Pitx2 modulates Fgf10 dosage to initiate asymmetric lung morphogenesis

Most of the visceral organs are anatomically asymmetric across the left-right axis. These asymmetries can be traced to a well-studied molecular cascade leading to left-sided gene expression, including Pitx2, in the mesoderm. Yet how these early differences in gene expression are converted into differential shaping of organs at later stages remains incompletely understood, and for many organs, such as the lung, the question has not even been explored. Meanwhile, the signaling pathways responsible for the morphogenesis of the lung have been intensively studied, but no insight has been reported regarding whether they should differ on the left and right sides. Here we identify Fgf10 as a Pitx2-sensitive signal in the mesenchyme of the developing mouse lung. Fgf10 expression increases as Pitx2 decreases, making the right lung, which lacks Pitx2 expression, grow faster than the left during the budding stage. Modulating Fgf10 dosage in the left mesenchyme is sufficient to alter lung budding asymmetry. At the cellular level, the faster growth of the right lung is established by increased levels of epithelial proliferation, without significant differences in directional migration into the mesenchyme. Conditional genetics further show that Pitx2 acts during the budding stage to establish later branching asymmetry. Thus, Pitx2 converts left-right mesenchymal identity into organ asymmetry by quantitatively tuning Fgf10-dependent epithelial growth during early organogenesis.