Epithelial stem/progenitor cells in lung postnatal growth, maintenance, and repair

Culture and expansion of murine proximal airway basal stem cells

BACKGROUND

The stem cell characteristic makes basal cells desirable for ex vivo modeling of airway diseases. However, to date, approaches allowing them extensively in vitro serial expansion and maintaining bona fide stem cell property are still awaiting to be established. This study aims to develop a feeder-free culture system of mouse airway basal stem cells (ABSCs) that sustain their stem cell potential in vitro, providing an experimental basis for further in-depth research and mechanism exploration.

METHODS

We used ROCK inhibitor Y-27632-containing 3T3-CM, MEF-CM, and RbEF-CM to determine the proper feeder-free culture system that could maintain in vitro stem cell morphology of mouse ABSCs. Immunocytofluorescence was used to identify the basal cell markers of obtained cells. Serial propagation was carried out to observe whether the stem cell morphology and basal cell markers could be preserved in this cultivation system. Next, we examined the in vitro expansion and self-renewal ability by evaluating population doubling time and colony-forming efficiency. Moreover, the differentiation potential was detected by an in vitro differentiation culture and a 3D tracheosphere assay.

RESULTS

When the mouse ABSCs were cultured using 3T3-CM containing ROCK inhibitor Y-27632 in combination with Matrigel-coated culture dishes, they could stably expand and maintain stem cell-like clones. We confirmed that the obtained clones comprised p63/Krt5 double-positive ABSCs. In continuous passage and maintenance culture, we found that it could be subculture to at least 15 passages in vitro, stably maintaining its stem cell morphology, basal cell markers, and in vitro expansion and self-renewal capabilities. Meanwhile, through in vitro differentiation culture and 3D tracheosphere culture, we found that in addition to maintaining self-renewal, mouse ABSCs could differentiate into other airway epithelial cells such as acetylated tubulin (Act-Tub) + ciliated and MUC5AC + mucus-secreting cells. However, they failed to differentiate into alveoli epithelial cells, including alveolar type I and alveolar type II.

CONCLUSION

We established an in vitro feeder-free culture system that allows mouse ABSCs to maintain their stem cell characteristics, including self-renewal and airway epithelium differentiation potential, while keeping up in vitro expansion stability.

Intrapulmonary distal airway stem cell transplantation repairs lung injury in chronic obstructive pulmonary disease

Objectives

Chronic obstructive pulmonary disease (COPD) is characterized by irreversible lung tissue damage including chronic bronchitis and emphysema, which could further develop into respiratory failure. Many studies have revealed a potential regenerative function of the distal airway stem/progenitor cells (DASCs) after lung injury.

Materials and Methods

Mouse and human DASCs were expanded, analysed, and engrafted into injured mouse lungs. Single‐cell analyses were performed to reveal the differentiation path of the engrafted cells. Finally, human DASCs were transplanted into COPD mice induced by porcine pancreatic elastase (PPE) and lipopolysaccharide (LPS) administration.

Results

We showed that isolated mouse and human DASCs could be indefinitely expanded and were able to further differentiate into mature alveolar structures in vitro. Single‐cell analysis indicated that the engrafted cells expressed typical cellular markers of type I alveolar cells as well as the specific secreted proteins. Interestingly, transplantation of human DASCs derived from COPD patients into the lungs of NOD‐SCID mice with COPD injury repaired the tissue damage and improved the pulmonary function.

Conclusions

The findings demonstrated that functional lung structure could be reconstituted by intrapulmonary transplantation of DASCs, suggesting a potential therapeutic role of DASCs transplantation in treatment for chronic obstructive pulmonary disease.

An NKX2-1GFP and TP63tdTomato dual fluorescent reporter for the investigation of human lung basal cell biology

Basal cells are multipotent stem cells responsible for the repair and regeneration of all the epithelial cell types present in the proximal lung. In mice, the elusive origins of basal cells and their contribution to lung development were recently revealed by high-resolution, lineage tracing studies. It however remains unclear if human basal cells originate and participate in lung development in a similar fashion, particularly with mounting evidence for significant species-specific differences in this process. To address this outstanding question, in the last several years differentiation protocols incorporating human pluripotent stem cells (hPSC) have been developed to produce human basal cells in vitro with varying efficiencies. To facilitate this endeavour, we introduced tdTomato into the human TP63 gene, whose expression specifically labels basal cells, in the background of a previously described hPSC line harbouring an NKX2-1GFP reporter allele. The functionality and specificity of the NKX2-1GFP;TP63tdTomato hPSC line was validated by directed differentiation into lung progenitors as well as more specialised lung epithelial subtypes using an organoid platform. This dual fluorescent reporter hPSC line will be useful for tracking, isolating and expanding basal cells from heterogenous differentiation cultures for further study.

