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The dogma that life without insulin is incompatible has recently been challenged by results showing the viability of insulin-deficient rodents undergoing leptin monotherapy. Yet, the mechanisms underlying these actions of leptin are unknown. Here, the metabolic outcomes of intracerebroventricular (i.c.v.) administration of leptin in mice devoid of insulin and lacking or re-expressing leptin receptors (LEPRs) only in selected neuronal groups were assessed. Our results demonstrate that concomitant re-expression of LEPRs only in hypothalamic γ-aminobutyric acid (GABA) and pro-opiomelanocortin (POMC) neurons is sufficient to fully mediate the lifesaving and antidiabetic actions of leptin in insulin deficiency. Our analyses indicate that enhanced glucose uptake by brown adipose tissue and soleus muscle, as well as improved hepatic metabolism, underlies these effects of leptin. Collectively, our data elucidate a hypothalamic-dependent pathway enabling life without insulin and hence pave the way for developing better treatments for diseases of insulin deficiency.

Energy and glucose homeostasis are regulated by central serotonin 2C receptors. These receptors are attractive pharmacological targets for the treatment of obesity; however, the identity of the serotonin 2C receptor-expressing neurons that mediate the effects of serotonin and serotonin 2C receptor agonists on energy and glucose homeostasis are unknown. Here, we show that mice lacking serotonin 2C receptors (Htr2c) specifically in pro-opiomelanocortin (POMC) neurons had normal body weight but developed glucoregulatory defects including hyperinsulinemia, hyperglucagonemia, hyperglycemia, and insulin resistance. Moreover, these mice did not show anorectic responses to serotonergic agents that suppress appetite and developed hyperphagia and obesity when they were fed a high-fat/high-sugar diet. A requirement of serotonin 2C receptors in POMC neurons for the maintenance of normal energy and glucose homeostasis was further demonstrated when Htr2c loss was induced in POMC neurons in adult mice using a tamoxifen-inducible POMC-cre system. These data demonstrate that serotonin 2C receptor-expressing POMC neurons are required to control energy and glucose homeostasis and implicate POMC neurons as the target for the effect of serotonin 2C receptor agonists on weight-loss induction and improved glycemic control., Introduction Pharmacological compounds that target central serotonin (5-hydroxytryptamine [5-HT]) signaling potently regulate energy homeostasis and have been used effectively as antiobesity drugs in humans (1). A prime example is d-fenfluramine [...]

The transcription factor FOXO1 plays a central role in metabolic homeostasis by regulating leptin and insulin activity in many cell types, including neurons. However, the neurons mediating these effects and the identity of the molecular targets through which FOXO1 regulates metabolism remain to be defined. Here, we show that the ventral medial nucleus of the hypothalamus (VMH) is a key site of FOXO1 action. We found that mice lacking FOXO1 in steroidogenic factor 1 (SF-1) neurons of the VMH are lean due to increased energy expenditure. The mice also failed to appropriately suppress energy expenditure in response to fasting. Furthermore, these mice displayed improved glucose tolerance due to increased insulin sensitivity in skeletal muscle and heart. Gene expression profiling and sequence analysis revealed several pathways regulated by FOXO1. In addition, we identified the nuclear receptor SF-1 as a direct FOXO1 transcriptional target in the VMH. Collectively, our data suggest that the transcriptional networks modulated by FOXO1 in VMH neurons are key components in the regulation of energy balance and glucose homeostasis., Introduction Defective regulation of energy balance results in obesity and components of metabolic syndrome, such as type 2 diabetes. Understanding the molecular and cellular mechanisms underlying the ability of the [...]

Leptin action on its receptor (LEPR) stimulates energy expenditure and reduces food intake, thereby lowering body weight. One leptin-sensitive target cell mediating these effects on energy balance is the proopiomelanocortin (POMC) neuron. Recent evidence suggests that the action of leptin on POMC neurons regulates glucose homeostasis independently of its effects on energy balance. Here, we have dissected the physiological impact of direct leptin action on POMC neurons using a mouse model in which endogenous LEPR expression was prevented by a LoxP-flanked transcription blocker (loxTB), but could be reactivated by Cre recombinase. Mice homozygous for the [Lepr.sup. loxTB] allele were obese and exhibited defects characteristic of LEPR deficiency. Reexpression of LEPR only in POMC neurons in the arcuate nucleus of the hypothalamus did not reduce food intake, but partially normalized energy expenditure and modestly reduced body weight. Despite the moderate effects on energy balance and independent of changes in body weight, restoring LEPR in POMC neurons normalized blood glucose and ameliorated hepatic insulin resistance, hyperglucagonemia, and dyslipidemia. Collectively, these results demonstrate that direct leptin action on POMC neurons does not reduce food intake, but is sufficient to normalize glucose and glucagon levels in mice otherwise lacking LEPR., Introduction Leptin is an adipose-derived hormone that acts on its cognate receptors (LEPR) expressed by multiple neuronal groups in distinct areas of the brain (1). The canonical effect of leptin [...]

