Differential expression of glutamate receptor subtypes in rat pancreatic islets

Restoring glutamate receptor signaling in pancreatic alpha cells rescues glucagon responses in type 1 diabetes

Glucagon secretion from pancreatic alpha cells is crucial to prevent hypoglycemia. People with type 1 diabetes lose this glucoregulatory mechanism and are susceptible to dangerous hypoglycemia for reasons still unclear. Here we determine that alpha cells in living pancreas slices from donors with type 1 diabetes do not mount an adequate glucagon response and cannot activate the positive autocrine feedback mediated by AMPA/kainate glutamate receptors. This feedback is required to elicit full glucagon responses in the healthy state. Reactivating residual AMPA/kainate receptor function with positive allosteric modulators restores glucagon secretion in human slices from donors with type 1 diabetes as well as glucose counterregulation in non-obese diabetic mice. Our study thus identifies a defect in autocrine signaling that contributes to alpha cell failure. The use of positive allosteric modulators of AMPA/kainate receptors overcomes this deficiency and prevents hypoglycemia, an effect that could be used to improve the management of diabetes.

NMDA Receptor Antagonists Increase the Release of GLP-1 From Gut Endocrine Cells

Type 2 diabetes mellitus (T2DM) remains one of the most pressing health issues facing modern society. Several antidiabetic drugs are currently in clinical use to treat hyperglycaemia, but there is a need for new treatments that effectively restore pancreatic islet function in patients. Recent studies reported that both murine and human pancreatic islets exhibit enhanced insulin release and β-cell viability in response to N-methyl-D-aspartate (NMDA) receptor antagonists. Furthermore, oral administration of dextromethorphan, an over-the-counter NMDA receptor antagonist, to diabetic patients in a small clinical trial showed improved glucose tolerance and increased insulin release. However, the effects of NMDA receptor antagonists on the secretion of the incretin hormone GLP-1 was not tested, and nothing is known regarding how NMDA receptor antagonists may alter the secretion of gut hormones. This study demonstrates for the first time that, similar to β-cells, the NMDA receptor antagonist MK-801 increases the release of GLP-1 from a murine L-cell enteroendocrine model cell line, GLUTag cells. Furthermore, we report the 3′ mRNA expression profiling of GLUTag cells, with a specific focus on glutamate-activated receptors. We conclude that if NMDA receptor antagonists are to be pursued as an alternative, orally administered treatment for T2DM, it is essential that the effects of these drugs on the release of gut hormones, and specifically the incretin hormones, are fully investigated.

Physiological effects of nutrients on insulin release by pancreatic beta cells

Obesity and type 2 diabetes (T2D) are growing health problems associated with a loss of insulin sensitivity. Both conditions arise from a long-term energy imbalance, and frequently, lifestyle measures can be useful in its prevention, including physical activity and a healthy diet. Pancreatic β-cells are determinant nutrient sensors that participate in energetic homeostasis needs. However, when pancreatic β-cells are incapable of secreting enough insulin to counteract the reduced sensitivity, the pathology evolves to an insulin resistance condition. The primary nutrient that stimulates insulin secretion is glucose, but also, there are multiple dietary and hormonal factors influencing that response. Many studies of the physiology of β-cells have highlighted the importance of glucose, fructose, amino acids, and free fatty acids on insulin secretion. The present review summarizes recent research on how β-cells respond to the most abundant nutrients that influence insulin secretion. Taken together, understand the subjacent mechanisms of each nutrient on β-cells can help to unravel the effects of mixed variables and complexity in the context of β-cell pathology.

Pancreatic α-cells - The unsung heroes in islet function

The pancreatic islets of Langerhans consist of several hormone-secreting cell types important for blood glucose control. The insulin secreting β-cells are the best studied of these cell types, but less is known about the glucagon secreting α-cells. The α-cells secrete glucagon as a response to low blood glucose. The major function of glucagon is to release glucose from the glycogen stores in the liver. In both type 1 and type 2 diabetes, glucagon secretion is dysregulated further exaggerating the hyperglycaemia, and in type 1 diabetes α-cells fail to counter regulate hypoglycaemia. Although glucagon has been recognized for almost 100 years, the understanding of how glucagon secretion is regulated and how glucagon act within the islet is far from complete. However, α-cell research has taken off lately which is promising for future knowledge. In this review we aim to highlight α-cell regulation and glucagon secretion with a special focus on recent discoveries from human islets. We will present some novel aspects of glucagon function and effects of selected glucose lowering agents on glucagon secretion.

Blood glutamate scavenging as a novel glutamate-based therapeutic approach for post-stroke depression

Post-stroke depression (PSD) is a major complication of stroke that significantly impacts functional recovery and quality of life. While the exact mechanism of PSD is unknown, recent attention has focused on the association of the glutamatergic system in its etiology and treatment. Minimizing secondary brain damage and neuropsychiatric consequences associated with excess glutamate concentrations is a vital part of stroke management. The blood glutamate scavengers, oxaloacetate and pyruvate, degrade glutamate in the blood to its inactive metabolite, 2-ketoglutarate, by the coenzymes glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT), respectively. This reduction in blood glutamate concentrations leads to a subsequent shift of glutamate down its concentration gradient from the blood to the brain, thereby decreasing brain glutamate levels. Although there are not yet any human trials that support blood glutamate scavengers for clinical use, there is increasing evidence from animal research of their efficacy as a promising new therapeutic approach for PSD. In this review, we present recent evidence in the literature of the potential therapeutic benefits of blood glutamate scavengers for reducing PSD and other related neuropsychiatric conditions. The evidence reviewed here should be useful in guiding future clinical trials.

The glutamate receptor GluK2 contributes to the regulation of glucose homeostasis and its deterioration during aging

OBJECTIVE

Islets secrete neurotransmitters including glutamate which participate in fine regulation of islet function. The excitatory ionotropic glutamate receptor GluK2 of the kainate receptor family is widely expressed in brain and also found in islets, mainly in α and γ cells. α cells co-release glucagon and glutamate and the latter increases glucagon release via ionotropic glutamate receptors. However, neither the precise nature of the ionotropic glutamate receptor involved nor its role in glucose homeostasis is known. As isoform specific pharmacology is not available, we investigated this question in constitutive GluK2 knock-out mice (GluK2-/-) using adult and middle-aged animals to also gain insight in a potential role during aging.

METHODS

We compared wild-type GluK2+/+ and knock-out GluK2-/- mice using adult (14-20 weeks) and middle-aged animals (40-52 weeks). Glucose (oral OGTT and intraperitoneal IPGTT) and insulin tolerance as well as pyruvate challenge tests were performed according to standard procedures. Parasympathetic activity, which stimulates hormones secretion, was measured by electrophysiology in vivo. Isolated islets were used in vitro to determine islet β-cell electrical activity on multi-electrode arrays and dynamic secretion of insulin as well as glucagon was determined by ELISA.

RESULTS

Adult GluK2-/- mice exhibit an improved glucose tolerance (OGTT and IPGTT), and this was also apparent in middle-aged mice, whereas the outcome of pyruvate challenge was slightly improved only in middle-aged GluK2-/- mice. Similarly, insulin sensitivity was markedly enhanced in middle-aged GluK2-/- animals. Basal and glucose-induced insulin secretion in vivo was slightly lower in GluK2-/- mice, whereas fasting glucagonemia was strongly reduced. In vivo recordings of parasympathetic activity showed an increase in basal activity in GluK2-/- mice which represents most likely an adaptive mechanism to counteract hypoglucagonemia rather than altered neuronal mechanism. In vitro recording demonstrated an improvement of glucose-induced electrical activity of β-cells in islets obtained from GluK2-/- mice at both ages. Finally, glucose-induced insulin secretion in vitro was increased in GluK2-/- islets, whereas glucagon secretion at 2 mmol/l of glucose was considerably reduced.

