Molecular biology and physiology of the melanocortin system in fish: a review

Triphenyltin induced darker body coloration by disrupting melanocortin system and pteridine metabolic pathway in a reef fish, Amphiprion ocellaris

Triphenyltin (TPT) is a typical persistent organic pollutant whose occurrence in coral reef ecosystems may threaten the survival of reef fishes. In this study, a brightly colored representative reef fish, Amphiprion ocellaris was used to explore the effects of TPT at environmental levels (1, 10, and 100 ng/L) on skin pigment synthesis. After the fish were exposed to TPT for 60 days, the skin became darker, owing to an increase in the relative area of black stripes, a decrease in orange color values while an increase in brown color values, and an increase in the number of melanocytes in the orange part of the skin tissues. To explore the mechanisms by which TPT induces darker body coloration, the enzymatic activity and gene expression levels of the members of melanocortin system that affect melanin synthesis were evaluated. Leptin levels and lepr expression were found to be increased after TPT exposure, which likely contributed to the increase found in pomc expression and α-melanocyte-stimulating hormone (α-MSH) levels. Then Tyr activity and mc1r, tyr, tyrp1, mitf, and dct were upregulated, ultimately increasing melanin levels. Importantly, RT-qPCR results were consistent with the transcriptome analysis of trends in lepr and pomc expression. Because the orange color values decreased, pterin levels and the pteridine metabolic pathway were also evaluated. The results showed that TPT induced BH4 levels and spr, xdh, and gch1 expression associated with pteridine synthesis decreased, ultimately decreasing the colored pterin content (sepiapterin). We conclude that TPT exposure interferes with the melanocortin system and pteridine metabolic pathway to increase melanin and decrease colored pterin levels, leading to darker body coloration in A. ocellaris. Given the importance of body coloration for the survival and reproduction of reef fishes, studies on the effects of pollutants (others alongside TPT) on body coloration are of high priority.

Divergent Pharmacology and Biased Signaling of the Four Melanocortin-4 Receptor Isoforms in Rainbow Trout (Oncorhynchus mykiss)

The melanocortin-4 receptor (MC4R) is essential for the modulation of energy balance and reproduction in both fish and mammals. Rainbow trout (Oncorhynchus mykiss) has been extensively studied in various fields and provides a unique opportunity to investigate divergent physiological roles of paralogues. Herein we identified four trout mc4r (mc4ra1, mc4ra2, mc4rb1, and mc4rb2) genes. Four trout Mc4rs (omMc4rs) were homologous to those of teleost and mammalian MC4Rs. Multiple sequence alignments, a phylogenetic tree, chromosomal synteny analyses, and pharmacological studies showed that trout mc4r genes may have undergone different evolutionary processes. All four trout Mc4rs bound to two peptide agonists and elevated intracellular cAMP levels dose-dependently. High basal cAMP levels were observed at two omMc4rs, which were decreased by Agouti-related peptide. Only omMc4rb2 was constitutively active in the ERK1/2 signaling pathway. Ipsen 5i, ML00253764, and MCL0020 were biased allosteric modulators of omMc4rb1 with selective activation upon ERK1/2 signaling. ML00253764 behaved as an allosteric agonist in Gs-cAMP signaling of omMc4rb2. This study will lay the foundation for future physiological studies of various mc4r paralogs and reveal the evolution of MC4R in vertebrates.

A Natural Gas Fermentation Bacterial Meal (FeedKind®) as a Functional Alternative Ingredient for Fishmeal in Diet of Largemouth Bass, Micropterus salmoides

A 10-week growth study was conducted to evaluate the effect of a natural gas fermentation bacterial meal (FeedKind^®, FK) as a fishmeal (FM) alternative in largemouth bass (Micropterus salmoides) (48.0 ± 0.03 g). Four isonitrogenous and isoenergetic diets were formulated including one commercial control (C, 42% FM) and three experimental diets with gradient FK of 3% (FK3, 29%FM), 6% (FK6, 26%FM) and 9% (FK9, 23%FM), respectively. FK-fed groups showed significantly higher SR than that of C group. The WGR and SGR of fish fed FK3 and FK6 were significantly higher than those of FK9, but not statistical different from the C group. FK-fed groups showed higher apparent digestibility coefficients of dry matter and nutrients. Further, FK-fed groups increased the ratio of SOD/MDA in the plasma and liver, and the upregulation of intestinal Keap1 and downregulation of HIF1α was found in FK3. Furthermore, FK-fed groups showed higher microbial richness and diversity. Pearson correlation analysis found that antioxidant relevant biomarkers were negatively correlated with the relative abundance of certain potential beneficial bacteria. In conclusion, supplemented up to 3–6% FK replacing FM in a low FM diet of largemouth bass could increase growth, survival rate, antioxidant capacity, and improve gut microbiota.

Neuropeptide Y and melanocortin receptors in fish: regulators of energy homeostasis

Energy homeostasis, which refers to the physiological processes that the energy intake is exquisitely coordinated with energy expenditure, is critical for survival. Therefore, multiple and complex mechanisms have been involved in the regulation of energy homeostasis. The central melanocortin system plays an important role in modulating energy homeostasis. This system includes the orexigenic neurons, expressing neuropeptide Y/Agouti-related protein (NPY/AgRP), and the anorexigenic neurons expressing proopiomelanocortin (POMC). The downstream receptors of NPY, AgRP and post-translational products of POMC are G protein-coupled receptors (GPCRs). This review summarizes the compelling evidence demonstrating that NPY and melanocortin receptors are involved in energy homeostasis. Subsequently, the comparative studies on physiology and pharmacology of NPY and melanocortin receptors in humans, rodents and teleosts are summarized. Also, we provide a strategy demonstrating the potential application of the new ligands and/or specific variants of melanocortin system in aquaculture.

Topmouth culter melanocortin-3 receptor: regulation by two isoforms of melanocortin-2 receptor accessory protein 2

Melanocortin-3 receptor (MC3R) is a regulator of energy homeostasis, and interaction of MC3R and melanocortin-2 receptor accessory protein 2 (MRAP2) plays a critical role in MC3R signaling of mammals. However, the physiological roles of MC3R in teleosts are not well understood. In this study, qRT-PCR was used to measure gene expression. Radioligand binding assay was used to study the binding properties of topmouth culter MC3R (caMC3R). Intracellular cAMP generation was determined by RIA, and caMC3R expression was quantified with flow cytometry. We showed that culter mc3r had higher expression in the CNS. All agonists could bind and stimulate caMC3R to increase dose dependently intracellular cAMP accumulation. Compared to human MC3R, culter MC3R showed higher constitutive activity, higher efficacies, and Rmax to alpha-melanocyte-stimulating hormone (α-MSH), des-α-MSH, and adrenocorticotrophic hormone. Both caMRAP2a and caMRAP2b markedly decreased caMC3R basal cAMP production. However, only caMRAP2a significantly decreased cell surface expression, Bmax, and Rmax of caMC3R. Expression analysis suggested that MRAP2a and MRAP2b might be more important in regulating MC3R/MC4R signaling during larval period, and reduced mc3r, mc4r, and pomc expression might be primarily involved in modulation of MC3R/MC4R in adults. These data indicated that the cloned caMC3R was a functional receptor. MRAP2a and MRAP2b had different effects on expression and signaling of caMC3R. In addition, expression analysis suggested that MRAP2s, receptors, and hormones might play different roles in regulating culter development and growth.

