Effects Of Dietary Zinc On The Growth Performance,Intestinal Health,Body Health,Gill Health And Flesh Quality,as Well As The Mechanisms In Young Grass Carp(Ctenopharyngodon Idella)

This study investigated the effects of dietary zinc(Zn)on disease resistance,intestinal health,body health,gill health and flesh quality,as well as its mechanisms in young grass carp(Ctenopharyngodon idella),and evaluated the Zn requirements(zinc lactate as Zn source)for young grass carp based on different indicators and the efficacy of zinc lactate relative to zinc sulfate in grass carp.A total of 630 grass carp(244.14 ± 0.40 g)were fed graded levels of zinc lactate(10.71,30.21,49.84,72.31,92.56,110.78 mg Zn/kg diet)and one zinc sulfate group(56.9 mg Zn/kg diet)for 60 days.At the end of the feeding trial,fish were challenged with Aeromonas hydrophila for 14 days and Flavobacteriurm columnare for 3 days to investigate the effects of Zn on the immune function,against oxidative damage capacity,apoptosis related parameters,tight junction protein of intestine,head kidney,spleen and skin and gill,as well as its potential mechanisms,respectively.The results indicated that optimal dietay Zn level increased percent weight gain(PWG),feed intake(FI),feed efficiency(FE)and specific growth rate(SGR)of young grass carp(P<0.05),suggesting that Zn could improve the growth performance of fish.Meanwhile,optimal dietary Zn level decreased enteritis,skin lesion and gill rot morbidity of young grass carp(P<0.05),suggesting that Zn could increase the capacity of against enteritis,skin lesion and gill rot morbidity of young grass carp.The capacity of against enteritis,skin hemorrhage and lesion and gill rot morbidity are related to the intestinal health,body health and gill health.Our results indicated that optimal dietary Zn level increased lysozyme(LZ)and acid phosphatase(ACP)activities,complement 3(C3),C4 and immunoglobulin M(IgM)contents,up-regulated mucin 2,hepcidin,liver-expressed antimicrobial peptide(LEAP)-2A,LEAP-2B,?-defensin-1 mRNA levels,down-regulated pro-inflammatory cytokines tumor necrosis factor ?(TNF-?),interferon y2(IFN-y2),interleukin 1?(IL-1?),IL-6?IL-8 and IL-12 p40,IL-15 and IL-17D as well as related signaling molecule nuclear factor kappa B(NF-?B)p65,c-Rel,inhibitor of ?B?(i?B?),I?B kinase ?(IKK?)and IKKy mRNA levels,up-regulated anti-inflammatory cytokines transforming growth factor ?2(TGF-?2)[not in the middle intestine(MI)and distal intestine(DI)],IL-4/13A,IL-10 and IL-11 mRNA levels,and related signaling molecule target of rapamycin(TOR),ribosomal protein S6 kinases 1(S6K1)and eIF4E-binding proteins 1(4E-BP1)(not in the skin)and 4E-BP 2(not in the gill)mRNA levels in the intestine,head kidney,spleen,skin and gill of young grass carp(P<0.05),suggesting that Zn might through NF-?B p65 and TOR regutating cytokine gene expression to alleviate the inflammation,and maintain intestinal,head kidney,spleen,skin and gill immune function of fish;optimal dietary Zn level decreased reactive oxygen species(ROS),malondialdehyde(MDA)and protein carbonyl(PC)contents,increased anti-superoxide anion(ASA),anti-hydroxy radical(AHR)activities and glutathione(GSH)content,increased antioxidant enzymes copper/zinc superoxide dismutase(CuZnSOD),manganese superoxide dismutase(MnSOD),catalase(CAT),glutathione peroxidase(GPx),glutathione S-transferase(GST)and glutathione reductase(GR)activities(P<0.05)and mRNA levels,and related signaling molecule NF-E2-related factor 2(Nrf2)and Kelch-like ECH-associated protein 1a(Keap1a)mRNA levels in the intestine,head kidney,spleen,skin and gill of young grass carp(P<0.05),improve antioxidant abibity of intestine,head kidney,spleen,skin