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

Asymptomatic Congenital Hyperinsulinism due to a Glucokinase-Activating Mutation, Treated as Adrenal Insufficiency for Twelve Years

Congenital hyperinsulinism (CHI) caused by a glucokinase- (GCK-) activating mutation shows autosomal dominant inheritance, and its severity ranges from mild to severe. A 43-year-old female with asymptomatic hypoglycemia (47 mg/dL) was diagnosed as partial adrenal insufficiency and the administration of hydrocortisone (10 mg/day) was initiated. Twelve years later, her 8-month-old grandchild was diagnosed with CHI. Heterozygosity of exon 6 c.590T>C (p.M197T) was identified in a gene analysis of GCK, which was also detected in her son and herself. The identification of GCK-activating mutations in hyperinsulinemic hypoglycemia patients may be useful for a deeper understanding of the pathophysiology involved and preventing unnecessary glucocorticoid therapy.

Gastric Bypass Reduces Symptoms and Hormonal Responses in Hypoglycemia

Gastric bypass (GBP) surgery, one of the most common bariatric procedures, induces weight loss and metabolic effects. The mechanisms are not fully understood, but reduced food intake and effects on gastrointestinal hormones are thought to contribute. We recently observed that GBP patients have lowered glucose levels and frequent asymptomatic hypoglycemic episodes. Here, we subjected patients before and after undergoing GBP surgery to hypoglycemia and examined symptoms and hormonal and autonomic nerve responses. Twelve obese patients without diabetes (8 women, mean age 43.1 years [SD 10.8] and BMI 40.6 kg/m(2) [SD 3.1]) were examined before and 23 weeks (range 19-25) after GBP surgery with hyperinsulinemic-hypoglycemic clamp (stepwise to plasma glucose 2.7 mmol/L). The mean change in Edinburgh Hypoglycemia Score during clamp was attenuated from 10.7 (6.4) before surgery to 5.2 (4.9) after surgery. There were also marked postsurgery reductions in levels of glucagon, cortisol, and catecholamine and the sympathetic nerve responses to hypoglycemia. In addition, growth hormone displayed a delayed response but to a higher peak level. Levels of glucagon-like peptide 1 and gastric inhibitory polypeptide rose during hypoglycemia but rose less postsurgery compared with presurgery. Thus, GBP surgery causes a resetting of glucose homeostasis, which reduces symptoms and neurohormonal responses to hypoglycemia. Further studies should address the underlying mechanisms as well as their impact on the overall metabolic effects of GBP surgery.

The incretin system ABCs in obesity and diabetes - novel therapeutic strategies for weight loss and beyond

Incretins are gastrointestinal-derived hormones released in response to a meal playing a key role in the regulation of postprandial secretion of insulin (incretin effect) and glucagon by the pancreas. Both incretins, glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 (GLP-1), have several other actions by peripheral and central mechanisms. GLP-1 regulates body weight by inhibiting appetite and delaying gastric, emptying actions that are dependent on central nervous system GLP-1 receptor activation. Several other hormones and gut peptides, including leptin and ghrelin, interact with GLP-1 to modulate appetite. GLP-1 is rapidly degraded by the multifunctional enzyme dipeptidyl peptidase-4 (DPP-4). DPP-4 is involved in adipose tissue inflammation, which is associated with insulin resistance and diabetes progression, being a common pathophysiological mechanism in obesity-related complications. Furthermore, the incretin system appears to provide the basis for understanding the high weight loss efficacy of bariatric surgery, a widely used treatment for obesity, often in association with diabetes. The present review brings together new insights into obesity pathogenesis, integrating GLP-1 and DPP-4 in the complex interplay between obesity and inflammation, namely, in diabetic patients. This in turn will provide the basis for novel incretin-based therapeutic strategies for obesity and diabetes with promising benefits in addition to weight loss. © 2016 World Obesity.

