Mechanisms of biphasic insulin-granule exocytosis - roles of the cytoskeleton, small GTPases and SNARE proteins

Gene targeting implicates Cdc42 GTPase in GPVI and non-GPVI mediated platelet filopodia formation, secretion and aggregation

Background

Cdc42 and Rac1, members of the Rho family of small GTPases, play critical roles in actin cytoskeleton regulation. We have shown previously that Rac1 is involved in regulation of platelet secretion and aggregation. However, the role of Cdc42 in platelet activation remains controversial. This study was undertaken to better understand the role of Cdc42 in platelet activation.

Methodology/Principal Findings

We utilized the Mx-cre;Cdc42^lox/lox inducible mice with transient Cdc42 deletion to investigate the involvement of Cdc42 in platelet function. The Cdc42-deficient mice exhibited a significantly reduced platelet count than the matching Cdc42^+/+ mice. Platelets isolated from Cdc42^−/−, as compared to Cdc42^+/+, mice exhibited (a) diminished phosphorylation of PAK1/2, an effector molecule of Cdc42, (b) inhibition of filopodia formation on immobilized CRP or fibrinogen, (c) inhibition of CRP- or thrombin-induced secretion of ATP and release of P-selectin, (d) inhibition of CRP, collagen or thrombin induced platelet aggregation, and (e) minimal phosphorylation of Akt upon stimulation with CRP or thrombin. The bleeding times were significantly prolonged in Cdc42^−/− mice compared with Cdc42^+/+ mice.

Conclusion/Significance

Our data demonstrate that Cdc42 is required for platelet filopodia formation, secretion and aggregation and therefore plays a critical role in platelet mediated hemostasis and thrombosis.

Bidirectional insulin granule turnover in the submembrane space during K(+) depolarization-induced secretion

Like primary mouse islets, MIN6 pseudoislets responded to the depolarization by 40 mm KCl and the resulting increase in the free cytosolic Ca(2+) concentration ([Ca(2+) ](i) ) with a massive increase in insulin secretion, whereas 15 mm KCl had little effect in spite of a clear increase in [Ca(2+) ](i) . Analysis of insulin-enhanced green fluorescent protein (EGFP)-labeled granules in MIN6 cells by total internal reflection fluorescence (TIRF) microscopy showed that 40 mm KCl increased the number of short-term resident granules (<1 second presence in the submembrane space), while the total granule number and the number of long-term resident granules decreased. The rates of granule arrival at and departure from the submembrane space changed in parallel and were two orders of magnitude higher than the release rates, suggesting a back-and-forth movement of the granules as the primary determinant of the submembrane granule number. The effect of 15 mm KCl resembled that of 40 mm but did not achieve significance. Both 15 and 40 mm KCl evoked a [Ca(2+) ](i) increase, which was antagonized by 10 µm nifedipine. Nifedipine also antagonized the effect on secretion and on granule number and mobility. In conclusion, during KCl depolarization L-type Ca(2+) channels seem to regulate two processes, insulin granule turnover in the submembrane space and granule exocytosis.

Rab27a in pancreatic beta-cells, a busy protein in membrane trafficking

The small GTPases have the 'active' GTP-bound and 'inactive' GDP-bound states, and thereby act as a molecular switch in cells. Rab27a is a member of this family and exists in T-lymphocytes, melanocytes and pancreatic beta-cells. Rab27a regulates secretion of cytolytic granules from cytotoxic T-lymphocytes and intracellular transport of melanosomes in melanocytes. In pancreatic beta-cells, Rab27a controls pre-exocytotic stages of insulin secretion. A few GTP-dependent Rab27a effectors are known to mediate these cellular functions. We recently found that Rab27a also possesses the GDP-dependent effector coronin 3. Coronin 3 regulates endocytosis in pancreatic beta-cells through its interaction with GDP-Rab27a. These results imply that GTP- and GDP-Rab27a actively regulate distinct stages in the insulin secretory pathway. In this review, we provide an overview of the roles of both GTP- and GDP-Rab27a in pancreatic beta-cells.

Regulation of retinoschisin secretion in Weri-Rb1 cells by the F-actin and microtubule cytoskeleton

Retinoschisin is encoded by the gene responsible for X-linked retinoschisis (XLRS), an early onset macular degeneration that results in a splitting of the inner layers of the retina and severe loss in vision. Retinoschisin is predominantly expressed and secreted from photoreceptor cells as a homo-oligomer protein; it then associates with the surface of retinal cells and maintains the retina cellular architecture. Many missense mutations in the XLRS1 gene are known to cause intracellular retention of retinoschisin, indicating that the secretion process of the protein is a critical step for its normal function in the retina. However, the molecular mechanisms underlying retinoschisin's secretion remain to be fully elucidated. In this study, we investigated the role of the F-actin cytoskeleton in the secretion of retinoschisin by treating Weri-Rb1 cells, which are known to secrete retinoschisin, with cytochalasin D, jasplakinolide, Y-27632, and dibutyryl cGMP. Our results show that cytochalasin D and jasplakinolide inhibit retinoschisin secretion, whereas Y-27632 and dibutyryl cGMP enhance secretion causing F-actin alterations. We also demonstrate that high concentrations of taxol, which hyperpolymerizes microtubules, inhibit retinoschisin secretion. Our data suggest that retinoschisin secretion is regulated by the F-actin cytoskeleton, that cGMP or inhibition of ROCK alters F-actin structure enhancing the secretion, and that the microtubule cytoskeleton is also involved in this process.

