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The glucagon-like peptide-1 receptor (GLP-1R) is a class B G protein–coupled receptor involved in the regulation of blood glucose levels and food intake. Stabilized agonists targeting GLP-1R are used in the treatment of type 2 diabetes and have recently become a breakthrough obesity therapy. Here, we revisit a classic article in Diabetes by Thorens et al. that described the cloning, sequencing, and functional expression of the human GLP-1R. The article also demonstrated that exendin4(1-39) was a full agonist of the human GLP-1R whereas exendin4(9-39) was a full antagonist. We discuss how the knowledge imparted by these studies has gone on to inform multiple strands of GLP-1R biology over the past three decades, including pharmacology, signaling, human genetics, structural biology, and chemical biology. [ABSTRACT FROM AUTHOR]

The article presents a study on the pharmacological treatments for type 2 diabetes. Topics discussed include information on the role of fibroblast growth factor one as paracrine regulators; discussions on the study supporting central nervous system as an important target for diabetes treatment; and the information on the challenges to the drugs in crossing the blood-brain barrier.

Abstract: GLUT proteins are encoded by the SLC2 genes and are members of the major facilitator superfamily of membrane transporters. Fourteen GLUT proteins are expressed in the human and they are categorized into three classes based on sequence similarity. All GLUTs appear to transport hexoses or polyols when expressed ectopically, but the primary physiological substrates for several of the GLUTs remain uncertain. GLUTs 1–5 are the most thoroughly studied and all have well established roles as glucose and/or fructose transporters in various tissues and cell types. The GLUT proteins are comprised of ∼500 amino acid residues, possess a single N-linked oligosaccharide, and have 12 membrane-spanning domains. In this review we briefly describe the major characteristics of the 14 GLUT family members. [Copyright &y& Elsevier]

Uric acid is the metabolic end product of purine metabolism in humans. It has antioxidant properties that may be protective but can also be pro-oxidant, depending on its chemical microenvironment. Hyperuricemia predisposes to disease through the formation of urate crystals that cause gout, but hyperuricemia, independent of crystal formation, has also been linked with hypertension, atherosclerosis, insulin resistance, and diabetes. We discuss here the biology of urate metabolism and its role in disease. We also cover the genetics of urate transport, including URAT1, and recent studies identifying SLC2A9, which encodes the glucose transporter family isoform Glut9, as a major determinant of plasma uric acid levels and of gout development. [ABSTRACT FROM AUTHOR]

The ability to take up and metabolize glucose at the cellular level is a property shared by the vast majority of existing organisms. Most mammalian cells import glucose by a process of facilitative diffusion mediated by members of the Glut (SLC2A) family of membrane transport proteins. Fourteen Glut proteins are expressed in the human and they include transporters for substrates other than glucose, including fructose, myoinositol, and urate. The primary physiological substrates for at least half of the 14 Glut proteins are either uncertain or unknown. The well-established glucose transporter isoforms, Gluts 1-4, are known to have distinct regulatory and/or kinetic properties that reflect their specific roles in cellular and whole body glucose homeostasis. Separate review articles on many of the Glut proteins have recently appeared in this journal. Here, we provide a very brief summary of the known properties of the 14 Glut proteins and suggest some avenues of future investigation in this area. [ABSTRACT FROM AUTHOR]

The SLC2 family of glucose and polyol transporters comprises 13 members, the glucose transporters (GLUT) 1–12 and the H+-myo-inositol cotransporter (HMIT). These proteins all contain 12 transmembrane domains with both the amino and carboxy-terminal ends located on the cytoplasmic side of the plasma membrane and a N-linked oligosaccharide side-chain located either on the first or fifth extracellular loop. Based on sequence comparison, the GLUT isoforms can be grouped into three classes: class I comprises GLUT1–4; class II, GLUT6, 8, 10, and 12 and class III, GLUT5, 7, 9, 11 and HMIT. Despite their sequence similarity and the presence of class-specific signature sequences, these transporters carry various hexoses and HMIT is a H+/myo-inositol co-transporter. Furthermore, the substrate transported by some isoforms has not yet been identified. Tissue- and cell-specific expression of the well-characterized GLUT isoforms underlies their specific role in the control of whole-body glucose homeostasis. Numerous studies with transgenic or knockout mice indeed support an important role for these transporters in the control of glucose utilization, glucose storage and glucose sensing. Much remains to be learned about the transport functions of the recently discovered isoforms (GLUT6–13 and HMIT) and their physiological role in the metabolism of glucose, myo-inositol and perhaps other substrates. [ABSTRACT FROM AUTHOR]

Investigates the mechanisms of glucose release from control and GLUT2-deficient hepatocytes by means of a glucose biosynthetic labeling protocol. Kinetics of glucose synthesis and release from hepatocytes of control; Effect of various inhibitors on glucose release measured at the end of continuous pulse labeling; Kinetics of release of intracellularly accumulated glucose.

During the last 2 years, several novel genes that encode glucose transporter-like proteins have been identified and characterized. Because of their sequence similarity with GLUT1, these genes appear to belong to the family of solute carriers 2A (SLC2A, protein symbol GLUT). Sequence comparisons of all 13 family members allow the definition of characteristic sugar/polyol transporter signatures: (1) the presence of 12 membrane-spanning helices, (2) seven conserved glycine residues in the helices, (3) several basic and acidic residues at the intracellular surface of the proteins, (4) two conserved tryptophan residues, and (5) two conserved tyrosine residues. On the basis of sequence similarities and characteristic elements, the extended GLUTfamily can be divided intothree subfamilies, namely class I (the previously known glucose transporters GLUT1-4), class II (the previously known fructose transporter GLUT5, the GLUT7, GLUT9 and GLUT11), and class III (GLUT6, 8, 10, 12, and the myoinositol transporter HMIT1). Functional characteristics have been reported for some of the novel GLUTs. Like GLUT1-4, they exhibit a tissue/cell-specific expression (GLUT6, leukocytes, brain; GLUT8, testis, blastocysts, brain, muscle, adipocytes; GLUT9, liver, kidney; GLUT10, liver, pancreas; GLUT11, heart, skeletal muscle). GLUT6 and GLUT8 appear to be regulated by sub-cellular redistribution, because they are targeted to intracellular compartments by dileucine motifs in a dynamin dependent manner. Sugar transport has been reported for GLUT6, 8, and 11; HMIT1 has been shown to be a H[sup +]/myo-inositol co-transporter. Thus, the members of the extended GLUT family exhibit a surprisingly diverse substrate specificity, and the definition of sequence elements determining this substrate specificity will require a full functional characterization of all members. [ABSTRACT FROM AUTHOR]