Differential Regulation of Gonadotropins as Revealed by Transcriptomes of Distinct LH and FSH Cells of Fish Pituitary

From mammals to fish, reproduction is driven by luteinizing hormone (LH) and follicle-stimulating hormone (FSH) temporally secreted from the pituitary gland. Teleost fish are an excellent model for addressing the unique regulation and function of each gonadotropin cell since, unlike mammals, they synthesize and secrete LH and FSH from distinct cells. Only very distant vertebrate classes (such as fish and birds) demonstrate the mono-hormonal strategy, suggesting a potential convergent evolution. Cell-specific transcriptome analysis of double-labeled transgenic tilapia expressing GFP and RFP in LH or FSH cells, respectively, yielded genes specifically enriched in each cell type, revealing differences in hormone regulation, receptor expression, cell signaling, and electrical properties. Each cell type expresses a unique GPCR signature that reveals the direct regulation of metabolic and homeostatic hormones. Comparing these novel transcriptomes to that of rat gonadotrophs revealed conserved genes that might specifically contribute to each gonadotropin activity in mammals, suggesting conserved mechanisms controlling the differential regulation of gonadotropins in vertebrates.

Alpha-melanocyte stimulating hormone immunoreactivity in the brain of the cichlid fish Oreochromis mossambicus

This study reports the distribution of a pro-opiomelanocortin-derived neuropeptide α-MSH in the brain of the cichlid fish Oreochromis mossambicus. α-MSH-ir fibres were found in the granule cell layer of the olfactory bulb, the medial olfactory tract, the pallium and the subpallium, whereas in the preoptic area of the telencephalon, few large α-MSH-ir perikarya along with extensively labeled fibres were observed close to the ventricular border. Dense network of α-MSH-ir fibres were seen in the hypothalamic areas such as the nucleus preopticus pars magnocellularis, the nucleus preopticus pars parvocellularis, the suprachiasmatic nucleus, the nucleus anterior tuberis, the paraventricular organ, the subdivisions of the nucleus recessus lateralis and the nucleus recessus posterioris. In the nucleus lateralis pars medialis, some α-MSH-ir perikarya and fibres were found along the ventricular margin. In the diencephalon, numerous α-MSH-ir fibres were detected in the nucleus posterior tuberis, the nucleus of the fasciculus longitudinalis medialis and the nucleus preglomerulosus medialis, whereas in the mesencephalon, α-MSH-ir fibres were located in the optic tectum, the torus semicircularis and the tegmentum. In the rhombencephalon, α-MSH-ir fibres were confined to the medial octavolateralis nucleus and the descending octaval nucleus. In the pituitary gland, densely packed α-MSH-ir cells were observed in the pars intermedia region. The widespread distribution of α-MSH-immunoreactivity throughout the brain and the pituitary gland suggests a role for α-MSH peptide in regulation of several neuroendocrine and sensorimotor functions as well as darkening of pigmentation in the tilapia.

Understanding neurobehavioral effects of acute and chronic stress in zebrafish

Stress is a common cause of neuropsychiatric disorders, evoking multiple behavioral, endocrine and neuro-immune deficits. Animal models have been extensively used to understand the mechanisms of stress-related disorders and to develop novel strategies for their treatment. Complementing rodent and clinical studies, the zebrafish (Danio rerio) is one of the most important model organisms in biomedicine. Rapidly becoming a popular model species in stress neuroscience research, zebrafish are highly sensitive to both acute and chronic stress, and show robust, well-defined behavioral and physiological stress responses. Here, we critically evaluate the utility of zebrafish-based models for studying acute and chronic stress-related CNS pathogenesis, assess the advantages and limitations of these aquatic models, and emphasize their relevance for the development of novel anti-stress therapies. Overall, the zebrafish emerges as a powerful and sensitive model organism for stress research. Although these fish generally display evolutionarily conserved behavioral and physiological responses to stress, zebrafish-specific aspects of neurogenesis, neuroprotection and neuro-immune responses may be particularly interesting to explore further, as they may offer additional insights into stress pathogenesis that complement (rather than merely replicate) rodent findings. Compared to mammals, zebrafish models are also characterized by increased availability of gene-editing tools and higher throughput of drug screening, thus being able to uniquely empower translational research of genetic determinants of stress and resilience, as well as to foster innovative CNS drug discovery and the development of novel anti-stress therapies.

Melanocortin Receptor 4 (MC4R) Signaling System in Nile Tilapia

The melanocortin receptor 4 (MC4R) signaling system consists of MC4R, MC4R ligands [melanocyte-stimulating hormone (MSH), adrenocorticotropin (ACTH), agouti-related protein (AgRP)], and melanocortin-2 receptor accessory protein 2 (MRAP2), and it has been proposed to play important roles in feeding and growth in vertebrates. However, the expression and functionality of this system have not been fully characterized in teleosts. Here, we cloned tilapia MC4R, MRAP2b, AgRPs (AgRP, AgRP2), and POMCs (POMCa1, POMCb) genes and characterized the interaction of tilapia MC4R with MRAP2b, AgRP, α-MSH, and ACTH in vitro. The results indicate the following. (1) Tilapia MC4R, MRAP2b, AgRPs, and POMCs share high amino acid identity with their mammalian counterparts. (2) Tilapia MRAP2b could interact with MC4R expressed in CHO cells, as demonstrated by Co-IP assay, and thus decrease MC4R constitutive activity and enhance its sensitivity to ACTH_1-40. (3) As in mammals, AgRP can function as an inverse agonist and antagonist of MC4R, either in the presence or absence of MRAP2b. These data, together with the co-expression of MC4R, MRAP2b, AgRPs, and POMCs in tilapia hypothalamus, suggest that as in mammals, ACTH/α-MSH, AgRP, and MRAP2 can interact with MC4R to control energy balance and thus play conserved roles in the feeding and growth of teleosts.

Biased signaling in fish melanocortin-4 receptors (MC4Rs): Divergent pharmacology of four ligands on spotted scat (Scatophagus argus) and grass carp (Ctenopharyngodon idella) MC4Rs

The melanocortin-4 receptor (MC4R) plays a critical role in the regulation of energy homeostasis in both mammals and fish. Several fish MC4Rs recently characterized have high constitutive activities, potentially associated with food intake and growth rate. In the present study, we systematically investigated the effects of four human MC4R (hMC4R) antagonists, including agouti-related peptide (AgRP), Ipsen 5i, ML00253764, and MCL0020, on the cAMP and ERK1/2 signaling of two fish MC4Rs: spotted scat (Scatophagus argus) MC4R (saMC4R) and grass carp (Ctenopharyngodon idella) MC4R (ciMC4R), with hMC4R as a control. We showed that both saMC4R and ciMC4R were constitutively active with significantly increased basal cAMP levels. AgRP acted as an inverse agonist in cAMP signaling pathway in both fish MC4Rs whereas MCL0020 functioned as an inverse agonist for ciMC4R but a weak neutral antagonist for saMC4R. Ipsen 5i and MCL0020 behaved as neutral allosteric modulators in the cAMP signaling of fish MC4Rs. The saMC4R and ciMC4R had similar basal pERK1/2 levels as hMC4R and the pERK1/2 levels of the two fish MC4Rs were significantly increased upon stimulation with all four ligands. In summary, our studies demonstrated the existence of biased signaling in fish MC4R. We also showed dramatic pharmacological differences of human and fish MC4Rs with synthetic ligands. Our data provided novel insights and led to a better understanding of fish MC4R pharmacology.

