TGF-beta3-null mutation does not abrogate fetal lung maturation in vivo by glucocorticoids

Targeted Disruption of Mouse Dip2B Leads to Abnormal Lung Development and Prenatal Lethality

Molecular and anatomical functions of mammalian Dip2 family members (Dip2A, Dip2B and Dip2C) during organogenesis are largely unknown. Here, we explored the indispensable role of Dip2B in mouse lung development. Using a LacZ reporter, we explored Dip2B expression during embryogenesis. This study shows that Dip2B expression is widely distributed in various neuronal, myocardial, endothelial, and epithelial cell types during embryogenesis. Target disruption of Dip2b leads to intrauterine growth restriction, defective lung formation and perinatal mortality. Dip2B is crucial for late lung maturation rather than early-branching morphogenesis. The morphological analysis shows that Dip2b loss leads to disrupted air sac formation, interstitium septation and increased cellularity. In BrdU incorporation assay, it is shown that Dip2b loss results in increased cell proliferation at the saccular stage of lung development. RNA-seq analysis reveals that 1431 genes are affected in Dip2b deficient lungs at E18.5 gestation age. Gene ontology analysis indicates cell cycle-related genes are upregulated and immune system related genes are downregulated. KEGG analysis identifies oxidative phosphorylation as the most overrepresented pathways along with the G2/M phase transition pathway. Loss of Dip2b de-represses the expression of alveolar type I and type II molecular markers. Altogether, the study demonstrates an important role of Dip2B in lung maturation and survival.

Mesenchyme-specific deletion of Tgf-β1 in the embryonic lung disrupts branching morphogenesis and induces lung hypoplasia

Proper lung development depends on the precise temporal and spatial expression of several morphogenic factors, including Fgf10, Fgf9, Shh, Bmp4, and Tgf-β. Over- or under-expression of these molecules often leads to aberrant embryonic or postnatal lung development. Herein, we deleted the Tgf-β1 gene specifically within the lung embryonic mesenchymal compartment at specific gestational stages to determine the contribution of this cytokine to lung development. Mutant embryos developed severe lung hypoplasia and died at birth due to the inability to breathe. Despite the markedly reduced lung size, proliferation and differentiation of the lung epithelium was not affected by the lack of mesenchymal expression of the Tgf-β1 gene, while apoptosis was significantly increased in the mutant lung parenchyma. Lack of mesenchymal expression of the Tgf-β1 gene was also associated with reduced lung branching morphogenesis, with accompanying inhibition of the local FGF10 signaling pathway as well as abnormal development of the vascular system. To shed light on the mechanism of lung hypoplasia, we quantified the phosphorylation of 226 proteins in the mutant E12.5 lung compared with control. We identified five proteins, Hrs, Vav2, c-Kit, the regulatory subunit of Pi3k (P85), and Fgfr1, that were over- or under-phosphorylated in the mutant lung, suggesting that they could be indispensable effectors of the TGF-β signaling program during embryonic lung development. In conclusion, we have uncovered novel roles of the mesenchyme-specific Tgf-β1 ligand in embryonic mouse lung development and generated a mouse model that may prove helpful to identify some of the key pathogenic mechanisms underlying lung hypoplasia in humans.

Trinucleotide repeat containing 6c (TNRC6c) is essential for microvascular maturation during distal airspace sacculation in the developing lung

