Conditional clara cell ablation reveals a self-renewing progenitor function of pulmonary neuroendocrine cells

The role of cancer stem cells in neoplasia of the lung: past, present and future

Through the identification and subsequent targeting of an exquisitely unique and phenotypically defined cancer stem-cell population exhibiting discrete therapeutic vulnerabilities (a potential source of tumor recurrence) better survival rates for these patients may be achieved. It is this impetus that is making the field of pulmonary stem cell biology a growing field in biomedicine. These efforts are leading to the steady identification of multi-potent, self-renewing and proliferative progenitor cell populations throughout the bronchopulmonary tree. These cells give rise to both transiently amplifying (TA) and terminally differentiated (TD) cells, which (like in many other organs) are crucial for tissue homeostasis. In leukemia, it has been shown that partially committed cells, which are normally responsible for tissue maintenance after trauma, may undergo transformation via mutations resulting in the selective expression of genes that accentuate and perpetuate these cells' self-renewal capabilities. It is therefore perhaps legitimate to consider stem cells as protumorigenic. It is when these cells undergo genetic mutations which make them acquire the ability to metastasize, that cancer occurs, rendering the concept of "cancer stem cells" a rather attractive one indeed.

Purinergic signaling in the pulmonary neuroepithelial body microenvironment unraveled by live cell imaging

Pulmonary neuroepithelial bodies (NEBs) are densely innervated groups of complex sensory airway receptors involved in the regulation of breathing. Together with their surrounding Clara-like cells, they exhibit stem cell potential through their capacity to regenerate depopulated areas of the epithelium following lung injury. We have employed confocal live cell imaging microscopy and novel electrophysiological techniques in a new ex vivo lung slice model to unravel potential purinergic signaling pathways within the NEB microenvironment. Quinacrine histochemistry indicated high amounts of vesicular ATP in NEB cells. Using a "reporter-patching" method adapted to create a uniquely sensitive and selective biosensor for the direct detection of ATP release from NEBs ex vivo, we demonstrated quantal ATP release from NEBs following their depolarization. Enhancing enzymatic extracellular ATP hydrolysis or inhibiting P2 receptors confirmed the central role of ATP in paracrine interactions between NEB cells and Clara-like cells. Combined calcium imaging, pharmacology, and immunohistochemistry showed that ligand-binding to functional P2Y(2) receptors underpins the activation of Clara-like cells. Hence, NEB cells communicate with their cellular neighbors in the NEB microenvironment by releasing ATP, which rapidly evokes purinergic activation of surrounding Clara-like cells. Besides ATP acting on the P2X(3) receptor expressing vagal sensory nerve terminals between NEB cells, local paracrine purinergic signaling within this potential stem cell niche may be important to both normal airway function, airway epithelial regeneration after injury, and/or the pathogenesis of small cell lung carcinomas.

Reparative capacity of airway epithelium impacts deposition and remodeling of extracellular matrix

Defective epithelial repair in the setting of chronic lung disease has been suggested to contribute to uncontrolled extracellular matrix (ECM) deposition and development of fibrosis. We sought to directly test this hypothesis through gene expression profiling of total lung RNA isolated from mouse models of selective epithelial cell injury that are associated with either productive or abortive repair. Analysis of gene expression in repairing lungs of naphthalene-exposed mice revealed prominent clusters of up-regulated genes with putative roles in regulation of the extracellular matrix and cellular proliferation. Further analysis of tenascin C (Tnc), a representative matrix protein, in total lung RNA revealed a transient 4.5-fold increase in mRNA abundance 1 day after injury and a return to steady-state levels by Recovery Day 3. Tnc was deposited by the peribronchiolar mesenchyme immediately after injury and was remodeled to basement membrane subtending the bronchiolar epithelium during epithelial repair. Epithelial restitution was accompanied by a decrease in Tnc mRNA and protein expression to steady-state levels. In contrast, abortive repair using a transgenic model allowing ablation of all reparative cells led to a progressive increase in Tnc mRNA within lung tissue and accumulation of its gene product within the subepithelial mesenchyme of both conducting airways and alveoli. These data demonstrate that the ECM is dynamically remodeled in response to selective epithelial cell injury and that this process is activated without resolution in the setting of defective airway epithelial repair.

The glandular stem/progenitor cell niche in airway development and repair

Airway submucosal glands (SMGs) are major secretory structures that lie beneath the epithelium of the cartilaginous airway. These glands are believed to play important roles in normal lung function and airway innate immunity by secreting antibacterial factors, mucus, and fluid into the airway lumen. Recent studies have suggested that SMGs may additionally serve as a protective niche for adult epithelial stem/progenitor cells of the proximal airways. As in the case of other adult stem cell niches, SMGs are believed to provide the localized environmental signals required to both maintain and mobilize stem/progenitor cells, in the setting of normal cellular turnover or injury. Aberrant proliferation and differentiation of glandular stem/progenitor cells may be associated with several hypersecretory lung diseases, including chronic bronchitis, asthma, and cystic fibrosis. To better understand the molecular mechanisms that regulate the specification and proliferation of glandular stem/progenitor cells in lung diseases associated with SMG hypertrophy and hyperplasia, researchers have begun to search for the molecular signals and cell types responsible for establishing the glandular stem/progenitor cell niche, and to dissect how these determinants of the niche change in the setting of proximal airway injury and repair. Such studies have revealed certain similarities between stem/progenitor cell niches of the distal conducting airways and the SMGs of the proximal airways.

Maintenance and repair of the bronchiolar epithelium

Bronchioles of the distal conducting airway are lined by a simple epithelium composed primarily of nonciliated secretory (Clara) cells and ciliated cells. These cells are long-lived in the normal lung; renewal is mediated by cells that constitute a nonclassical stem cell hierarchy. Within this type of hierarchy, facultative progenitor cells are responsible for normal epithelial maintenance and rare adult tissue-specific stem cells are activated only in response to depletion of the facultative progenitor cell pool. This organizational structure is a departure from the classical stem cell hierarchies that maintain rapidly renewing tissues such as the epithelium of the small intestine. This article compares cellular and molecular mechanisms of epithelial renewal in the relatively quiescent bronchiolar epithelium and in the mitotically active intestinal epithelium. Fundamental distinctions between stem cell hierarchies of slowly and rapidly renewing epithelia are highlighted and may provide insight into tissue-specific interpretation of signals that mediate repair in some tissues but lead to remodeling and chronic disease in other organ systems.