Hepatic glucagon action increases in response to accelerated metabolic demands and is associated with increased whole body substrate availability, including circulating lipids. The hypothesis that increases in hepatic glucagon action stimulate AMP-activated protein kinase (AMPK) signaling and peroxisome proliferator-activated receptor-[alpha] (PPAR[alpha] and fibroblast growth factor 21 (FGF21) expression in a manner modulated by fatty acids was tested in vivo. Wild-type ([gcgr.sup.+/+]) and glucagon receptor-null ([gcgr.sup.-/-]) littermate mice were studied using an 18-h fast, exercise, and hyperglucagonemic-euglycemic clamps plus or minus increased circulating lipids. Fasting and exercise in [gcgr.sup.+/+], but not [gcgr.sup.-/-] mice, increased hepatic phosphorylated AMPK[alpha] at threonine 172 ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) and PPAR[alpha] and FGF21 mRNA. Clamp results in [gcgr.sup.+/+] mice demonstrate that hyperlipidemia does not independently impact or modify glucagon-stimulated increases in hepatic AMP/ATP, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], or PPAR[alpha] and FGF21 mRNA. It blunted glucagon-stimulated acetyl-CoA carboxylase phosphorylation, a downstream target of AMPK, and accentuated PPAR[alpha] and FGF21 expression. All effects were absent in [gcgr.sup.-/-] mice. These findings demonstrate that glucagon exerts a critical regulatory role in liver to stimulate pathways linked to lipid metabolism in vivo and shows for the first time that effects of glucagon on PPARa and FGF21 expression are amplified by a physiological increase in circulating lipids. adenosine 5'-monophosphate-activated protein kinase; peroxisome proliferator-activated receptor-[alpha]; fibroblast growth factor 21 doi: 10.1152/ajpendo.00263.2010.

The hepatic energy state, defined by adenine nucleotide levels, couples metabolic pathways with energy requirements. This coupling is fundamental in the adaptive response to many conditions and is impaired in metabolic disease. We have found that the hepatic energy state is substantially reduced following exercise, fasting, and exposure to other metabolic stressors in C57BL/6 mice. Glucagon receptor signaling was hypothesized to mediate this reduction because increased plasma levels of glucagon are characteristic of metabolic stress and because this hormone stimulates energy consumption linked to increased gluconeogenic flux through cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) and associated pathways. We developed what we believe to be a novel hyperglucagonemic-euglycemic clamp to isolate an increment in glucagon levels while maintaining fasting glucose and insulin. Metabolic stress and a physiological rise in glucagon lowered the hepatic energy state and amplified AMP-activated protein kinase signaling in control mice, but these changes were abolished in glucagon receptor--null mice and mice with liver-specific PEPCK-C deletion. 129X1/Sv mice, which do not mount a glucagon response to hypoglycemia, displayed an increased hepatic energy state compared with C57BL/6 mice in which glucagon was elevated. Taken together, these data demonstrate in vivo that the hepatic energy state is sensitive to glucagon receptor activation and requires PEPCK-C, thus providing new insights into liver metabolism., Introduction The energy state in the cell is defined by adenine nucleotide levels and is critically coupled to nearly all metabolic processes (1). In the cell, the adenine nucleotides ATP, [...]

A portal venous 5-aminoimidazole-4-carboxamide-1-[beta]-D-fibofuranoside infusion that results in hepatic 5-aminoimidazole-4-carboxamide-1-[beta]-D-ribosyl-5-monophosphate (ZMP) concentrations of ~4 [micro]mol/g liver increases hepatic glycogenolysis and glucose output. ZMP is an AMP analog that mimics the regulatory actions of this nucleotide. The aim of this study was to measure hepatic AMP concentrations in response to increasing energy requirements to test the hypothesis that AMP achieves concentrations during exercise, consistent with a role in stimulation of hepatic glucose metabolism. Male C57BL/6J mice (27.4 [+ or -] 0.4 g) were subjected to 35 min of rest [sedentary (SED), n = 8], underwent short-term (ST, 35 min) moderate (20 m/min, 5% grade) exercise (n = 8), or underwent treadmill exercise under similar conditions but until exhaustion (EXH, n = 8). Hepatic AMP concentrations were 0.82 [+ or -] 0.05, 1.17 [+ or -] 0.11, and 2.52 [+ or -] 0.16 [micro]mol/g liver in SED, ST, and EXH mice, respectively (P < 0.05). Hepatic energy charge was 0.66 [+ or -] 0.01, 0.58 [+ or -] 0.02, and 0.33 [+ or -] 0.22 in SED, ST, and EXH mice, respectively (P < 0.05). Hepatic glycogen was 11.6 [+ or -] 1.0, 8.8 [+ or -] 2.2, and 0.0 [+ or -] 0.1 mg/g liver in SED, ST, and EXH mice, respectively (P < 0.05). Hepatic AMPK ([Thr.sup.172]) phosphorylation was 1.00 [+ or -] 0.14, 1.96 [+ or -] 0.16, and 7.44 [+ or -] 0.63 arbitrary units in SED, ST, and EXH mice, respectively (P < 0.05). Thus exercise increases hepatic AMP concentrations. These data suggest that the liver is highly sensitive to metabolic demands, as evidenced by dramatic changes in cellular energy indicators (AMP) and sensors thereof (AMP-activated protein kinase). In conclusion, AMP is sensitively regulated, consistent with it having an important role in hepatic metabolism. adenosine 5'-monophosphate; adenosine 5'-monophosphate-activated protein kinase; glycogen

