Using the human melanocortin-2 receptor as a model for analyzing hormone/receptor interactions between a mammalian MC2 receptor and ACTH(1-24)

Intraperitoneal administration of α-melanocyte stimulating hormone (α-MSH) suppresses food intake and induces anxiety-like behavior via the brain MC4 receptor-signaling pathway in goldfish

α-Melanocyte stimulating hormone (α-MSH) is a peptide hormone released from the intermediate lobe of the pituitary which regulates body pigmentation. In addition to the pituitary, α-MSH is also produced in the midbrain, and exerts both anorexigenic and an anxiogenic actions. Acyl ghrelin and cholecystokinin are peripheral hormones derived from the digestive tract which affect the brain to control food intake and feeding behavior in vertebrates. In the present study, hypothesizing that plasma α-MSH may also stimulate the brain and exert central effects, we examined whether peripherally administered α-MSH affects food intake and psychomotor activity using a goldfish model. Intraperitoneal (IP) administration of α-MSH at 100 pmol g-1 body weight (BW) reduced food consumption and enhanced thigmotaxis. These α-MSH-induced actions were blocked by intracerebroventricular administration of HS024, an antagonist of the melanocortin 4 receptor (MC4R), at 50 pmol g-1 BW, whereas these actions were not attenuated by pretreatment with an IP-injected excess amount of capsaicin, a neurotoxin that destroys primary sensory (vagal and splanchnic) afferents, at 160 nmol g-1 BW. Transcripts for the MC4R showed higher expression in the diencephalon in other regions of the brain. These results suggest that, in goldfish, IP administered α-MSH is taken up by the brain, and also acts as anorexigenic and anxiogenic factor via the MC4R signaling pathway.

Trends in the evolution of the elasmobranch melanocortin-2 receptor: Insights from structure/function studies on the activation of whale shark Mc2r

To understand the mechanism for activation of the melanocortin-2 receptor (Mc2r) of the elasmobranch, Rhincodon typus (whale shark; ws), wsmc2r was co-expressed with wsmrap1 in CHO cells, and the transfected cells were stimulated with alanine-substituted analogs of ACTH(1-24) at the "message" motif (H⁶F⁷R⁸W⁹) and the "address" motif (K¹⁵K¹⁶R¹⁷R¹⁸P¹⁹). Complete alanine substitution of the H⁶F⁷R⁸W⁹ motif blocked activation, whereas single alanine substitution at this motif indicated the following hierarchy of position importance for activation: W⁹ > R⁸, and substitution at F⁷ and H⁶ had no effect on activation. The same analysis was done on a representative bony vertebrate Mc2r ortholog (Amia calva; bowfin; bf) and the order of position importance for activation was W⁹ > R⁸ = F⁷, (alanine substitution at H⁶ was negligible). Complete alanine substitution at the K¹⁵K¹⁶R¹⁷R¹⁸P¹⁹ motif resulted in distinct outcomes for wsMc2r and bfMc2r. For bfMc2r, this analog blocked activation-an outcome typical for bony vertebrate Mc2r orthologs. For wsMc2r, this analog resulted in a shift in sensitivity to stimulation of the analog as compared to ACTH(1-24) by two orders of magnitude, but the dose response curve did reach saturation. To evaluate whether the EC2 domain of wsMc2r plays a role in activation, a chimeric wsMc2r was made in which the EC2 domain was replaced with the EC2 domain from a melanocortin receptor that does not interact with Mrap1 (i.e., Xenopus tropicalis Mc1r). This substitution did not negatively impact the activation of the chimeric receptor. In addition, alanine substitution at a putative activation motif in the N-terminal of wsMrap1 did not affect the sensitivity of wsMc2r to stimulation by ACTH(1-24). Collectively, these observations suggest that wsMc2r may only have a HFRW binding site for melanocortin-related ligand which would explain how wsMc2r could be activated by either ACTH or MSH-sized ligands.

Structural basis of signaling regulation of the human melanocortin-2 receptor by MRAP1

G protein-coupled receptors (GPCRs) are regulated by various downstream proteins, of which the melanocortin receptor accessory protein 1 (MRAP1) is closely involved in the regulation of melanocortin receptor 2 (MC2R). Assisted by MRAP1, MC2R responds to adrenocorticotropic hormone (ACTH) and stimulates glucocorticoid biogenesis and cortisol secretion. MC2R activation plays an essential role in the hypothalamic-pituitary-adrenal (HPA) axis that regulates stress response, while its dysfunction causes glucocorticoid insufficiency- or cortisol excess-associated disorders. Here, we present a cryo-electron microscopy (cryo-EM) structure of the ACTH-bound MC2R-Gs-MRAP1 complex. Our structure, together with mutagenesis analysis, reveals a unique sharp kink at the extracellular region of MRAP1 and the 'seat-belt' effect of MRAP1 on stabilizing ACTH binding and MC2R activation. Mechanisms of ACTH recognition by MC2R and receptor activation are also demonstrated. These findings deepen our understanding of GPCR regulation by accessory proteins and provide valuable insights into the ab initio design of therapeutic agents targeting MC2R.