Glucokinase activity in the arcuate nucleus regulates glucose intake

The brain relies on a constant supply of glucose, its primary fuel, for optimal function. A taste-independent mechanism within the CNS that promotes glucose delivery to the brain has been postulated to maintain glucose homeostasis; however, evidence for such a mechanism is lacking. Here, we determined that glucokinase activity within the hypothalamic arcuate nucleus is involved in regulation of dietary glucose intake. In fasted rats, glucokinase activity was specifically increased in the arcuate nucleus but not other regions of the hypothalamus. Moreover, pharmacologic and genetic activation of glucokinase in the arcuate nucleus of rodent models increased glucose ingestion, while decreased arcuate nucleus glucokinase activity reduced glucose intake. Pharmacologic targeting of potential downstream glucokinase effectors revealed that ATP-sensitive potassium channel and P/Q calcium channel activity are required for glucokinase-mediated glucose intake. Additionally, altered glucokinase activity affected release of the orexigenic neurotransmitter neuropeptide Y in response to glucose. Together, our results suggest that glucokinase activity in the arcuate nucleus specifically regulates glucose intake and that appetite for glucose is an important driver of overall food intake. Arcuate nucleus glucokinase activation may represent a CNS mechanism that underlies the oft-described phenomena of the "sweet tooth" and carbohydrate craving.

Behavioural profile of exendin-4/naltrexone dose combinations in male rats during tests of palatable food consumption

RATIONALE

The glucagon-like peptide 1 receptor (GLP-1R) agonist exendin-4 potently suppresses food intake in animals and humans. However, little is known about the behavioural specificity of this effect either when administered alone or when co-administered with another anorectic agent.

OBJECTIVES

The present study characterises the effects of exendin-4, both alone and in combination with naltrexone, on behaviours displayed by male rats during tests with palatable mash.

METHODS

Experiment 1 examined the dose-response effects of exendin-4 (0.025-2.5 μg/kg, IP), while experiment 2 profiled the effects of low-dose combinations of the peptide (0.025 and 0.25 μg/kg) and naltrexone (0.1 mg/kg).

RESULTS

In experiment 1, exendin-4 dose dependently suppressed food intake as well as the frequency and rate of eating. However, these effects were accompanied by dose-dependent reductions in all active behaviours and, at 2.5 μg/kg, a large increase in resting and disruption of the behavioural satiety sequence (BSS). In experiment 2, while exendin-4 (0.25 μg/kg) and naltrexone each produced a significant reduction in intake and feeding behaviour (plus an acceleration in the BSS), co-treatment failed to produce stronger effects than those seen in response to either compound alone.

CONCLUSION

Similarities between the behavioural signature of exendin-4 and that previously reported for the emetic agent lithium chloride would suggest that exendin-4 anorexia is related to the aversive effects of the peptide. Furthermore, as low-dose combinations of the peptide with naltrexone failed to produce an additive/synergistic anorectic effect, this particular co-treatment strategy would not appear to have therapeutic significance.

Expression Levels of Genes Likely Involved in Glucose-sensing in the Obese Zucker Rat Brain

It has been suggested that certain cells in the brain, like pancreatic beta-cells, use glucose transporter-2 (GLUT-2), glucokinase and glucagon-like peptide-1 receptor (GLP-1R) to sense glucose in the service of multiple aspects of energy balance. The obese Zucker rat displays numerous disturbances in energy homeostasis and may provide a model of dysfunctional expression of genes related to nutrient control systems. Using real-time RT-PCR we measured gene expression for three of the pancreatic glucose-sensing markers and neuropeptide Y (NPY) in the medial, lateral hypothalamus and hindbrain of lean and obese Zucker rats of both genders. Additionally, we measured circulating levels of glucose, leptin, insulin, corticosterone and glucagon. The results indicate that GLUT-2 mRNA expression is decreased, whereas glucokinase is increased in the hindbrain of obese rats. NPY mRNA level is significantly higher, whereas GLP-1R is significantly lower in the medial hypothalamus in obese individuals. Gender-related differences were found in the hindbrain and medial hypothalamus for GLUT-2 and in the lateral hypothalamus for GLP-1R and they may be related to the fact that the female Zucker rats do not develop diabetes as readily as males. Furthermore, the hindbrain may be an important site for glucose-sensing where major phenotypic changes occur for glucose-sensing genes expression.

