Ulcerative colitis is a chronic inflammatory disease that primarily impacts the colon and rectum [
2,
27]. While genetic factors, host immune system disorders, dysbiosis of the intestinal microbiota, and environmental factors have been associated with UC pathogenesis in recent research [
3‐
5], the molecular mechanisms underlying UC development remain incompletely understood. Consequently, additional investigation into potential therapeutic strategies may enhance patient prognosis.
The intestinal mucus barrier serves as the primary defense mechanism against environmental, physiological, and immune stimuli, and its impairment is a significant manifestation of UC [
17,
20]. The colonic mucus layer primarily comprises mucin, while proteoglycan components play a crucial role in preserving the mucus barrier, which undergoes significant alterations during the development of UC [
14,
15,
17]. In this study, we mainly observed MUC2 protein expression in colonic tissues of patients with mild UC, and no significant changes were observed. MUC2 serves as a marker for intestinal epithelial permeability [
28,
29], and the less severe manifestation of lesions in patients with mild UC may be attributed to the absence of significant structural changes. However, our findings indicated a significant reduction in the expression of the tight junction protein Occludin, which is indicative of mucosal damage and compromised gut epithelial barrier integrity [
30‐
32]. It was subsequently confirmed that protein glycosylation was markedly diminished in individuals with UC, indicating a potential association between UC, protein glycosylation, and mucosal permeability. Taken together, these findings suggest that mild UC patients exhibit heightened intestinal permeability despite preservation of the intestinal epithelial barrier.
The colonic mucus layer serves as the primary dwelling place for commensal bacteria. The composition of gut microbiota is under the regulation of the intestinal mucus layer, and the host mucus has the potential to shape the intestinal microbiota [
33‐
35]. The gut microbiota of individuals with IBD differs from that of healthy individuals and may play a crucial role in the pathogenesis of IBD [
33,
36]. In line with this, our study also revealed significant variations in the intestinal microbiota between healthy controls and patients with mild, severe, and severe UC. Next, we directed our attention toward
E. coli, an opportunistic pathogen, amidst the varied abundance of microbial taxa.
E. coli typically operates as a commensal bacterium in regular circumstances, but its role shifts to that of a harmful pathogen when the host's immune system weakens, leading to the onset of disease [
37‐
39]. The potential influence of O-glycans in promoting heightened virulence and invasiveness of
E. coli has been suggested [
14,
40]. Our data indicated that the treatment of HT29 cells with an O-glycan inhibitor enhanced the proliferative capacity and adhesion of
E. coli. While the specific molecular mechanism behind these effects remains unclear, our findings suggested that post-translational glycosylation modifications influenced the phenotypes of commensal gut microbiota, thereby impacting disease progression, including UC, through host interactions. Prior research has demonstrated that the elimination of glycosyltransferases resulted in a marked decrease in O-glycan manifestation, which subsequently led to the onset of intestinal inflammation and colorectal cancer in mice [
23‐
25]. Our animal experimentation similarly revealed that the administration of an O-glycan inhibitor induced epithelial thickening, and concurrent treatment with
E. coli intensified colonic inflammation in mice. As previously stated, the pathogenesis of UC is triggered by impairment of barrier integrity in intestinal mucosa and gut microbiota dysbiosis [
33,
34]. Our findings support this notion, as co-culturing
E. coli with epithelial cells resulted in significant activation of the NF-κB pathway and marked down-regulation of glycosyltransferase C1GALT1 protein expression in the epithelium, leading to a decrease in O-glycan expression. In contrast, the administration of an inhibitor targeting the NF-κB pathway resulted in the reversal of the aforementioned alterations, which were partially replicated in animal models. Prior investigations have demonstrated a strong correlation between the NF-κB pathway and IBD [
41‐
43], a finding that is supported by our current findings.