Detection of variations in blood glucose concentrations by pancreatic β-cells and a subsequent appropriate secretion of insulin are key events in the control of glucose homeostasis. Because a decreased capability to sense glycemic changes is a hallmark of type 2 diabetes, the glucose signalling pathway leading to insulin secretion in pancreatic β-cells has been extensively studied. This signalling mechanism depends on glucose metabolism and requires the presence of specific molecules such as GLUT2, glucokinase and the K[sub ATP] channel subunits Kir6.2 and SUR1. Other cells are also able to sense variations in glycemia or in local glucose concentrations and to modulate different physiological functions participating in the general control of glucose and energy homeostasis. These include cells forming the hepatoportal vein glucose sensor, which controls glucose storage in the liver, counterregulation, food intake and glucose utilization by peripheral tissues and neurons in the hypothalamus and brainstem whose firing rates are modulated by local variations in glucose concentrations or, when not protected by a blood-brain barrier, directly by changes in blood glucose levels. These glucose-sensing neurons are involved in the control of insulin and glucagon secretion, food intake and energy expenditure. Here, recent physiological studies performed with GLUT2[sup -/-] mice will be described, which indicate that this transporter is ess ential for glucose sensing by pancreatic β-cells, by the hepatoportal sensor and by sensors, probably located centrally, which control activity of the autonomic nervous system and stimulate glucagon secretion. These studies may pave the way to a fine dissection of the molecular and cellular components of extra-pancreatic glucose sensors involved in the control of glucose and energy homeostasis. [ABSTRACT FROM AUTHOR]

GLUT2-/- mice reexpressing GLUT1 or GLUT2 in their beta-cells (RIPGLUT1 x GLUT2-/- or RIPGLUT2 x GLUT2-/- mice) have nearly normal glucose-stimulated insulin secretion but show high glucagonemia in the fed state. Because this suggested impaired control of glucagon secretion, we set out to directly evaluate the control of glucagonemia by variations in blood glucose concentrations. Using fasted RIPGLUT1 x GLUT2-/- mice, we showed that glucagonemia was no longer increased by hypoglycemic (2.5 mmol/l glucose) clamps or suppressed by hyperglycemic (10 and 20 mmol/l glucose) clamps. However, an increase in plasma glucagon levels was detected when glycemia was decreased to < or =1 mmol/l, indicating preserved glucagon secretory ability, but of reduced sensitivity to glucopenia. To evaluate whether the high-fed glucagonemia could be due to an abnormally increased tone of the autonomic nervous system, fed mutant mice were injected with the ganglionic blockers hexamethonium and chlorisondamine. Both drugs lead to a rapid return of glucagonemia to the levels found in control fed mice. We conclude that 1) in the absence of GLUT2, there is an impaired control of glucagon secretion by low or high glucose; 2) this impaired glucagon secretory activity cannot be due to absence of GLUT2 from alpha-cells because these cells do not normally express this transporter; 3) this dysregulation may be due to inactivation of GLUT2-dependent glucose sensors located outside the endocrine pancreas and controlling glucagon secretion; and 4) because fed hyperglucagonemia is rapidly reversed by ganglionic blockers, this suggests that in the absence of GLUT2, there is an increased activity of the autonomic nervous system stimulating glucagon secretion during the fed state. [ABSTRACT FROM AUTHOR]

The article discusses the relationship between type 2 diabetes, the use of glucose-lowering sodium-coupled glucose transporter type 2 (SGLT2) inhibitors and glucose secretions. Topics covered include the glucagon peptide hormone, the hypoglycemic effect of sulfonylureas and glucagon-like peptide 1 (GLP-1) receptor agonists and low-dose sulfonylureas coprescribing.

Investigates the biosynthesis, intracellular transport and surface expression of the beta cell glucose transporter GLUT2 in isolated islets and insulinoma cells. Trypsin sensitivity assay; Surface reexpression of GLUT2 in pancreatic beta cells; Biosynthetic experiments; Brefeldin A inhibition of intracellular transport.

Presents a study on glucose transporters in the regulation of renal, intestinal, and liver glucose fluxes. Characterization of functional mammalian facilitated hexose carriers(GLUTs), by molecular cloning; Definition of their specificity and affinity for different hexoses by functional expression in heterologous systems; Identification of high-affinity and low-affinity transporters.

Presents a correction to the article "The SLC2 Family of Facilitated Hexose and Polyol Transporters," that was published in a previous issue of the periodical "Pflugers Archiv European Journal of Physiology."

Hyperglycemia increases glucose concentrations in the cerebrospinal fluid (CSF), activating glucose-sensing mechanisms and feeding behavior in the hypothalamus. Here, we discuss how hyperglycemia temporarily modifies ependymal cell ciliary beating to increase hypothalamic glucose sensing. A high level of glucose in the rat CSF stimulates glucose transporter 2 (GLUT2)-positive subcommissural organ (SCO) cells to release SCO-spondin into the dorsal third ventricle. Genetic inactivation of mice GLUT2 decreases hyperglycemia-induced SCO-spondin secretion. In addition, SCO cells secrete Wnt5a-positive vesicles; thus, Wnt5a and SCO-spondin are found at the apex of dorsal ependymal cilia to regulate ciliary beating. Frizzled-2 and ROR2 receptors, as well as specific proteoglycans, such as glypican/testican (essential for the interaction of Wnt5a with its receptors) and Cx43 coupling, were also analyzed in ependymal cells. Finally, we propose that the SCO-spondin/Wnt5a/Frizzled-2/Cx43 axis in ependymal cells regulates ciliary beating, a cyclic and adaptive signaling mechanism to control glucose sensing. Hyperglycemia increases glucose concentrations in the cerebrospinal fluid (CSF), activating glucose sensing and feeding behavior in the hypothalamus. This study shows that raised CSF glucose levels induce release of SCO-spondin and Wnt5a, controlling ciliary beating in the ependymal cells. [ABSTRACT FROM AUTHOR]

The article discusses the ways in which glucose triggers insulin secretion in regard to patients with type 2 diabetes mellitus. The processes for glucose-stimulated insulin secretions and other actions are explained. The author suggests that there may be a link between a high-fat diet and type 2 diabetes. The author also mentions that a mutation in certain glycosyltransferase genes may be linked with insulin secretion impairment and therefore cause type 2 diabetes. Several other medical studies focusing on diabetes were cited.

Aims: To describe the in vitro characteristics and antidiabetic in vivo efficacy of the novel glucagon‐like peptide‐1 receptor agonist (GLP‐1RA) GL0034. Materials and Methods: Glucagon‐like peptide‐1 receptor (GLP‐1R) kinetic binding parameters, cyclic adenosine monophosphate (cAMP) signalling, endocytosis and recycling were measured using HEK293 and INS‐1832/3 cells expressing human GLP‐1R. Insulin secretion was measured in vitro using INS‐1832/3 cells, mouse islets and human islets. Chronic administration studies to evaluate weight loss and glycaemic effects were performed in db/db and diet‐induced obese mice. Results: Compared to the leading GLP‐1RA semaglutide, GL0034 showed increased binding affinity and potency‐driven bias in favour of cAMP over GLP‐1R endocytosis and β‐arrestin‐2 recruitment. Insulin secretory responses were similar for both ligands. GL0034 (6 nmol/kg) led to at least as much weight loss and lowering of blood glucose as did semaglutide at a higher dose (14 nmol/kg). Conclusions: GL0034 is a G protein‐biased agonist that shows powerful antidiabetic effects in mice, and may serve as a promising new GLP‐1RA for obese patients with type 2 diabetes. [ABSTRACT FROM AUTHOR]