CONCLUSIONS

These observations indicate a general role for kainate receptors in glucose homeostasis and specifically suggest a negative effect of GluK2 on glucose homeostasis and preservation of islet function during aging. Our observations raise the possibility that blockade of GluK2 may provide benefits in glucose homeostasis especially during aging.

The Role of RNA Editing in Cancer Development and Metabolic Disorders

Numerous human diseases arise from alterations of genetic information, most notably DNA mutations. Thought to be merely the intermediate between DNA and protein, changes in RNA sequence were an afterthought until the discovery of RNA editing 30 years ago. RNA editing alters RNA sequence without altering the sequence or integrity of genomic DNA. The most common RNA editing events are A-to-I changes mediated by adenosine deaminase acting on RNA (ADAR), and C-to-U editing mediated by apolipoprotein B mRNA editing enzyme, catalytic polypeptide 1 (APOBEC1). Both A-to-I and C-to-U editing were first identified in the context of embryonic development and physiological homeostasis. The role of RNA editing in human disease has only recently started to be understood. In this review, the impact of RNA editing on the development of cancer and metabolic disorders will be examined. Distinctive functions of each RNA editase that regulate either A-to-I or C-to-U editing will be highlighted in addition to pointing out important regulatory mechanisms governing these processes. The potential of developing novel therapeutic approaches through intervention of RNA editing will be explored. As the role of RNA editing in human disease is elucidated, the clinical utility of RNA editing targeted therapies will be needed. This review aims to serve as a bridge of information between past findings and future directions of RNA editing in the context of cancer and metabolic disease.

NMDA receptors mediate leptin signaling and regulate potassium channel trafficking in pancreatic β-cells

NMDA receptors (NMDARs) are Ca²⁺-permeant, ligand-gated ion channels activated by the excitatory neurotransmitter glutamate and have well-characterized roles in the nervous system. The expression and function of NMDARs in pancreatic β-cells, by contrast, are poorly understood. Here, we report a novel function of NMDARs in β-cells. Using a combination of biochemistry, electrophysiology, and imaging techniques, we now show that NMDARs have a key role in mediating the effect of leptin to modulate β-cell electrical activity by promoting AMP-activated protein kinase (AMPK)-dependent trafficking of K_ATP and Kv2.1 channels to the plasma membrane. Blocking NMDAR activity inhibited the ability of leptin to activate AMPK, induce K_ATP and Kv2.1 channel trafficking, and promote membrane hyperpolarization. Conversely, activation of NMDARs mimicked the effect of leptin, causing Ca²⁺ influx, AMPK activation, and increased trafficking of K_ATP and Kv2.1 channels to the plasma membrane, and triggered membrane hyperpolarization. Moreover, leptin potentiated NMDAR currents and triggered NMDAR-dependent Ca²⁺ influx. Importantly, NMDAR-mediated signaling was observed in rat insulinoma 832/13 cells and in human β-cells, indicating that this pathway is conserved across species. The ability of NMDARs to regulate potassium channel surface expression and thus, β-cell excitability provides mechanistic insight into the recently reported insulinotropic effects of NMDAR antagonists and therefore highlights the therapeutic potential of these drugs in managing type 2 diabetes.

The Role of KV7.3 in Regulating Osteoblast Maturation and Mineralization

KCNQ (KV7) channels are voltage-gated potassium (KV) channels, and the function of KV7 channels in muscles, neurons, and sensory cells is well established. We confirmed that overall blockade of KV channels with tetraethylammonium augmented the mineralization of bone-marrow-derived human mesenchymal stem cells during osteogenic differentiation, and we determined that KV7.3 was expressed in MG-63 and Saos-2 cells at the mRNA and protein levels. In addition, functional KV7 currents were detected in MG-63 cells. Inhibition of KV7.3 by linopirdine or XE991 increased the matrix mineralization during osteoblast differentiation. This was confirmed by alkaline phosphatase, osteocalcin, and osterix in MG-63 cells, whereas the expression of Runx2 showed no significant change. The extracellular glutamate secreted by osteoblasts was also measured to investigate its effect on MG-63 osteoblast differentiation. Blockade of KV7.3 promoted the release of glutamate via the phosphorylation of extracellular signal-regulated kinase 1/2-mediated upregulation of synapsin, and induced the deposition of type 1 collagen. However, activation of KV7.3 by flupirtine did not produce notable changes in matrix mineralization during osteoblast differentiation. These results suggest that KV7.3 could be a novel regulator in osteoblast differentiation.

CFTR silencing in pancreatic β-cells reveals a functional impact on glucose-stimulated insulin secretion and oxidative stress response

Cystic fibrosis (CF)-related diabetes (CFRD) has become a critical complication that seriously affects the clinical outcomes of CF patients. Although CFRD has emerged as the most common nonpulmonary complication of CF, little is known about its etiopathogenesis. Additionally, whether oxidative stress (OxS), a common feature of CF and diabetes, influences CFRD pathophysiology requires clarification. The main objective of this study was to shed light on the role of the cystic fibrosis transmembrane conductance regulator (CFTR) in combination with OxS in insulin secretion from pancreatic β-cells. CFTR silencing was accomplished in MIN6 cells by stable expression of small hairpin RNAs (shRNA), and glucose-induced insulin secretion was evaluated in the presence and absence of the valuable prooxidant system iron/ascorbate (Fe/Asc; 0.075/0.75 mM) along with or without the antioxidant Trolox (1 mM). Insulin output from CFTR-silenced MIN6 cells was significantly reduced (∼ 70%) at basal and at different glucose concentrations compared with control Mock cells. Furthermore, CFTR silencing rendered MIN6 cells more sensitive to OxS as evidenced by both increased lipid peroxides and weakened antioxidant defense, especially following incubation with Fe/Asc. The decreased insulin secretion in CFTR-silenced MIN6 cells was associated with high levels of NF-κB (the major participant in inflammatory responses), raised apoptosis, and diminished ATP production in response to the Fe/Asc challenge. However, these defects were alleviated by the addition of Trolox, thereby pointing out the role of OxS in aggravating the effects of CFTR deficiency. Our findings indicate that CFTR deficiency in combination with OxS may contribute to endocrine cell dysfunction and insulin secretion, which at least in part may explain the development of CFRD.

Vitamin D3 supplementation increases insulin level by regulating altered IP3 and AMPA receptor expression in the pancreatic islets of streptozotocin-induced diabetic rat

Pancreatic islets, particularly insulin-secreting β cells, share common characteristics with neurons. Glutamate is one of the major excitatory neurotransmitter in the brain and pancreas, and its action is mediated through glutamate receptors. In the present work, we analysed the role of vitamin D3 in the modulation of AMPA receptor subunit and their functional role in insulin release. Radio receptor binding study in diabetic rats showed a significant increase in AMPA receptor density. Insulin AMPA colabelling study showed an altered AMPA GluR2 and GluR4 subunit expression in the pancreatic beta cells. We also found lowered IP3 content and decreased IP3 receptor in pancreas of diabetic rats. The alterations in AMPA and IP3 receptor resulted in reduced cytosolic calcium level concentration, which further blocks Ca(2+)-mediated insulin release. Vitamin D3 supplementation restored the alteration in vitamin D receptor expression, AMPA receptor density and AMPA and IP3 receptor expression in the pancreatic islets that helps to restore the calcium-mediated insulin secretion. Our study reveals the antidiabetic property of vitamin D3 that is suggested to have therapeutic role through regulating glutamatergic function in diabetic rats.

