Patterning the embryonic pulmonary mesenchyme

Biophysics of morphogenesis in the vertebrate lung

Morphogenesis is a physical process that sculpts the final functional forms of tissues and organs. Remarkably, the lungs of terrestrial vertebrates vary dramatically in form across species, despite providing the same function of transporting oxygen and carbon dioxide. These divergent forms arise from distinct physical processes through which the epithelium of the embryonic lung responds to the mechanical properties of its surrounding mesenchymal microenvironment. Here we compare the physical processes that guide folding of the lung epithelium in mammals, birds, and reptiles, and suggest a conceptual framework that reconciles how conserved molecular signaling generates divergent mechanical forces across these species.

Vangl-dependent mesenchymal thinning shapes the distal lung during murine sacculation

The planar cell polarity (PCP) complex is speculated to function in murine lung development, where branching morphogenesis generates an epithelial tree whose distal tips expand dramatically during sacculation. Here, we show that PCP is dispensable in the airway epithelium for sacculation. Rather, we find a Celsr1-independent role for the PCP component Vangl in the pulmonary mesenchyme: loss of Vangl1/2 inhibits mesenchymal thinning and expansion of the saccular epithelium. Further, loss of mesenchymal Wnt5a mimics sacculation defects observed in Vangl2-mutant lungs, implicating mesenchymal Wnt5a/Vangl signaling as a key regulator of late lung morphogenesis. A computational model predicts that sacculation requires a fluid mesenchymal compartment. Lineage-tracing and cell-shape analyses are consistent with the mesenchyme acting as a fluid tissue, suggesting that loss of Vangl1/2 impacts the ability of mesenchymal cells to exchange neighbors. Our data thus identify an explicit function for Vangl and the pulmonary mesenchyme in actively shaping the saccular epithelium.

Modulation of canonical Wnt signaling regulates peribiliary mesenchymal identity during homeostasis and injury

BACKGROUND

The matrix and associated mesenchyme of the extrahepatic bile ducts are distinct, which could drive diseases with a predilection for these ducts, such as primary sclerosing cholangitis. We aimed to understand the molecular drivers of peribiliary mesenchymal cell (PMC) identity in the extrahepatic bile ducts and dissect how this changed in the context of injury using an entirely in vivo approach with transcriptomic analysis.

METHODS AND RESULTS

Single-cell sequencing with a receptor-ligand analysis showed that PMCs had the most interactions with surrounding cells. Wnt4, Wnt5a, and Wnt7b were identified as the major ligands secreted from PMCs and cholangiocytes that interacted in both paracrine and autocrine fashion. Bile duct ligation caused an increase in all 3 Wingless/Integrated ligands and Axin2 with an associated increase in the transcription factors T-box transcription factor (Tbx)2 and Tbx3. Conversely, Indian hedgehog secretion decreased without an associated decrease in hedgehog signaling effectors. Loss of smoothened within PMCs did not impact hedgehog signaling effectors or cellular identity, whereas smoothened gain of function led to myofibroblast transdifferentiation with upregulation of Tbx2 and Tbx3 without injury. Loss of β-catenin caused a decrease in expression of all 3 Gli transcription factors and associated mesenchymal gene expression, which was phenocopied with compound Gli2 and Gli3 loss in uninjured PMCs. With injury, loss of β-catenin resulted in decreased myofibroblast transdifferentiation with reduced Tbx2 and Tbx3 expression.

CONCLUSIONS

Our results show how modulation of canonical Wingless/Integrated signaling in PMCs is important for regulating basal mesenchymal gene expression and initiating a myogenic gene transcriptional program during injury. They also highlight reciprocating interactions between the hedgehog and Wingless/Integrated signaling pathways within PMCs.

Spatial patterning of energy metabolism during tissue morphogenesis

Biophysical signaling organizes forces to drive tissue morphogenesis, a process co-opted during disease progression. The systematic buildup of forces at the tissue scale is energetically demanding. Just as mechanical forces, gene expression, and concentrations of morphogens vary spatially across a developing tissue, there might similarly be spatial variations in energy consumption. Recent studies have started to uncover the connections between spatial patterns of mechanical forces and spatial patterns of energy metabolism. Here, we define and review the concept of energy metabolism during tissue morphogenesis. We highlight experiments showing spatial variations in energy metabolism across several model systems, categorized by morphogenetic motif, including convergent extension, branching, and migration. Finally, we discuss approaches to further enable quantitative measurements of energy production and consumption during morphogenesis.

Organ function is preserved despite reorganization of niche architecture in the hair follicle

The ability of stem cells to build and replenish tissues depends on support from their niche. Although niche architecture varies across organs, its functional importance is unclear. During hair follicle growth, multipotent epithelial progenitors build hair via crosstalk with their remodeling fibroblast niche, the dermal papilla, providing a powerful model to functionally interrogate niche architecture. Through mouse intravital imaging, we show that dermal papilla fibroblasts remodel individually and collectively to form a morphologically polarized, structurally robust niche. Asymmetric TGF-β signaling precedes morphological niche polarity, and loss of TGF-β signaling in dermal papilla fibroblasts leads them to progressively lose their stereotypic architecture, instead surrounding the epithelium. The reorganized niche induces the redistribution of multipotent progenitors but nevertheless supports their proliferation and differentiation. However, the differentiated lineages and hairs produced by progenitors are shorter. Overall, our results reveal that niche architecture optimizes organ efficiency but is not absolutely essential for organ function.

