Widespread expression of Agouti-related protein (AGRP) in the chicken: a possible involvement of AGRP in regulating peripheral melanocortin systems in the chicken

Regulation of the central melanocortin system on energy balance in mammals and birds

Agouti-related protein/neuropeptide Y (AgRP/NPY) neurons promote feeding, while proopiomelanocortin/cocaine- and amphetamine-regulated transcript (POMC/CART) neurons and melanocortin receptor neurons inhibit feeding; these three types of neurons play vital roles in regulating feeding. The central melanocortin system composed of these neurons is critical for the regulation of food intake and energy metabolism. It regulates energy intake and consumption by activating or inhibiting the activities of AgRP/NPY neurons and POMC/CART neurons and then affects the feeding behaviour of animals to maintain the energy balance. Meanwhile, organisms can also positively or negatively regulate energy homeostasis through the negative feedback of the neuron system. With further studies, understanding of the process and factors involved in the energy balance regulation of mammals and birds can be improved, which will provide a favourable scientific basis to reduce costs and improve meat production in production and breeding.

Mutations in melanocortin-4 receptor: From fish to men

Melanocortin-4 receptor (MC4R), expressed abundantly in the hypothalamus, is a critical regulator of energy homeostasis, including both food intake and energy expenditure. Shortly after the publication in 1997 of the Mc4r knockout phenotypes in mice, including increased food intake and severe obesity, the first mutations in MC4R were reported in humans in 1998. Studies in the subsequent two decades have established MC4R mutation as the most common monogenic form of obesity, especially in early-onset severe obesity. Studies in animals, from fish to mammals, have established the conserved physiological roles of MC4R in all vertebrates in regulating energy balance. Drug targeting MC4R has been recently approved for treating morbid genetic obesity. How the MC4R can be exploited for animal production is highly worthy of active investigation.

Dietary phosphorus level regulates appetite through modulation of gut and hypothalamic expression of anorexigenic genes in broiler chickens

Two experiments were designed to elucidate gut and hypothalamic molecular regulation of appetite by dietary phosphorus (P) concentration in broiler chickens. Birds (192 Cobb-500 broiler chickens) were randomly assigned to 3 experimental diets in experiment 1 (Exp. 1) and 24 broiler chickens were randomly assigned to 3 treatment groups in Exp. 2. Each diet comprised 8 replicate cages, with either 8 birds (Exp. 1) or 1 bird (Exp. 2) per replicate cage. In Exp. 1, diets contained 1.2 (P-deficient), 2.8 (P-marginal) or 4.4 (P-adequate) g/kg non-phytate P (nPP). In Exp. 2, birds fed the P-adequate diet were pair-fed (PF) to the feed consumption levels of birds fed the P-deficient diet. Feed intake and BW gain (P < 0.001) decreased in birds fed the P-deficient diet in Exp. 1. Birds fed the P-deficient diet had similar feed intake and BW gain with PF group fed the P-adequate diet (Exp. 2) but was significantly lower (P < 0.001) than birds fed the P-adequate diets. Sodium-phosphate cotransporter (NaPi-IIb) mRNA was upregulated (P < 0.05) in both experiments. Conversely, cholecystokinin (CCK) mRNA was downregulated (P < 0.01) in birds fed P-deficient diets. Anorexia-related hypothalamic cholecystokinin receptor (CCKAR) and melanocortin receptors (MC3R and MC4R) were upregulated (P < 0.05) in birds fed P-deficient diets, in both experiments. The current data show that dietary P deficiency decreases feed intake in broiler chickens by altering the expression of anorexigenic genes in the gut and hypothalamus of broiler chickens.

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

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

Should I Lay or Should I Grow: Photoperiodic Versus Metabolic Cues in Chickens

While photoperiod has been generally accepted as the primary if not the exclusive cue to stimulate reproduction in photoperiodic breeders such as the laying hen, current knowledge suggests that metabolism, and/or body composition can also play an influential role to control the hypothalamic-pituitary gonadal (HPG)-axis. This review thus intends to first describe how photoperiodic and metabolic cues can impact the HPG axis, then explore and propose potential common pathways and mechanisms through which both cues could be integrated. Photostimulation refers to a perceived increase in day-length resulting in the stimulation of the HPG. While photoreceptors are present in the retina of the eye and the pineal gland, it is the deep brain photoreceptors (DBPs) located in the hypothalamus that have been identified as the potential mediators of photostimulation, including melanopsin (OPN4), neuropsin (OPN5), and vertebrate-ancient opsin (VA-Opsin). Here, we present the current state of knowledge surrounding these DBPs, along with their individual and relative importance and, their possible downstream mechanisms of action to initiate the activation of the HPG axis. On the metabolic side, specific attention is placed on the hypothalamic integration of appetite control with the stimulatory (Gonadotropin Releasing Hormone; GnRH) and inhibitory (Gonadotropin Inhibitory Hormone; GnIH) neuropeptides involved in the control of the HPG axis. Specifically, the impact of orexigenic peptides agouti-related peptide (AgRP), and neuropeptide Y (NPY), as well as the anorexigenic peptides pro-opiomelanocortin (POMC), and cocaine-and amphetamine regulated transcript (CART) is reviewed. Furthermore, beyond hypothalamic control, several metabolic factors involved in the control of body weight and composition are also presented as possible modulators of reproduction at all three levels of the HPG axis. These include peroxisome proliferator-activated receptor gamma (PPAR-γ) for its impact in liver metabolism during the switch from growth to reproduction, adiponectin as a potential modulator of ovarian development and follicular maturation, as well as growth hormone (GH), and leptin (LEP).

The interaction of MC3R and MC4R with MRAP2, ACTH, α-MSH and AgRP in chickens

The interaction of melanocortin-4 (MC4R) and melanocortin-3 (MC3R) receptors with proopiomelanocortin (POMC)-derived peptides (e.g. α-MSH), agouti-related protein (AgRP) and melanocortin-2 receptor accessory protein 2 (MRAP2) is suggested to play critical roles in energy balance of vertebrates. However, evidence on their interaction in birds remains scarce. Our study aims to reveal their interaction in chickens and the results showed that (1) chicken (c-)MC3R and cMC4R expressed in Chinese hamster ovary (CHO) cells can be activated by α-MSH and ACTH_1-39 equipotently, monitored by a pGL3-CRE-luciferase reporter system; (2) cMC3R and cMC4R, when co-expressed with cMRAP2 (or cMRAP, a cMRAP2 homolog), show increased sensitivity to ACTH treatment and thus likely act as ACTH-preferring receptors, and the interaction between cMC3R/cMC4R and cMRAP2 was demonstrated by co-immunoprecipitation assay; (3) both cMC3R and cMC4R display constitutive activity when expressed in CHO cells, as monitored by dual-luciferase reporter assay, and cMRAP2 (and cMRAP) can modulate their constitutive activity; (4) AgRP inhibits the constitutive activity of cMC3R/cMC4R, and it also antagonizes ACTH/α-MSH action on cMC4R/cMC3R, indicating that AgRP functions as the inverse agonist and antagonist for both receptors. These findings, together with the co-expression of cMC4R, cMC3R, cMRAP2, cAgRP and cPOMC in chicken hypothalamus detected by quantitative real-time PCR, suggest that within the hypothalamus, α-MSH/ACTH, AgRP and MRAP2 may interact at the MC4R(/MC3R) interface to control energy balance. Furthermore, our data provide novel proof for the involvement of MRAP2 (and MRAP) in fine-tuning the constitutive activity and ligand sensitivity and selectivity of both MC3R and MC4R in vertebrates.

