Suppression of sulfonylurea- and glucose-induced insulin secretion in vitro and in vivo in mice lacking the chloride transport protein ClC-3

Structural basis of adenine nucleotides regulation and neurodegenerative pathology in ClC-3 exchanger

The ClC-3 chloride/proton exchanger is both physiologically and pathologically critical, as it is potentiated by ATP to detect metabolic energy level and point mutations in ClC-3 lead to severe neurodegenerative diseases in human. However, why this exchanger is differentially modulated by ATP, ADP or AMP and how mutations caused gain-of-function remains largely unknow. Here we determine the high-resolution structures of dimeric wildtype ClC-3 in the apo state and in complex with ATP, ADP and AMP, and the disease-causing I607T mutant in the apo and ATP-bounded state by cryo-electron microscopy. In combination with patch-clamp recordings and molecular dynamic simulations, we reveal how the adenine nucleotides binds to ClC-3 and changes in ion occupancy between apo and ATP-bounded state. We further observe I607T mutation induced conformational changes and augments in current. Therefore, our study not only lays the structural basis of adenine nucleotides regulation in ClC-3, but also clearly indicates the target region for drug discovery against ClC-3 mediated neurodegenerative diseases.

Clcn3 deficiency ameliorates high-fat diet-induced obesity and improves metabolism in mice

Objective

Obesity is defined as excess body fat and is a current health epidemic associated with increased risk for type 2 diabetes and cardiovascular disease. The ClC-3 chloride channel/antiporter, encoded by the Clcn3, is associated with some diseases, like carcinoma, nervous system diseases, and metabolic diseases. To verify the relationship between the Clcn3 and weight including metabolic changes, searching for a new target for metabolic therapy of obesity, we designed the experiment.

Methods

The mice were divided into 4 different groups: Clcn3+/+ mice + high-fat diet (HFD), Clcn3-/- mice + HFD, Clcn3+/+ mice + normal diet (ND), Clcn3-/- mice + ND, and fed for 16 weeks. After the glucose tolerance test and insulin tolerance test, peripheral blood and adipose tissues were collected. Moreover, we performed transcriptome sequencing for the epididymal white adipose tissue from Clcn3+/+ and Clcn3-/- mice with the high-fat diet. Western blotting verified the changes in protein levels of relevant metabolic genes.

Results

We found that the Clcn3-/- mice had lower body weight and visceral fat, refining glucose and lipid metabolism in HFD-induced mice, but had no effect in normal diet mice. RNA-seq and Western blotting indicated that Clcn3 deficiency may inhibit obesity through the AMPK-UCP1 axis.

Conclusion

Modulation of Clcn3 may provide an appealing therapeutic target for obesity and associated metabolic syndrome.

Vesicular CLC chloride/proton exchangers in health and diseases

Chloride is one of the most abundant anions in the human body; it is implicated in several physiological processes such as the transmission of action potentials, transepithelial salt transport, maintenance of cellular homeostasis, regulation of osmotic pressure and intracellular pH, and synaptic transmission. The balance between the extracellular and intracellular chloride concentrations is controlled by the interplay of ion channels and transporters embedded in the cellular membranes. Vesicular members of the CLC chloride protein family (vCLCs) are chloride/proton exchangers expressed in the membrane of the intracellular organelles, where they control vesicular acidification and luminal chloride concentration. It is well known that mutations in CLCs cause bone, kidney, and lysosomal genetic diseases. However, the role of CLC exchangers in neurological disorders is only now emerging with the identification of pathogenic CLCN gene variants in patients with severe neuronal and intellectual dysfunctions. This review will provide an overview of the recent advances in understanding the role of the vesicular CLC chloride/proton exchangers in human pathophysiology.

Epigenetic and transcriptomic alterations in the ClC-3-deficient mice consuming a normal diet

