Effects Of Exogenous Taurine And Cysteine On Growth, Metabolism, Antioxidant And Immune Functions Of Juvenile Silver Car

Trachinotus ovatus is considered ideal for mariculture because of its high economic value and its adaptability to tasty meat.In recent years,the fluctuating decline in fishmeal resources has led to a rise in price fluctuations,and therefore partial substitution of fishmeal by animal and vegetable proteins is often used in large-scale economic fish farming.However,due to the generally unbalanced amino acid composition of animal proteins and the presence of anti-nutritional factors and poor palatability in plant proteins,replacing fishmeal with other plant and animal proteins can easily lead to reduced growth performance,reduced lipid and protein metabolism,oxidative stress,intestinal structural damage and dysbiosis,and reduced intestinal immunity in aquatic animals.Taurine,promotes growth,improves antioxidant capacity,and prevents inflammation.Cysteine,a precursor to taurine synthesis in living organisms,has important biological functions,including improving lipid and protein metabolism,and antioxidant and immune capacity.In summary,taurine and cysteine supplementation could theoretically address the negative effects of replacing fishmeal feeds with animal and plant proteins.This experiment aimed to investigate the effects of supplementation of low fishmeal diets(20 % fishmeal)with different levels of taurine and cysteine on the growth performance,antioxidant capacity,protein and lipid metabolism,intestinal microbiology,and immune function of juvenile T.ovatus,to provide a theoretical basis for the scientific application of taurine and cysteine in low fishmeal diets for T.ovatus.(1)In this section,we evaluated the effects of taurine added to low-fishmeal diets on growth,anti-oxidative stress,intestine immunity,and S.agalactiae resistance in juvenile T.ovatus,a basal diet of pomfret was prepared using plant and animal proteins(chicken meal,soybean protein concentrate,fermented soybean meal,etc.)as partial substitutes for fish meal,and 0(group T0),0.40 %(group T1),0.80 %(group T2),1.20 %(group T3)and 1.60 %(group T4)taurine was added to make five isonitrogenous and isoenergetic diets.The results of the 56-day feeding trial show that taurine supplementation of 1.20 % significantly enhanced the specific growth rate(SGR)of juvenile T.ovatus,significantly upregulating growth-related factor expression in the brain and liver,as well as the levels of growth-related parameters in the serum(P < 0.05).Polynomial regression analysis using SGR estimated the optimal dietary taurine level for T.ovatus at 1.18 %.Supplementation with 0.80-1.20 % taurine increased the diversity and relative abundance of the intestinal flora of T.ovatus.Additionally,supplementation with 0.80-1.60 % taurine maintained intestinal physical barrier stability and increased the muscular thickness and villus length within the intestine,activated the Nrf2/Keap-1/HO-1 signaling pathway,increased intestinal antioxidant enzyme gene expression and antioxidant enzyme activity in the serum,and increased immunoglobulin and complement levels in parallel with declining reactive oxygen species(ROS)levels in the serum.Antioxidant factor expression was also upregulated in the intestine.Furthermore,supplementation with 0.80-1.20 % taurine inhibited the NF-κB signaling pathway,down-regulated the expression levels of the IL-1β,IL-8,and TNF-α,up-regulated the expression levels of the IL-10,improved intestine immunity,and increased the survival rate of T.ovatus infected with S.agalactiae.at 1.20-1.60 % taurine supplementation.