Regulators of airway submucosal glands development and functions

Tracheobronchial submucosal glands (SMGs) develop from clusters of epithelial progenitor cells basally orientated within the surface airway epithelium called primordial glandular placodes (PGPs). Signal transduction events that coordinate the transitional process from PGPs into fully developed SMGs consisting of intricately branched networks of tubular secretary structures are still poorly understood. Wnt/β-catenin dependent induction of lymphoid enhancing factor-1 (Lef-1) expression in PGP progenitor/stem cells is required for SMG formation and maturation in the airway. In an effort to better understand the regulatory mechanisms that control Lef-1 during airway SMG development, I have studied its transcriptional regulation. I discovered that Sox2 expression is predominantly confined to the surface airway epithelium (SAE) and is repressed as Lef-1 is induced within PGPs. Deletion of Sox2 in polarized primary airway epithelia significantly enhances Lef-1 mRNA expression. Consequently, my hypothesis is that Sox2 functions as a negative regulator of Lef-1 expression in the SAE. I demonstrated that Sox2 modulates the expression of Lef-1 both independent and dependent on Wnt/β-catenin signaling. I discovered that a Sox2-binding site located in the Wnt Responsive Element (WRE) region of the 2.5Kb Lef-1 promoter is required for Sox2-mediated inhibition of β-catenin-dependent Lef-1 promoter transcription. It is important to understand the biology of SMG development because SMGs are the major mucus-producing structures in the proximal airway and are important in regulating the innate immunity of the lung in response to various neural signals. SMG ducts have also been proposed as a potential protective niche for slowly cycling progenitor cells (SCPCs). Hence, aberrant SMG function is thought to aggravate the pathoprogression of lung disease. Cystic fibrosis (CF) is a disease caused by a defect in the gene that encodes a chloride ion channel called cystic fibrosis transmembrane conductance regulator (CFTR). The absence of CFTR in serous cells within SMG ducts contributes to defective airway secretion, which alters the microenvironment within SMGs. I hypothesized that the glandular SCPC niche may be dysfunctional in CF. I reported that the neural peptide, calcitonin gene-related peptide (CGRP) activates CFTR-dependent SMG secretions and that this signaling pathway is hyperactivated in CF human, pig, ferret, and mouse SMGs. CFTR-deficient mice failed to maintain glandular SCPCs following airway injury, suggesting that the glandular SCPC niche may be dysfunctional in CF. CGRP levels increase following airway injury and function as an injury-inducible mitogen that stimulates progenitor cell proliferation. However, components of the receptor for CGRP (RAMP1 and CLR) were expressed in a very small subset of SCPCs, suggesting that CGRP indirectly stimulates SCPC proliferation through paracrine mechanisms. This discovery may have important implications for injury/repair mechanisms in the CF airway.