SUR1 regulates PKA-independent cAMP-induced granule priming in mouse pancreatic B-cells

Diabetes and the fabkin complex: A dual-edged sword

The Fabkin complex, composed of FABP4, ADK, and NDPKs, emerges as a novel regulator of insulin-producing beta cells, offering promising prospects for diabetes treatment. Our approach, which combines literature review and database analysis, sets the stage for future research. These findings hold significant implications for both diabetes treatment and research, as they present potential therapeutic targets for personalized treatment, leading to enhanced patient outcomes and a deeper comprehension of the disease. The multifaceted role of the Fabkin complex in glucose metabolism, insulin resistance, anti-inflammation, beta cell proliferation, and vascular function underscores its therapeutic potential, reshaping diabetes management and propelling advancements in the field.

FXR-mediated epigenetic regulation of GLP-1R expression contributes to enhanced incretin effect in diabetes after RYGB

In this study, we investigated how Roux-en-Y gastric bypass (RYGB) enhances glucagon-like peptide 1 (GLP-1) response in GK rats and explored the potential link between RYGB-stimulated BAs/FXR signalling and GLP-1R-linked signalling in β-cells, a key pathway that regulates glucose-stimulated insulin secretion (GSIS). Here we show that RYGB restores GLP-1R expression in GK rat islets. This involves increased total BAs as well as chenodeoxycholic acid (CDCA), leading to FXR activation, increasing FXR binding to the promoter of Glp-1r and enhancing occupancy of histone acetyltransferase steroid receptor coactivator-1 (SRC1), thus increasing histone H3 acetylation at the promoter. These coordinated events bring about increased GLP-1R expression, resulting in greater GLP-1 response in β-cells. Moreover, ablation of FXR suppressed the stimulatory effects of GLP-1. Thus, this study unravels the crucial role of the BAs/FXR/SRC1 axis-controlled GLP-1R expression in β-cells, which results in enhanced incretin effect and normalized blood glucose of GK rats after RYGB.

The triggering pathway, the metabolic amplifying pathway, and cellular transduction in regulation of glucose-dependent biphasic insulin secretion

Introduction: Insulin secretion is a highly regulated process critical for maintaining glucose homeostasis. This abstract explores the intricate interplay between three essential pathways: The Triggering Pathway, The Metabolic Amplifying Pathway, and Cellular Transduction, in orchestrating glucose-dependent biphasic insulin secretion.Mechanism: During the triggering pathway, glucose metabolism in pancreatic beta-cells leads to ATP production, closing ATP-sensitive potassium channels and initiating insulin exocytosis. The metabolic amplifying pathway enhances insulin secretion via key metabolites like NADH and glutamate, enhancing calcium influx and insulin granule exocytosis. Additionally, the cellular transduction pathway involves G-protein coupled receptors and cyclic AMP, modulating insulin secretion.Result and Conclusion: These interconnected pathways ensure a dynamic insulin response to fluctuating glucose levels, with the initial rapid phase and the subsequent sustained phase. Understanding these pathways' complexities provides crucial insights into insulin dysregulation in diabetes and highlights potential therapeutic targets to restore glucose-dependent insulin secretion.

The effect of forskolin and the role of Epac2A during activation, activity, and deactivation of beta cell networks

Beta cells couple stimulation by glucose with insulin secretion and impairments in this coupling play a central role in diabetes mellitus. Cyclic adenosine monophosphate (cAMP) amplifies stimulus-secretion coupling via protein kinase A and guanine nucleotide exchange protein 2 (Epac2A). With the present research, we aimed to clarify the influence of cAMP-elevating diterpene forskolin on cytoplasmic calcium dynamics and intercellular network activity, which are two of the crucial elements of normal beta cell stimulus-secretion coupling, and the role of Epac2A under normal and stimulated conditions. To this end, we performed functional multicellular calcium imaging of beta cells in mouse pancreas tissue slices after stimulation with glucose and forskolin in wild-type and Epac2A knock-out mice. Forskolin evoked calcium signals in otherwise substimulatory glucose and beta cells from Epac2A knock-out mice displayed a faster activation. During the plateau phase, beta cells from Epac2A knock-out mice displayed a slightly higher active time in response to glucose compared with wild-type littermates, and stimulation with forskolin increased the active time via an increase in oscillation frequency and a decrease in oscillation duration in both Epac2A knock-out and wild-type mice. Functional network properties during stimulation with glucose did not differ in Epac2A knock-out mice, but the presence of Epac2A was crucial for the protective effect of stimulation with forskolin in preventing a decline in beta cell functional connectivity with time. Finally, stimulation with forskolin prolonged beta cell activity during deactivation, especially in Epac2A knock-out mice.

Noncanonical Regulation of cAMP-Dependent Insulin Secretion and Its Implications in Type 2 Diabetes

Impaired glucose tolerance (IGT) and β-cell dysfunction in insulin resistance associated with obesity lead to type 2 diabetes (T2D). Glucose-stimulated insulin secretion (GSIS) from β-cells occurs via a canonical pathway that involves glucose metabolism, ATP generation, inactivation of K ATP channels, plasma membrane depolarization, and increases in cytosolic concentrations of [Ca 2+ ] c . However, optimal insulin secretion requires amplification of GSIS by increases in cyclic adenosine monophosphate (cAMP) signaling. The cAMP effectors protein kinase A (PKA) and exchange factor activated by cyclic-AMP (Epac) regulate membrane depolarization, gene expression, and trafficking and fusion of insulin granules to the plasma membrane for amplifying GSIS. The widely recognized lipid signaling generated within β-cells by the β-isoform of Ca 2+ -independent phospholipase A 2 enzyme (iPLA 2 β) participates in cAMP-stimulated insulin secretion (cSIS). Recent work has identified the role of a G-protein coupled receptor (GPCR) activated signaling by the complement 1q like-3 (C1ql3) secreted protein in inhibiting cSIS. In the IGT state, cSIS is attenuated, and the β-cell function is reduced. Interestingly, while β-cell-specific deletion of iPLA 2 β reduces cAMP-mediated amplification of GSIS, the loss of iPLA 2 β in macrophages (MØ) confers protection against the development of glucose intolerance associated with diet-induced obesity (DIO). In this article, we discuss canonical (glucose and cAMP) and novel noncanonical (iPLA 2 β and C1ql3) pathways and how they may affect β-cell (dys)function in the context of impaired glucose intolerance associated with obesity and T2D. In conclusion, we provide a perspective that in IGT states, targeting noncanonical pathways along with canonical pathways could be a more comprehensive approach for restoring β-cell function in T2D. © 2023 American Physiological Society. Compr Physiol 13:5023-5049, 2023.

Epac as a tractable therapeutic target

In 1957, cyclic adenosine monophosphate (cAMP) was identified as the first secondary messenger, and the first signaling cascade discovered was the cAMP-protein kinase A (PKA) pathway. Since then, cAMP has received increasing attention given its multitude of actions. Not long ago, a new cAMP effector named exchange protein directly activated by cAMP (Epac) emerged as a critical mediator of cAMP's actions. Epac mediates a plethora of pathophysiologic processes and contributes to the pathogenesis of several diseases such as cancer, cardiovascular disease, diabetes, lung fibrosis, neurological disorders, and others. These findings strongly underscore the potential of Epac as a tractable therapeutic target. In this context, Epac modulators seem to possess unique characteristics and advantages and hold the promise of providing more efficacious treatments for a wide array of diseases. This paper provides an in-depth dissection and analysis of Epac structure, distribution, subcellular compartmentalization, and signaling mechanisms. We elaborate on how these characteristics can be utilized to design specific, efficient, and safe Epac agonists and antagonists that can be incorporated into future pharmacotherapeutics. In addition, we provide a detailed portfolio for specific Epac modulators highlighting their discovery, advantages, potential concerns, and utilization in the context of clinical disease entities.

