ABSTRACT: Smooth muscle guides morphogenesis of epithelia during development of several organs, including the mammalian lung. However, it remains unclear how airway smooth-muscle differentiation is spatiotemporally patterned and whether it originates from distinct mesenchymal progenitors. Using single-cell RNA-sequencing of embryonic mouse lungs, we show that the pulmonary mesenchyme contains a continuum of cell identities, but no distinct progenitors. Transcriptional variability correlates with sub-epithelial and sub-mesothelial mesenchymal compartments that are regulated by Wnt signaling. Live-imaging and tension sensors reveal patterned migratory behaviors and cortical forces in each compartment, and show that sub-epithelial mesenchyme gives rise to airway smooth muscle. Differentiation trajectory reconstruction reveals that cytoskeleton, adhesion, and Wnt signaling pathways are activated early in differentiation. Finally, we show that Wnt activation stimulates the earliest stages of differentiation and induces local accumulation of mesenchymal F-actin, which influences epithelial morphology. Our work provides the first single-cell view of pulmonary mesenchymal patterning during branching morphogenesis.
Project description:Smooth muscle differentiation has been proposed to sculpt airway epithelial branches in mammalian lungs. Serum response factor (SRF) acts with its cofactor myocardin to promote the expression of contractile smooth muscle markers. However, smooth muscle cells exhibit a variety of phenotypes beyond contractile that are independent of SRF-myocardin-induced transcription. To determine whether airway smooth muscle exhibits phenotypic plasticity during embryonic development, we deleted Srf from the pulmonary mesenchyme. Srf-mutant lungs branch normally, and the mesenchyme exhibits normal cytoskeletal features and patterning. scRNA-seq revealed an Srf-null smooth muscle cluster wrapping the airways of mutant lungs that lacks contractile smooth muscle markers but retains many features of control smooth muscle. Srf-null airway smooth muscle exhibits a synthetic phenotype, compared to the contractile phenotype of wildtype airway smooth muscle. Our findings reveal plasticity in mesenchymal differentiation during lung development and demonstrate that a synthetic smooth muscle layer is sufficient for airway branching morphogenesis.
Project description:Wolffian duct maintenance and differentiation is predominantly driven by the androgen action, which is mediated by the androgen receptor (AR). It is well established that the mesenchyme indicates the fate and differentiation of epithelial cells. However, in vivo developmental requirement of mesenchymal AR in Wolffian duct development is still undefined. By designing a mesenchyme-specific Ar knockout (ARKO), we discovered that the loss of mesenchymal Ar led to the bilateral or unilateral degeneration of caudal Wolffian ducts and cystic formation at the cranial Wolffian ducts. Ex vivo culture of ARKO Wolffian ducts invariably resulted in bilateral defects, suggesting that some factor(s) originating from surrounding tissues in vivo might promote Wolffian duct survival and growth even in the absence of mesenchymal Ar. Mechanistically, we found cell proliferation was significantly reduced in both epithelial and mesenchymal compartments; but cell apoptosis was not affected. Transcriptomic analysis by RNA-seq revealed differentially expressed genes associated with morphological and cellular changes in ARKO male embryos (i.e. reduced cell proliferation and decreased number of epithelial cells). Mesenchymal differentiation into smooth muscle cells that are critical for morphogenesis was also impaired in ARKO male embryos. Taken together, our results demonstrate the crucial roles of the mesenchymal AR in Wolffian duct maintenance and morphogenesis in mice.
Project description:Most vertebrate organs are composed of epithelium surrounded by support and stromal tissues formed from mesenchyme cells, which are not generally thought to form organized progenitor pools. Here we use clonal cell labeling with multicolor reporters to characterize individual mesenchymal progenitors in the developing mouse lung. We observe a diversity of mesenchymal progenitor populations with different locations, movements, and lineage boundaries. Airway smooth muscle (ASM) progenitors map exclusively to mesenchyme ahead of budding airways. Progenitors recruited from these tip pools differentiate into ASM around airway stalks; flanking stalk mesenchyme can be induced to form an ASM niche by a lateral bud or by an airway tip plus focal Wnt signal. Thus, mesenchymal progenitors can be organized into localized and carefully controlled domains that rival epithelial progenitor niches in regulatory sophistication. Mesenchyme was microdissected from e11.5 and e12.5 lung branch tips and stalks and profiled directly. Another set of stalk dissections were cultured in either control media or media containing Wnt1. All experiments done in duplicate.
