Capacitance measurements of exocytosis in mouse pancreatic alpha-, beta- and delta-cells within intact islets of Langerhans

The impact of GLP-1 signalling on the energy metabolism of pancreatic islet β-cells and extrapancreatic tissues

Glucagon-like peptide-1 signalling impacts glucose homeostasis and appetite thereby indirectly affecting substrate availability at the whole-body level. The incretin canonically produces an insulinotropic effect, thereby lowering blood glucose levels by promoting the uptake and inhibiting the production of the sugar by peripheral tissues. Likewise, GLP-1 signalling within the central nervous system reduces the appetite and food intake, whereas its gastric effect delays the absorption of nutrients, thus improving glycaemic control and reducing the risk of postprandial hyperglycaemia. We review the molecular aspects of the GLP-1 signalling, focusing on its impact on intracellular energy metabolism. Whilst the incretin exerts its effects predominantly via a Gs receptor, which decodes the incretin signal into the elevation of intracellular cAMP levels, the downstream signalling cascades within the cell, acting on fast and slow timescales, resulting in an enhancement or an attenuation of glucose catabolism, respectively.

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.

Loss of tetraspanin-7 expression reduces pancreatic β-cell exocytosis Ca2+ sensitivity but has limited effect on systemic metabolism

BACKGROUND

Tetraspanin-7 (Tspan7) is an islet autoantigen involved in autoimmune type 1 diabetes and known to regulate β-cell L-type Ca²⁺ channel activity. However, the role of Tspan7 in pancreatic β-cell function is not yet fully understood.

METHODS

Histological analyses were conducted using immunostaining. Whole-body metabolism was tested using glucose tolerance test. Islet hormone secretion was quantified using static batch incubation or dynamic perifusion. β-cell transmembrane currents, electrical activity and exocytosis were measured using whole-cell patch-clamping and capacitance measurements. Gene expression was studied using mRNA-sequencing and quantitative PCR.

RESULTS

Tspan7 is expressed in insulin-containing granules of pancreatic β-cells and glucagon-producing α-cells. Tspan7 knockout mice (Tspan7^y/- mouse) exhibit reduced body weight and ad libitum plasma glucose but normal glucose tolerance. Tspan7^y/- islets have normal insulin content and glucose- or tolbutamide-stimulated insulin secretion. Depolarisation-triggered Ca²⁺ current was enhanced in Tspan7^y/- β-cells, but β-cell electrical activity and depolarisation-evoked exocytosis were unchanged suggesting that exocytosis was less sensitive to Ca²⁺ . TSPAN7 knockdown (KD) in human pseudo-islets led to a significant reduction in insulin secretion stimulated by 20 mM K⁺ . Transcriptomic analyses show that TSPAN7 KD in human pseudo-islets correlated with changes in genes involved in hormone secretion, apoptosis and ER stress. Consistent with rodent β-cells, exocytotic Ca²⁺ sensitivity was reduced in a human β-cell line (EndoC-βH1) following Tspan7 KD.

CONCLUSION

Tspan7 is involved in the regulation of Ca²⁺ -dependent exocytosis in β-cells. Its function is more significant in human β-cells than their rodent counterparts.

The changing view of insulin granule mobility: From conveyor belt to signaling hub

Before the advent of TIRF microscopy the fate of the insulin granule prior to secretion was deduced from biochemical investigations, electron microscopy and electrophysiological measurements. Since Calcium-triggered granule fusion is indisputably necessary to release insulin into the extracellular space, much effort was directed to the measure this event at the single granule level. This has also been the major application of the TIRF microscopy of the pancreatic beta cell when it became available about 20 years ago. To better understand the metabolic modulation of secretion, we were interested to characterize the entirety of the insulin granules which are localized in the vicinity of the plasma membrane to identify the characteristics which predispose to fusion. In this review we concentrate on how the description of granule mobility in the submembrane space has evolved as a result of progress in methodology. The granules are in a state of constant turnover with widely different periods of residence in this space. While granule fusion is associated +with prolonged residence and decreased lateral mobility, these characteristics may not only result from binding to the plasma membrane but also from binding to the cortical actin web, which is present in the immediate submembrane space. While granule age as such affects granule mobility and fusion probability, the preceding functional states of the beta cell leave their mark on these parameters, too. In summary, the submembrane granules form a highly dynamic heterogeneous population and contribute to the metabolic memory of the beta cells.

Gap junction coupling and islet delta-cell function in health and disease

The pancreatic islets contain beta-cells and alpha-cells, which are responsible for secreting two principal gluco-regulatory hormones; insulin and glucagon, respectively. However, they also contain delta-cells, a relatively sparse cell type that secretes somatostatin (SST). These cells have a complex morphology allowing them to establish an extensive communication network throughout the islet, despite their scarcity. Delta-cells are electrically excitable cells, and SST secretion is released in a glucose- and K_ATP-dependent manner. SST hyperpolarises the alpha-cell membrane and suppresses exocytosis. In this way, islet SST potently inhibits glucagon release. Recent studies investigating the activity of delta-cells have revealed they are electrically coupled to beta-cells via gap junctions, suggesting the delta-cell is more than just a paracrine inhibitor. In this Review, we summarize delta-cell morphology, function, and the role of SST signalling for regulating islet hormonal output. A distinguishing feature of this Review is that we attempt to use the discovery of this gap junction pathway, together with what is already known about delta-cells, to reframe the role of these cells in both health and disease. In particular, we argue that the discovery of gap junction communication between delta-cells and beta-cells provides new insights into the contribution of delta-cells to the islet hormonal defects observed in both type 1 and type 2 diabetes. This reappraisal of the delta-cell is important as it may offer novel insights into how the physiology of this cell can be utilised to restore islet function in diabetes.

Somatostatin receptors in congenital hyperinsulinism: Biology to bedside

Congenital hyperinsulinism (CHI), although a rare disease, is an important cause of severe hypoglycemia in early infancy and childhood, causing preventable morbidity and mortality. Prompt diagnosis and appropriate treatment is necessary to prevent hypoglycaemia mediated brain damage. At present, the medical treatment of CHI is limited to diazoxide as first line and synthetic somatostatin receptor ligands (SRLs) as second line options; therefore understanding somatostatin biology and treatment perspectives is important. Under healthy conditions, somatostatin secreted from pancreatic islet δ-cells reduces insulin release through somatostatin receptor induced cAMP-mediated downregulation and paracrine inhibition of β- cells. Several SRLs with extended duration of action are now commercially available and are being used off-label in CHI patients. Efficacy remains variable with the present generation of SRLs, with treatment effect often being compromised by loss of initial response and adverse effects such as bowel ischaemia and hepatobiliary dysfunction. In this review we have addressed the biology of the somatostatin system contexualised to CHI. We have discussed the clinical use, limitations, and complications of somatostatin agonists and new and emerging therapies for CHI.

