Neuraminidase-1 Desialylates the MUC1 Ectodomain to Release a Decoy Receptor that Protects against Lethal Pseudomonas aeruginosa Lung Infection

Abstract

Background: Pseudomonas aeruginosa (Pa) is a major opportunistic pathogen of human airways, but the host response to infection is incompletely understood. Epithelial cells lining the airways express numerous surface receptors that recognize infectious agents such as Pa. One such receptor, MUC1, recognizes Pa flagellin, the major structural protein of the bacterial flagellum. MUC1 consists of an NH2-terminal, highly O-glycosylated ectodomain (MUC1-ED) attached to the cell surface through a membrane-spanning domain. MUC1-ED is proteolytically processed and shed from the epithelial cell surface following cleavage at a juxtamembranous Gly-Ser peptide bond. We previously demonstrated that stimulation of human airway epithelial cells with Pa flagellin increased MUC1-ED shedding in vitro (Lillehoj et al., J. Biol. Chem. 290:18316, 2015). Using an intact, physiologically relevant murine model of Pa pneumonia, here we asked whether Pa and its flagellin might also stimulate NEU1-dependent MUC1-ED desialylation in vivo to release a hyperadhesive decoy receptor that provides a novel, protective host response to Pa lung infection. Results: Intranasal administration of e5.0x103 colony forming units of Pa strain K (PAK) to BALB/c mice increased MUC1-ED shedding into the bronchoalveolar compartment. MUC1-ED levels increased as early as 12 h, peaked at 24-48 h, reaching up to a 7.8-fold increase, and decreased by 72 h. The a-type flagellin-expressing PAK strain and the b-type flagellin-expressing PAO1 strain stimulated comparable levels of MUC1-ED shedding. A flagellindeficient isogenic PAK mutant provoked dramatically reduced MUC1-ED shedding compared with the wild-type strain, and purified flagellin recapitulated the wild-type effect. In lung tissues, Pa increased MUC1-ED desialylation by peanut agglutinin lectin blotting. NEU1-selective sialidase inhibition with C9-BA-DANA, or use of a catalytically-inactive NEU1-G68V mutant, protected against Pa-induced MUC1-ED desialylation and shedding. MUC1-ED inhibition of in vitro Pa adhesion and flagellin binding to airway epithelial cells was localized to its protein backbone and not to its glycans. Finally, co-administration of Pa with human recombinant (r)MUC1-ED expressed in E. coli diminished lung and BALF bacterial burden, proinflammatory cytokine levels, and pulmonary leukostasis, and enhanced 5-day survival from 0% to 75%. Conclusions: These combined data indicate that Pa flagellin provokes NEU1-mediated airway shedding of MUC1-ED as a decoy receptor that protects against lethal Pa lung infection. Human rMUC1-ED might someday be harnessed as a therapeutic intervention to target Pa lung infections, including those associated with multi-drug resistant organisms.