Introduction: Metabolic disorders are an important health concern that threatens life and burdens society severely. ClC-3 is a member of the chloride voltage-gated channel family, and ClC-3 deletion improved the phenotypes of dysglycemic metabolism and the impairment of insulin sensitivity. However, the effects of a healthy diet on transcriptome and epigenetics in ClC-3^-/- mice were not explained in detail. Methods: Here, we performed transcriptome sequencing and Reduced Representation Bisulfite Sequencing for the liver of 3 weeks old WT and ClC-3^-/- mice consuming a normal diet to insight into the epigenetic and transcriptomic alterations of ClC-3 deficient mice. Results: In the present study, we found that ClC-3^-/- mice that were younger than 8 weeks old had smaller bodies compared to ClC-3^+/+ mice with ad libitum self-feeding normal diet, and ClC-3^-/- mice that were older than 10 weeks old had a similar body weight. Except for the spleen, lung, and kidney, the average weight of the heart, liver, and brain in ClC-3^-/- mice was lower than that in ClC-3^+/+ mice. TG, TC, HDL, and LDL in fasting ClC-3^-/- mice were not significantly different from those in ClC-3^+/+ mice. Fasting blood glucose in ClC-3^-/- mice was lower than that in ClC-3^+/+ mice; the glucose tolerance test indicated the response to blood glucose increasing for ClC-3^-/- mice was torpid, but the efficiency of lowering blood glucose was much higher once started. Transcriptomic sequencing and reduced representation bisulfite sequencing for the liver of unweaned mice indicated that ClC-3 deletion significantly changed transcriptional expression and DNA methylation levels of glucose metabolism-related genes. A total of 92 genes were intersected between DEGs and DMRs-targeted genes, of which Nos3, Pik3r1, Socs1, and Acly were gathered in type II diabetes mellitus, insulin resistance, and metabolic pathways. Moreover, Pik3r1 and Acly expressions were obviously correlated with DNA methylation levels, not Nos3 and Socs1. However, the transcriptional levels of these four genes were not different between ClC-3^-/- and ClC-3^+/+ mice at the age of 12 weeks. Discussion: ClC-3 influenced the methylated modification to regulate glucose metabolism, of which the gene expressions could be driven to change again by a personalized diet-style intervention.

The mechanisms of chromogranin B-regulated Cl- homeostasis

Chloride is the most abundant inorganic anions in almost all cells and in human circulation systems. Its homeostasis is therefore important for systems physiology and normal cellular activities. This topic has been extensively studied with chloride loaders and extruders expressed in both cell surfaces and intracellular membranes. With the newly discovered, large-conductance, highly selective Cl- channel formed by membrane-bound chromogranin B (CHGB), which differs from all other known anion channels of conventional transmembrane topology, and is distributed in plasma membranes, endomembrane systems, endosomal, and endolysosomal compartments in cells expressing it, we will discuss the potential physiological importance of the CHGB channels to Cl- homeostasis, cellular excitability and volume control, and cation uptake or release at the cellular and subcellular levels. These considerations and CHGB's association with human diseases make the CHGB channel a possible druggable target for future molecular therapeutics.

Knockdown of CLC-3 may improve cognitive impairment caused by diabetic encephalopathy

BACKGROUND

Diabetic encephalopathy(DE) is a neurological complication of diabetes, and its pathogenesis is unclear. Current studies indicate that insulin receptors and downstream signaling pathways play a key role in the occurrence and development of DE. Additionally, CLC-3, a member of the CLC family of anion channels and transporters, is closely related to the secretion and processing of insulin. Here, we investigated the changes and putative roles of CLC-3 in diabetic encephalopathy.

RESULTS

To this aim, we combined lentivirus and adeno-associated virus gene transfer to change the expression level of CLC-3 in the HT-22 hippocampal cell line and hippocampal CA1. We studied the role of CLC-3 in DE through the Morris water maze test.CLC-3 expression increased significantly in HT-22 cells cultured with high glucose and STZ-induced DE model hippocampus. Moreover, Insulin receptor(IR) and downstream PI3K/AKT/GSK3β signaling pathways were also dysfunctional. After knocking down CLC-3, impaired cell proliferation, apoptosis, IR and the downstream PI3K/AKT/GSK3β signaling pathways were significantly improved. However, when CLC-3 was overexpressed, the neurotoxicity induced by high glucose was further aggravated. Rescue experiments found that through the use of inhibitors such as GSK3β, the PI3K/AKT/GSK3β signaling pathways pathway changes with the use of inhibition, and the expression of related downstream signaling molecules such as Tau and p-Tau also changes accordingly. Using adeno-associated virus gene transfer to knock down CLC-3 in the hippocampal CA1 of the DE model, the IR caused by DE and the dysfunction of the downstream PI3K/AKT/GSK3β signaling pathway were significantly improved. In addition, the impaired spatial recognition of DE was partially restored.

CONCLUSION

Our study proposes that CLC-3, as a key molecule, may regulate insulin receptor signaling and downstream PI3K/AKT/GSK3β signaling pathways and affect the pathogenesis of diabetic encephalopathy.

Inside the Insulin Secretory Granule

The pancreatic β-cell is purpose-built for the production and secretion of insulin, the only hormone that can remove glucose from the bloodstream. Insulin is kept inside miniature membrane-bound storage compartments known as secretory granules (SGs), and these specialized organelles can readily fuse with the plasma membrane upon cellular stimulation to release insulin. Insulin is synthesized in the endoplasmic reticulum (ER) as a biologically inactive precursor, proinsulin, along with several other proteins that will also become members of the insulin SG. Their coordinated synthesis enables synchronized transit through the ER and Golgi apparatus for congregation at the trans-Golgi network, the initiating site of SG biogenesis. Here, proinsulin and its constituents enter the SG where conditions are optimized for proinsulin processing into insulin and subsequent insulin storage. A healthy β-cell is continually generating SGs to supply insulin in vast excess to what is secreted. Conversely, in type 2 diabetes (T2D), the inability of failing β-cells to secrete may be due to the limited biosynthesis of new insulin. Factors that drive the formation and maturation of SGs and thus the production of insulin are therefore critical for systemic glucose control. Here, we detail the formative hours of the insulin SG from the luminal perspective. We do this by mapping the journey of individual members of the SG as they contribute to its genesis.