The Melanocortin System in Atlantic Salmon (Salmo salar L.) and Its Role in Appetite Control

The melanocortin system is a key neuroendocrine network involved in the control of food intake and energy homeostasis in vertebrates. Within the hypothalamus, the system comprises two main distinct neuronal cell populations that express the neuropeptides proopiomelanocortin (POMC; anorexigenic) or agouti-related protein (AGRP; orexigenic). Both bind to the melanocortin-4 receptor (MC4R) in higher order neurons that control both food intake and energy expenditure. This system is relatively well-conserved among vertebrates. However, in Atlantic salmon (Salmo salar L.), the salmonid-specific fourth round whole-genome duplication led to the presence of several paralog genes which might result in divergent functions of the duplicated genes. In the current study, we report the first comprehensive comparative identification and characterization of Mc4r and extend the knowledge of Pomc and Agrp in appetite control in Atlantic salmon. In silico analysis revealed multiple paralogs for mc4r (a1, a2, b1, and b2) in the Atlantic salmon genome and confirmed the paralogs previously described for pomc (a1, a2, and b) and agrp (1 and 2). All Mc4r paralogs are relatively well-conserved with the human homolog, sharing at least 63% amino acid sequence identity. We analyzed the mRNA expression of mc4r, pomc, and agrp genes in eight brain regions of Atlantic salmon post-smolt under two feeding states: normally fed and fasted for 4 days. The mc4ra2 and b1 mRNAs were predominantly and equally abundant in the hypothalamus and telencephalon, the mc4rb2 in the hypothalamus, and a1 in the telencephalon. All pomc genes were highly expressed in the pituitary, followed by the hypothalamus and saccus vasculosus. The agrp genes showed a completely different expression pattern from each other, with prevalent expression of the agrp1 in the hypothalamus and agrp2 in the telencephalon. Fasting did not induce any significant changes in the mRNA level of mc4r, agrp, or pomc paralogs in the hypothalamus or in other highly expressed regions between fed and fasted states. The identification and wide distribution of multiple paralogs of mc4r, pomc, and agrp in Atlantic salmon brain provide new insights and give rise to new questions of the melanocortin system in the appetite regulation in Atlantic salmon.

Regulation of Melanocortin-4 Receptor Pharmacology by Two Isoforms of Melanocortin Receptor Accessory Protein 2 in Topmouth Culter (Culter alburnus)

Melanocortin-4 receptor (MC4R) plays important roles in regulation of multiple physiological processes, and interaction of MC4R and melanocortin receptor accessory protein 2 (MRAP2) is suggested to play pivotal role in energy balance of vertebrates. Topmouth culter (Culter alburnus) is an economically important freshwater fish in China. Herein we cloned culter mc4r, mrap2a, and mrap2b. Culter mc4r consisted of a 981 bp open reading frame encoding a protein of 326 amino acids. qRT-PCR revealed that mc4r, mrap2a, and mrap2b were primarily expressed in the central nervous system. In the periphery, mc4r and mrap2b were expressed more widely in the male, while mrap2a was expressed more widely in the female. Culter MC4R could bind to four peptide agonists and increase intracellular cAMP production dose dependently. Culter MC4R was constitutively active in both cAMP and ERK1/2 pathways, which was differentially regulated by culter MRAP2a and MRAP2b. Culter MRAP2a significantly increased B_max and decreased agonist-stimulated cAMP, while MRAP2b increased cell surface and total expression but did not affect B_max and agonist-stimulated cAMP. These results will aid the investigation of the potential physiological processes that MC4R might be involved in topmouth culter.

Orange-spotted grouper melanocortin-4 receptor: Modulation of signaling by MRAP2

Melanocortin-4 receptor (MC4R) and melanocortin receptor accessory protein 2 (MRAP2) play important roles in the melanocortin system, and interaction of MC4R and MRAP2 is suggested to play pivotal role in energy balance of vertebrates. Orange-spotted grouper (Epinephelus coioides) is a widely cultured marine fish with high economic value in Asia. To explore potential interaction between grouper MC4R and MRAP2, herein we cloned grouper mc4r and mrap2. Grouper mc4r consisted of a 981 bp ORF encoding a putative protein of 327 amino acids, while the grouper mrap2 consisted of a 696 bp ORF encoding a putative protein of 232 amino acids. Sequence and phylogenetic analysis revealed that the grouper MC4R and MRAP2 were highly homologous at amino acid levels to several teleost MC4Rs and MRAP2s, respectively. qRT-PCR results showed that both mc4r and mrap2 were expressed primarily in the central nervous system. In the periphery, these genes were expressed more widely in male fish. The cloned grouper MC4R was functional, exhibiting high constitutive activity in cAMP pathway, capable of binding to three peptide agonists and increasing intracellular cAMP production dose-dependently. MRAP2 significantly decreased basal and agonist-stimulated cAMP signaling. MRAP2 also increased basal ERK1/2 activation but decreased ligand-induced stimulation when expressed at high levels. These data will facilitate future investigation of these molecules in regulating diverse physiological processes in orange-spotted grouper.

The effects of commonly used anaesthetics on colour measurements across body regions in the poeciliid fish, Girardinus metallicus

The effects of common anaesthetics on the hue, saturation and brightness measurements of the poeciliid fish Girardinus metallicus were investigated in two experiments. For both experiments the coloration of four body regions was measured from digital images of the same males obtained under three conditions: (1) control (in a water-filled chamber); (2) anaesthetised with MS-222; and (3) anaesthetised with eugenol (clove oil). In experiment 1 anaesthetised fish were photographed out of water. In experiment 2 all photographs were taken in a water-filled chamber. Anaesthetics altered coloration in both experiments. In the more methodologically consistent experiment 2 we found significantly different hue, increased saturation and decreased brightness in anaesthetic v. control conditions, consistent with darkening caused by the anaesthetics. The body regions differed in coloration consistent with countershading but did not differentially change in response to anaesthesia. These findings suggest that photographing fish in a water-filled chamber without anaesthetic is preferable for obtaining digital images for colour analysis and that multiple body regions of fish should be measured when assessing coloration patterns meaningful in behavioural contexts, to account for the gradients caused by countershading. We are encouraged that some researchers employ such methods already and caution against using anaesthetics except when absolutely necessary for immobilisation.