and gill in fish.Down-regulated cysteinyl aspartate specific proteinase(caspase)-3,-7,-8,-9 mRNA levels,pro-apoptotic factors fatty acid synthetase ligand(FasL),apoptotic protease activating factor-1(Apaf-1)and Bcl-2 associated X protein(Bax),up-regulated myeloid cell leukemia-1(Mcl-1),B-cell lymphoma protein-2(Bcl-2)and inhibitor of apoptosis proteins(IAP)mRNA levels,down-regutaled p38 mitogen activated protein(p38 MAPK)and c-Jun N-terminal protein kinase(JNK)mRNA levels in the intestine,head kidney,spleen,skin and gill of young grass carp(P<0.05),inhibit cell apoptosis of intestine,head kidney,spleen,skin and gill in fish,up-regulated tight junction complexes(TJs)occludin,Zonula occludens 1(ZO-1),ZO-2(not in the intestine)claudin-b,-c,-f,-3,-7a,-7b,-11,-12,-15a,-15b(not in the intestine)mRNA levels,down-regulated myosin light chain kinase(MLCK)mRNA levels in the intestine,head kidney,spleen,skin and gill of young grass carp(P<0.05),enhance structural integrity between cells of intestine,head kidney,spleen,skin and gill in fish.At the same time,optimal Zn level increased muscle protein and hydroxyproline contents,_EAA and fillet fatty acids composition,shear force and pH;decreased muscle cooking loss,lactate content,cathepsins B and L activities of young grass carp,suggesting that Zn could enhance the flesh quality.Meanwhile,optimal Zn level decreased reactive oxygen species,malondialdehyde and protein carbonyl contents(P<0.05),increased the activities and mRNA levels of CuZnSOD,MnSOD,ASA,AHR,GPx,GST and GR(P<0.05),and up-regulated the mRNA levels of Nrf2,CK2?,CK2?,TOR and S6K1(P<0.05),down-regulated the mRNA levels of Keapla and Keaplb in the muscle of young grass carp(P<0.05),suggesting that Zn could enhance the capacity of against oxidative damage of muscle and reduce the oxidative damage to ensure the structural integrity of muscle.In all,optimal Zn level could increase the growth performance and the capacity of against enteritis,skin lesion and gill rot morbidity.The increased capacity of against enteritis,skin lesion and gill rot morbidity are related to Zn enhanced intestinal health,body health ang gill health by improving immune function and structural integrity of intestine,head kidney,spleen,skin and gill in young grass carp.Optimal Zn level enhanced the intestinal mucocal immune,body immune and gill immune function were related to the increased antibacterial compounds production and attenuated inflammation by TOR signalling to up-regulate anti-inflammatory cytokines and NF-?B signalling todown-regulate pro-inflammatory cytokines in the intestine,head kidney,spleen,skin and gill of young grass carp.The improvement of structural integrity by Zn was related to the up-regulation of gene expression and activities of antioxidant enzyme by Nrf2 signalling,leading to the increased antioxidant capacity and decreased oxidative damage;by up-regulating anti-apoptosis factors and down-regulating pro-apoptosis factors gene expression to inhibit apoptosis;by MLCK signalling to affect the gene expression of tight junction protein,leading to the improvement of intercellular stractural integrity in the intestine,head kidney,spleen,skin and gill of young grass carp.Meanwhile,the improvement of flesh quality by Zn was partly related to the increase of antioxidant capacity in fish.Besides,based on PWG and enteritis,skin lesion and gill rot resistance,the Zn requirements(zinc lactate as Zn source)of young grass carp were estimated to be 61.2,61.4,65.3 and 66.0 mg/kg diet,respectively,and the effect of zinc lactate was superior to zinc sulfate on improving performance,enteritis,skin lesion and gill rot resistance of grass carp.