GW182 (also known asTNRC6) family members are critically involved in the final effector phase of miRNA-mediated mRNA repression. The three mammalian paralogs, TNRC6a, b and c, are thought to be redundant based on Argonaute (Ago) binding, tethering assays, and RNAi silencing of individual members in cell lines. To test this idea, we generated TNRC6a, b and c knockout mice. TNRC6a mutants die at mid-gestation, while b- and c- deleted mice are born at a Mendelian ratio. However, the majority of TNRC6b and all TNRC6c mutants die within 24h after birth, the latter with respiratory failure. Necropsy of TNRC6c mutants revealed normal-appearing airways that give rise to abnormally thick-walled distal gas exchange sacs. Immunohistological analysis of mutant lungs demonstrated a normal distribution of bronchiolar and alveolar cells, indicating that loss of TNRC6c did not abrogate epithelial cell differentiation. The cellular kinetics and relative proportions of endothelial, epithelial, and mesenchymal cells were also not altered. However, the underlying capillary network was simplified and endothelial cells had failed to become tightly apposed to the surface epithelium in TNRC6c mutants, presumably causing the observed respiratory failure. TGFβ family mutant mice exhibit a similar lung phenotype of thick-walled air sacs and neonatal lethality, and qRT-PCR confirmed dynamic downregulation of TGFβ1 and TGFβR2 in TNRC6c mutant lungs during sacculation. VEGFR, but not VEGF-A ligand, was also lower, likely reflecting the overall reduced capillary density in TNRC6c mutants. Together, these results demonstrate that GW182 paralogs are not functionally redundant in vivo. Surprisingly, despite regulating a general cellular process, TNRC6c is selectively required only in the distal lung and not until late in gestation for proper expression of the TGFβ family genes that drive sacculation. These results imply a complex and indirect mode of regulation of sacculation by TNRC6c, mediated in part by dynamic transcriptional repression of an inhibitor of TGFβ family gene expression.

BMP signaling is essential in neonatal surfactant production during respiratory adaptation

Deficiency in pulmonary surfactant results in neonatal respiratory distress, and the known genetic mutations in key components of surfactant only account for a small number of cases. Therefore, determining the regulatory mechanisms of surfactant production and secretion, particularly during the transition from prenatal to neonatal stages, is essential for better understanding of the pathogenesis of human neonatal respiratory distress. We have observed significant increase of bone morphogenetic protein (BMP) signaling in neonatal mouse lungs immediately after birth. Using genetically manipulated mice, we then studied the relationship between BMP signaling and surfactant production in neonates. Blockade of endogenous BMP signaling by deleting Bmpr1a (Alk3) or Smad1 in embryonic day 18.5 in perinatal lung epithelial cells resulted in severe neonatal respiratory distress and death, accompanied by atelectasis in histopathology and significant reductions of surfactant protein B and C, as well as Abca3, whereas prenatal lung development was not significantly affected. We then identified a new BMP-Smad1 downstream target, Nfatc3, which is known as an important transcription activator for surfactant proteins and Abca3. Furthermore, activation of BMP signaling in cultured lung epithelial cells was able to promote endogenous Nfatc3 expression and also stimulate the activity of an Nfatc3 promoter that contains a Smad1-binding site. Therefore, our study suggests that the BMP-Alk3-Smad1-Nfatc3 regulatory loop plays an important role in enhancing surfactant production in neonates, possibly helping neonatal respiratory adaptation from prenatal amniotic fluid environment to neonatal air breathing.

An overlapping set of genes is regulated by both NFIB and the glucocorticoid receptor during lung maturation

Background

Lung maturation is a late fetal developmental event in both mice and humans. Because of this, lung immaturity is a serious problem in premature infants. Disruption of genes for either the glucocorticoid receptor (Nr3c1) or the NFIB transcription factors results in perinatal lethality due to lung immaturity. In both knockouts, the phenotype includes excess cell proliferation, failure of saccularization and reduced expression of markers of epithelial differentiation. This similarity suggests that the two genes may co-regulate a specific set of genes essential for lung maturation.

Results

We analyzed the roles of these two transcription factors in regulating transcription using ChIP-seq data for NFIB, and RNA expression data and motif analysis for both. Our new ChIP-seq data for NFIB in lung at E16.5 shows that NFIB binds to a NFI motif. This motif is over-represented in the promoters of genes that are under-expressed in Nfib-KO mice at E18.5, suggesting an activator role for NFIB. Using available microarray data from Nr3c1-KO mice, we further identified 52 genes that are under-expressed in both Nfib and Nr3c1 knockouts, an overlap which is 13.1 times larger than what would be expected by chance. Finally, we looked for enrichment of 738 recently published transcription factor motifs in the promoters of these putative target genes and found that the NFIB and glucocorticoid receptor motifs were among the most enriched, suggesting that a subset of these genes may be directly activated by Nfib and Nr3c1.