Pharmacological properties of whale shark (Rhincodon typus) melanocortin-2 receptor and melancortin-5 receptor: Interaction with MRAP1 and MRAP2

In the current study, the whale shark (ws; Rhincodon typus) melanocortin-2 receptor (MC2R) co-expressed with wsMRAP1 in Chinese Hamster Ovary (CHO) Cells could be stimulated in a dose dependent manner by ACTH(1-24) with an EC₅₀ of 2.6 × 10^-10 M ± 9.7 × 10^-11. When the receptor was expressed alone, stimulation was only observed at [10^-6 M]. A comparable increase in sensitivity to stimulation by srDes-Ac-αMSH was also observed when the receptor was co-expressed with wsMRAP1. Furthermore, co-expression with wsMRAP1 significantly increased the trafficking of wsMC2R to the plasma membrane of CHO cells. Surprisingly, co-expression with wsMRAP2 also increased sensitivity to stimulation by ACTH(1-24) and srDes-Ac-αMSH, and increased trafficking of the receptor to the plasma membrane. These observations are in sharp contrast to the response of MC2R orthologs of bony vertebrates which have an obligate requirement for co-expression with MRAP1 for both trafficking to the plasma membrane and activation, whereas, co-expression with MRAP2 increases trafficking, but has minimal effects on activation. In addition, when comparing the activation features of wsMC2R with those of the elephant shark MC2R and red stingray MC2R orthologs, both similarities and differences are observed. The spectrum of features for cartilaginous fish MC2R orthologs will be discussed. A second objective of this study was to determine whether wsMC5R has features in common with wsMC2R in terms of ligand selectivity and interaction with wsMRAP paralogs. While wsMC5R can be activated by either srACTH(1-24) or srDes-Ac-αMSH, and co-expression with wsMRAP1 enhances this activation, wsMRAP1 had no effect on the trafficking of wsMC5R. In addition, co-expression with wsMRAP2 had no positive or negative effect on either ligand sensitivity or trafficking of wsMC5R.

Hypothesis and Theory: Evaluating the Co-Evolution of the Melanocortin-2 Receptor and the Accessory Protein MRAP1

The melanocortin receptors (MCRs) and the MRAP accessory proteins belong to distinct gene families that are unique to the chordates. During the radiation of the chordates, the melancortin-2 receptor paralog (MC2R) and the MRAP1 paralog (melanocortin-2 receptor accessory protein 1) have co-evolved to form a heterodimer interaction that can influence the ligand selectivity and trafficking properties of MC2R. This apparently spontaneous interaction may have begun with the ancestral gnathostomes and has persisted in both the cartilaginous fishes and the bony vertebrates. The ramifications of this interaction had profound effects on the hypothalamus/anterior pituitary/adrenal-interrenal axis of bony vertebrates resulting in MC2R orthologs that are exclusively selective for the anterior pituitary hormone, ACTH, and that are dependent on MRAP1 for trafficking to the plasma membrane. The functional motifs within the MRAP1 sequence and their potential contact sites with MC2R are discussed. The ramifications of the MC2R/MRAP1 interaction for cartilaginous fishes are also discussed, but currently the effects of this interaction on the hypothalamus/pituitary/interrenal axis is less clear. The cartilaginous fish MC2R orthologs have apparently retained the ability to be activated by either ACTH or MSH-sized ligands, and the effect of MRAP1 on trafficking varies by species. In this regard, the possible origin of the dichotomy between cartilaginous fish and bony vertebrate MC2R orthologs with respect to ligand selectivity and trafficking properties is discussed in light of the evolution of functional amino acid motifs within MRAP1.

