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

Agouti-Induced Anxiety-Like Behavior Is Mediated by Central Serotonergic Pathways in Zebrafish

Overexpression of the agouti-signaling protein (asip1), an endogenous melanocortin antagonist, under the control of a constitutive promoter in zebrafish [Tg(Xla.Eef1a1:Cau.Asip1]iim4] (asip1-Tg) increases food intake by reducing sensitivity of the central satiety systems and abolish circadian activity rhythms. The phenotype also shows increased linear growth and body weight, yet no enhanced aggressiveness in dyadic fights is observed. In fact, asip1-Tg animals choose to flee to safer areas rather than face a potential threat, thus suggesting a potential anxiety-like behavior (ALB). Standard behavioral tests, i.e., the open field test (OFT), the novel object test (NOT), and the novel tank dive test (NTDT), were used to investigate thigmotaxis and ALB in male and female zebrafish. Results showed that the asip1-Tg strain exhibited severe ALB in every test, mainly characterized by pronounced freezing behavior and increased linear and angular swimming velocities. asip1-Tg animals exhibited low central serotonin (5-HT) and dopamine (DA) levels and high turnover rates, thus suggesting that central monoaminergic pathways might mediate melanocortin antagonist-induced ALB. Accordingly, the treatment of asip1-Tg animals with fluoxetine, a selective serotonin reuptake inhibitor (SSRI), reversed the ALB phenotype in NTDT as well as 5-HT turnover. Genomic and anatomical data further supported neuronal interaction between melanocortinergic and serotonergic systems. These results suggest that inhibition of the melanocortin system by ubiquitous overexpression of endogenous antagonist has an anxiogenic effect mediated by serotonergic transmission.

Dark Morph of the Oriental Honey-Buzzard (Pernis ptilorhynchus orientalis) is Attributable to Specific MC1R Haplotypes

A thorough understanding of the development of complex plumages in birds necessitates the acquisition of genetic data pertaining to the mechanism underlying this phenomenon from various avian species. The oriental honey-buzzard (Pernis ptilorhynchus orientalis), a tropical summer migrant to Northeast Asia, including Japan, exemplifies this aspect owing to the diversity of its ventral coloration and intra-feather barring patterns. However, genetic polymorphism responsible for this diversity has not been identified yet. This study aimed to investigate the link between dark-plumed phenotypes of this subspecies and haplotypes of the melanocortin-1-receptor (MC1R) gene. A draft sequence of MC1R was constructed using next generation sequencing and subsequently amplified using designed polymerase chain reaction (PCR) primers. The genome sequences of 32 honey-buzzard individuals were determined using PCR, and 12 MC1R haplotype sequences were obtained. Among these haplotypes, we found that unique haplotypes with nine non-synonymous substitutions and four or five synonymous substitutions in the coding region had a perfect correlation with the dark-plumed phenotype. The lack of correlation between the genotype of ASIP coding region and plumage phenotype reiterated that the dark morph is attributable to specific MC1R haplotypes. The absence of a correlation between genetic polymorphisms of MC1R and the intra-feather barring patterns, as well as the diversity observed within lighter ground color classes (pale and intermediate), implies the involvement of alternative molecular mechanisms in the manifestation of the aforementioned phenotypes.

Identification of pigmentation genes in skin, muscle and tail of a Thai-flag variety of Siamese fighting fish Betta splendens

Pigmentation genes expressed in skin, body muscle and tail of Thai-flag compared with Blue, White and Red varieties of Siamese fighting fish Betta splendens were identified. In total, 22,919 new unigenes were found. Pearson correlation and PCA analysis revealed that expression profiles of genes in muscle, skin and tail across solid color variety were similar. In contrast, those in skin and red tail part of Thai-flag were closely related but they showed different expression profiles with the white tail part. Moreover, 21,347-64,965 SNPs were identified in exonic regions of identified genes. In total, 28,899 genes were differentially expressed between paired comparisons of libraries where 13,907 genes (48.12 %) were upregulated and 14,992 genes (51.88 %) were downregulated. DEGs between paired libraries were 106-5775 genes relative to the compared libraries (56-2982 and 50-2782 for upregulated and downregulated DEGs). Interestingly, 432 pigmentation genes of B. splendens were found. Of these, 297 DEGs showed differential expression between varieties. Many DEGs in melanogenesis (Bsmcr1r, Bsmcr5r, and Bsslc2a15b), tyrosine metabolism (Bstyr, Bstyrp1b and Bsdct), stripe repressor (BsAsip1 and BsAsip2b), pteridine (Bsgch2) and carotenoid (BsBco2) biosynthesis were downregulated in the Thai-flag compared with solid color varieties. Expression of Bsbco1l, Bsfrem2b, Bskcnj13, Bszic2a and Bspah in skin, muscle and tail of Thai-flag, Blue, Red and White varieties was analyzed by qRT-PCR and revealed differential expression between fish varieties and showed anatomical tissue-preferred expression patterns in the same fish variety. The information could be applied to assist genetic-based development of new B. splendens varieties in the future.

Functional characterization of melanocortin 2 receptor (Mc2r) from a lobe-finned fish (Protopterus annectens) and insights into the molecular evolution of melanocortin receptors

Recent studies from our group on melanocortin 2 receptors (Mc2r) from basal families of actinopterygians have served to resolve that Mrap1 dependence and ACTH selectivity are features of even the most basal ray-finned fishes. However, there have been no studies on Mc2r function of the basal sarcopterygians, the lobe-finned fishes, represented by the extant members coelacanths and lungfishes. Here, we offer the first molecular and functional characterization of an Mc2r from a lobe-finned fish, the West African lungfish (Protopterus annectens). Plasmids containing cDNA constructs of lungfish (lf) Mc2r and Mrap1 were expressed in mammalian and zebrafish cell lines. Cells were then stimulated by human ACTH(1-24) and melanocyte stimulating hormone (α-MSH), as well as alanine-substituted analogs of hACTH(1-24) targeting residues within the H6F7R8W9 and K15K16R17R18P19 motifs. Activation of lfMc2r was assessed using a cAMP-responsive luciferase reporter gene assay. In these assays, lfMc2r required co-expression with lfMrap1, was selective for ACTH over α-MSH at physiological concentrations of the ligands, and was completely inhibited by multiple-alanine substitutions of the HFRW (A6-9) and KKRRP (A15-19) motifs. Single- and partial-alanine substitutions of the HFRW and KKRRP motifs varied in their impacts on receptor-ligand affinity from having no effect to completely inhibiting lfMc2r activation. This characterization of the Mc2r of a lobe-finned fish fulfills the last major extant vertebrate group for which Mc2r function had yet to be characterized. In doing so, we resolve that all basal bony vertebrate groups exhibit Mc2r function that substantially differs from that of the cartilaginous fishes, indicating that rapid and dramatic shift in Mc2r function occurred between the radiation of cartilaginous fishes and the emergence of bony fishes. We support this interpretation with a molecular clock analysis of the melanocortin receptors, which demonstrates the uniquely high rate of sequence divergence in Mc2r. Much remains to be understood regarding the molecular evolution of Mc2r during the early radiation of vertebrates that resulted in the derived functional characteristics of Mrap1 dependence and exclusive selectivity for ACTH.

