Methionine hydroxy analogue enhanced fish immunity via modulation of NF-κB, TOR, MLCK, MAPKs and Nrf2 signaling in young grass carp (Ctenopharyngodon idella).

Our study investigated the effect of dietary methionine hydroxy analogue (MHA) on growth and immunity (head kidney, spleen and skin) of young grass carp (Ctenopharyngodon idella). A total of 630 grass carp (259.70 ± 0.47 g) were fed graded levels of MHA (0, 2.4, 4.4, 6.4, 8.5 and 10.5 g/kg diet) and one dl-methionine (DLM) group (6.4 g/kg diet) for 8 weeks. At the end of the feeding trial, fish were challenged with Aeromonas hydrophila for 14 days. The results indicated that optimal MHA increased lysozyme (LZ) and acid phosphatase (ACP) activities, complement 3 (C3), C4 and immunoglobulin M (IgM) contents and up-regulated mRNA levels of liver expressed antimicrobial peptide 2, hepcidin (head kidney), β-defensin-1 in the immune organs (P < 0.05), suggesting that MHA could enhance antimicrobial ability of fish. Meanwhile, optimal MHA enhanced the immune function of immune organs via down-regulating pro-inflammatory cytokines mRNA levels and up-regulated anti-inflammatory cytokines mRNA levels, which might be attributed to the down-regulation of nuclear factor κB p65, c-Rel, IκB kinase β, p38 mitogen activated protein kinase, eIF4E-binding protein1 (4E-BP1) and 4E-BP2 mRNA levels and up-regulation of inhibitor of κBα, ribosomal protein S6 kinase 1 and target of rapamycin mRNA levels (P < 0.05). In addition, optimal MHA improved cellular structure integrity of immune organs via repressing death receptor and mitochondria pathways induced apoptosis, which might be related to the down-regulation of c-Jun-N-terminal kinase mRNA levels (P < 0.05). Simultaneously, optimal MHA improved cellular structure integrity of immune organs via elevating glutathione contents, antioxidant enzymes activities and corresponding isoforms mRNA levels to attenuate oxidative damage, which might be to the up-regulation of NF-E2-related factor 2 mRNA levels and down-regulation of Kelch-like ECH-associating protein 1a mRNA levels (P < 0.05). Besides, optimal MHA improved intercellular structure integrity of immune organs via up-regulating the mRNA levels of intercellular tight junctions-related genes, which might be owing to the down-regulation of myosin light chain kinase mRNA levels (P < 0.05). In conclusion, MHA exerted a positive effect on the immune function and structural integrity of immune organs in fish. Furthermore, according to the positive effect, MHA was superior to DLM in grass carp. However, based on the growth performance, the efficacy of MHA relative to DLM was 97%. Finally, on the premise of the basal diet containing 4.01 g/kg methionine, the optimal MHA supplementation levels based on feed intake, PWG, defense against skin hemorrhage and lesion, LZ and ACP activities, IgM content, against malondialdehyde, protein carbonyl and ROS in the head kidney of young grass carp were 5.07, 5.21, 5.76, 5.90, 5.88, 5.80, 6.22, 5.68 and 6.85 g/kg diet, respectively.