Fgf10 dosage is critical for the amplification of epithelial cell progenitors and for the formation of multiple mesenchymal lineages during lung development

Transient overexpression of Gremlin results in epithelial activation and reversible fibrosis in rat lungs

Idiopathic pulmonary fibrosis (IPF) is a progressive fibrotic disease of the lung parenchyma, without curative treatment. Gremlin is a bone morphogenic protein (BMP) antagonist, its expression being increased in IPF lungs. It has been implicated in promoting myofibroblast accumulation, likely through inhibited fibroblast apoptosis and epithelial-to-mesenchymal transition. In the current study, we examined the effects of selective adenovirus-mediated overexpression of Gremlin in rat lungs. We show that transient Gremlin overexpression results in activation of alveolar epithelial cells with proliferation and apoptosis, as well as partly reversible lung fibrosis. We found myofibroblasts arranged in fibroblastic foci. Fibroblast proliferation occurred delayed as compared with epithelial changes. Fibrotic pathology significantly declined after Day 14, the reversal being associated with an increase of the epithelium-protective element, fibroblast growth factor (FGF)-10. Our data indicate that Gremlin-mediated BMP inhibition results in activation of epithelial cells and transient fibrosis, but also induction of epithelium-protective FGF10. A Gremlin-BMP-FGF10 loop may explain these results, and demonstrate that the interactions between different factors are quite complex in fibrotic lung disease. Increased Gremlin expression in human IPF tissue may be an expression of continuing epithelial injury, and Gremlin may be part of activated repair mechanisms.

Preparing for the first breath: genetic and cellular mechanisms in lung development

The mammalian respiratory system--the trachea and the lungs--arises from the anterior foregut through a sequence of morphogenetic events involving reciprocal endodermal-mesodermal interactions. The lung itself consists of two highly branched, tree-like systems--the airways and the vasculature--that develop in a coordinated way from the primary bud stage to the generation of millions of alveolar gas exchange units. We are beginning to understand some of the molecular and cellular mechanisms that underlie critical processes such as branching morphogenesis, vascular development, and the differentiation of multipotent progenitor populations. Nevertheless, many gaps remain in our knowledge, the filling of which is essential for understanding respiratory disorders, congenital defects in human neonates, and how the disruption of morphogenetic programs early in lung development can lead to deficiencies that persist throughout life.

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.

SPARC suppresses apoptosis of idiopathic pulmonary fibrosis fibroblasts through constitutive activation of beta-catenin

Idiopathic pulmonary fibrosis (IPF) is a poorly understood progressive disease characterized by the accumulation of scar tissue in the lung interstitium. A hallmark of the disease is areas of injury to type II alveolar epithelial cells with attendant accumulation of fibroblasts in areas called fibroblastic foci. In an effort to better characterize the lung fibroblast phenotype in IPF patients, we isolated fibroblasts from patients with IPF and looked for activation of signaling proteins, which could help explain the exaggerated fibrogenic response in IPF. We found that IPF fibroblasts constitutively expressed increased basal levels of SPARC, plasminogen activator inhibitor-1 (PAI-1), and active beta-catenin compared with control cells. Control of basal PAI-1 expression in IPF fibroblasts was regulated by SPARC-mediated activation of Akt, leading to inhibition of glycogen synthase kinase-3beta and activation of beta-catenin. Additionally, IPF fibroblasts (but not control fibroblasts) were resistant to plasminogen-induced apoptosis and were sensitized to plasminogen-mediated apoptosis by inhibition of SPARC or beta-catenin. These findings uncover a newly discovered regulatory pathway in IPF fibroblasts that is characterized by elevated SPARC, giving rise to activated beta-catenin, which regulates expression of downstream genes, such as PAI-1, and confers an apoptosis-resistant phenotype. Disruption of this pathway may represent a novel therapeutic target in IPF.

