Effects Of Supplementation Betaine(BET) On The Structure And Function Of Immune Organs And Related Mechanism In On-growing Grass Carp(Ctenopharyngodon Idellus)

This study was designed to explore the effects of betaine?BET?on growth,structure and function of immune organs?head kidney and spleen?as well as related mechanisms in on-growing grass carp?Ctenopharyngodon idella?and the efficacy between BET and DL-methionine.540 grass carp?210±0.68?g were randomly divided into 6 groups with 3replicates in each group.Six groups fed diets contain DLM(3.1 g kg^-1)?positive treatment group?and 5 graded levels of BET at 0,1.6,3.2,4.8 and 6.4 g kg^-11 respectively for 60 days.After the growth test,the challenge test of Aeromonas hydrophila was conducted for 14days.It was found that,compared with BET0 group,supplementation BET 1.6-6.4 g kg^-11 in diet boosted PWG,FI and FE after growth trial,indicating that BET enhanced the growth performance of on-growing grass carp.The growth performance of fish is related to the immune function of the immune organs.The results of challenge trial demonstrated that BET enhanced the ability to defense against disease in fish,which might be partly due to antimicrobial ability and inflammation response.BET facilitated the activities of lysozyme and acid phosphatase,the contents of complement 3?C3?,C4 and immunoglobulin M?P<0.05?,up-regulated the mRNA levels of antimicrobial peptides?-defensin-1,hepcidin?not in spleen?,liver expressed antimicrobial peptide 2A?LEAP-2A?,LEAP-2B and Mucin2?P<0.05?to improve immune function of immune organs;besides,BET down-regulated the pro-inflammatory cytokines TNF-?,IFN-?2,IL-6,IL-8?rather than spleen?,IL-17D?than spleen??P<0.05?and up-regulated anti-inflammatory cytokines(IL-10,IL-11 and IL-4/13A mRNA levels?P<0.05?,down-regulated pro-inflammatory signaling molecules I?B kinase??IKK??,IKK??rather than IKK??,nuclear factor-?B p65?NF-?B p65??except NF-?B p52?and c-Rel mRNA levels?P<0.05?while up-regulated inhibitor of?B??I?B??mRNA levels?P<0.05?to attenuate the inflammatory response.The immune function of immune organs is dependent on their structural integrity.This study revealed that supplementation BET in low methionine?Met?diets decreased the contents of reactive oxygen species,malondialdehyde and protein carbonyl in the head kidney and spleen of on-growing grass carp,indicating that BET had the ability to protected the oxidative damage,meanwhile,elevated MnSOD?not CuZnSOD?,CAT,GPx,GST,GR activities and mRNA levels?P<0.05?levels,up-regulated Nrf2 mRNA expression levels,down-regulated kelch-like-ECH-associated protein?Keap1a??rather than Keap1b?mRNA levels?P<0.05?and enhanced the ability to defend antioxidative damage of immune organ;BET down-regulated p38 mitogen-activated protein kinases?p38MAPK?,caspase-2,caspase-3,caspase-7,caspase-8,caspase-9,Apaf-1 as well as B-cell lymphoma protein 2associated X protein?Bax?mRNA levels?P<0.05?,and up-regulated Bcl-2,IAP and Mcl-1 mRNA level?P<0.05?in the head kidney and spleen of on-growing grass carp;BET up-regulated the mRNA levels of occludin,ZO-1?not in spleen?,ZO-2b?not in spleen?,claudin-b,claudin-c,claudin-7,claudin-11,down-regulated claudin-12a,claudin-12b,claudin-15a and claudin-15b?P<0.05?in immune organs,preserved intercellular structural integrity.Summarily,BET facilitated growth performance,reinforced structural integrity and immune function of the immune organs of on-growing grass carp.BET reinforced structural integrity of immune organs,which was partly related to up-regulate the levels of antioxidant enzymes,tight junction mRNA and down-regulate mRNA in the kidney and spleen via Nrf2,MLCK and p38 signaling pathways,respectively.Meanwhile,the up-regulation of anti-inflammatory cytokines and down-regulation of pro-inflammatory cytokine mRNA levels by TOR and NF-?B signaling pathways,respectively,increased the immune function of the immune organs of on-growing grass carp.Based on PWG,the immune related parameter LZ,the appropriate supplemental levels of BET in the diets of grass carps with the weight range from 210 to 776 g are 4.28 g kg^-1 and 4.43 g kg^-1,respectively.Besides,based on PWG,the efficacy?%?of BET?99%?relative to methionine?99%?in the diets was calculated?in kg?as 113.14%.