Conclusions

Our data provide the first evidence for Nfib and Nr3c1 co-regulating genes related to lung maturation. They also establish that the in vivo DNA-binding specificity of NFIB is the same as previously seen in vitro, and highly similar to that of the other NFI-family members NFIA, NFIC and NFIX.

Comparative molecular developmental aspects of the mammalian- and the avian lungs, and the insectan tracheal system by branching morphogenesis: recent advances and future directions

Gas exchangers fundamentally form by branching morphogenesis (BM), a mechanistically profoundly complex process which derives from coherent expression and regulation of multiple genes that direct cell-to-cell interactions, differentiation, and movements by signaling of various molecular morphogenetic cues at specific times and particular places in the developing organ. Coordinated expression of growth-instructing factors determines sizes and sites where bifurcation occurs, by how much a part elongates before it divides, and the angle at which branching occurs. BM is essentially induced by dualities of factors where through feedback- or feed forward loops agonists/antagonists are activated or repressed. The intricate transactions between the development orchestrating molecular factors determine the ultimate phenotype. From the primeval time when the transformation of unicellular organisms to multicellular ones occurred by systematic accretion of cells, BM has been perpetually conserved. Canonical signalling, transcriptional pathways, and other instructive molecular factors are commonly employed within and across species, tissues, and stages of development. While much still remain to be elucidated and some of what has been reported corroborated and reconciled with rest of existing data, notable progress has in recent times been made in understanding the mechanism of BM. By identifying and characterizing the morphogenetic drivers, and markers and their regulatory dynamics, the elemental underpinnings of BM have been more precisely explained. Broadening these insights will allow more effective diagnostic and therapeutic interventions of developmental abnormalities and pathologies in pre- and postnatal lungs. Conservation of the molecular factors which are involved in the development of the lung (and other branched organs) is a classic example of nature's astuteness in economically utilizing finite resources. Once purposefully formed, well-tested and tried ways and means are adopted, preserved, and widely used to engineer the most optimal phenotypes. The material and time costs of developing utterly new instruments and routines with every drastic biological change (e.g. adaptation and speciation) are circumvented. This should assure the best possible structures and therefore functions, ensuring survival and evolutionary success.

Dexamethasone normalizes aberrant elastic fiber production and collagen 1 secretion by Loeys-Dietz syndrome fibroblasts: a possible treatment?

Loeys-Dietz syndrome (LDS) is an autosomal dominant connective tissue disorder characterized by facial dysmorphism, cleft palate, dilation of the aortic arch, blood vessel tortuosity and a high risk of aortic dissection. It is caused by mutations in the transforming growth factor β-receptor 1 and 2 (TGFβ-R1 and TGFβ-R2) genes. Fibroblasts derived from 12 Loeys-Dietz syndrome patients, six with TGFB-R1 mutations and six with TGFB-R2 mutations, were analyzed using RT-PCR, biochemical assays, immunohistochemistry and electron microscopy for production of elastin, fibrillin 1, fibulin 1 and fibulin 4 and deposition of collagen type I. All LDS fibroblasts with TGFβ-R1 mutations demonstrated decreased expression of elastin and fibulin 1 genes and impaired deposition of elastic fibers. In contrast, fibroblasts with TGFβ-R2 mutations consistently demonstrated intracellular accumulation of collagen type I in the presence of otherwise normal elastic fiber production. Treatment of the cell cultures with dexamethasone induced remarkable upregulation in the expression of tropoelastin, fibulin 1- and fibulin 4-encoding mRNAs, leading to normalization of elastic fiber production in fibroblasts with TGFβ-R1 mutations. Treatment with dexamethasone also corrected the abnormal secretion of collagen type I from fibroblasts with TGFβ-R2 gene mutations. As the organogenesis-relevant elastic fiber production occurs exclusively in late fetal and early neonatal life, these findings may have implications for treatment in early life. Further studies are required to determine if dexamethasone treatment of fetuses prenatally diagnosed with LDS would prevent or alleviate the connective tissue and vascular defects seen in this syndrome.

