Chronic CNS administration of Agouti-related protein (Agrp) reduces energy expenditure

Hypoxia inducible factor-dependent upregulation of Agrp in glomus type I cells of the carotid body

Agouti-related peptide (AgRP) is a well-established potent orexigenic peptide primarily expressed in hypothalamic neurons. Nevertheless, the expression and functional significance of extrahypothalamic AgRP remain poorly understood. In this study, utilizing histological and molecular biology techniques, we have identified a significant expression of Agrp mRNA and AgRP peptide production in glomus type I cells within the mouse carotid body (CB). Furthermore, we have uncovered evidence supporting the expression of the AgRP receptor melanocortin receptor 3 (Mc3r) in adjacent sympathetic neurons, suggesting a potential local paracrine role for AgRP within the CB. Importantly, AgRP immunoreactivity was also identified in glomus type I cells of the human CB. Given the unexpected abundance of AgRP in glomus type I cells, a chemoreceptor cell specialized in oxygen sensing, we proceeded to investigate whether Agrp expression in the CB is regulated by hypoxemia and associated oxygen-sensing molecular mechanisms. In vitro luciferase assays reveal that hypoxia stimulates the human and mouse Agrp promoters in a Hypoxia Inducible Factor (HIF1/2)-dependent manner. Our in vivo experiments further demonstrate that exposure to environmental hypoxia (10%) robustly induces Agrp expression in type I glomus cells of mice. Furthermore, these findings collectively highlight the hitherto unknown source of AgRP in murine and human type I glomus cells and underscore the direct control of Agrp transcription by HIF signaling.

Dieting reverses histone methylation and hypothalamic AgRP regulation in obese rats

INTRODUCTION

Although dieting is a key factor in improving physiological functions associated with obesity, the role by which histone methylation modulates satiety/hunger regulation of the hypothalamus through weight loss remains largely elusive. Canonically, H3K9me2 is a transcriptional repressive post-translational epigenetic modification that is involved in obesity, however, its role in the hypothalamic arcuate nucleus (ARC) has not been thoroughly explored. Here we explore the role that KDM4D, a specific demethylase of residue H3K9, plays in energy balance by directly modulating the expression of AgRP, a key neuropeptide that regulates hunger response.

METHODS

We used a rodent model of diet-induced obesity (DIO) to assess whether histone methylation malprogramming impairs energy balance control and how caloric restriction may reverse this phenotype. Using ChIP-qPCR, we assessed the repressive modification of H3K9me2 at the site of AgRP. To elucidate the functional role of KDM4D in reversing obesity via dieting, a pharmacological agent, JIB-04 was used to inhibit the action of KDM4D in vivo.

RESULTS

In DIO, downregulation of Kdm4d mRNA results in both enrichment of H3K9me2 on the AgRP promoter and transcriptional repression of AgRP. Because epigenetic modifications are dynamic, it is possible for some of these modifications to be reversed when external cues are altered. The reversal phenomenon was observed in calorically restricted rats, in which upregulation of Kdm4d mRNA resulted in demethylation of H3K9 on the AgRP promoter and transcriptional increase of AgRP. In order to verify that KDM4D is necessary to reverse obesity by dieting, we demonstrated that in vivo inhibition of KDM4D activity by pharmacological agent JIB-04 in naïve rats resulted in transcriptional repression of AgRP, decreasing orexigenic signaling, thus inhibiting hunger.

DISCUSSION

We propose that the action of KDM4D through the demethylation of H3K9 is critical in maintaining a stable epigenetic landscape of the AgRP promoter, and may offer a target to develop new treatments for obesity.

Aberrant expression of agouti signaling protein (ASIP) as a cause of monogenic severe childhood obesity

Here we report a heterozygous tandem duplication at the ASIP (agouti signaling protein) gene locus causing ubiquitous, ectopic ASIP expression in a female patient with extreme childhood obesity. The mutation places ASIP under control of the ubiquitously active itchy E3 ubiquitin protein ligase promoter, driving the generation of ASIP in patient-derived native and induced pluripotent stem cells for all germ layers and hypothalamic-like neurons. The patient's phenotype of early-onset obesity, overgrowth, red hair and hyperinsulinemia is concordant with that of mutant mice ubiquitously expressing the homolog nonagouti. ASIP represses melanocyte-stimulating hormone-mediated activation as a melanocortin receptor antagonist, which might affect eating behavior, energy expenditure, adipocyte differentiation and pigmentation, as observed in the index patient. As the type of mutation escapes standard genetic screening algorithms, we rescreened the Leipzig Childhood Obesity cohort of 1,745 patients and identified four additional patients with the identical mutation, ectopic ASIP expression and a similar phenotype. Taken together, our data indicate that ubiquitous ectopic ASIP expression is likely a monogenic cause of human obesity.

The PACAP Paradox: Dynamic and Surprisingly Pleiotropic Actions in the Central Regulation of Energy Homeostasis

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP), a pleiotropic neuropeptide, is widely distributed throughout the body. The abundance of PACAP expression in the central and peripheral nervous systems, and years of accompanying experimental evidence, indicates that PACAP plays crucial roles in diverse biological processes ranging from autonomic regulation to neuroprotection. In addition, PACAP is also abundantly expressed in the hypothalamic areas like the ventromedial and arcuate nuclei (VMN and ARC, respectively), as well as other brain regions such as the nucleus accumbens (NAc), bed nucleus of stria terminalis (BNST), and ventral tegmental area (VTA) - suggesting that PACAP is capable of regulating energy homeostasis via both the homeostatic and hedonic energy balance circuitries. The evidence gathered over the years has increased our appreciation for its function in controlling energy balance. Therefore, this review aims to further probe how the pleiotropic actions of PACAP in regulating energy homeostasis is influenced by sex and dynamic changes in energy status. We start with a general overview of energy homeostasis, and then introduce the integral components of the homeostatic and hedonic energy balance circuitries. Next, we discuss sex differences inherent to the regulation of energy homeostasis via these two circuitries, as well as the activational effects of sex steroid hormones that bring about these intrinsic disparities between males and females. Finally, we explore the multifaceted role of PACAP in regulating homeostatic and hedonic feeding through its actions in regions like the NAc, BNST, and in particular the ARC, VMN and VTA that occur in sex- and energy status-dependent ways.

Neurohormonal Changes in the Gut–Brain Axis and Underlying Neuroendocrine Mechanisms following Bariatric Surgery

Obesity is a complex, multifactorial disease that is a major public health issue worldwide. Currently approved anti-obesity medications and lifestyle interventions lack the efficacy and durability needed to combat obesity, especially in individuals with more severe forms or coexisting metabolic disorders, such as poorly controlled type 2 diabetes. Bariatric surgery is considered an effective therapeutic modality with sustained weight loss and metabolic benefits. Numerous genetic and environmental factors have been associated with the pathogenesis of obesity, while cumulative evidence has highlighted the gut–brain axis as a complex bidirectional communication axis that plays a crucial role in energy homeostasis. This has led to increased research on the roles of neuroendocrine signaling pathways and various gastrointestinal peptides as key mediators of the beneficial effects following weight-loss surgery. The accumulate evidence suggests that the development of gut-peptide-based agents can mimic the effects of bariatric surgery and thus is a highly promising treatment strategy that could be explored in future research. This article aims to elucidate the potential underlying neuroendocrine mechanisms of the gut–brain axis and comprehensively review the observed changes of gut hormones associated with bariatric surgery. Moreover, the emerging role of post-bariatric gut microbiota modulation is briefly discussed.

