Characterization of voltage-dependent sodium and calcium channels in mouse pancreatic A- and B-cells

The endoplasmic reticulum plays a key role in α-cell intracellular Ca2+ dynamics and glucose-regulated glucagon secretion in mouse islets

Glucagon is secreted by pancreatic α-cells to counteract hypoglycaemia. How glucose regulates glucagon secretion remains unclear. Here, using mouse islets, we studied the role of transmembrane and endoplasmic reticulum (ER) Ca2+ on intrinsic α-cell glucagon secretion. Blocking isradipine-sensitive L-type voltage-gated Ca2+ (Cav) channels abolished α-cell electrical activity but had little impact on its cytosolic Ca2+ oscillations or low-glucose-stimulated glucagon secretion. In contrast, depleting ER Ca2+ with cyclopiazonic acid or blocking ER Ca2+-releasing ryanodine receptors abolished α-cell glucose sensitivity and low-glucose-stimulated glucagon secretion. ER Ca2+ mobilization in α-cells is regulated by intracellular ATP and likely to be coupled to Ca2+ influx through P/Q-type Cav channels. ω-Agatoxin IVA blocked α-cell ER Ca2+ release and cell exocytosis, but had no additive effect on glucagon secretion when combined with ryanodine. We conclude that glucose regulates glucagon secretion through the control of ER Ca2+ mobilization, a mechanism that can be independent of α-cell electrical activity.

Voltage-Gated T-Type Calcium Channel Modulation by Kinases and Phosphatases: The Old Ones, the New Ones, and the Missing Ones

Calcium (Ca²⁺) can regulate a wide variety of cellular fates, such as proliferation, apoptosis, and autophagy. More importantly, changes in the intracellular Ca²⁺ level can modulate signaling pathways that control a broad range of physiological as well as pathological cellular events, including those important to cellular excitability, cell cycle, gene-transcription, contraction, cancer progression, etc. Not only intracellular Ca²⁺ level but the distribution of Ca²⁺ in the intracellular compartments is also a highly regulated process. For this Ca²⁺ homeostasis, numerous Ca²⁺ chelating, storage, and transport mechanisms are required. There are also specialized proteins that are responsible for buffering and transport of Ca²⁺. T-type Ca²⁺ channels (TTCCs) are one of those specialized proteins which play a key role in the signal transduction of many excitable and non-excitable cell types. TTCCs are low-voltage activated channels that belong to the family of voltage-gated Ca²⁺ channels. Over decades, multiple kinases and phosphatases have been shown to modulate the activity of TTCCs, thus playing an indirect role in maintaining cellular physiology. In this review, we provide information on the kinase and phosphatase modulation of TTCC isoforms Cav3.1, Cav3.2, and Cav3.3, which are mostly described for roles unrelated to cellular excitability. We also describe possible potential modulations that are yet to be explored. For example, both mitogen-activated protein kinase and citron kinase show affinity for different TTCC isoforms; however, the effect of such interaction on TTCC current/kinetics has not been studied yet.

T-Type Ca2+ Channels Boost Neurotransmission in Mammalian Cone Photoreceptors

It is a commonly accepted view that light stimulation of mammalian photoreceptors causes a graded change in membrane potential instead of developing a spike. The presynaptic Ca2+ channels serve as a crucial link for the coding of membrane potential variations into neurotransmitter release. Cav1.4 L-type Ca2+ channels are expressed in photoreceptor terminals, but the complete pool of Ca2+ channels in cone photoreceptors appears to be more diverse. Here, we discovered, employing whole-cell patch-clamp recording from cone photoreceptor terminals in both sexes of mice, that their Ca2+ currents are composed of low- (T-type Ca2+ channels) and high- (L-type Ca2+ channels) voltage-activated components. Furthermore, Ca2+ channels exerted self-generated spike behavior in dark membrane potentials, and spikes were generated in response to light/dark transition. The application of fast and slow Ca2+ chelators revealed that T-type Ca2+ channels are located close to the release machinery. Furthermore, capacitance measurements indicated that they are involved in evoked vesicle release. Additionally, RT-PCR experiments showed the presence of Cav3.2 T-type Ca2+ channels in cone photoreceptors but not in rod photoreceptors. Altogether, we found several crucial functions of T-type Ca2+ channels, which increase the functional repertoire of cone photoreceptors. Namely, they extend cone photoreceptor light-responsive membrane potential range, amplify dark responses, generate spikes, increase intracellular Ca2+ levels, and boost synaptic transmission.SIGNIFICANCE STATEMENT Photoreceptors provide the first synapse for coding light information. The key elements in synaptic transmission are the voltage-sensitive Ca2+ channels. Here, we provide evidence that mouse cone photoreceptors express low-voltage-activated Cav3.2 T-type Ca2+ channels in addition to high-voltage-activated L-type Ca2+ channels. The presence of T-type Ca2+ channels in cone photoreceptors appears to extend their light-responsive membrane potential range, amplify dark response, generate spikes, increase intracellular Ca2+ levels, and boost synaptic transmission. By these functions, Cav3.2 T-type Ca2+ channels increase the functional repertoire of cone photoreceptors.

Role of High Voltage-Gated Ca2+ Channel Subunits in Pancreatic β-Cell Insulin Release. From Structure to Function

The pancreatic islets of Langerhans secrete several hormones critical for glucose homeostasis. The β-cells, the major cellular component of the pancreatic islets, secrete insulin, the only hormone capable of lowering the plasma glucose concentration. The counter-regulatory hormone glucagon is secreted by the α-cells while δ-cells secrete somatostatin that via paracrine mechanisms regulates the α- and β-cell activity. These three peptide hormones are packed into secretory granules that are released through exocytosis following a local increase in intracellular Ca2+ concentration. The high voltage-gated Ca2+ channels (HVCCs) occupy a central role in pancreatic hormone release both as a source of Ca2+ required for excitation-secretion coupling as well as a scaffold for the release machinery. HVCCs are multi-protein complexes composed of the main pore-forming transmembrane α1 and the auxiliary intracellular β, extracellular α2δ, and transmembrane γ subunits. Here, we review the current understanding regarding the role of all HVCC subunits expressed in pancreatic β-cell on electrical activity, excitation-secretion coupling, and β-cell mass. The evidence we review was obtained from many seminal studies employing pharmacological approaches as well as genetically modified mouse models. The significance for diabetes in humans is discussed in the context of genetic variations in the genes encoding for the HVCC subunits.

Insulin Release Mechanism Modulated by Toxins Isolated from Animal Venoms: From Basic Research to Drug Development Prospects

Venom from mammals, amphibians, snakes, arachnids, sea anemones and insects provides diverse sources of peptides with different potential medical applications. Several of these peptides have already been converted into drugs and some are still in the clinical phase. Diabetes type 2 is one of the diseases with the highest mortality rate worldwide, requiring specific attention. Diverse drugs are available (e.g., Sulfonylureas) for effective treatment, but with several adverse secondary effects, most of them related to the low specificity of these compounds to the target. In this context, the search for specific and high-affinity compounds for the management of this metabolic disease is growing. Toxins isolated from animal venom have high specificity and affinity for different molecular targets, of which the most important are ion channels. This review will present an overview about the electrical activity of the ion channels present in pancreatic β cells that are involved in the insulin secretion process, in addition to the diversity of peptides that can interact and modulate the electrical activity of pancreatic β cells. The importance of prospecting bioactive peptides for therapeutic use is also reinforced.

