Functional glucokinase isoforms are expressed in rat brain

Type 2 Diabetes Mellitus (T2DM) and Carbohydrate Metabolism in Relation to T2DM from Endocrinology, Neurophysiology, Molecular Biology, and Biochemistry Perspectives

Type 2 diabetes mellitus (T2DM) is a severe disease caused by metabolic disorders, particularly carbohydrate metabolism disorders. The disease is a fatal global trouble characterised by high prevalence rates, causing death, blindness, kidney failure, myocardial infarction, amputation of lower limps, and stroke. Biochemical metabolic pathways like glycolysis, gluconeogenesis, glycogenesis, and glycogenolysis are critical pathways that regulate blood glucose levels with the glucokinase (GK) enzyme playing a central role in glucose homeostasis. Any factor that perturbs the aforementioned biochemical pathways is detrimental. Endocrinological, neurophysiological, and molecular biological pathways that are linked to carbohydrate metabolism should be studied, grasped, and manipulated in order to alleviate T2DM global chaos. The challenge, howbeit, is that, since the body is an integration of systems that complement one another, studying one “isolated” system is not very useful. This paper serves to discuss endocrinology, neurophysiology, and molecular biology pathways that are involved in carbohydrate metabolism in relation to T2DM.

Significance of Brain Glucose Hypometabolism, Altered Insulin Signal Transduction, and Insulin Resistance in Several Neurological Diseases

Several neurological diseases share pathological alterations, even though they differ in their etiology. Neuroinflammation, altered brain glucose metabolism, oxidative stress, mitochondrial dysfunction and amyloidosis are biological events found in those neurological disorders. Altered insulin-mediated signaling and brain glucose hypometabolism are characteristic signs observed in the brains of patients with certain neurological diseases, but also others such as type 2 diabetes mellitus and vascular diseases. Thus, significant reductions in insulin receptor autophosphorylation and Akt kinase activity, and increased GSK-3 activity and insulin resistance, have been reported in these neurological diseases as contributing to the decline in cognitive function. Supporting this relationship is the fact that nasal and hippocampal insulin administration has been found to improve cognitive function. Additionally, brain glucose hypometabolism precedes the unmistakable clinical manifestations of some of these diseases by years, which may become a useful early biomarker. Deficiencies in the major pathways of oxidative energy metabolism have been reported in patients with several of these neurological diseases, which supports the hypothesis of their metabolic background. This review remarks on the significance of insulin and brain glucose metabolism alterations as keystone common pathogenic substrates for certain neurological diseases, highlighting new potential targets.

Storage and Utilization of Glycogen by Mouse Liver during Adaptation to Nutritional Changes Are GLP-1 and PASK Dependent

Glucagon-like peptide 1 (GLP-1) and PAS kinase (PASK) control glucose and energy homeostasis according to nutritional status. Thus, both glucose availability and GLP-1 lead to hepatic glycogen synthesis or degradation. We used a murine model to discover whether PASK mediates the effect of exendin-4 (GLP-1 analogue) in the adaptation of hepatic glycogen metabolism to nutritional status. The results indicate that both exendin-4 and fasting block the Pask expression, and PASK deficiency disrupts the physiological levels of blood GLP1 and the expression of hepatic GLP1 receptors after fasting. Under a non-fasted state, exendin-4 treatment blocks AKT activation, whereby Glucokinase and Sterol Regulatory Element-Binding Protein-1c (Srebp1c) expressions were inhibited. Furthermore, the expression of certain lipogenic genes was impaired, while increasing Glucose Transporter 2 (GLUT2) and Glycogen Synthase (GYS). Moreover, exendin-4 treatment under fasted conditions avoided Glucose 6-Phosphatase (G6pase) expression, while maintaining high GYS and its activation state. These results lead to an abnormal glycogen accumulation in the liver under fasting, both in PASK-deficient mice and in exendin-4 treated wild-type mice. In short, exendin-4 and PASK both regulate glucose transport and glycogen storage, and some of the exendin-4 effects could therefore be due to the blocking of the Pask expression.

Glucokinase activation as antidiabetic therapy: effect of nutraceuticals and phytochemicals on glucokinase gene expression and enzymatic activity

Diabetes represents an important public health problem. Recently, new molecular targets have been identified and exploited to treat this disease. Due to its pivotal role in glucose homeostasis, glucokinase (GCK) is a promising target for the development of novel antidiabetic drugs; however, pharmacological agents that modulate GCK activity have been linked to undesirable side-effects, limiting its use. Interestingly, plants might be a valuable source of new therapeutic compounds with GCK-activating properties and presumably no adverse effects. In this review, we describe biochemical characteristics related to the physiological and pathological importance of GCK, as well as the mechanisms involved in its regulation at different molecular levels. Posteriorly, we present a compendium of findings supporting the potential use of nutraceuticals and phytochemicals in the management of diabetes through modulation of GCK expression and activity. Finally, we propose critical aspects to keep in mind when designing experiments to evaluate GCK modulation properly.

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

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

When a Little Bit More Makes the Difference: Expression Levels of GKRP Determines the Subcellular Localization of GK in Tanycytes

Glucose homeostasis is performed by specialized cells types that detect and respond to changes in systemic glucose concentration. Hepatocytes, β-cells and hypothalamic tanycytes are part of the glucosensor cell types, which express several proteins involved in the glucose sensing mechanism such as GLUT2, Glucokinase (GK) and Glucokinase regulatory protein (GKRP). GK catalyzes the phosphorylation of glucose to glucose-6-phosphate (G-6P), and its activity and subcellular localization are regulated by GKRP. In liver, when glucose concentration is low, GKRP binds to GK holding it in the nucleus, while the rise in glucose concentration induces a rapid export of GK from the nucleus to the cytoplasm. In contrast, hypothalamic tanycytes display inverse compartmentalization dynamic in response to glucose: a rise in the glucose concentration drives nuclear compartmentalization of GK. The underlying mechanism responsible for differential GK subcellular localization in tanycytes has not been described yet. However, it has been suggested that relative expression between GK and GKRP might play a role. To study the effects of GKRP expression levels in the subcellular localization of GK, we used insulinoma 832/13 cells and hypothalamic tanycytes to overexpress the tanycytic sequences of Gckr. By immunocytochemistry and Western blot analysis, we observed that overexpression of GKRP, independently of the cellular context, turns GK localization to a liver-like fashion, as GK is mainly localized in the nucleus in response to low glucose. Evaluating the expression levels of GKRP in relation to GK through RT-qPCR, suggest that excess of GKRP might influence the pattern of GK subcellular localization. In this sense, we propose that the low expression of GKRP (in relation to GK) observed in tanycytes is responsible, at least in part, for the compartmentalization pattern observed in this cell type. Since GKRP behaves as a GK inhibitor, the regulation of GKRP expression levels or activity in tanycytes could be used as a therapeutic target to regulate the glucosensing activity of these cells and consequently to regulate feeding behavior.

PAS Kinase deficiency alters the glucokinase function and hepatic metabolism

The liver controls metabolic homeostasis in response to fasting and refeeding periods. Glucokinase (GCK) adjusts hepatic glucose phosphorylation to blood glucose levels, acting as a glucose sensor. Our objective was to determine whether PAS kinase (PASK), a nutrient sensor, could be affecting the expression or activity of liver GCK and the response to fasting and refeeding states of key hepatic metabolic pathways. PASK-deficient mice have impaired insulin signaling (AKT overactivation). Furthermore, PASK deficiency modified the expression of several transcription factors involved in the adjustment to fasting and refeeding. Foxo1 decreased under fasting conditions, while Ppara and Pparg were overexpressed in PASK-deficient mice. However, PEPCK protein levels were similar or higher, while the expression of Cpt1a decreased in PASK-deficient mice. By contrast, Lxra and Chrebp were overexpressed after refeeding, while the expression of Acc and Fas decreased in PASK-deficient mice. Likewise, with a decreased expression of Gck and increased nuclear location of the complex GCK-GCKR, GCK activity decreased in PASK-deficient mice. Therefore, PASK regulated some of the genes and proteins responsible for glucose sensing, such as glucokinase, and for insulin signalling, affecting glucose and lipid metabolism and consequently certain critical hepatic functions.