Dietary resistant starch improves selected brain and behavioral functions in adult and aged rodents

Resistant starch (RS) is a dietary fiber that exerts multiple beneficial effects. The current study explored the effects of dietary RS on selected brain and behavioral functions in adult and aged rodents. Because glucokinase (GK) expression in hypothalamic arcuate nucleus and area postrema of the brainstem is important for brain glucose sensing, GK mRNA was measured by brain nuclei microdissection and PCR. Adult RS-fed rats had a higher GK mRNA than controls in both brain nuclei, an indicator of improved brain glucose sensing. Next, we tested whether dietary RS improve selected behaviors in aged mice. RS-fed aged mice exhibited (i) an increased eating responses to fasting, a behavioral indicator of improvement in aged brain glucose sensing; (ii) a longer latency to fall from an accelerating rotarod, a behavioral indicator of improved motor coordination; and (iii) a higher serum active glucagon-like peptide-1 (GLP-1). Then, GLP-1 receptor null (GLP-1RKO) mice were used to test the role of GLP-1 in brain glucose sensing, and they exhibited impaired eating responses to fasting. We conclude that in rodents (i) dietary RS improves two important indicators of brain function: glucose sensing and motor coordination, and (ii) GLP-1 is important in the optimal feeding response to a fast.

Minireview: The value of looking backward: the essential role of the hindbrain in counterregulatory responses to glucose deficit

This review focuses on evidence indicating a key role for the hindbrain in mobilizing behavioral, autonomic and endocrine counterregulatory responses to acute and profound glucose deficit, and identifies hindbrain norepinephrine (NE) and epinephrine (E) neurons as essential mediators of some of these responses. It has become clear that hindbrain NE/E neurons are functionally diverse. However, considerable progress has been made in identifying the particular NE/E neurons important for particular glucoregulatory responses. Although it is not yet known whether NE/E neurons are directly activated by glucose deficit, compelling evidence indicates that if they are not, the primary glucoreceptor cells must be located in the immediate vicinity these neurons. Hindbrain studies identifying cellular markers associated with glucose-sensing functions in other brain regions are discussed, as are studies examining the relationship of these markers to counterregulatory responses of NE/E neurons. Further investigations to identify glucose-sensing cells (neurons, ependymocytes, or glia) controlling counterregulatory responses are crucial, as are studies to determine the specific functions of glucose-sensing cells throughout the brain. Likewise, examination of the roles (if any) of hindbrain counterregulatory systems in managing glucose homeostasis under basal, nonglucoprivic conditions will also be important for a full understanding of energy homeostasis. Nevertheless, the accumulated evidence demonstrates that hindbrain glucose sensors and NE/E neurons are essential players in triggering counterregulatory responses to emergencies of glucose deficit.

Obesity treatment: novel peripheral targets

Our knowledge of the complex mechanisms underlying energy homeostasis has expanded enormously in recent years. Food intake and body weight are tightly regulated by the hypothalamus, brainstem and reward circuits, on the basis both of cognitive inputs and of diverse humoral and neuronal signals of nutritional status. Several gut hormones, including cholecystokinin, glucagon-like peptide-1, peptide YY, oxyntomodulin, amylin, pancreatic polypeptide and ghrelin, have been shown to play an important role in regulating short-term food intake. These hormones therefore represent potential targets in the development of novel anti-obesity drugs. This review focuses on the role of gut hormones in short- and long-term regulation of food intake, and on the current state of development of gut hormone-based obesity therapies.

Evidence for the presence of a glucosensor in hypothalamus, hindbrain, and Brockmann bodies of rainbow trout