Dynamics of insulin secretion and the clinical implications for obesity and diabetes

Insulin secretion is a highly dynamic process regulated by various factors including nutrients, hormones, and neuronal inputs. The dynamics of insulin secretion can be studied at different levels: the single β cell, pancreatic islet, whole pancreas, and the intact organism. Studies have begun to analyze cellular and molecular mechanisms underlying dynamics of insulin secretion. This review focuses on our current understanding of the dynamics of insulin secretion in vitro and in vivo and discusses their clinical relevance.

Serum response factor expression is enriched in pancreatic β cells and regulates insulin gene expression

Serum response factor (SRF) is an essential regulator of myogenic and neurogenic genes and the ubiquitously expressed immediate-early genes. The purpose of this study is to determine SRF expression pattern in murine pancreas and examine the role of SRF in pancreatic gene expression. Immunohistochemical analysis of wild-type pancreas and LacZ staining of pancreas from SRF LacZ knock-in animals showed that SRF expression is restricted to β cells. SRF bound to the rat insulin promoter II (RIP II) serum response element, an element conserved in both rat I and murine I and II insulin promoters. SRF activated RIP II, and SRF binding to RIP II and the exon 5-encoded 64-aa subdomain of SRF was required for this activation. Transient or stable knockdown of SRF leads to down-regulation of insulin gene expression, suggesting that SRF is required for insulin gene expression. Further, SRF physically interacted with the pancreas and duodenum homeobox-1 (Pdx-1) and synergistically activated RIP II. Elevated glucose concentration down-regulated SRF binding to RIP II SRE, and this down-regulation was associated with decreased RIP II activity and increased SRF phosphorylation on serine 103. Together, our results demonstrate that SRF is a glucose concentration-sensitive regulator of insulin gene expression.

Arf nucleotide binding site opener [ARNO] promotes sequential activation of Arf6, Cdc42 and Rac1 and insulin secretion in INS 832/13 β-cells and rat islets

Glucose-stimulated insulin secretion [GSIS] involves interplay between small G-proteins and their regulatory factors. Herein, we tested the hypothesis that Arf nucleotide binding site opener [ARNO], a guanine nucleotide-exchange factor [GEF] for the small G-protein Arf6, mediates the functional activation of Arf6, and that ARNO/Arf6 signaling axis, in turn, controls the activation of Cdc42 and Rac1, which have been implicated in GSIS. Molecular biological [i.e., expression of inactive mutants or siRNA] and pharmacological approaches were employed to assess the roles for ARNO/Arf6 signaling pathway in insulin secretion in normal rat islets and INS 832/13 cells. Degrees of activation of Arf6 and Cdc42/Rac1 were quantitated by GST-GGA3 and PAK-1 kinase pull-down assays, respectively. ARNO is expressed in INS 832/13 cells, rat islets and human islets. Expression of inactive mutants of Arf6 [Arf6-T27N] or ARNO [ARNO-E156K] or siRNA-ARNO markedly reduced GSIS in isolated β-cells. SecinH3, a selective inhibitor of ARNO/Arf6 signaling axis, also inhibited GSIS in INS 832/13 cells and rat islets. Stimulatory concentrations of glucose promoted Arf6 activation, which was inhibited by secinH3 or siRNA-ARNO, suggesting that ARNO/Arf6 signaling cascade is necessary for GSIS. SecinH3 or siRNA-ARNO also inhibited glucose-induced activation of Cdc42 and Rac1 suggesting that ARNO/Arf6 might be upstream to Cdc42 and Rac1 activation steps, which are necessary for GSIS. Lastly, co-immunoprecipitation and confocal microscopic studies suggested increased association between Arf6 and ARNO in glucose-stimulated β-cells. These findings provide the first evidence to implicate ARNO in the sequential activation of Arf6, Cdc42 and Rac1 culminating in GSIS.

Friendly, and not so friendly, roles of Rac1 in islet β-cell function: lessons learnt from pharmacological and molecular biological approaches

Glucose-stimulated insulin secretion [GSIS] involves a sequence of metabolic events leading to small G-protein [e.g., Rac1]-mediated cytoskeletal remodeling to promote granule mobilization toward the plasma membrane for fusion and release of insulin. Existing evidence supports a positive modulatory role for Rac1 in GSIS. Specific regulatory factors of Rac1 function, including the guanine nucleotide exchange factors [e.g., Tiam1] have also been identified and studied in the islet. Inhibition of Tiam1/Rac1 signaling axis attenuates GSIS suggesting its pivotal role in insulin secretion. In addition to its positive [i.e., friendly] roles in GSIS, Rac1 also plays "non-friendly" role[s] in the islet function. For example, it up-regulates the intracellular reactive oxygen species [ROS] levels via activation of phagocyte-like NADPH oxidase [Nox]. Despite the emerging evidence that a tonic increase in intracellular ROS is necessary for GSIS, experimental evidence also suggests that chronic exposure of β-cells to high glucose, palmitate or cytokines results in the onset of oxidative stress leading to reduction in mitochondrial membrane potential, cytosolic accumulation of cytochrome C and activation of caspase-3 leading to β-cell apoptosis. Pharmacological and molecular biological inhibition of Rac1 activation affords partial protection against Nox-induced oxidative stress and mitochondrial dysfunction induced by elevated glucose, lipids or cytokines. Herein, we overview the existing evidence to suggest positive as well as negative modulatory roles of Rac1 in islet function. Potential avenues for future research including development of inhibitors to halt the Rac1-Nox activation and generation of oxidative stress leading to the metabolic dysfunction of the β-cell are discussed.