Lung epithelium damage in COPD - An unstoppable pathological event?

Chronic obstructive pulmonary disease (COPD) is a common term for alveolar septal wall destruction resulting in emphysema, and chronic bronchitis accompanied by conductive airway remodelling. In general, this disease is characterized by a disbalance of proteolytic/anti-proteolytic activity, augmented inflammatory response, increased oxidative/nitrosative stress, rise in number of apoptotic cells and decreased proliferation. As the first responder to the various environmental stimuli, epithelium occupies an important position in different lung pathologies, including COPD. Epithelium sequentially transitions from the upper airways in the direction of the gas exchange surface in the alveoli, and every cell type possesses a distinct role in the maintenance of the homeostasis. Basically, a thick ciliated structure of the airway epithelium has a major function in mucus secretion, whereas, alveolar epithelium which forms a thin barrier covered by surfactant has a function in gas exchange. Following this line, we will try to reveal whether or not the chronic bronchitis and emphysema, being two pathological phenotypes in COPD, could originate in two different types of epithelium. In addition, this review focuses on the role of lung epithelium in COPD pathology, and summarises underlying mechanisms and potential therapeutics.

Age-Dependent Translocation of Gold Nanoparticles across the Air-Blood Barrier

Do immature lungs have air-blood barriers that are more permeable to inhaled nanoparticles than those of fully developed mature lungs? Data supporting this notion and explaining the underlying mechanisms do not exist as far as we know. Using a rat model of postnatal lung development, here the data exactly supporting this notion, that is, significantly more gold nanoparticles (NPs) cross from the air space of the lungs to the rest of the body in neonates than in adults, are presented. Moreover, in neonates the translocation of gold NPs is not size dependent, whereas in adult animals smaller NPs cross the air-blood lung barrier much more efficiently than larger NPs. This difference in air-blood permeability in neonate versus adult animals suggests that NP translocation in the immature lungs may follow different rules than in mature lungs. Supporting this notion, we propose that the paracellular transport route may play a more significant role in NP translocation in immature animals, as suggested by protein expression studies. Findings from this study are critical to design optimal ways of inhalation drug delivery using NP nanocarriers for this age group, as well as for better understanding of the potential adverse health effects of nanoparticle exposures in infants and young children.

Distal airway stem cells ameliorate bleomycin-induced pulmonary fibrosis in mice

Background

Idiopathic pulmonary fibrosis is characterized by loss of lung epithelial cells and inexorable progression of fibrosis with no effective and approved treatments. The distal airway stem/progenitor cells (DASCs) have been shown to have potent regenerative capacity after lung injury. In this work, we aimed to define the role of mouse DASCs (mDASCs) in response to bleomycin-induced lung fibrosis in mice.

Methods

The mDASCs were isolated, expanded in vitro, and labeled with GFP by lentiviral infection. The labeled mDASCs were intratracheally instilled into bleomycin-induced pulmonary fibrosis mice on day 7. Pathological change, collagen content, α-SMA expression, lung function, and mortality rate were assessed at 7, 14, and 21 days after bleomycin administration. Tissue section and direct fluorescence staining was used to show the distribution and differentiation of mDASCs in lung.

Results

The transplanted mDASCs could incorporate, proliferate, and differentiate into type I pneumocytes in bleomycin-injured lung. They also inhibited fibrogenesis by attenuating the deposition of collagen and expression of α-SMA. In addition, mDASCs improved pulmonary function and reduce mortality in bleomycin-induced pulmonary fibrosis mice.

Conclusions

The data strongly suggest that mDASCs could ameliorate bleomycin-induced pulmonary fibrosis by promotion of lung regeneration and inhibition of lung fibrogenesis.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1257-2) contains supplementary material, which is available to authorized users.

