Plasticity in the airway epithelium

The Immune Function of Tuft Cells at Gut Mucosal Surfaces and Beyond

Tuft cells were first discovered in epithelial barriers decades ago, but their function remained unclear until recently. In the last 2 years, a series of studies has provided important advances that link tuft cells to infectious diseases and the host immune responses. Broadly, a model has emerged in which tuft cells use chemosensing to monitor their surroundings and translate environmental signals into effector functions that regulate immune responses in the underlying tissue. In this article, we review the current understanding of tuft cell immune function in the intestines, airways, and thymus. In particular, we discuss the role of tuft cells in type 2 immunity, norovirus infection, and thymocyte development. Despite recent advances, many fundamental questions about the function of tuft cells in immunity remain to be answered.

Transdifferentiation of outgrowth cells and cultured epithelial cells from swine trachea

The morphologic and functional properties of explant out-growth cells and epithelial cells isolated from swine trachea epithelium by proteolysis were examined. A mixed population of ciliated, serous, and basal cells, obtained from out-growths, from proteolysis of trachea epithelium, and from unattached explants in organ culture, all yielded cell cultures that werecomposed almost entirely of mucus-secreting cells. When the cells were grown in primary or secondary culture on a modified collagen matrix in supplemented HAM:DMEM (1:1) medium they expressed a mucus-secreting phenotype with numerous mucus granules at various stages of maturation and incorporated [3H]GlcN and 35SO4 into secreied mucin glycoproteins. Results obtained in these studies suggest that extensive transdifferentiation of ciliated and serous cells to mucus-secreting-cells occurs after the release and during subsequent attachment and culture. Ciliated cells containing mucus granules were seen in various stages of cilia resorption. Basal cells containing mucus granules were also frequently observed. The number of mucus-secreting cells and the synthesis of mucin glycoproteins increased dramatically with time of attachment and culture, whereas cell proliferation, population doubling time of 72 h, and incorporation of [3H]-thymidine into DNA increased much more slowly. The number of mucus-secreting cells correlated closely with the level of secretion of mucin glycoproteins. Taken collectively, these studies help to elucidate the transdifferentiation process, which dramatically increases the number of mucus-secreting cells after disruption and release of epithelial cells from swine tracheobronchial epithelium. A similar mechanism involving disruption of the extracellular matrix may be involved in the stimulation of hypersecretion of mucus and mucin glycoproteins by chemical and infections irritants.

Chemosensory brush cells of the trachea. A stable population in a dynamic epithelium

Tracheal brush cells (BCs) are specialized epithelial chemosensors that use the canonical taste transduction cascade to detect irritants. To test whether BCs are replaced at the same rate as other cells in the surrounding epithelium of adult mice, we used 5-bromo-2'-deoxyuridine (BrdU) to label dividing cells. Although scattered BrdU-labeled epithelial cells are present 5-20 days after BrdU, no BCs are labeled. These data indicate that BCs comprise a relatively static population. To determine how BCs are generated during development, we injected 5-day-old mice with BrdU and found labeled BCs and non-BC epithelial cells 5 days after BrdU. During the next 60 days, the percentage of labeled BCs increased, whereas the percentage of other labeled cell types decreased. These data suggest that BCs are generated from non-BC progenitor cells during postnatal tracheal growth. To test whether the adult epithelium retains the capacity to generate BCs, tracheal epithelial cells were recovered from adult mice and grown in an air-liquid interface (ALI) culture. After transition to differentiation conditions, BCs are detected, and comprise 1% of the total cell population by Day 14. BrdU added to cultures before the differentiation of BCs was chased into BCs, indicating that the increase in BC density is attributable to the proliferation of a non-BC progenitor. We conclude that: (1) BCs are normally a static population in adult mice; (2) BC progenitors proliferate and differentiate during neonatal development; and (3) BCs can be regenerated from a proliferative population resident in adult epithelium.