GLP-1 Localisation and Proglucagon Gene Expression in Healthy and Diabetic Mouse Ileum

Introduction

Glucagon-like peptide-1 (GLP-1) is a polypeptide that is mainly produced by intestinal L cells and is encoded by the proglucagon gene. In this study, GLP-1 localisation was investigated in the ileum of healthy and diabetic mice by immunohistochemistry and proglucagon gene expression was assayed by reverse transcription-polymerase chain reaction.

Material and Methods

This study included 18 male Balb/c mice that were divided into diabetic, sham, and control groups. Mice in the diabetic group received 100 mg/kg of streptozotocin. Immunohistochemical expression of GLP-1 was determined using the avidin–biotin–peroxidase complex technique, and proglucagon gene expression was determined by RT-PCR.

Results

Analysis of GLP-1 immunohistochemical localisation showed that GLP-1-immunopositive cells (L cells) were present between epithelial cells in the intestinal crypts. The intensity and localisation of GLP-1 immunoreactivity were similar among the mice in all the groups. Proglucagon gene expression levels were also statistically similar among the mice in all the groups.

Conclusion

No difference was demonstrated among the mice in the diabetic, sham, or control groups with respect to proglucagon gene expression and GLP-1 localisation in the ileum, suggesting that diabetes does not affect proglucagon gene expression in the ileum.

The inhibitor of connexin Cx36 channels, mefloquine, inhibits voltage-dependent Ca2+ channels and insulin secretion

The antimalarial agent, mefloquine, inhibits the function of connexin Cx36 gap junctions and hemichannels and has thus become a tool to investigate their physiological relevance in pancreatic islets. In view of earlier reports on a KATP channel-block by mefloquine, the specificity of mefloquine as a pharmacological tool was investigated. Mouse pancreatic islets and single beta cells were used to measure membrane potential, whole cell currents, Ca2+ channel activity, cytosolic Ca2+ concentration ([Ca2+]i) and insulin secretion. Mefloquine was tested in the concentration range of 5-50 μM 25 μM mefloquine was as effective as 500 μM tolbutamide to depolarize the plasma membrane of beta cells, but did not induce action potentials. Rather, it abolished tolbutamide-induced action potentials and the associated increase of [Ca2+]i. In the range of 5-50 μM mefloquine inhibited voltage-dependent Ca2+ currents in primary beta cells as effectively as 1 μM nisoldipine, a specific blocker of L-type Ca2+ channels. The Ca2+ channel opening effect of Bay K8644 was completely antagonized by mefloquine. Likewise, the increase of [Ca2+]i and of insulin secretion stimulated by 40 mM KCl, but not that by 30 mM glucose was antagonized by 50 μM mefloquine. Neither at 5 μM nor at 50 μM did mefloquin stimulate insulin secretion at basal glucose. In conclusion, mefloquine blocks KATP channels and L-type Ca2+ channels in pancreatic beta cells in the range from 5 to 50 μM. Thus it inhibits depolarization-induced insulin secretion, but in the presence of a stimulatory glucose concentration additional effects of mefloquine, possibly on intracellular Ca2+ mobilization, and the metabolic amplification by glucose permit a sustained rate of secretion.

Biphasic voltage-dependent inactivation of human NaV 1.3, 1.6 and 1.7 Na+ channels expressed in rodent insulin-secreting cells

KEY POINTS

Na+ current inactivation is biphasic in insulin-secreting cells, proceeding with two voltage dependences that are half-maximal at ∼-100 mV and -60 mV. Inactivation of voltage-gated Na+ (NaV ) channels occurs at ∼30 mV more negative voltages in insulin-secreting Ins1 and primary β-cells than in HEK, CHO or glucagon-secreting αTC1-6 cells. The difference in inactivation between Ins1 and non-β-cells persists in the inside-out patch configuration, discounting an involvement of a diffusible factor. In Ins1 cells and primary β-cells, but not in HEK cells, inactivation of a single NaV subtype is biphasic and follows two voltage dependences separated by 30-40 mV. We propose that NaV channels adopt different inactivation behaviours depending on the local membrane environment.

ABSTRACT

Pancreatic β-cells are equipped with voltage-gated Na+ channels that undergo biphasic voltage-dependent steady-state inactivation. A small Na+ current component (10-15%) inactivates over physiological membrane potentials and contributes to action potential firing. However, the major Na+ channel component is completely inactivated at -90 to -80 mV and is therefore inactive in the β-cell. It has been proposed that the biphasic inactivation reflects the contribution of different NaV α-subunits. We tested this possibility by expression of TTX-resistant variants of the NaV subunits found in β-cells (NaV 1.3, NaV 1.6 and NaV 1.7) in insulin-secreting Ins1 cells and in non-β-cells (including HEK and CHO cells). We found that all NaV subunits inactivated at 20-30 mV more negative membrane potentials in Ins1 cells than in HEK or CHO cells. The more negative inactivation in Ins1 cells does not involve a diffusible intracellular factor because the difference between Ins1 and CHO persisted after excision of the membrane. NaV 1.7 inactivated at 15--20 mV more negative membrane potentials than NaV 1.3 and NaV 1.6 in Ins1 cells but this small difference is insufficient to solely explain the biphasic inactivation in Ins1 cells. In Ins1 cells, but never in the other cell types, widely different components of NaV inactivation (separated by 30 mV) were also observed following expression of a single type of NaV α-subunit. The more positive component exhibited a voltage dependence of inactivation similar to that found in HEK and CHO cells. We propose that biphasic NaV inactivation in insulin-secreting cells reflects insertion of channels in membrane domains that differ with regard to lipid and/or membrane protein composition.

Calcium Channels in Postnatal Development of Rat Pancreatic Beta Cells and Their Role in Insulin Secretion

Pancreatic beta cells during the first month of development acquire functional maturity, allowing them to respond to variations in extracellular glucose concentration by secreting insulin. Changes in ionic channel activity are important for this maturation. Within the voltage-gated calcium channels (VGCC), the most studied channels are high-voltage-activated (HVA), principally L-type; while low-voltage-activated (LVA) channels have been poorly studied in native beta cells. We analyzed the changes in the expression and activity of VGCC during the postnatal development in rat beta cells. We observed that the percentage of detection of T-type current increased with the stage of development. T-type calcium current density in adult cells was higher than in neonatal and P20 beta cells. Mean HVA current density also increased with age. Calcium current behavior in P20 beta cells was heterogeneous; almost half of the cells had HVA current densities higher than the adult cells, and this was independent of the presence of T-type current. We detected the presence of α1G, α1H, and α1I subunits of LVA channels at all ages. The Cav 3.1 subunit (α1G) was the most expressed. T-type channel blockers mibefradil and TTA-A₂ significantly inhibited insulin secretion at 5.6 mM glucose, which suggests a physiological role for T-type channels at basal glucose conditions. Both, nifedipine and TTA-A₂, drastically decreased the beta-cell subpopulation that secretes more insulin, in both basal and stimulating glucose conditions. We conclude that changes in expression and activity of VGCC during the development play an important role in physiological maturation of beta cells.

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.

Sodium channel NaV1.3 is important for enterochromaffin cell excitability and serotonin release

In the gastrointestinal (GI) epithelium, enterochromaffin (EC) cells are enteroendocrine cells responsible for producing >90% of the body's serotonin (5-hydroxytryptamine, 5-HT). However, the molecular mechanisms of EC cell function are poorly understood. Here, we found that EC cells in mouse primary cultures fired spontaneous bursts of action potentials. We examined the repertoire of voltage-gated sodium channels (Na_V) in fluorescence-sorted mouse EC cells and found that Scn3a was highly expressed. Scn3a-encoded Na_V1.3 was specifically and densely expressed at the basal side of both human and mouse EC cells. Using electrophysiology, we found that EC cells expressed robust Na_V1.3 currents, as determined by their biophysical and pharmacologic properties. Na_V1.3 was not only critical for generating action potentials in EC cells, but it was also important for regulating 5-HT release by these cells. Therefore, EC cells use Scn3a-encoded voltage-gated sodium channel Na_V1.3 for electrical excitability and 5-HT release. Na_V1.3-dependent electrical excitability and its contribution to 5-HT release is a novel mechanism of EC cell function.

