Neuropeptides in Obesity and Metabolic Disease.
Neurons communicate with each other by virtue of chemical signals that are released by one cell and received by another. Neurons often synthesize a conventional neurotransmitter (such as glutamate, GABA), neuromodulators (such as serotonin, dopamine, acetylcholine), as well as one or more neuropeptides. Classically, neurotransmitters affect the excitability of their target neuron by depolarization or hyperpolarization. In contrast, neuropeptides have more diverse effects (2). They modulate neurotransmission by altering gene expression, local blood flow, and synaptogenesis, and they act as autocrine/paracrine regulators and as hormones. Their storage is tightly regulated and release follows demand. Almost all neuropeptides signal through G-protein coupled receptors and thus tend to have prolonged actions compared to rapid-acting neurotransmitters.
Much of the current knowledge of the role of these neuropeptides comes from the study of knockout animals and more recently neural pathway mapping through optogenetics and designer receptors exclusively activated by designer drugs (DREADD) techniques. Analytical biochemistry methods, qualitative and quantitative genetic methods, and more recently single-cell transcriptomics have helped to refine the neuropeptide signatures of specific neural populations involved in the control of food intake. In humans, research has been hampered by a lack of easily accessible analytical techniques to quantify neuropeptides in vivo. Although some neuropeptides such as brain-derived neurotrophic factor (BDNF) and oxytocin can be measured in the serum by direct enzyme-linked immunoassays (3, 4), this might not reflect central bioavailability.
This review provides an overview of some selected neuropeptides involved in the regulation of appetite and metabolism. The review begins by first describing peripheral hormones that regulate food intake, followed by a discussion of the role of the neuropeptides in arcuate nucleus (ARC). It then discusses downstream neuronal targets of the ARC such as the paraventricular nucleus (PVN), the LH, and the parabrachial nucleus (PBN) in the brainstem. Finally, neuropeptides involved in satiety and energy expenditure are highlighted.
PERIPHERAL HORMONES AND NUTRIENTS SIGNAL ENERGY STATE TO THE CENTRAL NERVOUS SYSTEM
The discovery of adipocyte-derived leptin illustrates the power of analytical chemistry combined with genetics. Seminal parabiosis experiments in inbred strains of mice with severe obesity (ob/ob and db/db rodents) led to the notion that a circulating factor, leptin, regulated body weight (5-7). Following this, a new leptin immunoassay played a role in the discovery of leptin's importance in human body-weight regulation. In 1997, 2 severely obese cousins from a highly consanguineous family of Pakistani origin were found to have undetectable serum leptin, and genetic studies revealed they were homozygous for a frameshift mutation in the LEP gene ([DELTA]G133), which resulted in a truncated protein that was not secreted (8).
Leptin finds itself among many other hormones, such as insulin and gut peptides, as well as nutrients (glucose, fatty acids, and peptides) that signal peripheral energy state to the central nervous system. Gut peptides such as ghrelin, peptide YY, and glucagon-like peptide 1 (GLP1) are secreted from entero-endocrine cells in response to meal ingestion and the presence of nutrients in the intestinal lumen (9, 10). Pioneering human infusion studies have demonstrated that a number of gut peptides modulate food intake when administered acutely in humans (11). The synthetic GLP1 receptor agonist liraglutide has recently been approved for the treatment of obesity alone by the FDA and several other gut peptide analogs, as well as gut hormone receptor agonists, are currently being studied in clinical trials.
Receptors for nutritional hormones are predominantly found in the brainstem and the hypothalamus. In addition, the nucleus of the solitary tract (NTS) in the brainstem also receives afferent nervous signals from mechanoreceptors, i.e., for gastric distension, and chemoreceptors indicating changes in nutrient composition via the vagus nerve. The ARC is an aggregation of neurons in the mediobasal hypothalamus adjacent to the third ventricle. It is known for its complex and unique anatomical relationship with the blood-brain barrier, ensuring privileged access to peripheral hormones and nutrients, and forms the first point-of-call for peripheral leptin.
NEUROPEPTIDES PRODUCED IN THE ARCUATE NUCLEUS OF THE HYPOTHALAMUS INVOLVED IN ENERGY HOMEOSTASIS
Two populations of neurons in the ARC have distinct biochemical signatures: neurons that produce the neuropeptides agouti-related peptide (AgRP)/neuropeptide Y (NPY) and neurons that produce proopiomelanocortin (POM)/cocaine- and amphetamine-regulated transcript (CART), which both produce the leptin receptor (LepR). These 2 neuronal populations are traditionally regarded as the key sensors of energy state. They relay information to second-order neurons, located in the paraventricular nucleus (PVN), the dorsomedial hypothalamus (DMN), LH, the brainstem, as well as wider brain areas such as the limbic system, nucleus accumbens, and prefrontal cortex. Collectively, they integrate and respond to peripheral signals from hormones and nutrients by altering food intake and energy expenditure (Fig. 1).
Leptin binds to the LepR on POMC-producing neurons in the ARC and stimulates POMC transcription. POMC is a 241-amino acid precursor polypeptide and is cleaved to produce multiple peptide hormones including [alpha]-melanocyte-stimulating hormone ([alpha]-MSH), the predominant anorexigenic neuropeptide of the ARC-PVN neurocircuitry. Alpha-MSH binds to the melanocortin-4-receptor (MC4R). Rodents that lack POMC or MC4R become severely obese (12). Disruption of this circuit in humans results in severe early-onset obesity, as in individuals with genetic variants in POMC,  prohormone convertase 1 (PC1), an enzyme involved in POMC processing, and MC4R (13, 14) (Table 1 and Table 2). In addition, genome-wide association studies have revealed more than 100 different candidate genes for body mass index. Pathway analyses on those genes support roles in the central nervous system, including many components of the melanocortin pathway (15). Therefore, disappointment followed the observation that early synthetic MC4R agonists increased blood pressure in humans, which revealed a key circuit linking body weight to blood pressure (16). However, renewed efforts from the pharmaceutical industry with the selective MC4R agonist (RM-493) do suggest that there might be a future for MC4R agonists in the treatment of obesity (17, 18).
