Evolution of the melanocortin system

Functional relation of agouti signaling proteins (ASIPs) to pigmentation and color change in the starry flounder, Platichthys stellatus

We investigated the involvement of agouti-signaling proteins (ASIPs) in morphological pigmentation and physiological color change in flatfishes. We isolated ASIP1 and 2 mRNAs from the skin of starry flounder (Platichthys stellatus), and compared their amino acid (aa) structures to those of other animals. Then, we examined the mRNA expression levels of two ASIPs (Sf-ASIPs) in the pigmented ocular body and in the unpigmented blind body, as well as in the ordinary skin and in albino skin, in flatfishes. To investigate the role of Sf-ASIPs in physiological color change (color camouflage), we compared the expression of the two genes in two background colors (dark-green and white). Sf-ASIP1 cDNA had a 375-bp open reading frame (ORF) that encoded a protein consisting of 125 aa residues, and Sf-ASIP2 cDNA had a 402-bp ORF that encoded a protein consisting of 132 aa residues. RT-PCR revealed that the strongest Sf-ASIP1 and Sf-ASIP2 expression levels were observed in the eye and blind-skin, respectively. In Sf-ASIP1, the gene expression did not differ between the ocular-side skin and blind-side skin, nor between ordinary skin and abnormal skin of the fish. However, in Sf-ASIP2, the expression level was significantly higher in blind-side skin, compared to ocular-side skin, suggesting that the ASIP2 gene is related to the countershading body pigment pattern of the fish. In addition, the Sf-ASIP2 gene expression level was lower in the pigmented spot regions than in the unpigmented spot regions of the malpigmented pseudo-albino skins on the ocular side, implying that ASIP2 is responsible for the ocular-side pseudo-albino. Additionally, ASIP2 gene expression in the blind-side skin of ordinary fish was enhanced by a white tank, implying that a bright background color could inhibit hypermelanosis in the blind-side skin of cultured flounder by increasing the activity of the Sf-ASIP2 gene. However, we did not find any relationship of ASIPs with camouflage color changes. In conclusion, the ASIP2 gene is related to the morphological pigmentation (countershading and malpigmentation) of the skin in starry flounder, but not with physiological color changes (color camouflage) in the ocular-side skin.

Role of melanocortin system in the locomotor activity rhythms and melatonin secretion as revealed by agouti-signalling protein (asip1) overexpression in zebrafish

Temporal signals such as light and temperature cycles profoundly modulate animal physiology and behaviour. Via endogenous timing mechanisms which are regulated by these signals, organisms can anticipate cyclic environmental changes and thereby enhance their fitness. The pineal gland in fish, through the secretion of melatonin, appears to play a critical role in the circadian system, most likely acting as an element of the circadian clock system. An important output of this circadian clock is the locomotor activity circadian rhythm which is adapted to the photoperiod and thus determines whether animals are diurnal or nocturnal. By using a genetically modified zebrafish strain known as Tg (Xla.Eef1a1:Cau.asip1)iim04, which expresses a higher level of the agouti signalling protein 1 (Asip1), an endogenous antagonist of the melanocortin system, we observed a complete disruption of locomotor activity patterns, which correlates with the ablation of the melatonin daily rhythm. Consistent with this, in vitro experiments also demonstrated that Asip1 inhibits melatonin secretion from the zebrafish pineal gland, most likely through the melanocortin receptors expressed in this gland. Asip1 overexpression also disrupted the expression of core clock genes, including per1a and clock1a, thus blunting circadian oscillation. Collectively, these results implicate the melanocortin system as playing an important role in modulating pineal physiology and, therefore, circadian organisation in zebrafish.

Mechanisms Underlying the Formation and Evolution of Vertebrate Color Patterns

Vertebrates exhibit a wide range of color patterns, which play critical roles in mediating intra- and interspecific communication. Because of their diversity and visual accessibility, color patterns offer a unique and fascinating window into the processes underlying biological organization. In this review, we focus on describing many of the general principles governing the formation and evolution of color patterns in different vertebrate groups. We characterize the types of patterns, review the molecular and developmental mechanisms by which they originate, and discuss their role in constraining or facilitating evolutionary change. Lastly, we outline outstanding questions in the field and discuss different approaches that can be used to address them. Overall, we provide a unifying conceptual framework among vertebrate systems that may guide research into naturally evolved mechanisms underlying color pattern formation and evolution.

Dimerization of melanocortin 4 receptor controls puberty onset and body size polymorphism

Xiphophorus fish exhibit a clear phenotypic polymorphism in puberty onset and reproductive strategies of males. In X. nigrensis and X. multilineatus, puberty onset is genetically determined and linked to a melanocortin 4 receptor (Mc4r) polymorphism of wild-type and mutant alleles on the sex chromosomes. We hypothesized that Mc4r mutant alleles act on wild-type alleles by a dominant negative effect through receptor dimerization, leading to differential intracellular signaling and effector gene activation. Depending on signaling strength, the onset of puberty either occurs early or is delayed. Here, we show by Förster Resonance Energy Transfer (FRET) that wild-type Xiphophorus Mc4r monomers can form homodimers, but also heterodimers with mutant receptors resulting in compromised signaling which explains the reduced Mc4r signaling in large males. Thus, hetero- vs. homo- dimerization seems to be the key molecular mechanism for the polymorphism in puberty onset and body size in male fish.

