Glucose- and hormone-induced cAMP oscillations in α- and β-cells within intact pancreatic islets

Reestablishment of Glucose Inhibition of Glucagon Secretion in Small Pseudoislets

Misregulated hormone secretion from the islet of Langerhans is central to the pathophysiology of diabetes. Although insulin plays a key role in glucose regulation, the importance of glucagon is increasingly acknowledged. However, the mechanisms that regulate glucagon secretion from α-cells are still unclear. We used pseudoislets reconstituted from dispersed islet cells to study α-cells with and without various indirect effects from other islet cells. Dispersed islet cells secrete aberrant levels of glucagon and insulin at basal and elevated glucose levels. When cultured, murine islet cells reassociate to form pseudoislets, which recover normal glucose-regulated hormone secretion, and human islet cells follow a similar pattern. We created small (∼40-µm) pseudoislets using all of the islet cells or only some of the cell types, which allowed us to characterize novel aspects of regulated hormone secretion. The recovery of regulated glucagon secretion from α-cells in small pseudoislets depends upon the combined action of paracrine factors, such as insulin and somatostatin, and juxtacrine signals between EphA4/7 on α-cells and ephrins on β-cells. Although these signals modulate different pathways, both appear to be required for proper inhibition of glucagon secretion in response to glucose. This improved understanding of the modulation of glucagon secretion can provide novel therapeutic routes for the treatment of some individuals with diabetes.

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

Transcriptome analysis of pancreatic cells across distant species highlights novel important regulator genes

BACKGROUND

Defining the transcriptome and the genetic pathways of pancreatic cells is of great interest for elucidating the molecular attributes of pancreas disorders such as diabetes and cancer. As the function of the different pancreatic cell types has been maintained during vertebrate evolution, the comparison of their transcriptomes across distant vertebrate species is a means to pinpoint genes under strong evolutionary constraints due to their crucial function, which have therefore preserved their selective expression in these pancreatic cell types.

RESULTS

In this study, RNA-sequencing was performed on pancreatic alpha, beta, and delta endocrine cells as well as the acinar and ductal exocrine cells isolated from adult zebrafish transgenic lines. Comparison of these transcriptomes identified many novel markers, including transcription factors and signaling pathway components, specific for each cell type. By performing interspecies comparisons, we identified hundreds of genes with conserved enriched expression in endocrine and exocrine cells among human, mouse, and zebrafish. This list includes many genes known as crucial for pancreatic cell formation or function, but also pinpoints many factors whose pancreatic function is still unknown. A large set of endocrine-enriched genes can already be detected at early developmental stages as revealed by the transcriptomic profiling of embryonic endocrine cells, indicating a potential role in cell differentiation. The actual involvement of conserved endocrine genes in pancreatic cell differentiation was demonstrated in zebrafish for myt1b, whose invalidation leads to a reduction of alpha cells, and for cdx4, selectively expressed in endocrine delta cells and crucial for their specification. Intriguingly, comparison of the endocrine alpha and beta cell subtypes from human, mouse, and zebrafish reveals a much lower conservation of the transcriptomic signatures for these two endocrine cell subtypes compared to the signatures of pan-endocrine and exocrine cells. These data suggest that the identity of the alpha and beta cells relies on a few key factors, corroborating numerous examples of inter-conversion between these two endocrine cell subtypes.

CONCLUSION

This study highlights both evolutionary conserved and species-specific features that will help to unveil universal and fundamental regulatory pathways as well as pathways specific to human and laboratory animal models such as mouse and zebrafish.

Human Beta Cells Produce and Release Serotonin to Inhibit Glucagon Secretion from Alpha Cells

In the pancreatic islet, serotonin is an autocrine signal increasing beta cell mass during metabolic challenges such as those associated with pregnancy or high-fat diet. It is still unclear whether serotonin is relevant for regular islet physiology and hormone secretion. Here, we show that human beta cells produce and secrete serotonin when stimulated with increases in glucose concentration. Serotonin secretion from beta cells decreases cyclic AMP (cAMP) levels in neighboring alpha cells via 5-HT1F receptors and inhibits glucagon secretion. Without serotonergic input, alpha cells lose their ability to regulate glucagon secretion in response to changes in glucose concentration, suggesting that diminished serotonergic control of alpha cells can cause glucose blindness and the uncontrolled glucagon secretion associated with diabetes. Supporting this model, pharmacological activation of 5-HT1F receptors reduces glucagon secretion and has hypoglycemic effects in diabetic mice. Thus, modulation of serotonin signaling in the islet represents a drug intervention opportunity.

Fluorescent protein vectors for pancreatic islet cell identification in live-cell imaging

The islets of Langerhans contain different types of endocrine cells, which are crucial for glucose homeostasis. β- and α-cells that release insulin and glucagon, respectively, are most abundant, whereas somatostatin-producing δ-cells and particularly pancreatic polypeptide-releasing PP-cells are more scarce. Studies of islet cell function are hampered by difficulties to identify the different cell types, especially in live-cell imaging experiments when immunostaining is unsuitable. The aim of the present study was to create a set of vectors for fluorescent protein expression with cell-type-specific promoters and evaluate their applicability in functional islet imaging. We constructed six adenoviral vectors for expression of red and green fluorescent proteins controlled by the insulin, preproglucagon, somatostatin, or pancreatic polypeptide promoters. After transduction of mouse and human islets or dispersed islet cells, a majority of the fluorescent cells also immunostained for the appropriate hormone. Recordings of the sub-plasma membrane Ca(2+) and cAMP concentrations with a fluorescent indicator and a protein biosensor, respectively, showed that labeled cells respond to glucose and other modulators of secretion and revealed a striking variability in Ca(2+) signaling among α-cells. The measurements allowed comparison of the phase relationship of Ca(2+) oscillations between different types of cells within intact islets. We conclude that the fluorescent protein vectors allow easy identification of specific islet cell types and can be used in live-cell imaging together with organic dyes and genetically encoded biosensors. This approach will facilitate studies of normal islet physiology and help to clarify molecular defects and disturbed cell interactions in diabetic islets.

Discovery of LRE1 as a specific and allosteric inhibitor of soluble adenylyl cyclase

The prototypical second messenger cAMP regulates a wide variety of physiological processes. It can simultaneously mediate diverse functions by acting locally in independently regulated microdomains. In mammalian cells, two types of adenylyl cyclase generate cAMP: G-protein-regulated transmembrane adenylyl cyclases and bicarbonate-, calcium- and ATP-regulated soluble adenylyl cyclase (sAC). Because each type of cyclase regulates distinct microdomains, methods to distinguish between them are needed to understand cAMP signaling. We developed a mass-spectrometry-based adenylyl cyclase assay, which we used to identify a new sAC-specific inhibitor, LRE1. LRE1 bound to the bicarbonate activator binding site and inhibited sAC via a unique allosteric mechanism. LRE1 prevented sAC-dependent processes in cellular and physiological systems, and it will facilitate exploration of the therapeutic potential of sAC inhibition.

Increased Melatonin Signaling Is a Risk Factor for Type 2 Diabetes

Type 2 diabetes (T2D) is a global pandemic. Genome-wide association studies (GWASs) have identified >100 genetic variants associated with the disease, including a common variant in the melatonin receptor 1 b gene (MTNR1B). Here, we demonstrate increased MTNR1B expression in human islets from risk G-allele carriers, which likely leads to a reduction in insulin release, increasing T2D risk. Accordingly, in insulin-secreting cells, melatonin reduced cAMP levels, and MTNR1B overexpression exaggerated the inhibition of insulin release exerted by melatonin. Conversely, mice with a disruption of the receptor secreted more insulin. Melatonin treatment in a human recall-by-genotype study reduced insulin secretion and raised glucose levels more extensively in risk G-allele carriers. Thus, our data support a model where enhanced melatonin signaling in islets reduces insulin secretion, leading to hyperglycemia and greater future risk of T2D. The findings also imply that melatonin physiologically serves to inhibit nocturnal insulin release.