The differential expression of MC1R regulators in dorsal and ventral quail plumages during embryogenesis: Implications for plumage pattern formation

Melanin pigmentation patterns are ubiquitous in animals and function in crypsis, physical protection, thermoregulation and signalling. In vertebrates, pigmentation patterns formed over large body regions as well as within appendages (hair/feathers) may be due to the differential distribution of pigment producing cells (melanocytes) and/or regulation of the melanin synthesis pathway. We took advantage of the pigmentation patterns of Japanese quail embryos (pale ventrum and patterned feathers dorsally) to explore the role of genes and their transcripts in regulating the function of the melanocortin-1-receptor (MC1R) via 1. activation: pro-opiomelanocortin (POMC), endoproteases prohormone convertase 1 (PC1) and 2 (PC2), and 2. inhibition—agouti signaling and agouti-related protein (ASIP and AGRP, respectively). Melanocytes are present in all feather follicles at both 8 and 12 days post-fertilisation (E8/E12), so differential deposition of melanocytes is not responsible for pigmentation patterns in embryonic quail. POMC transcripts expressed were a subset of those found in chicken and POMC expression within feather follicles was strong. PC1 was not expressed in feather follicles. PC2 was strongly expressed in all feather follicles at E12. ASIP transcript expression was variable and we report four novel ASIP transcripts. ASIP is strongly expressed in ventral feather follicles, but not dorsally. AGRP expression within feather follicles was weak. These results demonstrate that the pale-bellied quail phenotype probably involves inhibition of MC1R, as found previously. However, quail may require MC1R activation for eumelanogenesis in dorsal feathers which may have important implications for an understanding of colour pattern formation in vertebrates.

MC1R variants affect the expression of melanocortin and melanogenic genes and the association between melanocortin genes and coloration

The melanocortin-1 receptor (MC1R) gene influences coloration by altering the expression of genes acting downstream in the melanin synthesis. MC1R belongs to the melanocortin system, a genetic network coding for the ligands that regulate MC1R and other melanocortin receptors controlling different physiological and behavioural traits. The impact of MC1R variants on these regulatory melanocortin genes was never considered, even though MC1R mutations could alter the influence of these genes on coloration (e.g. by decreasing MC1R response to melanocortin ligands). Using barn owl growing feathers, we investigated the differences between MC1R genotypes in the (co)expression of six melanocortin and nine melanogenic-related genes and in the association between melanocortin gene expression and phenotype (feather pheomelanin content). Compared to the MC1R rufous allele, responsible for reddish coloration, the white allele was not only associated with an expected lower expression of melanogenic-related genes (TYR, TYRP1, OCA2, SLC45A2, KIT, DCT) but also with a lower MC1R expression and a higher expression of ASIP, the MC1R antagonist. More importantly, the expression of PCSK2, responsible for the maturation of the MC1R agonist, α-melanocyte-stimulating hormone, was positively related to pheomelanin content in MC1R white homozygotes but not in individuals carrying the MC1R rufous allele. These findings indicate that MC1R mutations not only alter the expression of melanogenic-related genes but also the association between coloration and the expression of melanocortin genes upstream of MC1R. This suggests that MC1R mutations can modulate the regulation of coloration by the pleiotropic melanocortin genes, potentially decoupling the often-observed associations between coloration and other phenotypes.

Pharmacologic analyses of four chicken melanocortin-4 receptor mutations

The melanocortin-4 receptor (MC4R) is a critical regulator of mammalian food intake and energy expenditure, with receptor activation resulting in decreased food intake and increased energy expenditure. Recently, studies on role of MC4R in regulation of food intake have been extended to other species, such as chicken. Functional study of mutant MC4Rs is important in proving the causal link between MC4R mutation and production traits. Herein, we cloned chicken MC4R (cMC4R) complementary DNA and generated 4 mutant cMC4Rs (Q18H, G21R, S76L, and L299P) by site-directed mutagenesis and measured their expression by flow cytometry. Pharmacologic characteristics were analyzed with binding and signaling assays using 3 agonists. We showed that G21R had decreased cell surface and total expression (P < 0.05), whereas the other 3 mutants had similar total and cell surface expression levels as wild-type cMC4R. The 4 mutants had either decreased (Q18H, G21R, S76L; P < 0.05) or no (L299P) binding to radiolabeled [Nle(4), D-Phe(7)]-α-melanocyte-stimulating hormone (MSH). In signaling assays, Q18H was constitutively active. Q18H, G21R, and S76L had decreased responses to α-MSH stimulation (P < 0.05). L299P had decreased basal and ligand-stimulated signaling (P < 0.01). Nle(4), D-Phe(7)-MSH was the most potent agonist for cMC4R and therefore would be better suited for further in vivo studies. We conclude that the cloned cMC4R was a functional receptor and provided detailed functional data for these mutations, contributing to a better understanding of cMC4R variants associated with production traits.

Regulation of the avian central melanocortin system and the role of leptin

The avian central melanocortin system is well conserved between birds and mammals in terms of the component genes, the localisation of their expression in the hypothalamic arcuate nucleus, the effects on feeding behaviour of their encoded peptides and the sensitivity of agouti-related protein (AGRP) and pro-opiomelanocortin (POMC) gene expression to changes in energy status. Our recent research has demonstrated that AGRP gene expression precisely differentiates between broiler breeder hens with different histories of chronic food restriction and refeeding. We have also shown that the sensitivity of AGRP gene expression to loss of energy stores is maintained even when food intake has been voluntarily reduced in chickens during incubation and in response to a stressor. However, the similarity between birds and mammals does not appear to extend to the way AGRP and POMC gene expression are regulated. In particular, the preliminary evidence from the discovery of the first avian leptin (LEP) genes suggests that LEP is more pleiotropic in birds and may not even be involved in regulating energy balance. Similarly, ghrelin exerts inhibitory, rather than stimulatory, effects on food intake. The fact that the importance of these prominent long-term regulators of AGRP and POMC expression in mammals appears diminished in birds suggests that the balance of regulatory inputs in birds may have shifted to more short-term influences such as the tone of cholecystokinin (CCK) signalling. This is likely to be related to the different metabolic fuelling required to support flight.

Signaling stress? An analysis of phaeomelanin-based plumage color and individual corticosterone levels at two temporal scales in North American barn swallows, Hirundo rustica erythrogaster

Sexually selected traits confer greater reproductive benefits to individuals with more elaborate forms of the signal. However, whether these signals convey reliable information about the physiology underlying trait development remains unknown in many species. The steroid hormone corticosterone (CORT) mediates important physiological and behavioral processes during the vertebrate stress response, and CORT secretion itself can be modulated by melanocortins. Thus, sexually selected melanin-based plumage coloration could function as an honest signal of an individual's ability to respond to stressors. This hypothesis was tested in North American barn swallows, Hirundo rustica erythrogaster, where males with darker ventral plumage color exhibit higher phaeomelanin content and are more successful at reproduction. Because reproductive behavior occurs months after plumage signals are developed, we also addressed the potential temporal disconnect of physiological state during trait development and trait advertisement by analyzing three different measurements of CORT levels in adult males during the breeding season (trait advertisement) and in nestling males while they were growing their feathers (trait development). Variation in adult plumage color did not predict baseline or stress-induced CORT, or stress responsiveness. Likewise, there was no relationship between nestling plumage color and any of the CORT measurements, but heavier nestlings had significantly lower baseline CORT. Our finding that a predominantly phaeomelanin-based trait is unrelated to circulating CORT suggests that phaeomelanin and eumelanin signals may convey different physiological information, and highlights the need for further study on the biochemical links between the hypothalamic-pituitary-adrenal (HPA) axis and the production of different melanin-based pigments.

