The study of O-GalNAcylation repair on MUC2 and E-cadherin of intestinal epithelial cells damaged by lipopolysaccharides

Technical Summary: The Study of O-GalNAcylation Repair on MUC2 and E-cadherin of Intestinal Epithelial Cells Damaged by Lipopolysaccharides

Problem Statement
The intestinal mucosal barrier serves as the primary defense against bacterial endotoxins, particularly lipopolysaccharides (LPS) from Gram-negative bacteria. Mucin-2 (MUC2) is the principal component of the mucus layer and has been shown to repair adherens junctions and reduce LPS translocation. However, the specific molecular mechanism by which MUC2 repairs the adherens junction structure under LPS-induced stress remains unclear. Specifically, it is unknown how LPS affects the O-glycosylation of MUC2 and E-cadherin, and whether restoring these glycosylation patterns can repair the intestinal barrier.

Methodology
The study utilized a co-culture model of Caco-2 (intestinal epithelial) and HT-29 (goblet cell) lines to simulate the human intestinal environment. The experimental design involved:

• LPS Treatment: Cells were exposed to LPS for varying durations (24–72 hours) to induce barrier damage.
• Genetic Manipulation:
  • Overexpression: Cells were transfected with a plasmid to overexpress GalNT3 (a key glycosyltransferase initiating O-GalNAcylation).
  • Silencing: MUC2 was silenced using siRNA to assess its necessity in barrier repair.
• Analytical Techniques:
  • Transmission Electron Microscopy (TEM): To visualize ultrastructural changes, specifically endoplasmic reticulum (ER) swelling and vacuolization.
  • Western Blotting and qPCR: To quantify protein and mRNA levels of MUC2, E-cadherin, tight junction proteins (ZO-1, Claudin, Occludin), and glycosyltransferases (GalNT3, B3GNT6, etc.).
  • Mass Spectrometry (MSFragger-Glyco): Used for comprehensive O-glycoproteomics to identify specific changes in O-glycan chain structures (e.g., HexNAc, Core-3, Core-4) on MUC2 and E-cadherin.
  • Co-Immunoprecipitation (Co-IP) and Structural Modeling: To confirm physical interactions between MUC2 and E-cadherin and predict binding interfaces using AlphaFold2 and SWISS-MODEL.
  • Calcium Assays: Measurement of intracellular Ca²⁺ concentrations and expression of calcium-regulating factors (STIM1, Orai1, IP3R, PMCA4, SERCA).
  • RNA-seq and Functional Enrichment: To analyze differentially expressed genes and map them to KEGG pathways (e.g., Adherens junction, Calcium-binding EGF domain) and Gene Ontology (GO) terms.

Key Results

1. LPS-Induced Damage Mechanism: LPS treatment caused significant ER swelling and vacuolization in intestinal epithelial cells. This damage correlated with a downregulation of GalNT3 and B3GNT6, leading to a loss of long-chain O-glycans and a reduction in HexNAc-containing O-glycan abundance on MUC2. Conversely, O-glycosylation on E-cadherin showed minimal changes under LPS stress alone.
2. MUC2-E-cadherin Interaction: Co-IP and structural modeling confirmed a direct physical interaction between MUC2 (central region, amino acids 1000–1500) and E-cadherin (N-terminal region, amino acids 0–500).
3. Restoration via GalNT3 Overexpression: Overexpressing GalNT3 in LPS-treated cells restored the O-glycan scaffold of MUC2. This included the re-addition of long-chain O-glycans (specifically Core-3 and Core-4 structures) and an increase in HexNAc abundance. While E-cadherin's own glycosylation remained largely unchanged, its protein expression levels increased and colocalized with MUC2, leading to tighter cell clustering.
4. Calcium and Adherens Junction Pathway: The repair mechanism was linked to intracellular calcium homeostasis. GalNT3 overexpression upregulated the expression of calcium-binding proteins and factors (STIM1, Orai1, IP3R1/3, PMCA4, SERCA2b). This restored intracellular Ca²⁺ levels, which are essential for the Ca²⁺-dependent homophilic adhesion of E-cadherin.
5. Barrier Function: The dual action of restored MUC2 O-glycosylation and reinforced E-cadherin junctions significantly reduced LPS translocation into cells, as evidenced by reduced FITC-LPS uptake and increased barrier integrity.

Significance and Claims
The paper claims to elucidate a specific molecular pathway where LPS damages the intestinal barrier by disrupting the initial phase of MUC2 O-glycosylation in the ER. The study posits that the repair of this barrier is not merely a result of increased MUC2 secretion, but specifically relies on the restoration of its O-GalNAc glycosylation scaffold.

The authors assert that:

• GalNT3 is a critical regulator that, when overexpressed, can reverse LPS-induced MUC2 glycosylation defects.
• Restored MUC2 O-glycans facilitate the interaction with E-cadherin, thereby reinforcing adherens junctions.
• This process is mediated through the Calcium-binding EGF domain and the maintenance of intracellular Ca²⁺ homeostasis, which is vital for E-cadherin function.
• The study provides a new research direction for treating intestinal mucosal barrier dysfunction by targeting O-GalNAc glycosylation and calcium signaling, offering potential therapeutic candidates that do not disrupt the microbiota.

The findings are presented as a mechanism explaining how healthy mucins attenuate bacterial pathogenicity and how the "dual actions" of MUC2 and E-cadherin jointly reduce cellular injury caused by endotoxin translocation.