Project description:Diverse functions of the homeodomain transcription factor BARX1 include Wnt-dependent, non-cell autonomous specification of the stomach epithelium, tracheo-bronchial septation, and Wnt-independent expansion of the spleen primordium. Tight spatio-temporal regulation of Barx1 levels in the mesentery and stomach mesenchyme suggests additional roles. To determine these functions, we forced constitutive BARX1 expression in the Bapx1 expression domain, which includes the mesentery and intestinal mesenchyme, and also examined Barx1-/- embryos in further detail. Transgenic embryos invariably showed intestinal truncation and malrotation, in part reflecting abnormal left-right patterning. Ectopic BARX1 expression did not affect intestinal epithelium, but intestinal smooth muscle developed with features typical of the stomach wall. BARX1, which is normally restricted to the developing stomach, drives robust smooth muscle expansion in this organ by promoting proliferation of myogenic progenitors at the expense of other sub-epithelial cells. Undifferentiated embryonic stomach and intestinal mesenchyme showed modest differences in mRNA expression and BARX1 was sufficient to induce much of the stomach profile in intestinal cells. However, limited binding at cis-regulatory sites implies that BARX1 may act principally through other transcription factors. Genes expressed ectopically in BARX1+ intestinal mesenchyme and reduced in Barx1-/- stomach mesenchyme include Isl1, Pitx1, Six2 and Pitx2, transcription factors known to control left-right patterning and influence smooth muscle development. The sum of evidence suggests that potent BARX1 functions in intestinal rotation and stomach myogenesis occur through this small group of intermediary transcription factors. To investigate how Barx1 regulates gut smooth muscle development in a cell-autonomous manner, we used Affymetrix arrays to profile genes enriched in wild-type stomach and BARX1-overexpressing intestinal mesenchyme, compared to wild-type intestinal mesenchyme.
Project description:Mesenchymal-epithelial interactions play a critical role in organ development, stem cells and disease. During intestinal development, pseudostratified epithelia undergo dramatic morphogenesis called villification, to form finger-like projections, in which mesenchymal cell clustering and muscle layers play a key role. In the adult, the gut mesenchyme is proposed as a key intestinal stem cell niche providing essential niche signals such as Wnt ligands, while the TGF beta signaling mediated gut stromal program is critical for cancer progression. However, how these signals are produced is currently unknown. In the gut, Hedgehog (Hh) signaling acts strictly in a paracrine manner: Hh ligands are expressed in the epithelium and activate signaling exclusively in the mesenchyme. Notably, Hh signaling is not only essential for mesenchymal clustering and muscle differentiation, it is also involved in intestinal tumorigenesis. To investigate Hh mediated mechanisms, we analyzed mice deleted for key Hh negative regulators, Sufu and/or Spop in the gut mesenchyme, and demonstrated their dosage dependent role in the negative regulation of Hh signaling. Although these mutants exhibit abnormal mesenchymal cell growth and functionally defective muscle layers, villification is completed with proper mesenchymal clustering, implying a permissive role for Hh signaling. These mesenchymal defects are partially rescued by Gli2 reduction, demonstrating the significance of its transcriptional regulation. Surprisingly, in contrast to its known inhibitory role in epithelial proliferation, abnormal Hh activation in the gut mesenchyme leads to increased epithelial proliferation. Corroborating this data, Sufu reduction is sufficient to promote intestinal tumorigenesis, while Gli2 heterozygosity suppresses it. To define GLI2-mediated downstream mechanisms, we mapped its binding sites and analyzed gene expression genome-wide, identifying one of the most robust Hh direct targetome data sets ever reported. This work reveals the GLI2 transcriptional regulation of Wnt and TGF beta pathways in stem cell proliferation and muscle differentiation, providing mechanistic insight into the intestinal stem cell niche in development and tumorigenesis.
Project description:Mesenchymal-epithelial interactions play a critical role in organ development, stem cells and disease. During intestinal development, pseudostratified epithelia undergo dramatic morphogenesis called villification, to form finger-like projections, in which mesenchymal cell clustering and muscle layers play a key role. In the adult, the gut mesenchyme is proposed as a key intestinal stem cell niche providing essential niche signals such as Wnt ligands, while the TGF beta signaling mediated gut stromal program is critical for cancer progression. However, how these signals are produced is currently unknown. In the gut, Hedgehog (Hh) signaling acts strictly in a paracrine manner: Hh ligands are expressed in the epithelium and activate signaling exclusively in the mesenchyme. Notably, Hh signaling is not only essential for mesenchymal clustering and muscle differentiation, it is also involved in intestinal tumorigenesis. To investigate Hh mediated mechanisms, we analyzed mice deleted for key Hh negative regulators, Sufu and/or Spop in the gut mesenchyme, and demonstrated their dosage dependent role in the negative regulation of Hh signaling. Although these mutants exhibit abnormal mesenchymal cell growth and functionally defective muscle layers, villification is completed with proper mesenchymal clustering, implying a permissive role for Hh signaling. These mesenchymal defects are partially rescued by Gli2 reduction, demonstrating the significance of its transcriptional regulation. Surprisingly, in contrast to its known inhibitory role in epithelial proliferation, abnormal Hh activation in the gut mesenchyme leads to increased epithelial proliferation. Corroborating this data, Sufu reduction is sufficient to promote intestinal tumorigenesis, while Gli2 heterozygosity suppresses it. To define GLI2-mediated downstream mechanisms, we mapped its binding sites and analyzed gene expression genome-wide, identifying one of the most robust Hh direct targetome data sets ever reported. This work reveals the GLI2 transcriptional regulation of Wnt and TGF beta pathways in stem cell proliferation and muscle differentiation, providing mechanistic insight into the intestinal stem cell niche in development and tumorigenesis.