Melanotan-II reverses memory impairment induced by a short-term HF diet

A high-fat (HF) diet has been shown to increase the risk of neurological impairments and neurodegenerative disorders. The melanotropins used in this study have been associated with diet-related disorders; however, there is an absence of studies on their effect on diet-induced neurobehavioral conditions. Here, we investigated the possible relationship among diet, Melanotan-II (MT-II) targeting melanotropin receptors, and the behavior of zebrafish (Danio rerio). Surprisingly, even a short-term HF diet lasting for ∼ 1 % of the zebrafish's life had a strong developmental effect. Zebrafish fed the HF diet showed an impairment in recognition memory, elevated anxiety levels, and reduced exploratory propensity after just three weeks compared to zebrafish fed the control diet. These HF diet-induced abnormalities were reversed by MT-II. Animals fed a HF diet and treated with MT-II demonstrated recognition memory, anxiety, and exploratory behavior similar to the control group. This study provides evidence that even a short-term HF diet has an impact on memory and emotions and is the first study to show that MT-II reverses these changes.

The multifunctional human ocular melanocortin system

Immune privilege in the eye involves physical barriers, immune regulation and secreted proteins that together limit the damaging effects of intraocular immune responses and inflammation. The neuropeptide alpha-melanocyte stimulating hormone (α-MSH) normally circulates in the aqueous humour of the anterior chamber and the vitreous fluid, secreted by iris and ciliary epithelium, and retinal pigment epithelium (RPE). α-MSH plays an important role in maintaining ocular immune privilege by helping the development of suppressor immune cells and by activating regulatory T-cells. α-MSH functions by binding to and activating melanocortin receptors (MC1R to MC5R) and receptor accessory proteins (MRAPs) that work in concert with antagonists, otherwise known as the melanocortin system. As well as controlling immune responses and inflammation, a broad range of biological functions is increasingly recognised to be orchestrated by the melanocortin system within ocular tissues. This includes maintaining corneal transparency and immune privilege by limiting corneal (lymph)angiogenesis, sustaining corneal epithelial integrity, protecting corneal endothelium and potentially enhancing corneal graft survival, regulating aqueous tear secretion with implications for dry eye disease, facilitating retinal homeostasis via maintaining blood-retinal barriers, providing neuroprotection in the retina, and controlling abnormal new vessel growth in the choroid and retina. The role of melanocortin signalling in uveal melanocyte melanogenesis however remains unclear compared to its established role in skin melanogenesis. The early application of a melanocortin agonist to downregulate systemic inflammation used adrenocorticotropic hormone (ACTH)-based repository cortisone injection (RCI), but adverse side effects including hypertension, edema, and weight gain, related to increased adrenal gland corticosteroid production, impacted clinical uptake. Compared to ACTH, melanocortin peptides that target MC1R, MC3R, MC4R and/or MC5R, but not adrenal gland MC2R, induce minimal corticosteroid production with fewer amdverse systemic effects. Pharmacological advances in synthesising MCR-specific targeted peptides provide further opportunities for treating ocular (and systemic) inflammatory diseases. Following from these observations and a renewed clinical and pharmacological interest in the diverse biological roles of the melanocortin system, this review highlights the physiological and disease-related involvement of this system within human eye tissues. We also review the emerging benefits and versatility of melanocortin receptor targeted peptides as non-steroidal alternatives for inflammatory eye diseases such as non-infectious uveitis and dry eye disease, and translational applications in promoting ocular homeostasis, for example, in corneal transplantation and diabetic retinopathy.

Agouti-Signalling Protein Overexpression Reduces Aggressiveness in Zebrafish

Feeding motivation plays a crucial role in food intake and growth. It closely depends on hunger and satiation, which are controlled by the melanocortin system. Overexpression of the inverse agonist agouti-signalling protein (ASIP) and agouti-related protein (AGRP) leads to enhanced food intake, linear growth, and weight. In zebrafish, overexpression of Agrp leads to the development of obesity, in contrast to the phenotype observed in transgenic zebrafish that overexpress asip1 under the control of a constitutive promoter (asip1-Tg). Previous studies have demonstrated that asip1-Tg zebrafish exhibit larger sizes but do not become obese. These fish display increased feeding motivation, resulting in a higher feeding rate, yet a higher food ration is not essential in order to grow larger than wild-type (WT) fish. This is most likely attributed to their improved intestinal permeability to amino acids and enhanced locomotor activity. A relationship between high feeding motivation and aggression has been previously reported in some other transgenic species showing enhanced growth. This study aims to elucidate whether the hunger observed in asip1-Tg is linked to aggressive behaviour. Dominance and aggressiveness were quantified using dyadic fights and mirror-stimulus tests, in addition to the analysis of basal cortisol levels. The results indicate that asip1-Tg are less aggressive than WT zebrafish in both dyadic fights and mirror-stimulus tests.

Hypothalamus-pituitary-interrenal (HPI) axis signaling in Atlantic sturgeon (Acipenser oxyrinchus) and sterlet (Acipenser ruthenus)

In vertebrates, the hypothalamic-pituitary-adrenal/interrenal (HPA/HPI) axis is a highly conserved endocrine axis that regulates glucocorticoid production via signaling by corticotropin releasing hormone (CRH) and adrenocorticotropic hormone (ACTH). Once activated by ACTH, G_s protein-coupled melanocortin 2 receptors (Mc2r) present in corticosteroidogenic cells stimulate expression of steroidogenic acute regulatory protein (Star), which initiates steroid biosynthesis. In the present study, we examined the tissue distribution of genes involved in HPI axis signaling and steroidogenesis in the Atlantic sturgeon (Acipenser oxyrinchus) and provided the first functional characterization of Mc2r in sturgeon. Mc2r of A. oxyrinchus and the sterlet sturgeon (Acipenser ruthenus) are co-dependent on interaction with the melanocortin receptor accessory protein 1 (Mrap1) and highly selective for human (h) ACTH over other melanocortin ligands. A. oxyrinchus expresses key genes involved in HPI axis signaling in a tissue-specific manner that is indicative of the presence of a complete HPI axis in sturgeon. Importantly, we co-localized mc2r, mrap1, and star mRNA expression to the head kidney, indicating that this is possibly a site of ACTH-mediated corticosteroidogenesis in sturgeon. Our results are discussed in the context of other studies on the HPI axis of basal bony vertebrates, which, when taken together, demonstrate a need to better resolve the evolution of HPI axis signaling in vertebrates.

What coloration brings: Implications of background adaptation to oxidative stress in anurans

BACKGROUND

Urban development results in habitat destruction, affecting populations of amphibians, the most fragile group of vertebrates. With changes in the environment, these animals become more exposed to light and predators. To enhance their chances of survival, they display plasticity of body coloration. Aside from adaptive benefits, animals exhibiting background matching meet the energetic costs and restrictions of changing body tones. To study the physiological consequences of Hyla arborea tadpole adaptation to background color, we followed oxidative stress parameters after rearing larvae on a constant background (black/white) and after changing the background color.

RESULTS

Larvae cultivated for 20 days on constant substrate color exhibited differences in body coloration but without differences in lipid peroxidation (LPO) concentration between dark and pale individuals, suggesting that coloration investment during this period did not induce higher oxidative damage in darker tadpoles. Prolonged exposure of larvae (37 days) to a dark habitat increased antioxidative system defense and LPO concentrations, compared to animals reared permanently in the white surroundings. The positive correlation of oxidative damage with color intensity of individuals points to the physiological consequences of higher investment in the number of pigment cells necessary for dark pigmentation. In individuals faced with non-matching background and change in body coloration, defense system declined and LPO occurred relative to individuals cultivated in white habitat.

