Dietary supplementation of leucine improved the flesh quality of largemouth bass, Micropterus salmoides through TOR, FoxO3a and MRFs regulation

The study was aimed to investigate the impacts of graded levels of dietary leucine on muscle composition, fillet texture and flesh quality related genes expression of largemouth bass, Micropterus salmoides. Four hundred and fifty fish (18.0 +/- 0.1 g) were fed six semi-purified diets supplemented with crystalline L-leucine of 0, 4.0, 8.0, 12.0, 16.0 and 20.0 g/kg (Control, Leu-4, Leu-8, Leu-12, Leu-16, Leu-20) for 56 days. Results showed that an increase (P < 0.05) in muscle crude protein, crude lipid and collagen contents as well as flesh shear force, hardness, chewiness and myofiber density was observed with the increasing leucine supplementation from 0 to 8.0 g/kg (P < 0.05). However, flesh steaming loss and drip loss6h exhibited a negative quadratic trend (P < 0.05), and Leu-8 diet presented the lowest values. Flesh amino acid compositions were quadratically influenced (P < 0.05) by dietary leucine except arginine, histidine, isoleucine, phenylalanine and alanine, and the highest concentrations of amino acids were noted in Leu-8 group. Moreover, incremental leucine in diet also significantly (P < 0.05) influenced muscle fatty acid values, except for C14:0, C15:0, C20:0, C18:1 and C20:3. Importantly, leucine addition had a quadratic response (P < 0.05) on the up-regulation of PI3K, AKT, TOR, MyoD, MyoG, Myf5 and Myf6, and down-regulation of FoxO3a and MSTN mRNA gene expression in muscle, reaching a maximum and a minimum in Leu-8 group, respectively. Conclusively, dietary leucine could improve the flesh quality of largemouth bass, attributed partly to the AKT/TOR-FoxO3a signaling pathways coupled with myogenic regulatory factors. Based on muscle nutritive composition and fillet texture, the recommended leucine supplementation is 8.0 g/kg, corresponding to 25.2 g/kg dietary leucine.