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

Abstract

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.

Keywords: ACTH; MC2R; MC5R; MRAP; constructive neutral evolution; evolutionary cell biology; melanocortin receptors; α-MSH.

Figure 1

Scheme for the evolution of opioid/orphanin precursors and melanocortin receptors. These evolutionary schemes assume that lampreys are 1R organisms. (A) Proposed evolution for the opioid/orphanin gene family. The scheme is modified from Dores and Baron (2011). (B) Proposed evolution of the melanocortin receptors. This scheme is modified from Baron et al. (2009). The dashed line indicates the origin of MC5R is not resolved. R, refers to the number of genome duplication events; Opioid, ancestral opioid precursor; Proto-Enk/Dyn, hypothetical proto-enkephalin/dynorphin gene; POMC, proopiomelanocortin; MCR, ancestral melanocortin receptor; MCaR, lamprey melanocortin-a receptor; MCbR, lamprey melanocortin-b receptor; MC1R, melanocortin-1 receptor; MC2R, melanocortin-2 receptor; MC3R, melanocortin-3 receptor, MC4R, melanocortin-4 receptor; MC5R, melanocortin-5 receptor.

Figure 2

Alternative schemes for the evolution of opioid/orphanin precursors and melanocortin receptors (MCRs). These evolutionary schemes assume that lampreys are 2R organisms. (A) Proposed evolution of the opioid/orphanin gene family. (B) Proposed evolution of the MCRs. The dashed line indicates the origin of MC5R is not resolved. R, refers to the number of genome duplication events; Opioid, ancestral opioid precursor; Prodyn, prodynorphin; Proenk, proenkephalin, Proorph, proorphanin; Enk/Dyn, hypothetical proto-enkephalin/dynorphin gene; POMC, proopiomelanocortin; POC, proopiocortin; POM, proopiomelanotropin; MCR, ancestral melanocortin receptor; MCaR, lamprey melanocortin-a receptor; MCbR, lamprey melanocortin-b receptor; MC1R, melanocortin-1 receptor; MC2R, melanocortin-2 receptor; MC3R, melanocortin-3 receptor, MC4R, melanocortin-4 receptor; MC5R, melanocortin-5 receptor.

Figure 3

Revised scheme for the evolution of the melanocortin receptors. This evolutionary scheme assumes that the melanocortin-4 receptor was the ancestral melanocortin receptor. The dashed box around a receptor indicates that the receptor is predicted but has not been identified. The dashed lines represent the hypothesis that the MC2R gene and the MC5R gene were the result of a local duplication of the proposed MC2R/MC5R gene.

Figure 4

Functional evolution of the melanocortin-2 receptor. (A) This summary of ligand selectivity of melanocortin-2 receptor orthologs indicates that teleost and tetrapod melanocortin-2 receptors can only be activated by ACTH, but not by any of the MSH-sized melanocortin ligands. The melanocortin-2 receptor ortholog of the cartilaginous fish, Callorhynchus milii can be activated by either ACTH or MSH-sized ligands (Reinick et al., 2012b). (B) This summary of the melanocortin-2 receptor interaction with MRAP1 indicates that teleost and tetrapod MC2R orthologs are dependent on MRAP1 for trafficking to the plasma membrane and for functional activation at the plasma membrane. The trafficking to the plasma membrane and the functional activation of the MC2R ortholog of the cartilaginous fish, C. milii is MRAP1 independent. (C) Following the divergence of the cartilaginous fishes and the bony fishes, an interaction developed between MC2R and MRAP1. As mutations occurred in the melanocortin-2 receptor, the receptor was rescued by MRAP1. In extant teleosts and tetrapods, the functional expression of MC2R is dependent on forming a complex with MRAP1.

Figure 5

Proposed Evolution of MRAP1 and MRAP2. It appears that MRAP2 may be the ancestral paralog in the MRAP gene family. This evolutionary scheme assumes that the MRAP2 gene was in the genome of the ancestral 2R vertebrates. This gene has been retained in the lamprey genome (Vastermark and Schiöth, 2011). In this scenario only a MRAP2 gene was present in the genome of the ancestral gnathostomes. Following the divergence of the cartilaginous fishes and the bony fishes, the MRAP2 gene duplicated in the bony fish lineage, and as a result MRAP2 and MRAP1 paralogs are present in the genomes of teleosts and tetrapods.

Figure A1

Alignment of cartilaginous fish melanocortin receptors. (A) The amino acid sequences of the elephant shark (C. milii) MC1R (AAVX01456471.1; http://esharkgenome.lmcb.a-star.edu.sg), elephant shark MC2R (accession number: FAA704.1), dogfish (Squalus acanthias) MC3R (accession number: AAS66720.1), dogfish MC4R (accession number: AA039833.1), and dogfish MC5R (accession number: AA867890.1) were aligned as described in Liang et al. (2011). In amino acids in red are the consensus amino acids for each position. Consensus was defined as two or more of the same amino acid at that position. Number below each consensus amino acid indicates the number of times that amino acid is present at the position. Positions marked by * there were two pairs of amino acids. These positions were not used in the analysis presented in Figure A2. Positions in which there was no consensus were left blank. The underlined amino acids indicate a N-linked glycosylation site. (B) A pair-wise comparison of sequence identity for these receptors is included.

Figure A2

Maximum parsimony analysis of cartilaginous fish melanocortin receptors. The sequences presented in Figure A1 including the consensus sequence were analyzed by maximum parsimony using the heuristic program in PAUP. The analysis yield a single tree and the jackknife values for 100 repetitions are included. The outgroup for the rooted tree was the lamprey MCb receptor (accession number: DQ213060) sequence. eMC1R, elephant shark MC1R; eMC2R, elephant shark MC2R; sMC3R, dogfish MC3R; sMC4R, dogfish MC4R; sMC5R, dogfish MC5R.