Insights into the role of neuronal glucokinase

Glucokinase is a key component of the neuronal glucose-sensing mechanism and is expressed in brain regions that control a range of homeostatic processes. In this review, we detail recently identified roles for neuronal glucokinase in glucose homeostasis and counterregulatory responses to hypoglycemia and in regulating appetite. We describe clinical implications from these advances in our knowledge, especially for developing novel treatments for diabetes and obesity. Further research required to extend our knowledge and help our efforts to tackle the diabetes and obesity epidemics is suggested.

The "metabolic sensor" function of rat supraoptic oxytocin and vasopressin neurons is attenuated during lactation but not in diet-induced obesity

The oxytocin (OT) and vasopressin (VP) neurons of the supraoptic nucleus (SON) demonstrate characteristics of "metabolic sensors". They express insulin receptors and glucokinase (GK). They respond to an increase in glucose and insulin with an increase in intracellular [Ca(2+)] and increased OT and VP release that is GK dependent. Although this is consistent with the established role of OT as an anorectic agent, how these molecules function relative to the important role of OT during lactation and whether deficits in this metabolic sensor function contribute to obesity remain to be examined. Thus, we evaluated whether insulin and glucose-induced OT and VP secretion from perifused explants of the hypothalamo-neurohypophyseal system are altered during lactation and by diet-induced obesity (DIO). In explants from female day 8 lactating rats, increasing glucose (Glu, 5 mM) did not alter OT or VP release. However, insulin (Ins; 3 ng/ml) increased OT release, and increasing the glucose concentration in the presence of insulin (Ins+Glu) resulted in a sustained elevation in both OT and VP release that was not prevented by alloxan, a GK inhibitor. Explants from male DIO rats also responded to Ins+Glu with an increase in OT and VP regardless of whether obesity had been induced by feeding a high-fat diet (HFD). The HFD-DIO rats had elevated body weight, plasma Ins, Glu, leptin, and triglycerides. These findings suggest that the role of SON neurons as metabolic sensors is diminished during lactation, but not in this animal model of obesity.

Ischemic preconditioning maintains the immunoreactivities of glucokinase and glucokinase regulatory protein in neurons of the gerbil hippocampal CA1 region following transient cerebral ischemia

Glucokinase (GK) is involved in the control of blood glucose homeostasis. In the present study, the effect of ischemic preconditioning (IPC) on the immunoreactivities of GK and its regulatory protein (GKRP) following 5 min of transient cerebral ischemia was investigated in gerbils. The gerbils were randomly assigned to four groups (sham-operated group, ischemia-operated group, IPC + sham-operated group and IPC + ischemia-operated group). IPC was induced by subjecting the gerbils to 2 min of ischemia, followed by 1 day of recovery. In the ischemia-operated group, a significant loss of neurons was observed in the stratum pyramidale (SP) of the hippocampal CA1 region (CA1) at 5 days post-ischemia; however, in the IPC+ischemia-operated group, the neurons in the SP were well protected. Following immunohistochemical investigation, the immunoreactivities of GK and GKRP in the neurons of the SP were markedly decreased in the CA1, but not the CA2/3, from 2 days post-ischemia, and were almost undetectable in the SP 5 days post-ischemia. In the IPC + ischemia-operated group, the immunoreactivities of GK and GKRP in the SP of the CA1 were similar to those in the sham-group. In brief, the findings of the present study demonstrated that IPC notably maintained the immunoreactivities of GK and GKRP in the neurons of the SP of CA1 following ischemia-reperfusion. This indicated that GK and GKRP may be necessary for neuron survival against transient cerebral ischemia.

The role of tanycytes in hypothalamic glucosensing

Tanycytes are elongated hypothalamic glial cells that cover the basal walls of the third ventricle; their apical regions contact the cerebrospinal fluid (CSF), and their processes reach hypothalamic neuronal nuclei that control the energy status of an organism. These nuclei maintain the balance between energy expenditure and intake, integrating several peripheral signals and triggering cellular responses that modify the feeding behaviour and peripheral glucose homeostasis. One of the most important and well-studied signals that control this process is glucose; however, the mechanism by which this molecule is sensed remains unknown. We along with others have proposed that tanycytes play a key role in this process, transducing changes in CSF glucose concentration to the neurons that control energy status. Recent studies have demonstrated the expression and function of monocarboxylate transporters and canonical pancreatic β cell glucose sensing molecules, including glucose transporter 2 and glucokinase, in tanycytes. These and other data, which will be discussed in this review, suggest that hypothalamic glucosensing is mediated through a metabolic interaction between tanycytes and neurons through lactate. This article will summarize the recent evidence that supports the importance of tanycytes in hypothalamic glucosensing, and discuss the possible mechanisms involved in this process. Finally, it is important to highlight that a detailed analysis of this mechanism could represent an opportunity to understand the evolution of associated pathologies, including diabetes and obesity, and identify new candidates for therapeutic intervention.

Regulation of gonadotropin secretion by monitoring energy availability

Nutrition is a principal environmental factor influencing fertility in animals. Energy deficit causes amenorrhea, delayed puberty, and suppression of copulatory behaviors by inhibiting gonadal activity. When gonadal activity is impaired by malnutrition, the signals originating from an undernourished state are ultimately conveyed to the gonadotropin-releasing hormone (GnRH) pulse generator, leading to suppressed secretion of GnRH and luteinizing hormone (LH). The mechanism responsible for energetic control of gonadotropin release is believed to involve metabolic signals, sensing mechanisms, and neuroendocrine pathways. The availabilities of blood-borne energy substrates such as glucose, fatty acids, and ketone bodies, which fluctuate in parallel with changes in nutritional status, act as metabolic signals that regulate the GnRH pulse generator activity and GnRH/LH release. As components of the specific sensing system, the ependymocytes lining the cerebroventricular wall in the lower brainstem integrate the information derived from metabolic signals to control gonadotropin release. One of the pathways responsible for the energetic control of gonadal activity consists of noradrenergic neurons from the solitary tract nucleus in the lower brainstem, projecting to the paraventricular nucleus of the hypothalamus. Further studies are needed to elucidate the mechanisms underlying energetic control of reproductive function.

Dynamic localization of glucokinase and its regulatory protein in hypothalamic tanycytes

Glucokinase (GK), the hexokinase involved in glucose sensing in pancreatic β cells, is also expressed in hypothalamic tanycytes, which cover the ventricular walls of the basal hypothalamus and are implicated in an indirect control of neuronal activity by glucose. Previously, we demonstrated that GK was preferentially localized in tanycyte nuclei in euglycemic rats, which has been reported in hepatocytes and is suggestive of the presence of the GK regulatory protein, GKRP. In the present study, GK intracellular localization in hypothalamic and hepatic tissues of the same rats under several glycemic conditions was compared using confocal microscopy and Western blot analysis. In the hypothalamus, increased GK nuclear localization was observed in hyperglycemic conditions; however, it was primarily localized in the cytoplasm in hepatic tissue under the same conditions. Both GK and GKRP were next cloned from primary cultures of tanycytes. Expression of GK by Escherichia coli revealed a functional cooperative protein with a S_0.5 of 10 mM. GKRP, expressed in Saccharomyces cerevisiae, inhibited GK activity in vitro with a K_i 0.2 µM. We also demonstrated increased nuclear reactivity of both GK and GKRP in response to high glucose concentrations in tanycyte cultures. These data were confirmed using Western blot analysis of nuclear extracts. Results indicate that GK undergoes short-term regulation by nuclear compartmentalization. Thus, in tanycytes, GK can act as a molecular switch to arrest cellular responses to increased glucose.

