Mice lacking pro-opiomelanocortin are sensitive to high-fat feeding but respond normally to the acute anorectic effects of peptide-YY(3-36)

Prolylcarboxypeptidase (PRCP) as a new target for obesity treatment

Recently, we serendipitously discovered that mice with the deficiency of the enzyme prolylcarboxypeptidase (PRCP) have elevated alpha-melanocyte-stimulating hormone (alpha-MSH) levels which lead to decreased food intake and weight loss. This suggests that PRCP is an endogenous inactivator of alpha-MSH and an appetite stimulant. Since a modest weight loss can have the most profound influence on reducing cardiovascular risk factors, the inhibitors of PRCP would be emerging as a possible alternative for pharmacotherapy in high-risk patients with obesity and obesity-related disorders. The discovery of a new biological activity of PRCP in the PRCP-deficient mice and studies of alpha-MSH function indicate the importance and complexity of the hypothalamic pro-opiomelanocortin (POMC) system in altering food intake. Identifying a role for PRCP in regulating alpha-MSH in the brain may be a critical step in enhancing our understanding of how the brain controls food intake and body weight. In light of recent findings, the potential role of PRCP in regulating fuel homeostasis is critically evaluated. Further studies of the role of PRCP in obesity are much needed.

Gene Expression and the Control of Food Intake by Hypothalamic POMC/CART Neurons

Neurons that express pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) in the arcuate nucleus of the hypothalamus suppress feeding and increase energy expenditure in response to circulating adiposity signals such as leptin. Alterations in gene expression may lead to long term modification of this circuit and alterations in body weight. Therefore, understanding how gene expression in these neurons is controlled is crucial to forming a complete picture of the central management of energy balance. This review outlines the heterogeneity of arcuate POMC/CART neurons, describes our current understanding of CART and POMC gene transcription in these neurons, and suggests future directions for extending the field.

Electrophysiological effects of MCH on neurons in the hypothalamus

Melanin concentrating hormone (MCH) has been implicated in many brain functions and behaviors essential to the survival of animals. The hypothalamus is one of the primary targets where MCH-containing nerve fibers and MCH receptors are extensively expressed and its actions in the brain are exerted. Since the identification of MCH receptors as orphan G protein coupled receptors, the cellular effects of MCH have been revealed in many non-neuronal expression systems (including Xenopus oocytes and cell lines), however, the mechanism by which MCH modulates the activity in the neuronal circuitry of the brain is still under investigation. This review summarizes our current knowledge of electrophysiological effects of MCH on neurons in the hypothalamus, particularly in the lateral hypothalamus. Generally, MCH exerts inhibitory effects on neurons in this structure and may serve as a homeostatic regulator in the lateral hypothalamic area. Given the contrast between the limited data on cellular functions of MCH in the hypothalamus versus a fast growing body of evidence on the vital role of MCH in animal behavior, further investigations of the former are warranted.

MCH and feeding behavior-interaction with peptidic network

Numerous works associate the MCH peptide, and the hypothalamic neurons that produce it, to the feeding behavior and energy homeostasis. It is commonly admitted that MCH is an orexigenic peptide, and MCH neurons could be under the control of arcuate NPY and POMC neurons. However, the literature data is not always concordant. In particular questions about the intrahypothalamic circuit involving other neuropeptides and about the mechanisms through which MCH could act are not yet clearly answered. For example, which receptors mediate a MCH response to NPY or alpha-MSH, does MCH act alone, is there any local anatomical organization within the tuberal LHA? A review of the current literature is then needed to help focus attention on these unresolved and often neglected issues.

Feeding signals and brain circuitry

Food intake is a major physiological function in animals and must be entrained to the circadian oscillations in food availability. In the last two decades a growing number of reports have shed light on the hormonal, cellular and molecular mechanisms involved in the regulation of food intake. Brain areas located in the hypothalamus have been shown to play a pivotal role in the regulation of energy metabolism, controlling energy balance. In these areas, neuronal plasticity has been reported that is dependent upon key hormones, such as leptin and ghrelin, that are produced by peripheral organs. This review will provide an overview of recent discoveries relevant to understanding these issues.

Role of central melanocortin pathways in energy homeostasis

The rise in the global prevalence of human obesity has emphasized the need for a greater understanding of the physiological mechanisms that underlie energy homeostasis. Numerous circulating nutritional cues and central neuromodulatory signals are integrated within the brain to regulate both short- and long-term nutritional state. The central melanocortin system represents a crucial point of convergence for these signals and, thus, has a fundamental role in regulating body weight. The melanocortin ligands, synthesized in discrete neuronal populations within the hypothalamus and brainstem, modulate downstream homeostatic signalling via their action at central melanocortin-3 and -4 receptors. Intimately involved in both ingestive behaviour and energy expenditure, the melanocortin system has garnered much interest as a potential therapeutic target for human obesity.

Pro-opiomelanocortin (POMC)-derived peptides and the regulation of energy homeostasis

Human genetic data indicate impaired synthesis or processing of POMC results in obesity. We have used a mouse model of POMC deficiency (Pomc null) to explore the role of POMC-derived peptides in energy homeostasis. The phenotype of Pomc null mice recapitulates the clinical syndrome seen in humans congenitally lacking POMC. Loss of only one copy of the Pomc gene is sufficient to render mice susceptible to the effects of high fat feeding, emphasizing an important gene-environment interaction predisposing to obesity. Our studies indicate that POMC-derived peptides have influences on the response to a high fat diet, including a major influence on the dietary preference for fat. Pomc null mice are unusual in that obesity and hyperphagia develop in the absence of circulating glucocorticoid (GC). To investigate the interaction between GCs and the melanocortin system, we administered corticosterone to Pomc null mice. They appear hypersensitive to the adverse metabolic effects of GCs, developing hypertension, an exacerbation of both hyperphagia and obesity and a profound insulin resistance. GC treatment of Pomc null mice significantly increases the expression of the melanocortin antagonist agouti-related protein (AgRP). On-going studies in mice lacking both AgRP and Pomc will determine whether the metabolic phenotype seen with this GC therapy is due to a lack of melanocortin peptide, the unopposed action of AgRP or a combination of both.

