KGF and FGF-10 stimulate liquid secretion in human fetal lung

Phenotypic spectrum of FGF10-related disorders: a systematic review

FGF10, as an FGFR2b-specific ligand, plays a crucial role during cell proliferation, multi-organ development, and tissue injury repair. The developmental importance of FGF10 has been emphasized by the identification of FGF10 abnormalities in human congenital disorders affecting different organs and systems. Single-nucleotide variants in FGF10 or FGF10-involving copy-number variant deletions have been reported in families with lacrimo-auriculo-dento-digital syndrome, aplasia of the lacrimal and salivary glands, or lethal lung developmental disorders. Abnormalities involving FGF10 have also been implicated in cleft lip and palate, myopia, or congenital heart disease. However, the exact developmental role of FGF10 and large phenotypic heterogeneity associated with FGF10 disruption remain incompletely understood. Here, we review human and animal studies and summarize the data on FGF10 mechanism of action, expression, multi-organ function, as well as its variants and their usefulness for clinicians and researchers.

A mesenchymal to epithelial switch in Fgf10 expression specifies an evolutionary-conserved population of ionocytes in salivary glands

Fibroblast growth factor 10 (FGF10) is well established as a mesenchyme-derived growth factor and a critical regulator of fetal organ development in mice and humans. Using a single-cell RNA sequencing (RNA-seq) atlas of salivary gland (SG) and a tamoxifen inducible Fgf10CreERT2:R26-tdTomato mouse, we show that FGF10pos cells are exclusively mesenchymal until postnatal day 5 (P5) but, after P7, there is a switch in expression and only epithelial FGF10pos cells are observed after P15. Further RNA-seq analysis of sorted mesenchymal and epithelial FGF10pos cells shows that the epithelial FGF10pos population express the hallmarks of ancient ionocyte signature Forkhead box i1 and 2 (Foxi1, Foxi2), Achaete-scute homolog 3 (Ascl3), and the cystic fibrosis transmembrane conductance regulator (Cftr). We propose that epithelial FGF10pos cells are specialized SG ionocytes located in ducts and important for the ionic modification of saliva. In addition, they maintain FGF10-dependent gland homeostasis via communication with FGFR2bpos ductal and myoepithelial cells.

Negative Transpulmonary Pressure Disrupts Airway Morphogenesis by Suppressing Fgf10

Mechanical forces are increasingly recognized as important determinants of cell and tissue phenotype and also appear to play a critical role in organ development. During the fetal stages of lung morphogenesis, the pressure of the fluid within the lumen of the airways is higher than that within the chest cavity, resulting in a positive transpulmonary pressure. Several congenital defects decrease or reverse transpulmonary pressure across the developing airways and are associated with a reduced number of branches and a correspondingly underdeveloped lung that is insufficient for gas exchange after birth. The small size of the early pseudoglandular stage lung and its relative inaccessibility in utero have precluded experimental investigation of the effects of transpulmonary pressure on early branching morphogenesis. Here, we present a simple culture model to explore the effects of negative transpulmonary pressure on development of the embryonic airways. We found that negative transpulmonary pressure decreases branching, and that it does so in part by altering the expression of fibroblast growth factor 10 (Fgf10). The morphogenesis of lungs maintained under negative transpulmonary pressure can be rescued by supplementing the culture medium with exogenous FGF10. These data suggest that Fgf10 expression is regulated by mechanical stress in the developing airways. Understanding the mechanical signaling pathways that connect transpulmonary pressure to FGF10 can lead to the establishment of novel non-surgical approaches for ameliorating congenital lung defects.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

FGF Signaling in Lung Development and Disease: Human Versus Mouse

Fibroblast growth factor 10 (FGF10) plays an important role in mouse lung development, injury, and repair. It is considered the main morphogen driving lung branching morphogenesis in rodents. While many studies have found FGF10 SNPs associated with COPD and branch variants in COPD smokers, there is no evidence of a causative role for FGF10 or these SNPs in human lung development and pediatric lung diseases. We and others have shown divergent roles for FGF10 in mouse lung development and early human lung development. Herein, we only review the existing literature on FGF signaling in human lung development and pediatric human lung diseases, comparing what is known in mouse lung to that in human lung.