Evidence of an epithelial stem/progenitor cell hierarchy in the adult mouse lung

The role of lung epithelial stem cells in maintenance and repair of the adult lung is ill-defined, and their identity remains contentious because of the lack of definitive markers for their prospective isolation and the absence of clonogenic assays able to measure their stem/progenitor cell potential. In this study, we show that replication of epithelial-mesenchymal interactions in a previously undescribed matrigel-based clonogenic assay enables the identification of lung epithelial stem/progenitor cells by their colony-forming potential in vitro. We describe a population of EpCAM(hi) CD49f(pos) CD104(pos) CD24(low) epithelial cfus that generate colonies comprising airway, alveolar, or mixed lung epithelial cell lineages when cocultured with EpCAM(neg) Sca-1(pos) lung mesenchymal cells. We show that soluble fibroblast growth factor-10 and hepatocyte growth factor partially replace the requirement for mesenchymal support of epithelial colony formation, allowing clonal passaging and demonstration of their capacity for self-renewal. These data support a model in which the adult mouse lung contains a minor population of multipotent epithelial stem/progenitor cells with the capacity for self-renewal and whose descendants give rise to airway and alveolar epithelial cell lineages in vitro.

Lung organogenesis

Developmental lung biology is a field that has the potential for significant human impact: lung disease at the extremes of age continues to cause major morbidity and mortality worldwide. Understanding how the lung develops holds the promise that investigators can use this knowledge to aid lung repair and regeneration. In the decade since the "molecular embryology" of the lung was first comprehensively reviewed, new challenges have emerged-and it is on these that we focus the current review. Firstly, there is a critical need to understand the progenitor cell biology of the lung in order to exploit the potential of stem cells for the treatment of lung disease. Secondly, the current familiar descriptions of lung morphogenesis governed by growth and transcription factors need to be elaborated upon with the reinclusion and reconsideration of other factors, such as mechanics, in lung growth. Thirdly, efforts to parse the finer detail of lung bud signaling may need to be combined with broader consideration of overarching mechanisms that may be therapeutically easier to target: in this arena, we advance the proposal that looking at the lung in general (and branching in particular) in terms of clocks may yield unexpected benefits.

Fgf10 gene expression is delayed in the embryonic lung mesenchyme in the adriamycin mouse model

BACKGROUND

The adriamycin mouse model is a well-established teratogenic model of esophageal atresia/tracheoesophageal fistula. Fibroblast growth factor 10 (Fgf10) plays a key role in branching of the lung buds during lung morphogenesis. Fgf10 knockout mice exhibit the absence of the lungs. Optical projection tomography (OPT) is a technique that allows three-dimensional (3D) imaging of gene expression in small tissue specimens in an anatomical context. The aim of this study was to investigate the temporo-spatial expression of Fgf10 during the critical period of separation of the trachea and esophagus in normal and adriamycin-treated embryos using OPT.

METHODS

Time-mated CBA/Ca mice received intraperitoneal injections of adriamycin (6 mg/kg) or saline on days 7 and 8 of gestation. Embryos were harvested on days 10-13, stained after whole mount in situ hybridization with labeled RNA probes to detect Fgf10 transcripts (n = 5 for each treatment/day of gestation). Immunolocalization with endoderm marker Hnf3 beta was used to visualize morphology. Embryos were scanned by OPT to obtain 3D representations of gene expression domains.

RESULTS

Computer reconstructed specimens allowed precise staging of developing embryos according to Theiler Staging (TS) criteria. OPT elegantly displayed Fgf10 gene expression in the pulmonary mesenchyme around the tip of the lung buds in both controls and treated embryos in the same spatial territory. Fgf10 gene expression was first detected in the control embryos at TS17. However, Fgf10 gene expression in adriamycin-treated embryos was first only observed at TS18 in 67% of the specimens.

CONCLUSION

Delayed Fgf10 gene expression during the critical period of separation of the trachea and esophagus may affect lung bud formation in the adriamycin model leading to tracheoesophageal malformations.