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

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

The effect of melanocortin (Mc3 and Mc4) antagonists on serotonin-induced food and water intake of broiler cockerels

The current study was designed to examine the effects of intracerebroventricular injections of SHU9119 [a nonselective melanocortin receptor (McR) antagonist] and MCL0020 (a selective McR antagonist) on the serotonin-induced eating and drinking responses of broiler cockerels deprived of food for 24 h (FD24). For Experiment 1, the chickens were intracerebroventricularly injected with 2.5, 5, and 10 µg serotonin. In Experiment 2, the chickens received 2 nmol SHU9119 before being injected with 10 µg serotonin. For Experiment 3, the chickens were given 10 µg serotonin after receiving 2 nmol MCL0020, and the level of food and water intake was determined 3 h post-injection. Results of this study showed that serotonin decreased food intake but increased water intake among the FD24 broiler cockerels and that these effects occurred in a dose-dependent manner. The inhibitory effect of serotonin on food intake was significantly attenuated by pretreatment with SHU9119 and MCL0020. However, the stimulatory effect of serotonin on water intake was not altered by this pretreatment. These results suggest that serotonin hypophagia and hyperdipsia were mediated by different mechanisms in the central nervous system, and that serotonin required downstream activation of McRs to promote hypophagia but not hyperdipsia in the FD24 chickens.

Polymorphisms of the pro-opiomelanocortin and agouti-related protein genes and their association with chicken production traits

Pro-opiomelanocortin (POMC) and agouti-related protein (AGRP) are hypothalamic neuropeptides that play a central role in the regulation of food intake and energy expenditure and for this reason the variations in the POMC and AGRP genes in chicken were examined by screening the DNA pools. Two silent cSNPs mutations in POMC gene and one silent cSNP mutation in AGRP gene were identified. PCR-restriction fragment length polymorphism (RFLP) was used to test the cSNPs c. C495T in the POMC and c. C9T in the AGRP gene in the F2 resource population of Gushi chicken crossed with Anak broiler. The association analysis on the polymorphisms of POMC, AGRP gene and production traits showed that the c. C495T mutation in the POMC gene was significantly linked to the pelvis breadth at 4 weeks of age (P = 0.035), body weight at 2 weeks of age (P = 0.013) and was highly significantly linked to the chest depth at 12 weeks of age (P = 0.006). The c. T9T genotype in the AGRP gene was associated with a low breast muscle water loss rate (P = 0.025), increased chest width at 12 weeks of age (P = 0.005) and body weight at 2 weeks of age (P = 0.036), a high slaughter rate (P = 0.049) and semi-evisceration weight (P = 0.019). These findings may have important implications for the molecular aspects of chicken breeding.

Molecular characterization of CART, AgRP, and MC4R genes and their expression with fasting and re-feeding in common carp (Cyprinus carpio)

Cocaine and amphetamine-regulated transcript (CART), agouti-related proteins (AgRP) and Melanocortin 4 Receptor (MC4R) involves in the control of appetite. The genes were cloned and characterized, and their regulation was studied in common carp. The CARTI and CARTII genes encode 117- and 120-amino acids, respectively. The AgRP-1 and AgRP-2 genes encode 128- and 136-amino acids, respectively. CARTI was principally expressed in the brain, eye and ovary, while CARTII was highly expressed in the brain. AgRP-1 was strongly expressed in the brain, intestine, testis and eye, while AgRP-2 was highly expressed only in the gill and eye. The MC4R gene, encoding 326-amino acids, was mainly expressed in the brain testis, pituitary and eye. Phylogenetic analysis had been conducted which implied that both CARTI/CARTII and AgRP-1/AgRP-2 might derived from gene duplication events during genome evolution of common carp. CART, AgRP and MC4R gene expression in brain were decreased after fasting treatment and increased sharply after refeeding comparing with normal fed controls, which suggested that CART, AgRP and MC4R are involved in appetite regulation in common carp.

Cloning and characterization of the 5'-flanking region of the pig AgRP gene

Agouti-related peptide (AgRP), a brain neuropeptide generated by AgRP/neuropeptide Y (NPY) neurons, plays a vital role in the hypothalamic regulation of energy homeostasis. RT-PCR and real-time PCR were carried out in various tissues to detect the AgRP expression pattern in pigs. Our RT-PCR results showed that the pig AgRP gene was ubiquitously expressed in all examined tissues including heart, liver, spleen, lung, kidney, stomach, bladder, m. longissimus, belly fat, brain, large intestine, lymph, back fat, skin, and hypothalamus. Real-time quantitative PCR experiments revealed that it is in the hypothalamus with the highest expression of AgRP both in adult Lantang and Landrace pigs compared to the back fat and m.longissimus muscle and the cDNA level of AgRP in the hypothalamus of adult Chinese indigenous Lantang pig (fat-type) is significantly higher than that of Landrace pig (lean-type). To understand the regulation of the pig AgRP gene, the 5'-flanking region was isolated from a pig bacterial artificial chromosome library and used in a luciferase reporter assay. A positive cis-acting element for efficient AgRP expression was identified at nucleotides -501 to -479, by 5'-serial deletion of the promoter. Electrophoretic mobility-shift assays (EMSA) with competing oligonucleotides revealed that the critical region contained a cis-acting element for Neurogenic Differentiation (NeuroD), which is a member of the NeuroD family of basic-helix-loop-helix transcription factors. This element has not been reported in human or mouse AgRP genes. Our results indicated that NeuroD might be an essential regulatory factor for transcription of pig AgRP, providing an important clue about energy homeostasis regulation in the porcine and human brain.

The early origin of melanocortin receptors, agouti-related peptide, agouti signalling peptide, and melanocortin receptor-accessory proteins, with emphasis on pufferfishes, elephant shark, lampreys, and amphioxus

There are conflicting theories about the evolution of melanocortin MC receptors while only few studies have addressed the evolution of agouti-related peptide (AgRP) and agouti signalling peptide (ASIP), which are antagonists at the melanocortin receptors (MCRs), or the melanocortin MC(2) receptor accessory proteins (MRAP1 and MRAP2). Previously we have cloned melanocortin MC receptors (MC(a) and MC(b)) genes in river lamprey and here we identify orthologues to these melanocortin MC receptor sequences in the sea lamprey. We investigate the putative presence of the melanocortin MC receptor genes in lancelet (amphioxus; Branchiostoma floridae) but we find it unlikely that such gene exists, due to a sharp drop in sequence similarity beyond sequence clusters of known receptors. We show the presence of AgRP and ASIP in elephant shark, a cartilaginous fish belonging to the subclass of Elasmobranchii. However, we do not find any of these genes in lamprey or lancelet after detailed analysis of both targeted and whole proteome regular expression scans. We found MRAP2, but not MRAP1, to be present in elephant shark and sea lamprey while Fugu (T. rubripes) has both genes. This study shows that the most ancient presence of these melanocortin-related sequences is found in elephant shark and lampreys considering the current available sequence data.