Mesenchymal Vangl facilitates airway elongation and widening independently of the planar cell polarity complex

A hallmark of mammalian lungs is the fractal nature of the bronchial tree. In the adult, 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. Achieving this pattern likely requires precise control of airway length and diameter, as the branches of the embryonic airways initially lack the fractal scaling observed in those of the adult lung. In epithelial monolayers and tubes, directional growth can be regulated by the planar cell polarity (PCP) complex. 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 branching morphogenesis proceeds independently of PCP-component expression 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. At the cellular level, we observe changes in the shape of smooth muscle cells that indicate a potential defect in collective mesenchymal rearrangements, which we hypothesize are necessary for lung morphogenesis. Our data thus reveal an explicit function for Vangl that is independent of the core PCP complex, suggesting a functional diversification of PCP components in vertebrate development. These data also reveal an essential role for the embryonic mesenchyme in generating the fractal structure of airways of the mature lung.

Single-cell RNA sequencing reveals the developmental program underlying proximal-distal patterning of the human lung at the embryonic stage

The lung is the primary respiratory organ in human, in which the proximal airway and the distal alveoli are responsible for air conduction and gas exchange, respectively. However, the regulation of proximal-distal patterning at the embryonic stage of human lung development is largely unknown. Here we investigated the early lung development of human embryos at weeks 4-8 post fertilization (Carnegie stages 12-21) using single-cell RNA sequencing, and obtained a transcriptomic atlas of 169,686 cells. We observed discernible gene expression patterns of proximal and distal epithelia at week 4, upon the initiation of lung organogenesis. Moreover, we identified novel transcriptional regulators of the patterning of proximal (e.g., THRB and EGR3) and distal (e.g., ETV1 and SOX6) epithelia. Further dissection revealed various stromal cell populations, including an early-embryonic BDNF⁺ population, providing a proximal-distal patterning niche with spatial specificity. In addition, we elucidated the cell fate bifurcation and maturation of airway and vascular smooth muscle progenitor cells at the early stage of lung development. Together, our study expands the scope of human lung developmental biology at early embryonic stages. The discovery of intrinsic transcriptional regulators and novel niche providers deepens the understanding of epithelial proximal-distal patterning in human lung development, opening up new avenues for regenerative medicine.

Analysis of Cre lines for targeting embryonic airway smooth muscle

During development of the embryonic mouse lung, the pulmonary mesenchyme differentiates into smooth muscle that wraps around the airway epithelium. Inhibiting smooth muscle differentiation leads to cystic airways, while enhancing it stunts epithelial branching. These findings support a conceptual model wherein the differentiation of smooth muscle sculpts the growing epithelium into branches at precise positions and with stereotyped morphologies. Unfortunately, most approaches to manipulate the differentiation of airway smooth muscle rely on pharmacological or physical perturbations that are conducted ex vivo. Here, we explored the use of diphtheria toxin-based genetic ablation strategies to eliminate airway smooth muscle in the embryonic mouse lung. Surprisingly, neither airway smooth muscle wrapping nor epithelial branching were affected in embryos in which the expression of diphtheria toxin or its receptor were driven by several different smooth muscle-specific Cre lines. Close examination of spatial patterns of Cre activity in the embryonic lung revealed that none of these commonly used Cre lines target embryonic airway smooth muscle robustly or specifically. Our findings demonstrate the need for airway smooth muscle-specific Cre lines that are active in the embryonic lung, and serve as a resource for researchers contemplating the use of these commonly used Cre lines for studying embryonic airway smooth muscle.

Plasticity in airway smooth muscle differentiation during mouse lung development

It has been proposed that smooth muscle differentiation may physically sculpt airway epithelial branches in mammalian lungs. Serum response factor (SRF) acts with its co-factor myocardin to activate the expression of contractile smooth muscle markers. In the adult, however, smooth muscle exhibits a variety of phenotypes beyond contractile, and these are independent of SRF/myocardin-induced transcription. To determine whether a similar phenotypic plasticity is exhibited during development, we deleted Srf from the mouse embryonic pulmonary mesenchyme. Srf-mutant lungs branch normally, and the mesenchyme displays mechanical properties indistinguishable from controls. scRNA-seq identified an Srf-null smooth muscle cluster, wrapping the airways of mutant lungs, which lacks contractile smooth muscle markers but retains many features of control smooth muscle. Srf-null embryonic airway smooth muscle exhibits a synthetic phenotype, compared with the contractile phenotype of mature wild-type airway smooth muscle. Our findings identify plasticity in embryonic airway smooth muscle and demonstrate that a synthetic smooth muscle layer promotes airway branching morphogenesis.

How to build an epithelial tree

Nature has evolved a variety of mechanisms to build epithelial trees of diverse architectures within different organs and across species. Epithelial trees are elaborated through branch initiation and extension, and their morphogenesis ends with branch termination. Each of these steps of the branching process can be driven by the actions of epithelial cells themselves (epithelial-intrinsic mechanisms) or by the cells of their surrounding tissues (epithelial-extrinsic mechanisms). Here, we describe examples of how these mechanisms drive each stage of branching morphogenesis, drawing primarily from studies of the lung, kidney, salivary gland, mammary gland, and pancreas, all of which contain epithelial trees that form through collective cell behaviors. Much of our understanding of epithelial branching comes from experiments using mice, but we also include examples here from avian and reptilian models. Throughout, we highlight how distinct mechanisms are employed in different organs and species to build epithelial trees. We also highlight how similar morphogenetic motifs are used to carry out conserved developmental programs or repurposed to support novel ones. Understanding the unique strategies used by nature to build branched epithelia from across the tree of life can help to inspire creative solutions to problems in tissue engineering and regenerative medicine.

Transmural pressure signals through retinoic acid to regulate lung branching

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.