PROJECT SUMMARY/ABSTRACT The airway is composed of luminal cells such as club and ciliated cells that moisturize and clean the airway, respectively. They are lined by basal cells that serve as progenitors for luminal cells in normal turnover and injury repair. Proper balance of progenitor and luminal/differentiated cell ratio is critical for airway function. While many genes have been identified that control individual cell fate, knowledge gaps remain in how the ratio of progenitors and differentiated cells are globally regulated. In this study, we will investigate how airway cell ratio is established in development, maintained in homeostasis and restored following injury. Our entry point is Lon protease 1 (LONP1), an ATP-dependent serine protease that functions in the mitochondria matrix to degrade oxidized and misfolded proteins, thereby control protein quality and mitochondria health. Mutations in LONP1 has been identified in congenital diaphragmatic hernia (CDH) patients. CDH carries a high mortality rate associated with lung hypoplasia and pulmonary hypertension. To address if Lonp1 plays a role in lung, we inactivated it in the developing lung epithelium and mesenchyme. While the mesenchymal mutants survived to adult with no discernable phenotype, the epithelial mutants exhibited lung hypoplasia and died at birth. Unexpectedly, these mutants also exhibited a striking increase of basal cells at the expense of club and ciliated cells. Further preliminary data revealed an increase in integrated stress response (ISR) pathway genes, and an increase of KDM6B, a key histone demethylase. In this study, we will investigate the role of this mitochondria factor LONP1 in controlling airway cell fate balance via ISR pathway (Aim 1), chromatin regulators (Aim 2) and in adult airway homeostasis and following influenza- induced injury (Aim 3). Our findings will delineate a novel pathway from a mitochondria protease to ER ISR to nuclear chromatin regulators in the fundamental control of airway cell fate.