Molecular mechanisms involved in the non-monotonic effect of bisphenol-a on ca2+ entry in mouse pancreatic β-cells

In regulatory toxicology, the dose-response relationship is a key element towards fulfilling safety assessments and satisfying regulatory authorities. Conventionally, the larger the dose, the greater the response, following the dogma “the dose makes the poison”. Many endocrine disrupting chemicals, including bisphenol-A (BPA), induce non-monotonic dose response (NMDR) relationships, which are unconventional and have tremendous implications in risk assessment. Although several molecular mechanisms have been proposed to explain NMDR relationships, they are largely undemonstrated. Using mouse pancreatic β-cells from wild-type and oestrogen receptor ERβ−/− mice, we found that exposure to increasing doses of BPA affected Ca²⁺ entry in an NMDR manner. Low doses decreased plasma membrane Ca²⁺ currents after downregulation of Cav2.3 ion channel expression, in a process involving ERβ. High doses decreased Ca²⁺ currents through an ERβ-mediated mechanism and simultaneously increased Ca²⁺ currents via oestrogen receptor ERα. The outcome of both molecular mechanisms explains the NMDR relationship between BPA and Ca²⁺ entry in β-cells.

Functional identification of islet cell types by electrophysiological fingerprinting

The α-, β- and δ-cells of the pancreatic islet exhibit different electrophysiological features. We used a large dataset of whole-cell patch-clamp recordings from cells in intact mouse islets (N = 288 recordings) to investigate whether it is possible to reliably identify cell type (α, β or δ) based on their electrophysiological characteristics. We quantified 15 electrophysiological variables in each recorded cell. Individually, none of the variables could reliably distinguish the cell types. We therefore constructed a logistic regression model that included all quantified variables, to determine whether they could together identify cell type. The model identified cell type with 94% accuracy. This model was applied to a dataset of cells recorded from hyperglycaemic βV59M mice; it correctly identified cell type in all cells and was able to distinguish cells that co-expressed insulin and glucagon. Based on this revised functional identification, we were able to improve conductance-based models of the electrical activity in α-cells and generate a model of δ-cell electrical activity. These new models could faithfully emulate α- and δ-cell electrical activity recorded experimentally.

β-arrestin-2 is an essential regulator of pancreatic β-cell function under physiological and pathophysiological conditions

β-arrestins are critical signalling molecules that regulate many fundamental physiological functions including the maintenance of euglycemia and peripheral insulin sensitivity. Here we show that inactivation of the β-arrestin-2 gene, barr2, in β-cells of adult mice greatly impairs insulin release and glucose tolerance in mice fed with a calorie-rich diet. Both glucose and KCl-induced insulin secretion and calcium responses were profoundly reduced in β-arrestin-2 (barr2) deficient β-cells. In human β-cells, barr2 knockdown abolished glucose-induced insulin secretion. We also show that the presence of barr2 is essential for proper CAMKII function in β-cells. Importantly, overexpression of barr2 in β-cells greatly ameliorates the metabolic deficits displayed by mice consuming a high-fat diet. Thus, our data identify barr2 as an important regulator of β-cell function, which may serve as a new target to improve β-cell function.

Beta-arrestins have key roles in development and metabolic functions as euglycaemic control and insulin sentitivity. Here Zhu et al. show that beta-arrestin-2 regulates insulin secretion and glucose tolerance in mice by promoting CAMKII functions in beta cells.

Roles of Voltage-Gated Tetrodotoxin-Sensitive Sodium Channels NaV1.3 and NaV1.7 in Diabetes and Painful Diabetic Neuropathy

Diabetes mellitus (DM) is a common chronic medical problem worldwide; one of its complications is painful peripheral neuropathy, which can substantially erode quality of life and increase the cost of management. Despite its clinical importance, the pathogenesis of painful diabetic neuropathy (PDN) is complex and incompletely understood. Voltage-gated sodium channels (VGSCs) link many physiological processes to electrical activity by controlling action potentials in all types of excitable cells. Two isoforms of VGSCs, Na_V1.3 and Na_V1.7, which are encoded by the sodium voltage-gated channel alpha subunit 3 and 9 (Scn3A and Scn9A) genes, respectively, have been identified in both peripheral nociceptive neurons of dorsal root ganglion (DRG) and pancreatic islet cells. Recent advances in our understanding of tetrodotoxin-sensitive (TTX-S) sodium channels Na_V1.3 and Na_V1.7 lead to the rational doubt about the cause–effect relation between diabetes and painful neuropathy. In this review, we summarize the roles of Na_V1.3 and Na_V1.7 in islet cells and DRG neurons, discuss the link between DM and painful neuropathy, and present a model, which may provide a starting point for further studies aimed at identifying the mechanisms underlying diabetes and painful neuropathy.

Modulation of Ionic Channels and Insulin Secretion by Drugs and Hormones in Pancreatic Beta Cells

Pancreatic beta cells, unique cells that secrete insulin in response to an increase in glucose levels, play a significant role in glucose homeostasis. Glucose-stimulated insulin secretion (GSIS) in pancreatic beta cells has been extensively explored. In this mechanism, glucose enters the cells and subsequently the metabolic cycle. During this process, the ATP/ADP ratio increases, leading to ATP-sensitive potassium (KATP) channel closure, which initiates depolarization that is also dependent on the activity of TRP nonselective ion channels. Depolarization leads to the opening of voltage-gated Na(+) channels (Nav) and subsequently voltage-dependent Ca(2+) channels (Cav). The increase in intracellular Ca(2+) triggers the exocytosis of insulin-containing vesicles. Thus, electrical activity of pancreatic beta cells plays a central role in GSIS. Moreover, many growth factors, incretins, neurotransmitters, and hormones can modulate GSIS, and the channels that participate in GSIS are highly regulated. In this review, we focus on the principal ionic channels (KATP, Nav, and Cav channels) involved in GSIS and how classic and new proteins, hormones, and drugs regulate it. Moreover, we also discuss advances on how metabolic disorders such as metabolic syndrome and diabetes mellitus change channel activity leading to changes in insulin secretion.

α-Cell Dysfunctions and Molecular Alterations in Male Insulinopenic Diabetic Mice Are Not Completely Corrected by Insulin

Glucagon and α-cell dysfunction are critical in the development of hyperglycemia during diabetes both in humans and rodents. We hypothesized that α-cell dysfunction leading to dysregulated glucagon secretion in diabetes is due to both a lack of insulin and intrinsic defects. To characterize α-cell dysfunction in diabetes, we used glucagon-Venus transgenic male mice and induced insulinopenic hyperglycemia by streptozotocin administration leading to alterations of glucagon secretion. We investigated the in vivo impact of insulinopenic hyperglycemia on glucagon-producing cells using FACS-sorted α-cells from control and diabetic mice. We demonstrate that increased glucagonemia in diabetic mice is mainly due to increases of glucagon release and biosynthesis per cell compared with controls without changes in α-cell mass. We identified genes coding for proteins involved in glucagon biosynthesis and secretion, α-cell differentiation, and potential stress markers such as the glucagon, Arx, MafB, cMaf, Brain4, Foxa1, Foxa3, HNF4α, TCF7L2, Glut1, Sglt2, Cav2.1, Cav2.2, Nav1.7, Kir6.2/Sur1, Pten, IR, NeuroD1, GPR40, and Sumo1 genes, which were abnormally regulated in diabetic mice. Importantly, insulin treatment partially corrected α-cell function and expression of genes coding for proglucagon, or involved in glucagon secretion, glucose transport and insulin signaling but not those coding for cMAF, FOXA1, and α-cell differentiation markers as well as GPR40, NEUROD1, CAV2.1, and SUMO1. Our results indicate that insulinopenic diabetes induce marked α-cell dysfunction and molecular alteration, which are only partially corrected by in vivo insulin treatment.

