Blocking airway mucous cell metaplasia by inhibiting EGFR antiapoptosis and IL-13 transdifferentiation signals

A transgenic FOXJ1-Cre system for gene inactivation in ciliated epithelial cells

Ciliated airway epithelial cells are critical for mucosal barrier function, including host defense against pathogens. This cell population is often the primary target and thereby the first line of defense against many common respiratory viruses. It is also the precursor for mucous cells and thereby promotes mucociliary clearance of infectious and other noxious agents. Cells with motile cilia in other organs (e.g., brain and reproductive organs) may also have roles in development and reproduction. However, definitive proof of ciliated cell function is hampered by the lack of strategies to specifically target this cell population for loss of function in vivo. To this end, cell type-specific gene promoters have been combined with the Cre/LoxP system to disrupt genes in airway and alveolar epithelial cell populations expressing surfactant protein C (SP-C) or Clara cell secretory protein (CCSP). By contrast, an analogous system to disrupt gene function in ciliated airway epithelial cells was still needed. Here we report the generation and analysis of mouse lines with a FOXJ1 promoter driving the Cre recombinase and show that this system mediates genomic recombination specifically in ciliated cells. The pattern of recombination recapitulates endogenous FOXJ1 promoter function, being restricted to ciliated cells present in pulmonary airways as well as choroid plexus, ependyma, oviduct, and testis. This transgenic mouse system thereby offers a new strategy for specific knockouts of genes in ciliated cells. It should prove extremely useful for defining ciliated cell function in airway mucosal immunity as well as development and reproduction.

Cell death, remodeling, and repair in chronic obstructive pulmonary disease?

Apoptotic cells can be detected in the parenchyma and airways of patients with chronic obstructive pulmonary disease (COPD) in greater numbers than seen in normal lungs or those from smokers without COPD. Implications include more apoptosis and/or decreased clearance of apoptotic cells. Both epithelial and endothelial cells become apoptotic. What role does the apoptosis play in the emphysema or small airway alterations seen in COPD? In simple terms, loss of cells by apoptosis would be expected to accompany, or perhaps initiate, the overall tissue destruction normally believed responsible. Indeed, direct induction of apoptosis in pulmonary endothelial or epithelial cells in rodents is accompanied by emphysematous changes. On the other hand, apoptotic cells are normally removed from tissues rapidly with minimal tissue response, to be followed by cell replacement to maintain homeostasis. The presence of detectable apoptotic cells, therefore, may imply defects in these clearance mechanisms, and, in keeping with this hypothesis, there is increasing evidence for such defects in patients with COPD. Mice with abnormalities in apoptotic cell removal also tend to develop spontaneous "emphysema." A reconciling hypothesis is that recognition of apoptotic cells not only leads to removal but also, normally, to signals for cell replacement. If this latter response is lacking in COPD-susceptible smokers, defects in normal alveolar or small airway repair could significantly contribute to the structural disruption. The concept puts emphasis on defective repair as well as initial injury (i.e., persistent alteration of dynamic tissue homeostasis, as a key contributor to COPD), with, it is hoped, additional approaches for mitigation.

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.

The histopathology of fatal untreated human respiratory syncytial virus infection

The pathology of respiratory syncytial virus (RSV) infection was evaluated 1 day after an outpatient diagnosis of RSV in a child who died in a motor vehicle accident. We then identified 11 children with bronchiolitis from the Vanderbilt University autopsy log between 1925 and 1959 who met criteria for possible RSV infection in the preintensivist era. Their tissue was re-embedded and evaluated by routine hematoxylin and eosin and PAS staining and immunostaining with RSV-specific antibodies. Tissue from three cases was immunostain-positive for RSV antigen and was examined in detail. Small bronchiole epithelium was circumferentially infected, but basal cells were spared. Both type 1 and 2 alveolar pneumocytes were also infected. Although, not possible for archival cases, tissue from the index case was evaluated by immunostaining with antibodies to define the cellular components of the inflammatory response. Inflammatory infiltrates were centered on bronchial and pulmonary arterioles and consisted of primarily CD69+ monocytes, CD3+ double-negative T cells, CD8+ T cells, and neutrophils. The neutrophil distribution was predominantly between arterioles and airways, while the mononuclear cell distribution was in both airways and lung parenchyma. Most inflammatory cells were concentrated submuscular to the airway, but many cells traversed the smooth muscle into the airway epithelium and lumen. Airway obstruction was a prominent feature in all cases attributed to epithelial and inflammatory cell debris mixed with fibrin, mucus, and edema, and compounded by compression from hyperplastic lymphoid follicles. These findings inform our understanding of RSV pathogenesis and may facilitate the development of new approaches for prevention and treatment.

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.