Molecular staging of epithelial maturation using secretory cell-specific genes as markers

Bronchiolar Clara cells undergo phenotypic changes during development and in disease. These changes are poorly described due to a paucity of molecular markers. We used chemical and transgenic approaches to ablate Clara cells, allowing identification of their unique gene expression profile. Flavin monooxygenase 3 (Fmo3), paraoxonase 1 (Pon1), aldehyde oxidase 3 (Aox3), and claudin 10 (Cldn10) were identified as novel Clara cell markers. New and existing Clara cell marker genes were categorized into three classes based on their unique developmental expression pattern. Cldn10 was uniformly expressed in the epithelium at Embryonic Day (E)14.5 and became restricted to secretory cells at E18.5. This transition was defined by induction of CCSP. Maturation of secretory cells was associated with progressive increases in the expression of Fmo3, Pon1, Aox3, and Cyp2f2 between late embryonic and postnatal periods. Messenger RNA abundance of all categories of genes was dramatically decreased after naphthalene-induced airway injury, and displayed a sequence of temporal induction during repair that suggested sequential secretory cell maturation. We have defined a broader repertoire of Clara cell-specific genes that allows staging of epithelial maturation during development and repair.

Cancer stem cells and the ontogeny of lung cancer

Lung cancer is the leading cause of cancer death in the world today and is poised to claim approximately 1 billion lives during the 21st century. A major challenge in treating this and other cancers is the intrinsic resistance to conventional therapies demonstrated by the stem/progenitor cell that is responsible for the sustained growth, survival, and invasion of the tumor. Identifying these stem cells in lung cancer and defining the biologic processes necessary for their existence is paramount in developing new clinical approaches with the goal of preventing disease recurrence. This review summarizes our understanding of the cellular and molecular mechanisms operating within the putative cancer-initiating cell at the core of lung neoplasia.

Methods for studying stem cells: adult stem cells for lung repair

Recent progress in lung biology includes the description of a series of pulmonary stem and progenitor cells involved in homeostasis and regeneration of the respiratory system. Moreover, the contribution of extrapulmonary stem cells to healthy and pathological lung tissue has been observed and the developmental biology of such processes should provide important hints for understanding maintenance and repair of adult lung structure and function. Despite such remarkable advances, the phenotypic and especially the functional characterization of these stem and progenitor cells, and their derivatives, along with an understanding of the molecular cues and pathways underlying differentiation into specific respiratory lineages is still in its infancy. Accordingly, the role of endogenous and extrapulmonary stem cells in normal tissue repair and pathogenesis is still largely mysterious and added basic knowledge is required in order to explore their potential for novel regenerative therapies. This review provides an overview of the current state of the art in adult lung stem cell biology including technical aspects of isolation, characterization and differentiation, and a discussion of perspectives for future regenerative therapies.

Rat aquaporin-5 4.3-kb 5'-flanking region differentially regulates expression in salivary gland and lung in vivo

We previously cloned a 4.3-kb genomic fragment encompassing 5'-flanking regulatory elements of rat aquaporin-5 (Aqp5) that demonstrated preferential transcriptional activity in lung and salivary cells in vitro. To investigate the ability of Aqp5 regulatory elements to direct transgene expression in vivo, transgenic (TG) mice and rats were generated in which the 4.3-kb Aqp5 fragment directed the expression of enhanced green fluorescent protein (EGFP). RT-PCR revealed relative promoter specificity for the lung and salivary glands in TG mice. Immunofluorescence microscopy showed strong EGFP expression in salivary acinar cells but not in lung type I (AT1) cells, both known sites of endogenous AQP5 expression. Similar results were obtained in TG rats generated by lentiviral transgenesis. EGFP mRNA was detected in both salivary glands and lung. Robust EGFP fluorescence was observed in frozen sections of the rat salivary gland but not in the lung or other tested tissues. The percentage of EGFP-positive acinar cells was increased in parotid and submandibular glands of TG rats receiving a chronic injection of the beta-adrenergic receptor agonist isoproterenol. EGFP-positive cells in the lung that were also reactive with the AT1-cell specific monoclonal antibody VIIIB2 were identified by flow cytometry. These findings demonstrate that the 4.3-kb Aqp5 promoter/enhancer directs strong cell-specific transgene expression in salivary gland and low-level AT1 cell-specific expression in the lung. While these Aqp5 regulatory elements should be useful for functional studies in salivary glands, additional upstream or intronic cis-active elements are likely required for robust expression in the lung.

Keratinocyte growth factor protects against Clara cell injury induced by naphthalene

Airway epithelial cells are exposed to environmental toxicants that result in airway injury. Naphthalene (NA) causes site-selective damage to Clara cells in mouse distal airways. N-terminally truncated recombinant human keratinocyte growth factor (DeltaN23-KGF) protects against acute lung injury. The present study investigated whether or not DeltaN23-KGF protects against NA-induced acute Clara cell damage by measuring airway responses specifically and in order to identify underlying molecular mechanisms. Mice were treated with DeltaN23-KGF or PBS 33 h prior to injection of 200 mg.kg body weight(-1) NA. Lung function was analysed by head-out body plethysmography. Distal airways isolated by microdissection were assessed for cell permeability using ethidium homodimer-1. Immunohistochemistry of Clara cell-specific protein in conjunction with a physical dissector was used to quantify Clara cell numbers. RNA was isolated from frozen airways in order to analyse gene expression using quantitative RT-PCR. DeltaN23-KGF prevented NA-induced airflow limitation and Clara cell permeability, and resulted in twice as many Clara cells compared with PBS pre-treatment. DeltaN23-KGF-pre-treated mice exhibited increased expression of proliferating cell nuclear antigen mRNA. Cytochrome P(450) isoform 2F2, which converts NA into its toxic metabolite, was reduced by approximately 50%. The present results demonstrate that pre-treatment with N-terminally truncated recombinant human keratinocyte growth factor protects against naphthalene-induced injury. This suggests that N-terminally truncated recombinant human keratinocyte growth factor exerts its beneficial effect through a decrease in the expression of cytochrome P(450) isoform 2F2.

Functional live cell imaging of the pulmonary neuroepithelial body microenvironment

Pulmonary neuroepithelial bodies (NEBs) are densely innervated groups of neuroendocrine cells invariably accompanied by Clara-like cells. Together with NEBs, Clara-like cells form the so-called "NEB microenvironment," which recently has been assigned a potential pulmonary stem cell niche. Conclusive data on the nature of physiological stimuli for NEBs are lacking. This study aimed at developing an ex vivo mouse lung vibratome slice model for confocal live cell imaging of physiological reactions in identified NEBs and surrounding epithelial cells. Immunohistochemistry of fixed slices demonstrated that NEBs are almost completely shielded from the airway lumen by tight junction-linked Clara-like cells. Besides the unambiguous identification of NEBs, the fluorescent dye 4-Di-2-ASP allowed microscopic identification of ciliated cells, Clara cells, and Clara-like cells in live lung slices. Using the mitochondrial uncoupler FCCP and a mitochondrial membrane potential indicator, JC-1, increases in 4-Di-2-ASP fluorescence in NEB cells and ciliated cells were shown to represent alterations in mitochondrial membrane potential. Changes in the intracellular free calcium concentration ([Ca2+](i)) in NEBs and surrounding airway epithelial cells were simultaneously monitored using the calcium indicator Fluo-4. Application (5 s) of 50 mM extracellular potassium ([K+](o)) evoked a fast and reproducible [Ca2+](i) increase in NEB cells, while Clara-like cells displayed a delayed (+/- 4 s) [Ca2+](i) increase, suggestive of an indirect, NEB-mediated activation. The presented approach opens interesting new perspectives for unraveling the functional significance of pulmonary NEBs in control lungs and disease models, and for the first time allows direct visualization of local interactions within the NEB microenvironment.