Effects of fasting on the central expression of appetite-regulating and reproductive hormones in wild-type and Casper zebrafish (Danio rerio)

Appetite and reproduction are closely related functions that are both regulated by brain hormones. Appetite stimulators include orexin and neuropeptide Y (NPY), and reproductive hormones include gonadotropin-releasing hormone (GnRH), gonadotropin-inhibitory hormone (GnIH), kisspeptin, and neurokinin B (NKB). GnRH stimulates the secretion of pituitary gonadotropes, and kisspeptin and GnIH modulate this action. Kisspeptin secretion is further controlled by neurokinin B (NKB) and dynorphin A (Dyn). To better understand the mechanisms regulating appetite and reproduction in fish, we examined the effects of fasting, reproductive stage, gender, and strain on the brain mRNA expression of appetite (orexin and NPY) and reproductive (GnRH, kisspeptin, GnIH, and NKB) hormones in zebrafish. In order to compare strains, we used both wild-type and transparent Casper zebrafish. In female wild-type zebrafish, fasting increased the expression of all hormones investigated, with the exception of Kiss2. Only NPY and Kiss2 were increased in male wild-type zebrafish during fasting. In Casper zebrafish, only GnIH and NKB in males were affected by fasting, suggesting that Casper fish may be more resistant to fasting than wild fish. Fasting increased expressions of orexin, GnRH2, Kiss1, GnIH and NKB in wild-type females with more eggs or larger eggs relative to body weight, compared to those with fewer or smaller eggs, suggesting that more mature females are more affected by fasting. No significant interactions of fasting and reproductive stage were noted in female Casper fish. To investigate whether differences between Casper and wild-type fish were due to genes involved in pigmentation, we compared the brain mRNA expressions of enzymes involved in melanin synthesis (tyrosinase and tyrosine hydroxylase - TH), melanocortin receptors (MC3R and MC4R), and the melanocortin precursor (proopiomelanocortin - POMC) between the two strains. Casper zebrafish had lower levels of MC3R, tyrosinase, TH1, TH2, and POMC than wild-type fish. Overall, our results suggest the existence of gender- and reproductive stage-specific, as well as strain-specific variations in the mechanisms regulating feeding and reproduction in zebrafish, and that the melanocortin system and melanin pathways may be in part responsible for these differences between strains.

Analyzing the signaling properties of gar (Lepisosteus oculatus) melanocortin receptors: Evaluating interactions with MRAP1 and MRAP2

RT-PCR analysis of gar pituitary and brain indicated that different combinations of gar melanocortin receptor mRNAs are present in the same tissues with mRNAs for gar mrap1 and gar mrap2. Against this background, an objective of this study was to determine whether the ligand sensitivity for either ACTH or α-MSH was affected when gar (g) melanocortin receptors (Mcrs) were co-expressed with either of the accessory proteins gMrap1 or gMrap2 in Chinese Hamster Ovary cells. The results indicated that gMc2r has an obligatory requirement for co-expression with gMrap1 in order for the receptor to be activated by hACTH(1-24). In addition, activation of gMc2r did not occur when the receptor was expressed alone or co-expressed with gMrap2. Furthermore, co-expression of gMc2r with gMrap1 followed by stimulation with NDP-MSH resulted in a low level of activation (only at 10-7 M and 10-6 M). However, gMc1r, gMc3r, gMc4r, and gMc5r responded to stimulation by NDP-MSH in a more robust manner. Co-expression of gMc1r, gMc3r, gMc4r, and gMc5r with gMRAP1 had no effect on sensitivity to stimulation by NDP-MSH or hACTH(1-24). Co-expression with gMRAP2 had no negative or positive effect on ligand sensitivity for gMc1r, gMc3r, and gMc5r, however this treatment did increase the activation of CHO cells transfected with gMc4r following stimulation with both hACTH(1-24) (p < 0.001), and NDP-MSH (p < 0.001). Co-expression of gMC5R with either gMRAP1 or gMRAP2 increased trafficking of gMC5R to the plasma membrane. These pharmacological observations are compared to the response of melanocortin receptors from other neopterygian fishes, cartilaginous fishes, and tetrapods to stimulation by ACTH(1-24) and forms of α-MSH.

Appetite-Controlling Endocrine Systems in Teleosts

Mammalian studies have shaped our understanding of the endocrine control of appetite and body weight in vertebrates and provided the basic vertebrate model that involves central (brain) and peripheral signaling pathways as well as environmental cues. The hypothalamus has a crucial function in the control of food intake, but other parts of the brain are also involved. The description of a range of key neuropeptides and hormones as well as more details of their specific roles in appetite control continues to be in progress. Endocrine signals are based on hormones that can be divided into two groups: those that induce (orexigenic), and those that inhibit (anorexigenic) appetite and food consumption. Peripheral signals originate in the gastrointestinal tract, liver, adipose tissue, and other tissues and reach the hypothalamus through both endocrine and neuroendocrine actions. While many mammalian-like endocrine appetite-controlling networks and mechanisms have been described for some key model teleosts, mainly zebrafish and goldfish, very little knowledge exists on these systems in fishes as a group. Fishes represent over 30,000 species, and there is a large variability in their ecological niches and habitats as well as life history adaptations, transitions between life stages and feeding behaviors. In the context of food intake and appetite control, common adaptations to extended periods of starvation or periods of abundant food availability are of particular interest. This review summarizes the recent findings on endocrine appetite-controlling systems in fish, highlights their impact on growth and survival, and discusses the perspectives in this research field to shed light on the intriguing adaptations that exist in fish and their underlying mechanisms.

α-MSH and melanocortin receptors at early ontogeny in European sea bass (Dicentrarchus labrax, L.)

Temporal patterns of whole-body α-MSH concentrations and of transcripts of melanocortin receptors during early development as well as the endocrine response (α-MSH, cortisol, MCR mRNAs) to stress at the end of the larval period were characterized in Dicentrarchus labrax. Immunohistochemistry showed α-MSH positive cells in the pituitary pars intermedia in all stages examined. As development proceeds, α-MSH content gradually increases; mRNA levels of mc2r and mc4r remain low until first feeding where peak values are observed. Mc1r expression was constant during development, pomc mRNA levels remain low until the stage of flexion after which a significant increase is observed. At the stage of the formation of all fins, whole-body cortisol and α-MSH concentrations responded with peak values at 2 h post stress. Additionally, the stress challenge resulted in elevated transcript levels of pomc, mc2r and mc4r but not in mc1r, with a pattern characterized by peak values at 1 h post stress and a strong correlation with whole body α-MSH concentrations was found. Our data provide for the first time a view on the importance of the α-MSH stress response in early development of European sea bass, an additional and relatively poorly understood signal involved in the stress response in teleosts.

ACTH Receptor (MC2R) Specificity: What Do We Know About Underlying Molecular Mechanisms?

Coincidentally, the release of this Research Topic in Frontiers in Endocrinology takes place 25 years after the discovery of the adrenocorticotropic hormone receptor (ACTHR) by Mountjoy and colleagues. In subsequent years, following the discovery of other types of mammalian melanocortin receptors (MCRs), ACTHR also became known as melanocortin type 2 receptor (MC2R). At present, five types of MCRs have been reported, all of which share significant sequence similarity at the amino acid level, and all of which specifically bind melanocortins (MCs)-a group of biologically active peptides generated by proteolysis of the proopiomelanocortin precursor. All MCs share an identical -H-F-R-W- pharmacophore sequence. α-Melanocyte-stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH) are the most extensively studied MCs and are derived from the same region. Essentially, α-MSH is formed from the first 13 amino acid residues of ACTH. ACTHR is unique among MCRs because it binds one sole ligand-ACTH, which makes it a very attractive research object for molecular pharmacologists. However, much research has failed, and functional studies of this receptor are lagging behind other MCRs. The reason for these difficulties has already been outlined by Mountjoy and colleagues in their publication on ACTHR coding sequence discovery where the Cloudman S91 melanoma cell line was used for receptor expression because it was a "more sensitive assay system." Subsequent work showed that ACTHR could be successfully expressed only in endogenous MCR-expressing cell lines, since in other cell lines it is retained within the endoplasmic reticulum. The resolution of this methodological problem came in 2005 with the discovery of melanocortin receptor accessory protein, which is required for the formation of functionally active ACTHR. The decade that followed this discovery was filled with exciting research that provided insight into the molecular mechanisms underlying the action of ACTHR. The purpose of this review is to summarize the advances in this fascinating research field.

