Abstract
Cocaine- and amphetamine-regulated transcript (CART) has been known to be involved in feeding and energy balance in mammals, acting as an anorexigenic neuropeptide in hypothalamus. In Atlantic salmon, little is known about Cart brain localization and its function. In this study, in silico analysis revealed the existence of 10 cart paralogs, here named cart1a, 1b1, 1b2, 2a, 2b1, 2b2, 3a1, 3a2, 3b, and 4. The Atlantic salmon Cart sequences shared from 19 to 50% of identity with the human homolog and between 25 and 90% of sequence identity among paralogs, except for Cart4 which only shared 18–23% of identity. We further explored cart mRNA expressions in 8 brain regions (Olfactory Bulb-OB, Telencephalon-TEL, Midbrain-MB, Cerebellum-CE, Hypothalamus-HYP, Saccus vasculosus-SV, Pituitary-PT, and Brain Stem-BS) of Atlantic salmon smolt under 4 days of fasting and continuous fed conditions. The cart paralogs analyzed were widely distributed among the brain regions and OB, TEL, HYP, MB, and BS seemed to be the major sites of expression. The expression of cart1a and 1b showed quite similar pattern in MB, HYP, and BS. The expression of cart2a had the highest in MB followed by HYP and TEL. The cart3a transcript was widely distributed in rostrocaudal regions of brain except in OB and SV whereas cart3b was predominantly expressed in BS followed by MB. Expression of cart4 was high in HYP followed by TEL. With regards to effect of feeding status the Atlantic salmon cart2b, which is the most abundant among the paralogs, was upregulated after 4 days of fasting in OB, MB, and HYP compared to fed group. This may suggest an unexpected, but possible orexigenic role of cart2b in Atlantic salmon or a fasting induced stress effect. No other significant effect was observed. Collectively, the differential expressions of the cart paralogs in different brain regions suggest that they may have roles in regional integration of appetite signals and are possibly involved in regulating other brain functions in Atlantic salmon. The fact that salmon has 10 cart paralogs, while mammalians only one, opens interesting perspectives for comparative research on evolutionary adaptations of gene function in the control of appetite and energy homeostasis.
Highlights
The cocaine- and amphetamine-regulated transcript (CART), initially discovered as an unknown peptide isolated from ovine hypothalamus by Spiess et al (1981), was found to be released in response to the psychomotor stimulants cocaine and amphetamine in rat brain by Douglass et al (1995) and, its name
Similar cart anorectic effects have been observed in Ya-fish, where the mRNA expression of the three identified cart genes decreased in the hypothalamus after fasting and increased with refeeding (Yuan et al, 2015), while in yellowtail, cart1b and 2a mRNA expression decreased in the telencephalon and hypothalamus following 8 days of fasting (Fukada et al, 2021)
Comparison of the Atlantic salmon cart gene structure indicated that gene organization is conserved with three exons encoding the preproCart sequence separated by two introns (Figure 1)
Summary
The cocaine- and amphetamine-regulated transcript (CART), initially discovered as an unknown peptide isolated from ovine hypothalamus by Spiess et al (1981), was found to be released in response to the psychomotor stimulants cocaine and amphetamine in rat brain by Douglass et al (1995) and, its name. Multiple cart transcripts have been identified in the brain including two genes in goldfish (Carassius auracius) (Volkoff and Peter, 2001), three in Ya-fish (Schizothorax prenanti) (Yuan et al, 2015), four in zebrafish (Danio rerio) (Nishio et al, 2012; Akash et al, 2014; Ahi et al, 2019), five in yellowtail (Seriola quinqueradiata) (Fukada et al, 2021), six in medaka (Oryzias latipes) (Murashita and Kurokawa, 2011), and seven in Senegalese sole (Solea senegalensis) (Bonacic et al, 2015). Cart2b (previously named cart ch3) expression decreased with 17 days of starvation (Murashita and Kurokawa, 2011)
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