Conditional stabilization of beta-catenin expands the pool of lung stem cells

Maintenance of classic stem cell hierarchies is dependent upon stem cell self-renewal mediated in part by Wnt/beta-catenin regulation of the cell cycle. This function is critical in rapidly renewing tissues due to the obligate role played by the tissue stem cell. However, the stem cell hierarchy responsible for maintenance of the conducting airway epithelium is distinct from classic stem cell hierarchies. The epithelium of conducting airways is maintained by transit-amplifying cells in the steady state; rare bronchiolar stem cells are activated to participate in epithelial repair only following depletion of transit-amplifying cells. Here, we investigate how signaling through beta-catenin affects establishment and maintenance of the stem cell hierarchy within the slowly renewing epithelium of the lung. Conditional potentiation of beta-catenin signaling in the embryonic lung results in amplification of airway stem cells through attenuated differentiation rather than augmented proliferation. Our data demonstrate that the differentiation-modulating activities of stabilized beta-catenin account for expansion of tissue stem cells.

Lung stem cells

The lung is a relatively quiescent tissue comprised of infrequently proliferating epithelial, endothelial, and interstitial cell populations. No classical stem cell hierarchy has yet been described for the maintenance of this essential tissue; however, after injury, a number of lung cell types are able to proliferate and reconstitute the lung epithelium. Differentiated mature epithelial cells and newly recognized local epithelial progenitors residing in specialized niches may participate in this repair process. This review summarizes recent discoveries and controversies, in the field of stem cell biology, that are not only challenging, but also advancing an understanding of lung injury and repair. Evidence supporting a role for the numerous cell types believed to contribute to lung epithelial homeostasis is reviewed, and initial studies employing cell-based therapies for lung disease are presented. As a detailed understanding of stem cell biology, lung development, lineage commitment, and epithelial differentiation emerges, an ability to modulate lung injury and repair is likely to follow.

Paving the road for lung stem cell biology: bronchioalveolar stem cells and other putative distal lung stem cells

New discoveries in stem cell biology are making the biology of solid tissues increasingly complex. Important seminal studies demonstrating the presence of damage-resistant cell populations together with new isolation and characterization techniques suggest that stem cells exist in the adult lung. More detailed in vivo molecular and cellular characterization of bronchioalveolar stem cells (BASCs), other putative lung stem and progenitor cells, and differentiated cells is needed to determine the lineage relationships in adult lung. Lung diseases such as cystic fibrosis or chronic obstructive pulmonary disease, as well as the most common form of lung cancer in the United States, all involve apparent bronchiolar and alveolar cell defects. It is likely that the delicate balance of stem, progenitor, and differentiated cell functions in the lung is critically affected in patients with these devastating diseases. Thus the discovery of BASCs and other putative lung stem cells will lay the foundation for new inroads to understanding lung biology related to lung disease.

Pulmonary expression of voltage-gated calcium channels: special reference to sensory airway receptors

Studying depolarisation induced calcium entry in our recently developed in situ lung slice model for molecular live cell imaging of selectively visualised pulmonary neuroepithelial bodies (NEBs), exemplified the need for information on the localisation of voltage-gated calcium channels (Ca(v)) in lungs in general, and related to sensory airway receptors more specifically. The present study therefore aimed at identifying the expression pattern of all major classes and subtypes of Ca(v) channels, using multiple immunostaining of rat lung cryosections. Ca(v) channel antibodies were combined with antibodies that selectively label NEBs, nerve fibre populations, smooth muscle, endothelium and Clara cells. Ca(v)2.1 (P/Q-type) was the only Ca(v) channel expressed in NEB cell membranes, and appeared to be restricted to the apical membrane of the slender NEB cell processes that reach the airway lumen. Subpopulations of the vagal but not the spinal sensory nerve fibres that contact NEBs showed immunoreactivity (IR) for Ca(v)1.2 (L-type) and Ca(v)2.1. Ca(v)2.3 (R-type) was selectively expressed by the so-called Clara-like cells that cover NEBs only, and appears to be a unique marker to discriminate this epithelial cell type from the much more extensive group of Clara cells in rat airways. The laminar nerve endings of smooth muscle-associated airway receptors (SMARs) revealed IR for both Ca(v)2.1 and Ca(v)2.2 (N-type). More generally, Ca(v)1.2 was seen to be expressed in vascular smooth muscle, Ca(v)2.3 and Ca(v)3.1 (T-type) in bronchial smooth muscle, Ca(v)3.1 and Ca(v)3.2 (T-type) in endothelial cells, and Ca(v)1.3 (L-type) in a limited number of epithelial cells. In conclusion, the present immunocytochemical study has demonstrated that the various subtypes of Ca(v) channels have distinct expression patterns in rat lungs. Special focus on morphologically/neurochemically characterised sensory airway receptors learned us that both NEBs and SMARs present Ca(v) channels. Knowledge of the identification and localisation of Ca(v) channels in airway receptors and surrounding tissues provides a solid basis for interpretation of the calcium mediated activation studied in our ex vivo lung slice model.

Stem cells in lung repair and regeneration

Repair or regeneration of defective lung tissue would be of great clinical use. Potential cellular sources for the regeneration of lung tissue in vivo or lung tissue engineering in vitro include endogenous pulmonary stem cells, extrapulmonary circulating stem cells and embryonic stem cells. This review summarizes the recent research on each of these stem cell types and their potential for use in the treatment of lung injury and disease.

Stem cells and chronic lung disease

Stem cells have been shown to contribute to the repair and regeneration of injured lungs. These stem cells are resident in specific protected niches in the lung, or they can be mobilized from the bone marrow and recruited from the circulation in the setting of severe injury. Normal repair of the airway involves regeneration of the airway epithelium by stem cells in both the proximal airway and distal airspace, whereas aberrant repair of the lung may result from stem cells that lead to fibrosis. The stem cell niche in the lung is probably critical in determining whether "good" or "bad" stem cells are involved in local repair, and therefore whether fibrosis predominates. There is much excitement about the possibility of harnessing stem cells for repair and regeneration of the lungs. This review highlights current knowledge of this area and identifies gaps in our understanding of this complicated process.