Dehydroepiandrosterone, its metabolites and ion channels

This review is focused on the physiological and pathophysiological relevance of steroids influencing the activities of the central and peripheral nervous systems with regard to their concentrations in body fluids and tissues in various stages of human life like the fetal development or pregnancy. The data summarized in this review shows that DHEA and its unconjugated and sulfated metabolites are physiologically and pathophysiologically relevant in modulating numerous ion channels and participate in vital functions of the human organism. DHEA and its unconjugated and sulfated metabolites including 5α/β-reduced androstane steroids participate in various physiological and pathophysiological processes like the management of GnRH cyclic release, regulation of glandular and neurotransmitter secretions, maintenance of glucose homeostasis on one hand and insulin insensitivity on the other hand, control of skeletal muscle and smooth muscle activities including vasoregulation, promotion of tolerance to ischemia and other neuroprotective effects. In respect of prevalence of steroid sulfates over unconjugated steroids in the periphery and the opposite situation in the CNS, the sulfated androgens and androgen metabolites reach relevance in peripheral organs. The unconjugated androgens and estrogens are relevant in periphery and so much the more in the CNS due to higher concentrations of most unconjugated steroids in the CNS tissues than in circulation and peripheral organs. This article is part of a Special Issue entitled "Essential role of DHEA".

Monosodium glutamate stimulates secretion of glucagon-like peptide-1 and reduces postprandial glucose after a lipid-containing meal

BACKGROUND

Monosodium l-glutamate (MSG) is known to influence the endocrine system and gastrointestinal (GI) motility. The mechanism of postprandial glycemic control by food in the GI tract is mostly unknown and of great interest.

AIM

To investigate the effect of MSG on glucose homeostasis, incretin secretion and gastric emptying in humans after a lipid-containing meal.

METHODS

Thirteen healthy male volunteers (mean age, 25.5 years) and with no Helicobcter pylori infection were enrolled. A 400 mL (520 kcal) liquid meal with MSG (2 g, 0.5% wt:vol) or NaCl (control) was ingested in a single-blind placebo-controlled cross-over study. Blood glucose, serum insulin, plasma glucagon, plasma glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide were measured. Gastric emptying was monitored by a 13C acetate breath test. Postprandial symptoms were assessed on a visual analogue scale.

RESULTS

The 30-min postprandial glucose concentration was significantly reduced by adding MSG to the test meal. The area under the glucose concentration vs. time curve (0-60 min) was also significantly reduced by adding MSG (40.6 ± 3.51 mg·1 hr/dL with MSG vs. 49.2 ± 3.86 mg·1 hr/dL with NaCl, P = 0.047), whereas, the 30-min postprandial plasma GLP-1 level was significantly increased (58.1 ± 15.8 pmol/L with MSG vs. 13.4 ± 15.8 pmol/L with NaCl, P = 0.035). MSG did not affect the half gastric emptying time or postprandial symptoms.

CONCLUSIONS

Monosodium l-glutamate improved early postprandial glycaemia after a lipid-containing liquid meal. This effect was not associated with a change in gastric emptying, but was possibly related to stimulation of glucagon-like peptide-1 secretion.

Blood glutamate scavenging: insight into neuroprotection

Brain insults are characterized by a multitude of complex processes, of which glutamate release plays a major role. Deleterious excess of glutamate in the brain's extracellular fluids stimulates glutamate receptors, which in turn lead to cell swelling, apoptosis, and neuronal death. These exacerbate neurological outcome. Approaches aimed at antagonizing the astrocytic and glial glutamate receptors have failed to demonstrate clinical benefit. Alternatively, eliminating excess glutamate from brain interstitial fluids by making use of the naturally occurring brain-to-blood glutamate efflux has been shown to be effective in various animal studies. This is facilitated by gradient driven transport across brain capillary endothelial glutamate transporters. Blood glutamate scavengers enhance this naturally occurring mechanism by reducing the blood glutamate concentration, thus increasing the rate at which excess glutamate is cleared. Blood glutamate scavenging is achieved by several mechanisms including: catalyzation of the enzymatic process involved in glutamate metabolism, redistribution of glutamate into tissue, and acute stress response. Regardless of the mechanism involved, decreased blood glutamate concentration is associated with improved neurological outcome. This review focuses on the physiological, mechanistic and clinical roles of blood glutamate scavenging, particularly in the context of acute and chronic CNS injury. We discuss the details of brain-to-blood glutamate efflux, auto-regulation mechanisms of blood glutamate, natural and exogenous blood glutamate scavenging systems, and redistribution of glutamate. We then propose different applied methodologies to reduce blood and brain glutamate concentrations and discuss the neuroprotective role of blood glutamate scavenging.

The effect of blood glutamate scavengers oxaloacetate and pyruvate on neurological outcome in a rat model of subarachnoid hemorrhage

Blood glutamate scavengers have been shown to effectively reduce blood glutamate concentrations and improve neurological outcome after traumatic brain injury and stroke in rats. This study investigates the efficacy of blood glutamate scavengers oxaloacetate and pyruvate in the treatment of subarachnoid hemorrhage (SAH) in rats. Isotonic saline, 250 mg/kg oxaloacetate, or 125 mg/kg pyruvate was injected intravenously in 60 rats, 60 minutes after induction of SAH at a rate of 0.1 ml/100 g/min for 30 minutes. There were 20 additional rats that were used as a sham-operated group. Blood samples were collected at baseline and 90 minutes after SAH. Neurological performance was assessed at 24 h after SAH. In half of the rats, glutamate concentrations in the cerebrospinal fluid were measured 24 h after SAH. For the remaining half, the blood brain barrier permeability in the frontal and parieto-occipital lobes was measured 48 h after SAH. Blood glutamate levels were reduced in rats treated with oxaloacetate or pyruvate at 90 minutes after SAH (p < 0.001). Cerebrospinal fluid glutamate was reduced in rats treated with pyruvate (p < 0.05). Neurological performance was significantly improved in rats treated with oxaloacetate (p < 0.05) or pyruvate (p < 0.01). The breakdown of the blood brain barrier was reduced in the frontal lobe in rats treated with pyruvate (p < 0.05) and in the parieto-occipital lobes in rats treated with either pyruvate (p < 0.01) or oxaloacetate (p < 0.01). This study demonstrates the effectiveness of blood glutamate scavengers oxaloacetate and pyruvate as a therapeutic neuroprotective strategy in a rat model of SAH.

The potential role of glutamate in the current diabetes epidemic

In the present article, we propose the perspective that abnormal glutamate homeostasis might contribute to diabetes pathogenesis. Previous reports and our recent data indicate that chronically high extracellular glutamate levels exert direct and indirect effects that might participate in the progressive loss of β-cells occurring in both T1D and T2D. In addition, abnormal glutamate homeostasis may impact all the three accelerators of the "accelerator hypothesis" and could partially explain the rising frequency of T1D and T2D.

AMPA receptors regulate exocytosis and insulin release in pancreatic β cells

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

Paracrine interactions within islets of Langerhans

Glucose supply fluctuates between meal and fasting periods and its consumption by the body varies greatly depending on bodily metabolism. Pancreatic islets of Langerhans secrete various endocrine hormones including insulin and glucagon to keep blood glucose level relatively constant. Additionally, islet hormones regulate activity of neighboring cells as local autocrine or paracrine modulators. Moreover, islet cells release neurotransmitters such as glutamate and γ-aminobutyric acid (GABA) to gain more precise regulation of hormones release kinetics. Excitatory glutamate is co-released with glucagon from α-cells and activates glutamate receptors in the neighboring cells. GABA released from β-cells was shown to inhibit α-cells but to activate β-cells by acting GABA(A) receptors. This review summarizes the recent progress in understanding the paracrine/autocrine interactions in islets.

