Functional characterization of two melanin-concentrating hormone genes in the color camouflage, hypermelanosis, and appetite of starry flounder

To investigate the involvement of two melanin-concentrating hormones (MCHs) in skin color change and appetite in flatfish, we isolated two forms of prepro-melanin concentrating hormone (pMCHs) mRNA in the starry flounder Platichthys stellatus and compared their amino acid structures to those of other animals. Then, we examined the relationship of the two starry flounder pMCH (sf-pMCH) with physiological color change, blind-side malpigmentation, and feeding by quantifying mRNA expression level. Sf-pMCH1 cDNA had a 387-bp open reading frame (ORF) that encoded a protein consisting of 129 amino acid residues. The sf-pMCH1 protein included a signal peptide composed of 24 amino acid residues; MCH1 encoded a protein consisting of 17 amino acids. The sf-pMCH2 cDNA had a 450-bp ORF that encoded a protein consisting of 150 amino acid residues, which included a signal peptide comprising 23 amino acid residues; MCH2 encoded a protein consisting of 23 amino acids that was structurally similar to mammalian MCH. Reverse transcription-polymerase chain reaction (RT-PCR) revealed that the strongest sf-pMCHs gene expression was observed in the brain and pituitary, but weak or no amplification was detected in other tissues. The expression of sf-pMCH1 was relatively high compared to that of sf-pMCH2 in the brain. The relative levels of mRNA were significantly lower in dark background-reared and hypermelanic fish, indicating that the two pMCHs and background color are related to the physiological and morphological color changes of skin. In term of feeding regulation, we found an obvious functional role of pMCH1 in appetite, whereas the pMCH2 gene was not found to play a role in feeding.