Intra-islet glucagon signalling regulates beta-cell connectivity, first-phase insulin secretion and glucose homoeostasis

BACKGROUND

Type 2 diabetes (T2D) is characterised by the loss of first-phase insulin secretion. We studied mice with β-cell selective loss of the glucagon receptor (Gcgrfl/fl X Ins-1Cre), to investigate the role of intra-islet glucagon receptor signalling on pan-islet calcium activity and insulin secretion.

METHODS

Metabolic profiling was conducted on Gcgrβ-cell-/- and littermate controls. Crossing with GCaMP6f (STOP flox) animals further allowed for β-cell specific expression of a fluorescent calcium indicator. These islets were functionally imaged in vitro and in vivo. Wild-type mice were transplanted with islets expressing GCaMP6f in β-cells into the anterior eye chamber and placed on a high fat diet. Part of the cohort received a glucagon analogue (GCG-analogue) for 40 days and the control group were fed to achieve weight matching. Calcium imaging was performed regularly during the development of hyperglycaemia and in response to GCG-analogue treatment.

RESULTS

Gcgrβ-cell-/- mice exhibited higher glucose levels following intraperitoneal glucose challenge (control 12.7 mmol/L ± 0.6 vs. Gcgrβ-cell-/- 15.4 mmol/L ± 0.0 at 15 min, p = 0.002); fasting glycaemia was not different to controls. In vitro, Gcgrβ-cell-/- islets showed profound loss of pan-islet [Ca2+]I waves in response to glucose which was only partially rescued in vivo. Diet induced obesity and hyperglycaemia also resulted in a loss of co-ordinated [Ca2+]I waves in transplanted islets. This was reversed with GCG-analogue treatment, independently of weight-loss (n = 8).

CONCLUSION

These data provide novel evidence for the role of intra-islet GCGR signalling in sustaining synchronised [Ca2+]I waves and support a possible therapeutic role for glucagonergic agents to restore the insulin secretory capacity lost in T2D.

Pancreatic β-cell imaging in humans: fiction or option?

Diabetes mellitus is a growing worldwide epidemic disease, currently affecting 1 in 12 adults. Treatment of disease complications typically consumes ∼10% of healthcare budgets in developed societies. Whilst immune-mediated destruction of insulin-secreting pancreatic β cells is responsible for Type 1 diabetes, both the loss and dysfunction of these cells underly the more prevalent Type 2 diabetes. The establishment of robust drug development programmes aimed at β-cell restoration is still hampered by the absence of means to measure β-cell mass prospectively in vivo, an approach which would provide new opportunities for understanding disease mechanisms and ultimately assigning personalized treatments. In the present review, we describe the progress towards this goal achieved by the Innovative Medicines Initiative in Diabetes, a collaborative public-private consortium supported by the European Commission and by dedicated resources of pharmaceutical companies. We compare several of the available imaging methods and molecular targets and provide suggestions as to the likeliest to lead to tractable approaches. Furthermore, we discuss the simultaneous development of animal models that can be used to measure subtle changes in β-cell mass, a prerequisite for validating the clinical potential of the different imaging tracers.

A red-shifted photochromic sulfonylurea for the remote control of pancreatic beta cell function

Azobenzene photoresponsive elements can be installed on sulfonylureas, yielding optical control over pancreatic beta cell function and insulin release. An obstacle to such photopharmacological approaches remains the use of ultraviolet-blue illumination. Herein, we synthesize and test a novel yellow light-activated sulfonylurea based on a heterocyclic azobenzene bearing a push-pull system.

Dorothy Hodgkin Lecture 2014. Understanding genes identified by genome-wide association studies for type 2 diabetes

Whilst the heritable nature of Type 2 diabetes has been recognized for many years, only in the past two decades have linkage analyses in families and genome-wide association studies in large populations begun to reveal the genetic landscape of the disease in detail. Whilst the former have provided a powerful means of identifying the genes responsible for monogenic forms of the disease, the latter highlight relatively large genomic regions. These often harbour multiple genes, whose relative contribution to exaggerated disease risk is uncertain. In the present study, the approaches that have been used to dissect the role of just a few (TCF7L2, SLC30A8, ADCY5, MTNR1B and CDKAL1) of the ~ 500 genes identified at dozens of implicated loci are described. These are usually selected based on the strength of their effect on disease risk, and predictions as to their likely biological role. Direct determination of the effects of identified polymorphisms on gene expression in disease-relevant tissues, notably the pancreatic islet, are then performed to identify genes whose expression is affected by a particular polymorphism. Subsequent functional analyses then involve perturbing gene expression in vitro in β-cell lines or isolated islets and in vivo in animal models. Although the majority of polymorphisms affect insulin production rather than action, and mainly affect the β cell, effects via other tissues may also contribute, requiring careful consideration in the design and interpretation of experiments in model systems. These considerations illustrate the scale of the task needed to exploit genome-wide association study data for the development of new therapeutic strategies.

Biologically targeted probes for Zn2+: a diversity oriented modular "click-SNAr-click" approach†Electronic supplementary information (ESI) available: Full experimental details including characterisation of all novel compounds can be found in the ESI. See DOI: 10.1039/c4sc01249f

We describe a one-pot strategy for the high yielding, operationally simple synthesis of fluorescent probes for Zn2+ that bear biological targeting groups and exemplify the utility of our method through the preparation of a small library of sensors. Investigation of the fluorescence behaviour of our library revealed that although all behaved as expected in MeCN, under biologically relevant conditions in HEPES buffer, a plasma membrane targeting sensor displayed a dramatic switch on response to excess Zn2+ as a result of aggregation phenomena. Excitingly, in cellulo studies in mouse pancreatic islets demonstrated that this readily available sensor was indeed localised to the exterior of the plasma membrane and clearly responded to the Zn2+ co-released when the pancreatic beta cells were stimulated to release insulin. Conversely, sensors that target intracellular compartments were unaffected. These results demonstrate that this sensor has the potential to allow the real time study of insulin release from living cells and exemplifies the utility of our simple synthetic approach.

Increased expression of miR-187 in human islets from individuals with type 2 diabetes is associated with reduced glucose-stimulated insulin secretion

AIMS/HYPOTHESIS

Type 2 diabetes is characterised by progressive beta cell dysfunction, with changes in gene expression playing a crucial role in its development. MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression and therefore alterations in miRNA levels may be involved in the deterioration of beta cell function.

METHODS

Global TaqMan arrays and individual TaqMan assays were used to measure islet miRNA expression in discovery (n = 20) and replication (n = 20) cohorts from individuals with and without type 2 diabetes. The role of specific dysregulated miRNAs in regulating insulin secretion, content and apoptosis was subsequently investigated in primary rat islets and INS-1 cells. Identification of miRNA targets was assessed using luciferase assays and by measuring mRNA levels.

