Foxp1, Foxp2 and Foxp4 transcription factors are conserved transcriptional repressors expressed in overlapping patterns in the epithelium and mesenchymal derivatives of the developing foregut

INTRODUCTION The tracheal and bronchiolar airways of mammals are essential for providing defense against inhaled pathogens and external environmental insults, such as cigarette smoke and pollution. The pseudostratified epithelium lining the trachea and proximal bronchiolar airways is composed of multiple cell lineages, including secretory and ciliated epithelium as well as basal cells, which act as progenitors for these lineages (reviewed by Morrisey and Hogan, 2010). The molecular pathways controlling cell fate and the precise allocation of epithelial lineages during both lung development and lung regeneration are poorly understood. Such insight is crucial for developing better therapeutic options for lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). The proximal airways of the respiratory system are lined with several differentiated cell lineages, including secretory Clara cells, ciliated epithelial cells, neuroendocrine cells and, in large mammals such as humans, goblet cells. Goblet cells are large morphologically distinct cells containing large vacuoles needed for the secretion of mucus in response to airway irritation or injury. Goblet cells are thought to differentiate from other secretory lineages in the proximal airways such as Clara secretory cells (Park et al., 2007; Chen et al., 2009). In normal proximal airways of mammals, only a small number of goblet cells are found and in mice this lineage is absent unless an injury or disease model is provoked. However, in diseases such as asthma and COPD, an increased numbers of goblet cells generate large amounts of mucus secretions in response to airway sensitization and to the chronic injury/repair cycle indicative of these diseases (reviewed by Fahy, 2001; Rogers, 2003; Tagaya and Tamaoki, 2007). The molecular pathways that regulate the cell fate of proximal airway epithelium, which generates the proper composition of the proximal airways, are poorly understood. However, previous studies have demonstrated important roles for the ETS-related transcription factor Spdef and the Notch signaling pathway (Park et al., 2007; Chen et al., 2009; Guseh et al., 2009; Tsao et al., 2009; Rock et al., 2011). Despite these findings, little is understood about how cell fate decisions are regulated in the developing and regenerating airway epithelium and whether there are factors that actively restrict cell lineage differentiation versus those that promote specific cell fates, such as Spdef (Park et al., 2007; Chen et al., 2009). Foxp1, Foxp2 and Foxp4 transcription factors are conserved transcriptional repressors expressed in overlapping patterns in the epithelium and mesenchymal derivatives of the developing foregut (Shu et al., 2001; Lu et al., 2002; Shu et al., 2007). Foxp1/2/4 are each expressed at high levels during lung development as well as in the adult lung with Foxp1/4 being the predominant paralogs expressed in proximal bronchiolar and tracheal epithelium (Lu et al., 2002). Previous studies have shown that Foxp1/2 cooperatively regulate distal lung epithelial development as well as esophageal development (Shu et al., 2007). Foxp2–/–:Foxp1+/– mutants express lower levels of the key transcription factors N-myc (Mycn – Mouse Genome Informatics) and Hopx, leading to perinatal demise (Shu et al., 2007). However, the role of Foxp1/2/4 in development and homeostasis of the epithelium in the proximal airways of the respiratory system was unknown. In this article, we show that conditional deletion of Foxp1/4 in developing lung epithelium leads to decreased secretory cell differentiation, in particular loss of Clara cells, but ectopic activation of the goblet cell program, thus indicating a key role for 1Department of Medicine, 2Department of Cell and Developmental Biology, 3Institute for Regenerative Medicine, 4Cardiovascular Institute, University of Pennsylvania, Philadelphia, PA 19104, USA. 5Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, TX 77030, USA. 6Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, USA.