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

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

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

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.

Characterization of the chicken melanocortin 5 receptor and its potential role in regulating hepatic glucolipid metabolism

Melanocortin receptors (MC1R-MC5R) and their accessory proteins (MRAPs) are involved in a variety of physiological processes, including pigmentation, lipolysis, adrenal steroidogenesis, and immunology. However, the physiological roles of MC5R are rarely characterized in vertebrates, particularly in birds. In this work, we cloned the full-length cDNA of chicken MC5R and identified its core promoter region. Functional studies revealed that cMC5R was more sensitive to ACTH/α-MSH than β-MSH/γ-MSH, and was coupled to the cAMP/PKA signaling pathway. We demonstrated that MRAP2 decreased MC5R sensitivity to α-MSH, whereas MRAP1 did not have a similar effect, and that both MRAPs significantly reduced MC5R expression on the cell membrane surface. Transcriptome and qPCR data showed that both MRAP1 and MC5R were highly expressed in chicken liver. Additionally, we observed that ACTH might increase hepatic glucose production and decrease lipogenesis in primary hepatocytes, and dose-dependently downregulated the expression levels of ELOVL6 and THRSPA genes. These findings indicated that ACTH may act directly on hepatocytes to regulate glucolipid metabolism, which will help to understand the function of MC5R in avian.

Analyzing the Hypothalamus/Pituitary/Interrenal axis of the neopterygian fish, Lepisosteus oculatus: Co-localization of MC2R, MC5R, MRAP1, and MRAP2 in interrenal cells

RT-PCR analysis indicated that steroidogenic tissues are located along the length of the kidney of the neopterygian fish, Lepisosteus oculatus (spotted gar; g). However, RT-PCR analysis of the distribution of mc2r mRNA and mrap1 mRNA, critical components of the gar hypothalamus/pituitary/interrenal (HPI) axis, was only associated with the anterior and medial regions of the kidney. Steroidogenic cells were designated as interrenal cells that possess star mRNA (in situ hybridization) and lipid vesicles (histological analysis) within the kidney. RT-PCR also detected mc5r mRNA along the length of the tissues associated with the kidney. In situ hybridization analysis of the putative interrenal cells revealed co-expression of mc2r, and mc5r mRNAs in the same steroidogenic cells. Co-expression of gar Mc2r (gMc2r) and Mrap1 (gMrap1) in Chinese Hamster Ovary (CHO) cells stimulated with ACTH(1-24) resulted in activation with an EC₅₀ value of 1.0 × 10^-11M +/- 4.6 × 10^-11); whereas stimulation of CHO cells co-expressed with gar Mc5r (gMc5r) and gMrap1 and stimulated with ACTH(1-24) resulted in an EC₅₀ value that was 3 orders of magnitude lower (2.1 × 10^-8 M +/- 3.5 × 10^-9). Interesting, when CHO cells were co-transfected with gMc2r, gMc5r, and gMrap1 there was a decline in activation as measured by the V_max values for CHO cells stimulated with either ACTH(1-24) or α-MSH. These results suggest that some interaction may occur between gMc2r and gMc5r when both receptors are expressed in the same cells. Phylogenetic and selection pressure analyses of vertebrate mc2r and mc5r genes concluded that the two genes are evolving at different rates after duplication from a proposed common ancestral gene.

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.

Evaluating interactions between the melanocortin-5 receptor, MRAP1, and ACTH(1-24): A phylogenetic study

The melanocortin-2 receptor (MC2R) and the melanocortin-5 receptor (MC5R) are found on the same chromosome in most vertebrate genomes, and for the species analyzed in this study, MC2R and MC5R are co-expressed in glucocorticoid-producing cells that also express the accessory protein MRAP1. Since MRAP1 affects the ligand sensitivity of MC2R orthologs, this study tested the hypothesis that co-expression of MC5R with MRAP1 would also affect the ligand sensitivity of MC5R. The hypothesis was confirmed for stingray, rainbow trout, and chicken, MC5R orthologs. However, elephant shark MC5R was not affected in the same way by co-expression of MRAP1. It appears that, for some MC5R orthologs (i.e., stingray, rainbow trout, and chicken), a docking site for the R/KKRRP motif of ACTH(1-24) may become exposed on the receptor following co-expression with MRAP1. However, for elephant shark MC5R co-expression with MRAP1 may not affect engagement ACTH(1-24). Hence during the radiation of the chordates, the interaction between MRAP1 and MC5R has diverged.