The Role of Leptin-Melanocortin System and Human Weight Regulation: Lessons from Experiments of Nature

Introduction: Common obesity is a multi-factorial trait, contributed by the “obesogenic” environment of caloric abundance and increasing automation, sedentary lifestyle and an underlying genetic susceptibility. There have been major advances in the past decade in our understanding of the human weight regulation mechanism and pathogenesis of obesity, abetted by discoveries of genetic defects which lead to human obesity. Materials and Methods: Reports of genetic mutations causing obesity in humans and murine models were reviewed Results: Humans with genetic defects resulting in leptin deficiency, leptin receptor deficiency, pro-opiomelanocortin deficiency (POMC), and melanocortin 4 receptor (MC4R) deficiency developed severe obesity as the dominant phenotypic feature, though these are rare autosomal recessive conditions, except MC4R deficiency which is inherited in an autosomal co-dominant fashion. Common and rare variants of the POMC and melanocortin 3 receptor genes may be pre-disposing factors in the development of common obesity. Recent reports of human obesity associated with thyrosine kinase B (TrkB) defect and brain derived neurotrophic factor (BDNF) disruption, coupled with other murine studies, supported the role of BDNF/TrkB as effectors downstream of the melanocortin receptors. Conclusions: Despite exciting discoveries of single gene mutations resulting in human obesity, most cases of obesity are likely the result of subtle interactions of several related genetic variants with environmental factors which favour the net deposition of calories as fat, culminating in the obese phenotype. The mechanisms of action of these genes in the development of obesity are now being examined, with the aim of eventually discovering a therapeutic intervention for obesity. Key words: Leptin, Melanocortin, Obesity

Dietary resistant starch increases hypothalamic POMC expression in rats

Resistant starch (RS) is fermentable dietary fiber. Inclusion of RS in the diet causes decreased body fat accumulation and altered gut hormone profile. This study investigates the effect of feeding RS on the neuropeptide messenger RNA (mRNA) expressions in the arcuate nucleus (ARC) of the hypothalamus and whether vagal afferent nerves are involved. The rats were injected intraperitoneally with capsaicin to destroy unmyelinated small vagal afferent nerve fibers. The cholecystokinin (CCK) food suppression test was performed to validate the effectiveness of the capsaicin treatment. Then, capsaicin-treated rats and vehicle-treated rats were subdivided into a control diet or a RS diet group, and fed the corresponding diet for 65 days. At the end of study, body fat, food intake, plasma peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), and hypothalamic pro-opiomelanocortin (POMC), neuropeptide Y (NPY), agouti-related peptide (AgRP) gene expressions were measured. RS-fed rats had decreased body fat, increased POMC expression in the hypothalamic ARC, and elevated plasma PYY and GLP-1 in both the capsaicin and vehicle-treated rats. Hypothalamic NPY and AgRP gene expressions were not changed by RS or capsaicin. Therefore, destruction of the capsaicin-sensitive afferent nerves did not alter the response to RS in rats. These findings suggest that dietary RS might reduce body fat through increasing the hypothalamic POMC expression and vagal afferent nerves are not involved in this process. This is the first study to show that dietary RS can alter hypothalamic POMC expression.

Contribution of the transmembrane domain 6 of melanocortin-4 receptor to peptide [Pro5, DNal (2')8]-gamma-MSH selectivity

The melanocortin receptor (MCR) subtype family is a member of the GPCR superfamily and each of them has a different pharmacological profile regarding the relative potency of the endogenous and synthetic melanocortin peptides. Substitution of Trp with DNal (2') in gamma-MSH resulted in the loss of binding affinity and potency at hMC4R. However, the molecular mechanism of this ligand selectivity is unclear. In this study, we utilized chimeric receptors and site-directed mutagenesis approaches to investigate the molecular basis of MC4R responsible for peptide [Pro5, DNal (2')8]-gamma-MSH selectivity. Cassette substitutions of the second, third, fourth, fifth, and sixth TM of the human MC4R (hMC4R) with the homologous regions of hMC1R were constructed and the binding affinity of peptide [Pro5, DNal (2')8]-gamma-MSH at these chimeric receptors was evaluated. Our results indicate that the cassette substitutions of TM2, TM3, TM4 and TM5 of hMC4R with homologous regions of the hMC1R did not significantly increase peptide [Pro5, DNal (2')8]-gamma-MSH binding affinity and potency but substitution of the TM6 of the hMC4R with the same region of the hMC1R significantly enhances [Pro5, DNal (2')8]-gamma-MSH binding affinity and potency. Further site-directed mutagenesis study indicates that four amino acid residues, Phe267, Tyr268, Ile269 and Ser270, in TM6 of the hMC4R may play an important role in [Pro5, DNal (2')-gamma-MSH selective activity at MC4R.

Hypothalamic regulation of appetite

The prevalence of obesity is steadily rising and has huge health and financial implications for society. Weight gain is due to an imbalance between dietary intake and energy expenditure and research has focused on trying to understand the complex pathways involved in controlling these aspects. This review highlights the key areas of research in the hypothalamic control of appetite. The hypothalamus consists of several nuclei that integrate peripheral signals, such as adiposity and caloric intake, to regulate important pathways within the CNS controlling food intake. The best characterized pathways are the orexigenic neuropeptide Y/Agouti-related protein and the anorexigenic pro-opiomelanocortin/cocaine- and amphetamine-related transcript neurons in the arcuate nucleus of the hypothalamus. These project from the arcuate nucleus to other key hypothalamic nuclei, such as the paraventricular, dorsomedial, ventromedial and lateral hypothalamic nuclei. There are also projections to and from the brainstem, cortical areas and reward pathways, all of which influence food intake. The challenge at present is to understand the complexity of these pathways and try to find ways of modulating them in order to find potential therapeutic targets.

Unraveling the obesity of OLETF rats

Cholecystokinin (CCK) is a brain gut peptide that plays an important role in satiety. CCK inhibits food intake by reducing meal size. CCK's satiety actions are mediating through its interaction with CCK1 receptors. Otsuka Long Evans Tokushima Fatty (OLETF) rats are a CCK1 receptor knockout model that allows the study of multiple CCK functions. OLETF rats are hyperphagic with the hyperphagia expressed as a significant increase in the size of meals. OLETF rat obesity is secondary to the hyperphagia and has been proposed to derive from two regulatory deficits. One is secondary to the loss of a feedback satiety signal. The other results from increased dorsomedial hypothalamic NPY expression. Recent studies have examined developmental aspects of altered feeding, body weight and orexigenic signaling in OLETF rats. OLETF rats demonstrate increases in meal size in independent ingestion tests as early as two days of age. OLETF pups are also more efficient in suckling situations. Consistent with such developmental differences, examinations of patterns of hypothalamic gene expression in OLETF pups indicate significant increases in DMH NPY expression as early as postnatal day 15. Access to a running wheel and the resulting exercise have age dependent effects on OLETF food intake and obesity. With running wheel access shortly after weaning, food intake decreases to the levels of LETO controls. When running wheel access is discontinued, food intake temporarily increases resulting in an intermediate phenotype and the absence of diabetes. Together these data demonstrate roles for peripheral CCK and CCK in feeding and body weight control and support the use of the OLETF rat as a model for examining obesity development and for investigating how interventions at critical developmental time points can alter genetic influences on food intake and body weight.