RESULTS

In the discovery and replication cohorts miR-187 expression was found to be significantly increased in islets from individuals with type 2 diabetes compared with matched controls. An inverse correlation between miR-187 levels and glucose-stimulated insulin secretion (GSIS) was observed in islets from normoglycaemic donors. This correlation paralleled findings in primary rat islets and INS-1 cells where overexpression of miR-187 markedly decreased GSIS without affecting insulin content or apoptotic index. Finally, the gene encoding homeodomain-interacting protein kinase-3 (HIPK3), a known regulator of insulin secretion, was identified as a direct target of miR-187 and displayed reduced expression in islets from individuals with type 2 diabetes.

CONCLUSIONS/INTERPRETATION

Our findings suggest a role for miR-187 in the blunting of insulin secretion, potentially involving regulation of HIPK3, which occurs during the pathogenesis of type 2 diabetes.

When less is more: the forbidden fruits of gene repression in the adult β-cell

Outside of the biological arena the term 'repression' often has a negative connotation. However, in the pancreatic β-cell a small group of genes, which are abundantly expressed in most if not all other mammalian tissues, are highly selectively repressed, with likely functional consequences. The two 'founder' members of this group, lactate dehydrogenase A (Ldha) and monocarboxylate transporter-1 (MCT-1/Slc16a1), are inactivated by multiple mechanisms including histone modifications and microRNA-mediated silencing. Their inactivation ensures that pyruvate and lactate, derived from muscle during exercise, do not stimulate insulin release inappropriately. Correspondingly, activating mutations in the MCT-1 promoter underlie 'exercise-induced hyperinsulinism' (EIHI) in man, a condition mimicked by forced over-expression of MCT-1 in the β-cell in mice. Furthermore, LDHA expression in the β-cell is upregulated in both human type 2 diabetes and in rodent models of the disease. Recent work by us and by others has identified a further ∼60 genes which are selectively inactivated in the β-cell, a list which we refine here up to seven by detailed comparison of the two studies. These genes include key regulators of cell proliferation and stimulus-secretion coupling. The present, and our earlier results, thus highlight the probable importance of shutting down a subset of 'disallowed' genes for the differentiated function of β-cells, and implicate previously unsuspected signalling pathways in the control of β-cell expansion and insulin secretion. Targeting of deregulated 'disallowed' genes in these cells may thus, in the future, provide new therapeutic avenues for type 2 diabetes.

Abnormal glucose tolerance and insulin secretion in pancreas-specific Tcf7l2-null mice

AIMS/HYPOTHESIS

Individuals carrying type 2 diabetes risk alleles in TCF7L2 display decreased beta cell levels of T cell factor 7 like-2 (TCF7L2) immunoreactivity, and impaired insulin secretion and beta cell sensitivity to glucagon-like peptide 1 (GLP-1). Here, we sought to determine whether selective deletion of Tcf7l2 in mouse pancreas impairs insulin release and glucose homeostasis.

METHODS

Pancreas-specific Tcf7l2-null (pTcf7l2) mice were generated by crossing mice carrying conditional knockout alleles of Tcf7l2 (Tcf7l2-flox) with mice expressing Cre recombinase under the control of the Pdx1 promoter (Pdx1.Cre). Gene expression was assessed by real-time quantitative PCR and beta cell mass by optical projection tomography. Glucose tolerance, insulin secretion from isolated islets, and plasma insulin, glucagon and GLP-1 content were assessed by standard protocols.

RESULTS

From 12 weeks of age, pTcf7l2 mice displayed decreased oral glucose tolerance vs control littermates; from 20 weeks they had glucose intolerance upon administration of glucose by the intraperitoneal route. pTcf7l2 islets displayed impaired insulin secretion in response to 17 (vs 3.0) mmol/l glucose (54.6 ± 4.6%, p < 0.01) or to 17 mmol/l glucose plus 100 nmol/l GLP-1 (44.3 ± 4.9%, p < 0.01) compared with control islets. Glp1r (42 ± 0.08%, p < 0.01) and Ins2 (15.4 ± 4.6%, p < 0.01) expression was significantly lower in pTcf7l2 islets than in controls. Maintained on a high-fat (but not on a normal) diet, pTcf7l2 mice displayed decreased expansion of pancreatic beta cell volume vs control littermates. No differences were observed in plasma insulin, proinsulin, glucagon or GLP-1 concentrations.

CONCLUSIONS/INTERPRETATION

Selective deletion of Tcf7l2 in the pancreas replicates key aspects of the altered glucose homeostasis in human carriers of TCF7L2 risk alleles, indicating the direct role of this factor in controlling beta cell function.

An alternative polyadenylation signal in TCF7L2 generates isoforms that inhibit T cell factor/lymphoid-enhancer factor (TCF/LEF)-dependent target genes

AIMS/HYPOTHESIS

Intronic single nucleotide polymorphisms within the transcription factor 7-like 2 (TCF7L2) gene are associated with risk of type 2 diabetes. It is widely hypothesised that the predisposing variation is involved in cis-regulation of TCF7L2 activity. The aim of this study was to seek evidence for the existence of novel TCF7L2 isoforms encoded within the type 2 diabetes-associated genomic region.

METHODS

We searched expressed sequence tag (EST) databases for novel TCF7L2 transcripts and sought to validate the function and integrity of any isoforms found using a combination of RT-PCR, western blotting and reporter gene techniques.

RESULTS

Analysis of EST databases suggested the presence of an alternative polyadenylation site located in intron 4 of TCF7L2. We used 3' rapid amplification of cDNA ends and real-time PCR to validate the integrity of this polyadenylation signal and show its wide use across human tissues. Western blotting results are consistent with the use of this polyadenylation signal to generate novel protein isoforms. The alternative polyadenylation signal results in the production of isoforms that retain the β-catenin binding domain but do not possess the high-mobility group box DNA-binding domain. Promoter-reporter gene assays suggest that these isoforms inhibit TCF7L2-dependent target genes by sequestering β-catenin.

CONCLUSIONS/INTERPRETATION

We have identified a novel polyadenylation signal within TCF7L2 that can result in the production of isoforms that act to repress TCF/LEF-dependent target genes. These findings may provide new insights into the association of TCF7L2 with susceptibility to type 2 diabetes.

Per-arnt-sim (PAS) domain-containing protein kinase is downregulated in human islets in type 2 diabetes and regulates glucagon secretion

AIMS/HYPOTHESIS

We assessed whether per-arnt-sim (PAS) domain-containing protein kinase (PASK) is involved in the regulation of glucagon secretion.

METHODS

mRNA levels were measured in islets by quantitative PCR and in pancreatic beta cells obtained by laser capture microdissection. Glucose tolerance, plasma hormone levels and islet hormone secretion were analysed in C57BL/6 Pask homozygote knockout mice (Pask-/-) and control littermates. Alpha-TC1-9 cells, human islets or cultured E13.5 rat pancreatic epithelia were transduced with anti-Pask or control small interfering RNAs, or with adenoviruses encoding enhanced green fluorescent protein or PASK.

