PROJECT SUMMARY Idiopathic pulmonary fibrosis (IPF) is a fatal fibrotic lung disease that is incurable and progressive due to fibroblast activation, and the formation of scar tissue. Approximately 130,000 Americans suffer from IPF, with an estimated 50,000 new cases diagnosed each year. Although it is well accepted that myofibroblast accumulation is a central component of pathogenesis in IPF, the transcriptional program(s) that orchestrate fibroblast activation including fibroblast-to-myofibroblast transformation (FMT), survival, migration and ECM organization are poorly defined and represent a significant knowledge gap in the field. Sox9 is a member of the HMG-box family of transcription factors that are selectively expressed by epithelial progenitor cells to modulate branching morphogenesis in the lung and the organized deposition of collagen as part of cartilage formation in multiple organs. However, the role of Sox9 in fibroblast activation has been poorly studied in adult fibrotic lung diseases. Our new findings have determined that Sox9 is upregulated in lung mesenchymal cells of IPF and functions as a positive regulator of FMT, myofibroblast survival, migration and ECM production. The loss of Sox9 expression has attenuated αSMA expression and myofibroblast transformation in mesenchymal cells isolated from IPF lungs and TGFα model. In support, a recent published study suggest Sox9 upregulation in patients with chronic liver disease that correlated with fibrosis severity and progression towards cirrhosis. Taken together, these findings lead us to postulate that Sox9 functions as a positive regulator of FMT, myofibroblast survival, migration, and ECM in the pathogenesis of pulmonary fibrosis. For this study, we propose three specific aims: 1) determine mechanisms by which Sox9 induces fibroblast activation; 2) establish in vivo the role of Sox9-expressing mesenchymal cells in the pathogenesis of pulmonary fibrosis; and 3) identify mechanisms by which Sox9 augments myofibroblast survival and the progression of established and ongoing pulmonary fibrosis. We will use advanced molecular methods and mouse transgenic approaches, coupled with detailed biochemical analysis of these Sox9-driven processes in vivo and in vitro. Completion of the proposed experiments is likely to impart a significant understanding of Sox9-driven fibroblast activation. The multidisciplinary team will facilitate a timely approach with expertise in all aspects of lung pathology and support future translational studies in IPF. Our approach is innovative due to the generation of novel transgenic mice to test mesenchymal cell-specific functions of Sox9 in pulmonary fibrosis. The proposed research is significant in that completion of this study will increase an understanding of the mechanisms causing fibroblast activation in IPF, which in turn will lead to advanced medical therapies for the treatment and possible cure or prevention of this debilitating lung disease.