Dynamin deficiency causes insulin secretion failure and hyperglycemia

Pancreatic β cells operate with a high rate of membrane recycling for insulin secretion, yet endocytosis in these cells is not fully understood. We investigate this process in mature mouse β cells by genetically deleting dynamin GTPase, the membrane fission machinery essential for clathrin-mediated endocytosis. Unexpectedly, the mice lacking all three dynamin genes (DNM1, DNM2, DNM3) in their β cells are viable, and their β cells still contain numerous insulin granules. Endocytosis in these β cells is severely impaired, resulting in abnormal endocytic intermediates on the plasma membrane. Although insulin granules are abundant, their release upon glucose stimulation is blunted in both the first and second phases, leading to hyperglycemia and glucose intolerance in mice. Dynamin triple deletion impairs insulin granule exocytosis and decreases intracellular Ca2+ responses and granule docking. The docking defect is correlated with reduced expression of Munc13-1 and RIM1 and reorganization of cortical F-actin in β cells. Collectively, these findings uncover the role of dynamin in dense-core vesicle endocytosis and secretory capacity. Insulin secretion deficiency in the absence of dynamin-mediated endocytosis highlights the risk of impaired membrane trafficking in endocrine failure and diabetes pathogenesis.

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.

Drp1-dependent mitochondrial fission in cardiovascular disease

Mitochondria are highly dynamic organelles undergoing cycles of fusion and fission to modulate their morphology, distribution, and function, which are referred as 'mitochondrial dynamics'. Dynamin-related protein 1 (Drp1) is known as the major pro-fission protein whose activity is tightly regulated to clear the damaged mitochondria via mitophagy, ensuring a strict control over the intricate process of cellular and organ dynamics in heart. Various posttranslational modifications (PTMs) of Drp1 have been identified including phosphorylation, SUMOylation, palmitoylation, ubiquitination, S-nitrosylation, and O-GlcNAcylation, which implicate a role in the regulation of mitochondrial dynamics. An intact mitochondrial homeostasis is critical for heart to fuel contractile function and cardiomyocyte metabolism, while defects in mitochondrial dynamics constitute an essential part of the pathophysiology underlying various cardiovascular diseases (CVDs). In this review, we summarize current knowledge on the critical role of Drp1 in the pathogenesis of CVDs including endothelial dysfunction, smooth muscle remodeling, cardiac hypertrophy, pulmonary arterial hypertension, myocardial ischemia-reperfusion, and myocardial infarction. We also highlight how the targeting of Drp1 could potentially contribute to CVDs treatments.

Calcium Oscillations in Pancreatic α-cells Rely on Noise and ATP-Driven Changes in Membrane Electrical Activity

In pancreatic α-cells, intracellular Ca²⁺ ([Ca²⁺]_i) acts as a trigger for secretion of glucagon, a hormone that plays a key role in blood glucose homeostasis. Intracellular Ca²⁺ dynamics in these cells are governed by the electrical activity of voltage-gated ion channels, among which ATP-sensitive K⁺ (K_ATP) channels play a crucial role. In the majority of α-cells, the global Ca²⁺ response to lowering external glucose occurs in the form of oscillations that are much slower than electrical activity. These Ca²⁺ oscillations are highly variable as far as inter-spike intervals, shapes and amplitudes are concerned. Such observations suggest that Ca²⁺ dynamics in α-cells are much influenced by noise. Actually, each Ca²⁺ increase corresponds to multiple cycles of opening/closing of voltage gated Ca²⁺ channels that abruptly become silent, before the occurrence of another burst of activity a few tens of seconds later. The mechanism responsible for this intermittent activity is currently unknown. In this work, we used computational modeling to investigate the mechanism of cytosolic Ca²⁺ oscillations in α-cells. Given the limited population of K_ATP channels in this cell type, we hypothesized that the stochastic activity of these channels could play a key role in the sporadic character of the action potentials. To test this assumption, we extended a previously proposed model of the α-cells electrical activity (Diderichsen and Göpel, 2006) to take Ca²⁺ dynamics into account. Including molecular noise on the basis of a Langevin type description as well as realistic dynamics of opening and closing of K_ATP channels, we found that stochasticity at the level of the activity of this channel is on its own not able to produce Ca²⁺ oscillations with a time scale of a few tens of seconds. However, when taking into account the intimate relation between Ca²⁺ and ATP changes together with the intrinsic noise at the level of the K_ATP channels, simulations displayed Ca²⁺ oscillations that are compatible with experimental observations. We analyzed the detailed mechanism and used computational simulations to identify the factors that can affect Ca²⁺ oscillations in α-cells.

Glucokinase intrinsically regulates glucose sensing and glucagon secretion in pancreatic alpha cells

The secretion of glucagon by pancreatic alpha cells is regulated by a number of external and intrinsic factors. While the electrophysiological processes linking a lowering of glucose concentrations to an increased glucagon release are well characterized, the evidence for the identity and function of the glucose sensor is still incomplete. In the present study we aimed to address two unsolved problems: (1) do individual alpha cells have the intrinsic capability to regulate glucagon secretion by glucose, and (2) is glucokinase the alpha cell glucose sensor in this scenario. Single cell RT-PCR was used to confirm that glucokinase is the main glucose-phosphorylating enzyme expressed in rat pancreatic alpha cells. Modulation of glucokinase activity by pharmacological activators and inhibitors led to a lowering or an increase of the glucose threshold of glucagon release from single alpha cells, measured by TIRF microscopy, respectively. Knockdown of glucokinase expression resulted in a loss of glucose control of glucagon secretion. Taken together this study provides evidence for a crucial role of glucokinase in intrinsic glucose regulation of glucagon release in rat alpha cells.

Cav2.3 channel function and Zn2+-induced modulation: potential mechanisms and (patho)physiological relevance

Voltage-gated calcium channels (VGCCs) are critical for Ca²⁺ influx into all types of excitable cells, but their exact function is still poorly understood. Recent reconstruction of homology models for all human VGCCs at atomic resolution provides the opportunity for a structure-based discussion of VGCC function and novel insights into the mechanisms underlying Ca²⁺ selective flux through these channels. In the present review, we use these data as a basis to examine the structure, function, and Zn²⁺-induced modulation of Ca_v2.3 VGCCs, which mediate native R-type currents and belong to the most enigmatic members of the family. Their unique sensitivity to Zn²⁺ and the existence of multiple mechanisms of Zn²⁺ action strongly argue for a role of these channels in the modulatory action of endogenous loosely bound Zn²⁺, pools of which have been detected in a number of neuronal, endocrine, and reproductive tissues. Following a description of the different mechanisms by which Zn²⁺ has been shown or is thought to alter the function of these channels, we discuss their potential (patho)physiological relevance, taking into account what is known about the magnitude and function of extracellular Zn²⁺ signals in different tissues. While still far from complete, the picture that emerges is one where Ca_v2.3 channel expression parallels the occurrence of loosely bound Zn²⁺ pools in different tissues and where these channels may serve to translate physiological Zn²⁺ signals into changes of electrical activity and/or intracellular Ca²⁺ levels.