Islet Function in the Pathogenesis of Cystic Fibrosis-Related Diabetes Mellitus

Cystic fibrosis-related diabetes mellitus (CFRD) is the most common non-pulmonary co-morbidity in cystic fibrosis (CF). CF is caused by mutations in the cystic fibrosis transmembrane conductance regulator gene (CFTR), which leads to aberrant luminal fluid secretions in organs such as the lungs and pancreas. How dysfunctional CFTR leads to CFRD is still under debate. Both intrinsic effects of dysfunctional CFTR in hormone secreting cells of the islets and effects of exocrine damage have been proposed. In the current review, we discuss these non-mutually exclusive hypotheses with a special focus on how dysfunctional CFTR in endocrine cells may contribute to an altered glucose homeostasis. We outline the proposed role of CFTR in the molecular pathways of β-cell insulin secretion and α-cell glucagon secretion, and touch upon the importance of the exocrine pancreas and intra-pancreatic crosstalk for proper islet function.

ClC-3: A Novel Promising Therapeutic Target for Atherosclerosis

Chloride channel 3 (ClC-3), a Cl^-/H⁺ antiporter, has been well established as a member of volume-regulated chloride channels (VRCCs). ClC-3 may be a crucial mediator for activating inflammation-associated signaling pathways by regulating protein phosphorylation. A growing number of studies have indicated that ClC-3 overexpression plays a crucial role in mediating increased plasma low-density lipoprotein levels, vascular endothelium dysfunction, pro-inflammatory activation of macrophages, hyper-proliferation and hyper-migration of vascular smooth muscle cells (VSMCs), as well as oxidative stress and foam cell formation, which are the main factors responsible for atherosclerotic plaque formation in the arterial wall. In the present review, we summarize the molecular structures and classical functions of ClC-3. We further discuss its emerging role in the atherosclerotic process. In conclusion, we explore the potential role of ClC-3 as a therapeutic target for atherosclerosis.

Chloride transporters and channels in β-cell physiology: revisiting a 40-year-old model

It is accepted that insulin-secreting β-cells release insulin in response to glucose even in the absence of functional ATP-sensitive K+ (KATP)-channels, which play a central role in a 'consensus model' of secretion broadly accepted and widely reproduced in textbooks. A major shortcoming of this consensus model is that it ignores any and all anionic mechanisms, known for more than 40 years, to modulate β-cell electrical activity and therefore insulin secretion. It is now clear that, in addition to metabolically regulated KATP-channels, β-cells are equipped with volume-regulated anion (Cl-) channels (VRAC) responsive to glucose concentrations in the range known to promote electrical activity and insulin secretion. In this context, the electrogenic efflux of Cl- through VRAC and other Cl- channels known to be expressed in β-cells results in depolarization because of an outwardly directed Cl- gradient established, maintained and regulated by the balance between Cl- transporters and channels. This review will provide a succinct historical perspective on the development of a complex hypothesis: Cl- transporters and channels modulate insulin secretion in response to nutrients.

Clcn3 deficiency ameliorates high-fat diet-induced obesity and adipose tissue macrophage inflammation in mice

Obesity induces accumulation of adipose tissue macrophages (ATMs) and ATM-driven inflammatory responses that promote the development of glucose and lipid metabolism disorders. ClC-3 chloride channel/antiporter, encoded by the Clcn3, is critical for some basic cellular functions. Our previous work has shown significant alleviation of type 2 diabetes in Clcn3 knockout (Clcn3^−/−) mice. In the present study we investigated the role of Clcn3 in high-fat diet (HFD)-induced obesity and ATM inflammation. To establish the mouse obesity model, both Clcn3^−/− mice and wild-type mice were fed a HFD for 4 or 16 weeks. The metabolic parameters were assessed and the abdominal total adipose tissue was scanned using computed tomography. Their epididymal fat pad tissue and adipose tissue stromal vascular fraction (SVF) cells were isolated for analyses. We found that the HFD-fed Clcn3^−/− mice displayed a significant decrease in obesity-induced body weight gain and abdominal visceral fat accumulation as well as an improvement of glucose and lipid metabolism as compared with HFD-fed wild-type mice. Furthermore, the Clcn3 deficiency significantly attenuated HFD-induced ATM accumulation, HFD-increased F4/80⁺ CD11c⁺ CD206⁻ SVF cells as well as HFD-activated TLR-4/NF-κB signaling in epididymal fat tissue. In cultured human THP-1 macrophages, adenovirus-mediated transfer of Clcn3 specific shRNA inhibited, whereas adenovirus-mediated cDNA overexpression of Clcn3 enhanced lipopolysaccharide-induced activation of NF-κB and TLR-4. These results demonstrate a novel role for Clcn3 in HFD-induced obesity and ATM inflammation.

