Regulation Of Glucan From Brown Seaweed On Gut Microbiota And Host Obesity

Glucan was the most widely distributed glycan which could be classified as α-and β-type according to the linkage sequence of the adjacent residues.Of the glucans,those linked by α-1,4 glycosidic bonds were the necessary energy storage sources for animals,plants,and microorganisms.In contrast,other glucans were not easily decomposed by animals and human,so they could not be used as their direct energy source.However,the gut microbiota encoded corresponding hydrolytic enzymes,which could utilize these glucans as energy substances,many of them belong to probiotics.In addition,glucans that were not easily decomposed could directly participate in the regulation of body health in various ways.Therefore,glucan was an important functional glycan.Brown seaweed was common aquatic organisms,were rich in glucan,had high edible and medicinal value,but the research on the regulation of gut mircobiota and host by Laminaria japonica glucan was not detailed enough.In this study,a brown seaweed Laminaria japonica-derived glucan was used as the research object.The primary structure was analysed by high-resolution mass spectrometry and nuclear magnetic resonance to clarify its main structural characteristics.Then,through animal and in vitro experiments,applied metagenomics,comparative genomics,and transcriptomics,we aimed to investigate the regulatory mechanism of glucan on gut microbiota and host obesity as well as the functional genes for bacterial utilization of glucan,and tried to establish the causal relationship between the structure of glucan and its efficacy,and provide data for related research.The main results of this study were as follows:(1)Laminaria japonica glucan(LJNP)was mainly composed of Laminaria japonica gluco-polysaccharide(LJGP,37300 Da)and Laminaria japonica gluco-oligosaccharide(LJGO,1100 Da).The ratio of the two components was about 3:7,which contained various linkage sequences including α-1,3-,α-1,4-,α-1,6-and β-1,3-linked glycosidic bonds.The main component,LJGO,was a mixture of gluco-oligosaccharide with a degree of polymerization(DP)of 2～8 and a structure that consisted of α-1,3-,α-1,4-linked glycosidic bonds.Both disaccharide and trisaccharide in LJGO were linked to adjacent glucose residues by α-1,4-glycosidic bonds.(2)Animal experiment and fecal metagenomic analysis suggested that LJNP had the potential to regulate gut microbiota,mainly increased the abundance of Bacteroides vulgatus and Bacteroides uniformis,and drove the above two to regulate gut amino acid metabolism(isoleucine,alanine,ornithine,histidine,homoserine,and methionine)and glycometabolism(pentose and glucuronic acid conversion,N-acetylglucosamine,glucuronic acid,mannose and fructose,starch and sucrose,glycolysis-related pathways,and polyketide sugar units).Fecal metabolomics further confirmed that some metabolites of the above metabolic pathways were significantly increased,indicating that LJNP had beneficial effect on gut metabolism.In addition,LJNP also showed weight loss effect and intestinal barrier protection,which may benefit host health.(3)Fermentation in vitro and gene-phenotype correlation analyses revealed that LJGO selectively stimulated the growth of lactic acid bacteria strains encoding specific ABC transport system components.This component consisted of one maltose/maltodextrin substrate-binding protein-encoding gene(cyc B)and two maltose/maltodextrin permease components encoding genes(gan P and gan Q).Meanwhile,comparative genomics further revealed that gene cluster in which the transport system components were located had species-specific and intra-species conservation,suggesting that this gene cluster may belong to core genes of lactic acid bacteria.(4)In vitro experiment showed that LJGO had a certain inhibitory effect on the jejunal α-glucosidase enzyme activity,suggesting hypoglycemic potential.Animal experiment further confirmed that LJGO could directly affect glucose metabolism by regulating blood glucose,glycated hemoglobin and insulin,thereby alleviating the pathological symptoms of obese mice.While LJGP was relatively weak in alleviating obesity.In addition,LJGO and LJGP could also reduce the levels of triglyceride and low-density lipoprotein to some extent,and significantly increased the level of fibroblast growth factor 21,had the potential for lipid metabolism regulation,which deserved further exploration.