Pharmacological properties of whale shark (Rhincodon typus) melanocortin-2 receptor and melancortin-5 receptor: Interaction with MRAP1 and MRAP2

In the current study, the whale shark (ws; Rhincodon typus) melanocortin-2 receptor (MC2R) co-expressed with wsMRAP1 in Chinese Hamster Ovary (CHO) Cells could be stimulated in a dose dependent manner by ACTH(1-24) with an EC₅₀ of 2.6 × 10^-10 M ± 9.7 × 10^-11. When the receptor was expressed alone, stimulation was only observed at [10^-6 M]. A comparable increase in sensitivity to stimulation by srDes-Ac-αMSH was also observed when the receptor was co-expressed with wsMRAP1. In addition, co-expression with wsMRAP1 significantly increased the trafficking of wsMC2R to the plasma membrane of CHO cells. Surprisingly, co-expression with wsMRAP2 also increased sensitivity to stimulation by ACTH(1-24) and srDes-Ac-αMSH, and increased trafficking of the receptor to the plasma membrane. These observations are in sharp contrast to the response of MC2R orthologs of bony vertebrates which have an obligate requirement for co-expression with MRAP1 for both trafficking to the plasma membrane and activation, and while co-expression with MRAP2 increases trafficking, it has minimal effects on activation. In addition, when comparing the activation features of wsMC2R with those of the elephant shark MC2R and red stingray MC2R orthologs, both similarities and differences are observed. The spectrum of features for cartilaginous fish MC2R orthologs will be discussed. A second objective of this study was to determine whether wsMC5R has features in common with wsMC2R in terms of ligand selectivity and interaction with wsMRAP paralogs. While wsMC5R can be activated by either srACTH(1-24) or srDes-Ac-αMSH, and co-expression with wsMRAP1 enhances this activation, wsMRAP1 had no effect on the trafficking of wsMC5R. Co-expression with wsMRAP2 had no positive or negative effect on either ligand sensitivity or trafficking of wsMC5R.

Molecular determinants of ACTH receptor for ligand selectivity

The adrenocorticotropic hormone (ACTH) receptor, known as the melanocortin-2 receptor (MC2R), plays a key role in regulating adrenocortical function. ACTH receptor is a subtype of the melanocortin receptor family which is a member of the G-protein coupled receptor (GPCR) superfamily. ACTH receptor has unique characteristics among MCRs. α-MSH, β-MSH, γ-MSH and ACTH are agonists for MCRs but only ACTH is the agonist for ACTH receptor. In addition, the melanocortin receptor accessory protein (MRAP) is required for ACTH receptor expression at cell surface and function. In this review, we summarized the information available on the relationship between ACTH and ACTH receptor and provide the latest understanding of the molecular basis of the ACTH receptor responsible for ligand selectivity and function.

Evidence for diversity in the activation of the melanocortin 2 receptor: A study on gar, elephant shark and stingray MC2Rs

The melanocortin-2 receptor (MC2R) is a critical component of the HPI and HPA axes of cartilaginous fishes, teleosts and tetrapods. Studies on teleost and tetrapod orthologs suggest two contact sites between ACTH and the receptor involving the following motifs on ACTH: H⁶F⁷R⁸W⁹ and K¹⁵K¹⁶R¹⁷R¹⁸P¹⁹. Using spotted gar (g) MC2R as a representative bony fish MC2R ortholog, we found that activation of gMC2R in Chinese Hamster Ovary (CHO) cells was diminished following stimulation of the transfected cells with hACTH(1-24) analogs substituted with alanine at either the H⁶F⁷R⁸W⁹ or K¹⁵K¹⁶R¹⁷R¹⁸P¹⁹ motifs compared to stimulation with hACTH(1-24). This observation suggests two ligand contact sites necessary for activation of the gMC2R. The same experiments were done with elephant shark (es) MC2R, however only the H⁶F⁷R⁸W⁹ analogs blocked activation, pointing to a single contact on esMC2R. Conversely, the red stingray (sr) MC2R activation was blocked by both the H⁶F⁷R⁸W⁹ and K¹⁵K¹⁶R¹⁷R¹⁸P¹⁹ alanine-substituted analogs. Together these results build a picture of the evolution of the ligand and receptor interaction between ACTH and MC2R orthologs of different taxa. These results will be discussed in light of the parallel evolution of MC2R orthologs in cartilaginous fishes and bony vertebrates.

Regulation of divergent cortisol responsiveness in European sea bass, Dicentrarchus labrax L

Mechanisms regulating differences in cortisol responsiveness between low (LR) and high response (HR) individuals have been poorly studied. In this context, we aimed to study key regulatory processes in cortisol dynamics at the head kidneys of LR and HR European sea bass. To do so, resting plasma cortisol and ACTH concentrations were quantified in these fish. Additionally, the head kidneys of these individuals were superfused through an in vitro superfusion system and stimulated with the same amount of ACTH to assess their cortisol biosynthetic capacity. Moreover, the expression of important genes in cortisol regulation was assessed. Results showed that LR fish had lower resting cortisol concentrations than HR, although no differences existed in the circulating levels of ACTH. Additionally, the biosynthetic capacity of HR was higher than that of LR fish when in vitro stimulated with ACTH. At the molecular level, a statistically significant 3.4-fold higher expression of the ACTH receptor, mc2r, and a 2.3-fold, though not significant, higher expression of 11β-hydroxylase (cyp11b1), an enzyme involved in cortisol biosynthesis, was observed in the HR fish. Finally, a statistically significant 1.3-fold lower expression of 11β-hydroxysteroid dehydrogenase 2 (hsd11b2), an enzyme involved in cortisol inactivation, was observed in HR when compared to LR fish. Therefore, it was for the first time indicated that cortisol dynamics can also be regulated at the post-production level in the head kidney. Collectively, our results highlight the crucial role of the interrenal tissue in the regulation of differences in cortisol response between LR and HR sea bass individuals.

