Respiratory tract infections are a major cause of morbidity and mortality in the elderly population. In this population, pneumonia is the leading cause of death due to infectious disease, and the incidence of community-acquired pneumonia increases with every decade of life. One of the most important innate defense mechanisms against respiratory infections is the mucociliary clearance (MCC) system. Individuals with genetic defects in MCC suffer from chronic pulmonary infections, demonstrating the importance of MCC to host defense. Importantly, there is good evidence to suggest that MCC declines with age. Therefore it is likely that a reduced rate of MCC in the elderly, increases their susceptibiliy to respiratory infections. However, none of the studies reported to date has identified the mechanism(s) responsible for the reduced rate of MCC in older subjects, nor have they explored the effectiveness of treatments to improve MCC. Surprisingly, there have been no studies of the effect of aging on MCC in a mouse model. The lack of an established mouse model is a significant barrier, both to studies of the mechanisms responsible for the reduced rate of MCC in the elderly and to the development and testing of therapeutics designed to improve MCC in this population. Therefore the goals of this proposal are to 1) accurately assess the effect of aging on MCC in an established mouse model of aging, and 2) to examine the effect of aging on the key components of MCC to begin to identify the mechanisms responsible for the decreased rate of MCC in older subjects.
Specific aim 1. To test the hypothesis that MCC is reduced in aged mice. Mucociliary clearance will be measured in the trachea and nasopharynx of 3, 6, 12, and 24 month old C57BL/6 mice by tracking the movement of endogenous mucus particles or instilled fluorescent beads.
Specific aim 2. To examine the effect of age on individual components of the MCC. 2a) To test the hypothesis that aged mice exhibit defects in the ciliary component of MCC. Ciliary beat frequency (CBF) will be measured by video microscopy in the trachea and nasal cavity of 3, 6, 12, and 24 month old mice. The number of ciliated cells will be determined by histology, and the incidence of axonemal structural defects will be quantified by electron microscopy. 2b) To test the hypothesis that aged mice exhibit defects in the mucus component of MCC. The number of mucous cells in the nasal cavity and trachea will be determined by histology in samples from mice of different ages. The amount of secreted and total airway mucin in the lung will be quantified by agarose gel electrophoresis and Western blotting. 2c) To test the hypothesis that aged mice exhibit defects in the regulation of ion transport. The ion transport properties of the upper and lower airways wil be determined, as these are reflective of the hydration status of the ASL. Tissue samples from mice 3, 6, 12, and 24 months old will be studied in Ussing chambers.
The long-term goal of this research is to understand the cause of reduced mucociliary clearance and the high incidence of respiratory infections (pneumonia) in the elderly population, and to develop effective treatments to improve mucociliary clearance and prevent respiratory disease. In these pilot studies, mucociliary clearance will be measured in young and old mice to determine if mice can be used as a model for studies of mucociliary clearance in humans. Additional studies will investigate some of the possible mechanisms that may contribute to the reduced mucociliary clearance and increased respiratory infections in the elderly.