Impaired insulin exocytosis in chronic hepatitis C infection: contributory role of p38δ MAPK-protein kinase D-golgi complex axis

Hepatitis C virus (HCV) infection and chronic hepatitis C (CHC) are associated with a measurable risk of insulin resistance (IR)/impaired glucose tolerance (IGT)/diabetes mellitus (DM). While loss of hepatic endocrine function contributes to liver cirrhosis in diabetic patients, onset and progression of IR/IGT to diabetes and exacerbation of incident hyperglycemia are ostensibly linked with chronic HCV infection. In this regard, the study by Chen J et al. appearing in Clinical Science (2020) (134(5) https://doi.org/10.1042/CS20190900) attempts to understand the mechanisms underlying the savaging effects of chronic HCV infection on insulin-producing pancreatic β-cells and hence diabetic onset. The study investigated the role of mitogen-activated protein kinase (MAPK) p38δ-protein kinase D (PKD)-golgi complex axis in impacting insulin exocytosis. It was inferred that an insulin secretory defect of pancreatic β-cells, owing to disrupted insulin exocytosis, to an extent explains β-cell dysfunction in HCV-infected or CHC milieu. HCV infection negatively regulates first-phase and second-phase insulin secretion by impinging on PKD-dependent insulin secretory granule fission at trans-golgi network and insulin secretory vesicle membrane fusion events. This commentary highlights the study in question, that deciphered the contribution of p38δ MAPK-PKD-golgi complex axis to β-cell dysfunction in CHC milieu. This pivotal axis proffers a formidable therapeutic opportunity for alleviation of double burden of glucose abnormalities/DM and CHC.

Cellular and systemic mechanisms for glucose sensing and homeostasis

Glucose is a major source of energy in animals. Maintaining blood glucose levels within a physiological range is important for facilitating glucose uptake by cells, as required for optimal functioning. Glucose homeostasis relies on multiple glucose-sensing cells in the body that constantly monitor blood glucose levels and respond accordingly to adjust its glycemia. These include not only pancreatic β-cells and α-cells that secrete insulin and glucagon, but also central and peripheral neurons regulating pancreatic endocrine function. Different types of cells respond distinctively to changes in blood glucose levels, and the mechanisms involved in glucose sensing are diverse. Notably, recent studies have challenged the currently held views regarding glucose-sensing mechanisms. Furthermore, peripheral and central glucose-sensing cells appear to work in concert to control blood glucose level and maintain glucose and energy homeostasis in organisms. In this review, we summarize the established concepts and recent advances in the understanding of cellular and systemic mechanisms that regulate glucose sensing and its homeostasis.

Exocytotic fusion pore under stress

Exocytosis is a universal process of eukaryotic cells, consisting of fusion between the vesicle and the plasma membranes, leading to the formation of a fusion pore, a channel through which vesicle cargo exits into the extracellular space. In 1986, Rand and Parsegian proposed several stages to explain the nature of membrane fusion. Following stimulation, it starts with focused stress destabilization of membranes in contact, followed by the coalescence of two membrane surfaces. In the next fraction of a millisecond, restabilization of fused membranes is considered to occur to maintain the cell's integrity. This view predicted that once a fusion pore is formed, it must widen abruptly, irreversibly and fully, whereby the vesicle membrane completely integrates with and collapses into the plasma membrane (full fusion exocytosis). However, recent experimental evidence has revealed that once the fusion pore opens, it may also reversibly close (transient or kiss-and-run exocytosis). Here, we present a historical perspective on understanding the mechanisms that initiate the membrane merger and fusion pore formation. Next, post-fusion mechanisms that regulate fusion pore stability are considered, reflecting the state in which the forces of widening and constriction of fusion pores are balanced. Although the mechanisms generating these forces are unclear, they may involve lipids and proteins, including SNAREs, which play a role not only in the pre-fusion but also post-fusion stages of exocytosis. How molecules stabilize the fusion pore in the open state is key for a better understanding of fusion pore physiology in health and disease.

'Resistance is futile?' - paradoxical inhibitory effects of KATP channel closure in glucagon-secreting α-cells

By secreting insulin and glucagon, the β- and α-cells of the pancreatic islets play a central role in the regulation of systemic metabolism. Both cells are equipped with ATP-regulated potassium (K_ATP ) channels that are regulated by the intracellular ATP/ADP ratio. In β-cells, K_ATP channels are active at low (non-insulin-releasing) glucose concentrations. An increase in glucose leads to K_ATP channel closure, membrane depolarization and electrical activity that culminates in elevation of [Ca²⁺ ]_i and initiation of exocytosis of the insulin-containing secretory granules. The α-cells are also equipped with K_ATP channels but they are under strong tonic inhibition at low glucose, explaining why α-cells are electrically active under hypoglycaemic conditions and generate large Na⁺ - and Ca²⁺ -dependent action potentials. Closure of residual K_ATP channel activity leads to membrane depolarization and an increase in action potential firing but this stimulation of electrical activity is associated with inhibition rather than acceleration of glucagon secretion. This paradox arises because membrane depolarization reduces the amplitude of the action potentials by voltage-dependent inactivation of the Na⁺ channels involved in action potential generation. Exocytosis in α-cells is tightly linked to the opening of voltage-gated P/Q-type Ca²⁺ channels, the activation of which is steeply voltage-dependent. Accordingly, the inhibitory effect of the reduced action potential amplitude exceeds the stimulatory effect resulting from the increased action potential frequency. These observations highlight a previously unrecognised role of the action potential amplitude as a key regulator of pancreatic islet hormone secretion.

The Role of α-Cells in Islet Function and Glucose Homeostasis in Health and Type 2 Diabetes

Pancreatic α-cells are the major source of glucagon, a hormone that counteracts the hypoglycemic action of insulin and strongly contributes to the correction of acute hypoglycemia. The mechanisms by which glucose controls glucagon secretion are hotly debated, and it is still unclear to what extent this control results from a direct action of glucose on α-cells or is indirectly mediated by β- and/or δ-cells. Besides its hyperglycemic action, glucagon has many other effects, in particular on lipid and amino acid metabolism. Counterintuitively, glucagon seems also required for an optimal insulin secretion in response to glucose by acting on its cognate receptor and, even more importantly, on GLP-1 receptors. Patients with diabetes mellitus display two main alterations of glucagon secretion: a relative hyperglucagonemia that aggravates hyperglycemia, and an impaired glucagon response to hypoglycemia. Under metabolic stress states, such as diabetes, pancreatic α-cells also secrete GLP-1, a glucose-lowering hormone, whereas the gut can produce glucagon. The contribution of extrapancreatic glucagon to the abnormal glucose homeostasis is unclear. Here, I review the possible mechanisms of control of glucagon secretion and the role of α-cells on islet function in healthy state. I discuss the possible causes of the abnormal glucagonemia in diabetes, with particular emphasis on type 2 diabetes, and I briefly comment the current antidiabetic therapies affecting α-cells.