Glucose control of glucagon secretion-'There's a brand-new gimmick every year'

Glucagon from the pancreatic α-cells is a major blood glucose-regulating hormone whose most important role is to prevent hypoglycaemia that can be life-threatening due to the brain's strong dependence on glucose as energy source. Lack of blood glucose-lowering insulin after malfunction or autoimmune destruction of the pancreatic β-cells is the recognized cause of diabetes, but recent evidence indicates that diabetic hyperglycaemia would not develop unless lack of insulin was accompanied by hypersecretion of glucagon. Glucagon release has therefore become an increasingly important target in diabetes management. Despite decades of research, an understanding of how glucagon secretion is regulated remains elusive, and fundamentally different mechanisms continue to be proposed. The autonomous nervous system is an important determinant of glucagon release, but it is clear that secretion is also directly regulated within the pancreatic islets. The present review focuses on pancreatic islet mechanisms involved in glucose regulation of glucagon release. It will be argued that α-cell-intrinsic processes are most important for regulation of glucagon release during recovery from hypoglycaemia and that paracrine inhibition by somatostatin from the δ-cells shapes pulsatile glucagon release in hyperglycaemia. The electrically coupled β-cells ultimately determine islet hormone pulsatility by releasing synchronizing factors that affect the α- and δ-cells.

Modeling of glucose-induced cAMP oscillations in pancreatic β cells: cAMP rocks when metabolism rolls

Recent advances in imaging technology have revealed oscillations of cyclic adenosine monophosphate (cAMP) in insulin-secreting cells. These oscillations may be in phase with cytosolic calcium oscillations or out of phase. cAMP oscillations have previously been modeled as driven by oscillations in calcium, based on the known dependence of the enzymes that generate cAMP (adenylyl cyclase) and degrade it (phosphodiesterase). However, cAMP oscillations have also been reported to occur in the absence of calcium oscillations. Motivated by similarities between the properties of cAMP and metabolic oscillations in pancreatic β cells, we propose here that in addition to direct control by calcium, cAMP is controlled by metabolism. Specifically, we hypothesize that AMP inhibits adenylyl cyclase. We incorporate this hypothesis into the dual oscillator model for β cells, in which metabolic (glycolytic) oscillations cooperate with modulation of ion channels and metabolism by calcium. We show that the combination of oscillations in AMP and calcium in the dual oscillator model can account for the diverse oscillatory patterns that have been observed, as well as for experimental perturbations of those patterns. Predictions to further test the model are provided.

Protein Kinase A Governs Oxidative Phosphorylation Kinetics and Oxidant Emitting Potential at Complex I

The mitochondrial electron transport system (ETS) is responsible for setting and maintaining both the energy and redox charges throughout the cell. Reversible phosphorylation of mitochondrial proteins, particularly via the soluble adenylyl cyclase (sAC)/cyclic AMP (cAMP)/Protein kinase A (PKA) axis, has recently been revealed as a potential mechanism regulating the ETS. However, the governance of cAMP/PKA signaling and its implications on ETS function are incompletely understood. In contrast to prior reports using exogenous bicarbonate, we provide evidence that endogenous CO₂ produced by increased tricarboxylic acid (TCA) cycle flux is insufficient to increase mitochondrial cAMP levels, and that exogenous addition of membrane permeant 8Br-cAMP does not enhance mitochondrial respiratory capacity. We also report important non-specific effects of commonly used inhibitors of sAC which preclude their use in studies of mitochondrial function. In isolated liver mitochondria, inhibition of PKA reduced complex I-, but not complex II-supported respiratory capacity. In permeabilized myofibers, inhibition of PKA lowered both the K_m and V_max for complex I-supported respiration as well as succinate-supported H₂O₂ emitting potential. In summary, the data provided here improve our understanding of how mitochondrial cAMP production is regulated, illustrate a need for better tools to examine the impact of sAC activity on mitochondrial biology, and suggest that cAMP/PKA signaling contributes to the governance of electron flow through complex I of the ETS.

Autocrine effect of Zn²⁺ on the glucose-stimulated insulin secretion

It is well known that zinc (Zn(2+)) is required for the process of insulin biosynthesis and the maturation of insulin secretory granules in pancreatic beta (β)-cells, and that changes in Zn(2+) levels in the pancreas have been found to be associated with diabetes. Glucose-stimulation causes a rapid co-secretion of Zn(2+) and insulin with similar kinetics. However, we do not know whether Zn(2+) regulates insulin availability and secretion. Here we investigated the effect of Zn(2+) on glucose-stimulated insulin secretion (GSIS) in isolated mouse pancreatic islets. Whereas Zn(2+) alone (control) had no effect on the basal secretion of insulin, it significantly inhibited GSIS. The application of CaEDTA, by removing the secreted Zn(2+) from the extracellular milieu of the islets, resulted in significantly increased GSIS, suggesting an overall inhibitory role of secreted Zn(2+) on GSIS. The inhibitory action of Zn(2+) was mostly mediated through the activities of KATP/Ca(2+) channels. Furthermore, during brief paired-pulse glucose-stimulated Zn(2+) secretion (GSZS), Zn(2+) secretion following the second pulse was significantly attenuated, probably by the secreted endogenous Zn(2+) after the first pulse. Such an inhibition on Zn(2+) secretion following the second pulse was completely reversed by Zn(2+) chelation, suggesting a negative feedback mechanism, in which the initial glucose-stimulated Zn(2+) release inhibits subsequent Zn(2+) secretion, subsequently inhibiting insulin co-secretion as well. Taken together, these data suggest a negative feedback mechanism on GSZS and GSIS by Zn(2+) secreted from β-cells, and the co-secreted Zn(2+) may act as an autocrine inhibitory modulator.

Rp-cAMPS Prodrugs Reveal the cAMP Dependence of First-Phase Glucose-Stimulated Insulin Secretion

cAMP-elevating agents such as the incretin hormone glucagon-like peptide-1 potentiate glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells. However, a debate has existed since the 1970s concerning whether or not cAMP signaling is essential for glucose alone to stimulate insulin secretion. Here, we report that the first-phase kinetic component of GSIS is cAMP-dependent, as revealed through the use of a novel highly membrane permeable para-acetoxybenzyl (pAB) ester prodrug that is a bioactivatable derivative of the cAMP antagonist adenosine-3',5'-cyclic monophosphorothioate, Rp-isomer (Rp-cAMPS). In dynamic perifusion assays of human or rat islets, a step-wise increase of glucose concentration leads to biphasic insulin secretion, and under these conditions, 8-bromoadenosine-3',5'-cyclic monophosphorothioate, Rp-isomer, 4-acetoxybenzyl ester (Rp-8-Br-cAMPS-pAB) inhibits first-phase GSIS by up to 80%. Surprisingly, second-phase GSIS is inhibited to a much smaller extent (≤20%). Using luciferase, fluorescence resonance energy transfer, and bioluminescence resonance energy transfer assays performed in living cells, we validate that Rp-8-Br-cAMPS-pAB does in fact block cAMP-dependent protein kinase activation. Novel effects of Rp-8-Br-cAMPS-pAB to block the activation of cAMP-regulated guanine nucleotide exchange factors (Epac1, Epac2) are also validated using genetically encoded Epac biosensors, and are independently confirmed in an in vitro Rap1 activation assay using Rp-cAMPS and Rp-8-Br-cAMPS. Thus, in addition to revealing the cAMP dependence of first-phase GSIS from human and rat islets, these findings establish a pAB-based chemistry for the synthesis of highly membrane permeable prodrug derivatives of Rp-cAMPS that act with micromolar or even nanomolar potency to inhibit cAMP signaling in living cells.

Effect of glucagon on insulin secretion through cAMP signaling pathway in MIN6 cells

OBJECTIVE

To explore the direct regulation effects and mechanisms of glucagon in insulin secretion of MIN6 cells that in the kind of the islet β cells. Methods ICUE3 and PCDNA3.1 plasmid were transfected to the MIN6 cells by electroporation transfection, and then treated with different concentrations of glucagon (Glg) and glucose (Glu). Biosensor technology that based on the fluorescence resonance energy transfer (FRET) was used to monitor the change of cAMP quantitatively and real-time. The level of cAMP and insulin were measured by the enzyme-linked immunosorbent assay (ELISA).