Lipopolysaccharide inhibits hypothalamic Agouti-related protein gene expression via activating mechanistic target of rapamycin signaling in chicks

Lipopolysaccharide (LPS) induces profound anorexia in birds. However, the neuronal regulatory network underlying LPS-provoked anorexia is unclear. To determine whether any cross talk occurs among hypothalamic mechanistic target of rapamycin (mTOR) and LPS in the regulation of appetite, we performed an intracerebroventricular injection of rapamycin (an mTOR inhibitor) on LPS-treated chicks. The results indicate that peripheral administrations of LPS decreased the agouti-related protein (AgRP) mRNA level, but increased the phosphorylated mTOR and nuclear factor-кB (NF-кB) protein level. Blocking mTOR significantly attenuated LPS-induced anorexia, AgRP suppression, and p-NF-кB increase. Thus, the results suggest that LPS causes anorexia via the mTOR-AgRP signaling pathway, and mTOR signaling is also associated with the regulation of LPS in p-NF-кB.

Hypothalamic GPCR Signaling Pathways in Cardiometabolic Control

Obesity is commonly associated with sympathetic overdrive, which is one of the major risk factors for the development of cardiovascular diseases, such as hypertension and heart failure. Over the past few decades, there has been a growing understanding of molecular mechanisms underlying obesity development with central origin; however, the relative contribution of these molecular changes to the regulation of cardiovascular function remains vague. A variety of G-protein coupled receptors (GPCRs) and their downstream signaling pathways activated in distinct hypothalamic neurons by different metabolic hormones, neuropeptides and monoamine neurotransmitters are crucial not only for the regulation of appetite and metabolic homeostasis but also for the sympathetic control of cardiovascular function. In this review, we will highlight the main GPCRs and associated hypothalamic nuclei that are important for both metabolic homeostasis and cardiovascular function. The potential downstream molecular mediators of these GPCRs will also be discussed.

Deciphering an AgRP-serotoninergic neural circuit in distinct control of energy metabolism from feeding

Contrasting to the established role of the hypothalamic agouti-related protein (AgRP) neurons in feeding regulation, the neural circuit and signaling mechanisms by which they control energy expenditure remains unclear. Here, we report that energy expenditure is regulated by a subgroup of AgRP neurons that send non-collateral projections to neurons within the dorsal lateral part of dorsal raphe nucleus (dlDRN) expressing the melanocortin 4 receptor (MC4R), which in turn innervate nearby serotonergic (5-HT) neurons. Genetic manipulations reveal a bi-directional control of energy expenditure by this circuit without affecting food intake. Fiber photometry and electrophysiological results indicate that the thermo-sensing MC4RdlDRN neurons integrate pre-synaptic AgRP signaling, thereby modulating the post-synaptic serotonergic pathway. Specifically, the MC4RdlDRN signaling elicits profound, bi-directional, regulation of body weight mainly through sympathetic outflow that reprograms mitochondrial bioenergetics within brown and beige fat while feeding remains intact. Together, we suggest that this AgRP neural circuit plays a unique role in persistent control of energy expenditure and body weight, hinting next-generation therapeutic approaches for obesity and metabolic disorders.

Adenosine 5'-monophosphate induces hypothermia and alters gene expressions in the brain and liver of chicks

The adenosine A1 receptor is important for body temperature regulation in mammals; however, little is known about its function in avian species. In this study, we investigated the effects of the adenosine A1 receptor agonist and antagonist (adenosine 5'-monophosphate [5'-AMP] and 8 p-sulfophenyl theophylline [8-SPT], respectively) on thermoregulation in chickens. Male chicks were used in this study. After administration of 5'-AMP and 8-SPT, the rectal temperature, plasma metabolites, and gene expressions in the hypothalamus and liver were measured. The rectal temperature was reduced by peripheral administration of 5'-AMP, and the hypothermic effect of 5'-AMP was attenuated by central injection of 8-SPT in chicks. In the hypothalamus, the mRNA level of the agouti-related protein (AgRP) was increased by 5'-AMP administration, whereas it was suppressed by 8-SPT. The plasma levels of free fatty acid were elevated in 5'-AMP-treated chicks and that elevation was suppressed by the 8-SPT treatment. The gene expression of proopiomelanocortin in the hypothalamus was affected by 8-SPT. Nevertheless, the gene expressions of the thermoregulation-related genes, such as the thyrotropin-releasing hormone, were not affected by 5'-AMP and 8-SPT. Hepatic gene expressions related to lipid intake and metabolism were suppressed by 5'-AMP. However, the gene expression of the uncoupling protein was upregulated by 5'-AMP. Based on these results, birds, like mammals, will undergo adenosine A1 receptor-induced hypothermia. In conclusion, it is suggested that 5'-AMP-mediated hypothermia via the adenosine A1 receptor may affect the central melanocortin system and suppress hepatic lipid metabolism in chickens.

Relationship Between Serum Agouti-Related Peptide Levels and Metabolic Syndrome in Euthymic Bipolar Patients

INTRODUCTION

Agouti-Related Peptide (AgRP) is expressed primarily in the hypothalamic arcuate nucleus, stimulates appetite and decreases metabolism and energy expenditure. The aim of our study is to evaluate the relationship between serum Agouti-Related Peptide (AgRP) levels and metabolic syndrome in euthymic bipolar patients.

METHODS

Forty euthymic bipolar patients who used only mood stabilizer for at least three months and 40 healthy volunteers as control group were included in the study. We measured fasting blood glucose levels and serum levels of AgRP, total cholesterol, triglyceride, high-density lipoprotein (HDL) and low-density lipoprotein (LDL) of all participants. The main outcome measure was the difference between patients and control groups in terms of metabolic syndrome frequency and the relationship between serum AgRP level and metabolic syndrome is also investigated.

RESULTS

The metabolic syndrome was significantly more common in euthymic bipolar patients than in control group (p=0.039). Additionally, levels of blood glucose and triglyceride were significantly higher in the patient group than in the control group (p=0.006 and 0.01 respectively). Serum AgRP levels did not differ between the patient and control groups (p=0.35). Also, in euthymic bipolar patients, there was no significant difference in serum AgRP levels between patients with metabolic syndrome and those without (p=0.754).

CONCLUSION

We found significantly higher frequency of metabolic syndrome in euthymic bipolar patients than in the control group. However, there was no significant difference in the levels of serum AgRP between bipolar patients with and without metabolic syndrome in either study groups.

Recent Advances in Hypertension: Intersection of Metabolic and Blood Pressure Regulatory Circuits in the Central Nervous System

Obesity represents the single greatest ongoing roadblock to improving cardiovascular health. Prolonged obesity is associated with fundamental changes in the integrative control of energy balance, including the development of selective leptin resistance, which is thought to contribute to obesity-associated hypertension, and adaptation of resting metabolic rate (RMR) when excess weight is reduced. Leptin and the melanocortin system within the hypothalamus contribute to the control of both energy balance and blood pressure. While the development of drugs to stimulate RMR and thereby reverse obesity through activation of the melanocortin system has been pursued, most of the resulting compounds simultaneously cause hypertension. Evidence supports the concept that although feeding behaviors, RMR, and blood pressure are controlled through mechanisms that utilize similar molecular mediators, these mechanisms exist in anatomically dissociable networks. New evidence supports a major change in molecular signaling within AgRP (Agouti-related peptide) neurons of the arcuate nucleus of the hypothalamus during prolonged obesity and the existence of multiple distinct subtypes of AgRP neurons that individually contribute to control of feeding, RMR, or blood pressure. Finally, ongoing work by our laboratory and others support a unique role for AT₁ (angiotensin II type 1 receptor) within one specific subtype of AgRP neuron for the control of RMR. We propose that understanding the unique biology of the AT₁-expressing, RMR-controlling subtype of AgRP neurons will help to resolve the selective dysfunctions in RMR control that develop during prolonged obesity and potentially point toward novel druggable antiobesity targets that will not simultaneously cause hypertension.

