Growth factor signaling in lung morphogenetic centers: automaticity, stereotypy and symmetry

Alteration in branching morphogenesis via YAP/TAZ in fibroblasts of fetal lungs in an LPS-induced inflammation model

BACKGROUND

Chorioamnionitis is a common cause of preterm birth and leads to serious complications in newborns. The objective of this study was to investigate the role of the Hippo signaling pathway in lung branching morphogenesis under a lipopolysaccharide (LPS)-induced inflammation model.

MATERIALS AND METHODS

IMR-90 cells and ex vivo fetal lungs were treated with 0, 10, 30, or 50 μg/ml LPS for 24 and 72 h. Supernatant levels of lactate dehydrogenase (LDH), interleukin (IL)-6, IL-8, Chemokine (C-X-C motif) ligand 1(CXCL1), branching and the surface area ratio, Yes-associated protein (YAP), transcription coactivator with PDZ-binding motif (TAZ), fibroblast growth factor 10 (FGF10), fibroblast growth factor receptor II (FGFR2), SRY-box transcription factor 2 (SOX2), SOX9, and sirtuin 1 (SIRT1) levels were examined. Differentially expressed genes in fetal lungs after LPS treatment were identified by RNA-sequencing.

RESULTS

LPS at 50 μg/ml increased IL-6 and IL-8 in IMR-90 cells and increased IL-6, CXCL1 and LDH in fetal lungs. The branching ratio significantly increased by LPS at 30 μg/ml compared to the control but the increased level had decreased by 50 μg/ml LPS exposure. Exposure to 50 μg/ml LPS increased phosphorylated (p)-YAP, p-YAP/YAP, and p-TAZ/TAZ in IMR-90 cells, whereas 50 μg/ml LPS decreased FGF10 and SOX2. Consistently, p-YAP/YAP and p-TAZ/TAZ were increased in fibronectin⁺ cells of fetal lungs. Moreover, results of RNA-sequencing in fetal lungs showed that SMAD, FGF, IκB phosphorylation, tissue remodeling and homeostasis was involved in branching morphogenesis following exposure to 50 μg/ml LPS. The p-SIRT1/SIRT1 ratio increased in IMR-90 cells by LPS treatment.

CONCLUSIONS

This study showed that regulation of the Hippo pathway in fibroblasts of fetal lungs was involved in branching morphogenesis under an inflammatory disease such as chorioamnionitis.

Conserved Mechanisms in the Formation of the Airways and Alveoli of the Lung

Branching is an intrinsic property of respiratory epithelium that can be induced and modified by signals emerging from the mesenchyme. However, during stereotypic branching morphogenesis of the airway, the relatively thick upper respiratory epithelium extrudes through a mesenchymal orifice to form a new branch, whereas during alveologenesis the relatively thin lower respiratory epithelium extrudes to form sacs or bubbles. Thus, both branching morphogenesis of the upper airway and alveolarization in the lower airway seem to rely on the same fundamental physical process: epithelial extrusion through an orifice. Here I propose that it is the orientation and relative stiffness of the orifice boundary that determines the stereotypy of upper airway branching as well as the orientation of individual alveolar components of the gas exchange surface. The previously accepted dogma of the process of alveologenesis, largely based on 2D microscopy, is that alveoli arise by erection of finger-like interalveolar septae to form septal clefts that subdivide pre-existing saccules, a process for which the contractile properties of specialized alveolar myofibroblasts are necessary. Here I suggest that airway tip splitting and stereotypical side domain branching are actually conserved processes, but modified somewhat by evolution to achieve both airway tip splitting and side branching of the upper airway epithelium, as well as alveologenesis. Viewed in 3D it is clear that alveolar “septal tips” are in fact ring or purse string structures containing elastin and collagen that only appear as finger like projections in cross section. Therefore, I propose that airway branch orifices as well as alveolar mouth rings serve to delineate and stabilize the budding of both airway and alveolar epithelium, from the tips and sides of upper airways as well as from the sides and tips of alveolar ducts. Certainly, in the case of alveoli arising laterally and with radial symmetry from the sides of alveolar ducts, the mouth of each alveolus remains within the plane of the side of the ductal lumen. This suggests that the thin epithelium lining these lateral alveolar duct buds may extrude or “pop out” from the duct lumen through rings rather like soap or gum bubbles, whereas the thicker upper airway epithelium extrudes through a ring like toothpaste from a tube to form a new branch.

The control of lung branching morphogenesis

Branching morphogenesis generates epithelial trees which facilitate gas exchange, filtering, as well as secretion processes with their large surface to volume ratio. In this review, we focus on the developmental mechanisms that control the early stages of lung branching morphogenesis. Lung branching morphogenesis involves the stereotypic, recurrent definition of new branch points, subsequent epithelial budding, and lung tube elongation. We discuss current models and experimental evidence for each of these steps. Finally, we discuss the role of the mesenchyme in determining the organ-specific shape.

Oral Primo-Colonizing Bacteria Modulate Inflammation and Gene Expression in Bronchial Epithelial Cells

The microbiota of the mouth disperses into the lungs, and both compartments share similar phyla. Considering the importance of the microbiota in the maturation of the immunity and physiology during the first days of life, we hypothesized that primo-colonizing bacteria of the oral cavity may induce immune responses in bronchial epithelial cells. Herein, we have isolated and characterized 57 strains of the buccal cavity of two human newborns. These strains belong to Streptococcus, Staphylococcus, Enterococcus, Rothia and Pantoea genera, with Streptococcus being the most represented. The strains were co-incubated with a bronchial epithelial cell line (BEAS-2B), and we established their impact on a panel of cytokines/chemokines and global changes in gene expression. The Staphylococcus strains, which appeared soon after birth, induced a high production of IL-8, suggesting they can trigger inflammation, whereas the Streptococcus strains were less associated with inflammation pathways. The genera Streptococcus, Enterococcus and Pantoea induced differential profiles of cytokine/chemokine/growth factor and set of genes associated with maturation of morphology. Altogether, our results demonstrate that the microorganisms, primo-colonizing the oral cavity, impact immunity and morphology of the lung epithelial cells, with specific effects depending on the phylogeny of the strains.

Lung development, regeneration and plasticity: From disease physiopathology to drug design using induced pluripotent stem cells

Lungs have a complex structure composed of different cell types that form approximately 17 million airway branches of gas-delivering bronchioles connected to 500 million gas-exchanging alveoli. Airways and alveoli are lined by epithelial cells that display a low rate of turnover at steady-state, but can regenerate the epithelium in response to injuries. Here, we review the key points of lung development, homeostasis and epithelial cell plasticity in response to injury and disease, because this knowledge is required to develop new lung disease treatments. Of note, canonical signaling pathways that are essential for proper lung development during embryogenesis are also involved in the pathophysiology of most chronic airway diseases. Moreover, the perfect control of these interconnected pathways is needed for the successful differentiation of induced pluripotent stem cells (iPSC) into lung cells. Indeed, differentiation of iPSC into airway epithelium and alveoli is based on the use of biomimetics of normal embryonic and fetal lung development. In vitro iPSC-based models of lung diseases can help us to better understand the impaired lung repair capacity and to identify new therapeutic targets and new approaches, such as lung cell therapy.

