This project is concerned with the structure and function of the protective mucus barrier in the large airways. Our work indicates that the layer should really be considered as two gels, one surrounding the cilia which we call the peri-ciliary gel layer (PGL) and the other flowing mucus layered upon it and moved by it. The gel surrounding the cilia and microvilli is a special gel formed by the epithelial mucins MUC1, 4 and 16, attached by their C-termini to these structures. Such a gel is called a grafted gel layer and it has special properties that control accessibility to the surface and its interaction with the layer above. The properties of the flowing mucus layer are controlled by two gel forming mucins termed MUC5B and MUC5AC. Our data both in cell culture and induced secretions suggests that MUC5B forms the basis of the normal flowing gel. However if the lung is irritated or provoked by acute infection a thick highly elastic gel enriched in MUC5AC is secreted by surface goblet cells. We hypothesise that this gel is important in the normal situation to help aid clearance by cough. During infection and/or chronic disease there is hyperplasia of goblet cells and an up-regulation of MUC5AC production.
The aims of our project are, using a human bronchial epithelial cell line and normal and pathological sputum to characterize the properties of the PGL and to identify the molecular mechanisms underlying the MUC5AC-rich gel and the MUC5B flowing mucus. We further hypothesise that cough and flow are controlled by differing adhesion strengths and mechanisms between these gels and the PGL. If this adhesion is too strong , e.g. malfunctioning hydration mechanisms such as occur in CF then neither of the maechanisms will function effectively and stasis, infection and inflammation will ensue. Relevance to Public Health The airway is defended from chemical, biological and physical insult by mucus, a gel comprised of greater than 90% water and associated cations, the remainder being proteins and glycoproteins. Diseases affecting the lung e.g. chronic bronchitis, asthma and cystic fibrosis are associated with an enhanced mucus secretion. In the normal lung a 10-20u thick mucus blanket flows from the alveolus to the esophagus over a bed of cilia which beat in a 7u thick layer that was just thought to be water but our work indicates is a special protective gel. We now call this protective layer the peri-ciliary gel layer (PGC). The long term objective of this project is to understand how these two layers are maintained and work together to protect the lung and thus help design better therapies.
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