Regulation of feeding behavior and food intake by appetite-regulating peptides in wild-type and growth hormone-transgenic coho salmon

Survival, competition, growth and reproductive success in fishes are highly dependent on food intake, food availability and feeding behavior and are all influenced by a complex set of metabolic and neuroendocrine mechanisms. Overexpression of growth hormone (GH) in transgenic fish can result in greatly enhanced growth rates, feed conversion, feeding motivation and food intake. The objectives of this study were to compare seasonal feeding behavior of non-transgenic wild-type (NT) and GH-transgenic (T) coho salmon (Oncorhynchus kisutch), and to examine the effects of intraperitoneal injections of the appetite-regulating peptides cholecystokinin (CCK-8), bombesin (BBS), glucagon-like peptide-1 (GLP-1), and alpha-melanocyte-stimulating hormone (α-MSH) on feeding behavior. T salmon fed consistently across all seasons, whereas NT dramatically reduced their food intake in winter, indicating the seasonal regulation of appetite can be altered by overexpression of GH in T fish. Intraperitoneal injections of CCK-8 and BBS caused a significant and rapid decrease in food intake for both genotypes. Treatment with either GLP-1 or α-MSH resulted in a significant suppression of food intake for NT but had no effect in T coho salmon. The differential response of T and NT fish to α-MSH is consistent with the melanocortin-4 receptor system being a significant pathway by which GH acts to stimulate appetite. Taken together, these results suggest that chronically increased levels of GH alter feeding regulatory pathways to different extents for individual peptides, and that altered feeding behavior in transgenic coho salmon may arise, in part, from changes in sensitivity to peripheral appetite-regulating signals.

Food intake and appetite control in a GH-transgenic zebrafish

The biological actions of growth hormone (GH) are pleiotropic, including growth promotion, energy mobilization, gonadal development, appetite, and social behavior. The regulatory network for GH is complex and includes many central and peripheral endocrine factors as well as that from the environment. It is known that GH transgenesis results in increased growth, food intake, and consequent metabolic rates in fishes. However, the manner in which GH transgenesis alters the energetic metabolism in fishes has not been well explored. In order to elucidate these consequences, we examined the effect of GH overexpression on appetite control mechanisms in a transgenic zebrafish (Danio rerio) model. To this, we analyzed feeding behavior and the expression of the main appetite-related genes in two different feeding periods (fed and fasting) in non-transgenic (NT) and transgenic (T) zebrafish as well as glycaemic parameters of them. Our initial results have shown that NT males and females present the same feeding behavior and expression of main appetite-controlling genes; therefore, the data of both sexes were properly grouped. Following grouped data analyses, we compared the same parameters in NT and T animals. Feeding behavior results have shown that T animals eat significantly more and faster than NT siblings. Gene expression results pointed out that gastrointestinal (GT) cholecystokinin has a substantial contribution to the communication between peripheral and central control of food intake. Brain genes expression analyses revealed that T animals have a down-regulation of two strong and opposite peptides related to food intake: the anorexigenic proopiomelanocortin (pomc) and the orexigenic neuropeptide Y (npy). The down-regulation of pomc in T when compared with NT is an expected result, since the decrease in an anorexigenic factor might keep the transgenic fish hungry. The down-regulation of npy seemed to be contradictory at first, but if we consider the GH's capacity to elevate blood glucose, and that NPY is able to respond to humoral factors like glucose, this down-regulation makes sense. In fact, our last experiment showed that transgenics presented elevated blood glucose levels, confirming that npy might responded to this humoral factor. In conclusion, we have shown that GT responds to feeding status without interference of transgenesis, whereas brain responds to GH transgenesis without any effect of treatment. It is clear that transgenic zebrafish eat more and faster, and it seems that it occurs due to pomc down-regulation, since npy might be under regulation of the humoral factor glucose.

Exposure to environmental levels of waterborne cadmium impacts corticosteroidogenic and metabolic capacities, and compromises secondary stressor performance in rainbow trout

The physiological responses to waterborne cadmium exposure have been well documented; however, few studies have examined animal performances at low exposure concentrations of this metal. We tested the hypothesis that longer-term exposure to low levels of cadmium will compromise the steroidogenic and metabolic capacities, and reduce the cortisol response to a secondary stressor in fish. To test this, juvenile rainbow trout (Oncorhynchus mykiss) were exposed to 0 (control), 0.75 or 2.0 μg/L waterborne cadmium in a flow-through system and were sampled at 1, 7 and 28 d of exposure. There were only very slight disturbances in basal plasma cortisol, lactate or glucose levels in response to cadmium exposure over the 28 d period. Chronic cadmium exposure significantly affected key genes involved in corticosteroidogenesis, including melanocortin 2 receptor, steroidogenic acute regulatory protein and cytochrome P450 side chain cleavage enzyme. At 28 d, the high cadmium exposure group showed a significant drop in the glucocorticoid receptor and mineralocorticoid receptor protein expressions in the liver and brain, respectively. There were also perturbations in the metabolic capacities in the liver and gill of cadmium-exposed trout. Subjecting these fish to a secondary handling disturbance led to a significant attenuation of the stressor-induced plasma cortisol, glucose and lactate levels in the cadmium groups. Collectively, although trout appears to adjust to subchronic exposure to low levels of cadmium, it may be at the cost of impaired interrenal steroidogenic and tissue-specific metabolic capacities, leading to a compromised secondary stress performance in rainbow trout.

The pituitary gland of the European eel reveals massive expression of genes involved in the melanocortin system

Hormones secreted from the pituitary gland regulate important processes such as development, growth and metabolism, reproduction, water balance, and body pigmentation. Synthesis and secretion of pituitary hormones are regulated by different factors from the hypothalamus, but also through feedback mechanisms from peripheral organs, and from the pituitary itself. In the European eel extensive attention has been directed towards understanding the different components of the brain-pituitary-gonad axis, but little is known about the regulation of upstream processes in the pituitary gland. In order to gain a broader mechanistic understanding of the eel pituitary gland, we have performed RNA-seq transcriptome profiling of the pituitary of prepubertal female silver eels. RNA-seq reads generated on the Illumina platform were mapped to the recently assembled European eel genome. The most abundant transcript in the eel pituitary codes for pro-opiomelanocortin, the precursor for hormones of the melanocortin system. Several genes putatively involved in downstream processing of pro-opiomelanocortin were manually annotated, and were found to be highly expressed, both by RNA-seq and by qPCR. The melanocortin system, which affects skin color, energy homeostasis and in other teleosts interacts with the reproductive system, has so far received limited attention in eels. However, since up to one third of the silver eel pituitary's mRNA pool encodes pro-opiomelanocortin, our results indicate that control of the melanocortin system is a major function of the eel pituitary.

