Abstract
Sensing of amino acids in fish brain, especially branched-chain amino acids (BCAA) like leucine, is involved in regulation of feed intake through different mechanisms. However, there is limited information regarding the possible involvement of mechanisms dependent on amino acid carriers of the solute carrier families (SLC) known to be key regulators of intracellular leucine concentration, namely L-type amino acid transporter 1 (LAT1), and sodium-dependent neutral amino acid transporter 2 (SNAT2) and 9,(SNAT9), for which evidence of their participation is available in mammals. Comparative analysis amongst sequences revealed a complex pattern of paralogues in Atlantic salmon, for LAT1 (slc7a5aa, slc7a5ab, slc7a5ba, slc7a5bb, slc7a5ca, and slc7a5cb), SNAT2 (slc38a2a and slc38a2b) and SNAT9 (slc38a9). After establishing phylogenetic relationships of the different paralogues evaluated, samples of the selected brain areas were taken from Atlantic salmon to assess tissue distribution of transcripts. In an additional experiment, fish were fed two diets with different levels of leucine (high leucine: 35 g/kg vs. control leucine: 27.3 g/kg). The high leucine diet resulted in lower feed intake and increased mRNA abundance of specific paralogues of LAT1 (slc7a5aa, slc7a5ab, and slc7a5bb) and SNAT2 (slc38a2a and slc38a2b) though apparently not for SNAT9 in brain areas like hypothalamus and telencephalon involved in food intake regulation. The results obtained suggest a role for members of the SLC family in the anorectic effect of leucine and thus their involvement as additional amino acid sensing mechanism not characterised so far in fish regulation of feed intake.
Highlights
In fish, feed intake is commonly regulated by both homeostatic and hedonic systems (Volkoff, 2016; Rønnestad et al, 2017; Soengas et al, 2018)
The amino acid transporters L-type amino acid transporter 1 (LAT1), sodium-dependent neutral amino acid transporter 2 (SNAT2), and SNAT9 are present in brain areas of Atlantic salmon
Elevated leucine level in the diet, resulted in decreased feed intake, and this occurred in parallel in the hypothalamus and the telencephalon with the up regulation of mRNA abundance of specific paralogues of LAT1 and SNAT2 though apparently not for SNAT9
Summary
In fish, feed intake is commonly regulated by both homeostatic and hedonic systems (Volkoff, 2016; Rønnestad et al, 2017; Soengas et al, 2018). Increased post-prandial circulating levels of leucine signals availability of protein/amino acids and cause a reduction in feed intake (Heeley and Blouet, 2016) This process occurs in the hypothalamus, and depends on several mechanisms not completely understood (Heeley and Blouet, 2016). These mechanisms include BCAA metabolism, glutamine metabolism, mechanistic target of rapamycin (mTOR), taste receptor signalling, general control non-derepressible 2 (GCN2) kinase signalling, and specific amino acid carriers (Efeyan et al, 2015; Heeley and Blouet, 2016). These transporters are key regulators of the intracellular concentration of leucine (Dodd and Tee, 2012; Wyant et al, 2017)
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