Mucin Glycosylation and Dysbiosis/Disease
The glycan expression changes from infancy to adulthood, and similar to how the gut microbiota differs among individuals, mucin glycans also differ56,150,151. Genetic factors play a major role in these differences as fucosylation of oligosaccharides is dependent on fucosyltransferase (FUT) secretor status and Lewis genes150. FUT is the enzyme that adds Fuc to epithelial glycan chains in conformations like α1-2, α1-3, and α1-6152. It has been demonstrated that α1-2 FUT (FUT1 and FUT2) polymorphism is highly associated with IBD susceptibility, and individuals with FUT non-secretor status (inactivating polymorphisms of FUT2) have an increased risk for Crohn’s disease as this deficiency inhibits Notch signaling, triggers spontaneous colitis, and possesses different microbiota properties than secreter status individuals153-155. The H antigen (Fucα1-2Gal) is formed by the addition of α1-2 Fuc to terminal Gal residues. FUT2 encodes the H antigen on intestinal epithelial cells, which allows for bacterial binding such as H. pylori 15,156,157. Increased epithelial α1-2 Fuc expression also helps promote the colonization of commensal bacteria like Bacteroides and Ruminococcaceae and, at the same time, reduce the colonization of opportunistic gut bacteria like Enterococcus faecalis (Fig. 1A)157. How fucosylation mediates this homeostatic gut environment has many aspects to it. Epithelial fucosylation can be negatively affected by IL-10-producing CD4+ T-cells (Fig. 2A)158. On the other hand, commensal and pathogenic bacteria (and their products like LPS) stimulate group 3 innate lymphoid cells (ILC3s), producing IL-22 and inducing α1-2 fucosylation of intra-epithelial cells159. In addition, while it has been demonstrated that loss of function mutations in FUT2 are in the group of IBD-associated genetic factors, increased FUT2 expression is detected in the mucosa of CD patients (Table 1)153,160.
Mucin glycans provide surfaces for bacteria to anchor themselves150,151. Bacterial populations that harbor enzymes such as glycoside hydrolases, sulfatases, and proteases can remove glycans from mucins, giving access to the anchor attachment points143,150,151,161. Mucin glycans act like decoys for epithelial surface glycans and confine bacteria to the mucus layer, preventing them from accessing epithelial surface glycans162. However, certain bacteria such as B. fragilis can directly attach to epithelial mucins, highlighting the significance of glycans in the diversity of gut microbiota163,164. Accordingly, the mucus layer in the gut protects the epithelial layer and helps prevent microbial invasion by separating microbes from the intestinal surface (Fig. 1A)81. This process helps control immune activation and maintains the balance in the host-microbial relationship81. However, pathogens may colonize the GI tract by binding to the fucosylated mucin glycans165. For instance, Bacillus subtilis YesU and Salmonella typhimurium Std fimbriae bind to these glycans to adhere to the gut166,167.
Mucin production results in a continuous flow of mucus, which can change during inflammation (Table 1)63,168,169. While Crohn’s disease is associated with increased mucus production, the mucus layer is thinner and discontinuous in ulcerative colitis with mucin glycosylation altered to shorter and less complex glycoforms (Fig. 3B)137,168. The properties of MUC2 and other mucins vary with the disease course, activity, and severity138,170,171. Impairment of the mucus barrier results in increased permeability, thereby allowing easier access for bacteria to the epithelial layer, and consequently inflammation168,169. Decreased O-glycosylation of mucin may cause faster digestion by bacteria. As a consequence, the mucus barrier malfunctions, increasing the susceptibility to diseases like IBDs172,173. A healthy gut microbiota helps maintain the integrity of mucus by preventing dysbiosis-induced changes to MUC2 production and thickness174.
The MUC2 monomer contains more than 5000 amino acids, rich in proline, serine, and threonine (Fig. 3A)175-177. It regulates the gut microbiota by providing nutrients, acting as ligands to microbial agents, and mediating host signaling135,178. Functional mucus layer is not possible in the absence of MUC2, as demonstrated by the development of bacterial overgrowth, spontaneous colitis, and progressive carcinomas in MUC2 deficient mice81,179. Additionally, changes in MUC2 glycosylation are associated with increased inflammation in ulcerative colitis as a result of the disrupted mucus layer, which leads to bacterial perfusion of the epithelial layer138,180. Similar to complete loss of MUC2, reduced MUC2 expression or MUC2 mutations have been found to cause spontaneous colitis179,181. Bacterial products like LPS, lipoteichoic acids, and flagellin can activate the expression of MUC2 via TLRs and trigger the secretion of mucin from goblet cells182-184. Germ-free mice show a decreased MUC2 expression and impaired mucosal layer due to their fewer and smaller goblet cells and less sialylated glycans in the mucus layer (Table 1)134,185.
The resident microbiota can affect the function of goblet cells and, thereby, the mucus layer properties via the release of bioactive compounds186. It has been established that LPS of gram-negative bacteria can stimulate the secretion of MUC5AC and MUC5B187. In addition, a gram-positive bacterium,Lactobacillus plantarum (L . plantarum ), has been shown to increase the secretion of MUC2 and MUC3188. These commensal bacteria not only stimulate the secretion of different mucin types, but they also play an essential role in preventing pathogenic bacteria from gaining access to the epithelial layer. For instance, increased expression of MUC3 can inhibit the attachment of enteropathogenic Escherichia coli (EPEC)189. In addition, a combination of probiotic bacteria Lactobacillus and Bifidobacterium spp. attenuates the pathogenicity of C. jejuni by stimulating the production of unique (i.e., low luminal pH) mucus layers144,190.
Mucin properties are also altered in CRC patients, as it was demonstrated that MUC1 expression is increased in these patients (Table 1)191. MUC1 is hyperglycosylated and expressed at very low levels in the colonic tissue of healthy individuals (up to 10%); conversely, it is hypoglycosylated and expressed in very high levels in the colonic tissue of CRC patients191,192. In addition, increased levels of MUC1 was associated with poor prognosis and metastasis193. To add to this, MUC2 expression levels decreased in patients with non-mucinous colon adenocarcinomas194. MUC5AC, a mucin that is normally found in gastric mucus and absent in the colon, was found to be expressed in CRCs195.
Mucins are a major source of sialic acid, and Neu5Ac is the most abundant sialic acid in the GI system. In adults, while the Fuc expression decreases from the proximal to the distal gut, sialic acid expression increases from the ileum to the colon196,197. A recent study has demonstrated that terminal sialylation of mucin glycans by ST6GALNAC1 (ST6) plays an important role in the integrity of the mucus layer by preventing excessive bacterial proteolytic degradation198. Furthermore, mutations of ST6 cause a defective mucus layer in patients with IBDs198. Bacterial sialidases can liberate the sialic acids that cap mucin glycans to be used by the same bacteria, other commensal bacteria, and/or pathogenic bacteria199. Besides the sialylation of mucin, sialylation of IgG also plays a part in the IBD pathogenesis as serum IgG sialylation levels decrease in patients with ulcerative colitis and Crohn’s disease200.