In addition to [alpha]-MSH, [beta]-endorphin is produced from POMC. Beta-endorphin autoinhibits hypothalamic POMC neurons through their [mu]-opioid receptors (19, 20). The [mu]-opioid receptor antagonist naloxone and the newer one GSK1521498 reduce both the consumption of palatable (high fat/high sugar) foods (21) and the hedonic responses and motivation for these foods (22). Several classical neurotransmitters and neuropeptides have been found to modulate the activity of the melanocortin system. POMC neurons produce the serotonin (5HT) 2C receptor (5HT2CR) and selective reinstatement of the 5HT2CR on POMC neurons rescues the obesity phenotype in 5HT2CR-null mice (23). This modulation of POMC neuronal activation seems to be the core mediator of the weight-reducing effects of lorcaserin, a selective 5HT2CR agonist approved by the FDA for the treatment of obesity (24).
Several neuropeptides have been found to modulate PC1 activity. proSAAS, a protein encoded by the mouse gene Pcsk1 and precursor to 5 different neuropeptides (big SAAS, little SAAS, PEN, big LEN, and little LEN), seems to act as an inhibitor of PC1 (25), and genetic overproduction of proSAAS leads to late-onset obesity in mice (26). The effects on PC1 might be specific to embryonic development (27, 28) and the mechanisms underlying the obesity need further exploration.
CART shows almost 100% colocalization with POMC in the ARC and complete segregation from the AgrP/NPY-producing neurons (29). However, differences with humans might exist here (30). In rodents, CART mRNA concentrations are regulated by circulating leptin (31) and injections of CART in nucleus accumbens inhibit feeding in rodents (32).
AgRP/NPY-producing neurons form the orexigenic arm of the ARC-PVN hypothalamic neurocircuitry; i.e., they send signals of energy deficit onwards through AgRP, NPY, and GABA release to increase food seeking and eating (33). AgRP is a 132-amino acid peptide and acts as an inverse agonist for the MC4R by suppressing its constitutive activity as well as promoting MC4R endocytosis (34). AgRP-producing neurons directly inhibit POMC neurons in energy deficit (35). AgRP-producing neurons send extensive axon projections to other brain areas including core forebrain nodes, which are part of an extended circuit that mediates feeding behavior (36). NPY colocalizes in the majority of AgRP-producing neurons. Intracerebroventricular injection of NPY potently stimulates food intake and continuous infusion of NPY readily leads to obesity in rodents (37). Leptin inhibits arcuate NPY production, and genetic knockout of NPY reduces hyperphagia and obesity in ob/ob mice (38). NPY Y1 and Y2 receptors are produced by POMC neurons, and their activation leads to inhibition of firing activity (39). The Y5 receptor has also been implicated in the regulation of spontaneous release of a-MSH from POMC neurons, further supporting the NPY-mediated inhibition of the melanocortin system (40). In addition to NPY and AgRP, these neurons also release GABA, and elegant studies using DREADD technology found that GABA and NPY are needed for the rapid stimulation of food intake, whereas AgRP through the MC4R receptors induces feeding over a delayed yet prolonged period (41). This elegantly illustrates that neurons can differentially employ classical neurotransmitters as well as neuropeptides to regulate food intake in a temporal manner.
Finally, AgRP neurons project to the PBN directly, a brainstem area important for feeding behavior, and activation of the PBN neurons by AgRP neurons inhibits food intake, an effect mediated by their release of GABA (42, 43).
NEUROPEPTIDES PRODUCED IN MC4R-POSITIVE, SECONDORDER NEURONS AND THEIR NEURONAL CONNECTIONS
Production of MC4R is abundant in the hypothalamus, including PVN, LH, VMH, and DMN as well as in anterior hypothalamic regions; the NTS; and the spinal cord (44). Several neuropeptides and neurons have been implicated in mediating the effects of the MC4R-positive, second-order neurons on food intake and bodyweight regulation. Single-minded homolog 1 (SIM1) is a transcription factor involved in the development of the PVN and supraoptic nucleus. SIM1 haplo-insufficiency in mice and loss-of-function mutations in humans cause severe obesity (45, 46). Reproduction of the MC4R receptor on SIM1-positive neurons only is sufficient to abolish the hyperphagia seen in MC4R-null mice (47). These neurons are glutamatergic but do not seem to produce oxytocin, corticotropin-releasing hormone, vasopressin, or prodynorphin, so their neuropeptide signature remains to be elucidated (47). They are synaptically connected to the PBN, and the loss of this inhibitory hypothalamic input to the PBN leads to starvation (47).
Alpha-MSH induces dendritic release of oxytocin in the PVN through the MC4R (48). Central administration of oxytocin in rodents is anorexigenic, and rodents that lack oxytocin or the oxytocin receptor become obese (49, 50). Magnocellular neurons of the PVN and supraoptic nucleus of the hypothalamus also produce a number of anorexigenic neuropeptides, including CART, pituitary adenylate cyclase-activating polypeptide, cholecystokinin (CCK), and nesfatin-1 (51), and are activated during feeding and by satiety peptides such as CCK and GLP1. In addition, these neurons produce LepR and are activated by leptin (52). Furthermore, AgRP neuron suppression of oxytocin neurons is critical for evoked feeding in rodents (33). The exact sites of locally released oxytocin that mediate the effects of oxytocin are the subjects ofintense study and most likely involve the VMH (53). Oxytocin release is negatively regulated by synaptotagmin-4 and genetic ablation of synaptotagmin-4 prevents against diet-induced obesity in mice (54). Oxytocin has attracted the attention of clinical researchers and several studies have explored the effects of oxytocin on food intake and some variably show minor effects on food intake (55). Therefore, it is conceivable that treatment with oxytocin, an oxytocin analog, or inhibitor of synaptotagmin-4 could form potential targets in the treatment of obesity.
Additionally, both POMC- and AgRP-producing neurons project to the VMH, where BDNF is abundantly produced. MC4R-null mice have decreased hypothalamic BDNF production, and administering an anti-BDNF antibody in the third ventricle blocks the anorexic effects of MC4R activation (56). Haplo-insufficient mice and mice in which BDNF has been deleted postnatally are obese (57), as are humans with genetic disruption of BDNF and its neurotrophic tyrosine kinase receptor type 2 (NTRK2) (58, 59).
Finally, MC4R neurons are involved in the regulation of sympathetic nervous system outflow. In this way, MC4R signaling mediates high-fat diet and cold-induced thermogenesis (60) and thus energy expenditure.