Insulin in the brain: its pathophysiological implications for States related with central insulin resistance, type 2 diabetes and Alzheimer's disease

Although the brain has been considered an insulin-insensitive organ, recent reports on the location of insulin and its receptors in the brain have introduced new ways of considering this hormone responsible for several functions. The origin of insulin in the brain has been explained from peripheral or central sources, or both. Regardless of whether insulin is of peripheral origin or produced in the brain, this hormone may act through its own receptors present in the brain. The molecular events through which insulin functions in the brain are the same as those operating in the periphery. However, certain insulin actions are different in the central nervous system, such as hormone-induced glucose uptake due to a low insulin-sensitive GLUT-4 activity, and because of the predominant presence of GLUT-1 and GLUT-3. In addition, insulin in the brain contributes to the control of nutrient homeostasis, reproduction, cognition, and memory, as well as to neurotrophic, neuromodulatory, and neuroprotective effects. Alterations of these functional activities may contribute to the manifestation of several clinical entities, such as central insulin resistance, type 2 diabetes mellitus (T2DM), and Alzheimer's disease (AD). A close association between T2DM and AD has been reported, to the extent that AD is twice more frequent in diabetic patients, and some authors have proposed the name "type 3 diabetes" for this association. There are links between AD and T2DM through mitochondrial alterations and oxidative stress, altered energy and glucose metabolism, cholesterol modifications, dysfunctional protein O-GlcNAcylation, formation of amyloid plaques, altered Aβ metabolism, and tau hyperphosphorylation. Advances in the knowledge of preclinical AD and T2DM may be a major stimulus for the development of treatment for preventing the pathogenic events of these disorders, mainly those focused on reducing brain insulin resistance, which is seems to be a common ground for both pathological entities.

Time-course changes in immunoreactivities of glucokinase and glucokinase regulatory protein in the gerbil hippocampus following transient cerebral ischemia

Glucose is a main energy source for normal brain functions. Glucokinase (GK) plays an important role in glucose metabolism as a glucose sensor, and GK activity is modulated by glucokinase regulatory protein (GKRP). In this study, we examined the changes of GK and GKRP immunoreactivities in the gerbil hippocampus after 5 min of transient global cerebral ischemia. In the sham-operated-group, GK and GKRP immunoreactivities were easily detected in the pyramidal neurons of the stratum pyramidale of the hippocampus. GK and GKRP immunoreactivities in the pyramidal neurons were distinctively decreased in the hippocampal CA1 region (CA), not CA2/3, 3 days after ischemia-reperfusion (I-R). Five days after I-R, GK and GKRP immunoreactivities were hardly detected in the CA1, not CA2/3, pyramidal neurons; however, at this point in time, GK and GKRP immunoreactivities were newly expressed in astrocytes, not microglia, in the ischemic CA1. In brief, GK and GKRP immunoreactivities are changed in pyramidal neurons and newly expressed in astrocytes in the ischemic CA1 after transient cerebral ischemia. These indicate that changes of GK and GKRP expression may be related to the ischemia-induced neuronal damage/death.

Insulin-receptor substrate-2 (irs-2) is required for maintaining glucokinase and glucokinase regulatory protein expression in mouse liver

Insulin receptor substrate (IRS) proteins play important roles in hepatic nutrient homeostasis. Since glucokinase (GK) and glucokinase regulatory protein (GKRP) function as key glucose sensors, we have investigated the expression of GK and GKRP in liver of Irs-2 deficient mice and Irs2(−/−) mice where Irs2 was reintroduced specifically into pancreatic β-cells [RIP-Irs-2/IRS-2(−/−)]. We observed that liver GK activity was significantly lower (p<0.0001) in IRS-2(−/−) mice. However, in RIP-Irs-2/IRS-2(−/−) mice, GK activity was similar to the values observed in wild-type animals. GK activity in hypothalamus was not altered in IRS-2(−/−) mice. GK and GKRP mRNA levels in liver of IRS-2(−/−) were significantly lower, whereas in RIP-Irs-2/IRS-2(−/−) mice, both GK and GKRP mRNAs levels were comparable to wild-type animals. At the protein level, the liver content of GK was reduced in IRS-2(−/−) mice as compared with controls, although GKRP levels were similar between these experimental models. Both GK and GKRP levels were lower in RIP-Irs-2/IRS-2(−/−) mice. These results suggest that IRS-2 signalling is important for maintaining the activity of liver GK. Moreover, the differences between liver and brain GK may be explained by the fact that expression of hepatic, but not brain, GK is controlled by insulin. GK activity was restored by the β-cell compensation in the RIP-Irs-2/IRS-2 mice. Interestingly, GK and GKRP protein expression remained low in RIP-Irs-2/IRS-2(−/−) mice, perhaps reflecting different mRNA half-lives or alterations in the process of translation and post-translational regulation.

Decreased glucokinase protein expression in the aged gerbil hippocampus

Glucokinase (GK) and its regulatory protein (GKRP) play roles in glucose utilization as well as glucose-sensing process in the brain. In the present study, we compared GK and GKRP protein expressions in the hippocampus of adult (postnatal month 6) and aged (postnatal month 24) gerbils using immunohistochemistry and western blot analysis. Both GK and GKRP immunoreactivities were observed primarily in the pyramidal cells of the hippocampus proper and in the granule cells of the dentate gyrus of the adult and aged hippocampus. GK, not GKRP, immunoreactivity was apparently decreased in the pyramidal and granule cells of the aged group compared with that in the adult group. In addition, western blot analysis also showed that the GK, not GKRP, protein level was significantly decreased in the aged hippocampus. These results indicate that the decrease of GK may be closely related to the reduction of glucose utilization and uptake, although the ability for regulation of GK is maintained in the aged hippocampus.

Essential elements for glucosensing by gastric vagal afferents: immunocytochemistry and electrophysiology studies in the rat

Glucosensing nodose ganglia neurons mediate the effects of hyperglycemia on gastrointestinal motility. We hypothesized that the glucose-sensing mechanisms in the nodose ganglia are similar to those of hypothalamic glucose excited neurons, which sense glucose through glycolysis. Glucose metabolism leads to ATP-sensitive potassium channel (K(ATP)) channel closure and membrane depolarization. We identified glucosensing elements in the form of glucose transporters (GLUTs), glucokinase (GK), and K(ATP) channels in rat nodose ganglia and evaluated their physiological significance. In vitro stomach-vagus nerve preparations demonstrated the gastric vagal afferent response to elevated glucose. Western blots and RT-PCR revealed the presence of GLUT1, GLUT3, GLUT4, GK, and Kir6.2 in nodose ganglia neurons and gastric branches of the vagus nerve. Immunocytochemistry confirmed the expression of GLUT3, GK, and Kir6.2 in nodose ganglia neurons (46.3 ± 3%). Patch-clamp studies detected glucose excitation in 30% (25 of 83) of gastric-projecting nodose ganglia neurons, which was abolished by GLUT3 or GK short hairpin RNA transfections. Silencing GLUT1 or GLUT4 in nodose ganglia neurons did not prevent the excitatory response to glucose. Elevated glucose elicited a response from 43% of in vitro nerve preparations. A dose-dependent response was observed, reaching maximum at a glucose level of 250 mg/dl. The gastric vagal afferent responses to glucose were inhibited by diazoxide, a K(ATP) channel opener. In conclusion, a subset of neurons in the nodose ganglia and gastric vagal afferents are glucoresponsive. Glucosensing requires a GLUT, GK, and K(ATP) channels. These elements are transported axonally to the gastric vagal afferents, which can be activated by elevated glucose through modulation of K(ATP) channels.

