Runx1 Promotes Myofibroblast Cell Fate during Development and Fibrosis

Alveologenesis is the final stage of lung maturation, during which alveolar saccules are subdivided into smaller alveoli through the process of secondary septation, responsible for generating ~95% of the gas exchange surface area (1). Secondary crest myofibroblasts (SCMFs), which are transiently present in the distal alveolar regions, are required for alveolar maturation due to their ability to contract (2). By the end of alveologenesis, SCMFs are eliminated through apoptosis (3). However, the molecular drivers for the initial SCMF cell fate specification remain poorly understood. Pulmonary fibrosis is characterized by aberrant activation of lung fibroblasts into fibrotic myofibroblast, which produce excessive extracellular matrix (ECM) among other hallmarks of the disease. In the bleomycin mouse model of pulmonary fibrosis, recent lineage tracing and single-cell RNA sequencing (scRNA-seq) analyses have shown that CTHRC1+ fibrotic myofibroblasts are primarily derived from alveolar fibroblasts (4). Although several signaling pathways, including TGFß, have been implicated in the transition of alveolar fibroblasts into fibrotic myofibroblasts (5), the downstream mediators, such as transcription factors (TFs), are not yet fully defined. Overall design: To identify mesenchymal cell type-specific TFs through motif enrichment in addition to gene expression, we performed single-cell ATAC-seq (scATAC-seq) to profile chromatin accessibility. Mesenchymal cells (EpCAM– CD45– CD31–) were sorted from postnatal day 8 (P8) mouse lungs and subjected to scATAC-seq.