The syntaxin 4 N terminus regulates its basolateral targeting by munc18c-dependent and -independent mechanisms

To generate and maintain epithelial cell polarity, specific sorting of proteins into vesicles destined for the apical and basolateral domain is required. Syntaxin 3 and 4 are apical and basolateral SNARE proteins important for the specificity of vesicle fusion at the apical and basolateral plasma membrane domains, respectively, but how these proteins are specifically targeted to these domains themselves is unclear. Munc18/SM proteins are potential regulators of this process. Like syntaxins, they are crucial for exocytosis and vesicle fusion. However, how munc18c and syntaxin 4 regulate the function of each other is unclear. Here, we investigated the requirement of syntaxin 4 in the delivery of basolateral membrane and secretory proteins, the basolateral targeting of syntaxin 4, and the role of munc18c in this targeting. Depletion of syntaxin 4 resulted in significant reduction of basolateral targeting, suggesting no compensation by other syntaxin forms. Mutational analysis identified amino acids Leu-25 and to a lesser extent Val-26 as essential for correct localization of syntaxin 4. Recently, it was shown that the N-terminal peptide of syntaxin 4 is involved in binding to munc18c. A mutation in this region that affects munc18c binding shows that munc18c binding is required for stabilization of syntaxin 4 at the plasma membrane but not for its correct targeting. We conclude that the N terminus serves two functions in membrane targeting. First, it harbors the sorting motif, which targets syntaxin 4 basolaterally in a munc18c-independent manner and second, it allows for munc18c binding, which stabilizes the protein in a munc18c-dependent manner.

Isoprenylcysteine carboxyl methyltransferase facilitates glucose-induced Rac1 activation, ROS generation and insulin secretion in INS 832/13 β-cells

Isoprenylcysteine carboxyl methyltransferase (ICMT) catalyzes the post-translational methylation of C-terminal cysteines of isoprenylated proteins, including small G-proteins and the γ-subunits of heterotrimeric G-proteins. It is widely felt that carboxymethylation promotes efficient membrane association of the methylated proteins and specific protein-protein interactions. In the current study, we tested the hypothesis that ICMT-mediated carboxymethylation of specific proteins (e.g., Rac1) plays a regulatory role in glucose-stimulated insulin secretion (GSIS). Western blot analysis indicated that lCMT is expressed and predominantly membrane associated in INS 832/13 β-cells. siRNA-mediated knockdown of endogenous expression of ICMT markedly attenuated glucose, but not KCl-induced insulin secretion. These findings were further supported by pharmacological observations, which suggested a marked reduction in glucose-, but not KCl-stimulated insulin secretion by acetyl farnesyl cysteine (AFC), a selective inhibitor of ICMT. In addition, glucose-induced Rac1 activation, a hallmark signaling step involved in glucose-stimulated insulin secretion, was markedly inhibited following pharmacological (AFC) or molecular biological (siRNA-ICMT) inhibition of ICMT. Lastly, we also noticed a marked reduction in glucose-induced acute increase in the generation of reactive oxygen species in INS 832/13 cells pre-treated with AFC or transfected with siRNA-ICMT. Together, these data suggest that ICMT regulates glucose-induced Rac1 activation, generation of reactive oxygen species and insulin secretion in pancreatic β-cells.

Metabolic amplification of insulin secretion by glucose is independent of β-cell microtubules

Glucose-induced insulin secretion (IS) by β-cells is controlled by two pathways. The triggering pathway involves ATP-sensitive potassium (KATP) channel-dependent depolarization, Ca2+ influx, and rise in the cytosolic Ca2+ concentration ([Ca2+]c), which triggers exocytosis of insulin granules. The metabolic amplifying pathway augments IS without further increasing [Ca2+]c. After exclusion of the contribution of actin microfilaments, we here tested whether amplification implicates microtubule-dependent granule mobilization. Mouse islets were treated with nocodazole or taxol, which completely depolymerized and polymerized tubulin. They were then perifused to measure [Ca2+]c and IS. Metabolic amplification was studied during imposed steady elevation of [Ca2+]c by tolbutamide or KCl or by comparing [Ca2+]c and IS responses to glucose and tolbutamide. Nocodazole did not alter [Ca2+]c or IS changes induced by the three secretagogues, whereas taxol caused a small inhibition of IS that is partly ascribed to a decrease in [Ca2+]c. When [Ca2+]c was elevated and controlled by KCl or tolbutamide, the amplifying action of glucose was unaffected by microtubule disruption or stabilization. Both phases of IS were larger in response to glucose than tolbutamide, although triggering [Ca2+]c was lower. This difference, due to amplification, persisted in nocodazole- or taxol-treated islets, even when IS was augmented fourfold by microfilament disruption with cytochalasin B or latrunculin B. In conclusion, metabolic amplification rapidly augments first and second phases of IS independently of insulin granule translocation along microtubules. We therefore extend our previous proposal that it does not implicate the cytoskeleton but corresponds to acceleration of the priming process conferring release competence to insulin granules.