Discordant roles for FGF ligands in lung branching morphogenesis between human and mouse

Fibroblast growth factor (FGF) signaling plays an important role in lung organogenesis. Over recent decades, FGF signaling in lung development has been extensively studied in animal models. However, little is known about the expression, localization, and functional roles of FGF ligands during human fetal lung development. Therefore, we aimed to determine the expression and function of several FGF ligands and receptors in human lung development. Using in situ hybridization (ISH) and RNA sequencing, we assessed their expression and distribution in native human fetal lung. Human fetal lung explants were treated with recombinant FGF7, FGF9, or FGF10 in air-liquid interface culture. Explants were analyzed grossly to observe differences in branching pattern as well as at the cellular and molecular level. ISH demonstrated that FGF7 is expressed in both the epithelium and mesenchyme; FGF9 is mainly localized in the distal epithelium, whereas FGF10 demonstrated diffuse expression throughout the parenchyma, with some expression in the smooth muscle cells (SMCs). FGFR2 expression was high in both proximal and distal epithelial cells as well as the SMCs. FGFR3 was expressed mostly in the epithelial cells, with lower expression in the mesenchyme, while FGFR4 was highly expressed throughout the mesenchyme and in the distal epithelium. Using recombinant FGFs, we demonstrated that FGF7 and FGF9 had similar effects on human fetal lung as on mouse fetal lung; however, FGF10 caused the human explants to expand and form cysts as opposed to inducing epithelial branching as seen in the mouse. In conjunction with decreased branching, treatment with recombinant FGF7, FGF9, and FGF10 also resulted in decreased double-positive SOX2/SOX9 progenitor cells, which are exclusively present in the distal epithelial tips in early human fetal lung. Although FGF ligand localization may be somewhat comparable between developing mouse and human lungs, their functional roles may differ substantially. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Lack of cystic fibrosis transmembrane conductance regulator disrupts fetal airway development in pigs

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) function causes cystic fibrosis (CF), predisposing the lungs to chronic infection and inflammation. In young infants with CF, structural airway defects are increasingly recognized before the onset of significant lung disease, which suggests a developmental origin and a possible role in lung disease pathogenesis. The role(s) of CFTR in lung development is unclear and developmental studies in humans with CF are not feasible. Young CF pigs have structural airway changes and develop spontaneous postnatal lung disease similar to humans; therefore, we studied lung development in the pig model (non-CF and CF). CF trachea and proximal airways had structural lesions detectable as early as pseudoglandular development. At this early developmental stage, budding CF airways had smaller, hypo-distended lumens compared to non-CF airways. Non-CF lung explants exhibited airway lumen distension in response to forskolin/IBMX as well as to fibroblast growth factor (FGF)-10, consistent with CFTR-dependent anion transport/secretion, but this was lacking in CF airways. We studied primary pig airway epithelial cell cultures and found that FGF10 increased cellular proliferation (non-CF and CF) and CFTR expression/function (in non-CF only). In pseudoglandular stage lung tissue, CFTR protein was exclusively localized to the leading edges of budding airways in non-CF (but not CF) lungs. This discreet microanatomic localization of CFTR is consistent with the site, during branching morphogenesis, where airway epithelia are responsive to FGF10 regulation. In summary, our results suggest that the CF proximal airway defects originate during branching morphogenesis and that the lack of CFTR-dependent anion transport/liquid secretion likely contributes to these hypo-distended airways.

Expression of chloride channels in trachea-occluded hyperplastic lungs and nitrofen-induced hypoplastic lungs in rats

BACKGROUND

Congenital diaphragmatic hernia is accompanied by pulmonary hypoplasia. Fetal lung growth is dependent on the secretion of lung liquid, in which Cl(-) secretion by the pulmonary epithelium plays a crucial role. A decrease of lung liquid production during fetal development renders marked pulmonary hypoplasia, while accelerated fetal lung growth in the form of pulmonary hyperplasia can be achieved by in utero tracheal occlusion (TO). Cl(-) secretion presumably involves NKCC-1, the primary basolateral Cl(-) entry pathway in airway epithelia, coupled to an apical Cl(-) exit pathway. The chloride channels ClC-2, -3 and -5, members of the CLC gene family, are all localized to the apical membrane of fetal respiratory epithelia, which makes them possible candidates for being mediators of fetal apical Cl(-) secretion. The aim of the study was to examine the potential of ClC-2, -3 and -5 as alternative apical airway epithelial Cl(-) channels in normal lung development and their possible role in the development of hypoplastic lungs in CDH. We also wanted to examine ClC-2, -3 and -5 together with the NKCC-1 in hyperplastic lungs created by TO.