CONCLUSION

Here, we have pointed to consequences related to background matching and stress that amphibians experienced during chromatic adaptations. Background color change causes a complex physiological response affecting the antioxidative defense parameters. This investigation elucidates the accompanying cost of amphibians' adjustment to an altered environment.

Loss-of-function mutations in melanocortin-1 receptor modulate immune response in teleost fishes

The melanocortin system is an ancient neuroendocrine system conserved from teleosts to mammals. The melanocortin system is a set of complex neuroendocrine signaling pathways involved in numerous physiological processes, and particularly associated with the hypothalamic-pituitary-interrenal (HPI) axis response. The melanocortin 1 receptor (MC1R) is the central melanocortin receptor involved in pigmentation in vertebrates, including fish. In order to assess the immune role of MC1R, this study used a homozygous Mc1r knockout zebrafish. Hence, skin cortisol levels, variations in the blood leucocyte population, as well as the expression levels of immune genes in various tissues of wild-type TU strain (Tübingen, Nüsslein-Volhard Lab) (WT) and homozygous mc1r knockout zebrafish (mc1r^K.O.) stimulated with LPS was carried out. Results show that the mc1r^K.O. mutant fish produce lower levels of cortisol in mucus and fewer macrophages in blood after exposure to LPS compared to control fish. Regarding the expression of immune genes, mutant fish show a significant increase in the expression of the anti-inflammatory interleukin il10. These results suggest that the mc1r^K.O. mutant fish may follow an alternative mechanism among the immune responses, where macrophages seem to have an anti-inflammatory function, attenuating nitric oxide (NO) production and providing an advantage through the mitigation of excessive or strong inflammatory reactions. Nonetheless, a lower number of this cell type could imply a reduced phagocytic potential in the face of an infection. At the same time, lower cortisol levels in the mc1r^K.O. mutant fish could be an advantage as for the lower susceptibility to stress and the physiological and metabolic consequences of high cortisol levels.

A basal actinopterygian melanocortin receptor: Molecular and functional characterization of an Mc2r ortholog from the Senegal bichir (Polypterus senegalus)

In bony vertebrates, melanocortin 2 receptor (Mc2r) specifically binds adrenocorticotropic hormone (ACTH) and is responsible for mediating anterior pituitary signaling that stimulates corticosteroid production in the adrenal gland/interrenal cells. In bony fishes Mc2r requires the chaperoning of an accessory protein (Mrap1) to traffic to the membrane surface and bind ACTH. Here, we evaluated the structure and pharmacological properties of Mc2r from the Senegal bichir (Polypterus senegalus), which represents the most basal bony fish from which an Mc2r has been pharmacologically studied to date. In our experiments, cDNA constructs of the Mc2r from the Senegal bichir (sbMc2r) and various vertebrate Mrap1s were heterologously co-expressed in Chinese hamster ovary (CHO) cells, stimulated by ACTH or melanocyte-stimulating hormone (α-MSH) ligands, and assessed using a luciferase reporter gene assay. When expressed without an Mrap1, sbMc2r was not activated by ACTH. When co-expressed with Mrap1 from either chicken (Gallus gallus) or bowfin (Amia calva), sbMc2r could be activated in a dose-dependent manner by ACTH, but not α-MSH. Co-expression of sbMrap2 with sbMc2r resulted in no detectable activation of the receptor. Collectively, these results demonstrate that sbMc2r has pharmacological properties similar to those of Mc2rs of later-evolved bony fishes, such as Mrap1 dependence and ACTH selectivity, indicating that these qualities of Mc2r function are ancestral to all bony fish Mc2rs.

Melanocortin receptor agonists suppress experimental autoimmune uveitis

The melanocortin system plays an essential role in the regulation of immune activity. The anti-inflammatory microenvironment of the eye is dependent on the expression of the melanocortin-neuropeptide alpha-melanocyte stimulating hormone (α-MSH). In addition, the melanocortin system may have a role in retinal development and retinal cell survival under conditions of retinal degeneration. We have found that treating experimental autoimmune uveitis (EAU) with α-MSH suppresses retinal inflammation. Also, this augmentation of the melanocortin system promotes immune tolerance and protection of the retinal structure. The benefit of α-MSH-therapy appears to be dependent on different melanocortin receptors. Therefore, we treated EAU mice with α-MSH-analogs with different melanocortin-receptor targets. This approach demonstrated which melanocortin-receptors suppress inflammation, preserve retinal structure, and induce immune tolerance in uveitis. At the chronic stage of EAU the mice were injected twice 1 day apart with 50 μg of α-MSH or an α-MSH-analog. The α-MSH-analogs were a pan-agonist PL8331, PL8177 (potent MC1r-only agonist), PL5000 (a pan-agonist with no MC5r functional activity), MT-II (same as PL5000) and PG901 (MC5r agonist, but also an antagonist to MC3r, and MC4r). Clinical EAU scores were measured until resolution in the α-MSH-treated mice, when the eyes were collected for histology, and spleen cells collected for retinal-antigen-stimulated cytokine production. Significant suppression of EAU was seen with α-MSH or PL8331 treatment. This was accompanied with significant preservation of retinal structure. A similar effect was seen in EAU-mice that were treated with PL8177, except the suppression of EAU was temporary. In EAU mice treated with PL5000, MTII, or PG901, there was no suppression of EAU with a significant loss in whole retina and outer-nuclear layer thickness. There was significant suppression of IL-17 with induction of IL-10 by retinal-antigen stimulated spleen T cells from EAU mice treated with α-MSH, PL8331, PL8177, or PL5000, but not from EAU mice treated with MT-II, or PG901. Our previous studies show the melanocortin system's importance in maintaining ocular immune privilege and that α-MSH-treatment accelerates recovery and induces retinal-antigen-specific regulatory immunity in EAU. Our current results show that this activity is centered around MC1r and MC5r. In addition, the results suggest that a therapeutic potential to target MC1r and MC5r together to suppress uveitis induces regulatory immunity with potentially maintaining a normal retinal structure.

Translational advances of melanocortin drugs: Integrating biology, chemistry and genetics

Melanocortin receptors have emerged as important targets with a very unusual versatility, as their widespread distribution on multiple tissues (e.g. skin, adrenal glands, brain, immune cells, exocrine glands) together with the variety of physiological processes they control (pigmentation, cortisol release, satiety mechanism, inflammation, secretions), place this family of receptors as genuine therapeutic targets for many disorders. This review focuses in the journey of the development of melanocortin receptors as therapeutic targets from the discovery of their existence in the early 1990 s to the approval of the first few drugs of this class. Two major areas of development characterise the current state of melanocortin drug development: their role in obesity, recently culminated with the approval of setmelanotide, and their potential for the treatment of chronic inflammatory and autoimmune diseases like rheumatoid arthritis, multiple sclerosis or fibrosis. The pro-resolving nature of these drugs offers the advantage of acting by mimicking the way our body naturally resolves inflammation, expecting fewer side effects and a more balanced (i.e. non-immunosuppressive) response from them. Here we also review the approaches followed for the design and development of novel compounds, the importance of the GPCR nature of these receptors in the process of drug development, therapeutic value, current challenges and successes, and the potential for the implementation of precision medicine approaches through the incorporation of genetics advances.

