The Cystic Fibrosis Transmembrane conductance Regulator (CFTR) is an ion channel primarily studied in the context of epithelial fluid homeostasis in the lung. CFTR is responsible for maintaining proper ionic composition and pH of the airway surface liquid through its transport of chloride, bicarbonate, and other anions. The airway surface liquid plays an important role in innate immunity, and when its homeostasis is disturbed, the lung becomes highly susceptible to infection. Cystic Fibrosis (CF), the disease after which CFTR is named, is a life-limiting autosomal-recessive disease caused by various loss-of-function mutations in CFTR. CF is a multi-organ disorder, but most of its associated morbidity and mortality are caused by recurrent lung infection and inflammation resulting in lung failure. While traditionally thought of as a childhood disease, many CF patients are reaching adulthood thanks to newborn screening and therapeutic advances. However, there remains no cure.
Another airway disease with no cure in which CFTR dysfunction is potentially implicated is Chronic Obstructive Pulmonary Disease (COPD). COPD is primarily caused by tobacco smoking, which has been shown to negatively regulate CFTR protein expression. CFTR expression has also been found to be low in the lungs of severe COPD patients compared to healthy controls. These findings, combined with phenotypic similarities between CF and COPD, have contributed to a growing body of research that aims to understand COPD as a disease of “acquired” CFTR dysfunction. Traditionally, COPD has been characterized by two primary phenotypes: chronic bronchitis and emphysema, although the disease itself is heterogeneous and there can be considerable overlap. Chronic bronchitis is characterized by airflow restriction in the conducting airways due to excessive mucus production, smooth muscle constriction, and tissue fibrosis. Emphysema encompasses alveolar cell death and tissue destruction, resulting in inefficient gas exchange and gas trapping.
Accordingly, the first chapter of this dissertation will address the question of whether reduced CFTR expression is a potential risk factor of COPD, or merely a consequence. This is important, because approximately 1 in 30 people in the US is an asymptomatic CF carrier. To answer this question, wild-type (WT), Cftr+/- (Het), and Cftr-/- (KO) mice were subjected to smoking or natural aging, two known COPD risk factors. Lungs were analyzed for histologic signs of COPD, and it was found that in response to smoking, both Het and KO mice displayed an increased mean linear intercept (LM), indicative of emphysema, compared to WT controls. On the other hand, aging led to increased LM in KO mice only. Results will be discussed in the context of the biology of emphysema and aging. Following that theme, in vitro data identifying a potential calcium-dependent DNA repair deficiency associated with CF will be discussed.
The second chapter will focus on therapeutic strategies to improve CFTR stability in the context of both CF and COPD. Importantly, no current therapeutics target CFTR stability. Literature highlighting the significance of the Epidermal Growth Factor Receptor (EGFR) – Extracellular Regulated Kinase (ERK) axis in both CF and COPD will be discussed. Data showing that this axis, dependent on the EGFR ligand amphiregulin, negatively regulates CFTR in CF will be presented. Next, it will be shown that ectoine, a natural osmolyte which can target EGFR signaling and benefit COPD patients, can improve CFTR stability and attenuate inflammatory signaling in CF. Another way to stabilize plasma membrane CFTR is by preventing its degradation by interfering with its binding to the CFTR-associated ligand (CAL) protein using PDZ domain peptide inhibitors.
The third chapter will discuss the transcriptome and proteome of CF primary bronchial epithelial cells, and the effects of the clinically approved CFTR corrector VX-661 and ectoine. These data will be used to provide a deeper understanding of CF lung pathology, especially in relation to chapters 1 and 2; and to identify potential opportunities for future therapeutic intervention.