Patched1 patterns Fibroblast growth factor 10 and Forkhead box F1 expression during pulmonary branch formation

Hedgehog (Hh) signalling, Fibroblast growth factor 10 (Fgf10) and Forkhead box F1 (Foxf1) are each individually important for directing pulmonary branch formation but their interactions are not well understood. Here we demonstrate that Hh signalling is vital in regulating Foxf1 and Fgf10 expression during branching. The Hedgehog receptor Patched1 (Ptch1) was conditionally inactivated in the lung mesenchyme by Dermo1-Cre in vivo or using a recombinant Cre recombinase protein (HNCre) in lung cultures resulting in cell autonomous activation of Hh signalling. Homozygous mesenchymal Ptch1 deleted embryos (Dermo1Cre+/-;Ptch1^lox/lox) showed secondary branching and lobe formation defects. Fgf10 expression is spatially reduced in the distal tip of Dermo1Cre+/-;Ptch1^lox/lox lungs and addition of Fgf10 recombinant protein to these lungs in culture has shown partial restoration of branching, indicating Ptch1 function patterns Fgf10 to direct lung branching. Foxf1 expression is upregulated in Dermo1Cre+/-;Ptch1^lox/lox lungs, suggesting Foxf1 may mediate Hh signalling effects in the lung mesenchyme. In vitro HNCre-mediated Ptch1 deleted lung explants support the in vivo observations, with evidence of mesenchyme hyperproliferation and this is consistent with the previously reported role of Hh signalling in maintaining mesenchymal cell survival. Consequently it is concluded that during early pseudoglandular stage of lung development Ptch1 patterns Fgf10 and regulates Foxf1 expression in the lung mesenchyme to direct branch formation and this is essential for proper lobe formation and lung function.

Age, Sexual Dimorphism, and Disease Associations in the Developing Human Fetal Lung Transcriptome

The fetal origins of disease hypothesis suggests that variations in the course of prenatal lung development may affect life-long pulmonary function growth, decline, and pathobiology. Many studies support the existence of differences in the developing lung trajectory in males and females, and sex-specific differences in the prevalence of chronic lung diseases, such as asthma and bronchopulmonary dysplasia. The objectives of this study were to investigate the early developing fetal lung for transcriptomic correlates of postconception age (maturity) and sex, and their associations with chronic lung diseases. We analyzed whole-lung transcriptome profiles of 61 females and 78 males at 54-127 days postconception (dpc) from nonsmoking mothers using unsupervised principal component analysis and supervised linear regression models. We identified dominant transcriptomic correlates for postconception age and sex with corresponding gene sets that were enriched for developing lung structural and functional ontologies. We observed that the transcriptomic sex difference was not a uniform global time shift/lag, rather, lungs of males appear to be more mature than those of females before 96 dpc, and females appear to be more mature than males after 96 dpc. The age correlate gene set was consistently enriched for asthma and bronchopulmonary dysplasia genes, but the sex correlate gene sets were not. Despite sex differences in the developing fetal lung transcriptome, postconception age appears to be more dominant than sex in the effect of early fetal lung developments on disease risk during this early pseudoglandular phase of development.

Overview of Lung Development in the Newborn Human

In human neonates rapid adaptation from an aqueous intrauterine environment to permanent air breathing is the rate-limiting step for extrauterine life, failure of which justifies the existence of neonatal intensive care units. The lung develops at about 4-6 weeks' gestation in humans as a ventral outpouching of the primitive foregut into the surrounding ventral mesenchyme, termed the laryngotracheal groove. At its posterior end lie progenitor cells that form a pair of bronchial tubes, from which arise all the distal epithelial structures of the lung. In humans, formation of the distal gas exchange surfaces begins in utero at about 20 weeks' gestation and is substantially established by term. Stereotypic branching of the proximal airway ends relatively early at 16-18 weeks at the bronchoalveolar duct junctions. Distally, about 5 finger-like alveolar ducts arise from each bronchoalveolar duct junction and ramify outwards towards the pleura. The majority of alveolar air sacs arise from the sides of the alveolar ducts and each of these alveoli can have up to 5 daughter alveoli arising from the outer surface as subsequent buds. At the end of each alveolar duct lie the mouths of 5 interconnected alveoli. Each family of alveoli arising from each bronchoalveolar duct junction has a different shape depending upon the limitations imposed by the pleural surface as well as the interstitial fascial planes.

Inactivation of Tsc2 in Mesoderm-Derived Cells Causes Polycystic Kidney Lesions and Impairs Lung Alveolarization

The tuberous sclerosis complex (TSC) proteins are critical negative regulators of the mammalian/mechanistic target of rapamycin complex 1 pathway. Germline mutations of TSC1 or TSC2 cause TSC, affecting multiple organs, including the kidney and lung, and causing substantial morbidity and mortality. The mechanisms of organ-specific disease in TSC remain incompletely understood, and the impact of TSC inactivation on mesenchymal lineage cells has not been specifically studied. We deleted Tsc2 specifically in mesoderm-derived mesenchymal cells of multiple organs in mice using the Dermo1-Cre driver. The Dermo1-Cre-driven Tsc2 conditional knockout mice had body growth retardation and died approximately 3 weeks after birth. Significant phenotypes were observed in the postnatal kidney and lung. Inactivation of Tsc2 in kidney mesenchyme caused polycystic lesions starting from the second week of age, with increased cell proliferation, tubular epithelial hyperplasia, and epithelial-mesenchymal transition. In contrast, Tsc2 deletion in lung mesenchyme led to decreased cell proliferation, reduced postnatal alveolarization, and decreased differentiation with reduced numbers of alveolar myofibroblast and type II alveolar epithelial cells. Two major findings thus result from this model: inactivation of Tsc2 in mesoderm-derived cells causes increased cell proliferation in the kidneys but reduced proliferation in the lungs, and inactivation of Tsc2 in mesoderm-derived cells causes epithelial-lined renal cysts. Therefore, Tsc2-mTOR signaling in mesenchyme is essential for the maintenance of renal structure and for lung alveolarization.

Genes associated with polymorphic variants predicting lung function are differentially expressed during human lung development

BACKGROUND

Recent meta-analyses of genome-wide association studies have identified single nucleotide polymorphisms (SNPs) within/near 54 genes associated with lung function measures. Current understanding of the contribution of these genes to human lung development is limited. We set out to further define i) the expression profile of these genes during human lung development using a unique set of resources to examine both mRNA and protein expression and ii) the link between key polymorphisms and genes using expression quantitative trait loci (eQTL) approaches.

METHODS

The mRNA expression profile of lung function associated genes across pseudoglandular and canalicular stages of lung development were determined using expression array data of 38 human fetal lungs. eQTLs were investigated for selected genes using blood cell and lung tissue data. Immunohistochemistry of the top 5 candidates was performed in a panel of 24 fetal lung samples.