METHODS

Pregnant Sprague-Dawley rat dams were given nitrofen on gestational day 9.5 to induce pulmonary hypoplasia. Controls were given only olive oil. The rat fetuses were removed on days 17, 19 and 21. Hyperplastic lungs were created by intrauterine TO of rat fetuses on day 19 and the lungs were harvested on day 21. The pulmonary expression of ClC-2, -3, -5 and NKCC-1 was then analyzed using Western blot.

RESULTS

We found that the temporal expression of ClC-2 and -3 in normal fetal lungs points toward a developmental regulation. ClC-2 and -3 were also both down-regulated on day 21 in hypoplastic CDH lungs. In TO induced hyperplastic lungs, the levels of ClC-2 were found to be significantly up-regulated. NKCC-1 showed a tendency toward up-regulation in hyperplastic lungs, while ClC-3 showed a tendency to be down-regulated, but no statistically significant changes could be seen. There was no difference between controls and any of the groups for the expression of ClC-5.

CONCLUSION

We show that the developmental changes in ClC-2 and ClC-3 protein expression are negatively affected in hypoplastic CDH lungs. Lung hyperplasia created by TO up-regulates the expression of ClC-2. ClC-2 is therefore an interesting potential target in the development of novel, non-invasive, therapies for CDH treatment.

Regulation of alveologenesis: clinical implications of impaired growth

During its development that begins in intrauterine life, the lung is transformed from a simple epithelial lined sac that emerges from the foregut into a complex arrangement of blood vessels, airways, and alveoli that make up the mature lung structure. This remarkable transformation that continues for several years postnatally, is achieved by the influence of several genes, transcription factors, growth factors and hormones upon the cells and proteins of the lung bud. A seminal event in this process is the formation of the air-blood barrier within the alveolar wall, an evolutionary modification that permits independent air-breathing existence in mammals. Molecular biological techniques have enabled elucidation of the mechanistic pathways contributing to alveologenesis and have provided probable molecular bases for examples of impaired alveologenesis encountered by the paediatric pathologist.

Elf5 is an epithelium-specific, fibroblast growth factor-sensitive transcription factor in the embryonic lung

Fibroblast growth factor (FGF) signaling has been shown to be essential for many aspects of normal lung development. To determine epithelial targets of FGF signaling, we cultured embryonic day (E) 11.5 mouse lungs for 24 hr in the presence or absence of the FGF receptor antagonist SU5402, which inhibited branching morphogenesis. Affymetrix gene chip analysis of treated and control epithelia identified several genes regulated by FGF signaling, including Elf5, a member of the Epithelial-specific Ets family of transcription factors. SU5402 reduced Elf5 expression in mesenchyme-free cultures of E12.5 epithelium, demonstrating that the inhibition was direct. In situ hybridization revealed that Elf5 had a dynamic pattern of expression during lung development. We found that expression of Elf5 was induced by FGF7 and FGF10, ligands that primarily bind FGFR2b. To further define the pathways by which FGFs activate Elf5 expression, we cultured E11.5 lung tips in the presence of compounds to inhibit FGF receptors (SU5402), PI3-Kinase/Akt-mediated signaling (LY294002), and MAP Kinase/Erk-mediated signaling (U0126). We found that SU5402 and LY294002 significantly reduced Elf5 expression, whereas U0126 had no effect. LY294002 also reduced Elf5 expression in cultures of purified epithelium. Finally, pAkt was coexpressed with Elf5 in the proximal epithelial airways of E17.5 lungs. These results demonstrate that Elf5 is an FGF-sensitive transcription factor in the lung with a dynamic pattern of expression and that FGF regulation of Elf5 by means of FGFR2b occurs through the PI3-Kinase/Akt pathway.