Binding, Thermodynamics, and Selectivity of a Non-peptide Antagonist to the Melanocortin-4 Receptor

The melanocortin-4 receptor (MC4R) is a potential drug target for treatment of obesity, anxiety, depression, and sexual dysfunction. Crystal structures for MC4R are not yet available, which has hindered successful structure-based drug design. Using microsecond-scale molecular-dynamics simulations, we have investigated selective binding of the non-peptide antagonist MCL0129 to a homology model of human MC4R (hMC4R). This approach revealed that, at the end of a multi-step binding process, MCL0129 spontaneously adopts a binding mode in which it blocks the agonistic-binding site. This binding mode was confirmed in subsequent metadynamics simulations, which gave an affinity for human hMC4R that matches the experimentally determined value. Extending our simulations of MCL0129 binding to hMC1R and hMC3R, we find that receptor subtype selectivity for hMC4R depends on few amino acids located in various structural elements of the receptor. These insights may support rational drug design targeting the melanocortin systems.

Regulation of Lipolytic Response and Energy Balance by Melanocortin 2 Receptor Accessory Protein (MRAP) in Adipocytes

Melanocortin 2 receptor accessory protein (MRAP) is highly expressed in adrenal gland and adipose tissue. In adrenal cells, MRAP is essential for adrenocorticotropic hormone (ACTH)-induced activation of the cAMP/protein kinase A (PKA) pathway by melanocortin 2 receptor (MC2R), leading to glucocorticoid production and secretion. Although ACTH was known to stimulate PKA-dependent lipolysis, the functional involvement of MRAP in adipocyte metabolism remains incompletely defined. Herein, we found that knockdown or overexpression of MRAP in 3T3-L1 adipocytes reduced or increased ACTH-induced lipolysis, respectively. Moreover, an unbiased proteomics screen and coimmunoprecipitation analysis identified Gαs as a novel interacting partner of MRAP. An MRAP mutant disabled in Gαs association failed to augment the activation of PKA and lipolytic response to ACTH. Furthermore, compared with wild-type mice, transgenic mice (aP2-MRAP) overexpressing MRAP fat specifically exhibited increased lipolytic response to ACTH. When fed a high-fat diet (HFD), the transgenic mice displayed a significant decrease in the gain of adiposity and body weight as well as an improvement in glucose and insulin tolerance. These phenotypes were accompanied by increased adipose expression of genes for mitochondrial fatty acid oxidation and thermogenesis, and overall energy expenditure. Collectively, our data strongly suggest that MRAP plays a critical role in the regulation of ACTH-induced adipose lipolysis and whole-body energy balance.

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

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

Design of cyclized selective melanotropins

This article describes the development of cyclic peptides for G-protein coupled receptors to enable structure-function knowledge and the design of novel therapeutics. One important property of cyclic peptides is that they tend to be resistant to the digestion, enabling them to survive in the human digestive tract. This trait makes them very important as drug leads or as scaffolds which, in theory, can be engineered to incorporate a peptide domain of medicinal value. This is especially important for delivery of peptides that would be destroyed without such implementation. The melanocortin system is the focus of this article, and includes melanotropin ligands and melanocortin receptors. We examine two strategies to constrain the melanotropin peptide backbone. The first is based on global constraint of peptides by cyclization using various kinds of linkers. In the second approach we describe the use of a natural cyclized template, the cyclotide, to graft the melanotropin phamacophore, -His-Phe-Arg-Trp-, to obtain selective drug leads. In these examples the conserved melanocyte stimulating hormone pharmacophore is examined and the modified peptides were synthesized by solid phase methodology. Biological studies confirmed the production of selective, potent and in some cases orally available ligands.

Stability and Pharmacological Effects of Gene-Recombinant Wild Type and Mutant Human Adrenocorticotropic Hormone

PURPOSE

Adrenocorticotropic hormone (ACTH) is the only medicine for treating infantile spasms, however, it is catabolized rapidly. In order to make an ACTH derivative with prolonged effects, we prepared genetically engineered wild type (WT) and mutant ACTH candidates based on protease database analysis, and compared their stability and pharmacological effects.