The effects of insulin, glucagon, glutamate, and glucose infusion on blood glutamate and plasma glucose levels in naive rats

BACKGROUND

Elevated levels of glutamate in brain fluids, in the context of several neurodegenerative conditions, are associated with a worsened neurological outcome. Because there is a clear relationship between brain glutamate levels and glutamate levels in the blood, and an association of the latter with stress, the purpose of this study was to investigate the effects of glucose, insulin, and glucagon on rat blood glutamate levels.

METHODS

Rats received either 1 mL/100 g of rat body weight (BW) intravenous isotonic saline (control), 150 mg/1 mL/100 g BW intravenous glucose, 75 mg/1 mL/100 g BW intravenous glutamate, 50 g/100 g BW intraparitoneal glucagon, or 0.2 UI/100 g BW intraparitoneal insulin. Blood samples were subsequently drawn at 0, 30, 60, 90, and 120 minutes for determination of blood glutamate and glucose levels.

RESULTS

We observed a significant decrease in blood glutamate levels at 30 minutes after injection of glucose (P<0.05), at 30 and 60 minutes after injection of insulin (P<0.05), and at 90 and 120 minutes after injection of glucagon. Plasma glucose levels were elevated after infusion of glutamate and glucose but were decreased after injection of insulin.

CONCLUSIONS

The results of this study demonstrate that glucose, insulin, and glucagon significantly reduce blood glutamate levels. The effect of insulin is immediate and transient, whereas the effect of glucagon is delayed but longer lasting, suggesting that the sensitivity of pancreatic glucagon and insulin-secreting cells to glutamate is dependent on glucose concentration. The results of this study provide insight into blood glutamate homeostasis and may assist in the implementation of new therapies for brain neuroprotection from excess glutamate.

A role for glutamate transporters in the regulation of insulin secretion

In the brain, glutamate is an extracellular transmitter that mediates cell-to-cell communication. Prior to synaptic release it is pumped into vesicles by vesicular glutamate transporters (VGLUTs). To inactivate glutamate receptor responses after release, glutamate is taken up into glial cells or neurons by excitatory amino acid transporters (EAATs). In the pancreatic islets of Langerhans, glutamate is proposed to act as an intracellular messenger, regulating insulin secretion from β-cells, but the mechanisms involved are unknown. By immunogold cytochemistry we show that insulin containing secretory granules express VGLUT3. Despite the fact that they have a VGLUT, the levels of glutamate in these granules are low, indicating the presence of a protein that can transport glutamate out of the granules. Surprisingly, in β-cells the glutamate transporter EAAT2 is located, not in the plasma membrane as it is in brain cells, but exclusively in insulin-containing secretory granules, together with VGLUT3. In EAAT2 knock out mice, the content of glutamate in secretory granules is higher than in wild type mice. These data imply a glutamate cycle in which glutamate is carried into the granules by VGLUT3 and carried out by EAAT2. Perturbing this cycle by knocking down EAAT2 expression with a small interfering RNA, or by over-expressing EAAT2 or a VGLUT in insulin granules, significantly reduced the rate of granule exocytosis. Simulations of granule energetics suggest that VGLUT3 and EAAT2 may regulate the pH and membrane potential of the granules and thereby regulate insulin secretion. These data suggest that insulin secretion from β-cells is modulated by the flux of glutamate through the secretory granules.

Reduction of plasma membrane glutamate transport potentiates insulin but not glucagon secretion in pancreatic islet cells

Glutamate is generated during nutrient stimulation of pancreatic islets and has been proposed to act both as an intra- and extra-cellular messenger molecule. We demonstrate that glutamate is not co-secreted with the hormones from intact islets or purified α- and β-cells. Fractional glutamate release was 5-50 times higher than hormone secretion. Furthermore, various hormone secretagogues did not elicit glutamate efflux. Interestingly, epinephrine even decreased glutamate release while increasing glucagon secretion. Rather than being co-secreted with hormones, we show that glutamate is mainly released via plasma membrane excitatory amino acid transporters (EAAT) by uptake reversal. Transcripts for EAAT1, 2 and 3 were present in both rat α- and β-cells. Inhibition of EAATs by L-trans-pyrrolidine-2,4-dicarboxylate augmented intra-cellular glutamate and α-ketoglutarate contents and potentiated glucose-stimulated insulin secretion from islets and purified β-cells without affecting glucagon secretion from α-cells. In conclusion, intra-cellular glutamate-derived metabolite pools are linked to glucose-stimulated insulin but not glucagon secretion.

The glial glutamate transporter 1 (GLT1) is expressed by pancreatic beta-cells and prevents glutamate-induced beta-cell death

Glutamate is the major excitatory neurotransmitter of the central nervous system (CNS) and may induce cytotoxicity through persistent activation of glutamate receptors and oxidative stress. Its extracellular concentration is maintained at physiological concentrations by high affinity glutamate transporters of the solute carrier 1 family (SLC1). Glutamate is also present in islet of Langerhans where it is secreted by the α-cells and acts as a signaling molecule to modulate hormone secretion. Whether glutamate plays a role in islet cell viability is presently unknown. We demonstrate that chronic exposure to glutamate exerts a cytotoxic effect in clonal β-cell lines and human islet β-cells but not in α-cells. In human islets, glutamate-induced β-cell cytotoxicity was associated with increased oxidative stress and led to apoptosis and autophagy. We also provide evidence that the key regulator of extracellular islet glutamate concentration is the glial glutamate transporter 1 (GLT1). GLT1 localizes to the plasma membrane of β-cells, modulates hormone secretion, and prevents glutamate-induced cytotoxicity as shown by the fact that its down-regulation induced β-cell death, whereas GLT1 up-regulation promoted β-cell survival. In conclusion, the present study identifies GLT1 as a new player in glutamate homeostasis and signaling in the islet of Langerhans and demonstrates that β-cells critically depend on its activity to control extracellular glutamate levels and cellular integrity.

The glutamate agonist homocysteine sulfinic acid stimulates glucose uptake through the calcium-dependent AMPK-p38 MAPK-protein kinase C zeta pathway in skeletal muscle cells

Homocysteine sulfinic acid (HCSA) is a homologue of the amino acid cysteine and a selective metabotropic glutamate receptor (mGluR) agonist. However, the metabolic role of HCSA is poorly understood. In this study, we showed that HCSA and glutamate stimulated glucose uptake in C2C12 mouse myoblast cells and increased AMP-activated protein kinase (AMPK) phosphorylation. RT-PCR and Western blot analysis revealed that C2C12 expresses mGluR5. HCSA transiently increased the intracellular calcium concentration. Although α-methyl-4-carboxyphenylglycine, a metabotropic glutamate receptor antagonist, blocked the action of HCSA in intracellular calcium response and AMPK phosphorylation, 6-cyano-7-nitroquinoxaline-2,3-dione, an AMPA antagonist, did not exhibit such effects. Knockdown of mGluR5 with siRNA blocked HCSA-induced AMPK phosphorylation. Pretreatment of cells with STO-609, a calmodulin-dependent protein kinase kinase (CaMKK) inhibitor, blocked HCSA-induced AMPK phosphorylation, and knockdown of CaMKK blocked HCSA-induced AMPK phosphorylation. In addition, HCSA activated p38 mitogen-activated protein kinase (MAPK). Expression of dominant-negative AMPK suppressed HCSA-mediated phosphorylation of p38 MAPK, and inhibition of AMPK and p38 MAPK blocked HCSA-induced glucose uptake. Phosphorylation of protein kinase C ζ (PKCζ) was also increased by HCSA. Pharmacologic inhibition or knockdown of p38 MAPK blocked HCSA-induced PKCζ phosphorylation, and knockdown of PKCζ suppressed the HCSA-induced increase of cell surface GLUT4. The stimulatory effect of HCSA on cell surface GLUT4 was impaired in FITC-conjugated PKCζ siRNA-transfected cells. Together, the above results suggest that HCSA may have a beneficial role in glucose metabolism in skeletal muscle cells via stimulation of AMPK.

