Acute overexpression of lactate dehydrogenase-A perturbs beta-cell mitochondrial metabolism and insulin secretion

Stronger control of ATP/ADP by proton leak in pancreatic beta-cells than skeletal muscle mitochondria

Pancreatic beta cells respond to rising blood glucose concentrations by increasing their oxidative metabolism, which leads to an increased ATP/ADP ratio, closure of K(ATP) channels, depolarization of the plasma membrane potential, influx of calcium and the eventual secretion of insulin. Such a signalling mechanism implies that the ATP/ADP ratio is flexible in beta cells (beta-cells), which is in contrast with other cell types (e.g. muscle and liver) that maintain a stable ATP/ADP poise while respiring at widely varying rates. To determine whether this difference in flexibility is accounted for by mitochondrial peculiarities, we performed a top-down metabolic control analysis to quantitatively assess how ATP/ADP is controlled in mitochondria isolated from rat skeletal muscle and cultured beta cells. We show that the ATP/ADP ratio is more strongly controlled (approx. 7.5-fold) by proton leak in beta cells than in muscle. The comparatively high importance of proton leak in beta cell mitochondria (relative to phosphorylation) is evidenced furthermore by its relatively high level of control over membrane potential and overall respiratory activity. Modular-kinetic analysis of oxidative phosphorylation reveals that these control differences can be fully explained by a higher relative leak activity in beta cell mitochondria, which results in a comparatively high contribution of proton leak to the overall respiratory activity in this system.

Different metabolic responses in alpha-, beta-, and delta-cells of the islet of Langerhans monitored by redox confocal microscopy

Blood glucose homeostasis is mainly achieved by the coordinated function of pancreatic alpha-, beta-, and delta-cells, which secrete glucagon, insulin, and somatostatin, respectively. Each cell type responds to glucose changes with different secretion patterns. Currently, considerable information can be found about the signal transduction mechanisms that lead to glucose-mediated insulin release in the pancreatic beta-cell, mitochondrial activation being an essential step. Increases in glucose stimulate the mitochondrial metabolism, activating the tricarboxylic acid cycle and raising the source of redox electron carrier molecules needed for respiratory ATP synthesis. However, little is known about the glucose-induced mitochondrial response of non-beta-cells and its role in the stimulus-secretion coupling process. This limited information is probably a result of the scarcity of these cells in the islet, the lack of identification patterns, and the technical limitations of conventional methods. In this study, we used flavin adenine dinucleotide redox confocal microscopy as a noninvasive technique to specifically monitor mitochondrial redox responses in immunoidentified alpha-, beta-, and delta-cells in freshly isolated intact islets and in dispersed cultured cells. We have shown that glucose provokes metabolic changes in beta- and delta-cell populations in a dose-dependent manner. Conversely, no significant responses were observed in alpha-cells, despite the sensitivity of their metabolism to drugs acting on the mitochondrial function, and their intact ability to develop Ca2+ signals. Identical results were obtained in islets and in cultures of dispersed cells. Our findings indicate metabolic differences in glucose utilization among the alpha-, beta-, and delta-cell populations, which might be important in the signal transduction events that lead to hormone release.

Glucose or insulin, but not zinc ions, inhibit glucagon secretion from mouse pancreatic alpha-cells

The mechanisms by which hypoglycemia stimulates glucagon release are still poorly understood. In particular, the relative importance of direct metabolic coupling versus paracrine regulation by beta-cell secretory products is unresolved. Here, we compare the responses to glucose of 1) alpha-cells within the intact mouse islet, 2) dissociated alpha-cells, and 3) clonal alphaTC1-9 cells. Free cytosolic concentrations of ATP ([ATP](c)) or Ca(2+) ([Ca(2+)](c)) were imaged using alpha-cell-targeted firefly luciferase or a green fluorescent protein-based Ca(2+) probe ("pericam"), respectively. Consistent with a direct effect of glucose on alpha-cell oxidative metabolism, an increase in glucose concentration (from 0 or 3 mmol/l to 20 mmol/l) increased [ATP](c) by 7-9% in alpha-cells within the intact islet and by approximately 4% in alphaTC1-9 cells. Moreover, glucose also dose-dependently decreased the frequency of [Ca(2+)](c) oscillations in both dissociated alpha-cells and alphaTC1-9 cells. Although the effects of glucose were mimicked by exogenous insulin, they were preserved when insulin signaling was blocked with wortmannin. Addition of ZnCl(2) slightly increased the frequency of [Ca(2+)](c) oscillations but failed to affect glucagon release from either islets or alphaTC1-9 cells under most conditions. We conclude that glucose and insulin, but not Zn(2+) ions, independently suppress glucagon secretion in the mouse.

Mammalian exocyst complex is required for the docking step of insulin vesicle exocytosis

Glucose stimulates insulin secretion from pancreatic beta cells by inducing the recruitment and fusion of insulin vesicles to the plasma membrane. However, little is currently known about the mechanism of the initial docking or tethering of insulin vesicles prior to fusion. Here, we examined the role of the SEC6-SEC8 (exocyst) complex, implicated in trafficking of secretory vesicles to fusion sites in the plasma membrane in yeast and in regulating glucose-stimulated insulin secretion from pancreatic MIN6 beta cells. We show first that SEC6 is concentrated on insulin-positive vesicles, whereas SEC5 and SEC8 are largely confined to the cytoplasm and the plasma membrane, respectively. Overexpression of truncated, dominant-negative SEC8 or SEC10 mutants decreased the number of vesicles at the plasma membrane, whereas expression of truncated SEC6 or SEC8 inhibited overall insulin secretion. When single exocytotic events were imaged by total internal reflection fluorescence microscopy, the fluorescence of the insulin surrogate, neuropeptide Y-monomeric red fluorescent protein brightened, diffused, and then vanished with kinetics that were unaffected by overexpression of truncated SEC8 or SEC10. Together, these data suggest that the exocyst complex serves to selectively regulate the docking of insulin-containing vesicles at sites of release close to the plasma membrane.

Mitochondrial localization as a determinant of capacitative Ca2+ entry in HeLa cells

Whether different subsets of mitochondria play distinct roles in shaping intracellular Ca2+ signals is presently unresolved. Here, we determine the role of mitochondria located beneath the plasma membrane in controlling (a) Ca2+ release from the endoplasmic reticulum (ER) and (b) capacitative Ca2+ entry. By over-expression of the dynactin subunit dynamitin, and consequent inhibition of the fission factor, dynamin-related protein (Drp-1), mitochondria were relocalised from the plasma membrane towards the nuclear periphery in HeLa cells. The impact of these changes on free calcium concentration in the cytosol ([Ca2+]c), mitochondria ([Ca2+]m) and ER ([Ca2+]ER) was then monitored with specifically-targeted aequorins. Whilst dynamitin over-expression increased the number of close contacts between the ER and mitochondria by >2.5-fold, assessed using organelle-targeted GFP variants, histamine-induced changes in organellar [Ca2+] were unaffected. By contrast, Ca2+ influx elicited significantly smaller increases in [Ca2+]c and [Ca2+]m in dynamitin-expressing than in control cells. These data suggest that the strategic localisation of a subset of mitochondria beneath the plasma membrane is required for normal Ca2+ influx, but that the transfer of Ca2+ ions between the ER and mitochondria is relatively insensitive to gross changes in the spatial relationship between these two organelles.