The melanocortin receptor signaling system and its role in neuroprotection against neurodegeneration: Therapeutic insights

The melanocortin signaling system consists of the melanocortin peptides, their distinctive receptors, accessory proteins, and endogenous antagonists. Melanocortin peptides are small peptide hormones that have been studied in a variety of physiological and pathological conditions. There are five types of melanocortin receptors, and they are distributed within the central nervous system and in several tissues of the periphery. The G protein-coupled melanocortin receptors typically signal through adenylyl cyclase and other downstream signaling pathways. Depending on the ligand, surface expression of melanocortin receptor, receptor occupancy period, related proteins, the type of cell, and other parameters, the signaling pathways are complicated and pleiotropic. While it is known that all five melanocortin receptors are coupled to Gs, they can also occasionally couple to Gq or Gi. Both direct and indirect neuroprotection are induced by the melanocortin signaling system. Targeting several of the components of the melanocortin signaling system (ligands, receptors, accessory proteins, signaling effectors, and regulators) may provide therapeutic opportunities. Activation of the melanocortin system improves different functional traits in neurodegenerative diseases. There is a potential for additional melanocortin system interventions by interfering with dimerization or dissociation. This review aims to discuss the melanocortin receptor signaling system and its role in neuroprotection, as well as its therapeutic potential.

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.

Role of the melanocortin system in zebrafish skin physiology

The agouti-signaling protein (ASIP) acts as both a competitive antagonist and inverse agonist of melanocortin receptors which regulate dorsal-ventral pigmentation patterns in fish. However, the potential role of ASIP in the regulation of additional physiological pathways in the skin is unknown. The skin plays a crucial role in the immune function, acting as a physical limitation against infestation and also as a chemical barrier due to its ability to synthesize and secrete mucus and many immune effector proteins. In this study, the putative role of ASIP in regulating the immune system of skin has been explored using a transgenic zebrafish model overexpressing the asip1 gene (ASIPzf). Initially, the structural changes in skin induced by asip1 overexpression were studied, revealing that the ventral skin of ASIPzf was thinner than that of wild type (WT) animals. A moderate hypertrophy of mucous cells was also found in ASIPzf. Histochemical studies showed that transgenic animals appear to compensate for the lower number of cell layers by modifying the mucus composition and increasing lectin affinity and mucin content in order to maintain or improve protection against microorganism adhesion. ASIPzf also exhibit higher protein concentration under crowding conditions suggesting an increased mucus production under stressful conditions. Exposure to bacterial lipopolysaccharide (LPS) showed that ASIPzf exhibit a faster pro-inflammatory response and increased mucin expression yet severe skin injures and a slight increase in mortality was observed. Electrophysiological measurements show that the ASIP1 genotype exhibits reduced epithelial resistance, an indicator of reduced tissue integrity and barrier function. Overall, not only are ASIP1 animals more prone to infiltration and subsequent infections due to reduced skin epithelial integrity, but also display an increased inflammatory response that can lead to increased skin sensitivity to external infections.

Role of the Melanocortin System in Gonadal Steroidogenesis of Zebrafish

In teleost, as in other vertebrates, stress affects reproduction. A key component of the stress response is the pituitary secretion of the adrenocorticotropic hormone (ACTH), which binds to the melanocortin 2 receptor (MC2R) in the adrenal glands and activates cortisol biosynthesis. In zebrafish, Mc2r was identified in male and female gonads, while ACTH has been shown to have a physiological role in modulating reproductive activity. In this study, the hypothesis that other melanocortins may also affect how the zebrafish gonadal function is explored, specifically steroid biosynthesis, given the presence of members of the melanocortin signaling system in zebrafish gonads. Using cell culture, expression analysis, and cellular localization of gene expression, our new observations demonstrated that melanocortin receptors, accessory proteins, antagonists, and agonists are expressed in both the ovary and testis of zebrafish (n = 4 each sex). Moreover, melanocortin peptides modulate both basal and gonadotropin-stimulated steroid release from zebrafish gonads (n = 15 for males and n = 50 for females). In situ hybridization in ovaries (n = 3) of zebrafish showed mc1r and mc4r in follicular cells and adjacent to cortical alveoli in the ooplasm of previtellogenic and vitellogenic oocytes. In zebrafish testes (n = 3), mc4r and mc1r were detected exclusively in germ cells, specifically in spermatogonia and spermatocytes. Our results suggest that melanocortins are, directly or indirectly, involved in the endocrine control of vitellogenesis in females, through modulation of estradiol synthesis via autocrine or paracrine actions in zebrafish ovaries. Adult zebrafish testes were sensitive to low doses of ACTH, eliciting testosterone production, which indicates a potential role of this peptide as a paracrine regulator of testicular function.

Improved Growth and High Inheritance of Melanocortin-4 Receptor (mc4r) Mutation in CRISPR/Cas-9 Gene-Edited Channel Catfish, Ictalurus punctatus

Effects of CRISPR/Cas9 knockout of the melanocortin-4 receptor (mc4r) gene in channel catfish, Ictalurus punctatus, were investigated. Three sgRNAs targeting the channel catfish mc4r gene in conjunction with Cas9 protein were microinjected in embryos and mutation rate, inheritance, and growth were studied. Efficient mutagenesis was achieved as demonstrated by PCR, Surveyor® assay, and DNA sequencing. An overall mutation rate of 33% and 33% homozygosity/bi-allelism was achieved in 2017. Approximately 71% of progeny inherited the mutation. Growth was generally higher in MC4R mutants than controls (CNTRL) at all life stages and in both pond and tank environments. There was a positive relationship between zygosity and growth, with F₁ homozygous/bi-allelic mutants reaching market size 30% faster than F₁ heterozygotes in earthen ponds (p = 0.022). At the stocker stage (~ 50 g), MC4R × MC4R mutants generated in 2019 were 40% larger than the mean of combined CNTRL × CNTRL families (p = 0.005) and 54% larger than F₁ MC4R × CNTRL mutants (p = 0.001) indicating mutation may be recessive. With a high mutation rate and inheritance of the mutation as well as improved growth, the use of gene-edited MC4R channel catfish appears to be beneficial for application on commercial farms.

