Neuromedin U-induced anorexigenic action is mediated by the corticotropin-releasing hormone receptor-signaling pathway in goldfish

The neuromedin U system: Pharmacological implications for the treatment of obesity and binge eating behavior

Neuromedin U (NMU) is a bioactive peptide produced in the gut and in the brain, with a role in multiple physiological processes. NMU acts by binding and activating two G protein coupled receptors (GPCR), the NMU receptor 1 (NMU-R1), which is predominantly expressed in the periphery, and the NMU receptor 2 (NMU-R2), mainly expressed in the central nervous system (CNS). In the brain, NMU and NMU-R2 are consistently present in the hypothalamus, commonly recognized as the main "feeding center". Considering its distribution pattern, NMU revealed to be an important neuropeptide involved in the regulation of food intake, with a powerful anorexigenic ability. This has been observed through direct administration of NMU and by studies using genetically modified animals, which revealed an obesity phenotype when the NMU gene is deleted. Thus, the development of NMU analogs or NMU-R2 agonists might represent a promising pharmacological strategy to treat obese individuals. Furthermore, NMU has been demonstrated to influence the non-homeostatic aspect of food intake, playing a potential role in binge eating behavior. This review aims to discuss and summarize the current literature linking the NMU system with obesity and binge eating behavior, focusing on the influence of NMU on food intake and the neuronal mechanisms underlying its anti-obesity properties. Pharmacological strategies to improve the pharmacokinetic profile of NMU will also be reported.

Distribution of neuromedin U (NMU)-like immunoreactivity in the goldfish brain, and effect of intracerebroventricular administration of NMU on emotional behavior in goldfish

Neuromedin U (NMU) is a multifunctional neuropeptide implicated in regulation of smooth muscle contraction in the circulatory and digestive systems, energy homeostasis and the stress response, but especially food intake in vertebrates. Recent studies have indicated the possible involvement of NMU in the regulation of psychomotor activity in rodents. We have identified four cDNAs encoding three putative NMU variants (NMU-21, -25 and -38) from the goldfish brain and intestine. Recently, we have also purified these NMUs and the truncated C-terminal form NMU-9 from these tissues, and demonstrated their anorexigenic action in goldfish. However, there is no information on the brain localization of NMU-like immunoreactivity and the psychophysiological roles of NMU in fish. Here, we investigated the brain distribution of NMU-like immunoreactivity and found that it was localized throughout the fore- and mid-brains. We subsequently examined the effect of intracerebroventricular (ICV) administration of NMU-21, which is abundant only in the brain on psychomotor activity in goldfish. As goldfish prefer the lower to the upper area of a tank, we developed an upper/lower area preference test in a tank for evaluating the psychomotor activity of goldfish using a personal tablet device without an automatic behavior-tracking device. ICV administration of NMU-21 at 10 pmol g^-1 body weight (BW) prolonged the time spent in the upper area of the tank, and this action mimicked that of ICV administration of the central-type benzodiazepine receptor (CBR) agonist tofisopam at 100 pmol g^-1 BW. These results suggest that NMU-21 potently induces anxiolytic-like action in the goldfish brain.

Appetite regulating genes in zebrafish gut; a gene expression study

The underlying molecular pathophysiology of feeding disorders, particularly in peripheral organs, is still largely unknown. A range of molecular factors encoded by appetite-regulating genes are already described to control feeding behaviour in the brain. However, the important role of the gastrointestinal tract in the regulation of appetite and feeding in connection to the brain has gained more attention in the recent years. An example of such inter-organ connection can be the signals mediated by leptin, a key regulator of body weight, food intake and metabolism, with conserved anorexigenic effects in vertebrates. Leptin signals functions through its receptor (lepr) in multiple organs, including the brain and the gastrointestinal tract. So far, the regulatory connections between leptin signal and other appetite-regulating genes remain unclear, particularly in the gastrointestinal system. In this study, we used a zebrafish mutant with impaired function of leptin receptor to explore gut expression patterns of appetite-regulating genes, under different feeding conditions (normal feeding, 7-day fasting, 2 and 6-hours refeeding). We provide evidence that most appetite-regulating genes are expressed in the zebrafish gut. On one hand, we did not observed significant differences in the expression of orexigenic genes (except for hcrt) after changes in the feeding condition. On the other hand, we found 8 anorexigenic genes in wild-types (cart2, cart3, dbi, oxt, nmu, nucb2a, pacap and pomc), as well as 4 genes in lepr mutants (cart3, kiss1, kiss1r and nucb2a), to be differentially expressed in the zebrafish gut after changes in feeding conditions. Most of these genes also showed significant differences in their expression between wild-type and lepr mutant. Finally, we observed that impaired leptin signalling influences potential regulatory connections between anorexigenic genes in zebrafish gut. Altogether, these transcriptional changes propose a potential role of leptin signal in the regulation of feeding through changes in expression of certain anorexigenic genes in the gastrointestinal tract of zebrafish.