Expression and network analysis of genes related to melanocyte development in the Silky Fowl and White Leghorn embryos

Silky Fowl is a natural mutant with hyperpigmentation of various internal tissues. Although the mechanism of hyperpigmentation remains unclear, recent studies have shown that the abnormal migration of melanoblast and the absence of environmental barrier molecules are responsible for the hyperpigmentation in Silky Fowl. In this study, 13 genes related to melanocyte development were selected to detect expression changes between Silky Fowl and White Leghorn [including SRY-box 10 (Sox10), paired box (Pax3), stem cell factor (Scf), v-kit Hardy-Zuckerman 4 feline sarcoma viral oncogene homolog (Kit), endothelin type-B receptor (Ednrb), endothelin 3 (Edn3), microphthalmia-associated transcription factor (Mitf), tyrosinase (Tyr), tyrosinase-related protein-1 (Trp1), tyrosinase-related protein-2 (Trp2), melanocortin-1 receptor (Mc1r), Agouti-related proteins (Agrp), and Proopiomelanocortin (Pomc)]. Transcript expression was detected in 11 stages from 2.5 to 15 days of incubation. In these embryonic periods, Mitf, Kit, Scf, and Agrp expressed earlier in Silky Fowl than in White Leghorn. Sox10, Ednrb, Kit, Mc1r, and Agrp, associating with the proliferation and differentiation of melanoblast, expressed higher (P < 0.05) in Silky Fowl than White Leghorn during 5-6 days of incubation. After day 8 of incubation, Mitf, Tyr, Trp1, Trp2, and Mc1r expressed higher (P < 0.05) in Silky Fowl than White Leghorn, while Agrp expressed higher (P < 0.05) in White Leghorn than Silky Fowl. Moreover, a regulatory network for melanocyte development was constructed based on the expression data. The network predicted novel regulatory relationships and confirmed relationships that have been reported. These results provide biological insight into the molecular mechanism of hyperpigmentation in the Silky Fowl. However, further investigation is needed to confirm these regulatory relationships.

Genetic diversity of the melanocortin 4 receptor (MC4R) gene and its association with slaughter traits in the Landes goose

Melanocortin 4 receptor (MC4R) plays a crucial part in regulating feeding behavior in humans and rodents. We detected two single nucleotide polymorphisms (SNPs; c.108G → A and c.627C → T) in the goose MC4R gene and genotyped 94 Landes geese for association analysis with several carcass traits. Significant associations (P < 0.05) were obtained for c.108G → A with carcass weight, breast muscle percentage, and leg muscle percentage, and for c.627C → T with body weight, carcass weight, semi-eviscerated weight, and eviscerated weight. We re-constructed haplotypes based on the two SNPs and analyzed diplotypes in association with carcass traits, obtaining significant associations with several of the traits. These results suggest that polymorphisms in the MC4R gene could have effects on carcass traits in Landes geese. More study is required to confirm these results.

Transcriptional and pathway analysis in the hypothalamus of newly hatched chicks during fasting and delayed feeding

Background

The hypothalamus plays a central role in regulating appetite and metabolism. However, the gene networks within the hypothalamus that regulate feed intake and metabolism, and the effects of fasting on those pathways are not completely understood in any species. The present experiment evaluated global hypothalamic gene expression in newly hatched chicks using microarray analysis to elucidate genes and pathways regulated by feeding, fasting, and delayed feeding. Ten groups of chicks were sampled over four days post-hatch, including fed, fasted, and 48 h fasted followed by access to feed for 4 h, 24 h, and 48 h. Hypothalamic samples were collected for microarray analysis (n = 4). Expression patterns of selected genes were confirmed by quantitative real-time PCR. Pathway analysis of the microarray results predicted a network of genes involved in neuropeptide or neurotransmitter signaling. To confirm the functionality of this predicted gene network, hypothalamic neurons from fed and fasted chicks were isolated and cultured in the presence of neuropeptide Y, somatostatin, α-melanocyte stimulating hormone, norepinephrine, and L-phospho-serine. Results confirmed functional relationships among members of the predicted gene network. Moreover, the effects observed were dependant upon the nutritional state of the animals (fed vs. fasted).

Results

Differences in gene expression (≥ 1.6 fold) were detected in 1,272 genes between treatments, and of those, 119 genes were significantly (P < 0.05) different. Pathway Miner analysis revealed that six genes (SSTR5, NPY5R, POMC, ADRB2, GRM8, and RLN3) were associated within a gene network. In vitro experiments with primary hypothalamic neurons confirmed that receptor agonists involved in this network regulated expression of other genes in the predicted network, and this regulation within the network was influenced by the nutritional status and age of the chick.

Conclusions

Microarray analysis of the hypothalamus during different nutritional states revealed that many genes are differentially regulated. We found that functional interactions exist among six differentially regulated genes associated within a putative gene network from this experiment. Considering that POMC, an important gene in controlling metabolism, was central to this network, this gene network may play an important role in regulation of feeding and metabolism in birds.

Acute circuit-resistance exercise increases expression of lymphocyte agouti-related protein in young women

Exercise-induced leukocytosis and lymphocytosis is accompanied by up-regulation and down-regulation of hundreds of genes in white blood cells (WBCs). Agouti-related protein (AgRP) is an orexigenic peptide secreted predominantly from the arcuate nucleus in the hypothalamus. AgRP affects feeding behavior and plays a role in energy and glucose homeostasis and adiposity. The purpose of the study was to determine effects of circuit resistance exercise (CRE) (9 exercises, 25 s per exercise) at different intensities on peripheral blood lymphocyte (PBL) AgRP mRNA expression and its concentrations in lymphocytes and plasma. Twenty-five young female college students were randomly divided into five groups: control, 40% 1-repetition maximum (1-RM), 60% 1-RM, 80% 1-RM and combined (40 + 60 + 80% 1-RM) loads. Peripheral blood mononuclear cells were isolated by a lymphocyte density gradient centrifugation method for AgRP mRNA expression. Lymphocyte ATP, glycogen, AgRP, growth hormone (GH), and plasma AgRP, GH and glucose concentrations were measured. CRE increased AgRP mRNA lymphocyte expression significantly ( P < 0.0001) at all intensities. A higher and significant ( P < 0.01) increase was found in the 60% 1-RM group when compared with the other groups. The CRE-induced lymphocyte AgRP expression was accompanied by elevations in plasma AgRP, glucose and GH levels as well as higher WBCs, lymphocytes and neutrophil counts. Lymphocyte AgRP and GH concentrations were significantly reduced ( P < 0.05). Lymphocyte ATP content was unchanged and glycogen was reduced in the combined group but not in the other groups. Data indicate that AgRP mRNA is expressed in PBLs and that CRE increases its expression. Data also reveal that the expression of AgRP was accompanied with higher plasma AgRP and GH concentrations. Findings suggest that AgRP may provide an important signal in the immune environment and that the lymphocyte may be considered as an extra-hypothalamic source of plasma AgRP following exercise stress.

