Effects Of Xylooligosaccharide Supplementation On Growth Performance And Intestinal Health And The Underlying Mechanisms For On-growing Grass Carp (Ctenopharyngodon Idellus)

Our study designed to research the effects of xylooligosaccharides(XOS)on growth performance,the health status of intestines and its underlying mechanisms in on-growing grass carp(Ctenopharyngodon idella)as well as estimate the appropriate levels of XOS supplementation based on different indices.A total of 540 healthy fish(167.46 ± 0.61 g)were randomly assigned to six treatments,each with three repetitions,which were offered with six diets containing graded levels of XOS(0,20,40,60,80 and 100 mg/kg diet)for 60 days.After the growth trial,a challenge trial was conducted by injecting 24 fish from each group with Aeromonas hydrophila for 6 days to ascertain the influences of XOS supplementation on the structural integrity of intestines(antioxidant ability,anti-apoptosis ability and apical junction complex)and the potential mechanisms in on-growing grass carp.The results showed that compared with the control group,XOS supplementation(40 mg/kg)significantly enhanced the growth performance(SGR and FE)(P<0.05),strengthened intestinal growth(IL,ILI,IW and ISI)(P<0.05),improved the microbial balance(increased the number of beneficial bacteria Lactobacillus and Bifidobacterium and decreased the number of harmful bacteria Aeromonas and E.coli)(P<0.05).XOS supplementation(60 mg/kg)significantly increased intestinal short-chain fatty acid(SCFAs)concentrations(P<0.05)and ameliorated intestinal histopathological injury as well as elevated resistance to enteritis,then promoted the intestinal health in on-growing grass carp in comparison with the control group.The improvement effects of appropriate levels of XOS on the intestinal health of on-growing grass carp are related to its enhancement of intestinal structural integrity.The results relevant intestinal structural integrity showed that appropriate levels of XOS,(1)decreased the contents of ROS(P<0.05)and oxidative damage production(MDA and PC)(P<0.05),increased the activities and gene expression of antioxidant enzymes SOD,CAT,GPx,GST and GR as well as up-regulated the Nrf2 signaling molecule m RNA levels in the three intestinal segments of fish(P<0.05);(2)decreased the gene expression of caspase-2,-3 and 7,caspase-8(rather than PI and MI),caspase-9(rather than DI),pro-apoptotic protein Bax(rather than DI)and apoptosis protein activating factor(Apaf-1)(rather than DI),apoptosis-related factor ligand(Fas L)(rather than PI and MI),JNK(rather than DI)and p38MAPK(P<0.05);up-regulated the m RNA levels of Mcl-1,Bcl-2(rather than PI)and IAP(P<0.05)as well as down-regulated the protein levels of cytochrome C and p38 MAPK signaling molecules in the three intestinal segments of fish(P<0.05)and then attenuated the death receptor and mitochondrial apoptosis pathways;(3)increased the gene expression of ZO-1,ZO-2,occludin(not in the PI and MI),claudin-3c(rather than PI),claudin-b,-c,-f,-7a,-7b,-11,α-caternin,β-caternin,afadin,JAM,E-cadherin,nectin;reduced claudin-12,-15a(rather than claudin-15b),MLCK,Rho A,NM Ⅱ and Rock gene expression as well as the protein levels of GTP-Rho A in the three intestinal segments of fish(P<0.05).In summary,the appropriate XOS supplementation enhanced the growth performance and improved intestinal health,which closely relied on the structural integrity of intestines in ongrowing grass carp.Further study in this experiment showed that the appropriate level of XOS supplementation might enhance the gene expression and activities of antioxidant enzymes by enhancing Keap1b/Nrf2 signals,thus strengthening intestinal antioxidant damage ability;depress JNK and p38 MAPK signaling pathway to attenuate intestinal apoptosis and inhibit Rho A/ROCK/ MLCK/NM II signaling pathway to strengthen tight junction and adherens junction which together reinforced intestinal structure integrity of fish.Finally,quadratic regression analysis demonstrated that the appropriate levels of XOS supplementation for ongrowing grass carp(167.46-623.78 g)for 60 days based on PWG(percent weight gain),FI(feed intake),the ability to resist enteritis and the content of MDA and PC in MI were estimated to be 51.81,51.61,57.20,65.48 and 57.70 mg/kg diet,respectively.