NEUROPEPTIDES PRODUCED IN THE LATERAL HYPOTHALAMUS INVOLVED IN FEEDING BEHAVIOR
Lesions of the LH cause severe anorexia and adipsia (1). The LH has 3 distinct populations of neurons: neurons that produce orexin, neurons that produce melanin-concentrating hormone (MCH), and neurons containing isoform 65 of glutamic acid decarboxylase (GAD65), named "GAD65 neurons."
Orexin-producing neurons receive direct projections from AgRP- and POMC-producing neurons (61) at the same time as sending direct projections to the ARC neurons (62), which suggests they are involved in coupling the drive for energy intake with energy demands. mRNA for the precursor of orexin, preproorexin, is abundantly and specifically produced in the LH and adjacent areas. Many AgRP/NPY- and POMC-producing neurons in the ARC coproduce the LepR and orexin 1 receptor (OX1R) (63), and orexin neurons can depolarize AgRP neurons (64) and POMC neurons (65). Genetic ablation of orexin neurons in mice leads to a phenotype very similar to human narcolepsy, including behavioral arrests, premature rapid eye movement sleep, and poorly consolidated sleep patterns as well as late-onset obesity despite hypophagia compared to littermates (66, 67).
MCH is a 19-amino acid neuropeptide encoded by the Pmch gene and can bind to 2 G-protein-coupled receptors, the MCH receptor 1 (MCHR1) and MCH receptor 2 (MCHR2). MCH is synthesized in the magnocellular neurons in the lateral hypothalamus. In contrast to orexin neurons, MCH neurons show increased activity when extracellular glucose concentrations increase (68). MCH mRNA is up-regulated in ob/ob mice and by fasting in wild-type mice (69). Mice genetically lacking MCH are hypophagic and lean (70), and central administration of MCH stimulates food intake (71). MCH neurons send dense projections to reward centers in the striatum and midbrain. In addition, there is some evidence to suggest MCH neurons might regulate a taste-independent preference for caloric feeding as mice lacking the sweet taste receptor by deletion of the long transient receptor potential channel 5 normally prefer sucrose over sucralose (72), but do not show this preference when they simultaneously lack MCH neurons (73).
NEUROPEPTIDES INVOLVED IN SATIETY AND ENERGY EXPENDITURE
While research has predominantly focused on the satiating properties of CCK and bombesin-like peptides released from the GI tract, these peptides are also produced in the central nervous system. The distribution and cell specificity of bioactive CCK species formed from processed preproCCK have been investigated with the use of sequence-specific immunoassays (74). Enteroendocrine cells contain a mixture of the medium-sized CCK-58, CCK-33, CCK-22, and CCK-8, whereas neurons mainly release CCK-8 and to some extent CCK-5. Intestinal CCK regulates satiety through its actions initiated in NTS in the brainstem as well as locally by slowing gastric emptying through its CCK-A receptors. The role of neural CCK in satiety has been less extensively investigated. CCK immunoreactivity is widespread across the central nervous system, with cortical, hypothalamic, and brainstem production. A high-fat diet downregulates central CCK in rodents (75), and fasting reduces cortical CCK but increases hypothalamic CCK (76). In addition, neural CCK has been implicated in a multitude of roles, regulating thermoregulation, sympathetic nervous system activation, anxiety, and sexual behavior in rodents (74).
As mentioned before, GLP1 is produced in enteroendocrine cells as well as preproglucagon neurons, located in the NTS in the brainstem, that project to hypothalamic targets in the ARC, PVN, and DMN. GLP1 is anorexigenic, and intracerebroventricular injection of a GLP1 receptor antagonist strongly increases food intake in satiated but not in fasted animals, which suggests that endogenous GLP1 tone alters with nutritional state (10).
The mammalian bombesin family consists of neuromedin B (NMB) and gastrin-releasing peptide (GRP). In situ hybridization studies have shown high NMB production in the human hypothalamus (77) and intracerebroventricular infusion of NMB decrease in food intake in rats (78). RIAs and immunohistochemical analyses have confirmed GRP immunoreactivity in a variety of tissues, including gastrointestinal tissues and central nervous system (especially the pituitary gland, spinal cord, and adrenal gland) tissues. GRP mRNA in the PVN decreases with food deprivation in rodents and increases after melanotan II, a nonselective agonist of the melanocortin 3 and 4 receptors, infusion, which suggests that GRP-producing neurons in the PVN are part of hypothalamic circuitry involved in energy homeostasis (79). Bombesin-like peptide receptors include the 7-transmembrane GRP receptor, the NMB receptor, and the bombesin-like receptor-3 (BRS3). BRS3 only interacts with low affinity with the naturally occurring bombesin-related peptides and has no known natural high-affinity ligand. Animals lacking BRS3, however, become obese (80), suggesting that this pathway may also be important in the regulation of body weight.
RNA sequencing experiments have revealed another neuropeptide involved in energy expenditure. Prodynorphin is abundantly produced in the hypothalamus, and approximately 40% of LepR-positive, prodynorphin-producing cells also produce POMC (81). Selective knockdown of prodynorphin in LepR-positive neurons leads to obesity on a high-fat diet due to decrease in energy expenditure (81).
A comprehensive review on the RFamide class of neuropeptides in energy homeostasis is available (82). Prolactin-releasing peptide (PrRP) is a member of this neuropeptide family and is predominantly centrally produced in the brainstem and the DMN. Mice lacking its receptor, the G-protein-coupled 10 receptor (GPR10), become obese (83). Intracerebroventricular injection of PrRP inhibits food intake and increases energy expenditure as it increases body temperature, O2 consumption, and UCP-1 production of brown adipose tissue in rodents (84). Likewise, GPR10 knockout animals have a lower basal metabolic rate than wild-type animals (83). Moreover, the DMN PrRP-producing neurons are activated by leptin, and genetic deletion of PrRP neurons blocks leptin's induction of thermogenesis (85). Therefore, modulation of PrRP and its receptor GPR10 would provide potential targets in the treatment of obesity.
