Project 3: Membrane-bound mucins on the airway surface ensure efficient mucus clearance and lung health

健康的气道上皮粘膜屏障是一个强大的先天免疫系统的中心，它保护呼吸道

The healthy airway epithelial mucosal barrier is the center of a powerful innate immune system that protects the pulmonary surfaces from the constant onslaught of inhaled infectious and noxious substances. However, abnormalities in the mucus clearance system characterize a number of airway diseases, including cystic fibrosis, chronic obstructive lung disease, and asthma. The pathogenesis of these diseases is multi-factorial, but appears to include a common disease-initiating step, a reduction of mucus clearance. We have developed a novel paradigm describing the mucus transport system, referred to as the “two-gel” model, which highlights the role of the two mucin-containing layers on the airway surface: 1) the mucus layer, containing secreted mucins which bind inhaled particles; and 2) membrane-spanning tethered mucins – a layer referred to as the periciliary layer- glycocalyx (or PCL-G) which lines the airway surface. We have previously shown that the failure to regulate both layers is associated with a reduction in mucociliary clearance. However, it is currently unknown how the cilia and PCL-G are organized to facilitate the efficient clearance of mucus out of the lungs and protect against inhaled pathogens. The overarching goal of Project 3 of this PPG is to apply novel theoretical concepts and experimental approaches to answer key questions about the multiple roles of the PCL-G with respect to both mucus transport and the host barrier defense activities required to protect the airway surfaces. First, we seek to understand how the mucus layer actually is transported, i.e., how momentum is transferred from cilia to the mucus layer. In Aim 1, we will test the hypothesis that in the distal airways, where mucins are too low to generate a viscoelastic gel, water is “pumped out” of the periciliary space, providing hydrodynamic force sufficient to transport the dilute mucus layer. Studies in this aim are directed at quantifying the water pumping forces generated by cilia beating, assess whether they are sufficient to propel a low-viscosity mucus across airway surfaces, and understand the impact of gravity. In Aim 2, we will test the hypothesis that in the larger airways, where mucin concentrations are sufficient to form a viscoelastic gel, a second mode of cilia-mucus momentum transfer is required for efficient mucus transport. Here, we hypothesize that the highly glycosylated tethered mucins on the tips of cilia form weak/transient interactions with the mucus gel during the cilia power stroke and physically “thrust” the mucus gel forward. Studies here will assess the impact of modulating tethered mucins on the efficacy of mucus gel transport and how it is altered in disease. Studies in Aim 3 are directed at understanding the role that the densely packed tethered mucins play in airway protection. We will test the hypothesis that tethered mucins lining the airway surface form a physical barrier to prevent inhaled pathogens, including viruses, from penetrating the PCL-G space. We anticipate that results from these studies will lead to paradigm shifts in our understanding of how mucus clearance is maintained in health and aid in the discovery of therapeutic strategies which can accelerate/restore clearance in persons with muco-obstructive lung diseases.