Summary Idiopathic pulmonary fibrosis (IPF) is a fatal disease characterized by progressive lung parenchymal scarring. IPF scars are caused by aberrant communication between injured alveolar epithelium and adjacent mesenchyme. Several pathways have been implicated in this communication but the complexity of their intersections has not been fully addressed. We propose that the Hedgehog (HH) and the Platelet-derived growth factor (PDGF) signaling pathways, both of relevance to fibrosis and aberrantly expressed in IPF, conspire to induce progressive scarring. Our goal is to define their interrelationship and to provide a framework for the design of combination therapy to manage this chronic disease for which current treatment options are limited. Our rationale for proposing that the HH and PDGF pathways intersect is based on their shared features and similar functions: i) both HH and PDGF signal from epithelium to mesenchyme; ii) both induce mesenchymal cell proliferation, differentiation and extracellular matrix deposition during lung development; iii) both target myofibroblasts expressing alpha smooth muscle actin (aSMA), a core feature of IPF scars; iv) embryonic overexpression of either pathway produces similar phenotypes; and v) HH inhibition reproduces the key phenotype of Pdgfa and Pdgfra knockout on myofibroblasts during secondary alveolar septum formation, namely simplified alveolar sacs and absent myofibroblasts, accompanied by decreased Pdgfra expression. These data, together with the presence of Gli binding sites in the Pdgfra and Acta2 (aSMA) promoters, indicate vital HH input into PDGFa signaling. The expansion of HH- and PDGFa- mesenchymal responding cells that generate aSMA+ alveolar septal myofibroblasts as well as the presence of aSMA+;Gli1+;PDGFRa+ myofibroblasts in postnatal lung, show these pathways stimulate the same cellular targets and give rise to a common lineage. In adult lung, these same lineages produce myofibroblasts in fibrotic scars, indicating the cellular recipient of HH and PDGFa pathway crosstalk essential for normal development participates in pathological fibrosis. Here we propose to delineate how HH and PDGFa signaling combine to instruct the myofibroblasts, the key cell-type in developing alveolar septa and in fibrosis scars. The central hypothesis is that specific mesenchymal cells integrate HH and PDGFa signaling, and these cells play a pivotal role in processes such as myofibroblast formation during lung development and injury. In Aim 1 we will delineate the molecular mechanisms underlying the HH–PDGF pathway interaction in vitro. In Aim 2 we will determine the role of HH–PDGF crosstalk during postnatal lung development. In Aim 3 we will establish the effect of the combined loss of HH and PDGF signaling in two disease-relevant lung injury models: (1) postnatal hyperoxia and (2) bleomycin-induced injury. This grant will establish the first evidence of HH-PDGFa crosstalk in lung development and lung injury/fi...