Loss of GFI1 impairs pulmonary neuroendorine cell proliferation, but the neuroendocrine phenotype has limited impact on post-naphthalene airway repair

Naphthalene exposure kills lung airway epithelial (Clara) cells, but is rapidly followed by Clara cell reconstitution coincident with proliferation of pulmonary neuroendocrine cells (PNEC). Although a role for mature PNEC in the reconstitution process has been excluded, the reconstituting progenitor cells have been suggested to enter a transient neuroendocrine (NE) differentiation phase before differentiating to Clara cells. Furthermore, these progenitors were suggested to be the target population for transformation to a NE tumor; small cell lung cancer (SCLC). Although the NE phenotype is central to SCLC oncogenesis, the relevance of NE differentiation to post naphthalene reconstitution remains to be determined. The Growth factor independent-1 (Gfi1) transcription factor is expressed in SCLC and is required for the NE differentiation of PNEC. Gfi1(-/-) mice display a 70% reduction in airway cells that express NE markers, and cells that stain for NE markers show weak expression of some markers. Therefore, to determine the relevance of the NE phenotype to post-naphthalene reconstitution, we examined post-naphthalene reconstitution in Gfi1(-/-) mice. Our analyses indicate that the post-naphthalene regeneration process includes both airway epithelial proliferation and apoptosis. Gfi1 deletion lowered both airway epithelial proliferation and apoptosis; however, the post-naphthalene rate of increase in growth and apoptosis was not significantly different between Gfi1(-/-) mice and wild-type littermates. Moreover, the timing and extent of CC10+ cell regeneration was unaffected by Gfi1 deletion. These data suggest that neither Gfi1 nor the NE phenotype play a dominant role in the regeneration process. However, the few Gfi1(-/-) cells capable of NE differentiation show a significant reduction in post-naphthalene proliferation. The modest proliferation seen in Gfi1(-/-) NE cells is consistent with the previously proposed role for Gfi1 in controlling neuroendocrine cancer growth.

Isolation of an adult mouse lung mesenchymal progenitor cell population

Contained within the adult lung are differentiated mesenchymal cell types (cartilage, smooth muscle, and myofibrobasts) that provide structural support for airways and vessels. Alterations in the number and phenotype of these cells figure prominently in the pathogenesis of a variety of lung diseases. While these cells are thought to arise locally, progenitors have yet to be purified. In previous work, we developed a method for isolating progenitors from lung tissue: this technique takes advantage of the unique ability of cell populations enriched for somatic stem and progenitor activity to efflux the vital dye Hoechst 33342, a feature that permits isolation by flow cytometry-based procedures. Using this method, we determined that a rare population of mesenchymal progenitors resides within the CD45- CD31- Hoechst low fraction of the adult murine lung. Similar to other mesenchymal progenitors, these cells express Sca-1, CD106, and CD44; can be serially passaged; and can differentiate to smooth muscle, cartilage, bone, and fat. Overall, these findings demonstrate that a phenotypically distinct mesenchymal progenitor resides within the adult murine lung, and provide a scheme for their isolation and study.

Airway epithelial stem cells and the pathophysiology of chronic obstructive pulmonary disease

Characteristic pathologic changes in chronic obstructive pulmonary disease (COPD) include an increased fractional volume of bronchiolar epithelial cells, fibrous thickening of the airway wall, and luminal inflammatory mucus exudates, which are positively correlated with airflow limitation and disease severity. The mechanisms driving general epithelial expansion, mucous secretory cell hyperplasia, and mucus accumulation must relate to the effects of initial toxic exposures on patterns of epithelial stem and progenitor cell proliferation and differentiation, eventually resulting in a self-perpetuating, and difficult to reverse, cycle of injury and repair. In this review, current concepts in stem cell biology and progenitor-progeny relationships related to COPD are discussed, focusing on the factors, pathways, and mechanisms leading to mucous secretory cell hyperplasia and mucus accumulation in the airways. A better understanding of alterations in airway epithelial phenotype in COPD will provide a logical basis for novel therapeutic approaches.

Lung development and repair: contribution of the ciliated lineage

The identity of the endogenous epithelial cells in the adult lung that are responsible for normal turnover and repair after injury is still controversial. In part, this is due to a paucity of highly specific genetic lineage tools to follow efficiently the fate of the major epithelial cell populations: the basal, secretory, ciliated, neuroendocrine, and alveolar cells. As part of a program to address this problem we have used a 1-kb FOXJ1 promoter to drive CreER in the ciliated cells of the embryonic and adult lung. Analysis of FOXJ1-GFP transgenic lungs shows that labeled cells appear in a proximal-distal pattern during embryogenesis and that the promoter drives expression in all ciliated cells. Using FOXJ1CreER adult mice, we have followed the fate of ciliated cells after epithelial injury by naphthalene or sulfur dioxide. From quantitative analysis and confocal microscopy we conclude that ciliated cells transiently change their morphology in response to lung injury but do not proliferate or transdifferentiate as part of the repair process.

Type II epithelial cells are critical target for hyperoxia-mediated impairment of postnatal lung development

Type II epithelial cells are essential for lung development and remodeling, as they are precursors for type I cells and can produce vascular mitogens. Although type II cell proliferation takes place after hyperoxia, it is unclear why alveolar remodeling occurs normally in adults whereas it is permanently disrupted in newborns. Using a line of transgenic mice whose type II cells could be identified by their expression of enhanced green fluorescent protein and endogenous expression of surfactant proteins, we investigated the age-dependent effects of hyperoxia on type II cell proliferation and alveolar repair. In adult mice, type II cell proliferation was low during room air and hyperoxia exposure but increased during recovery in room air and then declined to control levels by day 7. Eight weeks later, type II cell number and alveolar compliance were indistinguishable from those in room air controls. In newborn mice, type II cell proliferation markedly increased between birth and postnatal day 7 before declining by postnatal day 14. Exposure to hyperoxia between postnatal days 1 and 4 inhibited type II cell proliferation, which resumed during recovery and was aberrantly elevated on postnatal day 14. Eight weeks later, recovered mice had 70% fewer type II cells and 30% increased lung compliance compared with control animals. Recovered mice also had higher levels of T1alpha, a protein expressed by type I cells, with minimal changes detected in genes expressed by vascular cells. These data suggest that perinatal hyperoxia adversely affects alveolar development by disrupting the proper timing of type II cell proliferation and differentiation into type I cells.

Stem cells – potential for repairing damaged lungs and growing human lungs for transplant

Repair or regeneration of defective lung epithelium would be of great therapeutic potential. It is estimated by the British Lung Foundation that 1 in 7 people in the UK is affected by a lung disease and that 1 in 4 admissions to children's wards are as a result of respiratory problems. Potential cellular sources for the regeneration of lung tissue in vivo or lung tissue engineering in vitro include endogenous pulmonary epithelial stem cells, extrapulmonary circulating stem cells and embryonic stem cells. This article discusses the potential role of each of these stem cell types in future approaches to the treatment of lung injury and disease.

Epithelial stem cells of the lung: privileged few or opportunities for many?

Most reviews of adult stem cells focus on the relatively undifferentiated cells dedicated to the renewal of rapidly proliferating tissues, such as the skin, gut and blood. By contrast, there is mounting evidence that organs and tissues such as the liver and pancreatic islets, which turn over more slowly,use alternative strategies, including the self-renewal of differentiated cells. The response of these organs to injury may also reveal the potential of differentiated cells to act as stem cells. The lung shows both slow turnover and rapid repair. New experimental approaches, including those based on studies of embryonic development, are needed to identify putative lung stem cells and strategies of lung homeostasis and repair.

