| Our study conducted the effects of dietary lipids on the mucosal(intestine,gill and skin)and systemic immune organs(head kidney and spleen)immune function,structural integrity and flesh quality and its potential mechanism of young grass carp(Ctenopharyngodon idella),then evaluated the optimal dietary lipids level for young grass carp based on different indices.A total of 540 young grass carp(261.41 ± 0.53g)were fed six diets containing graded levels of lipid(5.9,21.4,36.0,50.2,66.6 and 80.2g lipids/kgdiets)for 8 weeks to explore the effect of dietary lipids on fish growth performance and flesh qulity.After growth trial,fish were challenged with Aeromonas hydrophila for 14 days and Flavobacterium columnare for 3 days to investigate the effects of lipids on the immune function,tight junction protein,antioxidative capacity and apoptosis of intestine,head kidney,spleen,skin and gill,as well as its potential mechanisms.The results indicated that compared with low or excess levels of lipids,optimal levels of lipids increased the percent weight gain(PWG),feed intake(FI)and specific growth rate(SGR)(P<0.05),suggesting that optimal levels of lipids could enhance the growth performance of young grass carp.Second,optimal levels of lipids decreased the enteritis morbidity,skin hemorrhage and lesion and gill rot morbidity(P<0.05),suggesting that lipids enhance the enteritis resistance ability,skin hemorrhage and lesion and gill rot morbidity of young grass carp.After that,we explored the mechanisms of optimal levels of lipids enchanced fish diseases resistance ability.The results showed that optimal levels of lipids increased lysozyme and acid phosphatase activity,complement 3 and 4 and IgM content(P<0.05),up-regulated the mRNA levels of liver-expressed antimicrobial peptide 2(LEAP-2),Hepcidin,P-defensin-1,anti-inflammatory cytokinesinterleukin 4/13A,IL-4/13B,IL-6,IL-1?transforming growth factor ?1(TGF-?1),inhibitor of ?KB?(I?B?),target of rapamycin(TOR)and ribosomal protein S6 kinases 1(S6K1)(P<0.05),whereas down-regulated pro-inflammatory cytokines tumor necrosis factor a(TNF-a),interleukin 1?(IL-1(3),IL-12 p40,IL-12p35 and interferon y2(IFN-y2),nuclear factor kappa B(NF-?B)p65,I?B kinase(IKK)and eIF4E-binding proteins(4E-BP)mRNA levels in the intestine,head kidney,spleen,skin and gill of young grass carp(P<0.05),suggesting that optimal levels of lipids couldenchance themucosal and systemic immune function.Third,optimal levels of lipids up-regulated the mRNA levels of tight junction protein occludin,zonula occludens 1(ZO-1),claudin b,3,11,7b,12 and 15a(P<0.05)and down-regulated mRNA levels of myosin light chain kinase(MLCK),MAPKK 6 and p-38 MAPK;declined the reactive oxygen species,malondialdehyde and protein carbonyl contents(P<0.05)and increased the activities and mRNA levels of antioxidant enzymes(P<0.05),whereas up-regulated the mRNA levels of NF-E2-related factor 2(Nrf2)(P<0.05)and down-regulated the mRNA levels of Kelch-like-ECH-associated protein 1(Keap1)(P<0.05);up-regulated the mRNA levels of B-cell lymphoma protein-2(Bcl-2)and inhibitor of apoptosis proteins(IAP)(P<0.05)and down-regulated the mRNA levels of cysteinyl aspartic acid-protease(Caspase 3,8,9),fatty acid synthetase ligand(FasL),apoptotic protease activating factor-1(Apaf-1)and Bcl-2 associated X protein(Bax)(P<0.05),suggesting that protein could inhibit the apoptosis to ensure the structural integrity of intestine,head kidney,spleen,skin and gill;partly through down-regulating the mRNA levels of myosin light chain kinase(MLCK)to up-regulate the mRNA levels of tight junction protein occludin,zonula occludens 1(ZO-1),claudin b,7a,7b,12 and 15a(P<0.05)(P<0.05),suggesting that optimal levels of lipids enhanced the tight junction gene expression and antioxidant ability and surpressed the apoptosis in the intestine,head kidney,spleen,skin and gill of young grass carp.At last,At the same time,optimal levels of lipids increasedthe contents of muscle protein,lipids,amino acid content(Aspartic acid,Threonine,Glycine,Valine,Methionine,Isoleucine and Lysine)andunsaturated fatty acids(C18:3n-3,C18:3n-6,C20:4,C20:5+ C22:0,C22:1 and C22:6),hydroxyproline,shear force and pH;decreased muscle cooking loss,lactate content,cathepsins B and L activities of young grass carp,suggesting that optimal levels of lipids could enhance the flesh quality.Meanwhile,optimal levels of lipidsdeecreased 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,CK2a,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),indicating that optimal levels of lipidscould enhance the antioxidant abilty of muscle and elevate the flesh qulity of young grass carp.In summary,optimal levels of lipids increased the growth performance,the capacity of against enteritis,skin hemorrhage and lesion,gill rot morbidity andflesh qulity of young grass carp.The potential mechanism may be related to the increased antibacterial compounds production and attenuated inflammation by TOR and NF-?B signalling molecules in the intestine,head kidney,spleen,skin and gill of young grass carp.The improvements of the intestine,head kidney,spleen,skin and gill structural integrity were related to the increased antioxidant ability by Nrf2 signalling,suppression of apoptosis,up-regulation of tight junction proteingene expression through MLCK signalling by dietary lipidsof young grass carp.In addition,the improvement of flesh quality by lipids was partly related to the enhancement of nutrient contents and antioxidant ability in fish.Finaly,in this research,based on thePWG,enteritis resistance ability,skin haemorrhage and lesions,gill rot morbidity and MDA content in mucsl,the optimal lipids levels for young grass carp(261-889 g)were estimated to be 43.7,60.2,54.5,57.0 and 48.5 g lipids/kg diet... |
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