Gene amplification and functional diversification of melanocortin 4 receptor at an extremely polymorphic locus controlling sexual maturation in the platyfish

In two swordtail species of the genus Xiphophorus, the onset of puberty has been shown to be modulated at the P locus by sequence polymorphism and gene copy-number variation affecting the type 4 melanocortin hormone receptor Mc4r. The system works through the interaction of two allelic types, one encoding wild type and the other dominant-negative receptors. We have analyzed the structure and evolution of the P locus in the platyfish Xiphophorus maculatus, where as many as nine alleles of P determining the onset of sexual maturity in males and females, fecundity in females, and adult size in males are located on both the X and Y chromosomes in a region linked to the master sex-determining locus. In this species, mc4r has been amplified to up to 10 copies on both the X and Y chromosomes through recent large serial duplications. Subsequently, mc4r paralogues have diverged considerably into many different subtypes. Certain copies have acquired new untranslated regions through genomic rearrangements, and transposable element insertions and other mutations have accumulated in promoter regions, possibly explaining observed deviations from the classical mc4r transcriptional pattern. In the mc4r-coding sequence, in-frame insertions and deletions as well as nonsense and missense mutations have generated a high diversity of Mc4r-predicted proteins. Most of these variants are expressed in embryos, adults, and/or tumors. Functional receptor characterization demonstrated major divergence in pharmacological behavior for Mc4r receptors encoded by different copies of platyfish mc4r, with differences in constitutive activity as well as binding and stimulation by hormones. The high degree of allelic and copy-number variation observed between individuals can explain the high level of polymorphism for sexual maturation, fecundity, and body size in the platyfish: multiple combinations of Mc4r variants with different biochemical properties might interact to modulate the melanocortin signaling that regulates the hypothalamus-pituitary-gonadal axis.

Melanocortin receptor 1 and black pigmentation in the Japanese ornamental carp (Cyprinus carpio var. Koi)

Colors and their patterns are fascinating phenotypes with great importance for fitness under natural conditions. For this reason and because pigmentation is associated with diseases, much research was devoted to study the genetics of pigmentation in animals. Considerable contribution to our understanding of color phenotypes was made by studies in domesticated animals that exhibit dazzling variation in color traits. Koi strains, the ornamental variants of the common carp, are a striking example for color variability that was selected by man during a very short period on an evolutionary timescale. Among several pigmentation genes, genetic variation in Melanocrtin receptor 1 was repeatedly associated with dark pigmentation phenotypes in numerous animals. In this study, we cloned Melanocrtin receptor 1 from the common carp. We found that alleles of the gene were not associated with the development of black color in Koi. However, the mRNA expression levels of the gene were higher during dark pigmentation development in larvae and in dark pigmented tissues of adult fish, suggesting that variation in the regulation of the gene is associated with black color in Koi. These regulatory differences are reflected in both the timing of the dark-pigmentation development and the different mode of inheritance of the two black patterns associated with them. Identifying the genetic basis of color and color patterns in Koi will promote the production of this valuable ornamental fish. Furthermore, given the rich variety of colors and patterns, Koi serves as a good model to unravel pigmentation genes and their phenotypic effects and by that to improve our understanding of the genetic basis of colors also in natural populations.

Zebrafish 20β-hydroxysteroid dehydrogenase type 2 is important for glucocorticoid catabolism in stress response

Stress, the physiological reaction to a stressor, is initiated in teleost fish by hormone cascades along the hypothalamus-pituitary-interrenal (HPI) axis. Cortisol is the major stress hormone and contributes to the appropriate stress response by regulating gene expression after binding to the glucocorticoid receptor. Cortisol is inactivated when 11β-hydroxysteroid dehydrogenase (HSD) type 2 catalyzes its oxidation to cortisone. In zebrafish, Danio rerio, cortisone can be further reduced to 20β-hydroxycortisone. This reaction is catalyzed by 20β-HSD type 2, recently discovered by us. Here, we substantiate the hypothesis that 20β-HSD type 2 is involved in cortisol catabolism and stress response. We found that hsd11b2 and hsd20b2 transcripts were up-regulated upon cortisol treatment. Moreover, a cortisol-independent, short-term physical stressor led to the up-regulation of hsd11b2 and hsd20b2 along with several HPI axis genes. The morpholino-induced knock down of hsd20b2 in zebrafish embryos revealed no developmental phenotype under normal culture conditions, but prominent effects were observed after a cortisol challenge. Reporter gene experiments demonstrated that 20β-hydroxycortisone was not a physiological ligand for the zebrafish glucocorticoid or mineralocorticoid receptor but was excreted into the fish holding water. Our experiments show that 20β-HSD type 2, together with 11β-HSD type 2, represents a short pathway in zebrafish to rapidly inactivate and excrete cortisol. Therefore, 20β-HSD type 2 is an important enzyme in stress response.

Spliceosomal intron insertions in genome compacted ray-finned fishes as evident from phylogeny of MC receptors, also supported by a few other GPCRs

BACKGROUND

Insertions of spliceosomal introns are very rare events during evolution of vertebrates and the mechanisms governing creation of novel intron(s) remain obscure. Largely, gene structures of melanocortin (MC) receptors are characterized by intron-less architecture. However, recently a few exceptions have been reported in some fishes. This warrants a systematic survey of MC receptors for understanding intron insertion events during vertebrate evolution.

METHODOLOGY/PRINCIPAL FINDINGS

We have compiled an extended list of MC receptors from different vertebrate genomes with variations in fishes. Notably, the closely linked MC2Rs and MC5Rs from a group of ray-finned fishes have three and one intron insertion(s), respectively, with conserved positions and intron phase. In both genes, one novel insertion was in the highly conserved DRY motif at the end of helix TM3. Further, the proto-splice site MAG↑R is maintained at intron insertion sites in these two genes. However, the orthologs of these receptors from zebrafish and tetrapods are intron-less, suggesting these introns are simultaneously created in selected fishes. Surprisingly, these novel introns are traceable only in four fish genomes. We found that these fish genomes are severely compacted after the separation from zebrafish. Furthermore, we also report novel intron insertions in P2Y receptors and in CHRM3. Finally, we report ultrasmall introns in MC2R genes from selected fishes.

CONCLUSIONS/SIGNIFICANCE

The current repository of MC receptors illustrates that fishes have no MC3R ortholog. MC2R, MC5R, P2Y receptors and CHRM3 have novel intron insertions only in ray-finned fishes that underwent genome compaction. These receptors share one intron at an identical position suggestive of being inserted contemporaneously. In addition to repetitive elements, genome compaction is now believed to be a new hallmark that promotes intron insertions, as it requires rapid DNA breakage and subsequent repair processes to gain back normal functionality.

The use of the zebrafish model in stress research

The study of the causes and mechanisms underlying psychiatric disorders requires the use of non-human models for the test of scientific hypotheses as well as for use in pre-clinical drug screening and discovery. This review argues in favor of the use of zebrafish as a novel animal model to study the impact of early (stressful) experiences on the development of differential stress phenotypes in later life. This phenomenon is evolutionary conserved among several vertebrate species and has relevance to the etiology of psychiatric disorders. Why do we need novel animal models? Although significant progress has been achieved with the use of traditional mammalian models, there are major pitfalls associated with their use that impedes progress on two major fronts: 1) uncovering of the molecular mechanisms underlying aspects of compromised (stress-exposed) brain development relevant to the etiology of psychiatric disorders, and 2) ability to develop high-throughput technology for drug discovery in the field of psychiatry. The zebrafish model helps resolve these issues. Here we present a conceptual framework for the use of zebrafish in stress research and psychiatry by addressing three specific domains of application: 1) stress research, 2) human disease mechanisms, and 3) drug discovery. We also present novel methodologies associated with the development of the zebrafish stress model and discuss how such methodologies can contribute to remove the main bottleneck in the field of drug discovery.

