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

Abstract

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 (Salmo salar 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. In silico analysis revealed multiple paralogs for mc4r (a1, a2, b1, and b2) in the Atlantic salmon genome and confirmed the paralogs previously described for pomc (a1, a2, and b) and agrp (1 and 2). 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 mc4r, pomc, and agrp genes in eight brain regions of Atlantic salmon post-smolt under two feeding states: normally fed and fasted for 4 days. The mc4ra2 and b1 mRNAs were predominantly and equally abundant in the hypothalamus and telencephalon, the mc4rb2 in the hypothalamus, and a1 in the telencephalon. All pomc genes were highly expressed in the pituitary, followed by the hypothalamus and saccus vasculosus. The agrp genes showed a completely different expression pattern from each other, with prevalent expression of the agrp1 in the hypothalamus and agrp2 in the telencephalon. Fasting did not induce any significant changes in the mRNA level of mc4r, agrp, or pomc paralogs in the hypothalamus or in other highly expressed regions between fed and fasted states. The identification and wide distribution of multiple paralogs of mc4r, pomc, and agrp in Atlantic salmon brain provide new insights and give rise to new questions of the melanocortin system in the appetite regulation in Atlantic salmon.

Mapping key neuropeptides involved in the melanocortin system in Atlantic salmon (Salmo salar) brain

“…( 2009 ) identified two Agouti‐like sequences, named agrp1 and agrp2 (also named asip2b , see Braasch and Postlethwait ( 2011 ) and NCBI GenBank 1 ). The orexigenic effect of Atlantic salmon agrp1 seems to be in line with those reported in mammals (Kalananthan, Murashita, et al., 2020 ). However, agrp2 seems to not be directly involved in appetite control in Atlantic salmon (Kalananthan, Lai, et al., 2020 ) but may play other functional roles, as demonstrated in the zebrafish, Danio rerio (Shainer et al., 2017 , 2019 ; Zhang et al., 2010 ).…”

“…The hypothalamic neuropeptides npya , agrp1 , cart2b , and pomca are involved in appetite control as their expression levels responded to a fed/fasted state in Atlantic salmon (Kalananthan et al., 2021 ; Kalananthan, Murashita, et al., 2020 ; Murashita et al., 2011 , 2009 ; Tolås et al., 2021 ; Valen et al., 2011 ). Here, we show the presence of these neuropeptides in the NLT region of the Atlantic salmon parr, the putative homolog to the mammalian arcuate nucleus ((Cerdá‐Reverter & Peter, 2003 ; Cerdá‐Reverter, Ringholm, et al., 2003 ) and reviewed in Biran et al.…”

“…POMC is a precursor peptide that is post‐translationally cleaved into several peptides with a wide range of functions, including α‐ and β‐melanocyte‐stimulating hormones, adrenocorticotropic hormone, and β‐endorphin (Takahashi & Mizusawa, 2013 ). Three pomc paralogs ( pomca1 , pomca2 , and pomcb ) have been previously identified in Atlantic salmon (Murashita et al., 2011 ; Valen et al., 2011 ) and are primarily expressed in the pituitary and hypothalamus (Kalananthan, Lai, et al., 2020 ).…”

Effects of Short-Term Fasting on mRNA Expression of Ghrelin and the Peptide Transporters PepT1 and 2 in Atlantic Salmon (Salmo salar)

“…This study is part of a larger experiment where the main data on fish growth and performance were assessed ( Kalananthan et al, 2020a ). A subgroup of fish was used to evaluate the transcriptional changes of ghrelin splicing variants and peptide transporters paralogs along the GIT segments.…”

Brain Distribution of 10 cart Transcripts and Their Response to 4 Days of Fasting in Atlantic Salmon (Salmo salar L.)

“…Six brains from fed and six from the fasted group of Atlantic salmon were dissected into 8 regions: olfactory bulb (OB), telencephalon (TEL), midbrain (MB), cerebellum (CE), hypothalamus (HYP), saccus vasculosus (SV), pituitary (PT), and medulla oblongata/brain stem (BS) using the same standard dissection protocol as illustrated by Kalananthan et al (2020a). To ensure high integrity and quality of total RNA, the brain was placed on an ice block during dissection under a zoom stereomicroscope (Olympus SZ51) and cleaned from blood vessels.…”