Figure 5. Healthy mucus protects against barrier dysfunction and disease.

Healthy mucus (center) protects epithelia by acting as a dynamic barrier that provides hydration, regulates diffusion and accommodates commensal microbes. Changes in mucin expression and identity are associated with disease. (A) Increased mucus permeability in various tissues potentially allows the outgrowth of pathogenic microbes leading to intra-amniotic infections and stomach ulcers. (B) Decreased mucus permeability impairs mucociliary clearance, leading to increased microbial colonization, as observed in cystic fibrosis and chronic obstructive pulmonary disease. (C) Altered mucin glycosylation is associated with various diseases, including Sjögren’s syndrome, cancer, and cystic fibrosis. Loss of sialylated glycans in Sjögren’s syndrome reduces the hydrophilicity of mucin glycans, causing dry mouth. (D) Altered mucin expression can lead to cancer. Membrane-associated MUC1 promotes the survival of epithelial cells, but MUC1 overexpression can lead to aberrant regulation of growth factor signaling.