Comparative ontogenetic and transcriptomic analyses shed light on color pattern divergence in cichlid fishes

Stripe patterns are a striking example for a repeatedly evolved color pattern. In the African adaptive radiations of cichlid fishes, stripes evolved several times independently. Previously, it has been suggested that regulatory evolution of a single gene, agouti-related-peptide 2 (agrp2), explains the evolutionary lability of this trait. Here, using a comparative transcriptomic approach, we performed comparisons between (adult) striped and nonstriped cichlid fishes of representatives of Lake Victoria and the two major clades of Lake Malawi (mbuna and non-mbuna lineage). We identify agrp2 to be differentially expressed across all pairwise comparisons, reaffirming its association with stripe pattern divergence. We therefore also provide evidence that agrp2 is associated with the loss of the nonstereotypic oblique stripe of Mylochromis mola. Complementary ontogenetic data give insights into the development of stripe patterns as well as vertical bar patterns that both develop postembryonically. Lastly, using the Lake Victoria species pair Haplochromis sauvagei and Pundamilia nyererei, we investigated the differences between melanic and non-melanic regions to identify additional genes that contribute to the formation of stripes. Expression differences-that most importantly also do not include agrp2-are surprisingly small. This suggests, at least in this species pair, that the stripe phenotype might be caused by a combination of more subtle transcriptomic differences or cellular changes without transcriptional correlates. In summary, our comprehensive analysis highlights the ontogenetic and adult transcriptomic differences between cichlids with different color patterns and serves as a basis for further investigation of the mechanistic underpinnings of their diversification.

Melanocortin-5 Receptor: Pharmacology and Its Regulation of Energy Metabolism

As the most recent melanocortin receptor (MCR) identified, melanocortin-5 receptor (MC5R) has unique tissue expression patterns, pharmacological properties, and physiological functions. Different from the other four MCR subtypes, MC5R is widely distributed in both the central nervous system and peripheral tissues and is associated with multiple functions. MC5R in sebaceous and preputial glands regulates lipid production and sexual behavior, respectively. MC5R expressed in immune cells is involved in immunomodulation. Among the five MCRs, MC5R is the predominant subtype expressed in skeletal muscle and white adipose tissue, tissues critical for energy metabolism. Activated MC5R triggers lipid mobilization in adipocytes and glucose uptake in skeletal muscle. Therefore, MC5R is a potential target for treating patients with obesity and diabetes mellitus. Melanocortin-2 receptor accessory proteins can modulate the cell surface expression, dimerization, and pharmacology of MC5R. This minireview summarizes the molecular and pharmacological properties of MC5R and highlights the progress made on MC5R in energy metabolism. We poInt. out knowledge gaps that need to be explored in the future.

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.

Interdependence of Thyroid and Corticosteroid Signaling in Vertebrate Developmental Transitions

Post-embryonic acute developmental processes mainly allow the transition from one life stage in a specific ecological niche to the next life stage in a different ecological niche. Metamorphosis, an emblematic type of these post-embryonic developmental processes, has occurred repeatedly and independently in various phylogenetic groups throughout metazoan evolution, such as in cnidarian, insects, molluscs, tunicates, or vertebrates. This review will focus on metamorphoses and developmental transitions in vertebrates, including typical larval metamorphosis in anuran amphibians, larval and secondary metamorphoses in teleost fishes, egg hatching in sauropsids and birth in mammals. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in the regulation of these life transitions. The review will address the molecular and functional evolution of these axes and their interactions. Mechanisms of integration of internal and environmental cues, and activation of these neuroendocrine axes represent key questions in an “eco-evo-devo” perspective of metamorphosis. The roles played by developmental transitions in the innovation, adaptation, and plasticity of life cycles throughout vertebrates will be discussed. In the current context of global climate change and habitat destruction, the review will also address the impact of environmental factors, such as global warming and endocrine disruptors on hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal/interrenal axes, and regulation of developmental transitions.

A 1-bp deletion in Mc1r in a Norway rat (Rattus norvegicus) from Sado Island, Japan gives rise to a yellowish color variant: an insight into mammalian MC1R variants

The melanocortin-1 receptor gene (MC1R) controls production of the pigments eumelanin and pheomelanin. Changes in MC1R lead to variation in coat color in mammals, which can range from entirely black (melanism) to yellowish. In this study, we report a case of a wild-caught Norway rat (Rattus norvegicus) from Sado Island, Japan with a yellowish coat color. Upon sequencing the whole coding region of the Mc1r gene (954 bp), we found a 1-bp deletion at site 337 (c.337del), indicative of a frameshift mutation, which was characterized as a severe loss-of-function or null mutation. A spectrophotometer was used to measure coat color, revealing that the rat had a distinctly lighter coat, based on lightness score, than mice with homozygous similar loss-of-function mutations. This implies that loss-of-function mutations can yield different phenotypes in murine rodents. The loss-of-function-mutant rat exhibited a contrasting coat pattern consisting of darker and lighter colors along its dorsal and ventral sides, respectively. Similar patterns have been observed in homozygous MC1R-deficient mutants in other mammals, implying that the countershading pattern can still be expressed despite the absence of MC1R in the melanocyte.