Pharmacological modulation of the cAMP signaling of two isoforms of melanocortin-3 receptor by melanocortin receptor accessory proteins in the tetrapod Xenopus laevis

As a member of the seven-transmembrane rhodopsin-like G protein-coupled receptor superfamily, the melanocortin-3 receptor (MC3R) is vital for the regulation of energy homeostasis and rhythms synchronizing in mammals, and its pharmacological effect could be directly influenced by the presence of melanocortin receptor accessory proteins (MRAPs), MRAP1 and MRAP2. The tetrapod amphibian Xenopus laevis (xl) retains higher duplicated genome than extant teleosts and serves as an ideal model system for embryonic development and physiological studies. However, the melanocortin system of the Xenopus laevis has not yet been thoroughly evaluated. In this work, we performed sequence alignment, phylogenetic tree, and synteny analysis of two xlMC3Rs. Co-immunoprecipitation and immunofluorescence assay further confirmed the co-localization and in vitro interaction of xlMC3Rs with xlMRAPs on the plasma membrane. Our results demonstrated that xlMRAP2.L/S could improve α-MSH-stimulated xlMC3Rs signaling and suppress their surface expression. Moreover, xlMC3R.L showed a similar profile on the ligands and surface expression in the presence of xlMRAP1.L. Overall, the distinct pharmacological modulation of xlMC3R.L and xlMC3R.S by dual MRAP2 proteins elucidated the functional consistency of melanocortin system during genomic duplication of tetrapod vertebrates.

Dorsal Pigmentation and Its Association with Functional Variation in MC1R in a Lizard from Different Elevations on the Qinghai-Tibetan Plateau

Identification of the role of the MC1R gene has provided major insights into variation in skin pigmentation in several organisms, including humans, but the evolutionary genetics of this variation is less well established. Variation in this gene and its relationship with degree of melanism was analyzed in one of the world’s highest-elevation lizards, Phrynocephalus theobaldi from the Qinghai–Tibetan Plateau. Individuals from the low-elevation group were shown to have darker dorsal pigmentation than individuals from a high-elevation group. The existence of climatic variation across these elevations was quantified, with lower elevations exhibiting higher air pressure, temperatures, and humidity, but less wind and insolation. Analysis of the MC1R gene in 214 individuals revealed amino acid differences at five sites between intraspecific sister lineages from different elevations, with two sites showing distinct fixed residues at low elevations. Three of the four single-nucleotide polymorphisms that underpinned these amino acid differences were highly significant outliers, relative to the generalized MC1R population structuring, suggestive of selection. Transfection of cells with an MC1R allele from a lighter high-elevation population caused a 43% reduction in agonist-induced cyclic AMP accumulation, and hence lowered melanin synthesis, relative to transfection with an allele from a darker low-elevation population. The high-elevation allele led to less efficient integration of the MC1R protein into melanocyte membranes. Our study identifies variation in the degree of melanism that can be explained by four or fewer MC1R substitutions. We establish a functional link between these substitutions and melanin synthesis and demonstrate elevation-associated shifts in their frequencies.

The effects of corticosterone and background colour on tadpole physiological plasticity

Corticosterone (CORT)-mediated adaptive plasticity improves animal fitness in stressful environments. Although it brings ecological benefits, the cost potentially constrains its expression and evolution. Revealing the factors affecting plasticity costs is of great ecological and evolutionary significance. Evidence indicates that both CORT and background colour can induce metabolic changes in animals, which in turn determine phenotypic plasticity. However, whether and/or how CORT and background colour jointly act on plastic responses has not been studied. Here, this question has been investigated in amphibian tadpoles (Microhyla fissipes) exposed to CORT at different background colours (white or black) using integrated morphological, histological, and transcriptomic analyses. The results showed that CORT exposure increased relative tail length, immune function, and metabolic maintenance (i.e., transcription of substrate catabolism and oxidative phosphorylation) at the expense of reduction in growth rate and skin melanin level. The black background also increased relative tail length and metabolic maintenance (i.e., transcription of oxidative phosphorylation) at the cost of reduction in growth rate, but increased skin melanin level. The expression of critical pigmentation genes indicated that black background activated a distinct and opposite pigmentation regulating route to CORT. Although there was no interactive effect of background colour and CORT on phenotypic and metabolic variations, their additive effects further impact the trade-off between somatic growth, metabolic maintenance, and pigmentation in terms of resource allocation. In conclusion, the individual and additive effects of background colour and CORT exposure on tadpole plasticity were revealed. These results likely provide new insights into the environmental adaptation of animals.

Melanocortin 5 Receptor Expression and Recovery of Ocular Immune Privilege after Uveitis

Purpose: The therapeutic use of the RPE-neuropeptide α-MSH suppresses experimental autoimmune uveitis (EAU). This suppression is partially through the α-MSH melanocortin 5 receptor (MC5r). Therefore, we examined the possible role of MC5r-expression in the recovery of RPE suppression of phagolysosome-activation in macrophages following α-MSH-treatment of EAUMethods: The conditioned media of cultured in situ RPE-eyecup from α-MSH-treated EAU wild-type and MC5r(-/-) mice were used to treat macrophages to assay for phagolysosome activation.Results: MC5r(-/-) mice treated with α-MSH recovered from EAU, but with greater retinal damage, and the RPE suppressed phagolysosome activation in wild type but not in MC5r(-/-) macrophages. In addition, α-MSH did not suppress phagolysosome activation in MC5r(-/-) macrophages, and resting-MC5r(-/-) macrophages had augmented phagocytic activity.Conclusion: α-MSH treatment of EAU mediates a MC5r-dependent recovery of RPE suppression of phagolysosome activation in macrophages possibly altering antigen processing and presentation. Also, MC5r-expression helps protect the retina from inflammatory damage. In addition, MC5r-expression is important in the homeostatic maintenance of phagosome-maturation within macrophages.

Hypothalamic Pomc Neurons Innervate the Spinal Cord and Modulate the Excitability of Premotor Circuits

Locomotion requires energy, yet animals need to increase locomotion in order to find and consume food in energy-deprived states. While such energy homeostatic coordination suggests brain origin, whether the central melanocortin 4 receptor (Mc4r) system directly modulates locomotion through motor circuits is unknown. Here, we report that hypothalamic Pomc neurons in zebrafish and mice have long-range projections into spinal cord regions harboring Mc4r-expressing V2a interneurons, crucial components of the premotor networks. Furthermore, in zebrafish, Mc4r activation decreases the excitability of spinal V2a neurons as well as swimming and foraging, while systemic or V2a neuron-specific blockage of Mc4r promotes locomotion. In contrast, in mice, electrophysiological recordings revealed that two-thirds of V2a neurons in lamina X are excited by the Mc4r agonist α-MSH, and acute inhibition of Mc4r signaling reduces locomotor activity. In addition, we found other Mc4r neurons in spinal lamina X that are inhibited by α-MSH, which is in line with previous studies in rodents where Mc4r agonists reduced locomotor activity. Collectively, our studies identify spinal V2a interneurons as evolutionary conserved second-order neurons of the central Mc4r system, providing a direct anatomical and functional link between energy homeostasis and locomotor control systems. The net effects of this modulatory system on locomotor activity can vary between different vertebrate species and, possibly, even within one species. We discuss the biological sense of this phenomenon in light of the ambiguity of locomotion on energy balance and the different living conditions of the different species.

