Fig. 4: Cell-autonomous requirement of Fgfr2 for AT2 fate selection in vivo.

a Alveolar region of lungs from Nkx2.1-Cre; Rosa26-mTmG; Fgfr2^fl/+ (upper panels) or Nkx2.1-Cre; Rosa26-mTmG; Fgfr2^fl/delta (lower) mice at postnatal day 0 with Cre expressed in developing lung epithelial cells to delete conditional Fgfr2 allele (Fgfr2^fl) and activate a farnesylated GFP protein reporter, which targets all membranes including cytoplasmic vesicles (mTmG, green). Lungs were immunostained for GFP, E-cadherin (E-cad), and AT2 marker Muc1 as indicated. Note mosaic GFP expression showing Cre is active in some but not all alveolar epithelial cells, and that Fgfr2 is required cell autonomously for AT2 cell development because GFP^pos control cells carrying wild-type Fgfr2 allele (Fgfr2^fl/+, upper panels) became either cuboidal Muc1^pos AT2 cells (arrowheads) or squamous AT1 cells, whereas GFP^pos cells lacking Fgfr2 (Fgfr2^fl/delta, lower panels) almost exclusively became AT1 cells. Bar, 50 µm. b Quantification of a. n number of GFP^pos cells scored in three lungs. **p = 0.0022 (Student’s two-sided t-test, data as mean ± SD). c Close-up of a rare GFP^pos AT2 cell (boxed) from Nkx2.1-Cre; Rosa26-mTmG; Fgfr2^fl/delta lung, as in bottom panels of a, stained for Fgfr2 and markers indicated. Note that Fgfr2 has not yet been lost from the GFP^pos cell, implying the recent deletion of conditional allele Fgfr2^fl and perdurance of Fgfr2 that promoted AT2 fate selection. Bar, 10 µm. Stain repeated in biological triplicate. All experiments were repeated at least three times.