High-content screening identifies a role for Na(+) channels in insulin production

Insulin production is the central feature of functionally mature and differentiated pancreatic β-cells. Reduced insulin transcription and dedifferentiation have been implicated in type 2 diabetes, making drugs that could reverse these processes potentially useful. We have previously established ratiometric live-cell imaging tools to identify factors that increase insulin promoter activity and promote β-cell differentiation. Here, we present a single vector imaging tool with eGFP and mRFP, driven by the Pdx1 and Ins1 promoters, respectively, targeted to the nucleus to enhance identification of individual cells in a high-throughput manner. Using this new approach, we screened 1120 off-patent drugs for factors that regulate Ins1 and Pdx1 promoter activity in MIN6 β-cells. We identified a number of compounds that positively modulate Ins1 promoter activity, including several drugs known to modulate ion channels. Carbamazepine was selected for extended follow-up, as our previous screen also identified this use-dependent sodium channel inhibitor as a positive modulator of β-cell survival. Indeed, carbamazepine increased Ins1 and Ins2 mRNA in primary mouse islets at lower doses than were required to protect β-cells. We validated the role of sodium channels in insulin production by examining Nav1.7 (Scn9a) knockout mice and remarkably islets from these animals had dramatically elevated insulin content relative to wild-type controls. Collectively, our experiments provide a starting point for additional studies aimed to identify drugs and molecular pathways that control insulin production and β-cell differentiation status. In particular, our unbiased screen identified a novel role for a β-cell sodium channel gene in insulin production.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

Mathematical modelling of local calcium and regulated exocytosis during inhibition and stimulation of glucagon secretion from pancreatic alpha-cells

Glucagon secretion from pancreatic alpha-cells is dysregulated in diabetes. Despite decades of investigations of the control of glucagon release by glucose and hormones, the underlying mechanisms are still debated. Recently, mathematical models have been applied to investigate the modification of electrical activity in alpha-cells as a result of glucose application. However, recent studies have shown that paracrine effects such as inhibition of glucagon secretion by glucagon-like peptide 1 (GLP-1) or stimulation of release by adrenaline involve cAMP-mediated effects downstream of electrical activity. In particular, depending of the intracellular cAMP concentration, specific types of Ca(2+) channels are inhibited or activated, which interacts with mobilization of secretory granules. To investigate these aspects of alpha-cell function theoretically, we carefully developed a mathematical model of Ca(2+) levels near open or closed Ca(2+) channels of various types, which was linked to a description of Ca(2+) below the plasma membrane, in the bulk cytosol and in the endoplasmic reticulum. We investigated how the various subcellular Ca(2+) compartments contribute to control of glucagon-exocytosis in response to glucose, GLP-1 or adrenaline. Our studies refine previous modelling studies of alpha-cell function, and provide deeper insight into the control of glucagon secretion.

CaV1.2 and CaV1.3 channel hyperactivation in mouse islet β cells exposed to type 1 diabetic serum

The voltage-gated Ca(2+) (CaV) channel acts as a key player in β cell physiology and pathophysiology. β cell CaV channels undergo hyperactivation subsequent to exposure to type 1 diabetic (T1D) serum resulting in increased cytosolic free Ca(2+) concentration and thereby Ca(2+)-triggered β cell apoptosis. The present study was aimed at revealing the subtypes of CaV1 channels hyperactivated by T1D serum as well as the biophysical mechanisms responsible for T1D serum-induced hyperactivation of β cell CaV1 channels. Patch-clamp recordings and single-cell RT-PCR analysis were performed in pancreatic β cells from CaV1 channel knockout and corresponding control mice. We now show that functional CaV1.3 channels are expressed in a subgroup of islet β cells from CaV1.2 knockout mice (CaV1.2(-/-)). T1D serum enhanced whole-cell CaV currents in islet β cells from CaV1.3 knockout mice (CaV1.3(-/-)). T1D serum increased the open probability and number of functional unitary CaV1 channels in CaV1.2(-/-) and CaV1.3(-/-) β cells. These data demonstrate that T1D serum hyperactivates both CaV1.2 and CaV1.3 channels by increasing their conductivity and number. These findings suggest CaV1.2 and CaV1.3 channels as potential targets for anti-diabetes therapy.

The organ-protective effect of N-type Ca(2+) channel blockade

The six subtypes of voltage-dependent Ca(2+) channels (VDCCs) mediate a wide range of physiological responses. N-type VDCCs (NCCs) were originally identified as a high voltage-activated Ca(2+) channel selectively blocked by omega-conotoxin (ω-CTX)-GVIA. Predominantly localized in the nervous system, NCCs are key regulators of neurotransmitter release. Both pharmacological blockade with ω-CTX-GVIA and, more recently, mice lacking CNCNA1B, encoding the α1B subunit of NCC, have been used to assess the physiological and pathophysiological functions of NCCs, revealing in part their significant roles in sympathetic nerve activation and nociceptive transmission. The evidence now available indicates that NCCs are a potentially useful therapeutic target for the treatment of several pathological conditions. Efforts are therefore being made to develop effective NCC blockers, including both synthetic ω-CTX-GVIA derivatives and small-molecule inhibitors. Cilnidipine, for example, is a dihydropyridine L-type VDCC blocking agent that also possesses significant NCC blocking ability. As over-activation of the sympathetic nervous system appears to contribute to the pathological processes underlying cardiovascular, renal and metabolic diseases, NCC blockade could be a useful approach to treating these ailments. In this review article, we provide an overview of what is currently known about the physiological and pathophysiological activities of NCCs and the potentially beneficial effects of NCC blockade in several disease conditions, in particular cardiovascular diseases.

Ionic mechanisms in pancreatic β cell signaling

The function and survival of pancreatic β cells critically rely on complex electrical signaling systems composed of a series of ionic events, namely fluxes of K(+), Na(+), Ca(2+) and Cl(-) across the β cell membranes. These electrical signaling systems not only sense events occurring in the extracellular space and intracellular milieu of pancreatic islet cells, but also control different β cell activities, most notably glucose-stimulated insulin secretion. Three major ion fluxes including K(+) efflux through ATP-sensitive K(+) (KATP) channels, the voltage-gated Ca(2+) (CaV) channel-mediated Ca(2+) influx and K(+) efflux through voltage-gated K(+) (KV) channels operate in the β cell. These ion fluxes set the resting membrane potential and the shape, rate and pattern of firing of action potentials under different metabolic conditions. The KATP channel-mediated K(+) efflux determines the resting membrane potential and keeps the excitability of the β cell at low levels. Ca(2+) influx through CaV1 channels, a major type of β cell CaV channels, causes the upstroke or depolarization phase of the action potential and regulates a wide range of β cell functions including the most elementary β cell function, insulin secretion. K(+) efflux mediated by KV2.1 delayed rectifier K(+) channels, a predominant form of β cell KV channels, brings about the downstroke or repolarization phase of the action potential, which acts as a brake for insulin secretion owing to shutting down the CaV channel-mediated Ca(2+) entry. These three ion channel-mediated ion fluxes are the most important ionic events in β cell signaling. This review concisely discusses various ionic mechanisms in β cell signaling and highlights KATP channel-, CaV1 channel- and KV2.1 channel-mediated ion fluxes.

