Maintenance of classic stem cell hierarchies is dependent upon stem cell self-renewal mediated in part by Wnt/␤-catenin regulation of the cell cycle. This function is critical in rapidly renewing tissues due to the obligate role played by the tissue stem cell. However, the stem cell hierarchy responsible for maintenance of the conducting airway epithelium is distinct from classic stem cell hierarchies. The epithelium of conducting airways is maintained by transit-amplifying cells in the steady state; rare bronchiolar stem cells are activated to participate in epithelial repair only following depletion of transit-amplifying cells. Here, we investigate how signaling through ␤-catenin affects establishment and maintenance of the stem cell hierarchy within the slowly renewing epithelium of the lung. Conditional potentiation of ␤-catenin signaling in the embryonic lung results in amplification of airway stem cells through attenuated differentiation rather than augmented proliferation. Our data demonstrate that the differentiation-modulating activities of stabilized ␤-catenin account for expansion of tissue stem cells. STEM CELLS

Air spaces of the mammalian lung are lined by a specialized epithelium that is maintained by endogenous progenitor cells. Within bronchioles, the abundance and distribution of progenitor cells that contribute to epithelial homeostasis change as a function of maintenance versus repair. It is unclear whether functionally distinct progenitor pools or a single progenitor cell type maintain the epithelium and how the behavior is regulated in normal or disease states. To address these questions, we applied fractionation methods for the enrichment of distal airway progenitors. We show that bronchiolar progenitor cells can be subdivided into two functionally distinct populations that differ in their susceptibility to injury and contribution to repair. The proliferative capacity of these progenitors is confirmed in a novel in vitro assay. We show that both populations give rise to colonies with a similar dependence on stromal cell interactions and regulation by TGF-β. These findings provide additional insights into mechanisms of epithelial remodeling in the setting of chronic lung disease and offer hope that pharmacologic interventions may be developed to mitigate tissue remodeling.

Bronchiolar stem cells have been functionally defined in vivo on the basis of their resistance to chemical (naphthalene) injury, their infrequent proliferation relative to other progenitor cell types, and their coexpression of the airway and alveolar secretory cell markers Clara cell secretory protein and pro-surfactant protein C, respectively. Cell surface markers that have previously been used for their prospective isolation included Sca-1 and CD34. Using transgenic animal models associated with stem cell expansion, ablation, and lineage tracing, we demonstrate that CD34 pos cells do not belong to the airway epithelial lineage and that cell surface Sca-1 immunoreactivity does not distinguish between bronchiolar stem and facultative transitamplifying (Clara) cell populations. Furthermore, we show that high autofluorescence (AF high ) is a distinguishing characteristic of Clara cells allowing for the fractionation of AF low bronchiolar stem cells. On the basis of these data we show that the defining phenotype of the bronchiolar stem cell is CD45 neg CD31 neg CD34 neg Sca-l low AF low . This refinement in the definition of bronchiolar stem cells provides a critical tool by which to assess functional and molecular distinctions between bronchiolar stem cells and the more abundant pool of facultative transit-amplifying (Clara) cells.