MicroRNAs in Type 2 Diabetes: Focus on MicroRNA Profiling in Islets of Langerhans

Differential expression of microRNAs (miRNAs) is observed in many diseases including type 2 diabetes (T2D). Insulin secretion from pancreatic beta cells is central for the regulation of blood glucose levels and failure to release enough insulin results in hyperglycemia and T2D. The importance in T2D pathogenesis of single miRNAs in beta cells has been described; however, to get the full picture, high-throughput miRNA sequencing is necessary. Here we describe a method using small RNA sequencing, from sample preparation to expression analysis using bioinformatic tools. In the end, a tutorial on differential expression analysis is presented in R using publicly available data.

Secretory granule exocytosis and its amplification by cAMP in pancreatic β-cells

The sequence of events for secreting insulin in response to glucose in pancreatic β-cells is termed "stimulus-secretion coupling". The core of stimulus-secretion coupling is a process which generates electrical activity in response to glucose uptake and causes Ca²⁺ oscillation for triggering exocytosis of insulin-containing secretory granules. Prior to exocytosis, the secretory granules are mobilized and docked to the plasma membrane and primed for fusion with the plasma membrane. Together with the final fusion with the plasma membrane, these steps are named the exocytosis process of insulin secretion. The steps involved in the exocytosis process are crucial for insulin release from β-cells and considered indispensable for glucose homeostasis. We recently confirmed a signature of defective exocytosis process in human islets and β-cells of obese donors with type 2 diabetes (T2D). Furthermore, cyclic AMP (cAMP) potentiates glucose-stimulated insulin secretion through mechanisms including accelerating the exocytosis process. In this mini-review, we aimed to organize essential knowledge of the secretory granule exocytosis and its amplification by cAMP. Then, we suggest the fatty acid translocase CD36 as a predisposition in β-cells for causing defective exocytosis, which is considered a pathogenesis of T2D in relation to obesity. Finally, we propose potential therapeutics of the defective exocytosis based on a CD36-neutralizing antibody and on Apolipoprotein A-I (ApoA-I), for improving β-cell function in T2D.

Glycine Release Is Potentiated by cAMP via EPAC2 and Ca2+ Stores in a Retinal Interneuron

Neuromodulation via the intracellular second messenger cAMP is ubiquitous at presynaptic nerve terminals. This modulation of synaptic transmission allows exocytosis to adapt to stimulus levels and reliably encode information. The AII amacrine cell (AII-AC) is a central hub for signal processing in the mammalian retina. The main apical dendrite of the AII-AC is connected to several lobular appendages that release glycine onto OFF cone bipolar cells and ganglion cells. However, the influence of cAMP on glycine release is not well understood. Using membrane capacitance measurements from mouse AII-ACs to directly measure exocytosis, we observe that intracellular dialysis of 1 mm cAMP enhances exocytosis without affecting the L-type Ca²⁺ current. Responses to depolarizing pulses of various durations show that the size of the readily releasable pool of vesicles nearly doubles with cAMP, while paired-pulse depression experiments suggest that release probability does not change. Specific agonists and antagonists for exchange protein activated by cAMP 2 (EPAC2) revealed that the cAMP-induced enhancement of exocytosis requires EPAC2 activation. Furthermore, intact Ca²⁺ stores were also necessary for the cAMP potentiation of exocytosis. Postsynaptic recordings from OFF cone bipolar cells showed that increasing cAMP with forskolin potentiated the frequency of glycinergic spontaneous IPSCs. We propose that cAMP elevations in the AII-AC lead to a robust enhancement of glycine release through an EPAC2 and Ca²⁺ store signaling pathway. Our results thus contribute to a better understanding of how AII-AC crossover inhibitory circuits adapt to changes in ambient luminance.SIGNIFICANCE STATEMENT The mammalian retina operates over a wide dynamic range of light intensities and contrast levels. To optimize the signal-to-noise ratio of processed visual information, both excitatory and inhibitory synapses within the retina must modulate their gain in synaptic transmission to adapt to different levels of ambient light. Here we show that increases of cAMP concentration within AII amacrine cells produce enhanced exocytosis from these glycinergic interneurons. Therefore, we propose that light-sensitive neuromodulators may change the output of glycine release from AII amacrine cells. This novel mechanism may fine-tune the amount of tonic and phasic synaptic inhibition received by bipolar cell terminals and, consequently, the spiking patterns that ganglion cells send to the upstream visual areas of the brain.

Phytochemicals modulate pancreatic islet β cell function through glucagon-like peptide-1-related mechanisms

Glucagon-like peptide-1 (GLP-1) receptor-based therapies have been developed and extensively applied in clinical practice. GLP-1 plays an important role in improving glycemic homeostasis by stimulating insulin biosynthesis and secretion, suppressing glucagon activity, delaying gastric emptying, and reducing appetite and food ingestion. Furthermore, GLP-1 has positive effects on β-cell function by promoting β-cell proliferation and neogenesis while simultaneously reducing apoptosis. Here, we summarize possible mechanisms of action of GLP-1 upon pancreatic islets as well as describe phytochemicals that modulate pancreatic islet β cell function through glucagon-like peptide-1-related mechanisms. Together, this information provides potential lead compound candidates against diabetes that function as GLP-1 receptor-based pharmacotherapy.

The Role of cAMP in Beta Cell Stimulus-Secretion and Intercellular Coupling

Pancreatic beta cells secrete insulin in response to stimulation with glucose and other nutrients, and impaired insulin secretion plays a central role in development of diabetes mellitus. Pharmacological management of diabetes includes various antidiabetic drugs, including incretins. The incretin hormones, glucagon-like peptide-1 and gastric inhibitory polypeptide, potentiate glucose-stimulated insulin secretion by binding to G protein-coupled receptors, resulting in stimulation of adenylate cyclase and production of the secondary messenger cAMP, which exerts its intracellular effects through activation of protein kinase A or the guanine nucleotide exchange protein 2A. The molecular mechanisms behind these two downstream signaling arms are still not fully elucidated and involve many steps in the stimulus-secretion coupling cascade, ranging from the proximal regulation of ion channel activity to the central Ca2+ signal and the most distal exocytosis. In addition to modifying intracellular coupling, the effect of cAMP on insulin secretion could also be at least partly explained by the impact on intercellular coupling. In this review, we systematically describe the possible roles of cAMP at these intra- and inter-cellular signaling nodes, keeping in mind the relevance for the whole organism and translation to humans.

