The improved growth performance and enhanced immune function by DL-methionyl-DL-methionine are associated with NF-κB and TOR signalling in intestine of juvenile grass carp (Ctenopharyngodon idella).

The present study investigated the effects of dietary DL-methionyl-DL-methionine (Met-Met) on growth performance, intestinal immune function and the underlying signalling molecules in juvenile grass carp (Ctenopharyngodon idella). Fish were fed one DL-methionine (DL-Met) group (2.50 g/kg diet) and six graded levels of Met-Met groups (0, 0.79, 1.44, 1.84, 2.22 and 2.85 g/kg diet) for 10 weeks, and then challenged with Aeromonas hydrophila for 14 days. Results indicated that the optimal Met-Met supplementation: (1) increased fish growth performance, intestinal lysozyme (LZ) and acid phosphatase (ACP) activities, complement (C3 and C4) and immunoglobulin M (IgM) contents, up-regulated hepcidin, liver expressed antimicrobial peptide 2A (LEAP-2A), LEAP-2B, β-defensin-1 and Mucin2 mRNA levels; (2) down-regulated tumour necrosis factor α (TNF-α), interferon γ2 (IFN-γ2), interleukin 1β (IL-1β), IL-8 [only in the distal intestine (DI)], IL-12p35, IL-12p40 and IL-15 (not IL-17D) mRNA levels partially related to the down-regulation of IκB kinase β (IKKβ) and IKKγ (rather than IKKα), nuclear factor kappa B (NF-κB) p65 and c-Rel (rather than NF-κB p52) mRNA levels and the up-regulation of inhibitor of κBα (IκBα) mRNA levels; (3) up-regulated IL-4/13A, IL-4/13B, IL-6, IL-10, IL-11 and transforming growth factor (TGF)-β1 (not TGF-β2) mRNA levels partially associated with the target of rapamycin (TOR) signalling pathway [TOR/ribosomal protein S6 kinases 1 (S6K1), eIF4E-binding proteins (4E-BP)] in three intestinal segments of juvenile grass carp. These results suggest that Met-Met supplementation improves growth and intestinal immune function in fish. Furthermore, according to a positive effect, the optimal Met-Met supplementation was superior to the optimal DL-Met supplementation at improving the growth performance and enhancing the intestinal immune function in fish. Finally, based on percent weight gain (PWG), protection against enteritis morbidity and immune index (LZ activity), the optimal Met-Met supplementation for juvenile grass carp was estimated as 1.61, 1.64 and 1.68 g/kg diet, respectively, as the basal diet contains 8.03 g/kg total sulfur amino acids (TSAA) (4.26 g methionine/kg and 3.77 g cysteine/kg).