Embryonic stem cells form glandular structures and express surfactant protein C following culture with dissociated fetal respiratory tissue

Mouse embryonic stem cells (MESCs) are pluripotent, theoretically immortal cells derived from the inner cell mass of developing blastocysts. The respiratory epithelium develops from the primitive foregut endoderm as a result of inductive morphogenetic interactions with the surrounding visceral mesoderm. After dissociation of the explanted fetal lung into single cells, these morphogenetic signaling pathways instruct reconstitution of the developing lung according to a process known as organotypic regeneration. Data presented here demonstrate that such fetal lung morphogenetic cues induce MESC derivatives to incorporate into the reforming pseudoglandular-like tubular ducts, display pan-keratin and surfactant protein C (Sftpc) immunoreactivity, and express Sftpc transcripts while displaying a normal diploid karyotype in coculture. The Sftpc inductive capacity of dissociated fetal lung tissue shows stage specificity with 24% of all MESC derivatives displaying Sftpc immunoreactivity after coculture with embryonic day 11.5 (E11.5) lung buds compared with 6% and 0.02% following coculture with E12.5 and E13.5 lung buds, respectively. MESC derivative Sftpc immunoreactivity follows a spatial and temporal specific maturation profile with an initially ubiquitous cellular Sftpc immunostaining pattern becoming apically polarized with time. Directing differentiation of MESCs into respiratory lineages has important implications for cell replacement therapeutics aimed at treating respiratory-specific diseases such as cystic fibrosis and idiopathic pulmonary fibrosis.

Neuroendocrine-like differentiation of non-small cell lung carcinoma cells: regulation by cAMP and the interaction of mac25/IGFBP-rP1 and 25.1

The need to develop more effective therapies for lung cancer has led to investigations in understanding the molecular mechanisms of the differentiation process, in particular neuroendocrine (NE) differentiation. Recent studies have demonstrated that NE differentiation in non-small cell lung carcinoma (NSCLC) is not uncommon. Those NSCLCs with NE differentiation are considered a form of in transition NE carcinoma and show a more aggressive clinical course compared with NSCLC without NE differentiation. 25.1, a novel protein interacting with mac25/insulin-like growth factor-binding protein-related protein 1 (mac25/IGFBP-rP1), induced NE-like differentiation when collectively overexpressed in M12 prostate cancer cells. We have examined mac25/IGFBP-rP1 and 25.1 as potential molecular regulators in vitro of the NE-differentiation process in lung cancer. In a panel of SCLC and NSCLC cell lines, mac25/IGFBP-rP1 and 25.1 were expressed at higher levels in SCLC. An increase and sustained activation of adenosine 3',5'-cyclic monophosphate (cAMP) levels induced NE-like differentiation in NSCLC cell lines, and a concomitant increase in the expression of mac25/IGFBP-rP1 and 25.1 was observed during the cAMP-regulated differentiation of NCI-H157 cells, suggesting the involvement of these proteins. Furthermore, the collective overexpression of mac25/IGFBP-rP1 and 25.1 in NSCLC cells induced NE-like differentiation as early as 6 h postinfection. The present data suggest that mac25/IGFBP-rP1 and 25.1 may play a functional role in the NE differentiation of NSCLC cell lines and may provide a novel therapeutic target for treating lung cancers, in particular NSCLC with NE differentiation.

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.

Differential expression of chitinases identify subsets of murine airway epithelial cells in allergic inflammation

The mammalian chitinase family includes members both with and without enzymatic activity against chitin, a product of fungal cell walls, exoskeletons of crustaceans and insects, and the microfilarial sheaths of parasitic nematodes. Two members of that family, Ym1 and acidic mammalian chitinase (AMCase), are strongly upregulated in pulmonary T helper (Th) 2 inflammation but not in Th1 inflammation. The sites of expression of these products are incompletely known. We show here that, in two different models of Th2 inflammation, Ym1 and AMCase are mutually exclusively expressed in proximal vs. distal airway epithelium, respectively, whereas both are expressed in alveolar macrophages. This regional difference along the airway corresponds to the previously noted distinction between mucus positive proximal cells and mucus negative distal cells under the same conditions. Among distal cells, AMCase colocalizes with epithelial cells expressing the Clara cell marker Clara cell secretory protein. These AMCase-expressing cells retain expression of FOXA2, a transcription factor whose downregulation in association with IL-13 signaling has previously been associated with production of mucus in proximal airway epithelial cells. These results provide evidence that secretory cells of proximal and distal airways undergo fundamentally different gene expression programs in response to allergic inflammation. Furthermore, AMCase provides the first positive molecular marker of distal Clara cell secretory protein-expressing cells under these conditions.

Stem cells and cystic fibrosis

Although cystic fibrosis at first sight appears to be one of the most obvious human diseases to treat with gene therapy, since it is caused by a single-gene defect and the main affected organ is the lung which is relatively easily accessible, clinical results have thus far been disappointingly limited. At least one cause for this lack of success is the failure to permanently correct the gene defect in addition to the rapid turnover of lung epithelial cells. Alternative approaches therefore involve the search for and use of stem cell populations. This review presents an overview of recent attempts to identify lung- or bone marrow-derived populations of stem cells or progenitor cells and to apply such cells, heterologous or gene-corrected autologous, to colonize the airways while differentiating into functional respiratory columnar epithelial cells. The most successful approaches thus far appear to be obtained with bone marrow-derived cells such as mesenchymal stem cells, although the transdifferentiation rate thus far has been limited to below the 1% level. As an alternative the proven multipotent nature of bronchioalveolar stem cells isolated from lung tissue may provide another promising approach for successful stem cell therapy.

Stem cells in the lung

The lung is composed of two major anatomically distinct regions-the conducting airways and gas-exchanging airspaces. From a cell biology standpoint, the conducting airways can be further divided into two major compartments, the tracheobronchial and bronchiolar airways, while the alveolar regions of the lung make up the gas-exchanging airspaces. Each of these regions consists of distinct epithelial cell types with unique cellular physiologies and stem cell compartments. This chapter focuses on model systems with which to study stem cells in the adult tracheobronchial airways, also referred to as the proximal airway of the lung. Important in such models is an appreciation for the diversity of stem cell niches in the conducting airways that provide localized environmental signals to both maintain and mobilize stem cells in the setting of airway injury and normal cellular turnover. Because cellular turnover in airways is relatively slow, methods for analysis of stem cells in vivo have required prior injury to the lung. In contrast, ex vivo and in vitro models for analysis of airway stem cells have used genetic markers to track lineage relationships together with reconstitution systems that mimic airway biology. Over the past decades, several widely acceptable methods have been developed and used in the characterization of adult airway stem/progenitor cells. These include localization of label-retaining cells (LRCs), retroviral tagging of epithelial cells seeded into xenografts, air-liquid interface cultures to track clonal proliferative potential, and multiple transgenic mouse models. This chapter reviews the biologic context and use of these models while providing detailed methods for several of the more broadly useful models for studying adult airway stem/progenitor cell types.