Characterizing single-channel behavior of GluA3 receptors

AMPA receptors play a major role in excitatory neurotransmission in the CNS and are involved in numerous neurological disorders. Agonists bind to each of four bilobed LBDs of this tetrameric receptor, and upon binding, the lobes close to envelope the agonist, leading to channel activation. However, AMPA receptors exhibit complex activation kinetics, the mechanism of which has not yet been determined. We report here single-channel studies of a homomeric AMPA receptor (GluA3) activated by the full agonist, glutamate, and a partial agonist, fluorowillardiine. Both agonists activate the channel to the same three open conductance levels but with different open probabilities in each level. The closed probability (P(c)) varied within records, particularly at low agonist concentrations. By sorting discrete segments of the record according to P(c) using the X-means algorithm, we defined five modes of activity. The kinetic behavior could then be analyzed for both agonists over a range of agonist concentrations with a relatively simple model (three closed states and two open states for each open conductance level). The structural mechanism underlying the modal behavior is not clear; however, it occurs on a timescale consistent with hydrogen bonding across the lobe interface in the LBD.

An AP-3-dependent mechanism drives synaptic-like microvesicle biogenesis in pancreatic islet beta-cells

Pancreatic islet beta-cells contain synaptic-like microvesicles (SLMVs). The origin, trafficking, and role of these SLMVs are poorly understood. In neurons, synaptic vesicle (SV) biogenesis is mediated by two different cytosolic adaptor protein complexes, a ubiquitous AP-2 complex and the neuron-specific AP-3B complex. Mice lacking AP-3B subunits exhibit impaired GABAergic (inhibitory) neurotransmission and reduced neuronal vesicular GABA transporter (VGAT) content. Since beta-cell maturation and exocytotic function seem to parallel that of the inhibitory synapse, we predicted that AP-3B-associated vesicles would be present in beta-cells. Here, we test the hypothesis that AP-3B is expressed in islets and mediates beta-cell SLMV biogenesis. A secondary aim was to test whether the sedimentation properties of INS-1 beta-cell microvesicles are identical to those of bona fide SLMVs isolated from PC12 cells. Our results show that the two neuron-specific AP-3 subunits beta3B and mu3B are expressed in beta-cells, the first time these proteins have been found to be expressed outside the nervous system. We found that beta-cell SLMVs share the same sedimentation properties as PC12 SLMVs and contain SV proteins that sort specifically to AP-3B-associated vesicles in the brain. Brefeldin A, a drug that interferes with AP-3-mediated SV biogenesis, inhibits the delivery of AP-3 cargoes to beta-cell SLMVs. Consistent with a role for AP-3 in the biogenesis of GABAergic SLMV in beta-cells, INS-1 cell VGAT content decreases upon inhibition of AP-3 delta-subunit expression. Our findings suggest that beta-cells and neurons share molecules and mechanisms important for mediating the neuron-specific membrane trafficking pathways that underlie synaptic vesicle formation.

Characteristics and functions of {alpha}-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors expressed in mouse pancreatic {alpha}-cells

Pancreatic islet cells use neurotransmitters such as l-glutamate to regulate hormone secretion. We determined which cell types in mouse pancreatic islets express ionotropic glutamate receptor channels (iGluRs) and describe the detailed biophysical properties and physiological roles of these receptors. Currents through iGluRs and the resulting membrane depolarization were measured with patch-clamp methods. Ca(2+) influx through voltage-gated Ca(2+) channels and Ca(2+)-evoked exocytosis were detected by Ca(2+) imaging and carbon-fiber microamperometry. Whereas iGluR2 glutamate receptor immunoreactivity was detected using specific antibodies in immunocytochemically identified mouse alpha- and beta-cells, functional iGluRs were detected only in the alpha-cells. Fast application of l-glutamate to cells elicited rapidly activating and desensitizing inward currents at -60 mV. By functional criteria, the currents were identified as alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. They were activated and desensitized by AMPA, and were activated only weakly by kainate. The desensitization by AMPA was inhibited by cyclothiazide, and the currents were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Islet iGluRs showed nonselective cation permeability with a low Ca(2+) permeability (P(Ca)/P(Na) = 0.16). Activation of the AMPA receptors induced a sequence of cellular actions in alpha-cells: 1) depolarization of the membrane by 27 +/- 3 mV, 2) rise in intracellular Ca(2+) mainly mediated by voltage-gated Ca(2+) channels activated during the membrane depolarization, and 3) increase of exocytosis by the Ca(2+) rise. In conclusion, iGluRs expressed in mouse alpha-cells resemble the low Ca(2+)-permeable AMPA receptor in brain and can stimulate exocytosis.

Complementary expression of SN1 and SAT2 in the islets of Langerhans suggests concerted action of glutamine transport in the regulation of insulin secretion

Insulin and glucagon secretion from the islets of Langerhans is highly regulated. Although an increased plasma glucose level is the major stimulus for insulin exocytosis, roles for glutamine and glutamate have been suggested. Interestingly, the islet cells display elements associated with synaptic transmission. In the central nervous system (CNS), glutamine transport by SN1 and SAT2 sustain the generation of neurotransmitter glutamate. We hypothesized that the same transporters are essential for glutamine transport into the islet cells and for subsequent formation of glutamate acting as an intracellular signaling molecule. We demonstrate that islet cells express several transporters which can mediate glutamine transport. In particular, we show pronounced expression of SN1 and SAT2 in B-cells and A-cells, respectively. The cell-specific expression of these transporters together with their functional characteristics suggest an important role for glutamine in the regulation of insulin secretion.

Novel aspects of glutamatergic signalling in the neuroendocrine system

L-glutamate, the main excitatory neurotransmitter, influences virtually all neurones of the neuroendocrine hypothalamus via synaptic mechanisms. Vesicular glutamate transporters (VGLUT1-3), which selectively accumulate L-glutamate into synaptic vesicles, provide markers with which to visualise glutamatergic neurones in histological preparations; excitatory neurones in the endocrine hypothalamus synthesise the VGLUT2 isoform. Results of recent dual-label in situ hybridisation studies indicate that glutamatergic neurones in the preoptic area and the hypothalamic paraventricular, supraoptic and periventricular nuclei include parvocellular and magnocellular neurosecretory neurones which secrete peptide neurohormones into the bloodstream to regulate endocrine functions. Neurosecretory terminals of GnRH, TRH, CRF-, somatostatin-, oxytocin- and vasopressin-secreting neurones contain VGLUT2 immunoreactivity, suggesting the co-release of glutamate with hypophysiotrophic peptides. The presence of VGLUT2 also indicates glutamate secretion from non-neuronal endocrine cells, including gonadotrophs and thyrotrophs of the anterior pituitary. Results of in vitro studies show that ionotropic glutamate receptor analogues can elicit hormone secretion at neuroendocrine/endocrine release sites. Structural constituents of the median eminence, adenohypophysis and neurohypophysis contain elements of glutamatergic transmission, including glutamate receptors and enzymes of the glutamate/glutamine cycle. The synthesis of VGLUT2 exhibits robust up-regulation in response to certain endocrine challenges, indicating that altered glutamatergic signalling may represent an important adaptive mechanism. This review article discusses the newly emerged non-synaptic role of glutamate in neuroendocrine and endocrine communication.