Glibenclamide-Induced Autophagy Inhibits Its Insulin Secretion-Improving Function in β Cells

Diabetes is a metabolic disease, partly due to hypoinsulinism, which affects ∼8% of the world's adult population. Glibenclamide is known to promote insulin secretion by targeting β cells. Autophagy as a self-protective mechanism of cells has been widely studied and has particular physiological effects in different tissues or cells. However, the interaction between autophagy and glibenclamide is unclear. In this study, we investigated the role of autophagy in glibenclamide-induced insulin secretion in pancreatic β cells. Herein, we showed that glibenclamide promoted insulin release and further activated autophagy through the adenosine 5'-monophosphate (AMP) activated protein kinase (AMPK) pathway in MIN-6 cells. Inhibition of autophagy with autophagy inhibitor 3-methyladenine (3-MA) potentiated the secretory function of glibenclamide further. These results suggest that glibenclamide-induced autophagy plays an inhibitory role in promoting insulin secretion by activating the AMPK pathway instead of altering the mammalian target of rapamycin (mTOR).

Glutamate as intracellular and extracellular signals in pancreatic islet functions

l-Glutamate is one of the most abundant amino acids in the body and is a constituent of proteins and a substrate in metabolism. It is well known that glutamate serves as a primary excitatory neurotransmitter and a critical neuromodulator in the brain. Recent studies have shown that in addition to its pivotal role in neural functions, glutamate plays many important roles in a variety of cellular functions, including those as intracellular and extracellular signals. In pancreatic islets, glutamate is now known to be required for the normal regulation of insulin secretion, such as incretin-induced insulin secretion. In this review, we primarily discuss the physiological and pathophysiological roles of glutamate as intracellular and extracellular signals in the functions of pancreatic islets.

Activation of GLP-1 receptor signalling alleviates cellular stresses and improves beta cell function in a mouse model of Wolfram syndrome

AIMS/HYPOTHESIS

Loss of functional beta cells results in a gradual progression of insulin insufficiency in Wolfram syndrome caused by recessive WFS1 mutations. However, beta cell dysfunction in Wolfram syndrome has yet to be fully characterised, and there are also no specific treatment recommendations. In this study, we aimed to characterise beta cell secretory defects and to examine the potential effects of a glucagon-like peptide-1 (GLP-1) receptor agonist on diabetes in Wolfram syndrome.

METHODS

Insulin secretory function was assessed by the pancreatic perfusion method in mice used as a model of Wolfram syndrome. In addition, granule dynamics in living beta cells were examined using total internal reflection fluorescence microscopy. Acute and chronic effects of exendin-4 (Ex-4) on glucose tolerance and insulin secretion were examined in young Wfs1^-/- mice without hyperglycaemia. Molecular events associated with Ex-4 treatment were investigated using pancreatic sections and isolated islets. In addition, we retrospectively observed a woman with Wolfram syndrome who had been treated with liraglutide for 24 weeks.

RESULTS

Treatment with liraglutide ameliorated our patient's glycaemic control and resulted in a 20% reduction of daily insulin dose along with an off-drug elevation of fasting C-peptide immunoreactivity. Glucose-stimulated first-phase insulin secretion and potassium-stimulated insulin secretion decreased by 53% and 59%, respectively, in perfused pancreases of 10-week-old Wfs1^-/- mice compared with wild-type (WT) mice. The number of insulin granule fusion events in the first phase decreased by 41% in Wfs1^-/- beta cells compared with WT beta cells. Perfusion with Ex-4 increased insulin release in the first and second phases by 3.9-fold and 5.6-fold, respectively, in Wfs1^-/- mice compared with perfusion with saline as a control. The physiological relevance of the effects of Ex-4 was shown by the fact that a single administration potentiated glucose-stimulated insulin secretion and improved glucose tolerance in Wfs1^-/- mice. Four weeks of administration of Ex-4 resulted in an off-drug amelioration of glucose excursions after glucose loading in Wfs1^-/- mice, with insulin secretory dynamics that were indistinguishable from those in WT mice, despite the fact that there was no alteration in beta cell mass. In association with the functional improvements, Ex-4 treatment reversed the increases in phosphorylated eukaryotic initiation factor (EIF2α) and thioredoxin interacting protein (TXNIP), and the decrease in phosphorylated AMP-activated kinase (AMPK), in the beta cells of the Wfs1^-/- mice. Furthermore, Ex-4 treatment modulated the transcription of oxidative and endoplasmic reticulum stress-related markers in isolated islets, implying that it was able to mitigate the cellular stresses resulting from Wfs1 deficiency.

