Effects Of Dietary Taurine Supplementation On The Growth Performance And Health Status Of Intestinal,Gill And Body,and Flesh Quality Of Young Grass Carp(Ctenopharyngodon Idella) And The Related Mechanisms

This study investigated the effects of optimal taurine supplementation in a plant protein sources based diet on growth,intestinal health,gill health,body health and flesh quality,as well as the related mechanisms in young grass carp(Ctenopharyngodon idella).A total of 540 healthy grass carp(255.74 ± 0.65 g)were fed one fish meal diet(containing 0.42 g taurine kg-1 diet)and all-plant protein sources based diets with graded levels of taurine(0,0.50,0.99,1.46 and 1.98 g kg-1 diet)for 60 days,respectively.To study the relationship between taurine and fish intestinal,gill and body health status and its mechanisms,we selected 30 young grass carp per treatment,and divided into two groups,then conducted two challenge trails with Aeromonas hydrophila infection for 14 days,and Flavobacterium Columnare infection for 3 days,respectively,after the feeding trial.Results indicated that optimal taurine supplementation increased the specific growth rate(SGR),percent weight gain(PWG),feed intake(FI)and feed efficiency(FE),suggesting that optimal taurine supplementation could increase growth performance of young grass carp.Futher study found that optimal taurine supplementation increased the ability to against enteritis,gill rot and skin lesion,suggesting that optimal supplementation could enhance the intestinal,gill and body health.The enhancement of the intestinal,gill and body health might be related to the improved immune function and structure integrity in intestine,head kidney,spleen,skin and gill:(1)optimal taurine increased the lysozyme(LZ)and acid phosphatase(ACP)activities,the complement 3(C3)and C4,as well as IgM contents,up-regulated the mRNA levels of antimicrobial peptides:liver-expressed antimicrobial peptide-2A(LEAP-2A),LEAP-2B,hepcidin,?-defensin and Mucin 2(only in intestine)in the intestine,head kidney,spleen,skin and gill,leading to the increased antimicrobial ability in young grass carp.Meanwhile,optimal taurine down-regulated the nuclear factor ?B p65(NF-?B p65)(rather than NF-?B p52)and up-regulated the target of rapamycin signaling(TOR)mRNA levels,resulting in the down-regulation of major pro-inflammatory cytokines tumor necrosis factor a(TNF-a)(except DI),interferon ?2(IFN-?2)(except gill),interleukin 1?(IL-1?),IL-8(except PI and MI),IL-17D(except gill)and IL-12p40(not IL-12p35)mRNA levels,and up-regulation of major anti-inflammatory cytokines IL-10(except DI),IL-11,transforming growth factor ?1(TGF-?1)(rather than TGF-?1)mRNA levels,which lead to the inhibited inflammatory response and improved immune function in the intestine,head kidney,spleen,skin and gill of young grass carp;(2)In one hand:optimal taurine supplementation up-regulated the mRNA levels of the signaling molecule NF-E2-related factor 2(Nrf2)and down-regulated the mRNA levels of signalling molecule Keap1a(rather than Keap1b),leading to the up-regulation mRNA levels of antioxidant enzymes copper/zinc superoxide dismutase(CuZnSOD),manganese superoxide dismutase(MnSOD),catalase(CAT),glutathione peroxidase(GPx),glutathione S-transferases(GST)and glutathione reductase(GR)and increased CuZnSOD,MnSOD,CAT(except MI and DI),GPx,GST and GR activities and GSH contents,which resulting in the improved antioxidant ability and decreased oxidative damage in the intestine,head kidney,spleen,skin and gill of young grass carp.Meanwhile,optimal taurine supplementation inhibited p38 MAPK and JNK(except the intestine and gill)signalings to down-regulate caspase-3,-7,-8 and-9,Bcl-2 associated X protein(Bax),apoptotic protease activating facter-1(Apaf-1),Fas ligand(FasL)(except the spleen and skin)and up-regulate the relative mRNA levels of myeloid cell leukemia-1(Mcl-1)and B cell lymphoma-2 protein(Bcl-2)mRNA levels,suggesting that optimal taurine could inhibit cell apoptosis in intestine,head kidney,spleen,skin and gill of young grass carp.These observations indicated that optimal taurine could maintain the cell structural integrity in the intestine,head kidney,spleen,skin and gill of young grass carp.On the other hand,optimal taurine supplementation down-regulated the signal molecule myosin light-chain kinase(MLCK)mRNA levels to up-regulate the ZO-1,ZO-2(except gill),occludin(except head kidney,spleen and DI),claudin-b and claudin-c mRNA levels,and down-regulate the claudin-12 and-15 mRNA levels,leading to the strengthened structural integrity of intercellular in the intestine,head kidney,spleen,skin and gill of young grass carp.In addition,optimal taurine supplementation increased fish muscle nutrient(protein,essential amino acids,flavor amino acids and PUFA)contents,pH value and hydroxyproline content,and decreased lipid,shear force,cooking lose and lactate content,as well as cathepsin B and L activities,leading to the increased fish flesh quality.Further study found that optimal taurine increased fish flesh quality might be related to the reduction oxidative injury,which may be partly associated with the elevated activities and mRNA levels of antioxidant enzymes such as copper/zinc superoxide dismutase(CuZnSOD)referring to the up-regulation of TOR,S6K1,Nrf2,CK2a and CK2? mRNA levels.Together,optimal taurine supplementation could increase growth performance,intestinal,gill and body health,and flesh quality.The increased intestinal,gill and bodyhealth might be related to the enhancement of the immune function and structure integrity in the intestine,head kidney,spleen,skin and gill of young grass carp:(1)optimal taurine increased antibacterial ability by increasing the activities and contents of major antibacterial compounds,and up-regulating the mRNA levels of antibacterial peptides.Meanwhile,optimal taurine down-regulated the NF-?B p65 and up-regulated TOR mRNA levels to down-regulate pro-inflammatory cytokines and up-regulate anti-inflammatory cytokines,resulting in the decreased inflammation response and increased immune function.(2)optimal taurine strengthened Nrf2 signaling pathway to increase the activitiesand mRNA levels of antioxidant enzymes such as CuZnSOD,leading to the decreased oxidative injury,as well as inhibited the p38 MAPK or JNK(except intestine and gill)signaling pathways to down-regulate pro-apoptotic protein,like caspase-3 mRNA levels,resulting in the decreased apoptosis,which maintained the structure integrity of cellular.Meanwhile,optimal taurine inhibited MLCK signaling to regulate tight junction protein mRNA levels and maintained the structural integrity of intercellular.Optimal taurine increased fish flesh quality might be related to reducing oxidative injury,which might be partly related to the elevated mRNA levels and activities of antioxidant enzymes such as CuZnSOD referring to up-regulating Nrf2 signaling.Based on the PWG,ability to against enteritis,skin lesion and gill rot,and muscle hydroxyproline content,the optimal levels of taurine supplementation for young grass carp(255-740 g)were estimated to be 1.05,1.08,1.12,1.09 and 1.02 g kg-1 diet,respectively.