A Brain-Melanocortin-Vagus Axis Mediates Adipose Tissue Expansion Independently of Energy Intake

The melanocortin system is a brain circuit that influences energy balance by regulating energy intake and expenditure. In addition, the brain-melanocortin system controls adipose tissue metabolism to optimize fuel mobilization and storage. Specifically, increased brain-melanocortin signaling or negative energy balance promotes lipid mobilization by increasing sympathetic nervous system input to adipose tissue. In contrast, calorie-independent mechanisms favoring energy storage are less understood. Here, we demonstrate that reduction of brain-melanocortin signaling actively promotes fat mass gain by activating the lipogenic program and adipocyte and endothelial cell proliferation in white fat depots independently of caloric intake via efferent nerve fibers conveyed by the common hepatic branch of the vagus nerve. Those vagally regulated obesogenic signals also contribute to the fat mass gain following chronic high-fat diet feeding. These data reveal a physiological mechanism whereby the brain controls energy stores that may contribute to increased susceptibility to obesity.

Brain-melanocortin signaling controls fat mass indirectly by regulating energy balance and by direct control of lipid mobilization from adipose tissue via sympathetic nervous system activity. Holland et al. show that reduced brain-melanocortin signaling promotes white adipose tissue expansion via signals conveyed by efferent innervation of the vagus nerve.

Effects of corticosterone within the hypothalamic arcuate nucleus on food intake and body weight in male rats

OBJECTIVE

Obesity is a major cause of morbidity and mortality. Few weight-reducing medications are available, and these have limited efficacy. Cushing's Syndrome (caused by elevated glucocorticoid levels) and obesity have similar metabolic features. Though circulating glucocorticoid levels are not elevated in obesity, tissue-specific glucocorticoid levels have been implicated in the development of the metabolic phenotype of obesity. Tissue glucocorticoid levels are regulated by 11β-hydroxysteroid dehydrogenase type1 (11βHSD1), which increases the local concentration of active glucocorticoids by the production of corticosterone from 11-dehydrocorticosterone. 11βHSD1 is expressed in the hypothalamic arcuate nucleus (ARC), a major weight and appetite-regulating centre, and therefore represents a target for novel anti-obesity therapeutic agents. Thus, we sought to investigate the effect of chronic alterations of ARC corticosterone levels (mediated by 11βHSD1) on food intake and body weight in adult male rats.

METHODS

Recombinant adeno-associated virus particles bearing sense 11βHSD1 (rAAV-S11βHSD1) and small interfering 11βHSD1 (rAAV-si11βHSD1), respectively, were stereotactically injected into the ARC (bilaterally) of adult male Wistar rats. rAAV-GFP was injected into control groups of male Wistar rats. Food intake and body weight were measured three times a week for 70 days. Terminal brain, plasma and intrascapular brown adipose tissue (iBAT) samples were taken for measurement of mRNA expression and hormone levels.

RESULTS

Compared to controls, rAAV-S11βHSD1 injection resulted in higher ARC corticosterone levels, hyperphagia and increased weight gain. Conversely, rAAV-si11βHSD1 injection (compared to controls) resulted in lower ARC corticosterone levels, higher iBAT uncoupling protein-1 mRNA expression and less weight gain despite similar food intake.

CONCLUSIONS

Therefore ARC corticosterone, regulated by 11βHSD1, may play a role in food intake and body weight regulation. These data have important implications for the development of centrally-acting 11βHSD1 inhibitors, which are currently being developed for the treatment of obesity, metabolic disorders, and other conditions.

The role of epigenetics in hypothalamic energy balance control: implications for obesity

Despite enormous social and scientific efforts, obesity rates continue to increase worldwide. While genetic factors contribute to obesity development, genetics alone cannot explain the current epidemic. Obesity is essentially the consequence of complex genetic-environmental interactions. Evidence suggests that contemporary lifestyles trigger epigenetic changes, which can dysregulate energy balance and thus contribute to obesity. The hypothalamus plays a pivotal role in the regulation of body weight, through a sophisticated network of neuronal systems. Alterations in the activity of these neuronal pathways have been implicated in the pathophysiology of obesity. Here, we review the current knowledge on the central control of energy balance with a focus on recent studies linking epigenetic mechanisms in the hypothalamus to the development of obesity and metabolic disorders.

Metabolic Abnormalities of Chronic High-Dose Glucocorticoids Are Not Mediated by Hypothalamic AgRP in Male Mice

Glucocorticoids are potent and widely used medicines but often cause metabolic side effects. A murine model of corticosterone treatment resulted in increased hypothalamic expression of the melanocortin antagonist AgRP in parallel with obesity and hyperglycemia. We investigated how these adverse effects develop over time, with particular emphasis on hypothalamic involvement. Wild-type and Agrp-/- male mice were treated with corticosterone for 3 weeks. Phenotypic, biochemical, protein, and mRNA analyses were undertaken on central and peripheral tissues, including white and brown adipose tissue, liver, and muscle, to determine the metabolic consequences. Corticosterone treatment induced hyperphagia within 1 day in wild-type mice, which persisted for 3 weeks. Despite this early increase in food intake, the body weight only started to increase after 10 days. Hyperinsulinemia occurred at day 1. Also, although after 2 days, alterations were present in the genes often associated with insulin resistance in several peripheral tissues, hyperglycemia only developed at 3 weeks. Throughout, sustained elevation in hypothalamic Agrp expression was present. Mice with Agrp deleted [using clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9, Agrp-/-] were partially protected against corticosterone-induced hyperphagia. However, Agrp-/- mice still had corticosterone-induced increases in body weight and adiposity similar to those of the Agrp+/+ mice. Loss of Agrp did not diminish corticosterone-induced hyperinsulinemia or correct changes in hepatic gluconeogenic genes. Chronic glucocorticoid treatment in mice mimics many of the metabolic side effects seen in patients and leads to a robust increase in Agrp. However, AgRP does not appear to be responsible for most of the glucocorticoid-induced adverse metabolic effects.

MicroRNAs are involved in the hypothalamic leptin sensitivity

The central nervous system monitors modifications in metabolic parameters or hormone levels (leptin) and elicits adaptive responses such as food intake and glucose homeostasis regulation. Particularly, within the hypothalamus, pro-opiomelanocortin (POMC) neurons are crucial regulators of energy balance. Consistent with a pivotal role of the melanocortin system in the control of energy homeostasis, disruption of the Pomc gene causes hyperphagia and obesity. Pomc gene expression is tightly controlled by different mechanisms. Interestingly, recent studies pointed to a key role for micro ribonucleic acid (miRNAs) in the regulation of gene expression. However, the role of miRNAs in the leptin sensitivity in hypothalamic melanocortin system has never been assessed. We developed a transgenic mouse model (PDKO) with a partial deletion of the miRNA processing enzyme DICER specifically in POMC neurons. PDKO mice exhibited a normal body weight but a decrease of food intake. Interestingly, PDKO mice had decreased metabolic rate by reduction of VO₂ consumption and CO₂ production which could explain that PDKO mice have normal weight while eating less. Interestingly, we observed an increase of leptin sensitivity in the POMC neurons of PDKO mice which could explain the decrease of food intake in this model. We also observed an increase in the expression of genes involved in the function of brown adipose tissue that is in polysynaptic contact with the POMC neurons. In summary, these results support the hypothesis that Dicer-derived miRNAs may be involved in the effect of leptin on POMC neurons activity.