The Role of Epac in Cancer Progression

Cancer continues to be a prime contributor to global mortality. Despite tremendous research efforts and major advances in cancer therapy, much remains to be learned about the underlying molecular mechanisms of this debilitating disease. A better understanding of the key signaling events driving the malignant phenotype of cancer cells may help identify new pharmaco-targets. Cyclic adenosine 3',5'-monophosphate (cAMP) modulates a plethora of biological processes, including those that are characteristic of malignant cells. Over the years, most cAMP-mediated actions were attributed to the activity of its effector protein kinase A (PKA). However, studies have revealed an important role for the exchange protein activated by cAMP (Epac) as another effector mediating the actions of cAMP. In cancer, Epac appears to have a dual role in regulating cellular processes that are essential for carcinogenesis. In addition, the development of Epac modulators offered new routes to further explore the role of this cAMP effector and its downstream pathways in cancer. In this review, the potentials of Epac as an attractive target in the fight against cancer are depicted. Additionally, the role of Epac in cancer progression, namely its effect on cancer cell proliferation, migration/metastasis, and apoptosis, with the possible interaction of reactive oxygen species (ROS) in these phenomena, is discussed with emphasis on the underlying mechanisms and pathways.

A Membrane Permeable Prodrug of S223 for Selective Epac2 Activation in Living Cells

Signalling by cyclic adenosine monophosphate (cAMP) occurs via various effector proteins, notably protein kinase A and the guanine nucleotide exchange factors Epac1 and Epac2. These proteins are activated by cAMP binding to conserved cyclic nucleotide binding domains. The specific roles of the effector proteins in various processes in different types of cells are still not well defined, but investigations have been facilitated by the development of cyclic nucleotide analogues with distinct selectivity profiles towards a single effector protein. A remaining challenge in the development of such analogues is the poor membrane permeability of nucleotides, which limits their applicability in intact living cells. Here, we report the synthesis and characterisation of S223-AM, a cAMP analogue designed as an acetoxymethyl ester prodrug to overcome limitations of permeability. Using total internal reflection imaging with various fluorescent reporters, we show that S223-AM selectively activates Epac2, but not Epac1 or protein kinase A, in intact insulin-secreting β-cells, and that this effect was associated with pronounced activation of the small G-protein Rap. A comparison of the effects of different cAMP analogues in pancreatic islet cells deficient in Epac1 and Epac2 demonstrates that cAMP-dependent Rap activity at the β-cell plasma membrane is exclusively dependent on Epac2. With its excellent selectivity and permeability properties, S223-AM should get broad utility in investigations of cAMP effector involvement in many different types of cells.

Arsenite and its trivalent methylated metabolites inhibit glucose-stimulated calcium influx and insulin secretion in murine pancreatic islets

Chronic exposure to inorganic arsenic (iAs), a common drinking water and food contaminant, has been associated with an increased risk of type 2 diabetes in population studies worldwide. Several mechanisms underlying the diabetogenic effects of iAs have been proposed through laboratory investigations. We have previously shown that exposure to arsenite (iAs(III)) or its methylated trivalent metabolites, methylarsonite (MAs(III)) and dimethylarsinite (DMAs(III)), inhibits glucose-stimulated insulin secretion (GSIS) in pancreatic islets, without significant effects on insulin expression or insulin content. The goal of the present study was to determine if iAs(III) and/or its metabolites inhibit Ca²⁺ influx, an essential mechanism that regulates the release of insulin from β cells in response to glucose. We found that in vitro exposures for 48 h to non-cytotoxic concentrations of iAs(III), MAs(III), and DMAs(III) impaired Ca²⁺ influx in isolated murine pancreatic islets stimulated with glucose. MAs(III) and DMAs(III) were more potent inhibitors of Ca²⁺ influx than iAs(III). These arsenicals also inhibited Ca²⁺ influx and GSIS in islets treated with depolarizing levels of potassium chloride in the absence of glucose. Treatment with Bay K8644, a Ca_v1.2 channel agonist, did not restore insulin secretion in arsenical-exposed islets. Tolbutamide, a K_ATP channel blocker, prevented inhibition of insulin secretion in MAs(III)- and DMAs(III)-exposed islets, but only marginally in islets exposed to iAs(III). Our findings suggest that iAs(III), MAs(III), and DMAs(III) inhibit glucose-stimulated Ca²⁺ influx in pancreatic islets, possibly by interfering with K_ATP and/or Ca_v1.2 channel function. Notably, the mechanisms underlying inhibition of GSIS by iAs(III) may differ from those of its trivalent methylated metabolites.

Fusion pore regulation by cAMP/Epac2 controls cargo release during insulin exocytosis

Regulated exocytosis establishes a narrow fusion pore as initial aqueous connection to the extracellular space, through which small transmitter molecules such as ATP can exit. Co-release of polypeptides and hormones like insulin requires further expansion of the pore. There is evidence that pore expansion is regulated and can fail in diabetes and neurodegenerative disease. Here, we report that the cAMP-sensor Epac2 (Rap-GEF4) controls fusion pore behavior by acutely recruiting two pore-restricting proteins, amisyn and dynamin-1, to the exocytosis site in insulin-secreting beta-cells. cAMP elevation restricts and slows fusion pore expansion and peptide release, but not when Epac2 is inactivated pharmacologically or in Epac2^-/- (Rapgef4^-/-) mice. Consistently, overexpression of Epac2 impedes pore expansion. Widely used antidiabetic drugs (GLP-1 receptor agonists and sulfonylureas) activate this pathway and thereby paradoxically restrict hormone release. We conclude that Epac2/cAMP controls fusion pore expansion and thus the balance of hormone and transmitter release during insulin granule exocytosis.

Insulin is the hormone that signals to the body to take up sugar from the blood. Specialized cells in the pancreas – known as β-cells – release insulin after a meal. Before that, insulin molecules are stored in tiny granules inside the β-cells; these granules must fuse with the cells’ surface membranes to release their contents. The first step in this process creates a narrow pore that allows small molecules, but not the larger insulin molecules, to seep out. The pore then widens to release the insulin. Since the small molecules are known to act locally in the pancreas, it is possible that this “molecular sieve” is biologically important. Yet it is not clear how the pore widens.

One of the problems for people with type 2 diabetes is that they release less insulin into the bloodstream. Two kinds of drugs used to treat these patients work by stimulating β-cells to release their insulin. One way to achieve this is by raising the levels of a small molecule called cAMP, which is well known to help prepare insulin granules for release. The cAMP molecule also seems to slow the widening of the pore, and Gucek et al. have now investigated how this happens at a molecular level.

By observing individual granules of human β-cells using a special microscope, Gucek et al. could watch how different drugs affect pore widening and content release. They also saw that cAMP activated a protein called Epac2, which then recruited two other proteins – amisyn and dynamin – to the small pores. These two proteins together then closed the pore, rather than expanding it to let insulin out. Type 2 diabetes patients sometimes have high levels of amisyn in their β-cells, which could explain why they do not release enough insulin. The microscopy experiments also revealed that two common anti-diabetic drugs activate Epac2 and prevent the pores from widening, thereby counteracting their positive effect on insulin release. The combined effect is likely a shift in the balance between insulin and the locally acting small molecules.

These findings suggest that two common anti-diabetic drugs activate a common mechanism that may lead to unexpected outcomes, possibly even reducing how much insulin the β-cells can release. Future studies in mice and humans will have to investigate these effects in whole organisms.