The aim of this study was to evaluate the existence of a glucosensor in different regions of the brain and in the Brockmann bodies (BB) of the rainbow trout, Oncorhynchus mykiss. Five groups ( n = 12) of trout were injected intraperitoneally with saline alone (control) or saline-containing bovine glucagon (100 μg/kg), bovine insulin (4 mg/kg), 2-deoxy-d-glucose (100 mg/kg), or d-glucose (500 mg/kg) to promote hyperglycemia (glucagon, d-glucose, 2-deoxy-d-glucose) or hypoglycemia (insulin). Six hours after injection, samples from four brain regions (hypothalamus, telencephalon, hindbrain, and midbrain) and the entire BB were taken. Our results demonstrate within the BB and both the hypothalamus and hindbrain a metabolic response different to that observed in other tissues (midbrain, telencephalon) but similar to that described in tissues known to be glucosensors in mammals. The metabolic responses of these areas to changes in plasma glycemia were characterized by parallel changes in GLUT-2 expression, hexokinase-IV, or glucokinase activity and expression, glycolytic potential, and levels of glycogen and glucose. These changes are similar to those reported in mammalian pancreatic β-cells and glucose-excited (GE) neurons, two cell types containing glucosensors. This study provides evidence for the presence of glucosensors responsive to hyper- and hypoglycemia in rainbow trout BB, hypothalamus, and hindbrain.

Effects of resistant starch, a non-digestible fermentable fiber, on reducing body fat

OBJECTIVE

To assess the effects of energy dilution with non-fermentable and fermentable fibers on abdominal fat and gut peptide YY (PYY) and glucagon-like peptide (GLP)-1 expressions, three rat studies were conducted to: determine the effects of energy dilution with a non-fermentable fiber, compare similar fiber levels of fermentable and non-fermentable fibers, and compare similar metabolizable energy dilutions with fermentable and non-fermentable fibers.

RESEARCH METHODS AND PROCEDURES

In Study 1, rats were fed one of three diets with different metabolizable energy densities. In Study 2, rats were fed diets with similar fiber levels using high amylose-resistant cornstarch (RS) or methylcellulose. In Study 3, rats were fed diets with a similar dilution of metabolizable energy using cellulose or RS. Measurements included food intake, body weight, abdominal fat, plasma PYY and GLP-1, gastrointestinal tract weights, and gene transcription of PYY and proglucagon.

RESULTS

Energy dilution resulted in decreased abdominal fat in all studies. In Study 2, rats fed fermentable RS had increased cecal weights and plasma PYY and GLP-1, and increased gene transcription of PYY and proglucagon. In Study 3, RS-fed rats had increased short-chain fatty acids in cecal contents, plasma PYY (GLP-1 not measured), and gene transcription for PYY and proglucagon.

DISCUSSION

Inclusion of RS in the diet may affect energy balance through its effect as a fiber or a stimulator of PYY and GLP-1 expression. Increasing gut hormone signaling with a bioactive functional food such as RS may be an effective natural approach to the treatment of obesity.

Changes in extracellular hypothalamic glucose in relation to feeding

The aim of the present in vivo microdialysis study was to investigate the relation between feeding and changes in glucose concentrations in the rat ventromedial hypothalamus (VMH). Absolute ambient glucose concentrations in VMH were 1.43 mm in non-deprived rats as compared to 0.94 mm after 24-h food deprivation. To examine whether feeding influences hypothalamic glucose, changes of glucose concentration over time were determined relative to a baseline. Experiments were conducted in relation to both, nutritional state (food-deprived rats vs. non-deprived rats) and feeding conditions throughout the experiment (freely feeding rats vs. rats without access to food). The results of this microdialysis study show clearly that glucose concentration in the VMH of rats increases significantly in relation to food intake. The data demonstrate that a 24-h food deprivation before the experiment further augments this increase (up to 350% from baseline) as compared to non-deprived conditions (up to 60% from baseline). However, the magnitude of food related increase in VMH glucose does not correlate with the individual amount of food eaten. In conclusion, the present study shows for the first time that VMH glucose concentrations increase with food intake in the early dark phase, indicating that such changes do not only occur after pharmacological treatment, but also under physiological feeding conditions. The results further indicate that the feeding related increase in VMH glucose depends on the nutritional state of the organism.