Treadmill training enhances rat agouti-related protein in plasma and reduces ghrelin levels in plasma and soleus muscle

Ghrelin and agouti-related protein (AgRP) are orexigenic peptides secreted from stomach mucosa and the arcuate nucleus of the hypothalamus, respectively. Both peptides affect feeding behavior and play a role in energy balance, glucose homeostasis, and adiposity. The purpose of the current study was to determine the effects of moderate-term (6 weeks) running regimen on resting levels of ghrelin, AgRP, adenosine triphosphate, and glycogen in soleus muscle as well as plasma concentrations of the orexigenic hormones. Eighteen adult Wistar male rats (12 weeks old, 235-255 g) were randomly assigned to training (n = 10) and control (n = 8) groups. The training group ran for 60 min/d, 5d/wk at 25 m/min and 0% grade for 6 weeks. Forty-eight hours after the last exercise session, rats were killed; and soleus muscle and plasma were collected and frozen in liquid nitrogen for later analysis. Results demonstrated that 6 weeks of treadmill exercise reduced ghrelin and increased AgRP levels in plasma. Trained rat soleus muscle had higher levels of glycogen but not adenosine triphosphate or AgRP compared with untrained controls. Data indicate that training lowers ghrelin levels in rat soleus and plasma, which is accompanied by higher plasma AgRP and soleus glycogen content.

Characterization, tissue distribution, and regulation of agouti-related protein (AgRP), cocaine- and amphetamine-regulated transcript (CART) and neuropeptide Y (NPY) in Atlantic salmon (Salmo salar)

Key peptide hormones involved in the control of appetite in vertebrates were identified, their genes characterized and their regulation studied in Atlantic salmon: two agouti-related proteins (AgRP), cocaine- and amphetamine-regulated transcript (CART) and neuropeptide Y (NPY). The AgRP-1 and AgRP-2 genes encode prepro-proteins of 142- and 117-amino acids, respectively. The deduced AgRP-2 protein has 10 cysteine residues in the C-terminal polycysteine domain, while the AgRP-1 lacks the 6th and 7th cysteine residues observed in other species. AgRP-1 was principally expressed in the pituitary and skin, while AgRP-2 was highly expressed in the mid-gut, red muscle and gonads. The CART gene, encoding 118-amino acids, was strongly expressed in the brain and eye. In addition to salmon CART, we identified three to six variants of the CART gene in lower vertebrates by mining available databases. The salmon NPY gene, encoding 100-amino acids, was mainly expressed in the brain and eye. AgRP-1 and CART mRNA levels in the brain decreased after 6 days of fasting while AgRP-2 and NPY showed no significant change, suggesting that AgRP-1 and CART are involved in feeding regulation in Atlantic salmon. The identification of multiple variants of these appetite-regulating genes emphasizes the importance to further investigate the complex regulation of these genes.

Polymorphisms in uncoupling protein, melanocortin 3 receptor, melanocortin 4 receptor, and pro-opiomelanocortin genes and association with production traits in a commercial broiler line

Because avian uncoupling protein (avUCP), melanocortin 3 receptor (MC3R), melanocortin 4 receptor (MC4R), and pro-opiomelanocortin (POMC) genes may be associated with production traits [e.g., BW, weight gain (WG), and feed conversion ratio (FCR)], male and female broilers from an elite broiler line were screened for polymorphisms in these genes. The PCR-restriction fragment length polymorphism (RFLP) tests were developed to type the missense polymorphisms UCPAla118Val, MC4RSer76Leu, MC3R-Met54Leu, and Gly104Ser and POMCPro61Leu. Of 39 single nucleotide polymorphisms identified in all 4 genes, 24/39 were transitions with 11 having a C to T change. Of the 23 polymorphisms in UCP, 17 represented at least 7 haplotypes in this pedigreed broiler line. The UCP Ala-118Val allele was associated with a) high feed efficiency (FE; P = 0.03) and WG (P = 0.053) in selected males, and b) high BW in selected females (P = 0.07) and unselected males (P = 0.015). The UCPVal118Val allele was found in approximately 10% of the birds that were screened. Five silent substitutions, 3 in MC3R and 2 in MC4R, were also identified. Thirteen polymorphisms were identified in the POMC gene representing at least 3 different alleles. A missense Pro61Leu heterozygote was associated with greater BW in females. The heterozygote MC3R Gly104Ser polymorphism was associated with greater FE in selected males (P = 0.03) and greater BW in unselected males (P = 0.007). The MC4R Ser76Leu heterozygote polymorphism was associated with greater BW than the Leu76 homozygote in females (P = 0.05). From these findings, we hypothesize that UCP, MC3R, MC4R and POMC genes may play important roles and could be candidate loci for production traits such as feed conversion and BW in commercial broiler breeding stock.

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

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

Characterization of Japanese quail yellow as a genomic deletion upstream of the avian homolog of the mammalian ASIP (agouti) gene

ASIP is an important pigmentation gene responsible for dorsoventral and hair-cycle-specific melanin-based color patterning in mammals. We report some of the first evidence that the avian ASIP gene has a role in pigmentation. We have characterized the genetic basis of the homozygous lethal Japanese quail yellow mutation as a >90-kb deletion upstream of ASIP. This deletion encompasses almost the entire coding sequence of two upstream loci, RALY and EIF2B, and places ASIP expression under control of the RALY promoter, leading to the presence of a novel transcript. ASIP mRNA expression was upregulated in many tissues in yellow compared to wild type but was not universal, and consistent differences were not observed among skins of yellow and wild-type quail. In a microarray analysis on developing feather buds, the locus with the largest downregulation in yellow quail was SLC24A5, implying that it is regulated by ASIP. Finally, we document the presence of ventral skin-specific isoforms of ASIP mRNA in both wild-type quails and chickens. Overall, there are remarkable similarities between yellow in quail and lethal yellow in mouse, which involve a deletion in a similar genomic position. The presence of ventral-specific ASIP expression in birds shows that this feature is conserved across vertebrates.

Central insulin suppresses feeding behavior via melanocortins in chicks

Growing evidence suggests that insulin interacts with both orexigenic and anorexigenic peptides in the brain for the control of feeding behavior in mammals. However, the action of central insulin in chicks has not yet been identified. In the present study, we investigated the effects of central injection of insulin on feeding behavior in chicks. Intracerebroventricular (ICV) administration of insulin, at doses that do not influence peripheral glucose levels, significantly inhibited food intake in chicks. Central injection of insulin in chicks significantly increased expression of pro-opiomelanocortin (POMC) mRNA, and decreased that of neuropeptide Y (NPY) mRNA. Finally, co-injection of the melanocortin antagonist (SHU9119 or HS014) prevented the reduction in food intake caused by ICV administration of insulin. These data suggest that insulin functions in chicks as an appetite-suppressive peptide in the central nervous system, and that the central melanocortin system mediates this anorexic effect of insulin, as in mammals.

Chronic consumption of a low-fat diet leads to increased hypothalamic agouti-related protein and reduced leptin

OBJECTIVE

This study examined the hypothesis that dietary fat under ad libitum feeding conditions influences expression levels (mRNA) of the mouse agouti-related protein (AgRP), leptin, leptin receptor (OBRb), and neuropeptide Y (NPY) at early stages of development.

METHODS

C57Bl/6J male mice were placed on a high-fat diet (HFD) or a low-fat diet (LFD) shortly after weaning. Groups of mice were euthanized at various ages and real-time one-step reverse transcriptase polymerase chain reaction was used to analyze gene expression in the hypothalamus (AgRP, NPY, OBRb), the adrenal gland (AgRP), the testis (AgRP), and epididymal fat (leptin).

RESULTS

Leptin expression increased linearly with age but only under the HFD despite body weight gain under both diets. This pattern of expression coincided with reduced expression of hypothalamic AgRP under an HFD, whereas OBRb and NPY did not fluctuate in response to diet. By contrast, consumption of an LFD (i.e., high carbohydrate) increased hypothalamic AgRP and suppressed adipose leptin, which is consistent with the notion that leptin could regulate AgRP centrally. In contrast, AgRP expression in the adrenal gland initially decreased and then increased with age under both diets.

