Transgenic models for study of lung morphogenesis and repair: Parker B. Francis lecture

High-efficiency type II cell-enhanced green fluorescent protein expression facilitates cellular identification, tracking, and isolation

We have developed a transgenic mouse expressing enhanced green fluorescent protein (EGFP) in virtually all type II (TII) alveolar epithelial cells. The CBG mouse (SPC-BAC-EGFP) contains a bacterial artificial chromosome modified to express EGFP within the mouse surfactant protein (SP)-C gene 3' untranslated region. EGFP mRNA expression is limited to the lung. EGFP fluorescence is both limited to and exhibited by all cells expressing pro-SP-C; fluorescence is uniform throughout all lobes of the lung and does not change as mice age. EGFP(+) cells also express SP-B but do not express podoplanin, a type I (TI) cell marker. CBG mice show no evidence of lung disease with aging. In 3 hours, TII cells can be isolated in >99% purity from CBG mice by FACS; the yield of 3.7 ± 0.6 × 10(6) cells represents approximately 25 to 60% of the TII cells in the lung. By FACS analysis, approximately 0.9% of TII cells are in mitosis in uninjured lungs; after bleomycin injury, 4.1% are in mitosis. Because EGFP fluorescence can be detected for >14 days in culture, at a time that SP-C mRNA expression is essentially nil, this line may be useful for tracking TII cells in culture and in vivo. When CBG mice are crossed to transgenic mice expressing rat podoplanin, TI and TII cells can be easily simultaneously identified and isolated. When bred to other strains of mice, EGFP expression can be used to identify TII cells without the need for immunostaining for SP-C. These mice should be useful in models of mouse pulmonary disease and in studies of TII cell biology, biochemistry, and genetics.

Tetracycline-regulated mouse models of cancer

Genetically engineered mouse models (GEMMs) have proven essential to the study of mammalian gene function in both development and disease. However, traditional constitutive transgenic mouse model systems are limited by the temporal and spatial characteristics of the experimental promoter used to drive transgene expression. To address this limitation, considerable effort has been dedicated to developing conditional and inducible mouse model systems. Although a number of approaches to generating inducible GEMMs have been pursued, several have been restricted by toxic or undesired physiological side effects of the compounds used to activate gene expression. The development of tetracycline (tet)-dependent regulatory systems has allowed for circumvention of these issues resulting in the widespread adoption of these systems as an invaluable tool for modeling the complex nature of cancer progression.

Transgenic overexpression of interleukin-1β induces persistent lymphangiogenesis but not angiogenesis in mouse airways

These studies used bi-transgenic Clara cell secretory protein (CCSP)/IL-1β mice that conditionally overexpress IL-1β in Clara cells to determine whether IL-1β can promote angiogenesis and lymphangiogenesis in airways. Doxycycline treatment induced rapid, abundant, and reversible IL-1β production, influx of neutrophils and macrophages, and conspicuous and persistent lymphangiogenesis, but surprisingly no angiogenesis. Gene profiling showed many up-regulated genes, including chemokines (Cxcl1, Ccl7), cytokines (tumor necrosis factor α, IL-1β, and lymphotoxin-β), and leukocyte genes (S100A9, Aif1/Iba1). Newly formed lymphatics persisted after IL-1β overexpression was stopped. Further studies examined how IL1R1 receptor activation by IL-1β induced lymphangiogenesis. Inactivation of vascular endothelial growth factor (VEGF)-C and VEGF-D by adeno-associated viral vector-mediated soluble VEGFR-3 (VEGF-C/D Trap) completely blocked lymphangiogenesis, showing its dependence on VEGFR-3 ligands. Consistent with this mechanism, VEGF-C immunoreactivity was present in some Aif1/Iba1-immunoreactive macrophages. Because neutrophils contribute to IL-1β-induced lung remodeling in newborn mice, we examined their potential role in lymphangiogenesis. Triple-transgenic CCSP/IL-1β/CXCR2(-/-) mice had the usual IL-1β-mediated lymphangiogenesis but no neutrophil recruitment, suggesting that neutrophils are not essential. IL1R1 immunoreactivity was found on some epithelial basal cells and neuroendocrine cells, suggesting that these cells are targets of IL-1β, but was not detected on lymphatics, blood vessels, or leukocytes. We conclude that lymphangiogenesis triggered by IL-1β overexpression in mouse airways is driven by VEGF-C/D from macrophages, but not neutrophils, recruited by chemokines from epithelial cells that express IL1R1.

