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Membrane-bound mucins of the airway mucosal surfaces are densely decorated with keratan sulfate: revisiting their role in the Lung's innate defense.
Understanding the basic elements of the airway mucosal surfaces and how they form a functional barrier is essential in understanding disease initiation, progression, pathogenesis and ultimately treating chronic lung diseases. Using primary airway epithelial cell cultures, atomic force microscopy (AFM), multiangle light scattering, and quartz crystal micro balance with dissipation monitoring (QCMD) techniques, here we report that the membrane bound mucins (MBMs) found in the periciliary layer (PCL) of the airway surface are densely decorated with keratan sulfate (KS). AFM and immunoblotting show that the KS sidechains can be removed enzymatically with keratanase II (KII) treatment and the antibody accessibility for B2729 (MUC1), MUCH4 (MUC4), OC125 (MUC16) was substantially enhanced. Light scattering analysis confirmed that KII treatment removed ~ 40% of the mass from the mucin fractions. Surface binding experiments indicated that MBMs were able to pack into a tighter conformation following KS removal, suggesting that negatively charged KS sidechains play a role in mucin-mucin repulsion and contribute to "space filling" in the PCL. We also observed that soluble filtrate from the common airway pathogen Pseudomonas aeruginosa is capable of stripping KS from MBMs. Together, our findings indicate that KS glycosylation of MBMs may play an important role in the integrity of the airway mucosal barrier and its compromise in disease.
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