The impact of androgen actions in neurons on metabolic health and disease

The male hormone testosterone exerts different effects on glucose and energy homeostasis in males and females. Testosterone deficiency predisposes males to visceral obesity, insulin resistance and type 2 diabetes. However, testosterone excess predisposes females to similar metabolic dysfunction. Here, we review the effects of testosterone actions in the central nervous system on metabolic function in males and females. In particular, we highlight changes within the hypothalamus that control glucose and energy homeostasis. We distinguish the organizational effects of testosterone in the programming of neural circuitry during development from the activational effects of testosterone during adulthood. Finally, we explore potential sites where androgen might be acting to impact metabolism within the central nervous system.

Central and peripheral leptin and agouti-related protein during and after pregnancy in relation to weight change

OBJECTIVE

To study changes of neuropeptides and adipokines in cerebrospinal fluid (CSF) and serum from pregnancy to postpregnancy in relation to weight changes, fat mass and glucose metabolism.

CONTEXT

With high postpartum weight retention being a risk factor in future pregnancies and of lifelong obesity, we evaluated neuropeptide and adipokine changes in women who either gained weight or were weight stable.

DESIGN

Women were followed for 5 ± 1 years after pregnancy and divided into two groups, weight stable and weight gain, by weight change from start of pregnancy.

PATIENTS

Twenty-five women (BMI 27 ± 5 kg/m² ) recruited at admission for elective caesarean section.

MEASUREMENTS

CSF and serum levels of agouti-related protein (AgRP), leptin and insulin, and serum levels of adiponectin and soluble leptin receptor were measured during and after pregnancy. These measurements were further related to fat mass and insulin sensitivity (HOMA-IR).

RESULTS

S-AgRP levels during pregnancy were lower in the weight stable group and a 1 unit increase in s-AgRP was associated with 24% higher odds of pertaining to the weight gain group. After pregnancy, s-AgRP increased in the weight stable group but decreased in the weight gain group. Decreased transport of leptin into CSF during pregnancy was reversed by an increased CSF:serum leptin ratio after pregnancy. In women who returned to their prepregnancy weight, serum adiponectin increased after pregnancy and correlated negatively with HOMA-IR.

CONCLUSION

S-AgRP concentration in late pregnancy may be one factor predicting weight change after pregnancy, and circulating AgRP may be physiologically important in the long-term regulation of body weight.

Integrated insights into the role of alpha-melanocyte stimulatory hormone in the control of food intake and glycaemia

Identifying peptide hormones with multipotent actions on both weight and glycaemia can have a significant impact on therapeutic options in the treatment of obesity and diabetes. This has been exemplified by recent advances involving pharmacological exploitation of glucagon-like peptide 1 biology. Herein, we summarise evidence supporting the potential candidacy in this light of alpha-melanocyte stimulatory hormone, an endogenous peptide hormone and a breakdown product of the neuropeptide pro-opiomelanocortin. We reference its well described central actions in the control of food intake and moreover highlight new data pointing to an important role for this peptide hormone in the periphery, in relation to glycaemic control.

Arcuate nucleus neurons are not essential for the preprandial peak in plasma ghrelin after neonatal monosodium glutamate treatment

The aim of the present study was to determine whether arcuate nucleus (ARC) lesions affect the ghrelin level in the plasma and the stomach in monosodium glutamate (MSG)‑treated mice. The aim of the present study was to investigate whether the ARC was destroyed in mice treated neonatally with MSG, and whether the ARC lesions affect the ghrelin level in the plasma and lipid mobilization in MSG‑treated mice. The results revealed that MSG led to a marked reduction in ARC cresyl violet staining, tyrosine hydroxylase-immunoreactive (IR) neurons and neuropeptide Y‑IR fibers, compared with saline controls. MSG‑treated mice exhibited significantly increased body mass compared with saline controls, and MSG treatment did not prevent food deprivation‑induced decrease in white adipose tissue mass compared with controls. Plasma ghrelin levels were significantly increased in MSG‑treated mice that were fasted for 48 h, compared with the levels prior to fasting and re‑feeding, and the preprandial peak of plasma ghrelin persisted in MSG‑treated mice. In summary, the ARC was not found to be essential for food deprivation‑induced lipid mobilization and preprandial peak in MSG‑treated mice. However, this finding does not mean that ARC neurons do not contribute to food sensing and lipid mobilization under normal conditions, as compensatory mechanisms may have emerged after the ablation of ARC neurons.

mTORC1 in AGRP neurons integrates exteroceptive and interoceptive food-related cues in the modulation of adaptive energy expenditure in mice

Energy dissipation through interscapular brown adipose tissue (iBAT) thermogenesis is an important contributor to adaptive energy expenditure. However, it remains unresolved how acute and chronic changes in energy availability are detected by the brain to adjust iBAT activity and maintain energy homeostasis. Here, we provide evidence that AGRP inhibitory tone to iBAT represents an energy-sparing circuit that integrates environmental food cues and internal signals of energy availability. We establish a role for the nutrient-sensing mTORC1 signaling pathway within AGRP neurons in the detection of environmental food cues and internal signals of energy availability, and in the bi-directional control of iBAT thermogenesis during nutrient deficiency and excess. Collectively, our findings provide insights into how mTORC1 signaling within AGRP neurons surveys energy availability to engage iBAT thermogenesis, and identify AGRP neurons as a neuronal substrate for the coordination of energy intake and adaptive expenditure under varying physiological and environmental contexts.

DOI:http://dx.doi.org/10.7554/eLife.22848.001

Losing weight through dieting can be difficult. Weight loss strategies often prove ineffective because the body works like a thermostat and couples what we eat to the number of calories we burn. When we eat less, our bodies compensate and burn fewer calories, which makes losing weight harder. The brain is the master regulator of this caloric thermostat, but it is not clear how it adjusts our energy expenditure to account for how much we have eaten.

A structure deep within the brain called the hypothalamus, which helps regulate appetite, is thought to be involved in the caloric thermostat. Activating a group of neurons within the hypothalamus called the agouti-related neuropeptide (AGRP) neurons causes animals to consume large quantities of food. By contrast, inhibiting AGRP neurons causes animals to stop eating almost entirely.

Burke et al. studied AGRP neurons in mice. The experiments show that artificially activating the neurons in mice that don’t have access to food increases the animals’ activity levels but reduces the rate at which they burn calories, which helps the mice to maintain their existing weight. Allowing the mice to eat, or even just to see and smell food, switches off this effect and returns energy expenditure to normal. Finally, exposing mice to a high-fat diet for several days inhibits their AGRP neurons, and causes the animals to burn calories at a faster rate. By using up excess calories, this change also helps the animals maintain their existing body weight.

The findings of Burke et al. show that AGRP neurons are a key component of the caloric thermostat. By adjusting the rate at which the body burns calories, AGRP neurons can compensate for any changes in food intake and so limit changes in body weight. This work opens up the possibility of developing therapies that disconnect energy expenditure from energy intake to help maintain long-term weight loss.

