Heparan sulfate determines myofibroblast identity during alveolar morphogenesis (ATAC-seq)

Postnatal respiration requires the bulk formation of alveoli that produces extensive surface area for gas diffusion from epithelium to the circulatory system. Alveolar morphogenesis initiates at late gestation or postnatal stage during mammalian development and is mediated by coordination among epithelial, mesenchymal and endothelial cells. Here we show that stromal cells produce Heparan Sulfate Glycosaminoglycan (HS-GAG) to maintain a niche for alveolar development by modulating both the biophysical and biochemical cues. HS is predominantly enriched in myofibroblasts in postnatal murine lungs, and genetic ablation of HS synthase gene Ext1 in fibroblast populations results in enlarged and simplified alveolar structures. HS is selectively required for the cell identity of a subset of alveolar myofibroblasts (PDGFRαhi αSMA+) residing in the distal alveolar region, which exhibit contractile properties and maintain WNT signaling activity to support normal proliferation and differentiation of alveolar epithelial cells. HS is essential to prevent precocious apoptosis of alveolar myofibroblasts. We show that these processes are dependent upon FGF/MAPK signaling and forced activation of MAPK/ERK signaling partially corrected alveolar simplification along with restoration of alveolar myofibroblast identity and AT2 cell proliferation in HS deficient mice. These data reveal HS as an essential orchestrator for developing alveolar niche critical for generation of gas exchange units.