Mechanisms of lung development: contribution to adult lung disease and relevance to chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) results in major remodeling of the distal airspaces and changes in the differentiation profile of the airway epithelium. The cellular and molecular mechanisms involved in initiation and progression of this disease are little understood. Although environmental factors, including cigarette smoke, have been directly implicated in the pathogenesis of COPD, genetic risk factors also appear to play a fundamental role in the individual's susceptibility to this disease. Lung development depends on precise coordination of signals, such as fibroblast growth factors (Fgf), Sonic Hedgehog (Shh), retinoic acid, Notch, and Tgf beta. Dramatic changes in the pattern of branching and differentiation of the lung epithelium results from disruption of these signals in genetically altered mice. Recent studies, including whole-genome expression and genome-wide association analyses, suggest that some molecular regulators originally described in developmental processes may be altered in patients with COPD. Whether disturbances in the molecular and cellular events mediated by these genes during development participate in the initiation or exacerbation of COPD, needs further investigation. The role of selected pathways, including Sonic hedgehog, Notch, retinoid, and Tgf beta in the developing lung and the potential association with COPD are discussed.

Development, repair and fibrosis: what is common and why it matters

The complex structure of the lung is developed sequentially, initially by epithelial tube branching and later by septation of terminal air sacs with accompanying coordinated growth of a variety of lung epithelial and mesenchymal cells. Groups of transcriptional factors, peptide growth factors and their intracellular signaling regulators, as well as extracellular matrix proteins are programmed to be expressed at appropriate levels in the right place at the right time to control normal lung formation. Studies of lung development and lung repair/fibrosis to date have discovered that many of the same factors that control normal development are also key players in lung injury repair and fibrosis. Transforming growth factor-beta (TGF-beta) family peptide signaling is a prime example. Lack of TGF-beta signaling results in abnormal lung branching morphogenesis and alveolarization during development, whereas excessive amounts of TGF-beta signaling cause severe hypoplasia in the immature lung and fibrosis in mature lung. This leads us to propose the 'Goldilocks' hypothesis of regulatory signaling in lung development and injury repair that everything must be done just right!

TGF-beta receptor II in epithelia versus mesenchyme plays distinct roles in the developing lung

Transforming growth factor (TGF)-beta signalling plays important roles in regulating lung development. However, the specific regulatory functions of TGF-beta signalling in developing lung epithelial versus mesenchymal cells are still unknown. By immunostaining, the expression pattern of the TGF-beta type II receptor (TbetaRII) was first determined in the developing mouse lung. The functions of TbetaRII in developing lung were then determined by conditionally knocking out TbetaRII in the lung epithelium of floxed-TbetaRII/surfactant protein C-reverse tetracycline transactivator/TetO-Cre mice versus mesenchyme of floxed-TbetaRII/Dermo1-Cre mice. TbetaRII was expressed only in distal airway epithelium at early gestation (embryonic day (E)11.5), but in both airway epithelium and mesenchyme from mid-gestation (E14.5) to post-natal day 14. Abrogation of TbetaRII in mouse lung epithelium resulted in retardation of post-natal lung alveolarisation, with markedly decreased type I alveolar epithelial cells, while no abnormality in prenatal lung development was observed. In contrast, blockade of TbetaRII in mesoderm-derived tissues, including lung mesenchyme, resulted in mildly abnormal lung branching and reduced cell proliferation after mid-gestation, accompanied by multiple defects in other organs, including diaphragmatic hernia. The primary lung branching defect was verified in embryonic lung explant culture. The novel findings of the present study suggest that transforming growth factor-beta type II receptor-mediated transforming growth factor-beta signalling plays distinct roles in lung epithelium versus mesenchyme to differentially control specific stages of lung development.