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.

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.

Identifying Common Features in the Activation of Melanocortin-2 Receptors: Studies on the Xenopus tropicalis Melanocortin-2 Receptor

The interaction between the pituitary hormone, adrenocorticotropin (ACTH), and melanocortin-2 receptor (MC2R) orthologs involves the H6 F7 R8 W9 and R/K15 K16 R17 R18 motifs in ACTH making contact with corresponding contact sites on MC2R. Earlier studies have localized the common HFRW binding site of all melanocortin receptors to residues in TM2, TM3, and TM6 that are located close to the extracellular space. The current study has identified residues in Xenopus tropicalis (xt) MC2R in TM4 (I158, F161), in EC2 (M166), and in TM5 (V172) that also are involved in activation of xtMC2R, and may be in the R/KKRR contact site of xtMC2R. These results are compared to earlier studies on the corresponding domains of human MC2R and rainbow trout MC2R in an effort to identify common features in the activation of teleost and tetrapod MC2R orthologs following stimulation with ACTH.

Elephant shark melanocortin receptors: Novel interactions with MRAP1 and implication for the HPI axis

The presence of Mrap1 and Mrap2 orthologs in the genome of the elephant shark (es), a cartilaginous fish, presented an opportunity to evaluate the potential interactions between these accessory proteins and melanocortin receptors of a cartilaginous fish. RT-PCR analysis indicated that Mrap1 mRNA was present in interrenal, brain, and pituitary tissue with mRNA for Mc2R, Mc3R, Mc4R, and Mc5r. Co-expression of esMrap1 cDNA with esMc2r cDNA or esMc5r cDNA in CHO cells increased sensitivity to stimulation with ACTH(1-24) 10 fold and 100 fold, respectfully, but had no effect on sensitivity to stimulation with DesAc-αMSH [i.e., ACTH(1-13)NH₂] for either receptor, and had no effect on the ligand sensitivity of either esMc3r or esMc4r. Fluorescence image analysis indicated co-localization of esMrap1/esMc2r, and esMrap1/esMc5r on the plasma membrane; however, cell surface ELISA analysis indicated that co-expression with esMrap1 had no effect, positive or negative, on the trafficking of either esMc2r or esMc5r to the plasma membrane. RT-PCR analysis also indicated that Mrap2 mRNA, as well as, mRNAs for Mc2r, Mc3r, Mc4r, and Mc5r could be detected in brain tissue, however no Mrap2 mRNA was detected in interrenal tissue. Co-expression of esMrap2 in CHO cells with, respectively, esMc2r, esMc4r, or esMc5r had no effect on ligand sensitivity. However, co-expression of esMrap2 with esMc3r did lower sensitivity to stimulation by DesAc-αMSH 10 fold. These observations are discussed in the context of the parallel evolution of melanocortin receptors and their accessory proteins, and the hypothalamus/pituitary/adrenal axis and the hypothalamus/pituitary/interrenal axis in bony vertebrates and cartilaginous fishes.

The melanocortin-2 receptor of the rainbow trout: Identifying a role for critical positions in transmembrane domain 4, extracellular loop 2, and transmembrane domain 5 in the activation of rainbow trout MC2R

The activation of either teleost or tetrapod melanocortin-2 receptor (MC2R) orthologs requires interaction between the HFRW motif and R/KKRRP motif in the primary sequence of ACTH, and two corresponding sites on the melanocortin 2 receptor. While the HFRW contact site on MC2R appears to involve residues in TM2, TM3, and TM6, several studies on human MC2R point to the EC2/TM5 region of MC2R as a possible location for the R/KKRRP contact site. In this study nineteen single-alanine mutants of rainbow trout (rt) MC2R were made beginning at V¹⁵³ in TM4, at all positions in EC2 (extracellular loop 2), to F¹⁷⁵ in TM5. For twelve of these alanine mutants (i.e., V¹⁵³, G¹⁵⁵, C¹⁶², D¹⁶³, T¹⁶⁵, V¹⁶⁶, I¹⁶⁷, H¹⁶⁹, F¹⁷⁰, H¹⁷², V¹⁷³, L¹⁷⁴), alanine substitution did not have a statistically significant effect on activation of the receptor. For four of these alanine mutations (i.e., V¹⁵⁷, M¹⁵⁸, F¹⁶¹, K¹⁶⁸), while the negative shift in ligand sensitivity was statistically significant, the magnitude of the negative shift in activation was fivefold or less. However, for substitution at V¹⁵⁹ in TM4 (negative shift in activation: 110 fold), F¹⁷¹ in TM5 (negative shift in activation: 48-fold), and F¹⁷⁵ in TM5 (negative shift in activation: 100 fold), the effect on activation was both statistically significant and may be physiologically relevant. To support this conclusion, a triple alanine mutant of rtMC2R (V¹⁵⁹/A, F¹⁷¹/A, F¹⁷⁵/A), and this mutant receptor could not be activated by ACTH at concentrations as high as 10^-6M. A Cell Surface ELISA analysis indicated that the trafficking of the triple alanine mutant rtMC2R to the plasma membrane was not impaired by the alanine substitutions. Collectively, these observations point to a critical role for TM4 and TM5 in the activation of the rainbow trout melanocortin-2 receptor.

