A Genome-Wide Transcription Factor Atlas of Mouse Lung Development

At its inception, our work was directed at understanding the transcriptional mechanisms used to construct the lung during embryogenesis. A genome-wide transcription factor screen was performed with greater than 1100 transcription factors to identify lung progenitor cells by their discrete and specific patterns of transcription factor expression. To our surprise, embryonic progenitor cell populations within the developing lung are identified by a small number of stereotyped patterns of transcription factor expression. These patterns establish a “molecular atlas” for identifying specific lung progenitor cells. The recognizable progenitors include undifferentiated lung bud tip epithelial progenitors and putative progenitor cells for the mesenchyme, vasculature, and smooth muscle. After establishing the discrete nature of these putative progenitor cell domains, we chose to examine signaling pathways that are specifically activated in each of the domains.

Anatomically distinct domains of transcription factor expression identified in a screen of over 1100 known transcription factors on embryonic lung tissue 

The Identification of Wnt7b as a Lung Size Regulator

Wnt7b is expressed in the lung bud tip, which is thought to harbor the epithelial progenitor cells. Since Wnts are known to regulate stem cell proliferation, we tested the hypothesis that Wnt7b regulates tip epithelial progenitor cells by conditionally deleting Wnt7b from mouse lungs. This resulted in mice with small-sized lungs that otherwise possessed normal differentiation. This pattern of proportionate small size is exceptionally rare among mutant hypoplastic lungs. We showed that mechanistically, Wnt7b coordinately regulates progenitor cell replication in the embryonic lung epithelium and mesenchyme. This coordinate regulation of progenitor cell populations permits proportionate changes in embryonic lung size. This mechanistic insight now forms a basis for exploring the role of Wnts in adult lung diseases and epithelial regeneration. Indeed, Wnts are already known to be aberrantly activated in lung cancer and interstitial lung diseases. Furthermore, Wnt7b is now a candidate growth factor for supporting the replication of lung stem cells ex vivo.

 Expression of Wnt-Target gene axin2 in embryonic mouse lung, identified by whole mount in situ hybridization

Notch and Mucous Metaplasia

We have also explored the developmental cascades that regulate lung cell fate. We demonstrated that Notch target genes are expressed in the airway epithelium, but not in the alveolus. Our results show that Notch prevents the differentiation of lung progenitor cells into alveoli. Indeed, the misexpression of Notch traps lung progenitor cells in a “pre-alveolar” airway state. Although the airway itself is appropriately formed during Notch misexpression, the number and type of airway epithelial cells is altered. In addition, Notch misexpression leads to mucous metaplasia, the single defining pathology of chronic human airways disease. Interestingly, we show that the Notch pathway is also reutilized in adult tissue turnover and during tracheal regeneration. In these cases, Notch modulates cell fates in adult tracheal epithelium in a manner identical to its role in embryogenesis. Indeed, pharmacologic inhibition of Notch blocks the induction of mucous metaplasia by factors already known to induce mucous metaplasia in human disease. These studies demonstrate that knowledge of developmental mechanisms can serve as a foundation for the identification of therapeutic targets that modulate adult lung disease. Future work will directly explore the role of Notch signaling in human mucous metaplasia and the clinical utility of inhaled Notch antagonists in ameliorating airways disease.

Expression of Notch-target gene Hes1 in embryonic mouse lung is restricted to epithelial cells proximal to the airway-alveolar junction 

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