RESULTS

PASK expression was significantly lower in islets from human type 2 diabetic than control participants. PASK mRNA was present in alpha and beta cells from mouse islets. In Pask-/- mice, fasted blood glucose and plasma glucagon levels were 25 ± 5% and 50 ± 8% (mean ± SE) higher, respectively, than in control mice. At inhibitory glucose concentrations (10 mmol/l), islets from Pask-/- mice secreted 2.04 ± 0.2-fold (p < 0.01) more glucagon and 2.63 ± 0.3-fold (p < 0.01) less insulin than wild-type islets. Glucose failed to inhibit glucagon secretion from PASK-depleted alpha-TC1-9 cells, whereas PASK overexpression inhibited glucagon secretion from these cells and human islets. Extracellular insulin (20 nmol/l) inhibited glucagon secretion from control and PASK-deficient alpha-TC1-9 cells. PASK-depleted alpha-TC1-9 cells and pancreatic embryonic explants displayed increased expression of the preproglucagon (Gcg) and AMP-activated protein kinase (AMPK)-alpha2 (Prkaa2) genes, implying a possible role for AMPK-alpha2 downstream of PASK in the control of glucagon gene expression and release.

CONCLUSIONS/INTERPRETATION

PASK is involved in the regulation of glucagon secretion by glucose and may be a useful target for the treatment of type 2 diabetes.

Beta cell-specific Znt8 deletion in mice causes marked defects in insulin processing, crystallisation and secretion

AIMS/HYPOTHESIS

Zinc is highly concentrated in pancreatic beta cells, is critical for normal insulin storage and may regulate glucagon secretion from alpha cells. Zinc transport family member 8 (ZnT8) is a zinc efflux transporter that is highly abundant in beta cells. Polymorphisms of ZnT8 (also known as SLC30A8) gene in man are associated with increased risk of type 2 diabetes. While global Znt8 knockout (Znt8KO) mice have been characterised, ZnT8 is also present in other islet cell types and extra-pancreatic tissues. Therefore, it is important to find ways of understanding the role of ZnT8 in beta and alpha cells without the difficulties caused by the confounding effects of ZnT8 in these other tissues.

METHODS

We generated mice with beta cell-specific (Znt8BKO) and alpha cell-specific (Znt8AKO) knockout of Znt8, and performed in vivo and in vitro characterisation of the phenotypes to determine the functional and anatomical impact of ZnT8 in these cells. Thus we assessed zinc accumulation, insulin granule morphology, insulin biosynthesis and secretion, and glucose homeostasis.

RESULTS

Znt8BKO mice are glucose-intolerant, have reduced beta cell zinc accumulation and atypical insulin granules. They also display reduced first-phase glucose-stimulated insulin secretion, reduced insulin processing enzyme transcripts and increased proinsulin levels. In contrast, Znt8AKO mice show no evident abnormalities in plasma glucagon and glucose homeostasis.

CONCLUSIONS/INTERPRETATION

This is the first report of specific beta and alpha cell deletion of Znt8. Our data indicate that while, under the conditions studied, ZnT8 is absolutely essential for proper beta cell function, it is largely dispensable for alpha cell function.

Rapid three-dimensional imaging of individual insulin release events by Nipkow disc confocal microscopy

Minute-to-minute control of the release of insulin by pancreatic beta-cells in response to glucose or other stimuli requires the precise delivery of large dense-core vesicles to the plasma membrane and regulated exocytosis. At present, the precise spatial organization at the cell surface and the nature of these events ('transient' versus 'full fusion') are debated. In order to monitor secretory events simultaneously over most of the surface of clusters of single MIN6 beta-cells, we have expressed recombinant neuropeptide Y-Venus (an enhanced and vesicle-targeted form of yellow fluorescent protein) as an insulin surrogate. Individual exocytotic events were monitored using Nipkow spinning disc confocal microscopy, with acquisition of a three-dimensional complete image (eight to twelve confocal slices) in <1 s, in response to cell depolarization. Corroborating earlier studies using TIRF (total internal reflection fluorescence) microscopy, this approach indicates that events occur with roughly equal probability over the entire cell surface, with only minimal clustering in individual areas, and provides no evidence for multiple events at the same site. Nipkow disc confocal imaging may thus provide a useful tool to determine whether event types occur at different sites at the cell surface and to explore the role of endocytic proteins including dynamin-1 and -2 in terminating individual exocytotic events.

Regulation by Per-Arnt-Sim (PAS) kinase of pancreatic duodenal homeobox-1 nuclear import in pancreatic beta-cells

The transcription factor PDX-1 (pancreatic duodenal homeobox-1) is required for normal pancreatic development and for the function of insulin-producing islet beta-cells in mammals. We have shown previously that glucose regulates insulin gene expression in part through the activation and translocation of PDX-1 from the nuclear periphery to the nucleoplasm. We have also found that PASK [PAS (Per-Arnt-Sim) kinase], a member of the nutrient-regulated family of protein kinases, is activated in response to glucose challenge in beta-cells and is involved in the regulation of expression of PDX-1. Purified PASK efficiently phosphorylated recombinant PDX-1 in vitro on a single site (Thr-152). To determine the impact of phosphorylation at this site, we generated wild-type and mutant (T152A, T152D and T152E) forms of PDX-1 and examined the distribution of each of these in clonal MIN6 beta-cells by immunocytochemical analysis. Unexpectedly, only the T152D mutation significantly affected subcellular distribution, increasing the ratio of nuclear/cytosolic labelling at low and high glucose concentrations, suggesting that phosphorylation at Thr-152 inhibits nuclear uptake in response to glucose. Based on these results, experiments to examine the contribution of Thr-152 to the overall phosphorylation of PDX-1 in intact cells will be undertaken.

Inhibition by glucose or leptin of hypothalamic neurons expressing neuropeptide Y requires changes in AMP-activated protein kinase activity

AIMS/HYPOTHESIS

Changes in the activity of glucose-excited and glucose-inhibited neurons within the basomedial hypothalamus are key to the central regulation of satiety. However, the molecular mechanisms through which these cells respond to extracellular stimuli remain poorly understood. Here, we investigate the role of 5'-AMP-activated protein kinase (AMPK), a trimeric complex encoded by seven distinct genes of the PRKA family, in the responses to glucose and leptin of each cell type.

METHODS

The activity of isolated rat basomedial hypothalamic neurons was assessed by: (1) recording cellular voltage responses under current clamp; (2) measuring intracellular free Ca(2+) with fluo-3 or fura-2; and (3) developing a neuropeptide Y (NPY) promoter-driven adenovirally produced ratiometric 'pericam' (a green fluorescent protein-based Ca(2+) sensor) to monitor [Ca(2+)] changes selectively in NPY-positive neurons.