Sp1 is required for glucose-induced transcriptional regulation of mouse vesicular glutamate transporter 2 gene

BACKGROUND & AIMS

Vesicular glutamate transporter (VGLUT) has been reported to be involved in glucose-induced insulin secretion. It has been shown that glucose stimulates the expression of VGLUT isoform 2 (VGLUT2) in beta cells via transcriptional mechanism. In this study, we identified the mouse VGLUT2 (mVGLUT2) promoter and characterized the transcriptional mechanism of glucose-stimulated mVGLUT2 expression in beta-cells.

METHODS

A promoter region of mVGLUT2 was cloned by genomic polymerase chain reaction. The mechanism of Sp1 in glucose-induced transactivation of mVGLUT2 was investigated by luciferase assay, electrophoretic mobility shift assay, chromatin immunoprecipitation assay, and Western blot analysis.

RESULTS

A promoter containing 2133 base pairs of upstream sequence of the 5'-flanking region of mVGLUT2 complementary DNA was cloned. Transient transfection of various 5'-end deletion constructs of the mVGLUT2 promoter/luciferase reporter indicated that the region between -96 to +68 base pair contains the basal promoter for mVGLUT2. Mutational analysis and electromobility shift assay showed an important role for the transcription factor Sp1 in both basal and glucose-induced mVGLUT2 transcription. The interaction between Sp1 and mVGLUT2 was confirmed by chromatin immunoprecipitation assays. Glucose stimulates the phosphorylation of Sp1 via mitogen-activated protein kinase P38 and P44/42. This leads to increased binding activity of Sp1 to the mVGLUT2 promoter and results in activation of the gene.

CONCLUSIONS

We cloned the mouse VGLUT2 promoter and showed a novel molecular mechanism of glucose-induced mVGLUT2 transcription.

Glutamate is a positive autocrine signal for glucagon release

An important feature of glucose homeostasis is the effective release of glucagon from the pancreatic alpha cell. The molecular mechanisms regulating glucagon secretion are still poorly understood. We now demonstrate that human alpha cells express ionotropic glutamate receptors (iGluRs) that are essential for glucagon release. A lowering in glucose concentration results in the release of glutamate from the alpha cell. Glutamate then acts on iGluRs of the AMPA/kainate type, resulting in membrane depolarization, opening of voltage-gated Ca(2+) channels, increase in cytoplasmic free Ca(2+) concentration, and enhanced glucagon release. In vivo blockade of iGluRs reduces glucagon secretion and exacerbates insulin-induced hypoglycemia in mice. Hence, the glutamate autocrine feedback loop endows the alpha cell with the ability to effectively potentiate its own secretory activity. This is a prerequisite to guarantee adequate glucagon release despite relatively modest changes in blood glucose concentration under physiological conditions.

Glutaminase activity is confined to the mantle of the islets of Langerhans

Glutamatergic signalling plays an important role in the coordination of hormone secretion from the endocrine pancreas. Thus, glutamate production must be a process exquisitely regulated to ensure a proper transmitter function. Recently we have reported that the endocrine pancreas co-expresses two isoforms of the protein glutaminase (GA), denoted as kidney-type (KGA) and liver-type (LGA). However, how GA activity, and therefore glutamate production, is regulated in the islets represents a critical issue that remains unresolved. Since the purification of these enzymes from rat islets is a daunting task, in order to characterize each isoform we have taken advantage of the spatial segregation of these isoenzymes in pancreas. To assist us with this goal, we have developed a new procedure that enables us to assay GA activity in situ. The assay is highly specific for GA as indicated by its dependence on glutamine and orthophosphate. Surprisingly, LGA, which is abundantly expressed by beta-cells, did not show detectable activity under the assay conditions. All the GA activity detected in pancreatic islets was attributed to KGA and was confined to the mantle of the islets. Double labelling analyses strongly suggested that alpha-cells should be regarded as the site of glutamate production in the endocrine pancreas.

Alpha-cells of the endocrine pancreas: 35 years of research but the enigma remains

Glucagon, a hormone secreted from the alpha-cells of the endocrine pancreas, is critical for blood glucose homeostasis. It is the major counterpart to insulin and is released during hypoglycemia to induce hepatic glucose output. The control of glucagon secretion is multifactorial and involves direct effects of nutrients on alpha-cell stimulus-secretion coupling as well as paracrine regulation by insulin and zinc and other factors secreted from neighboring beta- and delta-cells within the islet of Langerhans. Glucagon secretion is also regulated by circulating hormones and the autonomic nervous system. In this review, we describe the components of the alpha-cell stimulus secretion coupling and how nutrient metabolism in the alpha-cell leads to changes in glucagon secretion. The islet cell composition and organization are described in different species and serve as a basis for understanding how the numerous paracrine, hormonal, and nervous signals fine-tune glucagon secretion under different physiological conditions. We also highlight the pathophysiology of the alpha-cell and how hyperglucagonemia represents an important component of the metabolic abnormalities associated with diabetes mellitus. Therapeutic inhibition of glucagon action in patients with type 2 diabetes remains an exciting prospect.

Competitive AMPA receptor antagonists

Glutamic acid (Glu) is the major excitatory neurotransmitter in the mammalian central nervous system (CNS) where it is involved in the physiological regulation of different processes. It has been well established that excessive endogenous Glu is associated with many acute and chronic neurodegenerative disorders such as cerebral ischaemia, epilepsy, amiotrophic lateral sclerosis, Parkinson's, and Alzheimer's disease. These data have consequently added great impetus to the research in this field. In fact, many Glu receptor antagonists acting at the N-methyl-D-aspartic acid (NMDA), 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA), and/or kainic acid (KA) receptors have been developed as research tools and potential therapeutic agents. Ligands showing competitive antagonistic action at the AMPA type of Glu receptors were first reported in 1988, and the systemically active 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo[f]quinoxaline (NBQX) was first shown to have useful therapeutic effects in animal models of neurological disease in 1990. Since then, the quinoxaline template has represented the backbone of various competitive AMPA receptor antagonists belonging to different classes which had been developed in order to increase potency, selectivity and water solubility, but also to prolong the "in vivo" action. Compounds that present better pharmacokinetic properties and less serious adverse effects with respect to the others previously developed are undergoing clinical evaluation. In the near future, the most important clinical application for the AMPA receptor antagonists will probably be as neuroprotectant in neurodegenerative diseases, such as epilepsy, for the treatment of patients not responding to current therapies. The present review reports the history of competitive AMPA receptor antagonists from 1988 up to today, providing a systematic coverage of both the open and patent literature.