Growth Performance After Agouti-Signaling Protein 1 (Asip1) Overexpression in Transgenic Zebrafish

The melanocortin system is a key structure in the regulation of energy balance. Overexpression of inverse agonists, agouti-signaling protein (ASIP), and agouti-related protein (AGRP) results in increased food intake, linear growth, and body weight. ASIP regulates dorsal-ventral pigment polarity through melanocortin 1 receptor (MC1R) and overexpression induces obesity in mice by binding to central MC4R. Asip1 overexpression in transgenic zebrafish (asip1-Tg) enhances growth, yet experiments show fish overexpressing Asip1 do not develop obesity even under severe feeding regimes. Asip1-Tg fish do not need to eat more to grow larger and faster; thus, increased food efficiency can be observed. In addition, asip1-Tg fish reared at high density are able to grow far more than wild-type (WT) fish reared at low density, although asip1-Tg fish seem to be more sensitive to crowding stress than WT fish, thus making the melanocortin system a target for sustainable aquaculture, especially as the U.S. Food and Drug Association has recently approved transgenic fish trading.

Use of Adrenocorticotropic Hormone in Ophthalmology

The ability of the adrenocorticotropic hormone (ACTH) to induce steroidogenesis and upregulate anti-inflammatory processes has long been known. More recently, however, extrasteroidal mechanisms, through which ACTH exerts anti-inflammatory processes, have been described. This has renewed hope that ACTH can combat inflammatory conditions even when resistant to steroids. This review article summarizes the literature on the use of ACTH in ocular disease. Unfortunately, much of the data regarding the clinical utility of ACTH are outdated, with many studies published in the 1950s and 1960s. Many of these older studies are inconsistent or incomplete with their reporting, making it difficult to ascertain the meaning of the outcomes. Despite the limitations, 2 important trends are evident. First, when used to treat an inflammatory disease, ACTH can be effective at decreasing or eliminating ocular inflammation, even in a refractory disease resistant to multiple treatment modalities. Second, adverse effects of ACTH are rare and are most likely to be reported with relatively high doses of ACTH therapy. Taken as a whole, these studies offer initial promising data that ACTH may be a safe and effective alternative in refractory ocular inflammatory disease. However, they highlight an important lack of prospective data to more rigorously understand the true safety and efficacy of this therapy.

New perspectives on GPCRs: GPCR heterodimer formation (melanocortin receptor) and GPCR on primary cilia (melanin concentrating hormone receptor)

GPCRs are the largest family of receptors accounting for about 30% of the current drug targets. However, it is difficult to fully elucidate the mechanisms regulating intracellular GPCR signal regulation. It is thus important to consider and investigate GPCRs with respect to endogenous situations. Our group has been investigating GPCRs involved in body color (teleost and amphibian) and eating (vertebrate). Here, I review two independent GPCR systems (heterodimer formation and primary ciliated GPCR) that can be breakthroughs in GPCR research. In teleosts, MCRs form heterodimers, which significantly reduce their affinity for acetylated ligands. In mammals, MCHR1 is localized in the ciliary membrane and shortens the length of the primary cilia through a unique signal from the ciliary membrane. Considering these two new GPCR concepts is expected to advance the overall view of the GPCR system.

Pharmacological modulation of melanocortin-4 receptor by melanocortin receptor accessory protein 2 in Nile tilapia

The melanocortin-4 receptor (MC4R) acts as a member of G-protein coupled receptors and participate in food intake and energy expenditure. Melanocortin 2 receptor accessory protein 2 (MRAP2) plays a critical role in regulating MC4R signaling in mammals and zebrafish. However, evidence on their interaction in other teleost species remains elusive. Here, we cloned and assessed the evolutionary aspect and pharmacological modulation of MRAP2 on MC4R signaling in Nile tilapia (Oreochromis niloticus). Tissue distribution analysis of tmc4r and tmrap2 confirmed their co-expression in the brain region. tMRAP2 protein could form antiparallel homo-dimer and directly interacted with tMC4R in vitro and presence of tMRAP2 led to the reduction of agonist response and surface expression of tMC4R. Overall, our findings provide a comparative overview on the evolutionary conservation, genomic distribution, tissue-specific expression and pharmacological profile of the MC4R and MRAP2 in another non-mammalian teleost.

Behavioral, energetic, and color trait integration in male guppies: testing the melanocortin hypothesis

Individuals of the same population differ consistently from each other in the average expression of behavioral and physiological traits. Often, such traits are integrated and thus correlated with each other. However, the underlying proximate mechanisms generating and maintaining this among-individual covariation are still poorly understood. The melanocortin hypothesis suggests that the melanocortin pathways can have pleiotropic effects linking the expression of melanin-based coloration with physiological and behavioral traits. In the present study, we test this hypothesis in adult male guppies (Poecilia reticulata), by estimating among individual correlations between behaviors (activity, feeding, boldness, display, and chase during courtship), stress response (peak metabolic rate), and coloration (black spot, fuzzy black, and orange). The lack of correlation of any behavior or metabolism with black coloration indicates that the melanocortin hypothesis is not supported in this species. However, we observed covariation among coloration traits, as well as among behavioral traits. Our findings suggest that, although there appear to be constraints within sets of related traits, coloration, physiology, and behaviors can potentially evolve as independent modules in response to selection in this species.

Molecular characterization of feline melanocortin 4 receptor and melanocortin 2 receptor accessory protein 2

Melanocortin 4 receptor (MC4R), which is a member of the G protein-coupled receptor (GPCR) family, mediates regulation of energy homeostasis upon the binding of α-melanocyte-stimulating hormone (α-MSH) in the central nervous system (CNS). Melanocortin 2 receptor accessory protein 2 (MRAP2) modulates the function of MC4R. We performed cDNA cloning of cat MC4R and MRAP2 and characterized their amino acid sequences, mRNA expression patterns in cat tissues, protein-protein interactions, and functions. We found high sequence homology (>88%) with other mammalian MC4R and MRAP2 encoding 332 and 206 amino acid residues, respectively. Reverse transcription-polymerase chain reaction analysis revealed that cat MC4R and MRAP2 mRNA were expressed highly in the CNS. In CHO-K1 cells transfected with cat MC4R, stimulation with α-MSH increased intracellular cyclic adenosine monophosphate (cAMP) concentration in a dose-dependent manner. Furthermore, the presence of MRAP2 enhanced the cat MC4R-mediated cAMP production. These results suggested that cat MC4R acts as a neuronal mediator in the CNS and that its function is modulated by MRAP2. In addition, our NanoBiT study showed the dynamics of their interactions in living cells; stimulation with α-MSH slightly affected the interaction between MC4R and MRAP2, and did not affect MC4R homodimerization, suggesting that they interact in the basal state and that structural change of MC4R by activation may affect the interaction between MC4R and MRAP2.

