A CELSR-INDEPENDENT ROLE FOR MESENCHYMAL VANGL IN EMBRYONIC LUNG DEVELOPMENT

Abstract

A hallmark of mammalian lungs is the fractal nature of the bronchial tree. In adult lungs, each successive generation of airways is a fraction of the size of the parental branch. This fractal structure is physiologically beneficial, as it minimizes the energy needed for breathing. To achieve this fractal pattern, airway width and diameter must be tightly regulated but the processes that undergird the generation of the fractal bronchial tree are deeply understudied. In epithelial monolayers and tubes, directional growth can be regulated by the planar cell polarity (PCP) complex, which drives cellular and cytoskeletal organization throughout vertebrate development. Here, we comprehensively characterized the roles of PCP complex components in airway initiation, elongation, and widening during branching morphogenesis of the murine lung. Using tissue-specific knockout mice, we surprisingly found that the PCP complex is dispensable for branching morphogenesis in the developing airway epithelium. Instead, we found a novel, Celsr1-independent role for the PCP component Vangl in the pulmonary mesenchyme. Specifically, mesenchymal loss of Vangl1/2 leads to defects in branch initiation, elongation, and widening. We further assessed the role PCP is during lung sacculation, when distal epithelial tips expand dramatically in preparation for gas exchange after birth. Here, we use tissue-specific knockout mice to show that the PCP pathway is dispensable for sacculation in the developing airway epithelium. Rather, we find a novel, Celsr1-independent role for the PCP component Vangl in the pulmonary mesenchyme: loss of Vangl1/2 inhibits mesenchymal thinning and expansion of the epithelium. Further, loss of mesenchymal Wnt5a mimics the sacculation defects observed in Vangl2-mutant lungs, implicating mesenchymal Wnt5a/Vangl signaling as a key regulator of morphogenesis in the embryonic lung. By modeling sacculation in silico, we predict that mesenchymal cell motility is integral to normal sacculation. Finally, using a combination of lineage-tracing and cell shape analysis, we show that the pulmonary mesenchyme is a fluid and actively motile tissue and that loss of Vangl2 likely impacts motility. Our data thus reveal an explicit function for Vangl and the pulmonary mesenchyme in actively shaping the saccular epithelium during late lung morphogenesis.