Feeding behavior is a complex process that depends on the ability of the brain to integrate hormonal and nutritional signals, such as glucose. One glucosensing mechanism relies on the glucose transporter 2 (GLUT2) in the hypothalamus, especially in radial glia-like cells called tanycytes. Here, we analyzed whether a GLUT2-dependent glucosensing mechanism is required for the normal regulation of feeding behavior in GFAP-positive tanycytes. Genetic inactivation of Glut2 in GFAP-expressing tanycytes was performed using Cre/Lox technology. The efficiency of GFAP-tanycyte targeting was analyzed in the anteroposterior and dorsoventral axes by evaluating GFP fluorescence. Feeding behavior, hormonal levels, neuronal activity using c-Fos, and neuropeptide expression were also analyzed in the fasting-to-refeeding transition. In basal conditions, Glut2-inactivated mice had normal food intake and meal patterns. Implementation of a preceeding fasting period led to decreased total food intake and a delay in meal initiation during refeeding. Additionally, Glut2 inactivation increased the number of c-Fos-positive cells in the ventromedial nucleus in response to fasting and a deregulation of Pomc expression in the fasting-to-refeeding transition. Thus, a GLUT2-dependent glucose-sensing mechanism in GFAP-tanycytes is required to control food consumption and promote meal initiation after a fasting period. [ABSTRACT FROM AUTHOR]

Feeding behavior is a complex process that depends on the ability of the brain to integrate hormonal and nutritional signals, such as glucose. One glucosensing mechanism relies on the glucose transporter 2 (GLUT2) in the hypothalamus, especially in radial glia-like cells called tanycytes. Here, we analyzed whether a GLUT2-dependent glucosensing mechanism is required for the normal regulation of feeding behavior in GFAP-positive tanycytes. Genetic inactivation of Glut2 in GFAP-expressing tanycytes was performed using Cre/Lox technology. The efficiency of GFAP-tanycyte targeting was analyzed in the anteroposterior and dorsoventral axes by evaluating GFP fluorescence. Feeding behavior, hormonal levels, neuronal activity using c-Fos, and neuropeptide expression were also analyzed in the fasting-to-refeeding transition. In basal conditions, Glut2-inactivated mice had normal food intake and meal patterns. Implementation of a preceeding fasting period led to decreased total food intake and a delay in meal initiation during refeeding. Additionally, Glut2 inactivation increased the number of c-Fos-positive cells in the ventromedial nucleus in response to fasting and a deregulation of Pomc expression in the fasting-to-refeeding transition. Thus, a GLUT2-dependent glucose-sensing mechanism in GFAP-tanycytes is required to control food consumption and promote meal initiation after a fasting period. [ABSTRACT FROM AUTHOR]

Glucose homeostasis as well as homeostatic and hedonic control of feeding is regulated by hormonal, neuronal, and nutrient-related cues. Glucose, besides its role as a source of metabolic energy, is an important signal controlling hormone secretion and neuronal activity, hence contributing to whole-body metabolic integration in coordination with feeding control. Brain glucose sensing plays a key, but insufficiently explored, role in these metabolic and behavioral controls, which when deregulated may contribute to the development of obesity and diabetes. The recent introduction of innovative transgenic, pharmacogenetic, and optogenetic techniques allows unprecedented analysis of the complexity of central glucose sensing at the molecular, cellular, and neuronal circuit levels, which will lead to a new understanding of the pathogenesis of metabolic diseases. [ABSTRACT FROM AUTHOR]

The thalamic paraventricular nucleus (PVT) is a structure highly interconnected with several nuclei ranging from forebrain to hypothalamus and brainstem. Numerous rodent studies have examined afferent and efferent connections of the PVT and their contribution to behavior, revealing its important role in the integration of arousal cues. However, the majority of these studies used a region‐oriented approach, without considering the neuronal subtype diversity of the nucleus. In the present study, we provide the anatomical and transcriptomic characterization of a subpopulation of PVT neurons molecularly defined by the expression of glucokinase (Gck). Combining a genetically modified mouse model with viral tracing approaches, we mapped both the anterograde and the retrograde projections of Gck‐positive neurons of the anterior PVT (GckaPVT). Our results demonstrated that GckaPVT neurons innervate several nuclei throughout the brain axis. The strongest connections are with forebrain areas associated with reward and stress and with hypothalamic structures involved in energy balance and feeding regulation. Furthermore, transcriptomic analysis of the Gck‐expressing neurons revealed that they are enriched in receptors for hypothalamic‐derived neuropeptides, adhesion molecules, and obesity and diabetes susceptibility transcription factors. Using retrograde labeling combined with immunohistochemistry and in situ hybridization, we identify that GckaPVT neurons receive direct inputs from well‐defined hypothalamic populations, including arginine‐vasopressin‐, melanin‐concentrating hormone‐, orexin‐, and proopiomelanocortin‐expressing neurons. This detailed anatomical and transcriptomic characterization of GckaPVT neurons provides a basis for functional studies of the integration of homeostatic and hedonic aspects of energy homeostasis, and for deciphering the potential role of these neurons in obesity and diabetes development. [ABSTRACT FROM AUTHOR]

Cystathionine beta synthase (CBS) catalyzes the first step of the transsulfuration pathway from homocysteine to cystathionine, and its deficiency leads to hyperhomocysteinemia (HHcy) in humans and rodents. To date, scarce information is available about the HHcy effect on insulin secretion, and the link between CBS activity and the setting of type 2 diabetes is still unknown. We aimed to decipher the consequences of an inborn defect in CBS on glucose homeostasis in mice. We used a mouse model heterozygous for CBS (CBS+/−) that presented a mild HHcy. Other groups were supplemented with methionine in drinking water to increase the mild to intermediate HHcy, and were submitted to a high-fat diet (HFD). We measured the food intake, body weight gain, body composition, glucose homeostasis, plasma homocysteine level, and CBS activity. We evidenced a defect in the stimulated insulin secretion in CBS+/− mice with mild and intermediate HHcy, while mice with intermediate HHcy under HFD presented an improvement in insulin sensitivity that compensated for the decreased insulin secretion and permitted them to maintain a glucose tolerance similar to the CBS+/+ mice. Islets isolated from CBS+/− mice maintained their ability to respond to the elevated glucose levels, and we showed that a lower parasympathetic tone could, at least in part, be responsible for the insulin secretion defect. Our results emphasize the important role of Hcy metabolic enzymes in insulin secretion and overall glucose homeostasis. [ABSTRACT FROM AUTHOR]