RESULTS

The receptor of Glg was mainly located on the cell membrane in MIN6 cells. Compared with the 0 ng/L Glg group in the Glu-free state, the average value of CFP/YFP increased 4%±0.02 in the 500 ng/L Glg group, and the value in the 1000 ng/L Glg group increased 6%±0.03 (P>0.05). While in the high-Glu (16.7 mmol/L) state, the value increased 11%±0.02 in the 500 ng/L Glg group, and increased 23%±0.06 in the 1000 ng/L Glg group when compared with the 0 ng/L Glg group (P<0.01). The levels of the cAMP of 1000 ng/L and 500 ng/L Glg group were higher than those of the 100 ng/L and 0 ng/L Glg group in the condition of Glu-free (81.27±6.29, 76.73±2.10,39.45±2.83, 40.36±4.20; P<0.01). The levels of the cAMP of 1000 ng/L, 500 ng/L and 100 ng/L Glg group were higher than those of the 0 ng/L Glg group, at the meanwhile, the levels of the cAMP of 1000 ng/L and 500 ng/L Glg group were also higher than 100 ng/L Glg group in the condition of low-Glu (2.8 mmol/L) (92.91±7.35, 90.36±3.15, 65.82±10.49, 46.73±1.05; P<0.01). And this trend in the condition of high-Glu was almost to the low-Glu (106.75±7.26, 94.18±2.99, 83.09±1.16, 55.60±5.51, P<0.01). The levels of the insulin of 1000 ng/L, 500 ng/L and 100 ng/L Glg group were higher than those of the 0 ng/L Glg group. While 1000 ng/L Glg group was higher than that of the 500 ng/L and 100 ng/L Glg group in the condition of Glu-free (1844.02±200.93, 1387.94±483.12, 1251.817±60.30, 787.33±81.72; P<0.01). The levels of the insulin of 1000 ng/L and 500 ng/L Glg group were higher than those of the 100 ng/L and 0 ng/L Glg group, and the 1000 ng/L and was also higher than 500 ng/L Glg group in the condition of low-Glu (1552.31±81.20, 1285.62±131.67, 1020.85±42.60, 762.89±26.94, P<0.01). And this trend in the condition of high-Glu was almost to the low-Glu (1898.337±169.03, 1399.30±148.66, 1061.735±9.13, 972.89±22.19; P<0.01). The levels of cAMP and insulin secretion of MIN6 cells had a positive correlation in different Glu conditions (r2=0.559, P<0.01).

CONCLUSION

Glg may stimulate insulin secretion by increasing cAMP levels in the way of concentration gradient within the islet β cell lines--MIN6 cells. And the increasing trend was Glu dependent.

Submembrane ATP and Ca2+ kinetics in α-cells: unexpected signaling for glucagon secretion

Cytoplasmic ATP and Ca(2+) are implicated in current models of glucose's control of glucagon and insulin secretion from pancreatic α- and β-cells, respectively, but little is known about ATP and its relation to Ca(2+) in α-cells. We therefore expressed the fluorescent ATP biosensor Perceval in mouse pancreatic islets and loaded them with a Ca(2+) indicator. With total internal reflection fluorescence microscopy, we recorded subplasma membrane concentrations of Ca(2+) and ATP ([Ca(2+)]pm; [ATP]pm) in superficial α- and β-cells of intact islets and related signaling to glucagon and insulin secretion by immunoassay. Consistent with ATP's controlling glucagon and insulin secretion during hypo- and hyperglycemia, respectively, the dose-response relationship for glucose-induced [ATP]pm generation was left shifted in α-cells compared to β-cells. Both cell types showed [Ca(2+)]pm and [ATP]pm oscillations in opposite phase, probably reflecting energy-consuming Ca(2+) transport. Although pulsatile insulin and glucagon release are in opposite phase, [Ca(2+)]pm synchronized in the same phase between α- and β-cells. This paradox can be explained by the overriding of Ca(2+) stimulation by paracrine inhibition, because somatostatin receptor blockade potently stimulated glucagon release with little effect on Ca(2+). The data indicate that an α-cell-intrinsic mechanism controls glucagon in hypoglycemia and that paracrine factors shape pulsatile secretion in hyperglycemia.

Integrative function of adrenaline receptors for glucagon-like peptide-1 exocytosis in enteroendocrine L cell line GLUTag

Adrenaline reacts with three types of adrenergic receptors, α1, α2 and β-adrenergic receptors (ARs), inducing many physiological events including exocytosis. Although adrenaline has been shown to induce glucagon-like peptide-1 (GLP-1) secretion from intestinal L cells, the precise molecular mechanism by which adrenaline regulates GLP-1 secretion remains unknown. Here we show by live cell imaging that all types of adrenergic receptors are stimulated by adrenaline in enteroendocrine L cell line GLUTag cells and are involved in GLP-1 exocytosis. We performed RT-PCR analysis and found that α1B-, α2A-, α2B-, and β1-ARs were expressed in GLUTag cells. Application of adrenaline induced a significant increase of intracellular Ca(2+) and cAMP concentration ([Ca(2+)]i and [cAMP]i, respectively), and GLP-1 exocytosis in GLUTag cells. Blockade of α1-AR inhibited adrenaline-induced [Ca(2+)]i increase and exocytosis but not [cAMP]i increase, while blockade of β1-AR inhibited adrenaline-induced [cAMP]i increase and exocytosis but not [Ca(2+)]i increase. Furthermore, overexpression of α2A-AR suppressed the adrenaline-induced [cAMP]i increase and exocytosis. These results suggest that the fine-turning of GLP-1 secretion from enteroendocrine L cells is established by the balance between α1-, α2-, and β-ARs activation.

Impaired cAMP generation contributes to defective glucose-stimulated insulin secretion after long-term exposure to palmitate

Chronic palmitate exposure impairs glucose-stimulated insulin secretion and other aspects of β-cell function, but the underlying mechanisms are not known. Using various live-cell fluorescence imaging approaches, we show here that long-term palmitate treatment influences cAMP signaling in pancreatic β-cells. Glucose stimulation of mouse and human β-cells induced oscillations of the subplasma-membrane cAMP concentration, but after 48 h exposure to palmitate, most β-cells failed to increase cAMP in response to glucose. In contrast, GLP-1-triggered cAMP formation and glucose- and depolarization-induced increases in cytoplasmic Ca2+ concentration were unaffected by the fatty acid treatment. Insulin secretion from control β-cells was pulsatile, but the response deteriorated after long-term palmitate exposure. Palmitate-treated mouse islets showed reduced expression of adenylyl cyclase 9, and knockdown of this protein in insulinoma cells reduced the glucose-stimulated cAMP response and insulin secretion. We conclude that impaired glucose-induced generation of cAMP is an important determinant of defective insulin secretion after chronic palmitate exposure.

Calcium influx activates adenylyl cyclase 8 for sustained insulin secretion in rat pancreatic beta cells

AIMS/HYPOTHESIS

Insulin is a key metabolic regulator in health and diabetes. In pancreatic beta cells, insulin release is regulated by the major second messengers Ca(2+) and cAMP: exocytosis is triggered by Ca(2+) and mediated by the cAMP/protein kinase A (PKA) signalling pathway. However, the causal link between these two processes in primary beta cells remains undefined.

METHODS

Time-resolved confocal imaging of fluorescence resonance energy transfer signals was performed to visualise PKA activity, and combined membrane capacitance recordings were used to monitor insulin secretion from patch-clamped rat beta cells.

RESULTS

Membrane depolarisation-induced Ca(2+) influx caused an increase in cytosolic PKA activity via activating a Ca(2+)-sensitive adenylyl cyclase 8 (ADCY8) subpool. Glucose stimulation triggered coupled Ca(2+) oscillations and PKA activation. ADCY8 knockdown significantly reduced the level of depolarisation-evoked PKA activation and impaired replenishment of the readily releasable vesicle pool. Pharmacological inhibition of PKA by two inhibitors reduced depolarisation-induced PKA activation to a similar extent and reduced the capacity for sustained vesicle exocytosis and insulin release.

CONCLUSIONS/INTERPRETATION

Our findings suggest that depolarisation-induced Ca(2+) influx plays dual roles in regulating exocytosis in rat pancreatic beta cells by triggering vesicle fusion and replenishing the vesicle pool to support sustained insulin release. Therefore, Ca(2+) influx may be important for glucose-stimulated insulin secretion.

Somatostatin and insulin mediate glucose-inhibited glucagon secretion in the pancreatic α-cell by lowering cAMP

The dysregulation of glucose-inhibited glucagon secretion from the pancreatic islet α-cell is a critical component of diabetes pathology and metabolic disease. We show a previously uncharacterized [Ca(2+)]i-independent mechanism of glucagon suppression in human and murine pancreatic islets whereby cAMP and PKA signaling are decreased. This decrease is driven by the combination of somatostatin, which inhibits adenylyl cyclase production of cAMP via the Gαi subunit of the SSTR2, and insulin, which acts via its receptor to activate phosphodiesterase 3B and degrade cytosolic cAMP. Our data indicate that both somatostatin and insulin signaling are required to suppress cAMP/PKA and glucagon secretion from both human and murine α-cells, and the combination of these two signaling mechanisms is sufficient to reduce glucagon secretion from isolated α-cells as well as islets. Thus, we conclude that somatostatin and insulin together are critical paracrine mediators of glucose-inhibited glucagon secretion and function by lowering cAMP/PKA signaling with increasing glucose.

