Methionine Promoted the Growth of Golden Pompano by Activating Intestinal Transport and TOR Signaling Pathway

This experiment aimed to study the effects of methionine (Met) on growth performance, intestinal structure and transport, Met metabolism, and target of rapamycin (TOR) signaling pathway in golden pompano (<i>Trachinotus ovatus</i>). Fish (9.12 ± 0.03 g) were fed with six diets with 0.79%, 0.90%, 1.01%, 1.12%, 1.23%, and 1.34% DL-Met for 8 weeks. Our study showed that Met supplementation significantly increased fish growth, villus length, crypt depth, and the mRNA levels of intestinal amino acid transporters including asc-type amino acid transporter 1 (<i>asc-1</i>), sodium-dependent neutral amino acid transporter B (<i>asct2</i>), and cationic amino acid transporter1 (<i>cat1</i>) (<i>p</i> < 0.05). Liver histological analysis revealed that Met addition improved cell swelling, nuclear migration, and hepatic vacuolation. Appropriate Met supplementation significantly increased mRNA level of key genes (methionine adenosyl transferase [<i>mat</i>], cystathionine beta-synthase [<i>cbs</i>], cystathionine γ-lyase [<i>cse</i>], and cysteine dioxygenase [<i>cdo</i>]) involved in transmethylation, transsulfuration metabolism, and taurine synthesis pathways. The level of 1.12%-1.23%, 1.12%, and 1.23% Met significantly upregulated the mRNA expression of S-adenosyl methionine/target of rapamycin (<i>samtor</i>), regulatory-associated protein of TOR (<i>raptor</i>), and ribosomal protein s6 (<i>s6</i>), respectively. The protein levels of protein kinase B (AKT), TOR, p-TOR, S6K, p-S6K, and p-S6 increased firstly and then decreased with increasing dietary Met supplementation. In conclusion, Met supplementation may promote growth of golden pompano by improving intestinal structure and amino acid transport, increasing transmethylation and transsulfuration metabolism, and activating TOR signaling pathways through SAMTOR and AKT.