IGF2 is an autocrine ligand for the beta cell IGF1R receptor and GLP-1 increases the activity of this autocrine loop by enhancing IGF1R expression, a mechanism that mediates the trophic effects of GLP-1 on beta cell mass and function. Here, we investigated the regulation of IGF2 biosynthesis and secretion. We showed that glutamine rapidly and strongly induced IGF2 mRNA translation using reporter constructs transduced in MIN6 cells and primary islet cells. This was followed by rapid secretion of IGF2 via the regulated pathway, as revealed by the presence of mature IGF2 in insulin granule fractions and by inhibition of secretion by nimodipine and diazoxide. When maximally stimulated by glutamine, the amount of secreted IGF2 rapidly exceeded its initial intracellular pool and tolbutamide, and high K+ increased IGF2 secretion only marginally. This indicates that the intracellular pool of IGF2 is small and that sustained secretion requires de novo synthesis. The stimulatory effect of glutamine necessitates its metabolism but not mTOR activation. Finally, exposure of insulinomas or beta cells to glutamine induced Akt phosphorylation, an effect that was dependent on IGF2 secretion, and reduced cytokine-induced apoptosis. Thus, glutamine controls the activity of the beta cell IGF2/IGF1R autocrine loop by increasing the biosynthesis and secretion of IGF2. This autocrine loop can thus integrate changes in feeding and metabolic state to adapt beta cell mass and function. [ABSTRACT FROM AUTHOR]

Role of GLP-1 and GIP in beta cell compensatory responses to beta cell attack and insulin resistance were examined in C57BL/6 mice lacking functional receptors for GLP-1 and GIP. Mice were treated with multiple low dose streptozotocin or hydrocortisone. Islet parameters were assessed by immunohistochemistry and hormone measurements were determined by specific enzyme linked immunoassays. Wild-type streptozotocin controls exhibited severe diabetes, irregularly shaped islets with lymphocytic infiltration, decreased Ki67/TUNEL ratio with decreased beta cell and increased alpha cell areas. GLP-1 and GIP were co-expressed with glucagon and numbers of alpha cells mainly expressing GLP-1 were increased. In contrast, hydrocortisone treatment and induction of insulin resistance increased islet numbers and area, with enhanced beta cell replication, elevated mass of beta and alpha cells, together with co-expression of GLP-1 and GIP with glucagon in islets. The metabolic responses to streptozotocin in GLP-1RKO and GIPRKO mice were broadly similar to C57BL/6 controls, although decreases in islet numbers and size were more severe. In contrast, both groups of mice lacking functional incretin receptors displayed substantially impaired islet adaptations to insulin resistance induced by hydrocortisone, including marked curtailment of expansion of islet area, beta cell mass and islet number. Our observations cannot be explained by simple changes in circulating incretin concentrations, suggesting that intra-islet GLP-1 and GIP make a significant contribution to islet adaptation, particularly expansion of beta cell mass and compensatory islet compensation to hydrocortisone and insulin resistance. [ABSTRACT FROM AUTHOR]

Islet adaptations to pregnancy were explored in C57BL6/J mice lacking functional receptors for glucagon-like peptide 1 (GLP-1) and gastric inhibitory polypeptide (GIP). Pregnant wild type mice and GIPRKO mice exhibited marked increases in islet and beta cell area, numbers of medium/large sized islets, with positive effects on Ki67/Tunel ratio favouring beta cell growth and enhanced pancreatic insulin content. Alpha cell area and glucagon content were unchanged but prohormone convertases PC2 and PC1/3 together with significant amounts of GLP-1 and GIP were detected in alpha cells. Knockout of GLP-1R abolished these islet adaptations and paradoxically decreased pancreatic insulin, GLP-1 and GIP. This was associated with abolition of normal pregnancy-induced increases in plasma GIP, L-cell numbers, and intestinal GIP and GLP-1 stores. These data indicate that GLP-1 but not GIP is a key mediator of beta cell mass expansion and related adaptations in pregnancy, triggered in part by generation of intra-islet GLP-1. [ABSTRACT FROM AUTHOR]

Plac8 belongs to an evolutionary conserved family of proteins, mostly abundant in plants where they control fruit weight through regulation of cell number. In mice, Plac8 is expressed both in white and brown adipose tissues and we previously showed that Plac8-/- mice develop late-onset obesity, with abnormal brown fat differentiation and reduced thermogenic capacity. We also showed that in brown adipocytes, Plac8 is an upstream regulator of C/EBPβ expression. Here, we first assessed the role of Plac8 in white adipogenesis in vitro. We show that Plac8 is induced early after induction of 3T3-L1 adipocytes differentiation, a process that is prevented by Plac8 knockdown; similarly, embryonic fibroblasts obtained from Plac8 knockout mice failed to form adipocytes upon stimulation of differentiation. Knockdown of Plac8 in 3T3-L1 was associated with reduced expression of C/EBPβ, Krox20, and Klf4, early regulators of the white adipogenic program, and we show that Plac8 could transactivate the C/EBPβ promoter. In vivo, we show that absence of Plac8 led to increased white fat mass with enlarged adipocytes but reduced total number of adipocytes. Finally, even though Plac8-/- mice showed impaired thermogenesis due to brown fat dysfunction, this was not associated with changes in glycemia or plasma free fatty acid and triglyceride levels. Collectively, these data indicate that Plac8 is an upstream regulator of C/EBPβ required for adipogenesis in vitro. However, in vivo, Plac8 is dispensable for the differentiation of white adipocytes with preserved fat storage capacity but is required for normal fat cell number regulation. [ABSTRACT FROM AUTHOR]

The NLRP3 inflammasome has a major role in regulating innate immunity. Deregulated inflammasome activity is associated with several inflammatory diseases, yet little is known about the signaling pathways that lead to its activation. Here we show that NLRP3 interacted with thioredoxin (TRX)-interacting protein (TXNIP), a protein linked to insulin resistance. Inflammasome activators such as uric acid crystals induced the dissociation of TXNIP from thioredoxin in a reactive oxygen species (ROS)-sensitive manner and allowed it to bind NLRP3. TXNIP deficiency impaired activation of the NLRP3 inflammasome and subsequent secretion of interleukin 1β (IL-1β). Akin to Txnip−/− mice, Nlrp3−/− mice showed improved glucose tolerance and insulin sensitivity. The participation of TXNIP in the NLRP3 inflammasome activation may provide a mechanistic link to the observed involvement of IL-1β in the pathogenesis of type 2 diabetes. [ABSTRACT FROM AUTHOR]

The pancreatic β-cell paradigm for glucose sensing has been proposed to apply to brain glucose sensors controlling counterregulation to hypoglycemia and feeding behavior. Over recent years, we tested this model in mice by first showing that inactivation of the GLUT2 gene suppressed glucose sensing and correctly regulated insulin secretion by pancreatic β-cells. Then, we restored the function of the β-cell in GLUT2-null mice by transgenic expression of a glucose transporter under the control of the rat insulin promoter. Using these rescued mice, we showed that GLUT2-dependent sensors are present in several anatomical sites, including the hepatoportal vein and the central nervous system. When these extrapancreatic glucose sensors are inactivated, the mice display loss of first-phase insulin secretion and hyperglucagonemia in the fed state, and they eat more than control mice--defects characteristic of developing obesity/diabetes. By gene complementation experiments, we further showed that glucose sensors controlling glucagon secretion require GLUT2 expression in glial cells. However, transgenic expression of GLUT2 in astrocytes or neurons failed to restore the normal control of feeding, indicating that different classes of glucose sensors control the response to hypoglycemia and food intake. [ABSTRACT FROM AUTHOR]