α-Synuclein binds the K(ATP) channel at insulin-secretory granules and inhibits insulin secretion

α-Synuclein has been studied in numerous cell types often associated with secretory processes. In pancreatic β-cells, α-synuclein might therefore play a similar role by interacting with organelles involved in insulin secretion. We tested for α-synuclein localizing to insulin-secretory granules and characterized its role in glucose-stimulated insulin secretion. Immunohistochemistry and fluorescent sulfonylureas were used to test for α-synuclein localization to insulin granules in β-cells, immunoprecipitation with Western blot analysis for interaction between α-synuclein and K(ATP) channels, and ELISA assays for the effect of altering α-synuclein expression up or down on insulin secretion in INS1 cells or mouse islets, respectively. Differences in cellular phenotype between α-synuclein knockout and wild-type β-cells were found by using confocal microscopy to image the fluorescent insulin biosensor Ins-C-emGFP and by using transmission electron microscopy. The results show that anti-α-synuclein antibodies labeled secretory organelles within β-cells. Anti-α-synuclein antibodies colocalized with K(ATP) channel, anti-insulin, and anti-C-peptide antibodies. α-Synuclein coimmunoprecipitated in complexes with K(ATP) channels. Expression of α-synuclein downregulated insulin secretion at 2.8 mM glucose with little effect following 16.7 mM glucose stimulation. α-Synuclein knockout islets upregulated insulin secretion at 2.8 and 8.4 mM but not 16.7 mM glucose, consistent with the depleted insulin granule density at the β-cell surface membranes observed in these islets. These findings demonstrate that α-synuclein interacts with K(ATP) channels and insulin-secretory granules and functionally acts as a brake on secretion that glucose stimulation can override. α-Synuclein might play similar roles in diabetes as it does in other degenerative diseases, including Alzheimer's and Parkinson's diseases.

Phagocyte-like NADPH oxidase generates ROS in INS 832/13 cells and rat islets: role of protein prenylation

Recent evidence suggests that an acute increase in the generation of phagocyte-like NADPH-oxidase (Nox)-mediated reactive oxygen species (ROS) may be necessary for glucose-stimulated insulin secretion. Using rat islets and INS 832/13 cells, we tested the hypothesis that activation of specific G proteins is necessary for nutrient-mediated intracellular generation of ROS. Stimulation of β-cells with glucose or a mixture of mitochondrial fuels (mono-methylsuccinate plus α-ketoisocaproic acid) markedly elevated intracellular accumulation of ROS, which was attenuated by selective inhibitors of Nox (e.g., apocynin or diphenyleneiodonium chloride) or short interfering RNA-mediated knockdown of p47(phox), one of the subunits of Nox. Selective inhibitors of protein prenylation (FTI-277 or GGTI-2147) markedly inhibited nutrient-induced ROS generation, suggesting that activation of one (or more) prenylated small G proteins and/or γ-subunits of trimeric G proteins is involved in this signaling axis. Depletion of endogenous GTP levels with mycophenolic acid significantly reduced glucose-induced activation of Rac1 and ROS generation in these cells. Other immunosuppressants, like cyclosporine A or rapamycin, which do not deplete endogenous GTP levels, failed to affect glucose-induced ROS generation, suggesting that endogenous GTP is necessary for glucose-induced Nox activation and ROS generation. Treatment of INS 832/13 cells or rat islets with pertussis toxin (Ptx), which ADP ribosylates and inhibits inhibitory class of trimeric G proteins (i.e., G(i) or G(o)), significantly attenuated glucose-induced ROS generation in these cells, implicating activation of a Ptx-sensitive G protein in these signaling cascade. Together, our findings suggest a prenylated Ptx-sensitive signaling step couples Rac1 activation in the signaling steps necessary for glucose-mediated generation of ROS in the pancreatic β-cells.

Syntaxin 1A regulates surface expression of beta-cell ATP-sensitive potassium channels

The pancreatic ATP-sensitive potassium (K(ATP)) channel consisting of four inwardly rectifying potassium channel 6.2 (Kir6.2) and four sulfonylurea receptor SUR1 subunits plays a key role in insulin secretion by linking glucose metabolism to membrane excitability. Syntaxin 1A (Syn-1A) is a plasma membrane protein important for membrane fusion during exocytosis of insulin granules. Here, we show that Syn-1A and K(ATP) channels endogenously expressed in the insulin-secreting cell INS-1 interact. Upregulation of Syn-1A by overexpression in INS-1 leads to a decrease, whereas downregulation of Syn-1A by small interfering RNA (siRNA) leads to an increase, in surface expression of K(ATP) channels. Using COSm6 cells as a heterologous expression system for mechanistic investigation, we found that Syn-1A interacts with SUR1 but not Kir6.2. Furthermore, Syn-1A decreases surface expression of K(ATP) channels via two mechanisms. One mechanism involves accelerated endocytosis of surface channels. The other involves decreased biogenesis and processing of channels in the early secretory pathway. This regulation is K(ATP) channel specific as Syn-1A has no effect on another inward rectifier potassium channel Kir3.1/3.4. Our results demonstrate that in addition to a previously documented role in modulating K(ATP) channel gating, Syn-1A also regulates K(ATP) channel expression in β-cells. We propose that physiological or pathological changes in Syn-1A expression may modulate insulin secretion by altering glucose-secretion coupling via changes in K(ATP) channel expression.