RESULTS

Twenty-nine lung function associated genes were differentially expressed during lung development at the mRNA level. The greatest magnitude of effect was observed for 5 genes; TMEM163, FAM13A and HHIP which had increasing expression and CDC123 and PTCH1 with decreased expression across developmental stages. Focussed eQTL analyses investigating SNPs in these five loci identified several cis-eQTL's. Protein expression of TMEM163 increased and CDC123 decreased with fetal lung age in agreement with mRNA data. Protein expression in FAM13A, HHIP and PTCH1 remained relatively constant throughout lung development.

CONCLUSIONS

We have identified that > 50 % of lung function associated genes show evidence of differential expression during lung development and we show that in particular TMEM163 and CDC123 are differentially expressed at both the mRNA and protein levels. Our data provides a systematic evaluation of lung function associated genes in this context and offers some insight into the potential role of several of these genes in contributing to human lung development.

Regulation by a TGFβ-ROCK-actomyosin axis secures a non-linear lumen expansion that is essential for tubulogenesis

Regulation of lumen growth is crucial to ensure the correct morphology, dimensions and function of a tubular structure. How this is controlled is still poorly understood. During Ciona intestinalis notochord tubulogenesis, single extracellular lumen pockets grow between pairs of cells and eventually fuse into a continuous tube. Here, we show that lumen growth exhibits a lag phase, during which the luminal membranes continue to grow but the expansion of the apical/lateral junction pauses for ∼30 min. Inhibition of non-muscle myosin II activity abolishes this lag phase and accelerates expansion of the junction, resulting in the formation of narrower lumen pockets partially fusing into a tube of reduced size. Disruption of actin dynamics, conversely, causes a reversal of apical/lateral junction expansion, leading to a dramatic conversion of extracellular lumen pockets to intracellular vacuoles and a tubulogenesis arrest. The onset of the lag phase is correlated with a de novo accumulation of actin that forms a contractile ring at the apical/lateral junctions. This actin ring actively restricts the opening of the lumen in the transverse plane, allowing sufficient time for lumen growth via an osmotic process along the longitudinal dimension. The dynamics of lumen formation is controlled by the TGFβ pathway and ROCK activity. Our findings reveal a TGFβ-ROCK-actomyosin contractility axis that coordinates lumen growth, which is powered by the dynamics of luminal osmolarity. The regulatory system may function like a sensor/checkpoint that responds to the change of luminal pressure and fine-tunes actomyosin contractility to effect proper tubulogenesis.

Secreted phosphoprotein 1 is a determinant of lung function development in mice

Secreted phosphoprotein 1 (Spp1) is located within quantitative trait loci associated with lung function that was previously identified by contrasting C3H/HeJ and JF1/Msf mouse strains that have extremely divergent lung function. JF1/Msf mice with diminished lung function had reduced lung SPP1 transcript and protein during the peak stage of alveologenesis (postnatal day [P]14-P28) as compared with C3H/HeJ mice. In addition to a previously identified genetic variant that altered runt-related transcription factor 2 (RUNX2) binding in the Spp1 promoter, we identified another promoter variant in a putative RUNX2 binding site that increased the DNA protein binding. SPP1 induced dose-dependent mouse lung epithelial-15 cell proliferation. Spp1((-/-)) mice have decreased specific total lung capacity/body weight, higher specific compliance, and increased mean airspace chord length (Lm) compared with Spp1((+/+)) mice. Microarray analysis revealed enriched gene ontogeny categories, with numerous genes associated with lung development and/or respiratory disease. Insulin-like growth factor 1, Hedgehog-interacting protein, wingless-related mouse mammary tumor virus integration site 5A, and NOTCH1 transcripts decreased in the lung of P14 Spp1((-/-)) mice as determined by quantitative RT-PCR analysis. SPP1 promotes pneumocyte growth, and mice lacking SPP1 have smaller, more compliant lungs with enlarged airspace (i.e., increased Lm). Microarray analysis suggests a dysregulation of key lung developmental transcripts in gene-targeted Spp1((-/-)) mice, particularly during the peak phase of alveologenesis. In addition to its known roles in lung disease, this study supports SPP1 as a determinant of lung development in mice.

On the evolution of development

Perhaps development is more than just morphogenesis. We now recognize that the conceptus expresses epigenetic marks that heritably affect it phenotypically, indicating that the offspring are to some degree genetically autonomous, and that ontogeny and phylogeny may coordinately determine the fate of such marks. This scenario mechanistically links ecology, ontogeny and phylogeny together as an integrated mechanism for evolution for the first time. As a functional example, the Parathyroid Hormone-related Protein (PTHrP) signaling duplicated during the Phanerozoic water-land transition. The PTHrP signaling pathway was critical for the evolution of the skeleton, skin barrier, and lung function, based on experimental evidence, inferring that physiologic stress can profoundly affect adaptation through internal selection, giving seminal insights to how and why vertebrates were able to evolve from water to land. By viewing evolution from its inception in unicellular organisms, driven by competition between pro- and eukaryotes, the emergence of complex biologic traits from the unicellular cell membrane offers a novel way of thinking about the process of evolution from its beginnings, rather than from its consequences as is traditionally done. And by focusing on the epistatic balancing mechanisms for calcium and lipid homeostasis, the evolution of unicellular organisms, driven by competition between pro- and eukaryotes, gave rise to the emergence of complex biologic traits derived from the unicellular plasma lemma, offering a unique way of thinking about the process of evolution. By exploiting the cellular-molecular mechanisms of lung evolution as ontogeny and phylogeny, the sequence of events for the evolution of the skin, kidney and skeleton become more transparent. This novel approach to the evolution question offers equally novel insights to the primacy of the unicellular state, hologenomics and even a priori bioethical decisions.

Pulmonary FGF-18 gene expression is downregulated during the canalicular-saccular stages in nitrofen-induced hypoplastic lungs

PURPOSE

Pulmonary hypoplasia (PH) associated with congenital diaphragmatic hernia (CDH) represents one of the major challenges in neonatal intensive care. However, the molecular pathogenesis of PH is still poorly understood. In developing fetal lungs, fibroblast growth factor 18 (FGF-18) plays a crucial role in distal airway maturation. FGF-18 knockouts show smaller lung sizes with reduced alveolar spaces and thicker interstitial mesenchymal compartments, highlighting its important function for fetal lung growth and differentiation. We hypothesized that pulmonary FGF-18 gene expression is downregulated during late gestation in nitrofen-induced hypoplastic lungs.

METHODS

Pregnant rats were exposed to either olive oil or nitrofen on day 9 of gestation (D9). Fetuses were harvested on D18 and D21, and lungs were divided into three groups: controls, hypoplastic lungs without CDH [CDH(-)], and hypoplastic lungs with CDH [CDH(+)] (n = 24 at each time-point). Pulmonary FGF-18 gene expression levels were analyzed by qRT-PCR. Immunohistochemistry was performed to investigate FGF-18 protein expression/distribution.