Phenotypic Analysis of BrdU Label-Retaining Cells during the Maturation of Conducting Airway Epithelium in a Porcine Lung

Stem/progenitor cells have recently been demonstrated to play key roles in the maturation, injury repair, and regeneration of distinct organs or tissues. Porcine has spurred an increased interest in biomedical research models and xenotransplantation, owing to most of its organs share similarities in physiology, cellular composition and size to humans. Therefore, characterization of stem/progenitor cells in porcine organs or tissues may provide a novel avenue to better understand the biology and function of stem cells in humans. In the present study, potential stem/progenitor cells in conducting airway epithelium of a porcine lung were characterized by morphometric analysis of bromodeoxyuridine (BrdU) label-retaining cells (LRCs) during the maturation of the lung. The results showed a pseudostratified mucociliary epithelium comprised of basal, ciliated, goblet, and columnar cells in the conducting airway of a porcine lung. In addition, the majority of primary epithelial cells able to proliferate in vitro expressed keratin 5, a subpopulation of these keratin 5-positive cells, also expressed CD117 (c-Kit) or CD49f (integrin alpha 6, ITGA6), implying that they might be potential epithelial stem/progenitor cells in conducting airway of a porcine lung. Lineage tracing analysis with a BrdU-labeled neonatal piglet showed that the proportion of BrdU-labeled cells in conducting airways decreased over the 90-day period of lung maturation. The BrdU-labeled epithelial cells also expressed keratin 14, mucin 5AC, or prosurfactant protein C (ProSP-C); among them, the keratin 14-positive cells were the most frequent BrdU-labeled epithelial cell type as determined by immunohistochemical and immunofluorescence staining. This study may provide valuable information on the biology and function of epithelial stem/progenitor cells in conducting airway of pigs and humans.

Selective activation and proliferation of a quiescent stem cell population in the neuroepithelial body microenvironment

BACKGROUND

The microenvironment (ME) of neuroepithelial bodies (NEBs) harbors densely innervated groups of pulmonary neuroendocrine cells that are covered by Clara-like cells (CLCs) and is believed to be important during development and for adult airway epithelial repair after severe injury. Yet, little is known about its potential stem cell characteristics in healthy postnatal lungs.

METHODS

Transient mild lung inflammation was induced in mice via a single low-dose intratracheal instillation of lipopolysaccharide (LPS). Bronchoalveolar lavage fluid (BALF), collected 16 h after LPS instillation, was used to challenge the NEB ME in ex vivo lung slices of control mice. Proliferating cells in the NEB ME were identified and quantified following simultaneous LPS instillation and BrdU injection.

RESULTS

The applied LPS protocol induced very mild and transient lung injury. Challenge of lung slices with BALF of LPS-treated mice resulted in selective Ca²⁺-mediated activation of CLCs in the NEB ME of control mice. Forty-eight hours after LPS challenge, a remarkably selective and significant increase in the number of divided (BrdU-labeled) cells surrounding NEBs was observed in lung sections of LPS-challenged mice. Proliferating cells were identified as CLCs.

CONCLUSIONS

A highly reproducible and minimally invasive lung inflammation model was validated for inducing selective activation of a quiescent stem cell population in the NEB ME. The model creates new opportunities for unraveling the cellular mechanisms/pathways regulating silencing, activation, proliferation and differentiation of this unique postnatal airway epithelial stem cell population.

Stem cell therapies for chronic obstructive pulmonary disease: current status of pre-clinical studies and clinical trials

Chronic obstructive pulmonary disease (COPD) is a respiratory disease that has a major impact worldwide. The currently-available drugs mainly focus on relieving the symptoms of COPD patients. Novel regenerative therapeutic approaches have been investigated with the aim of repairing or replacing the injured functional structures of the respiratory system. We summarized the progress made by regenerative therapies for COPD by analyzing results from both pre-clinical studies and completed clinical trials. These approaches include the application of exogenous stem cells or small molecules to stimulate the regeneration by endogenous lung stem/progenitor cells. Exogenous mesenchymal stem cells (MSCs) have been reported to repair the structure and improve the function of the injured respiratory system in COPD models. However, the studies that used MSCs in patients with moderate-to-severe COPD patients did not lead to clear respiratory functional improvements. Exogenous human lung stem cells applied to cryo-injured (CI) lungs of mice have been shown to organize into human-like pulmonary structures, indicating a new property of stem cells that is potentially capable of curing COPD patients. Small molecules like retinoic acid has been shown to lead to regeneration and repair of the damaged lung structures in COPD mouse models probably by activation of endogenous lung stem/progenitor cells. However, retinoic acid or agonists of retinoic acid receptor administered to moderate or severe COPD patients did not improve the density and function of the damaged lung. These novel regenerative approaches have failed in preliminary clinical trials, possibly due to the advanced severity of the disease. Further work should be done to develop the current regenerative approaches for curing patients at different stages of COPD. We suggest that some modifications of the approach in the clinical studies may lead to more successful outcomes of regenerative therapy for COPD.