Transcriptional study of appetite regulating genes in the brain of zebrafish (Danio rerio) with impaired leptin signalling

The hormone leptin is a key regulator of body weight, food intake and metabolism. In mammals, leptin acts as an anorexigen and inhibits food intake centrally by affecting the appetite centres in the hypothalamus. In teleost fish, the regulatory connections between leptin and other appetite-regulating genes are largely unknown. In the present study, we used a zebrafish mutant with a loss of function leptin receptor to investigate brain expression patterns of 12 orexigenic and 24 anorexigenic genes under different feeding conditions (normal feeding, 7-day fasting, 2 and 6-hours refeeding). Expression patterns were compared to wild-type zebrafish, in order to identify leptin-dependent differentially expressed genes under different feeding conditions. We provide evidence that the transcription of certain orexigenic and anorexigenic genes is influenced by leptin signalling in the zebrafish brain. We found that the expression of orexigenic genes was not affected by impaired leptin signalling under normal feeding conditions; however, several orexigenic genes showed increased transcription during fasting and refeeding, including agrp, apln, galr1a and cnr1. This suggests an inhibitory effect of leptin signal on the transcription of these orexigenic genes during short-term fasting and refeeding in functional zebrafish. Most pronounced effects were observed in the group of anorexigenic genes, where the impairment of leptin signalling resulted in reduced gene expression in several genes, including cart family, crhb, gnrh2, mc4r, pomc and spx, in the control group. This suggests a stimulatory effect of leptin signal on the transcription of these anorexigenic genes under normal feeding condition. In addition, we found multiple gain and loss in expression correlations between the appetite-regulating genes, in zebrafish with impaired leptin signal, suggesting the presence of gene regulatory networks downstream of leptin signal in zebrafish brain. The results provide the first evidence for the effects of leptin signal on the transcription of various appetite-regulating genes in zebrafish brain, under different feeding conditions. Altogether, these transcriptional changes suggest an anorexigenic role for leptin signal, which is likely to be mediated through distinct set of appetite-regulating genes under different feeding conditions.

Identification and immunoregulatory function of neuromedin U (Nmu) in the Japanese pufferfish Takifugu rubripes

In this study, immunoregulatory function of neuromedin U (Nmu) in the teleost fish Fugu (Takifugu rubripes) was characterized. Three splicing variants of nmu mRNA encoding preproNMUs consisting of 164 (Nmu1), 139 (Nmu2), and 129 (Nmu3) amino acid residues were found in Fugu.The biologically active C-terminal region of Fugu Nmu showed high homology among fish and other vertebrate NMUs. The genomic organization of Fugu nmu differed from those of zebrafish and mammals. However, in phylogenetic analysis, Fugu Nmu formed a cluster with NMUs of other vertebrates, in addition to neuromedin S. The splicing variants of mRNA were identified in various tissues. Nmu-21 and Nmu-9 were purified as endogenous peptides from Fugu intestine. The synthetic Nmu-21 peptide activated phagocytic cells, and elevated the expression of cytokine mRNA in peripheral blood leukocytes.

The Neuroendocrine Regulation of Food Intake in Fish: A Review of Current Knowledge

Fish are the most diversified group of vertebrates and, although progress has been made in the past years, only relatively few fish species have been examined to date, with regards to the endocrine regulation of feeding in fish. In fish, as in mammals, feeding behavior is ultimately regulated by central effectors within feeding centers of the brain, which receive and process information from endocrine signals from both brain and peripheral tissues. Although basic endocrine mechanisms regulating feeding appear to be conserved among vertebrates, major physiological differences between fish and mammals and the diversity of fish, in particular in regard to feeding habits, digestive tract anatomy and physiology, suggest the existence of fish- and species-specific regulating mechanisms. This review provides an overview of hormones known to regulate food intake in fish, emphasizing on major hormones and the main fish groups studied to date.