Insulin/phosphoinositide 3-kinase pathway accelerates the glucose-induced first-phase insulin secretion through TrpV2 recruitment in pancreatic β-cells

Functional insulin receptor and its downstream effector PI3K (phosphoinositide 3-kinase) have been identified in pancreatic β-cells, but their involvement in the regulation of insulin secretion from β-cells remains unclear. In the present study, we investigated the physiological role of insulin and PI3K in glucose-induced biphasic insulin exocytosis in primary cultured β-cells and insulinoma Min6 cells using total internal reflection fluorescent microscopy. The pretreatment of β-cells with insulin induced the rapid increase in intracellular Ca2+ levels and accelerated the exocytotic response without affecting the second-phase insulin secretion. The inhibition of PI3K not only abolished the insulin-induced rapid development of the exocytotic response, but also potentiated the second-phase insulin secretion. The rapid development of Ca2+ and accelerated exocytotic response induced by insulin were accompanied by the translocation of the Ca2+-permeable channel TrpV2 (transient receptor potential V2) in a PI3K-dependent manner. Inhibition of TrpV2 by the selective blocker tranilast, or the expression of shRNA (short-hairpin RNA) against TrpV2 suppressed the effect of insulin in the first phase, but the second phase was not affected. Thus our results demonstrate that insulin treatment induced the acceleration of the exocytotic response during the glucose-induced first-phase response by the insertion of TrpV2 into the plasma membrane in a PI3K-dependent manner.

Rab GTPases implicated in inherited and acquired disorders

The endocytotic machinery imports, transports and exports receptors and associated molecules between the plasma membrane and various cytoplasmic chambers resulting in selective recycling, degradation, or secretion of molecules and signaling complexes. Trafficking of receptors, growth factors, nutrients, cytokines, integrins as well as pathogens dictates the kinetics and magnitude of signal transduction cascades. Understandably, alterations in the 'fate' of such cargo complexes have profound physiologic and pathophysiologic implications. Rab GTPases regulate endocytosis by decorating intracellular vesicles and targeting these vesicles along with their cargoes to appropriate subcellular compartments. In the last decade, the number of genetic diseases driven by germline mutations in Rab GTPases or their interacting proteins, has increased and there is growing evidence of aberrant Rab GTPase function in acquired pathophysiologies such as immune deficiency, infection, obesity, diabetes and cancer.

Chronic high-fat diet in fathers programs β-cell dysfunction in female rat offspring

The global prevalence of obesity is increasing across most ages in both sexes. This is contributing to the early emergence of type 2 diabetes and its related epidemic. Having either parent obese is an independent risk factor for childhood obesity. Although the detrimental impacts of diet-induced maternal obesity on adiposity and metabolism in offspring are well established, the extent of any contribution of obese fathers is unclear, particularly the role of non-genetic factors in the causal pathway. Here we show that paternal high-fat-diet (HFD) exposure programs β-cell 'dysfunction' in rat F(1) female offspring. Chronic HFD consumption in Sprague-Dawley fathers induced increased body weight, adiposity, impaired glucose tolerance and insulin sensitivity. Relative to controls, their female offspring had an early onset of impaired insulin secretion and glucose tolerance that worsened with time, and normal adiposity. Paternal HFD altered the expression of 642 pancreatic islet genes in adult female offspring (P < 0.01); genes belonged to 13 functional clusters, including cation and ATP binding, cytoskeleton and intracellular transport. Broader pathway analysis of 2,492 genes differentially expressed (P < 0.05) demonstrated involvement of calcium-, MAPK- and Wnt-signalling pathways, apoptosis and the cell cycle. Hypomethylation of the Il13ra2 gene, which showed the highest fold difference in expression (1.76-fold increase), was demonstrated. This is the first report in mammals of non-genetic, intergenerational transmission of metabolic sequelae of a HFD from father to offspring.

Gαo represses insulin secretion by reducing vesicular docking in pancreatic beta-cells

OBJECTIVE

Pertussis toxin uncoupling-based studies have shown that Gαi and Gαo can inhibit insulin secretion in pancreatic β-cells. Yet it is unclear whether Gαi and Gαo operate through identical mechanisms and how these G-protein-mediated signals inhibit insulin secretion in vivo. Our objective is to examine whether/how Gαo regulates islet development and insulin secretion in β-cells.

RESEARCH DESIGN AND METHODS

Immunoassays were used to analyze the Gαo expression in mouse pancreatic cells. Gαo was specifically inactivated in pancreatic progenitor cells by pancreatic cell-specific gene deletion. Hormone expression and insulin secretion in response to different stimuli were assayed in vivo and in vitro. Electron microscope and total internal reflection fluorescence-based assays were used to evaluate how Gαo regulates insulin vesicle docking and secretion in response to glucose stimulation.