DOI:http://dx.doi.org/10.7554/eLife.22848.002

The Use of Silk as a Scaffold for Mature, Sustainable Unilocular Adipose 3D Tissue Engineered Systems

There is a critical need for monitoring physiologically relevant, sustainable, human adipose tissues in vitro to gain new insights into metabolic diseases. To support long-term culture, a 3D silk scaffold assisted culture system is developed that maintains mature unilocular adipocytes ex vivo in coculture with preadipocytes, endothelial cells, and smooth muscle cells obtained from small volumes of liquefied adipose samples. Without the silk scaffold, adipose tissue explants cannot be sustained in long-term culture (3 months) due to their fragility. Adjustments to media components are used to tune lipid metabolism and proliferation, in addition to responsiveness to an inflammatory stimulus. Interestingly, patient specific responses to TNFα stimulation are observed, providing a proof-of-concept translational technique for patient specific disease modeling in the future. In summary, this novel 3D scaffold assisted approach is required for establishing physiologically relevant, sustainable, human adipose tissue systems from small volumes of lipoaspirate, making this methodology of great value to studies of metabolism, adipokine-driven diseases, and other diseases where the roles of adipocytes are only now becoming uncovered.

Npvf: Hypothalamic Biomarker of Ambient Temperature Independent of Nutritional Status

The mechanism by which mice, exposed to the cold, mobilize endogenous or exogenous fuel sources for heat production is unknown. To address this issue we carried out experiments using 3 models of obesity in mice: C57BL/6J+/+ (wild-type B6) mice with variable susceptibility to obesity in response to being fed a high-fat diet (HFD), B6. Ucp1-/- mice with variable diet-induced obesity (DIO) and a deficiency in brown fat thermogenesis and B6. Lep-/- with defects in thermogenesis, fat mobilization and hyperphagia. Mice were exposed to the cold and monitored for changes in food intake and body composition to determine their energy balance phenotype. Upon cold exposure wild-type B6 and Ucp1-/- mice with diet-induced obesity burned endogenous fat in direct proportion to their fat reserves and changes in food intake were inversely related to fat mass, whereas leptin-deficient and lean wild-type B6 mice fed a chow diet depended on increased food intake to fuel thermogenesis. Analysis of gene expression in the hypothalamus to uncover a central regulatory mechanism revealed suppression of the Npvf gene in a manner that depends on the reduced ambient temperature and degree of exposure to the cold, but not on adiposity, leptin levels, food intake or functional brown fat.

Current knowledge does not provide a clear, definite view of central mechanisms controlling energy balance upon cold-activated thermogenesis. Here we show that upon cold exposure lean mice maintain body composition but increase food intake to fuel thermogenesis, whereas cold-exposed mice with DIO utilize endogenous fat stores and then transition to increased food intake as body composition approaches that of the lean controls. Using knockout mice with leptin and Ucp1 gene deficiency our study indicates that the relative energy utilization from food intake and endogenous energy reserves to maintain body temperature during cold exposure is independent of both leptin action and brown fat-linked thermogenesis. Using a combination of genetic and biological approaches, we demonstrate that Npvf gene expression in the hypothalamus is regulated by changes in ambient temperature in a manner independent of the nutritional status of the mouse.

Unravelling the mysterious roles of melanocortin-3 receptors in metabolic homeostasis and obesity using mouse genetics

The central nervous melanocortin system maintains body mass and adiposity within a 'healthy' range by regulating satiety and metabolic homeostasis. Neural melanocortin-4 receptors (MC4R) modulate satiety signals and regulate autonomic outputs governing glucose and lipid metabolism in the periphery. The functions of melanocortin-3 receptors (MC3R) have been less well defined. We have observed that food anticipatory activity (FAA) is attenuated in Mc3r-/- mice housed in light:dark or constant dark conditions. Mc3r-/- mice subjected to the restricted feeding protocol that was used to induce FAA also developed insulin resistance, dyslipidaemia, impaired glucose tolerance and evidence of a cellular stress response in the liver. MC3Rs may thus function as modulators of oscillator systems that govern circadian rhythms, integrating signals from nutrient sensors to facilitate synchronizing peak foraging behaviour and metabolic efficiency with nutrient availability. To dissect the functions of MC3Rs expressed in hypothalamic and extra-hypothalamic structures, we inserted a 'lox-stop-lox' (TB) sequence into the Mc3r gene. Mc3r (TB/TB) mice recapitulate the phenotype reported for Mc3r-/- mice: increased adiposity, accelerated diet-induced obesity and attenuated FAA. The ventromedial hypothalamus exhibits high levels of Mc3r expression; however, restoring the expression of the LoxTB Mc3r allele in this nucleus did not restore FAA. However, a surprising outcome came from studies using Nestin-Cre to restore the expression of the LoxTB Mc3r allele in the nervous system. These data suggest that 'non-neural' MC3Rs have a role in the defence of body weight. Future studies examining the homeostatic functions of MC3Rs should therefore consider actions outside the central nervous system.

Hypothalamic and brainstem neuronal circuits controlling homeostatic energy balance

Alterations in adequate energy balance maintenance result in serious metabolic disturbances such as obesity. In mammals, this complex process is orchestrated by multiple and distributed neuronal circuits. Hypothalamic and brainstem neuronal circuits are critically involved in the sensing of circulating and local factors conveying information about the energy status of the organism. The integration of these signals culminates in the generation of specific and coordinated physiological responses aimed at regulating energy balance through the modulation of appetite and energy expenditure. In this article, we review current knowledge on the homeostatic regulation of energy balance, emphasizing recent advances in mouse genetics, electrophysiology, and optogenetic techniques that have greatly contributed to improving our understanding of this central process.

Sodium meta-arsenite ameliorates hyperglycemia in obese diabetic db/db mice by inhibition of hepatic gluconeogenesis

Sodium meta-arsenite (SA) is implicated in the regulation of hepatic gluconeogenesis-related genes in vitro; however, the effects in vivo have not been studied. We investigated whether SA has antidiabetic effects in a type 2 diabetic mouse model. Diabetic db/db mice were orally intubated with SA (10 mg kg(-1) body weight/day) for 8 weeks. We examined hemoglobin A1c (HbA1c), blood glucose levels, food intake, and body weight. We performed glucose, insulin, and pyruvate tolerance tests and analyzed glucose production and the expression of gluconeogenesis-related genes in hepatocytes. We analyzed energy metabolism using a comprehensive animal metabolic monitoring system. SA-treated diabetic db/db mice had reduced concentrations of HbA1c and blood glucose levels. Exogenous glucose was quickly cleared in glucose tolerance tests. The mRNA expressions of genes for gluconeogenesis-related enzymes, glucose 6-phosphatase (G6Pase), and phosphoenolpyruvate carboxykinase (PEPCK) were significantly reduced in the liver of SA-treated diabetic db/db mice. In primary hepatocytes, SA treatment decreased glucose production and the expression of G6Pase, PEPCK, and hepatocyte nuclear factor 4 alpha (HNF-4α) mRNA. Small heterodimer partner (SHP) mRNA expression was increased in hepatocytes dependent upon the SA concentration. The expression of Sirt1 mRNA and protein was reduced, and acetylated forkhead box protein O1 (FoxO1) was induced by SA treatment in hepatocytes. In addition, SA-treated diabetic db/db mice showed reduced energy expenditure. Oral intubation of SA ameliorates hyperglycemia in db/db mice by reducing hepatic gluconeogenesis through the decrease of Sirt1 expression and increase in acetylated FoxO1.

Characterization, tissue distribution and regulation of agouti-related protein (AgRP) in a cyprinid fish (Schizothorax prenanti)

Agouti-related protein (AgRP) is an important neuropeptide involved in the regulation of feeding in both mammals and fish. In this study, we have cloned the full-length cDNA sequence for AgRP in a cyprinid fish (Schizothorax prenanti). The AgRP gene, encoding 126-amino acids, was strongly expressed in the brain. The AgRP gene was detected in embryos at developmental stages. Further, its mRNA was detectable in unfertilized eggs. An experiment was conducted to determine the expression profile of AgRP during short-term and long-term fasting of the hypothalamus. The expression level of AgRP in unfed fish was significantly increased at 3 and 4h post-fasting than in fed fish but did not affect AgRP mRNA expression after 14 days fasting. Overall, our results suggest that AgRP is a conserved peptide that might be involved in the regulation of short-term feeding and other physiological function in Schizothorax prenanti.