Visualising insulin secretion. The Minkowski Lecture 2004

Insulin secretion from pancreatic islet beta cells is a tightly regulated process, under the close control of blood glucose concentrations, neural inputs and circulating hormones. Defects in glucose-triggered insulin secretion, possibly exacerbated by a decrease in beta cell mass, are ultimately responsible for the development of type 2 diabetes. A full understanding of the mechanisms by which glucose and other nutrients trigger insulin secretion will probably be essential to allow for the development of new therapies of type 2 diabetes and for the derivation of "artificial" beta cells from embryonic stem cells as a treatment for type 1 diabetes. I focus here on recent developments in our understanding of beta cell glucose sensing, achieved in part through the development of recombinant targeted probes (luciferase, green fluorescent protein) that allow islet beta cell metabolism and Ca(2+) handling to be imaged in situ in the intact islet with single cell resolution. Combined with classical biochemistry, these techniques show that the beta cell is uniquely poised, thanks to the expression of low levels of lactate dehydrogenase and plasma membrane lactate/monocarboxylate transporters, to channel glucose carbons towards oxidative metabolism, ATP synthesis and inhibition of AMP-activated protein kinase, a newly defined regulator of insulin release. I also discuss the molecular basis of the recruitment of secretory vesicles to the cell surface, analysed by the use of new imaging techniques including total internal reflection of fluorescence, as well as the "nanomechanics" of the exocytotic event itself.

Impact of PPARgamma overexpression and activation on pancreatic islet gene expression profile analyzed with oligonucleotide microarrays

Peroxisome proliferator-activated receptor-gamma (PPARgamma) serves as a target for the thiazolidinedione class of antidiabetic drugs and is an important regulator of adipose tissue differentiation. By contrast, the principal target genes for PPARgamma in the pancreatic islet and the impact of their induction on insulin secretion are largely undefined. Here, we show that mRNAs encoding both isoforms of rodent PPARgamma, gamma1 and gamma2, are expressed in primary rat islets and are upregulated by overexpresssion of the lipogenic transcription factor sterol response element-binding protein 1c. Unexpectedly, however, oligonucleotide microarray analysis demonstrates that graded activation of PPARgamma achieved with 1) the thiazolidinedione GW-347845, 2) transduction with adenoviral PPARgamma1, or 3) a combination of both treatments progressively enhances the expression of genes involved in fatty acid oxidation and transport. Moreover, maximal activation of PPARgamma1 reduces islet triglyceride levels and enhances the oxidation of exogenous palmitate while decreasing glucose oxidation, cellular ATP content, and glucose-, but not depolarization-stimulated, insulin secretion. We conclude that, in the context of the pancreatic islet, the principal response to PPARgamma expression and activation is the activation of genes involved in the disposal, rather than the synthesis, of fatty acids. Although fatty acid oxidation may have beneficial effects on beta-cell function in the longer term by countering beta-cell "lipotoxicity," the acute response to this metabolic shift is a marked inhibition of insulin secretion.

Cytoplasmic dynein regulates the subcellular distribution of mitochondria by controlling the recruitment of the fission factor dynamin-related protein-1

While the subcellular organisation of mitochondria is likely to influence many aspects of cell physiology, its molecular control is poorly understood. Here, we have investigated the role of the retrograde motor protein complex, dynein-dynactin, in mitochondrial localisation and morphology. Disruption of dynein function, achieved in HeLa cells either by over-expressing the dynactin subunit, dynamitin (p50), or by microinjection of an anti-dynein intermediate chain antibody, resulted in (a) the redistribution of mitochondria to the nuclear periphery, and (b) the formation of long and highly branched mitochondrial structures. Suggesting that an alteration in the balance between mitochondrial fission and fusion may be involved in both of these changes, overexpression of p50 induced the translocation of the fission factor dynamin-related protein (Drp1) from mitochondrial membranes to the cytosol and microsomes. Moreover, a dominant-negative-acting form of Drp1 mimicked the effects of p50 on mitochondrial morphology, while wild-type Drp1 almost completely restored normal mitochondrial distribution in p50 over-expressing cells. Thus, the dynein/dynactin complex plays an unexpected role in the regulation of mitochondrial morphology in living cells, by controlling the recruitment of Drp1 to these organelles.

Impaired glucose homeostasis in transgenic mice expressing the human transient neonatal diabetes mellitus locus, TNDM

Transient neonatal diabetes mellitus (TNDM) is a rare inherited diabetic syndrome apparent in the first weeks of life and again during early adulthood. The relative contributions of reduced islet beta cell number and impaired beta cell function to the observed hypoinsulinemia are unclear. The inheritance pattern of this imprinted disorder implicates overexpression of one or both genes within the TNDM locus: ZAC, which encodes a proapoptotic zinc finger protein, and HYMAI, which encodes an untranslated mRNA. To investigate the consequences for pancreatic function, we have developed a high-copy transgenic mouse line, TNDM29, carrying the human TNDM locus. TNDM29 neonates display hyperglycemia, and older adults, impaired glucose tolerance. Neonatal hyperglycemia occurs only on paternal transmission, analogous to paternal dependence of TNDM in humans. Embryonic pancreata of TNDM29 mice showed reductions in expression of endocrine differentiation factors and numbers of insulin-staining structures. By contrast, beta cell mass was normal or elevated at all postnatal stages, whereas pancreatic insulin content in neonates and peak serum insulin levels after glucose infusion in adults were reduced. Expression of human ZAC and HYMAI in these transgenic mice thus recapitulates key features of TNDM and implicates impaired development of the endocrine pancreas and beta cell function in disease pathogenesis.

Impaired glucose homeostasis in transgenic mice expressing the human transient neonatal diabetes mellitus locus, TNDM

Transient neonatal diabetes mellitus (TNDM) is a rare inherited diabetic syndrome apparent in the first weeks of life and again during early adulthood. The relative contributions of reduced islet beta cell number and impaired beta cell function to the observed hypoinsulinemia are unclear. The inheritance pattern of this imprinted disorder implicates overexpression of one or both genes within the TNDM locus: ZAC, which encodes a proapoptotic zinc finger protein, and HYMAI, which encodes an untranslated mRNA. To investigate the consequences for pancreatic function, we have developed a high-copy transgenic mouse line, TNDM29, carrying the human TNDM locus. TNDM29 neonates display hyperglycemia, and older adults, impaired glucose tolerance. Neonatal hyperglycemia occurs only on paternal transmission, analogous to paternal dependence of TNDM in humans. Embryonic pancreata of TNDM29 mice showed reductions in expression of endocrine differentiation factors and numbers of insulin-staining structures. By contrast, beta cell mass was normal or elevated at all postnatal stages, whereas pancreatic insulin content in neonates and peak serum insulin levels after glucose infusion in adults were reduced. Expression of human ZAC and HYMAI in these transgenic mice thus recapitulates key features of TNDM and implicates impaired development of the endocrine pancreas and beta cell function in disease pathogenesis.