RESULTS

Relative mRNA levels of pulmonary FGF-18 gene expression were significantly decreased in CDH(-) and CDH(+) on D18 and D21 compared to controls (p < 0.05 and p < 0.01, respectively). Immunoreactivity of FGF-18 was markedly diminished in mesenchymal cells surrounding the airway epithelium on D18 and D21 compared to controls.

CONCLUSION

Downregulation of FGF-18 gene expression in nitrofen-induced hypoplastic lungs suggests that decreased FGF-18 expression during the canalicular-saccular stages may interfere with saccular-alveolar differentiation and distal airway maturation resulting in PH.

Influence of age on wound healing and fibrosis

The incidence and severity of fibrotic lung diseases increase with age, but very little is known about how age-related changes affect the mechanisms that underlie disease emergence and progression. Normal ageing includes accumulation of DNA mutations, oxidative and cell stresses, mitochondria dysfunction, increased susceptibility to apoptosis, telomere length dysfunction and differential gene expression as a consequence of epigenetic changes and miR regulation. These inevitable ageing-related phenomena may cause dysfunction and impaired repair capacity of lung epithelial cells, fibroblasts and MSCs. As a consequence, the composition of the extracellular matrix changes and the dynamic interaction between cells and their environment is damaged, resulting ultimately in predisposition for several diseases. This review summarizes what is known about age-related molecular changes that are implicated in the pathobiology of lung fibrosis in lung tissue.

Activation of the canonical bone morphogenetic protein (BMP) pathway during lung morphogenesis and adult lung tissue repair

Signaling by Bone Morphogenetic Proteins (BMP) has been implicated in early lung development, adult lung homeostasis and tissue-injury repair. However, the precise mechanism of action and the spatio-temporal pattern of BMP-signaling during these processes remains inadequately described. To address this, we have utilized a transgenic line harboring a BMP-responsive eGFP-reporter allele (BRE-eGFP) to construct the first detailed spatiotemporal map of canonical BMP-pathway activation during lung development, homeostasis and adult-lung injury repair. We demonstrate that during the pseudoglandular stage, when branching morphogenesis progresses in the developing lung, canonical BMP-pathway is active mainly in the vascular network and the sub-epithelial smooth muscle layer of the proximal airways. Activation of the BMP-pathway becomes evident in epithelial compartments only after embryonic day (E) 14.5 primarily in cells negative for epithelial-lineage markers, located in the proximal portion of the airway-tree, clusters adjacent to neuro-epithelial-bodies (NEBs) and in a substantial portion of alveolar epithelial cells. The pathway becomes activated in isolated E12.5 mesenchyme-free distal epithelial buds cultured in Matrigel suggesting that absence of reporter activity in these regions stems from a dynamic cross-talk between endoderm and mesenchyme. Epithelial cells with activated BMP-pathway are enriched in progenitors capable of forming colonies in three-dimensional Matrigel cultures.As lung morphogenesis approaches completion, eGFP-expression declines and in adult lung its expression is barely detectable. However, upon tissue-injury, either with naphthalene or bleomycin, the canonical BMP-pathways is re-activated, in bronchial or alveolar epithelial cells respectively, in a manner reminiscent to early lung development and in tissue areas where reparatory progenitor cells reside. Our studies illustrate the dynamic activation of canonical BMP-pathway during lung development and adult lung tissue-repair and highlight its involvement in two important processes, namely, the early development of the pulmonary vasculature and the management of epithelial progenitor pools both during lung development and repair of adult lung tissue-injury.

Light-dark oscillations in the lung transcriptome: implications for lung homeostasis, repair, metabolism, disease, and drug action

Diurnal-nocturnal, or circadian-like, rhythms are 24-h variations in biological processes, evolved for the efficient functioning of living organisms. Such oscillations and their regulation in many peripheral tissues are still unclear. In this study, we used Affymetrix gene chips in a rich time-series experiment involving 54 animals killed at 18 time points within the 24-h cycle to examine light-dark cycle patterns of gene expression in rat lungs. Data mining identified 646 genes (represented by 1,006 probe sets) showing robust oscillations in expression in lung that were parsed into 8 distinct temporal clusters. Surprisingly, more than two-thirds of the probe sets showing cyclic expression peaked during the animal's light/inactive period. Six core clock genes and nine clock-related genes showed rhythmic oscillations in their expression in lung. Many of the genes that peaked during the inactive period included genes related to extracellular matrix, cytoskeleton, and protein processing and trafficking, which appear to be mainly involved in the repair and remodeling of the organ. Genes coding for growth factor ligands and their receptors, which play important roles in maintaining normal lung function, also showed rhythmic expression. In addition, genes involved in the metabolism and transport of endogenous compounds, xenobiotics, and therapeutic drugs, along with genes that are biomarkers or potential therapeutic targets for many lung diseases, also exhibited 24-h cyclic oscillations, suggesting an important role for such rhythms in regulating various aspects of the physiology and pathophysiology of lung.

Overexpression of transforming growth factor-beta1 in fetal monkey lung results in prenatal pulmonary fibrosis

Altered transforming growth factor (TGF)-β expression levels have been linked to a variety of human respiratory diseases, including bronchopulmonary dysplasia and pulmonary fibrosis. However, a causative role for aberrant TGF-β in neonatal lung diseases has not been defined in primates. Exogenous and transient TGF-β1 overexpression in fetal monkey lung was achieved by transabdominal ultrasound-guided fetal intrapulmonary injection of adenoviral vector expressing TGF-β1 at the second or third trimester of pregnancy. The lungs were then harvested near term, and fixed for histology and immunohistochemistry. Lung hypoplasia was observed where TGF-β1 was overexpressed during the second trimester. The most clearly marked phenotype consisted of severe pulmonary and pleural fibrosis, which was independent of the gestational time point when TGF-β1 was overexpressed. Increased cell proliferation, particularly in α-smooth muscle actin-positive myofibroblasts, was detected within the fibrotic foci. But epithelium to mesenchyme transdifferentiation was not detected. Massive collagen fibres were deposited on the inner and outer sides of the pleural membrane, with an intact elastin layer in the middle. This induced fibrotic pathology persisted even after adenoviral-mediated TGF-β1 overexpression was no longer evident. Therefore, overexpression of TGF-β1 within developing fetal monkey lung results in severe and progressive fibrosis in lung parenchyma and pleural membrane, in addition to pulmonary hypoplasia.