[Astrogliopathy as a loss of astroglial protective function against glycoxidative stress under hyperglycemia]

Reactive oxygen species (ROS) derived from mitochondria play an essential role in stroke as well as in neurodegenerative disorders. Although hyperglycemia associated with diabetes mellitus is well known to enhance ROS production in vascular endothelial cells, the effects of either acute or chronic high glucose environments on neurons and glial cells remain unclear. Astroglia play a pivotal role in glucose metabolism. Thus, the astroglial metabolic response to high glucose environments is an interesting subject. In particular, the glutathione/pentose phosphate pathway (PPP) system, which is a major defense mechanism against ROS in the brain, contributes to glucose metabolism and is more active in astroglia. We propose that high glucose environments activate PPP through an increased flux to the hexosamine biosynthetic pathway (HBP). HBP is known to induce endoplasmic reticulum (ER) stress under hyperglycemia, resulting in the nuclear translocation of nuclear factor-erythroid-2-related factor 2 (Nrf2), a master regulator of phase 2 detoxifying enzymes including glucose-6-phosphate dehydrogenase that regulates PPP activity, as Nrf2 is reported to be a direct substrate of protein kinase RNA (PKR)-like ER kinase (PERK), a transducer of ER stress. Therefore, the phosphorylation of Nrf2 by hyperglycemia-induced ER stress facilitates Nrf2 translocation through PERK, thus activating the PPP. If acute or chronic hyperglycemia induces PPP activation in astroglia to reduce ROS, reducing the glucose concentration may be accompanied by a risk, which may explain the lack of evidence that strict glycemic control during the acute phase of stroke conveys no beneficial effect.

Fueling and imaging brain activation

Metabolic signals are used for imaging and spectroscopic studies of brain function and disease and to elucidate the cellular basis of neuroenergetics. The major fuel for activated neurons and the models for neuron–astrocyte interactions have been controversial because discordant results are obtained in different experimental systems, some of which do not correspond to adult brain. In rats, the infrastructure to support the high energetic demands of adult brain is acquired during postnatal development and matures after weaning. The brain's capacity to supply and metabolize glucose and oxygen exceeds demand over a wide range of rates, and the hyperaemic response to functional activation is rapid. Oxidative metabolism provides most ATP, but glycolysis is frequently preferentially up-regulated during activation. Underestimation of glucose utilization rates with labelled glucose arises from increased lactate production, lactate diffusion via transporters and astrocytic gap junctions, and lactate release to blood and perivascular drainage. Increased pentose shunt pathway flux also causes label loss from C1 of glucose. Glucose analogues are used to assay cellular activities, but interpretation of results is uncertain due to insufficient characterization of transport and phosphorylation kinetics. Brain activation in subjects with low blood-lactate levels causes a brain-to-blood lactate gradient, with rapid lactate release. In contrast, lactate flooding of brain during physical activity or infusion provides an opportunistic, supplemental fuel. Available evidence indicates that lactate shuttling coupled to its local oxidation during activation is a small fraction of glucose oxidation. Developmental, experimental, and physiological context is critical for interpretation of metabolic studies in terms of theoretical models.

Astroglial pentose phosphate pathway rates in response to high-glucose environments

ROS (reactive oxygen species) play an essential role in the pathophysiology of diabetes, stroke and neurodegenerative disorders. Hyperglycaemia associated with diabetes enhances ROS production and causes oxidative stress in vascular endothelial cells, but adverse effects of either acute or chronic high-glucose environments on brain parenchymal cells remain unclear. The PPP (pentose phosphate pathway) and GSH participate in a major defence mechanism against ROS in brain, and we explored the role and regulation of the astroglial PPP in response to acute and chronic high-glucose environments. PPP activity was measured in cultured neurons and astroglia by determining the difference in rate of ¹⁴CO₂ production from [1-¹⁴C]glucose and [6-¹⁴C]glucose. ROS production, mainly H₂O₂, and GSH were also assessed. Acutely elevated glucose concentrations in the culture media increased PPP activity and GSH level in astroglia, decreasing ROS production. Chronically elevated glucose environments also induced PPP activation. Immunohistochemical analyses revealed that chronic high-glucose environments induced ER (endoplasmic reticulum) stress (presumably through increased hexosamine biosynthetic pathway flux). Nuclear translocation of Nrf2 (nuclear factor-erythroid 2 p45 subunit-related factor 2), which regulates G6PDH (glyceraldehyde-6-phosphate dehydrogenase) by enhancing transcription, was also observed in association with BiP (immunoglobulin heavy-chain-binding protein) expression. Acute and chronic high-glucose environments activated the PPP in astroglia, preventing ROS elevation. Therefore a rapid decrease in glucose level seems to enhance ROS toxicity, perhaps contributing to neural damage when insulin levels given to diabetic patients are not properly calibrated and plasma glucose levels are not adequately maintained. These findings may also explain the lack of evidence for clinical benefits from strict glycaemic control during the acute phase of stroke.

Glucagon-like peptide 1 (GLP-1) can reverse AMP-activated protein kinase (AMPK) and S6 kinase (P70S6K) activities induced by fluctuations in glucose levels in hypothalamic areas involved in feeding behaviour

The anorexigenic peptide, glucagon-like peptide-1 (GLP-1), reduces glucose metabolism in the human hypothalamus and brain stem. The brain activity of metabolic sensors such as AMP-activated protein kinase (AMPK) responds to changes in glucose levels. The mammalian target of rapamycin (mTOR) and its downstream target, p70S6 kinase (p70S6K), integrate nutrient and hormonal signals. The hypothalamic mTOR/p70S6K pathway has been implicated in the control of feeding and the regulation of energy balances. Therefore, we investigated the coordinated effects of glucose and GLP-1 on the expression and activity of AMPK and p70S6K in the areas involved in the control of feeding. The effect of GLP-1 on the expression and activities of AMPK and p70S6K was studied in hypothalamic slice explants exposed to low- and high-glucose concentrations by quantitative real-time RT-PCR and by the quantification of active-phosphorylated protein levels by immunoblot. In vivo, the effects of exendin-4 on hypothalamic AMPK and p70S6K activation were analysed in male obese Zucker and lean controls 1 h after exendin-4 injection to rats fasted for 48 h or after re-feeding for 2-4 h. High-glucose levels decreased the expression of Ampk in the lateral hypothalamus and treatment with GLP-1 reversed this effect. GLP-1 treatment inhibited the activities of AMPK and p70S6K when the activation of these protein kinases was maximum in both the ventromedial and lateral hypothalamic areas. Furthermore, in vivo s.c. administration of exendin-4 modulated AMPK and p70S6K activities in those areas, in both fasted and re-fed obese Zucker and lean control rats.

Brain glucose sensors play a significant role in the regulation of pancreatic glucose-stimulated insulin secretion

As patients decline from health to type 2 diabetes, glucose-stimulated insulin secretion (GSIS) typically becomes impaired. Although GSIS is driven predominantly by direct sensing of a rise in blood glucose by pancreatic β-cells, there is growing evidence that hypothalamic neurons control other aspects of peripheral glucose metabolism. Here we investigated the role of the brain in the modulation of GSIS. To examine the effects of increasing or decreasing hypothalamic glucose sensing on glucose tolerance and insulin secretion, glucose or inhibitors of glucokinase, respectively, were infused into the third ventricle during intravenous glucose tolerance tests (IVGTTs). Glucose-infused rats displayed improved glucose handling, particularly within the first few minutes of the IVGTT, with a significantly lower area under the excursion curve within the first 10 min (AUC0-10). This was explained by increased insulin secretion. In contrast, infusion of the glucokinase inhibitors glucosamine or mannoheptulose worsened glucose tolerance and decreased GSIS in the first few minutes of IVGTT. Our data suggest a role for brain glucose sensors in the regulation of GSIS, particularly during the early phase. We propose that pharmacological agents targeting hypothalamic glucose-sensing pathways may represent novel therapeutic strategies for enhancing early phase insulin secretion in type 2 diabetes.