Food intake, growth, and expression of neuropeptides regulating appetite in clown anemonefish (Amphiprion ocellaris) exposed to predicted climate changes

The clown anemonefish (Amphiprion ocellaris) is a common model species in studies assessing the impact of climate changes on tropical coral fish physiology, metabolism, growth, and stress. However, the basic endocrine principles for the control of food intake and energy homeostasis, under normal and elevated sea temperatures, in this species remain unknown. In this work, we studied food intake and growth in clown anemonefish reared at different temperatures and with different food availability. We also analyzed expression of genes in the melanocortin system, which is believed to be involved in the control of appetite and feeding behavior. These were two paralogues of pomc: pomca and pomcb; two paralogs of agrp: agrp1 and agrp2; and one mc4r-like. Groups of juvenile clown anemonefish were exposed to four experimental treatments combining (orthogonal design) two rearing temperatures: 28 °C (T28; normal) and 32 °C (T32; high) and two feeding regimes: one (1 M; 08:00) or three (3 M; 08:00, 12:00, 15:00) meals per day, fed to satiety by hand. The results showed that high temperature (T32) did not affect the average growth rate but induced a stronger asymmetrical individual body weight of the fish within the population (tank). Lower feeding frequency (1 M) resulted in lower growth rates at both rearing temperatures. Fish reared at high temperature had higher total daily food intake, which correlated with a lower expression of pomca, supporting an anorexigenic role of this gene. High temperature combined with restricted feeding induced higher agrp1 levels and resulted in a higher food intake in the morning meal compared to the control. This supports an orexigenic role for agrp1. mRNA levels of agrp2 responded differently from agrp1, supporting different roles for the paralogues. Levels of mc4r-like inversely correlated with fish body weight, indicating a possible size/stage dependence of gene expression. In conclusion, our results indicate that the melanocortin system is involved in adjusting appetite and food intake of clown anemonefish in response to elevated temperature and low food availability.

Melanocortin System in Kidney Homeostasis and Disease: Novel Therapeutic Opportunities

Melanocortin peptides, melanocortin receptors, melanocortin receptor accessory proteins, and endogenous antagonists of melanocortin receptors are the key components constituting the melanocortin hormone system, one of the most complex and important hormonal systems in our body. A plethora of evidence suggests that melanocortins possess a protective activity in a variety of kidney diseases in both rodent models and human patients. In particular, the steroidogenic melanocortin peptide adrenocorticotropic hormone (ACTH), has been shown to exert a beneficial effect in a number of kidney diseases, possibly via a mechanism independent of its steroidogenic activity. In patients with steroid-resistant nephrotic glomerulopathy, ACTH monotherapy is still effective in inducing proteinuria remission. This has inspired research on potential implications of the melanocortin system in glomerular diseases. However, our understanding of the role of the melanocortinergic pathway in kidney disease is very limited, and there are still huge unknowns to be explored. The most controversial among these is the identification of effector cells in the kidney as well as the melanocortin receptors responsible for conveying the renoprotective action. This review article introduces the melanocortin hormone system, summarizes the existing evidence for the expression of melanocortin receptors in the kidney, and evaluates the potential strategy of melanocortin therapy for kidney disease.

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.

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.

Melanocortin 4 receptor signaling and puberty onset regulation in Xiphophorus swordtails

Fish of the genus Xiphophorus provide a prominent example of genetic control of male body size and reproductive tactics. In X.nigrensis and X.multilineatus, puberty onset and body length are determined by melanocortin 4 receptor (Mc4r) allelic and copy number variations which were proposed to fine-tune the signaling output of the system. Accessory protein Mrap2 is required for growth across species by affecting Mc4r signaling. The molecular mechanism how Mc4r signaling controls puberty regulation in Xiphophorus and whether the interaction with Mrap2 is also involved was so far unclear. Hence, we examined Mc4r and Mrap2 in X.nigrensis and X.multilineatus, in comparison to a more distantly related species, X.hellerii. mc4r and mrap2 transcripts co-localized in the hypothalamus and preoptic regions in large males, small males and females of X.nigrensis, with similar signal strength for mrap2 but higher expression of mc4r in large males. This overexpression is constituted by wild-type and one subtype of mutant alleles. In vitro studies revealed that Mrap2 co-expressed with Mc4r increased cAMP production but did not change EC50. Cells co-expressing the wild-type and one mutant allele showed lower cAMP signaling than Mc4r wild-type cells. This indicates a role of Mc4r alleles, but not Mrap2, in puberty signaling. Different from X.nigrensis and X.multilineatus, X.hellerii has only wild-type alleles, but also shows a puberty onset and body length polymorphism, despite the absence of mutant alleles. Like in the two other species, mc4r and mrap2 transcripts colocalized and mc4r is expressed at substantially higher levels in large males. This demonstrates that puberty and growth regulation mechanism may not be identical even within same genus.

Screening and analysis of agouti signaling protein interaction partners in Pelodiscus sinensis suggests a role in lipid metabolism

Agouti signaling protein (ASP) is a secreted paracrine protein that has been widely reported to function in melanogenesis and obesity and could potentially be a core protein that regulates the color and fatty phenotype of P. sinensis. In this study, we screened out interacting proteins of ASP by combined co-immunoprecipitation mass spectrometry (CoIP-MS), yeast two hybrid (Y2H) analysis, and computational predictions. We performed docking of ASP with its well-known receptor melanocortin receptor 4 (MC4R) to predict the binding capacity and to screen out actual ASP interacting proteins, CoIP-MS was performed where identified 32 proteins that could bind with ASP and Y2H confirmed seven proteins binding with ASP directly. CoIP-MS and Y2H screening results including PPI prediction revealed that vitronectin (VTN), apolipoprotein A1 (APOA1), apolipoprotein B (APOB), and filamin B (FLNB) were the key interacting proteins of ASP. VTN, APOA1, and APOB are functional proteins in lipid metabolism and various skin disorders, suggesting ASP may function in lipid metabolism through these partners. This study provided protein-protein interaction information of ASP, and the results will promote further research into the diverse roles of ASP, as well as its binding partners, and their function in different strains of P. sinensis.