CONCLUSIONS/INTERPRETATION

Our study provides deeper insights into the pathophysiology of beta cell dysfunction caused by WFS1 deficiency and implies that activation of the GLP-1 receptor signal may alleviate insulin insufficiency and aid glycaemic control in Wolfram syndrome.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

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.

Inactivation of SPAK kinase reduces body weight gain in mice fed a high-fat diet by improving energy expenditure and insulin sensitivity

The STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) controls the activity of the electroneutral cation-chloride cotransporters (SLC12 family) and thus physiological processes such as modulation of cell volume, intracellular chloride concentration [Cl^-]_i, and transepithelial salt transport. Modulation of SPAK kinase activity may have an impact on hypertension and obesity, as STK39, the gene encoding SPAK, has been suggested as a hypertension and obesity susceptibility gene. In fact, the absence of SPAK activity in mice in which the activating threonine in the T loop was substituted by alanine (SPAK-KI mice) is associated with decreased blood pressure; however its consequences in metabolism have not been explored. Here, we fed wild-type and homozygous SPAK-KI mice a high-fat diet for 17 wk to evaluate weight gain, circulating substrates and hormones, energy expenditure, glucose tolerance, and insulin sensitivity. SPAK-KI mice exhibit resistance to HFD-induced obesity and hepatic steatosis associated with increased energy expenditure, higher thermogenic activity in brown adipose tissue, increased mitochondrial activity in skeletal muscle, and reduced white adipose tissue hypertrophy mediated by augmented whole body insulin sensitivity and glucose tolerance. Our data reveal a previously unrecognized role for the SPAK kinase in the regulation of energy balance, thermogenesis, and insulin sensitivity, suggesting that this kinase could be a new drug target for the treatment of obesity and the metabolic syndrome.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Regulation of Pancreatic Beta Cell Stimulus-Secretion Coupling by microRNAs

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

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.

Glutamate acts as a key signal linking glucose metabolism to incretin/cAMP action to amplify insulin secretion

Incretins, hormones released by the gut after meal ingestion, are essential for maintaining systemic glucose homeostasis by stimulating insulin secretion. The effect of incretins on insulin secretion occurs only at elevated glucose concentrations and is mediated by cAMP signaling, but the mechanism linking glucose metabolism and cAMP action in insulin secretion is unknown. We show here, using a metabolomics-based approach, that cytosolic glutamate derived from the malate-aspartate shuttle upon glucose stimulation underlies the stimulatory effect of incretins and that glutamate uptake into insulin granules mediated by cAMP/PKA signaling amplifies insulin release. Glutamate production is diminished in an incretin-unresponsive, insulin-secreting β cell line and pancreatic islets of animal models of human diabetes and obesity. Conversely, a membrane-permeable glutamate precursor restores amplification of insulin secretion in these models. Thus, cytosolic glutamate represents the elusive link between glucose metabolism and cAMP action in incretin-induced insulin secretion.

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Glutamate is derived from the malate-aspartate shuttle upon glucose stimulation

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Shuttle-derived glutamate is crucial for incretin-induced insulin secretion

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Cytosolic glutamate is transported into insulin granules via cAMP/PKA signaling

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Glutamate production by glucose is defective in incretin-unresponsive β cells

Glucose-induced electrical activities and insulin secretion in pancreatic islet β-cells are modulated by CFTR

The cause of insulin insufficiency remains unknown in many diabetic cases. Up to 50% adult patients with cystic fibrosis (CF), a disease caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR), develop CF-related diabetes (CFRD) with most patients exhibiting insulin insufficiency. Here we show that CFTR is a regulator of glucose-dependent electrical acitivities and insulin secretion in β-cells. We demonstrate that glucose elicited whole-cell currents, membrane depolarization, electrical bursts or action potentials, Ca(2+) oscillations and insulin secretion are abolished or reduced by inhibitors or knockdown of CFTR in primary mouse β-cells or RINm5F β-cell line, or significantly attenuated in CFTR mutant (DF508) mice compared with wild-type mice. VX-809, a newly discovered corrector of DF508 mutation, successfully rescues the defects in DF508 β-cells. Our results reveal a role of CFTR in glucose-induced electrical activities and insulin secretion in β-cells, shed light on the pathogenesis of CFRD and possibly other idiopathic diabetes, and present a potential treatment strategy.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Metabolic regulation of insulin secretion

Regulation of metabolic fuel homeostasis is a critical function of β-cells, which are located in the islets of Langerhans of the animal pancreas. Impairment of this β-cell function is a hallmark of pancreatic β-cell failure and may lead to development of type 2 diabetes mellitus. β-Cells are essentially "fuel sensors" that monitor and react to elevated nutrient load by releasing insulin. This response involves metabolic activation and generation of metabolic coupling factors (MCFs) that relay the nutrient signal throughout the cell and induce insulin biosynthesis and secretion. Glucose is the most important insulin secretagogue as it is the primary fuel source in food. Glucose metabolism is central to generation of MCFs that lead to insulin release, most notably ATP. In addition, other classes of nutrients are able to augment insulin secretion and these include members of the lipid and amino acid family of nutrients. Therefore, it is important to investigate the interplay between glucose, lipid, and amino acid metabolism, as it is this mixed nutrient sensing that generate the MCFs required for insulin exocytosis. The mechanisms by which these nutrients are metabolized to generate MCFs, and how they impact on β-cell insulin release and function, are discussed in detail in this article.