Defective exocytosis and processing of insulin in a cystic fibrosis mouse model

Cystic fibrosis-related diabetes (CFRD) is a common complication for patients with cystic fibrosis (CF), a disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR). The cause of CFRD is unclear, but a commonly observed reduction in first-phase insulin secretion suggests defects at the beta cell level. Here we aimed to examine beta- and alpha-cell function in the Cftrtm1EUR/F508del mouse model (C57BL/6J), which carries the most common human mutation in CFTR, the F508del mutation. CFTR expression, beta cell mass, insulin granule distribution, hormone secretion and single cell capacitance changes were evaluated using islets (or beta cells) from F508del mice and age-matched wild-type mice aged 7-10 weeks. Granular pH was measured with DND-189 fluorescence. Serum glucose, insulin and glucagon levels were measured in vivo, and glucose tolerance was assessed using IPGTT. We show increased secretion of proinsulin and concomitant reduced secretion of C-peptide in islets from F508del mice compared to WT mice. Exocytosis and number of docked granules was reduced. We confirmed reduced granular pH by CFTR stimulation. We detected decreased pancreatic beta cell area, but unchanged beta cell number. Moreover, the F508del mutation caused failure to suppress glucagon secretion leading to hyperglucagonemia. In conclusion, F508del mice have beta cell defects resulting in 1) reduced number of docked insulin granules and reduced exocytosis, and 2) potential defective proinsulin cleavage and secretion of immature insulin. These observations provide insight into the functional role of CFTR in pancreatic islets and contribute to increased understanding of the pathogenesis of CFRD.

Molecular aspects of pancreatic β-cell dysfunction: Oxidative stress, microRNA, and long noncoding RNA

Metabolic syndrome is known as a frequent precursor of type 2 diabetes mellitus (T2D). This disease could affect 8% of the people worldwide. Given that pancreatic β-cell dysfunction and loss have central roles in the initiation and progression of the disease, the understanding of cellular and molecular pathways associated with pancreatic β-cell dysfunction can provide more information about the underlying pathways involved in T2D. Multiple lines evidence indicated that oxidative stress, microRNA, and long noncoding RNA play significant roles in various steps of diseases. Oxidative stress is one of the important factors involved in T2D pathogenesis. This could affect the function and survival of the β cell via activation or inhibition of several processes and targets, such as receptor-signal transduction, enzyme activity, gene expression, ion channel transport, and apoptosis. Besides oxidative stress, microRNAs and noncoding RNAs have emerged as epigenetic regulators that could affect pancreatic β-cell dysfunction. These molecules exert their effects via targeting a variety of cellular and molecular pathways involved in T2D pathogenesis. Here, we summarized the molecular aspects of pancreatic β-cell dysfunction. Moreover, we highlighted the roles of oxidative stress, microRNAs, and noncoding RNAs in pancreatic β-cell dysfunction.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

Interaction of Epac with Non-canonical Cyclic Nucleotides

Epac1 and Epac2 are cyclic nucleotide-binding (CNB) domain containing proteins, which were originally identified as cAMP-regulated guanine nucleotide exchange factors (GEFs) for the small G-protein Rap. Therefore, Epac proteins founded next to protein kinase A (PKA) and cyclic nucleotide-regulated ion channels the third group of cAMP-responsive proteins in higher organisms. Epac proteins are involved in the regulation of several physiological processes. In particular Epac1 mediates the regulation of molecular processes underlying cell adhesion and mobility. In the pancreas activation of Epac2 potentiates the release of glucose-induced insulin secretion and received attention as a putative target for antidiabetic treatment. While the regulation of Epac by cAMP has been analysed in structural and biochemical detail, less is known on the interaction of Epac with non-canonical cyclic nucleotides. This chapter will discuss to what extent other cyclic purines than cAMP or cyclic pyrimidine could act as Epac agonists or antagonists. The focus will be on the biophysical analysis of the interaction between Epac and these cyclic nucleotides.

Recruitment of Epac2A to Insulin Granule Docking Sites Regulates Priming for Exocytosis

Epac is a cAMP-activated guanine nucleotide exchange factor that mediates cAMP signaling in various types of cells, including β-cells, where it is involved in the control of insulin secretion. Upon activation, the protein redistributes to the plasma membrane, but the underlying molecular mechanisms and functional consequences are unclear. Using quantitative high-resolution microscopy, we found that cAMP elevation caused rapid binding of Epac2A to the β-cell plasma membrane, where it accumulated specifically at secretory granules and rendered them more prone to undergo exocytosis. cAMP-dependent membrane binding required the high-affinity cyclic nucleotide-binding (CNB) and Ras association domains, but not the disheveled-Egl-10-pleckstrin domain. Although the N-terminal low-affinity CNB domain (CNB-A) was dispensable for the translocation to the membrane, it was critical for directing Epac2A to the granule sites. Epac1, which lacks the CNB-A domain, was recruited to the plasma membrane but did not accumulate at granules. We conclude that Epac2A controls secretory granule release by binding to the exocytosis machinery, an effect that is enhanced by prior cAMP-dependent accumulation of the protein at the plasma membrane.

cAMP signalling in insulin and glucagon secretion

The "second messenger" archetype cAMP is one of the most important cellular signalling molecules with central functions including the regulation of insulin and glucagon secretion from the pancreatic β- and α-cells, respectively. cAMP is generally considered as an amplifier of insulin secretion triggered by Ca²⁺ elevation in the β-cells. Both messengers are also positive modulators of glucagon release from α-cells, but in this case cAMP may be the important regulator and Ca²⁺ have a more permissive role. The actions of cAMP are mediated by protein kinase A (PKA) and the guanine nucleotide exchange factor Epac. The present review focuses on how cAMP is regulated by nutrients, hormones and neural factors in β- and α-cells via adenylyl cyclase-catalysed generation and phosphodiesterase-mediated degradation. We will also discuss how PKA and Epac affect ion fluxes and the secretory machinery to transduce the stimulatory effects on insulin and glucagon secretion. Finally, we will briefly describe disturbances of the cAMP system associated with diabetes and how cAMP signalling can be targeted to normalize hypo- and hypersecretion of insulin and glucagon, respectively, in diabetic patients.

The Role of PYY in Pancreatic Islet Physiology and Surgical Control of Diabetes

Bariatric surgery in obese individuals leads to rapid and lasting remission of type 2 diabetes (T2D). This phenomenon occurs independently of weight loss possibly via a combination of factors. The incretin hormone GLP-1 has so far been recognised as a critical factor. However, recent data have indicated that elevation in another gut hormone, peptide tyrosine tyrosine (PYY), may drive the beneficial effects of surgery. Here we discuss recent findings on PYY-mediated control of glucose homeostasis and its role in diabetes, in the context of what is known for GLP-1. Identification of factors that increase the expression of PYY following bariatric surgery and elucidation of its role in diabetes reversal may have clinical relevance as a nonsurgical therapy for T2D.

A Ratiometric Sensor for Imaging Insulin Secretion in Single β Cells

Despite the urgent need for assays to visualize insulin secretion there is to date no reliable method available for measuring insulin release from single cells. To address this need, we developed a genetically encoded reporter termed RINS1 based on proinsulin superfolder GFP (sfGFP) and mCherry fusions for monitoring insulin secretion. RINS1 expression in MIN6 β cells resulted in proper processing yielding single-labeled insulin species. Unexpectedly, glucose or drug stimulation of insulin secretion in β cells led to the preferential release of the insulin-sfGFP construct, while the mCherry-fused C-peptide remained trapped in exocytic granules. This physical separation was used to monitor glucose-stimulated insulin secretion ratiometrically by total internal reflection fluorescence microscopy in single MIN6 and primary mouse β cells. Further, RINS1 enabled parallel monitoring of pulsatile insulin release in tolbutamide-treated β cells, demonstrating the potential of RINS1 for investigations of antidiabetic drug candidates at the single-cell level.

