COPD is a progressive, debilitating chronic lung disease with high morbidity and mortality rate. It is a major unmet medical need in the United States and worldwide and, in western society, is impressively associated with cigarette smoke (CS) exposure. A number of studies strongly support that CS causes apoptotic cell death response, especially in epithelial cells and endothelial cells, that lead to inflammation and emphysematous destruction in the lung. However, the molecular mechanism of CS-induced emphysema is still largely elusive.
Recent studies from our laboratory and others reported dysregulated expression of Chi3l1 in the serum and lungs of patients with COPD and their expression levels were correlated with CS exposure and disease progression, suggesting an important role of Chi3l1 in the COPD pathogenesis. Chi3l1 is a prototype of chitinase like proteins in mammals and implicated in various diseases characterized by inflammation and tissue remodeling. Our laboratory further identified that Chi3l1 forms a receptor complex called “chitosome” that contains IL-13R2 and its co-receptor TMEM219 (TMEM). We have also shown that each component of chitosome mediates specific biological function of Chi3l1 via interaction with other molecules such as galectin-3. Among these, TMEM plays a central role in effector function of Chi3l1 since null mutation of TMEM abrogate the Chi3l1-stimulated ERK, AKT signaling and anti-apoptotic effects on oxidative stress-induced epithelial cell death. Chi3l1 was reported to promote angiogenesis through VEGF-dependent and independent pathways, however, specific regulatory function of Chi3l1 in CS-stimulated endothelial cell death response related to COPD pathogenesis has never been explored.
Previously we demonstrated that CS increased the expression of Chi3l1 in the murine lung and that CS-induced alveolar destruction and epithelial cell death was significantly enhanced in Chi3l1-/- mice compared to WT controls. Apparently, there is a paradoxical relationship between the level of Chi3l1 expression and the lack of protective role of Chi3l1 in CS-induced alveolar cell death response that resulted into emphysematous destruction. Studies were undertaken to understand this paradox. We exposed primary human small airway epithelial cells (SAECs) to CS extract (CSE) and evaluated the expression of each chitosome component. Surprisingly, these studies revealed that TMEM expression was significantly reduced with increase of Chi3l1 and IL-13R2 expression after CSE exposure compared to vehicle treated ones. As a result, Chi3l1 mediated MAPK and Akt signaling was decreased with CSE exposure and forced over-expression of TMEM restored Chi3l1-stimulated signaling and cell death response. The epigenetic studies identified that the promoter region of TMEM containing CpG islands was significantly methylated with CSE exposure, and the treatment of AZA (a demethylase, AZA deoxy cytidine) restored the expression of TMEM as well as a protective role of Chi3l1 against CSE-induced cell death response. These findings lead us to hypothesize that epigenetic control of TMEM plays a critical role in CS-stimulated epithelial and endothelial cell death response leading to emphysema. The specific hypotheses that will be evaluated and the aims that will be employed are detailed below.

Hypothesis #1. TMEM plays a critical role in CS-induced emphysematous alveolar destruction and remodeling.
Aim 1. Characterize the specific role of TMEM in CS-induced inflammatory, remodeling, cell death and repair responses in the lung. WT, TMEM-/- and TMEM overexpressing knock-in transgenic (TMEM KI Tg) mice will be used to determine the TMEM expression and the cell-specific role of TMEM in CS-induced cellular and tissue responses in the lung after Room Air (RA) or CS exposure of these mice.

Hypothesis #2. CS enhances epithelial and endothelial cell death response via suppression of TMEM expression that results into the loss of chitosome signaling.
Aim 2. Determine the specific mechanism that CS uses to regulate TMEM expression and TMEM regulation of CS- induced epi- and endothelial cell death response. We will determine 1) the mechanisms that CS uses to regulate TMEM expression, 2) the signaling pathway mediating anti-apoptotic role of chitosome in CS -induced epithelial and endothelial cell death.

The achievement of these aims will provide the specific role and mechanism of TMEM in lung epithelial and endothelial cells as a component of chitosome in the pathogenesis of COPD and also offers a translational potential to use TMEM as a biomarker and therapeutic target of COPD patients.