PROJECT SUMMARY Significance. Many lung diseases, including the acute respiratory distress syndrome, pulmonary fibrosis, and emphysema, result from a failure of the alveolar epithelium to regenerate normally after injury. Unfortunately, no therapies exist to promote lung regeneration, in large part because of our limited understanding of the underlying molecular mechanisms. Regeneration of the alveolar epithelium is orchestrated principally by alveolar type 2 epithelial cells (AEC2s). Surviving AEC2s proliferate to replace lost cells, after which proliferation halts and some AEC2s differentiate into AEC1s to restore normal alveolar architecture and gas exchange function. Historically, investigations of lung regeneration have explored the mechanisms of AEC2 proliferation. The molecular signals that induce a switch from the proliferation phase to the differentiation phase are poorly understood. Hypothesis. Based on our preliminary data, we hypothesize that during regeneration of the alveolar epithelium after lung injury, TGFβ halts AEC2 proliferation whereas deactivation of TGFβ induces BMP- dependent AEC2 to AEC1 differentiation. Research Plan. Aim 1 will test the hypothesis that TGFβ induces the termination of AEC2 cell proliferation during regeneration after lung injury. Aim 2 will test the hypothesis that TGFβ deactivation drives BMP-dependent differentiation during regeneration after lung injury. Mechanisms of epithelial-fibroblast crosstalk during alveolar regeneration will also be examined. Lung injury will be induced in AEC2- and fibroblast-specific gene deficient mice, and proliferation and differentiation will be quantitated using stringent stereologic techniques. In vivo studies will be directly linked to mechanistic experiments in rodent and human AEC2s grown in 2-dimentional and organoid culture. Conclusion. This work will enhance our understanding of fundamental mechanisms of lung regeneration. Specifically, we will investigate the molecular mechanisms underlying the termination of proliferation and initiation of differentiation, coordinated processes that are critical for restoration of normal alveolar structure and function after injury. These studies may identify novel therapeutic targets to accelerate epithelial regeneration during the pathogenesis of diverse lung diseases.