Sphingosine kinase 1-interacting protein is a novel regulator of glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) is essential in keeping blood glucose levels within normal range. GSIS is impaired in type 2 diabetes, and its recovery is crucial in treatment of the disease. We find here that sphingosine kinase 1-interacting protein (SKIP, also called Sphkap) is highly expressed in pancreatic β-cells but not in α-cells. Intraperitoneal glucose tolerance test showed that plasma glucose levels were decreased and insulin levels were increased in SKIP^−/− mice compared to SKIP^+/+ mice, but exendin-4-enhanced insulin secretion was masked. GSIS was amplified more in SKIP^−/− but exendin-4-enhanced insulin secretion was masked compared to that in SKIP^+/+ islets. The ATP and cAMP content were similarly increased in SKIP^+/+ and SKIP^−/− islets; depolarization-evoked, PKA and cAMP-mediated insulin secretion were not affected. Inhibition of PDE activity equally augmented GSIS in SKIP^+/+ and SKIP^−/− islets. These results indicate that SKIP modulates GSIS by a pathway distinct from that of cAMP-, PDE- and sphingosine kinase-dependent pathways.

Sulfonylurea Blockade of KATP Channels Unmasks a Distinct Type of Glucose-Induced Ca2+ Decrease in Pancreatic β-Cells

OBJECTIVES

This study aimed to explore how sulfonylurea blockade of KATP channels affects the early Ca signals for glucose generation of insulin release.

METHODS

Cytoplasmic Ca was measured with ratiometric microfluorometry in isolated mouse islets loaded with Fura-PE3.

RESULTS

After sulfonylurea blockade of the KATP channels (50 μM-1 mM tolbutamide or 1 μM-1 mM gliclazide), increase of glucose from 3 to 20 mM resulted in suppression of elevated Ca during a 3- to 5-minute period. The Ca decrease was shorter after inhibition of the Na/K pump with ouabain (10 and 100 μM) but prolonged when the α2A adrenoceptors were activated with clonidine (1 and 10 nM) or epinephrine (10 nM). Inhibition of the sarco/endoplasmic reticulum Ca-ATPase pump with 10 μM cyclopiazonic acid counteracted the action of 10 nM clonidine, making the Ca decrease shorter than in controls. Extended superfusion of islets with a medium containing 20 mM glucose and 1 mM tolbutamide sometimes resulted in delayed appearance of Ca oscillations mediated by periodic interruption of elevated Ca.

CONCLUSIONS

Increase of glucose generates prompt suppression of cytoplasmic Ca in β-cells lacking functional KATP channels. Activation of α2A adrenoceptors markedly prolongs the period of glucose-induced Ca decrease, an effect counteracted by cyclopiazonic acid.

Lessons from basic pancreatic beta cell research in type-2 diabetes and vascular complications

The changes in life-style with increased access of food and reduced physical activity have resulted in the global epidemic of obesity. Consequently, individuals with type 2 diabetes and cardiovascular disease have also escalated. A central organ in the development of diabetes is the pancreas, and more specifically the pancreatic beta cells within the islets of Langerhans. Beta cells have been assigned the important task of secreting insulin when blood glucose is increased to lower the glucose level. An early sign of diabetes pathogenesis is lack of first phase insulin response and reduced second phase secretion. In this review, which is based on the foreign investigator award lecture given at the JSDC meeting in Sendai in October 2016, we discuss a possible cellular explanation for the reduced first phase insulin response and how this can be influenced by lipids. Moreover, since patients with cardiovascular disease and high levels of cholesterol are often treated with statins, we summarize recent data regarding effects on statins on glucose homeostasis and insulin secretion. Finally, we suggest microRNAs (miRNAs) as central players in the adjustment of beta cell function during the development of diabetes. We specifically discuss miRNAs regarding their involvement in insulin secretion regulation, differential expression in type 2 diabetes, and potential as biomarkers for prediction of diabetes and cardiovascular complications.

HDAC7 is overexpressed in human diabetic islets and impairs insulin secretion in rat islets and clonal beta cells

AIMS/HYPOTHESIS

Pancreatic beta cell dysfunction is a prerequisite for the development of type 2 diabetes. Histone deacetylases (HDACs) may affect pancreatic endocrine function and glucose homeostasis through alterations in gene regulation. Our aim was to investigate the role of HDAC7 in human and rat pancreatic islets and clonal INS-1 beta cells (INS-1 832/13).

METHODS

To explore the role of HDAC7 in pancreatic islets and clonal beta cells, we used RNA sequencing, mitochondrial functional analyses, microarray techniques, and HDAC inhibitors MC1568 and trichostatin A.

RESULTS

Using RNA sequencing, we found increased HDAC7 expression in human pancreatic islets from type 2 diabetic compared with non-diabetic donors. HDAC7 expression correlated negatively with insulin secretion in human islets. To mimic the situation in type 2 diabetic islets, we overexpressed Hdac7 in rat islets and clonal beta cells. In both, Hdac7 overexpression resulted in impaired glucose-stimulated insulin secretion. Furthermore, it reduced insulin content, mitochondrial respiration and cellular ATP levels in clonal beta cells. Overexpression of Hdac7 also led to changes in the genome-wide gene expression pattern, including increased expression of Tcf7l2 and decreased expression of gene sets regulating DNA replication and repair as well as nucleotide metabolism. In accordance, Hdac7 overexpression reduced the number of beta cells owing to enhanced apoptosis. Finally, we found that inhibiting HDAC7 activity with pharmacological inhibitors or small interfering RNA-mediated knockdown restored glucose-stimulated insulin secretion in beta cells that were overexpressing Hdac7.

CONCLUSIONS/INTERPRETATION

Taken together, these results indicate that increased HDAC7 levels caused beta cell dysfunction and may thereby contribute to defects seen in type 2 diabetic islets. Our study supports HDAC7 inhibitors as a therapeutic option for the treatment of type 2 diabetes.

Sulphonylurea receptor-1, sulphonylureas and amplification of insulin secretion by Epac activation in β cells

Amplification of insulin secretion by cyclic AMP involves activation of protein kinase A (PKA) and Epac2 in pancreatic β cells. Recent hypotheses suggest that sulphonylurea receptor-1 (SUR1), the regulatory subunit of ATP-sensitive potassium channels, is implicated in Epac2 effects and that direct activation of Epac2 by hypoglycaemic sulphonylureas contributes to the stimulation of insulin secretion by these drugs. In the present experiments, using islets from Sur1KO mice, we show that dibutyryl-cAMP and membrane-permeant selective activators of Epac or PKA normally amplify insulin secretion in β cells lacking SUR1. In contrast to Epac activator, sulphonylureas (glibenclamide and tolbutamide) did not increase insulin secretion in Sur1KO islets, as would be expected if they were activating Epac2 directly. Furthermore, glibenclamide and tolbutamide did not augment the amplification of insulin secretion produced by Epac activator or dibutyryl-cAMP. Collectively, the results show that SUR1 is dispensable for amplification of insulin secretion by Epac2 activation and that direct activation of Epac2 is unimportant for the action of therapeutic concentrations of sulphonylureas in β cells.