Directed expression of transgenes to alveolar type I cells in the mouse

Podoplanin (RTI40, aggrus, T1alpha, hT1alpha-2, E11, PA2.26, RANDAM-2, gp36, gp38, gp40, OTS8) is a type I cell marker in rat lung. We show that a bacterial artificial chromosome vector containing the rat podoplanin gene (RTIbac) delivers a pattern of transgene expression in lung that is more restricted to mouse type I cells than that of the endogenous mouse podoplanin gene. RTIbac-transgenic mice expressed rat podoplanin in type I cells; type II cells, airways, and vascular endothelium were negative. A modified bacterial artificial chromosome containing internal ribosome entry site (IRES)-green fluorescent protein (GFP) sequences in the podoplanin 3'UTR expressed rat podoplanin and transgenic GFP in type I cells. RTIbac transgene expression was absent or reduced in pulmonary pleura, lymphatic endothelium, and putative lymphoid-associated stromal tissue, all of which contained abundant mouse podoplanin. Rat podoplanin mRNA levels in normal rat lung and RTIbac transgenic lung were 25-fold higher than in corresponding kidney and brain samples. On Western blots, transgenic rat and endogenous mouse podoplanin displayed very similar patterns of protein expression in various organs. Highest protein levels were observed in lung with 10- to 20-fold less in brain; there were low levels in thymus and kidney. Both GFP and rat podoplanin transgenes were expressed at extrapulmonary sites of endogenous mouse podoplanin gene expression, including choroid plexus, eye ciliary epithelium, and renal glomerulus. Because their pulmonary expression is more restricted than endogenous mouse podoplanin, RTIbac derivatives should be useful for mouse type I cell-specific transgene delivery.

Targeted knockout of BRG1 potentiates lung cancer development

Brahma-related gene 1 (BRG1) is a catalytic subunit of the switch in mating type/sucrose nonfermentation complex and plays an important role in cancer development. Mouse homozygous knockout experiments testing the role of BRG1 in tumorigenesis have been hampered because BRG1 inactivation is embryonic lethal. To bypass this constraint, we developed a lung-specific conditional knockout of BRG1 and examined the effect of BRG1 inactivation in an ethyl carbamate lung carcinogenesis mouse model. We found that the heterozygous loss of BRG1 resulted in increases in both the number and size of tumors when compared with controls. In contrast, when both BRG1 alleles were inactivated, neither the number nor the size of tumors increased compared with controls. In mouse lung tissue where BRG1 was homozygously inactivated, immunostaining for apoptotic markers showed significant increase in Apo-BrdUrd and cleaved caspase-3. These data indicate that a loss of cell viability underlies why biallelic inactivation of BRG1 does not increase tumorigenesis. We also examined mice when exposed to the carcinogen ethyl carbamate and then subjected to BRG1 inactivation. In these cells, loss of BRG1 after carcinogen exposure potentiated tumor development. A subset of tumors retained BRG1 expression, whereas others showed either partial or complete loss of BRG1 expression. Tumors completely devoid of BRG1 expression were significantly larger and expressed higher levels of two markers of proliferation, proliferating cell nuclear antigen and Ki67. Although biallelic inactivation of BRG1 could not initiate tumor development in untransformed cells, our results indicate that transformation and tumor progression are greatly affected by loss of BRG1.

Genetic Abnormalities of Surfactant Metabolism

Pulmonary surfactant is the complex mixture of lipids and proteins needed to reduce alveolar surface tension at the air-liquid interface and prevent alveolar collapse at the end of expiration. It has been recognized for almost 50 years that a deficiency in surfactant production due to pulmonary immaturity is the principal cause of the respiratory distress syndrome (RDS) observed in prematurely born infants.1 Secondary surfactant deficiency due to injury to the cells involved in its production and functional inactivation of surfactant is also important in the pathophysiology of acute respiratory distress syndrome (ARDS) observed in older children and adults.2,3 In the past 15 years, it has been recognized that surfactant deficiency may result from genetic mechanisms involving mutations in genes encoding critical components of the surfactant system or proteins involved in surfactant metabolism.4,5 Although rare, these single gene disorders provide important insights into normal surfactant metabolism and into the genes in which frequently occurring allelic variants may be important in more common pulmonary diseases.

Genetically engineered mice in understanding the basis of neonatal lung disease

Advances in genetic engineering have allowed the creation of animals with additional or deleted genes. New genes may be inserted in mice, specific genes inactivated or "knocked out," and more complex animals created in which genes can be turned on or off at different times in development or in different tissues. These animal models allow for more detailed studies of the proteins encoded by the manipulated gene, an improved understanding of the pathophysiology of diseases resulting from the genetic alterations, and model organisms in which to study potential new therapies. Multiple mouse models involving genes important in surfactant production and regulation relevant to lung disease observed in human newborns have been created. This review will discuss the creation of such animals and illustrate their utility in understanding human disease.

Functional facets of the pulmonary neuroendocrine system

Pulmonary neuroendocrine cells (PNECs) have been around for 60 years in the scientific literature, although phylogenetically they are ancient. Their traditionally ascribed functions include chemoreception and regulation of lung maturation and growth. There is recent evidence that neuroendocrine (NE) differentiation in the lung is regulated by genes and pathways that are conserved in the development of the nervous system from Drosophila to humans (such as achaete-scute homolog-1), or implicated in the carcinogenesis of the nervous or NE system (such as the retinoblastoma tumor suppressor gene). In addition, complex neural networks are in place to regulate chemosensory and other functions. Even solitary PNECs appear to be innervated. For the first time ever, we have mouse models for lung NE carcinomas, including the most common and virulent small cell lung carcinoma. Moreover, PNECs may be important for inflammatory responses, and pivotal for lung stem cell niches. These discoveries signify an exciting new era for PNECs and are likely to have therapeutic and diagnostic applications.