Over-expression of sterol-regulatory-element-binding protein-1c (SREBP1c) in rat pancreatic islets induces lipogenesis and decreases glucose-stimulated insulin release: modulation by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR)

Accumulation of intracellular lipid by pancreatic islet beta-cells has been proposed to inhibit normal glucose-regulated insulin secretion ('glucolipotoxicity'). In the present study, we determine whether over-expression in rat islets of the lipogenic transcription factor SREBP1c (sterol-regulatory-element-binding protein-1c) affects insulin release, and whether changes in islet lipid content may be reversed by activation of AMPK (AMP-activated protein kinase). Infection with an adenovirus encoding the constitutively active nuclear fragment of SREBP1c resulted in expression of the protein in approx. 20% of islet cell nuclei, with a preference for beta-cells at the islet periphery. Real-time PCR (TaqMan) analysis showed that SREBP1c up-regulated the expression of FAS (fatty acid synthase; 6-fold), acetyl-CoA carboxylase-1 (2-fold), as well as peroxisomal-proliferator-activated receptor-gamma (7-fold), uncoupling protein-2 (1.4-fold) and Bcl2 (B-cell lymphocytic-leukaemia proto-oncogene 2; 1.3-fold). By contrast, levels of pre-proinsulin, pancreatic duodenal homeobox-1, glucokinase and GLUT2 (glucose transporter isoform-2) mRNAs were unaltered. SREBP1c-transduced islets displayed a 3-fold increase in triacylglycerol content, decreased glucose oxidation and ATP levels, and a profound inhibition of glucose-, but not depolarisation-, induced insulin secretion. Culture of islets with the AMPK activator 5-amino-4-imidazolecarboxamide riboside decreased the expression of the endogenous SREBP1c and FAS genes, and reversed the effect of over-expressing active SREBP1c on FAS mRNA levels and cellular triacylglycerol content. We conclude that SREBP1c over-expression, even when confined to a subset of beta-cells, leads to defective insulin secretion from islets and may contribute to some forms of Type II diabetes.

Involvement of Per-Arnt-Sim (PAS) kinase in the stimulation of preproinsulin and pancreatic duodenum homeobox 1 gene expression by glucose

Per-Arnt-Sim (PAS) domain-containing kinases are common in prokaryotes, but a mammalian counterpart has only recently been described. Although the PAS domain of the mammalian PAS kinase (PASK) is closely related to the bacterial oxygen sensor FixL, it is unclear whether PASK activity is changed in mammalian cells in response to nutrients and might therefore contribute to signal transduction by these or other stimuli. Here, we show that elevated glucose concentrations rapidly increase PASK activity in pancreatic islet beta cells, an event followed by the accumulation of both PASK mRNA and protein. Demonstrating a physiological role for PASK activation, comicroinjection into clonal beta cells of cDNA encoding wild-type PASK, or PASK protein itself, mimics the induction of preproinsulin promoter activity by high glucose concentrations. Conversely, anti-PASK antibodies block promoter activation by the sugar, and the silencing of PASK expression by RNA interference suppresses the up-regulation by glucose of preproinsulin and pancreatic duodenum homeobox 1 gene expression, without affecting glucose-induced changes in the levels of mRNAs encoding glucokinase or uncoupling protein 2. We conclude that PASK is an important metabolic sensor in nutrient-sensitive mammalian cells and plays an unexpected role in the regulation of key genes involved in maintaining the differentiated phenotype of pancreatic beta cells.

Role for plasma membrane-related Ca2+-ATPase-1 (ATP2C1) in pancreatic beta-cell Ca2+ homeostasis revealed by RNA silencing

Changes in intracellular Ca(2+) concentration play a key role in the regulation of insulin secretion by glucose and other secretagogues. Here, we explore the importance of the secretory pathway Ca(2+)-ATPase, plasma membrane-related Ca(2+)-ATPase-1 (PMR1; human orthologue ATP2C1) in intracellular Ca(2+) homeostasis in pancreatic islet beta-cells. Endogenous PMR1 mRNA and protein were detected in both isolated rat islets and beta-cell-derived lines (MIN6 and INS1). Subcellular fractionation of the cell lines revealed PMR1 immunoreactivity in both microsomal and dense-core secretory vesicle-enriched fractions. Correspondingly, depletion of cellular PMR1 with small interfering RNAs inhibited Ca(2+) uptake into the endoplasmic reticulum and secretory vesicles by approximately 20%, as assessed using organelle-targeted aequorins in permeabilized INS1 cells. In intact cells, PMR1 depletion markedly enhanced flux though L-type Ca(2+) channels and augmented glucose-stimulated, but not basal, insulin secretion. Whereas average cytosolic [Ca(2+)] increases in response to 30.0 mmol/l glucose were unaffected by PMR1 depletion, [Ca(2+)] oscillation shape, duration, and decay rate in response to glucose plus tetraethylammonium were modified in PMR1-depleted single cells, imaged using fluo-3-acetoxymethylester. PMR1 thus plays an important role, which is at least partially nonoverlapping with that of sarco(endo-)plasmic reticulum Ca(2+)-ATPases, in the control of beta-cell Ca(2+) homeostasis and insulin secretion.

Severe transient neonatal hyperinsulinism associated with hyperlactataemia in non-asphyxiated infants

Transient hyperinsulinism (HI) occurs in infants born to diabetic mothers, in infants experiencing perinatal asphyxia and in infants with intrauterine growth retardation. The precise mechanism of transient HI in these different aetiologies is not fully understood. Lactic acidosis is commonly seen in neonates as a secondary phenomenon due to hypoxia, hypovolaemia, anaemia and infection. The combination of transient HI and lactic acidosis is rare. We present the clinical and biochemical features of five infants presenting with transient HI associated with hyperlactataemia in the absence of markers of perinatal stress. This combination lasted for 3-4 weeks with complete resolution except in one patient in whom the hyperinsulinism lasted until 6 months before resolution. The precise mechanism of this association is not clear but may be related either to immaturity of the pyruvate dehydrogenase complex or to the accumulation of abnormal intramitochondrial intermediary metabolites. Infants presenting with HI should have a free flowing blood sample drawn for the measurement of plasma lactate levels.

Distinct roles for insulin and insulin-like growth factor-1 receptors in pancreatic beta-cell glucose sensing revealed by RNA silencing

The importance of the insulin receptor (IR) and the insulin-like growth factor-1 receptor (IGF-1R) for glucose-regulated insulin secretion and gene expression in pancreatic islet beta-cells is at present unresolved. Here, we have used small interfering RNAs (siRNAs) to silence the expression of each receptor selectively in clonal MIN6 beta-cells. Reduction of IR levels by >90% completely inhibited glucose (30 mM compared with 3 mM)-induced insulin secretion, but had no effect on depolarization-stimulated secretion. IR depletion also blocked the accumulation of preproinsulin (PPI), pancreatic duodenum homoeobox-1 (PDX-1) and glucokinase (GK) mRNAs at elevated glucose concentrations, as assessed by quantitative real-time PCR analysis (TaqMan). Similarly, depletion of IGF-1R inhibited glucose-induced insulin secretion but, in contrast with the effects of IR silencing, had little impact on the regulation of gene expression by glucose. Moreover, loss of IGF-1R, but not IR, markedly inhibited glucose-stimulated increases in cytosolic and mitochondrial ATP, suggesting a role for IGF-1R in the maintenance of oxidative metabolism and in the generation of mitochondrial coupling factors. RNA silencing thus represents a useful tool for the efficient and selective inactivation of receptor tyrosine kinases in isolated beta-cells. By inhibiting glucose-stimulated insulin secretion through the inactivation of IGF-1R, this approach also demonstrates the existence of insulin-independent mechanisms whereby elevated glucose concentrations regulate PPI, PDX-1 and GK gene expression in beta-cells.