Pulmonary epithelium

Repair or regeneration of defective lung epithelium would be of great therapeutic potential. Cellular sources for such repair have long been searched for within the lung, but the identification and characterization of stem or progenitor cells have been hampered by the complexity and cellular heterogeneity of the organ. In recent years, various pulmonary cells have been identified that meet the criteria for stem cells but it remains to be seen how far manipulation of these tissue-specific cell pools can upregulate epithelial repair. The initial excitement that greeted the results of animal experiments showing cells of bone marrow origin in murine lung has been tempered by more recent data suggesting that the cells do not repair pulmonary epithelium. However, there are reports of engraftment of bone marrow-derived cells in human lung, albeit at a low level, so the administration of cell therapy via the circulation, for repair and/or gene delivery, needs further investigation. The potential of human embryonic stem cells to generate any cell, tissue, or organ on demand for tissue repair or replacement is promising to revolutionize the treatment of human disease. Although some headway has been made into making pulmonary epithelium from these stem cells, human embryonic stem cell technology is still in its infancy and many technical, safety, and ethical hurdles must be cleared before clinical trials can begin. This chapter focuses on the potential role of stem cells in future approaches to lung repair and regeneration.

ASH1Gene Is a Specific Therapeutic Target for Lung Cancers with Neuroendocrine Features

Lung cancers with neuroendocrine features are usually aggressive, although the underlying molecular mechanisms largely remain to be determined. The basic helix-loop-helix protein, achaete-scute complex-like 1/achaete-scute homologue 1 (ASH1), is expressed in normal fetal pulmonary neuroendocrine cells and lung cancers with neuroendocrine elements and is suggested to be involved in lung carcinogenesis. In the present study, we show inhibition of ASH1 expression by plasmid-based RNA interference (RNAi) to significantly suppress growth of lung cancer cells with ASH1 expression through G2-M cell cycle arrest and accumulation of sub-G1 populations, possibly linked to cleavage of caspase-9 and caspase-7. However, lung cancer cell lines without ASH1 expression and immortalized normal BEAS2B bronchial epithelial cells were not affected. The RNAi-resistant mutant ASH1 clearly induced rescue from G2-M arrest, suggesting a target-specific effect of RNAi. An ASH1-RNAi adenovirus was also established and significantly inhibited not only in vitro cell proliferation but also in vivo xenograft growth of ASH1-positive NCI-H460 cells. Elevated levels of apoptosis were also observed in NCI-H460 xenografts with the ASH1-RNAi adenovirus. The present study therefore suggests that ASH1 plays a crucial role in lung cancer development and may be an effective therapeutic target in lung cancers with neuroendocrine features.

Identification of bronchioalveolar stem cells in normal lung and lung cancer

Injury models have suggested that the lung contains anatomically and functionally distinct epithelial stem cell populations. We have isolated such a regional pulmonary stem cell population, termed bronchioalveolar stem cells (BASCs). Identified at the bronchioalveolar duct junction, BASCs were resistant to bronchiolar and alveolar damage and proliferated during epithelial cell renewal in vivo. BASCs exhibited self-renewal and were multipotent in clonal assays, highlighting their stem cell properties. Furthermore, BASCs expanded in response to oncogenic K-ras in culture and in precursors of lung tumors in vivo. These data support the hypothesis that BASCs are a stem cell population that maintains the bronchiolar Clara cells and alveolar cells of the distal lung and that their transformed counterparts give rise to adenocarcinoma. Although bronchiolar cells and alveolar cells are proposed to be the precursor cells of adenocarcinoma, this work points to BASCs as the putative cells of origin for this subtype of lung cancer.

Identification of novel resident pulmonary stem cells: form and function of the lung side population

Resident lung stem cells function to replace all lineages of pulmonary tissue, including mesenchyme, epithelium, and vasculature. The phenotype of the lung side population (SP) cells is currently under investigation; their function is currently unknown. Recent data suggest lung SP cells are an enriched tissue-specific source of organ-specific pulmonary precursors and, therefore, a source of adult stem cells. The adult lung SP cell population has been isolated and characterized for expression of markers indicative of stem cell, epithelial, and mesenchymal lineages. These studies determined that the adult mouse lung SP has epithelial and mesenchymal potential that resides within a CD45- mesenchymal subpopulation, as well as limited hematopoietic ability, which resides in the bone marrow-derived CD45+ subpopulation. The ability to identify these adult lung precursor cells allows us to further study the potential of these cells and their role in the regulation of tissue homeostasis and response to injury. The identification of this target population will potentially allow earlier treatment and, long term, a functional restoration of injured pulmonary tissue and lung health.

Stem Cells for Lung Cancer?

Stem cells are believed to be crucial players in tumor development. There is much interest in identifying those compartments that harbor stem cells involved in lung cancer, given the high incidence and recurrence rate of this disease. In this issue of Cell, Kim and colleagues describe a niche in the bronchioalveolar duct junction of adult mouse lung that harbors stem cells from which adenocarcinomas are likely to arise. They enriched, propagated, and differentiated these stem cells in vitro and found that they were activated by the oncogenic protein K-ras. This study provides exciting insights into how the stem cell compartment operates during both normal lung-tissue homeostasis and the development of lung cancer. The new work offers perspectives on possible therapeutic interventions to combat lung cancer.

Airway glandular development and stem cells

Submucosal glands in the lung play important roles in several hypersecretory lung disease processes, including chronic bronchitis, asthma, and cystic fibrosis. In this context, submucosal glands undergo abnormal growth and differentiation through processes that are poorly understood. To better understand the pathophysiological mechanisms that lead to submucosal gland hypertrophy and hyperplasia in the adult human lung, efforts have been made to dissect the molecular signals and cell types responsible for normal submucosal gland development in the airway. Such studies have revealed a close relationship between progenitor?stem cell phenotypes in the surface airway epithelia and submucosal glands, and thus it has been suggested that submucosal glands serve as a protective niche for surface airway epithelial stem cells. Furthermore, the pluripotent progenitor cells that exist in the surface airway epithelium, which have the capacity to differentiate into ciliated, secretory, intermediate, and basal cells, also have a developmental capacity for submucosal glands. This putative adult stem cell compartment of the airway epithelium has been the focus of research attempting to identify molecular markers for signaling pathways that control stem cell phenotypes and their capacity for proliferation and differentiation following airway injury.

