In the adult, lung AT2 cells both self-renew and serve as progenitors for AT1 cells during normal maintenance and following injury. Understanding mechanisms that regulate normal AT2 to AT1 cell differentiation is key to elucidating mechanisms of repair following injury. We have developed methods for isolation of rat, mouse and human AT2 cells (and more recently AT1 cells) and in vitro models in which AT2 to AT1 cell differentiation can be modulated. We demonstrated that AT2 cells in primary culture undergo transition to an AT1 cell phenotype and that both exogenous soluble factors and changes in cell shape induce AT1-like cells to revert to an AT2 cell phenotype, indicating both active regulation of AEC phenotype and phenotypic plasticity. Ongoing projects include:
The alveolar epithelium lining the distal airspaces of the lung is comprised of type I (ATI) and type II (AT2 cells) labeled by Nkx2.1 (green) and aquaporin-5 (AQP5, red), respectively
Freshly isolated AT1 cells are labeled with aquaporin-5 (green).
Nuclei are labeled with propidium iodide
Idiopathic pulmonary fibrosis (IPF) is a fatal disease of unknown etiology characterized by progressive fibrosis leading to lung destruction. AT2 cells in lungs of IPF patients are morphologically abnormal, demonstrating a hyperplastic phenotype with increased proliferation and impaired differentiation to AT1 cell phenotype. We were among the first to show that AEC in IPF express mesenchymal markers, consistent with EMT, suggesting that the epithelium in IPF is ‘reprogrammed’ perhaps as a result of chronic injury. We further showed that epithelial abnormalities in IPF are abrogated by treatment with the selective inhibitor of catenin/CBP interactions, ICG-001 (which normalizes epithelial differentiation), and that interactions between catenin and TGF mediate EMT. These findings were key to development of a new paradigm supporting a central role for epithelial abnormalities in lung fibrosis that may be amenable to combinatorial treatment with inhibitors of catenin and TGF signaling. Ongoing projects include:
Willis eta/., Am. J. Pathol. 166:1321-32, 2005
Co-localization of myofibroblast (a-SMA) and AEC proteins (Nkx2.1 and pro-SP-B) in AEC in IPF lung tissue
Claudins are a family of integral tight junction (TJ) proteins that regulate paracellular permeability to ions and solutes. We recently generated constitutive Cldn18 knockout (KO) mice that demonstrate increased solute permeability and alveolar fluid clearance (AFC) with normal survival compared to wild type (WT) controls. Increased AFC in Cldn18 mice was associated with increased β-adrenergic signaling, with activation of cystic fibrosis transmembrane conductance regulator (CFTR), higher epithelial sodium channel (ENaC) and Na-K-ATPase (Na pump) activity, and increased Na-K-ATPase β1 subunit expression. CLDN18 KO AEC monolayers exhibited lower transepithelial resistance (Rt) and increased solute and ion permeability with unchanged ion selectivity, demonstrating a crucial role for CLDN18 in alveolar epithelial TJ composition and permeability properties. CLDN4 has been reported to function as an epithelial paracellular Na transport barrier. Unexpectedly Cldn4 KO mice exhibited a generally normal phenotype although Cldn4 KO mice showed increased susceptibility to ventilator-induced or hyperoxic lung injury under these conditions.