CONCLUSIONS

Dietary fat can have a tissue-dependent effect on AgRP that may be unfettered by leptin under an HFD.

Mechanisms Regulating Feed Intake, Energy Expenditure, and Body Weight in Poultry

To achieve energy balance and maintain a constant BW, changes in feed intake and energy expenditure must be coordinated and tightly regulated. This may not hold true for some poultry species intensively selected for such economically important traits as growth and meat production. For example, the modern commercial broiler breeder does not adequately control voluntary feed intake to meet its energy requirements and maintain energy balance. As a consequence, feeding must be limited in these birds to avoid overconsumption and excessive fattening during production. It is important to determine a genetic basis to help explain this situation and to offer potential strategies for producing more efficient poultry. This review summarizes what is currently known about the control of feed intake and energy expenditure at the gene level in birds. Highly integrated regulatory systems have been identified that link the control of feeding with the sensing of energy status. How such systems function in poultry is currently being explored. One example recently identified in chickens is the adenosine monophosphate-activated protein kinase pathway that links energy sensing with modulation of metabolic activity to maintain energy homeostasis at the cellular level. In the hypothalamus, this same pathway may also play an important role in regulating feed intake and energy expenditure commensurate with perceived whole body energy needs. Genes encoding key regulatory factors such as hormones, neuropeptides, receptors, enzymes, and transcription factors produce the molecular components that make up intricate and interconnected neural, endocrine, and metabolic pathway networks linking peripheral tissues with the central nervous system. Moreover, coordinate expression of specific gene groups can establish functional pathways that respond to and are regulated by such factors as hormones, nutrients, and metabolites. Thus, with a better understanding of the genetic and molecular basis for regulating feed intake and energy expenditure in birds important progress can be made in developing, evaluating, and managing more efficient commercial poultry lines.

Characterization and tissue distribution of multiple agouti-family genes in pufferfish, Takifugu rubripes

Four types of agouti-family genes (AGRP1, AGRP2, ASIP1 and ASIP2) were obtained from torafugu, Takifugu rubripes. Their characterization and structure were analyzed to elucidate the relationship among the torafugu agouti-family genes. Both AGRP1 and AGRP2 showed genomic synteny with the human AGRP gene. Phylogenetic tree analysis showed that AGRP1 formed a cluster with human AGRP. We inferred that torafugu AGRP1 and AGRP2 are orthologs of human AGRP and that they are paralogous genes derived from genome duplication occurred in the teleost phylogeny. Torafugu ASIP1 showed genomic synteny with the human ASIP, but ASIP2 did not. The ASIP1 expression level was about five times higher in the white ventral skin than in the black dorsal skin. Therefore, we concluded that torafugu ASIP1 is an ortholog of human ASIP, nevertheless, we are unable to determine if torafugu ASIP2 is a paralog of ASIP1 or not.

Selective tissue uptake of agouti-related protein(82-131) and its modulation by fasting

The blood concentration of agouti-related protein (AgRP), a protein related to hyperphagia and obesity, is increased in obese human and fasted lean subjects. Because there is no saturable transport system at the blood-brain barrier for circulating AgRP to reach its central nervous system target, uptake of AgRP by peripheral organs might be physiologically meaningful. Using the biologically active fragment AgRP(82-131), we determined the pharmacokinetics of its radioactively labeled tracer after iv bolus injection and compared it with that of the vascular marker albumin. AgRP enters peripheral organs at different influx rates, all of which were higher than into brain and spinal cord. At 10 min after iv injection, the radioactivity recovered in the liver, which had the fastest influx rate for AgRP, represented intact (125)I-AgRP. The adrenal gland had a moderately fast uptake (but the highest initial volume of distribution), followed by the heart, lungs, and skeletal muscle. By comparison, epididymal fat, testis, and pancreas had low permeability to AgRP. Saturation of influx was determined by coadministration of excess unlabeled AgRP and was shown to be present in the liver and adrenal gland. The influx rate and initial volume of distribution did not show a linear correlation with vascular permeability or regional blood flow. AgRP uptake by the liver and epididymal fat was significantly increased by overnight fasting, whereas that by the adrenal gland was significantly decreased in fasted mice. Thus, the differential uptake of AgRP by peripheral organs could be a regulated process that is modulated by food deprivation.

The agouti-related protein and its role in energy homeostasis

The melanocortin system plays an important role in the regulation of energy homeostasis. The Agouti-related protein (AGRP) is a natural antagonist of the action of alpha-melanocyte stimulating hormone (alpha-MSH) at the melanocortin receptors (MCR). AGRP is upregulated by fasting while intracerebroventricular injections of synthetic AGRP lead to increased appetite and food intake. Transgenic mice overexpressing AGRP are also hyperphagic and eventually become obese. AGRP is, therefore, a significant regulator of energy balance and a candidate gene for human fatness. Indeed, humans with common single nucleotide polymorphisms (SNPs) in the promoter or the coding region are leaner and resistant to late-onset obesity than wild-type individuals. AGRP is also expressed in the periphery. Recent studies show that AGRP in the adrenal gland is upregulated by fasting as much as it is in the hypothalamus. These data open up the possibility for a wider role by AGRP not only in food intake but also in the regulation of energy balance through its actions on peripheral tissues. This review summarizes recent advances in the biochemical and physiological properties of AGRP in an effort to enhance our understanding of the role this powerful neuropeptide plays in mammalian energy homeostasis.

Recent developments in our understanding of the avian melanocortin system: its involvement in the regulation of pigmentation and energy homeostasis

The mammalian melanocortin system has been established as a crucial regulatory component in an extraordinarily diverse number of physiological functions. In contrast, comparatively little is known about the avian melanocortin system: interest in the physiological role of alpha-MSH in birds has been limited by the fact that birds lack the intermediate lobe of the pituitary, the main source of circulating alpha-MSH in most vertebrates. Recently, however, the main avian melanocortin system genes, including POMC, AGRP, and all the melanocortin receptors, have been cloned and their physiological roles are the beginning to be elucidated. This review outlines our improved understanding of the avian melanocortin system, particularly in relation to two of the most widely studied physiological functions of the melanocortin system in mammals, the regulation of pigmentation and energy homeostasis. The data reviewed here indicate that the melanocortin system has been strongly conserved during vertebrate evolution and that alpha-MSH is produced locally in birds to act as an autocrine/paracrine hormone.

Evolutionary conservation of the structural, pharmacological, and genomic characteristics of the melanocortin receptor subtypes

We have cloned melanocortin receptors (MCRs) from several species of fish. The MC4R and MC5R subtypes arose early in vertebrate evolution and their primary structure is remarkably conserved. Expression and pharmacological characterization of the MCRs in fish has revealed that they bind and respond to melanocortin peptides with high potency. Detailed characterization of the binding properties of the different subtypes suggests that MCRs in early vertebrates had preference for adrenocorticotropic hormone (ACTH) peptides, while the high sensitivity for the shorter proopiomelanocortin (POMC) products, such as the alpha-, beta-, and gamma-melanocyte-stimulating hormone (MSH), has appeared later, perhaps as the MCR subtypes gained more specialized functions. The MCR repertoire shows in general high similarities in their primary structures, while they are however not similar in terms of functional roles. The MCRs serve therefore as an interesting model family to understand the molecular mechanisms of how functions of the genes can diverge during evolution. In this review, we provide an overview of our recent studies on the cloning, expression, pharmacology, 3D modeling, and genomic studies of the MCRs in non-mammalian species.