RESULTS

The stimulatory effects of decreased (0 or 1.0 vs 15 mmol/l) glucose on glucose-inhibited neurons were mimicked by the AMPK activator, 5-amino-imidazole-4-carboxamide riboside (AICAR) and blocked by the inhibitor Compound C. Similarly, AICAR reversed the inhibitory effects of leptin in the majority of glucose-inhibited neurons. The responses to glucose of Npy-expressing cells, which represented approximately 40 % of all glucose-inhibited neurons, were also sensitive to Compound C or AICAR. Forced changes in AMPK activity had no effect on glucose-excited and non-glucose-responsive neurons.

CONCLUSIONS/INTERPRETATION

Changes in AMPK activity are involved in the responses of glucose-inhibited neurons to large fluctuations in glucose concentration, and possibly also to leptin. This mechanism may contribute to the acute reduction of electrical activity and Ca(2+) oscillation frequency in these, but not other neurons, in the basomedial hypothalamus.

Variance stabilization and normalization for one-color microarray data using a data-driven multiscale approach

MOTIVATION

Many standard statistical techniques are effective on data that are normally distributed with constant variance. Microarray data typically violate these assumptions since they come from non-Gaussian distributions with a non-trivial mean-variance relationship. Several methods have been proposed that transform microarray data to stabilize variance and draw its distribution towards the Gaussian. Some methods, such as log or generalized log, rely on an underlying model for the data. Others, such as the spread-versus-level plot, do not. We propose an alternative data-driven multiscale approach, called the Data-Driven Haar-Fisz for microarrays (DDHFm) with replicates. DDHFm has the advantage of being 'distribution-free' in the sense that no parametric model for the underlying microarray data is required to be specified or estimated; hence, DDHFm can be applied very generally, not just to microarray data.

RESULTS

DDHFm achieves very good variance stabilization of microarray data with replicates and produces transformed intensities that are approximately normally distributed. Simulation studies show that it performs better than other existing methods. Application of DDHFm to real one-color cDNA data validates these results.

AVAILABILITY

The R package of the Data-Driven Haar-Fisz transform (DDHFm) for microarrays is available in Bioconductor and CRAN.

Intracellular ATP-sensitive K+ channels in mouse pancreatic beta cells: against a role in organelle cation homeostasis

AIMS/HYPOTHESIS

ATP-sensitive K(+) (K(ATP)) channels located on the beta cell plasma membrane play a critical role in regulating insulin secretion and are targets for the sulfonylurea class of antihyperglycaemic drugs. Recent reports suggest that these channels may also reside on insulin-containing dense-core vesicles and mitochondria. The aim of this study was to explore these possibilities and to test the hypothesis that vesicle-resident channels play a role in the control of organellar Ca(2+) concentration or pH.

METHODS

To quantify the subcellular distribution of the pore-forming subunit Kir6.2 and the sulfonylurea binding subunit SUR1 in isolated mouse islets and clonal pancreatic MIN6 beta cells, we used four complementary techniques: immunoelectron microscopy, density gradient fractionation, vesicle immunopurification and fluorescence-activated vesicle isolation. Intravesicular and mitochondrial concentrations of free Ca(2+) were measured in intact or digitonin-permeabilised MIN6 cells using recombinant, targeted aequorins, and intravesicular pH was measured with the recombinant fluorescent probe pHluorin.

RESULTS

SUR1 and Kir6.2 immunoreactivity were concentrated on dense-core vesicles and on vesicles plus the endoplasmic reticulum/Golgi network, respectively, in both islets and MIN6 cells. Reactivity to neither subunit was detected on mitochondria. Glibenclamide, tolbutamide and diazoxide all failed to affect Ca(2+) uptake into mitochondria, and K(ATP) channel regulators had no significant effect on intravesicular free Ca(2+) concentrations or vesicular pH.

CONCLUSIONS/INTERPRETATION

A significant proportion of Kir6.2 and SUR1 subunits reside on insulin-secretory vesicles and the distal secretory pathway in mouse beta cells but do not influence intravesicular ion homeostasis. We propose that dense-core vesicles may serve instead as sorting stations for the delivery of channels to the plasma membrane.

Insulin secretion in health and disease: genomics, proteomics and single vesicle dynamics

Defective insulin secretion from pancreatic islet beta-cells is a sine qua non of Type II (non-insulin-dependent) diabetes. Digital imaging analysis of the nanomechanics of individual exocytotic events, achieved using total internal reflection fluorescence microscopy, has allowed us to demonstrate that insulin is released via transient or 'cavicapture' events whereby the vesicle and plasma membranes fuse transiently and reversibly. Such studies reveal that an increase in the number of abortive fusion events contributes to defective insulin secretion in in vitro models of Type II diabetes. Complementary analyses of genome-wide changes in beta-cell gene expression, at both the mRNA and protein levels, are now facilitating the identification of key molecular players whose altered expression may contribute to the secretory defects in the diabetic beta-cell.

Moving Ca2+ from the endoplasmic reticulum to mitochondria: is spatial intimacy enough?

A number of studies in recent years have demonstrated that the ER (endoplasmic reticulum) makes intimate contacts with mitochondria, the latter organelles existing both as individual organelles and occasionally as a more extensive interconnected network. Demonstrations that mitochondria take up Ca2+ more avidly upon its mobilization from the ER than when delivered to permeabilized cells as a buffered solution also indicate that a shielded conduit for Ca2+ may exist between the two organelle types, perhaps comprising the inositol 1,4,5-trisphosphate receptor and mitochondrial outer membrane proteins including the VDAC (voltage-dependent anion channel). Although the existence of such intracellular ER–mitochondria ‘synapses’, or of an ER–mitochondria Ca2+ ‘translocon’, is an exciting idea, more definitive experiments are needed to test this possibility.

Over-expression of AMP-activated protein kinase impairs pancreatic {beta}-cell function in vivo

Treatment of type 1 diabetes by islet transplantation is currently limited by loss of functional beta-cell mass after transplantation. We investigated here whether adenovirus-mediated changes in AMP-activated protein kinase (AMPK) activity, previously shown to affect insulin secretion in vitro, might affect islet graft function in vivo. In isolated mouse and rat islets, insulin secretion stimulated by 17 (vs 3) mmol/l glucose was inhibited by 36.5% (P<0.01) and 43% (P<0.02) respectively after over-expression of constitutively-active AMPK- (AMPK CA) versus null (eGFP-expressing) viruses, and glucose oxidation was decreased by 38% (P<0.05) and 26.6% (P<0.05) respectively. Increases in apoptotic index (terminal deoxynucleotide transferase-mediated deoxyuridine trisphosphate biotin nick end-labelling) (TUNEL)) were also observed in AMPK CA- (22.8 +/- 3.6% TUNEL-positive cells, P<0.001), but not AMPK DN- (2.72 +/- 3.9%, positive cells, P=0.05) infected islets, versus null adenovirus-treated islets (0.68 +/- 0.36% positive cells). Correspondingly, transplantation of islets expressing AMPK CA into streptozotocin-diabetic C57 BL/6 mice improved glycaemic control less effectively than transplantation with either null (P<0.02) or AMPK-DN-infected (P<0.01) islets. We conclude that activation of AMPK inhibits beta-cell function in vivo and may represent a target for therapeutic intervention during islet transplantation.