Development of the Pituitary Gland

The development of the anterior pituitary gland occurs in distinct sequential developmental steps, leading to the formation of a complex organ containing five different cell types secreting six different hormones. During this process, the temporal and spatial expression of a cascade of signaling molecules and transcription factors plays a crucial role in organ commitment, cell proliferation, patterning, and terminal differentiation. The morphogenesis of the gland and the emergence of distinct cell types from a common primordium are governed by complex regulatory networks involving transcription factors and signaling molecules that may be either intrinsic to the developing pituitary or extrinsic, originating from the ventral diencephalon, the oral ectoderm, and the surrounding mesenchyme. Endocrine cells of the pituitary gland are organized into structural and functional networks that contribute to the coordinated response of endocrine cells to stimuli; these cellular networks are formed during embryonic development and are maintained or may be modified in adulthood, contributing to the plasticity of the gland. Abnormalities in any of the steps of pituitary development may lead to congenital hypopituitarism that includes a spectrum of disorders from isolated to combined hormone deficiencies including syndromic disorders such as septo-optic dysplasia. Over the past decade, the acceleration of next-generation sequencing has allowed for rapid analysis of the patient genome to identify novel mutations and novel candidate genes associated with hypothalmo-pituitary development. Subsequent functional analysis using patient fibroblast cells, and the generation of stem cells derived from patient cells, is fast replacing the need for animal models while providing a more physiologically relevant characterization of novel mutations. Furthermore, CRISPR-Cas9 as the method for gene editing is replacing previous laborious and time-consuming gene editing methods that were commonly used, thus yielding knockout cell lines in a fraction of the time. © 2020 American Physiological Society. Compr Physiol 10:389-413, 2020.

Melanocortin 3 receptor gene polymorphism is associated with polycystic ovary syndrome in Turkish population

Polycystic ovary syndrome (PCOS) is a frequent complex disorder with an ill-defined etiology. Genetic factors seem rather effective at the occurrence of the disease, however, the evidence of established various studies results are unsatisfied. We aimed to make a contribution to the genetic baseline of the disease by investigating melanocortin 3 receptor gene polymorphism in affected patients. 101 PCOS patients and 162 age-matched healthy volunteered control subjects recruited to the study. PCOS patients classified according to their BMI class and insulin resistance situation. Anthropometric measurements, physical examination results, laboratory findings, and hormone levels were recorded for each participant and analysis of two SNPs on the MC3R gene; rs3746619 and rs3827103 were performed. Although no significant difference was observed in rs3827103 polymorphism between PCOS patients and controls; rs3746619 polymorphism was determined associated with PCOS in the heritage of dominant (AA + AC) and co-dominant (AA) genotypes. Two polymorphisms did not found related to obesity and insulin resistance in PCOS subgroups analysis. MC3R gene rs 3746619 polymorphism was found associated with PCOS in the Turkish population and may make a contribution to the genetic baseline of the disease.

Arctic charr brain transcriptome strongly affected by summer seasonal growth but only subtly by feed deprivation

Background

The Arctic charr (Salvelinus alpinus) has a highly seasonal feeding cycle that comprises long periods of voluntary fasting and a short but intense feeding period during summer. Therefore, the charr represents an interesting species for studying appetite-regulating mechanisms in fish.

Results

In this study, we compared the brain transcriptomes of fed and feed deprived charr over a 4 weeks trial during their summer feeding season. Despite prominent differences in body condition between fed and feed deprived charr at the end of the trial, feed deprivation affected the brain transcriptome only slightly. In contrast, the transcriptome differed markedly over time in both fed and feed deprived charr, indicating strong shifts in basic cell metabolic processes possibly due to season, growth, temperature, or combinations thereof. The GO enrichment analysis revealed that many biological processes appeared to change in the same direction in both fed and feed deprived fish. In the feed deprived charr processes linked to oxygen transport and apoptosis were down- and up-regulated, respectively. Known genes encoding for appetite regulators did not respond to feed deprivation. Gene expression of Deiodinase 2 (DIO2), an enzyme implicated in the regulation of seasonal processes in mammals, was lower in response to season and feed deprivation. We further found a higher expression of VGF (non-acronymic) in the feed deprived than in the fed fish. This gene encodes for a neuropeptide associated with the control of energy metabolism in mammals, and has not been studied in relation to regulation of appetite and energy homeostasis in fish.

Conclusions

In the Arctic charr, external and endogenous seasonal factors for example the increase in temperature and their circannual growth cycle, respectively, evoke much stronger responses in the brain than 4 weeks feed deprivation. The absence of a central hunger response in feed deprived charr give support for a strong resilience to the lack of food in this high Arctic species. DIO2 and VGF may play a role in the regulation of energy homeostasis and need to be further studied in seasonal fish.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5874-z) contains supplementary material, which is available to authorized users.

Evolution of proopiomelanocortin

Proopiomelanocortin (POMC) belongs to the opioid/orphanin gene family whose peptide precursors include either opioid (YGGF) or the orphanin/nociceptin core sequences (FGGF). In addition to POMC the family includes the proenkephalin (PENK), prodynorphin (PDYN), and nociceptin/proorphanin (PNOC) precursors. The opioid core sequence in POMC is incorporated by the β-endorphin that occupies the C-terminal region but this propeptide also exhibits at least two "alien" melanocortin core sequences (HFRW). An ACTH/MSH fragment merged into the opioid fragment not earlier than the two tetraploidizations of the vertebrate genome. Therefore, POMC exhibit a complex "evolutionary life" since the gene has coevolved together with two different receptor systems, i.e., opioid and melanocortin following a horse trading system. In this article, we summarize the different evolutionary hypotheses proposed for POMC evolution.