BAD modulates counterregulatory responses to hypoglycemia and protective glucoprivic feeding

Hypoglycemia or glucoprivation triggers protective hormonal counterregulatory and feeding responses to aid the restoration of normoglycemia. Increasing evidence suggests pertinent roles for the brain in sensing glucoprivation and mediating counterregulation, however, the precise nature of the metabolic signals and molecular mediators linking central glucose sensing to effector functions are not fully understood. Here, we demonstrate that protective hormonal and feeding responses to hypoglycemia are regulated by BAD, a BCL-2 family protein with dual functions in apoptosis and metabolism. BAD-deficient mice display impaired glycemic and hormonal counterregulatory responses to systemic glucoprivation induced by 2-deoxy-D-glucose. BAD is also required for proper counterregulatory responses to insulin-induced hypoglycemia as evident from significantly higher glucose infusion rates and lower plasma epinephrine levels during hyperinsulinemic hypoglycemic clamps. Importantly, RNA interference-mediated acute knockdown of Bad in the brain provided independent genetic evidence for its relevance in central glucose sensing and proper neurohumoral responses to glucoprivation. Moreover, BAD deficiency is associated with impaired glucoprivic feeding, suggesting that its role in adaptive responses to hypoglycemia extends beyond hormonal responses to regulation of feeding behavior. Together, these data indicate a previously unappreciated role for BAD in the control of central glucose sensing.

PREB regulates transcription of pancreatic glucokinase in response to glucose and cAMP

Prolactin regulatory element binding (PREB) is a transcription factor that regulates prolactin promoter activity in rat anterior pituitary. The PREB protein is not only expressed in the anterior pituitary but also in the pancreas. We have recently reported that in pancreatic beta-cells, PREB regulates the transcription of the insulin gene in response to glucose stimulation. In the current study, we have examined the role of PREB in regulating glucokinase (GK) in pancreatic beta-cells. To analyse the effects of PREB on GK gene transcription, we employed a reporter gene assay. In the cells expressing or with knocked down PREB, GK expression was determined. GK expression was regulated by glucose and cAMP, and both glucose and cAMP stimulated the expression of PREB in a dose-dependent manner. Conversely, overexpression of PREB using a PREB-expressing adenovirus increased the expression of the GK protein. GK enzymatic activity was also significantly increased in the cells that stably expressed PREB. In addition, PREB induced GK promoter activity. Chromatin immunoprecipitation (ChIP) analyses showed that PREB mediated its transcriptional effect by binding to the PREB-responsive cis-element of the GK promoter. Finally, we used siRNA to inhibit PREB expression in cells and demonstrated that the knockdown of PREB attenuated the effects of glucose and cAMP on GK expression. Our data show that in pancreatic -cells, PREB regulates the transcription of the GK gene in response to glucose and cAMP.

Developmental gene x environment interactions affecting systems regulating energy homeostasis and obesity

Most human obesity is inherited as a polygenic trait which is largely refractory to medical therapy because obese individuals avidly defend their elevated body weight set-point. This set-point is mediated by an integrated neural network that controls energy homeostasis. Epidemiological studies suggest that perinatal and pre-pubertal environmental factors can promote offspring obesity. Rodent studies demonstrate the important interactions between genetic predisposition and environmental factors in promoting obesity. This review covers issues of development and function of neural systems involved in the regulation of energy homeostasis and the roles of leptin and insulin in these processes, the ways in which interventions at various phases from gestation, lactation and pre-pubertal stages of development can favorably and unfavorably alter the development of obesity n offspring. These studies suggest that early identification of obesity-prone humans and of the factors that can prevent them from becoming obese could provide an effective strategy for preventing the world-wide epidemic of obesity.

Antibodies against the voltage-dependent anion channel (VDAC) and its protective ligand hexokinase-I in children with autism

Autistic children show elevated serum levels of autoantibodies to several proteins essential for the function of normal brains. The voltage-dependent anion channel (VDAC) and hexokinase-I, a VDAC protective ligand, were identified as targets of this autoimmunity in autistic children. These autoantibodies were purified using immunoaffinity chromatographic techniques. Both antibodies induce apoptosis of cultured human neuroblastoma cells. Because VDAC and hexokinase-I are essential for brain protection from ischemic damage, the presence of these autoantibodies suggests a possible causal role in the neurologic pathogenesis of autism.

Glial glucokinase expression in adult and post-natal development of the hypothalamic region

It has recently been proposed that hypothalamic glial cells sense glucose levels and release lactate as a signal to activate adjacent neurons. GK (glucokinase), the hexokinase involved in glucose sensing in pancreatic β-cells, is also expressed in the hypothalamus. However, it has not been clearly determined if glial and/or neuronal cells express this protein. Interestingly, tanycytes, the glia that cover the ventricular walls of the hypothalamus, are in contact with CSF (cerebrospinal fluid), the capillaries of the arcuate nucleus and adjacent neurons; this would be expected for a system that can detect and communicate changes in glucose concentration. Here, we demonstrated by Western-blot analysis, QRT–PCR [quantitative RT–PCR (reverse transcription–PCR)] and in situ hybridization that GK is expressed in tanycytes. Confocal microscopy and immunoultrastructural analysis revealed that GK is localized in the nucleus and cytoplasm of β1-tanycytes. Furthermore, GK expression increased in these cells during the second week of post-natal development. Based on this evidence, we propose that tanycytes mediate, at least in part, the mechanism by which the hypothalamus detects changes in glucose concentrations.

Glucokinase and glucokinase regulatory proteins are functionally coexpressed before birth in the rat brain

Our previous description of functional glucokinase (GK) isoforms and their interactions with glucokinase regulatory protein (GKRP) in adult rat and human brains suggested that both participate in glucose sensing in the central nervous system. To determine whether both proteins are coexpressed and active before birth or during early post-natal life, we characterised these molecules in the brains of foetal and post-natal pup rats. We found GK and GKRP mRNAs that were similar to those previously reported in the adult rat brain. Likewise, GK and GKRP gene expression gave rise to proteins of 52 and 69 kDa, respectively. Immunohistochemistry experiments showed the colocalisation of both GK and GKRP proteins in the same brain cells of 21-day-old rat foetuses. Furthermore, coprecipitation of GK and GKRP in the presence of fructose 6-phosphate suggests interactions between both proteins. The presence of GK phosphorylating activity was detected in different brain areas of 21-day-old foetuses with a contribution to the total glucose-phosphorylating activity of between 17.2 +/- 1.7% and 12.4 +/- 3.7%, with the hypothalamus being the region of maximum activity. The hypothalamic GK activity in 21-day-old foetuses has a high apparent K(m) for glucose and no product inhibition by glucose 6-phosphate. Our findings indicate that both proteins may be functionally active before birth and that they can act within a glucose sensor system involved in controlling food intake.

Immunohistochemical localization of glucokinase in rainbow trout brain

In mammals, glucosensing neurons reside in brain areas known to play a critical regulatory role in energy homeostasis and neuroendocrine function. In fish, although no glucosensing neurons have been characterized to date, glucokinase (GCK; the main glucosensing marker in mammals) activity and expression were found in hypothalamus and hindbrain of rainbow trout where they related to food intake regulation and glucose homeostasis. However, there are no available studies in literature regarding GCK distribution in brain areas as well as the cell types expressing that protein. In the present study we hypothesize that, as occurs in mammals, GCK would be localized where glucosensing areas have been described. In this sense, we have found GCK immunoreactivity in several areas of trout hypothalamus, of which some of them are related to glucose metabolism, energy homeostasis and food intake, including the lateral hypothalamus, anterior tuberal nucleus, posterior tuberal nucleus and lateral tuberal nucleus. On the other hand, GCKimmunoreactivity was also observed in other areas where the glucosensor system is probably functional,such as the preoptic area and the oculomotor nucleus. Therefore, in this study using immunoreactive techniques, we have demonstrated in those specific areas of the rainbow trout brain previously described as glucosensor the presence of GCK in different cell types.