Behind melanocortin antagonist overexpression in the zebrafish brain: A behavioral and transcriptomic approach

Melanocortin signaling is regulated by the binding of naturally occurring antagonists, agouti-signaling protein (ASIP) and agouti-related protein (AGRP) that compete with melanocortin peptides by binding to melanocortin receptors to regulate energy balance and growth. Using a transgenic model overexpressing ASIP, we studied the involvement of melanocortin system in the feeding behaviour, growth and stress response of zebrafish. Our data demonstrate that ASIP overexpression results in enhanced growth but not obesity. The differential growth is explained by increased food intake and feeding efficiency mediated by a differential sensitivity of the satiety system that seems to involve the cocaine- and amphetamine- related transcript (CART). Stress response was similar in both genotypes. Brain transcriptome of transgenic (ASIP) vs wild type (WT) fish was compared using microarrays. WT females and males exhibited 255 genes differentially expressed (DEG) but this difference was reduced to 31 after ASIP overexpression. Statistical analysis revealed 1122 DEG when considering only fish genotype but 1066 and 981 DEG when comparing ASIP males or females with their WT counterparts, respectively. Interaction between genotype and sex significantly affected the expression of 97 genes. Several neuronal systems involved in the control of food intake were identified which displayed a differential expression according to the genotype of the fish that unravelling the flow of melanocortinergic information through the central pathways that controls the energy balance. The information provided herein will help to elucidate new central systems involved in control of obesity and should be of invaluable use for sustaining fish production systems.

Regulation of feeding behavior and psychomotor activity by corticotropin-releasing hormone (CRH) in fish

Corticotropin-releasing hormone (CRH) is a hypothalamic neuropeptide belonging to a family of neuropeptides that includes urocortins, urotensin I, and sauvagine in vertebrates. CRH and urocortin act as anorexigenic factors for satiety regulation in fish. In a goldfish model, intracerebroventricular (ICV) administration of CRH has been shown to affect not only food intake, but also locomotor and psychomotor activities. In particular, CRH elicits anxiety-like behavior as an anxiogenic neuropeptide in goldfish, as is the case in rodents. This paper reviews current knowledge of CRH and its related peptides derived from studies of teleost fish, as representative non-mammals, focusing particularly on the role of the CRH system, and examines its significance from a comparative viewpoint.

The role of neuromedin U during inflammatory response in the common carp

In the current study, we cloned and characterized the neuromedin U (NMU) gene from the common carp Cyprinus carpio L., and identified its participation in immune responses in the teleost. Five isoforms of the preproNMU genes were generated by alternative splicing and isolated from carp. The longest form of the carp preproNMU1 (isoform 1) cDNA was composed of 803 bp, and contained an 18 bp 5'-UTR, a 212 bp 3'-UTR and a 573 bp open reading frame, which translates into a peptide comprising 190 amino acid (aa) residues. The remaining carp preproNMU isoforms were composed of 175 (preproNMU2), 158 (preproNMU3), 150 (preproNMU4) and 133 (preproNMU5) aa residues. Isoforms 1-3 contained four processing signals (KR or RR), while isoforms 4 and 5 contained only two processing signals. High homology was demonstrated among fish and other vertebral NMU at the biologically active C-terminal region (aa position 175-182). Carp preproNMU transcript variants were identified in various tissues, and the expression pattern has been shown to change depending on feeding status. Moreover, it was shown that the expression of preproNMU3 and preproNMU5 was increased following treatment with bacterial or viral mimics. Finally, we investigated the functional aspect of carp NMU using a synthetic NMU peptide. The peptide was found to increase the expression of inflammation-related cytokine genes in intestinal cells within 1 h of treatment. In addition, the activation of phagocytic cells was also stimulated by the NMU peptide. The discovery of NMU in carp allows for a further understanding of immune regulation by biologically active substances.

Inhibitory effect of corticotropin-releasing factor on food intake in the bullfrog, Aquarana catesbeiana

Corticotropin-releasing factor (CRF) and CRF-related peptides exert hypophysiotropic and anorexigenic effects in mammals and teleost fish. In anuran amphibians, CRF acts as a potent stimulator of thyrotropin release from the pituitary. According to our recent study, CRF also acts as an anorexigenic factor for the cessation of food intake in the metamorphosing bullfrog larvae. However, the anorexigenic action of CRF has not been confirmed in adult bullfrogs. In this context, we examined the effect of feeding status on the expression level of the CRF transcript in the hypothalamus of the adult bullfrog. Levels of CRF mRNA in the hypothalami from bullfrogs fasted for 7 days were lower than in those from the bullfrogs that had been fed normally. Subsequently, we developed a method for measuring food intake in adult bullfrogs, and then investigated the effect of CRF on their food consumption in these animals. Intracerebroventricular (ICV) administration of CRF at 1 and 10pmol/g body weight (BW) induced a significant decrease of food intake during 60min. The CRF-induced anorexigenic action was blocked by treatment with a CRF receptor 1/CRF receptor 2 antagonist, α-helical CRF((9-41)), at 100pmol/g BW. These results provide direct evidence for the inhibitory effect of CRF on food intake, and suggest the involvement of CRF in the regulation of feeding through a CRF receptor-signaling pathway in the adult bullfrog.