Negative regulators that mediate ocular immune privilege

The ocular microenvironment has adapted several negative regulators of inflammation to maintain immune privilege and health of the visual axis. Several constitutively produced negative regulators within the eye TGF-β2, α-melanocyte stimulating hormone (α-MSH), Fas ligand (FasL), and PD-L1 standout because of their capacity to influence multiple pathways of inflammation, and that they are part of promoting immune tolerance. These regulators demonstrate the capacity of immune privilege to prevent the activation of inflammation, and to suppress activation of effector immune cells even under conditions of ocular inflammation induced by endotoxin and autoimmune disease. In addition, these negative regulators promote and expand immune cells that mediate regulatory and tolerogenic immunity. This in turn makes the immune cells themselves negative regulators of inflammation. This provides for a greater understanding of immune privilege in that it includes both molecular and cellular negative regulators of inflammation. This would mean that potentially new approaches to the treatment of autoimmune disease can be developed through the use of molecules and cells as negative regulators of inflammation.

Hypothalamic Integration of the Endocrine Signaling Related to Food Intake

Hypothalamic integration of gastrointestinal and adipose tissue-derived hormones serves as a key element of neuroendocrine control of food intake. Leptin, adiponectin, oleoylethanolamide, cholecystokinin, and ghrelin, to name a few, are in a constant "cross talk" with the feeding-related brain circuits that encompass hypothalamic populations synthesizing anorexigens (melanocortins, CART, oxytocin) and orexigens (Agouti-related protein, neuropeptide Y, orexins). While this integrated neuroendocrine circuit successfully ensures that enough energy is acquired, it does not seem to be equally efficient in preventing excessive energy intake, especially in the obesogenic environment in which highly caloric and palatable food is constantly available. The current review presents an overview of intricate mechanisms underlying hypothalamic integration of energy balance-related peripheral endocrine input. We discuss vulnerabilities and maladaptive neuroregulatory processes, including changes in hypothalamic neuronal plasticity that propel overeating despite negative consequences.

Melanocortins, Melanocortin Receptors and Multiple Sclerosis

The melanocortins and their receptors have been extensively investigated for their roles in the hypothalamo-pituitary-adrenal axis, but to a lesser extent in immune cells and in the nervous system outside the hypothalamic axis. This review discusses corticosteroid dependent and independent effects of melanocortins on the peripheral immune system, central nervous system (CNS) effects mediated through neuronal regulation of immune system function, and direct effects on endogenous cells in the CNS. We have focused on the expression and function of melanocortin receptors in oligodendroglia (OL), the myelin producing cells of the CNS, with the goal of identifying new therapeutic approaches to decrease CNS damage in multiple sclerosis as well as to promote repair. It is clear that melanocortin signaling through their receptors in the CNS has potential for neuroprotection and repair in diseases like MS. Effects of melanocortins on the immune system by direct effects on the circulating cells (lymphocytes and monocytes) and by signaling through CNS cells in regions lacking a mature blood brain barrier are clear. However, additional studies are needed to develop highly effective MCR targeted therapies that directly affect endogenous cells of the CNS, particularly OL, their progenitors and neurons.

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.

Molecular cloning, tissue distribution, and pharmacological characterization of melanocortin-4 receptor in grass carp (Ctenopharyngodon idella)

Melanocortin-4 receptor (MC4R) plays a pivotal role in the mediation of leptin action on food intake and energy expenditure in mammals. The MC4R has also been identified in several teleosts, and its importance in the regulation of fish energy homeostasis is emerging. We herein reported on the molecular cloning, tissue distribution, and pharmacological characterization of MC4R in grass carp (Ctenopharyngodon idella), an economically and ecologically important fish. We showed that grass carp MC4R (ciMC4R) consisted of a 981 bp open reading frame encoding a protein of 326 amino acids, highly homologous (>95%) to several teleost MC4Rs. Phylogenetic and synteny analysis further indicated ciMC4R was closely related to piscine MC4Rs. Using reverse transcription PCR, we found that mc4r messenger RNA was expressed in the brain as well as various peripheral tissues in grass carp. The pharmacological properties of ciMC4R were investigated using 4 agonists, including α-melanocyte stimulating hormone (α-MSH), β-MSH, [Nle⁴, D-Phe⁷]-MSH (NDP-MSH), and adrenocorticotropic hormone (ACTH). We showed that all 4 ligands could bind to ciMC4R and initiate dose-dependent intracellular cyclic adenosine monophosphate (cAMP) accumulation. Grass carp MC4R had the highest affinity for NDP-MSH. Both NDP-MSH and ACTH (1-24) exhibited higher potencies compared to the other 2 endogenous agonists. The ciMC4R was constitutively active, with significantly increased basal cAMP level compared with that of human MC4R (P < 0.01). The availability of ciMC4R and its pharmacologic characteristics provide a basis for future investigation of its functional roles in regulating diverse physiological processes and novel insights into understanding the mechanism of food habit transition in grass carp.

Multiple beneficial effects of melanocortin MC4 receptor agonists in experimental neurodegenerative disorders: Therapeutic perspectives

Melanocortin peptides induce neuroprotection in acute and chronic experimental neurodegenerative conditions. Melanocortins likewise counteract systemic responses to brain injuries. Furthermore, they promote neurogenesis by activating critical signaling pathways. Melanocortin-induced long-lasting improvement in synaptic activity and neurological performance, including learning and memory, sensory-motor orientation and coordinated limb use, has been consistently observed in experimental models of acute and chronic neurodegeneration. Evidence indicates that the neuroprotective and neurogenic effects of melanocortins, as well as the protection against systemic responses to a brain injury, are mediated by brain melanocortin 4 (MC₄) receptors, through an involvement of the vagus nerve. Here we discuss the targets and mechanisms underlying the multiple beneficial effects recently observed in animal models of neurodegeneration. We comment on the potential clinical usefulness of melanocortin MC₄ receptor agonists as neuroprotective and neuroregenerative agents in ischemic stroke, subarachnoid hemorrhage, traumatic brain injury, spinal cord injury, and Alzheimer's disease.

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

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

60 YEARS OF POMC: Melanocortin receptors: evolution of ligand selectivity for melanocortin peptides

The evolution of the melanocortin receptors (MCRs) is linked to the evolution of adrenocorticotrophic hormone (ACTH), the melanocyte-stimulating hormones (MSHs), and their common precursor pro-opiomelanocortin (POMC). The origin of the MCRs and POMC appears to be grounded in the early radiation of the ancestral protochordates. During the genome duplications that have occurred during the evolution of the chordates, the organization plan for POMC was established, and features that have been retained include, the high conservation of the amino acid sequences of α-MSH and ACTH, and the presence of the HFRW MCR activation motif in all of the melanocortin peptides (i.e. ACTH, α-MSH, β-MSH, γ-MSH, and δ-MSH). For the MCRs, the chordate genome duplication events resulted in the proliferation of paralogous receptor genes, and a divergence in ligand selectivity. While most gnathostome MCRs can be activated by either ACTH or the MSHs, teleost and tetrapod MC2R orthologs can only be activated by ACTH. The appearance of the accessory protein, MRAP1, paralleled the emergence of teleost and tetrapods MC2R ligand selectivity, and the dependence of these orthologs on MRAP1 for trafficking to the plasma membrane. The accessory protein, MRAP2, does not affect MC2R ligand selectivity, but does influence the functionality of MC4R orthologs. In this regard, the roles that these accessory proteins may play in the physiology of the five MCRs (i.e. MC1R, MC2R, MC3R, MC4R, and MC5R) are discussed.