Leptin extends the anorectic effects of chronic PYY(3-36) administration in ad libitum-fed rats

Acute administration of peptide YY(3-36) [PYY(3-36)] results in a reduction in food intake in several different vertebrates. However, long-term continuous administration of PYY(3-36) causes only a transient reduction in food intake, thus potentially limiting its therapeutic efficacy. We hypothesized that a fall in leptin levels associated with reduced food intake could contribute to the transient anorectic effects of continuous PYY(3-36) infusion and thus that leptin replacement might prolong the anorectic effects of PYY(3-36). Seven-day administration of 100 microg x kg body wt(-1) x day(-1) PYY(3-36) using osmotic minipumps caused a significant reduction in food intake of ad libitum-fed rats, but only for the first 2 days postimplantation. Circulating levels of leptin were reduced 1 day following continuous infusion of PYY(3-36), and combined leptin infusion at a dose of leptin that had no anorectic effects on its own (100 microg x kg body wt(-1) x day(-1)) prolonged the anorectic actions of PYY(3-36) in ad libitum-fed rats for up to 6 days postimplantation and yielded reduced weight gain compared with either peptide alone. The inhibitory effects of 100 microg x kg body wt(-1) x day(-1) PYY(3-36) on food intake were absent in rats refed after a 24-h fast and substantially reduced at a dose of 1,000 microg x kg body wt(-1) x day(-1) PYY(3-36). Leptin replacement was unable to recover the anorectic effects of PYY(3-36) in fasted rats. Our results suggest that an acute fall in leptin levels is not solely responsible for limiting duration of action of chronic PYY(3-36) infusion, yet chronic coadministration of a subanorectic dose of leptin can extend the anorectic effects of PYY(3-36).

PYY3-36 injection in mice produces an acute anorexigenic effect followed by a delayed orexigenic effect not observed with other anorexigenic gut hormones

Peptide YY (PYY) is secreted postprandially from the endocrine L cells of the gastrointestinal tract. PYY(3-36), the major circulating form of the peptide, is thought to reduce food intake in humans and rodents via high-affinity binding to the autoinhibitory neuropeptide Y (NPY) receptor within the arcuate nucleus. We studied the effect of early light-phase injection of PYY(3-36) on food intake in mice fasted for 0, 6, 12, 18, 24, and 30 h and show that PYY(3-36) produces an acute anorexigenic effect regardless of the duration of fasting. We also show evidence of a delayed orexigenic effect in ad libitum-fed mice injected with PYY(3-36) in the early light phase. This delayed orexigenic effect also occurs in mice administered a potent analog of PYY(3-36), d-Allo Ile(3) PYY(3-36), but not following injection of other anorectic agents (glucagon-like-peptide 1, oxyntomodulin, and lithium chloride). Early light-phase injection of PYY(3-36) to ad libitum-fed mice resulted in a trend toward increased levels of hypothalamic NPY and agouti-related peptide mRNA and a decrease in proopiomelanocortin mRNA at the beginning of the dark phase. Furthermore, plasma levels of ghrelin were increased significantly, and there was a trend toward decreased plasma PYY(3-36) levels at the beginning of the dark phase. These data indicate that PYY(3-36) injection results in an acute anorexigenic effect followed by a delayed orexigenic effect.

Serotonin 5-HT2C receptor agonist promotes hypophagia via downstream activation of melanocortin 4 receptors

The neurotransmitter serotonin (5-hydroxytryptamine) is a well-established modulator of energy balance. Both pharmacological and genetic evidence implicate the serotonin 2C receptor (5-HT(2C)R) as a critical receptor mediator of serotonin's effects on ingestive behavior. Here we characterized the effect of the novel and selective 5-HT(2C)R agonist BVT.X on energy balance in obese and lean mice and report that BVT.X significantly reduces acute food intake without altering locomotor activity or oxygen consumption. In an effort to elucidate the mechanism of this effect, we examined the chemical phenotype of 5-HT(2C)R-expressing neurons in a critical brain region affecting feeding behavior, the arcuate nucleus of the hypothalamus. We show that 5-HT(2C)Rs are coexpressed with neurons containing proopiomelanocortin, known to potently affect appetite, in the arcuate nucleus of the hypothalamus of the mouse. We then demonstrate that prolonged infusion with BVT.X in obese mice significantly increases Pomc mRNA and reduces body weight, percent body fat, and initial food intake. To evaluate the functional importance of melanocortin circuitry in the effect of BVT.X on ingestive behavior, we assessed mice with disrupted melanocortin pathways. We report that mice lacking the melanocortin 4 receptor are not responsive to BVT.X-induced hypophagia, demonstrating that melanocortins acting on melanocortin 4 receptor are a requisite downstream pathway for 5-HT(2C)R agonists to exert effects on food intake. The data presented here not only indicate that the novel 5-HT(2C)R agonist BVT.X warrants further investigation as a treatment for obesity but also elucidate specific neuronal pathways potently affecting energy balance through which 5-HT(2C)R agonists regulate ingestive behavior.

The role of the thyrotropin-releasing hormone (TRH) neuron as a metabolic sensor

Thyroid hormone (TH) plays a critical role in mediating changes in development and metabolism in humans. Thus, circulating TH levels are regulated by a number of distinct mechanisms to allow them to remain at physiologic levels. The central regulation of the thyroid axis by thyrotropin-releasing hormone (TRH) neurons in the paraventricular nucleus of the hypothalamus (PVH) is absolutely required for normal function of the axis. Remarkably, the TRH neurons in the PVH are regulated by multiple pathways that allow for the set point of TRH production to be determined. The following review will focus on how the TRH neuron is regulated by TH as well as key pathways that regulate energy expenditure. By integrating these inputs, the TRH neuron is able to set the thyroid axis at the appropriate level given the physiologic demands present.

Peptide YY

PURPOSE OF REVIEW

This review discusses recent studies examining the effects of peptide YY on energy homeostasis, highlights the emerging hedonic effects of peptide YY and evaluates the therapeutic potential of the peptide YY system.

RECENT FINDINGS

A role for exogenous PYY3-36 as an anorectic agent in obese humans and rodents has been established and weight loss effects demonstrated in obese rodents. New lines of evidence support a role for endogenous peptide YY in regulating energy homeostasis. The NPY-Y2 receptor mediates the anorectic actions of PYY3-36 with rodent studies implicating the hypothalamus, vagus and brainstem as key target sites. Functional imaging in humans has confirmed that PYY3-36 activates brainstem and hypothalamic regions. The greatest effects, however, were observed within the orbitofrontal cortex, a brain region involved in reward processing. Further evidence for a hedonic role for PYY3-36 is supported by rodent studies showing that PYY3-36 decreases the motivation to seek high-fat food. Rodent studies using selective Y2 agonists and strategies combining PYY3-36/Y2 agonists with other anorectic agents have revealed increased anorectic and weight-reducing effects.