[Advances in Classification and Research Methods of Lung Epithelial Stem and Progenitor Cells]

分离和鉴定肺上皮干/祖细胞，深入了解他们在肺脏生理病理条件下的具体作用机理，对于防治包括肺癌在内的肺脏疾病有重要意义。本综述介绍了已鉴定的肺上皮干/祖细胞种类和肺上皮干/祖细胞研究方法的最新进展，前者具有区域特异性，主要包括位近端气道的基底细胞和导管细胞，位细支气管的Clara细胞、变异Clara细胞、细支气管肺泡干细胞和诱导出的krt5⁺细胞及位肺泡的Ⅱ型肺泡上皮细胞和Ⅱ型肺泡上皮祖细胞；后者主要包括肺损伤模型、谱系示踪技术、三维培养技术、移植、慢性标记细胞法及单细胞转录组学分析等。最后简述了肺上皮干/祖细胞与肺癌的关系以及肺癌干细胞靶向药物治疗进展。

Lung Regeneration: Endogenous and Exogenous Stem Cell Mediated Therapeutic Approaches

The tissue turnover of unperturbed adult lung is remarkably slow. However, after injury or insult, a specialised group of facultative lung progenitors become activated to replenish damaged tissue through a reparative process called regeneration. Disruption in this process results in healing by fibrosis causing aberrant lung remodelling and organ dysfunction. Post-insult failure of regeneration leads to various incurable lung diseases including chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis. Therefore, identification of true endogenous lung progenitors/stem cells, and their regenerative pathway are crucial for next-generation therapeutic development. Recent studies provide exciting and novel insights into postnatal lung development and post-injury lung regeneration by native lung progenitors. Furthermore, exogenous application of bone marrow stem cells, embryonic stem cells and inducible pluripotent stem cells (iPSC) show evidences of their regenerative capacity in the repair of injured and diseased lungs. With the advent of modern tissue engineering techniques, whole lung regeneration in the lab using de-cellularised tissue scaffold and stem cells is now becoming reality. In this review, we will highlight the advancement of our understanding in lung regeneration and development of stem cell mediated therapeutic strategies in combating incurable lung diseases.

Lung regeneration: mechanisms, applications and emerging stem cell populations

Recent studies have shown that the respiratory system has an extensive ability to respond to injury and regenerate lost or damaged cells. The unperturbed adult lung is remarkably quiescent, but after insult or injury progenitor populations can be activated or remaining cells can re-enter the cell cycle. Techniques including cell-lineage tracing and transcriptome analysis have provided novel and exciting insights into how the lungs and trachea regenerate in response to injury and have allowed the identification of pathways important in lung development and regeneration. These studies are now informing approaches for modulating the pathways that may promote endogenous regeneration as well as the generation of exogenous lung cell lineages from pluripotent stem cells. The emerging advances, highlighted in this Review, are providing new techniques and assays for basic mechanistic studies as well as generating new model systems for human disease and strategies for cell replacement.

Clinical potentials of human pluripotent stem cells in lung diseases

Lung possesses very limited regenerative capacity. Failure to maintain homeostasis of lung epithelial cell populations has been implicated in the development of many life-threatening pulmonary diseases leading to substantial morbidity and mortality worldwide, and currently there is no known cure for these end-stage pulmonary diseases. Embryonic stem cells (ESCs) and somatic cell-derived induced pluripotent stem cells (iPSCs) possess unlimited self-renewal capacity and great potential to differentiate to various cell types of three embryonic germ layers (ectodermal, mesodermal, and endodermal). Therapeutic use of human ESC/iPSC-derived lung progenitor cells for regeneration of injured or diseased lungs will have an enormous clinical impact. This article provides an overview of recent advances in research on pluripotent stem cells in lung tissue regeneration and discusses technical challenges that must be overcome for their clinical applications in the future.