Testosterone and 11-ketotestosterone have different regulatory effects on electric communication signals of male Brachyhypopomus gauderio

The communication signals of electric fish can be dynamic, varying between the sexes on a circadian rhythm and in response to social and environmental cues. In the gymnotiform fish Brachyhypopomus gauderio waveform shape of the electric organ discharge (EOD) is regulated by steroid and peptide hormones. Furthermore, EOD amplitude and duration change on different timescales and in response to different social stimuli, suggesting that they are regulated by different mechanisms. Little is known about how androgen and peptide hormone systems interact to regulate signal waveform. We investigated the relationship between the androgens testosterone (T) and 11-ketotestosterone (11-KT), the melanocortin peptide hormone α-MSH, and their roles in regulating EOD waveform of male B. gauderio. Males were implanted with androgen (T, 11-KT, or blank), and injected with α-MSH before and at the peak of androgen effect. We compared the effects of androgen implants and social interactions by giving males a size-matched male stimulus with which they could interact electrically. Social stimuli and both androgens increased EOD duration, but only social stimuli and 11-KT elevated amplitude. However, no androgen enhanced EOD amplitude to the extent of a social stimulus, suggesting that a yet unidentified hormonal pathway regulates this signal parameter. Additionally, both androgens increased response of EOD duration to α-MSH, but only 11-KT increased response of EOD amplitude to α-MSH. Social stimuli had no effect on EOD response to α-MSH. The finding that EOD amplitude is preferentially regulated by 11-KT in B. gauderio may provide the basis for independent control of amplitude and duration.

Vertebrate melanophores as potential model for drug discovery and development: a review

Drug discovery in skin pharmacotherapy is an enormous, continually expanding field. Researchers are developing novel and sensitive pharmaceutical products and drugs that target specific receptors to elicit concerted and appropriate responses. The pigment-bearing cells called melanophores have a significant contribution to make in this field. Melanophores, which contain the dark brown or black pigment melanin, constitute an important class of chromatophores. They are highly specialized in the bidirectional and coordinated translocation of pigment granules when given an appropriate stimulus. The pigment granules can be stimulated to undergo rapid dispersion throughout the melanophores, making the cell appear dark, or to aggregate at the center, making the cell appear light. The major signals involved in pigment transport within the melanophores are dependent on a special class of cell surface receptors called G-protein-coupled receptors (GPCRs). Many of these receptors of adrenaline, acetylcholine, histamine, serotonin, endothelin and melatonin have been found on melanophores. They are believed to have clinical relevance to skin-related ailments and therefore have become targets for high throughput screening projects. The selective screening of these receptors requires the recognition of particular ligands, agonists and antagonists and the characterization of their effects on pigment motility within the cells. The mechanism of skin pigmentation is incredibly intricate, but it would be a considerable step forward to unravel its underlying physiological mechanism. This would provide an experimental basis for new pharmacotherapies for dermatological anomalies. The discernible stimuli that can trigger a variety of intracellular signals affecting pigment granule movement primarily include neurotransmitters and hormones. This review focuses on the role of the hormone and neurotransmitter signals involved in pigment movement in terms of the pharmacology of the specific receptors.

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.

Selection for a dominant oncogene and large male size as a risk factor for melanoma in the Xiphophorus animal model

Adult height is a risk factor in numerous human cancers that involve aberrant receptor tyrosine kinase (RTK) signalling. However, its importance is debated due to conflicting epidemiological studies and the lack of useful in vivo models. In Xiphophorus fishes (Platyfishes/Swordtails), a functional RTK, Xiphophorus melanoma receptor kinase (Xmrk), serves as the dominant oncogene and has been maintained for several million years despite being deleterious and in an extremely unstable genomic region. Here we show that the Xmrk genotype is positively correlated with standard length in male and female wild caught Xiphophorus cortezi sampled throughout their phylogeographic distribution. Histopathology confirms the occurrence of malignant melanomas in both sexes; however, melanoma incidence was extremely male biased. Furthermore, males collected with malignant melanomas in the field were significantly larger than both Xmrk males collected without melanomas and wildtype (Xmrk deficient) males. These results not only provide a novel selective mechanism for the persistence of the germline Xmrk oncogene but also create an innovative avenue of melanoma research within the Xiphophorus fishes. Wildlife cancer in natural systems is a growing concern, therefore, future research investigating life history characteristics associated with certain phenotypes and genotypes that predispose an individual to cancer will be fundamental to increasing our understanding of the evolutionary biology of cancer in nature as well as in humans.

The genetic and evolutionary basis of colour variation in vertebrates

Variation in pigmentation is one of the most conspicuous phenotypic traits in vertebrates. Although mammals show less variation in body pigmentation than other vertebrate groups, the genetics of colour determination and variation is best understood for them. More than 150 genes have been identified that influence pigmentation, and in many cases, the cause for variation in pigmentation has been identified down to the underlying nucleotide changes. These studies show that while some genes are often responsible for deviating pigmentation, similar or almost identical phenotypes even in the same species may be due to mutations in different genes. In this review we will first discuss the current knowledge about the genes and their functions underlying the biochemical pathways that determine pigmentation and then give examples where the mutations responsible for colour variation have been determined. Finally, we will discuss potential evolutionary causes for and consequences of differences in pigmentation between individuals.

Differential expressions of melanocortin receptor subtypes in melanophores and xanthophores of barfin flounder

alpha-Melanocyte-stimulating hormone (alpha-MSH) is a member of the melanocortin (MC) family, and the MC receptor (MCR) is a member of the G protein-coupled receptor (GPCR) superfamily. We previously found that in barfin flounder, a flatfish, alpha-MSH with an acetyl group at the N-terminus stimulated pigment dispersion in xanthophores; however, this effect was not observed in melanophores. Therefore, the present study was undertaken to find which MCR subtypes are expressed in these pigment cells in order to elucidate how acetylation regulates activities of alpha-MSH-related peptides. Here, we also report the cloning of Mc1r and Mc5r from barfin flounder. Three types of cells-melanophores, xanthophores, and nonchromatophoric dermal cells-were isolated from the skin samples collected from the dorsal fin. These cells were then tested for the expression of Mc1r and Mc5r as well as Mc2r and Mc4r that we had previously cloned. Mc1r and Mc5r transcripts were detected in melanophores, and a sole Mc5r transcript was detected in xanthophores. We had previously found that the efficiency of alpha-MSH was higher than that of desacetyl-alpha-MSH for pigment dispersion in xanthophores. Acetylated MSH peptide may have increased binding affinity to MC5R, whereas alpha-MSH lacks melanin-dispersing activity. Increasing evidences indicate that many GPCRs form heterodimers, and this may affect the affinity of the ligand for the corresponding GPCR. Taken together, the expression of two different Mcr subtypes in melanophores may suggest that a heterodimer consisting of MC1R and MC5R may have a low binding affinity toward alpha-MSH. The present results clarify the types of MCRs that are expressed in melanophores and xanthophores of barfin flounder; furthermore, the results provide important clues about the functional regulation of alpha-MSH-related peptides.

