Transmural pressure signals through retinoic acid to regulate lung branching

Abstract

During development, the mammalian lung undergoes several rounds of branching, the rate of which is tuned by the relative pressure of the fluid within the lumen of the lung. We carried out bioinformatics analysis of RNA-sequencing of embryonic mouse lungs cultured under physiologic or sub-physiologic transmural pressure and identified transcription factor-binding motifs near genes whose expression changes in response to pressure. Surprisingly, we found retinoic acid (RA) receptor binding sites significantly overrepresented in the promoters and enhancers of pressure-responsive genes. Consistently, increasing transmural pressure activates RA signaling, and pharmacologically inhibiting RA signaling decreases airway epithelial branching and smooth muscle wrapping. We found that pressure activates RA signaling through the mechanosensor Yap. A computational model predicts that mechanical signaling through Yap and RA affects lung branching by altering the balance between epithelial proliferation and smooth muscle wrapping, which we test experimentally. Our results reveal that transmural pressure signals through RA to balance the relative rates of epithelial growth and smooth muscle differentiation in the developing mouse lung and identify RA as a previously unreported component in the mechanotransduction machinery of embryonic tissues.

Focal sources of FGF-10 promote the buckling morphogenesis of the embryonic airway epithelium

“…lateral branches, bifurcations) are affected differently by changes in luminal pressure, or that transmural pressure somehow regulates smooth muscle differentiation, which then constrains any FGF-10-mediated buckling. In support of this second possibility, recent work has shown that luminal fluid pressure can impact smooth muscle wrapping in embryonic mouse lungs via changes in retinoic acid signaling ( Jaslove et al, 2022 ). Future experiments, which use more direct means to modulate fluid pressure, will be needed to investigate the possibility of species-specific differences in the role of transmural pressure during lung development.…”

“…Several recent studies, however, have shown that mechanical forces can also influence airway branching ( Varner and Nelson, 2014 ; Varner et al, 2015 ; Goodwin and Nelson, 2020 ; Nelson and Gleghorn, 2012 ; Warburton et al, 2010 ; Hogan, 2018 ; Tang et al, 2018 ). Microfluidic control of luminal fluid pressure, for instance, alters rates of branching morphogenesis in cultured embryonic mouse lungs ( Nelson et al, 2017 ), perhaps by modulating levels of FGF-10 expression ( Stanton et al, 2021 ) or retinoic acid signaling ( Jaslove et al, 2022 ). But it is still unclear how the cellular behaviors downstream of FGF-10 act in concert with exogeneous mechanical forces to sculpt new epithelial branches.…”

Exploiting mechanisms for hierarchical branching structure of lung airway

“…Mouse lungs genetically deficient in epithelial YAP, a mediator of Hippo signaling, have been shown to become cystic without branching, supporting the crucial role of mechanical signaling during lung development (55). YAP is also required for the differentiation of smooth muscle covering the epithelium, depending on luminal pressure (56).…”

“…Smooth muscle that wraps the lung epithelium is thought to mediate duct diameter (57) and promote tip splitting by sculpting the cleft in the epithelial bud tip (58), which is still controversial (56,57). The active involvement of smooth muscle in morphogenesis has not been demonstrated in organs with randomly branched elongated serpentine ducts, such as the pancreas, mammary gland, and thyroid gland (59)(60)(61), suggesting that the regularly branching structure from the widened lumen is characteristic of the airways sculpted by smooth muscle differentiation.…”

Plasticity in airway smooth muscle differentiation during mouse lung development

“…Disrupting the smooth muscle differentiation program leads to defective branching morphogenesis of the epithelium, with a loss of smooth muscle differentiation associated with cystic branches and a gain associated with stunted branches (He et al, 2017, Boucherat et al, 2015, Yi et al, 2009, Goodwin et al, 2019, Goodwin et al, 2022, Jaslove et al, 2022, Kim et al, 2015). These observations have led to a conceptual model wherein the pulmonary mesenchyme sculpts the morphology and positions of branches that form in the growing airway epithelium (Jaslove and Nelson, 2018).…”