Genetic influences on asthma susceptibility in the developing lung

Asthma is the leading serious pediatric chronic illness in the United States, affecting 7.1 million children. The prevalence of asthma in children under 4 years of age has increased dramatically in the last 2 decades. Existing evidence suggests that this increase in prevalence derives from early environmental exposures acting on a pre-existing asthma-susceptible genotype. We studied the origins of asthma susceptibility in developing lung in rat strains that model the distinct phenotypes of airway hyperresponsiveness (Fisher rats) and atopy (brown Norway [BN] rats). Postnatal BN rat lungs showed increased epithelial proliferation and tracheal goblet cell hyperplasia. Fisher pups showed increased lung resistance at age 2 weeks, with elevated neutrophils throughout the postnatal period. Diverse transcriptomic signatures characterized the distinct respiratory phenotypes of developing lung in both rat models. Linear regression across age and strain identified developmental variation in expression of 1,376 genes, and confirmed both strain and temporal regulation of lung gene expression. Biological processes that were heavily represented included growth and development (including the T Box 1 transcription factor [Tbx5], the epidermal growth factor receptor [Egfr], the transforming growth factor beta-1-induced transcript 1 [Tgfbr1i1]), extracellular matrix and cell adhesion (including collagen and integrin genes), and immune function (including lymphocyte antigen 6 (Ly6) subunits, IL-17b, Toll-interacting protein, and Ficolin B). Genes validated by quantitative RT-PCR and protein analysis included collagen III alpha 1 Col3a1, Ly6b, glucocorticoid receptor and Importin-13 (specific to the BN rat lung), and Serpina1 and Ficolin B (specific to the Fisher lung). Innate differences in patterns of gene expression in developing lung that contribute to individual variation in respiratory phenotype are likely to contribute to the pathogenesis of asthma.

Congenital airway lesions and lung disease

Structural upper and lower airway disorders and parenchymal disorders are uncommon in pediatric practice, but many pediatricians will encounter them and be responsible for the ongoing care of these patients. Pediatricians need to be cognizant of these diagnoses because, even though management of these disorders generally lacks an evidence base, existing principles of good care surrounding accurate diagnosis, classifications of severity, judicious use of investigations, medication, and surgical approaches are essential to good outcomes.

Cited2 is required for fetal lung maturation

Lung maturation at the terminal sac stage of lung development is characterized by a coordinated increase in terminal sac formation and vascular development in conjunction with the differentiation of alveolar type I and type II epithelial cells. The Cited2-Tcfap2a/c complex has been shown to activate transcription of Erbb3 and Pitx2c during mouse development. In this study, we show that E17.5 to E18.5 Cited2-null lungs had significantly reduced terminal sac space due to an altered differentiation of type I and type II alveolar epithelial cells. In addition, E17 Cited2-null lungs exhibited a decrease in the number of apoptotic cells, contributing to the loss in airspace. Consistent with the phenotype, genes associated with alveolar cell differentiation and survival were differentially expressed in Cited2-null fetal lungs compared to those of wild-type littermates. Moreover, expression of Cebpa, a key regulator of airway epithelial maturation, was significantly decreased in Cited2-null fetal lungs. Cited2 and Tcfap2c were present on the Cebpa promoter in E18.5 lungs to activate Cebpa transcription. We propose that the Cited2-Tcfap2c complex controls lung maturation by regulating Cebpa expression. Understanding the function of this complex may provide novel therapeutic strategies for patients with respiratory distress syndromes.

Misexpression of MIA disrupts lung morphogenesis and causes neonatal death

Microarray experiments designed to identify genes differentially expressed in the E11.5 lung and trachea showed that melanoma inhibitory activity (Mia1) was expressed only in the lung. Mia1 was abundantly expressed during early lung development, but was virtually absent by the end of gestation. Distal embryonic lung epithelium showed high levels of Mia1 expression, which was suppressed by treatment with either retinoic acid or the FGF signaling antagonist SU5402. Late-gestation fetuses in which lung epithelial hyperplasia was induced by misexpression of FGF7 or FGF10 showed continued expression of Mia1 in areas of aberrant morphogenesis. Mia1 expression was also significantly increased in urethane-induced lung adenomas. Treatment of E18.5 lung explants with exogenous MIA caused significant reductions in the expression of the lung differentiation markers Sftpa, Sftpb, Sftpc, and Abca3. Bitransgenic mice expressing MIA under the control of the SFTPC promoter after E16.5, the age when Mia1 is normally silenced, died from respiratory failure at birth with morphologically immature lungs associated with reduced levels of saturated phosphatidylcholine and mature SP-B. Microarray analysis showed significant reductions in the expression of Sftpa, Sftpb, Abca3, Aqp5, Lzp-s, Scd2, and Aytl2 in lungs misexpressing MIA. These results suggest that the silencing of Mia1 that occurs in late gestation may be required for maturation of the surfactant system.

Lung growth and development

Human lung growth starts as a primitive lung bud in early embryonic life and undergoes several morphological stages which continue into postnatal life. Each stage of lung growth is a result of complex and tightly regulated events governed by physical, environmental, hormonal and genetic factors. Fetal lung liquid and fetal breathing movements are by far the most important determinants of lung growth. Although timing of the stages of lung growth in animals do not mimic that of human, numerous animal studies, mainly on sheep and rat, have given us a better understanding of the regulators of lung growth. Insight into the genetic basis of lung growth has helped us understand and improve management of complex life threatening congenital abnormalities such as congenital diaphragmatic hernia and pulmonary hypoplasia. Although advances in perinatal medicine have improved survival of preterm infants, premature birth is perhaps still the most important factor for adverse lung growth.

Lung development and adult lung diseases

Adult respiratory diseases are caused by many factors, including genetic-environmental interaction. Genetic abnormalities can impact early fetal lung development, postnatal lung maturation, as well as adult lung injury and repair. Studies suggest that abnormally developed lung structure and function may contribute as a susceptibility factor for several adult lung diseases. This review focuses on the relationship between lung development and pathogenesis of several lung diseases including COPD, cystic fibrosis (CF), and asthma. COPD with emphysema has been considered to be an accelerated involutional disease of aging smokers. However, since only a proportion (approximately 15%) of smokers get COPD with emphysema, clearly genetic susceptibility must play a significant part in determining both the age of onset and the rapidity of decline in lung function. In mice, interference with key genes either by null mutation, hypomorphism, or gain or loss of function results in phenotypes comprising either neonatal lethal respiratory distress if the structural effect is severe, or reduced alveolarization and/or early onset emphysema if the effect is milder. Reported susceptibility candidate genes are therefore discussed in some detail, including elastin, lysyl oxidase, fibrillin, the transforming growth factor-beta-Smad3 pathway, as well as extracellular matrix proteases. In the case of CF, the Cftr gene has been shown to regulate fetal lung epithelial cell differentiation and maturation. Subtle abnormalities of lung structure and function are found in clinically asymptomatic CF infants. Finally, airway remodeling due to chronic inflammation is important in infants who later acquire asthma.

Integrated proteomic and transcriptomic profiling of mouse lung development and Nmyc target genes

Although microarray analysis has provided information regarding the dynamics of gene expression during development of the mouse lung, no extensive correlations have been made to the levels of corresponding protein products. Here, we present a global survey of protein expression during mouse lung organogenesis from embryonic day E13.5 until adulthood using gel-free two-dimensional liquid chromatography coupled to shotgun tandem mass spectrometry (MudPIT). Mathematical modeling of the proteomic profiles with parallel DNA microarray data identified large groups of gene products with statistically significant correlation or divergence in coregulation of protein and transcript levels during lung development. We also present an integrative analysis of mRNA and protein expression in Nmyc loss- and gain-of-function mutants. This revealed a set of 90 positively and negatively regulated putative target genes. These targets are evidence that Nmyc is a regulator of genes involved in mRNA processing and a repressor of the imprinted gene Igf2r in the developing lung.