Blockade of Na+ channels in pancreatic α-cells has antidiabetic effects

Pancreatic α-cells express voltage-gated Na(+) channels (NaChs), which support the generation of electrical activity leading to an increase in intracellular calcium, and cause exocytosis of glucagon. Ranolazine, a NaCh blocker, is approved for treatment of angina. In addition to its antianginal effects, ranolazine has been shown to reduce HbA1c levels in patients with type 2 diabetes mellitus and coronary artery disease; however, the mechanism behind its antidiabetic effect has been unclear. We tested the hypothesis that ranolazine exerts its antidiabetic effects by inhibiting glucagon release via blockade of NaChs in the pancreatic α-cells. Our data show that ranolazine, via blockade of NaChs in pancreatic α-cells, inhibits their electrical activity and reduces glucagon release. We found that glucagon release in human pancreatic islets is mediated by the Nav1.3 isoform. In animal models of diabetes, ranolazine and a more selective NaCh blocker (GS-458967) lowered postprandial and basal glucagon levels, which were associated with a reduction in hyperglycemia, confirming that glucose-lowering effects of ranolazine are due to the blockade of NaChs. This mechanism of action is unique in that no other approved antidiabetic drugs act via this mechanism, and raises the prospect that selective Nav1.3 blockers may constitute a novel approach for the treatment of diabetes.

Modeling the pancreatic α-cell: dual mechanisms of glucose suppression of glucagon secretion

The mechanism by which glucose induces insulin secretion in β-cells is fairly well understood. Despite years of research, however, the mechanism of glucagon secretion in α-cells is still not well established. It has been proposed that glucose regulates glucagon secretion by decreasing the conductance of either outward ATP-dependent potassium channels (K(ATP)) or an inward store-operated current (SOC). We have developed a mathematical model based on mouse data to test these hypotheses and found that both mechanisms are possible. Glucose metabolism closes K(ATP) channels, which depolarizes the cell but paradoxically reduces calcium influx by inactivating voltage-dependent calcium and sodium channels and decreases secretion. Glucose metabolism also activates SERCA pumps, which fills the endoplasmic reticulum and hyperpolarizes the cells by reducing the inward current through SOC channels and again suppresses glucagon secretion. We find further that the two mechanisms can combine to account for the nonmonotonic dependence of secretion on glucose observed in some studies, an effect that cannot be obtained with either mechanism alone.

Na+ current properties in islet α- and β-cells reflect cell-specific Scn3a and Scn9a expression

Mouse pancreatic β- and α-cells are equipped with voltage-gated Na⁺ currents that inactivate over widely different membrane potentials (half-maximal inactivation (V_0.5) at −100 mV and −50 mV in β- and α-cells, respectively). Single-cell PCR analyses show that both α- and β-cells have Na_v1.3 (Scn3) and Na_v1.7 (Scn9a) α subunits, but their relative proportions differ: β-cells principally express Na_v1.7 and α-cells Na_v1.3. In α-cells, genetically ablating Scn3a reduces the Na⁺ current by 80%. In β-cells, knockout of Scn9a lowers the Na⁺ current by >85%, unveiling a small Scn3a-dependent component. Glucagon and insulin secretion are inhibited in Scn3a^−/− islets but unaffected in Scn9a-deficient islets. Thus, Na_v1.3 is the functionally important Na⁺ channel α subunit in both α- and β-cells because Na_v1.7 is largely inactive at physiological membrane potentials due to its unusually negative voltage dependence of inactivation. Interestingly, the Na_v1.7 sequence in brain and islets is identical and yet the V_0.5 for inactivation is >30 mV more negative in β-cells. This may indicate the presence of an intracellular factor that modulates the voltage dependence of inactivation.

Channelopathies, painful neuropathy, and diabetes: which way does the causal arrow point?

Diabetes mellitus, a major global health problem, is commonly associated with painful peripheral neuropathy, which can substantially erode quality of life. Despite its clinical importance, the pathophysiology of painful diabetic neuropathy is incompletely understood. It has traditionally been thought that diabetes may cause neuropathy in patients with appropriate genetic makeup. Here, we propose a hypothesis whereby painful neuropathy is not a complication of diabetes, but rather occurs as a result of mutations that, in parallel, confer vulnerability to injury in pancreatic β cells and pain-signaling dorsal root ganglion (DRG) neurons. We suggest that mutations of sodium channel NaV1.7, which is present in both cell types, may increase susceptibility for development of diabetes via β cell injury and produce painful neuropathy via a distinct effect on DRG neurons.

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.

Insulinotropic effect of high potassium concentration beyond plasma membrane depolarization

The question whether K⁺ depolarization is an appropriate experimental substitute for the physiological nutrient-induced depolarization of the β-cell plasma membrane was investigated using primary mouse β-cells and islets. At basal glucose 40 mM K⁺ induced a massive monophasic response, whereas 15 mM K⁺ had only a minimal insulinotropic effect, even though the increase in the cytosolic Ca²⁺ concentration ([Ca²⁺]i) was not inferior to that by 20 mM glucose. In voltage-clamp experiments, Ca²⁺ influx appeared as nifedipine-inhibitable inward action currents in the presence of sulfonylurea plus TEA to block compensatory outward K⁺ currents. Under these conditions, 15 mM K⁺ induced prolonged action currents and 40 mM K⁺ transformed the action current pattern into a continuous inward current. Correspondingly, 15 mM K⁺ led to an oscillatory increase and 40 mM K⁺ to a plateau of [Ca²⁺]i superimposed on the [Ca²⁺]i elevated by sulfonylurea plus TEA. Raising K⁺ to 15 or 40 mM in the presence of sulfonylurea (±TEA) led to a fast further increase of insulin secretion. This was reduced to basal levels by nifedipine or CoCl₂. The effects of 15 mM K⁺ on depolarization, action currents, and insulin secretion were mimicked by adding 35 mM Cs⁺ and those of 40 mM K⁺ by adding 35 mM Rb⁺, in parallel with their ability to substitute for K⁺ as permeant cation. In conclusion, the alkali metals K⁺, Rb⁺, or Cs⁺ concentration-dependently transform the pattern of Ca²⁺ influx into the β-cell and may thus generate stimuli of supraphysiological strength for insulin secretion.

Multiparameter screening reveals a role for Na+ channels in cytokine-induced β-cell death

Pancreatic β-cell death plays a role in both type 1 and type 2 diabetes, but clinical treatments that specifically target β-cell survival have not yet been developed. We have recently developed live-cell imaging-based, high-throughput screening methods capable of identifying factors that modulate pancreatic β-cell death, with the hope of finding drugs that can intervene in this process. In the present study, we used a high-content screen and the Prestwick Chemical Library of small molecules to identify drugs that block cell death resulting from exposure to a cocktail of cytotoxic cytokines (25 ng/mL TNF-α, 10 ng/mL IL-1β, and 10 ng/mL IFN-γ). Data analysis with self-organizing maps revealed that 19 drugs had profiles similar to that of the no cytokine condition, indicating protection. Carbamazepine, an antiepileptic Na(+) channel inhibitor, was particularly interesting because Na(+) channels are not generally considered targets for antiapoptotic therapy in diabetes and because the function of these channels in β-cells has not been well studied. We analyzed the expression and characteristics of Na(+) currents in mature β-cells from MIP-GFP mice. We confirmed the dose-dependent protective effects of carbamazepine and another use-dependent Na(+) channel blocker in cytokine-treated mouse islet cells. Carbamazepine down-regulated the proapoptotic and endoplasmic reticulum stress signaling induced by cytokines. Together, these studies point to Na(+) channels as a novel therapeutic target in diabetes.

