The lungs are divided, both structurally and functionally, into two distinct components, the proximal airways, which conduct air, and the peripheral airways, which mediate gas exchange. The mechanisms that control the specification of these two structures during lung development are currently unknown. Here we show that ␤-catenin signaling is required for the formation of the distal, but not the proximal, airways. When the gene for ␤-catenin was conditionally excised in epithelial cells of the developing mouse lung prior to embryonic day 14.5, the proximal lung tubules grew and differentiated appropriately. The mice, however, died at birth because of respiratory failure. Analysis of the lungs by in situ hybridization and immunohistochemistry, using molecular markers of the epithelial and mesenchymal components of both proximal and peripheral airways, showed that the lungs were composed primarily of proximal airways. These observations establish, for the first time, both the sites and timing of specification of the proximal and peripheral airways in the developing lung, and that ␤-catenin is one of the essential components of this specification.Lung morphogenesis depends upon precise regulation of reciprocal interactions between the endodermally derived respiratory epithelium and the surrounding lung mesenchyme. The primordial lung buds, derived from the foregut endoderm, invade the splanchnic mesenchyme at approximately embryonic (E) 1 9 to E9.5 in the developing mouse embryo. During the embryonic stage of development (E9.5 to E11.0), the buds undergo stereotypic branching to form the main stem and lobar bronchi. Extensive branching and budding of the airways continues throughout the pseudoglandular stage (E11.5 to E16.5), during which the intrapulmonary conducting airways and peripheral lung are formed. With advancing gestation, cytodifferentiation of distinct respiratory epithelial cell types occurs, producing the various cells lining the conducting (basal, ciliated, non-ciliated columnar, and neuroendocrine cells) and peripheral (alveolar Type I and Type II cells) airways. During the canalicular and saccular stages of lung development (E16.5 to E17.5 and E17.5 to postnatal day 4, respectively), the acinar tubules dilate into terminal alveolar saccules and the mesenchyme thins in association with formation of an extensive capillary network, forming the gas exchange region required for respiration after birth (1). Thus, formation of the lung is dependent upon precise temporal and spatial control of cell proliferation, migration, and differentiation, processes that are mediated by complex reciprocal interactions between cell types.

An activated form of beta-catenin [Catnb(Delta(ex3))] was expressed in respiratory epithelial cells of the developing lung. Although morphogenesis was not altered at birth, air space enlargement and epithelial cell dysplasia were observed in the early postnatal period and persisted into adulthood. The Catnb(Delta(ex3)) protein caused squamous, cuboidal, and goblet cell dysplasia in intrapulmonary conducting airways. Atypical epithelial cells that stained for surfactant pro protein C (pro-SP-C) and had morphological characteristics of alveolar type II cells were observed in bronchioles of the transgenic mice. Catnb(Delta(ex3)) inhibited expression of Foxa2 and caused goblet cell hyperplasia associated with increased staining for mucins and the MUC5A/C protein. In vitro, both wild type and activated beta-catenin negatively regulated the expression of the Foxa2 promoter. Catnb(Delta(ex3)) also caused pulmonary tumors in adult mice. Activation of beta-catenin caused ectopic differentiation of alveolar type II-like cells in conducting airways, goblet cell hyperplasia, and air space enlargement, demonstrating a critical role for the Wnt/beta-catenin signal transduction pathway in the differentiation of the respiratory epithelium in the postnatal lung.