Differential response of the epithelium and interstitium in developing human fetal lung explants to hyperoxia

Hyperoxia is closely linked with the development of chronic lung disease of prematurity (CLD), but the exact mechanisms whereby hyperoxia alters the lung architecture in the developing lung remain largely unknown. We developed a fetal human lung organ culture model to investigate (a) the morphologic changes induced by hyperoxia and (b) whether hyperoxia resulted in differential cellular responses in the epithelium and interstitium. The effects of hyperoxia on lung morphometry were analyzed using computer-assisted image analysis. The lung architecture remained largely unchanged in normoxia lasting as long as 4 d. In contrast, hyperoxic culture of pseudoglandular fetal lungs resulted in significant dilatation of airways, thinning of the epithelium, and regression of the interstitium including the pulmonary vasculature. Although there were no significant differences in Ki67 between normoxic and hyperoxic lungs, activated caspase-3 was significantly increased in interstitial cells, but not epithelial cells, under hyperoxic conditions. These changes show that exposure of pseudoglandular lungs to hyperoxia modulates the lung architecture to resemble saccular lungs.

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.

Developmental Regulation of Lung Liquid Transport

The developing distal lung epithelium displays an evolving liquid transport phenotype, reflecting a changing and dynamic balance between Cl- ion secretion and Na+ ion absorption, which in turn reflects changing functional requirements. Thus in the fetus, Cl--driven liquid secretion predominates throughout gestation and generates a distending pressure to stretch the lung and stimulate growth. Increasing Na+ absorptive capacity develops toward term, anticipating the switch to an absorptive phenotype at birth and beyond. There is some empirical evidence of ligand-gated regulation of Cl- transport and of regulation via changes in the driving force for Cl- secretion. Epinephrine, O2, glucocorticoid, and thyroid hormones interact to stimulate Na+ absorption by increasing Na+ pump activity and apical Na+ conductance (GNa+) to bring about the switch from net secretion to net absorption as lung liquid is cleared from the lung at birth. Postnatally, the lung lumen contains a small Cl--based liquid secretion that generates a surface liquid layer, but the lung retains a large absorptive capacity to prevent alveolar flooding and clear edema fluid. This review explores the mechanisms underlying the functional development of the lung epithelium and draws upon evidence from classic integrative physiological studies combined with molecular physiology approaches.

Gene expression of fibroblast growth factors 10 and 7 is downregulated in the lung of nitrofen-induced diaphragmatic hernia in rats

BACKGROUND/PURPOSE

Newborns with congenital diaphragmatic hernia (CDH) still have a high mortality rate, which has been attributed to pulmonary hypoplasia and pulmonary hypertension. Fibroblast growth factors (FGFs) are essential components of the gene network that regulates lung development. Recent studies suggest that the new member of FGF family, FGF-10, plays a fundamental role in branching morphogenesis and is essential for lung formation. FGF-10-deficient mice exhibit complete absence of lungs. FGF-7 promotes epithelial proliferation and expansion leading to the formation of cystlike structures. The aim of this study was to determine the gene level expression of FGF-10 and FGF-7 in the lung of nitrofen-induced CDH.

METHODS

Congenital diaphragmatic hernia (CDH) was induced in pregnant rats after administration of 100 mg of nitrofen on day 9.5 of gestation (term, 22 days). In control animals, the same dose of olive oil was given without nitrofen. Cesarean section was performed on day 21 of gestation. The fetuses were divided into 3 groups: normal controls (n = 16), nitrofen induced without CDH (n = 16), and nitrofen-induced CDH (n = 16). Total RNA and DNA were extracted from the lung in each group and measured. mRNA was extracted from total RNA. Reverse transcription polymerase chain reaction (RT-PCR) was performed to evaluate mRNA expressions of FGF-10 and FGF-7. Levels of mRNA were expressed as a ratio of the band density divided by that of beta-actin, a house-keeping gene.