Effects of second hand smoke on airway secretion and mucociliary clearance

The airway acts as the first defense against inhaled pathogens and particulate matter from the environment. One major way for the airway to clear inhaled foreign objects is through mucociliary clearance (MCC), an important component of the respiratory innate immune defense against lung disease. MCC is characterized by the upward movement of mucus by ciliary motion that requires a balance between the volume and composition of the mucus, adequate periciliary liquid (PCL) volume, and normal ciliary beat frequency (CBF). Airway surface fluid (ASL) is a thin layer liquid that consists of the highly viscous mucus upper “gel” layer, and the watery lubricating lower “sol” layer. Mucus production, secretion and clearance are considered to play a critical role in maintenance of airway health because it maintains hydration in the airway and traps particulates, bacteria, and viruses. Different types of epithelial cells, including secretory cells, and ciliated cells, contribute to the MCC function. Cigarette smoke (CS) contains chemicals and particulates that significantly affect airway secretion. Active and passive CS-induced chronic obstructive pulmonary disease (COPD) is frequently associated with hyperplasia of goblet cells and submucosal glands (SMGs), thus increasing the secretory capacity of the airways that impairs MCC.

{alpha}7 nicotinic acetylcholine receptor regulates airway epithelium differentiation by controlling basal cell proliferation

Airway epithelial basal cells are known to be critical for regenerating injured epithelium and maintaining tissue homeostasis. Recent evidence suggests that the alpha7 nicotinic acetylcholine receptor (nAChR), which is highly permeable to Ca(2+), is involved in lung morphogenesis. Here, we have investigated the potential role of the alpha7 nAChR in the regulation of airway epithelial basal cell proliferation and the differentiation of the human airway epithelium. In vivo during fetal development and in vitro during the regeneration of the human airway epithelium, alpha7 nAChR expression coincides with epithelium differentiation. Inactivating alpha7 nAChR function in vitro increases cell proliferation during the initial steps of the epithelium regeneration, leading to epithelial alterations such as basal cell hyperplasia and squamous metaplasia, remodeling observed in many bronchopulmonary diseases. The regeneration of the airway epithelium after injury in alpha7(-/-) mice is delayed and characterized by a transient hyperplasia of basal cells. Moreover, 1-year-old alpha7(-/-) mice more frequently present basal cells hyperplasia. Modulating nAChR function or expression shows that only alpha7 nAChR, as opposed to heteropentameric alpha(x)beta(y) nAChRs, controls the proliferation of human airway epithelial basal cells. These findings suggest that alpha7 nAChR is a key regulator of the plasticity of the human airway epithelium by controlling basal cell proliferation and differentiation pathway and is involved in airway remodeling during bronchopulmonary diseases.

Identification of proteomic differences between squamous cell carcinoma of the lung and bronchial epithelium

Proteins that exhibit different expression levels in normal and malignant lung cells are good candidate biomarkers to improve early diagnosis and intervention. We used a quantitative approach and compared the proteome of microdissected cells from normal human bronchial epithelium and squamous cell carcinoma tumors of histopathological grades G2 and G3. DIGE analysis and subsequent MS-based protein identification revealed that 32 non-redundant proteins were differentially regulated between the respective tissue types. These proteins are mainly involved in energy pathways, cell growth or maintenance mechanisms, protein metabolism, and the regulation of DNA and RNA metabolism. The expression of some of these proteins was analyzed by immunohistochemistry using tissue microarrays containing tissue specimen of 55 patients, including normal bronchial epithelium, squamous cell carcinomas, adenocarcinomas, and large cell carcinomas. The results of the immunohistochemical studies correlated with the proteome study data and revealed that particularly HSP47 and a group of cytokeratins (i.e. cytokeratins 6a, 16, and 17) are significantly co-regulated in squamous cell carcinoma. Furthermore cytokeratin 17 showed significantly higher abundance in G2 grade compared with G3 grade squamous cell carcinomas in both the gel-based and the immunohistochemical analysis. Therefore this protein might be used as a marker for stratification between different tumor grades.