Glucose stimulates somatostatin secretion in pancreatic δ-cells by cAMP-dependent intracellular Ca2+ release

Somatostatin secretion from pancreatic islet δ-cells is stimulated by elevated glucose levels, but the underlying mechanisms have only partially been elucidated. Here we show that glucose-induced somatostatin secretion (GISS) involves both membrane potential-dependent and -independent pathways. Although glucose-induced electrical activity triggers somatostatin release, the sugar also stimulates GISS via a cAMP-dependent stimulation of CICR and exocytosis of somatostatin. The latter effect is more quantitatively important and in mouse islets depolarized by 70 mM extracellular K+ , increasing glucose from 1 mM to 20 mM produced an ∼3.5-fold stimulation of somatostatin secretion, an effect that was mimicked by the application of the adenylyl cyclase activator forskolin. Inhibiting cAMP-dependent pathways with PKI or ESI-05, which inhibit PKA and exchange protein directly activated by cAMP 2 (Epac2), respectively, reduced glucose/forskolin-induced somatostatin secretion. Ryanodine produced a similar effect that was not additive to that of the PKA or Epac2 inhibitors. Intracellular application of cAMP produced a concentration-dependent stimulation of somatostatin exocytosis and elevation of cytoplasmic Ca2+ ([Ca2+]i). Both effects were inhibited by ESI-05 and thapsigargin (an inhibitor of SERCA). By contrast, inhibition of PKA suppressed δ-cell exocytosis without affecting [Ca2+]i Simultaneous recordings of electrical activity and [Ca2+]i in δ-cells expressing the genetically encoded Ca2+ indicator GCaMP3 revealed that the majority of glucose-induced [Ca2+]i spikes did not correlate with δ-cell electrical activity but instead reflected Ca2+ release from the ER. These spontaneous [Ca2+]i spikes are resistant to PKI but sensitive to ESI-05 or thapsigargin. We propose that cAMP links an increase in plasma glucose to stimulation of somatostatin secretion by promoting CICR, thus evoking exocytosis of somatostatin-containing secretory vesicles in the δ-cell.

Cyclic AMP links glucose stimulation to somatostatin secretion in δ-cells

Tengholm reflects on new work providing insight into the mechanisms of glucose-stimulated somatostatin secretion from δ-cells.

Glucose controls glucagon secretion by directly modulating cAMP in alpha cells

AIMS/HYPOTHESIS

Glucagon is critical for normal glucose homeostasis and aberrant secretion of the hormone aggravates dysregulated glucose control in diabetes. However, the mechanisms by which glucose controls glucagon secretion from pancreatic alpha cells remain elusive. The aim of this study was to investigate the role of the intracellular messenger cAMP in alpha-cell-intrinsic glucose regulation of glucagon release.

METHODS

Subplasmalemmal cAMP and Ca²⁺ concentrations were recorded in isolated and islet-located alpha cells using fluorescent reporters and total internal reflection microscopy. Glucagon secretion from mouse islets was measured using ELISA.

RESULTS

Glucose induced Ca²⁺-independent alterations of the subplasmalemmal cAMP concentration in alpha cells that correlated with changes in glucagon release. Glucose-lowering-induced stimulation of glucagon secretion thus corresponded to an elevation in cAMP that was independent of paracrine signalling from insulin or somatostatin. Imposed cAMP elevations stimulated glucagon secretion and abolished inhibition by glucose elevation, while protein kinase A inhibition mimicked glucose suppression of glucagon release.

CONCLUSIONS/INTERPRETATION

Glucose concentrations in the hypoglycaemic range control glucagon secretion by directly modulating the cAMP concentration in alpha cells independently of paracrine influences. These findings define a novel mechanism for glucose regulation of glucagon release that underlies recovery from hypoglycaemia and may be disturbed in diabetes.

GLP-1 suppresses glucagon secretion in human pancreatic alpha-cells by inhibition of P/Q-type Ca2+ channels

Glucagon is the body's main hyperglycemic hormone, and its secretion is dysregulated in type 2 diabetes mellitus (T2DM). The incretin hormone glucagon-like peptide-1 (GLP-1) is released from the gut and is used in T2DM therapy. Uniquely, it both stimulates insulin and inhibits glucagon secretion and thereby lowers plasma glucose levels. In this study, we have investigated the action of GLP-1 on glucagon release from human pancreatic islets. Immunocytochemistry revealed that only <0.5% of the α-cells possess detectable GLP-1R immunoreactivity. Despite this, GLP-1 inhibited glucagon secretion by 50-70%. This was due to a direct effect on α-cells, rather than paracrine signaling, because the inhibition was not reversed by the insulin receptor antagonist S961 or the somatostatin receptor-2 antagonist CYN154806. The inhibitory effect of GLP-1 on glucagon secretion was prevented by the PKA-inhibitor Rp-cAMPS and mimicked by the adenylate cyclase activator forskolin. Electrophysiological measurements revealed that GLP-1 decreased action potential height and depolarized interspike membrane potential. Mathematical modeling suggests both effects could result from inhibition of P/Q-type Ca2+ channels. In agreement with this, GLP-1 and ω-agatoxin (a blocker of P/Q-type channels) inhibited glucagon secretion in islets depolarized by 70 mmol/L [K+ ]o , and these effects were not additive. Intracellular application of cAMP inhibited depolarization-evoked exocytosis in individual α-cells by a PKA-dependent (Rp-cAMPS-sensitive) mechanism. We propose that inhibition of glucagon secretion by GLP-1 involves activation of the few GLP-1 receptors present in the α-cell membrane. The resulting small elevation of cAMP leads to PKA-dependent inhibition of P/Q-type Ca2+ channels and suppression of glucagon exocytosis.

Chronic fractalkine administration improves glucose tolerance and pancreatic endocrine function

We have previously reported that the fractalkine (FKN)/CX3CR1 system represents a novel regulatory mechanism for insulin secretion and β cell function. Here, we demonstrate that chronic administration of a long-acting form of FKN, FKN-Fc, can exert durable effects to improve glucose tolerance with increased glucose-stimulated insulin secretion and decreased β cell apoptosis in obese rodent models. Unexpectedly, chronic FKN-Fc administration also led to decreased α cell glucagon secretion. In islet cells, FKN inhibited ATP-sensitive potassium channel conductance by an ERK-dependent mechanism, which triggered β cell action potential (AP) firing and decreased α cell AP amplitude. This results in increased glucose-stimulated insulin secretion and decreased glucagon secretion. Beyond its islet effects, FKN-Fc also exerted peripheral effects to enhance hepatic insulin sensitivity due to inhibition of glucagon action. In hepatocytes, FKN treatment reduced glucagon-stimulated cAMP production and CREB phosphorylation in a pertussis toxin-sensitive manner. Together, these results raise the possibility of use of FKN-based therapy to improve type 2 diabetes by increasing both insulin secretion and insulin sensitivity.