Transcriptional regulation of the miR-212/miR-132 cluster in insulin-secreting β-cells by cAMP-regulated transcriptional co-activator 1 and salt-inducible kinases

MicroRNAs are central players in the control of insulin secretion, but their transcriptional regulation is poorly understood. Our aim was to investigate cAMP-mediated transcriptional regulation of the miR-212/miR-132 cluster and involvement of further upstream proteins in insulin secreting β-cells. cAMP induced by forskolin+IBMX or GLP-1 caused increased expression of miR-212/miR-132, and elevated phosphorylation of cAMP-response-element-binding-protein (CREB)/Activating-transcription-factor-1 (ATF1) and Salt-Inducible-Kinases (SIKs). CyclicAMP-Regulated Transcriptional Co-activator-1 (CRTC1) was concomitantly dephosphorylated and translocated to the nucleus. Silencing of miR-212/miR-132 reduced, and overexpression of miR-212 increased, glucose-stimulated insulin secretion. Silencing of CRTC1 expression resulted in decreased insulin secretion and miR-212/miR-132 expression, while silencing or inhibition of SIKs was associated with increased expression of the microRNAs and dephosphorylation of CRTC1. CRTC1 protein levels were reduced after silencing of miR-132, suggesting feed-back regulation. Our data propose cAMP-dependent co-regulation of miR-212/miR-132, in part mediated through SIK-regulated CRTC1, as an important factor for fine-tuned regulation of insulin secretion.

Albiglutide: Is a better hope against diabetes mellitus?

Type-2 diabetes mellitus (T2DM) is the chronic metabolic disorder which provokes several pitfall signalling. Though, a series of anti-diabetic drugs are available in the market but T2DM is still a huge burden on the developed and developing countries. Numerous studies and survey predict the associated baleful circumstances in near future due to incessant increase in this insidious disorder. The novelty of recent explored anti-diabetic drugs including glitazone, glitazaar and gliflozines seems to be vanished due to their associated toxic side effects. Brown and Dryburgh (1970) isolated an intestinal amino acid known as gastric inhibitory peptide (GIP) which had insulinotropic activity. Subsequently in 1985, another incretin glucagon likes peptide 1 (GLP-1) having potent insulinotropic properties was discovered by Schmidt and his co-workers. On the basis of results' obtained by Phase III Harmony program FDA approved (14 April, 2014) new GLP-1 agonist 'Albiglutide (ALB)', in addition to exiting components Exenatide (Eli Lilly, 2005) and Liraglutide (Novo Nordisk, 2010). ALB stimulates the release of protein kinase A (PKA) via different mechanisms which ultimately leads to increase in intracellular Ca(2+) levels. This increased intracellular Ca(2+) releases insulin vesicle from β-cells. In-addition, ALB being resistant to degradation by dipeptidyl peptidase-4 (DPP-4) and has longer half life. DPP-4 can significantly degrade the level of GLP-1 agonist by hydrolysis. In spite of potent anti-hypergycemic activity, ALB has pleiotropic action of improving cardiovascular physiology. In light of these viewpoints we reveal the individual pharmacological profile of ALB and the critical analyse about its future perspective in present review.

Simvastatin Impairs Insulin Secretion by Multiple Mechanisms in MIN6 Cells

Statins are widely used in the treatment of hypercholesterolemia and are efficient in the prevention of cardiovascular disease. Molecular mechanisms explaining statin-induced impairment in insulin secretion remain largely unknown. In the current study, we show that simvastatin decreased glucose-stimulated insulin secretion in mouse pancreatic MIN6 β-cells by 59% and 79% (p<0.01) at glucose concentration of 5.5 mmol/l and 16.7 mmol/l, respectively, compared to control, whereas pravastatin did not impair insulin secretion. Simvastatin induced decrease in insulin secretion occurred through multiple targets. In addition to its established effects on ATP-sensitive potassium channels (p = 0.004) and voltage-gated calcium channels (p = 0.004), simvastatin suppressed insulin secretion stimulated by muscarinic M3 or GPR40 receptor agonists (Tak875 by 33%, p = 0.002; GW9508 by 77%, p = 0.01) at glucose level of 5.5 mmol/l, and inhibited calcium release from the endoplasmic reticulum. Impaired insulin secretion caused by simvastatin treatment were efficiently restored by GPR119 or GLP-1 receptor stimulation and by direct activation of cAMP-dependent signaling pathways with forskolin. The effects of simvastatin treatment on insulin secretion were not affected by the presence of hyperglycemia. Our observation of the opposite effects of simvastatin and pravastatin on glucose-stimulated insulin secretion is in agreement with previous reports showing that simvastatin, but not pravastatin, was associated with increased risk of incident diabetes.

Membrane Potential and Calcium Dynamics in Beta Cells from Mouse Pancreas Tissue Slices: Theory, Experimentation, and Analysis

Beta cells in the pancreatic islets of Langerhans are precise biological sensors for glucose and play a central role in balancing the organism between catabolic and anabolic needs. A hallmark of the beta cell response to glucose are oscillatory changes of membrane potential that are tightly coupled with oscillatory changes in intracellular calcium concentration which, in turn, elicit oscillations of insulin secretion. Both membrane potential and calcium changes spread from one beta cell to the other in a wave-like manner. In order to assess the properties of the abovementioned responses to physiological and pathological stimuli, the main challenge remains how to effectively measure membrane potential and calcium changes at the same time with high spatial and temporal resolution, and also in as many cells as possible. To date, the most wide-spread approach has employed the electrophysiological patch-clamp method to monitor membrane potential changes. Inherently, this technique has many advantages, such as a direct contact with the cell and a high temporal resolution. However, it allows one to assess information from a single cell only. In some instances, this technique has been used in conjunction with CCD camera-based imaging, offering the opportunity to simultaneously monitor membrane potential and calcium changes, but not in the same cells and not with a reliable cellular or subcellular spatial resolution. Recently, a novel family of highly-sensitive membrane potential reporter dyes in combination with high temporal and spatial confocal calcium imaging allows for simultaneously detecting membrane potential and calcium changes in many cells at a time. Since the signals yielded from both types of reporter dyes are inherently noisy, we have developed complex methods of data denoising that permit for visualization and pixel-wise analysis of signals. Combining the experimental approach of high-resolution imaging with the advanced analysis of noisy data enables novel physiological insights and reassessment of current concepts in unprecedented detail.

Salt-Inducible Kinase 1 Terminates cAMP Signaling by an Evolutionarily Conserved Negative-Feedback Loop in β-Cells

Pancreatic β-cells are critical in the regulation of glucose homeostasis by controlled secretion of insulin in mammals. Activation of protein kinase A by cAMP is shown to be responsible for enhancing this pathway, which is countered by phosphodiesterase (PDE) that converts cAMP to AMP and turns off the signal. Salt-inducible kinases (SIKs) were also known to inhibit cAMP signaling, mostly by promoting inhibitory phosphorylation on CREB-regulated transcription coactivators. Here, we showed that SIK1 regulates insulin secretion in β-cells by modulating PDE4D and cAMP concentrations. Haploinsufficiency of SIK1 led to the improved glucose tolerance due to the increased glucose-stimulated insulin secretion. Depletion of SIK1 promoted higher cAMP concentration and increased insulin secretion from primary islets, suggesting that SIK1 controls insulin secretion through the regulation of cAMP signaling. By using a consensus phosphorylation site of SIK1, we identified PDE4D as a new substrate for this kinase family. In vitro kinase assay as well as mass spectrometry analysis revealed that the predicted Ser(136) and the adjacent Ser(141) of PDE4D are critical in SIK1-mediated phosphorylation. We found that overexpression of either SIK1 or PDE4D in β-cells reduced insulin secretion, while inhibition of PDE4 activity by rolipram or knockdown of PDE4D restored it, showing indeed that SIK1-dependent phosphorylation of PDE4D is critical in reducing cAMP concentration and insulin secretion from β-cells. Taken together, we propose that SIK1 serves as a part of a self-regulatory circuit to modulate insulin secretion from pancreatic β-cells by controlling cAMP concentration through modulation of PDE4D activity.