Pulmonary lesions in female Harlan Sprague-Dawley rats following two-year oral treatment with dioxin-like compounds

Dioxin and dioxin-related compounds have been associated with high incidences of pulmonary dysfunctions and/or cancers in humans. To evaluate the relative potencies of effects of these compounds, the National Toxicology Program completed a series of two-year bioassays which were conducted using female Harlan Sprague-Dawley rats. The rats were treated orally for up to 2 years with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3',4,4',5-pentachlorobiphenyl (PCB126), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), and a ternary mixture of TCDD, PCB126 and PeCDF. In addition to treatment-related effects reported in other organs, a variety of pulmonary lesions were observed that were related to exposure. Pulmonary CYP1A1-associated 7-ethoxyresorufin-O-deethylase (EROD) activity was increased in all dosed groups. The most common non-neoplastic lesions, which occurred in all studies, were bronchiolar metaplasia and squamous metaplasia of the alveolar epithelium. Cystic keratinizing epithelioma was the most commonly observed neoplasm which occurred in all studies. A low incidence of squamous cell carcinoma was associated only with PCB126 treatment. Potential mechanisms leading to altered differentiation and/or proliferation of bronchiolar and alveolar epithelia may be through CYP1A1 induction or disruption of retinoid metabolism.

Hepatocyte growth factor and other fibroblast secretions modulate the phenotype of human bronchial epithelial cells

The luminal airway surface is lined with epithelial cells that provide a protective barrier from the external environment and clear inhaled pathogens from the lung. To accomplish this important function, human bronchial epithelial (HBE) cells must be able to rapidly regenerate a mucociliary layer of cells following epithelial injury. Whereas epithelial-fibroblast interactions are known to modulate the airway architecture during lung development and repair, little is known about how these two cells interact. Using a primary HBE and lung fibroblast coculture system, we demonstrate that 1) subepithelial fibroblasts provide a suitable environment for differentiation of HBE cells into a polarized ciliated phenotype despite being cultured in media that induces terminal squamous differentiation and growth arrest in the absence of fibroblasts, 2) HBE cells cocultured with subepithelial fibroblasts exhibit augmented ciliogenesis, accelerated wound repair, and diminished polarized ion transport compared with cells grown in control conditions, and 3) hepatocyte growth factor (HGF) is important for subepithelial fibroblast modulation of HBE cell differentiation. These results provide a model to study fibroblast modulation of epithelial phenotype and indicate that HGF secreted by subepithelial fibroblasts contributes to HBE cell differentiation.

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.

Roles of apoptosis in airway epithelia

The airway epithelium functions primarily as a barrier to foreign particles and as a modulator of inflammation. Apoptosis is induced in airway epithelial cells (AECs) by viral and bacterial infections, destruction of the cytoskeleton, or by exposure to toxins such as high oxygen and polycyclic hydrocarbons. Various growth factors and cytokines including TGF-beta, IFN-gamma, or the activators of the death receptors, TNF-alpha and FasL, also induce apoptosis in AECs. However, cell death is observed in maximally 15% of AECs after 24 h of treatment. Preincubation with IFN-gamma or a zinc deficiency increases the percentage of apoptotic AECs in response to TNF-alpha or FasL, suggesting that AECs have mechanisms to protect them from cell death. Apoptosis of AECs is a major mechanism in reducing cell numbers after hyperplastic changes in airway epithelia that may arise due to major injuries in response to LPS or allergen exposures. Resolution of hyperplastic changes or changes during prolonged exposure to an allergen is primarily regulated by the Bcl-2 family of proteins. Fas and FasL are both expressed in AECs, and their main function may be to control inflammation by inducing Fas-induced death in inflammatory cells without inducing apoptosis in neighboring cells. Furthermore, AECs engulf dying eosinophils to clear them by phagocytosis. Therefore, in the airway epithelium apoptosis serves three main roles: (1) to eliminate damaged cells; (2) to restore homeostasis following hyperplastic changes; and (3) to control inflammation, and thereby support the barrier and anti-inflammatory functions.

Solitary chemoreceptor cell proliferation in adult nasal epithelium

Nasal trigeminal chemosensitivity in mice and rats is mediated in part by solitary chemoreceptor cells (SCCs) in the nasal epithelium (Finger et al., 2003). Many nasal SCCs express the G-protein alpha-gustducin as well as other elements of the bitter-taste signaling cascade including phospholipase Cbeta2, TRPM5 and T2R bitter-taste receptors. While some populations of sensory cells are replaced throughout life (taste and olfaction), others are not (hair cells and carotid body chemoreceptors). These experiments were designed to test whether new SCCs are generated within the epithelium of adult mice. Wild type C57/B6 mice were injected with the thymidine analog 5-bromo-2'-deoxyuridine (BrdU) to label dividing cells. At various times after injection (1-40 days), the mice were perfused with 4% paraformaldehyde and prepared for dual-label immunocytochemistry. Double labeled cells were detected as early as 3 days post BrdU injection and remained for as long as 12 days post-injection suggesting that SCCs do undergo turnover like the surrounding nasal epithelium. No BrdU labeled cells were detected after 24 days suggesting relatively rapid replacement of the SCCs.