Glucokinase and hexokinase expression and activities in rainbow trout tissues: changes with food deprivation and refeeding

The expression and activities of glucokinase (GK) and hexokinase (HK) were assessed in different tissues of rainbow trout (Oncorhynchus mykiss) under different feeding conditions (fed, fasted for 14 days, and refed for 7 days). Two different HK-I cDNAs were identified with different tissue distributions. One transcript named heart or H-HK-I was observed in the four brain regions assessed, white muscle, kidney, and gills but not in liver or erythrocytes. A second transcript named liver or L-HK-I was found in all tissues surveyed. GK mRNA was identified only in liver and the four brain regions. GK expression was altered by feeding conditions, especially in liver and hypothalamus where food deprivation decreased and re-feeding increased expression; changes in expression reflected activity changes and changes in tissue glycogen levels. In contrast, feeding conditions did not alter expression of either HK-I transcript but did alter tissue HK activities. The reduced phosphorylating capacity noted with food deprivation correlates primarily with changes in tissue HK, whereas increased capacity, as with refeeding, was associated with changes in GK; these changes fit with the different K(m) values of the GK and HK enzymes. These results provide evidence for the hypothalamus acting as a glucosensor in trout, as hyperglycemia produced increased GK expression and activity, as well as increased glycogen levels. Thus, even though trout use glucose poorly, none of the parameters tested here relate to this inability to use glucose and suggest that, at least, rainbow trout, if given an appropriate carbohydrate diet, could metabolically adjust to such a diet.

Peptide YY and proglucagon mRNA expression patterns and regulation in the gut

OBJECTIVE

Peptide YY (PYY) and glucagon-like peptide-1 are important in the control of energy homeostasis and are both secreted from the gut in response to ingested nutrients. However, more studies are needed on nutrient regulation of their gene expression patterns in specific areas of the gut. This study detailed PYY and proglucagon (the gene that encodes glucagon-like peptide-1) gene expression patterns and regulation in the gut. We further examined the regulation of PYY and proglucagon mRNA by a diet containing fermentation-resistant starch (in vivo) and butyrate (in vitro).

RESEARCH METHODS AND PROCEDURES

Quantitative real time reverse transcriptase-polymerase chain reaction was used to measure PYY and proglucagon gene expression in epithelial cells collected from the duodenum, jejunum, cecum, and colon in normal Sprague-Dawley rats and in rats fed a resistant starch diet for 4 weeks. The same measurements were also performed in primary epithelial cells collected from the cecum and colon of normal rats after the cells were incubated with butyrate for 3 hours.

RESULTS

The gene expression patterns for PYY and proglucagon are similar to their peptide distribution patterns in the gut. Also, PYY and proglucagon mRNA expression were up-regulated in the cecum and colon in resistant-starch-fed rats. Butyrate increased PYY and proglucagon gene expression in a dose-dependent manner in vitro.

DISCUSSION

Our data provide evidence that the distal part of the gut has the ability to sense nutrients such as butyrate, resulting in the up-regulation of PYY and proglucagon gene expression.

Role of neuronal energy status in the regulation of adenosine 5'-monophosphate-activated protein kinase, orexigenic neuropeptides expression, and feeding behavior

Nutrient sensing in the hypothalamus is tightly related to food intake regulation. However, the mechanisms by which the nutrient-sensing cells of the brain translate this signal of energy need into feeding behavior via regulation of neuropeptide expression are not known. To address this issue, we investigated two neuronal cell lines expressing agouti-related protein (AgRP), ex vivo hypothalamic tissues, and in vivo whole animals. Maintaining cells in a low cellular ATP concentration generated by low glucose, 2-deoxyglucose (2-DG), ATP synthesis inhibitor, and 5-aminoimidazole-4-carboxamide 1-beta-d-ribofuranoside increased phosphorylation of AMP-activated protein kinase (AMPK) and increased AgRP expression, whereas maintaining cells in high ATP status by high glucose and pyruvate supplementation in 2-DG-treated cells decreased phosphorylation of AMPK and decreased AgRP expression. Overexpression of a dominant-inhibitory mutant of AMPK significantly decreased low-glucose- or 2-DG-induced AgRP expression. Furthermore, ex vivo hypothalamus culture in high glucose concentrations decreased both expression and phosphorylation of AMPK and expression of both AgRP and neuropeptide Y, whereas pyruvate supplementation suppressed a 2-DG-induced AgRP expression. Finally, our in vivo studies clearly show that central administration of pyruvate dramatically delayed 2-DG-induced food intake. These data indicate that modulation of ATP levels in neuronal cells triggers a cascade of events via AMPK that modulate feeding behavior to restore energy status of cells.