Fig 7. Protective functions of MUC1 and MUC16 at the respiratory surface.

(A) Immunofluorescence confocal microscopy analysis of expression and localization of ACE2 (green) and TM mucin MUC1 (214D4, red) in Calu-3 cells. (B) Proximity ligation assay (PLA) for MUC1-ED (214D4; light blue) and ACE2 (green) (left) and MUC1-SEA (232A1; light blue) and ACE2 (green) (right) in Calu-3 cells. A positive PLA signal (red) was detectable for both combinations in double positive cells, demonstrating that MUC1 and ACE2 are in close proximity. (C) Immunofluorescence confocal microscopy to determine expression of MUC1 (214D4; red) in ciliated cells (AcTub; green). MUC1 is highly expressed in non-ciliated cells and is expressed in cells with short cilia. (D) Immunofluorescence confocal microscopy for MUC16 (red) and cilia (AcTub; green). MUC16 is expressed in some non-ciliated cells and ciliated cells. (E) Immunofluorescence confocal microscopy for MUC16 (red) and goblet cells (MUC5AC; green). Several goblet cells are positive for MUC16. White scale bars represent 20 μm for A and B and 50 μm for C-E. (F) Schematic model describing the expression and localization of transmembrane mucins MUC1 and MUC16 in different cell types within the respiratory epithelium. MUC1 is highly expressed in non-ciliated cells and MUC16 is expressed in some ciliated cells and enriched in goblet cells. (G) Schematic model describing the protective functions of the extracellular domains of transmembrane mucins MUC1 and MUC16 during SARS-CoV-2 infection. The extended glycosylated extracellular domains prevent access of the virus to the ACE2 receptor (left). Enzymatic removal of the glycosylated part of the extracellular domains with the StcE mucinase allows the viral spike protein to connect with the ACE2 receptor resulting in viral entry into lung epithelial cells (right).