Microbiota is separated from the intestinal epithelium by a barrier that consists of a dense inner layer devoid of bacteria and an outer mucous layer. The main component of this mucous layer is mucin 2, a glycoprotein that is heavily glycosylated with over 100 different complex O-glycan structures – sugars that are added to the amino acids serine or threonine.
Assessing the role of microbiota enzymes in intestinal mucous composition
Some gut bacteria possess the ability to degrade mucin glycans, contributing to the development of inflammatory bowel diseases (IBD), such as ulcerative colitis (UC). The precise mechanism of mucin degradation remains unclear. The MUC project, undertaken with the support of the Marie Skłodowska-Curie Actions (MSCA), investigated the implication of specific bacterial enzymes in mucin degradation. “Understanding how microbiota can alter mucin composition in the gut will have important ramifications for human health,” outlines MSCA research fellow Ana Sofia Luis. The research focused on Bacteroides thetaiotaomicron (B. theta), a bacterium that persists in the gut because of its ability to utilise polysaccharides present in our diet as well as different endogenous host glycans. Pre-clinical data have indicated that B. theta can induce ulcerative colitis in an animal model through its sulfatase enzymes. The research team determined the activity of 38 enzymes (sulfatases and glycoside hydrolases) required to sequentially degrade O-glycans. Most of the project’s experimental work was devoted to delineating the role of specific B. theta sulfatases on mucin degradation. Structure determination of some of these enzymes provided important insight into their substrate specificity. Microbiology analysis combined with biochemical characterisation and mass spectrometry analysis of complex O-glycans allowed Luis to identify specific enzymes that drive B. theta fitness during gut colonisation. Interestingly, nearly half of these enzymes were found to be related to mucin metabolism.
Targeting microbiota enzymes for gut health
Undoubtedly, the state of the gastrointestinal tract mucous layer is involved in the progression of several diseases and can be influenced, directly and indirectly by the activity of gut bacteria. If the degradation of the mucous layer exceeds mucus production, the bacteria can reach the epithelium and cause inflammation. The MUC project identified a single sulfatase with a critical role in O glycan utilisation by B. theta. This improved our understanding of how gut bacteria modify the mucous layer and shed light into the mechanism of mucin degradation. “Importantly, our findings create the opportunity to develop specific inhibitors of this sulfatase to block microbiota activity and prevent mucus disruption,” outlines Luis. This will have important ramifications for IBD and other inflammatory diseases which are increasing in incidence in developed countries. Such key enzymes implicated in mucin degradation are encoded in the genome of other members of the human microbiota and warrant further investigation. Furthermore, the complexity of mucin glycans implies a coordinated enzyme activity by multiple bacteria. Future research directions for Luis will focus on delineating the mechanism of host mucin O-glycan utilisation by different microbiota species and the association with IBD.
MUC, mucin, microbiota, B. theta, O-glycan, sulfatase, IBD, UC, inflammatory bowel diseases, ulcerative colitis