(2)In this section,we assessed the effect of supplementation with different levels of taurine in the low fishmeal diet on the lipid and protein metabolic capacity of juvenile T.ovatus(10.05 ±0.05 g).It was shown that taurine supplementation in a low fishmeal diet may enhance protein anabolism and increase protein deposition in the muscles of T.ovatus by activating the S6K/PI3K/TOR/4E-BP1 pathway.Supplementation with 0.80-1.60 % taurine increased muscle,liver,and serum total protein(TP)levels and decreased urea nitrogen(UN)and blood ammonia(SA)levels in T.ovatus.At the same time,taurine supplementation in the low fishmeal diet upregulated the expression of peroxisome proliferator-activated receptor alpha(PPARα)in the muscle of T.ovatus,resulting in high expression of hormone-sensitive lipase(HSL)and carnitine palmitoyltransferase 1(CPT1)and an accompanying increase in enzyme activity,which in turn promoted β-oxidation of fatty acids in the muscle,reduced the consumption of protein for energy supply due to catabolism and promoted the deposition of protein in the muscle.In addition,taurine supplementation down-regulated the expression of peroxisome proliferator-activated receptor γ(PPARγ)in the muscles of T.ovatus,reducing the transcript levels of acetyl coenzyme A carboxylase(ACC)and fatty acid synthase(FAS)and accompanying the decrease of ACC and FAS enzyme activity levels in muscle,liver,and serum,inhibiting lipid anabolism in muscle,which in turn reduced triglycerides(TG),total cholesterol(T-CHO),free fatty acids(FFA)and muscle crude lipid levels in muscle,liver,and serum.(3)In this section,we evaluated the effects of different levels of cysteine supplementation in low fishmeal diets on the growth,antioxidant capacity,intestinal immunity,and resistance to S.agalactiae in juvenile T.ovatus(10.05 ± 0.05 g).The results of the 56-day feeding experiment showed that supplementation with 0.60-1.20 % cysteine significantly increased the FBW and SGR of T.ovatus compared to the control group(P < 0.05).Under the present experimental conditions,the optimal dietary cysteine level in a 20 % fishmeal diet for T.ovatus was 0.91 % according to SGR polynomial regression analysis.Meanwhile,cysteine activated the Nrf2/Keap1/HO-1signaling pathway,up-regulated serum antioxidant enzyme activity and intestinal gene expression,and reduced serum ROS levels in T.ovatus.In addition,exogenous cysteine significantly improved muscle thickness and villi length(P < 0.05),suggesting that cysteine strengthened the intestinal physical barrier.In addition,cysteine increased the diversity and relative abundance of intestinal flora in T.ovatus.Cysteine inhibited intestinal NF-κB/IKK/IκB signaling and pro-inflammatory cytokine m RNA levels,increased intestinal anti-inflammatory cytokine gene expression and serum immune parameters,and improved intestine immunity.In addition,cysteine supplementation significantly reduced mortality in T.ovatus infected with S.agalactiae(P < 0.05).(4)To investigate the effect of cysteine on the metabolism of juvenile T.ovatus(10.05 ±0.05g).The results of the 56-day feeding trial show that: 1)Cysteine enhances protein anabolism in T.ovatus by activating the S6K/PI3K/TOR/4E-BP1 pathway.Compared to the C0 group,supplementation with 0.60 % cysteine(C2 group)significantly up-regulated the m RNA levels of S6 K,PI3K,and 4E-BP1 in muscle(P < 0.05),increased TP in muscle and serum as well as muscle crude protein content,significantly reduced SA and UN in muscle and liver,promoting protein deposition in muscle(P < 0.05).2)Cysteine inhibits lipid anabolism by down-regulating the expression level of PPARγ in muscle,decreasing the transcript level and enzyme activity level of ACC and FAS;meanwhile,up-regulating the expression level of PPARα in muscle,resulting in high HSL and CPT1 expression with increased enzyme activity,which in turn promotes β-oxidation of fatty acids in muscle,reducing protein consumption due to catabolism for energy supply This promotes protein deposition in muscle.In summary,taurine and cysteine supplementation in low fishmeal diets inhibited inflammatory responses,improved intestinal physical barrier and intestinal flora and increased intestinal immunity through inhibition of NF-κB/IKK/IκB pathway;improved antioxidant capacity of ovine pompano trevally by activating Nrf2/Keap-1 pathway and increasing the level of antioxidant enzyme activity and gene expression;could inhibit muscle It can inhibit fat and formation,promote fatty acid β-oxidation for energy supply,and thus conserve protein;activate PI3K/S6K/4E-BP1 pathway,regulate protein synthesis,increase protein deposition,improve weight gain rate and specific growth rate,and improve growth performance.