Imaging sub-plasma membrane cAMP dynamics with fluorescent translocation reporters

Imaging cAMP dynamics in single cells and tissues can provide important insights into the regulation of a variety of cellular processes. In recent years, a large number of tools for cAMP measurements have been developed. While most cAMP reporters are designed to undergo changes in fluorescence resonance energy transfer (FRET), there are alternative techniques with advantages for certain applications. Here, we describe protocols for cAMP measurements in the sub-plasma membrane space based on the detection of the cAMP-induced translocation of engineered fluorescent protein-tagged subunits of protein kinase A between the cytoplasm and the plasma membrane. Total internal reflection fluorescence (TIRF) imaging of the changes in reporter localization yields robust signal changes and has contributed to the discovery of cAMP oscillations in the sub-plasma membrane space of insulin-secreting β-cells stimulated with glucose and gluco-incretin hormones. We also demonstrate how the technique can be combined with measurements of the cytosolic Ca(2+) concentration or with recordings of the subcellular localization of the cAMP effector protein Epac2. The translocation reporter approach provides a valuable complement to other methods for imaging sub-membrane cAMP dynamics in various types of cells.

Absence of Shb impairs insulin secretion by elevated FAK activity in pancreatic islets

The Src homology-2 domain containing protein B (SHB) has previously been shown to function as a pleiotropic adapter protein, conveying signals from receptor tyrosine kinases to intracellular signaling intermediates. The overexpression of Shb in β-cells promotes β-cell proliferation by increased insulin receptor substrate (IRS) and focal adhesion kinase (FAK) activity, whereas Shb deficiency causes moderate glucose intolerance and impaired first-peak insulin secretion. Using an array of techniques, including live-cell imaging, patch-clamping, immunoblotting, and semi-quantitative PCR, we presently investigated the causes of the abnormal insulin secretory characteristics in Shb-knockout mice. Shb-knockout islets displayed an abnormal signaling signature with increased activities of FAK, IRS, and AKT. β-catenin protein expression was elevated and it showed increased nuclear localization. However, there were no major alterations in the gene expression of various proteins involved in the β-cell secretory machinery. Nor was Shb deficiency associated with changes in glucose-induced ATP generation or cytoplasmic Ca(2+) handling. In contrast, the glucose-induced rise in cAMP, known to be important for the insulin secretory response, was delayed in the Shb-knockout compared with WT control. Inhibition of FAK increased the submembrane cAMP concentration, implicating FAK activity in the regulation of insulin exocytosis. In conclusion, Shb deficiency causes a chronic increase in β-cell FAK activity that perturbs the normal insulin secretory characteristics of β-cells, suggesting multi-faceted effects of FAK on insulin secretion depending on the mechanism of FAK activation.

Adenylate cyclase-centred microdomains

Recent advances in the AC (adenylate cyclase)/cAMP field reveal overarching roles for the ACs. Whereas few processes are unaffected by cAMP in eukaryotes, ranging from the rapid modulation of ion channel kinetics to the slowest developmental effects, the large number of cellular processes modulated by only three intermediaries, i.e. PKA (protein kinase A), Epacs (exchange proteins directly activated by cAMP) and CNG (cyclic nucleotide-gated) channels, poses the question of how selectivity and fine control is achieved by cAMP. One answer rests on the number of differently regulated and distinctly expressed AC species. Specific ACs are implicated in processes such as insulin secretion, immunological responses, sino-atrial node pulsatility and memory formation, and specific ACs are linked with particular diseased conditions or predispositions, such as cystic fibrosis, Type 2 diabetes and dysrhythmias. However, much of the selectivity and control exerted by cAMP lies in the sophisticated properties of individual ACs, in terms of their coincident responsiveness, dynamic protein scaffolding and organization of cellular microassemblies. The ACs appear to be the centre of highly organized microdomains, where both cAMP and Ca2+, the other major influence on ACs, change in patterns quite discrete from the broad cellular milieu. How these microdomains are organized is beginning to become clear, so that ACs may now be viewed as fundamental signalling centres, whose properties exceed their production of cAMP. In the present review, we summarize how ACs are multiply regulated and the steps that are put in place to ensure discrimination in their signalling. This includes scaffolding of targets and modulators by the ACs and assembling of signalling nexuses in discrete cellular domains. We also stress how these assemblies are cell-specific, context-specific and dynamic, and may be best addressed by targeted biosensors. These perspectives on the organization of ACs uncover new strategies for intervention in systems mediated by cAMP, which promise far more informed specificity than traditional approaches.

Fluorescence recovery after photobleaching reveals regulation and distribution of connexin36 gap junction coupling within mouse islets of Langerhans

The pancreatic islets are central to the maintenance of glucose homeostasis through insulin secretion. Glucose‐stimulated insulin secretion is tightly linked to electrical activity in β cells within the islet. Gap junctions, composed of connexin36 (Cx36), form intercellular channels between β cells, synchronizing electrical activity and insulin secretion. Loss of gap junction coupling leads to altered insulin secretion dynamics and disrupted glucose homeostasis. Gap junction coupling is known to be disrupted in mouse models of pre‐diabetes. Although approaches to measure gap junction coupling have been devised, they either lack cell specificity, suitable quantification of coupling or spatial resolution, or are invasive. The purpose of this study was to develop fluorescence recovery after photobleaching (FRAP) as a technique to accurately and robustly measure gap junction coupling in the islet. The cationic dye Rhodamine 123 was used with FRAP to quantify dye diffusion between islet β cells as a measure of Cx36 gap junction coupling. Measurements in islets with reduced Cx36 verified the accuracy of this technique in distinguishing between distinct levels of gap junction coupling. Analysis of individual cells revealed that the distribution of coupling across the islet is highly heterogeneous. Analysis of several modulators of gap junction coupling revealed glucose‐ and cAMP‐dependent modulation of gap junction coupling in islets. Finally, FRAP was used to determine cell population specific coupling, where no functional gap junction coupling was observed between α cells and β cells in the islet. The results of this study show FRAP to be a robust technique which provides the cellular resolution to quantify the distribution and regulation of Cx36 gap junction coupling in specific cell populations within the islet. Future studies utilizing this technique may elucidate the role of gap junction coupling in the progression of diabetes and identify mechanisms of gap junction regulation for potential therapies.

Involvement of cAMP/EPAC/TRPM2 activation in glucose- and incretin-induced insulin secretion

In pancreatic β-cells, closure of the ATP-sensitive K(+) (K(ATP)) channel is an initial process triggering glucose-stimulated insulin secretion. In addition, constitutive opening of background nonselective cation channels (NSCCs) is essentially required to effectively evoke depolarization as a consequence of K(ATP) channel closure. Thus, it is hypothesized that further opening of NSCC facilitates membrane excitability. We identified a class of NSCC that was activated by exendin (ex)-4, GLP-1, and its analog liraglutide at picomolar levels. This NSCC was also activated by increasing the glucose concentration. NSCC activation by glucose and GLP-1 was a consequence of the activated cAMP/EPAC-mediated pathway and was attenuated in TRPM2-deficient mice. The NSCC was not activated by protein kinase A (PKA) activators and was activated by ex-4 in the presence of PKA inhibitors. These results suggest that glucose- and incretin-activated NSCC (TRPM2) works in concert with closure of the KATP channel to effectively induce membrane depolarization to initiate insulin secretion. The current study reveals a new mechanism for regulating electrical excitability in β-cells and for mediating the action of glucose and incretin to evoke insulin secretion, thereby providing an innovative target for the treatment of type 2 diabetes.

ATP-regulated potassium channels and voltage-gated calcium channels in pancreatic alpha and beta cells: similar functions but reciprocal effects on secretion

Closure of ATP-regulated K(+) channels (K(ATP) channels) plays a central role in glucose-stimulated insulin secretion in beta cells. K(ATP) channels are also highly expressed in glucagon-producing alpha cells, where their function remains unresolved. Under hypoglycaemic conditions, K(ATP) channels are open in alpha cells but their activity is low and only ~1% of that in beta cells. Like beta cells, alpha cells respond to hyperglycaemia with K(ATP) channel closure, membrane depolarisation and stimulation of action potential firing. Yet, hyperglycaemia reciprocally regulates glucagon (inhibition) and insulin secretion (stimulation). Here we discuss how this conundrum can be resolved and how reduced K(ATP) channel activity, via membrane depolarisation, paradoxically reduces alpha cell Ca(2+) entry and glucagon exocytosis. Finally, we consider whether the glucagon secretory defects associated with diabetes can be attributed to impaired K(ATP) channel regulation and discuss the potential for remedial pharmacological intervention using sulfonylureas.