Analyzing the effects of co-expression of chick (Gallus gallus) melanocortin receptors with either chick MRAP1 or MRAP2 in CHO cells on sensitivity to ACTH(1-24) or ACTH(1-13)NH2: Implications for the avian HPA axis and avian melanocortin circuits in the hypothalamus

In order to better understand the roles that melanocortin receptors (cMCRs) and melanocortin-2 receptor accessory proteins (cMRAP1 and cMRAP2) play in the HPA axis and hypothalamus, adrenal gland and hypothalamus mRNA from 1day-old white leghorn chicks (Gallus gallus), were analyzed by real-time PCR. mRNA was also made for kidney, ovary, and liver. Mrap1 mRNA could be detected in adrenal tissue, but not in any of the other tissues, and mrap2 mRNA was also detected in the adrenal gland. Finally, all five melanocortin receptors mRNAs could be detected in the adrenal gland; mc2r and mc5r mRNAs were the most abundant. To evaluate any potential interactions between MRAP1 and the MCRs that may occur in adrenal cells, individual chick mcr cDNA constructs were transiently expressed in CHO cells either in the presence or absence of a chick mrap1 cDNA, and the transfected cells were stimulated with hACTH(1-24) at concentrations ranging from 10^-13M to 10^-6M. As expected, MC2R required co-expression with MRAP1 for functional expression; whereas, co-expression of cMC3R with cMRAP1 had no statistically significant effect on sensitivity to hACTH(1-24). However, co-expression of MC4R and MC5R with MRAP1, increased sensitivity for ACTH(1-24) by approximately 35 fold and 365 fold, respectively. However, co-expressing of cMRAP2 with these melanocortin receptors had no effect on sensitivity to hACTH(1-24). Since the real-time PCR analysis detected mrap2 mRNA and mc4r mRNA in the hypothalamus, the interaction between cMC4R and cMRAP2 with respect to sensitivity to ACTH(1-13)NH₂ stimulation was also evaluated. However, no effect, either positive or negative, was observed. Finally, the highest levels of mc5r mRNA were detected in liver cells. This observation raises the possibility that in one-day old chicks, activation of the HPA axis may also involve a physiological response from liver cells.

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

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

Identifying the activation motif in the N-terminal of rainbow trout and zebrafish melanocortin-2 receptor accessory protein 1 (MRAP1) orthologs

The activation of mammalian melanocortin-2 receptor (MC2R) orthologs is dependent on a four-amino acid activation motif (LDYL/I) located in the N-terminal of mammalian MRAP1 (melanocortin-2 receptor accessory protein). Previous alanine substitution analysis had shown that the Y residue in this motif appears to be the most important for mediating the activation of mammalian MC2R orthologs. Similar, but not identical amino acid motifs were detected in rainbow trout MRAP1 (YDYL) and zebrafish MRAP1 (YDYV). To determine the importance of these residues in the putative activation motifs, rainbow trout and zebrafish MRAP1 orthologs were individually co-expressed in CHO cells with rainbow trout MC2R, and the activation of this receptor with either the wild-type MRAP1 ortholog or alanine-substituted analogs of the two teleost MRAP1s was analyzed. Alanine substitutions at all four amino acid positions in rainbow trout MRAP1 blocked activation of the rainbow trout MC2R. Single alanine substitutions of the D and Y residues in rainbow trout and zebrafish MRAP1 indicate that these two residues play a significant role in the activation of rainbow trout MC2R. These observations indicate that there are subtle differences in the way that teleost and mammalian MRAPs are involved in the activation of their corresponding MC2R orthologs.

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

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

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

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

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

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