Cell biology and physiology of CLC chloride channels and transporters

Proteins of the CLC gene family assemble to homo- or sometimes heterodimers and either function as Cl(-) channels or as Cl(-)/H(+)-exchangers. CLC proteins are present in all phyla. Detailed structural information is available from crystal structures of bacterial and algal CLCs. Mammals express nine CLC genes, four of which encode Cl(-) channels and five 2Cl(-)/H(+)-exchangers. Two accessory β-subunits are known: (1) barttin and (2) Ostm1. ClC-Ka and ClC-Kb Cl(-) channels need barttin, whereas Ostm1 is required for the function of the lysosomal ClC-7 2Cl(-)/H(+)-exchanger. ClC-1, -2, -Ka and -Kb Cl(-) channels reside in the plasma membrane and function in the control of electrical excitability of muscles or neurons, in extra- and intracellular ion homeostasis, and in transepithelial transport. The mainly endosomal/lysosomal Cl(-)/H(+)-exchangers ClC-3 to ClC-7 may facilitate vesicular acidification by shunting currents of proton pumps and increase vesicular Cl(-) concentration. ClC-3 is also present on synaptic vesicles, whereas ClC-4 and -5 can reach the plasma membrane to some extent. ClC-7/Ostm1 is coinserted with the vesicular H(+)-ATPase into the acid-secreting ruffled border membrane of osteoclasts. Mice or humans lacking ClC-7 or Ostm1 display osteopetrosis and lysosomal storage disease. Disruption of the endosomal ClC-5 Cl(-)/H(+)-exchanger leads to proteinuria and Dent's disease. Mouse models in which ClC-5 or ClC-7 is converted to uncoupled Cl(-) conductors suggest an important role of vesicular Cl(-) accumulation in these pathologies. The important functions of CLC Cl(-) channels were also revealed by human diseases and mouse models, with phenotypes including myotonia, renal loss of salt and water, deafness, blindness, leukodystrophy, and male infertility.

GLP-1 receptor activated insulin secretion from pancreatic β-cells: mechanism and glucose dependence

The major goal in the treatment of type 2 diabetes mellitus is to control the hyperglycaemia characteristic of the disease. However, treatment with common therapies such as insulin or insulinotrophic sulphonylureas (SU), while effective in reducing hyperglycaemia, may impose a greater risk of hypoglycaemia, as neither therapy is self-regulated by ambient blood glucose concentrations. Hypoglycaemia has been associated with adverse physical and psychological outcomes and may contribute to negative cardiovascular events; hence minimization of hypoglycaemia risk is clinically advantageous. Stimulation of insulin secretion from pancreatic β-cells by glucagon-like peptide 1 receptor (GLP-1R) agonists is known to be glucose-dependent. GLP-1R agonists potentiate glucose-stimulated insulin secretion and have little or no activity on insulin secretion in the absence of elevated blood glucose concentrations. This 'glucose-regulated' activity of GLP-1R agonists makes them useful and potentially safer therapeutics for overall glucose control compared to non-regulated therapies; hyperglycaemia can be reduced with minimal hypoglycaemia. While the inherent mechanism of action of GLP-1R agonists mediates their glucose dependence, studies in rats suggest that SUs may uncouple this dependence. This hypothesis is supported by clinical studies showing that the majority of events of hypoglycaemia in patients treated with GLP-1R agonists occur in patients treated with a concomitant SU. This review aims to discuss the current understanding of the mechanisms by which GLP-1R signalling promotes insulin secretion from pancreatic β-cells via a glucose-dependent process.

Chloride in vesicular trafficking and function

Luminal acidification is of pivotal importance for the physiology of the secretory and endocytic pathways and its diverse trafficking events. Acidification by the proton-pumping V-ATPase requires charge compensation by counterion currents that are commonly attributed to chloride. The molecular identification of intracellular chloride transporters and the improvement of methodologies for measuring intraorganellar pH and chloride have facilitated the investigation of the physiology of vesicular chloride transport. New data question the requirement of chloride for pH regulation of various organelles and furthermore ascribe functions to chloride that are beyond merely electrically shunting the proton pump. This review surveys the currently established and proposed intracellular chloride transporters and gives an overview of membrane-trafficking steps that are affected by the perturbation of chloride transport. Finally, potential mechanisms of membrane-trafficking modulation by chloride are discussed and put into the context of organellar ion homeostasis in general.