Activators of PKA and Epac distinctly influence insulin secretion and cytosolic Ca2+ in female mouse islets stimulated by glucose and tolbutamide

Amplification of insulin secretion by cAMP is mediated by protein kinase A (PKA) and exchange protein directly activated by cAMP (Epac). Using selective activators, we determined how each effector influences the cytosolic free Ca(2+) concentration ([Ca(2+)]c) and insulin secretion in mouse islets. Alone PKA activator amplified glucose- and tolbutamide-induced insulin secretion, with a greater impact on second than first phase. Epac activator strongly amplified both phases in response to either secretagogue. Amplification was even greater when activators were combined. Although both activators similarly amplified glucose-induced insulin secretion, Epac activator was particularly efficient on tolbutamide-induced insulin secretion. That greater efficacy is attributed to higher [Ca(2+)]c rather than interaction of tolbutamide with Epac, because it was also observed during KCl stimulation. Moreover, in contrast to Epac activator, tolbutamide was inactive when insulin secretion was increased by gliclazide, and its effect on glucose-induced insulin secretion was unaffected by an inhibitor of Epac2. PKA activator increased [Ca(2+)]c during acute or steady-state glucose stimulation, whereas Epac activator had no effect alone or in combination. Neither activator affected [Ca(2+)]c response to tolbutamide or KCl. Metabolic (glucose-mediated) amplification of insulin secretion was unaffected by PKA activator. It was attenuated when insulin secretion was augmented by Epac activator but insensitive to Epac2 inhibitor, which suggests distinct although somewhat overlapping mechanisms. In conclusion, activators of PKA and Epac amplify insulin secretion by augmenting the action of Ca(2+) on exocytosis and, for PKA only, slightly increasing glucose-induced [Ca(2+)]c rise. The influence of Epac seems more important than that of PKA during first phase.

Soluble adenylyl cyclase in health and disease

The second messenger cAMP is integral for many physiological processes. Soluble adenylyl cyclase (sAC) was recently identified as a widely expressed intracellular source of cAMP in mammalian cells. sAC is evolutionary, structurally, and biochemically distinct from the G-protein-responsive transmembranous adenylyl cyclases (tmAC). The structure of the catalytic unit of sAC is similar to tmAC, but sAC does not contain transmembranous domains, allowing localizations independent of the membranous compartment. sAC activity is stimulated by HCO(3)(-), Ca²⁺ and is sensitive to physiologically relevant ATP fluctuations. sAC functions as a physiological sensor for carbon dioxide and bicarbonate, and therefore indirectly for pH. Here we review the physiological role of sAC in different human tissues with a major focus on the lung. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease, guest edited by J. Buck and L.R. Levin.

Activation of GPR40 attenuates chronic inflammation induced impact on pancreatic β-cells health and function

Background

Chronic inflammation-mediated β-cell apoptosis is known to decrease β-cell mass in diabetes leading to reduced insulin secretion. Exposure to pro-inflammatory cytokines can stimulate apoptosis in pancreatic β-cells. The G protein coupled receptor 40 (GPR40) is implicated for glucose induced insulin secretion. We hypothesized that GPR40 activation can protect β-cells from inflammation-induced apoptosis and restore glucose stimulated insulin secretion.

Results

By exposing NIT1 insulinoma cells and rat islets to a cocktail of pro-inflammatory cytokines (TNFα and IL1β), we mimicked inflammatory signaling as seen by JNK and NFκB activation and increased mRNA levels of TNFα, IL1β and NOS2a. These changes were reversed by pharmacological activation of GPR40 by a specific, small molecule, CNX-011-67. Further, GPR40 activation reduced inflammation-mediated oxidative and endoplasmic reticulum (ER) stresses. Importantly, GPR40 activation decreased inflammation-induced apoptosis as measured by key markers. These impacts of GPR40 were mediated through activation of PLC, CaMKII, calcineurin and cAMP. Cell survival was also enhanced by GPR40 activation as seen from the increased phosphorylation of Akt/PKB and enhanced expression of BCL2 and PDX1 genes. Interestingly, GPR40 activation restored both, inflammation-mediated inhibition on insulin secretion and intracellular insulin content.

Conclusions

In this study, we provide evidences that CNX-011-67, a GPR40 agonist, reduces inflammatory signaling and apoptosis in pancreatic β-cells while promoting insulin secretion and synthesis. Activation of GPR40 leads to attenuation of β-cell dysfunction caused by chronic inflammation and thus could be of immense clinical value to improve insulin secretion and β-cell survival.

New insights concerning the molecular basis for defective glucoregulation in soluble adenylyl cyclase knockout mice

Recently published findings indicate that a knockout (KO) of soluble adenylyl cyclase (sAC, also known as AC-10) gene expression in mice leads to defective glucoregulation that is characterized by reduced pancreatic insulin secretion and reduced intraperitoneal glucose tolerance. Summarized here are current concepts regarding the molecular basis for this phenotype, with special emphasis on the potential role of sAC as a determinant of glucose-stimulated insulin secretion. Highlighted is new evidence that in pancreatic beta cells, oxidative glucose metabolism stimulates mitochondrial CO₂production that in turn generates bicarbonate ion (HCO(3)(-)). Since HCO(3)(-) binds to and directly stimulates the activity of sAC, we propose that glucose-stimulated cAMP production in beta cells is mediated not simply by transmembrane adenylyl cyclases (TMACs), but also by sAC. Based on evidence that sAC is expressed in mitochondria, there exists the possibility that beta-cell glucose metabolism is linked to mitochondrial cAMP production with consequent facilitation of oxidative phosphorylation. Since sAC is also expressed in the cytoplasm, sAC catalyzed cAMP production may activate cAMP sensors such as PKA and Epac2 to control ion channel function, intracellular Ca²⁺ handling, and Ca²⁺-dependent exocytosis. Thus, we propose that the existence of sAC in beta cells provides a new and unexpected explanation for previously reported actions of glucose metabolism to stimulate cAMP production. It seems possible that alterations of sAC activity might be of importance when evaluating new strategies for the treatment of type 2 diabetes (T2DM), or when evaluating why glucose metabolism fails to stimulate insulin secretion in patients diagnosed with T2DM. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

SUMO1 enhances cAMP-dependent exocytosis and glucagon secretion from pancreatic α-cells

Post-translational modification by the small ubiquitin-like modifier-1 (SUMO1) limits insulin secretion from β-cells by inhibiting insulin exocytosis and glucagon-like peptide-1 (GLP-1) receptor signalling. The secretion of glucagon from α-cells is regulated in a manner opposite to that of insulin; it is inhibited by elevated glucose and GLP-1, and increased by adrenergic signalling. We therefore sought to determine whether SUMO1 modulates mouse and human α-cell function. Action potentials (APs), ion channel function and exocytosis in single α-cells from mice and humans, identified by glucagon immunostaining, and glucagon secretion from intact islets were measured. The effects of SUMO1 on α-cell function and the respective inhibitory and stimulatory effects of exendin 4 and adrenaline were examined. Upregulation of SUMO1 increased α-cell AP duration, frequency and amplitude, in part as a result of increased Ca(2+) channel activity that led to elevated exocytosis. The ability of SUMO1 to enhance α-cell exocytosis was cAMP-dependent and resulted from an increased L-type Ca(2+) current and a shift away from exocytosis dependent on non-L-type channels, an effect that was mimicked by knockdown of the deSUMOylating enzyme sentrin/SUMO-specific protease-1 (SENP1). Finally, although SUMO1 prevented GLP-1 receptor-mediated inhibition of α-cell Na(+) channels and single-cell exocytosis, it failed to prevent the exendin 4-mediated inhibition of glucagon secretion. Consistent with its cAMP dependence, however, SUMO1 enhanced α-cell exocytosis and glucagon secretion stimulated by adrenaline. Thus, by contrast with its inhibitory role in β-cell exocytosis, SUMO1 is a positive regulator of α-cell exocytosis and glucagon secretion under conditions of elevated cAMP.