The effects of agouti-related protein gene transfer in vivo by electroporation in mice

In the present study, the cDNA encoding agouti-related protein (AGRP) gene known as an orexigenic factor was transferred in vivo to test whether food intake and body weight gain is improved in mice. When the expression plasmid of AGRP gene driven by mouse beta-actin, pActAGRP, was transferred into leg muscle by electroporation, body weight of gene-transferred mice was significantly increased at 14 days and afterwards compared with that of control counterparts (p < 0.05). Likewise, daily food intake was also significantly higher in the AGRP gene-transferred mice than in the control mice at 4 days and afterwards (p < 0.05). A significant increase in serum AGRP concentration of the AGRP gene-transferred group was detected compared with the control group at 1 week (p < 0.01), but the difference quickly disappeared at 3 weeks. However, the hypothalamic NPY mRNA abundance of AGRP gene-transferred mice was significantly higher than that of the control mice at 3 weeks (p < 0.05). These results suggested that instead of hormone administration per se, in vivo AGPR gene transfer into skeletal muscle was found to mimic hormonal effects. The present methodology of in vivo gene transfer by electroporation might be useful to promote growth and food intake in farm livestock as well as experimental animals.

Expression of agouti-related protein (AgRP) in the hypothalamus and adrenal gland of the duck ( Anas platyrhynchos)

The presence and distribution of agouti-related protein (AgRP) immunoreactivity were investigated in the hypothalamus and adrenal gland of the duck using immunohistochemistry and Western blot analysis. Expression of AgRP mRNA was also studied using reverse transcriptase polymerase chain reaction (RT-PCR). A partial coding sequence (cds) of the duck AgRP gene was identified. Western blot analysis showed the presence of an AgRP-like peptide having a molecular weight consistent with the number of predicted amino acids of the avian AgRP. In the hypothalamus, AgRP immunoreactivity was found in neurons of the nucleus infundibularis and in fibers projecting to the median eminence. In the adrenals, AgRP immunoreactivity was observed in medullary cells. These findings suggest that in the duck, AgRP may play a role in regulating energy homeostasis and adrenal endocrine functions.

Concordant evolution of plumage colour, feather microstructure and a melanocortin receptor gene between mainland and island populations of a fairy-wren

Studies of the patterns of diversification of birds on islands have contributed a great deal to the development of evolutionary theory. In white-winged fairy-wrens, Malurus leucopterus, mainland males develop a striking blue nuptial plumage whereas those on nearby islands develop black nuptial plumage. We explore the proximate basis for this divergence by combining microstructural feather analysis with an investigation of genetic variation at the melanocortin-1 receptor locus (MC1R). Fourier analysis revealed that the medullary keratin matrix (spongy layer) of the feather barbs of blue males was ordered at the appropriate nanoscale to produce the observed blue colour by coherent light scattering. Surprisingly, the feather barbs of black males also contained a spongy layer that could produce a similar blue colour. However, black males had more melanin in their barbs than blue males, and this melanin may effectively mask any structural colour produced by the spongy layer. Moreover, the presence of this spongy layer suggests that black island males evolved from a blue-plumaged ancestor. We also document concordant patterns of variation at the MC1R locus, as five amino acid substitutions were perfectly associated with the divergent blue and black plumage phenotypes. Thus, with the possible involvement of a melanocortin receptor locus, increased melanin density may mask the blue-producing microstructure in black island males, resulting in the divergence of plumage coloration between mainland and island white-winged fairy-wrens. Such mechanisms may also be responsible for plumage colour diversity across broader geographical and evolutionary scales.

Central and peripheral interactions between the agouti-related protein and leptin

The agouti-related protein (AgRP) is a powerful appetite modulator expressed in the hypothalamus and the adrenal gland and regulated by leptin. Here we report the robust expression of AgRP in epididymal fat and its upregulation in this tissue by feeding rather than by fasting. This was observed in both the obesity-susceptible C57BL/6J and the obesity-resistant CAST/Ei mouse strains. Surprisingly, AgRP expression was higher in the hypothalamus and the adrenal gland in the leaner and obesity-resistant CAST/Ei strain. In vitro leptin treatment upregulated endogenous AgRP in mouse hypothalamus and adrenal cells, after an acute 6-h exposure, but it downregulated AgRP after a long-term 60-h exposure. AgRP, on the other hand, upregulated its own endogenous expression in the hypothalamus and the adrenal cells and also upregulated endogenous leptin in the adrenal cells. These results reveal a novel feedback loop and reciprocal transcriptional regulation between AgRP and leptin centrally and peripherally.

The melanocortin system in Fugu: determination of POMC/AGRP/MCR gene repertoire and synteny, as well as pharmacology and anatomical distribution of the MCRs

The G-protein-coupled melanocortin receptors (MCRs) play an important role in a variety of essential functions such as the regulation of pigmentation, energy homeostasis, and steroid production. We performed a comprehensive characterization of the MC system in Fugu (Takifugu rubripes). We show that Fugu has an AGRP gene with high degree of conservation in the C-terminal region in addition to a POMC gene lacking gamma-MSH. The Fugu genome contains single copies of four MCRs, whereas the MC3R is missing. The MC2R and MC5R are found in tandem and remarkably contain one and two introns, respectively. We suggest that these introns were inserted through a reverse splicing mechanism into the DRY motif that is widely conserved through GPCRs. We were able to assemble large blocks around the MCRs in Fugu, showing remarkable synteny with human chromosomes 16 and 18. Detailed pharmacological characterization showed that ACTH had surprisingly high affinity for the Fugu MC1R and MC4R, whereas alpha-MSH had lower affinity. We also showed that the MC2R gene in Fugu codes for an ACTH receptor, which did not respond to alpha-MSH. All the Fugu receptors were able to couple functionally to cAMP production in line with the mammalian orthologs. The anatomical characterization shows that the MC2R is expressed in the brain in addition to the head-kidney, whereas the MC4R and MC5R are found in both brain regions and peripheral tissues. This is the first comprehensive genomic and functional characterization of a GPCR family within the Fugu genome. The study shows that some parts of the MC system are highly conserved through vertebrate evolution, such as regions in POMC coding for ACTH, alpha-MSH, and beta-MSH, the C-terminal region of AGRP, key binding units within the MC1R, MC2R, MC4R, and MC5R, synteny blocks around the MCRs, pharmacological properties of the MC2R, whereas other parts in the system are either missing, such as the MC3R and gamma-MSH, or different as compared to mammals, such as the affinity of ACTH and MSH peptides to MC1R and MC4R and the anatomical expression pattern of the MCRs.