Mucin is produced by clara cells in the proximal airways of antigen-challenged mice

Airway mucus hypersecretion is a prominent feature of many obstructive lung diseases. We thus determined the ontogeny and exocytic phenotype of mouse airway mucous cells. In naive mice, ciliated (approximately 40%) and nonciliated (approximately 60%) epithelial cells line the airways, and > 95% of the nonciliated cells are Clara cells that contain Clara cell secretory protein (CCSP). Mucous cells comprise < 5% of the nonciliated cells. After sensitization and a single aerosol antigen challenge, alcian blue-periodic acid Schiff's positive mucous cell numbers increase dramatically, appearing 6 h after challenge (21% of nonciliated/nonbasal cells), peaking from Days 1-7 (99%), and persisting at Day 28 (65%). Throughout the induction and resolution of mucous metaplasia, ciliated and Clara cell numbers identified immunohistochemically change only slightly. Intracellular mucin content peaks at Day 7, and mucin expression is limited specifically to a Clara cell subset in airway generations 2-4 that continue to express CCSP. Functionally, Clara cells are secretory cells that express the regulated exocytic marker Rab3D and, in antigen-challenged mice, rapidly secrete mucin in response to inhaled ATP in a dose-dependent manner. Thus, Clara cells show great plasticity in structure and secretory products, yet have molecular and functional continuity in their identity as specialized apical secretory cells.

Airway injury in lung disease pathophysiology: selective depletion of airway stem and progenitor cell pools potentiates lung inflammation and alveolar dysfunction

Identification of early events that contribute to the establishment of chronic lung disease has been complicated by the variable involvement of the airway and alveolar compartments in the complex physiology of end-stage disease. In particular, the impact of airway injury on alveolar integrity and function has not been addressed and would be facilitated by development of animal models of lung disease that specifically target a single cell type within the airway epithelium. We have previously demonstrated that ganciclovir treatment of CCtk transgenic mice, which express the herpes simplex thymidine kinase gene under regulation of the mouse Clara cell secretory protein (CCSP) promoter, results in elimination of the airway progenitor and stem cell pools and a consequent failure of airway regeneration that is associated with rapid morbidity and mortality. In this study, we used the CCtk model to test the hypothesis that selective airway injury initiates profound lung dysfunction through mechanisms that compromise alveolar integrity. Results demonstrate that elimination of the CCSP-expressing cell population results in secondary alveolar inflammation, edema, and depletion of the alveolar type II cell population. On the basis of these data we conclude that selective airway injury can serve as the inciting injury in diseases characterized by severely compromised alveolar function.

Lung stem cells: new paradigms

The intrinsic anatomical complexity of the lung, its slow cell turnover, and the lack of regenerative models are among the factors that have complicated the study and isolation of adult lung stem cells. Despite this, several endogenous lung progenitor cells have been identified in the proximal and distal lung. However, there is limited data regarding the lineage relationships, self-renewal properties, and clonality of these specific lung cell progenitors. More recent work showing that marrow cells can engraft as differentiated cells of solid organs has suggested new stem cell paradigms for the lung. In this review, we explore the implications of these new studies for lung stem cell biology. We also summarize and discuss the ongoing controversies that these studies have generated.

In vivo differentiation potential of tracheal basal cells: evidence for multipotent and unipotent subpopulations

The composition of the conducting airway epithelium varies significantly along the proximal to distal axis, with that of the tracheal epithelium exhibiting the greatest complexity. A number of progenitor cells have been proposed to contribute to the maintenance of this cellular diversity both in the steady state and in response to injury. However, individual roles for each progenitor cell type are poorly defined in vivo. The present study was undertaken to investigate the hypothesis that basal cells represent a multipotent progenitor cell type for renewal of the injured tracheal epithelium. To understand their contribution to epithelial repair, mice were exposed to naphthalene to induce airway injury and depletion of the secretory cell progenitor pool. Injury resulted in a rapid induction of cytokeratin 14 (K14) expression among the majority of GSI-B4-reactive cells and associated hyperplasia of basal cells. Restoration of depleted secretory cells occurred after 6 days of recovery and was associated with regression of the basal cell hyperplasia, suggesting a progenitor-progeny relationship. Multipotent differentiation of basal cells was confirmed using a bitransgenic ligand-regulated Cre-loxP reporter approach in which expression of a ubiquitously expressed LacZ reporter was activated within K14-expressing progenitor cells during airway repair. With the use of this approach, it was determined that K14-expressing cells include subsets capable of either multipotent or unipotent differentiation in vivo. We conclude that basal cells have the capacity for restoration of a fully differentiated epithelium.

Molecular phenotype of airway side population cells

Lung epithelial-specific stem cells have been localized to discrete microenvironments throughout the adult conducting airway. Properties of these cells include pollutant resistance, multipotent differentiation, and infrequent proliferation. Goals of the present study were to use Hoechst 33342 efflux, a property of stem cells in other tissues, to purify and further characterize airway stem cells. Hoechst 33342 effluxing lung cells were identified as a verapamil-sensitive side population by flow cytometry. Lung side population cells were further subdivided on the basis of hematopoietic (CD45 positive) or nonhematopoietic (CD45 negative) origin. Nonhematopoietic side population cells were enriched for stem cell antigen-1 reactivity and expressed molecular markers specific to both airway and mesenchymal lineages. Analysis of the molecular phenotype of airway-derived side population cells indicates that they are similar to neuroepithelial body-associated variant Clara cells. Taken together, these data suggest that the nonhematopoietic side population isolated from lung is enriched for previously identified airway stem cells.

Basal cells are a multipotent progenitor capable of renewing the bronchial epithelium

Commitment of the pulmonary epithelium to bronchial and bronchiolar airway lineages occurs during the transition from pseudoglandular to cannalicular phases of lung development, suggesting that regional differences exist with respect to the identity of stem and progenitor cells that contribute to epithelial maintenance in adulthood. We previously defined a critical role for Clara cell secretory protein-expressing (CE) cells in renewal of bronchiolar airway epithelium following injury. Even though CE cells are also the principal progenitor for maintenance of the bronchial airway epithelium, CE cell injury is resolved through a mechanism involving recruitment of a second progenitor cell population that we now identify as a GSI-B(4) reactive, cytokeratin-14-expressing basal cell. These cells exhibit multipotent differentiation capacity as assessed by analysis of cellular phenotype within clones of LacZ-tagged cells. Clones were derived from K14-expressing cells tagged in a cell-type-specific fashion by ligand-regulable Cre recombinase-mediated genomic rearrangement of the ROSA26 recombination substrate allele. We conclude that basal cells represent an alternative multipotent progenitor cell population of bronchial airways and that progenitor cell selection is dictated by the type of airway injury.

Time and pressure dependence of transvascular Clara cell protein, albumin, and IgG transport during ventilator-induced lung injury in mice

We compared the transport of three proteins with different hydrodynamic radii with ultrastructural changes in lungs of intact mice ventilated at peak inflation pressures (PIP) of 15, 35, 45, and 55 cmH2O for 2 h and PIP of 55 cmH2O for 0.5 and 1 h. After 2 h of ventilation, significant increases were observed in plasma Clara cell secretory protein (1.9 nm radius) at 35 cmH2O PIP and in bronchoalveolar lavage fluid albumin (3.6 nm radius) at 45 cmH2O PIP and IgG (5.6 nm radius) at 55 cmH2O PIP. Increased concentrations of all three proteins and lung wet-to-dry weight ratios were significantly correlated with PIP and ventilation time. Clara cell secretory protein and albumin increased significantly after 0.5 h of 55 cmH2O PIP, but IgG increased only after 2 h. Separation of endothelium or epithelium to form blebs was apparent only in small vessels (15-30 μm diameter) at 45 cmH2O PIP and after 0.5 h at 55 cmH2O PIP but became extensive after 2 h of ventilation at 55 cmH2O PIP. Junctional gaps between cells were rarely observed. Ultrastructural lung injury and protein clearances across the air-blood barrier were related to ventilation time and PIP levels. Protein clearances increased in relation to molecular size, consistent with increasing dimensions and frequency of transmembrane aqueous pathways.