Molecular physiology of mammalian glucokinase

The glucokinase (GCK) gene was one of the first candidate genes to be identified as a human “diabetes gene". Subsequently, important advances were made in understanding the impact of GCK in the regulation of glucose metabolism. Structure elucidation by crystallography provided insight into the kinetic properties of GCK. Protein interaction partners of GCK were discovered. Gene expression studies revealed new facets of the tissue distribution of GCK, including in the brain, and its regulation by insulin in the liver. Metabolic control analysis coupled to gene overexpression and knockout experiments highlighted the unique impact of GCK as a regulator of glucose metabolism. Human GCK mutants were studied biochemically to understand disease mechanisms. Drug development programs identified small molecule activators of GCK as potential antidiabetics. These advances are summarized here, with the aim of offering an integrated view of the role of GCK in the molecular physiology and medicine of glucose homeostasis.

Ventromedial hypothalamic glucokinase is an important mediator of the counterregulatory response to insulin-induced hypoglycemia

OBJECTIVE

The counterregulatory response to insulin-induced hypoglycemia is mediated by the ventromedial hypothalamus (VMH), which contains specialized glucosensing neurons, many of which use glucokinase (GK) as the rate-limiting step in glucose's regulation of neuronal activity. Since conditions associated with increased VMH GK expression are associated with a blunted counterregulatory response, we tested the hypothesis that increasing VMH GK activity would similarly attenuate, while decreasing GK activity would enhance the counterregulatory response to insulin-induced hypoglycemia.

RESEARCH DESIGN AND METHODS

The counterregulatory response to insulin-induced hypoglycemia was evaluated in Sprague-Dawley rats after bilateral VMH injections of 1) a GK activator drug (compound A) to increase VMH GK activity, 2) low-dose alloxan (4 mug) to acutely inhibit GK activity, 3) high-dose alloxan (24 microg), or 4) an adenovirus expressing GK short hairpin RNA (shRNA) to chronically reduce GK expression and activity.

RESULTS

Compound A increased VMH GK activity sixfold in vitro and reduced the epinephrine, norepinephrine, and glucagon responses to insulin-induced hypoglycemia by 40-62% when injected into the VMH in vivo. On the other hand, acute and chronic reductions of VMH GK mRNA or activity had a lesser and more selective effect on increasing primarily the epinephrine response to insulin-induced hypoglycemia by 23-50%.

CONCLUSIONS

These studies suggest that VMH GK activity is an important regulator of the counterregulatory response to insulin-induced hypoglycemia and that a drug that specifically inhibited the rise in hypothalamic GK activity after insulin-induced hypoglycemia might improve the dampened counterregulatory response seen in tightly controlled diabetic subjects.

Expression of hexokinase isoforms in the dorsal root ganglion of the adult rat and effect of experimental diabetes

The effect of streptozotocin (STZ)-induced diabetes on expression and activity of hexokinase, the first enzyme and rate-limiting step in glycolysis, was studied in sensory neurons of lumbar dorsal root ganglia (DRG). The DRG and sciatic nerve of adult rats expressed the hexokinase I isoform only. Immunofluorescent staining of lumbar DRG demonstrated that small-medium neurons and satellite cells exhibited high levels of expression of hexokinase I. Large, mainly proprioceptive neurons, had very low or negative staining for hexokinase I. Intracellular localization and biochemical studies on intact DRG from adult rats and cultured adult rat sensory neurons revealed that hexokinase I was almost exclusively found in the mitochondrial compartment. Duration of STZ-diabetes of 6 or 12 weeks diminished hexokinase activity by 28% and 30%, respectively, in lumbar DRG compared with age matched controls (P<0.05). Quantitative Western blotting showed no effect of diabetes on hexokinase I protein expression in homogenates or mitochondrial preparations from DRG. Immunofluorescent staining for hexokinase I showed no diabetes-dependent change in small-medium neuron expression in DRG, however, large neurons became positive for hexokinase I (P<0.05). Such complex effects of diabetes on hexokinase I expression in the DRG may be due to glucose-driven up-regulation of expression or the result of impaired axonal transport and perikaryal accumulation in the large neuron sub-population. Because hexokinase is the rate-limiting enzyme of glycolysis these results imply that metabolic flux through the glycolytic pathway is reduced in diabetes. This finding, therefore, questions the role of high glucose-induced metabolic flux as a key driving force in reactive oxygen species generation by mitochondria.

Cell biology assessment of glucokinase mutations V62M and G72R in pancreatic beta-cells: evidence for cellular instability of catalytic activity

Mutations in the glucokinase (GK) gene cause defects in blood glucose homeostasis. In some cases (V62M and G72R), the phenotype cannot be explained by altered enzyme kinetics or protein instability. We used transient and stable expression of green fluorescent protein (GFP) GK chimaeras in MIN6 beta-cells to study the phenotype defect of V62M and G72R. GK activity in lysates of MIN6 cell lines stably expressing wild-type or mutant GFP GK showed the expected affinity for glucose and response to pharmacological activators, indicating the expression of catalytically active enzymes. MIN6 cells stably expressing GFP V62M or GFP G72R had a lower GK activity-to-GK immunoreactivity ratio and GK activity-to-GK mRNA ratio but not GK immunoreactivity-to-GK mRNA ratio than wild-type GFP GK. Heterologous expression of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2/FDP2) in cell lines increased GK activity for wild-type GK and V62M but not for G72R, whereas expression of liver GK regulatory protein (GKRP) increased GK activity for wild type but not V62M or G72R. Lack of interaction of these mutants with GKRP was also evident in hepatocyte transfections from the lack of nuclear accumulation. These results suggest that cellular loss of GK catalytic activity rather than impaired translation or enhanced protein degradation may account for the hyperglycemia in subjects with V62M and G72R mutations.

Glucose sensing by hypothalamic neurones and pancreatic islet cells: AMPle evidence for common mechanisms?

A fuller understanding of the central mechanisms involved in controlling food intake and metabolism is likely to be crucial for developing treatments to combat the growing problem of obesity in Westernised societies. Within the hypothalamus, specialized neurones respond to both appetite-regulating hormones and circulating metabolites to regulate feeding behaviour accordingly. Thus, the activity of hypothalamic glucose-excited and glucose-inhibited neurones is increased or decreased, respectively, by an increase in local glucose concentration. These 'glucose-sensing' neurones may therefore play a key role in the central regulation of food intake and potentially in the regulation of blood glucose concentrations. Whilst the intracellular signalling mechanisms through which glucose-sensing neurones detect changes in the concentration of the sugar have been investigated quite extensively, many elements remain poorly understood. Furthermore, the similarities, or otherwise, with other nutrient-sensing cells, including pancreatic islet cells, are not completely resolved. In this review, we discuss recent advances in this field and explore the potential involvement of AMP-activated protein kinase and other nutrient-regulated protein kinases.

Glucokinase is a critical regulator of ventromedial hypothalamic neuronal glucosensing

To test the hypothesis that glucokinase is a critical regulator of neuronal glucosensing, glucokinase activity was increased, using a glucokinase activator drug, or decreased, using RNA interference combined with calcium imaging in freshly dissociated ventromedial hypothalamic nucleus (VMN) neurons or primary ventromedial hypothalamus (VMH; VMN plus arcuate nucleus) cultures. To assess the validity of our approach, we first showed that glucose-induced (0.5-2.5 mmol/l) changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations, using fura-2 and changes in membrane potential (using a membrane potential-sensitive dye), were highly correlated in both glucose-excited and -inhibited neurons. Also, glucose-excited neurons increased (half-maximal effective concentration [EC(50)] = 0.54 mmol/l) and glucose-inhibited neurons decreased (half-maximal inhibitory concentration [IC(50)] = 1.12 mmol/l) [Ca(2+)](i) oscillations to incremental changes in glucose from 0.3 to 5 mmol/l. In untreated primary VMH neuronal cultures, the expression of glucokinase mRNA and the number of demonstrable glucosensing neurons fell spontaneously by half over 12-96 h without loss of viable neurons. Transfection of neurons with small interfering glucokinase RNA did not affect survival but did reduce glucokinase mRNA by 90% in association with loss of all demonstrable glucose-excited neurons and a 99% reduction in glucose-inhibited neurons. A pharmacological glucokinase activator produced a dose-related increase in [Ca(2+)](i) oscillations in glucose-excited neurons (EC(50) = 0.98 mmol/l) and a decrease in glucose-inhibited neurons (IC(50) = 0.025 micromol/l) held at 0.5 mmol/l glucose. Together, these data support a critical role for glucokinase in neuronal glucosensing.