Exacerbations of chronic obstructive pulmonary disease and chronic mucus hypersecretion

Chronic obstructive pulmonary disease (COPD) exacerbations are an important cause of the considerable morbidity and mortality found in COPD. COPD exacerbations increase with increasing severity of COPD, and some patients are prone to frequent exacerbations leading to hospital admission and readmission. These frequent exacerbations may have considerable impact on quality of life and activities of daily living. Factors that increase the risk for COPD exacerbations are associated with increased airway inflammation caused by common pollutants and bacterial and/or viral infections. These inflammatory responses cause mucus hypersecretion and, thereby, airway obstruction and associated exacerbations. While chronic mucus hypersecretion is a significant risk factor for frequent and severe exacerbations, patients with chronic mucus hypersecretion have a lower rate of relapse after initial treatment for acute exacerbation. The benefit of antibiotics for treatment of COPD exacerbations is small but significant. While the mechanisms of actions are not clear, mucolytic agents reduce the number of days of disability in subjects with exacerbations. Reducing mucous cell numbers in small airways could be a useful way to reduce chronic mucus hypersecretion. Our studies suggest that programmed cell death is crucial in the resolution of metaplastic mucous cells, and understanding these mechanisms may provide novel therapies to reduce the risk of COPD exacerbations.

Matrix metalloproteinase/epidermal growth factor receptor/mitogen-activated protein kinase signaling regulate fra-1 induction by cigarette smoke in lung epithelial cells

Exposure to cigarette smoke (CS) can lead to the development of lung cancer, but the molecular mechanisms underlying this process remain unclear. Given that activator protein 1 (AP-1) regulates genes involved in both physiologic and pathophysiologic processes, we have investigated the effects of CS on Jun and Fos family member expression and regulation using a nonmalignant human bronchial epithelial cell line, 1HAEo. Exposure to CS caused a marked upregulation of c-Jun, c-Fos, and Fra-1, but not of Fra-2, Jun-B, and Jun-D expression. Because Fra-1 is overexpressed in various tumors and upregulates genes associated with tumor progression, we further elucidated the mechanisms that control CS-stimulated fra-1 induction. CS stimulated fra-1 induction primarily at the transcriptional level. However, epidermal growth factor receptor (EGFR)-specific inhibitor, AG1478, completely suppressed CS-stimulated fra-1 expression. Similarly, the specific inhibitors of extracellular signal-regulated kinase (ERK), c-Jun NH2 terminal kinase (JNK), and p38 kinase signaling markedly suppressed fra-1 induction. Consistent with this finding, AG1478 blocked CS-stimulated ERK, JNK, and p38 phosphorylation. These results suggest that EGFR-activated multiple kinase signaling is essential for fra-1 induction. Furthermore, treatment of cells with GM6001, which inhibits matrix metalloproteinase activity, significantly suppressed CS-stimulated EGF shedding, EGFR and ERK kinase phosphorylation, and subsequent fra-1 induction. Collectively, our findings indicate an obligatory role for metalloproteinase-EGFR-mediated mitogen-activated protein kinase signaling in controlling CS-induced fra-1 expression.

Mechanism of repression of squamous differentiation marker, SPRR1B, in malignant bronchial epithelial cells: role of critical TRE-sites and its transacting factors