METHODS

For analysis of stability, serum concentration of WT and mutant ACTH candidates were tested at different time after intravenous injection, and elimination curves were calculated to compare pharmacokinetic properties of WT and E5D-mutant ACTH. For comparison of their pharmacological effects, levels of glucocorticoids (GC) in the blood serum and secreted from cultured Y1 mouse adrenal cells were tested, and their effects on the signaling pathway mediating the expression of genes critical for GC synthesis were analyzed. The effects of ACTHs on transcription levels of the genes involved in GC synthesis were tested by qPCR.

RESULTS

The blood concentration of E5D ACTH is higher than the WT after injection, and E5D mutation increased the t_1/2 and AUC of ACTH. Pharmacological experiments showed that the effects of E5D and Y2S mutant ACTH on the production of GC and the critical signal transduction were equivalent to those of WT. WT, E5D and Y2S ACTH also have similar effects on the transcriptional levels of the genes for GC synthesis, including STAR, P450-scc, 3β-HSD, and SF-1.

CONCLUSION

The stability of E5D mutant ACTH is higher than WT ACTH. The pharmacological effects of E5D ACTH is equivalent to those of WT ACTH.

ACTH Antagonists

Adrenocorticotropin (ACTH) acts via a highly selective receptor that is a member of the melanocortin receptor subfamily of type 1 G protein-coupled receptors. The ACTH receptor, also known as the melanocortin 2 receptor (MC2R), is unusual in that it is absolutely dependent on a small accessory protein, melanocortin receptor accessory protein (MRAP) for cell surface expression and function. ACTH is the only known naturally occurring agonist for this receptor. This lack of redundancy and high degree of ligand specificity suggests that antagonism of this receptor could provide a useful therapeutic aid and a potential investigational tool. Clinical situations in which this could be useful include (1) Cushing's disease and ectopic ACTH syndrome - especially while preparing for definitive treatment of a causative tumor, or in refractory cases, or (2) congenital adrenal hyperplasia - as an adjunct to glucocorticoid replacement. A case for antagonism in other clinical situations in which there is ACTH excess can also be made. In this article, we will explore the scientific and clinical case for an ACTH antagonist, and will review the evidence for existing and recently described peptides and modified peptides in this role.

60 YEARS OF POMC: Adrenal and extra-adrenal functions of ACTH

The pituitary adrenocorticotropic hormone (ACTH) plays a pivotal role in homeostasis and stress response and is thus the major component of the hypothalamo-pituitary-adrenal axis. After a brief summary of ACTH production from proopiomelanocortin (POMC) and on ACTH receptor properties, the first part of the review covers the role of ACTH in steroidogenesis and steroid secretion. We highlight the mechanisms explaining the differential acute vs chronic effects of ACTH on aldosterone and glucocorticoid secretion. The second part summarizes the effects of ACTH on adrenal growth, addressing its role as either a mitogenic or a differentiating factor. We then review the mechanisms involved in steroid secretion, from the classical Cyclic adenosine monophosphate second messenger system to various signaling cascades. We also consider how the interaction between the extracellular matrix and the cytoskeleton may trigger activation of signaling platforms potentially stimulating or repressing the steroidogenic potency of ACTH. Finally, we consider the extra-adrenal actions of ACTH, in particular its role in differentiation in a variety of cell types, in addition to its known lipolytic effects on adipocytes. In each section, we endeavor to correlate basic mechanisms of ACTH function with the pathological consequences of ACTH signaling deficiency and of overproduction of ACTH.

Adrenocorticotropic Hormone (ACTH) Responses Require Actions of the Melanocortin-2 Receptor Accessory Protein on the Extracellular Surface of the Plasma Membrane

The melanocortin-2 (MC2) receptor is a G protein-coupled receptor that mediates responses to ACTH. The MC2 receptor acts in concert with the MC2 receptor accessory protein (MRAP) that is absolutely required for ACTH binding and signaling. MRAP has a single transmembrane domain and forms a highly unusual antiparallel homodimer that is stably associated with MC2 receptors at the plasma membrane. Despite the physiological importance of the interaction between the MC2 receptor and MRAP, there is little understanding of how the accessory protein works. The dual topology of MRAP has made it impossible to determine whether highly conserved and necessary regions of MRAP are required on the intracellular or extracellular face of the plasma membrane. The strategy used here was to fix the orientation of two antiparallel MRAP molecules and then introduce inactivating mutations on one side of the membrane or the other. This was achieved by engineering proteins containing tandem copies of MRAP fused to the amino terminus of the MC2 receptor. The data firmly establish that only the extracellular amino terminus (Nout) copy of MRAP, oriented with critical segments on the extracellular side of the membrane, is essential. The transmembrane domain of MRAP is also required in only the Nout orientation. Finally, activity of MRAP-MRAP-MC2-receptor fusion proteins with inactivating mutations in either MRAP or the receptor was rescued by co-expression of free wild-type MRAP or free wild-type receptor. These results show that the basic MRAP-MRAP-receptor signaling unit forms higher order complexes and that these multimers signal.