SUMMARY

Peptide YY plays a role in the integrative regulation of metabolism. The emerging hedonic effects of peptide YY together with the weight-reducing effects observed in obese rodents suggest that targeting the peptide YY system may offer a therapeutic strategy for obesity.

Can gut hormones control appetite and prevent obesity?

The current obesity epidemic is fuelled by the availability of highly palatable, calorie-dense food, and the low requirement for physical activity in our modern environment. If energy intake exceeds energy use, the excess calories are stored as body fat. Although the body has mechanisms that act to maintain body weight over time, they primarily defend against starvation and are less robust in preventing the development of obesity. Knowledge of this homeostatic system that controls body weight has increased exponentially over the last decade and has revealed new possibilities for the treatment of obesity and its associated comorbidities. One therapeutic target is the development of agents based on the gastrointestinal hormones that control appetite. This review discusses the hormones oxyntomodulin, peptide YY, glucagon-like peptide 1, pancreatic polypeptide, and ghrelin and their emerging potential as anti-obesity treatments.

Counterintuitive effects of double-heterozygous null melanocortin-4 receptor and leptin genes on diet-induced obesity and insulin resistance in C57BL/6J mice

Circulating levels of leptin correlate with food intake and adiposity. A decline in serum leptin associated with calorie restriction instigates behavioral and metabolic adaptation, increasing appetite and conserving energy. Brain melanocortin-4 receptors (Mc4rs) are important mediators of leptin's effects on appetite and energy expenditure. Because subtle changes in function associated with heterozygous null mutations for either the Leptin (Lep-HET) or Mc4r genes (Mc4r-HET) increase adiposity, we tested the hypothesis that combined heterozygous mutations (Dbl-HET) would severely exacerbate diet-induced obesity (DIO) and insulin resistance in C57BL/6J mice. Serum leptin levels were lower as a function of adiposity in heterozygous Leptin mutants (Lep-HET, Dbl-HET) matched with mice homozygous for the wild-type (WT) Lep gene (Mc4r-HET). Evidence for an additive interaction on adiposity in Dbl-HET mice maintained on a low-fat diet was observed at 10 wk of age. Male but not female mice developed DIO and insulin resistance on a high-fat diet. Compared with WT mice, DIO was more severe in Mc4r-HET but not Lep-HET mice, regardless of sex. However, the response of male and female Dbl-HET mice was different, with males being less and females being more responsive relative to Mc4r-HET. Glucose tolerance of Dbl-HET mice was not significantly different from WT mice in either sex. These results show a complex interaction between the Leptin and Mc4r genes that is influenced by age, gender, and diet. Remarkably, while heterozygous Lep mutations initially exacerbate obesity, in situations of severe obesity, reduced leptin levels may act oppositely and have beneficial effects on energy homeostasis.

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.

The serine protease Corin is a novel modifier of the Agouti pathway

The hair follicle is a model system for studying epithelial-mesenchymal interactions during organogenesis. Although analysis of the epithelial contribution to these interactions has progressed rapidly, the lack of tools to manipulate gene expression in the mesenchymal component, the dermal papilla, has hampered progress towards understanding the contribution of these cells. In this work, Corin was identified in a screen to detect genes specifically expressed in the dermal papilla. It is expressed in the dermal papilla of all pelage hair follicle types from the earliest stages of their formation, but is not expressed elsewhere in the skin. Mutation of the Corin gene reveals that it is not required for morphogenesis of the hair follicle. However, analysis of the ;dirty blonde' phenotype of these mice reveals that the transmembrane protease encoded by Corin plays a critical role in specifying coat color and acts downstream of agouti gene expression as a suppressor of the agouti pathway.

Gastrointestinal satiety signals

The increasing prevalence of obesity worldwide has imparted renewed impetus to the study of the mechanisms of appetite regulation. Digestion and nutrient absorption take place in the gastrointestinal (GI) tract, whereas food intake is controlled by neuronal circuits in the central nervous system. The need for gut-brain cross talk is therefore clear. It is now recognized that hormones released into the circulation from the GI tract in response to nutritional stimuli form a key component of this gut-brain axis. Peptides such as glucagon-like peptide-1, oxyntomodulin, pancreatic polypeptide, and peptide YY3-36 reduce food intake in both animal models and in humans. Physiologically, such peptides are thought to act as satiety signals and meal terminators. Here, we review the current state of the field of the effects of gut hormone action on appetite control.

Combination therapy with amylin and peptide YY[3-36] in obese rodents: anorexigenic synergy and weight loss additivity

Circulating levels of the pancreatic beta-cell peptide hormone amylin and the gut peptide PYY[3-36] increase after nutrient ingestion. Both have been implicated as short-term signals of meal termination with anorexigenic and weight-reducing effects. However, their combined effects are unknown. We report that the combination of amylin and PYY[3-36] elicited greater anorexigenic and weight-reducing effects than either peptide alone. In high-fat-fed rats, a single ip injection of amylin (10 microg/kg) plus PYY[3-36] (1000 microg/kg) reduced food intake for 24 h (P < 0.05 vs. vehicle), whereas the anorexigenic effects of either PYY[3-36] or amylin alone began to diminish 6 h after injection. These anorexigenic effects were dissociable from changes in locomotor activity. Subcutaneous infusion of amylin plus PYY[3-36] for 14 d suppressed food intake and body weight to a greater extent than either agent alone in both rat and mouse diet-induced obesity (DIO) models (P < 0.05). In DIO-prone rats, 24-h metabolic rate was maintained despite weight loss, and amylin plus PYY[3-36] (but not monotherapy) increased 24-h fat oxidation (P < 0.05 vs. vehicle). Finally, a 4 x 3 factorial design was used to formally describe the interaction between amylin and PYY[3-36]. DIO-prone rats were treated with amylin (0, 4, 20, and 100 microg/kg.d) and PYY[3-36] (0, 200, 400 microg/kg.d) alone and in combination for 14 d. Statistical analyses revealed that food intake suppression with amylin plus PYY[3-36] treatment was synergistic, whereas body weight reduction was additive. Collectively, these observations highlight the importance of studying peptide hormones in combination and suggest that integrated neurohormonal approaches may hold promise as treatments for obesity.