Exploring differentially expressed genes associated with coat color in goat skin using RNA-seq

Fur color in domestic goats is an important, genetically determined characteristic that is associated with economic value. This study was designed to perform a comprehensive expression profiling of genes expressed in the skin tissues from Laiwu Black goat and Lubei White goat. Comparisons of black and white goat skin transcriptomes revealed 102 differentially expressed genes (DEGs), of which 38 were upregulated and 64 downregulated in black skin compared with white skin. Among the DEGs, we identified six genes involved in pigmentation, including agouti signaling protein (ASIP), CAMP responsive element binding protein 3-like 1 (CREB3L1), dopachrome tautomerase (DCT), premelanosome protein (PMEL), transient receptor potential cation channel subfamily M member 1 (TRPM1), and tyrosinase-related protein 1 (TYRP1). Notably, there were no significant differences in the expression of melanocortin 1 receptor, microphthalmia-associated transcription factor, tyrosinase, and KIT proto-oncogene receptor tyrosine kinase between the black and white skin samples, whereas ASIP expression was detected only in white skin. PMEL, TRPM1, TYRP1, and DCT showed higher expression in black goat skin, but ASIP and CREB3L1 had higher expression in white goat skin. Quantitative polymerase chain reaction results for PMEL, TRPM1, DCT, TYRP1, and CREB3L1 expression were consistent with those for RNA-seq. These results will expand our understanding of the complex molecular mechanisms of skin physiology and melanogenesis in goats, and provide a foundation for future studies.

Pharmacological modulation of MRAP2 protein on melanocortin receptors in the sea lamprey

Melanocortin receptors (MCRs) and their accessory proteins (MRAPs) evolutionarily first appear in the genome of sea lamprey. The most ancient melanocortin system consists of only two melanocortin receptors (slMCa and slMCb) and one MRAP2 (slMRAP2) protein, but the physiological roles have not been fully explored in this primitive species. Here, we synthesize and characterize the pharmacological features of slMRAP2 protein on two slMCRs. Our results show that the slMRAP2 protein lacks the long carboxyl terminus; it directly interacts and decreases the surface expression but enhances the α-MSH-induced agonism of slMCa and slMCb. In comparison with higher organisms such as elephant shark and zebrafish, we also demonstrate the constantly evolving regulatory function of the carboxyl terminus of MRAP2 protein, the unique antiparallel topology of slMRAP2 dimer and the homo- and hetero-dimerization of two slMCRs. This study elucidates the presence and modulation of melanocortin receptor by the accessory protein of the agnathans for the first time, which provides a better insight of the melanocortin system in ancient species of chordates.

Plasticity for colour adaptation in vertebrates explained by the evolution of the genes pomc, pmch and pmchl

Different camouflages work best with some background matching colour. Our understanding of the evolution of skin colour is based mainly on the genetics of pigmentation ("background matching"), with little known about the evolution of the neuroendocrine systems that facilitate "background adaptation" through colour phenotypic plasticity. To address the latter, we studied the evolution in vertebrates of three genes, pomc, pmch and pmchl, that code for α-MSH and two melanin-concentrating hormones (MCH and MCHL). These hormones induce either dispersion/aggregation or the synthesis of pigments. We find that α-MSH is highly conserved during evolution, as is its role in dispersing/synthesizing pigments. Also conserved is the three-exon pmch gene that encodes MCH, which participates in feeding behaviours. In contrast, pmchl (known previously as pmch), is a teleost-specific intron-less gene. Our data indicate that in zebrafish, pmchl-expressing neurons extend axons to the pituitary, supportive of an MCHL hormonal role, whereas zebrafish and Xenopus pmch+ neurons send axons dorsally in the brain. The evolution of these genes and acquisition of hormonal status for MCHL explain different mechanisms used by vertebrates to background-adapt.

Analysis of the Role of the Mc4r System in Development, Growth, and Puberty of Medaka

In mammals the melanocortin 4 receptor (Mc4r) signaling system has been mainly associated with the regulation of appetite and energy homeostasis. In fish of the genus Xiphophorus (platyfish and swordtails) puberty onset is genetically determined by a single locus, which encodes the mc4r. Wild populations of Xiphophorus are polymorphic for early and late-maturing individuals. Copy number variation of different mc4r alleles is responsible for the difference in puberty onset. To answer whether this is a special adaptation of the Mc4r signaling system in the lineage of Xiphophorus or a more widely conserved mechanism in teleosts, we studied the role of Mc4r in reproductive biology of medaka (Oryzias latipes), a close relative to Xiphophorus and a well-established model to study gonadal development. To understand the potential role of Mc4r in medaka, we characterized the major features of the Mc4r signaling system (mc4r, mrap2, pomc, agrp1). In medaka, all these genes are expressed before hatching. In adults, they are mainly expressed in the brain. The transcript of the receptor accessory protein mrap2 co-localizes with mc4r in the hypothalamus in adult brains indicating a conserved function of modulating Mc4r signaling. Comparing growth and puberty between wild-type and mc4r knockout medaka revealed that absence of Mc4r does not change puberty timing but significantly delays hatching. Embryonic development of knockout animals is retarded compared to wild-types. In conclusion, the Mc4r system in medaka is involved in regulation of growth rather than puberty.

Melanocortin overexpression limits diet-induced inflammation and atherosclerosis in LDLR-/- mice

Atherosclerosis is a chronic inflammatory disease of the arteries. The disease is initiated by endothelial dysfunction that allows the transport of leukocytes and low-density lipoprotein into the vessel wall forming atherosclerotic plaques. The melanocortin system is an endogenous peptide system that regulates, for example, energy homeostasis and cardiovascular function. Melanocortin treatment with endogenous or synthetic melanocortin peptides reduces body weight, protects the endothelium and alleviates vascular inflammation, but the long-term effects of melanocortin system activation on atheroprogression remain largely unknown. In this study, we evaluated the effects of transgenic melanocortin overexpression in a mouse model of atherosclerosis. Low-density lipoprotein receptor-deficient mice overexpressing alpha- and gamma₃-MSH (MSH-OE) and their wild-type littermates were fed either a regular chow or Western-style diet for 16 weeks. During this time, their metabolic parameters were monitored. The aortae were collected for functional analysis, and the plaques in the aortic root and arch were characterised by histological and immunohistochemical stainings. The aortic expression of inflammatory mediators was determined by quantitative PCR. We found that transgenic MSH-OE improved glucose tolerance and limited atherosclerotic plaque formation particularly in Western diet-fed mice. In terms of aortic vasoreactivity, MSH-OE blunted alpha₁-adrenoceptor-mediated vasoconstriction and enhanced relaxation response to acetylcholine, indicating improved endothelial function. In addition, MSH-OE markedly attenuated Western diet-induced upregulation of proinflammatory cytokines (Ccl2, Ccl5 and Il6) that contribute to the pathogenesis of atherosclerosis. These results show that the activation of the melanocortin system improves glucose homeostasis and limits diet-induced vascular inflammation and atherosclerotic plaque formation.