Following a former immunohistochemical study in the rat brain [Arluison, M., Quignon, M., Nguyen, P., Thorens, B., Leloup, C., Pénicaud, L. Distribution and anatomical localization of the glucose transporter 2 (GLUT2) in the adult rat brain. I. Immunohistochemical study. J. Chem. Neuroanat., in press], we have analyzed the ultrastructural localization of GLUT2 in representative and/or critical areas of the forebrain and hindbrain. In agreement with previous results, we observe few oligodendrocyte and astrocyte cell bodies discretely labeled for GLUT2 in large myelinated fibre bundles and most brain areas examined, whereas the reactive glial processes are more numerous and often localized in the vicinity of nerve terminals and/or dendrites or dendritic spines forming synaptic contacts. Only some of them appear closely bound to unlabeled nerve cell bodies and dendrites. Furthermore, the nerve cell bodies prominently immunostained for GLUT2 are scarce in the brain nuclei examined, whereas the labeled dendrites and dendritic spines are relatively numerous and frequently engaged in synaptic junctions. In conformity with the observation of GLUT2-immunoreactive rings at the periphery of numerous nerve cell bodies in various brain areas (see previous paper), we report here that some neuronal perikarya of the dorsal endopiriform nucleus/perirhinal cortex exhibit some patches of immunostaining just below the plasma membrane. However, the presence of many GLUT2-immunoreactive nerve terminals and/or astrocyte processes, some of them being occasionally attached to nerve cell bodies and dendrites, could also explain the pericellular labeling observed. The results here reported support the idea that GLUT2 may be expressed by some cerebral neurones possibly involved in glucose sensing, as previously discussed. However, it is also possible that this transporter participate in the regulation of neurotransmitter release and, perhaps, in the release of glucose by glial cells. [Copyright &y& Elsevier]

In the preceding article, we demonstrated that activation of the hepatoportal glucose sensor led to a paradoxical development of hypoglycemia that was associated with increased glucose utilization by a subset of tissues. In this study, we tested whether GLUT2 plays a role in the portal glucose-sensing system that is similar to its involvement in pancreatic beta-cells. Awake RIPGLUT1 x GLUT2-/- and control mice were infused with glucose through the portal (Po-) or the femoral (Fe-) vein for 3 h at a rate equivalent to the endogenous glucose production rate. Blood glucose and plasma insulin concentrations were continuously monitored. Glucose turnover, glycolysis, and glycogen synthesis rates were determined by the 3H-glucose infusion technique. We showed that portal glucose infusion in RIPGLUT1 x GLUT24-/- mice did not induce the hypoglycemia observed in control mice but, in contrast, led to a transient hyperglycemic state followed by a return to normoglycemia; this glycemic pattern was similar to that observed in control Fe-mice and RIPGLUT1 x GLUT2-/- Fe-mice. Plasma insulin profiles during the infusion period were similar in control and RIPGLUT1 x GLUT2-/- Po- and Fe-mice. The lack of hypoglycemia development in RIPGLUT1 x GLUT2-/- mice was not due to the absence of GLUT2 in the liver. Indeed, reexpression by transgenesis of this transporter in hepatocytes did not restore the development of hypoglycemia after initiating portal vein glucose infusion. In the absence of GLUT2, glucose turnover increased in Po-mice to the same extent as that in RIPGLUT1 x GLUT2-/- or control Fe-mice. Finally, co-infusion of somatostatin with glucose prevented development of hypoglycemia in control Po-mice, but it did not affect the glycemia or insulinemia of RIPGLUT1 x GLUT2-/- Po-mice. Together, our data demonstrate that GLUT2 is required for the function of the hepatoportal glucose sensor and that somatostatin could inhibit the glucose signal by interfering with GLUT2-expressing sensing units. [ABSTRACT FROM AUTHOR]

To analyze the role of the murine hepatoportal glucose sensor in the control of whole-body glucose metabolism, we infused glucose at a rate corresponding to the endogenous glucose production rate through the portal vein of conscious mice (Po-mice) that were fasted for 6 h. Mice infused with glucose at the same rate through the femoral vein (Fe-mice) and mice infused with a saline solution (Sal-mice) were used as controls. In Po-mice, hypoglycemia progressively developed until glucose levels dropped to a nadir of 2.3 +/- 0.1 mmol/l, whereas in Fe-mice, glycemia rapidly and transiently developed, and glucose levels increased to 7.7 +/- 0.6 mmol/l before progressively returning to fasting glycemic levels. Plasma insulin levels were similar in both Po- and Fe-mice during and at the end of the infusion periods (21.2 +/- 2.2 vs. 25.7 +/- 0.9 microU/ml, respectively, at 180 min of infusion). The whole-body glucose turnover rate was significantly higher in Po-mice than in Fe-mice (45.9 +/- 3.8 vs. 37.7 +/- 2.0 mg x kg(-1) x min)-1), respectively) and in Sal-mice (24.4 +/- 1.8 mg x kg(-1) x min(-1)). Somatostatin co-infusion with glucose in Po-mice prevented hypoglycemia without modifying the plasma insulin profile. Finally, tissue glucose clearance, which was determined after injecting 14C-2-deoxyglucose, increased to a higher level in Po-mice versus Fe-mice in the heart, brown adipose tissue, and the soleus muscle. Our data show that stimulation of the hepatoportal glucose sensor induced hypoglycemia and increased glucose utilization by a combination of insulin-dependent and insulin-independent or -sensitizing mechanisms. Furthermore, activation of the glucose sensor and/or transmission of its signal to target tissues can be blocked by somatostatin. [ABSTRACT FROM AUTHOR]

We previously reported that pancreatic islet beta-cells from GLUT2-null mice lost the first phase but preserved the second phase of glucose-stimulated insulin secretion (GSIS). Furthermore, we showed that the remaining secretory activity required glucose uptake and metabolism because it can be blocked by inhibition of oxidative phosphorylation. Here, we extend these previous studies by analyzing, in GLUT2-null islets, glucose transporter isoforms and glucokinase expression and by measuring glucose usage, GSIS, and glucose-stimulated insulin mRNA biosynthesis. We show that in the absence of GLUT2, no compensatory expression of either GLUT1 or GLUT3 is observed and that glucokinase is expressed at normal levels. Glucose usage by isolated islets was increased between 1 and 6 mmol/l glucose but was not further increased between 6 and 20 mmol/l glucose. Parallel GSIS measurements showed that insulin secretion was not stimulated between 2.8 and 6 mmol/l glucose but was increased by >4-fold between 6 and 20 mmol/l glucose. Stimulation by glucose of total protein and insulin biosynthesis was also markedly impaired in the absence of GLUT2. Finally, we re-expressed GLUT2 in GLUT2-null beta-cells using recombinant lentiviruses and demonstrated a restoration of normal GSIS. Together, these data show that in the absence of GLUT2, glucose can still be taken up by beta-cells, albeit at a low rate, and that this transport activity is unlikely to be attributed to GLUT1 or GLUT3. This uptake activity, however, is limiting for normal glucose utilization and signaling to secretion and translation. These data further demonstrate the key role of GLUT2 in murine beta-cells for glucose signaling to insulin secretion and biosynthesis. [ABSTRACT FROM AUTHOR]