Potential therapies based on antidiabetic peptides

Since adipose tissue was shown to be more than a storage organ, the many cytokines it produces have been identified, along with their roles in energy homeostasis, appetite, and insulin resistance. Concurrently, numerous gut hormones with a diversity of effects have been discovered. They include, amongst many others, peptide YY, ghrelin and oxyntomodulin. As these peptides have been investigated, the potential for their use as novel anti-obesity and antidiabetic therapies has been realized. In this chapter we describe the actions of four of the peptides that have been proposed as the basis for promising new therapies for diabetes: leptin, adiponectin, obestatin and peptide YY. They each have an effect on appetite and, directly or indirectly, on glucose metabolism. We synthesize available data for these peptides and consider the therapeutic potential of each.

Changes in adiponectin and inflammatory genes in response to hormonal imbalances in female mice and exacerbation of depot selective visceral adiposity by high-fat diet: implications for insulin resistance

Early obesity and late onset of insulin resistance associated with hormonal imbalances occur in FSH receptor-deficient follitropin receptor knockout female mice. This study tests the hypothesis that chronic high-fat diet aggravates obesogenic changes in a depot-specific manner and explores some molecular links of hormone imbalances with insulin resistance. In SV 129 mice, hormonal imbalances seem obligatory for exacerbation of diet-induced obesity. Visceral adiposity, glucose intolerance, and lipid disturbances in 9-month follitropin receptor knockout females were associated with decrease in adiponectin signaling. High-molecular-weight plasma adiponectin and adipose tissue adiponectin mRNA were decreased. Adiponectin receptors R1 and R2 mRNA was selectively altered in mesenteric fat but not periuterine fat. R2 decreased in the liver and R1 was higher in muscle. Whereas hepatic adenosine monophosphate T-activated protein kinase activity was down-regulated, both phosphoenolpyruvate carboxykinase and glucose-6-phosphatase enzymes were up-regulated. Longitudinally, diminishing sex hormone signaling in adipose tissue was associated with progressive down-regulation of adiponectin activity and gradual impaired glucose tolerance. Chronic high-fat diet in SV129 wild-type mice did not produce overt obesity but induced visceral fat depot changes accompanied by liver lipid accumulation, high cholesterol, and up-regulation of inflammation gene mRNAs. Thus, TNF-alpha, C-C motif chemokine receptor-2, and C-C motif chemokine ligand-2 were selectively elevated in mesenteric fat without altering glucose tolerance and adiponectin signaling. Our study highlights adiponectin signaling and regulation to be involved in hormone imbalance-induced insulin resistance and demonstrates selective visceral adipose depot alterations by chronic high-fat diet and induction of inflammatory genes.

A -defensin mutation causes black coat color in domestic dogs

Genetic analysis of mammalian color variation has provided fundamental insight into human biology and disease. In most vertebrates, two key genes, Agouti and Melanocortin 1 receptor (Mc1r), encode a ligand-receptor system that controls pigment type-switching, but in domestic dogs, a third gene is implicated, the K locus, whose genetic characteristics predict a previously unrecognized component of the melanocortin pathway. We identify the K locus as beta-defensin 103 (CBD103) and show that its protein product binds with high affinity to the Mc1r and has a simple and strong effect on pigment type-switching in domestic dogs and transgenic mice. These results expand the functional role of beta-defensins, a protein family previously implicated in innate immunity, and identify an additional class of ligands for signaling through melanocortin receptors.

Melanocortin 2 receptor is required for adrenal gland development, steroidogenesis, and neonatal gluconeogenesis

ACTH (i.e., corticotropin) is the principal regulator of the hypothalamus-pituitary-adrenal axis and stimulates steroidogenesis in the adrenal gland via the specific cell-surface melanocortin 2 receptor (MC2R). Here, we generated mice with an inactivation mutation of the MC2R gene to elucidate the roles of MC2R in adrenal development, steroidogenesis, and carbohydrate metabolism. These mice, the last of the knockout (KO) mice to be generated for melanocortin family receptors, provide the opportunity to compare the phenotype of proopiomelanocortin KO mice with that of MC1R-MC5R KO mice. We found that the MC2R KO mutation led to neonatal lethality in three-quarters of the mice, possibly as a result of hypoglycemia. Those surviving to adulthood exhibited macroscopically detectable adrenal glands with markedly atrophied zona fasciculata, whereas the zona glomerulosa and the medulla remained fairly intact. Mutations of MC2R have been reported to be responsible for 25% of familial glucocorticoid deficiency (FGD) cases. Adult MC2R KO mice resembled FGD patients in several aspects, such as undetectable levels of corticosterone despite high levels of ACTH, unresponsiveness to ACTH, and hypoglycemia after prolonged (36 h) fasting. However, MC2R KO mice differ from patients with MC2R-null mutations in several aspects, such as low aldosterone levels and unaltered body length. These results indicate that MC2R is required for postnatal adrenal development and adrenal steroidogenesis and that MC2R KO mice provide a useful animal model by which to study FGD.

Gut hormones as mediators of appetite and weight loss after Roux-en-Y gastric bypass

OBJECTIVE

To evaluate the physiologic importance of the satiety gut hormones.

BACKGROUND

Controversy surrounds the physiologic role of gut hormones in the control of appetite. Bariatric surgery remains the most effective treatment option for obesity, and gut hormones are implicated in the reduction of appetite and weight after Roux-en-Y gastric bypass.

METHODS

We correlated peptide YY (PYY) and glucagon-like peptide 1 (GLP-1) changes within the first week after gastric bypass with changes in appetite. We also evaluated the gut hormone responses of patients with good or poor weight loss after gastric bypass. Finally, we inhibited the gut hormone responses in gastric bypass patients and then evaluated appetite and food intake.

RESULTS

Postprandial PYY and GLP-1 profiles start rising as early as 2 days after gastric bypass (P < 0.05). Changes in appetite are evident within days after gastric bypass surgery (P < 0.05), and unlike other operations, the reduced appetite continues. However, in patients with poor weight loss after gastric bypass associated with increased appetite, the postprandial PYY and GLP-1 responses are attenuated compared with patients with good weight loss (P < 0.05). Inhibiting gut hormone responses, including PYY and GLP-1 after gastric bypass, results in return of appetite and increased food intake (P < 0.05).

CONCLUSION

The attenuated appetite after gastric bypass is associated with elevated PYY and GLP-1 concentrations, and appetite returns when the release of gut hormones is inhibited. The results suggest a role for gut hormones in the mechanism of weight loss after gastric bypass and may have implications for the treatment of obesity.