Project description:Most vertebrate organs are composed of epithelium surrounded by support and stromal tissues formed from mesenchyme cells, which are not generally thought to form organized progenitor pools. Here we use clonal cell labeling with multicolor reporters to characterize individual mesenchymal progenitors in the developing mouse lung. We observe a diversity of mesenchymal progenitor populations with different locations, movements, and lineage boundaries. Airway smooth muscle (ASM) progenitors map exclusively to mesenchyme ahead of budding airways. Progenitors recruited from these tip pools differentiate into ASM around airway stalks; flanking stalk mesenchyme can be induced to form an ASM niche by a lateral bud or by an airway tip plus focal Wnt signal. Thus, mesenchymal progenitors can be organized into localized and carefully controlled domains that rival epithelial progenitor niches in regulatory sophistication.
Project description:Diverse functions of the homeodomain transcription factor BARX1 include Wnt-dependent, non-cell autonomous specification of the stomach epithelium, tracheo-bronchial septation, and Wnt-independent expansion of the spleen primordium. Tight spatio-temporal regulation of Barx1 levels in the mesentery and stomach mesenchyme suggests additional roles. To determine these functions, we forced constitutive BARX1 expression in the Bapx1 expression domain, which includes the mesentery and intestinal mesenchyme, and also examined Barx1-/- embryos in further detail. Transgenic embryos invariably showed intestinal truncation and malrotation, in part reflecting abnormal left-right patterning. Ectopic BARX1 expression did not affect intestinal epithelium, but intestinal smooth muscle developed with features typical of the stomach wall. BARX1, which is normally restricted to the developing stomach, drives robust smooth muscle expansion in this organ by promoting proliferation of myogenic progenitors at the expense of other sub-epithelial cells. Undifferentiated embryonic stomach and intestinal mesenchyme showed modest differences in mRNA expression and BARX1 was sufficient to induce much of the stomach profile in intestinal cells. However, limited binding at cis-regulatory sites implies that BARX1 may act principally through other transcription factors. Genes expressed ectopically in BARX1+ intestinal mesenchyme and reduced in Barx1-/- stomach mesenchyme include Isl1, Pitx1, Six2 and Pitx2, transcription factors known to control left-right patterning and influence smooth muscle development. The sum of evidence suggests that potent BARX1 functions in intestinal rotation and stomach myogenesis occur through this small group of intermediary transcription factors.
Project description:Loss of contractility and acquisition of an epithelial phenotype of vascular smooth muscle cells (VSMCs) are key events in proliferative vascular pathologies such as atherosclerosis and post-angioplastic restenosis. There is no proper cell culture system allowing VSMC differentiation so that it is difficult to delineate the molecular mechanism responsible for proliferative vasculopathy. We investigated whether a micro-patterned substrate could restore the contractile phenotype of VSMCs in vitro. To induce and maintain the differentiated VSMC phenotype in vitro, we introduced a micro-patterned groove substrate to modulate the morphology and function of VSMCs.
Project description:Reciprocal signaling between an epithelium and its surrounding mesenchyme is common during morphogenesis. These epithelial-mesenchymal interactions are particularly evident in tissues that undergo branching morphogenesis, such as the airway epithelium of the lung. Here, we found that reciprocal interactions between the epithelium and mesenchyme drive remodeling of the extracellular matrix (ECM) during morphogenesis of the embryonic chicken lung. RNA-Seq analysis revealed changes in the expression of genes associated with integrin signaling and ECM remodeling. Consistently, we found that prior to branching, the basement membrane is a spatially uniform sheath that wraps the airway epithelium. After branch initiation, however, the basement membrane is significantly depleted from the tip of extending branches. Culturing embryonic lung explants revealed that this basement membrane thinning is mediated by matrix metalloproteinase-2 (MMP2), which is expressed in the mesenchyme. Inhibiting MMP activity suppresses branch extension but has no effect on branch initiation. As branches extend, we found that tenascin-C (TNC) accumulates in the mesenchyme several cell diameters away from the branch tip. Despite its pattern of accumulation, this mesenchymal ECM protein is expressed exclusively by airway epithelial cells, which activate focal adhesion kinase (FAK) to induce TNC expression. We found that branch extension coincides with the deformation of adjacent mesenchymal cells into elongated geometries, which correlates with an increase in the fluidity of the mesenchyme at branch tips. This local increase in mesenchymal movement transports TNC from the epithelial surface into the mesenchyme. These data reveal novel epithelial-mesenchymal interactions that direct ECM remodeling during airway branching morphogenesis.