Mesenchymal maintenance of distal epithelial cell phenotype during late fetal lung development

Classical tissue recombination experiments have reported that at early gestation both tracheal and distal lung epithelium have the plasticity to respond to mesenchymal signals. Herein we examined the role of epithelial-mesenchymal interactions in maintaining epithelial differentiation at late (E19-E21, term = 22 days) fetal gestation in the rat. Isolated distal lung epithelial cells were recombined with mesenchymal cells from lung, skin, and intestine, and the homotypic or heterotypic recombinant cell aggregates were cultured for up to 5 days. Recombining lung epithelial cells with mesenchyme from various sources induced a morphological pattern that was specific to the type of inducing mesenchyme. In situ analysis of surfactant protein (SP)-C, SP-B, and Clara cell secretory protein (CCSP) expression, as well as SP-C and CCSP promoter transactivation experiments, revealed that distal lung epithelium requires lung mesenchyme to maintain the alveolar, but not bronchiolar, phenotype. Incubation of lung recombinants with an anti-FGF7 antibody resulted in a partial inhibition of mesenchyme-induced SP-C promoter transactivation. Immunoreactivity for Delta and Lunatic fringe, components of the Notch pathway that regulates cell differentiation, was downregulated in the heterotypic recombinants. In contrast, Hes1 mRNA expression was increased in these recombinants. Cumulatively, these results suggest that at late fetal gestation, distal lung epithelial cells are not fully committed to a specific phenotype and still have the plasticity to respond to various signals. Their alveolar phenotype is likely maintained by Notch/Notch ligand interactions and mesenchymal factors, including FGF7.

Faulty bone morphogenetic protein signaling in esophageal atresia with tracheoesophageal fistula

BACKGROUND

The organogenesis of esophageal atresia with tracheoesophageal fistula remains unclear. We have previously demonstrated that the fistula tract develops from a trifurcation of the embryonic lung bud and displays pulmonary lineage traits. Unlike the lung, the fistula grows without branching. Bone morphogenetic proteins (BMPs) are known to be important in lung branching. We studied possible BMP signaling defects as a potential cause for the absence of branching in the fistula tract.

METHODS

Adriamycin was administered to pregnant rats on days 6-9 of gestation to induce tracheoesophageal fistula. Microdissection was performed at E13 and E17 isolating the foregut. Tissues were analyzed using immunohistochemistry for BMP ligand (BMP2, BMP4, BMP7) and receptor (BMPRIA, BMPRIB, BMPRII) expression.

RESULTS

Immunohistochemistry revealed the presence of all 3 BMP ligands at E13, localized specifically to the esophageal mucosa but absent in the fistula and lung. At E17, the ligands were again present in the esophageal mucosa, and additionally in the fistula tract mucosa, but remained absent in the lung. At E17, all of the BMP receptors were also localized to the luminal surface of esophagus and fistula. However, in the lung epithelium, only BMPRII was found, whereas BMPRIA and BMPRIB remained absent.

CONCLUSIONS

The normal expression pattern of BMP4 was increased at the branch tips and low between branches. Among other results, we show here a constant expression level of BMP ligands throughout the entire epithelium of the fistula tract. This diffuse expression suggests defective BMP signaling in the fistula tract and explains its nonbranching phenotype.

Separate respiratory phenotypes in methyl-CpG-binding protein 2 (Mecp2) deficient mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2) that encodes a DNA binding protein involved in gene silencing. Selective deletion of Mecp2 in post-mitotic neurons in mice results in a Rett-like phenotype characterized by disturbances in motor activity and body weight, suggesting that these symptoms are exclusively caused by neuronal deficiency. Included in the RTT phenotype are episodes of respiratory depression that follow hyperventilation. Here we show that the respiratory phenotype depends on the organ distribution of Mecp2 deficiency. Both female mice heterozygous for a null mutation in Mecp2 (Mecp2+/-) and those with selective deletion of the protein in neurons (Mecp2+/nestin-Cre lox), showed an initial response to hypoxia that exceeded that in wild type (WT). However, marked respiratory depression following hypoxic hyperventilation was only seen in Mecp2+/- animals. Addition of carbon dioxide to the hypoxic exposure eliminated the respiratory depression. Tidal volume and lung volume were larger in Mecp2+/- and respiratory depression was directly related to tidal volume. Taken together these results indicate that the depression is due to hypocapnia. Respiratory depression in this mouse model of Rett Syndrome is seen in with ubiquitous deficiency in Mecp2 but not when it is confined to neurons.

Growth factors in lung development

Organized and coordinated lung development follows transcriptional regulation of a complex set of cell-cell and cell-matrix interactions resulting in a blood-gas interface ready for physiologic gas exchange at birth. Transcription factors, growth factors, and various other signaling molecules regulate epithelial-mesenchymal interactions by paracrine and autocrine mechanisms. Transcriptional control at the earliest stages of lung development results in cell differentiation and cell commitment in the primitive lung bud, in essence setting up a framework for pattern formation and branching morphogenesis. Branching morphogenesis results in the formation of the conductive airway system, which is critical for alveolization. Lung development is influenced at all stages by spatial and temporal distribution of various signaling molecules and their receptors and also by the positive and negative control of signaling by paracrine, autocrine, and endocrine mechanisms. Lung bud formation, cell differentiation, and its interaction with the splanchnic mesoderm are regulated by HNF-3beta, Shh, Nkx2.1, HNF-3/Forkhead homolog-8 (HFH-8), Gli, and GATA transcription factors. HNF-3beta regulates Nkx2.1, a transcription factor critical to the formation of distal pulmonary structures. Nkx2.1 regulates surfactant protein genes that are important for the development of alveolar stability at birth. Shh, produced by the foregut endoderm, regulates lung morphogenesis signaling through Gli genes expressed in the mesenchyme. FGF10, produced by the mesoderm, regulates branching morphogenesis via its receptors on the lung epithelium. Alveolization and formation of the capillary network are influenced by various factors that include PDGF, vascular endothelial growth factor (VEGF), and retinoic acid. Epithelial-endothelial interactions during lung development are important in establishing a functional blood-gas interface. The effects of various growth factors on lung development have been demonstrated by gain- or loss-of-function studies in null mutant and transgenic mice models. Understanding the role of growth factors and various other signaling molecules and their cellular interactions in lung development will provide us with new insights into the pathogenesis of bronchopulmonary dysplasia and disorders of lung morphogenesis.