The counterregulatory response to hypoglycemia is an essential survival function. It is controlled by an integrated network of glucose-responsive neurons, which trigger endogenous glucose production to restore normoglycemia. The complexity of this glucoregulatory network is, however, only partly characterized. In a genetic screen of a panel of recombinant inbred mice we previously identified Fgf15, expressed in neurons of the dorsomedial hypothalamus (DMH), as a negative regulator of glucagon secretion. Here, we report on the generation of Fgf15CretdTomato mice and their use to further characterize these neurons. We show that they were glutamatergic and comprised glucose-inhibited and glucose-excited neurons. When activated by chemogenetics, Fgf15 neurons prevented the increase in vagal nerve firing and the secretion of glucagon normally triggered by insulin-induced hypoglycemia. On the other hand, they increased the activity of the sympathetic nerve in the basal state and prevented its silencing by glucose overload. Higher sympathetic tone increased hepatic Creb1 phosphorylation, Pck1 mRNA expression, and hepatic glucose production leading to glucose intolerance. Thus, Fgf15 neurons of the DMH participate in the counterregulatory response to hypoglycemia by a direct adrenergic stimulation of hepatic glucose production while suppressing vagally induced glucagon secretion. This study provides new insights into the complex neuronal network that prevents the development of hypoglycemia. [ABSTRACT FROM AUTHOR]

Proopiomelanocortin (POMC) neurons are major regulators of energy balance and glucose homeostasis. In addition to being regulated by hormones and nutrients, POMC neurons are controlled by glutamatergic input originating from multiple brain regions. However, the factors involved in the formation of glutamatergic inputs and how they contribute to bodily functions remain largely unknown. Here, we show that during the development of glutamatergic inputs, POMC neurons exhibit enriched expression of the Efnb1 (EphrinB1) and Efnb2 (EphrinB2) genes, which are known to control excitatory synapse formation. In vivo loss of Efnb1 in POMC-expressing progenitors decreases the amount of glutamatergic inputs, associated with a reduced number of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor subunits and excitability of these cells. We found that mice lacking Efnb1 in POMC-expressing progenitors display impaired glucose tolerance due to blunted vagus nerve activity and decreased insulin secretion. However, despite reduced excitatory inputs, mice lacking Efnb2 in POMC-expressing progenitors showed no deregulation of insulin secretion and only mild alterations in feeding behavior and gluconeogenesis. Collectively, our data demonstrate the role of ephrins in controlling excitatory input amount into POMC-expressing progenitors and show an isotype-specific role of ephrins on the regulation of glucose homeostasis and feeding. Proopiomelanocortin- (POMC-) expressing neurons are major regulators of energy balance and glucose homeostasis. Arcuate POMC neurons primarily receive glutamatergic inputs, but the mechanisms underlying their formation are still unknown. This study reveals a role for EphrinB1 and EphrinB2 in the development and function of glutamatergic inputs into POMC-expressing progenitors. [ABSTRACT FROM AUTHOR]

The ventromedial nucleus of the hypothalamus (VMN) is involved in the counterregulatory response to hypoglycemia. VMN neurons activated by hypoglycemia (glucose-inhibited [GI] neurons) have been assumed to play a critical although untested role in this response. Here, we show that expression of a dominant negative form of AMPK or inactivation of AMPK α1 and α2 subunit genes in Sf1 neurons of the VMN selectively suppressed GI neuron activity. We found that Txn2, encoding a mitochondrial redox enzyme, was strongly downregulated in the absence of AMPK activity and that reexpression of Txn2 in Sf1 neurons restored GI neuron activity. In cell lines, Txn2 was required to limit glucopenia-induced reactive oxygen species production. In physiological studies, absence of GI neuron activity after AMPK suppression in the VMN had no impact on the counterregulatory hormone response to hypoglycemia or on feeding. Thus, AMPK is required for GI neuron activity by controlling the expression of the antioxidant enzyme Txn2. However, the glucose-sensing capacity of VMN GI neurons is not required for the normal counterregulatory response to hypoglycemia. Instead, it may represent a fail-safe system in case of impaired hypoglycemia sensing by peripherally located glucose detection systems that are connected to the VMN. [ABSTRACT FROM AUTHOR]

Glucose transporter 2 (Glut2)-positive cells are sparsely distributed in brain and play an important role in the stimulation of glucagon secretion in response to hypoglycemia. We aimed to determine if Glut2 -positive cells can influence another response to hypoglycemia, i.e. increased cerebral blood flow (CBF). CBF of adult male mice devoid of Glut2, either globally (ripglut1:glut2−/−) or in the nervous system only (NG2KO), and their respective controls were studied under basal glycemia and insulin-induced hypoglycemia using quantitative perfusion magnetic resonance imaging at 9.4 T. The effect on CBF of optogenetic activation of hypoglycemia responsive Glut2 -positive neurons of the paraventricular thalamic area was measured in mice expressing channelrhodopsin2 under the control of the Glut2 promoter. We found that in both ripglut1:glut2−/− mice and NG2KO mice, CBF in basal conditions was higher than in their respective controls and not further activated by hypoglycemia, as measured in the hippocampus, hypothalamus and whole brain. Conversely, optogenetic activation of Glut2 -positive cells in the paraventricular thalamic nucleus induced a local increase in CBF similar to that induced by hypoglycemia. Thus, Glut2 expression in the nervous system is required for the control of CBF in response to changes in blood glucose concentrations. [ABSTRACT FROM AUTHOR]

Uric acid (UA) is a metabolite of purine degradation and is involved in gout flairs and kidney stones formation. GLUT9 (SLC2A9) was previously shown to be a urate transporter in vitro. In vivo, humans carrying GLUT9 loss-of-function mutations have familial renal hypouricemia type 2, a condition characterized by hypouricemia, UA renal wasting associated with kidney stones, and an increased propensity to acute renal failure during strenuous exercise. Mice carrying a deletion of GLUT9 in the whole body are hyperuricemic and display a severe nephropathy due to intratubular uric acid precipitation. However, the precise role of GLUT9 in the kidney remains poorly characterized. We developed a mouse model in which GLUT9 was deleted specifically along the whole nephron in a tetracycline-inducible manner (subsequently called kidney-inducible KO or kiKO). The urate/creatinine ratio was increased as early as 4 days after induction of the KO and no GLUT9 protein was visible on kidney extracts. kiKO mice are morphologically identical to their wild-type littermates and had no spontaneous kidney stones. Twenty-four-hour urine collection revealed a major increase of urate urinary excretion rate and of the fractional excretion of urate, with no difference in urate concentration in the plasma. Polyuria was observed, but kiKO mice were still able to concentrate urine after water restriction. KiKO mice displayed lower blood pressure accompanied by an increased heart rate. Overall, these results indicate that GLUT9 is a crucial player in renal handling of urate in vivo and a putative target for uricosuric drugs. [ABSTRACT FROM AUTHOR]