Endogenous melanocortin antagonist in fish: structure, brain mapping, and regulation by fasting of the goldfish agouti-related protein gene

Agouti-related protein (AGRP) is a naturally occurring antagonist of melanocortin. In mammals, central AGRP expression is restricted to the arcuate nucleus in which it plays a key role in the control of energy balance by antagonizing melanocortin effects at melanocortin 4 receptors. In goldfish, melanocortin 4 receptor is profusely expressed within the main brain areas for the control of energy balance, and central administration of agonist or antagonist analogs inhibits or stimulates food intake, respectively. Here we demonstrate that the goldfish genome has a homologous gene to mammalian AGRP. Detailed brain mapping by in situ hybridization shows that AGRP is exclusively expressed in the ventrobasal hypothalamic lateral tuberal nucleus, the teleostean homolog of the arcuate nucleus. Fasting up-regulates its mRNA levels in the lateral tuberal nucleus. In the periphery, AGRP is expressed in several tissues including ovary, muscle, and ventral skin, suggesting that AGRP might regulate peripheral actions of melanocortin peptides. The results provide the first evidence for an endogenous melanocortin antagonist in nontetrapod species and suggest that hypothalamic overexpression during fasting might regulate the inhibitory effects of melanocortin peptides on food intake in goldfish.

Genetic regulation of feed intake and energy balance in poultry

Intensive selection by poultry breeders over many generations for economically important production traits such as growth rate and meat production has been accompanied by significant changes in feed intake and energy balance. For example, the modern commercial broiler, selected for rapid growth and enhanced muscle mass, does not adequately regulate voluntary feed intake to achieve energy balance. When given unrestricted access to feed, broilers exhibit hyperphagia leading to an excessive accumulation of energy (fat) stores, making these birds prone to obesity and other health-related problems. Humoral and neural pathways have been identified and studied in mammals that link appetite and energy balance. A series of highly integrated regulatory mechanisms exists for both of these processes involving complex interactions between peripheral tissues and the central nervous system. Within the central nervous system, the brainstem and the hypothalamus play critical roles in the regulation of feed intake and energy balance. Genes encoding key regulatory factors such as hormones, neuropeptides, receptors, enzymes, transcription factors, and binding/transport proteins constitute the molecular basis for regulatory systems that derive from integrated sensing, signaling, and metabolic pathways. However, we do not yet have a complete understanding of the genetic basis for this regulation in poultry. This review examines what is currently known about the regulation of feed intake and energy balance in poultry. A better understanding of the genes associated with controlling feed intake and energy balance and how their expression is regulated by nutritional and hormonal stimuli will offer new insights into current poultry breeding and management practices.

Avian melanocortin system: alpha-MSH may act as an autocrine/paracrine hormone: a minireview

The interest in the physiological role of alpha-MSH in birds has been limited because they lack the intermediate lobe of the pituitary, the main source of circulating alpha-MSH in most vertebrates. Recent studies have improved our understanding of the avian melanocortin system. We have cloned and characterized all five MC-R subtypes, POMC, and AGRP in chicken. Analyses of the tissue distribution of expression of these genes revealed widespread expression throughout the body, corresponding to the situation in mammals in which alpha-MSH exerts a multiplicity of effects in different tissues by acting as a local mediator. We showed that the extended black locus controlling feather pigmentation in the chicken encodes MC1-R. Moreover, black chickens carrying the dominant allele, the extended black, express the MC1-R with ligand-independent activity as the somber-3J black mice. alpha-MSH and AGRP were expressed in the infundibular nucleus of POMC and NPY neurons, respectively, in the brain of Japanese quail. Furthermore, fasting stimulated AGRP expression and lowered POMC expression. These data indicate that at least two of the major melanocortin systems reported in mammals, that is, regulation of pigmentation and energy homeostasis, was developed in a common ancestor to chicken and mammals at least 300 million years ago. Furthermore, alpha-MSH peptide was identified in developing chicken eye, suggesting a possible involvement of alpha-MSH in regulation of ocular development. Collectively, the data reviewed here indicate that alpha-MSH is produced locally and acts as an autocrine/paracrine hormone in birds.

Association of feather colour with constitutively active melanocortin 1 receptors in chicken

Seven alleles of the chicken melanocortin (MC) 1 receptor were cloned into expression vectors, expressed in mammalian cells and pharmacologically characterized. Four of the clones e(+R), e(+B&D), e(wh)/e(y), E(Rfayoumi) gave receptors to which melanocortin stimulating hormone (alpha-MSH) and NDP-MSH bound with similar IC50 values and responded to alpha-MSH by increasing intracellular cAMP levels in a dose-dependent manner. Three of the cMC1 receptors; e(b), E and E(R), did not show any specific binding to the radioligand, but were found to be constitutively active in the cAMP assay. The E and E(R) alleles are associated with black feather colour in chicken while the eb allele gives rise to brownish pigmentation. The three constitutively active receptors share a mutation of Glu to Lys in position 92. This mutation was previously found in darkly pigmented sombre mice, but constitutively active MC receptors have not previously been shown in any nonmammalian species. We also inserted the Glu to Lys mutation in the human MC1 and MC4 receptors. In contrast with the chicken clones, the hMC1-E94K receptor bound to the ligand, but was still constitutively active independently of ligand concentration. The hMC4-E100K receptor did not bind to the MSH ligand and was not constitutively active. The results indicate that the structural requirements that allow the receptor to adapt an active conformation without binding to a ligand, as a consequence of this E/K mutation, are not conserved within the MC receptors. The results are discussed in relationship to feather colour in chicken, molecular receptor structures and evolution. We suggest that properties for the 'E92K switch' mechanism may have evolved in an ancestor common to chicken and mammals and were maintained over long time periods through evolutionary pressure, probably on closely linked structural features.

The role of the melanocortin system and the melanocortin-4 receptor in ring dove (Streptopelia risoria) feeding behavior

The melanocortin-4 receptor (MC4-R) is an important mediator of the effects of two melanocortin system ligands, alpha melanocyte stimulating hormone (alpha-MSH) and agouti-related peptide (AGRP), on feeding behavior and energy balance in mammals. Although an avian homologue of the mammalian MC4-R has recently been identified, there is little information on the role of this receptor and the melanocortin system in avian feeding and body weight regulation. In these studies, we measured changes in feeding behavior in ring doves (Streptopelia risoria) following intracerebroventricular (i.c.v.) injection of various melanocortin receptor agonists and antagonists. The selective MC4-R antagonist HS014 elevated food intake within 4 h at all three doses tested (0.02, 0.2, and 2 nmol). A 1 nmol dose of the endogenous antagonist AGRP also stimulated feeding but only after a post-injection interval of 10 h. Surprisingly, the MC3-R and MC4-R antagonist SHU9119 not only failed to stimulate food intake at the same doses as HS014, but actually inhibited food intake at 8 h after injection. Whether this was due to toxicity effects or differences in the pharmacology of avian and mammalian melanocortin receptors remains to be determined. Food-deprived doves showed a fourfold increase in the number of AGRP-immunoreactive cells in the tuberal region of the hypothalamus and 5 ng of the MC3-R and MC4-R agonist MTII significantly attenuated the amount of food consumed by food-deprived birds that were allowed to re-feed. These data support a role for the melanocortin system and the melanocortin-4 receptor in the ring dove feeding behavior.

Plasma agouti-related protein level: a possible correlation with fasted and fed states in humans and rats

We measured plasma concentrations of agouti-related protein (AGRP) in humans and rats and determined whether these were affected by ingestion of a meal after fasting. In 17 healthy human subjects, the mean plasma concentration of AGRP was lower in the fed state than in the fasted state. Two hours after a breakfast meal, AGRP levels dropped by 39%. By contrast, a continued fast for 2 h increased the average AGRP concentration by 73%. In rats with diet-induced obesity, refeeding resulted in a 50% decrease in plasma AGRP concentrations following a fasting-refeeding protocol. Our results support the notion that plasma AGRP may serve as a biomarker for the transition from a fasted to the satiated state.