American Journal Of Respiratory Cell And Molecular Biology
The alveolar epithelium consists of squamous alveolar type (AT)I and cuboidal ATII cells. ATI cells cover 95-98% of the alveolar surface, thereby playing a critical role in barrier integrity, and are extremely thin, thus permitting efficient gas exchange. During lung injury, ATI cells die, resulting in increased epithelial permeability. ATII cells reepithelialize the alveolar surface via proliferation and transdifferentiation into ATI cells. Transdifferentiation is characterized by downregulation of ATII cell markers, upregulation of ATI cell markers, and cell spreading resulting in a change in morphology from cuboidal to squamous, thus restoring normal alveolar architecture and function. The mechanisms underlying ATII to ATI cell transdifferentiation have not been studied in vivo. A prerequisite for mechanistic investigation is a rigorous, unbiased method to quantitate this process. Here, we used SPCCreERT2;mTmG mice, in which ATII cells and their progeny express GFP, and applied stereologic techniques to measure transdifferentiation during repair after injury induced by LPS. Transdifferentiation was quantitated as the percent of alveolar surface area covered by ATII-derived (GFP+) cells expressing ATI but not ATII cell markers. Using this methodology, the time course and magnitude of transdifferentiation during repair was determined. We found that ATI cell loss and epithelial permeability occurred by Day 4, and ATII to ATI cell transdifferentiation begins by Day 7 and continues until Day 16. Notably, transdifferentiation and barrier restoration are temporally correlated. This methodology can be applied to investigate the molecular mechanisms underlying transdifferentiation, ultimately revealing novel therapeutic targets to accelerate repair after lung injury.