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.

Glutamine potently stimulates glucagon-like peptide-1 secretion from GLUTag cells

AIMS/HYPOTHESIS

Glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) are secreted from enteroendocrine L cells in response to nutrient ingestion. As glutamine is an important metabolic fuel for the gut, the aim of this study was to investigate the effect of glutamine on the GLP-1-secreting cell line, GLUTag.

METHODS

GLP-1 release was measured following incubation of GLUTag cells under a range of conditions. Single cells were studied by electrophysiology, calcium imaging and cytosolic ATP measurement using recombinant luciferase.

RESULTS

Glutamine was a more potent GLP-1 secretagogue than glucose or other amino acids, increasing GLP-1 release 7.1+/-0.7-fold ( n=19) at 10 mmol/l, with an estimated median effective concentration of between 0.1 and 1 mmol/l. Glutamine (10 mmol/l) induced a sodium-dependent inward current of 3.2+/-1.2 pA per cell ( n=9), which triggered membrane depolarisation and an increase in intracellular calcium. Asparagine and alanine produced electrophysiological and calcium changes that were at least as large as those caused by glutamine, but they were less effective GLP-1 secretagogues, suggesting that glutamine also potentiates secretion downstream of the calcium signal. This was confirmed by measuring secretion in the presence of 30 mmol/l KCl + diazoxide, or in alpha-haemolysin-permeabilised cells. Glutamine increased cytosolic ATP, but was less effective than glucose.

CONCLUSIONS/INTERPRETATION

Glutamine acts as a trigger and potentiator of GLP-1 release, consistent with its role as the major metabolic fuel for the gut. The results suggest that nutritional agents like glutamine might have beneficial effects in diabetes and obesity.

Insulin secretion by 'kiss-and-run' exocytosis in clonal pancreatic islet beta-cells

Exocytotic release of neuropeptides and hormones is generally believed to involve the complete merger of the secretory vesicle with the plasma membrane. However, recent data have suggested that 'kiss-and-run' mechanisms may also play a role. To analyse secretory events in neuroendocrine beta-cells, we imaged chimaeric reporters targeted to either the vesicle membrane [chimaeras of synaptobrevin-2 and pH-sensitive green fluorescent protein (synapto.pHluorin) or of phogrin (phosphatase on the granule of insulinoma) and enhanced green fluorescent protein (EGFP) (phogrin.EGFP)] or the lumen [neuropeptide Y (NPY).pH-insensitive yellow fluorescent protein (Venus)] by evanescent wave microscopy. Unexpectedly, the frequency of NPY.Venus release events was only 17-27% of that of vesicle fusion reported with synapto.pHluorin, but not phogrin.EGFP, indicating that exocytosis of cargo peptides that is likely to require complete collapse of the vesicle into the plasma membrane is relatively rare. However, both the frequency and the kinetics of NPY.Venus release were modulated by stimulus strength or by overexpression of synaptotagmin IV, demonstrating the plasticity of 'kiss-and-run' fusion.

Molecular mechanisms involved in secretory vesicle recruitment to the plasma membrane in beta-cells

Glucose stimulates the release of insulin in part by activating the recruitment of secretory vesicles to the cell surface. While this movement is known to be microtubule-dependent, the molecular motors involved are undefined. Active kinesin was found to be essential for vesicle translocation in live beta-cells, since microinjection of cDNA encoding dominant-negative KHC(mut) (motor domain of kinesin heavy chain containing a Thr(93)-->Asn point mutation) blocked vesicular movements. Moreover, expression of KHC(mut) strongly inhibited the sustained, but not acute, stimulation of secretion by glucose. Thus, vesicles released during the first phase of insulin secretion exist largely within a translocation-independent pool. Kinesin-driven anterograde movement of vesicles is then necessary for the sustained (second phase) of insulin release. Kinesin may, therefore, represent a novel target for increases in intracellular ATP concentrations in response to elevated extracellular glucose and may be involved in the ATP-sensitive K+channel-independent stimulation of secretion by the sugar.

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.

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.

Diabetes: the importance of the liver

Insulin resistance, the hallmark of non-insulin dependent diabetes mellitus, is characterized by the failure of tissues to take up and store glucose in response to insulin. Two recent studies shed new light on the importance of insulin signalling in the liver and how this may become defective in diabetes.

Simultaneous evanescent wave imaging of insulin vesicle membrane and cargo during a single exocytotic event

The classical model of secretory vesicle recycling after exocytosis involves the retrieval of membrane (the omega figure) at a different site. An alternative model involves secretory vesicles transiently fusing with the plasma membrane (the 'kiss and run' mechanism) [1,2]. No continuous observation of the fate of a single secretory vesicle after exocytosis has been made to date. To study the dynamics of fusion immediately following exocytosis of insulin-containing vesicles, enhanced green fluorescent protein (EGFP) fused to the vesicle membrane protein phogrin [3] was delivered to the secretory vesicle membrane of INS-1 beta-cells using an adenoviral vector. The behaviour of the vesicle membrane during single exocytotic events was then examined using evanescent wave microscopy [4-6]. In unstimulated cells, secretory vesicles showed only slow Brownian movement. After a depolarizing pulse, most vesicles showed a small decrease in phogrin-EGFP fluorescence, and some moved laterally over the plasma membrane for approximately 1 microm. In contrast, secretory vesicles loaded with acridine orange all showed a transient (33-100 ms) increase in fluorescence intensity followed by rapid disappearance. Simultaneous observations of phogrin-EGFP and acridine orange indicated that the decrease in EGFP fluorescence occurred at the time of the acridine orange release, and that the lateral movement of EGFP-expressing vesicles occurred after this. Post-exocytotic retrieval of the vesicle membrane in INS-1 cells is thus slow, and can involve the movement of empty vesicles under the plasma membrane ('kiss and glide').

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

Islet beta-cells express low levels of lactate dehydrogenase and have high glycerol phosphate dehydrogenase activity. To determine whether this configuration favors oxidative glucose metabolism via mitochondria in the beta-cell and is important for beta-cell metabolic signal transduction, we have determined the effects on glucose metabolism and insulin secretion of acute overexpression of the skeletal muscle isoform of lactate dehydrogenase (LDH)-A. Monitored in single MIN6 beta-cells, LDH hyperexpression (achieved by intranuclear cDNA microinjection or adenoviral infection) diminished the response to glucose of both phases of increases in mitochondrial NAD(P)H, as well as increases in mitochondrial membrane potential, cytosolic free ATP, and cystolic free Ca2+. These effects were observed at all glucose concentrations, but were most pronounced at submaximal glucose levels. Correspondingly, adenoviral vector-mediated LDH-A overexpression reduced insulin secretion stimulated by 11 mmol/l glucose and the subsequent response to stimulation with 30 mmol/l glucose, but it was without significant effect when the concentration of glucose was raised acutely from 3 to 30 mmol/l. Thus, overexpression of LDH activity interferes with normal glucose metabolism and insulin secretion in the islet beta-cell type, and it may therefore be directly responsible for insulin secretory defects in some forms of type 2 diabetes. The results also reinforce the view that glucose-derived pyruvate metabolism in the mitochondrion is critical for glucose-stimulated insulin secretion in the beta-cell.