Pulmonary epithelial stem cells

Classically, the stem/progenitor cells of the pulmonary epithelium are considered to be the basal and mucous cells of the proximal airways, Clara cells in the bronchioles and type II pneumocytes in the alveoli. Recent data suggest that there is a variant of Clara cells, lying in pulmonary neuroendocrine bodies, that meets several stem cell criteria and that type II pneumocytes exist in at least two populations, one of which is more resistant to injury. However, a complete revision of our understanding of pulmonary stem cell biology is underway as a result of the discovery of pulmonary epithelium derived from blood-borne cells. In addition, the existence in the lung of a 'universal' pluripotent cell has long been speculated upon and now some initial evidence has emerged with the identification of a spore-like cell that can differentiate in vitro to bronchiolar tissue.

Expression of Clara cell secretory protein in the lungs of rats exposed to silicon carbide whisker in vivo

Intratracheal instillation studies have shown that exposure to silicon carbide whiskers (SiCW), an asbestos substitute, produces pulmonary fibrotic changes, suggesting that SiCW might have fibrogenic potential. It has been theorized that Clara cell secretory protein (CCSP) plays a critical role in regulating the acute inflammatory response in the lung. The present study was conducted to investigate the time course of the expression of CCSP in lungs exposed to SiCW in vivo. Male Wistar rats were administered 2 mg or 10 mg of SiCW suspended in saline by a single intratracheal instillation and were sacrificed at 3 days, 1 week, 1 month, 3 months and 6 months of recovery time. The expression of CCSP was observed by reverse transcriptase-polymerase chain reaction (RT-PCR), Western blot and immunostaining. Exposure to 10 mg of SiCW decreased in levels of CCSP mRNA at 3 days, 1 week, 1 month and 6 months following intratracheal instillation. The protein level of CCSP in SiCW-exposed rats was decreased at 1 day, 3 days and 1 month after a single instillation of 2 and 10 mg. These findings suggest that CCSP are involved not only in the acute injury but also in the chronic injury of the lung exposed to SiCW.

Multiple mechanisms for oxygen-induced regulation of the Clara cell secretory protein gene

The Clara cell secretory protein (CCSP) imparts a protective effect to the lung during oxidant injury. However, exposure to supplemental oxygen, a common therapeutic modality for lung disease, represses the expression of CCSP in the adult mouse lung. We investigated the mechanisms of hyperoxia-induced repression of the mouse CCSP promoter. Deletion experiments in vivo and in vitro indicated that the hyperoxia-responsive elements are localized to the proximal -166 bp of the CCSP promoter. Electrophoretic mobility shift and supershift analyses demonstrated increased binding of c-Jun at the activator protein-1 site, increased binding of CCAAT/enhancer binding protein (C/EBP) beta at the C/EBP sites, and decreased binding at the Nkx2.1 sites. Western analyses revealed that hyperoxia exposure induced an increase in the expression of the C/EBPbeta isoform liver-inhibiting protein (LIP) and an increase in cytoplasmic Nkx2.1. Cotransfection of LIP or c-Jun expression plasmids decreased the transcriptional activity of the proximal -166-bp CCSP promoter. These observations suggest that hyperoxia-induced repression of the CCSP gene is mediated, at least in part, at the level of transcription and that multiple mechanisms mediate this repression. Moreover, these novel observations may provide insights for generation of therapeutic interventions for the amelioration of oxidant-induced lung injury.

Induction of small cell lung cancer by somatic inactivation of both Trp53 and Rb1 in a conditional mouse model

Small cell lung cancer (SCLC) is a highly aggressive human tumor with a more than 95% mortality rate. Its ontogeny and molecular pathogenesis remains poorly understood. We established a mouse model for neuroendocrine (NE) lung tumors by conditional inactivation of Rb1 and Trp53 in mouse lung epithelial cells. Mice carrying conditional alleles for both Rb1 and Trp53 developed with high incidence aggressive lung tumors with striking morphologic and immunophenotypic similarities to SCLC. Most of these tumors, which we designate MSCLC (murine small cell lung carcinoma), diffusely spread through the lung and gave rise to extrapulmonary metastases. In our model, inactivation of both Rb1 and p53 was a prerequisite for the pathogenesis of SCLC.

Identification and isolation of mouse type II cells on the basis of intrinsic expression of enhanced green fluorescent protein

The unique morphology and cell-specific expression of surfactant genes have been used to identify and isolate alveolar type II epithelial cells. Because these attributes can change during lung injury, a novel method was developed for detecting and isolating mouse type II cells on the basis of transgenic expression of enhanced green fluorescence protein (EGFP). A line of transgenic mice was created in which EGFP was targeted to type II cells under control of the human surfactant protein (SP)-C promoter. Green fluorescent cells that colocalized by immunostaining with endogenous pro-SP-C were scattered throughout the parenchyma. EGFP was not detected in Clara cell secretory protein-expressing airway epithelial cells or other nonlung tissues. Pro-SP-C immunostaining diminished in lungs exposed to hyperoxia, consistent with decreased expression and secretion of intracellular precursor protein. In contrast, type II cells could still be identified by their intrinsic green fluorescence, because EGFP is not secreted. Type II cells could also be purified from single-cell suspensions of lung homogenates using fluorescence-activated cell sorting. Less than 1% of presorted cells exhibited green fluorescence compared with >95% of the sorted population. As expected for type II cells, ultrastructural analysis revealed that the sorted cells contained numerous lamellar bodies. SP-A, SP-B, and SP-C mRNAs were detected in the sorted population, but T1alpha and CD31 (platelet endothelial cell adhesion molecule) were not, indicating enrichment of type II epithelial cells. This method will be invaluable for detecting and isolating mouse type II cells under a variety of experimental conditions.

Stem cells in adult tissues

In recent years the concept of a stem cell has evolved to encompass the hypotheses that stem cells exist within many adult tissues, and that a common 'interchangeable' progenitor cell may exist within the bone marrow capable of regenerating and repairing tissues throughout the body. As more knowledge is gained about stem cells, their potential roles in disease processes, including the development and progression of cancer, have moved to the forefront. The underlying hypothesis of this review is that cell fate is determined by a combination of intrinsic and extrinsic factors; growth and differentiation are regulated through intracellular integration of a multitude of signals initiated by internal and external stimuli. The development of successful stem cell based therapies may depend on experimental approaches that consider both the intrinsic and extrinsic factors that control cell fate.