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.

Bone morphogenetic protein 4 inhibits insulin secretion from rodent beta cells through regulation of calbindin1 expression and reduced voltage-dependent calcium currents

AIMS/HYPOTHESIS

Type 2 diabetes is characterised by progressive loss of pancreatic beta cell mass and function. Therefore, it is of therapeutic interest to identify factors with the potential to improve beta cell proliferation and insulin secretion. Bone morphogenetic protein 4 (BMP4) expression is increased in diabetic animals and BMP4 reduces glucose-stimulated insulin secretion (GSIS). Here, we investigate the molecular mechanism behind this inhibition.

METHODS

BMP4-mediated inhibition of GSIS was investigated in detail using single cell electrophysiological measurements and live cell Ca(2+) imaging. BMP4-mediated gene expression changes were investigated by microarray profiling, quantitative PCR and western blotting.

RESULTS

Prolonged exposure to BMP4 reduced GSIS from rodent pancreatic islets. This inhibition was associated with decreased exocytosis due to a reduced Ca(2+) current through voltage-dependent Ca(2+) channels. To identify proteins involved in the inhibition of GSIS, we investigated global gene expression changes induced by BMP4 in neonatal rat pancreatic islets. Expression of the Ca(2+)-binding protein calbindin1 was significantly induced by BMP4. Overexpression of calbindin1 in primary islet cells reduced GSIS, and the effect of BMP4 on GSIS was lost in islets from calbindin1 (Calb1) knockout mice.

CONCLUSIONS/INTERPRETATION

We found BMP4 treatment to markedly inhibit GSIS from rodent pancreatic islets in a calbindin1-dependent manner. Calbindin1 is suggested to mediate the effect of BMP4 by buffering Ca(2+) and decreasing Ca(2+) channel activity, resulting in diminished insulin exocytosis. Both BMP4 and calbindin1 are potential pharmacological targets for the treatment of beta cell dysfunction.

Impairment of adenylyl cyclase-mediated glutamatergic synaptic plasticity in the periaqueductal grey in a rat model of neuropathic pain

KEY POINTS

Long-lasting neuropathic pain has been attributed to elevated neuronal plasticity changes in spinal, peripheral and cortical levels. Here, we found that reduced neuronal plasticity in the ventrolateral periaqueductal grey (vlPAG), a midbrain region important for initiating descending pain inhibition, may also contribute to neuropathic pain. Forskolin- and isoproterenol (isoprenaline)-elicited EPSC potentiation was impaired in the vlPAG of a rat model of neuropathic pain induced by spinal nerve injury. Down-regulation of adenylyl cyclase-cAMP- PKA signalling, due to impaired adenylyl cyclase, but not phosphodiesterase, in glutamatergic terminals may contribute to the hypofunction of excitatory synaptic plasticity in the vlPAG of neuropathic rats and the subsequent descending pain inhibition, ultimately leading to long-lasting neuropathic pain. Our results suggest that drugs that activate adenylyl cyclase in the vlPAG have the potential for relieving neuropathic pain.

ABSTRACT

Neuropathic pain has been attributed to nerve injury-induced elevation of peripheral neuronal discharges and spinal excitatory synaptic plasticity while little is known about the contribution of neuroplasticity changes in the brainstem. Here, we examined synaptic plasticity changes in the ventrolateral (vl) periaqueductal grey (PAG), a crucial midbrain region for initiating descending pain inhibition, in spinal nerve ligation (SNL)-induced neuropathic rats. In vlPAG slices of sham-operated rats, forskolin, an adenylyl cyclase (AC) activator, produced long-lasting enhancement of EPSCs. This is a presynaptic effect since forskolin decreased the paired-pulse ratio and failure rate of EPSCs, and increased the frequency, but not the amplitude, of miniature EPSCs. Forskolin-induced EPSC potentiation was mimicked by a β-adrenergic agonist (isoproterenol (isoprenaline)), and prevented by an AC inhibitor (SQ 22536) and a cAMP-dependent protein kinase (PKA) inhibitor (H89), but not by a phosphodiesterase (PDE) inhibitor (Ro 20-1724) or an A1 -adenosine antagonist (DPCPX). Both forskolin- and isoproterenol-induced EPSC potentiation was impaired in PAG slices of SNL rats. The SNL group had lower AC, but not PDE, activity in PAG synaptosomes than the sham group. Conversely, IPSCs in vlPAG slices were not different between SNL and sham groups. Intra-vlPAG microinjection of forskolin alleviated SNL-induced mechanical allodynia in rats. These results suggest that SNL leads to inadequate descending pain inhibition resulting from impaired glutamatergic synaptic plasticity mediated by the AC-cAMP-PKA signalling cascade, possibly due to AC down-regulation in the PAG, leading to long-term neuropathic pain.

Glucotoxicity inhibits cAMP-protein kinase A-potentiated glucose-stimulated insulin secretion in pancreatic β-cells

BACKGROUND

The effect of incretin is markedly blunted in patients with type 2 diabetes (T2D), and this reduced effect of incretin is correlated with a diminished insulintropic potency of glucagon-like peptide-1 (GLP-1). We reported recently that GLP-1 potentiates glucose-stimulated insulin secretion (GSIS) mainly via activation of the cAMP-protein kinase A (PKA) signaling pathway in INS-1E cells under hyperglycemic conditions. In the present study, we further explored whether glucotoxicity impairs cAMP-PKA-mediated effects and its relevance to the reduced insulinotropic action of GLP-1 in hyperglycemia.

METHODS

Mouse islets and INS-1E cells were cultured in 30 mmol/L glucose for 72 h. The effects of glucotoxicity on cAMP-PKA-linked pathways and its insulinotropic action were then evaluated.

RESULTS

Chronic exposure of INS-1E cells and primary mouse islets to 30 mmol/L glucose almost abolished GSIS. The cAMP-elevating agent forskolin produced an approximate 1.9-fold increase in GSIS, significantly lower than that observed with 5.5 mmol/L glucose (~3.3-fold). Moreover, 72 h culture in the presence of 30 mmol/L glucose reduced forskolin-stimulated cAMP accumulation in β-cells. Notably, glucotoxicity reduced the expression and activity of PKA, as well as PKA-mediated effects. In contrast, glucotoxicity had no effect on the expression of Epac2, another cAMP effector.

CONCLUSIONS

Glucotoxicity-induced reductions in PKA and its signaling account, at least in part, for the decreased incretin effect under conditions of glucotoxicity.

P2Y purinergic receptor-regulated insulin secretion is mediated by a cAMP/Epac/Kv channel pathway

Enhancement of insulin secretion is a major therapeutic approach for type 2 diabetes (T2D). Activation of P2Y purinergic receptor (P2YR) causes potentiation of insulin secretion in a glucose-dependent manner, making it a promising therapeutic target for T2D. Here we show that activation of P2YR to potentiate insulin secretion is mediated by adenylyl cyclase/cyclic AMP (cAMP) and the downstream effector, exchange protein directly activated by cAMP (Epac), leading to inhibition of voltage-dependent potassium (Kv) channels. P2YR-mediated Kv channel inhibition results in prolongation of action potential duration, and in turn elevates intracellular Ca(2+) level and insulin secretion. Taken together, the data indicate that cAMP/Epac/Kv channel pathway mediates P2YR-regulated insulin secretion, which may have important therapeutic implications for T2D.

