Aims/hypothesisPancreatic beta cells play a central role in the control of glucose homeostasis by secreting insulin to stimulate glucose uptake by peripheral tissues. Understanding the molecular mechanisms that control beta cell function and plasticity has critical implications for the pathophysiology and therapy of major forms of diabetes. Selective gene inactivation in pancreatic beta cells, using the Cre-lox system, is a powerful approach to assess the role of particular genes in beta cells and their impact on whole body glucose homeostasis. Several Cre recombinase (Cre) deleter mice have been established to allow inactivation of genes in beta cells, but many show non-specific recombination in other cell types, often in the brain.MethodsWe describe the generation of Ins1Cre and Ins1CreERT2 mice in which the Cre or Cre-oestrogen receptor fusion protein (CreERT2) recombinases have been introduced at the initiation codon of the Ins1 gene.ResultsWe show that Ins1Cre mice induce efficient and selective recombination of floxed genes in beta cells from the time of birth, with no recombination in the central nervous system. These mice have normal body weight and glucose homeostasis. Furthermore, we show that tamoxifen treatment of adult Ins1CreERT2 mice crossed with Rosa26-tdTomato mice induces efficient recombination in beta cells.Conclusions/interpretationThese two strains of deleter mice are useful new resources to investigate the molecular physiology of pancreatic beta cells.Electronic supplementary materialThe online version of this article (doi:10.1007/s00125-014-3468-5) contains peer-reviewed but unedited supplementary material, which is available to authorised users.

Ripglut1;glut2 -/-mice have no endogenous glucose transporter type 2 (glut2) gene expression but rescue glucoseregulated insulin secretion. Control of glucagon plasma levels is, however, abnormal, with fed hyperglucagonemia and insensitivity to physiological hypo-or hyperglycemia, indicating that GLUT2-dependent sensors control glucagon secretion. Here, we evaluated whether these sensors were located centrally and whether GLUT2 was expressed in glial cells or in neurons. We showed that ripglut1;glut2 -/-mice failed to increase plasma glucagon levels following glucoprivation induced either by i.p. or intracerebroventricular 2-deoxy-D-glucose injections. This was accompanied by failure of 2-deoxy-D-glucose injections to activate c-Fos-like immunoreactivity in the nucleus of the tractus solitarius and the dorsal motor nucleus of the vagus. When glut2 was expressed by transgenesis in glial cells but not in neurons of ripglut1;glut2 -/-mice, stimulated glucagon secretion was restored as was c-Fos-like immunoreactive labeling in the brainstem. When ripglut1;glut2 -/-mice were backcrossed into the C57BL/6 genetic background, fed plasma glucagon levels were also elevated due to abnormal autonomic input to the a cells; glucagon secretion was, however, stimulated by hypoglycemic stimuli to levels similar to those in control mice. These studies identify the existence of central glucose sensors requiring glut2 expression in glial cells and therefore functional coupling between glial cells and neurons. These sensors may be activated at different glycemic levels depending on the genetic background.

Feeding behavior is governed by homeostatic needs and motivational drive to obtain palatable foods. Here, we identify a population of glutamatergic neurons in the paraventricular thalamus, which express the glucose transporter Glut2 (Scl2a2) and project to the nucleus accumbens. These neurons are activated by hypoglycemia and, in freely moving mice, their activation by optogenetics or Slc2a2 inactivation increases motivated sucrose but not saccharin-seeking behavior. These neurons may control sugar overconsumption in obesity and diabetes.