RESULTS

FGF-10 mRNA expression was decreased significantly in CDH lung (2.914 +/- 0.320) compared with controls (4.062 +/- 0.307; P <.05) and nitrofen induced without CDH lung (3.923 +/- 0.250; P <.01). FGF-7 mRNA expression was decreased significantly in CDH lung (0.777 +/- 0.097) compared with controls (1.028 +/- 0.093; P <.01).

CONCLUSIONS

Decreased gene expression of FGF-10 and FGF-7 in the hypoplastic lung suggests that pulmonary hypoplasia in nitrofen-induced CDH rat may be caused by reduced synthesis of FGF-10 and FGF-7 during lung morphogenesis.

Developmental paradigms in terminal lung development

Late lung development comprises the formation of the terminal sac followed by the subdivision of the terminal sac by septa into alveoli and results in the formation of the gas-exchange surface of the lung. This developmentally regulated process involves a complex epithelium-mesenchyme interaction via evolutionarily conserved molecular signaling pathways. In addition, there is a continuous process of vascular growth and development. Currently there are large gaps in our understanding of the molecular mechanisms involved in the formation of the gas-exchange surface. In this review, we attempt to integrate and reconcile the morphologic features in late lung development with what is known about the molecular basis for these processes. We describe the formation of the terminal sac and the subsequent formation of the septa, which divide the terminal sac into alveoli, in terms of the classically described developmental stages of induction, morphogenesis and differentiation. We believe that evolutionarily conserved pathways regulate this process and that morphogen gradients are likely to be a central mechanism. In addition, we highlight the importance of the molecular mechanisms involved in the simultaneous development of the vascular bed and its importance in the late development of the lungs.

Keratinocyte and hepatocyte growth factors in the lung: roles in lung development, inflammation, and repair

A growing body of evidence indicates that the epithelial-specific growth factors keratinocyte growth factor (KGF), fibroblast growth factor (FGF)-10, and hepatocyte growth factor (HGF) play important roles in lung development, lung inflammation, and repair. The therapeutic potential of these growth factors in lung disease has yet to be fully explored. KGF has been best studied and has impressive protective effects against a wide variety of injurious stimuli when given as a pretreatment in animal models. Whether this protective effect could translate to a treatment effect in humans with acute lung injury needs to be investigated. FGF-10 and HGF may also have therapeutic potential, but more extensive studies in animal models are needed. Because HGF lacks true epithelial specificity, it may have less potential than KGF and FGF-10 as a targeted therapy to facilitate lung epithelial repair. Regardless of their therapeutic potential, studies of the unique roles played by these growth factors in the pathogenesis and the resolution of acute lung injury and other lung diseases will continue to enhance our understanding of the complex pathophysiology of inflammation and repair in the lung.

KGF alters gene expression in human airway epithelia: potential regulation of the inflammatory response

Keratinocyte growth factor (KGF) regulates several functions in adult and developing lung epithelia; it causes proliferation, stimulates secretion of fluid and electrolytes, enhances repair, and may minimize injury. To gain insight into the molecular processes influenced by KGF, we applied KGF to primary cultures of well-differentiated human airway epithelia and used microarray hybridization to assess the abundance of gene transcripts. Of 7,069 genes tested, KGF changed expression levels of 910. Earlier studies showed that KGF causes epithelial proliferation, and as expected, treatment altered expression of numerous genes involved in cell proliferation. We found that KGF stimulated transepithelial Cl(-) transport, but the number of cystic fibrosis (CF) transmembrane conductance regulator (CFTR) transcripts fell. Although transcripts for ClC-1 and ClC-7 Cl(-) channels increased, KGF failed to augment transepithelial Cl(-) transport in CF epithelia, suggesting that KGF-stimulated Cl(-) transport in differentiated airway epithelia depends on the CFTR Cl(-) channel. Interestingly, KGF decreased transcripts for many interferon (IFN)-induced genes. IFN causes trafficking of Stat dimers to the nucleus, where they activate transcription of IFN-induced genes. We found that KGF prevented the IFN-stimulated trafficking of Stat1 from the cytosol to the nucleus, suggesting a molecular mechanism for KGF-mediated suppression of the IFN-signaling pathway. These results suggest that in addition to stimulating proliferation and repair of damaged airway epithelia, KGF stimulates Cl(-) transport and may dampen the response of epithelial cells to inflammatory mediators.