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.

Decrease of γ-aminobutyric acid and zinc ions in the islet periportal circulation stimulates glucagon secretion during hypoglycemia

The present study assessed the effects of γ-aminobutyric acid (GABA) from β-cells on glucose levels and glucagon secretion, and identified channels via which glucagon secretion is initiated. An in vivo experiment was performed containing three groups: Intrapancreatic artery infusion of GABA alone, GABA plus insulin or insulin alone in rats with diabetes. Rats infused with GABA and insulin were also subdivided in groups receiving additional infusion of K⁺-channel activator diazoxide (DIA), K⁺-channel blocker tolbutamide (TLB) and calcium channel blocker nifedipine (NIF). In the hypoglycemic state, termination of infusion of insulin and insulin plus GABA resulted in signaling to the α-cells to secrete glycogen, while that of GABA alone did not. However, intrapancreatic artery infusion of K⁺-channel activator DIA, K⁺-channel blocker TLB or calcium channel blocker NIF in addition to GABA and insulin had no effect on glucagon secretion. In conclusion, if the delivery of insulin or GABA plus insulin in rats with hypoglycemia is terminated, β-cells are stimulated and signal the α-cells to secrete glucagon. Thus, the detection of a sudden decrease in zinc levels by β-cells as well as a decrease in GABA in the periportal circulation induces signaling to α-cells to stimulate them to secrete glucagon.

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.

Dapagliflozin stimulates glucagon secretion at high glucose: experiments and mathematical simulations of human A-cells

Glucagon is one of the main regulators of blood glucose levels and dysfunctional stimulus secretion coupling in pancreatic A-cells is believed to be an important factor during development of diabetes. However, regulation of glucagon secretion is poorly understood. Recently it has been shown that Na⁺/glucose co-transporter (SGLT) inhibitors used for the treatment of diabetes increase glucagon levels in man. Here, we show experimentally that the SGLT2 inhibitor dapagliflozin increases glucagon secretion at high glucose levels both in human and mouse islets, but has little effect at low glucose concentrations. Because glucagon secretion is regulated by electrical activity we developed a mathematical model of A-cell electrical activity based on published data from human A-cells. With operating SGLT2, simulated glucose application leads to cell depolarization and inactivation of the voltage-gated ion channels carrying the action potential, and hence to reduce action potential height. According to our model, inhibition of SGLT2 reduces glucose-induced depolarization via electrical mechanisms. We suggest that blocking SGLTs partly relieves glucose suppression of glucagon secretion by allowing full-scale action potentials to develop. Based on our simulations we propose that SGLT2 is a glucose sensor and actively contributes to regulation of glucagon levels in humans which has clinical implications.

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

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

Alpha-, Delta- and PP-cells: Are They the Architectural Cornerstones of Islet Structure and Co-ordination?

Islet non-β-cells, the α- δ- and pancreatic polypeptide cells (PP-cells), are important components of islet architecture and intercellular communication. In α-cells, glucagon is found in electron-dense granules; granule exocytosis is calcium-dependent via P/Q-type Ca(2+)-channels, which may be clustered at designated cell membrane sites. Somatostatin-containing δ-cells are neuron-like, creating a network for intra-islet communication. Somatostatin 1-28 and 1-14 have a short bioactive half-life, suggesting inhibitory action via paracrine signaling. PP-cells are the most infrequent islet cell type. The embryologically separate ventral pancreas anlage contains PP-rich islets that are morphologically diffuse and α-cell deficient. Tissue samples taken from the head region are unlikely to be representative of the whole pancreas. PP has anorexic effects on gastro-intestinal function and alters insulin and glucagon secretion. Islet architecture is disrupted in rodent diabetic models, diabetic primates and human Type 1 and Type 2 diabetes, with an increased α-cell population and relocation of non-β-cells to central areas of the islet. In diabetes, the transdifferentiation of non-β-cells, with changes in hormone content, suggests plasticity of islet cells but cellular function may be compromised. Understanding how diabetes-related disordered islet structure influences intra-islet cellular communication could clarify how non-β-cells contribute to the control of islet function.

Two-photon fluorescence lifetime imaging of primed SNARE complexes in presynaptic terminals and β cells

It remains unclear how readiness for Ca(2+)-dependent exocytosis depends on varying degrees of SNARE complex assembly. Here we directly investigate the SNARE assembly using two-photon fluorescence lifetime imaging (FLIM) of Förster resonance energy transfer (FRET) between three pairs of neuronal SNAREs in presynaptic boutons and pancreatic β cells in the islets of Langerhans. These FRET probes functionally rescue their endogenous counterparts, supporting ultrafast exocytosis. We show that trans-SNARE complexes accumulated in the active zone, and estimate the number of complexes associated with each docked vesicle. In contrast, SNAREs were unassembled in resting state, and assembled only shortly prior to insulin exocytosis, which proceeds slowly. We thus demonstrate that distinct states of fusion readiness are associated with SNARE complex formation. Our FRET/FLIM approaches enable optical imaging of fusion readiness in both live and chemically fixed tissues.

Synaptotagmin-7 phosphorylation mediates GLP-1-dependent potentiation of insulin secretion from β-cells

Glucose stimulates insulin secretion from β-cells by increasing intracellular Ca(2+). Ca(2+) then binds to synaptotagmin-7 as a major Ca(2+) sensor for exocytosis, triggering secretory granule fusion and insulin secretion. In type-2 diabetes, insulin secretion is impaired; this impairment is ameliorated by glucagon-like peptide-1 (GLP-1) or by GLP-1 receptor agonists, which improve glucose homeostasis. However, the mechanism by which GLP-1 receptor agonists boost insulin secretion remains unclear. Here, we report that GLP-1 stimulates protein kinase A (PKA)-dependent phosphorylation of synaptotagmin-7 at serine-103, which enhances glucose- and Ca(2+)-stimulated insulin secretion and accounts for the improvement of glucose homeostasis by GLP-1. A phospho-mimetic synaptotagmin-7 mutant enhances Ca(2+)-triggered exocytosis, whereas a phospho-inactive synaptotagmin-7 mutant disrupts GLP-1 potentiation of insulin secretion. Our findings thus suggest that synaptotagmin-7 is directly activated by GLP-1 signaling and may serve as a drug target for boosting insulin secretion. Moreover, our data reveal, to our knowledge, the first physiological modulation of Ca(2+)-triggered exocytosis by direct phosphorylation of a synaptotagmin.