The overexpression of SPRR1B in bronchial epithelium is a marker for early metaplastic changes and the loss of its expression is associated with an irreversible malignant transformation. In the present study, we have used a model system consisting of normal and malignant bronchial epithelial (BE) cells to elucidate the differential transcriptional control of SPRR1B. SPRR1B expression is either detectable or PMA (phorbol 13-myristate 12-acetate) -inducible in several malignant BE cells including squamous, adeno, small and large cell carcinomas. Loss of SPRR1B expression is correlated well with the lack of strong in vivo protein-DNA interactions at the -152 bp promoter, which contains two functional TRE sites. Even though the basal level AP-1 protein DNA binding pattern is different between normal and malignant cells, PMA significantly enhances Jun and Fos binding to the consensus TRE site in both cell types. Intriguingly, the composition of AP-1 protein binding to the -152 to -86 bp SPRR1B promoter is quite different. In untreated cells, SPRR1B promoter is predominantly occupied by JunD and Fra2. PMA significantly induced binding of JunB and Fra1 in normal cells, while JunB and Fra2 bound to TREs in the malignant cells. Overexpression of fra1 in malignant cells significantly enhanced SPRR1B promoter activity. In contrast, overexpression of fra2, but not fra1, strongly reduced both basal and PMA-inducible promoter activities in normal cells. Together, these results indicate that either temporal expression and/or differential activation of AP-1 proteins, especially Fra1 and Fra2, might contribute to the dysregulation of terminal differentiation marker, SPRR1B, expression in various BE cells.

Human airway xenograft models of epithelial cell regeneration

Regeneration and restoration of the airway epithelium after mechanical, viral or bacterial injury have a determinant role in the evolution of numerous respiratory diseases such as chronic bronchitis, asthma and cystic fibrosis. The study in vivo of epithelial regeneration in animal models has shown that airway epithelial cells are able to dedifferentiate, spread, migrate over the denuded basement membrane and progressively redifferentiate to restore a functional respiratory epithelium after several weeks. Recently, human tracheal xenografts have been developed in immunodeficient severe combined immunodeficiency (SCID) and nude mice. In this review we recall that human airway cells implanted in such conditioned host grafts can regenerate a well-differentiated and functional human epithelium; we stress the interest in these humanized mice in assaying candidate progenitor and stem cells of the human airway mucosa.

Calcitonin driven v-Ha-ras induces multilineage pulmonary epithelial hyperplasias and neoplasms

We initiated a transgenic model for primary pulmonary neuroendocrine cell (PNEC) hyperplasia/neoplasia using v-Ha-ras driven by the neural/neuroendocrine (NE)-specific calcitonin promoter (rascal). Previously, we showed that nitrosamine treated rodents develop PNEC hyperplasia but non-NE lung tumors, with variable outcomes presumably reflecting ras activation in multiple cell lineages. Interestingly, all rascal transgenic mouse lineages develop hyperplasias of NE and non-NE cells but mostly non-NE lung carcinomas, with rascal mRNA in differentiated PNECs and tumor cells. Analyses of embryonic lung demonstrate rascal mRNA in undifferentiated epithelium, consistent with expression in a common pluripotent precursor cell. These unexpected observations indicate that v-Ha-ras can lead to both NE and non-NE hyperplasia/neoplasia in vivo, opening new avenues for studies of lung carcinogenesis.

Mechanisms and function of intercellular calcium signaling

Intercellular Ca2+ waves initiated by mechanical or chemical stimuli propagate between cells via gap junctions. The ability of a wide diversity of cells to display intercellular Ca2+ waves suggests that these Ca2+ waves may represent a general mechanism by which cells communicate. Although Ca2+ may permeate gap junctions, the intercellular movement of Ca2+ is not essential for the propagation of Ca2+ waves. The messenger that moves from one cell to the next through gap junctions appears to be IP3 and a regenerative mechanism for IP3 may be required to effect multicellular communication. Extracellularly mediated Ca2+ signaling also exists and this could be employed to supplement or replace gap junctional communication. The function of intercellular Ca2+ waves may be the coordination of cooperative cellular responses to local stimuli.

Mucin gene expression in rat airways following infection and irritation

Airway mucus hypersecretion occurs in response to infection and irritation and poses an important and poorly understood clinical problem. In order to gain insight into its pathogenesis, we have focused on an mRNA encoding the major mucus glycoprotein, mucin. Northern blots showed that mucin mRNA was abundant in the intestine of specific pathogen free rats whereas it was undetectable in the airways of these rats until pathogen-free conditions were suspended and rats acquired Sendai (Parainfluenza I) virus infections. Airway mucin hybridization signals in rats that were both infected with Sendai virus and exposed to SO2 were more intense than those in rats with infection alone. These results suggest that pathogen-and irritant-induced hypersecretion may be partly controlled at the level of mucin mRNA.