ATP-dependent interaction of the cytosolic domains of the inwardly rectifying K+ channel Kir6.2 revealed by fluorescence resonance energy transfer

ATP-sensitive K(+) (K(ATP)) channels play important roles in the regulation of membrane excitability in many cell types. ATP inhibits channel activity by binding to a specific site formed by the N and C termini of the pore-forming subunit, Kir6.2, but the structural changes associated with this interaction remain unclear. Here, we use fluorescence resonance energy transfer (FRET) to study the ATP-dependent interaction between the N and C termini of Kir6.2 using a construct bearing fused cyan and yellow fluorescent proteins (ECFP-Kir6.2-EYFP). When expressed in human embryonic kidney cells, ECFP-Kir6.2-EYFP/SUR1 channels displayed FRET that was augmented by agonist stimulation and diminished by metabolic poisoning. Addition of ATP to permeabilized cells or isolated plasma membrane sheets increased FRET. FRET changes were abolished by Kir6.2 mutations that altered ATP-dependent channel closure and channel gating. In the wild-type channel, the ATP concentrations, which increased FRET (EC(50) = 1.36 mM), were significantly higher than those causing channel inhibition (IC(50) = 0.29 mM). Demonstrating the existence of intermolecular interactions, a dimeric construct comprising two molecules of Kir6.2 linked head-to-tail (ECFP-Kir6.2-Kir6.2-EYFP) displayed less FRET than the monomer in the absence of nucleotide but still exhibited ATP-dependent FRET increases (EC(50) = 1.52 mM) and channel inhibition. We conclude that binding of ATP to Kir6.2, (i). alters the interaction between the N- and C-terminal domains, (ii). probably involves both intrasubunit and intersubunit interactions, (iii). reflects ligand binding not channel gating, and (iv). occurs in intact cells when subplasmalemmal [ATP] changes in the millimolar range.

5'-AMP-activated protein kinase controls insulin-containing secretory vesicle dynamics

Changes in 5'-AMP-activated protein kinase (AMPK) activity have recently been implicated in the control of insulin secretion by glucose (da Silva Xavier, G., Leclerc, I., Varadi, A., Tsuboi, T., Moule, S. K., and Rutter, G. A. (2003) Biochem. J. 371, 761-774). Here, we examine the possibility that activation of AMPK may regulate distal steps in insulin secretion, including vesicle movement and fusion with the plasma membrane. Vesicle dynamics were imaged in single pancreatic MIN6 beta-cells expressing lumen-targeted pH-insensitive yellow fluorescent protein, neuropeptide Y.Venus, or monomeric red fluorescent protein by total internal reflection fluorescence and Nipkow disc confocal microscopy. Overexpression of a truncated, constitutively active form of AMPK (AMPKalpha1, 1-312, T172D; AMPK CA), inhibited glucose-stimulated (30 versus 3.0 mM) vesicle movements, and decreased the number of vesicles docked or fusing at the plasma membrane, while having no effect on the kinetics of individual secretory events. Expression of the activated form of AMPK also prevented dispersal of the cortical actin network at high glucose concentrations. Monitored in permeabilized cells, where the effects of AMPK CA on glucose metabolism and ATP synthesis were bypassed, AMPK CA inhibited Ca2+ and ATP-induced insulin secretion, and decreased ATP-dependent vesicle movements. These findings suggest that components of the vesicle transport network, including vesicle-associated motor proteins, may be targets of AMPK in beta-cells, dephosphorylation of which is required for vesicle mobilization at elevated glucose concentrations.

Roles of 5'-AMP-activated protein kinase (AMPK) in mammalian glucose homoeostasis

AMPK (5'-AMP-activated protein kinase) is emerging as a metabolic master switch, by which cells in both mammals and lower organisms sense and decode changes in energy status. Changes in AMPK activity have been shown to regulate glucose transport in muscle and glucose production by the liver. Moreover, AMPK appears to be a key regulator of at least one transcription factor linked to a monogenic form of diabetes mellitus. As a result, considerable efforts are now under way to explore the usefulness of AMPK as a therapeutic target for other forms of this disease. Here we review this topic, and discuss new findings which suggest that AMPK may play roles in regulating insulin release and the survival of pancreatic islet beta-cells, and nutrient sensing by the brain.

Role for AMP-activated protein kinase in glucose-stimulated insulin secretion and preproinsulin gene expression

AMP-activated protein kinase (AMPK) has recently been implicated in the control of preproinsulin gene expression in pancreatic islet beta-cells [da Silva Xavier, Leclerc, Salt, Doiron, Hardie, Kahn and Rutter (2000) Proc. Natl. Acad. Sci. U.S.A. 97, 4023-4028]. Using pharmacological and molecular strategies to regulate AMPK activity in rat islets and clonal MIN6 beta-cells, we show here that the effects of AMPK are exerted largely upstream of insulin release. Thus forced increases in AMPK activity achieved pharmacologically with 5-amino-4-imidazolecarboxamide riboside (AICAR), or by adenoviral overexpression of a truncated, constitutively active form of the enzyme (AMPK alpha 1.T(172)D), blocked glucose-stimulated insulin secretion. In MIN6 cells, activation of AMPK suppressed glucose metabolism, as assessed by changes in total, cytosolic or mitochondrial [ATP] and NAD(P)H, and reduced increases in intracellular [Ca(2+)] caused by either glucose or tolbutamide. By contrast, inactivation of AMPK by expression of a dominant-negative form of the enzyme mutated in the catalytic site (AMPK alpha 1.D(157)A) did not affect glucose-stimulated increases in [ATP], NAD(P)H or intracellular [Ca(2+)], but led to the unregulated release of insulin. These results indicate that inhibition of AMPK by glucose is essential for the activation of insulin secretion by the sugar, and may contribute to the transcriptional stimulation of the preproinsulin gene. Modulation of AMPK activity in the beta-cell may thus represent a novel therapeutic strategy for the treatment of type 2 diabetes mellitus.

Ryanodine receptor type I and nicotinic acid adenine dinucleotide phosphate receptors mediate Ca2+ release from insulin-containing vesicles in living pancreatic beta-cells (MIN6)

We have demonstrated recently (Mitchell, K. J., Pinton, P., Varadi, A., Tacchetti, C., Ainscow, E. K., Pozzan, T., Rizzuto, R., and Rutter, G. A. (2001) J. Cell Biol. 155, 41-51) that ryanodine receptors (RyR) are present on insulin-containing secretory vesicles. Here we show that pancreatic islets and derived beta-cell lines express type I and II, but not type III, RyRs. Purified by subcellular fractionation and membrane immuno-isolation, dense core secretory vesicles were found to possess a similar level of type I RyR immunoreactivity as Golgi/endoplasmic reticulum (ER) membranes but substantially less RyR II than the latter. Monitored in cells expressing appropriately targeted aequorins, dantrolene, an inhibitor of RyR I channels, elevated free Ca(2+) concentrations in the secretory vesicle compartment from 40.1 +/- 6.7 to 90.4 +/- 14.8 microm (n = 4, p < 0.01), while having no effect on ER Ca(2+) concentrations. Furthermore, nicotinic acid adenine dinucleotide phosphate (NAADP), a novel Ca(2+)-mobilizing agent, decreased dense core secretory vesicle but not ER free Ca(2+) concentrations in permeabilized MIN6 beta-cells, and flash photolysis of caged NAADP released Ca(2+) from a thapsigargin-insensitive Ca(2+) store in single MIN6 cells. Because dantrolene strongly inhibited glucose-stimulated insulin secretion (from 3.07 +/- 0.51-fold stimulation to no significant glucose effect; n = 3, p < 0.01), we conclude that RyR I-mediated Ca(2+)-induced Ca(2+) release from secretory vesicles, possibly potentiated by NAADP, is essential for the activation of insulin secretion.