Regulation of insulin secretion in human pancreatic islets

Pancreatic β cells secrete insulin, the body's only hormone capable of lowering plasma glucose levels. Impaired or insufficient insulin secretion results in diabetes mellitus. The β cell is electrically excitable; in response to an elevation of glucose, it depolarizes and starts generating action potentials. The electrophysiology of mouse β cells and the cell's role in insulin secretion have been extensively investigated. More recently, similar studies have been performed on human β cells. These studies have revealed numerous and important differences between human and rodent β cells. Here we discuss the properties of human pancreatic β cells: their glucose sensing, the ion channel complement underlying glucose-induced electrical activity that culminates in exocytotic release of insulin, the cellular control of exocytosis, and the modulation of insulin secretion by circulating hormones and locally released neurotransmitters. Finally, we consider the pathophysiology of insulin secretion and the interactions between genetics and environmental factors that may explain the current diabetes epidemic.

Epac2: a sulfonylurea receptor?

Sulfonylureas are widely used oral drugs in the treatment of diabetes mellitus. They function by the inhibition of ATP-sensitive K+ channels in pancreatic β-cells, which are thus considered the 'classical' sulfonylurea receptor. Next to the ATP-sensitive K+ channels, additional sulfonylurea-interacting proteins were identified, which might contribute to the physiological effects of this drug family. Most recently, Epac2 (exchange protein directly activated by cAMP 2) was added to the list of sulfonylurea receptors. However, this finding caused controversy in the literature. The critical discussion of the present paper comes to the conclusion that sulfonylureas are not able to activate Epac2 directly and are unlikely to bind to Epac2. Increased blood glucose levels after food intake result in the secretion of insulin from pancreatic β-cells. Glucose levels are detected 'indirectly' by β-cells: owing to increased glycolysis rates, the ratio of cellular ATP/ADP increases and causes the closure of ATP-sensitive K+ channels. In consequence, cells depolarize and voltage-dependent Ca2+ channels open to cause an increase in the cellular Ca2+ concentration. Finally, Ca2+ induces the fusion of insulin-containing granules with the plasma membrane. Sulfonylureas, such as tolbutamide, glibenclamide or acetohexamide, form a class of orally applicable drugs used in the treatment of non-insulin-dependent diabetes mellitus.

A new role for aquaporin 7 in insulin secretion

BACGROUNS/AIMS: Several insulinotropic agents were recently reported to cause β-cell swelling. The possible participation of AQP7 to water transport was investigated in AQP7(+/+) or AQP7(-/-) mice.

METHODS

Aquaporin expression, insulin secretion, cell swelling and electrical activity were investigated in pancreatic islets.

RESULTS

RT-PCR revealed the expression of AQP5 and AQP8 mRNA. Double immunofluorescent labeling indicated their presence in β-cells. Whilst basal insulin release from isolated pancreatic islets incubated at 2.8 mM D-glucose did not differ between AQP7(+/+) or AQP7(-/-) mice, the secretion of insulin evoked by the omission of 50 mM NaCl, the substitution of 50 mM NaCl by 100 mM glycerol or a rise in D-glucose concentration to 8.3 mM and 16.7 mM was severely impaired in the islets from AQP7(-/-) mice. Yet, exposure of β-cells to either the hypotonic medium or a rise in D-glucose concentration caused a similar degree of swelling and comparable pattern of electrical activity in cells from AQP7(+/+) and AQP7(-/-) mice. Both the cell swelling and change in membrane potential were only impaired in AQP7(-/-) cells when exposed to 50 mM glycerol.

CONCLUSION

It is proposed, therefore, that AQP7 may, directly or indirectly, play a role at a distal site in the exocytotic pathway.

Molecular mechanisms of endolysosomal Ca2+ signalling in health and disease

Endosomes, lysosomes and lysosome-related organelles are emerging as important Ca2+ storage cellular compartments with a central role in intracellular Ca2+ signalling. Endocytosis at the plasma membrane forms endosomal vesicles which mature to late endosomes and culminate in lysosomal biogenesis. During this process, acquisition of different ion channels and transporters progressively changes the endolysosomal luminal ionic environment (e.g. pH and Ca2+) to regulate enzyme activities, membrane fusion/fission and organellar ion fluxes, and defects in these can result in disease. In the present review we focus on the physiology of the inter-related transport mechanisms of Ca2+ and H+ across endolysosomal membranes. In particular, we discuss the role of the Ca2+-mobilizing messenger NAADP (nicotinic acid adenine dinucleotide phosphate) as a major regulator of Ca2+ release from endolysosomes, and the recent discovery of an endolysosomal channel family, the TPCs (two-pore channels), as its principal intracellular targets. Recent molecular studies of endolysosomal Ca2+ physiology and its regulation by NAADP-gated TPCs are providing exciting new insights into the mechanisms of Ca2+-signal initiation that control a wide range of cellular processes and play a role in disease. These developments underscore a new central role for the endolysosomal system in cellular Ca2+ regulation and signalling.