A Transcriptional Profiling Study of CCAAT/Enhancer Binding Protein Targets Identifies Hepatocyte Nuclear Factor 3β as a Novel Tumor Suppressor in Lung Cancer

We showed previously that CCAAT/enhancer binding protein alpha (C/EBP alpha), a tissue-specific transcription factor, is a candidate tumor suppressor in lung cancer. In the present study, we have performed a transcriptional profiling study of C/EBP alpha target genes using an inducible cell line system. This study led to the identification of hepatocyte nuclear factor 3beta (HNF3 beta), a transcription factor known to play a role in airway differentiation, as a downstream target of C/EBP alpha. We found down-regulation of HNF3 beta expression in a large proportion of lung cancer cell lines examined and identified two novel mutants of HNF3 beta, as well as hypermethylation of the HNF3 beta promoter. We also developed a tetracycline-inducible cell line model to study the cellular consequences of HNF3 beta expression. Conditional expression of HNF3 beta led to significant growth reduction, proliferation arrest, apoptosis, and loss of clonogenic ability, suggesting additionally that HNF3 beta is a novel tumor suppressor in lung cancer. This is the first study to show genetic abnormalities of lung-specific differentiation pathways in the development of lung cancer.

Genetically engineered mouse models for lung cancer

The lung is a complex organ consisting of numerous cell types that function to ensure sufficient gas exchange to oxygenate the blood. In order to accomplish this function, the lung must be exposed to the external environment and at the same time maintain a homeostatic balance between its function in gas exchange and the maintenance of inflammatory balance. During the past two decades, as molecular methodologies have evolved with the sequencing of entire genomes, the use of in vivo models to elucidate the molecular mechanisms involved in pulmonary physiology and disease have increased. The mouse has emerged as a potent model to investigate pulmonary physiology due to the explosion in molecular methods that now allow for the developmental and tissue-specific regulation of gene transcription. Initial efforts to manipulate gene expression in the mouse genome resulted in the generation of transgenic mice characterized by the constitutive expression of a specific gene and knockout mice characterized by the ablation of a specific gene. The utility of these original mouse models was limited, in many cases, by phenotypes resulting in embryonic or neonatal lethality that prevented analysis of the impact of the genetic manipulation on pulmonary biology. Second-generation transgenic mouse models employ multiple strategies that can either activate or silence gene expression thereby providing extensive temporal and spatial control of the experimental parameters of gene expression. These highly regulated mouse models are intended to serve as a foundation for further investigation of the molecular basis of human disease such as tumorigenesis. This review describes the principles, progress, and application of systems that are currently employed in the conditional regulation of gene expression in the investigation of lung cancer.

Protective effect of keratinocyte growth factor against lung abnormalities associated with hyperoxia in prematurely born rats

We used a prematurely born rat/hyperoxia model of bronchopulmonary dysplasia (BPD) to test whether keratinocyte growth factor (KGF) treatment would protect against the development of several serious (cardio-)pulmonary complications of early life exposure to hyperoxia. KGF significantly protected against hyperoxic lethality (13-day survival rate = 50/64 (78%) for the O(2)-KGF vs. 29/66 (44%) for the O(2)-saline group, p < 0.001). Although KGF failed to protect against hyperoxic inhibition of normal postnatal alveoli formation and early pulmonary fibrosis, KGF consistently had a significant protective/preventive effect against the development of pulmonary hypertension during hyperoxia as reflected in comparative right ventricular hypertrophy: mean increase = +35% above normal for the O(2)-saline group vs. +3% for the O(2)-KGF premature rat group (p < 0.01).

Pulmonary phosphatidic acid phosphatase and lipid phosphate phosphohydrolase

The lung contains two distinct forms of phosphatidic acid phosphatase (PAP). PAP1 is a cytosolic enzyme that is activated through fatty acid-induced translocation to the endoplasmic reticulum, where it converts phosphatidic acid (PA) to diacylglycerol (DAG) for the biosynthesis of phospholipids and neutral lipids. PAP1 is Mg(2+) dependent and sulfhydryl reagent sensitive. PAP2 is a six-transmembrane-domain integral protein localized to the plasma membrane. Because PAP2 degrades sphingosine-1-phosphate (S1P) and ceramide-1-phosphate in addition to PA and lyso-PA, it has been renamed lipid phosphate phosphohydrolase (LPP). LPP is Mg(2+) independent and sulfhydryl reagent insensitive. This review describes LPP isoforms found in the lung and their location in signaling platforms (rafts/caveolae). Pulmonary LPPs likely function in the phospholipase D pathway, thereby controlling surfactant secretion. Through lowering the levels of lyso-PA and S1P, which serve as agonists for endothelial differentiation gene receptors, LPPs regulate cell division, differentiation, apoptosis, and mobility. LPP activity could also influence transdifferentiation of alveolar type II to type I cells. It is considered likely that these lipid phosphohydrolases have critical roles in lung morphogenesis and in acute lung injury and repair.