Deficiency in type 1 insulin-like growth factor receptor in mice protects against oxygen-induced lung injury

Background

Cellular responses to aging and oxidative stress are regulated by type 1 insulin-like growth factor receptor (IGF-1R). Oxidant injury, which is implicated in the pathophysiology of a number of respiratory diseases, acutely upregulates IGF-1R expression in the lung. This led us to suspect that reduction of IGF-1R levels in lung tissue could prevent deleterious effects of oxygen exposure.

Methods

Since IGF-1R null mutant mice die at birth from respiratory failure, we generated compound heterozygous mice harboring a hypomorphic (Igf-1r^neo) and a knockout (Igf-1r^-) receptor allele. These IGF-1R^neo/- mice, strongly deficient in IGF-1R, were subjected to hyperoxia and analyzed for survival time, ventilatory control, pulmonary histopathology, morphometry, lung edema and vascular permeability.

Results

Strikingly, after 72 h of exposure to 90% O₂, IGF-1R^neo/- mice had a significantly better survival rate during recovery than IGF-1R^+/+ mice (77% versus 53%, P < 0.05). The pulmonary injury was consistently, and significantly, milder in IGF-1R^neo/- mice which developed conspicuously less edema and vascular extravasation than controls. Also, hyperoxia-induced abnormal pattern of breathing which precipitated respiratory failure was elicited less frequently in the IGF-1R^neo/- mice.

Conclusion

Together, these data demonstrate that a decrease in IGF-1R signaling in mice protects against oxidant-induced lung injury.

Interstitial lung disease in children -- genetic background and associated phenotypes

Interstitial lung disease in children represents a group of rare chronic respiratory disorders. There is growing evidence that mutations in the surfactant protein C gene play a role in the pathogenesis of certain forms of pediatric interstitial lung disease. Recently, mutations in the ABCA3 transporter were found as an underlying cause of fatal respiratory failure in neonates without surfactant protein B deficiency. Especially in familiar cases or in children of consanguineous parents, genetic diagnosis provides an useful tool to identify the underlying etiology of interstitial lung disease. The aim of this review is to summarize and to describe in detail the clinical features of hereditary interstitial lung disease in children. The knowledge of gene variants and associated phenotypes is crucial to identify relevant patients in clinical practice.

Cellular and molecular mechanisms involved in branching morphogenesis of the Drosophila tracheal system

Recent comparative studies have shown that, in many instances, the genetic network underlying the development of distinct organ systems is similar in invertebrate and vertebrate organisms. Genetically well-characterized, simple invertebrate model systems, such as Caenorhabditis elegans and Drosophila melanogaster, can thus provide useful insight for understanding more complex organ systems in vertebrates. Here, we summarize recent progress in the genetic analysis of tracheal development in Drosophila and compare the results to studies aimed at a better understanding of lung development in mouse and man. Clearly, both striking similarities and important differences are apparent, but it might still be too early to conclude whether the former or the latter prevail.

The Molecular Basis for Abnormal Human Lung Development

Our understanding of lung development in the past two decades has moved from an anatomical to a histological basis and, most recently, to a molecular basis. Tissue interactions specify tracheal and lung primordia formation, program branching morphogenesis of the airway epithelium and regulate epithelial differentiation. In addition, lung development is influenced by mechanical and humoral factors. The regulatory molecules involved in morphogenetic signaling include growth and transcription factors and extracellular matrix molecules. These morphogenetic signals are responsible for lung patterning and differentiation. We will provide a brief overview of molecular signaling during early respiratory formation, airway branching, pulmonary vascularization and epithelial differentiation. We will then review aberrant morphogenetic signaling in human lung abnormalities, such as tracheoesophageal fistula, congenital diaphragmatic hernia, pulmonary hyperplasia, alveolar capillary dysplasia, congenital cystic adenomatoid malformation and bronchopulmonary dysplasia.

Overexpression of lunatic fringe does not affect epithelial cell differentiation in the developing mouse lung

The Notch/Notch-ligand pathway regulates cell fate decisions and patterning in various tissues. Several of its components are expressed in the developing lung, suggesting that this pathway is important for airway cellular patterning. Fringe proteins, which modulate Notch signaling, are crucial for defining morphogenic borders in several organs. Their role in controlling cellular differentiation along anterior-posterior axis of the airways is unknown. Herein, we report the temporal-spatial expression patterns of Lunatic fringe (Lfng) and Notch-regulated basic helix-loop-helix factors, Hes1 and Mash-1, during murine lung development. Lfng was only expressed during early development in epithelial cells lining the larger airways. Those epithelial cells also expressed Hes1, but at later gestation Hes1 expression was confined to epithelium lining the terminal bronchioles. Mash-1 displayed a very characteristic expression pattern. It followed neural crest migration in the early lung, whereas at later stages Mash-1 was expressed in lung neuroendocrine cells. To clarify whether Lfng influences airway cell differentiation, Lfng was overexpressed in distal epithelial cells of the developing mouse lung. Overexpression of Lfng did not affect spatial or temporal expression of Hes1 and Mash-1. Neuroendocrine CGRP and protein gene product 9.5 expression was not altered by Lfng overexpression. Expression of proximal ciliated (β-tubulin IV), nonciliated ( CCSP), and distal epithelial cell ( SP-C, T1α) markers also was not influenced by Lfng excess. Overexpression of Lfng had no effect on mesenchymal cell marker (α-sma, vWF, PECAM-1) expression. Collectively, the data suggest that Lunatic fringe does not play a significant role in determining cell fate in fetal airway epithelium.

Smad1 expression and function during mouse embryonic lung branching morphogenesis

Bone morphogenetic protein (BMP) 4 plays very important roles in regulating developmental processes of many organs, including lung. Smad1 is one of the BMP receptor downstream signaling proteins that transduce BMP4 ligand signaling from cell surface to nucleus. The dynamic expression patterns of Smad1 in embryonic mouse lungs were examined using immunohistochemistry. Smad1 protein was predominantly detected in peripheral airway epithelial cells of early embryonic lung tissue [embryonic day 12.5 (E12.5)], whereas Smad1 protein expression in mesenchymal cells increased during mid-late gestation. Many Smad1-positive mesenchymal cells were localized adjacent to large airway epithelial cells and endothelial cells of blood vessels, which colocalized with a molecular marker of smooth muscle cells (alpha-smooth muscle actin). The biological function of Smad1 in early lung branching morphogenesis was then studied in our established E11.5 lung explant culture model. Reduction of endogenous Smad1 expression was achieved by adding a Smad1-specific antisense DNA oligonucleotide, causing approximately 20% reduction of lung epithelial branching. Furthermore, airway epithelial cell proliferation and differentiation were also inhibited when endogenous Smad1 expression was knocked down. Therefore, these data indicate that Smad1, acting as an intracellular BMP signaling pathway component, positively regulates early mouse embryonic lung branching morphogenesis.