National Heart, Lung, and Blood Institute perspective: lung progenitor and stem cells--gaps in knowledge and future opportunities

Because the lung stem cell field is so new, there remain many unanswered questions that are being addressed regarding the identification, location, and role of exogenous and endogenous stem and progenitor cell populations in growth, regeneration, and repair of the lung. Advancing lung stem cell biology will require multidisciplinary teams and a long term effort to unravel the biologic processes of stem cells in the lung. While no clinical research in lung stem cell therapies are currently funded by NHLBI, the knowledge gained by understanding the basic biology of the lung stem cell populations will be needed to translate to diagnostic and therapeutic strategies in the future.

Conditional gene inactivation reveals roles for Fgf10 and Fgfr2 in establishing a normal pattern of epithelial branching in the mouse lung

Fibroblast growth factor 10 (FGF10) signaling through FGF receptor 2 (FGFR2) is required for lung initiation. While studies indicate that Fgf10 and Fgfr2 are also important at later stages of lung development, their roles in early branching events remain unclear. We addressed this question through conditional inactivation of both genes in mouse subsequent to lung initiation. Inactivation of Fgf10 in lung mesenchyme resulted in smaller lobes with a reduced number of branches. Inactivation of Fgfr2 in lung epithelium resulted in disruption of lobes and small epithelial outgrowths that arose arbitrarily along the main bronchi. In both mutants, there was an increase in cell death. Also, the expression patterns of key signaling molecules implicated in branching morphogenesis were altered and a proximal lung marker was expanded distally. Our results indicate that both Fgf10 and Fgfr2 are required for a normal branching program and for proper proximal-distal patterning of the lung.

Overexpression of fibroblast growth factor-10 during both inflammatory and fibrotic phases attenuates bleomycin-induced pulmonary fibrosis in mice

RATIONALE

Fibroblast growth factor-10 (FGF10) controls survival, proliferation, and differentiation of distal-alveolar epithelial progenitor cells during lung development.

OBJECTIVES

To test for the protective and regenerative effect of Fgf10 overexpression in a bleomycin-induced mouse model of pulmonary inflammation and fibrosis.

METHODS

In SP-C-rtTA; tet(O)Fgf10 double-transgenic mice, lung fibrosis was induced in 2-month-old transgenic mice by subcutaneous delivery of bleomycin (BLM), using an osmotic minipump for 1 week. Exogenous Fgf10 expression in the alveolar epithelium was induced for 7 days with doxycycline during the first, second, and third weeks after bleomycin pump implantation, and lungs were examined at 28 days.

MEASUREMENTS AND MAIN RESULTS

Fgf10 overexpression during Week 1 (inflammatory phase) resulted in increased survival and attenuated lung fibrosis score and collagen deposition. In these Fgf10-overexpressing mice, an increase in regulatory T cells and a reduction in both transforming growth factor-beta(1) and matrix metalloproteinase-2 activity were observed in bronchoalveolar lavage fluids whereas the number of surfactant protein C (SP-C)-positive, alveolar epithelial type II cells (AEC2) was markedly elevated. Analysis of SP-C and TUNEL (terminal deoxynucleotidyltransferase dUTP nick end labeling) double-positive cells and isolation of AEC2 from lungs overexpressing Fgf10 demonstrated increased AEC2 survival. Expression of Fgf10 during Weeks 2 and 3 (fibrotic phase) showed significant attenuation of the lung fibrosis score and collagen deposition.

CONCLUSIONS

In the bleomycin model of lung inflammation and fibrosis, Fgf10 overexpression during both the inflammatory and fibrotic phases results in a greatly reduced extent of lung fibrosis, suggesting that FGF10 may be useful as a novel approach to the treatment of pulmonary fibrosis.

Fibroblast growth factor 9 signaling inhibits airway smooth muscle differentiation in mouse lung

In mammalian lungs, airway smooth muscle cells (airway SMCs) are present in the proximal lung adjacent to bronchi and bronchioles, but are absent in the distal lung adjacent to terminal sacs that expand during gas exchange. Evidence suggests that this distribution is essential for the formation of a functional respiratory tree, but the underlying genetic mechanism has not been elucidated. In this study, we test the hypothesis that fibroblast growth factor 9 (Fgf9) signaling is essential to restrict SMC differentiation to the proximal lung. We show that loss of Fgf9 or conditional inactivation of Fgf receptors (Fgfr) 1 and 2 in mouse lung mesenchyme results in ectopic SMCs. Our data support a model where FGF9 maintains a SMC progenitor population by suppressing differentiation and promoting growth. This model also represents our findings on the genetic relationship between FGF9 and sonic hedgehog (SHH) in the establishment of airway SMC pattern.