Incretin-modulated beta cell energetics in intact islets of Langerhans

Incretins such as glucagon-like peptide 1 (GLP-1) are released from the gut and potentiate insulin release in a glucose-dependent manner. Although this action is generally believed to hinge on cAMP and protein kinase A signaling, up-regulated beta cell intermediary metabolism may also play a role in incretin-stimulated insulin secretion. By employing recombinant probes to image ATP dynamically in situ within intact mouse and human islets, we sought to clarify the role of GLP-1-modulated energetics in beta cell function. Using these techniques, we show that GLP-1 engages a metabolically coupled subnetwork of beta cells to increase cytosolic ATP levels, an action independent of prevailing energy status. We further demonstrate that the effects of GLP-1 are accompanied by alterations in the mitochondrial inner membrane potential and, at elevated glucose concentration, depend upon GLP-1 receptor-directed calcium influx through voltage-dependent calcium channels. Lastly, and highlighting critical species differences, beta cells within mouse but not human islets respond coordinately to incretin stimulation. Together, these findings suggest that GLP-1 alters beta cell intermediary metabolism to influence ATP dynamics in a species-specific manner, and this may contribute to divergent regulation of the incretin-axis in rodents and man.

CFTR and Anoctamin 1 (ANO1) contribute to cAMP amplified exocytosis and insulin secretion in human and murine pancreatic beta-cells

BACKGROUND

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene lead to the disease cystic fibrosis (CF). Although patients with CF often have disturbances in glucose metabolism including impaired insulin release, no previous studies have tested the hypothesis that CFTR has a biological function in pancreatic beta-cells.

METHODS

Experiments were performed on islets and single beta-cells from human donors and NMRI-mice. Detection of CFTR was investigated using PCR and confocal microscopy. Effects on insulin secretion were measured with radioimmunoassay (RIA). The patch-clamp technique was used to measure ion channel currents and calcium-dependent exocytosis (as changes in membrane capacitance) on single cells with high temporal resolution. Analysis of ultrastructure was done on transmission electron microscopy (TEM) images.

RESULTS

We detected the presence of CFTR and measured a small CFTR conductance in both human and mouse beta-cells. The augmentation of insulin secretion at 16.7 mM glucose by activation of CFTR by cAMP (forskolin (FSK) or GLP-1) was significantly inhibited when CFTR antagonists (GlyH-101 and/or CFTRinh-172) were added. Likewise, capacitance measurements demonstrated reduced cAMP-dependent exocytosis upon CFTR-inhibition, concomitant with a decreased number of docked insulin granules. Finally, our studies demonstrate that CFTR act upstream of the chloride channel Anoctamin 1 (ANO1; TMEM16A) in the regulation of cAMP- and glucose-stimulated insulin secretion.

CONCLUSION

Our work demonstrates a novel function for CFTR as a regulator of pancreatic beta-cell insulin secretion and exocytosis, and put forward a role for CFTR as regulator of ANO1 and downstream priming of insulin granules prior to fusion and release of insulin. The pronounced regulatory effect of CFTR on insulin secretion is consistent with impaired insulin secretion in patients with CF.

Chronic exposure to GLP-1 increases GLP-1 synthesis and release in a pancreatic alpha cell line (α-TC1): evidence of a direct effect of GLP-1 on pancreatic alpha cells

Aims/Hypothesis

Incretin therapies, which are used to treat diabetic patients, cause a chronic supra-physiological increase in GLP-1 circulating levels. It is still unclear how the resulting high hormone concentrations may affect pancreatic alpha cells. The present study was designed to investigate the effects of chronic exposure to high GLP-1 levels on a cultured pancreatic alpha cell line.

Methods

α-TC1-6 cell line was cultured in the presence or absence of GLP-1 (100 nmol/l) for up to 72 h. In our model GLP-1 receptor (GLP-1R) was measured. After the cells were exposed to GLP-1 the levels of glucagon secretion were measured. Because GLP-1 acts on intracellular cAMP production, the function of GLP-1R was studied. We also investigated the effects of chronic GLP-1 exposure on the cAMP/MAPK pathway, Pax6 levels, the expression of prohormone convertases (PCs), glucagon gene (Gcg) and protein expression, glucagon and GLP-1 production.

Results

In our model, we were able to detect GLP-1R. After GLP-1 exposure we found a reduction in glucagon secretion. During further investigation of the function of GLP-1R, we found an activation of the cAMP/MAPK/Pax6 pathway and an increase of Gcg gene and protein expression. Furthermore we observed a significant increase in PC1/3 protein expression, GLP-1 intracellular content and GLP-1 secretion.

Conclusions/Interpretation

Our data indicate that the chronic exposure of pancreatic alpha cells to GLP-1 increases the ability of these cells to produce and release GLP-1. This phenomenon occurs through the stimulation of the transcription factor Pax6 and the increased expression of the protein convertase PC1/3.

New insights into the role of connexins in pancreatic islet function and diabetes

Multi-cellular systems require complex signaling mechanisms for proper tissue function, to mediate signaling between cells in close proximity and at distances. This holds true for the islets of Langerhans, which are multicellular micro-organs located in the pancreas responsible for glycemic control, through secretion of insulin and other hormones. Coupling of electrical and metabolic signaling between islet β-cells is required for proper insulin secretion and effective glycemic control. β-cell specific coupling is established through gap junctions composed of connexin36, which results in coordinated insulin release across the islet. Islet connexins have been implicated in both Type-1 and Type-2 diabetes; however a clear link remains to be determined. The goal of this review is to discuss recent discoveries regarding the role of connexins in regulating insulin secretion, the regulation of connexins within the islet, and recent studies which support a role for connexins in diabetes. Further studies which investigate the regulation of connexins in the islet and their role in diabetes may lead to novel diabetes therapies which regulate islet function and β-cell survival through modulation of gap junction coupling.

Glucose regulation of glucagon secretion

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

CO2/HCO3(-)- and calcium-regulated soluble adenylyl cyclase as a physiological ATP sensor

The second messenger molecule cAMP is integral for many physiological processes. In mammalian cells, cAMP can be generated from hormone- and G protein-regulated transmembrane adenylyl cyclases or via the widely expressed and structurally and biochemically distinct enzyme soluble adenylyl cyclase (sAC). sAC activity is uniquely stimulated by bicarbonate ions, and in cells, sAC functions as a physiological carbon dioxide, bicarbonate, and pH sensor. sAC activity is also stimulated by calcium, and its affinity for its substrate ATP suggests that it may be sensitive to physiologically relevant fluctuations in intracellular ATP. We demonstrate here that sAC can function as a cellular ATP sensor. In cells, sAC-generated cAMP reflects alterations in intracellular ATP that do not affect transmembrane AC-generated cAMP. In β cells of the pancreas, glucose metabolism generates ATP, which corresponds to an increase in cAMP, and we show here that sAC is responsible for an ATP-dependent cAMP increase. Glucose metabolism also elicits insulin secretion, and we further show that sAC is necessary for normal glucose-stimulated insulin secretion in vitro and in vivo.

Pharmacological distinction between soluble and transmembrane adenylyl cyclases

The second messenger cAMP is involved in a number of cellular signaling pathways. In mammals, cAMP is produced by either the hormonally responsive, G protein-regulated transmembrane adenylyl cyclases (tmACs) or by the bicarbonate- and calcium-regulated soluble adenylyl cyclase (sAC). To develop tools to differentiate tmAC and sAC signaling, we determined the specificity and potency of commercially available adenylyl cyclase inhibitors. In cellular systems, two inhibitors, KH7 and catechol estrogens, proved specific for sAC, and 2',5'-dideoxyadenosine proved specific for tmACs. These tools provide a means to define the specific contributions of the different families of adenylyl cyclases in cells and tissues, which will further our understanding of cell signaling.

Lipotoxicity disrupts incretin-regulated human β cell connectivity

Pancreatic β cell dysfunction is pathognomonic of type 2 diabetes mellitus (T2DM) and is driven by environmental and genetic factors. β cell responses to glucose and to incretins such as glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are altered in the disease state. While rodent β cells act as a coordinated syncytium to drive insulin release, this property is unexplored in human islets. In situ imaging approaches were therefore used to monitor in real time the islet dynamics underlying hormone release. We found that GLP-1 and GIP recruit a highly coordinated subnetwork of β cells that are targeted by lipotoxicity to suppress insulin secretion. Donor BMI was negatively correlated with subpopulation responses to GLP-1, suggesting that this action of incretin contributes to functional β cell mass in vivo. Conversely, exposure of mice to a high-fat diet unveiled a role for incretin in maintaining coordinated islet activity, supporting the existence of species-specific strategies to maintain normoglycemia. These findings demonstrate that β cell connectedness is an inherent property of human islets that is likely to influence incretin-potentiated insulin secretion and may be perturbed by diabetogenic insults to disrupt glucose homeostasis in humans.