Glucokinase (Gck) is a critical regulator of glucose-induced insulin secretion by pancreatic β-cells. It has been suggested to also play an important role in glucose signaling in neurons of the ventromedial hypothalamic nucleus (VMN), a brain nucleus involved in the control of glucose homeostasis and feeding. To test the role of Gck in VMN glucose sensing and physiological regulation, we studied mice with genetic inactivation of the Gck gene in Sf1 neurons of the VMN (Sf1Gck(-/-) mice). Compared with control littermates, Sf1Gck(-/-) mice displayed increased white fat mass and adipocyte size, reduced lean mass, impaired hypoglycemia-induced glucagon secretion, and a lack of parasympathetic and sympathetic nerve activation by neuroglucopenia. However, these phenotypes were observed only in female mice. To determine whether Gck was required for glucose sensing by Sf1 neurons, we performed whole-cell patch clamp analysis of brain slices from control and Sf1Gck(-/-) mice. Absence of Gck expression did not prevent the glucose responsiveness of glucose-excited or glucose-inhibited Sf1 neurons in either sex. Thus Gck in the VMN plays a sex-specific role in the glucose-dependent control of autonomic nervous activity; this is, however, unrelated to the control of the firing activity of classical glucose-responsive neurons. [ABSTRACT FROM AUTHOR]

Key points Glucose transporters are central players in glucose homeostasis., There are two major classes of glucose transporters in the body, the passive facilitative glucose transporters (GLUTs) and the secondary active sodium-coupled glucose transporters (SGLTs)., In the present study, we report the use of a non-invasive imaging technique, positron emission tomography, in mice aiming to evaluate the role of GLUTs and SGLTs in controlling glucose distribution and utilization., We show that GLUTs are most significant for glucose uptake into the brain and liver, whereas SGLTs are important in glucose recovery in the kidney., This work provides further support for the use of SGLT imaging in the investigation of the role of SGLT transporters in human physiology and diseases such as diabetes and cancer., Abstract The importance of sodium-coupled glucose transporters (SGLTs) and facilitative glucose transporters (GLUTs) in glucose homeostasis was studied in mice using fluorine-18 labelled glucose molecular imaging probes and non-invasive positron emission tomography (PET) imaging. The probes were: α-methyl-4-[F-18]-fluoro-4-deoxy- d-glucopyranoside (Me-4FDG), a substrate for SGLTs; 4-deoxy-4-[F-18]-fluoro- d-glucose (4-FDG), a substrate for SGLTs and GLUTs; and 2-deoxy-2-[F-18]-fluoro- d-glucose (2-FDG), a substrate for GLUTs. These radiolabelled imaging probes were injected i.v. into wild-type, Sglt1-/-, Sglt2-/- and Glut2-/- mice and their dynamic whole-body distribution was determined using microPET. The distribution of 2-FDG was similar to that reported earlier (i.e. it accumulated in the brain, heart, liver and kidney, and was excreted into the urinary bladder). There was little change in the distribution of 2-FDG in Glut2-/- mice, apart from a reduction in the rate of uptake into liver. The major differences between Me-4FDG and 2-FDG were that Me-4FDG did not enter the brain and was not excreted into the urinary bladder. There was urinary excretion of Me-4FDG in Sglt1-/- and Sglt2-/- mice. However, Me-4FDG was not reabsorbed in the kidney in Glut2-/- mice. There were no differences in Me-4FDG uptake into the heart of wild-type, Sglt1-/- and Sglt2-/- mice. We conclude that GLUT2 is important in glucose liver transport and reabsorption of glucose in the kidney along with SGLT2 and SGLT1. Complete reabsorption of Me-4FDG from the glomerular filtrate in wild-type mice and the absence of reabsorption in the kidney in Glut2-/- mice confirm the importance of GLUT2 in glucose absorption across the proximal tubule. [ABSTRACT FROM AUTHOR]

Preserving β-cell function during the development of obesity and insulin resistance would limit the worldwide epidemic of type 2 diabetes. Endoplasmic reticulum (ER) calcium (Ca(2+)) depletion induced by saturated free fatty acids and cytokines causes β-cell ER stress and apoptosis, but the molecular mechanisms behind these phenomena are still poorly understood. Here, we demonstrate that palmitate-induced sorcin downregulation and subsequent increases in glucose-6-phosphatase catalytic subunit-2 (G6PC2) levels contribute to lipotoxicity. Sorcin is a calcium sensor protein involved in maintaining ER Ca(2+) by inhibiting ryanodine receptor activity and playing a role in terminating Ca(2+)-induced Ca(2+) release. G6PC2, a genome-wide association study gene associated with fasting blood glucose, is a negative regulator of glucose-stimulated insulin secretion (GSIS). High-fat feeding in mice and chronic exposure of human islets to palmitate decreases endogenous sorcin expression while levels of G6PC2 mRNA increase. Sorcin-null mice are glucose intolerant, with markedly impaired GSIS and increased expression of G6pc2 Under high-fat diet, mice overexpressing sorcin in the β-cell display improved glucose tolerance, fasting blood glucose, and GSIS, whereas G6PC2 levels are decreased and cytosolic and ER Ca(2+) are increased in transgenic islets. Sorcin may thus provide a target for intervention in type 2 diabetes. [ABSTRACT FROM AUTHOR]

Insulin-like growth factor 2 (IGF2), produced and secreted by adult β-cells, functions as an autocrine activator of the β-cell insulin-like growth factor 1 receptor signaling pathway. Whether this autocrine activity of IGF2 plays a physiological role in β-cell and whole-body physiology is not known. Here, we studied mice with β-cell-specific inactivation of Igf2 (βIGF2KO mice) and assessed β-cell mass and function in aging, pregnancy, and acute induction of insulin resistance. We showed that glucose-stimulated insulin secretion (GSIS) was markedly reduced in old female βIGF2KO mice; glucose tolerance was, however, normal because of increased insulin sensitivity. While on a high-fat diet, both male and female βIGF2KO mice displayed lower GSIS compared with control mice, but reduced β-cell mass was observed only in female βIGF2KO mice. During pregnancy, there was no increase in β-cell proliferation and mass in βIGF2KO mice. Finally, β-cell mass expansion in response to acute induction of insulin resistance was lower in βIGF2KO mice than in control mice. Thus, the autocrine action of IGF2 regulates adult β-cell mass and function to preserve in vivo GSIS in aging and to adapt β-cell mass in response to metabolic stress, pregnancy hormones, and acute induction of insulin resistance. [ABSTRACT FROM AUTHOR]