Extracellular factors and immunosuppressive drugs influencing insulin secretion of murine islets

Approximately 60% of transplanted islets undergo apoptosis within the first week post-transplantation into the liver attributed to poor engraftment, immune rejection and toxicity of immunosuppressive drugs. Understanding how extracellular matrix (ECM) components, immunosuppressive drugs and proinflammatory cytokines affect insulin secretion will contribute to an improved clinical outcome of islet transplantations. In this study, functional activity of isolated murine islets was measured by glucose-stimulated insulin secretion (GSIS) and by electrophysiological measurements using patch-clamp. Cultivating islets with soluble fibronectin or laminin, as opposed to with coated laminin, markedly increased GSIS. Addition of cyclosporin A reduced GSIS and suppressed glucose-induced spike activity. Tacrolimus affected neither GSIS nor spike activity, indicating a different mechanism. To evaluate the influence of proinflammatory cytokines, islets were incubated with interleukin (IL)-1β, tumour necrosis factor (TNF)-α or with supernatants from cultured Kupffer cells, the main mediators of inflammation in the hepatic sinusoids. IL-1β exerted a bimodal effect on insulin secretion, stimulating below 2 ng/ml and suppressing above 10 ng/ml. Soluble laminin in combination with a stimulatory IL-1β concentration further increased insulin secretion by 20% compared to IL-1β alone, while with high IL-1β concentrations soluble laminin slightly attenuated GSIS inhibition. TNF-α alone did not affect GSIS, but with stimulatory IL-1β concentrations completely abolished it. Similarly, supernatants derived from Kupffer cells exerted a bimodal effect on GSIS. Our data suggest that improved insulin secretion of transplanted islets could be achieved by including soluble laminin and low IL-1β concentrations in the islet cultivation medium, and by a simultaneous inhibition of cytokine secretion from Kupffer cells.

The human L-type calcium channel Cav1.3 regulates insulin release and polymorphisms in CACNA1D associate with type 2 diabetes

AIMS/HYPOTHESIS

Voltage-gated calcium channels of the L-type have been shown to be essential for rodent pancreatic beta cell function, but data about their presence and regulation in humans are incomplete. We therefore sought to elucidate which L-type channel isoform is functionally important and its association with inherited diabetes-related phenotypes.

METHODS

Beta cells of human islets from cadaver donors were enriched using FACS to study the expression of the genes encoding voltage-gated calcium channel (Cav)1.2 and Cav1.3 by absolute quantitative PCR in whole human and rat islets, as well as in clonal cells. Single-cell exocytosis was monitored as increases in cell capacitance after treatment with small interfering (si)RNA against CACNA1D (which encodes Cav1.3). Three single nucleotide polymorphisms (SNPs) were genotyped in 8,987 non-diabetic and 2,830 type 2 diabetic individuals from Finland and Sweden and analysed for associations with type 2 diabetes and insulin phenotypes.

RESULTS

In FACS-enriched human beta cells, CACNA1D mRNA expression exceeded that of CACNA1C (which encodes Cav1.2) by approximately 60-fold and was decreased in islets from type 2 diabetes patients. The latter coincided with diminished secretion of insulin in vitro. CACNA1D siRNA reduced glucose-stimulated insulin release in INS-1 832/13 cells and exocytosis in human beta cells. Phenotype/genotype associations of three SNPs in the CACNA1D gene revealed an association between the C allele of the SNP rs312480 and reduced mRNA expression, as well as decreased insulin secretion in vivo, whereas both rs312486/G and rs9841978/G were associated with type 2 diabetes.

CONCLUSION/INTERPRETATION

We conclude that the L-type calcium channel Cav1.3 is important in human glucose-induced insulin secretion, and common variants in CACNA1D might contribute to type 2 diabetes.

ATP regulates sodium channel kinetics in pancreatic islet beta cells

Pancreatic beta cells act as glucose sensors, in which intracellular ATP ([ATP](i)) are altered with glucose concentration change. The characterization of voltage-gated sodium channels under different [ATP](i) remains unclear. Here, we demonstrated that increasing [ATP](i) within a certain range of concentrations (2-8 mM) significantly enhanced the voltage-gated sodium channel currents, compared with 2 mM cytosolic ATP. This enhancement was attenuated by even high intracellular ATP (12 mM). Furthermore, elevated ATP modulated the sodium channel kinetics in a dose-dependent manner. Increased [ATP](i) shifted both the current-voltage curve and the voltage-dependent inactivation curve of sodium channel to the right. Finally, the sodium channel recovery from inactivation was significantly faster when the intracellular ATP level was increased, especially in 8 mM [ATP](i), which is an attainable concentration by the high glucose stimulation. In summary, our data suggested that elevated cytosolic ATP enhanced the activity of Na(+) channels, which may play essential roles in modulating β cell excitability and insulin release when blood glucose concentration increases.

Regulation of insulin synthesis and secretion and pancreatic Beta-cell dysfunction in diabetes

Pancreatic β-cell dysfunction plays an important role in the pathogenesis of both type 1 and type 2 diabetes. Insulin, which is produced in β-cells, is a critical regulator of metabolism. Insulin is synthesized as preproinsulin and processed to proinsulin. Proinsulin is then converted to insulin and C-peptide and stored in secretary granules awaiting release on demand. Insulin synthesis is regulated at both the transcriptional and translational level. The cis-acting sequences within the 5' flanking region and trans-activators including paired box gene 6 (PAX6), pancreatic and duodenal homeobox- 1(PDX-1), MafA, and β-2/Neurogenic differentiation 1 (NeuroD1) regulate insulin transcription, while the stability of preproinsulin mRNA and its untranslated regions control protein translation. Insulin secretion involves a sequence of events in β-cells that lead to fusion of secretory granules with the plasma membrane. Insulin is secreted primarily in response to glucose, while other nutrients such as free fatty acids and amino acids can augment glucose-induced insulin secretion. In addition, various hormones, such as melatonin, estrogen, leptin, growth hormone, and glucagon like peptide-1 also regulate insulin secretion. Thus, the β-cell is a metabolic hub in the body, connecting nutrient metabolism and the endocrine system. Although an increase in intracellular [Ca2+] is the primary insulin secretary signal, cAMP signaling- dependent mechanisms are also critical in the regulation of insulin secretion. This article reviews current knowledge on how β-cells synthesize and secrete insulin. In addition, this review presents evidence that genetic and environmental factors can lead to hyperglycemia, dyslipidemia, inflammation, and autoimmunity, resulting in β-cell dysfunction, thereby triggering the pathogenesis of diabetes.