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.

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

Structure/function studies on the activation of the rainbow trout melanocortin-2 receptor

Functional expression of the rainbow trout (rt) melanocortin-2 receptor (MC2R) in CHO cells requires co-expression with a teleost melanocortin-2 receptor accessory protein (MRAP) such as zebrafish (zf) MRAP. Transiently transfected rtMC2R/zfMRAP1 CHO cells were used to evaluate the efficacy of alanine substituted analogs of hACTH(1-24) in three motifs in the ligand: H(6)F(7)R(8)W(9), G(10)K(11)P(12)V(13)G(14), and K(15)K(16)R(17)R(18)P(19). Alanine substitution at all positions in each motif either completely blocked activation of the receptor (H(6)F(7)R(8)W(9) and K(15)K(16)R(17)R(18)P(19)) or resulted in just over 400 fold increase in EC50 value (G(10)K(11)P(12)V(13)G(14)). Single alanine substitutions in the H(6)F(7)R(8)W(9) motif indicated that substitution at either W(9) or R(8) resulted in a much larger increase in EC50 values as compared to substitutions at either F(7) or W(9). Alanine substitution at either K(15)K(16) or R(17)R(18)P(19) in the K(15)K(16)R(17)R(18)P(19) motif resulted in a statistically equivalent increase in EC50 value of at least 600 fold. Finally, alanine substitutions in the G(10)K(11)P(12)V(13)G(14) motif resulted in increases in EC50 values presumably as a result of altering the secondary structure of the ligand. However, truncated analogs of hACTH(1-24) in which either G(10)G(14) (ACTH(1-22), or K(11)P(12)V(13) (ACTH(1-21) were removed had no stimulatory activity. Finally, some of the hACTH(1-24) analogs were tested using rainbow trout head kidney pieces in vitro to confirm whether the response to analogs seen with the transient transfected rtMC2R CHO cells was similar to that of trout interrenal cells. The results of these alanine substitution analog studies are used to construct a multistep hypothetical model for the activation of teleost and tetrapod MC2Rs to account for the unique ligand selectivity of this receptor.

Analysis of the pharmacological properties of chicken melanocortin-2 receptor (cMC2R) and chicken melanocortin-2 accessory protein 1 (cMRAP1)

The chicken (Gallus gallus) melanocortin-2 receptor (cMC2R) can be functionally expressed in CHO cells when chicken melanocortin-2 receptor accessory protein 1 (cMRAP1) is co-expressed. The transiently transfected CHO cells responded in a robust manner to stimulation by hACTH(1-24) (EC50 value=2.7 × 10(-12)M +/- 1.3 × 10(-12)), but the transfected CHO cells could not be stimulated by NDP-MSH at concentrations as high as 10(-7)M. Incubation of cMC2R/cMRAP1 transfected cells with alanine substituted analogs of hACTH(1-24) at amino acid positions F(7) or W(9) completely blocked stimulation of the transfected cells. Similarly, incubation of cMC2R/cMRAP1 transfected cells with an analog of hACTH(1-24) with alanine substitutions at amino acid positions R(17)R(18)P(19) resulted in a 276 fold shift in EC50 value relative to the positive control (p<0.004). Collectively these observations suggest that cMC2R has binding sites for the HFRW motif and KKRRP motif of hACTH(1-24), and both motifs are required for full activation of the receptor. While previous studies had shown that Anolis carolinensis MC2R and Xenopus tropicalis MC2R could be functionally expressed in CHO cells that co-expressed mouse MRAP1, co-expression of these non-mammalian tetrapod MC2Rs with cMRAP1 resulted in a significant increase in sensitivity to hACTH(1-24), as measured by EC50 value, for A. carolinensis MC2R (p<0.005) and X. tropicalis MC2R (p<0.007). The implications of these observations are discussed.

