Hydrogel properties on airway surfaces in health and muco-obstructive lung disease

Airway mucus has important homeostatic functions that are essential for lung health. Mucus dysfunction has been implicated as a key factor in the pathogenesis of chronic lung disease in patients with the severe genetic disorder cystic fibrosis (CF, also known as mucoviscidosis), that is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene encoding for an epithelial Cl- channel implicated in salt and water transport across airway surfaces, as well as a spectrum of other muco-obstructive lung diseases such as chronic obstructive pulmonary disease (COPD), which has emerged as a leading cause of death worldwide. The mucus layer covering healthy airway surfaces is a complex hydrogel formed mainly by the two secreted mucins constituting large and complex glycoproteins (MUC5B and MUC5AC), salt, and water. The main biological function of this hydrogel at the airway interface is to bind constantly inhaled pathogens and irritants and transport them out of the lung by mucociliary clearance. Proper functioning of airway mucus is critically determined by its rheological properties that are in turn controlled by its mesoscopic structure and dynamics. However, i) the rheological properties and mesoscopic structure of airway mucus in health; ii) the changes that determine formation of abnormal mucus and impaired mucociliary clearance during the onset and progression of mucoobstructive lung disease; and iii) if these abnormalities can be corrected therapeutically remains poorly understood. In this project, we will therefore perform a comprehensive characterization of the biophysical and structural properties of airway mucus in health in order to properly understand the desired state of mucus. Based on this understanding, we will then study mucus dysfunction in CF as a model of muco-obstructive lung diseases. Specifically, we will i) determine effects of hydration and mucin crosslinking, ii) use genetic approaches in mouse models (knockout and overexpression) to elucidate the relative roles of the two secreted airway mucins, MUC5B and MUC5AC, iii) evaluate effects of current “mucolytic” therapies and novel CFTR-directed therapeutics that target the CF basic defect on rheological properties, mucus transport, mesoscopic structure and dynamics in healthy and CF airways. To achieve these goals, we will analyze airway mucus specimens obtained from transgenic mice with CF-like lung disease (βENaC-overexpressing mouse), primary airway epithelial cultures, and sputum from healthy individuals and patients with CF. We will focus on the viscoelastic properties determined by classical rheology and micro-rheological approaches, which will be correlated with the mesoscopic structure of the mucus hydrogel. We expect these quantitative studies of airway mucus properties from the “macro” to the “micro” scale performed in this interdisciplinary project to yield a comprehensive understanding of mucus (dys)function in health and disease on a sound physico-chemical basis and thereby to provide a knowledge base for future development of effective therapies for patients with CF and other mucoobstructive lung diseases.