Inhibition of insulin secretion from rat pancreatic islets by dexmedetomidine and medetomidine, two sedatives frequently used in clinical settings

The aim of this study was to determine whether dexmedetomidine (DEX) and medetomidine (MED), α2-adrenergic agonists clinically used as sedatives, influence insulin secretion from rat pancreatic islets. Islets were isolated from adult male Wistar rats after collagenase digestion. Static incubation was used to determine effects of DEX or MED on insulin secretion and ionic-channel currents of β-cells. Results indicate that both drugs dose-dependently inhibit insulin secretion, DEX more potently than MED. The inhibitory effects were attenuated by addition of yohimbine or by pretreatment of rats with pertussis toxin (PTX). 10 nM DEX decreased the current amplitude of voltage-dependent Ca2+ channels, but this did not occur when the N-type Ca2+ channel blocker ω-conotoxin was added. In the presence of tetraethylammonium, a classical voltage-gated K+ channel (Kv channel) blocker, the magnitude of inhibition of insulin secretion by MED was reduced. However, when tolbutamide, a specific blocker of the ATP-sensitive K+ channel (KATP channel), was present, the magnitude of MED inhibition of insulin secretion was not influenced, suggesting that Kv-channel activity alteration, but not that of KATP channels, is involved in MED-associated insulin secretory inhibition. The Kv-channel currents were increased during 1 nM MED exposure at membrane potentials ranging from -30 mV to -10 mV, where action potentials were generated in response to glucose stimulation. These results indicate that DEX and MED inhibit insulin secretion through an α2-adrenoceptor and PTX-sensitive GTP-binding protein pathway that eventually involves Kv channel activation and Ca2+ channel inhibition.

Regulation of insulin synthesis and secretion and pancreatic Beta-cell dysfunction in diabetes

Pancreatic β-cell dysfunction plays an important role in the pathogenesis of both type 1 and type 2 diabetes. Insulin, which is produced in β-cells, is a critical regulator of metabolism. Insulin is synthesized as preproinsulin and processed to proinsulin. Proinsulin is then converted to insulin and C-peptide and stored in secretary granules awaiting release on demand. Insulin synthesis is regulated at both the transcriptional and translational level. The cis-acting sequences within the 5' flanking region and trans-activators including paired box gene 6 (PAX6), pancreatic and duodenal homeobox- 1(PDX-1), MafA, and β-2/Neurogenic differentiation 1 (NeuroD1) regulate insulin transcription, while the stability of preproinsulin mRNA and its untranslated regions control protein translation. Insulin secretion involves a sequence of events in β-cells that lead to fusion of secretory granules with the plasma membrane. Insulin is secreted primarily in response to glucose, while other nutrients such as free fatty acids and amino acids can augment glucose-induced insulin secretion. In addition, various hormones, such as melatonin, estrogen, leptin, growth hormone, and glucagon like peptide-1 also regulate insulin secretion. Thus, the β-cell is a metabolic hub in the body, connecting nutrient metabolism and the endocrine system. Although an increase in intracellular [Ca2+] is the primary insulin secretary signal, cAMP signaling- dependent mechanisms are also critical in the regulation of insulin secretion. This article reviews current knowledge on how β-cells synthesize and secrete insulin. In addition, this review presents evidence that genetic and environmental factors can lead to hyperglycemia, dyslipidemia, inflammation, and autoimmunity, resulting in β-cell dysfunction, thereby triggering the pathogenesis of diabetes.

Early development of electrical excitability in the mouse enteric nervous system

Neural activity is integral to the development of the enteric nervous system (ENS). A subpopulation of neural crest-derived cells expresses pan-neuronal markers at early stages of ENS development (at E10.5 in the mouse). However, the electrical activity of these cells has not been previously characterized, and it is not known whether all cells expressing neuronal markers are capable of firing action potentials (APs). In this study, we examined the activity of "neuron"-like cells (expressing pan-neuronal markers or with neuronal morphology) in the gut of E11.5 and E12.5 mice using whole-cell patch-clamp electrophysiology and compared them to the activity of neonatal and adult enteric neurons. Around 30-40% of neuron-like cells at E11.5 and E12.5 fired APs, some of which were very similar to those of adult enteric neurons. All APs were sensitive to tetrodotoxin (TTX), indicating that they were driven by voltage-gated Na+ currents. Expression of mRNA encoding several voltage-gated Na+ channels by the E11.5 gut was detected using RT-PCR. The density of voltage-gated Na+ currents increased from E11.5 to neonates. Immature active responses, mediated in part by TTX- and lidocaine-insensitive channels, were observed in most cells at E11.5 and E12.5, but not in P0/P1 or adult neurons. However, some cells expressing neuronal markers at E11.5 or E12.5 did not exhibit an active response to depolarization. Spontaneous depolarizations resembling excitatory postsynaptic potentials were observed at E12.5. The ENS is one of the earliest parts of the developing nervous system to exhibit mature forms of electrical activity.

Glucose decouples intracellular Ca2+ activity from glucagon secretion in mouse pancreatic islet alpha-cells

The mechanisms of glucagon secretion and its suppression by glucose are presently unknown. This study investigates the relationship between intracellular calcium levels ([Ca²⁺]_i) and hormone secretion under low and high glucose conditions. We examined the effects of modulating ion channel activities on [Ca²⁺]_i and hormone secretion from ex vivo mouse pancreatic islets. Glucagon-secreting α-cells were unambiguously identified by cell specific expression of fluorescent proteins. We found that activation of L-type voltage-gated calcium channels is critical for α-cell calcium oscillations and glucagon secretion at low glucose levels. Calcium channel activation depends on K_ATP channel activity but not on tetrodotoxin-sensitive Na⁺ channels. The use of glucagon secretagogues reveals a positive correlation between α-cell [Ca²⁺]_i and secretion at low glucose levels. Glucose elevation suppresses glucagon secretion even after treatment with secretagogues. Importantly, this inhibition is not mediated by K_ATP channel activity or reduction in α-cell [Ca²⁺]_i. Our results demonstrate that glucose uncouples the positive relationship between [Ca²⁺]_i and secretory activity. We conclude that glucose suppression of glucagon secretion is not mediated by inactivation of calcium channels, but instead, it requires a calcium-independent inhibitory pathway.

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.

AMPA receptors regulate exocytosis and insulin release in pancreatic β cells

Ionotropic glutamate receptors (iGluRs) are expressed in islets and insulinoma cells and involved in insulin secretion. However, the exact roles that iGluRs play in β cells remain unclear. Here, we demonstrated that GluR2-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) were expressed in mouse β cells. Glutamate application increased both cytosolic calcium and the number of docked insulin-containing granules, which resulted in augmentation of depolarization-induced exocytosis and high-glucose-stimulated insulin release. While glutamate application directly depolarized β cells, it also induced an enormous depolarization when K(ATP) channels were available. Glutamate application reduced the conductance of K(ATP) channels and increased voltage oscillations. Moreover, actions of AMPARs were absent in Kir6.2 knock-out mice. The effects of AMPARs on K(ATP) channels were mediated by cytosolic cGMP. Taken together, our experiments uncovered a novel mechanism by which AMPARs participate in insulin release.

Regulation of calcium in pancreatic α- and β-cells in health and disease

The glucoregulatory hormones insulin and glucagon are released from the β- and α-cells of the pancreatic islets. In both cell types, secretion is secondary to firing of action potentials, Ca²⁺-influx via voltage-gated Ca²⁺-channels, elevation of [Ca²⁺]_i and initiation of Ca²⁺-dependent exocytosis. Here we discuss the mechanisms that underlie the reciprocal regulation of insulin and glucagon secretion by changes in plasma glucose, the roles played by different types of voltage-gated Ca²⁺-channel present in α- and β-cells and the modulation of hormone secretion by Ca²⁺-dependent and -independent processes. We also consider how subtle changes in Ca²⁺-signalling may have profound impact on β-cell performance and increase risk of developing type-2 diabetes.