SUMMARY AND CONCLUSION
Targets for pharmacotherapy in obesity include the central and peripheral regulation of food intake, but also energy expenditure and physical activity. Neuropeptides regulate this, and drugs that mimic neuropeptides (MC4R agonists) or act as neuromodulators, such as lorcaserin or naltrexone/bupropion, induce weight loss in humans (24). Future drugs will need to be directed at highly specific targets and may consist of combinations of compounds that target different mechanisms, as illustrated by recent studies demonstrating the efficacy of dual MC4R and GLP1 receptor agonism (86) However, for more successful therapeutic strategies, we need more in-depth knowledge of the neuronal circuits in which they are working, the downstream targets, and potential compensatory mechanisms not only in rodents but, critically, also in humans.
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
Authors' Disclosures or Potential Conflicts of Interest: Upon manuscript submission, all authors completed the author disclosure form. Disclosures and/or potential conflicts of interest:
Employment or Leadership: None declared.
Consultant or Advisory Role: None declared.
Stock Ownership: None declared.
Honoraria: None declared.
Research Funding: A. van der Klaauw, the Wellcome Trust Early Postdoctoral Fellowship for Clinician Scientists (099038/Z/12/Z).
Expert Testimony: None declared.
Patents: None declared.
Acknowledgments: The author would like to thank Professor I Sadaf Farooqi, Ms. Naomi Clark, Dr. Adrian Park and Dr. Fleur Talbot for their guidance and support in writing this manuscript.
(1.) Anand BK, Brobeck JR. Localization of a "feeding center" in the hypothalamus of the rat. Proc Soc Exp Biol Med 1951;77:323-4.
(2.) Burbach JP. What are neuropeptides? Meth Mol Biol 2011;789:1-36.
(3.) Han JC, Muehlbauer MJ, Cui HN, Newgard CB, Haqq AM. Lower brain-derived neurotrophic factor in patients with Prader-Willi syndrome compared to obese and lean control subjects. J Clin Endocrinol Metab 2010;95: 3532-6.
(4.) Gupta SL, Dhiman V, Jayasekharan T, Sahoo NK. Analysis of argentinated peptide complexes using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: Peptide = oxytocin, arg(8)-vasopressin, bradykinin, bombesin, somatostatin, neurotensin. Rap Commun Mass Spectrom 2016;30:1313-22.
(5.) Coleman DL. Effects of parabiosis of obese with diabetes and normal mice. Diabetologia 1973;9:294-8.
(6.) Coleman DL, Hummel KP. Effects of parabiosis of normal with genetically diabetic mice. Am J Physiol 1969; 217:1298-1304.
(7.) Zhang Y, Proenca R, Maffei M, Barone M, Leopold L, Friedman JM. Positional cloning of the mouse obese gene and its human homologue. Nature 1994;372:425-32.
(8.) Montague CT, Farooqi IS, Whitehead JP, Soos MA, Rau H, Wareham NJ, et al. Congenital leptin deficiency is associated with severe early-onset obesity in humans. Nature 1997;387:903-8.
(9.) Batterham RL, Cowley MA, Small CJ, Herzog H, Cohen MA, Dakin CL, Wren AM, et al. Gut hormone pyy(3-36) Physiologically inhibits food intake. Nature 2002;418: 650-4.
(10.) Turton MD, O'Shea D, Gunn I, Beak SA, Edwards CM, Meeran K, et al. A role for glucagon-like peptide-1 in the central regulation of feeding. Nature 1996;379: 69-72.
(11.) Tan T, Bloom S. Gut hormones as therapeutic agents in treatment of diabetes and obesity. Curr Opin Pharmacol 2013;13:996-1001.
(12.) Huszar D, Lynch CA, Fairchild-Huntress V, Dunmore JH, Fang Q, Berkemeier LR, et al. Targeted disruption of the melanocortin-4 receptor results in obesity in mice. Cell 1997;88:131-41.
(13.) Krude H, Biebermann H, Luck W, Horn R, Brabant G, Gruters A. Severe early-onset obesity, adrenal insufficiency And red hair pigmentation caused by POMC mutations in humans. Nat Genet 1998;19:155-7.
(14.) Farooqi IS, Yeo GS, Keogh JM, Aminian S, Jebb SA, Butler G, et al. Dominant and recessive inheritance of morbid obesity associated with melanocortin 4 receptor deficiency. J Clin Invest 2000;106:271-9.
(15.) Locke AE, Kahali B, Berndt SI, Justice AE, Pers TH, Day FR, et al. Genetic studies of body mass index yield new insights for obesity biology. Nature 2015;518: 197-206.
(16.) Greenfield JR, Miller JW, Keogh JM, Henning E, Satterwhite JH, Cameron GS, et al. Modulation of blood pressure by central melanocortinergic pathways. N Engl J Med 2009;360:44-52.
(17.) Kuhnen P, Clement K, Wiegand S, Blankenstein O, Gottesdiener K, Martini LL, et al. Proopiomelanocortin deficiency treated with a melanocortin-4 receptor agonist. N Engl J Med 2016;375:240-6.
(18.) Chen KY, Muniyappa R, Abel BS, Mullins KP, Staker P, Brychta RJ, et al. RM-493, a melanocortin-4 receptor (MC4R) agonist, increases resting energy expenditure in obese individuals. J Clin Endocrinol Metab 2015; 100:1639-45.
(19.) Cowley MA, Cone R, Enriori P, Louiselle I, Williams SM, Evans AE. Electrophysiological actions of peripheral hormones on melanocortin neurons. Ann NY Acad Sci 2003;994:175-86.
(20.) Cowley MA, Smart JL, Rubinstein M, Cerdan MG, Diano S, Horvath TL, et al. Leptin activates anorexigenic POMC neurons through a neural network in the arcuate nucleus. Nature 2001;411:480-4.
(21.) Nathan PJ, O'Neill BV, Bush MA, Koch A, Tao WX, Maltby K, et al. Opioid receptor modulation of hedonic taste preference and food intake: a single-dose safety, pharmacokinetic, and pharmacodynamic investigation with GSK1521498, a novel mu-opioid receptor inverse agonist. J Clin Pharmacol 2012;52: 464-74.
(22.) Ziauddeen H, Chamberlain SR, Nathan PJ, Koch A, Maltby K, Bush M, et al. Effects of the mu-opioid receptor antagonist GSK1521498 on hedonic and consummatory eating behaviour: a proof of mechanism study in binge-eating obese subjects. Mol Psych 2013;18: 1287-93.