Modulation of feeding by chronic rAAV expression of a relaxin-3 peptide agonist in rat hypothalamus

Relaxin-3 is a neuropeptide that is abundantly expressed by discrete brainstem neuron populations that broadly innervate forebrain areas rich in the relaxin-3 G-protein-coupled-receptor, RXFP3. Acute and subchronic central administration of synthetic relaxin-3 or an RXFP3-selective agonist peptide, R3/I5, increase feeding and body weight in rats. Intrahypothalamic injection of relaxin-3 also increases feeding. In this study, we developed a recombinant adeno-associated virus 1/2 (rAAV1/2) vector that drives expression and constitutive secretion of bioactive R3/I5 and assessed the effect of intrahypothalamic injections on daily food intake and body weight gain in adult male rats over 8 weeks. In vitro testing revealed that the vector rAAV1/2-fibronectin (FIB)-R3/I5 directs the constitutive secretion of bioactive R3/I5 peptide. Bilateral injection of rAAV1/2-FIB-R3/I5 vector into the paraventricular nucleus produced an increase in daily food intake and body weight gain (P<0.01, ~23%, respectively), relative to control treatment. In a separate cohort of rats, quantitative polymerase chain reaction analysis of hypothalamic mRNA revealed strong expression of R3/I5 transgene at 3 months post-rAAV1/2-FIB-R3/I5 infusion. Levels of mRNA transcripts for the relaxin-3 receptor RXFP3, the hypothalamic 'feeding' peptides neuropeptide Y, AgRP and POMC, and the reproductive hormone, GnRH, were all similar to control, whereas vasopressin and oxytocin (OT) mRNA levels were reduced by ~25% (P=0.051) and ~50% (P<0.005), respectively, in rAAV1/2-FIB-R3/I5-treated rats (at 12 weeks, n=9/8 rats per group). These data demonstrate for the first time that R3/I5 is effective in modulating feeding in the rat by chronic hypothalamic RXFP3 activation and suggest a potential underlying mechanism involving altered OT signalling. Importantly, there was no desensitization of the feeding response over the treatment period and no apparent deleterious health effects, indicating that targeting the relaxin-3-RXFP3 system may be an effective long-term therapy for eating disorders.

Central control of thermogenesis

In mammals and birds, conservation of body heat at around 37 °C is vital to life. Thermogenesis is the production of this heat which can be obligatory, as in basal metabolic rate, or it can be facultative such as the response to cold. A complex regulatory system has evolved which senses environmental or core temperature and integrates this information in hypothalamic regions such as the preoptic area and dorsomedial hypothalamus. These areas then send the appropriate signals to generate and conserve heat (or dissipate it). In this review, the importance of the sympathetic nervous system is discussed in relation to its role in basal metabolic rate and adaptive thermogenesis with a particular emphasis to human obesity. The efferent sympathetic pathway does not uniformly act on all tissues; different tissues can receive different levels of sympathetic drive at the same time. This is an important concept in the discussion of the pharmacotherapy of obesity. Despite decades of work the medicine chest contains only one pill for the long term treatment of obesity, orlistat, a lipase inhibitor that prevents the absorption of lipid from the gut and is itself not systemically absorbed. The central controlling system for thermogenesis has many potential intervention points. Several drugs, previously marketed, awaiting approval or in the earlier stages of development may have a thermogenic effect via activation of the sympathetic nervous system at some point in the thermoregulatory circuit and are discussed in this review. If the balance is weighted to the "wrong" side there is the burden of increased cardiovascular risk while a shift to the "right" side, if possible, will afford a thermogenic benefit that is conducive to weight loss maintenance. This article is part of a Special Issue entitled 'Central Control Food Intake'

Interleukin-6 regulates the expression of hypothalamic neuropeptides involved in body weight in a gender-dependent way

Interleukin (IL)-6 has been involved in the control of body weight and body fat. Nevertheless, the mechanisms underlying these effects are not completely understood because central and peripheral actions of IL-6 are plausible. To gain further insight into the central effects of IL-6, we used transgenic mice expressing the IL-6 gene under the control of the glial fibrillary acidic protein (GFAP) promoter (GFAP-IL-6 mice), therefore with central nervous system-restricted over-expression of IL-6, and we studied the expression of the main neuropeptides responsible for energy homeostasis in specific hypothalamic nuclei. Neuropeptide Y (NPY), agouti-related peptide (AgRP), melanin-concentrating hormone (MCH), prepro-orexin (preproOX) (orexigenic and anabolic neuropeptides), pro-opiomelanocortin (POMC) and corticotrophin-releasing hormone (CRH) (anorexigenic and catabolic peptides) mRNA levels were determined using in situ hybridisation in young (2-4 month-old) and old (10-12 month-old) female and male mice under different feeding conditions: normal diet (control) and high-fat diet (HFD), and 24 h-food deprivation. In GFAP-IL-6 females fed a control diet (GFAP-IL-6-control), we showed a significant decrease in NPY and AgRP mRNA levels at all ages, and a late increase in POMC expression (only significant in older animals). These differences were blunted in HFD mice. By contrast, GFAP-IL-6-control males showed a decrease in CRH mRNA content at early ages (2-4 months), and an increase in older mice (10-12 months). Interestingly, these differences were again blunted in HFD mice. Finally, central IL-6 was not able to counteract the effects of 24 h of fasting on body weight, plasma glucose levels and the mRNA content of the peptides evaluated in the present study. Our results demonstrate that IL-6 may regulate the expression of hypothalamic neuropeptides involved in the control of body weight and body fat acting at the central level in a gender- and age-dependent way.

Fibroblast growth factor 21 is a metabolic regulator that plays a role in the adaptation to ketosis

Fibroblast growth factor 21 (FGF21) was originally identified as a member of the FGF family in homology studies and is a member of the endocrine FGF subfamily that lacks heparin binding domains and is released into the circulation. A potential role as a metabolic regulator emerged when FGF21 was shown to increase glucose uptake in adipocytes. Subsequently, marked elevations in FGF21 expression were observed in mice that ate a ketogenic diet and when fasting, which suggests that FGF21 expression plays a role in the adaptation to metabolic states that require increased fatty acid oxidation. Consistent with this evidence, FGF21 knockout mice were not able to respond appropriately to consumption of a ketogenic diet. FGF21 expression is downstream of peroxisome proliferator-activated receptor (PPAR) α in the liver and PPARγ in adipose tissue. FGF21 concentrations are higher in both rodent and human obesity, and recent data suggest that obesity may be an FGF21-resistant state. Recent data increasingly suggest that FGF21 is an important metabolic regulator that may have potential clinical implications.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

Deficiency of lipoprotein lipase in neurons modifies the regulation of energy balance and leads to obesity

Free fatty acids (FFAs) suppress appetite when injected into the hypothalamus. To examine whether lipoprotein lipase (LPL), a serine hydrolase that releases FFAs from circulating triglyceride (TG)-rich lipoproteins, might contribute to FFA-mediated signaling in the brain, we created neuron-specific LPL-deficient mice. Homozygous mutant (NEXLPL-/-) mice were hyperphagic and became obese by 16 weeks of age. These traits were accompanied by elevations in the hypothalamic orexigenic neuropeptides, AgRP and NPY, and were followed by reductions in metabolic rate. The uptake of TG-rich lipoprotein fatty acids was reduced in the hypothalamus of 3-month-old NEXLPL-/- mice. Moreover, deficiencies in essential fatty acids in the hypothalamus were evident by 3 months, with major deficiencies of long-chain n-3 fatty acids by 12 months. These results indicate that TG-rich lipoproteins are sensed in the brain by an LPL-dependent mechanism and provide lipid signals for the central regulation of body weight and energy balance.