Isolation and characterisation of two cDNAs encoding the neuromedin U receptor from goldfish brain

Intracerebroventricular administration of neuromedin U (NMU) exerts an anorexigenic effect in a goldfish model. However, little is known about the NMU receptor and its signalling system in fish. In the present study, we isolated and cloned two cDNAs encoding different proteins comprising 429 and 388 amino acid residues from the goldfish brain based on the nucleotide sequences of human NMU receptor 1 (NMU-R1) and receptor 2 (NMU-R2). Hydropathy and phylogenetic analyses suggested that these two proteins were orthologues of NMU-R1 and -R2 of goldfish. We established two human embryonic kidney 293 cell lines stably expressing putative NMU-R1 and -R2, respectively, and showed that NMU induced an increase in intracellular calcium concentration ([Ca(2+)](i)) in these cells. We examined the presence of NMU-R1 and -R2 in the goldfish brain by western blotting analysis using affinity-purified antisera raised against peptide fragments derived from these receptors. NMU-R1-specific and NMU-R2-specific antisera detected a 49-kDa and 45-kDa immunopositive bands, respectively, in the brain extract. The mass of each band corresponded to that of the deduced respective primary structures. Reverse transcriptase-polymerase chain reaction analysis showed that NMU-R1 and -R2 transcripts were detected in several tissues. In particular, both mRNAs were strongly expressed in the goldfish brain. By contrast, NMU-R2 mRNA was also expressed in the gut. These results indicate for the first time that NMU-R orthologues exist in goldfish, and suggest physiological roles of NMU and its receptor system in fish.

Gonadotropin-releasing hormone II (GnRH II) mediates the anorexigenic actions of α-melanocyte-stimulating hormone (α-MSH) and corticotropin-releasing hormone (CRH) in goldfish

Intracerebroventricular (ICV) administration of gonadotropin-releasing hormone II (GnRH II), which plays a crucial role in the regulation of reproduction in vertebrates, markedly reduces food intake in goldfish. However, the neurochemical pathways involved in the anorexigenic action of GnRH II and its interaction with other neuropeptides have not yet been identified. Alpha-melanocyte-stimulating hormone (α-MSH), corticotropin-releasing hormone (CRH) and CRH-related peptides play a major role in feeding control as potent anorexigenic neuropeptides in goldfish. However, our previous study has indicated that the GnRH II-induced anorexigenic action is not blocked by treatment with melanocortin 4 receptor (MC4R) and CRH receptor antagonists. Therefore, in the present study, we further examined whether the anorexigenic effects of α-MSH and CRH in goldfish could be mediated through the GnRH receptor neuronal pathway. ICV injection of the MC4R agonist, melanotan II (80 pmol/g body weight; BW), significantly reduced food intake, and its anorexigenic effect was suppressed by ICV pre-administration of the GnRH type I receptor antagonist, antide (100 pmol/gBW). The CRH-induced (50 pmol/gBW) anorexigenic action was also blocked by treatment with antide. ICV injection of CRH (50 pmol/gBW) induced a significant increase of the GnRH II mRNA level in the hypothalamus, while ICV injection of melanotan II (80 pmol/gBW) had no effect on the level of GnRH II mRNA. These results indicate that, in goldfish, the anorexigenic actions of α-MSH and CRH are mediated through the GnRH type I receptor-signaling pathway, and that the GnRH II system regulates feeding behavior.

