The Melanocortin System in Fugu: Determination of POMC/AGRP/MCR Gene Repertoire and Synteny, As Well As Pharmacology and Anatomical Distribution of the MCRs

Abstract

The G-protein-coupled melanocortin receptors (MCRs) play an important role in a variety of essential functions such as the regulation of pigmentation, energy homeostasis, and steroid production. We performed a comprehensive characterization of the MC system in Fugu (Takifugu rubripes). We show that Fugu has an AGRP gene with high degree of conservation in the C-terminal region in addition to a POMC gene lacking gamma-MSH. The Fugu genome contains single copies of four MCRs, whereas the MC3R is missing. The MC2R and MC5R are found in tandem and remarkably contain one and two introns, respectively. We suggest that these introns were inserted through a reverse splicing mechanism into the DRY motif that is widely conserved through GPCRs. We were able to assemble large blocks around the MCRs in Fugu, showing remarkable synteny with human chromosomes 16 and 18. Detailed pharmacological characterization showed that ACTH had surprisingly high affinity for the Fugu MC1R and MC4R, whereas alpha-MSH had lower affinity. We also showed that the MC2R gene in Fugu codes for an ACTH receptor, which did not respond to alpha-MSH. All the Fugu receptors were able to couple functionally to cAMP production in line with the mammalian orthologs. The anatomical characterization shows that the MC2R is expressed in the brain in addition to the head-kidney, whereas the MC4R and MC5R are found in both brain regions and peripheral tissues. This is the first comprehensive genomic and functional characterization of a GPCR family within the Fugu genome. The study shows that some parts of the MC system are highly conserved through vertebrate evolution, such as regions in POMC coding for ACTH, alpha-MSH, and beta-MSH, the C-terminal region of AGRP, key binding units within the MC1R, MC2R, MC4R, and MC5R, synteny blocks around the MCRs, pharmacological properties of the MC2R, whereas other parts in the system are either missing, such as the MC3R and gamma-MSH, or different as compared to mammals, such as the affinity of ACTH and MSH peptides to MC1R and MC4R and the anatomical expression pattern of the MCRs.

Molecular cloning and pharmacological characterization of porcine melanocortin-3 receptor

“…Previously, the MC3Rs have been cloned from a number of species, including human (Gantz et al 1993a), rat (Roselli-Rehfuss et al 1993), mouse (Desarnaud et al 1994), and elephant (Rompler et al 2006). In lower vertebrates, the MC3Rs have been cloned from the chicken (Takeuchi & Takahashi 1999) and zebrafish (Logan et al 2003) but was found to be absent in some other fishes such as the pufferfishes (Klovins et al 2004a). The numbers we reported herein for pMC3R are roughly in line with previous studies.…”

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

“…In this regard, a number of studies have been done on the pharmacological properties of mammalian (Gantz & Fong 2003), avian (Ling et al 2004), amphibian , and teleost (Klovins et al 2004a) MC1R, MC3R, MC4R, and MC5R. Collectively, these studies all show that these paralogs can all be activated by either ACTH or α-MSH with varying degrees of efficacy.…”

Pineal-specific agouti protein regulates teleost background adaptation

“…In mammals, the MC1R is expressed primarily in skin (31), where it acts directly to regulate the eumelaninpheomelanin switch (32). In contrast, MC1R expression has been reported in the brain of several fish species (33,34). AgRP2 is most potently an MC1R antagonist, with little activity at the MC4R and no activity at the MC3R (Fig.…”