Optimization of adeno-associated viral vector-mediated gene delivery to the hypothalamus

To efficiently deliver genes and short hairpin RNAs to the hypothalamus we aimed to optimize the transduction efficiency of adeno-associated virus (AAV) in the rat hypothalamus. We compared the transduction efficiencies of AAV2 vectors pseudotyped with AAV1, AAV8, and mosaic AAV1/2 and AAV2/8 coats with that of an AAV2 coated vector after injection into the lateral hypothalamus of rats. In addition, we determined the transduction areas and the percentage of neurons infected after injection of various titers and volumes of two AAV1-pseudotyped vectors in the paraventricular hypothalamus (PVN). Successful gene delivery to the hypothalamus was achieved with AAV1-pseudotyped AAV vectors. The optimal approach to transduce an area, with the size of the PVN, was to inject 1 x 10(9) genomic copies of an AAV1-pseudotyped vector in a volume of 1 microl. At a radius of 0.05 mm from the injection site almost all neurons were transduced. In addition, overexpression of AgRP with the optimal approach resulted in an increase in food intake and body weight when compared with AAV-GFP.

Neuropeptide delivery to the brain: a von Willebrand factor signal peptide to direct neuropeptide secretion

BACKGROUND

Multiple neuropeptides, sometimes with opposing functions, can be produced from one precursor gene. To study the roles of the different neuropeptides encoded by one large precursor we developed a method to overexpress minigenes and establish local secretion.

RESULTS

We fused the signal peptide from the Von Willebrand Factor (VWF) to a furin site followed by a processed form of the Agouti related protein (AgRP), AgRP(83-132) or alpha-melanocyte stimulating hormone. In vitro, these minigenes were secreted and biologically active. Additionally, the proteins of the minigenes were not transported into projections of primary neurons, thereby ensuring local release. In vivo administration of VWF-AgRP(83-132), using an adeno-associated viral vector as a delivery vehicle, into the paraventricular hypothalamus increased body weight and food intake of these rats compared to rats which received a control vector.

CONCLUSIONS

This study demonstrated that removal of the N-terminal part of full length AgRP and addition of a VWF signal peptide is a successful strategy to deliver neuropeptide minigenes to the brain and establish local neuropeptide secretion.

Short-days induce weight loss in Siberian hamsters despite overexpression of the agouti-related peptide gene

Many vertebrates express profound annual cycles of body fattening, although it is not clear whether these represent differential activity of the central pathways known to mediate homeostatic control of food intake and energy expenditure, or whether the recent discovery of a major role for pars tuberalis-ependymal signalling points towards novel mechanisms. We examined this in the Siberian hamster (Phodopus sungorus) by using gene transfection to up-regulate a major orexigenic peptide, agouti-related peptide (AgRP), and then determined whether this increased anabolic drive could prevent the short-day induced winter catabolic state. Infusions of a recombinant adeno-associated virus encoding an AgRP construct into the hypothalamus of hamsters in the long-day obese phase of their seasonal cycle produced a 20% gain in body weight over 6 weeks compared to hamsters receiving a control reporter construct, reflecting a significant increase in food intake and a significant decrease in energy expenditure. However, all hamsters showed a significant, prolonged decrease in body weight when exposed to short photoperiods, despite the hamsters expressing the AgRP construct maintaining a higher food intake and lower energy expenditure relative to the control hamsters. Visualisation of the green fluorescent protein reporter and analysis of AgRP-immunoreactivity confirmed widespread expression of the construct in the hypothalamus, which was maintained for the 21-week duration of the study. In conclusion, the over-expression of AgRP in the hypothalamus produced a profoundly obese state but did not block the seasonal catabolic response, suggesting a separation of rheostatic mechanisms in seasonality from those maintaining homeostasis of energy metabolism.

An intrinsic gut leptin-melanocortin pathway modulates intestinal microsomal triglyceride transfer protein and lipid absorption

Fat is delivered to tissues by apoB-containing lipoproteins synthesized in the liver and intestine with the help of an intracellular chaperone, microsomal triglyceride transfer protein (MTP). Leptin, a hormone secreted by adipose tissue, acts in the brain and on peripheral tissues to regulate fat storage and metabolism. Our aim was to identify the role of leptin signaling in MTP regulation and lipid absorption using several mouse models deficient in leptin receptor (LEPR) signaling and downstream effectors. Mice with spontaneous LEPR B mutations or targeted ablation of LEPR B in proopiomelanocortin (POMC) or agouti gene related peptide (AGRP) expressing cells had increased triglyceride in plasma, liver, and intestine. Furthermore, melanocortin 4 receptor (MC4R) knockout mice expressed a similar triglyceride phenotype, suggesting that leptin might regulate intestinal MTP expression through the melanocortin pathway. Mechanistic studies revealed that the accumulation of triglyceride in the intestine might be secondary to decreased expression of MTP and lipid absorption in these mice. Surgical and chemical blockade of vagal efferent outflow to the intestine in wild-type mice failed to alter the triglyceride phenotype, demonstrating that central neural control mechanisms were likely not involved in the observed regulation of intestinal MTP. Instead, we found that enterocytes express LEPR, POMC, AGRP, and MC4R. We propose that a peripheral, local gut signaling mechanism involving LEPR B and MC4R regulates intestinal MTP and controls intestinal lipid absorption.

Tail suspension increases energy expenditure independently of the melanocortin system in miceThis article is one of a selection of papers published in a special issue celebrating the 125th anniversary of the Faculty of Medicine at the University of Manitoba

Space travelers experience anorexia and body weight loss in a microgravity environment, and microgravity-like situations cause changes in hypothalamic activity. Hypothalamic melanocortins play a critical role in the regulation of metabolism. Therefore, we hypothesized that microgravity affects metabolism through alterations in specific hypothalamic signaling pathways, including melanocortin signaling. To address this hypothesis, the microgravity-like situation was produced by an antiorthostatic tail suspension in wild-type and agouti mice, and the effect of tail suspension on energy expenditure and hypothalamic gene expression was examined. Energy expenditure was measured using indirect calorimetry before and during the tail suspension protocol. Hypothalamic tissues were collected for gene expression analysis at the end of the 3 h tail suspension period. Tail suspension significantly increased oxygen consumption, carbon dioxide production, and heat production in wild-type mice. Tail suspension-induced increases in energy expenditure were not attenuated in agouti mice. Although tail suspension did not alter hypothalamic proopiomelanocortin (POMC) and agouti-related protein (AGRP) mRNA levels, it significantly increased hypothalamic interleukin 6 (Il-6) mRNA levels. These data are consistent with the hypothesis that microgravity increases energy expenditure and suggest that these effects are mediated through hypothalamic signaling pathways that are independent of melanocortins, but possibly used by Il-6.

Central mechanisms controlling appetite and food intake in a cancer setting: an update

PURPOSE OF REVIEW

Cachexia, also known as disease-associated wasting, is an important factor in the mortality of many patients with diseases such as cancer, as well as renal and congestive heart failure. Yet the syndrome is not yet well defined, making diagnosis difficult and often subjective on the part of the physician. Nor are the central mechanisms of cachexia fully elucidated. Recent studies have begun to address these gaps by focusing on three areas: the role of cytokines in cachexia, other proteins and peptides that might be involved, and potential treatments for this devastating syndrome.