Glucose-stimulated preproinsulin gene expression and nuclear trans-location of pancreatic duodenum homeobox-1 require activation of phosphatidylinositol 3-kinase but not p38 MAPK/SAPK2

Exposure of islet beta-cells to elevated glucose concentrations (30 versus 3 mm) prompts enhanced preproinsulin (PPI) gene transcription and the trans-location to the nucleoplasm of pancreatic duodenum homeobox-1 (PDX-1; Rafiq, I., Kennedy, H., and Rutter, G. A. (1998) J. Biol. Chem. 273, 23241-23247). Here, we show that in MIN6 beta-cells, over-expression of p110.CAAX, a constitutively active form of phosphatidylinositol 3-kinase (PI3K) mimicked the activatory effects of glucose on PPI promoter activity, whereas Deltap85, a dominant negative form of the p85 subunit lacking the p110-binding domain, and the PI3K inhibitor LY 294002, blocked these effects. Similarly, glucose-stimulated nuclear trans-location of endogenous PDX-1 was blocked by Deltap85 expression, and wortmannin or LY 294002 blocked the trans-location from the nuclear membrane to the nucleoplasm of epitope-tagged PDX-1.c-myc. By contrast, SB 203580, an inhibitor of stress-activated protein kinase-2 (SAPK2)/p38 MAP kinase, had no effect on any of the above parameters, and PPI promoter activity and PDX-1.c-myc localization were unaffected by over-expression of the upstream kinase MKK6 (MAP kinase kinase-6) or wild-type p38/SAPK2, respectively. Furthermore, no change in the activity of extracted p38/SAPK2 could be detected after incubation of cells at either 3 or 30 mm glucose. These data suggest that stimulation of PI3K is necessary and sufficient for the effects of glucose on PPI gene transcription, acting via a downstream signaling pathway that does not involve p38/SAPK2.

Regulation of mitochondrial metabolism by ER Ca2+ release: an intimate connection

New live-cell imaging techniques indicate that mitochondria exist in the living cell as a continuous interconnected mitochondrial reticulum, or 'MR', closely associated with the endoplasmic reticulum (ER). Ca2+ ions released from the ER in response to hormonal stimulation might thus be preferentially transferred into the mitochondrial matrix causing the local activation of ATP synthesis. Ca2+ uptake into the MR might also subtly modify the activity of ER Ca2+ release channels and thus the dynamics of cytosolic Ca2+ oscillations and waves.

Role of AMP-activated protein kinase in the regulation by glucose of islet beta cell gene expression

Elevated glucose concentrations stimulate the transcription of the pre-proinsulin (PPI), L-type pyruvate kinase (L-PK), and other genes in islet beta cells. In liver cells, pharmacological activation by 5-amino-4-imidazolecarboxamide riboside (AICAR) of AMP-activated protein kinase (AMPK), the mammalian homologue of the yeast SNF1 kinase complex, inhibits the effects of glucose, suggesting a key signaling role for this kinase. Here, we demonstrate that AMPK activity is inhibited by elevated glucose concentrations in MIN6 beta cells and that activation of the enzyme with AICAR prevents the activation of the L-PK gene by elevated glucose. Furthermore, microinjection of antibodies to the alpha2- (catalytic) or beta2-subunits of AMPK complex, but not to the alpha1-subunit or extracellular stimulus-regulated kinase, mimics the effects of elevated glucose on the L-PK and PPI promoter activities as assessed by single-cell imaging of promoter luciferase constructs. In each case, injection of antibodies into the nucleus and cytosol, but not the nucleus alone, was necessary, indicating the importance of either a cytosolic phosphorylation event or the subcellular localization of the alpha2-subunits. Incubation with AICAR diminished, but did not abolish, the effect of glucose on PPI transcription. These data suggest that glucose-induced changes in AMPK activity are necessary and sufficient for the regulation of the L-PK gene by the sugar and also play an important role in the regulation of the PPI promoter.

Present and potential future use of gene therapy for the treatment of non-insulin dependent diabetes mellitus (Review)

This review describes the latest approaches towards using gene therapy as a treatment for non-insulin dependent diabetes mellitus (NIDDM; Type 2 diabetes). We examine attempts to directly deliver the insulin gene to non-beta-cells, to improve insulin secretion from existing beta-cells and to develop ex vivo approaches to implanting genetically modified cells. Future research into the pathology of non-insulin dependent diabetes, combined with the latest developments in gene delivery systems, may potentially make gene therapy an attractive alternative NIDDM treatment in the future.

Regulation of mitochondrial ATP synthesis by calcium: evidence for a long-term metabolic priming

In recent years, mitochondria have emerged as important targets of agonist-dependent increases in cytosolic Ca(2+) concentration. Here, we analyzed the significance of Ca(2+) signals for the modulation of organelle function by directly measuring mitochondrial and cytosolic ATP levels ([ATP](m) and [ATP](c), respectively) with specifically targeted chimeras of the ATP-dependent photoprotein luciferase. In both HeLa cells and primary cultures of skeletal myotubes, stimulation with agonists evoking cytosolic and mitochondrial Ca(2+) signals caused increases in [ATP](m) and [ATP](c) that depended on two parameters: (i) the amplitude of the Ca(2+) rise in the mitochondrial matrix, and (ii) the availability of mitochondrial substrates. Moreover, the Ca(2+) elevation induced a long-lasting priming that persisted long after agonist washout and caused a major increase in [ATP](m) upon addition of oxidative substrates. These results demonstrate a direct role of mitochondrial Ca(2+) in driving ATP production and unravel a form of cellular memory that allows a prolonged metabolic activation in stimulated cells.

Glucose enhances insulin promoter activity in MIN6 beta-cells independently of changes in intracellular Ca2+ concentration and insulin secretion

Recent studies have suggested that glucose may activate insulin gene transcription through increases in intracellular Ca(2+) concentration, possibly acting via the release of stored insulin. We have investigated this question by dynamic photon-counting imaging of insulin- and c-fos-promoter-firefly luciferase reporter construct activity. Normalized to constitutive viral promoter activity, insulin promoter activity in MIN6 beta-cells was increased 1.6-fold after incubation at 30 mM compared with 3 mM glucose, but was unaltered at either glucose concentration by the presence of insulin (100 nM) or the Ca(2+) channel inhibitor, verapamil (100 microM). Increases in intracellular [Ca(2+)] achieved by plasma membrane depolarization with KCl failed to enhance either insulin or c-fos promoter activity in MIN6 cells, but increased c-fos promoter activity 5-fold in AtT20 cells. Together, these results demonstrate that glucose can exert a direct effect on insulin promoter activity in islet beta-cells, via a signalling pathway which does not require increases in intracellular [Ca(2+)] nor insulin release and insulin receptor activation.

