Project 4: Biophysical and structural characterization of airway submucosal gland mucus in health and disease

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Abstract The contribution of submucosal gland (SMG) mucus to the host defense functions of airway surfaces is not well understood. SMG mucus emerges from the SMG onto airway surfaces in the form of micron long bundles. However, it is unknown how SMG mucus bundles are formed, what regulates their structure, and what is their functional role. The minimal understanding of SMG mucus structure has limited our ability to predict how SMG mucus promotes lung health and contributes to disease. The prevalence of SMGs in the proximal airways suggests SMG mucus defense functions include clearing large, aspirated, or inhaled particles by cough. However, understanding how SMG bundles are configured for this role requires a comprehensive characterization of SMG gland mucus as well as engineering of novel models to elucidate the mechanisms of SMG bundle formation. We have identified three key gaps in our understanding of the structure and function/dysfunction of SMG mucus for investigation, encapsulated in our three specific aims. Specific Aim 1: Do SMGs secrete a different mucus in basal versus stimulated conditions? This aim will test the hypothesis that SMG basal secretions constitute a soluble gel that dissolves into the air surface liquid, whereas stimulated SMG secretions consists of both soluble and insoluble fractions, the insoluble component constituting a “permanent” mucus bundle. Specific Aim 2: How do SMG bundles form and what is their functional role? This aim will engineer gland mimetic systems to test the hypothesis that glands are configured to produce strong fluid flows to extrude bundles in a two-level hierarchical process involving primary strand formation by mucin stretching and sticking through open globular domains in peripheral gland ‘nozzle’ junctions, followed by their extrusion as bundles through the ciliated duct nozzle. This aim will also test how bundles are configured to bind and clear large particles by cough. Specific Aim 3: How and why SMGs fail in CF? This aim will test the hypothesis that reduced fluid flows in the CF SMG is the critical failure, generating insufficient mucin extensional forces, producing disorganized mucus bundles that stick to cell surfaces and reduce cilia-dependent transport. Resolution of the hypotheses advanced in this proposal will establish a new paradigm of how glands produce mucus bundles in response to stimulation and complement superficial epithelial responses to clear large irritants by cough.