Chronic obstructive pulmonary disease (COPD) is the 3rd leading cause of death in the US with >90% of cases caused by cigarette smoking. Currently there is no cure for COPD. Remodeling of airway epithelial architecture in response to cigarette smoke exposure plays a critical role in the pathogenesis of COPD. Goblet cell hyperplasia or metaplasia is an epithelial remodeling phenotype characterized by increased numbers of mucus-producing (“goblet”) secretory cells. In association with decreased ciliated cell numbers, these changes culminate in excess mucus production and impaired mucociliary clearance, which contribute significantly to COPD morbidity and mortality. Therefore, identifying the mechanisms by which cigarette smoke dysregulates secretory cell differentiation in the airway epithelium is critical to developing novel therapeutics to treat COPD.
Basal cells (BC) function as tissue-specific stem/progenitor cells in the human airway epithelium. Notch signaling is a developmental pathway that plays a key role in regulating BC stem/progenitor function in both humans and mice. This includes regulating the balance of BC differentiation into either secretory or ciliated cells. In vivo human studies have identified alterations in Notch signaling components at the mRNA, protein and epigenetic levels in the small airway epithelium (SAE) of smokers with and without COPD relative to nonsmokers. These include increased levels of activated NOTCH1 receptor and altered expression of NOTCH3 and multiple Notch downstream effector genes (e.g., HES5, HEY1 and HEY2). We demonstrated in vitro that constitutive activation via the NOTCH1 and 3 receptors in nonsmoker-BC activates a unique transcriptional response relative to NOTCH2 and 4, with increased expression of the downstream effectors HES5, HEY1, HEY2 and HEYL. Furthermore, NOTCH1 and 3 signaling activation increased secretory cell differentiation with a corresponding decrease in ciliated cell differentiation characteristic of goblet cell hyperplasia/metaplasia. Therefore, we hypothesize that cigarette smoking increases Notch signaling activity in BC stem/progenitors resulting in development of goblet cell hyperplasia/metaplasia in the airways of smokers with COPD.
Our long-term goal is to uncover pathogenic mechanisms and treatment strategies for COPD. Although Notch regulates BC differentiation and is implicated in COPD, there is limited insight into how Notch signaling contributes to COPD. Therefore, despite Notch providing a potential therapeutic target to treat COPD-associated goblet cell hyperplasia/metaplasia, important questions remain. First, what is the effect of cigarette smoking on Notch activity in the airway epithelium (Aim 1)? Second, as BC from COPD patients have altered regenerative capacity compared to BC from nonsmokers, does modulation of Notch activity in COPD-BC have the same outcome as in nonsmoker-BC (Aim 2)? Third, what are the downstream effectors that regulate Notch-mediated goblet cell differentiation (Aim 3)? To address these questions we propose the following Aims:
1) Determine whether cigarette smoke exposure activates Notch signaling in the airway epithelium in vitro and in vivo.
2) Determine whether BC stem/progenitor cells from nonsmokers vs. patients with COPD respond differentially to Notch signaling modulation as they differentiate into a mucociliated epithelium.
3) Determine whether specific Notch downstream effectors can induce goblet cell differentiation in vitro and whether targeting these genes can inhibit Notch-induced goblet cell hyperplasia/metaplasia.
The outcomes of these studies will advance our understanding of the impact of cigarette smoke exposure on Notch activity in the airway epithelium and identify Notch effectors that regulate goblet cell differentiation. Further, they will provide candidate therapeutic targets to suppress COPD-associated goblet cell hyperplasia/metaplasia.
Matthew Walters, PhD
Associate Professor of Medicine