Decline in Glucokinase Activity in the Arcuate Nucleus of Streptozotocin-Induced Diabetic Rats

Glucokinase (GK) is known to be the critical glucose sensor of pancreatic B-cells. However, the localization and functional role of GK in the brain remains to be elucidated. In this study, we measured both the activity and mRNA level of GK in the hypothalamic nuclei and the cortex of rats injected intraperitoneally with streptozotocin or vehicle. GK activity was measured by a fluorometric assay; and the GK mRNA level, by use of the real-time reverse transcription polymerase chain reaction. GK activity in vehicle-treated rats was high in the arcuate nucleus, moderate or low in the ventromedial nucleus, lateral hypothalamic area, and paraventricular nucleus, and very low in the cortex. The order of GK mRNA level was almost the same as that of GK activity. GK activity and GK mRNA level only in the arcuate nucleus of streptozotocin-treated rats at 7 d, but not at 2 d, after treatment were lower than those of vehicle-treated rats. The results suggest that prolonged hyperglycemia induced by diabetes decreased the activity of GK in the arcuate nucleus.

Developmental expression of glucokinase in rat hypothalamus

Neurons in the hypothalamus sense changes in glucose concentration. Glucokinase (GK), a key enzyme for pancreatic (beta)-cell glucose sensing, was found in both the embryonic and adult hypothalamus. GK activity accounted for approximately 20% of total hexokinase (HK) activity in both embryonic and adult hypothalamus with no activity measured in cortical samples, indicating that glucose sensing in the hypothalamus initiates early in development and precedes the maturation of glucose signaling in liver.

Distribution and anatomical localization of the glucose transporter 2 (GLUT2) in the adult rat brain--an immunohistochemical study

The aim of this work was to study the distribution and cellular localization of GLUT2 in the rat brain by light and electron microscopic immunohistochemistry, whereas our ultrastructural observations will be reported in a second paper. Confirming previous results, we show that GLUT2-immunoreactive profiles are present throughout the brain, especially in the limbic areas and related nuclei, whereas they appear most concentrated in the ventral and medial regions close to the midline. Using cresyl violet counterstaining and double immunohistochemical staining for glial or neuronal markers (GFAp, CAII and NeuN), we show that two limited populations of oligodendrocytes and astrocytes cell bodies and processes are immunoreactive for GLUT2, whereas a cross-reaction with GLUT1 cannot be ruled out. In addition, we report that the nerve cell bodies clearly immunostained for GLUT2 were scarce (although numerous in the dentate gyrus granular layer in particular), whereas the periphery of numerous nerve cells appeared labeled for this transporter. The latter were clustered in the dorsal endopiriform nucleus and neighboring temporal and perirhinal cortex, in the dorsal amygdaloid region, and in the paraventricular and reuniens thalamic nuclei, whereas they were only a few in the hypothalamus. Moreover, a group of GLUT2-immunoreactive nerve cell bodies was localized in the dorsal medulla oblongata while some large multipolar nerve cell bodies peripherally labeled for GLUT2 were scattered in the caudal ventral reticular formation. This anatomical localization of GLUT2 appears characteristic and different from that reported for the neuronal transporter GLUT3 and GLUT4. Indeed, the possibility that GLUT2 may be localized in the sub-plasmalemnal region of neurones and/or in afferent nerve fibres remains to be confirmed by ultrastructural observations. Because of the neuronal localization of GLUT2, and of its distribution relatively similar to glucokinase, it may be hypothesized that this transporter is, at least partially, involved in cerebral glucose sensing.

Immunocytochemical localization of the glucose transporter 2 (GLUT2) in the adult rat brain. II. Electron microscopic study

Following a former immunohistochemical study in the rat brain [Arluison, M., Quignon, M., Nguyen, P., Thorens, B., Leloup, C., Penicaud, 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.

Obesity-prone rats have preexisting defects in their counterregulatory response to insulin-induced hypoglycemia

Rats that develop diet-induced obesity (DIO) on a 31% fat [high-energy (HE)] diet have defective sensing and responding to altered glucose levels compared with diet-resistant (DR) rats. Thus we postulated that they would also have defective counterregulatory responses (CRR) to insulin-induced hypoglycemia (IIH). Chow-fed selectively bred DIO and DR rats underwent three sequential 60-min bouts of IIH separated by 48 h. Glucose levels fell comparably, but DIO rats had 22–29% lower plasma epinephrine (Epi) levels during the first two bouts than DR rats. By the third trial, despite comparable Epi levels, DIO rats had lower 30-min glucose levels and rebounded less than DR rats 85 min after intravenous glucose. Although DIO rats gained more carcass and fat weight after 4 wk on an HE diet than DR rats, they were unaffected by prior IIH. Compared with controls, DR rats with prior IIH and HE diet had higher arcuate nucleus neuropeptide Y (50%) and proopiomelanocortin (POMC; 37%) mRNA and an inverse correlation ( r = 0.85; P = 0.004) between POMC expression and body weight gain on the HE diet. These data suggest that DIO rats have a preexisting defect in their CRR to IIH but that IIH does not affect the expression of their hypothalamic neuropeptides or weight gain as it does in DR rats.

Neuronal glucosensing: what do we know after 50 years?

Glucosensing neurons are specialized cells that use glucose as a signaling molecule to alter their action potential frequency in response to variations in ambient glucose levels. Glucokinase (GK) appears to be the primary regulator of most neuronal glucosensing, but other regulators almost certainly exist. Glucose-excited neurons increase their activity when glucose levels rise, and most use GK and an ATP-sensitive K(+) channel as the ultimate effector of glucose-induced signaling. Glucose-inhibited (GI) neurons increase their activity at low glucose levels. Although many use GK, it is unclear what the final pathway of GI neuronal glucosensing is. Glucosensing neurons are located in brain sites and respond to and integrate a variety of hormonal, metabolic, transmitter, and peptide signals involved in the regulation of energy homeostasis and other biological functions. Although it is still uncertain whether daily fluctuations in blood glucose play a specific regulatory role in these physiological functions, it is clear that large decreases in glucose availability stimulate food intake and counterregulatory responses that restore glucose levels to sustain cerebral function. Finally, glucosensing is altered in obesity and after recurrent bouts of hypoglycemia, and this altered sensing may contribute to the adverse outcomes of these conditions. Thus, although much is known, much remains to be learned about the physiological function of brain glucosensing neurons.

The role of glucosensing neurons in the detection of hypoglycemia

Hypoglycemia is a life-threatening side effect of intensive insulin therapy in Type 1 diabetic patients. The ability to detect hypoglycemia and restore blood glucose levels to normal is of critical concern to the brain since glucose is its preferred fuel. When plasma glucose levels fall, powerful hormonal and sympathoadrenal mechanisms respond to restore blood glucose levels to normal. These mechanisms are believed to be initiated by diverse populations of glucose sensors, which are located centrally as well as peripherally. The exact contribution of each of these individual glucose sensors to the regulation of glucose homeostasis is not known at this time. This review focuses on the diversity of central and peripheral glucose sensors and the mechanisms by which they sense glucose.