Tgfb1 expressed in the Tgfb3 locus partially rescues the cleft palate phenotype of Tgfb3 null mutants

Although TGF-beta isoforms (TGF-beta1-3) display very similar biochemical characteristics in vitro, it has been determined that they demonstrate different or even opposing effects in vivo. During embryogenesis, TGF-betas play important roles in several developmental processes. Tgfb3 is strongly expressed in the prefusion palatal epithelium, and mice lacking Tgfb3 display a cleft of the secondary palate. To test whether the effect of TGF-beta3 in palatogenesis is isoform-specific in vivo, we generated a knockin mouse by replacing the coding region of exon1 in the Tgfb3 gene with the full-length Tgfb1 cDNA, which resulted in the expression of Tgfb1 in the Tgfb3 expressing domain. The homozygote knockin mice display a complete fusion at the mid-portion of the secondary palate, while the most anterior and posterior regions fail to fuse appropriately indicating that in vivo replacement of TGF-beta3 with TGF-beta1 can only partially correct the epithelial fusion defect of Tgfb3 knockout embryos. Palatal shelves of Tgfb1 knockin homozygote mice adhere, intercalate, and form characteristic epithelial triangles. However, decreased apoptosis in the midline epithelium, slower breakdown of the basement membrane and a general delay in epithelial fusion were observed when compared to control littermates. These results demonstrate an isoform-specific role for TGF-beta3 in the palatal epithelium during palate formation, which cannot be fully substituted with TGF-beta1.

Abnormal mouse lung alveolarization caused by Smad3 deficiency is a developmental antecedent of centrilobular emphysema

Transforming growth factor-β (TGF-β) signaling plays an important regulatory role during lung development and remodeling. Smad3 is a major downstream signal transducer in the TGF-β pathway from the cell membrane to the nucleus. In Smad3 null mutant mice, we have observed retarded lung alveolarization from postnatal day 7 to day 28, and subsequently centrilobular emphysema starting from day 28, as determined by morphometric analysis. In addition to the morphological changes, peripheral lung cell proliferation in Smad3 knockout mice was reduced compared with the wild-type control between postnatal days 7 and 28. Expression of tropoelastin at the mRNA level was also dramatically decreased in Smad3 knockout lungs from postnatal day 28 through adulthood. Furthermore, increased matrix metalloproteinase-9 protein expression and activity were detected in the Smad3 knockout mouse lung tissue and the bronchoalveolar lavage fluid at postnatal day 28 when the centrilobular emphysema pathology was just beginning to appear. Therefore, these results indicate that Smad3 not only has a positive regulatory impact on neonatal lung alveolarization but also potentially plays a protective role against the occurrence of centrilobular emphysema later on in life.

The transcription factor gene Nfib is essential for both lung maturation and brain development

The phylogenetically conserved nuclear factor I (NFI) gene family encodes site-specific transcription factors essential for the development of a number of organ systems. We showed previously that Nfia-deficient mice exhibit agenesis of the corpus callosum and other forebrain defects, whereas Nfic-deficient mice have agenesis of molar tooth roots and severe incisor defects. Here we show that Nfib-deficient mice possess unique defects in lung maturation and exhibit callosal agenesis and forebrain defects that are similar to, but more severe than, those seen in Nfia-deficient animals. In addition, loss of Nfib results in defects in basilar pons formation and hippocampus development that are not seen in Nfia-deficient mice. Heterozygous Nfib-deficient animals also exhibit callosal agenesis and delayed lung maturation, indicating haploinsufficiency at the Nfib locus. The similarity in brain defects in Nfia- and Nfib-deficient animals suggests that these two genes may cooperate in late fetal forebrain development, while Nfib is essential for late fetal lung maturation and development of the pons.