Analysis of signaling pathways controlling flagellar movements in mammalian spermatozoa

The mammalian sperm flagellum is an example of organelles where sensory and signaling functions are integrated with motility. Structurally related to other ciliary appendages, it has unique cytoskeletal structures that serve to assemble signaling complexes as well as components of metabolic pathways. Flagellar motility is regulated by signaling events that control sperm ion milieu, energy production, and classical second messenger-dependent phosphorylation cascades. Here, we will concentrate on the cAMP signaling pathway associated with flagellar motility. We will describe methods to analyze the properties of a unique adenylyl cyclase termed sAC (gene name Sacy, Adcy10), which plays an essential function in cAMP accumulation in spermatozoa. This soluble adenylyl cyclase (sAC) is a sensor for bicarbonate, pH, and Ca(2+) and is likely involved in coupling energy homeostasis with signaling events. We will also describe methods to investigate the macromolecular complexes that bring together sAC and other signaling molecules.

Oscillations of sub-membrane ATP in glucose-stimulated beta cells depend on negative feedback from Ca(2+)

AIMS/HYPOTHESIS

ATP links changes in glucose metabolism to electrical activity, Ca(2+) signalling and insulin secretion in pancreatic beta cells. There is evidence that beta cell metabolism oscillates, but little is known about ATP dynamics at the plasma membrane, where regulation of ion channels and exocytosis occur.

METHODS

The sub-plasma-membrane ATP concentration ([ATP]pm) was recorded in beta cells in intact mouse and human islets using total internal reflection microscopy and the fluorescent reporter Perceval.

RESULTS

Glucose dose-dependently increased [ATP]pm with half-maximal and maximal effects at 5.2 and 9 mmol/l, respectively. Additional elevations of glucose to 11 to 20 mmol/l promoted pronounced [ATP]pm oscillations that were synchronised between neighbouring beta cells. [ATP]pm increased further and the oscillations disappeared when voltage-dependent Ca(2+) influx was prevented. In contrast, K(+)-depolarisation induced prompt lowering of [ATP]pm. Simultaneous recordings of [ATP]pm and the sub-plasma-membrane Ca(2+) concentration ([Ca(2+)]pm) during the early glucose-induced response revealed that the initial [ATP]pm elevation preceded, and was temporarily interrupted by the rise of [Ca(2+)]pm. During subsequent glucose-induced oscillations, the increases of [Ca(2+)]pm correlated with lowering of [ATP]pm.

CONCLUSIONS/INTERPRETATION

In beta cells, glucose promotes pronounced oscillations of [ATP]pm, which depend on negative feedback from Ca(2+) . The bidirectional interplay between these messengers in the sub-membrane space generates the metabolic and ionic oscillations that underlie pulsatile insulin secretion.

X-box binding protein 1 is essential for insulin regulation of pancreatic α-cell function

Patients with type 2 diabetes (T2D) often exhibit hyperglucagonemia despite hyperglycemia, implicating defective α-cell function. Although endoplasmic reticulum (ER) stress has been suggested to underlie β-cell dysfunction in T2D, its role in α-cell biology remains unclear. X-box binding protein 1 (XBP1) is a transcription factor that plays a crucial role in the unfolded protein response (UPR), and its deficiency in β-cells has been reported to impair insulin secretion, leading to glucose intolerance. To evaluate the role of XBP1 in α-cells, we created complementary in vivo (α-cell-specific XBP1 knockout [αXBPKO] mice) and in vitro (stable XBP1 knockdown α-cell line [αXBPKD]) models. The αXBPKO mice exhibited glucose intolerance, mild insulin resistance, and an inability to suppress glucagon secretion after glucose stimulation. αXBPKD cells exhibited activation of inositol-requiring enzyme 1, an upstream activator of XBP1, leading to phosphorylation of Jun NH2-terminal kinase. Interestingly, insulin treatment of αXBPKD cells reduced tyrosine phosphorylation of insulin receptor substrate 1 (IRS1) (pY(896)) and phosphorylation of Akt while enhancing serine phosphorylation (pS(307)) of IRS1. Consequently, the αXBPKD cells exhibited blunted suppression of glucagon secretion after insulin treatment in the presence of high glucose. Together, these data indicate that XBP1 deficiency in pancreatic α-cells induces altered insulin signaling and dysfunctional glucagon secretion.

Inhibitory effect of zinc on glucose-stimulated zinc/insulin secretion in an insulin-secreting β-cell line

Diminished or inappropriate secretion of insulin is associated with type II diabetes. The cellular/molecular mechanism coupled with the regulation of insulin secretion is still under intense investigation. Divalent ion zinc (Zn(2+)) is co-packaged and co-secreted with insulin and is intimately involved in the process of insulin biosynthesis and the maturation of insulin secretory granules. The study reported here investigated glucose-stimulated zinc secretion (GSZS) and the effect of zinc on glucose-stimulated insulin secretion (GSIS) in the HIT-T15 pancreatic β-cell line. Zinc secretion was measured using a newly developed fluorescent zinc imaging approach, and the insulin secretion was measured using an enzyme-linked immunosorbent assay. There was apparent granular-like zinc staining in β-cells. The application of glucose induced detectable zinc secretion or GSZS. Like GSIS, GSZS was dependent on the glucose concentration (5-20 mm) and the presence of extracellular calcium. The application of a zinc chelator enhanced GSZS. When brief paired-pulse glucose stimulations, which involve the initial glucose stimulation followed by a second round of glucose stimulation, were applied, zinc secretion or GSZS that followed the first pulse was inhibited. This inhibition was reversed by zinc chelation, suggesting a feedback mechanism on GSZS by zinc secreted from β-cells. Finally, the application of zinc (50 μm) strongly inhibited GSIS as measured by enzyme-linked immunosorbent assay. The present study suggests that insulin secretion is regulated by co-secreted zinc that may act as an autocrine inhibitory modulator.

Glucagon regulates its own synthesis by autocrine signaling

Peptide hormones are powerful regulators of various biological processes. To guarantee continuous availability and function, peptide hormone secretion must be tightly coupled to its biosynthesis. A simple but efficient way to provide such regulation is through an autocrine feedback mechanism in which the secreted hormone is "sensed" by its respective receptor and initiates synthesis at the level of transcription and/or translation. Such a secretion-biosynthesis coupling has been demonstrated for insulin; however, because of insulin's unique role as the sole blood glucose-decreasing peptide hormone, this coupling is considered an exception rather than a more generally used mechanism. Here we provide evidence of a secretion-biosynthesis coupling for glucagon, one of several peptide hormones that increase blood glucose levels. We show that glucagon, secreted by the pancreatic α cell, up-regulates the expression of its own gene by signaling through the glucagon receptor, PKC, and PKA, supporting the more general applicability of an autocrine feedback mechanism in regulation of peptide hormone synthesis.

Cyclic AMP dynamics in the pancreatic β-cell

Insulin secretion from pancreatic β-cells is tightly regulated by glucose and other nutrients, hormones, and neural factors. The exocytosis of insulin granules is triggered by an elevation of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) and is further amplified by cyclic AMP (cAMP). Cyclic AMP is formed primarily in response to glucoincretin hormones and other G(s)-coupled receptor agonists, but generation of the nucleotide is critical also for an optimal insulin secretory response to glucose. Nutrient and receptor stimuli trigger oscillations of the cAMP concentration in β-cells. The oscillations arise from variations in adenylyl cyclase-mediated cAMP production and phosphodiesterase-mediated degradation, processes controlled by factors like cell metabolism and [Ca(2+)](i). Protein kinase A and the guanine nucleotide exchange factor Epac2 mediate the actions of cAMP in β-cells and operate at multiple levels to promote exocytosis and pulsatile insulin secretion. The cAMP signaling system contains important targets for pharmacological improvement of insulin secretion in type 2 diabetes.

cAMP-mediated and metabolic amplification of insulin secretion are distinct pathways sharing independence of β-cell microfilaments