MicroRNA-127 modulates fetal lung development

MicroRNAs (miRNAs) are small endogenous RNAs and are widely regarded as one of the most important regulators of gene expression in both plants and animals. To define the roles of miRNAs in fetal lung development, we profiled the miRNA expression pattern during lung development with a miRNA microarray. We identified 21 miRNAs that showed significant changes in expression during lung development. These miRNAs were grouped into four distinct clusters based on their expression pattern. Cluster 1 contained miRNAs whose expression increased as development progressed, while clusters 2 and 3 showed the opposite trend of expression. miRNAs in cluster 4 including miRNA-127 (miR-127) had the highest expression at the late stage of fetal lung development. Quantitative real-time PCR validated the microarray results of six selected miRNAs. In situ hybridization demonstrated that miR-127 expression gradually shifted from mesenchymal cells to epithelial cells as development progressed. Overexpression of miR-127 in fetal lung organ culture significantly decreased the terminal bud count, increased terminal and internal bud sizes, and caused unevenness in bud sizes, indicating improper development. These findings suggest that miR-127 may have an important role in fetal lung development.

Resident cellular components of the lung: developmental aspects

In a recent workshop organized by the NIH-NHLBI, investigators working on different aspects of lung biology met to discuss recent progress regarding the origin, development, and characterization of the various cell lineages present in the lung in both normal and disease states. The workshop was entitled "Resident Cellular Components of the Human Lung: Current Knowledge and Goals for Research on Cell Phenotyping and Function." In this article we will highlight some of the developmental aspects of the lung discussed at the meeting. We will review information about developmental signals that are possibly reactivated during lung regeneration/repair and disease processes, and we will pose the questions and challenges viewed to be relevant to further advance the field.

Lung epithelial progenitor cells: lessons from development

The current enthusiasm for stem cell research stems from the hope that damaged or diseased tissues may one day be repaired through the manipulation of endogenous or exogenous stem cells. The postnatal human respiratory system is highly accessible and provides unique opportunities for the application of such techniques. Several putative adult lung epithelial stem cells have been identified in the mouse model system. However, their in vivo capabilities to contribute to different lineages, and their control mechanisms, remain unclear. If stem cell-based therapies are to be successful in the lung, it is vitally important that we understand the normal behavior of adult lung stem cells, and how this is regulated. Lung embryonic progenitor cells are much better defined and characterized than their adult counterparts. Moreover, experiments on a variety of developing tissues are beginning to uncover general mechanisms by which embryonic progenitors influence final organ size and structure. This provides a framework for the study of lung embryonic progenitor cells, facilitating experimental design and interpretation. A similar approach to investigating adult lung stem cells could produce rapid advances in the field.

Stem/progenitor cells in lung development, injury repair, and regeneration

At least two populations of epithelial stem/progenitor cells give rise to the lung anlage, comprising the laryngo-tracheal complex versus the distal lung below the first bronchial bifurcation. Amplification of the distal population requires FGF9-FGF10-FGFR2b-Sprouty signaling. Residual pools of adult stem cells are hypothesized to be the source of lung regeneration and repair. These pools have been located within the basal layer of the upper airways, within or near pulmonary neuroendocrine cell rests, at the bronchoalveolar junction as well as within the alveolar epithelial surface. Rapid repair of the denuded alveolar surface after injury is clearly key to survival. Strategies to enhance endogenous alveolar epithelial repair could include protection of epithelial progenitors from injury and/or stimulation of endogenous progenitor cell function. Protection with inosine or FGF signaling are possible small molecule therapeutic options. Alternatively, exogenous stem/progenitor cells can be delivered into the lung either intravenously, intratracheally, or by direct injection. Sources of exogenous stem/progenitor cells that are currently under evaluation in the context of acute lung injury repair include embryonic stem cells, bone marrow- or fat-derived mesenchymal stem cells, circulating endothelial progenitors, and, recently, amniotic fluid stem/progenitor cells. Further work will be needed to translate stem/progenitor cell therapy for the lung.