CNS sensing and regulation of peripheral glucose levels

It is clear that the brain has evolved a mechanism for sensing levels of ambient glucose. Teleologically, this is likely to be a function of its requirement for glucose as a primary metabolic substrate. There is no question that the brain can sense and mount a counterregulatory response to restore very low levels of plasma and brain glucose. But it is less clear that the changes in glucose associated with normal diurnal rhythms and feeding cycles are sufficient to influence either ingestive behavior or the physiologic responses involved in regulating plasma glucose levels. Glucosensing neurons are clearly a distinct class of metabolic sensors with the capacity to respond to a variety of intero- and exteroceptive stimuli. This makes it likely that these glucosensing neurons do participate in physiologically relevant homeostatic mechanisms involving energy balance and the regulation of peripheral glucose levels. It is our challenge to identify the mechanisms by which these neurons sense and respond to these metabolic cues.

Functional metabotropic glutamate receptors are expressed in oligodendrocyte progenitor cells

We investigated the expression of metabotropic glutamate receptor (mGluR) isoforms in CG-4 rodent oligodendroglial progenitor cells (OPC) and rat brain oligodendrocytes. Our RT-PCR analysis detected mRNAs for mGluR3 and mGluR5 isoforms in OPCs. Although neurons express both mGluR5a and mGluR5b splice variants, only mGluR5a was identified in OPCs. Antibodies to mGluR2/3 and mGluR5 detected the corresponding receptor proteins in immunoblots of OPC membrane fractions. Furthermore, immunocytochemical analysis identified mGluR5 in oligodendrocyte marker O4-positive OPCs. The expression of mGluR5 was also demonstrated in oligodendrocyte marker (O4 and O1) positive cells in white matter of postnatal 4- and 7-day-old rat brain sections using immunofluorescent double labelling and confocal microscopy. The mGluR5 receptor function was assessed in CG-4 OPCs with fura-2 microfluorometry. Application of the mGluR1/5 specific agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induced calcium oscillations, which were inhibited by the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP). The DHPG induced calcium oscillations required Ca2+ release from intracellular stores. In OPCs the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) decreased forskolin-stimulated cAMP synthesis, indicating the presence of functional mGluR3. The newly identified mGluR3 and mGluR5a may be involved in the differentiation of oligodendrocytes, myelination and the development of white matter damage.

Critical reduction in beta-cell mass results in two distinct outcomes over time. Adaptation with impaired glucose tolerance or decompensated diabetes

We have proposed that hyperglycemia-induced dedifferentiation of beta-cells is a critical factor for the loss of insulin secretory function in diabetes. Here we examined the effects of the duration of hyperglycemia on gene expression in islets of partially pancreatectomized (Px) rats. Islets were isolated, and mRNA was extracted from rats 4 and 14 weeks after Px or sham Px surgery. Px rats developed different degrees of hyperglycemia; low hyperglycemia was assigned to Px rats with fed blood glucose levels less than 150 mg/dl, and high hyperglycemia was assigned above 150 mg/dl. beta-Cell hypertrophy was present at both 4 and 14 weeks. At the same time points, high hyperglycemia rats showed a global alteration in gene expression with decreased mRNA for insulin, IAPP, islet-associated transcription factors (pancreatic and duodenal homeobox-1, BETA2/NeuroD, Nkx6.1, and hepatocyte nuclear factor 1 alpha), beta-cell metabolic enzymes (glucose transporter 2, glucokinase, mitochondrial glycerol phosphate dehydrogenase, and pyruvate carboxylase), and ion channels/pumps (Kir6.2, VDCC beta, and sarcoplasmic reticulum Ca(2+)-ATPase 3). Conversely, genes normally suppressed in beta-cells, such as lactate dehydrogenase-A, hexokinase I, glucose-6-phosphatase, stress genes (heme oxygenase-1, A20, and Fas), and the transcription factor c-Myc, were markedly increased. In contrast, gene expression in low hyperglycemia rats was only minimally changed at 4 weeks but significantly changed at 14 weeks, indicating that even low levels of hyperglycemia induce beta-cell dedifferentiation over time. In addition, whereas 2 weeks of correction of hyperglycemia completely reverses the changes in gene expression of Px rats at 4 weeks, the changes at 14 weeks were only partially reversed, indicating that the phenotype becomes resistant to reversal in the long term. In conclusion, chronic hyperglycemia induces a progressive loss of beta-cell phenotype with decreased expression of beta-cell-associated genes and increased expression of normally suppressed genes, these changes being present with even minimal levels of hyperglycemia. Thus, both the severity and duration of hyperglycemia appear to contribute to the deterioration of the beta-cell phenotype found in diabetes.

Glucagon-like peptide-1 mobilizes intracellular Ca2+ and stimulates mitochondrial ATP synthesis in pancreatic MIN6 beta-cells

Glucagon-like peptide-1 (GLP-1) is a potent regulator of glucose-stimulated insulin secretion whose mechanisms of action are only partly understood. In the present paper, we show that at low (3 mM) glucose concentrations, GLP-1 increases the free intramitochondrial concentrations of both Ca(2+) ([Ca(2+)](m)), and ATP ([ATP](m)) in clonal MIN6 beta-cells. Suggesting that cAMP-mediated release of Ca(2+) from intracellular stores is responsible for these effects, increases in [ATP](m) that were induced by GLP-1 were completely blocked by the Rp isomer of adenosine-3',5'-cyclic monophosphothioate (Rp-cAMPS), or by chelation of intracellular Ca(2+). Furthermore, inhibition of Ins(1,4,5) P (3) (IP(3)) receptors with xestospongin C, or application of ryanodine, partially inhibited GLP-1-induced [ATP](m) increases, and the simultaneous blockade of both IP(3) and ryanodine receptors (RyR) completely eliminated the rise in [ATP](m). GLP-1 appeared to prompt Ca(2+)-induced Ca(2+) release through IP(3) receptors via a protein kinase A (PKA)-mediated phosphorylation event, since ryanodine-insensitive [ATP](m) increases were abrogated with the PKA inhibitor, H89. In contrast, the effects of GLP-1 on RyR-mediated [ATP](m) increases were apparently mediated by the cAMP-regulated guanine nucleotide exchange factor cAMP-GEFII, since xestospongin C-insensitive [ATP](m) increases were blocked by a dominant-negative form of cAMP-GEFII (G114E,G422D). Taken together, these results demonstrate that GLP-1 potentiates glucose-stimulated insulin release in part via the mobilization of intracellular Ca(2+), and the stimulation of mitochondrial ATP synthesis.

Involvement of conventional kinesin in glucose-stimulated secretory granule movements and exocytosis in clonal pancreatic beta-cells

Recruitment of secretory vesicles to the cell surface is essential for the sustained secretion of insulin in response to glucose. At present, the molecular motors involved in this movement, and the mechanisms whereby they may be regulated, are undefined. To investigate the role of kinesin family members, we labelled densecore vesicles in clonal beta-cells using an adenovirally expressed, vesicle-targeted green fluorescent protein (phogrin.EGFP), and employed immunoadsorption to obtain highly purified insulin-containing vesicles. Whereas several kinesin family members were expressed in this cell type, only conventional kinesin heavy chain (KHC) was detected in vesicle preparations. Expression of a dominant-negative KHC motor domain (KHC(mut)) blocked all vesicular movements with velocity >0.4 micro m second(-1), which demonstrates that kinesin activity was essential for vesicle motility in live beta-cells. Moreover, expression of KHC(mut) strongly inhibited the sustained, but not acute, stimulation of secretion by glucose. Finally, vesicle movement was stimulated by ATP dose-dependently in permeabilized cells, which suggests that glucose-induced increases in cytosolic [ATP] mediate the effects of the sugar in vivo, by enhancing kinesin activity. These data therefore provide evidence for a novel mechanism whereby glucose may enhance insulin release.