RESULTS

Islet cells differentiate properly in Gαo(-/-) mutant mice. Gαo inactivation significantly enhances insulin secretion both in vivo and in isolation. Gαo nullizygous β-cells contain an increased number of insulin granules docked on the cell plasma membrane, although the total number of vesicles per β-cell remains unchanged.

CONCLUSIONS

Gαo is not required for endocrine islet cell differentiation, but it regulates the number of insulin vesicles docked on the β-cell membrane.

Metabolic amplifying pathway increases both phases of insulin secretion independently of β-cell actin microfilaments

Two pathways control glucose-induced insulin secretion (IS) by β-cells. The triggering pathway involves ATP-sensitive potassium (KATP) channel-dependent depolarization, Ca2+ influx, and a rise in the cytosolic Ca2+ concentration ([Ca2+]c), which triggers exocytosis of insulin granules. The metabolic amplifying pathway augments IS without further increasing [Ca2+]c. The underlying mechanisms are unknown. Here, we tested the hypothesis that amplification implicates actin microfilaments. Mouse islets were treated with latrunculin B and cytochalasin B to depolymerize actin or jasplakinolide to polymerize actin. They were then perifused to measure [Ca2+]c and IS. Metabolic amplification was studied during imposed steady elevation of [Ca2+]c by tolbutamide or KCl or by comparing the magnitude of [Ca2+]c and IS changes produced by glucose and tolbutamide. Both actin polymerization and depolymerization augmented IS triggered by all stimuli without increasing (sometimes decreasing) [Ca2+]c, which indicates a predominantly inhibitory function of microfilaments in exocytosis at a step distal to [Ca2+]c increase. When [Ca2+]c was elevated and controlled by KCl or tolbutamide, the amplifying action of glucose was facilitated by actin depolymerization and unaffected by polymerization. Both phases of IS were larger in response to high-glucose than to tolbutamide in low-glucose, although triggering [Ca2+]c was lower. This difference in IS, due to amplification, persisted when the IS rate was doubled by actin depolymerization or polymerization. In conclusion, metabolic amplification is rapid and influences the first as well as the second phase of IS. It is a late step of stimulus-secretion coupling, which does not require functional actin microfilaments and could correspond to acceleration of the priming process conferring release competence to insulin granules.

An intracellular role for ABCG1-mediated cholesterol transport in the regulated secretory pathway of mouse pancreatic beta cells

Cholesterol is a critical component of cell membranes, and cellular cholesterol levels and distribution are tightly regulated in mammals. Recent evidence has revealed a critical role for pancreatic beta cell-specific cholesterol homeostasis in insulin secretion as well as in beta cell dysfunction in diabetes and the metabolic response to thiazolidinediones (TZDs), which are antidiabetic drugs. The ATP-binding cassette transporter G1 (ABCG1) has been shown to play a role in cholesterol efflux, but its role in beta cells is currently unknown. In other cell types, ABCG1 expression is downregulated in diabetes and upregulated by TZDs. Here we have demonstrated an intracellular role for ABCG1 in beta cells. Loss of ABCG1 expression impaired insulin secretion both in vivo and in vitro, but it had no effect on cellular cholesterol content or efflux. Subcellular localization studies showed the bulk of ABCG1 protein to be present in insulin granules. Loss of ABCG1 led to altered granule morphology and reduced granule cholesterol levels. Administration of exogenous cholesterol restored granule morphology and cholesterol content and rescued insulin secretion in ABCG1-deficient islets. These findings suggest that ABCG1 acts primarily to regulate subcellular cholesterol distribution in mouse beta cells. Furthermore, islet ABCG1 expression was reduced in diabetic mice and restored by TZDs, implicating a role for regulation of islet ABCG1 expression in diabetes pathogenesis and treatment.

Regulation of insulin secretion: role of mitochondrial signalling

Pancreatic beta cells are specialised endocrine cells that continuously sense the levels of blood sugar and other fuels and, in response, secrete insulin to maintain normal fuel homeostasis. During postprandial periods an elevated level of plasma glucose rapidly stimulates insulin secretion to decrease hepatic glucose output and promote glucose uptake into other tissues, principally muscle and adipose tissues. Beta cell mitochondria play a key role in this process, not only by providing energy in the form of ATP to support insulin secretion, but also by synthesising metabolites (anaplerosis) that can act, both intra- and extramitochondrially, as factors that couple glucose sensing to insulin granule exocytosis. ATP on its own, and possibly modulated by these coupling factors, triggers closure of the ATP-sensitive potassium channel, resulting in membrane depolarisation that increases intracellular calcium to cause insulin secretion. The metabolic imbalance caused by chronic hyperglycaemia and hyperlipidaemia severely affects mitochondrial metabolism, leading to the development of impaired glucose-induced insulin secretion in type 2 diabetes. It appears that the anaplerotic enzyme pyruvate carboxylase participates directly or indirectly in several metabolic pathways which are important for glucose-induced insulin secretion, including: the pyruvate/malate cycle, the pyruvate/citrate cycle, the pyruvate/isocitrate cycle and glutamate-dehydrogenase-catalysed alpha-ketoglutarate production. These four pathways enable 'shuttling' or 'recycling' of these intermediate(s) into and out of mitochondrion, allowing continuous production of intracellular messenger(s). The purpose of this review is to present an account of recent progress in this area of central importance in the realm of diabetes and obesity research.

