Effects Of Dietary Bile Acids On Growth And Intestinal Health And Its Potential Mechanisms Of On-growing Grass Carp(Ctenopharyngodon Idella)

The aim of this study was to investigate the effects of dietary bile acids(BAs)on the immune function and intestinal physical barrier and its potential mechanisms in on-growing grass carp(Ctenopharyngodon idella).There were 6 dietary treatments that included a normal protein and lipid(NPNL)diet containing 29% crude protein(CP)and 5% ether extract(EE),and the low-protein and high-lipid(LPHL)diets(26% CP,6% EE)supplemented with graded levels of BAs at 0,80,160,240 and 320 mg/kg diet.A total of 540 grass carp(179.85±1.34 g)were fed six diets for 50 days and then challenged with Aeromonas hydrophila for 14 days to investigate the effects of BAs on the fish growth performance and intestinal development,intestinal immune function and intestinal physical barrier as well as its potential mechanisms.The results showed that LPHL diet could decrease percent weight gain(PWG),specific growth rate,feed intake,feed efficiency,enteritis resistance,impare partly intestinal immune function [e.g.lysozyme(LZ)and acid phosphatase(ACP)activities in the PI] and physical barrier(e.g.activities of CAT,SOD and ASA,and mRNA levels of claudin-f)of on-growing grass carp.BAs supplematation in LPHL diet,improved growth performance and enteritis resistance of fish.The enhance growth performance is partly associated with immune function and intestinal physical barrier.(1)BAs could increase contents of complement 3(C3),C4 and immunoglobulin M(IgM),activities of LZ and ACP,up-regulated the mRNA levels of LEAP-2A,LEAP-2B,hepcidin,Mucin 2 and ?-defensin-1,and down-regulate the mRNA levels of pro-inflammation cytokine [IL-1?,TNF-?,IFN-?2,IL-6,IL-8,IL-12p35,IL-12p40(only DI),IL-15 and IL-17D] through nuclear factor kappa B(NF-?B)signalling [I?B kinase ?(IKK?)and IKK?/inhibitor of ?B?(I?B?)/NF-?B(p65 and c-Rel)] and up-regulate the mRNA levels of anti-inflammation cytokine(IL-10,IL-11,IL-4/13 A,TGF-?1 and TGF-?2)by rapamycin(TOR)signalling [TOR/ribosomal protein S6 kinases 1(S6K1)and eIF4E-binding proteins(4E-BP)],mitigating inflammatory response.(2)BAs could up-regulate the gene expression and activity of SOD,CAT,GPx,GST and GR in the intestine of grass carp,which might be partly related to the activation of Nrf2/Keap1b(not Keap1a)signaling pathway;(3)BAs could inhibit cell apoptosis by up-regulated the mRNA levels of anti-apoptosis protein Bcl-2,Mcl-b and IAP,and down-regulated the mRNA levels of pro-apoptosis protein caspase-2,-3,-7,-8,-9,Apaf1,Bax and FasL,which might partly be regulated by inhibiting p38 MAPK signaling pathway,inhibiting the apoptosis;(4)BAs could strengthen the tight junctions by up-regulated mRNA levels of occluding(only DI),ZO-1,claudin-c,-f,-7a,-7b and-11,down-regulated the mRNA levels of claudin-12,-15 a and-15 b,which might be associated with the inhibition of MLCK signaling pathway.As stated above,LPHL diet could decrease the growth performance,immune function and structural integrity of the intestine in on-growing grass carp,which could be reversed by BAs.Finally,based on PWG and disease resistance capacity,immune index(LZ and ACP activities in DI)and structural integrity index(contents of PC and MDA in DI),the optimal BAs levels in LPHL diet for on-growing grass carp(180-533 g)were estimated to be 168.98,166.67,177.87,191.97,181.25 and 170 mg/kg,respectively.