Dynamic imaging of free cytosolic ATP concentration during fuel sensing by rat hypothalamic neurones: evidence for ATP-independent control of ATP-sensitive K(+) channels

Glucose-responsive (GR) neurons from hypothalamic nuclei are implicated in the regulation of feeding and satiety. To determine the role of intracellular ATP in the closure of ATP-sensitive K(+) (K(ATP)) channels in these cells and associated glia, the cytosolic ATP concentration ([ATP](c)) was monitored in vivo using adenoviral-driven expression of recombinant targeted luciferases and bioluminescence imaging. Arguing against a role for ATP in the closure of K(ATP) channels in GR neurons, glucose (3 or 15 mM) caused no detectable increase in [ATP](c), monitored with cytosolic luciferase, and only a small decrease in the concentration of ATP immediately beneath the plasma membrane, monitored with a SNAP25-luciferase fusion protein. In contrast to hypothalamic neurons, hypothalamic glia responded to glucose (3 and 15 mM) with a significant increase in [ATP](c). Both neurons and glia from the cerebellum, a glucose-unresponsive region of the brain, responded robustly to 3 or 15 mM glucose with increases in [ATP](c). Further implicating an ATP-independent mechanism of K(ATP) channel closure in hypothalamic neurons, removal of extracellular glucose (10 mM) suppressed the electrical activity of GR neurons in the presence of a fixed, high concentration (3 mM) of intracellular ATP. Neurons from both brain regions responded to 5 mM lactate (but not pyruvate) with an oligomycin-sensitive increase in [ATP](c). High levels of the plasma membrane lactate-monocarboxylate transporter, MCT1, were found in both cell types, and exogenous lactate efficiently closed K(ATP) channels in GR neurons. These data suggest that (1) ATP-independent intracellular signalling mechanisms lead to the stimulation of hypothalamic neurons by glucose, and (2) these effects may be potentiated in vivo by the release of lactate from neighbouring glial cells.

Dynamics of glucose-induced membrane recruitment of protein kinase C beta II in living pancreatic islet beta-cells

The mechanisms by which glucose may affect protein kinase C (PKC) activity in the pancreatic islet beta-cell are presently unclear. By developing adenovirally expressed chimeras encoding fusion proteins between green fluorescent protein and conventional (betaII), novel (delta), or atypical (zeta) PKCs, we show that glucose selectively alters the subcellular localization of these enzymes dynamically in primary islet and MIN6 beta-cells. Examined by laser scanning confocal or total internal reflection fluorescence microscopy, elevated glucose concentrations induced oscillatory translocations of PKCbetaII to spatially confined regions of the plasma membrane. Suggesting that increases in free cytosolic Ca(2+) concentrations ([Ca(2+)](c)) were primarily responsible, prevention of [Ca(2+)](c) increases with EGTA or diazoxide completely eliminated membrane recruitment, whereas elevation of cytosolic [Ca(2+)](c) with KCl or tolbutamide was highly effective in redistributing PKCbetaII both to the plasma membrane and to the surface of dense core secretory vesicles. By contrast, the distribution of PKCdelta.EGFP, which binds diacylglycerol but not Ca(2+), was unaffected by glucose. Measurement of [Ca(2+)](c) immediately beneath the plasma membrane with a ratiometric "pericam," fused to synaptic vesicle-associated protein-25, revealed that depolarization induced significantly larger increases in [Ca(2+)](c) in this domain. These data demonstrate that nutrient stimulation of beta-cells causes spatially and temporally complex changes in the subcellular localization of PKCbetaII, possibly resulting from the generation of Ca(2+) microdomains. Localized changes in PKCbetaII activity may thus have a role in the spatial control of insulin exocytosis.

Stimulation of acetyl-CoA carboxylase gene expression by glucose requires insulin release and sterol regulatory element binding protein 1c in pancreatic MIN6 beta-cells

Acetyl-CoA carboxylase I (ACCI) is a key lipogenic enzyme whose induction in islet beta-cells may contribute to glucolipotoxicity. Here, we provide evidence that enhanced insulin release plays an important role in the activation of this gene by glucose. Glucose (30 vs. 3 mmol/l) increased ACCI mRNA levels approximately 4-fold and stimulated ACCI (pII) promoter activity >30-fold in MIN6 cells. The latter effect was completely suppressed by blockade of insulin release or of insulin receptor signaling. However, added insulin substantially, but not completely, mimicked the effects of glucose, suggesting that intracellular metabolites of glucose may also contribute to transcriptional stimulation. Mutational analysis of the ACCI promoter, and antibody microinjection, revealed that the effect of glucose required sterol response element binding protein (SREBP)-1c. Moreover, adenoviral transduction with dominant-negative-acting SREBP1c blocked ACCI gene induction, whereas constitutively active SREBP1c increased ACCI mRNA levels. Finally, glucose also stimulated SREBP1c transcription, although this effect was independent of insulin release. These data suggest that glucose regulates ACCI gene expression in the beta-cell by complex mechanisms that may involve the covalent modification of SREBP1c. However, overexpression of SREBP1c also decreased glucose-stimulated insulin release, implicating SREBP1c induction in beta-cell lipotoxicity in some forms of type 2 diabetes.

Succinate is a preferential metabolic stimulus-coupling signal for glucose-induced proinsulin biosynthesis translation

The secondary signals emanating from increased glucose metabolism, which lead to specific increases in proinsulin biosynthesis translation, remain elusive. It is known that signals for glucose-stimulated insulin secretion and proinsulin biosynthesis diverge downstream of glycolysis. Consequently, the mitochondrial products ATP, Krebs cycle intermediates, glutamate, and acetoacetate were investigated as candidate stimulus-coupling signals specific for glucose-induced proinsulin biosynthesis in rat islets. Decreasing ATP levels by oxidative phosphorylation inhibitors showed comparable effects on proinsulin biosynthesis and total protein synthesis. Although it is a cofactor, ATP is unlikely to be a metabolic stimulus-coupling signal specific for glucose-induced proinsulin biosynthesis. Neither glutamic acid methyl ester nor acetoacetic acid methyl ester showed a specific effect on glucose-stimulated proinsulin biosynthesis. Interestingly, among Krebs cycle intermediates, only succinic acid monomethyl ester specifically stimulated proinsulin biosynthesis. Malonic acid methyl ester, an inhibitor of succinate dehydrogenase, also specifically increased glucose-induced proinsulin biosynthesis without affecting islet ATP levels or insulin secretion. Glucose caused a 40% increase in islet intracellular succinate levels, but malonic acid methyl ester showed no further effect, probably due to efficient conversion of succinate to succinyl-CoA. In this regard, a GTP-dependent succinyl-CoA synthetase activity was found in cytosolic fractions of pancreatic islets. Thus, succinate and/or succinyl-CoA appear to be preferential metabolic stimulus-coupling factors for glucose-induced proinsulin biosynthesis translation.

