Effects Of Succinate-supplemented Diet On Intestinal Barrier Function And Bile Acid Metabolism In Pigs

In modern large-scale and intensive pig production,the high incidence of intestinal diseases severely restricts the efficient production of the pig industry.Animal intestinal mucosa has an important barrier against the invasion of harmful substances and pathogens in the external environment,which can maintain the intestinal homeostasis and intestinal health,and keep the normal physiological activities.Therefore,maintaining the integrity of intestinal barrier function is the key to ensure the intestinal health of pigs and achieve efficient production.The rich and diverse metabolites produced by intestinal microbial metabolism can participate in regulating intestinal barrier function.Succinate is a metabolic intermediate of the tricarboxylic acid cycle in all aerobic organisms and is also a vital microbial metabolite in the gut.In recent years,the effect of succinate on intestinal immune function and glycolipid metabolism has become a new research hotspot.However,the effect of succinate on intestinal epithelial barrier function has not been reported.In addition,related research suggests that succinate and bile acid may have potential connections in function in the lumen.Bile acid is an important component of the intestinal-hepatic circulation of the body,which is involved in regulating the intestinal immune response and intestinal barrier function.Therefore,we fed pigs with sodium succinate-supplemented diet,and aimed to explore the effect of sodium succinate on the intestinal barrier function and bile acid metabolism and analyze its potential regulatory mechanism.This study is mainly divided into the following three parts:1 Effects of succinate-supplemented diet on intestinal barrier function of pigsIn this part,we aimed to evaluate the effects of succinate on intestinal epithelial barrier function and intestinal inflammation in pigs.Twenty-four growing pigs(15.44 kg ±0.16)were randomly distributed into three groups(n=8)and received either a basal diet(control group)or the same diet supplemented with 0.1%sodium succinate or 1%sodium succinate.The experimental period is 28 days.The results showed that diet supplemented with 1%sodium succinate significantly increased the villus height and decreased the crypt depth in the jejunum of growing pigs(P<0.05).We confirmed in vitro that 5 mM sodium succinate treatment modulated intestinal epithelial permeability by increased transepithelial electrical resistance in IPEC-J2 cells(P<0.05).Furthermore,sodium succinate treatment increased the abundance of tight junction proteins Claudin-1,ZO-1,and ZO-2 in the jejunum in vivo and in vitro(P<0.05).In addition,dietary sodium succinate supplementation promoted the secretin of inflammatory cytokines IL-25,IL-10,IL-8,and IL-18 in the jejunum;5 mM sodium succinate significantly promoted the secretion of IL-6,IL-8 and IL-18 in IPEC-J2 cells(P<0.05).These findings indicate that succinate changed the intestinal morphology,reduced the intestinal epithelial permeability,improved the intestinal epithelial barrier,and modulated the intestinal inflammatory response of pigs.2 Effects of succinate-supplemented diet on bile acid metabolism of pigsThe results of the first section indicate that succinate can regulate intestinal epithelial barrier.Recently,it has been pointed out that certain intestinal microbiota,which regulate host metabolism via producing succinate,have the ability of bile acid transformation.It is reported that bile acids can regulate intestinal epithelial barrier.Therefore,in this part,we aim to explore the effect of succinate on bile acid metabolism.The results showed that 1%sodium succinate significantly increased the level of TBA in the jejunum,ileum and liver,but decreased the level of TBA in plasma(P<0.05).At the same time,sodium succinate significantly increased the level of glucose in plasma(P<0.05).Further analysis of the bile acid profile showed that sodium succinate increased the concentrations of HCA,GCDCA and GHDCA in the liver(P<0.05);meanwhile,it increased the concentrations of CDCA,GCDCA,TCDCA,HDCA,GHDCA,UDCA,GLCA,TLCA,GLCA and THCA in the ileum(P<0.05);however,it decreased the concentrations of CDCA,GCDCA,UDCA,HCA,GHCA,TCDCA,THDCA and THCA in the plasma(P<0.05),These findings indicate that succinate is involved in the regulation of bile acid metabolism.3 Effects of succinate-supplemented diet on bile acid metabolic pathway of pigsBased on the previous two sections,it was found that dietary sodium succinate enhanced intestinal barrier function and changed bile acid metabolism.In order to explore the mechanism of succinate regulating the bile acid metabolism,we analyzed the composition of intestinal microbiota,the pathway of bile acid metabolism in liver and the signal of ileal bile acid in this part.The results were as follows:dietary sodium succinate had no significant effect on the structure of ileal microflora,and had no effect on bile acid metabolism related microflora at the level of phylum and genus(P>0.05);however,the results of liver transcriptome showed that sodium succinate promoted the secretion of bile acid in the liver,and significantly upregulated genes related to bile acid secretion,including CYP7A1,AMACR,SLC22A7,OSTβ and NTCP,etc.(P<0.05);at the same time,the expression of FXR signal related genes in ileum showed that dietary sodium succinate supplementation significantly reduced the expression level of FXR,FGFR4 and SHP in the ileum,and decreased the concentration of FGF19 in plasma(P<0.05).In addition,sodium succinate significantly increased the expression of GPR91 in the intestine(P<0.05).These findings indicate that succinate promote the secretion of bile acids in the liver via inhibiting FXR-FGF19 signal pathway in the ileum,and then affect the bile acid metabolism in the host.In conclusion,dietary sodium succinate supplementation changed intestinal morphology,decreased intestinal epithelial permeability,enhanced intestinal barrier function,and had a certain impact on the occurrence of intestinal inflammation.Dietary sodium succinate supplementation had no significant effect on the structure of ileal microflora,but it inhibited the signal pathway of FXR-FGF19 in the ileum,promoted the secretion of bile acids in the liver,and changed the composition of bile acids in the intestine.