Fig. 7: Robust regeneration following focal AT2 loss.

a, b Alveolar regions of adult control LyzM-Cre;Rosa26-mTmG;Fgfr2^fl/+ (a) and AT2 conditional deletion LyzM-Cre;Rosa26-mTmG;Fgfr2^fl/fl (b) mice immunostained as indicated. Note loss in b of lineage-labeled AT2 but preserved alveolar architecture. Mac2, macrophage marker. Bars, 50 µm. c Quantification of a, b. AT2 density is maintained in Fgfr2^fl/fl deletion by the compensatory expansion of unlabeled AT2 (with unrecombined Fgfr^fl alleles). n = 300 cells scored/condition (six animals, mean ± SD); p = 0.65 (not significant, Student’s two-sided t-test) for density difference. d AT2 proliferation in control adult Sftpc-CreER;Rosa26-mTmG;Fgfr2^fl/+ and AT2 conditional Fgfr2 deletion Sftpc-CreER;Rosa26-mTmG;Fgfr2^fl/delta by EdU labeling for 2 weeks following tamoxifen. n = 400 AT2 cells scored/condition (six lungs, mean ± SD). ***p = 5.4 × 10⁻⁶ (Student’s two-sided t-test). e Alveolar region of adult Sftpc-CreER;Rosa26-mTmG tamoxifen-induced to label AT2 and instilled with Fgfr inhibitor FIIN-1 then stained 48 h later for Ki67 and markers indicated. Arrowheads, proliferating AT2 (Ki67^pos,GFP^pos). Bar, 50 µm. f Quantification of e. n = 800 AT2 cells scored/condition (six animals, mean ± SD). **p = 2.6 × 10⁻³ (Student’s two-sided t-test). g Time course of AT2 apoptosis (cCasp3^pos), compensatory AT2 proliferation (Ki67^pos), and conversion to AT1 (Pdpn^pos) after FIIN-1 instillation in adult Sftpc^CreER/+;Rosa26^mTmG/+ lungs (see Fig. 6k–n) and subsequent staining (AT2 lineage mGFP, cCasp3, Ki67, Pdpn). mGFP^pos (AT2 lineage) cells were scored in instilled (WGA^pos) regions; uninstilled regions and vehicle-instilled lungs showed no changes. n = 300 mGFP^pos cells scored per stain/timepoint (21 animals, mean ± SD). h Vehicle and FIIN-1-treated alveolar regions 2 weeks after instillation. Note clusters of lineage-labeled AT2 (asterisks) and conversion to AT1 (arrowheads) in FIIN-1 lung, and overall restoration of the alveolar structure. Bar, 100 µm. Stain repeated in biological triplicate. i Sequential roles of Fgf/Fgfr2 pathway in alveolus: AT2 selection and 2^o induction of AT1 (embryo), AT2 fate consolidation and prevention of reprogramming to AT1 (juvenile), and AT2 survival (adult, mediated by Akt). AT2 loss triggers regeneration by AT2^stem. j Source and dynamics of FGF signaling. After AT2 selection in development (left), a 2° signal induces other progenitors to AT1 fate. FGF signaling remains active in adults, maintaining AT2 (middle). Loss of FGF in the stressed alveolus (right) triggers AT2 apoptosis and rapid replacement. k FGF signaling in adults might prevent AT2 overgrowth/tumors by depriving daughters (dashed) moving away from the FGF source of survival signal. All experiments were repeated at least three times.