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

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

High extracellular Ca2+ stimulates Ca2+-activated Cl- currents in frog parathyroid cells through the mediation of arachidonic acid cascade

Elevation of extracellular Ca(2+) concentration induces intracellular Ca(2+) signaling in parathyroid cells. The response is due to stimulation of the phospholipase C/Ca(2+) pathways, but the direct mechanism responsible for the rise of intracellular Ca(2+) concentration has remained elusive. Here, we describe the electrophysiological property associated with intracellular Ca(2+) signaling in frog parathyroid cells and show that Ca(2+)-activated Cl(-) channels are activated by intracellular Ca(2+) increase through an inositol 1,4,5-trisphophate (IP(3))-independent pathway. High extracellular Ca(2+) induced an outwardly-rectifying conductance in a dose-dependent manner (EC(50) ∼6 mM). The conductance was composed of an instantaneous time-independent component and a slowly activating time-dependent component and displayed a deactivating inward tail current. Extracellular Ca(2+)-induced and Ca(2+) dialysis-induced currents reversed at the equilibrium potential of Cl(-) and were inhibited by niflumic acid (a specific blocker of Ca(2+)-activated Cl(-) channel). Gramicidin-perforated whole-cell recording displayed the shift of the reversal potential in extracellular Ca(2+)-induced current, suggesting the change of intracellular Cl(-) concentration in a few minutes. Extracellular Ca(2+)-induced currents displayed a moderate dependency on guanosine triphosphate (GTP). All blockers for phospholipase C, diacylglycerol (DAG) lipase, monoacylglycerol (MAG) lipase and lipoxygenase inhibited extracellular Ca(2+)-induced current. IP(3) dialysis failed to induce conductance increase, but 2-arachidonoylglycerol (2-AG), arachidonic acid and 12S-hydroperoxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (12(S)-HPETE) dialysis increased the conductance identical to extracellular Ca(2+)-induced conductance. These results indicate that high extracellular Ca(2+) raises intracellular Ca(2+) concentration through the DAG lipase/lipoxygenase pathway, resulting in the activation of Cl(-) conductance.

Wolfram syndrome 1 gene (WFS1) product localizes to secretory granules and determines granule acidification in pancreatic beta-cells

Wolfram syndrome is an autosomal recessive disorder characterized by juvenile-onset insulin-dependent diabetes mellitus and optic atrophy. The gene responsible for the syndrome (WFS1) encodes an endoplasmic reticulum (ER) resident transmembrane protein. The Wfs1-null mouse exhibits progressive insulin deficiency causing diabetes. Previous work suggested that the function of the WFS1 protein is connected to unfolded protein response and to intracellular Ca(2+) homeostasis. However, its precise molecular function in pancreatic β-cells remains elusive. In our present study, immunofluorescent and electron-microscopic analyses revealed that WFS1 localizes not only to ER but also to secretory granules in pancreatic β-cells. Intragranular acidification was assessed by measuring intracellular fluorescence intensity raised by the acidotrophic agent, 3-[2,4-dinitroanilino]-3'-amino-N-methyldipropyramine. Compared with wild-type β-cells, there was a 32% reduction in the intensity in WFS1-deficient β-cells, indicating the impairment of granular acidification. This phenotype may, at least partly, account for the evidence that Wfs1-null islets have impaired proinsulin processing, resulting in an increased circulating proinsulin level. Morphometric analysis using electron microscopy evidenced that the density of secretory granules attached to the plasma membrane was significantly reduced in Wfs1-null β-cells relative to that in wild-type β-cells. This may be relevant to the recent finding that granular acidification is required for the priming of secretory granules preceding exocytosis and may partly explain the fact that glucose-induced insulin secretion is profoundly impaired in young prediabetic Wfs1-null mice. These results thus provide new insights into the molecular mechanisms of β-cell dysfunction in patients with Wolfram syndrome.

The insulin secretory granule as a signaling hub

The insulin granule was previously thought of as merely a container, but accumulating evidence suggests that it also acts as a signaling node. Regulatory pathways intersect at but also originate from the insulin granule membrane. Examples include the small G-proteins Rab3a and Rab27a, which influence granule movement, and the transmembrane proteins (tyrosine phosphatase receptors type N) PTPRN and PTPRN2, which upregulate β-cell transcription and proliferation. In addition, many cosecreted compounds possess regulatory functions, often related to energy metabolism. For instance, ATP and γ-amino butyric acid (GABA) modulate insulin and glucagon secretion, respectively; C-peptide protects β-cells and kidney cells; and amylin reduces gastric emptying and food intake via the brain. In this paper, we review the current knowledge of the insulin granule proteome and discuss its regulatory functions.