AUTHOR=Kalananthan Tharmini , Lai Floriana , Gomes Ana S. , Murashita Koji , Handeland Sigurd , Rønnestad Ivar 

TITLE=The Melanocortin System in Atlantic Salmon (Salmo salar L.) and Its Role in Appetite Control

JOURNAL=Frontiers in Neuroanatomy

VOLUME=14

YEAR=2020

URL=https://www.frontiersin.org/journals/neuroanatomy/articles/10.3389/fnana.2020.00048

DOI=10.3389/fnana.2020.00048

ISSN=1662-5129

ABSTRACT=<p>The melanocortin system is a key neuroendocrine network involved in the control of food intake and energy homeostasis in vertebrates. Within the hypothalamus, the system comprises two main distinct neuronal cell populations that express the neuropeptides proopiomelanocortin (POMC; anorexigenic) or agouti-related protein (AGRP; orexigenic). Both bind to the melanocortin-4 receptor (MC4R) in higher order neurons that control both food intake and energy expenditure. This system is relatively well-conserved among vertebrates. However, in Atlantic salmon (<italic>Salmo salar</italic> L.), the salmonid-specific fourth round whole-genome duplication led to the presence of several paralog genes which might result in divergent functions of the duplicated genes. In the current study, we report the first comprehensive comparative identification and characterization of Mc4r and extend the knowledge of Pomc and Agrp in appetite control in Atlantic salmon. <italic>In silico</italic> analysis revealed multiple paralogs for <italic>mc4r</italic> (<italic>a1</italic>, <italic>a2</italic>, <italic>b1</italic>, and <italic>b2</italic>) in the Atlantic salmon genome and confirmed the paralogs previously described for <italic>pomc</italic> (<italic>a1</italic>, <italic>a2</italic>, and <italic>b</italic>) and <italic>agrp</italic> (<italic>1</italic> and <italic>2</italic>). All Mc4r paralogs are relatively well-conserved with the human homolog, sharing at least 63% amino acid sequence identity. We analyzed the mRNA expression of <italic>mc4r</italic>, <italic>pomc</italic>, and <italic>agrp</italic> genes in eight brain regions of Atlantic salmon post-smolt under two feeding states: normally fed and fasted for 4 days. The <italic>mc4ra2</italic> and <italic>b1</italic> mRNAs were predominantly and equally abundant in the hypothalamus and telencephalon, the <italic>mc4rb2</italic> in the hypothalamus, and <italic>a1</italic> in the telencephalon. All <italic>pomc</italic> genes were highly expressed in the pituitary, followed by the hypothalamus and saccus vasculosus. The <italic>agrp</italic> genes showed a completely different expression pattern from each other, with prevalent expression of the <italic>agrp1</italic> in the hypothalamus and <italic>agrp2</italic> in the telencephalon. Fasting did not induce any significant changes in the mRNA level of <italic>mc4r</italic>, <italic>agrp</italic>, or <italic>pomc</italic> paralogs in the hypothalamus or in other highly expressed regions between fed and fasted states. The identification and wide distribution of multiple paralogs of <italic>mc4r</italic>, <italic>pomc</italic>, and <italic>agrp</italic> in Atlantic salmon brain provide new insights and give rise to new questions of the melanocortin system in the appetite regulation in Atlantic salmon.</p>