Project Summary The majority of preterm births occur while the lungs are in their final embryonic developmental stage, sacculation. However, our understanding of how the delicate architecture of the distal lung develops during this stage is extremely sparse, limiting our capacity to develop therapeutic interventions for neonatal infants affected by respiratory diseases. The planar cell polarity (PCP) pathway has recently been shown to play a pivotal role in sacculation, a developmental process during which the epithelial surface area of distal airways expands while the mesenchyme between adjacent airways thins. In vertebrates, the PCP pathway regulates convergent-extension during key developmental processes such as gastrulation and neural tube closure. My preliminary data reveal that, although epithelial PCP is not required for lung morphogenesis, the core PCP gene Vangl2 is specifically required in the pulmonary mesenchyme to achieve normal sacculation. I hypothesize that Vangl2 regulates cytoskeletal machinery to drive mesenchymal thinning in a way that parallels how convergent-extension intercalations elongate the body axis. Confirming this hypothesis would support a model in which cell rearrangements in the pulmonary mesenchyme actively shape the distal lung during sacculation and would delineate a novel mesenchymal PCP pathway. To test this hypothesis, I will determine the cellular mechanisms by which Vangl2 promotes mesenchymal thinning during sacculation through a live- imaging approach using transgenic mice (Aim 1). I will then use a joint genetic and biochemical approach to map the molecular players through which Vangl2 drives this mesenchyme-specific process (Aim 2). Successfully completing these aims will deepen our understanding of how Vangl2 functions at both the cellular and molecular level to facilitate dramatic changes in mesenchyme morphology during lung development. Moreover, they will elucidate, for the first time, a specific mechanism for mesenchymal thinning during sacculation. This knowledge will not only inform future research into therapies that may enhance or supplement Vangl2 function to treat preterm infants born with severely underdeveloped lungs, but will also illuminate a developmental pathway that may prove useful in engineering lung tissues to treat additional respiratory conditions.