Seeing the light to change colour: An evolutionary perspective on the role of melanopsin in neuroendocrine circuits regulating light-mediated skin pigmentation

Melanopsin photopigments, Opn4x and Opn4m, were evolutionary selected to "see the light" in systems that regulate skin colour change. In this review, we analyse the roles of melanopsins, and how critical evolutionary developments, including the requirement for thermoregulation and ultraviolet protection, the emergence of a background adaptation mechanism in land-dwelling amphibian ancestors and the loss of a photosensitive pineal gland in mammals, may have helped sculpt the mechanisms that regulate light-controlled skin pigmentation. These mechanisms include melanopsin in skin pigment cells directly inducing skin darkening for thermoregulation/ultraviolet protection; melanopsin-expressing eye cells controlling neuroendocrine circuits to mediate background adaptation in amphibians in response to surface-reflected light; and pineal gland secretion of melatonin phased to environmental illuminance to regulate circadian and seasonal variation in skin colour, a process initiated by melanopsin-expressing eye cells in mammals, and by as yet unknown non-visual opsins in the pineal gland of non-mammals.

Selection on the Major Color Gene Melanocortin-1-Receptor Shaped the Evolution of the Melanocortin System Genes

Modular genetic systems and networks have complex evolutionary histories shaped by selection acting on single genes as well as on their integrated function within the network. However, uncovering molecular coevolution requires the detection of coevolving sites in sequences. Detailed knowledge of the functions of each gene in the system is also necessary to identify the selective agents driving coevolution. Using recently developed computational tools, we investigated the effect of positive selection on the coevolution of ten major genes in the melanocortin system, responsible for multiple physiological functions and human diseases. Substitutions driven by positive selection at the melanocortin-1-receptor (MC1R) induced more coevolutionary changes on the system than positive selection on other genes in the system. Contrarily, selection on the highly pleiotropic POMC gene, which orchestrates the activation of the different melanocortin receptors, had the lowest coevolutionary influence. MC1R and possibly its main function, melanin pigmentation, seems to have influenced the evolution of the melanocortin system more than functions regulated by MC2-5Rs such as energy homeostasis, glucocorticoid-dependent stress and anti-inflammatory responses. Although replication in other regulatory systems is needed, this suggests that single functional aspects of a genetic network or system can be of higher importance than others in shaping coevolution among the genes that integrate it.

Novel hypophysiotropic AgRP2 neurons and pineal cells revealed by BAC transgenesis in zebrafish

The neuropeptide agouti-related protein (AgRP) is expressed in the arcuate nucleus of the mammalian hypothalamus and plays a key role in regulating food consumption and energy homeostasis. Fish express two agrp genes in the brain: agrp1, considered functionally homologous with the mammalian AgRP, and agrp2. The role of agrp2 and its relationship to agrp1 are not fully understood. Utilizing BAC transgenesis, we generated transgenic zebrafish in which agrp1- and agrp2-expressing cells can be visualized and manipulated. By characterizing these transgenic lines, we showed that agrp1-expressing neurons are located in the ventral periventricular hypothalamus (the equivalent of the mammalian arcuate nucleus), projecting throughout the hypothalamus and towards the preoptic area. The agrp2 gene was expressed in the pineal gland in a previously uncharacterized subgroup of cells. Additionally, agrp2 was expressed in a small group of neurons in the preoptic area that project directly towards the pituitary and form an interface with the pituitary vasculature, suggesting that preoptic AgRP2 neurons are hypophysiotropic. We showed that direct synaptic connection can exist between AgRP1 and AgRP2 neurons in the hypothalamus, suggesting communication and coordination between AgRP1 and AgRP2 neurons and, therefore, probably also between the processes they regulate.

A Life without Hunger: The Ups (and Downs) to Modulating Melanocortin-3 Receptor Signaling

Melanocortin neurons conserve body mass in hyper- or hypo-caloric conditions by conveying signals from nutrient sensors into areas of the brain governing appetite and metabolism. In mice, melanocortin-3 receptor (MC3R) deletion alters nutrient partitioning independently of hyperphagia, promoting accumulation of fat over muscle mass. Enhanced rhythms in insulin and insulin-responsive metabolic genes during hypocaloric feeding suggest partial insulin resistance and enhanced lipogenesis. However, exactly where and how MC3Rs affect metabolic control to alter nutrient partitioning is not known. The behavioral phenotypes exhibited by MC3R-deficient mice suggest a contextual role in appetite control. The impact of MC3R-deficiency on feeding behavior when food is freely available is minor. However, homeostatic responses to hypocaloric conditioning involving increased expression of appetite-stimulating (orexigenic) neuropeptides, binge-feeding, food anticipatory activity (FAA), entrainment to nutrient availability and enhanced feeding-related motivational responses are compromised with MC3R-deficiency. Rescuing Mc3r transcription in hypothalamic and limbic neurons improves appetitive responses during hypocaloric conditioning while having minor effects on nutrient partitioning, suggesting orexigenic functions. Rescuing hypothalamic MC3Rs also restores responses of fasting-responsive hypothalamic orexigenic neurons in hypocaloric conditions, suggesting actions that sensitize fasting-responsive neurons to signals from nutrient sensors. MC3R signaling in ventromedial hypothalamic SF1(+ve) neurons improves metabolic control, but does not restore appetitive responses or nutrient partitioning. In summary, desensitization of fasting-responsive orexigenic neurons may underlie attenuated appetitive responses of MC3R-deficient mice in hypocaloric situations. Further studies are needed to identify the specific location(s) of MC3Rs controlling appetitive responses and partitioning of nutrients between fat and lean tissues.