In vitro increase in intracellular calcium concentrations induced by low or high extracellular glucose levels in ependymocytes and serotonergic neurons of the rat lower brainstem

Pancreatic glucokinase (GK)-like immunoreactivities are located in ependymocytes and serotonergic neurons of the rat brain. The present study investigated in vitro changes in intracellular calcium concentrations ([Ca(2+)](i)) in response to low (2 mm) or high (20 mm) extracellular glucose concentrations in isolated cells from the wall of the central canal (CC), raphe obscurus nucleus (ROb), ventromedial hypothalamus (VMH), and lateral hypothalamic area (LHA) in male rats. An increase in [Ca(2+)](i) was found in cells from the CC (21.1% or 9.8% of ependymocytes), ROb (10.9% or 14.5% of serotonergic neurons), VMH (7.8% and 25.2% of neurons), and LHA (20% or 15.7% of neurons), when extracellular glucose levels were changed from 10 to either 2 or 20 mm, respectively. Most of the ependymocytes and serotonergic neurons responding to the glucose changes were immunoreactive to the anti-GK in the CC (96.8% for low glucose and 100% for high glucose) and ROb (100% for low and high glucose). The [Ca(2+)](i) increase was blocked with calcium-free medium or L-type calcium channel blocker. Cells with an increase in [Ca(2+)](i) in response to low glucose did not respond to high glucose and vice versa. Inhibition of GK activity with acute alloxan treatment blocked low or high glucose-induced [Ca(2+)](i) increases in most GK-immunoreactive cells from the CC or ROb. The glucose-sensitive [Ca(2+)](i) increase in neurons of the VMH and LHA was also alloxan-sensitive, but no cells taken from the VMH and LHA were immunoreactive to the antibody used. The present study further indicates that ependymocytes of the CC and serotonergic neurons in the ROb are also sensitive to the changes in extracellular glucose in a GK-dependent manner, but that the subtype of GK in these cells could be different from that in the VMH and LHA.

Third ventricular alloxan reversibly impairs glucose counterregulatory responses

Glucokinase (GK) is hypothesized to be the critical glucosensor of pancreatic beta-cells and hypothalamic glucosensing neurons. To understand the role of GK in glucoprivic counterregulatory responses, we injected alloxan, a GK inhibitor and toxin, into the third ventricle (3v) to target nearby GK-expressing neurons. Four and 6 days after 3v, but not 4v, alloxan injection, alloxan-treated rats ate only 30% and their blood glucose area under the curve was only 28% of saline controls' after systemic 2-deoxy-D-glucose. In addition, their hyperglycemic response to hindbrain glucoprivation induced with 5-thio-glucose was impaired, whereas fasting blood glucose levels and food intake after an overnight fast were elevated. These impaired responses were associated with the destruction of 3v tanycytes, reduced glial fibrillary acidic protein-immunoreactivity surrounding the 3v, neuronal swelling, and decreased arcuate nucleus neuropeptide Y (NPY) mRNA. Nevertheless, hypothalamic GK mRNA was significantly elevated. Two weeks after alloxan injection, 3v tanycyte destruction was reversed along with restoration of feeding and hyperglycemic responses to both systemic and hindbrain glucoprivation. At this time there were significant decreases in GK, NPY, and proopiomelanocortin mRNA. Thus, neural substrates near and around the 3v affected by alloxan may be critically involved in the expression of these glucoprivic responses.

Expression of glucose transporter isoform GLUT-2 and glucokinase genes in human brain

The glucose transporter isoform-2 (GLUT-2) and glucokinase are considered to be components of a glucose sensor system controlling several key processes, and hence may modulate feeding behaviour. We have found GLUT-2 and glucokinase mRNAs in several brain regions, including the ventromedial and arcuate nuclei of the hypothalamus. GLUT-2, glucokinase and glucokinase regulatory protein mRNAs and proteins were present in these areas as determined by biochemical approaches. In addition, glucose-phosphorylating activity with a high apparent Km for glucose that displayed no product inhibition by glucose-6-phosphate was observed. Increased glycaemia after meals may be recognized by specific hypothalamic neurones due to the high Km of GLUT-2 and glucokinase. This enzyme is considered to be the true glucose sensor because it catalyses the rate-limiting step of glucose catabolism its activity being regulated by interaction with glucokinase regulatory protein, that functions as a metabolic sensor.

Physiological and molecular characteristics of rat hypothalamic ventromedial nucleus glucosensing neurons

To evaluate potential mechanisms for neuronal glucosensing, fura-2 Ca(2+) imaging and single-cell RT-PCR were carried out in dissociated ventromedial hypothalamic nucleus (VMN) neurons. Glucose-excited (GE) neurons increased and glucose-inhibited (GI) neurons decreased intracellular Ca(2+) ([Ca(2+)](i)) oscillations as glucose increased from 0.5 to 2.5 mmol/l. The Kir6.2 subunit mRNA of the ATP-sensitive K(+) channel was expressed in 42% of GE and GI neurons, but only 15% of nonglucosensing (NG) neurons. Glucokinase (GK), the putative glucosensing gatekeeper, was expressed in 64% of GE, 43% of GI, but only 8% of NG neurons and the GK inhibitor alloxan altered [Ca(2+)](i) oscillations in approximately 75% of GK-expressing GE and GI neurons. Insulin receptor and GLUT4 mRNAs were coexpressed in 75% of GE, 60% of GI, and 40% of NG neurons, although there were no statistically significant intergroup differences. Hexokinase-I, GLUT3, and lactate dehydrogenase-A and -B were ubiquitous, whereas GLUT2, monocarboxylate transporters-1 and -2, and leptin receptor and GAD mRNAs were expressed less frequently and without apparent relationship to glucosensing capacity. Thus, although GK may mediate glucosensing in up to 60% of VMN neurons, other regulatory mechanisms are likely to control glucosensing in the remaining ones.

Glucosensing neurons in the ventromedial hypothalamic nucleus (VMN) and hypoglycemia-associated autonomic failure (HAAF)

Hypoglycemia is a profound threat to the brain since glucose is its preferred fuel. Thus, decreases in plasma glucose must be sensed and appropriate hormonal and neuroendocrine responses generated to restore glucose to safe levels (i.e. counterregulatory responses (CRR) to hypoglycemia). Recurrent hypoglycemia impairs these protective mechanisms, resulting in a potentially life-threatening condition known as hypoglycemia-associated autonomic failure (HAAF). During HAAF, the glycemic threshold is reset so that glucose levels must fall further before the CRR is initiated. The brain plays a critical role in sensing hypoglycemia and initiating the CRR. Additionally, many neurons may sense changes in plasma and extracellular glucose. However, the way in which central glucose sensing is integrated to lead to effective initiation of the CRR is unknown. Furthermore, the mechanisms by which this system becomes impaired during HAAF are also unknown. Glucosensing neurons in the ventromedial hypothalamic nucleus (VMN) are poised to serve an integrative function in glucose homeostasis. First, they sense glucose. Second, the VMN receives input from other glucose-sensing areas. Finally, the VMN projects to areas linked to the regulation of the sympathoadrenal system that mediates the CRR. This review discusses VMN glucosensing neurons relative to their capacity to play a role in the regulation of the CRR and the generation of HAAF. Glucosensing neurons in the hindbrain as well as peripheral glucosensors are also considered.

Short-term food restriction and refeeding alter expression of genes likely involved in brain glucosensing

Several genes involved in glucosensing of the endocrine pancreas have been proposed to serve a similar function in the brain. These genes include the glucose transporter-2 (Glut-2) and glucokinase (GK). In addition, the glucagon-like peptide 1 receptor, which serves as a downstream signal modulator in pancreatic glucosensing and centrally alters feeding, is also of interest. We used quantitative real-time RT-PCR to measure changes in hypothalamic and brainstem Glut-2, GK, and Glp-1R expression of these genes induced by food restriction and refeeding. Sprague-Dawley rats were 50% food restricted for 1 day; one-half of the food-restricted rats were refed with chow for 1 hr before sacrifice. In both hypothalamus and brainstem, gene expression of Glut-2, GK, and Glp-1R was significantly lower in refed rats compared with food-restricted rats. The measures of gene expression in two feeding control groups (ad libitum and voluntarily overfed animals) were intermediate between the food-restricted and refed groups, but were not significantly different from each other. The results indicate that putative glucosensing (GK, Glut-2, and Glp-1R) gene expression in the hypothalamus and brainstem is reduced in response to food intake, depending on prior nutritional status.