The anorexigenic effect of cholecystokinin octapeptide in a goldfish model is mediated by the vagal afferent and subsequently through the melanocortin- and corticotropin-releasing hormone-signaling pathways

We have been extensively investigating the mechanisms by which neuropeptides regulate feeding behavior by using a goldfish (Carassius auratus) model. In this species, the anorexigenic action of melanocortin peptide is centrally mediated via the corticotropin-releasing hormone (CRH)/CRH receptor neuronal system, whereas sulfated cholecystokinin octapeptide (CCK-8s) is involved in the appetite regulation as a peripheral anorexigenic factor. The aim of the present study was to identify the mechanism of the anorexigenic effect of peripherally injected CCK-8s, which has not yet been identified in goldfish. Co-administration of capsaicin, a neurotoxin that destroys primary sensory afferents, at 100 nmol/g BW, blocked the anorexigenic action of intraperitoneally injected CCK-8s (100 pmol/g BW), whereas the anorexigenic action of intracerebroventricularly injected CCK-8s (5 pmol/g BW) was not blocked by co-administration of capsaicin. Pre-treatment with a specific CRH receptor antagonist, α-helical CRH((9-41)), attenuated the anorexigenic action of CCK-8s. The expression level of CRH mRNA in the diencephalic tissue of the CCK-8s-injected group was not changed, but the level of proopiomelanocortin mRNA was significantly increased at 1h after treatment. Therefore, we have identified for the first time that the reduction of appetite induced by peripherally injected CCK-8s in goldfish appears to be mediated by the vagal afferent and subsequently through the melanocortin- and corticotropin-releasing hormone-signaling pathways.

Changes in the distribution of corticotropin-releasing factor (CRF)-like immunoreactivity in the larval bullfrog brain and the involvement of CRF in the cessation of food intake during metamorphosis

In submammalian vertebrates, corticotropin-releasing factor (CRF) acts as an anorexigenic neuropeptide as well as a potent stimulator of corticotropin and thyrotropin release from the pituitary. As a step for demonstrating the involvement of CRF in the feeding regulation of anuran larvae, which are known to stop feeding toward the metamorphic climax, we studied firstly the changes in the distribution of CRF-like immunoreactivity (CRF-LI) in the brain of metamorphosing bullfrog larvae. Neuronal cell bodies showing CRF-LI were invariably present in the thalamic regions throughout larval development. Cells with CRF-LI were also found in the hypothalamus. The number of cells with CRF-LI in the hypothalamus, but not in the thalamus, showed a significant increase as metamorphosis progressed. Immunoreactive nerve fibers were observed mainly in the median eminence, and became abundant as metamorphosis proceeded. The number of cells showing CRF-LI in the hypothalamus as well as the density of immunoreactive fibers in the median eminence decreased at the end of metamorphosis. Secondly, we examined the effect of intracerebroventricular (ICV) injection of CRF on the food intake in the premetamorphic larvae. ICV injection of CRF at 10 pmol/g body weight (BW) induced a significant decrease of food intake during 15 min. The CRF-induced anorexigenic action was blocked by the treatment with a CRF receptor antagonist [alpha-helical CRF(9-41)] at 100 pmol/g BW. The results suggest the involvement of CRF in the accomplishment of metamorphosis through the pituitary and in the feeding restriction that occurs during the later stages of metamorphosis through the central nervous system.

Relationship between alpha-melanocyte-stimulating hormone- and neuropeptide Y-containing neurons in the goldfish hypothalamus

Intracerebroventricular (ICV) injection of alpha-melanocyte-stimulating hormone (alpha-MSH) inhibits, whereas ICV injection of neuropeptide Y (NPY) stimulates food intake in the goldfish. However, there is little information about the functional relationship between alpha-MSH-induced anorexigenic and NPY-induced orexigenic actions in the goldfish. In this study we examined the relationship between alpha-MSH- and NPY-containing neurons in the goldfish hypothalamus to investigate whether these alpha-MSH- and NPY-containing neurons have direct mutual inputs. alpha-MSH- and NPY-like immunoreactivities were distributed throughout the brain, especially in the diencephalon. In particular, alpha-MSH-containing nerve fibers or endings lay in close apposition to NPY-containing neurons in a specific region of the hypothalamus, the nucleus posterioris periventricularis (NPPv). NPY-containing nerve fibers or endings also lay in close apposition to alpha-MSH-containing neurons specifically in the interior part of the nucleus lateralis tuberis (NLTi). We also investigated the effect of ICV injection of melanocortin 4 receptor agonist (melanotan II) at 100 pmol/g body weight (BW), which is enough to suppress food intake, or NPY at 10 pmol/g BW, which is enough to enhance food intake, on expression levels of mRNA for NPY or proopiomelanocortin (POMC) in the hypothalamus. ICV injection of melanotan II and NPY induced a significant decrease in the expression levels for NPY and POMC mRNA, respectively. These observations suggest that alpha-MSH- and NPY-containing neurons share direct mutual inputs in the NPPv and the NLTi of the hypothalamus, and that alpha-MSH and NPY functionally interact or exhibit mutual inhibition to regulate feeding behavior in the goldfish.