(23.) Xu Y, Jones JE, Kohno D, Williams KW, Lee CE, Choi MJ, et al. 5-HT2CRs expressed by pro-opiomelanocortin neurons regulate energy homeostasis. Neuron 2008; 60:582-9.
(24.) Burke LK, Doslikova B, D'Agostino G, Garfield AS, Farooq G, Burdakov D, Low MJ, et al. 5-HT obesity medication efficacy via POMC activation is maintained during aging. Endocrinol 2014;155:3732-8.
(25.) Lee SN, Prodhomme E, Lindberg I. Prohormone convertase 1 (PC1) processing and sorting: Effect of PC1 propeptide and proSAAS. J Endocrinol 2004;182: 353-64.
(26.) Wei S, Feng Y, Che FY, Pan H, Mzhavia N, Devi LA, et al. Obesity and diabetes in transgenic mice expressing proSAAS. J Endocrinol 2004;180:357-68.
(27.) Feng Y, Reznik SE, Fricker LD. ProSAAS and prohormone convertase 1 are broadly expressed during mouse development. Brain Res Gene Expr Patterns 2002;1:135-40.
(28.) Morgan DJ, Wei S, Gomes I, Czyzyk T, Mzhavia N, Pan H, Devi LA, et al. The propeptide precursor proSAAS is involved in fetal neuropeptide processing and body weight regulation. J Neurochem 2010;113: 1275-84.
(29.) Fekete C, Wittmann G, Liposits Z, Lechan RM. Origin of cocaine- and amphetamine-regulated transcript (CART)-immunoreactive innervation of the hypothalamic paraventricular nucleus. J Compar Neurol 2004;469:340-50.
(30.) Menyhert J, Wittmann G, Lechan RM, Keller E, Liposits Z, Fekete C. Cocaine- and amphetamine-regulated transcript (CART) is colocalized with the orexigenic neuropeptide Y and agouti-related protein and absent from the anorexigenic alpha-melanocyte-stimulating hormone neurons in the infundibular nucleus of the human hypothalamus. Endocrinol 2007;148:4276-81.
(31.) Kristensen P, Judge ME, Thim L, Ribel U, Christjansen KN, Wulff BS, et al. Hypothalamic cart is a new anorectic peptide regulated by leptin. Nature 1998;393:72-6.
(32.) Jean A, Conductier G, Manrique C, Bouras C, Berta P, Hen R, et al. Anorexia induced by activation of serotonin 5-HT4 receptors is mediated by increases in cart in the nucleus accumbens. Proc Natl Acad Sci U S A2007; 104:16335-40.
(33.) Sternson SM, Atasoy D. Agouti-related protein neuron circuits that regulate appetite. Neuroendocrinol 2014; 100:95-102.
(34.) Nijenhuis WA, Oosterom J, Adan RA. AgRP(83-132) acts as an inverse agonist on the human-melanocortin-4 receptor. Mol Endocrinol 2001;15:164-71.
(35.) Sato E, Miyamoto Y. [Cultivation of mouse oocytes in vitro: the ability to resume meiosis]. Jikken Dobutsu 1988;37:231-8.
(36.) Betley JN, Cao ZF, Ritola KD, Sternson SM. Parallel, redundant circuit organization for homeostatic control of feeding behavior. Cell 2013;155:1337-50.
(37.) Raposinho PD, Pierroz DD, Broqua P, White RB, Pedrazzini T, Aubert ML. Chronic administration of neuropeptide Y into the lateral ventricle of C57BL/6J male mice produces an obesity syndrome including hyperphagia, hyperleptinemia, insulin resistance, and hypogonadism. Mol Cell Endocrinol 2001;185: 195-204.
(38.) Erickson JC, Hollopeter G, Palmiter RD. Attenuation of the obesity syndrome of ob/ob mice by the loss of neuropeptide Y. Science 1996;274:1704-7.
(39.) Mercer RE, Chee MJ, Colmers WF. The role of NPY in hypothalamic mediated food intake. Front Neuroendocrinol 2011;32:398-415.
(40.) Galas L, Tonon MC, Beaujean D, Fredriksson R, Larhammar D, Lihrmann I, et al. Neuropeptide Y inhibits spontaneous alpha-melanocyte-stimulating hormone (alpha-MSH)release via a Y(5) receptor and suppresses thyrotropin-releasing hormone-induced alpha-MSH secretion via a Y(1) receptor in frog melanotrope cells. Endocrinol 2002;143:1686-94.
(41.) Krashes MJ, Shah BP, Koda S, Lowell BB. Rapid versus delayed stimulation of feeding by the endogenously released AgRP neuron mediators GABA, NPY, and AgRP. Cell Metab 2013;18:588-95.
(42.) Wu Q, Boyle MP, Palmiter RD. Lossof GABAergic signaling by AgRP neuronsto the parabrachial nucleus leads to starvation. Cell 2009;137:1225-34.
(43.) Wu Q, Clark MS, Palmiter RD. Deciphering a neuronal circuit that mediates appetite. Nature 2012;483: 594-7.
(44.) Kishi T, Aschkenasi CJ, Lee CE, Mountjoy KG, Saper CB, Elmquist JK. Expression of melanocortin 4 receptor mRNA in the central nervous system of the rat. J Comp Neurol 2003;457:213-35.
(45.) Bonnefond A, Raimondo A, Stutzmann F, Ghoussaini M, Ramachandrappa S, Bersten DC, et al. Loss-of-function mutations in SIM1 contribute to obesity and Prader-Willi-like features. J Clin Invest 2013;123: 3037-41.
(46.) Ramachandrappa S, Raimondo A, Cali AM, Keogh JM, Henning E, Saeed S, Thompson A, et al. Rare variants in single-minded 1 (SIM1) are associated with severe obesity. J Clin Invest 2013;123:3042-50.
(47.) Shah BP, Vong L, Olson DP, Koda S, Krashes MJ, Ye C, et al. MC4R-expressing glutamatergic neurons in the paraventricular hypothalamus regulate feeding and are synaptically connected to the parabrachial nucleus. Proc Natl Acad Sci USA 2014;111:13193-8.
