Unravelling the molecular mechanisms controlling respiratory tract mucus gel formation in health and disease

The lungs produce the jelly-like substance mucus in order to defend the body from environmental challenges (pollution or bacteria and viruses present in the air). This mucus traps these environmental agents and they are moved out of the lung by what is called the 'mucociliary escalator'; this is made up from many cells lining the surface of the upper airways, which have hair-like protrusions (cilia). The cilia beat and move the mucus up to the throat where it is swallowed. One of the major struggles faced by individuals suffering from airway diseases such as asthma, cystic fibrosis and chronic recurrent obstruction of their airway is that they make too much mucus, and the mucus is stickier than normal. The consequence of this is that the mucus cannot be moved by cilia and it blocks the respiratory system, which leads to infection and risk of death. The framework of the mucus is provided extremely large molecules called mucins. Unlike many other molecules in mucus, mucins are made and then stored within special structures inside cells awaiting the appropriate signal to be released and, via interactions with themselves and other molecules, form mucus. During their release the structure of mucins changes from a highly condensed form to an expanded, rope-like form. There are two types of mucin (MUC5AC and MUC5B) found in airways mucus and we know that in disease larger amounts of these molecules are produced; in particular overproduction of one of these (MUC5B) with an abnormal structural form (more condensed and web-like rather than the more normal rope-like form) contributes to obstruction of the airways. Importantly, recent research suggests that MUC5B is crucial for the generation of mucus that is easily moved by cilia. Therefore, the principal question of our research proposal is how abnormal mucus is generated; is it defective condensation, defective expansion, or altered interactive properties (or a combination of these factors) of the main gel-forming molecule, MUC5B. To answer this question we are employing state-of-the-art methods spanning biology and physics. Our goal in this project is to understand the mechanisms that control the production of mucins with properties that cause the failure of mucus to protect the lung. This new knowledge of mucus biology in the lung will help design novel treatments for people suffering with obstruction of their airways.

Respiratory tract mucus in lung diseases such as asthma, chronic obstructive pulmonary disease and cystic fibrosis has abnormal properties that contribute to obstruction of the airways. The failure in function of mucus in airway clearance and pathogen protection leads to chronic infection and risk of death. Polymeric mucins provide the structural framework of the airway mucus gel. Mucins are packaged in a highly organised and condensed form within secretory granules inside specialised secretory cells, and after the appropriate stimulus mucins are released and expand to form mucus. MUC5B the dominant mucin in healthy airways is indispensable for transport by cilia of mucus out of the lung Paradoxically MUC5B is associated with mucus obstruction in airways disease where it has been shown to have an altered structure. The planned studies will test the hypothesis that a combination of defective packaging, defective expansion and altered interactive properties yields mucus gels with aberrant transport properties that compromise the protection of the lung. Using a combination of biochemical, biophysical and structural approaches we will investigate three interlinked questions that are key to understanding this process. (1) What is the molecular mechanism that condenses MUC5B into a packaged form in secretory granules? (2) What is the molecular mechanism that controls the transition during secretion from the condensed packaged mucin to the expanded gel form with viscoelastic flow properties? (3) What are the intra- and intermolecular interactions of MUC5B involved in its self-organisation, and its interactions with other components of the secretion and the epithelial surface? It is by unravelling the molecular mechanisms that control mucin supramolecular structure and viscoelastic properties that we will gain insight into what determines aberrant mucus gel production. This will help us identify novel ways to combat the accumulation of aberrant mucus in diseased airways.

Mucus has a vital role in protecting the lung from infection and its ability to be transported out of the airways by the action of cilia is critical for health. Obstruction of the airways by mucus with abnormal properties is a key feature of lung disease and can lead to chronic infection and risk of death. Studying the mechanisms of mucus gel formation is important to understand the production of normal mucus, which can be transported by the action of cilia, and abnormal mucus that is unable to be transported by cilia. The goal of our research is to identify critical steps that result in mucus with failed functional properties, which may identify new clinical targets for the development of therapeutics to alleviate mucus obstruction of the lungs. This unmet clinical need would ultimately benefit patients suffering from debilitating lung diseases such as asthma, cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD).

The research outlined in this proposal will have broad impact since the Identification of the mechanisms underlying the formation of mucus gels will be of interest to:

1. Basic scientists: Our data will be of interest to a wide community of researchers studying innate defence of mucosal surfaces (e.g. lungs, eyes, gastrointestinal tract and urinogenital tracts), in man and animals. The new insights gained from our studies on mucin/mucus structure and function will inform studies on how microbes, both pathogens and commensal organisms, interact with this protective barrier in health and disease. (Years 1-3)

2. Biomedical scientists and clinicians: Our data will increase understanding of mucus associated pathologies in mucosal diseases such as asthma, CF, COPD, inflammatory bowel disease, gastric ulcers, dry eye and dry mouth and inform on therapeutic strategies to modulate mucus properties. (Years 1-3 and beyond the end of the grant)

3. Pharmaceutical and food industries: Our findings will provide better understanding of mucus barrier organisation in health and disease may aid improved delivery of drugs and CF gene therapy agents across mucosal surfaces, and the absorption and bioavailability of nutrients in the gastrointestinal tract. (Years 1-3)

4. General Public: During the project, we expect to have societal impact, particularly in the local area, with active participation in public engagement events. This will increase awareness of how mucus protects the body against infection and what goes wrong with mucus in common diseases. The opportunity to interact with the general public will raise awareness of how scientists perform research and why research is important for everyone in society. (Years 1-3)