Cisco (Coregonus artedi) once dominated fish communities in the Laurentian Great Lakes. Restoring the abundance and distribution of this species has emerged as a management priority, yet our understanding of Cisco spawning habitat use is insufficient to characterise habitat needs for these populations and assess whether availability of suitable spawning habitat could be a constraint to recovery. We characterised the distribution of incubating Cisco eggs in situ across gradients of depth and substrate types to describe the spawning habitat used by three Great Lakes populations. In Chaumont Bay, Lake Ontario, eggs were concentrated on shallow bedrock shoals and not found on deeper silt or sand substrate. In contrast, eggs in Thunder Bay, Lake Superior, and Elk Rapids, Lake Michigan, were found on deeper fine grain sediments with low utilisation of shallow rocky and cobble habitats. These patterns of egg incubation habitat use suggest a broad spawning habitat niche at the species level but distinct spawning habitat preferences at the population level. While our results indicate some historical diversity in spawning habitat use has been maintained across the species' range in the Great Lakes, comparisons of contemporary spawning habitat utilisation against historical accounts raise questions as to whether some spawning habitat use behaviours may no longer be prevalent within specific lakes. Thus, characterising the portfolio of spawning strategies remaining within lakes may improve our understanding of habitat needs and identify opportunities to maintain population diversity while supporting Cisco rehabilitation. [ABSTRACT FROM AUTHOR]

Insulin-induced hypoglycemia leads to far-ranging negative consequences in patients with diabetes. Components of the counterregulatory response (CRR) system that help minimize and reverse hypoglycemia and coordination between those components are well studied but not yet fully characterized. Here, we tested the hypothesis that acyl-ghrelin, a hormone that defends against hypoglycemia in a preclinical starvation model, is permissive for the normal CRR to insulin-induced hypoglycemia. Ghrelin knockout (KO) mice and wild-type (WT) littermates underwent an insulin bolus-induced hypoglycemia test and a low-dose hyperinsulinemic-hypoglycemic clamp procedure. Clamps also were performed in ghrelin-KO mice and C57BL/6N mice administered the growth hormone secretagogue receptor agonist HM01 or vehicle. Results show that hypoglycemia, as induced by an insulin bolus, was more pronounced and prolonged in ghrelin-KO mice, supporting previous studies suggesting increased insulin sensitivity upon ghrelin deletion. Furthermore, during hyperinsulinemic-hypoglycemic clamps, ghrelin-KO mice required a 10-fold higher glucose infusion rate (GIR) and exhibited less robust corticosterone and growth hormone responses. Conversely, HM01 administration, which reduced the GIR required by ghrelin-KO mice during the clamps, increased plasma corticosterone and growth hormone. Thus, our data suggest that endogenously produced acyl-ghrelin not only influences insulin sensitivity but also is permissive for the normal CRR to insulin-induced hypoglycemia. [ABSTRACT FROM AUTHOR]

Leptin resistance is a hallmark of obesity with unclear etiology. Celastrol, a compound found in the roots of the Tripterygium wilfordii and known to reduce endoplasmic reticulum (ER) stress, has recently emerged as a promising candidate to treat obesity by improving leptin sensitivity. However, the underlying neural mechanisms by which celastrol reduces obesity remain unclear. Using three different mouse models of obesity-diet-induced obesity (DIO), leptin receptor (LepR)-null, and melanocortin 4 receptor (MC4R)-null mice-in this study, we show that systemic celastrol administration substantially reduces food intake and body weight in MC4R-null comparable to DIO, proving the MC4R-independent antiobesity effect of celastrol. Body weight reduction was due to decreases in both fat and lean mass, and modest but significant body weight reduction was also observed in nonobese wild-type and LepR-null mice. Unexpectedly, celastrol upregulated proinflammatory cytokines without affecting genes involved in ER stress. Importantly, celastrol steadily increased sympathetic nerve activity to the brown fat and kidney with concordant increases of resting metabolic rate and arterial pressure. Our results suggest a previously unappreciated mechanism of action of celastrol in the regulation of energy homeostasis and highlight the need for careful consideration of its development as a safe antiobesity medication. [ABSTRACT FROM AUTHOR]