Regulatory Effects And Mechanisms Of Sargassum Pallidum Polysaccharides On Obesity And Intestinal Inflammation By Mediating Gut Microbiota In Mice Induced By High-fat Diet

Sargassum pallidum(Turn.)C.Agardh is an edible brown alga,which has been reported to have a large content of bioactive polysaccharides.S.pallidum polysaccharide(SPP)is demonstrated to possess anti-oxidant,hypoglycemic,hypolipidemia,and prebiotics properties.However,the impacts and regulatory mechanisms of SPP on obesity and intestinal inflammation by mediating gut microbiota are still unclear.Based on our previous research,SPP was selected as the research objective in this study.Firstly,the degradation pattern,structural characteristics,bioactive properties as well as in vitro digestion and fermentation properties of SPP degraded by ultrasound degradation were investigated.After that,the effects and mechanisms of degraded SPP(UD-SPP)on obesity symptoms,intestinal inflammation,composition and metabolites of gut microbiota were systematically studied.The obtained results will provide theoretical basis for the research and development of SPP used as a functional food additive.The main results are as follows:(1)Ultrasonic wave was utilized to degrade the SPP,and three degraded polysaccharide fractions including UD-SPP-1h,UD-SPP-2h,and UD-SPP-5h were obtained.Results manifested that ultrasonic wave treatment significantly reduced the average molecule weight(Mw)and particle size of native SPP.Degradation kinetics analysis showed that the degradation pattern of SPP was fitted to the random chain scission.The primary structure of SPP was not changed by the ultrasonic treatment.Rheological analysis showed that the degraded fractions solutions exhibited lower apparent viscosity when compared to native SPP solution,while the elasticity was enhanced to a certain extent.Furthermore,appropriate degradation fraction(UD-SPP-1h)exhibited stronger DPPH and ABTS scavenging activities.(2)The digestion and fermentation profiles of SPP,UD-SPP-1h,UD-SPP-2h and UD-SPP-5h and their impacts on human fecal microbiota were studied via in vitro digestion and fermentation experiment models.The physical and chemical properties of SPP and its degraded fractions were significantly different.The antioxidant activity,α-glucosidase inhibitory activity,and bile acid-binding capacity of SPP and its degraded fractions were decreased after digestion.Furthermore,SPP and its degraded fractions showed good fermentability,and the consumption rates of SPP,UD-SPP-1h,UD-SPP-2h and UD-SPP-5h by gut microbiota were 75.63%,76.16%,75.40%,and 73.54%,respectively.The degraded SPP fractions increased the utilization rate rather than the utilization degree of the gut microbes to polysaccharides.The monosaccharides including arabinose,galactose,glucose,xylose,and glucuronic acid were thoroughly consumed,while fucose,mannose and galacturonic acid were partially consumed.The SPP and its degraded fractions could modulate gut microenvironment by lowering p H,decreasing the Firmicutes/Bacteroidetes ratio(F/B),and increasing the relative abundances of some beneficial genera,such as Prevotella,Dialister,Phascolarctobacterium,Ruminococcus,and Bacteroides.SPP and its degraded fractions promoted the production of acetic acid,propionic acid,and valeric acid by promoting the growth of Prevotella,Phascolarctobacterium,Bacteroides,Dorea and Oscillospira.(3)Administration of the optimal degraded SPP fraction(UD-SPP)not only dramatically suppressed body weight gain,reduced fasting blood glucose level,but also lowered the levels of serum and hepatic lipids in vivo obese mice model.Histopathological analysis showed that UD-SPP significantly prevented fat accumulation in the liver,and reduced white adipose hypertrophy and adipocyte size.Real-time quantitative polymerase chain reaction(RT-q PCR)analysis indicated that UD-SPP significantly down-regulated the expressions of adipogenesis genes.The peroxisome-proliferator-activated receptor(PPAR)-γ,sterol regulatory element-binding protein(Srebp)-1c,acetyl Co A carboxylase(ACC1)and fatty acid synthase(FAS)in the liver of obese mice were decreased by 68%,53%,73% and 78%,respectively,as compared to the model group.Furthermore,Western blot,RT-q PCR,and histopathological analysis confirmed that UD-SPP enhanced the proliferation of goblet cells and abundance of crypt.The expression levels of tight junctions including Claudin-1,ZO-1and Occludin of UD-SPP intervention at 200 mg/kg were up-regulated by 3,2,and 5-folds,respectively in the colon tissue of model mice.Meanwhile,UD-SPP intervention at 200 mg/kg remarkably inhibited the inflammatory signaling pathway of Toll-like receptor 4(TLR),myeloid differentiation factor 88(MYD)and nuclear transcription factor(NF-κB).The secretion of downstream pro-inflammatory cytokines including IL-1β,IL-6 and TNF-α were also reduced(p < 0.05).(4)The metabonomic analysis demonstrated that the metabolisms of short chain fatty acids(SCFAs),polysaturated fatty acids,branched-chain and aromatic amino acids,and bile acids in HFD-induced obese mice were significantly changed.The F/B ratios of low,medium and high dosages of UD-SPP intervention group were reduced by 3.25,3,3.8-folds,respectively.The abundance of obesity positively related to Lachnospiraceae and opportunistic pathogen Desulfovibrionacea were reduced by 8% and 2.4%,respectively.The abundances of some opportunistic pathogen microbiota genera including Odoribacter,Desulfovibrio,Mucispirillum,AF12 and Rikenella were significantly decreased,while some beneficial genera including Oscillospira,Lactobacillus,Bacteroides,Prevotella and Roseburia were increased,which were positively related with the production of SCFAs.Spearman’s correlation analysis between microbiome and non-targeted metabolome indicated that UD-SPP modulated a healthy gut microbiota structure,and on this basis affects the obesity intestinal inflammation process in HFD-feeding mice indirectly.UD-SPP down-regulated metabolic level of fecal bile acids by increasing the abundance of some genera concerning fiber consumption and bile acids metabolism.UD-SPP alleviated metabolism of fecal polysaturated fatty acids in HFD-induced obesity mice by increasing the abundance of acetic acid producing genera.UD-SPP modulated the abundance of some genera regarding amino acids metabolism,resulting in an improvement of TCA cycle and inflammatory response.The results of this study suggest that SPP can reduce obesity and intestinal inflammation by regulating the composition of intestinal flora and metabolites,which can be developed as a food ingredient to improve obesity and intestinal health.