Pancreatic alpha-cells from female mice undergo morphofunctional changes during compensatory adaptations of the endocrine pancreas to diet-induced obesity

Obesity is frequently associated with insulin resistance. To compensate for this situation and maintain normoglycaemia, pancreatic beta-cells undergo several morphofunctional adaptations, which result in insulin hypersecretion and hyperinsulinaemia. However, no information exists about pancreatic alpha-cells during this compensatory stage of obesity. Here, we studied alpha-cells in mice fed a high-fat diet (HFD) for 12 weeks. These animals exhibited hyperinsulinaemia and normoglycaemia compared with control animals in addition to hypoglucagonaemia. While the in vivo response of glucagon to hypoglycaemia was preserved in the obese mice, the suppression of glucagon secretion during hyperglycaemia was impaired. Additionally, in vitro glucagon release at low glucose levels and glucagon content in isolated islets were decreased, while alpha-cell exocytosis remained unchanged. Assessment of morphological parameters revealed that alpha-cell area was reduced in the pancreas of the obese mice in association with alpha-cell hypotrophy, increased apoptosis and decreased proliferation. HFD feeding for 24 weeks led to significant deterioration in beta-cell function and glucose homeostasis. Under these conditions, the majority of alpha-cell changes were reversed and became comparable to controls. These findings indicate that pancreatic compensatory adaptations during obesity may also involve pancreatic alpha-cells. Additionally, defects in alpha-cell function during obesity may be implicated in progression to diabetes.

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

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

Enhanced glucose-induced intracellular signaling promotes insulin hypersecretion: pancreatic beta-cell functional adaptations in a model of genetic obesity and prediabetes

Obesity is associated with insulin resistance and is known to be a risk factor for type-2 diabetes. In obese individuals, pancreatic beta-cells try to compensate for the increased insulin demand in order to maintain euglycemia. Most studies have reported that this adaptation is due to morphological changes. However, the involvement of beta-cell functional adaptations in this process needs to be clarified. For this purpose, we evaluated different key steps in the glucose-stimulated insulin secretion (GSIS) in intact islets from female ob/ob obese mice and lean controls. Obese mice showed increased body weight, insulin resistance, hyperinsulinemia, glucose intolerance and fed hyperglycemia. Islets from ob/ob mice exhibited increased glucose-induced mitochondrial activity, reflected by enhanced NAD(P)H production and mitochondrial membrane potential hyperpolarization. Perforated patch-clamp examination of beta-cells within intact islets revealed several alterations in the electrical activity such as increased firing frequency and higher sensitivity to low glucose concentrations. A higher intracellular Ca(2+) mobilization in response to glucose was also found in ob/ob islets. Additionally, they displayed a change in the oscillatory pattern and Ca(2+) signals at low glucose levels. Capacitance experiments in intact islets revealed increased exocytosis in individual ob/ob beta-cells. All these up-regulated processes led to increased GSIS. In contrast, we found a lack of beta-cell Ca(2+) signal coupling, which could be a manifestation of early defects that lead to beta-cell malfunction in the progression to diabetes. These findings indicate that beta-cell functional adaptations are an important process in the compensatory response to obesity.

Increased biogenesis of glucagon-containing secretory granules and glucagon secretion in BIG3-knockout mice

OBJECTIVE

Although both insulin and glucagon are intimately involved in the regulation of glucose homeostasis, the intrinsic control of glucagon secretion, including the biogenesis and exocytosis of glucagon-containing granules, is far less understood compared with that of insulin. As Brefeldin A-inhibited guanine nucleotide exchange protein 3 (BIG3) is a negative regulator of insulin-granule biogenesis and insulin secretion, we investigated whether BIG3 plays any role in alpha-cells and glucagon secretion.

METHODS

We examined the expression of BIG3 in islet cells by immuno-fluorescence and confocal microscopy, and measured glucagon production and secretion in BIG3-depleted and wild-type mice, islets and cells.

RESULTS

BIG3 is highly expressed in pancreatic alpha-cells in addition to beta-cells, but is absent in delta-cells. Depletion of BIG3 in alpha-cells leads to elevated glucagon production and secretion. Consistently, BIG3-knockout (BKO) mice display increased glucagon release under hypoglycemic conditions.

CONCLUSIONS

Together with our previous studies, the current data reveal a conserved role for BIG3 in regulating alpha- and beta-cell functions. We propose that BIG3 negatively regulates hormone production at the secretory granule biogenesis stage and that such regulatory mechanism may be used in secretory pathways of other endocrine cells.

Differential stimulation of insulin secretion by GLP-1 and Kisspeptin-10

β-cells in the pancreatic islet respond to elevated plasma glucose by secreting insulin to maintain glucose homeostasis. In addition to glucose stimulation, insulin secretion is modulated by numerous G-protein coupled receptors (GPCRs). The GPCR ligands Kisspeptin-10 (KP) and glucagon-like peptide-1 (GLP-1) potentiate insulin secretion through G_q and G_s-coupled receptors, respectively. Despite many studies, the signaling mechanisms by which KP and GLP-1 potentiate insulin release are not thoroughly understood. We investigated the downstream signaling pathways of these ligands and their affects on cellular redox potential, intracellular calcium activity ([Ca²⁺]_i), and insulin secretion from β-cells within intact murine islets. In contrast to previous studies performed on single β-cells, neither KP nor GLP-1 affect [Ca²⁺]_i upon stimulation with glucose. KP significantly increases the cellular redox potential, while no effect is observed with GLP-1, suggesting that KP and GLP-1 potentiate insulin secretion through different mechanisms. Co-treatment with KP and the G_βγ-subunit inhibitor gallein inhibits insulin secretion similar to that observed with gallein alone, while co-treatment with gallein and GLP-1 does not differ from GLP-1 alone. In contrast, co-treatment with the G_βγ activator mSIRK and either KP or GLP-1 stimulates insulin release similar to mSIRK alone. Neither gallein nor mSIRK alter [Ca²⁺]_i activity in the presence of KP or GLP-1. These data suggest that KP likely alters insulin secretion through a G_βγ-dependent process that stimulates glucose metabolism without altering Ca²⁺ activity, while GLP-1 does so, at least partly, through a G_α-dependent pathway that is independent of both metabolism and Ca²⁺.

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.

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

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

Neurotransmitter control of islet hormone pulsatility

Pulsatile secretion is an inherent property of hormone-releasing pancreatic islet cells. This secretory pattern is physiologically important and compromised in diabetes. Neurotransmitters released from islet cells may shape the pulses in auto/paracrine feedback loops. Within islets, glucose-stimulated β-cells couple via gap junctions to generate synchronized insulin pulses. In contrast, α- and δ-cells lack gap junctions, and glucagon release from islets stimulated by lack of glucose is non-pulsatile. Increasing glucose concentrations gradually inhibit glucagon secretion by α-cell-intrinsic mechanism/s. Further glucose elevation will stimulate pulsatile insulin release and co-secretion of neurotransmitters. Excitatory ATP may synchronize β-cells with δ-cells to generate coinciding pulses of insulin and somatostatin. Inhibitory neurotransmitters from β- and δ-cells can then generate antiphase pulses of glucagon release. Neurotransmitters released from intrapancreatic ganglia are required to synchronize β-cells between islets to coordinate insulin pulsatility from the entire pancreas, whereas paracrine intra-islet effects still suffice to explain coordinated pulsatile release of glucagon and somatostatin. The present review discusses how neurotransmitters contribute to the pulsatility at different levels of integration.