Development of potent selective competitive-antagonists of the melanocortin type 2 receptor

Cushing's disease, a hypercortisolemic state induced by an ACTH overexpressing pituitary adenoma, causes increased morbidity and mortality. Selective antagonism of the melanocortin type 2 receptor (MC2R) may be a novel treatment modality. Five structurally related peptides with modified HFRW sites but intact putative MC2R binding sites were tested for antagonistic activity at MC1R, MC2R/MRAP, MC3R, MC4R and MC5R. Two of these peptides (GPS1573 and GPS1574) dose-dependently antagonized ACTH-stimulated MC2R activity (IC50s of 66±23 nM and 260±1 nM, respectively). GPS1573 and 1574 suppressed the Rmax but not EC50 of ACTH on MC2R, indicating non-competitive antagonism. These peptides did not antagonize α-MSH stimulation of MC1R and antagonized MC3, 4 and 5R at markedly lower potency. GP1573 and GPS1574 antagonize MC4R with IC50s of 950 nM and 3.7 μM, respectively. In conclusion, two peptide antagonists were developed with selectivity for MC2R, forming a platform for development of a medical treatment for Cushing's disease.

Analyzing the activation of the melanocortin-2 receptor of tetrapods

Following the biochemical characterization of the pituitary hormone, adrenocorticotropin (ACTH), in the 1950's, a number of structure/function studies were done which identifies two amino acid motifs in ACTH, the HFRW motif and KKRR motif, as critical for the activation of the "ACTH" receptor on adrenal cortex cells. In the 1990's the "ACTH" receptor was identified as a member of the melanocortin receptor gene family, and given the name melanocortin-2 receptor (MC2R). Since that time a number of studies on both tetrapod and teleost MC2R orthologs have established that these orthologs can only be activated by ACTH, but not by any of the MSH-sized melanocortin ligands, and these orthologs require interaction with the melanocortin-2 receptor accessory protein (MRAP) for functional expression. This review summarizes recent structure/function studies on human ACTH, and points out the importance of the GKPVG motif in ACTH for the activation of the receptor. In this regard, a multiple-step model for the activation of tetrapod and teleost MC2R orthologs is presented, and the evolution of gnathostome MC2R ligand selectivity and the requirement for MRAP interaction is discussed in light of a recent study on a cartilaginous fish MC2R ortholog. This review contains excerpts from the Gorbman/Bern Lecture presented at the Second Meeting of the North American Society for Comparative Endocrinology (NASCE).

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).

The use of amphipols for NMR structural characterization of 7-TM proteins

While amphipols have been proven useful for refolding of seven transmembrane helical (7-TM) proteins including G-protein-coupled receptors (GPCRs) and it could be shown that an amphipol environment is in principle suitable for NMR structural studies of the embedded protein, high-resolution NMR insights into amphipol refolded and isotopically labeled GPCRs are still very limited. Here we report on the recent progress toward NMR structural studies of the melanocortin-2 and -4 receptors, two class A GPCRs which so far have not been reported to be incorporated into an amphipol environment. Making use of the established 7-TM protein bacteriorhodopsin (BR) we initially tested and optimized amphipol refolding conditions. Most promising conditions were transferred to the refolding of the two melanocortin receptors. Analytical-scale refolding experiments on the melanocortin-2 receptor show very similar behavior to the results obtained on BR. Using cell-free protein expression we could generate sufficient amounts of isotopically labeled bacteriorhodopsin as well as melanocortin-2 and -4 receptors for an initial NMR analysis. Upscaling of the amphipol refolding protocol to protein amounts needed for NMR structural studies was, however, not straightforward and impeded detailed NMR insights for the two GPCRs. While well-resolved and dispersed NMR spectra could only be obtained for bacteriorhodopsin, a comparison of NMR data recorded on the melanocortin-4 receptor in SDS and in an amphipol environment indicates that amphipol refolding induces larger structural modifications in the receptor.

Modeling the interactions between MC2R and ACTH models from human

Melanocortin system is composed of four peptide hormones namely α-, β-, -γ, and adrenocorticotropic hormone (ACTH), derived from post-translational cleavage of a polypeptide precursor 'proopiomelanocortin (POMC).' Among these hormones, ACTH, a 38 amino acid residue peptide fragment is an important hormone as it is involved in steroid secretion. In addition to this, to cite a few, this hormone is also known to induce variety of other effects, such as alterations in motor/sexual behavior, improvement in memory, and anti-inflammatory effects. To date, five melanocortin receptors (MC1R-MC5R) have been characterized with tissue-specific expression patterns and different binding affinities for each of the melanocortin hormones to regulate various biological functions. In the present work, three-dimensional (3D) models of MC2R and ACTH from human have been predicted, followed by docking and molecular dynamics simulation. While the 3D model of MC2R receptor has been predicted through threading approach, structure of ACTH was built based on ab initio technique. The MC2R model was later successfully docked onto the ACTH structure. Molecular dynamics (MD) simulation for 20 ns was used to compute the binding free energy of MC2R with ACTH model under implicit solvent conditions.