Imaging Ca2+ concentration changes at the secretory vesicle surface with a recombinant targeted cameleon

Regulated exocytosis involves the Ca(2+)-triggered fusion of secretory vesicles with the plasma membrane, by activation of vesicle membrane Ca(2+)-binding proteins [1]. The Ca(2+)-binding sites of these proteins are likely to lie within 30 nm of the vesicle surface, a domain in which changes in Ca2+ concentration cannot be resolved by conventional fluorescence microscopy. A fluorescent indicator for Ca2+ called a yellow 'cameleon' (Ycam2) - comprising a fusion between a cyan-emitting mutant of the green fluorescent protein (GFP), calmodulin, the calmodulin-binding peptide M13 and an enhanced yellow-emitting GFP - which is targetable to specific intracellular locations, has been described [2]. Here, we generated a fusion between phogrin, a protein that is localised to secretory granule membranes [3], and Ycam2 (phogrin-Ycam2) to monitor changes in Ca2+ concentration ([Ca2+]) at the secretory vesicle surface ([Ca2+]gd) through alterations in fluorescence resonance energy transfer (FRET) between the linked cyan and yellow fluorescent proteins (CFP and YFP, respectively) in Ycam2. In both neuroendocrine PC12 and MIN6 pancreatic beta cells, apparent resting values of cytosolic [Ca2+] and [Ca2+](gd) were similar throughout the cell. In MIN6 cells following the activation of Ca2+ influx, the minority of vesicles that were within approximately 1 microm of the plasma membrane underwent increases in [Ca2+](gd) that were significantly greater than those experienced by deeper vesicles, and greater than the apparent cytosolic [Ca2+] change. The ability to image both global and compartmentalised [Ca2+] changes with recombinant targeted cameleons should extend the usefulness of these new Ca2+ probes.

Insulin secretion: feed-forward control of insulin biosynthesis?

It has long been accepted wisdom that insulin secreted from islet beta cells has either no effect, or an inhibitory feedback effect, on insulin synthesis and secretion. Recent work suggests, instead, that secreted insulin acts directly on beta cells, via its own receptor, to enhance insulin production in an autocrine feed-forward loop.

Glucose generates sub-plasma membrane ATP microdomains in single islet beta-cells. Potential role for strategically located mitochondria

Increases in the concentration of free ATP within the islet beta-cell may couple elevations in blood glucose to insulin release by closing ATP-sensitive K+ (KATP) channels and activating Ca2+ influx. Here, we use recombinant targeted luciferases and photon counting imaging to monitor changes in free [ATP] in subdomains of single living MIN6 and primary beta-cells. Resting [ATP] in the cytosol ([ATP]c), in the mitochondrial matrix ([ATP]m), and beneath the plasma membrane ([ATP]pm) were similar ( approximately 1 mM). Elevations in extracellular glucose concentration (3-30 mM) increased free [ATP] in each domain with distinct kinetics. Thus, sustained increases in [ATP]m and [ATP]pm were observed, but only a transient increase in [ATP]c. However, detectable increases in [ATP]c and [ATP]pm, but not [ATP]m, required extracellular Ca2+. Enhancement of glucose-induced Ca2+ influx with high [K+] had little effect on the apparent [ATP]c and [ATP]m increases but augmented the [ATP]pm increase. Underlying these changes, glucose increased the mitochondrial proton motive force, an effect mimicked by high [K+]. These data support a model in which glucose increases [ATP]m both through enhanced substrate supply and by progressive Ca2+-dependent activation of mitochondrial enzymes. This may then lead to a privileged elevation of [ATP]pm, which may be essential for the sustained closure of KATP channels. Luciferase imaging would appear to be a useful new tool for dynamic in vivo imaging of free ATP concentration.

Real-time imaging of gene expression in single living cells

Recent advances in reporter gene technologies are now allowing us to image gene transcription at the single cell level, using either fluorescence or luminescence microscopy. Here, the basis of these techniques is outlined and their advantages and disadvantages in various biological systems are discussed.

Glucose-dependent translocation of insulin promoter factor-1 (IPF-1) between the nuclear periphery and the nucleoplasm of single MIN6 beta-cells

Using laser-scanning confocal microscopy, we have monitored glucose-induced changes in the subcellular localization of insulin promoter factor-1 (IPF-1) labeled with a c-myc epitope tag. This construct trans-activated the insulin promoter in single living MIN6-beta-cells as assessed by luciferase-based promoter analysis. IPF-1.c-myc expression also enhanced the response of the insulin promoter to elevations in extracellular glucose concentration. In the majority (148/235, 63%) of cells maintained at low (3 mM) extracellular glucose concentration, IPF-1.c-myc immunoreactivity was confined to the nuclear periphery. Incubation of cells at stimulatory (30 mM) glucose concentrations caused a rapid redistribution of the chimera to the nucleoplasm (775/958, 81% of cells). By contrast, the irrelevant transcription factor c-Fos, tagged with either c-myc or as a chimera with luciferase, was localized exclusively to the nucleoplasm irrespective of the glucose concentration. Furthermore, IPF-1 extended with the bulky (27 kDa) enhanced green fluorescent protein (EGFP) group was confined largely to the nucleoplasm at all glucose concentrations tested and did not support trans-activation of the insulin promoter by glucose. Movement of endogenous IPF-1 from the nuclear periphery to the nucleoplasm may therefore increase the trans-activational capacity of this factor in native beta-cells exposed to high extracellular glucose concentrations.

Integrating cytosolic calcium signals into mitochondrial metabolic responses

Stimulation of hepatocytes with vasopressin evokes increases in cytosolic free Ca2+ ([Ca2+]c) that are relayed into the mitochondria, where the resulting mitochondrial Ca2+ ([Ca2+]m) increase regulates intramitochondrial Ca2+-sensitive targets. To understand how mitochondria integrate the [Ca2+]c signals into a final metabolic response, we stimulated hepatocytes with high vasopressin doses that generate a sustained increase in [Ca2+]c. This elicited a synchronous, single spike of [Ca2+]m and consequent NAD(P)H formation, which could be related to changes in the activity state of pyruvate dehydrogenase (PDH) measured in parallel. The vasopressin-induced [Ca2+]m spike evoked a transient increase in NAD(P)H that persisted longer than the [Ca2+]m increase. In contrast, PDH activity increased biphasically, with an initial rapid phase accompanying the rise in [Ca2+]m, followed by a sustained secondary activation phase associated with a decline in cellular ATP. The decline of NAD(P)H in the face of elevated PDH activity occurred as a result of respiratory chain activation, which was also manifest in a calcium-dependent increase in the membrane potential and pH gradient components of the proton motive force (PMF). This is the first direct demonstration that Ca2+-mobilizing hormones increase the PMF in intact cells. Thus, Ca2+ plays an important role in signal transduction from cytosol to mitochondria, with a single [Ca2+]m spike evoking a complex series of changes to activate mitochondrial oxidative metabolism.

