet cell survival and function and their glucotoxic alterations were almost identical in both types of islets, except for a lower level of apoptosis in Fn3k(-/-) islets cultured for 3 wk in G30. In comparison, d-ribose (1 wk) similarly decreased preproinsulin expression and beta-cell glucose responsiveness in both types of islets, whereas it increased apoptosis to a larger extent in Fn3k(-/-) vs. WT islets. We conclude that, despite its ability to reduce the glycation of intracellular islet proteins, FN3K is neither required for the maintenance of beta-cell survival and function under control conditions nor involved in protection against beta-cell glucotoxicity. The latter, therefore, occurs independently from the associated increase in the level of intracellular fructose-epsilon-lysines.

Glucose and pharmacological modulators of ATP-sensitive K+ channels control [Ca2+]c by different mechanisms in isolated mouse alpha-cells

OBJECTIVE

We studied how glucose and ATP-sensitive K(+) (K(ATP)) channel modulators affect alpha-cell [Ca(2+)](c).

RESEARCH DESIGN AND METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

The GluCre-ROSA26EYFP mouse: a new model for easy identification of living pancreatic alpha-cells

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.

Glucose Acutely Decreases pH of Secretory Granules in Mouse Pancreatic Islets

Glucose-induced insulin secretion requires a rise in beta-cell cytosolic Ca2+ ([Ca2+]c) that triggers exocytosis and a mechanistically unexplained amplification of the action of [Ca2+]c. Insulin granules are kept acidic by luminal pumping of protons with simultaneous Cl- uptake to maintain electroneutrality. Experiments using patched, dialyzed beta-cells prompted the suggestion that acute granule acidification by glucose underlies amplification of insulin secretion. However, others found glucose to increase granular pH in intact islets. In this study, we measured islet granular pH with Lysosensor DND-160, a fluorescent dye that permits ratiometric determination of pH < 6 in acidic compartments. Stimulation of mouse islets with glucose reversibly decreased granular pH by mechanisms that are dependent on metabolism and Cl- ions but independent of changes in [Ca2+]c and protein kinase A or C activity. Granular pH was increased by concanamycin (blocker of the vesicular type H+-ATPase) > methylamine (weak base) > Cl- omission. Concanamycin and methylamine did not alter glucose-induced [Ca2+]c increase in islets but strongly inhibited the two phases of insulin secretion. Omission of Cl- did not affect the first phase but decreased the second phase of both [Ca2+]c and insulin responses. Neither experimental condition affected the [Ca2+]c rise induced by 30 mM KCl, but the insulin responses were inhibited by concanamycin > methylamine and not affected by Cl- omission. The amplification of insulin secretion by glucose was not suppressed. We conclude that an acidic granular pH is important for insulin secretion but that the acute further acidification produced by glucose is not essential for the augmentation of secretion via the amplifying pathway.

Glucose-induced mixed [Ca2+]c oscillations in mouse beta-cells are controlled by the membrane potential and the SERCA3 Ca2+-ATPase of the endoplasmic reticulum

Stimulatory concentrations of glucose induce two patterns of cytosolic Ca2+ concentration ([Ca2+]c) oscillations in mouse islets: simple or mixed. In the mixed pattern, rapid oscillations are superimposed on slow ones. In the present study, we examined the role of the membrane potential in the mixed pattern and the impact of this pattern on insulin release. Simultaneous measurement of [Ca2+]c and insulin release from single islets revealed that mixed [Ca2+]c oscillations triggered synchronous oscillations of insulin secretion. Simultaneous recordings of membrane potential in a single beta-cell within an islet and of [Ca2+]c in the whole islet demonstrated that the mixed pattern resulted from compound bursting (i.e., clusters of membrane potential oscillations separated by prolonged silent intervals) that was synchronized in most beta-cells of the islet. Each slow [Ca2+]c increase during mixed oscillations was due to a progressive summation of rapid oscillations. Digital image analysis confirmed the good synchrony between subregions of an islet. By contrast, islets from sarco(endo)plasmic reticulum Ca2+-ATPase isoform 3 (SERCA3)-knockout mice did not display typical mixed [Ca2+]c oscillations in response to glucose. This results from a lack of progressive summation of rapid oscillations and from altered spontaneous electrical activity, i.e., lack of compound bursting, and membrane potential oscillations characterized by lower-frequency but larger-depolarization phases than observed in SERCA3+/+ beta-cells. We conclude that glucose-induced mixed [Ca2+]c oscillations result from compound bursting in all beta-cells of the islet. Disruption of SERCA3 abolishes mixed [Ca2+]c oscillations and augments beta-cell depolarization. This latter observation indicates that the endoplasmic reticulum participates in the control of the beta-cell membrane potential during glucose stimulation.

Increased glucose sensitivity of both triggering and amplifying pathways of insulin secretion in rat islets cultured for 1 wk in high glucose

Chronic hyperglycemia has been shown to induce either a lack of response or an increased sensitivity to glucose in pancreatic beta-cells. We reinvestigated this controversial issue in a single experimental model by culturing rat islets for 1 wk in 10 or 30 mmol/l glucose (G10, Controls; or G30, High-glucose islets) before testing the effect of stepwise glucose stimulation from G0.5 to G20 on key beta-cell stimulus-secretion coupling events. Compared with Controls, the glucose sensitivity of High-glucose islets was markedly increased, leading to maximal stimulation of oxidative metabolism and both triggering and amplifying pathways of insulin secretion in G6 rather than G20, hence to loss of glucose effect above G6. This enhanced glucose sensitivity occurred despite an approximately twofold increase in islet uncoupling protein 2 mRNA expression. Besides this increased glucose sensitivity, the maximal glucose stimulation of insulin secretion in High-glucose islets was reduced by approximately 50%, proportionally to the reduction of insulin content. In High-glucose islets, changes in (45)Ca(2+) influx induced by glucose and diazoxide were qualitatively similar but quantitatively smaller than in Control islets and, paradoxically, did not lead to detectable changes in the intracellular Ca(2+) concentration measured by microspectrofluorimetry (fura PE 3). In conclusion, after 1 wk of culture in G30, the loss of glucose stimulation of insulin secretion in the physiological range of glucose concentrations (G5-G10) results from the combination of an increased sensitivity to glucose of both triggering and amplifying pathways of insulin secretion and an approximately 50% reduction in the maximal glucose stimulation of insulin secretion.