Epithelial progenitor cells of the embryonic lung and the role of microRNAs in their proliferation

The entire epithelium of the lung is generated from a small pool of undifferentiated progenitor cells. At least during the early stages of development these reside in the distal tips of the embryonic lung. They respond to multiple signals from the surrounding mesenchyme and play a critical role as morphogenetic organizing centers. In addition, they proliferate rapidly and give rise to daughter cells that differentiate into all the specialized epithelial cells types of the newborn lung. Despite the importance of the progenitor cells, we still know relatively little about the mechanisms controlling their proliferation, morphogenesis, and developmental fate. Here, we discuss new data on the potential role of microRNAs in co-coordinately regulating multiple signaling pathways in embryonic progenitor cells. In particular, our recent transgenic experiments suggest that microRNAs encoded by the miR-17-92 cluster positively promote proliferation of epithelial progenitor cells and inhibit their differentiation. We speculate on how this information might be exploited therapeutically in the long term.

Misexpression of ELF5 disrupts lung branching and inhibits epithelial differentiation

ELF5, an Ets family transcription factor found exclusively in epithelial cells, is expressed in the distal lung epithelium during embryogenesis, then becomes restricted to proximal airways at the end of gestation and postnatally. To test the hypothesis that ELF5 represses distal epithelial differentiation, we generated a transgenic mouse model in which a doxycycline inducible HA-tagged mouse Elf5 transgene was placed under the control of the lung epithelium-specific human SFTPC promoter. We found that expressing high levels of ELF5 during early lung development disrupted branching morphogenesis and produced a dilated epithelium. The effects of ELF5 on morphogenesis were stage-dependent, since inducing the transgene on E16.5 had no effect on branching. ELF5 reduced expression of the distal lung epithelial differentiation markers Erm, Napsa and Sftpc, and type II cell ultrastructural differentiation was immature. ELF5 overexpression did not induce the proximal airway epithelial markers Ccsp and Foxj1, but did induce expression of p63, a marker of basal cells in the trachea and esophagus. High ELF5 levels also induced the expression of genes found in other endodermal epithelia but not normally associated with the lung. These results suggest that precise levels of ELF5 regulate the specification and differentiation of epithelial cells in the lung.

FGF10 maintains distal lung bud epithelium and excessive signaling leads to progenitor state arrest, distalization, and goblet cell metaplasia

BACKGROUND

Interaction with the surrounding mesenchyme is necessary for development of endodermal organs, and Fibroblast growth factors have recently emerged as mesenchymal-expressed morphogens that direct endodermal morphogenesis. The fibroblast growth factor 10 (Fgf10) null mouse is characterized by the absence of lung bud development. Previous studies have shown that this requirement for Fgf10 is due in part to its role as a chemotactic factor during branching morphogenesis. In other endodermal organs Fgf10 also plays a role in regulating differentiation.

RESULTS

Through gain-of-function analysis, we here find that FGF10 inhibits differentiation of the lung epithelium and promotes distalization of the embryonic lung. Ectopic expression of FGF10 in the lung epithelium caused impaired lung development and perinatal lethality in a transgenic mouse model. Lung lobes were enlarged due to increased interlobular distance and hyperplasia of the airway epithelium. Differentiation of bronchial and alveolar cell lineages was inhibited. The transgenic epithelium consisted predominantly of proliferating progenitor-like cells expressing Pro-surfactant protein C, TTF1, PEA3 and Clusterin similarly to immature distal tip cells. Strikingly, goblet cells developed within this arrested epithelium leading to goblet cell hyperplasia.

CONCLUSION

We conclude that FGF10 inhibits terminal differentiation in the embryonic lung and maintains the distal epithelium, and that excessive levels of FGF10 leads to metaplastic differentiation of goblet cells similar to that seen in chronic inflammatory diseases.