Leucine improves dietary protein use efficiency by regulating protein synthesis by activating amino acid transporters and the mTORC1 pathways in Chinese mitten crab (Eriocheir sinensis)

Leucine plays a critical role in protein metabolism and energy homeostasis. This study determines whether leucine supplementation increases body protein synthesis in juvenile E. sinensis while consuming less protein than their requirements. It also investigates the effects of leucine on molecular mechanisms regulating protein synthesis. The 8-week growth trial included three dietary protein levels (22%, 30% or 38%) crossing three leucine levels (1.1, 2.5 or 5.0%). In the trial, 1440 crabs (0.40 ± 0.05 g) were allocated to a 3 × 3 factorial experiment with four replicates of 40 crabs each. The WG and SGR of the NP group were significantly higher than those of the LLP group when the diet contained 1.1% leucine (P < 0.05). The WG, SGR and whole-body protein content of the LP group were significantly higher than those of the LLP group with 2.5% leucine added to the diet (P < 0.05) but not significantly different from those of the NP group (P > 0.05). However, when 5% leucine was added to the diet, the whole-body protein content of the LP group was significantly higher than that of the LLP group (P < 0.05). Dietary leucine supplementation significantly improved growth performance, increased crude protein and decreased crude lipids in the crabs. When the diet contained 30% crude protein, the highest WG, SGR, PER and PRR appeared in the LP-2.5 group (P > 0.05). The PER and PRR of the NP-2.5 group were significantly higher than those of the NP-1.1 group (P < 0.05) and were not significantly different from those of the NP-5.0 group (P > 0.05). In addition, the whole-body protein content of the NP-2.5 group was significantly higher than that of the NP-1.1 and NP-5.0 groups (P < 0.05). Growth hormone and ecdysterone significantly increased with dietary protein levels. The highest growth hormone and ecdysterone in the hemolymph occurred in the 2.5% leucine groups. Moreover, leucine supplementation significantly increased lipase and amylase activity in the hepatopancreas (P < 0.05). The 2.5% leucine supplementation significantly increased CAT activity and decreased MDA in the hepatopancreas. Dietary leucine significantly increased hemocyanin in the hemolymph, and the expression of amino acid transporter genes was significantly increased with dietary leucine supplementation. The crabs fed 2.5% leucine supplementation had the highest gene expression of the mTOR pathway, indicating that dietary leucine significantly promotes amino acid transport and protein synthesis in the hepatopancreas and muscles. This study suggests that a low-protein diet can inhibit animal protein synthesis and decrease energy accumulation and growth. The 2.5% leucine supplementation in the 30% protein diet effectively could promote protein synthesis and utilization and improve crabs growth and protein utlization.