Coupling between cytosolic and mitochondrial calcium oscillations: role in the regulation of hepatic metabolism

Mitochondria are strategically localized at sites of Ca2+ release, such that increases in cytosolic free Ca2+ ([Ca2+]c) from either internal Ca2+ stores or Ca2+ influx across the plasma membrane can be rapidly transported into the mitochondrial matrix. The consequent elevation in mitochondrial Ca2+ ([Ca2+]m) stimulates the Ca2+-sensitive intramitochondrial dehydrogenases, resulting in elevation of NAD(P)H. The preferential coupling between increases in [Ca2+]c and [Ca2+]m is one proposed mechanism to coordinate mitochondrial ATP production with cellular energy demand. In liver cells, hormones that act through the second messenger inositol 1,4, 5-trisphosphate (IP3) generate oscillatory [Ca2+]c signals, which result from a periodic Ca2+- and IP3-mediated activation/deactivation of intracellular Ca2+ release channels. The [Ca2+]c spiking frequency increases with agonist dose, whereas the amplitude of each [Ca2+]c spike is constant. This frequency modulation of [Ca2+]c spiking encodes the signal from the extracellular agonist, which is then decoded by the internal Ca2+-sensitive proteins such as the Ca2+-sensitive intramitochondrial dehydrogenases. Our studies have investigated the relationship between IP3-dependent [Ca2+]c signals and [Ca2+]m in primary cultured hepatocytes. In addition, the changes in cellular [Ca2+] levels have been correlated with the regulation of intramitochondrial NAD(P)H levels, pyruvate dehydrogenase activity and the magnitude of the mitochondrial proton motive force.

Mitochondrial calcium transporting pathways during hypoxia and reoxygenation in single rat cardiomyocytes

OBJECTIVE

Mitochondrial [Ca2+] ([Ca2+]m) rises in parallel with cytosolic [Ca2+] ([Ca2+]c) following ATP-depletion rigor contracture induced by hypoxia in isolated cardiomyocytes. We investigated the pathways involved in the hypoxia induced changes in [Ca2+]m by using known inhibitors of mitochondrial Ca2+ transport, namely ruthenium red, an inhibitor of the Ca2+ uniporter (the normal influx route) and clonazepam, an inhibitor of Na+/Ca2+ exchange, (the normal efflux route).

METHODS

[Ca2+]m was determined from indo-1/am loaded rat myocytes where the cytosolic fluorescence signal had been quenched by superfusion with Mn2+. [Ca2+]c was measured by loading myocytes with indo-1 pentapotassium salt during the isolation procedure. Cells were placed in a specially developed chamber for induction of hypoxia and reoxygenated 40 min after rigor development.

RESULTS

50% of control cells hypercontracted upon reoxygenation; this correlated with a [Ca2+]m or [Ca2+]c higher than approximately 350 nM at the end of rigor. Clonazepam completely abolished the rigor-induced rise in [Ca2+]m but not [Ca2+]c. On reoxygenation [Ca2+]m increased over the first 5 min and remained elevated whereas [Ca2+]c fell. In the presence of ruthenium red a dramatic increase in [Ca2+]m occurred 5-10 min after rigor development (the indo-1 fluorescence signal was saturated); [Ca2+]c also increased but to a lesser extent. On reoxygenation, [Ca2+]m fell rapidly even though cells hypercontracted and [Ca2+]c remained elevated.

CONCLUSIONS

During hypoxia following rigor development Ca2+ uptake into mitochondria occurs largely via the Na+/Ca2+ exchanger rather than the Ca2+ uniporter whereas on reoxygenation the transporters resume their normal directionality.

Overexpression of lactate dehydrogenase A attenuates glucose-induced insulin secretion in stable MIN-6 beta-cell lines

Since islet beta-cells express little L-lactate dehydrogenase (LDH) activity, we have examined the effects on these cells of LDH overexpression. In mock-transfected MIN6 beta-cells, LDH activity was 38 nmol/min/mg protein, and 30 mM glucose stimulated secretion 10.4-fold. In two MIN6 cell clones stably overexpressing human LDH-A cDNA, insulin secretion was stimulated only 2.7- and 2.1-fold by high glucose. K+-stimulated insulin secretion was unaffected, and leucine stimulation enhanced, by LDH-A overexpression. LDH-A-overexpressing clones displayed unaltered activities of hexokinase, glucokinase, and malate dehydrogenase, slightly elevated plasma membrane lactate transport activity, and lowered insulin content. Low LDH activity would therefore appear important in beta-cell glucose sensing.

Secretory-granule dynamics visualized in vivo with a phogrin-green fluorescent protein chimaera

To image the behaviour in real time of single secretory granules in neuroendocrine cells we have expressed cDNA encoding a fusion construct between the dense-core secretory-granule-membrane glycoprotein, phogrin (phosphatase on the granule of insulinoma cells), and enhanced green fluorescent protein (EGFP). Expressed in INS-1 beta-cells and pheochromocytoma PC12 cells, the chimaera was localized efficiently (up to 95%) to dense-core secretory granules (diameter 200-1000 nm), identified by co-immunolocalization with anti-(pro-)insulin antibodies in INS-1 cells and dopamine beta-hydroxylase in PC12 cells. Using laser-scanning confocal microscopy and digital image analysis, we have used this chimaera to monitor the effects of secretagogues on the dynamics of secretory granules in single living cells. In unstimulated INS-1 beta-cells, granule movement was confined to oscillatory movement (dithering) with period of oscillation 5-10 s and mean displacement <1 microm. Both elevated glucose concentrations (30 mM), and depolarization of the plasma membrane with K+, provoked large (5-10 microm) saltatory excursions of granules across the cell, which were never observed in cells maintained at low glucose concentration. By contrast, long excursions of granules occurred in PC12 cells without stimulation, and occurred predominantly from the cell body towards the cell periphery and neurite extensions. Purinergic-receptor activation with ATP provoked granule movement towards the membrane of PC12 cells, resulting in the transfer of fluorescence to the plasma membrane consistent with fusion of the granule and diffusion of the chimaera in the plasma membrane. These results illustrate the potential use of phogrin-EGFP chimeras in the study of secretory-granule dynamics, the regulation of granule-cytoskeletal interactions and the trafficking of a granule-specific transmembrane protein during the cycle of exocytosis and endocytosis.