Complex structural and regulatory evolution of the pro-opiomelanocortin gene family

The melanocortin system is a neuroendocrine machinery that has been associated with phenotypic diversification in a number of vertebrate lineages. Central to the highly pleiotropic melanocortin system is the pro-opiomelanocortin (pomc) gene family, a family of pre-prohormones that each give rise to melanocyte stimulating hormone (MSH), adrenocorticotropic releasing hormone (ACTH), β-lipotropin hormone, and β-endorphin. Here we examine the structure, tissue expression profile, and pattern of cis transcriptional regulation of the three pomc paralogs (α1, α2, and β) in the model cichlid fish Astatotilapia burtoni and other cichlids, teleosts, and mammals. We found that the hormone-encoding regions of pomc α1, pomc α2 and pomc β are highly conserved, with a few notable exceptions. Surprisingly, the pomc β gene of cichlids and pomacentrids (damselfish) encodes a novel melanocortin peptide, ε-MSH, as a result of a tandem duplication of the segment encoding ACTH. All three genes are expressed in the brain and peripheral tissues, but pomc α1 and α2 show a more spatially restricted expression profile than pomc β. In addition, the promoters of each pomc gene have diverged in nucleotide sequence, which may have facilitated the diverse tissue-specific expression profiles of these paralogs across species. Increased understanding of the mechanisms regulating pomc gene expression will be invaluable to the study of pomc in the context of phenotypic evolution.

Development and in vivo quantitative magnetic resonance imaging of polymer micelles targeted to the melanocortin 1 receptor

Recent emphasis has focused on the development of rationally designed polymer-based micelle carriers for drug delivery. The current work tests the hypothesis that target specificity can be enhanced by micelles with cancer-specific ligands. In particular, we describe the synthesis and characterization of a new gadolinium texaphyrin (Gd-Tx) complex encapsulated in an IVECT micellar system, stabilized through Fe(III) cross-linking and targeted with multiple copies of a specific ligand for the melanocortin 1 receptor (MC1R), which has been evaluated as a cell-surface marker for melanoma. On the basis of comparative MRI experiments, we have been able to demonstrate that these Gd-Tx micelles are able to target MC1R-expressing xenograft tumors in vitro and in vivo more effectively than various control systems, including untargeted or un-cross-linked Gd-Tx micelles. Taken in concert, the findings reported herein support the conclusion that appropriately designed micelles are able to deliver contrast agent payloads to tumors expressing the MC1R.

Evolution of the melanocortin-2 receptor in tetrapods: studies on Xenopus tropicalis MC2R and Anolis carolinensis MC2R

The tetrapods are a diverse assemblage of vertebrates, and that diversity is reflected in the sequences of tetrapod melanocortin-2 receptors (MC2Rs). In this review, the features common to human (mammal), Gallus gallus (bird), Anolis carolinensis (reptile), and Xenopus tropicalis (amphibian) MC2Rs in terms of ligand selectivity, requirements for interaction with MRAP1, and the effects of alanine substitutions to three amino acid motifs in the ligand hACTH(1-24) are discussed. Analysis of the effects of alanine substitutions to the H(6)F(7)R(8)W(9) and the K(15)K(16)R(17)R(18)P(19) motifs of hACTH(1-24) indicated that activation of A. carolinensis MC2R and X. tropicalis MC2R was more adversely affected by alanine substitutions at these positions as compared to the response of human MC2R to these same analogs. Furthermore, single alanine substitutions in the G(10)K(11)P(12)V(13)G(14) motif of hACTH(1-24) had negative affects on activation of both A. carolinensis MC2R and X. tropicalis MC2R that were not observed for human MC2R. The implications of responses to the various analogs of hACTH(1-24) in terms of the mechanism for mediating the activation of these various tetrapod melanocortin-2 receptors are discussed.

Observations on the evolution of the melanocortin receptor gene family: distinctive features of the melanocortin-2 receptor

The melanocortin receptors (MCRs) are a gene family in the rhodopsin class of G protein-coupled receptors. Based on the analysis of several metazoan genome databases it appears that the MCRs are only found in chordates. The presence of five genes in the family (i.e., mc1r, mc2r, mc3r, mc4r, mc5r) in representatives of the tetrapods indicates that the gene family is the result of two genome duplication events and one local gene duplication event during the evolution of the chordates. The MCRs are activated by melanocortin ligands (i.e., ACTH, α-MSH, β-MSH, γ-MSH, δ-MSH) which are all derived from the polypeptide hormone/neuropeptide precursor, POMC, and as a result the functional evolution of the MCRs is intimately associated with the co-evolution of POMC endocrine and neuronal circuits. This review will consider the origin of the MCRs, and discuss the evolutionary relationship between MC2R, MC5R, and MC4R. In addition, this review will analyze the functional evolution of the mc2r gene in light of the co-evolution of the MRAP (Melanocortin-2 Receptor Accessory Protein) gene family.