FGF-10 induces SP-C and Bmp4 and regulates proximal-distal patterning in embryonic tracheal epithelium

The induction, growth, and differentiation of epithelial lung buds are regulated by the interaction of signals between the lung epithelium and its surrounding mesenchyme. Fibroblast growth factor-10 (FGF-10), which is expressed in the mesenchyme near the distal tips, and bone morphogenetic protein 4 (BMP4), which is expressed in the most distal regions of the epithelium, are important molecules in lung morphogenesis. In the present study, we used two in vitro systems to examine the induction, growth, and differentiation of lung epithelium. Transfilter cultures were used to determine the effect of diffusible factors from the distal lung mesenchyme (LgM) on epithelial branching, and FGF-10 bead cultures were used to ascertain the effect of a high local concentration of a single diffusible molecule on the epithelium. Embryonic tracheal epithelium (TrE) was induced to grow in both culture systems and to express the distal epithelial marker surfactant protein C at the tips nearest the diffusible protein source. TrE cultured on the opposite side of a filter to LgM branched in a pattern resembling intact lungs, whereas TrE cultured in apposition to an FGF-10 bead resembled a single elongating epithelial bud. Examination of the role of BMP4 on lung bud morphogenesis revealed that BMP4 signaling suppressed expression of the proximal epithelial genes Ccsp and Foxj1 in both types of culture and upregulated the expression of Sprouty 2 in TrE cultured with an FGF-10 bead. Antagonizing BMP signaling with Noggin, however, increased expression of both Ccsp and Foxj1.

Matrix GLA protein stimulates VEGF expression through increased transforming growth factor-beta1 activity in endothelial cells

Matrix GLA protein (MGP) is expressed in endothelial cells (EC), and MGP deficiency results in developmental defects suggesting involvement in EC function. To determine the role of MGP in EC, we cultured bovine aortic EC with increasing concentrations of human MGP (hMGP) for 24 h. The results showed increased proliferation, migration, tube formation, and increased release of vascular endothelial growth factor-A (VEGF-A) and basic fibroblast growth factor (bFGF). HMGP, added endogenously or transiently expressed, increased VEGF gene expression dose-dependently as determined by real-time PCR. To determine the mechanism by which hMGP increased VEGF expression, we studied the effect of MGP on the activity of transforming growth factor (TGF)-beta1 compared with that of bone morphogenetic protein (BMP)-2 using transfection assays with TGF-beta- and BMP-response element reporter genes. Our results showed a strong enhancement of TGF-beta1 activity by hMGP, which was paralleled by increased VEGF expression. BMP-2 activity, on the other hand, was inhibited by hMGP. Neutralizing antibodies to TGF-beta blocked the effect of MGP on VEGF expression. The enhanced TGF-beta1 activity specifically activated the Smad1/5 pathway indicating that the TGF-beta receptor activin-like kinase 1 (ALK1) had been stimulated. It occurred without changes in expression of TGF-beta1 or ALK1 and was mimicked by transfection of constitutively active ALK1, which increased VEGF expression. Expression of VEGF and MGP was induced by TGF-beta1, but the induction of MGP preceded that of VEGF, consistent with a promoting effect on VEGF expression. Together, the results suggest that MGP plays a role in EC function, altering the response to TGF-beta superfamily growth factors.

Hh pathway expression in human gut tissues and in inflammatory gut diseases

Sonic hedgehog (Shh) directs early gut patterning via epithelial-mesenchymal signaling and remains expressed in endoderm-derived tissues into the adult period. In human adult gut epithelium SHH/SHH expression is strongest in basal layers, which suggests that SHH may function in the maintenance of gut epithelial stem or progenitor cells. Recent publications suggest a role for aberrant SHH/SHH expression in gut epithelial neoplasias. We hypothesized that the regenerating gut epithelium in inflammatory gut disorders would show an upregulation of SHH/SHH signaling and this abnormal signal may explain the increased incidence of neoplasia in these diseases. Archived healthy gut and inflammatory gut diseased tissues were analyzed by RNA in situ hybridization and immunohistochemistry to describe location and levels of SHH signaling. We show that SHH/SHH and its receptor PTCH1/PTCH1 expression is restricted to the glandular epithelium of the gut, in an antiluminal pattern (strongest in basal layers and weak to absent in luminal epithelium). Inflammatory diseases of the gut show dramatic increases in epithelial SHH signaling. Expression increases in inflamed glandular epithelium (including metaplastic glandular epithelium), losing its radial (crypt-villous) polarity, and expression appears upregulated and present in all epithelial cells. We also describe strong SHH/SHH and PTCH1/PTCH1 expression in intraepithelial and mucosal inflammatory cells. We suggest that SHH signaling in inflammatory diseases of the gut acts to ensure stem cell restitution of damaged mucosal epithelium. However, such signaling may also present a risk for neoplastic transformation.

Fgf18 is required for embryonic lung alveolar development

Fgf18 is abundantly expressed in mouse embryonic lungs. To elucidate the roles of Fgf18 in mouse embryonic lung development, we examined the Fgf18-/- embryonic lungs. Although the sizes of the Fgf18-/- lungs were a little smaller in appearance than those of wild-type lungs, neither proximal nor distal airway branching in the Fgf18-/- lungs was impaired. However, the Fgf18-/- lungs at E18.5 had reduced alveolar space, thicker interstitial mesenchymal compartments, and many embedded capillaries. Cell proliferation in the Fgf18-/- lungs was also transiently reduced around E17.5, although the expression of marker genes for lung epithelial cells in the Fgf18-/- lungs was not impaired. The present findings indicate that the Fgf18 plays roles in lung alveolar development during late embryonic lung development stages. The cell proliferation during the terminal saccular stage stimulated by Fgf18 might play roles in the remodeling of the distal lung.

Overexpression of Smurf1 negatively regulates mouse embryonic lung branching morphogenesis by specifically reducing Smad1 and Smad5 proteins

Early embryonic lung branching morphogenesis is regulated by many growth factor-mediated pathways. Bone morphogenetic protein 4 (BMP4) is one of the morphogens that stimulate epithelial branching in mouse embryonic lung explant culture. To further understand the molecular mechanisms of BMP4-regulated lung development, we studied the biological role of Smad-ubiquitin regulatory factor 1 (Smurf1), an ubiquitin ligase specific for BMP receptor-regulated Smads, during mouse lung development. The temporo-spatial expression pattern of Smurf1 in mouse embryonic lung was first determined by quantitative real-time PCR and immunohistochemistry. Overexpression of Smurf1 in airway epithelial cells by intratracheal introduction of recombinant adenoviral vector dramatically inhibited embryonic day (E) 11.5 lung explant growth in vitro. This inhibition of lung epithelial branching was restored by coexpression of Smad1 or by addition of soluble BMP4 ligand into the culture medium. Studies at the cellular level show that overexpression of Smurf1 reduced epithelial cell proliferation and differentiation, as documented by reduced PCNA-positive cell index and by reduced mRNA levels for surfactant protein C and Clara cell protein 10 expression. Further studies found that overexpression of Smurf1 reduced BMP-specific Smad1 and Smad5, but not Smad8, protein levels. Thus overexpression of Smurf1 specifically promotes Smad1 and Smad5 ubiquitination and degradation in embryonic lung epithelium, thereby modulating the effects of BMP4 on embryonic lung growth.