Pro-opiomelanocortin modulates the thermogenic and physical activity responses to high-fat feeding and markedly influences dietary fat preference

Complete proopiomelanocortin (POMC) deficiency causes a human syndrome of hypoadrenalism, altered skin and hair pigmentation, and severe hyperphagic obesity. Heterozygote carriers of nonsense mutations are strongly predisposed to obesity. Pomc(+/-) mice have normal body weight on a chow diet but increase food intake and become more obese than wild-type littermates when placed on a high-fat diet. To further explore the mechanisms whereby dietary fat interacts with Pomc genotype to produce obesity, we examined Pomc-null, Pomc(+/-), and wild-type mice for changes in the components of energy balance in response to provision of a high-fat diet and macronutrient preference when presented with a selection of dietary choices. In contrast to wild-type mice, Pomc null mice did not increase their resting energy expenditure or their spontaneous physical activity when given a high-fat diet. Pomc(+/-) mice increased resting energy expenditure similarly to wild types, but their increase in physical activity was significantly less than that seen in wild-type mice. In two independent experimental tests of macronutrient preference, Pomc genotype was a strong predictor of dietary fat preference with Pomc null animals choosing to eat approximately twice as much fat, but similar amounts of carbohydrate and protein, as wild-type animals. Pomc(+/-) mice showed an intermediate response. In summary, POMC-derived peptides have influences on multiple aspects of the organism's response to the presentation of high-fat diet. This includes a major influence, readily discernible even in heterozygote animals, on the dietary preference for fat.

The role of melanocortin 3 receptor gene in childhood obesity

OBJECTIVE

Melanocortin 3 receptor (MC3R) plays a critical role in weight regulation of rodents, but its role in humans remains unclear. The objective of this study was to identify genetic variants of the MC3R gene and determine its association with childhood obesity.

RESEARCH DESIGN AND METHODS

We screened 201 obese children for MC3R gene mutations with anthropometric measurements, blood tests, feeding behavior, and body composition assessment. We identified three novel heterozygous mutations (Ile183Asn, Ala70Thr, and Met134Ile) in three unrelated subjects, which were not found in 188 control subjects, and two common polymorphisms Thr6Lys and Val81Ile.

RESULTS

In vitro functional studies of the resultant mutant receptors revealed impaired signaling activity but normal ligand binding and cell surface expression. The heterozygotes demonstrated higher leptin levels and adiposity and less hunger compared with obese control subjects, reminiscent of the MC3R knockout mice. Family studies showed that these mutations may be associated with childhood or early-onset obesity. The common variants Thr6Lys and Val81Ile were in complete linkage disequilibrium, and in vitro studies revealed reduced signaling activity compared with wild-type MC3R. Obese subjects with the 6Lys/81Ile haplotype had significantly higher leptin levels, percentage body fat, and insulin sensitivity, and the causative role of the 6Lys/81Ile variants is supported by the presence of an additive effect in which heterozygotes had an intermediate phenotype compared with homozygotes.

CONCLUSIONS

MC3R mutations may not result in autosomal dominant forms of obesity but may contribute as a predisposing factor to childhood obesity and exert an effect on the human phenotype. Our report supports the role of MC3R in human weight regulation.

The role of proopiomelanocortin (POMC) neurones in feeding behaviour

The precursor protein, proopiomelanocortin (POMC), produces many biologically active peptides via a series of enzymatic steps in a tissue-specific manner, yielding the melanocyte-stimulating hormones (MSHs), corticotrophin (ACTH) and β-endorphin. The MSHs and ACTH bind to the extracellular G-protein coupled melanocortin receptors (MCRs) of which there are five subtypes. The MC3R and MC4R show widespread expression in the central nervous system (CNS), whilst there is low level expression of MC1R and MC5R. In the CNS, cell bodies for POMC are mainly located in the arcuate nucleus of the hypothalamus and the nucleus tractus solitarius of the brainstem. Both of these areas have well defined functions relating to appetite and food intake. Mouse knockouts (ko) for pomc, mc4r and mc3r all show an obese phenotype, as do humans expressing mutations of POMC and MC4R. Recently, human subjects with specific mutations in β-MSH have been found to be obese too, as have mice with engineered β-endorphin deficiency. The CNS POMC system has other functions, including regulation of sexual behaviour, lactation, the reproductive cycle and possibly central cardiovascular control. However, this review will focus on feeding behaviour and link it in with the neuroanatomy of the POMC neurones in the hypothalamus and brainstem.

Orexin expression is regulated by alpha-melanocyte-stimulating hormone

The hypothalamic melanocortin system plays a fundamental role in the regulation of energy homeostasis. Orexins (hypocretins) are also involved in a diverse range of physiological processes, including food intake. Previous evidence has suggested that hypothalamic orexin expression may be influenced by the central melanocortin system. Here, we studied orexin mRNA levels in pro-opiomelanocortin-deficient (Pomc(-/-)) mice, a mouse model lacking all endogenously produced melanocortin peptides. Orexin expression in the lateral hypothalamus was significantly increased in corticosterone deficient Pomc(-/-) mice. Furthermore, when circulating glucocorticoids were restored to levels within the physiological range, orexin expression remained elevated. However, i.c.v. administration of the melanocortin alpha-melanocyte-stimulating hormone (MSH) to Pomc(-/-) mice reduced orexin expression back down to wild-type levels. This was independent of the effects of alpha-MSH on food intake because elevated orexin expression persisted in Pomc(-/-) mice pairfed to alpha-MSH-treated animals. These data indicate that alpha-MSH may play a role in the regulation of orexin expression in Pomc(-/-), with an elevation in orexin levels contributing to the hyperphagia seen in these animals.

Modulation by high-fat diets of gastrointestinal function and hormones associated with the regulation of energy intake: implications for the pathophysiology of obesity

The presence of fat in the small intestine slows gastric emptying, stimulates the release of many gastrointestinal hormones, and suppresses appetite and energy intake as a result of the digestion of fats into free fatty acids; the effects of free fatty acids are, in turn, dependent on their chain length. Given these effects of fat, it is paradoxical that high dietary fat intakes have been linked to increased energy intake and body weight and are considered to play a significant role in the pathogenesis of obesity. However, increasing evidence indicates that a chronic increase in dietary fat is associated with an attenuation of the feedback signals arising from the small intestine induced by fat, with a consequent relative acceleration of gastric emptying, modulation of gastrointestinal hormone secretion, and attenuation of the suppression of energy intake. This review addresses the gastrointestinal factors involved in the regulation of appetite and energy intake, with a particular focus on 1) the gastrointestinal mechanisms triggered by small intestinal fat that modulate energy intake, 2) the potential role of a high dietary fat intake in the development of obesity, and 3) implications for the prevention and management of obesity.

Effects of pro-opiomelanocortin (POMC) on food intake and body weight: mechanisms and therapeutic potential?

POMC (pro-opiomelanocortin) is a complex polypeptide precursor which is cleaved into smaller biologically active peptides such as the melanocortins, alpha-, beta- and gamma-melanocyte-stimulating hormone. Data from human genetic and murine studies convincingly show that an intact central melanocortin signalling pathway is critical for normal energy homoeostasis. Not only does a loss of normal melanocortin signalling lead to obesity, but there are also data implicating increased melanocortin activity in the pathogenesis of cachexia. The study of POMC biology has lead to some fundamental insights into the mechanisms controlling food intake and body weight. This increased understanding of the physiological roles of the melanocortin system has opened up the potential for the design and development of rational therapies to treat perturbations in energy homoeostasis.