Dimerization of melanocortin receptor 1 (MC1R) and MC5R creates a ligand-dependent signal modulation: Potential participation in physiological color change in the flounder

Vertebrates produce α-melanocyte-stimulating hormone (α-MSH), which contains an N-terminal acetyl group, and desacetyl-α-MSH, which does not contain an N-terminal acetyl group. In teleosts and amphibians, α-MSH-related peptides stimulate pigment dispersion via melanocortin receptors 1-5 (MC1R-MC5R), which are members of the G-protein-coupled receptor (GPCR) family. We previously reported an interesting phenomenon associated with physiological color changes in the skin of a flatfish, barfin flounder (bf). Specifically, pigments in xanthophores expressing only the bfMC5R gene were dispersed by both α-MSH and desacetyl-α-MSH, whereas those in melanophores expressing both the bfMC1R and bfMC5R genes were dispersed by desacetyl-α-MSH, but not by α-MSH. In this study, we examined whether heterodimers of bfMC1R and bfMC5R can act as significant inhibitory receptors for the N-terminal acetylation of α-MSH in mammalian Chinese hamster ovary cells. Immunofluorescence analyses showed that bfMC1R and bfMC5R were localized together at the plasma membrane when expressed in the same cells. Indeed, after coexpression of Flag-bfMC1R and HA-bfMC5R, immunoprecipitation with anti-Flag antibodies resulted in the presence of anti-HA immunoreactivity in the precipitate, and vice versa. Importantly, cyclic AMP assays showed that cotransfection of bfMC1R with bfMC5R inhibited the cyclic AMP accumulation induced by α-MSH to a greater extent than that observed after transfection of bfMC1R alone. Of note, this inhibitory response was not caused by desacetyl-α-MSH. Thus, we show a ligand-dependent signaling through functional heterodimerization of MC1R and MC5R in mammalian cells. The ligand-selective receptor complex also provide the first mechanistic implication that may play a role in the control of color change in teleosts.

Molecular cloning, tissue distribution, and pharmacological characterization of melanocortin-4 receptor in spotted scat, Scatophagus argus

Melanocortin-4 receptor (MC4R) plays an important role in the regulation of food intake and energy expenditure in mammals. The functions of the MC4R in fish have not been investigated extensively. We herein reported on the cloning, tissue distribution, and pharmacological characterization of spotted scat (Scatophagus argus) MC4R (SAMC4R). It consisted of a 984bp open reading frame predicted to encode a protein of 327 amino acids. Sequence analysis revealed that SAMC4R was highly homologous (>80%) at amino acid levels to several teleost MC4Rs. Phylogenetic analyses showed that SAMC4R was closely related to piscine MC4R. Using RT-PCR, we showed that in addition to brain, pituitary, and gonads, mc4r mRNA was also widely expressed in peripheral tissues of spotted scat in sexually divergent pattern. With human MC4R (hMC4R) as a control, several agonists including α-melanocyte stimulating hormone (α-MSH), [Nle(4), D-Phe(7)]-α-MSH (NDP-MSH), adrenocorticotropic hormone (ACTH) and THIQ (N-[(3R)-1,2,3,4-tetrahydroisoquinolinium3-ylcarbonyl]-(1R)-1-(4-chlorobenzyl)-2-[4-cyclohexyl-4-(1H-1,2,4-triazol-1-ylmethyl)piperidin-1-yl]-2-oxoethylamine), were used to investigate the binding and signaling properties of SAMC4R. The results showed that SAMC4R bound NDP-MSH with the highest affinity followed by ACTH (1-24) and α-MSH. Similar ranking was also found for hMC4R, although SAMC4R had two to five-fold higher affinities for these ligands. THIQ did not displace NDP-MSH from SAMC4R, different from hMC4R. α-MSH, NDP-MSH, and ACTH (1-24) were identified as potent agonists to stimulate cAMP generation followed by THIQ in SAMC4R. The availability of SAMC4R and its pharmacological characteristics will facilitate the investigation of its function in regulating diverse physiological processes in spotted scat.

The Role of Alpha-MSH as a Modulator of Ocular Immunobiology Exemplifies Mechanistic Differences between Melanocortins and Steroids

Melanocortins are a highly conserved family of peptides and receptors that includes multiple proopiomelanocortin-derived peptides and five defined melanocortin receptors. The melanocortins have an important role in maintaining immune homeostasis and in suppressing inflammation. Within the healthy eye, the melanocortins have a central role in preventing inflammation and maintaining immune privilege. A central mediator of the anti-inflammatory activity is the non-steroidogenic melanocortin peptide alpha-melanocyte stimulating hormone. In this review we summarize the major findings of melanocortin regulation of ocular immunobiology with particular interest in the ability of melanocortin to induce immune tolerance and cytoprotection. The melanocortins have therapeutic potential because their mechanisms of action in regulating immunity are distinctly different from the actions of steroids.

Hypothesis and Theory: Revisiting Views on the Co-evolution of the Melanocortin Receptors and the Accessory Proteins, MRAP1 and MRAP2

The evolution of the melanocortin receptors (MCRs) is closely associated with the evolution of the melanocortin-2 receptor accessory proteins (MRAPs). Recent annotation of the elephant shark genome project revealed the sequence of a putative MRAP1 ortholog. The presence of this sequence in the genome of a cartilaginous fish raises the possibility that the mrap1 and mrap2 genes in the genomes of gnathostome vertebrates were the result of the chordate 2R genome duplication event. The presence of a putative MRAP1 ortholog in a cartilaginous fish genome is perplexing. Recent studies on melanocortin-2 receptor (MC2R) in the genomes of the elephant shark and the Japanese stingray indicate that these MC2R orthologs can be functionally expressed in CHO cells without co-expression of an exogenous mrap1 cDNA. The novel ligand selectivity of these cartilaginous fish MC2R orthologs is discussed. Finally, the origin of the mc2r and mc5r genes is reevaluated. The distinctive primary sequence conservation of MC2R and MC5R is discussed in light of the physiological roles of these two MCR paralogs.

Views on the co-evolution of the melanocortin-2 receptor, MRAPs, and the hypothalamus/pituitary/adrenal-interrenal axis

A critical regulatory component of the hypothalamus/pituitary/adrenal axis (HPA) in mammals, reptiles and birds, and in the hypothalamus/pituitary/interrenal (HPI) axis of amphibians and teleosts (modern bony fishes) is the strict ligand selectivity of the melanocortin-2 receptor (MC2R). Tetrapod and teleost MC2R orthologs can only be activated by the anterior pituitary hormone, ACTH, but not by any of the MSH-sized ligands coded in POMC. In addition, both tetrapod and teleost MC2R orthologs require co-expression with the accessory protein, MRAP. However, the MC2R ortholog of the elephant shark, a cartilaginous fish, can be activated by either ACTH or the MSH-sized ligands, and the elephant shark MC2R ortholog does not require co-expression with an MRAP for activation. Given these observations, this review will provide a scenario for the co-evolution of MC2R and MRAP, based on the assumption that the obligate interaction between MC2R and MRAP evolved during the early radiation of the ancestral bony fishes.