A cancer-causing gene is positively correlated with male aggression in Xiphophorus cortezi

The persistence of seemingly maladaptive genes in organisms challenges evolutionary biological thought. In Xiphophorus fishes, certain melanin patterns form malignant melanomas because of a cancer-causing gene (Xiphophorus melanoma receptor kinase; Xmrk), which arose several millions years ago from unequal meiotic recombination. Xiphophorus melanomas are male biased and induced by androgens however male behaviour and Xmrk genotype has not been investigated. This study found that male X. cortezi with the spotted caudal (Sc) pattern, from which melanomas originate, displayed increased aggression in mirror image trials. Furthermore, Xmrk males (regardless of Sc phenotype) bit and performed more agonistic displays than Xmrk deficient males. Male aggressive response decreased when males viewed their Sc image as compared with their non-Sc image. Collectively, these results indicate that Xmrk males experience a competitive advantage over wild-type males and that intrasexual selection could be an important component in the evolutionary maintenance of this oncogene within Xiphophorus.

New ligands for melanocortin receptors

Named originally for their effects on peripheral end organs, the melanocortin system controls a diverse set of physiological processes through a series of five G-protein-coupled receptors and several sets of small peptide ligands. The central melanocortin system plays an essential role in homeostatic regulation of body weight, in which two alternative ligands, alpha-melanocyte-stimulating hormone and agouti-related protein, stimulate and inhibit receptor signaling in several key brain regions that ultimately affect food intake and energy expenditure. Much of what we know about the relationship between central melanocortin signaling and body weight regulation stems from genetic studies. Comparative genomic studies indicate that melanocortin receptors used for controlling pigmentation and body weight regulation existed more than 500 million years ago in primitive vertebrates, but that fine-grained control of melanocortin receptors through neuropeptides and endogenous antagonists developed more recently. Recent studies based on dog coat-color genetics revealed a new class of melanocortin ligands, the beta-defensins, which reveal the potential for cross talk between the melanocortin and the immune systems.

The zebrafish stress axis: molecular fallout from the teleost-specific genome duplication event

The teleost-specific whole genome duplication event 350 million years ago resulted in a variety of duplicated genes that exist in fish today. In this review, we examine whether molecular components involved in the functioning of the hypothalamus-pituitary-interrenal (HPI) axis are present as single or duplicate genes. Specifically, we looked at corticotropin releasing hormone (CRH), adrenocorticotropic hormone (ACTH) and the glucocorticoid receptor (GR). The focus is on zebrafish but a variety of species are covered whenever data is available through literature or genomic database searches. Duplicate CRH genes are retained in the salmoniformes and cypriniformes, and the peptide sequences are very similar or identical. Zebrafish, along with the Acanthopterygii, are the exceptions as they have a single CRH gene. Also, two copies of the proopiomelanocortin (POMC) gene, which encodes for ACTH and other peptides, have been observed in all teleosts except tilapia and sea bass. In zebrafish, ACTH is derived from only one POMC gene, since the cleavage site is mutated in the other gene. All teleosts examined to date have two GRs, including the recent discoveries of duplicate GRs in two species of cyprinids (carp and fathead minnow). Zebrafish are the only known exception with one GR gene. The loss of duplicate genes is not a general feature of the zebrafish genome, but zebrafish have lost the duplicate CRH, ACTH and GR genes in the past 33 million years, after possessing two of each for the previous 300 million years. The evolutionary pressures underlying the rapid loss of these HPI axis genes, and the implications on the development and the functioning of the evolutionarily conserved cortisol stress response in zebrafish are currently unknown.

Melanocortin peptides affect the motivation to feed in rainbow trout (Oncorhynchus mykiss)

In this study, we investigated the effects of one melanocortin receptor (MCR) agonist and two antagonists on food intake in juvenile rainbow trout. Baseline food intake was established prior to 1 microl intracerebroventricular injection (ICV) of the non-specific agonist MTII, the MC4R antagonist HS024 and the MC3/4R antagonist SHU9119 at concentrations of 0.3, 1 or 3 nM. Saline-injected fish and untreated fish served as controls. Changes in food intake were observed 1h after the ICV injections. Our results showed that treatment with MTII significantly decreased food intake at 3 nM compared to control, HS024 significantly increased food intake at 3 nM compared to control and saline-treated fish, and SHU9119 significantly increased food intake at 3 nM compared to saline-treated fish. In conclusion, our study provides further evidence, and hence strengthens the hypothesis, that MC4R participates in the control of energy balance in fish in the same manner as in mammals. Our findings that HS024 is more potent than SHU9119 in increasing food intake suggest that the effects of melanocortin on energy balance in rainbow trout are mainly regulated by activation of MC4R. Hence, HS024 seems an excellent tool as a MC4R antagonist in rainbow trout.

A novel role for Mc1r in the parallel evolution of depigmentation in independent populations of the cavefish Astyanax mexicanus

The evolution of degenerate characteristics remains a poorly understood phenomenon. Only recently has the identification of mutations underlying regressive phenotypes become accessible through the use of genetic analyses. Focusing on the Mexican cave tetra Astyanax mexicanus, we describe, here, an analysis of the brown mutation, which was first described in the literature nearly 40 years ago. This phenotype causes reduced melanin content, decreased melanophore number, and brownish eyes in convergent cave forms of A. mexicanus. Crosses demonstrate non-complementation of the brown phenotype in F₂ individuals derived from two independent cave populations: Pachón and the linked Yerbaniz and Japonés caves, indicating the same locus is responsible for reduced pigmentation in these fish. While the brown mutant phenotype arose prior to the fixation of albinism in Pachón cave individuals, it is unclear whether the brown mutation arose before or after the fixation of albinism in the linked Yerbaniz/Japonés caves. Using a QTL approach combined with sequence and functional analyses, we have discovered that two distinct genetic alterations in the coding sequence of the gene Mc1r cause reduced pigmentation associated with the brown mutant phenotype in these caves. Our analysis identifies a novel role for Mc1r in the evolution of degenerative phenotypes in blind Mexican cavefish. Further, the brown phenotype has arisen independently in geographically separate caves, mediated through different mutations of the same gene. This example of parallelism indicates that certain genes are frequent targets of mutation in the repeated evolution of regressive phenotypes in cave-adapted species.

As we approach the 150th year since publication of On the Origin of Species, understanding the genetic architecture underlying evolutionary change remains an important challenge. When an organism enters a completely new environment or ecological niche, certain traits are no longer necessary for survival, while other new traits become critical for maintaining fitness. An example of such a transition is provided by cave animals. Many disparate taxa (e.g., crustaceans, salamanders, fish) have colonized caves, presumably to escape predation or expand populations into an unexploited niche. Strikingly, similar traits evolve convergently despite significant phylogenetic distance between these organisms. Caves provide a unique environment including the absence of light, few predators, few sources of food, etc. Under these conditions, one observes striking changes in morphology including reduction in eyes, expansion of non-visual sensory systems, and a suite of metabolic and behavioral changes. To understand the genetic underpinnings of these shifts, we have established the blind Mexican cave tetra, A. mexicanus, as a genetic system. In this paper, we use this system to investigate a classic morphological feature in these animals, depigmentation. We identify the gene Mc1r as being responsible for reduction in melanin content in multiple caves.