Effect of lung fluid composition on type II cellular activity after tracheal occlusion in the fetal lamb

BACKGROUND/PURPOSE

Fetal tracheal occlusion (TO) causes accelerated lung growth. However, prolonged TO is associated with a decline in the type II cell number. Type II cell function after TO is unclear. Herein, the authors examine type II cell function after TO and the role of tracheal fluid.

METHODS

Fetal lambs (term, 145 days) underwent TO at 122 days. Tracheal pressure was recorded daily. In one group of animals (TF; n = 6), lung fluid was aspirated, measured, and reinfused daily. In their respective twins, NS group, lung fluid was replaced milliliter per milliliter with normal saline (NS; n = 6). At death near term, lung weight was obtained, and tissues were processed for stereologic volumetry. Type II cells were quantitated using antisurfactant protein B immunohistochemistry. Surfactant protein B-mRNA expression was studied by Northern analysis. Wilcoxon signed rank test and single factor analysis of variance (ANOVA) were used for statistical analysis (P<.05 was significant).

RESULTS

In both experimental groups, intratracheal pressure rose from 1.9+/-1.0 torr to 3.7 to 4.8 torr by day 1, and remained constant thereafter. Lung fluid volume increased from 11.9+/-4.2 on day 0 to 36.8+/-8.0 mL/kg in TF, and to 28.4+/-9.3 mL/kg in NS by day 1 (P<.05). At death, lung weight/body weight ratio was higher in TF (5.45% +/- 0.91%) than in NS (4.40% +/- 0. 67%) or control (3.83%+/-0.58%; P<.05). Type II numerical density was substantially reduced after TO: 57.7+/-12.8 x 10(6)/mL (TF) and 45.0 +/-25.9 x 10(6)/mL (NS), versus 82.3+/-13.6 x 10(6)/mL in controls. Ultrastructurally, remaining type II cells in TF were enlarged and engorged with lamellar bodies; in NS, they were smaller and contained fewer lamellar bodies. Surfactant protein B mRNA expression was significantly decreased in NS, but not in TF, compared with controls.

CONCLUSIONS

Type II cell function as well as overall lung growth are stimulated by TO. Lung growth after TO is therefore not unavoidably detrimental to type II cells. After isobaric saline exchange of lung fluid, type II cell function is severely inhibited, confirming the role of tracheal fluid composition in type II stimulating type II cell function.

Conditional expression of fibroblast growth factor-7 in the developing and mature lung

Effects of fibroblast growth factor-7 (FGF-7) on lung morphogenesis, respiratory epithelial cell differentiation, and proliferation were assessed in transgenic mice in which the human FGF-7 cDNA was controlled by a conditional promoter under the direction of regulatory elements from either the human surfactant protein-C (SP-C) or rat Clara cell secretory protein (ccsp) genes. Expression of FGF-7 was induced in respiratory epithelial cells of the fetal lung by administration of doxycycline to the dam. Prenatally, doxycycline induced FGF-7 mRNA in respiratory epithelial cells in both Sp-c and Ccsp transgenic lines, increasing lung size and causing cystadenomatoid malformation. Postnatally, mice bearing both Ccsp-rtta and (Teto)(7)-cmv-fgf-7 transgenes survived, and lung morphology was normal. Induction of FGF-7 expression by doxycycline in the Ccsp-rtta x (Teto)(7)-cmv-fgf-7 mice caused marked epithelial cell proliferation, adenomatous hyperplasia, and pulmonary infiltration with mononuclear cells. Epithelial cell hyperplasia caused by FGF-7 was largely resolved after removal of doxycycline. Surfactant proteins, TTF-1, and aquaporin 5 expression were conditionally induced by doxycycline. The Sp-c-rtta and Ccsp-rtta activator mice provide models in which expression is conditionally controlled in respiratory epithelial cells in the developing and mature lung, altering lung morphogenesis, differentiation, and proliferation.