RECENT FINDINGS

Cachexia can be caused, in the absence of disease, by inflammatory stimuli and some chemotherapy drugs, suggesting possible central mechanisms in cachexia. Promising treatments include melanocortin antagonism and some hormones.

SUMMARY

While more research is necessary to illuminate causal mechanisms and uncover potential therapies of cachexia, several of its major molecular pathways have become elucidated, suggesting directions for therapeutic approaches.

The role of the Agouti-Related Protein in energy balance regulation

The Agouti-Related Protein (AgRP) is a powerful orexigenic peptide that increases food intake when ubiquitously overexpressed or when administered centrally. AgRP-deficiency, on the other hand, leads to increased metabolic rate and a longer lifespan when mice consume a high fat diet. In humans, AgRP polymorphisms have been consistently associated with resistance to fatness in Blacks and Whites and resistance to the development of type-2 diabetes in African Blacks. Systemically administered AgRP accumulates in the liver, the adrenal gland and fat tissue while recent findings suggest that AgRP may also have inverse agonist effects, both centrally and peripherally. AgRP could thus modulate energy balance via different actions. Its absence or reduced functionality may offer a benefit both in terms of bringing about negative energy balance in obesigenic environments, as well as leading to an increased lifespan.

Food deprivation-induced changes in body fat mobilization after neonatal monosodium glutamate treatment

The reversal of obesity is a difficult feat at best and is a growing problem as the obesity epidemic increases worldwide. Considerable focus has been made on the arcuate nucleus (Arc) in the control of body and lipid mass and food intake. To test the role of the Arc in body fat mobilization, we compared the effects of food deprivation on white adipose tissue (WAT) mass in adult Siberian hamsters by making exocytotic lesions of the Arc via neonatal subcutaneous injections of monosodium glutamate (MSG). MSG-treated hamsters had significantly increased body mass, total and individual WAT pad masses, and serum leptin concentrations compared with their vehicle-injected counterparts. MSG produced marked reductions in Arc Nissl staining, tyrosine hydroxylase-immunoreactive (ir) neurons, and neuropeptide Y (NPY)- and agouti-related protein (AgRP)-ir fibers compared with controls. MSG significantly decreased hypothalamic paraventricular nucleus (PVN) NPY- and AgRP fiber-ir compared with controls, likely because of Arc projections to this nucleus. MSG treatment also reduced area postrema (AP) tyrosine hydroxylase (TH)-ir fibers compared with controls. MSG treatment did not, however, block food deprivation-induced decreases in WAT pad mass compared with controls. Thus, despite considerable damage to the Arc and some of its projections to the PVN, as well as the AP, body fat was mobilized apparently normally, bringing into question the necessity of these structures for food deprivation-induced lipid mobilization. These data support recent evidence that chronically decerebrate rats, in which the forebrain is surgically isolated from the caudal brainstem, show normal food deprivation responses, including lipid mobilization.

In vivo evidence for inverse agonism of Agouti-related peptide in the central nervous system of proopiomelanocortin-deficient mice

OBJECTIVE

Melanocyte-stimulating hormone (MSH) peptides processed from proopiomelanocortin (POMC) regulate energy homeostasis by activating neuronal melanocortin receptor (MC-R) signaling. Agouti-related peptide (AgRP) is a naturally occurring MC-R antagonist but also displays inverse agonism at constitutively active melanocortin-4 receptor (MC4-R) expressed on transfected cells. We investigated whether AgRP functions similarly in vivo using mouse models that lack all neuronal MSH, thereby precluding competitive antagonism of MC-R by AgRP.

RESEARCH DESIGN AND METHODS

Feeding and metabolic effects of the MC-R agonist melanotan II (MTII), AgRP, and ghrelin were investigated after intracerebroventricular injection in neural-specific POMC-deficient (Pomc(-/-)Tg/+) and global POMC-deficient (Pomc(-/-)) mice. Gene expression was quantified by RT-PCR.

RESULTS

Hyperphagic POMC-deficient mice were more sensitive than wild-type mice to the anorectic effects of MTII. Hypothalamic melanocortin-3 (MC3)/4-R mRNAs in POMC-deficient mice were unchanged, suggesting increased receptor sensitivity as a possible mechanism for the heightened anorexia. AgRP reversed MTII-induced anorexia in both mutant strains, demonstrating its ability to antagonize MSH agonists at central MC3/4-R, but did not produce an acute orexigenic response by itself. The action of ghrelin was attenuated in Pomc(-/-)Tg/+ mice, suggesting decreased sensitivity to additional orexigenic signals. However, AgRP induced delayed and long-lasting modifications of energy balance in Pomc(-/-)Tg/+, but not glucocorticoid-deficient Pomc(-/-) mice, by decreasing oxygen consumption, increasing the respiratory exchange ratio, and increasing food intake.

CONCLUSIONS

These data demonstrate that AgRP can modulate energy balance via a mechanism independent of MSH and MC3/4-R competitive antagonism, consistent with either inverse agonist activity at MC-R or interaction with a distinct receptor.

Role of mTOR in physiology and pathology of the nervous system

Mammalian target of rapamycin (mTOR) is a serine-threonine protein kinase that regulates several intracellular processes in response to extracellular signals, nutrient availability, energy status of the cell and stress. mTOR regulates survival, differentiation and development of neurons. Axon growth and navigation, dendritic arborization, as well as synaptogenesis, depend on proper mTOR activity. In adult brain mTOR is crucial for synaptic plasticity, learning and memory formation, and brain control of food uptake. Recent studies reveal that mTOR activity is modified in various pathologic states of the nervous system, including brain tumors, tuberous sclerosis, cortical displasia and neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases. This review presents current knowledge about the role of mTOR in the physiology and pathology of the nervous system, with special focus on molecular targets acting downstream of mTOR that potentially contribute to neuronal development, plasticity and neuropathology.

Central blockade of melanocortin receptors attenuates the metabolic and locomotor responses to peripheral interleukin-1beta administration

Loss of appetite and cachexia is an obstacle in the treatment of chronic infection and cancer. Proinflammatory cytokines released from activated immune cells and acting in the central nervous system (CNS) are prime candidates for mediating these metabolic changes, potentially affecting both energy intake as well as energy expenditure. The effect of intravenous administration of two proinflammatory cytokines, interleukin (IL)-1beta (15 microg/kg) and tumor necrosis factor (TNF)-alpha (10 microg/kg), on food and water intake, locomotor activity, oxygen consumption (VO2), and respiratory exchange ratio (RER) was evaluated. The two cytokines elicited a comparable decrease in food intake and activated similar numbers of cells in the paraventricular nucleus of the hypothalamus (PVH), a region that plays a critical role in the regulation of appetite and metabolism (determined via expression of the immediate early gene, c-fos). However, only IL-1beta reduced locomotion and RER, and increased VO2, while TNF-alpha was without effect. To examine the role of the melanocortins in mediating IL-1beta- induced metabolic changes, animals were pretreated centrally with a melanocortin receptor antagonist, HS014. Pretreatment with HS014 blocked the effect of IL-1beta on food intake and RER at later time points (beyond 8 h post injection), as well as the hypoactivity and increased metabolic rate. Further, HS014 blocked the induction of Fos-ir in the PVH. These data highlight the importance of the melanocortin system, particularly within the PVH, in mediating a broad range of metabolic responses to IL-1beta.