Engineering the melanocortin-4 receptor to control constitutive and ligand-mediated G(S) signaling in vivo

The molecular and functional diversity of G protein-coupled receptors is essential to many physiological processes. However, this diversity presents a significant challenge to understanding the G protein-mediated signaling events that underlie a specific physiological response. To increase our understanding of these processes, we sought to gain control of the timing and specificity of G(s) signaling in vivo. We used naturally occurring human mutations to develop two G(s)-coupled engineered receptors that respond solely to a synthetic ligand (RASSLs). Our G(s)-coupled RASSLs are based on the melanocortin-4 receptor, a centrally expressed receptor that plays an important role in the regulation of body weight. These RASSLs are not activated by the endogenous hormone alpha-melanocyte-stimulating hormone but respond potently to a selective synthetic ligand, tetrahydroisoquinoline. The RASSL variants reported here differ in their intrinsic basal activities, allowing the separation of the effects of basal signaling from ligand-mediated activation of the G(s) pathway in vivo. These RASSLs can be used to activate G(s) signaling in any tissue, but would be particularly useful for analyzing downstream events that mediate body weight regulation in mice. Our study also demonstrates the use of human genetic variation for protein engineering.

The role of oxyntomodulin and peptide tyrosine-tyrosine (PYY) in appetite control

Oxyntomodulin and peptide tyrosine-tyrosine (PYY) are released from intestinal enteroendocrine cells in response to a meal. These circulating hormones are considered to be satiety signals, as they have been found to decrease food intake, body weight and adiposity in rodents. Their effect on energy homeostasis is mediated by the hypothalamus and brainstem, and several studies have demonstrated alterations in neuropeptide signaling within the arcuate nucleus. The weight loss that has been observed in animal models after repeated administration of oxyntomodulin and PYY has led to interest in developing these peptides as antiobesity therapies in humans. Indeed, preliminary studies have found that oxyntomodulin or PYY administration reduces food intake and body weight effectively in overweight human volunteers. This research suggests that modulation of these gut hormones could prove to be effective long-term therapies in the quest to combat the obesity epidemic.

Differential body weight and feeding responses to high-fat diets in rats and mice lacking cholecystokinin 1 receptors

Prior data demonstrated differential roles for cholecystokinin (CCK)1 receptors in maintaining energy balance in rats and mice. CCK1 receptor deficiency results in hyperphagia and obesity of Otsuka Long-Evans Tokushima Fatty (OLETF) rats but not in mice. To ascertain the role of CCK1 receptors in high-fat-diet (HFD)-induced obesity, we compared alterations in food intake, body weight, fat mass, plasma glucose, and leptin levels, and patterns of hypothalamic gene expression in OLETF rats and mice lacking CCK1 receptors in response to a 10-wk exposure to HFD. Compared with Long-Evans Tokushima Otsuka (LETO) control rats, OLETF rats on HFD had sustained overconsumption over the 10-wk period. High fat feeding resulted in greater increases in body weight and plasma leptin levels in OLETF than in LETO rats. In situ hybridization determinations revealed that, while HFD reduced neuropeptide Y (NPY) mRNA expression in both the arcuate nucleus (Arc) and the dorsomedial hypothalamus (DMH) of LETO rats, HFD resulted in decreased NPY expression in the Arc but not in the DMH of OLETF rats. In contrast to these results in OLETF rats, HFD increased food intake and induced obesity to an equal degree in both wild-type and CCK1 receptor(-/-) mice. NPY gene expression was decreased in the Arc in response to HFD, but was not detectable in the DMH in both wild-type and CCK1 receptor(-/-) mice. Together, these data provide further evidence for differential roles of CCK1 receptors in the controls of food intake and body weight in rats and mice.

The leptin melanocortin pathway and the control of body weight: lessons from human and murine genetics

The recent rapid increase in the prevalence of obesity across the world is undoubtedly due to changes in diet and lifestyle. However, it is also indisputable that different people react differently to this change in environment and this variation in response is likely to be genetically determined. While for the majority of people this effect is presumed to be polygenic in origin, there is now strong evidence for a small number of genes having a large effect in some families with severe obesity. Studies of these families, coupled with parallel studies in murine models, have provided novel insights into the molecules involved in the regulation of appetite, energy expenditure and nutrient partitioning. We review here the lessons we have learnt from mouse models of obesity, both naturally occurring and artificially generated through targeted gene deletions, and more importantly from human monogenic syndromes of obesity. These have illuminated the critical role in which the central leptin melanocortin pathway plays in the control of mammalian food intake and body weight.

Peripheral mechanisms contributing to the glucocorticoid hypersensitivity in proopiomelanocortin null mice treated with corticosterone

Proopiomelanocortin (POMC) deficiency causes severe obesity through hyperphagia of hypothalamic origin. However, low glucocorticoid levels caused by adrenal insufficiency mitigate against insulin resistance, hyperphagia and fat accretion in Pomc-/- mice. Upon exogenous glucocorticoid replacement, corticosterone-supplemented (CORT) Pomc-/- mice show exaggerated responses, including excessive fat accumulation, hyperleptinaemia and insulin resistance. To investigate the peripheral mechanisms underlying this glucocorticoid hypersensitivity, we examined the expression levels of key determinants and targets of glucocorticoid action in adipose tissue and liver. Despite lower basal expression of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1), which generates active glucocorticoids within cells, CORT-mediated induction of 11beta-HSD1 mRNA levels was more pronounced in adipose tissues of Pomc-/- mice. Similarly, CORT treatment increased lipoprotein lipase mRNA levels in all fat depots in Pomc-/- mice, consistent with exaggerated fat accumulation. Glucocorticoid receptor (GR) mRNA levels were selectively elevated in liver and retroperitoneal fat of Pomc-/- mice but were corrected by CORT in the latter depot. In liver, CORT increased phosphoenolpyruvate carboxykinase mRNA levels specifically in Pomc-/- mice, consistent with their insulin-resistant phenotype. Furthermore, CORT induced hypertension in Pomc-/- mice, independently of adipose or liver renin-angiotensin system activation. These data suggest that CORT-inducible 11beta-HSD1 expression in fat contributes to the adverse cardiometabolic effects of CORT in POMC deficiency, whereas higher GR levels may be more important in liver.