(48.) Sabatier N, Caquineau C, Dayanithi G, Bull P, Douglas AJ, Guan XM, Jiang M, et al. Alpha-melanocyte stimulating hormone stimulates oxytocin release from the dendrites of hypothalamic neurons while inhibiting oxytocin release from their terminals in the neurohypophysis. J Neurosci 2003;23:10351-8.
(49.) Olson BR, Drutarosky MD, Chow MS, Hruby VJ, Stricker EM, Verbalis JG. Oxytocin and an oxytocin agonist administered centrally decrease food intake in rats. Peptides 1991;12:113-8.
(50.) Camerino C. Low sympathetic tone and obese phenotype in oxytocin-deficient mice. Obesity 2009;17: 980-4.
(51.) Bundzikova J, Pirnik Z, Zelena D, Mikkelsen JD, Kiss A. Response of substances co-expressed in hypothalamic magnocellular neurons to osmotic challenges in normal and Brattleboro rats. Cell Mol Neurobiol 2008;28:1033-47.
(52.) Blevins JE, Schwartz MW, Baskin DG. Evidence that paraventricular nucleus oxytocin neurons link hypothalamic leptin action to caudal brain stem nuclei control ling meal size. Am J Physiol Regul Integr Comp Physiol 2004;287:R87-96.
(53.) Noble EE, Billington CJ, Kotz CM, Wang C. Oxytocin in the ventromedial hypothalamic nucleus reduces feeding and acutely increases energy expenditure. Am J Physiol Regul Integr Comp Physiol 2014;307:R737-45.
(54.) Zhang G, Bai H, Zhang H, Dean C, Wu Q, Li J, et al. Neuropeptide exocytosis involving synaptotagmin-4 and oxytocin in hypothalamic programming of body weight and energy balance. Neuron 2011;69:523-35.
(55.) OttV, Finlayson G, LehnertH, Heitmann B, Heinrichs M, Born J, Hallschmid M. Oxytocin reduces reward driven food intake in humans. Diabetes 2013;62: 3418-25.
(56.) Xu B, Goulding EH, Zang K, Cepoi D, Cone RD, Jones KR, et al. Brain-derived neurotrophic factor regulates energy balance downstream of melanocortin-4 receptor. Nature Neurosci 2003;6:736 -42.
(57.) Lyons WE, Mamounas LA, Ricaurte GA, Coppola V, Reid SW, Bora SH, et al. Brain-derived neurotrophic factor deficient mice develop aggressiveness and hyperphagia in conjunction with brain serotonergic abnormalities. Proc Natl Acad Sci USA 1999;96:15239 -44.
(58.) Gray J, Yeo GS, Cox JJ, Morton J, Adlam AL, Keogh JM, et al. Hyperphagia, severe obesity, impaired cognitive function, and hyperactivity associated with functional loss of one copy of the brain-derived neurotrophic factor (BDNF) gene. Diabetes 2006;55:3366-71.
(59.) Gray J, Yeo G, Hung C, Keogh J, Clayton P, Banerjee K, et al. Functional characterization of human NTRK2 mutations identified in patients with severe early-onset obesity. Int J Obes 2007;31:359-64.
(60.) Voss-Andreae A, Murphy JG, Ellacott KL, Stuart RC, Nillni EA, Cone RD, Fan W. Role of the central melanocortin circuitry in adaptive thermogenesis of brown adipose tissue. Endocrinology 2007;148:1550-60.
(61.) Elias CF, Aschkenasi C, Lee C, Kelly J, Ahima RS, Bjorbaek C, et al. Leptin differentially regulates NPY and POMC neurons projecting to the lateral hypothalamic area. Neuron 1999;23:775-86.
(62.) de Lecea L, Kilduff TS, Peyron C, Gao X, Foye PE, Danielson PE, et al. The hypocretins: Hypothalamus-specific peptides with neuroexcitatory activity. Proc Natl Acad Sci USA 1998;95:322-7.
(63.) Funahashi H, Yamada S, Kageyama H, Takenoya F, Guan JL, Shioda S. Co-existence of leptin- and orexin-receptors in feeding-regulating neurons in the hypothalamic arcuate nucleus-a triple labeling study. Peptides 2003;24:687-94.
(64.) van den Top M, Lee K, WhymentAD, Blanks AM, Spanswick D. Orexigen-sensitive NPY/AgRP pacemaker neurons in the hypothalamic arcuate nucleus. Nature Neurosci 2004;7:493-4.
(65.) Burdakov D, Liss B, Ashcroft FM. Orexin excites GABAergic neurons of the arcuate nucleus by activating the sodium-calcium exchanger. J Neurosci 2003;23:4951-7.
(66.) Hara J, Beuckmann CT, Nambu T, Willie JT, Chemelli RM, Sinton CM, et al. Genetic ablation of orexin neurons in mice results in narcolepsy, hypophagia, and obesity. Neuron 2001;30:345-54.
(67.) Gonzalez JA, Jensen LT, Iordanidou P, Strom M, Fugger L, Burdakov D. Inhibitory interplay between orexin neurons and eating. Curr Biol 2016;26:2486-91.
(68.) Burdakov D, Luckman SM, Verkhratsky A. Glucose-sensing neurons of the hypothalamus. Philos Trans Royal Soc B 2005;360:2227-35.
(69.) Qu D, Ludwig DS, Gammeltoft S, Piper M, Pelley-mounter MA, Cullen MJ, et al. A role for melanin-concentrating hormone in the central regulation of feeding behaviour. Nature 1996;380:243-7.
(70.) Shimada M, Tritos NA, Lowell BB, Flier JS, Maratos-Flier E. Mice lacking melanin-concentrating hormone are hypophagic and lean. Nature 1998;396:670-4.
(71.) Morens C, Norregaard P, Receveur JM, van Dijk G, Scheurink AJ. Effects of MCH and a MCH1-receptor antagonist on (palatable)food and water intake. Brain Res 2005;1062:32-8.
(72.) de Araujo IE, Oliveira-Maia AJ, Sotnikova TD, Gainetdinov RR, Caron MG, Nicolelis MA, Simon SA. Food reward in the absence of taste receptor signaling. Neuron 2008;57:930-41.
(73.) Domingos AI, Sordillo A, Dietrich MO, Liu ZW, Tellez LA, Vaynshteyn J, et al. Hypothalamic melanin concentrating hormone neurons communicate the nutrient value of sugar. eLife 2013;2:e01462.