Protein farnesylation-dependent Raf/extracellular signal-related kinase signaling links to cytoskeletal remodeling to facilitate glucose-induced insulin secretion in pancreatic beta-cells

OBJECTIVE

Posttranslational prenylation (e.g., farnesylation) of small G-proteins is felt to be requisite for cytoskeletal remodeling and fusion of secretory vesicles with the plasma membrane. Here, we investigated roles of protein farnesylation in the signaling steps involved in Raf-1/extracellular signal-related kinase (ERK1/2) signaling pathway in glucose-induced Rac1 activation and insulin secretion in the pancreatic beta-cell.

RESEARCH DESIGN AND METHODS

These studies were carried out in INS 832/13 cells and normal rat islets. Molecular biological (e.g., overexpression or small interfering RNA [siRNA]-mediated knockdown) and pharmacologic approaches were used to determine roles for farnesylation in glucose-mediated activation of ERK1/2, Rac1, and insulin secretion. Activation of ERK1/2 was determined by Western blotting. Rac1 activation (i.e., Rac1.GTP) was quantitated by p21-activated kinase pull-down assay. Insulin release was quantitated by enzyme-linked immunosorbent assay.

RESULTS

Coprovision of structure-specific inhibitors of farnesyl transferase (FTase; e.g., FTI-277 or FTI-2628) or siRNA-mediated knockdown of FTase beta-subunit resulted in a significant inhibition of glucose-stimulated ERK1/2 and Rac1 activation and insulin secretion. Pharmacologic inhibition of Raf-1 kinase using GW-5074 markedly reduced the stimulatory effects of glucose on ERK1/2 phosphorylation, Rac1 activation, and insulin secretion, suggesting that Raf-1 kinase activation may be upstream to ERK1/2 and Rac1 activation leading to glucose-induced insulin release. Lastly, siRNA-mediated silencing of endogenous expression of ERK1/2 markedly attenuated glucose-induced Rac1 activation and insulin secretion.

CONCLUSIONS

Together, our findings provide the first evidence of a role for protein farnesylation in glucose-mediated regulation of the Raf/ERK signaling pathway culminating in the activation of Rac1, which has been shown to be necessary for cytoskeletal reorganization and exocytotic secretion of insulin.

Cellular modeling: insight into oral minimal models of insulin secretion

The oral glucose tolerance test and meal tolerance test are common clinical tests of the glucose-insulin system. Several mathematical models have been suggested as means to extract information about beta-cell function from data from oral tolerance tests. Any such model needs to be fairly simple but should at the same time be linked to the underlying biology of the insulin-secreting beta-cells. The scope of the present work is to present a way to make such a connection using a recent model describing intracellular mechanisms. We show how the three main components of oral minimal secretion models, derivative control, proportional control, and delay, are related to subcellular events, thus providing mechanistic underpinning of the assumptions of the minimal models.

Small G proteins in islet beta-cell function

Glucose-stimulated insulin secretion from the islet beta-cell involves a sequence of metabolic events and an interplay between a wide range of signaling pathways leading to the generation of second messengers (e.g., cyclic nucleotides, adenine and guanine nucleotides, soluble lipid messengers) and mobilization of calcium ions. Consequent to the generation of necessary signals, the insulin-laden secretory granules are transported from distal sites to the plasma membrane for fusion and release of their cargo into the circulation. The secretory granule transport underlies precise changes in cytoskeletal architecture involving a well-coordinated cross-talk between various signaling proteins, including small molecular mass GTP-binding proteins (G proteins) and their respective effector proteins. The purpose of this article is to provide an overview of current understanding of the identity of small G proteins (e.g., Cdc42, Rac1, and ARF-6) and their corresponding regulatory factors (e.g., GDP/GTP-exchange factors, GDP-dissociation inhibitors) in the pancreatic beta-cell. Plausible mechanisms underlying regulation of these signaling proteins by insulin secretagogues are also discussed. In addition to their positive modulatory roles, certain small G proteins also contribute to the metabolic dysfunction and demise of the islet beta-cell seen in in vitro and in vivo models of impaired insulin secretion and diabetes. Emerging evidence also suggests significant insulin secretory abnormalities in small G protein knockout animals, further emphasizing vital roles for these proteins in normal health and function of the islet beta-cell. Potential significance of these experimental observations from multiple laboratories and possible avenues for future research in this area of islet research are highlighted.