Genetic regulation of metabolic pathways in beta-cells disrupted by hyperglycemia

In models of type 2 diabetes the expression of beta-cell genes is altered, but these changes have not fully explained the impairment in beta-cell function. We hypothesized that changes in beta-cell phenotype and global alterations in both carbohydrate and lipid pathways are likely to contribute to secretory abnormalities. Therefore, expression of genes involved in carbohydrate and lipid metabolism were analyzed in islets 4 weeks after 85-95% partial pancreatectomy (Px) when beta-cells have impaired glucose-induced insulin secretion and ATP synthesis. Px rats after 1 week developed mild to severe hyperglycemia that was stable for the next 3 weeks, whereas neither plasma triglyceride, non-esterified fatty acid, or islet triglyceride levels were altered. Expression of peroxisome proliferator-activated receptors (PPARs), with several target genes, were reciprocally regulated; PPARalpha was markedly reduced even at low level hyperglycemia, whereas PPARgamma was progressively increased with increasing hyperglycemia. Uncoupling protein 2 (UCP-2) was increased as were other genes barely expressed in sham islets including lactate dehydrogenase-A (LDH-A), lactate (monocarboxylate) transporters, glucose-6-phosphatase, fructose-1,6-bisphosphatase, 12-lipoxygenase, and cyclooxygenase 2. On the other hand, the expression of beta-cell-associated genes, insulin, and GLUT2 were decreased. Treating Px rats with phlorizin normalized hyperglycemia without effecting plasma fatty acids and reversed the changes in gene expression implicating the importance of hyperglycemia per se in the loss of beta-cell phenotype. In addition, parallel changes were observed in beta-cell-enriched tissue dissected by laser capture microdissection from the central core of islets. In conclusion, chronic hyperglycemia leads to a critical loss of beta-cell differentiation with altered expression of genes involved in multiple metabolic pathways diversionary to normal beta-cell glucose metabolism. This global maladaptation in gene expression at the time of increased secretory demand may contribute to the beta-cell dysfunction found in diabetes.

Glucose-stimulated insulin secretion does not require activation of pyruvate dehydrogenase: impact of adenovirus-mediated overexpression of PDH kinase and PDH phosphate phosphatase in pancreatic islets

Glucose-stimulated increases in mitochondrial metabolism are generally thought to be important for the activation of insulin secretion. Pyruvate dehydrogenase (PDH) is a key regulatory enzyme, believed to govern the rate of pyruvate entry into the citrate cycle. We show here that elevated glucose concentrations (16 or 30 vs 3 mM) cause an increase in PDH activity in both isolated rat islets, and in a clonal beta-cell line (MIN6). However, increases in PDH activity elicited with either dichloroacetate, or by adenoviral expression of the catalytic subunit of pyruvate dehydrogenase phosphatase, were without effect on glucose-induced increases in mitochondrial pyridine nucleotide levels, or cytosolic ATP concentration, in MIN6 cells, and insulin secretion from isolated rat islets. Similarly, the above parameters were unaffected by blockade of the glucose-induced increase in PDH activity by adenovirus-mediated over-expression of PDH kinase (PDK). Thus, activation of the PDH complex plays an unexpectedly minor role in stimulating glucose metabolism and in triggering insulin release.

Glucose-stimulated oscillations in free cytosolic ATP concentration imaged in single islet beta-cells: evidence for a Ca2+-dependent mechanism

Normal glucose-stimulated insulin secretion is pulsatile, but the molecular mechanisms underlying this pulsatility are poorly understood. Oscillations in the intracellular free [ATP]/[ADP] ratio represent one possible mechanism because they would be expected to cause fluctuations in ATP-sensitive K(+) channel activity and hence oscillatory Ca(2+) influx. After imaging recombinant firefly luciferase, expressed via an adenoviral vector in single human or mouse islet beta-cells, we report here that cytosolic free ATP concentrations oscillate and that these oscillations are affected by glucose. In human beta-cells, oscillations were observed at both 3 and 15 mmol/l glucose, but the oscillations were of a longer wavelength at the higher glucose concentration (167 vs. 66 s). Mouse beta-cells displayed oscillations in both cytosolic free [Ca(2+)] and [ATP] only at elevated glucose concentrations, both with a period of 120 s. To explore the causal relationship between [Ca(2+)] and [ATP] oscillations, the regulation of each was further investigated in populations of MIN6 beta-cells. Incubation in Ca(2+)-free medium lowered cytosolic [Ca(2+)] but increased [ATP] in MIN6 cells at both 3 and 30 mmol/l glucose. Removal of external Ca(2+) increased [ATP], possibly by decreasing ATP consumption by endoplasmic reticulum Ca(2+)-ATPases. These results allow a model to be constructed of the beta-cell metabolic oscillator that drives nutrient-induced insulin secretion.

Dynamic imaging of endoplasmic reticulum Ca2+ concentration in insulin-secreting MIN6 Cells using recombinant targeted cameleons: roles of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)-2 and ryanodine receptors

The endoplasmic reticulum (ER) plays a pivotal role in the regulation of cytosolic Ca(2+) concentrations ([Ca(2+)](cyt)) and hence in insulin secretion from pancreatic beta-cells. However, the molecular mechanisms involved in both the uptake and release of Ca(2+) from the ER are only partially defined in these cells, and the presence and regulation of ER ryanodine receptors are a matter of particular controversy. To monitor Ca(2+) fluxes across the ER membrane in single live MIN6 beta-cells, we have imaged changes in the ER intralumenal free Ca(2+) concentration ([Ca(2+)](ER)) using ER-targeted cameleons. Resting [Ca(2+)](ER) (approximately 250 micromol/l) was markedly reduced after suppression (by approximately 40%) of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)-2b but not the SERCA3 isoform by microinjection of antisense oligonucleotides, implicating SERCA2b as the principle ER Ca(2+)-ATPase in this cell type. Nutrient secretagogues that elevated [Ca(2+)](cyt) also increased [Ca(2+)](ER), an effect most marked at the cell periphery, whereas inositol 1,4,5-trisphosphate-generating agents caused a marked and homogenous lowering of [Ca(2+)](ER). Demonstrating the likely presence of ryanodine receptors (RyRs), caffeine and 4-chloro-3-ethylphenol both caused an almost complete emptying of ER Ca(2+) and marked increases in [Ca(2+)](cyt). Furthermore, photolysis of caged cyclic ADP ribose increased [Ca(2+)](cyt), and this effect was largely abolished by emptying ER/Golgi stores with thapsigargin. Expression of RyR protein in living MIN6, INS-1, and primary mouse beta-cells was also confirmed by the specific binding of cell-permeate BODIPY TR-X ryanodine. RyR channels are likely to play an important part in the regulation of intracellular free Ca(2+) changes in the beta-cell and thus in the regulation of insulin secretion.

Nutrient-secretion coupling in the pancreatic islet beta-cell: recent advances

Insulin secretion from pancreatic islet beta-cells is a tightly regulated process, under the close control of blood glucose concentrations, and several hormones and neurotransmitters. Defects in glucose-triggered insulin secretion are ultimately responsible for the development of type II diabetes, a condition in which the total beta-cell mass is essentially unaltered, but beta-cells become progressively "glucose blind" and unable to meet the enhanced demand for insulin resulting for peripheral insulin resistance. At present, the mechanisms by which glucose (and other nutrients including certain amino acids) trigger insulin secretion in healthy individuals are understood only in part. It is clear, however, that the metabolism of nutrients, and the generation of intracellular signalling molecules including the products of mitochondrial metabolism, probably play a central role. Closure of ATP-sensitive K+(K(ATP)) channels in the plasma membrane, cell depolarisation, and influx of intracellular Ca2+, then prompt the "first phase" on insulin release. However, recent data indicate that glucose also enhances insulin secretion through mechanisms which do not involve a change in K(ATP) channel activity, and seem likely to underlie the second, sustained phase of glucose-stimulated insulin secretion. In this review, I will discuss recent advances in our understanding of each of these signalling processes.