(74.) Rehfeld JF. Cholecystokinin-from local gut hormone to ubiquitous messenger. Front Endocrinol 2017;8:47.
(75.) Morris MJ, Chen H, Watts R, Shulkes A, Cameron-Smith D. Brain neuropeptide Y and CCK and peripheral adipokine receptors: temporal response in obesity induced by palatable diet. Int J Obes 2008;32:249-58.
(76.) Saito A, Williams JA, Goldfine ID. Alterations in brain cholecystokinin receptors after fasting. Nature 1981; 289:599-600.
(77.) Krane IM, Naylor SL, Helin-Davis D, Chin WW, Spindel ER. Molecular cloning of cDNAs encoding the human bombesin-like peptide neuromedin B. Chromosomal localization and comparison to cDNAs encoding its amphibian homolog ranatensin. J Biol Chem 1988;263: 13317-23.
(78.) Sayegh AI. The role of bombesin and bombesin-related peptides in the short-term control of food intake. Prog Mol Biol Transl Sci 2013;114:343-70.
(79.) Ladenheim EE, Behles RR, Bi S, Moran TH. Gastrin-releasing peptide messenger ribonucleic acid expression in the hypothalamic paraventricular nucleus is altered by melanocortin receptor stimulation and food deprivation. Endocrinology 2009;150:672-8.
(80.) Brommage R, Desai U, Revelli JP, Donoviel DB, Fontenot GK, Dacosta CM, et al. High-throughput screening of mouse knockout lines identifies true lean and obese phenotypes. Obesity 2008;16:2362-7.
(81.) Allison MB, Patterson CM, Krashes MJ, Lowell BB, Myers MG, Jr., Olson DP. TRAP-seq defines markers for novel populations of hypothalamic and brainstem LepRb neurons. Mol Metab 2015;4:299-309.
(82.) Bechtold DA, Luckman SM. The role of RFamide peptides in feeding. J Endocrinol 2007;192:3-15.
(83.) Bjursell M, Lenneras M, Goransson M, Elmgren A, Bohlooly YM. GPR10 deficiency in mice results in altered energy expenditure and obesity. Biochem Biophys Res Comm 2007;363:633-8.
(84.) Lawrence CB, Liu YL, Stock MJ, Luckman SM. Anorectic actions of prolactin-releasing peptide are mediated by corticotropin-releasing hormone receptors. Am J Physiol Regul Integr Comp Physiol 2004;286: R101-7.
(85.) Dodd GT, Worth AA, Nunn N, Korpal AK, Bechtold DA, Allison MB, Myers MG, Jr., et al. The thermogenic effect of leptin is dependent on a distinct population of prolactin-releasing peptide neurons in the dorsomedial hypothalamus. Cell Metab 2014;20:639-49.
(86.) Clemmensen C, Finan B, Fischer K, Tom RZ, Legutko B, Sehrer L, et al. Dual melanocortin-4 receptor and GLP-1 receptor agonism amplifies metabolic benefits in diet-induced obese mice. EMBO Mol Med 2015;7:288-98.
(87.) Farooqi IS, Keogh JM, Yeo GS, Lank EJ, Cheetham T, O'Rahilly S. Clinical spectrum of obesity and mutations in the melanocortin 4 receptor gene. New Engl J Med 2003;348:1085-95.
(88.) Yeo GS, Connie Hung CC, Rochford J, Keogh J, Gray J, Sivaramakrishnan S, et al. A de novo mutation affecting human TrkB associated with severe obesity and developmental delay. Nat Neurosci 2004;7:1187-9.
(89.) Walters RG, Jacquemont S, Valsesia A, de Smith AJ, Martinet D, Andersson J, et al. A new highly penetrant form of obesity due to deletions on chromosome 16p11.2. Nature 2010;463:671-5.
(90.) Sahoo T, del Gaudio D, German JR, Shinawi M, Peters SU, Person RE, et al. Prader-Willi phenotype caused by paternal deficiency for the HBII-85 C/D boxsmall nucleolar RNA cluster. Nat Genet 2008;40:719 -21.
(91.) Morton GJ, Meek TH, Schwartz MW. Neurobiology of food intake in health and disease. Nat Rev Neurosci 2014;15:367-78.
(92.) Trapp S, Richards JE. The gut hormone glucagon-like peptide-1 produced in brain: is this physiologically relevant? Curr Opin Pharmacol 2013;13:964-9.
Agatha A. van der Klaauw  *
 Department of Clinical Biochemistry, Metabolic Research Laboratories--Institute of Metabolic Science, University of Cambridge, Cambridge, England.
* Address correspondence to the author at: University of Cambridge, Department of Clinical Biochemistry--Metabolic Research Laboratories, Level 4, Wellcome Trust-MRC Institute of Metabolic Science, Box 289, Addenbrooke's Hospital, Cambridge, CB2 0QQ. Fax 1223-762-657; e-mail email@example.com.
Received September 8, 2017; accepted October 18, 2017.
Previously published online at DOI: 10.1373/clinchem.2017.281568 [c]2017 American Association for Clinical Chemistry
 Nonstandard abbreviations: DREADD, designer receptors exclusively activated by designer drugs; ARC, arcuate nucleus; PVN, paraventricular nucleus; VMH, ventromedial hypothalamus; LH, lateral hypothalamus; POMC, proopiomelanocortin; CART, cocaine- and amphetamine-related transcript; NPY, neuropeptide Y; AgRP, agouti-related peptide; SIM 1, single-minded homolog 1; [alpha]-MSH, alpha-melanocyte-stimulating hormone; MCH, melanin-concentrating hormone; BDNF, brain-derived neurotrophic factor; NTS, nucleus of the solitary tract; PBN, parabrachial nucleus.
 Genes: POMC, proopiomelanocortin; PC1, prohormone convertase 1; MC4R, melanocortin 4 receptor; Pcskl, proprotein convertase subtilisin/kexintype 1; SIM1, single-minded family bHLH transcription factor 1; BDNF, brain-derived neurotrophic factor; Pmch, promelanin concentrating hormone; NTRK2, neurotrophic receptor tyrosine kinase 2; SNRPN, small nuclear ribonucleoprotein polypeptide N; SNURF, SNRPN upstream reading frame.