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

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

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: POMC: an evolutionary perspective

Proopiomelanocortin (POMC) is a complex precursor that comprises several peptidic hormones, including melanocyte-stimulating hormones (MSHs), adrenocorticotropic hormone (ACTH), and β-endorphin. POMC belongs to the opioid/orphanin gene family, whose precursors include either opioid (YGGF) or the orphanin/nociceptin core sequences (FGGF). This gene family diversified during early tetraploidizations of the vertebrate genome to generate four different precursors: proenkephalin (PENK), prodynorphin (PDYN), and nociceptin/proorphanin (PNOC) as well as POMC, although both PNOC and POMC seem to have arisen due to a local duplication event. POMC underwent complex evolutionary processes, including internal tandem duplications and putative coevolutionary events. Controversial and conflicting hypotheses have emerged concerning the sequenced genomes. In this article, we summarize the different evolutionary hypotheses proposed for POMC evolution.

60 YEARS OF POMC: Regulation of feeding and energy homeostasis by α-MSH

The melanocortin peptides derived from pro-opiomelanocortin (POMC) were originally understood in terms of the biological actions of α-melanocyte-stimulating hormone (α-MSH) on pigmentation and adrenocorticotrophic hormone on adrenocortical glucocorticoid production. However, the discovery of POMC mRNA and melanocortin peptides in the CNS generated activities directed at understanding the direct biological actions of melanocortins in the brain. Ultimately, discovery of unique melanocortin receptors expressed in the CNS, the melanocortin-3 (MC3R) and melanocortin-4 (MC4R) receptors, led to the development of pharmacological tools and genetic models leading to the demonstration that the central melanocortin system plays a critical role in the regulation of energy homeostasis. Indeed, mutations in MC4R are now known to be the most common cause of early onset syndromic obesity, accounting for 2-5% of all cases. This review discusses the history of these discoveries, as well as the latest work attempting to understand the molecular and cellular basis of regulation of feeding and energy homeostasis by the predominant melanocortin peptide in the CNS, α-MSH.

Dual Topology of the Melanocortin-2 Receptor Accessory Protein Is Stable

Melanocortin 2 receptor accessory protein (MRAP) facilitates trafficking of melanocortin 2 (MC2) receptors and is essential for ACTH binding and signaling. MRAP is a single transmembrane domain protein that forms antiparallel homodimers. These studies ask when MRAP first acquires this dual topology, whether MRAP architecture is static or stable, and whether the accessory protein undergoes rapid turnover. To answer these questions, we developed an approach that capitalizes on the specificity of bacterial biotin ligase, which adds biotin to lysine in a short acceptor peptide sequence; the distinct mobility of MRAP protomers of opposite orientations based on their N-linked glycosylation; and the ease of identifying biotin-labeled proteins. We inserted biotin ligase acceptor peptides at the N- or C-terminal ends of MRAP and expressed the modified proteins in mammalian cells together with either cytoplasmic or endoplasmic reticulum-targeted biotin ligase. MRAP assumed dual topology early in biosynthesis in both CHO and OS3 adrenal cells. Once established, MRAP orientation was stable. Despite its conformational stability, MRAP displayed a half-life of under 2 h in CHO cells. The amount of MRAP was increased by the proteasome inhibitor MG132 and MRAP underwent ubiquitylation on lysine and other amino acids. Nonetheless, when protein synthesis was blocked with cycloheximide, MRAP was rapidly degraded even when MG132 was included and all lysines were replaced by arginines, implicating non-proteasomal degradation pathways. The results show that although MRAP does not change orientations during trafficking, its synthesis and degradation are dynamically regulated.

Deficiency in Melanocortin 1 Receptor Signaling Predisposes to Vascular Endothelial Dysfunction and Increased Arterial Stiffness in Mice and Humans

Objective—

The melanocortin 1 receptor (MC1-R) is expressed by vascular endothelial cells and shown to enhance nitric oxide (NO) availability and vasodilator function on pharmacological stimulation. However, the physiological role of MC1-R in the endothelium and its contribution to vascular homeostasis remain unresolved. We investigated whether a lack of functional MC1-R signaling carries a phenotype with predisposition to vascular abnormalities.

Approach and Results—

Recessive yellow mice (MC1R e/e ), deficient in MC1-R signaling, and their wild-type littermates were studied for morphology and functional characteristics of the aorta. MC1R e/e mice showed increased collagen deposition and arterial stiffness accompanied by an elevation in pulse pressure. Contractile capacity and NO-dependent vasodilatation were impaired in the aorta of MC1R e/e mice supported by findings of decreased NO availability. These mice also displayed elevated levels of systemic and local cytokines. Exposing the mice to high-sodium diet or acute endotoxemia revealed increased susceptibility to inflammation-driven vascular dysfunction. Finally, we investigated whether a similar phenotype can be found in healthy human subjects carrying variant MC1-R alleles known to attenuate receptor function. In a longitudinal analysis of 2001 subjects with genotype and ultrasound data (The Cardiovascular Risk in Young Finns Study), weak MC1-R function was associated with lower flow-mediated dilatation response of the brachial artery and increased carotid artery stiffness.

Conclusions—

The present study demonstrates that deficiency in MC1-R signaling is associated with increased arterial stiffness and impairment in endothelium-dependent vasodilatation, suggesting a physiological role for MC1-R in the regulation of arterial tone.