Insulin secretion is triggered by an increase in the cytosolic calcium concentration ([Ca(2+)](c)) in β-cells. Ca(2+)-induced exocytosis of insulin granules can be augmented by metabolic amplification (unknown signals generated through glucose metabolism) or neurohormonal amplification (in particular cAMP mediated). Functional actin microfilaments are not required for metabolic amplification, but their possible role in cAMP-mediated amplification is unknown. It is also uncertain whether cAMP (generated in response to glucose) is implicated in metabolic amplification. These questions were addressed using isolated mouse islets. cAMP levels were increased by phosphodiesterase inhibition (with isobutylmethylxanthine) and adenylate-cyclase stimulation (with forskolin or glucagon-like peptide-1, 7-36 amide). Raising cAMP levels had no steady-state impact on actin polymerization in control islets. Neither disruption (depolymerization by latrunculin) nor stabilization (polymerization by jasplakinolide) of actin microfilaments was counteracted by cAMP. Both changes increased both phases of glucose- or tolbutamide-induced insulin secretion but did not prevent further amplification by cAMP. These large changes in secretion were not caused by changes in [Ca(2+)](c), which was only slightly increased by cAMP. Both phases of insulin secretion were larger in response to glucose than tolbutamide, although [Ca(2+)](c) was lower. This difference in secretion, which reflects metabolic amplification, was independent of microfilaments, was not attributable to differences in cAMP, and persisted in presence of dibutyryl-cAMP or when cAMP levels were variably raised by isobutylmethylxanthine + forskolin or glucagon-like peptide-1, 7-36 amide. We conclude that metabolic and cAMP-mediated amplification of insulin secretion are distinct pathways that accelerate acquisition of release competence by insulin granules that can access exocytotic sites without intervention of microfilaments.

Role of phosphodiesterases in the shaping of sub-plasma-membrane cAMP oscillations and pulsatile insulin secretion

Specificity and versatility in cyclic AMP (cAMP) signalling are governed by the spatial localisation and temporal dynamics of the signal. Phosphodiesterases (PDEs) are important for shaping cAMP signals by hydrolyzing the nucleotide. In pancreatic β-cells, glucose triggers sub-plasma-membrane cAMP oscillations, which are important for insulin secretion, but the mechanisms underlying the oscillations are poorly understood. Here, we investigated the role of different PDEs in the generation of cAMP oscillations by monitoring the concentration of cAMP in the sub-plasma-membrane space ([cAMP](pm)) with ratiometric evanescent wave microscopy in MIN6 cells or mouse pancreatic β-cells expressing a fluorescent translocation biosensor. The general PDE inhibitor IBMX increased [cAMP](pm), and whereas oscillations were frequently observed at 50 µM IBMX, 300 µM-1 mM of the inhibitor caused a stable increase in [cAMP](pm). The [cAMP](pm) was nevertheless markedly suppressed by the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine, indicating IBMX-insensitive cAMP degradation. Among IBMX-sensitive PDEs, PDE3 was most important for maintaining a low basal level of [cAMP](pm) in unstimulated cells. After glucose induction of [cAMP](pm) oscillations, inhibitors of PDE1, PDE3 and PDE4 inhibitors the average cAMP level, often without disturbing the [cAMP](pm) rhythmicity. Knockdown of the IBMX-insensitive PDE8B by shRNA in MIN6 cells increased the basal level of [cAMP](pm) and prevented the [cAMP](pm)-lowering effect of 2',5'-dideoxyadenosine after exposure to IBMX. Moreover, PDE8B-knockdown cells showed reduced glucose-induced [cAMP](pm) oscillations and loss of the normal pulsatile pattern of insulin secretion. It is concluded that [cAMP](pm) oscillations in β-cells are caused by periodic variations in cAMP generation, and that several PDEs, including PDE1, PDE3 and the IBMX-insensitive PDE8B, are required for shaping the sub-membrane cAMP signals and pulsatile insulin release.

High glucose and free fatty acids induce beta cell apoptosis via autocrine effects of ADP acting on the P2Y(13) receptor

While high levels of glucose and saturated fatty acids are known to have detrimental effects on beta cell function and survival, the signalling pathways mediating these effects are not entirely known. In a previous study, we found that ADP regulates beta cell insulin secretion and beta cell apoptosis. Using MIN6c4 cells as a model system, we investigated if autocrine/paracrine mechanisms of ADP and purinergic receptors are involved in this process. High glucose (16.7 mmol/l) and palmitate (100 μmol/l) rapidly and potently elevated the extracellular ATP levels, while mannitol was without effect. Both tolbutamide and diazoxide were without effect, while the calcium channel blocker nifedipine, the volume-regulated anion channels (VRAC) inhibitor NPPB, and the pannexin inhibitor carbenoxolone could inhibit both effects. Similarly, silencing the MDR1 gene also blocked nutrient-generated ATP release. These results indicate that calcium channels and VRAC might be involved in the ATP release mechanism. Furthermore, high glucose and palmitate inhibited cAMP production, reduced cell proliferation in MIN6c4 and increased activated Caspase-3 cells in mouse islets and in MIN6c4 cells. The P2Y(13)-specific antagonist MRS2211 antagonized all these effects. Further studies showed that blocking the P2Y(13) receptor resulted in enhanced CREB, Bad and IRS-1 phosphorylation, which are known to be involved in beta cell survival and insulin secretion. These findings provide further support for the concept that P2Y(13) plays an important role in beta cell apoptosis and suggest that autocrine/paracrine mechanisms, related to ADP and P2Y(13) receptors, contribute to glucolipotoxicity.

Anchored phosphatases modulate glucose homeostasis

AKAP150 knockout- and mutant knock-in alleles reveal an unexpected role of the adaptor in anchoring phosphatase 2B for efficient insulin secretion from pancreatic β-cells and thus glucose homeostasis.

Endocrine release of insulin principally controls glucose homeostasis. Nutrient-induced exocytosis of insulin granules from pancreatic β-cells involves ion channels and mobilization of Ca²⁺ and cyclic AMP (cAMP) signalling pathways. Whole-animal physiology, islet studies and live-β-cell imaging approaches reveal that ablation of the kinase/phosphatase anchoring protein AKAP150 impairs insulin secretion in mice. Loss of AKAP150 impacts L-type Ca²⁺ currents, and attenuates cytoplasmic accumulation of Ca²⁺ and cAMP in β-cells. Yet surprisingly AKAP150 null animals display improved glucose handling and heightened insulin sensitivity in skeletal muscle. More refined analyses of AKAP150 knock-in mice unable to anchor protein kinase A or protein phosphatase 2B uncover an unexpected observation that tethering of phosphatases to a seven-residue sequence of the anchoring protein is the predominant molecular event underlying these metabolic phenotypes. Thus anchored signalling events that facilitate insulin secretion and glucose homeostasis may be set by AKAP150 associated phosphatase activity.

A computational systems analysis of factors regulating α cell glucagon secretion

Glucagon, a peptide hormone secreted from the α-cells of the pancreatic islets, is critical for blood glucose homeostasis. We reviewed the literature and employed a computational systems analysis of intracellular metabolic and electrical regulation of glucagon secretion to better understand these processes. The mathematical model of α-cell metabolic parameters is based on our previous model for pancreatic β-cells. We also formulated an ionic model for action potentials that incorporates Ca ( 2+) , K (+) , Na (+) and Cl (-) currents. Metabolic and ionic models are coupled to the equations describing Ca ( 2+) homeostasis and glucagon secretion that depends on activation of specific voltage-gated Ca ( 2+) channels. Paracrine and endocrine regulations were analyzed with an emphasis on their effects on a hyperpolarization of membrane potential. This general model simulates and gives insight into the mechanisms of regulation of glucagon secretion under a wide range of experimental conditions. We also reviewed and analyzed dysfunctional mechanisms in α-cells to determine key pharmacological targets for modulating glucagon secretion in type 1 and 2 diabetes.

Roles of Ca2+ ions in the control of ChREBP nuclear translocation

Carbohydrate-responsive element binding protein (ChREBP (MLXIPL)) is emerging as an important mediator of glucotoxity both in the liver and in the pancreatic β-cells. Although the regulation of its nuclear translocation and transcriptional activation by glucose has been the subject of intensive research, it is still not fully understood. We have recently uncovered a novel mechanism in the excitable pancreatic β-cell where ChREBP interacts with sorcin, a penta-EF-hand Ca(2)(+)-binding protein, and is sequestered in the cytosol at low glucose concentrations. Upon stimulation with glucose and activation of Ca(2)(+) influx, or application of ATP as an intracellular Ca(2)(+)-mobilising agent, ChREBP rapidly translocates to the nucleus. In sorcin-silenced cells, ChREBP is constitutively present in the nucleus, and both glucose and Ca(2)(+) are ineffective in stimulating further ChREBP nuclear shuttling. Whether an active Ca(2)(+)-sorcin element of ChREBP activation also exists in non-excitable cells is discussed.