dl-Methionine supplementation in a low-fishmeal diet affects the TOR/S6K pathway by stimulating ASCT2 amino acid transporter and insulin-like growth factor-I in the dorsal muscle of juvenile cobia (Rachycentron canadum)

An 8-week feeding experiment was conducted to investigate the effects of dl-methionine (Met) supplementation in a low-fishmeal diet on growth, key gene expressions of amino acid transporters and target of rapamycin (TOR) pathway in juvenile cobia, Rachycentron canadum. Seven isonitrogenous and isolipidic diets were formulated, containing 0 center dot 72, 0 center dot 90, 1 center dot 00, 1 center dot 24, 1 center dot 41, 1 center dot 63 and 1 center dot 86 % Met. Weight gain and specific growth rates increased gradually with Met levels of up to 1 center dot 24 % and then decreased gradually. In dorsal muscle, mRNA levels of ASCT2 in the 1 center dot 00 % Met group were significantly up-regulated compared with 0 center dot 72, 1 center dot 63, and 1 center dot 86 %. The insulin-like growth factor-I (IGF-I) mRNA levels in the dorsal muscle of fish fed 1 center dot 00 and 1 center dot 24 % Met were higher than those in fish fed other Met levels. In addition, fish fed 1 center dot 24 % Met showed the highest mRNA levels of TOR and phosphorylation of TOR on Ser(2448). The phosphorylation of ribosomal p70-S6 kinase (S6K) on Ser(371) in the dorsal muscle of fish fed 1 center dot 86 % Met was higher than those in the 0 center dot 72 % group. In conclusion, straight broken-line analysis of weight gain rate against dietary Met level indicates that the optimal Met requirement for juvenile cobia is 1 center dot 24 % (of DM, or 2 center dot 71 % dietary protein). Met supplementation in a low-fishmeal diet increased cobia growth via a mechanism that can partly be attributed to Met's ability to affect the TOR/S6K signalling pathway by enhancing ASCT2 and IGF-I transcription in cobia dorsal muscle.