Transgenic MSH overexpression attenuates the metabolic effects of a high-fat diet

To determine whether long-term melanocortinergic activation can attenuate the metabolic effects of a high fat diet, mice overexpressing an NH(2)-terminal POMC transgene that includes alpha- and gamma(3)-MSH were studied on either a 10% low-fat diet (LFD) or 45% high-fat diet (HFD). Weight gain was modestly reduced in transgenic (Tg-MSH) male and female mice vs. wild type (WT) on HFD (P < 0.05) but not LFD. Substantial reductions in body fat percentage were found in both male and female Tg-MSH mice on LFD (P < 0.05) and were more pronounced on HFD (P < 0.001). These changes occurred in the absence of significant feeding differences in most groups, consistent with effects of Tg-MSH on energy expenditure and partitioning. This is supported by indirect calorimetry studies demonstrating higher resting oxygen consumption and lower RQ in Tg-MSH mice on the HFD. Tg-MSH mice had lower fasting insulin levels and improved glucose tolerance on both diets. Histological and biochemical analyses revealed that hepatic fat accumulation was markedly reduced in Tg-MSH mice on the HFD. Tg-MSH also attenuated the increase in corticosterone induced by the HFD. Higher levels of Agrp mRNA, which might counteract effects of the transgene, were measured in Tg-MSH mice on LFD (P = 0.02) but not HFD. These data show that long-term melanocortin activation reduces body weight, adiposity, and hepatic fat accumulation and improves glucose metabolism, particularly in the setting of diet-induced obesity. Our results suggest that long-term melanocortinergic activation could serve as a potential strategy for the treatment of obesity and its deleterious metabolic consequences.

Peripheral tissue-brain interactions in the regulation of food intake

More than 70 years ago the glucostatic, lipostatic and aminostatic hypotheses proposed that the central nervous system sensed circulating levels of different metabolites, changing feeding behaviour in response to the levels of those molecules. In the last 20 years the rapid increase in obesity and associated pathologies in developed countries has involved a substantial increase in the knowledge of the physiological and molecular mechanism regulating body mass. This effort has resulted in the recent discovery of new peripheral signals, such as leptin and ghrelin, as well as new neuropeptides, such as orexins, involved in body-weight homeostasis. The present review summarises research into energy balance, starting from the original classical hypotheses proposing metabolite sensing, through peripheral tissue-brain interactions and coming full circle to the recently-discovered role of hypothalamic fatty acid synthase in feeding regulation. Understanding these molecular mechanisms will provide new pharmacological targets for the treatment of obesity and appetite disorders.

Appetite regulation: an overview

Obesity is a major public health problem associated with morbidity and mortality and continues to increase worldwide. This review focuses on the regions of the brain that are important in appetite regulation and the circulating factors implicated in the control of food intake. The hypothalamus is critical in the regulation of food intake containing neural circuits, which produce a number of peptides that influence food intake. The arcuate nucleus of the hypothalamus produces both orexigenic peptides (agouti-related protein and neuropeptide Y) and anorectic peptides (alpha-melanocyte-stimulating hormone and cocaine- and amphetamine-related transcript). The lateral hypothalamus produces the orexigenic peptides (melanin-concentrating hormone and orexins). Other hypothalamic factors recently implicated in appetite regulation include the endocannabinoids, brain-derived neurotrophic factor, nesfatin-1, AMP-activated protein kinase, mammalian target of rapamycin protein, and protein tyrosine phosphatase. Circulating factors that affect food intake mediate their effects by signaling to the hypothalamus and/or brainstem. A number of circulating factors are produced by peripheral organs, for example, leptin by adipose tissue, insulin and pancreatic polypeptide by the pancreas, gut hormones (e.g., ghrelin, obestatin, glucagon-like peptide-1, oxyntomodulin, peptide YY), and triiodothyronine by the thyroid gland. Circulating carbohydrates, lipids, and amino acids also affect appetite regulation. Knowledge regarding appetite regulation has vastly expanded in recent years providing targets for antiobesity drug design.

Gastrointestinal regulation of food intake

Despite substantial fluctuations in daily food intake, animals maintain a remarkably stable body weight, because overall caloric ingestion and expenditure are exquisitely matched over long periods of time, through the process of energy homeostasis. The brain receives hormonal, neural, and metabolic signals pertaining to body-energy status and, in response to these inputs, coordinates adaptive alterations of energy intake and expenditure. To regulate food consumption, the brain must modulate appetite, and the core of appetite regulation lies in the gut-brain axis. This Review summarizes current knowledge regarding the neuroendocrine regulation of food intake by the gastrointestinal system, focusing on gastric distention, intestinal and pancreatic satiation peptides, and the orexigenic gastric hormone ghrelin. We highlight mechanisms governing nutrient sensing and peptide secretion by enteroendocrine cells, including novel taste-like pathways. The increasingly nuanced understanding of the mechanisms mediating gut-peptide regulation and action provides promising targets for new strategies to combat obesity and diabetes.

Y2, Y4 receptors and obesity

The neuropeptide Y system - comprising neuropeptide Y, peptide YY, pancreatic polypeptide and the Y receptors through which they act (Y1, Y2, Y4, Y5 and y6) - has been at the center of attention with regards to regulation of feeding behavior and its possible involvement in obesity. In the past, research has focused mainly on the orexigenic and obesogenic action of this system, with Y1 and Y5 receptors being prime candidates as mediators of neuropeptide Y-induced hyperphagia and obesity. However, in recent years, the role of other members of the neuropeptide Y family, peptide YY, pancreatic polypeptide and the Y2 and Y4 receptors through which they predominantly act, have commanded increasing attention on account of their effects to mediate satiety and promote weight loss via actions in key brain structures, such as the arcuate nucleus of the hypothalamus and the brain stem. This review focuses on the role of peptide YY- and pancreatic polypeptide-like compounds as possible antiobesity drugs, taking into account their effects, not only on energy balance, but also in the regulation of bone formation, and highlights potential benefits of using Y2 and/or Y4 antagonists (as opposed to agonists such as peptide YY or pancreatic polypeptide) in the treatment of obesity.

The role of the central melanocortin system in the regulation of food intake and energy homeostasis: lessons from mouse models

A little more than a decade ago, the molecular basis of the lipostat was largely unknown. At that time, many laboratories were at work attempting to clone the genes encoding the obesity, diabetes, fatty, tubby and agouti loci, with the hope that identification of these obesity genes would help shed light on the process of energy homeostasis, appetite and energy expenditure. Characterization of obesity and diabetes elucidated the nature of the adipostatic hormone leptin and its receptor, respectively, while cloning of the agouti gene eventually led to the identification and characterization of one of the key neural systems upon which leptin acts to regulate intake and expenditure. In this review, we describe the neural circuitry known as the central melanocortin system and discuss the current understanding of its role in feeding and other processes involved in energy homeostasis.