Urate is the metabolic end point of purines in humans. Although supra-physiological plasma urate levels are associated with obesity, insulin resistance, dyslipidemia, and hypertension, a causative role is debated. We previously established a mouse model of hyperuricemia by liver-specific deletion of Glut9, a urate transporter that provides urate to the hepatocyte enzyme uricase. These LG9 knockout mice show mild hyperuricemia (120 μmol/l), which can be further increased by the urate precursor inosine. Here, we explored the role of progressive hyperuricemia on the cardiovascular function. Arterial blood pressure and heart rate were periodically measured by telemetry over 6 months in LG9 knockout mice supplemented with incremental amounts of inosine in a normal chow diet. This long-term inosine treatment elicited a progressive increase in uricemia up to 300 μmol/l; however, it did not modify heart rate or mean arterial blood pressure in LG9 knockout compared with control mice. Inosine treatment did not alter cardiac morphology or function measured by ultrasound echocardiography. However, it did induce mild renal dysfunction as revealed by higher plasma creatinine levels, lower glomerular filtration rate, and histological signs of chronic inflammation and fibrosis. Thus, in LG9 knockout mice, inosine-induced hyperuricemia was not associated with hypertension despite partial renal deficiency. This does not support a direct role of urate in the control of blood pressure. [ABSTRACT FROM AUTHOR]

In the nervous system, NMDA receptors (NMDARs) participate in neurotransmission and modulate the viability of neurons. In contrast, little is known about the role of NMDARs in pancreatic islets and the insulin-secreting beta cells whose functional impairment contributes to diabetes mellitus. Here we found that inhibition of NMDARs in mouse and human islets enhanced their glucose-stimulated insulin secretion (GSIS) and survival of islet cells. Further, NMDAR inhibition prolonged the amount of time that glucose-stimulated beta cells spent in a depolarized state with high cytosolic Ca2+ concentrations. We also noticed that, in vivo, the NMDAR antagonist dextromethorphan (DXM) enhanced glucose tolerance in mice, and that in vitro dextrorphan, the main metabolite of DXM, amplified the stimulatory effect of exendin-4 on GSIS. In a mouse model of type 2 diabetes mellitus (T2DM), long-term treatment with DXM improved islet insulin content, islet cell mass and blood glucose control. Further, in a small clinical trial we found that individuals with T2DM treated with DXM showed enhanced serum insulin concentrations and glucose tolerance. Our data highlight the possibility that antagonists of NMDARs may provide a useful adjunct treatment for diabetes. [ABSTRACT FROM AUTHOR]

Fully differentiated pancreatic β cells are essential for normal glucose homeostasis in mammals. Dedifferentiation of these cells has been suggested to occur in type 2 diabetes, impairing insulin production. Since chronic fuel excess ("glucotoxicity") is implicated in this process, we sought here to identify the potential roles in β-cell identity of the tumor suppressor liver kinase B1 (LKB1/STK11) and the downstream fuel-sensitive kinase, AMP-activated protein kinase (AMPK). Highly β-cell-restricted deletion of each kinase in mice, using an Ins1-controlled Cre, was therefore followed by physiological, morphometric, and massive parallel sequencing analysis. Loss of LKB1 strikingly (2.0-12-fold, E<0.01) increased the expression of subsets of hepatic (Alb, Iyd, Elovl2) and neuronal (Nptx2, Dlgap2, Cartpt, Pdyn) genes, enhancing glutamate signaling. These changes were partially recapitulated by the loss of AMPK, which also up-regulated β-ceU "disallowed" genes (Slc16a1, Ldha, Mgst1, Pdgfra) 1.8- to 3.4-fold (E<0.01). Correspondingly, targeted promoters were enriched for neuronal (Zfp206; P=1.3 x 10-33) and hypoxia-regulated (HIF1; P=2.5 x 10-16) transcription factors. In summary, LKB1 and AMPK, through only partly overlapping mechanisms, maintain β-cell identity by suppressing alternate pathways leading to neuronal, hepatic, and other characteristics. Selective targeting of these enzymes may provide a new approach to maintaining β-cell function in some forms of diabetes. [ABSTRACT FROM AUTHOR]

Background/Aims: Gluco-incretin hormones increase the glucose competence of pancreatic beta-cells by incompletely characterized mechanisms. Methods: We searched for genes that were differentially expressed in islets from control and Glp1r−/−; Gipr−/− (dKO) mice, which show reduced glucose competence. Overexpression and knockdown studies; insulin secretion analysis; analysis of gene expression in islets from control and diabetic mice and humans as well as gene methylation and transcriptional analysis were performed. Results: Fxyd3 was the most up-regulated gene in glucose incompetent islets from dKO mice. When overexpressed in beta-cells Fxyd3 reduced glucose-induced insulin secretion by acting downstream of plasma membrane depolarization and Ca++ influx. Fxyd3 expression was not acutely regulated by cAMP raising agents in either control or dKO adult islets. Instead, expression of Fxyd3 was controlled by methylation of CpGs present in its proximal promoter region. Increased promoter methylation reduced Fxyd3 transcription as assessed by lower abundance of H3K4me3 at the transcriptional start site and in transcription reporter assays. This epigenetic imprinting was initiated perinatally and fully established in adult islets. Glucose incompetent islets from diabetic mice and humans showed increased expression of Fxyd3 and reduced promoter methylation. Conclusions/Interpretation: Because gluco-incretin secretion depends on feeding the epigenetic regulation of Fxyd3 expression may link nutrition in early life to establishment of adult beta-cell glucose competence; this epigenetic control is, however, lost in diabetes possibly as a result of gluco-incretin resistance and/or de-differentiation of beta-cells that are associated with the development of type 2 diabetes. [ABSTRACT FROM AUTHOR]

Intestinal glucose absorption is mediated by SGLT1 whereas GLUT2 is considered to provide basolateral exit. Recently, it was proposed that GLUT2 can be recruited into the apical membrane after a high luminal glucose bolus allowing bulk absorption of glucose by facilitated diffusion. Moreover, SGLT1 and GLUT2 are suggested to play an important role in intestinal glucose sensing and incretin secretion. In mice that lack either SGLT1 or GLUT2 we re-assessed the role of these transporters in intestinal glucose uptake after radiotracer glucose gavage and performed Western blot analysis for transporter abundance in apical membrane fractions in a comparative approach. Moreover, we examined the contribution of these transporters to glucose-induced changes in plasma GIP, GLP-1 and insulin levels. In mice lacking SGLT1, tissue retention of tracer glucose was drastically reduced throughout the entire small intestine whereas GLUT2-deficient animals exhibited higher tracer contents in tissue samples than wild type animals. Deletion of SGLT1 resulted also in reduced blood glucose elevations and abolished GIP and GLP-1 secretion in response to glucose. In mice lacking GLUT2, glucose-induced insulin but not incretin secretion was impaired. Western blot analysis revealed unchanged protein levels of SGLT1 after glucose gavage. GLUT2 detected in apical membrane fractions mainly resulted from contamination with basolateral membranes but did not change in density after glucose administration. SGLT1 is unequivocally the prime intestinal glucose transporter even at high luminal glucose concentrations. Moreover, SGLT1 mediates glucose-induced incretin secretion. Our studies do not provide evidence for GLUT2 playing any role in either apical glucose influx or incretin secretion. [ABSTRACT FROM AUTHOR]