Caption: Fig. 1. Hypothalamic control of energy homeostasis.
Table 1. Human genetic obesity syndromes due to perturbation of neuropeptide synthesis, secretion, orsignaling. Gene name Function Genetics and phenotype Proopiomelanocortin Precursor of Homozygous or compound (POMC) neuropeptides heterozygous complete loss-of- function variants severe phenotype of early-onset obesity, adrenal insufficiency, and red hair Heterozygous loss-of-function mutations in [alpha]-and [beta]-melanocyte-stimulating hormone ([alpha]-and [beta]- MSH) significantly increase obesity risk Proconvertase Impaired POMC Compound heterozygote loss-of- 1 (PCSK1) processing function mutations Phenotype: early-onset obesity, adrenal insufficiency, elevated proinsulin, reactive hypoglycemia Melanocortin-4 Receptor for Homozygous and heterozygous receptor (MC4R) POMC-derived loss of function, severity of neuropeptides obesity dependent on degree of preserved signaling (87) Brain-derived BDNF Haploinsufficiency (de novo neurotrophic chromosomal inversion) (58) factor (BDNF) Severe obesity with hyperphagia Neurotrophic Receptor Heterozygous missense mutations tyrosine kinase for BDNF (88) Early-onset obesity receptor type 2 (TrkB) Single-minded Likely Haploinsufficiency and rare homolog 1 (SIM1) oxytocin heterozygous variants (46, 89) Early-onset obesity, developmental delay Prader-Willi Reduced number Deficiency for one or more syndrome of oxytocin paternally expressed imprinted neurons in PVN transcripts within chromosome in postmortem 15q11-q13, including the studies bicistronic SNURF-SnRpN gene and multiple small nucleolar RNAs (90) Neonatal hypotonia and poor feeding, obesity, hyperphagia, behavioral problems MSH, melanocyte-stimulating hormone; POMC, proopiomelanocortin; PVN, paraventricular nucleus; SNURF, SNRPN upstream readingframe; SNRPN, small nuclear ribonucleopro-tein polypeptide N. Table 2. Overview of neuropeptides discussed. Neuropeptide Action Neuronal circuit Agouti-related Orexigenic AgRP is released by ARC peptide (AgRP) neurons; AgRP is a natural inverse agonist of the MC4R receptor and acts in the PVN, LH, and VMH AgRP neurons also send GABAergic projection to the PBN Neuropeptide Orexigenic NPY colocalizes in the Y (NPY) majority of AgRP-producing neurons and binds to its NPYY1-5 receptors. NPY regulates a-MSH release from POMC neurons through the Y5 receptor Proopiomelanocortin Anorexigenic POMC is cleaved to multiple (POMC) peptide hormones in ARC neurons. a-MSH is a core anorexigenic peptide, which binds to the MC4R that is abundantly produced throughout the hypothalamus. The MC4R, SIM1-positive, glutamatergic neurons in the PVN are critically involved in regulating food intake Cocaine- and Anorexigenic Sites of action include amphetamine-related hypothalamus, but injections transcript (CART) of CART in the nucleus accumbens also inhibits food intake in rodents Oxytocin Anorexigenic Oxytocin is released from magnocellular neurons in the PVN and most likely acts as an anorexigenic neuropeptide through its receptors in the VMH Brain-derived Anorexigenic BDNF is produced in large neurotrophic quantities in the VMH and acts factor (BDNF) downstream of the MC4R Orexin Orexigenic Orexin/producing neurons are located in the LH and regulate wakefulness/arousal. Orexin neurons are synaptically connected to ARC POMC and AgRP neurons Mice lacking orexin develop a phenotype similar to narcolepsy and late-onset obesity despite hypophagia Melanin-concentrating Orexigenic MCH neurons are located in the hormone (MCH) LH and project within the hypothalamus and to reward centers in the brain such as striatum/midbrain. Mice lacking MCH are hypophagic Cholecystokinin (CCK) Anorexigenic Bioactive CCK species are processed in a tissue- specific manner. CCK receptors are found in cortical areas and in the hypothalamus Glucagon-like Anorexigenic Produced by enteroendocrine peptide 1 (GLP1) cells as well as neurons in the NTS. Receptors are produced in the hypothalamus and amygdala Neuromedin B (NMB) Anorexigenic NMB is abundantly produced in the human hypothalamus and signals through the NMB- receptor. ICV NMB inhibits food intake in rodents Gastrin-releasing Anorexigenic GRP is produced in the PVN and peptide (GRP) increases after infusion of the MC4R agonist MTII, suggesting its role is downstream of the MC4R Prolactin-releasing Anorexigenic, PrRP acts through the GPR10. peptide (PrRP) increase in ICV PrRP decreases food intake energy and increases energy expenditure expenditure. PrRP-producing neurons in the DMN are critically involved in leptin's effects on thermogenesis Neuropeptide References Agouti-related (91) peptide (AgRP) Neuropeptide (91) Y (NPY) Proopiomelanocortin (91) (POMC) Cocaine- and (32) amphetamine-related transcript (CART) Oxytocin (48-52) Brain-derived (56-59) neurotrophic factor (BDNF) Orexin (61-64) Melanin-concentrating (69-71) hormone (MCH) Cholecystokinin (CCK) (74-76) Glucagon-like (92) peptide 1 (GLP1) Neuromedin B (NMB) (77-78) Gastrin-releasing (79) peptide (GRP) Prolactin-releasing (83-85) peptide (PrRP) MC4R, melanocortin-4-receptor; PVN, paraventricular nucleus; LH, lateral hypothalamus; VMH, ventromedial hypothalamus; PBN, parabrachial nucleus; ARC-arcuate nucleus; SIM1, single-minded homolog 1; ICV, intracerebroventricular; MTII, melanotan II; GPR10, G-protein coupled 10 receptor; DMN, dorsomedial hypothalamus.
|Printer friendly Cite/link Email Feedback|
|Author:||van der Klaauw, Agatha A.|
|Date:||Jan 1, 2018|
|Previous Article:||Policies to Prevent Obesity and Promote Healthier Diets: A Critical Selective Review.|
|Next Article:||Dairy Consumption and Body Mass Index Among Adults: Mendelian Randomization Analysis of 184802 Individuals from 25 Studies.|