The primitive foregut is patterned in a manner that spatially promotes proper organ specification along the anterior-posterior foregut axis. However, the molecular pathways that specify foregut endoderm progenitors are poorly understood. We show that Wnt2/2b signaling is required to specify lung endoderm progenitors within the anterior foregut. Embryos lacking Wnt2/2b expression exhibit complete lung agenesis and do not express Nkx2.1, the earliest marker of the lung endoderm. In contrast, other foregut endoderm derived organs including the thyroid, liver, and pancreas are correctly specified in Wnt2/2b null animals. We show that this phenotype is recapitulated by an endoderm restricted deletion of β-catenin, demonstrating that Wnt2/2b signaling through the canonical Wnt pathway is required to specify lung endoderm progenitors within the foregut. Moreover, activation of canonical Wnt/β-catenin signaling results in reprogramming of esophagus and stomach endoderm to a lung endoderm progenitor fate. Together, these data reveal that canonical Wnt2/2b signaling is uniquely required for specification of lung endoderm progenitors in the developing foregut.

Epithelial organs including the lung are known to possess regenerative abilities through activation of endogenous stem cell populations but the molecular pathways regulating stem cell expansion and regeneration are not well understood. Here we show that Gata6 regulates the temporal appearance and number of bronchioalveolar stem cells (BASCs) in the lung leading to the precocious appearance of BASCs and concurrent loss in epithelial differentiation in Gata6 null lung epithelium. This expansion of BASCs is the result of a dramatic increase in canonical Wnt signaling in lung epithelium upon loss of Gata6. Expression of the non-canonical Wnt receptor Fzd2 is down-regulated in Gata6 mutants and increased Fzd2 or decreased (β-catenin expression rescues, in part, the lung epithelial defects in Gata6 mutants. During lung epithelial regeneration, we show that canonical Wnt signaling is activated in the niche containing BASCs and forced activation of Wnt signaling leads to a dramatic increase in BASC numbers. Moreover, Gata6 is required for proper lung epithelial regeneration and postnatal loss of Gata6 leads to increased BASC expansion and decreased differentiation. Together, these data demonstrate that Gata6 regulated Wnt signaling controls the balance between stem/ progenitor expansion and epithelial differentiation required for both lung development and regeneration.

Although the molecular pathways governing the development of the anterior pole of the heart have been the subject of intense investigation, little is understood about the molecular mechanisms underlying the morphogenesis of the posterior pole of the heart which generates the atria, pulmonary veins and portions of the atrio-ventricular canal. Here we show that Wnt2 is expressed specifically in the developing inflow tract mesoderm in a domain encompassing the dorsal mesocardium and dorsal mesenchymal protrusion which generates portions of the atria and atrio-ventricular cushions. Loss of Wnt2 results in defective development of the atrial myocardium, atrio-ventricular canal, and pulmonary veins resulting in a phenotype resembling the human congenital heart syndrome complete common atrio-ventricular canal. The dorsal mesocardium and dorsal mesenchymal protrusion overlaps spatially with posterior second heart field progenitors and we show that the number and proliferation of these progenitors is reduced in Wnt2-/- mutants. Remarkably, these defects can be rescued in vivo in a temporally restricted manner through pharmacological inhibition of Gsk-3β, indicating that Wnt2 regulates canonical Wnt signaling in the posterior cardiac mesoderm. Molecular and genetic analysis shows that Wnt2 works in a feed-forward transcriptional loop with Gata6 to regulate posterior cardiac development. These data reveal an important new molecular pathway regulating cardiac inflow tract development and demonstrates that such defects in the second heart field can be rescued pharmacologically in vivo.

Although mutations in Kras are present in 21% of lung tumors, there is a high level of heterogeneity in phenotype and outcome among patients with lung cancer bearing similar mutations, suggesting that other pathways are important. Wnt/β-catenin signaling is a known oncogenic pathway that plays a well-defined role in colon and skin cancer; however, its role in lung cancer is unclear. We have shown here that activation of Wnt/β-catenin in the bronchiolar epithelium of the adult mouse lung does not itself promote tumor development. However, concurrent activation of Wnt/β-catenin signaling and expression of a constitutively active Kras mutant (KrasG12D) led to a dramatic increase in both overall tumor number and size compared with KrasG12D alone. Activation of Wnt/β-catenin signaling altered the KrasG12D tumor phenotype, resulting in a phenotypic switch from bronchiolar epithelium to the highly proliferative distal progenitors found in the embryonic lung. This was associated with decreased E-cadherin expression at the cell surface, which may underlie the increased metastasis of tumors with active Wnt/β-catenin signaling. Together, these data suggest that activation of Wnt/β-catenin signaling can combine with other oncogenic pathways in lung epithelium to produce a more aggressive tumor phenotype by imposing an embryonic distal progenitor phenotype and by decreasing E-cadherin expression. IntroductionLung cancer is the second most common cancer in both men and women, accounting for approximately 15% of all newly diagnosed cancers (1, 2). Despite extensive investment of research dollars into developing new treatments for lung cancer, it remains responsible for 29% of cancer deaths in the United States, which is more than breast, colon, and prostate cancer combined. The overall 5-year survival rate is only 15.8%, making lung cancer one of the most deadly and difficult to diagnose cancers in humans.Although smoking is the primary cause of most lung cancer, approximately 10%-15% of lung cancers are found in nonsmokers, suggesting additional genetic causes of this disease. Extensive research into the underlying molecular insults leading to lung epithelial oncogenesis has resulted in the finding that Kras and EGFR mutations account for a large number of pulmonary cancers (3-7). However, there is significant heterogeneity among patients with similar mutations, suggesting the contribution of additional molecular pathways in the regulation of lung oncogenesis.Wnt/β-catenin signaling is a well-described oncogenic pathway that plays important roles in several cancers, including colon cancer, where it is the most common genetic cause of oncogenesis (8, 9). The role of Wnt/β-catenin signaling in lung cancer remains somewhat unclear. Mutations in β-catenin and adenomatous polyposis coli (APC), the most commonly mutated factors found in other cancers, are found only rarely in human lung tumors, and previous mouse models of Wnt/β-catenin activation in the postnatal

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