On depolarization-evoked exocytosis as a function of calcium entry: possibilities and pitfalls

Secretion from many endocrine cells is a result of calcium-regulated exocytosis due to Ca²⁺ influx. Using the patch-clamp technique, voltage pulses can be applied to the cells to open Ca²⁺ channels, resulting in a measurable Ca²⁺ current, and evoke exocytosis, which can be seen as an increase in membrane capacitance. A common tool for evaluating the relation between Ca²⁺ influx and exocytosis is to plot the increase in capacitance (ΔC(m)) as a function of the integral of the measured Ca²⁺current (Q). When depolarizations of different lengths are imposed, the rate of exocytosis is typically higher for shorter than for longer pulses, which has been suggested to result from depletion of a granule pool or from Ca²⁺ current inactivation. It is here demonstrated that ΔC(m) as a function of Q can reveal whether Ca²⁺ current inactivation masquerades as pool depletion. Moreover, it is shown that a convex, cooperativity-like, relation between ΔC(m) and Q surprisingly cannot occur as a result of cooperative effects, but can result from delays in the exocytotic process or in Ca²⁺dynamics. An overview of expected ΔC(m)-versus-Q relations for a range of explicit situations is given, which should help in the interpretation of data of depolarization-evoked exocytosis in endocrine cells.

Ionic mechanisms and Ca2+ dynamics underlying the glucose response of pancreatic β cells: a simulation study

To clarify the mechanisms underlying the pancreatic β-cell response to varying glucose concentrations ([G]), electrophysiological findings were integrated into a mathematical cell model. The Ca²⁺ dynamics of the endoplasmic reticulum (ER) were also improved. The model was validated by demonstrating quiescent potential, burst–interburst electrical events accompanied by Ca²⁺ transients, and continuous firing of action potentials over [G] ranges of 0–6, 7–18, and >19 mM, respectively. These responses to glucose were completely reversible. The action potential, input impedance, and Ca²⁺ transients were in good agreement with experimental measurements. The ionic mechanisms underlying the burst–interburst rhythm were investigated by lead potential analysis, which quantified the contributions of individual current components. This analysis demonstrated that slow potential changes during the interburst period were attributable to modifications of ion channels or transporters by intracellular ions and/or metabolites to different degrees depending on [G]. The predominant role of adenosine triphosphate–sensitive K⁺ current in switching on and off the repetitive firing of action potentials at 8 mM [G] was taken over at a higher [G] by Ca²⁺- or Na⁺-dependent currents, which were generated by the plasma membrane Ca²⁺ pump, Na⁺/K⁺ pump, Na⁺/Ca²⁺ exchanger, and TRPM channel. Accumulation and release of Ca²⁺ by the ER also had a strong influence on the slow electrical rhythm. We conclude that the present mathematical model is useful for quantifying the role of individual functional components in the whole cell responses based on experimental findings.

Cyclic GMP kinase I modulates glucagon release from pancreatic α-cells

OBJECTIVE

The physiologic significance of the nitric oxide (NO)/cGMP signaling pathway in islets is unclear. We hypothesized that cGMP-dependent protein kinase type I (cGKI) is directly involved in the secretion of islet hormones and glucose homeostasis.

RESEARCH DESIGN AND METHODS

Gene-targeted mice that lack cGKI in islets (conventional cGKI mutants and cGKIα and Iβ rescue mice [α/βRM] that express cGKI only in smooth muscle) were studied in comparison to control (CTR) mice. cGKI expression was mapped in the endocrine pancreas by Western blot, immuno-histochemistry, and islet-specific recombination analysis. Insulin, glucagon secretion, and cytosolic Ca²(+) ([Ca²(+)](i)) were assayed by radioimmunoassay and FURA-2 measurements, respectively. Serum levels of islet hormones were analyzed at fasting and upon glucose challenge (2 g/kg) in vivo.

RESULTS

Immunohistochemistry showed that cGKI is present in α- but not in β-cells in islets of Langerhans. Mice that lack α-cell cGKI had significantly elevated fasting glucose and glucagon levels, whereas serum insulin levels were unchanged. High glucose concentrations strongly suppressed the glucagon release in CTR mice, but had only a moderate effect on islets that lacked cGKI. 8-Br-cGMP reduced stimulated [Ca²(+)](i) levels and glucagon release rates of CTR islets at 0.5 mmol/l glucose, but was without effect on [Ca²(+)](i) or hormone release in cGKI-deficient islets.

CONCLUSIONS

We propose that cGKI modulates glucagon release by suppression of [Ca²(+)](i) in α-cells.

Different roles attributed to Cav1 channel subtypes in spontaneous action potential firing and fine tuning of exocytosis in mouse chromaffin cells

J. Neurochem. (2011) 116, 105–121.

Abstract

This study examines the Cav1 isoforms expressed in mouse chromaffin cells and compares their biophysical properties and roles played in cell excitability and exocytosis. Using immunocytochemical and electrophysiological techniques in mice lacking the Cav1.3α1 subunit (Cav1.3^−/−) or the high sensitivity of Cav1.2α1 subunits to dihydropyridines, Cav1.2 and Cav1.3 channels were identified as the only Cav1 channel subtypes expressed in mouse chromaffin cells. Cav1.3 channels were activated at more negative membrane potentials and inactivated more slowly than Cav1.2 channels. Cav1 channels, mainly Cav1.2, control cell excitability by functional coupling to BK channels, revealed by nifedipine blockade of BK channels in wild type (WT) and Cav1.3^−/− cells (53% and 35%, respectively), and by the identical change in the shape of the spontaneous action potentials elicited by the dihydropyridine in both strains of mice. Cav1.2 channels also play a major role in spontaneous action potential firing, supported by the following evidence: (i) a similar percentage of WT and Cav1.3^−/− cells fired spontaneous action potentials; (ii) firing frequency did not vary between WT and Cav1.3^−/− cells; (iii) mostly Cav1.2 channels contributed to the inward current preceding the action potential threshold; and (iv) in the presence of tetrodotoxin, WT or Cav1.3^−/− cells exhibited spontaneous oscillatory activity, which was fully abolished by nifedipine perfusion. Finally, Cav1.2 and Cav1.3 channels were essential for controlling the exocytotic process at potentials above and below −10 mV, respectively. Our data reveal the key yet differential roles of Cav1.2 and Cav1.3 channels in mediating action potential firing and exocytotic events in the neuroendocrine chromaffin cell.

Unperturbed islet α-cell function examined in mouse pancreas tissue slices

Critical investigation into α-cell biology in health and diabetes has been sparse and at times inconsistent because of the technical difficulties with employing conventional strategies of isolated islets and dispersed single cells. An acute pancreas slice preparation was developed to overcome the enzymatic and mechanical perturbations inherent in conventional islet cell isolation procedures. This preparation preserves intra-islet cellular communication and islet architecture in their in situ native state. α-Cells within tissue slices were directly assessed by patch pipette and electrophysiologically characterized. The identity of the patched cells was confirmed by biocytin dye labelling and immunocytochemistry. α-Cells in mouse pancreas slices exhibited well-described features of I(Na) (excitable at physiological membrane potential), I(KATP), small cell size, low resting membrane conductance, and inducible low and high voltage-activated I(Ca), the latter correlating with exocytosis determined by capacitance measurements. In contrast to previous reports, our large unbiased sampling of α-cells revealed a wide range distribution of all of these parameters, including the amount of K(ATP) conductance, Na+ and Ca2+ current amplitudes, and capacitance changes induced by a train of depolarization pulses. The proposed pancreas slice preparation in combination with standard patch-clamping technique allowed large sampling and rapid assessment of α-cells, which revealed a wide distribution in α-cell ion channel properties. This specific feature explains the apparent inconsistency of previous reports on these α-cell ion channel properties. Our innovative approach will enable future studies into elucidating islet α-cell dysregulation occurring during diabetes.