Dense core secretory vesicles revealed as a dynamic Ca(2+) store in neuroendocrine cells with a vesicle-associated membrane protein aequorin chimaera

The role of dense core secretory vesicles in the control of cytosolic-free Ca(2+) concentrations ([Ca(2+)](c)) in neuronal and neuroendocrine cells is enigmatic. By constructing a vesicle-associated membrane protein 2-synaptobrevin.aequorin chimera, we show that in clonal pancreatic islet beta-cells: (a) increases in [Ca(2+)](c) cause a prompt increase in intravesicular-free Ca(2+) concentration ([Ca(2+)]SV), which is mediated by a P-type Ca(2+)-ATPase distinct from the sarco(endo) plasmic reticulum Ca(2+)-ATPase, but which may be related to the PMR1/ATP2C1 family of Ca(2+) pumps; (b) steady state Ca(2+) concentrations are 3-5-fold lower in secretory vesicles than in the endoplasmic reticulum (ER) or Golgi apparatus, suggesting the existence of tightly bound and more rapidly exchanging pools of Ca(2+); (c) inositol (1,4,5) trisphosphate has no impact on [Ca(2+)](SV) in intact or permeabilized cells; and (d) ryanodine receptor (RyR) activation with caffeine or 4-chloro-3-ethylphenol in intact cells, or cyclic ADPribose in permeabilized cells, causes a dramatic fall in [Ca(2+)](SV). Thus, secretory vesicles represent a dynamic Ca(2+) store in neuroendocrine cells, whose characteristics are in part distinct from the ER/Golgi apparatus. The presence of RyRs on secretory vesicles suggests that local Ca(2+)-induced Ca(2+) release from vesicles docked at the plasma membrane could participate in triggering exocytosis.

Use of fluorescent Ca2+ dyes with green fluorescent protein and its variants: problems and solutions

We have studied the degree to which fluorescent Ca(2+) indicator dyes, and green fluorescent protein and its variants, can be used together. We find that the most commonly used fluorescent protein, enhanced green fluorescent protein (EGFP), seriously contaminates fura 2 signals. We suggest two alternative combinations for which there is no detectable contamination of the Ca(2+) indicator signal by the fluorescent protein. Blue fluorescent protein can be used with the Ca(2+) indicator Fura Red; EGFP can be used with the Ca(2+) indicator X-Rhod 1. The use of these combinations will permit the accurate measurement of Ca(2+) signals in cells transfected with fluorescent proteins.

Expression and distribution of lactate/monocarboxylate transporter isoforms in pancreatic islets and the exocrine pancreas

Transport of lactate across the plasma membrane of pancreatic islet beta-cells is slow, as described by Sekine et al. (J Biol Chem 269:4895-4902, 1994), which is a feature that may be important for normal nutrient-induced insulin secretion. Although eight members of the monocarboxylate transporter (MCT) family have now been identified, the expression of these isoforms within the exocrine and endocrine pancreas has not been explored in detail. Using immunocytochemical analysis of pancreatic sections fixed in situ, we demonstrated three phenomena. First, immunoreactivity of the commonly expressed lactate transporter isoform MCT1 is near zero in both alpha- and beta-cells but is abundant in the pancreatic acinar cell plasma membrane. No MCT2 or MCT4 was detected in any pancreatic cell type. Second, Western analysis of purified beta- and non-beta-cell membranes revealed undetectable levels of MCT1 and MCT4. In derived beta-cell lines, MCT1 was absent from MIN6 cells and present in low amounts in INS-1 cell membranes and at high levels in RINm5F cells. MCT4 was weakly expressed in MIN6 beta-cells. Third, CD147, an MCT-associated chaperone protein, which is closely colocalized with MCT1 on acinar cell membranes, was absent from islet cell membranes. CD147 was also largely absent from MIN6 and INS-1 cells but abundant in RINm5F cells. Low expression of MCT1, MCT2, and MCT4 contributes to the enzymatic configuration of beta-cells, which is poised to ensure glucose oxidation and the generation of metabolic signals and may also be important for glucose sensing in islet non-beta-cells. MCT overexpression throughout the islet could contribute to deranged hormone secretion in some forms of type 2 diabetes.

Mitochondrial priming modifies Ca2+ oscillations and insulin secretion in pancreatic islets

Increases in mitochondrial [Ca(2+)] ([Ca(2+)](m)) have recently been reported to cause long-term alterations in cellular ATP production [Jouaville, Bastianutto, Rutter and Rizzuto (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 13807-13812]. We have determined the importance of this phenomenon for nutrient sensing in pancreatic islets and beta-cells by imaging adenovirally expressed Ca(2+) and ATP sensors (aequorin and firefly luciferase). [Ca(2+)](m) increases provoked by KCl or tolbutamide evoked an immediate increase in cytosolic and mitochondrial free ATP concentration ([ATP](c) and [ATP](m) respectively) at 3 mM glucose. Subsequent increases in [glucose] (to 16 or 30 mM) then caused a substantially larger increase in [ATP](c) and [ATP](m) than in naïve cells, and pre-stimulation with tolbutamide led to a larger secretory response in response to glucose. Whereas pre-challenge of islets with KCl altered the response to high [glucose] of [Ca(2+)](m) from periodic oscillations to a sustained elevation, oscillations in [ATP](c) were observed neither in naïve nor in stimulated islets. Hence, long-term potentiation of mitochondrial ATP synthesis is a central element in nutrient recognition by pancreatic islets.

Regulation of gene expression by glucose in pancreatic beta -cells (MIN6) via insulin secretion and activation of phosphatidylinositol 3'-kinase

Increases in glucose concentration control the transcription of the preproinsulin (PPI) gene and several other genes in the pancreatic islet beta-cell. Although recent data have demonstrated that secreted insulin may regulate the PPI gene (Leibiger, I. B., Leibiger, B., Moede, T., and Berggren, P. O. (1998) Mol. Cell 1, 933-938), the role of insulin in the control of other beta-cell genes is unexplored. To study the importance of insulin secretion in the regulation of the PPI and liver-type pyruvate kinase (L-PK) genes by glucose, we have used intranuclear microinjection of promoter-luciferase constructs into MIN6 beta-cells and photon-counting imaging. The activity of each promoter was increased either by 30 (versus 3) mm glucose or by 1-20 nm insulin. These effects of insulin were not due to enhanced glucose metabolism since culture with the hormone had no impact on the stimulation of increases in intracellular ATP concentration caused by 30 mm glucose. Furthermore, the islet-specific glucokinase promoter and cellular glucokinase immunoreactivity were unaffected by 30 mm glucose or 20 nm insulin. Inhibition of insulin secretion with the Ca(2+) channel blocker verapamil, the ATP-sensitive K(+) channel opener diazoxide, or the alpha(2)-adrenergic agonist clonidine blocked the effects of glucose on L-PK gene transcription. Similarly, 30 mm glucose failed to induce the promoter after inhibition of phosphatidylinositol 3'-kinase activity with LY294002 and the expression of dominant negative-acting phosphatidylinositol 3'-kinase (Deltap85) or the phosphoinositide 3'-phosphatase PTEN (phosphatase and tensin homologue). LY294002 also diminished the activation of the L-PK gene caused by inhibition of 5'-AMP-activated protein kinase with anti-5'-AMP-activated protein kinase alpha2 antibodies. Conversely, stimulation of insulin secretion with 13 mm KCl or 10 microm tolbutamide strongly activated the PPI and L-PK promoters. These data indicate that, in MIN6 beta-cells, stimulation of insulin secretion is important for the activation by glucose of L-PK as well as the PPI promoter, but does not cause increases in glucokinase gene expression or glucose metabolism.