Induced trefoil factor family 1 expression by trans-differentiating Clara cells in a murine asthma model

Asthma is a chronic inflammatory disease of the airways that is accompanied by goblet cell metaplasia and mucus hypersecretion. Trefoil factor family (TFF) peptides represent major secretory products of the respiratory tract and are synthesized together with mucins. In the murine lung, TFF2 is mainly expressed, whereas TFF1 transcripts represent only a minor species. TFF peptides are well known for their motogenic and anti-apoptotic effects, and they modulate the inflammatory response of bronchial epithelial cells. Here, an established mouse model of asthma was investigated (i.e., exposure to Aspergillus fumigatus [AF] antigens). RT-PCR analysis of lung tissue showed elevated levels particularly of TFF1 transcripts in AF-sensitized/challenged animals. In contrast, transcripts encoding Clara cell secretory protein (CCSP/CC10) were strongly diminished in these animals. For comparison, the expression of the goblet cell secretory granule marker mCLCA3/Gob-5, the mucins Muc1-Muc6 and Muc19, and the secretoglobins ScgB3A1 and ScgB3A2, as well as the mammalian ependymin-related gene MERP2, were monitored. Immunohistochemistry localized TFF1 mainly in cells with a mixed phenotype (e.g., TFF1-positive cells stain with the lectin wheat germ agglutinin (WGA), which recognizes mucins characteristic of goblet cells). In addition, these cells express CCSP/CC10, a Clara cell marker. When compared with mucins or CCSP/CC10, TFF1 was stored in a different population of secretory granules localized at the more basolateral portion of these cells. Thus, the results presented indicate for the first time that allergen exposure leads to the trans-differentiation of Clara cells toward a TFF1-expressing mucous phenotype.

IL-13 and epidermal growth factor receptor have critical but distinct roles in epithelial cell mucin production

Overproduction of mucus is a central feature of asthma. The cytokine, IL-13, epidermal growth factor receptor (EGFR), and transcription factor, FOXA2, have each been implicated in mucus production, but the mechanistic relationships between these molecules are not yet well understood. To address this, we established a primary normal human bronchial epithelial cell culture system with IL-13-induced mucus production and gene transcript expression changes similar to those seen in vivo in mice. IL-13 did not stimulate release of the EGFR ligand, transforming growth factor (TGF)-alpha. However, there was constitutive release of TGF-alpha from normal human bronchial epithelial cells, and inhibition of TGF-alpha or EGFR reduced both constitutive and IL-13-induced mucin production. Microarray analysis revealed that IL-13 and the EGFR pathway appear to have almost completely independent effects on transcript expression. IL-13 induced a relatively small set of transcripts, including several novel transcripts that might play a role in pathogenesis of allergic airway disease. In contrast, EGFR activity had extensive effects, including altered expression of many transcripts associated with cell metabolism, survival, transcription, and differentiation. One of the few common effects of IL-13 and EGFR signaling was decreased expression of FOXA2, which is known to prevent mucus production. We conclude that the IL-13 and EGFR pathways make critical but quite distinct contributions to gene regulation in airway epithelial cells, and that both pathways affect expression of the key transcription factor, FOXA2, a known regulator of mucus production.

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.

Genetic segregation of airway disease traits despite redundancy of calcium-activated chloride channel family members

Complex airway diseases such as asthma and chronic obstructive pulmonary disease exhibit stereotyped traits (especially airway hyperreactivity and mucous cell metaplasia) that are variably expressed in each patient. Here, we used a mouse model for virus-induced long-term expression of these traits to determine whether individual traits can be genetically segregated and thereby linked to separate determinants. We showed that an F2 intercross population derived from susceptible and nonsusceptible mouse strains can manifest individual phenotypic extremes that exhibit one or the other disease trait. Functional genomic analysis of these extremes further indicated that a member of the calcium-activated chloride channel (CLCA) gene family designated mClca3 was inducible with mucous cell metaplasia but not airway hyperreactivity. In confirmation of this finding, we found that mClca3 gene transfer to mouse airway epithelium was sufficient to induce mucous cell metaplasia but not airway hyperreactivity. However, newly developed mClca3(-/-) mice exhibited the same degree of mucous cell metaplasia and airway hyperreactivity as wild-type mice. Bioinformatic analysis of the Clca locus led to the identification of mClca5, and gene transfer indicated that mClca5 also selectively drives mucous cell metaplasia. Thus, in addition to the capacity of CLCA family members to exhibit diverse functional activities, there is also preserved function so that more than one family member mediates mucous cell metaplasia. Nonetheless, Clca expression appears to be a selective determinant of mucous cell metaplasia so that shared homologies between CLCA family members may still represent a useful target for focused therapeutic intervention in hypersecretory airway disease.

Mucus in chronic airway diseases: sorting out the sticky details

Mucous hypersecretion is a major cause of airway obstruction in asthma, chronic obstructive pulmonary disease, and cystic fibrosis. EGFR ligands and IL-13 are known to stimulate mucous induction, but the detailed mechanisms of epithelial mucous regulation have not been well defined. In this issue of the JCI, Tyner et al. show, in a mouse model of chronic mucous hypersecretion, that ciliated epithelial cell apoptosis is inhibited by EGFR activation, allowing IL-13 to stimulate the differentiation of these cells into goblet cells, which secrete mucus. In defining this coordinated, 2-step process, we can consider the therapeutic effects of blocking mucous production. This begs the question, Is it possible to reduce airway obstruction in chronic lung disease by inhibiting EGFR activation and/or by inhibiting IL-13?