A role for glucose in the control of feeding has been proposed, but its precise physiological importance is unknown. Here, we evaluated feeding behavior in glut2-null mice, which express a transgenic glucose transporter in their ␤-cells to rescue insulin secretion (ripglut1;glut2 Ϫ/Ϫ mice). We showed that in the absence of GLUT2, daily food intake was increased and feeding initiation and termination following a fasting period were abnormal. This was accompanied by suppressed regulation of hypothalamic orexigenic and anorexigenic neuropeptides expression during the fast-to-refed transition. In these conditions, however, there was normal regulation of the circulating levels of insulin, leptin, or glucose but a loss of regulation of plasma ghrelin concentrations. To evaluate whether the abnormal feeding behavior was due to suppressed glucose sensing, we evaluated feeding in response to intraperitoneal or intracerebroventricular glucose or 2-deoxy-D-glucose injections. We showed that in GLUT2-null mice, feeding was no longer inhibited by glucose or activated by 2-deoxy-D-glucose injections and the regulation of hypothalamic neuropeptide expression by intracerebroventricular glucose administration was lost. Together, these data demonstrate that absence of GLUT2 suppresssed the function of central glucose sensors, which control feeding probably by regulating the hypothalamic melanocortin pathway. Futhermore, inactivation of these glucose sensors causes overeating. Diabetes 55: 988 -995, 2006 T he control of body weight depends on the balance between food intake and energy expenditure. The current epidemic of obesity, which represents a major risk factor for the development of type 2 diabetes and cardiovascular diseases, is caused by a dysregulation of this homeostatic process (1). Both internal and environmental signals cooperate to trigger or terminate food intake and to stimulate anabolic or catabolic pathways. The internal signals are hormones derived from the gut, such as ghrelin, cholecystokinin, glucagon-like peptide-1, or peptide YY 3-36 , from adipocytes (leptin), and pancreatic ␤ cells (insulin) but also nutrients such as glucose and lipids. These signals are integrated by the central nervous system to control feeding and energy expenditure (2,3). In this integrative function, the melanocortin pathway of the hypothalamus plays a critical role, as it is directly regulated by hormones and nutrients (4 -9). This pathway consists of neurons of the arcuate nucleus, which synthesize either orexigenic (neuropeptide Y [NPY] and agouti-related peptide [AgRP]) or anorexigenic (proopiomelanocortin [POMC] and cocaine-and amphetaminerelated transcript [CART]) neuropeptides. These then regulate second-order neurons, in particular those located in the paraventricular hypothalamic nucleus (PVN) or the lateral hypothalamus (LH). Whereas the PVN neurons express anorexigenic peptides such as thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH), LH neurons express the orexigenic peptides orexins and...

Liver glucose metabolism plays a central role in glucose homeostasis and may also regulate feeding and energy expenditure. Here we assessed the impact of glucose transporter 2 (Glut2) gene inactivation in adult mouse liver (LG2KO mice). Loss of Glut2 suppressed hepatic glucose uptake but not glucose output. In the fasted state, expression of carbohydrate-responsive element-binding protein (ChREBP) and its glycolytic and lipogenic target genes was abnormally elevated. Feeding, energy expenditure, and insulin sensitivity were identical in LG2KO and control mice. Glucose tolerance was initially normal after Glut2 inactivation, but LG2KO mice exhibited progressive impairment of glucose-stimulated insulin secretion even though β cell mass and insulin content remained normal. Liver transcript profiling revealed a coordinated downregulation of cholesterol biosynthesis genes in LG2KO mice that was associated with reduced hepatic cholesterol in fasted mice and reduced bile acids (BAs) in feces, with a similar trend in plasma. We showed that chronic BAs or farnesoid X receptor (FXR) agonist treatment of primary islets increases glucose-stimulated insulin secretion, an effect not seen in islets from Fxr -/-mice. Collectively, our data show that glucose sensing by the liver controls β cell glucose competence and suggest BAs as a potential mechanistic link. IntroductionHepatic glucose metabolism is highly regulated during the fed-tofast transition by changes in plasma levels of insulin and glucagon, but also by the changes in blood glucose concentrations. In the fed state, the presence of high insulin concentrations in the portal circulation favors storage of glucose in the form of glycogen and the use of glucose through the glycolytic pathway for its conversion into fatty acids. Important regulatory events activated during the absorptive phase include the transcriptional induction of glucokinase by insulin and of L-pyruvate kinase by the carbohydrate-responsive element-binding protein (ChREBP), which translocates to the nucleus following its dephosphorylation by a glucose metabolite-activated phosphatase (1). At the same time, glucose inhibits glycogen phosphorylase through inhibition of glycogen phosphorylase phosphatase, whereas glucose-6-phosphate activates glycogen synthase (2), thus favoring glycogen biosynthesis. The combination of insulin-dependent Srebp-1c and glucose-dependent ChREBP activation then induces the expression of lipogenic genes, including Acc, Fas, and Scd1 (1, 3).In the fasted state, the decrease in glycemia reduces the intracellular levels of glucose and glucose-6-phosphate, thereby favoring glycogen degradation and reducing the activation of ChREBP and the expression of L-pyruvate kinase and lipogenic genes. Higher glucagon levels favor the gluconeogenic pathway by inducing the expression of PEPCK and G6Pase that catalyzes the hydrolysis of glucose-6-phosphate into glucose, a reaction that takes place in the lumen of the ER. The last steps of glucose output