RNA editing by ADAR2 is metabolically regulated in pancreatic islets and beta-cells

RNA editing via the conversion of adenosine (A) to inosine (I) is catalyzed by two major families of adenosine deaminases acting on RNA (ADARs), ADAR1 and ADAR2. This genetic recoding process is known to play essential roles in the brain, due in part to changes in functional activities of edited neurotransmitter receptors and ion channels. Little is known, however, about the physiological regulation and function of A to I RNA editing in peripheral tissues and other biological processes. Here, we report that both ADAR1 and ADAR2 are expressed in the murine pancreatic islets, and ADAR2 is primarily localized in the islet endocrine cells. In contrast to ADAR1, ADAR2 transcripts in the pancreatic islets exhibit a nearly 2-fold increase in insulin-resistant mice chronically fed a high fat diet. Concurrent with this diet-induced metabolic stress, RNA editing in the islets is dramatically enhanced for the RNA transcripts encoding the ionotropic glutamate receptor subunit B. Moreover, ADAR2 protein expression is repressed in the islets under fuel deficiency condition during fasting, and this repression can be completely reversed by refeeding. We also show that, specifically in pancreatic beta-cell lines, not only the expression of ADAR2 but also the glutamate receptor subunit B editing and ADAR2 self-editing are markedly augmented in response to glucose at the physiological concentration for insulin secretion stimulation. Thus, RNA editing by ADAR2 in pancreatic islets and beta-cells is metabolically regulated by nutritional and energy status, suggesting that A to I RNA editing is most likely involved in the modulation of pancreatic islet and beta-cell function.

The glutamatergic system outside the CNS and in cancer biology

Glutamate is a major excitatory neurotransmitter in the CNS. The signalling machinery consists of: glutamate receptors, which are responsible for signal input; plasma glutamate transporters, which are responsible for signal termination; and vesicular glutamate transporters for signal output through exocytic release. Recently, data have suggested that the glutamatergic system plays an important role in non-neuronal tissues. In addition, the expression of glutamatergic system has been implicated in tumour biology. This review outlines the evidence, which suggests that the glutamatergic system may have an important role in cancer biology.

Molecular physiology of vesicular glutamate transporters in the digestive system

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system (CNS). Packaging and storage of glutamate into glutamatergic neuronal vesicles require ATP-dependent vesicular glutamate uptake systems, which utilize the electrochemical proton gradient as a driving force. Three vesicular glutamate transporters (VGLUT1-3) have been recently identified from neuronal tissue where they play a key role to maintain the vesicular glutamate level. Recently, it has been demonstrated that glutamate signaling is also functional in peripheral neuronal and non-neuronal tissues, and occurs in sites of pituitary, adrenal, pineal glands, bone, GI tract, pancreas, skin, and testis. The glutamate receptors and VGLUTs in digestive system have been found in both neuronal and endocrinal cells. The glutamate signaling in the digestive system may have significant relevance to diabetes and GI tract motility disorders. This review will focus on the most recent update of molecular physiology of digestive VGLUTs.

NMDA receptor-mediated calcium influx plays an essential role in myoblast fusion

Ca2+ influx is known to be prerequisite for myoblast fusion during skeletal muscle differentiation. Here, we show that the N-methyl-D-aspartate (NMDA) receptor is involved in the Ca2+ influx of C2C12 myoblasts. NMDA receptor (NR) 1 and NR2D were expressed in the myoblasts during muscle differentiation. Using Ca2+ imaging analysis, Ca2+ influx through NRs was directly measured at a single-cell level. l-Glutamate increased myoblast fusion as well as intracellular Ca2+ levels, and both effects were completely blocked by MK801, a selective antagonist of NRs. Furthermore, treatment with the Ca2+ ionophore A23187 recovered MK801-mediated inhibition of myoblast fusion. These results suggest that the NRs may play an important role in myoblast fusion by mediating Ca2+ influx.

A novel variant of ionotropic glutamate receptor regulates somatostatin secretion from delta-cells of islets of Langerhans

Many metabolic factors affect the secretion of insulin from beta-cells and glucagon from alpha-cells of the islets of Langerhans to regulate blood glucose. Somatostatin from delta-cells, considered a local inhibitor of islet function, reduces insulin and glucagon secretion by activating somatostatin receptors in islet cells. Somatostatin secretion from delta-cells is increased by high glucose via glucose metabolism in a similar way to insulin secretion from beta-cells. However, it is unknown how low glucose triggers somatostatin secretion. Because L-glutamate is cosecreted with glucagon from alpha-cells under low-glucose conditions and acts as a primary intercellular messenger, we hypothesized that glutamate signaling triggers the secretion of somatostatin. In this study, we showed that delta-cells express GluR4c-flip, a newly identified splicing variant of GluR4, an (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type ionotropic glutamate receptor of rat. After treatment with L-glutamate, AMPA, or kainate, secretion of somatostatin from isolated islets was significantly stimulated under low-glucose conditions. The glutamate-dependent somatostatin secretion was Ca(2+) dependent and blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Somatostatin in turn inhibited the secretion of L-glutamate and glucagon from alpha-cells. These results indicate that L-glutamate triggers somatostatin secretion from delta-cells by way of the GluR4c-flip receptor under low-glucose conditions. The released somatostatin may complete the feedback inhibition of alpha-cells. Thus, alpha- and delta-cells may communicate with each other through L-glutamate and somatostatin signaling.

Metabotropic glutamate receptor type 4 is involved in autoinhibitory cascade for glucagon secretion by alpha-cells of islet of Langerhans

In islets of Langerhans, L-glutamate is stored in glucagon-containing secretory granules of alpha-cells and cosecreted with glucagon under low-glucose conditions. The L-glutamate triggers secretion of gamma-aminobutyric acid (GABA) from beta-cells, which in turn inhibits glucagon secretion from alpha-cells through the GABAA receptor. In the present study, we tested the working hypothesis that L-glutamate functions as an autocrine/paracrine modulator and inhibits glucagon secretion through a glutamate receptor(s) on alpha-cells. The addition of L-glutamate at 1 mmol/l; (R,S)-phosphonophenylglycine (PPG) and (S)-3,4-dicarboxyphenylglycine (DCPG), specific agonists for class III metabotropic glutamate receptor (mGluR), at 100 micromol/l; and (1S,3R,4S)-1-aminocyclopentane-1,3,4-tricarboxylic acid (ACPT-I) at 50 micromol/l inhibited the low-glucose-evoked glucagon secretion by 87, 81, 73, and 87%, respectively. This inhibition was dose dependent and was blocked by (R,S)-cyclopropyl-4-phosphonophenylglycine (CPPG), a specific antagonist of class III mGluR. Agonists of other glutamate receptors, including kainate and quisqualate, had little effectiveness. RT-PCR and immunological analyses indicated that mGluR4, a class III mGluR, was expressed and localized with alpha- and F cells, whereas no evidence for expression of other mGluRs, including mGluR8, was obtained. L-Glutamate, PPG, and ACPT-I decreased the cAMP content in isolated islets, which was blocked by CPPG. Dibutylyl-cAMP, a nonhydrolyzable cAMP analog, caused the recovery of secretion of glucagon. Pertussis toxin, which uncouples adenylate cyclase and inhibitory G-protein, caused the recovery of both the cAMP content and secretion of glucagon. These results indicate that alpha- and F cells express functional mGluR4, and its stimulation inhibits secretion of glucagon through an inhibitory cAMP cascade. Thus, L-glutamate may directly interact with alpha-cells and inhibit glucagon secretion.

Glutamatergic Chemical Transmission: Look! Here, There, and Anywhere

Vesicular glutamate transporter (VGLUT) is responsible for the active transport of L-glutamate in synaptic vesicles and thus plays an essential role in the glutamatergic chemical transmission in the central nervous system. VGLUT comprises three isoforms, VGLUT1, 2, and 3, and is a potential marker for the glutamatergic phenotype. Recent studies indicated that VGLUT is also expressed in non-neuronal cells, and localized with various organelles such as synaptic-like microvesicles in the pineal gland, and hormone-containing secretory granules in endocrine cells. L-Glutamate is stored in these organelles, secreted upon various forms of stimulation, and then acts as a paracrine-like modulator. Thus, VGLUTs highlight a novel framework of glutamatergic signaling and reveal its diverse modes of action.