Specific ablation of the nidogen-binding site in the laminin γ1 chain interferes with kidney and lung development

Basement membrane assembly is of crucial importance in the development and function of tissues and during embryogenesis. Nidogen 1 was thought to be central in the assembly processes, connecting the networks formed by collagen type IV and laminins, however, targeted inactivation of nidogen 1 resulted in no obvious phenotype. We have now selectively deleted the sequence coding for the 56 amino acid nidogen-binding site, γ1III4, within the Lamc1 gene by gene targeting. Here, we show that mice homozygous for the deletion die immediately after birth, showing renal agenesis and impaired lung development. These developmental defects were attributed to locally restricted ruptures in the basement membrane of the elongating Wolffian duct and of alveolar sacculi. These data demonstrate that an interaction between two basement membrane proteins is required for early kidney morphogenesis in vivo.

Sim2 mutants have developmental defects not overlapping with those of Sim1 mutants

The mouse genome contains two Sim genes, Sim1 and Sim2. They are presumed to be important for central nervous system (CNS) development because they are homologous to the Drosophila single-minded (sim) gene, mutations in which cause a complete loss of CNS midline cells. In the mammalian CNS, Sim2 and Sim1 are coexpressed in the paraventricular nucleus (PVN). While Sim1 is essential for the development of the PVN (J. L. Michaud, T. Rosenquist, N. R. May, and C.-M. Fan, Genes Dev. 12:3264-3275, 1998), we report here that Sim2 mutant has a normal PVN. Analyses of the Sim1 and Sim2 compound mutants did not reveal obvious genetic interaction between them in PVN histogenesis. However, Sim2 mutant mice die within 3 days of birth due to lung atelectasis and breathing failure. We attribute the diminished efficacy of lung inflation to the compromised structural components surrounding the pleural cavity, which include rib protrusions, abnormal intercostal muscle attachments, diaphragm hypoplasia, and pleural mesothelium tearing. Although each of these structures is minimally affected, we propose that their combined effects lead to the mechanical failure of lung inflation and death. Sim2 mutants also develop congenital scoliosis, reflected by the unequal sizes of the left and right vertebrae and ribs. The temporal and spatial expression patterns of Sim2 in these skeletal elements suggest that Sim2 regulates their growth and/or integrity.

Gremlin negatively modulates BMP-4 induction of embryonic mouse lung branching morphogenesis

Bone morphogenetic protein-4 (BMP-4) is a key morphogen for embryonic lung development that is expressed at high levels in the peripheral epithelium, but the mechanisms that modulate BMP-4 function in early mouse lung branching morphogenesis are unclear. Here, we studied the BMP-4 antagonist Gremlin, which is a member of the DAN family of BMP antagonists that can bind and block BMP-2/4 activity. The expression level of gremlin in embryonic mouse lungs is highest in the early embryonic pseudoglandular stage [embryonic days (E) 11.5-14.5] and is reduced during fetal lung maturation (E18.5 to postnatal day 1). In situ hybridization indicates that gremlin is diffusely expressed in peripheral lung mesenchyme and epithelium, but relatively high epithelial expression occurs in branching buds at E11.5 and in large airways after E16.5. In E11.5 lung organ culture, we found that exogenous BMP-4 dramatically enhanced peripheral lung epithelial branching morphogenesis, whereas reduction of endogenous gremlin expression with antisense oligonucleotides achieved the same gain-of-function phenotype as exogenous BMP-4, including increased epithelial cell proliferation and surfactant protein C expression. On the other hand, adenoviral overexpression of gremlin blocked the stimulatory effects of exogenous BMP-4. Therefore, our data support the hypothesis that Gremlin is a physiologically negative regulator of BMP-4 in lung branching morphogenesis.