Melatonin influences somatostatin secretion from human pancreatic δ-cells via MT1 and MT2 receptors

Melatonin is an effector of the diurnal clock on pancreatic islets. The membrane receptor-transmitted inhibitory influence of melatonin on insulin secretion is well established and contrasts with the reported stimulation of glucagon release from α-cells. Virtually, nothing is known concerning the melatonin-mediated effects on islet δ-cells. Analysis of a human pancreatic δ-cell model, the cell line QGP-1, and the use of a somatostatin-specific radioimmunoassay showed that melatonin primarily has an inhibitory effect on somatostatin secretion in the physiological concentration range. In the pharmacological range, melatonin elicited slightly increased somatostatin release from δ-cells. Cyclic adenosine monophosphate (cAMP) is the major second messenger dose-dependently stimulating somatostatin secretion, in experiments employing the membrane-permeable 8-Br-cAMP. 8-Br-cyclic guanosine monophosphate proved to be of only minor relevance to somatostatin release. As the inhibitory effect of 1 nm melatonin was reversed after incubation of QGP-1 cells with the nonselective melatonin receptor antagonist luzindole, but not with the MT2-selective antagonist 4-P-PDOT (4-phenyl-2-propionamidotetraline), an involvement of the MT1 receptor can be assumed. Somatostatin release from the δ-cells at low glucose concentrations was significantly inhibited during co-incubation with 1 nm melatonin, an effect which was less pronounced at higher glucose levels. Transient expression experiments, overexpressing MT1, MT2, or a deletion variant as a control, indicated that the MT1 and not the MT2 receptor was the major transmitter of the inhibitory melatonin effect. These data point to a significant influence of melatonin on pancreatic δ-cells and on somatostatin release.

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.

Structure-guided design of selective Epac1 and Epac2 agonists

The second messenger cAMP is known to augment glucose-induced insulin secretion. However, its downstream targets in pancreatic β-cells have not been unequivocally determined. Therefore, we designed cAMP analogues by a structure-guided approach that act as Epac2-selective agonists both in vitro and in vivo. These analogues activate Epac2 about two orders of magnitude more potently than cAMP. The high potency arises from increased affinity as well as increased maximal activation. Crystallographic studies demonstrate that this is due to unique interactions. At least one of the Epac2-specific agonists, Sp-8-BnT-cAMPS (S-220), enhances glucose-induced insulin secretion in human pancreatic cells. Selective targeting of Epac2 is thus proven possible and may be an option in diabetes treatment.

cAMP is a small molecule produced by cells that activates proteins involved in a wide range of biological processes, including olfaction, pacemaker activity, regulation of gene expression, insulin secretion, and many others. In the case of insulin secretion, cAMP seems to impinge on different stages of the signalling cascade to regulate secretory activity in pancreatic β-cells. Here we have developed a chemically modified version of cAMP that specifically only activates Epac2, one of the cAMP-responsive proteins in this cascade. Furthermore, our cAMP analogue activates Epac2 more potently than cAMP itself does. We have determined several crystal structures of Epac2 in complex with cAMP analogues to help us explain the molecular basis of the observed selectivity and the strong activation potential. In addition, we were able to show that the analogue is able to potentiate glucose-induced secretion of insulin from human pancreatic islets. The principal challenge during this study was identifying and understanding small differences in the cAMP-binding domains of cAMP-regulated proteins and matching these differences with suitable modifications of the cAMP molecule.

A newly developed analogue of cAMP that selectively activates Epac2 can potentiate glucose-induced insulin secretion from human pancreatic β-cells.

Glucagon-like peptide 1 stimulates insulin secretion via inhibiting RhoA/ROCK signaling and disassembling glucotoxicity-induced stress fibers

Chronic hyperglycemia leads to pancreatic β-cell dysfunction characterized by diminished glucose-stimulated insulin secretion (GSIS), but the precise cellular processes involved are largely unknown. Here we show that pancreatic β-cells chronically exposed to a high glucose level displayed substantially increased amounts of stress fibers compared with β-cells cultured at a low glucose level. β-Cells at high glucose were refractory to glucose-induced actin cytoskeleton remodeling and insulin secretion. Importantly, F-actin depolymerization by either cytochalasin B or latrunculin B restored glucotoxicity-diminished GSIS. The effects of glucotoxicity on increasing stress fibers and reducing GSIS were reversed by Y-27632, a Rho-associated kinase (ROCK)-specific inhibitor, which caused actin depolymerization and enhanced GSIS. Notably, glucagon-like peptide-1-(7-36) amide (GLP-1), a peptide hormone that stimulates GSIS at both normal and hyperglycemic conditions, also reversed glucotoxicity-induced increase of stress fibers and reduction of GSIS. In addition, GLP-1 inhibited glucotoxicity-induced activation of RhoA/ROCK and thereby resulted in actin depolymerization and potentiation of GSIS. Furthermore, this effect of GLP-1 was mimicked by cAMP-increasing agents forskolin and 3-isobutyl-1-methylxanthine as well as the protein kinase A agonist 6-Bnz-cAMP-AM whereas it was abolished by the protein kinase A inhibitor Rp-Adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt. To establish a clinical relevance of our findings, we examined the association of genetic variants of RhoA/ROCK with metabolic traits in homeostasis model assessment index of insulin resistance. Several single-nucleotide polymorphisms in and around RHOA were associated with elevated fasting insulin and homeostasis model assessment index of insulin resistance, suggesting a possible role in metabolic dysregulation. Collectively these findings unravel a novel mechanism whereby GLP-1 potentiates glucotoxicity-diminished GSIS by depolymerizing F-actin cytoskeleton via protein kinase A-mediated inhibition of the RhoA-ROCK signaling pathway.

Regulation of Pancreatic Beta Cell Stimulus-Secretion Coupling by microRNAs

Increased blood glucose after a meal is countered by the subsequent increased release of the hypoglycemic hormone insulin from the pancreatic beta cells. The cascade of molecular events encompassing the initial sensing and transport of glucose into the beta cell, culminating with the exocytosis of the insulin large dense core granules (LDCVs) is termed "stimulus-secretion coupling." Impairment in any of the relevant processes leads to insufficient insulin release, which contributes to the development of type 2 diabetes (T2D). The fate of the beta cell, when exposed to environmental triggers of the disease, is determined by the possibility to adapt to the new situation by regulation of gene expression. As established factors of post-transcriptional regulation, microRNAs (miRNAs) are well-recognized mediators of beta cell plasticity and adaptation. Here, we put focus on the importance of comprehending the transcriptional regulation of miRNAs, and how miRNAs are implicated in stimulus-secretion coupling, specifically those influencing the late stages of insulin secretion. We suggest that efficient beta cell adaptation requires an optimal balance between transcriptional regulation of miRNAs themselves, and miRNA-dependent gene regulation. The increased knowledge of the beta cell transcriptional network inclusive of non-coding RNAs such as miRNAs is essential in identifying novel targets for the treatment of T2D.