Glutamate signaling in peripheral tissues

The hypothesis that l-glutamate (Glu) is an excitatory amino acid neurotransmitter in the mammalian central nervous system is now gaining more support after the successful cloning of a number of genes coding for the signaling machinery required for this neurocrine at synapses in the brain. These include Glu receptors (signal detection), Glu transporters (signal termination) and vesicular Glu transporters (signal output through exocytotic release). Relatively little attention has been paid to the functional expression of these molecules required for Glu signaling in peripheral neuronal and non-neuronal tissues; however, recent molecular biological analyses show a novel function for Glu as an extracellular signal mediator in the autocrine and/or paracrine system. Emerging evidence suggests that Glu could play a dual role in mechanisms underlying the maintenance of cellular homeostasis - as an excitatory neurotransmitter in the central neurocrine system and an extracellular signal mediator in peripheral autocrine and/or paracrine tissues. In this review, the possible Glu signaling methods are outlined in specific peripheral tissues including bone, testis, pancreas, and the adrenal, pituitary and pineal glands.

Emerging structural explanations of ionotropic glutamate receptor function

High-resolution studies of ionotropic glutamate receptor (iGluR) extracellular domains are beginning to bridge the gap between structure and function. Crystal structures have defined the ligand binding pocket well beyond what was suggested by mutational analysis and homology models alone, providing initial suggestions about the mechanisms of channel gating and desensitization. NMR-derived backbone dynamics and molecular dynamics simulations have added further insights into the role of protein dynamics in receptor function. As a whole, the current knowledge of iGluR structure in conjunction with new advances in the understanding of K+ channels provides a vastly improved understanding of iGluR function. This review focuses on structural and dynamic studies of the extracellular ligand binding domain of iGluRs and the pore region of K+ channels that have contributed to mechanistic insights into the processes of iGluR gating and desensitization

Ionotropic Glutamate Receptor Recognition and Activation

Ionotropic glutamate receptors are the major excitatory neurotransmitters in mammalian brain but are found throughout the animal kingdom as well as in plants and bacteria. A great deal of progress in understanding the structure of these essential neurotransmitter receptors has been made since the first examples were cloned and sequenced in 1989. The atomic structure of the ligand-binding domain of several ionotropic glutamate receptors has been determined, and a great deal of progress has been made in relating the structural properties of the binding site to the function of the intact receptor. In addition, the identification of glutamate receptors from a wide variety of organisms ranging from several types of bacteria to Arabidopsis to a range of animal species has made glutamate receptors a molecular laboratory for studying the evolution of proteins. The fact that glutamate receptors are a particularly ancient intercellular signaling molecule suggests a potential role in the transition from single celled to multicellular organisms. This review focuses on the structure and dynamics of ionotropic glutamate receptors and their relation to the function and evolution of these proteins.

Amino acid transporters: roles in amino acid sensing and signalling in animal cells

Amino acid availability regulates cellular physiology by modulating gene expression and signal transduction pathways. However, although the signalling intermediates between nutrient availability and altered gene expression have become increasingly well documented, how eukaryotic cells sense the presence of either a nutritionally rich or deprived medium is still uncertain. From recent studies it appears that the intracellular amino acid pool size is particularly important in regulating translational effectors, thus, regulated transport of amino acids across the plasma membrane represents a means by which the cellular response to amino acids could be controlled. Furthermore, evidence from studies with transportable amino acid analogues has demonstrated that flux through amino acid transporters may act as an initiator of nutritional signalling. This evidence, coupled with the substrate selectivity and sensitivity to nutrient availability classically associated with amino acid transporters, plus the recent discovery of transporter-associated signalling proteins, demonstrates a potential role for nutrient transporters as initiators of cellular nutrient signalling. Here, we review the evidence supporting the idea that distinct amino acid "receptors" function to detect and transmit certain nutrient stimuli in higher eukaryotes. In particular, we focus on the role that amino acid transporters may play in the sensing of amino acid levels, both directly as initiators of nutrient signalling and indirectly as regulators of external amino acid access to intracellular receptor/signalling mechanisms.

Characterization of vesicular glutamate transporter in pancreatic alpha - and beta -cells and its regulation by glucose

Glutamate has been suggested to play an important role in the release of insulin and glucagon from pancreatic cells via exocytosis. Vesicular glutamate transporter is a rate-limiting step for glutamate release and is involved in the glutamate-evoked exocytosis. Two vesicular glutamate transporters (VGLUT1 and -2) have recently been cloned from the brain. In this report, we first functionally characterized vesicular glutamate transporter in cultured pancreatic alpha- and beta-cells, and then detected mRNA expression of VGLUT1 and -2 in these cells. We also investigated the effect of high or low level of glucose on vesicular glutamate transport in cultured pancreas cells. Our results suggest that both alpha- and beta-cells contain functional vesicular glutamate transporter. The transport characteristics are similar to the cloned neuronal VGLUT1 and -2 in regard to ATP dependence, substrate specificity, kinetics, and chloride dependence. VGLUT2 mRNA is expressed in both alpha- and beta-cells, whereas VGLUT1 is only expressed in beta-cells. High (12.8 mM) and low (2.8 mM) concentrations of glucose increased vesicular glutamate transport in beta- and alpha-cells, respectively. VGLUT2 mRNA was significantly increased in beta- and alpha-cells by high and low glucose concentration, respectively. This increase in VGLUT2 mRNA was suppressed by actinomycin D. We conclude that both alpha- and beta-cells possess functional vesicular glutamate transporters regulated by alteration in glucose concentration, partly via the transcriptional mechanism.

Secretory granule-mediated co-secretion of L-glutamate and glucagon triggers glutamatergic signal transmission in islets of Langerhans

L-Glutamate is believed to function as an intercellular transmitter in the islets of Langerhans. However, critical issues, i.e. where, when and how L-glutamate appears, and what happens upon stimulation of glutamate receptors in the islets, remain unresolved. Vesicular glutamate transporter 2 (VGLUT2), an isoform of the vesicular glutamate transporter essential for neuronal storage of L-glutamate, is expressed in alpha cells (Hayashi, M., Otsuka, M., Morimoto, R., Hirota, S., Yatsushiro, S., Takeda, J., Yamamoto, A., and Moriyama, Y. (2001) J. Biol. Chem. 276, 43400-43406). Here we show that VGLUT2 is specifically localized in glucagon-containing secretory granules but not in synaptic-like microvesicles in alpha TC6 cells, clonal alpha cells, and islet alpha cells. VGLUT1, another VGLUT isoform, is also expressed and localized in secretory granules in alpha cells. Low glucose conditions triggered co-secretion of stoichiometric amounts of L-glutamate and glucagon from alpha TC6 cells and isolated islets, which is dependent on temperature and Ca(2+) and inhibited by phentolamine. Similar co-secretion of L-glutamate and glucagon from islets was observed upon stimulation of beta-adrenergic receptors with isoproterenol. Under low glucose conditions, stimulation of glutamate receptors facilitates secretion of gamma-aminobutyric acid from MIN6 m9, clonal beta cells, and isolated islets. These results indicate that co-secretion of L-glutamate and glucagon from alpha cells under low glucose conditions triggers GABA secretion from beta cells and defines the mode of action of L-glutamate as a regulatory molecule for the endocrine function. To our knowledge, this is the first example of secretory granule-mediated glutamatergic signal transmission.