Obesity and genetics regulate microRNAs in islets, liver, and adipose of diabetic mice

Type 2 diabetes results from severe insulin resistance coupled with a failure of b cells to compensate by secreting sufficient insulin. Multiple genetic loci are involved in the development of diabetes, although the effect of each gene on diabetes susceptibility is thought to be small. MicroRNAs (miRNAs) are noncoding 19-22-nucleotide RNA molecules that potentially regulate the expression of thousands of genes. To understand the relationship between miRNA regulation and obesity-induced diabetes, we quantitatively profiled approximately 220 miRNAs in pancreatic islets, adipose tissue, and liver from diabetes-resistant (B6) and diabetes-susceptible (BTBR) mice. More than half of the miRNAs profiled were expressed in all three tissues, with many miRNAs in each tissue showing significant changes in response to genetic obesity. Furthermore, several miRNAs in each tissue were differentially responsive to obesity in B6 versus BTBR mice, suggesting that they may be involved in the pathogenesis of diabetes. In liver there were approximately 40 miRNAs that were downregulated in response to obesity in B6 but not BTBR mice, indicating that genetic differences between the mouse strains play a critical role in miRNA regulation. In order to elucidate the genetic architecture of hepatic miRNA expression, we measured the expression of miRNAs in genetically obese F2 mice. Approximately 10% of the miRNAs measured showed significant linkage (miR-eQTLs), identifying loci that control miRNA abundance. Understanding the influence that obesity and genetics exert on the regulation of miRNA expression will reveal the role miRNAs play in the context of obesity-induced type 2 diabetes.

Glucose activates prenyltransferases in pancreatic islet beta-cells

A growing body of evidence implicates small G-proteins [e.g., Cdc42 and Rac1] in glucose-stimulated insulin secretion [GSIS] in the islet beta-cell. These signaling proteins undergo post-translational modifications [e.g., prenylation] at their C-terminal cysteine residue and appear to be essential for the transport and fusion of insulin-containing secretory granules with the plasma membrane and the exocytotic secretion of insulin. However, potential regulation of the prenylating enzymes by physiological insulin secretogues [e.g., glucose] has not been investigated thus far. Herein, we report immunological localization, sub-cellular distribution and regulation of farnesyltransferases [FTases] and geranylgeranyltransferase [GGTase] by glucose in insulin-secreting INS 832/13 beta-cells and normal rat islets. Our findings suggest that an insulinotropic concentration of glucose [20mM] markedly stimulated the expression of the alpha-subunits of FTase/GGTase-1, but not the beta-subunits of FTase or GGTase-1 without significantly affecting the predominantly cytosolic distribution of these holoenzymes in INS 832/13 cells and rodent islets. Under these conditions, glucose significantly stimulated [2.5- to 4.0-fold over basal] the activities of both FTase and GGTase-1 in both cell types. Together, these findings provide the first evidence to suggest that GSIS involves activation of the endogenous islet prenyltransferases by glucose, culminating in the activation of their respective G-protein substrates, which is necessary for cytoskeletal rearrangement, vesicular transport, fusion and secretion of insulin.

Cholesterol regulates glucose-stimulated insulin secretion through phosphatidylinositol 4,5-bisphosphate

Membrane cholesterol modulates the ability of glucose to stimulate insulin secretion from pancreatic beta-cells. The molecular mechanism by which this occurs is not understood. Here, we show that in cultured beta-cells, cholesterol acts through phosphatidylinositol 4,5-bisphosphate (PIP(2)) to regulate actin dynamics, plasma membrane potential, and glucose-stimulated insulin secretion. Cholesterol-overloaded beta-cells exhibited decreased PIP(2) hydrolysis, with diminished glucose-induced actin reorganization, membrane depolarization, and insulin secretion. The converse findings were observed in cholesterol-depleted cells. These results support a model in which cholesterol depletion is coupled through PIP(2) to enhance both plasma membrane Ca2+ influx from the extracellular space, as well as inositol 1,4,5-triphosphate-stimulated Ca2+ efflux from intracellular stores. The inability to increase cytosolic Ca2+ may be the main underlying factor to account for impaired glucose-stimulated insulin secretion in cholesterol-overloaded beta-cells.

Obesity and genetics regulate microRNAs in islets, liver, and adipose of diabetic mice

Type 2 diabetes results from severe insulin resistance coupled with a failure of b cells to compensate by secreting sufficient insulin. Multiple genetic loci are involved in the development of diabetes, although the effect of each gene on diabetes susceptibility is thought to be small. MicroRNAs (miRNAs) are noncoding 19-22-nucleotide RNA molecules that potentially regulate the expression of thousands of genes. To understand the relationship between miRNA regulation and obesity-induced diabetes, we quantitatively profiled approximately 220 miRNAs in pancreatic islets, adipose tissue, and liver from diabetes-resistant (B6) and diabetes-susceptible (BTBR) mice. More than half of the miRNAs profiled were expressed in all three tissues, with many miRNAs in each tissue showing significant changes in response to genetic obesity. Furthermore, several miRNAs in each tissue were differentially responsive to obesity in B6 versus BTBR mice, suggesting that they may be involved in the pathogenesis of diabetes. In liver there were approximately 40 miRNAs that were downregulated in response to obesity in B6 but not BTBR mice, indicating that genetic differences between the mouse strains play a critical role in miRNA regulation. In order to elucidate the genetic architecture of hepatic miRNA expression, we measured the expression of miRNAs in genetically obese F2 mice. Approximately 10% of the miRNAs measured showed significant linkage (miR-eQTLs), identifying loci that control miRNA abundance. Understanding the influence that obesity and genetics exert on the regulation of miRNA expression will reveal the role miRNAs play in the context of obesity-induced type 2 diabetes.