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.

Carbachol-induced colonic mucus formation requires transport via NKCC1, K⁺ channels and CFTR

The colonic mucosa protects itself from the luminal content by secreting mucus that keeps the bacteria at a distance from the epithelium. For this barrier to be effective, the mucus has to be constantly replenished which involves exocytosis and expansion of the secreted mucins. Mechanisms involved in regulation of mucus exocytosis and expansion are poorly understood, and the aim of this study was to investigate whether epithelial anion secretion regulates mucus formation in the colon. The muscarinic agonist carbachol was used to induce parallel secretion of anions and mucus, and by using established inhibitors of ion transport, we studied how inhibition of epithelial transport affected mucus formation in mouse colon. Anion secretion and mucin exocytosis were measured by changes in membrane current and epithelial capacitance, respectively. Mucus thickness measurements were used to determine the carbachol effect on mucus growth. The results showed that the carbachol-induced increase in membrane current was dependent on NKCC1 co-transport, basolateral K(+) channels and Cftr activity. In contrast, the carbachol-induced increase in capacitance was partially dependent on NKCC1 and K(+) channel activity, but did not require Cftr activity. Carbachol also induced an increase in mucus thickness that was inhibited by the NKCC1 blocker bumetanide. However, mice that lacked a functional Cftr channel did not respond to carbachol with an increase in mucus thickness, suggesting that carbachol-induced mucin expansion requires Cftr channel activity. In conclusion, these findings suggest that colonic epithelial transport regulates mucus formation by affecting both exocytosis and expansion of the mucin molecules.

Insulin secretion from beta cells in intact mouse islets is targeted towards the vasculature

AIMS/HYPOTHESIS

We set out to test the hypothesis that insulin secretion from beta cells is targeted towards the vasculature.

METHODS

The spatial location of granule fusion was identified by live-cell two-photon imaging of mouse pancreatic beta cells within intact islets, using sulforhodamine B labelling. Three-dimensional (3D) immunofluorescence of pancreatic slices was used to identify the location of proteins associated with neuronal synapses.

RESULTS

We demonstrated an asymmetric, non-random, distribution of sites of insulin granule fusion in response to glucose and focal targeting of insulin granule secretion to the beta cell membrane facing the vasculature. 3D immunofluorescence of islets showed that structural proteins, such as liprin, piccolo and Rab2-interacting molecule, normally associated with neuronal presynaptic targeting, were present in beta cells and enriched at the vascular face. In contrast, we found that syntaxin 1A and synaptosomal-associated protein 25 kDa (SNAP25) were relatively evenly distributed across the beta cells.

CONCLUSIONS/INTERPRETATION

Our results show that beta cells in situ, within intact islets, are polarised and target insulin secretion. This evidence for an 'endocrine synapse' has wide implications for our understanding of stimulus-secretion coupling in healthy islets and in disease.

The secretory deficit in islets from db/db mice is mainly due to a loss of responding beta cells

AIMS/HYPOTHESIS

We used the db/db mouse to determine the nature of the secretory defect in intact islets.

METHODS

Glucose tolerance was compared in db/db and wild-type (WT) mice. Isolated islets were used: to measure insulin secretion and calcium in a two-photon assay of single-insulin-granule fusion; and for immunofluorescence of soluble N-ethylmaleimide-sensitive factor attachment proteins (SNAREs).

RESULTS

The 13-18-week-old db/db mice showed a diabetic phenotype. Isolated db/db islets showed a 77% reduction in insulin secretion induced by 15 mmol/l glucose and reductions in the amplitude and rise-time of the calcium response to glucose. Ionomycin-induced insulin secretion in WT but not db/db islets. Immunofluorescence showed an increase in the levels of the SNAREs synaptosomal-associated protein 25 (SNAP25) and vesicle-associated membrane protein 2 (VAMP2) in db/db islets, but reduced syntaxin-1A. Therefore, db/db islets have both a compromised calcium response to glucose and a compromised secretory response to calcium. Two-photon microscopy of isolated islets determined the number and distribution of insulin granule exocytic events. Compared with WT, db/db islets showed far fewer exocytic events (an 83% decline at 15 mmol/l glucose). This decline was due to a 73% loss of responding cells and, in the remaining responsive cells, a 50% loss of exocytic responses per cell. An assay measuring granule re-acidification showed evidence for more recaptured granules in db/db islets compared with WT.

CONCLUSIONS/INTERPRETATION

We showed that db/db islets had a reduced calcium response to glucose and a reduction in syntaxin-1A. Within the db/db islets, changes were manifest as both a reduction in responding cells and a reduction in fusing insulin granules per cell.

Distinct actions of Rab3 and Rab27 GTPases on late stages of exocytosis of insulin

Rab GTPases associated with insulin-containing secretory granules (SGs) are key in targeting, docking and assembly of molecular complexes governing pancreatic β-cell exocytosis. Four Rab3 isoforms along with Rab27A are associated with insulin granules, yet elucidation of the distinct roles of these Rab families on exocytosis remains unclear. To define specific actions of these Rab families we employ Rab3GAP and/or EPI64A GTPase-activating protein overexpression in β-cells from wild-type or Ashen mice to selectively transit the entire Rab3 family or Rab27A to a GDP-bound state. Ashen mice carry a spontaneous mutation that eliminates Rab27A expression. Using membrane capacitance measurements we find that GTP/GDP nucleotide cycling of Rab27A is essential for generation of the functionally defined immediately releasable pool (IRP) and central to regulating the size of the readily releasable pool (RRP). By comparison, nucleotide cycling of Rab3 GTPases, but not of Rab27A, is essential for a kinetically rapid filling of the RRP with SGs. Aside from these distinct functions, Rab3 and Rab27A GTPases demonstrate considerable functional overlap in building the readily releasable granule pool. Hence, while Rab3 and Rab27A cooperate to generate release-ready SGs in β-cells, they also direct unique kinetic and functional properties of the exocytotic pathway.