Evolution of the melanocortin system

The melanocortin system is one of the most complex of the hormonal systems. It involves different agonists encoded in the multiplex precursor proopiomelanocortin (POMC) or in different genes as β-defensins, endogenous antagonist, like agouti-signalling protein (ASIP) or agouti-related protein (AGRP), and five different melanocortin receptors (MCRs). Rounds of whole genome duplication events have preceded the functional and molecular diversification of the family in addition some co-evolutionary and tandem duplication processes have been proposed. The evolutionary patterns of the different partners are controversial and different hypotheses have emerged from a study of the sequenced genomes. In this review, we summarize the different evolutionary hypotheses proposed for the different melanocortin partners.

Characterization, tissue distribution and regulation by fasting of the agouti family of peptides in the sea bass (Dicentrarchus labrax)

The melanocortin system is one of the most complex hormonal systems in vertebrates. Atypically, the signaling of melanocortin receptors is regulated by the binding of endogenous antagonists, named agouti-signaling protein (ASIP) and agouti-related protein (AGRP). Teleost specific genome duplication (TSGD) rendered new gene copies in teleost fish and up to four different genes of the agouti family of peptides have been characterized. In this paper, molecular cloning was used to characterize mRNA of the agouti family of peptides in sea bass. Four different genes were identified: AGRP1, ASIP1, AGRP2 and ASIP2. The AGRP1 gene is mainly expressed in the brain whereas ASIP1 is mainly expressed in the ventral skin. Both ASIP2 and AGRP2 are expressed in the brain and the pineal gland but also in some peripheral tissues. Immunocytochemical studies demonstrated that AGRP1 is exclusively expressed within the lateral tuberal nucleus, the homologue of the mammalian arcuate nucleus in fish. Long-term fasting (8-29 days) increased the hypothalamic expression of AGRP1 but depressed AGRP2 expression (15-29 days). In contrast, the hypothalamic expression of ASIP2 was upregulated during short-term fasting suggesting that this peptide could be involved in the short term regulation of food intake in the sea bass.

Adrenocorticotropin hormone 1-39 promotes proliferation and differentiation of oligodendroglial progenitor cells and protects from excitotoxic and inflammation-related damage

Oligodendroglia (OL) are highly susceptible to damage and, like neurons, are terminally differentiated. It is important to protect OL precursors (OPC) because they are reservoirs of differentiating cells capable of myelination following perinatal insult and remyelination in white matter diseases, including multiple sclerosis (MS). Patients with relapsing-remitting MS are commonly treated with high-dose corticosteroids (CS) when experiencing an exacerbation. Adrenocorticotropin hormone (ACTH), a primary component of another approved MS exacerbation treatment, is a melanocortin peptide that stimulates production of CS by the adrenals. Melanocortin receptors are also found in the central nervous system (CNS) and on immune cells. ACTH is produced within the CNS and may have CS-independent effects on glia. We found that ACTH 1-39 stimulated proliferation of OPC, and to a lesser extent astroglia (AS) and microglia (MG), in rat glial cultures. ACTH accelerated differentiation of PDGFRα(+) OPC to a later stage marked by galactolipid expression and caused greater expansion of OL myelin-like sheets compared with untreated cells. Protective effects of ACTH on OPC were assessed by treating cultures with selected toxic agents, with or without ACTH. At 200 nM, ACTH protected OPC from death induced by staurosporine, glutamate, NMDA, AMPA, kainate, quinolinic acid, H2 O2 , and slow NO release, but not against kynurenic acid or rapid NO release. These agents and ACTH were not toxic to AS or MG. Our findings indicate that ACTH 1-39 provides benefits by increasing the number of OPC, accelerating their development into mature OL, and reducing OPC death from toxic insults.

Molecular evolution of GPCRs: Melanocortin/melanocortin receptors

The melanocortin receptors (MCRs) are a family of G protein-coupled receptors that are activated by melanocortin ligands derived from the proprotein, proopiomelanocortin (POMC). During the radiation of the gnathostomes, the five receptors have become functionally segregated (i.e. melanocortin 1 receptor (MC1R), pigmentation regulation; MC2R, glucocorticoid synthesis; MC3R and MC4R, energy homeostasis; and MC5R, exocrine gland physiology). A focus of this review is the role that ligand selectivity plays in the hypothalamus/pituitary/adrenal-interrenal (HPA-I) axis of teleosts and tetrapods as a result of the exclusive ligand selectivity of MC2R for the ligand ACTH. A second focal point of this review is the roles that the accessory proteins melanocortin 2 receptor accessory protein 1 (MRAP1) and MRAP2 are playing in, respectively, the HPA-I axis (MC2R) and the regulation of energy homeostasis by neurons in the hypothalamus (MC4R) of teleosts and tetrapods. In addition, observations are presented on trends in the ligand selectivity parameters of cartilaginous fish, teleost, and tetrapod MC1R, MC3R, MC4R, and MC5R paralogs, and the modeling of the HFRW motif of ACTH(1-24) when compared with α-MSH. The radiation of the MCRs during the evolution of the gnathostomes provides examples of how the physiology of endocrine and neuronal circuits can be shaped by ligand selectivity, the intersession of reverse agonists (agouti-related peptides (AGRPs)), and interactions with accessory proteins (MRAPs).

Melanocortin receptor accessory protein 2 (MRAP2) interplays with the zebrafish melanocortin 1 receptor (MC1R) but has no effect on its pharmacological profile

The melanocortin system is probably one of the most complex hormonal systems since it integrates agonist, encoded in the proopiomelanocortin precursor, endogenous antagonist, agouti signaling protein and agouti-related protein, five different G-protein coupled receptors and two accessory proteins. These accessory proteins interact with melanocortin receptors to allow traffic to the plasma membrane or to regulate the pharmacological profile. The MC1R fill the extension locus, which is primarily responsible for the regulation of pigmentation. In zebrafish, both MC1R and MRAP2 system are expressed in the skin. We demonstrate that zebrafish MC1R physically, or closely, interacts with the MRAP2 system, although this interaction did not result in modification of the studied pharmacological profile. However, progressive fasting induced skin darkening but also an upregulation of the MRAP2 expression in the skin, suggesting an unknown role for MRAP2a that could involve receptor desensitization processes. We also demonstrate that crowding stress induces skin darkening and a downregulation of MC1R expression in the skin.

Fish in behavior research: unique tools with a great promise!

Fish represent the most diverse class of vertebrates on Earth and also an unprecedented, but as of yet still largely untapped, resource for comparative analyses that can illuminate answers to questions about both how organisms work and how they evolved. The current review is a general discussion of some of the basic principles of why adding new species such as fish to the short list of biomedical model organisms (mainly the house mouse and the rat) has merit. In addition to the general points, it also reviews some questions about a newcomer, the zebrafish, which is rapidly gaining popularity in brain and behavior research. It discusses some examples demonstrating the advantages and disadvantages of the zebrafish mainly in the context of biomedical research. It is followed by other articles that further elaborate on these questions.

Synaptic changes induced by melanocortin signalling

The melanocortin system has a well-established role in the regulation of energy homeostasis, but there is growing evidence of its involvement in memory, nociception, mood disorders and addiction. In this Review, we focus on the role of the melanocortin 4 receptor and provide an integrative view of the molecular mechanisms that lead to melanocortin-induced changes in synaptic plasticity within these diverse physiological systems. We also highlight the importance of melanocortin peptides and receptors in chronic pain syndromes, memory impairments, depression and drug abuse, and the possibility of targeting them for therapeutic purposes.