Nutrient regulation of glucagon secretion: involvement in metabolism and diabetes

Glucose homeostasis is precisely regulated by glucagon and insulin, which are released by pancreatic α- and β-cells, respectively. While β-cells have been the focus of intense research, less is known about α-cell function and the actions of glucagon. In recent years, the study of this endocrine cell type has experienced a renewed drive. The present review contains a summary of established concepts as well as new information about the regulation of α-cells by glucose, amino acids, fatty acids and other nutrients, focusing especially on glucagon release, glucagon synthesis and α-cell survival. We have also discussed the role of glucagon in glucose homeostasis and in energy and lipid metabolism as well as its potential as a modulator of food intake and body weight. In addition to the well-established action on the liver, we discuss the effects of glucagon in other organs, where the glucagon receptor is expressed. These tissues include the heart, kidneys, adipose tissue, brain, small intestine and the gustatory epithelium. Alterations in α-cell function and abnormal glucagon concentrations are present in diabetes and are thought to aggravate the hyperglycaemic state of diabetic patients. In this respect, several experimental approaches in diabetic models have shown important beneficial results in improving hyperglycaemia after the modulation of glucagon secretion or action. Moreover, glucagon receptor agonism has also been used as a therapeutic strategy to treat obesity.

A novel framework for segmentation of secretory granules in electron micrographs

It is still a standard practice for biologists to manually analyze transmission electron microscopy images. This is not only time consuming but also not reproducible and prone to induce subjective bias. For large-scale studies of insulin granules inside beta cells of the islet of Langerhans, an automated method for analysis is essential. Due to the complex structure of the images, standard microscopy segmentation techniques cannot be applied. We present a new approach to segment and measure transmission electron microscopy images of insulin granule cores and membranes from beta cells of rat islets of Langerhans. The algorithm is separated into two broad components, core segmentation and membrane segmentation. Core segmentation proceeds through three steps: pre-segmentation using a novel level-set active contour, morphological cleaning and a refining segmentation on each granule using a novel dual level-set active contour. Membrane segmentation is achieved in four steps: morphological cleaning, membrane sampling and scaling, vector field convolution for gap filling and membrane verification using a novel convergence filter. We show results from our algorithm alongside popular microscopy segmentation methods; the advantages of our method are demonstrated. Our algorithm is validated by comparing automated results to a manually defined ground truth. When the number of granules detected is compared to the number of granules in the ground truth a precision of 91% and recall of 87% is observed. The average granule areas differ by 13.35% and 6.08% for core and membranes respectively, when compared to the average areas of the ground truth. These results compare favorably to previously published data.

The somatostatin receptor in human pancreatic β-cells

The peptide hormone somatostatin (SST) is produced in the brain, the gut, and in δ-cells in pancreatic islets of Langerhans. SST secretion from δ-cells is stimulated by glucose, amino acids, and glucagon-like peptide-1. Exogenous SST strongly inhibits the secretion of the blood glucose-regulating hormones insulin and glucagon from pancreatic β-cells and α-cells, respectively. Endogenous SST secreted from δ-cells is a paracrine regulator of insulin and glucagon secretion, although the exact physiological significance of this regulation is unclear. Secreted SST binds to specific receptors (SSTRs), which are coupled to Gi/o proteins. In both β- and α-cells, activation of SSTRs suppresses hormone secretion by reducing cAMP levels, inhibiting electrical activity, decreasing Ca²⁺ influx through voltage-gated Ca²⁺ channels and directly reducing exocytosis in a Ca²⁺ and cAMP-independent manner. In rodents, β-cells express predominantly SSTR5, whereas α-cells express SSTR2. In human islets, SSTR2 is the dominant receptor in both β- and α-cells, but other isoforms also contribute to the SST effects. Evidence from rodent models suggests that SST secretion from δ-cells is dysregulated in diabetes mellitus, which may contribute to the metabolic disturbances in this disease. SST analogues are currently used for the treatment of hyperinsulinism and other endocrine disorders, including acromegaly and Cushing's syndrome.

Enhanced insulin secretion responsiveness and islet adrenergic desensitization after chronic norepinephrine suppression is discontinued in fetal sheep

Intrauterine growth-restricted (IUGR) fetuses experience prolonged hypoxemia, hypoglycemia, and elevated norepinephrine (NE) concentrations, resulting in hypoinsulinemia and β-cell dysfunction. Previously, we showed that acute adrenergic blockade revealed enhanced insulin secretion responsiveness in the IUGR fetus. To determine whether chronic exposure to NE alone enhances β-cell responsiveness afterward, we continuously infused NE into fetal sheep for 7 days and, after terminating the infusion, evaluated glucose-stimulated insulin secretion (GSIS) and glucose-potentiated arginine-induced insulin secretion (GPAIS). During treatment, NE-infused fetuses had greater (P < 0.05) plasma NE concentrations and exhibited hyperglycemia (P < 0.01) and hypoinsulinemia (P < 0.01) compared with controls. GSIS during the NE infusion was also reduced (P < 0.05) compared with pretreatment values. GSIS and GPAIS were approximately fourfold greater (P < 0.01) in NE fetuses 3 h after the 7 days that NE infusion was discontinued compared with age-matched controls or pretreatment GSIS and GPAIS values of NE fetuses. In isolated pancreatic islets from NE fetuses, mRNA concentrations of adrenergic receptor isoforms (α1D, α2A, α2C, and β1), G protein subunit-αi-2, and uncoupling protein 2 were lower (P < 0.05) compared with controls, but β-cell regulatory genes were not different. Our findings indicate that chronic exposure to elevated NE persistently suppresses insulin secretion. After removal, NE fetuses demonstrated a compensatory enhancement in insulin secretion that was associated with adrenergic desensitization and greater stimulus-secretion coupling in pancreatic islets.

Glucose regulation of glucagon secretion

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

Optogenetic control of insulin secretion in intact pancreatic islets with β-cell-specific expression of Channelrhodopsin-2

Insulin is secreted from the pancreatic β-cells in response to elevated glucose. In intact islets the capacity for insulin release is determined by a complex interplay between different cell types. This has made it difficult to specifically assess the role of β-cell defects to the insulin secretory impairment in type 2 diabetes. Here we describe a new approach, based on optogenetics, that enables specific investigation of β-cells in intact islets. We used transgenic mice expressing the light-sensitive cation channel Channelrhodopsin-2 (ChR2) under control of the insulin promoter. Glucose tolerance in vivo was assessed using intraperitoneal glucose tolerance tests, and glucose-induced insulin release was measured from static batch incubations. ChR2 localization was determined by fluorescence confocal microscopy. The effect of ChR2 stimulation with blue LED light was assessed using Ca(2+) imaging and static islet incubations. Light stimulation of islets from transgenic ChR2 mice triggered prompt increases in intracellular Ca(2+). Moreover, light stimulation enhanced insulin secretion in batch-incubated islets at low and intermediate but not at high glucose concentrations. Glucagon release was not affected. Beta-cells from mice rendered diabetic on a high-fat diet exhibited a 3.5-fold increase in light-induced Ca(2+) influx compared with mice on a control diet. Furthermore, light enhanced insulin release also at high glucose in these mice, suggesting that high-fat feeding leads to a compensatory potentiation of the Ca(2+) response in β-cells. The results demonstrate the usefulness and versatility of optogenetics for studying mechanisms of perturbed hormone secretion in diabetes with high time-resolution and cell-specificity.