Paracrine functions of fibrocytes to promote lung fibrosis

Fibrocytes are derived from the bone marrow and are found in the circulation. They can be recruited to sites of injury and contribute to repair/remodeling. In vitro evidence suggests that fibrocytes may differentiate into fibroblasts to promote lung fibrosis. However, in vivo evidence for this is sparse. This review summarizes recent literature which may suggest that fibrocytes function to promote fibrosis via paracrine actions. In this way, secretion of growth factors, proteases and matricellular proteins may strongly influence the actions of resident epithelial and mesenchymal cells to promote repair and resolution or to tip the scale toward pathologic remodeling.

Integrin α6β4 identifies human distal lung epithelial progenitor cells with potential as a cell-based therapy for cystic fibrosis lung disease

To develop stem/progenitor cell-based therapy for cystic fibrosis (CF) lung disease, it is first necessary to identify markers of human lung epithelial progenitor/stem cells and to better understand the potential for differentiation into distinct lineages. Here we investigated integrin α6β4 as an epithelial progenitor cell marker in the human distal lung. We identified a subpopulation of α6β4⁺ cells that localized in distal small airways and alveolar walls and were devoid of pro-surfactant protein C expression. The α6β4⁺ epithelial cells demonstrated key properties of stem cells ex vivo as compared to α6β4^- epithelial cells, including higher colony forming efficiency, expression of stem cell-specific transcription factor Nanog, and the potential to differentiate into multiple distinct lineages including basal and Clara cells. Co-culture of α6β4⁺ epithelial cells with endothelial cells enhanced proliferation. We identified a subset of adeno-associated virus (AAVs) serotypes, AAV2 and AAV8, capable of transducing α6β4⁺ cells. In addition, reconstitution of bronchi epithelial cells from CF patients with only 5% normal α6β4⁺ epithelial cells significantly rescued defects in Cl^- transport. Therefore, targeting the α6β4⁺ epithelial population via either gene delivery or progenitor cell-based reconstitution represents a potential new strategy to treat CF lung disease.

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.

Prevention of the second stage of epithelial loss is a potential novel treatment for bronchiolitis obliterans

OBJECTIVES

Loss of epithelial cells is one of the key factors that lead to airway fibrosis. Loss of epithelial cells may decrease the barrier to host cell infiltration into the lumen, allowing deposition of extracellular matrix, with subsequent obliteration of the airway. The objective of this study was to determine whether injection of epithelial cells/progenitor cells from the recipient into the lumen of the donor trachea could prevent bronchiolitis obliterans (BO) in a mouse heterotopic tracheal transplantation (HTT) model.

METHODS

A major histocompatibility complex class I and class II mismatch of mouse HTT model of BO was used. Epithelial cells from recipient mice were isolated and reinjected into the lumen of the allografts on day 3 after transplantation. Rag-1 knock-out and isografts were also performed as controls. The grafts were analyzed by immunohistochemistry and densitometric analysis.

RESULTS

The results demonstrated that tracheal epithelium was lost by day 3, regenerated between 3 to 7 days, and was lost again in all allografts, but not in the isografts or in Rag-1 knock-out groups by day 12. The reconstituted epithelium was donor originated on day 7 based on green fluorescent protein staining. Furthermore, with the injection of recipient cells into the tracheal lumen, loss of the epithelium was not observed and the luminal obliteration was significantly less in the allografts.

CONCLUSIONS

Injection of recipient epithelial cells prevents the second phase of epithelial loss and significantly decreases BO development in an HTT model. Clinically, the use of injected recipient epithelial cells could be a novel treatment for BO.

Understanding cellular mechanisms underlying airway epithelial repair: selecting the most appropriate animal models

Understanding the mechanisms underlying the process of regeneration and repair of airway epithelial structures demands close characterization of the associated cellular and molecular events. The choice of an animal model system to study these processes and the role of lung stem cells is debatable since ideally the chosen animal model should offer a valid comparison with the human lung. Species differences may include the complex three-dimensional lung structures, cellular composition of the lung airway as well as transcriptional control of the molecular events in response to airway epithelium regeneration, and repair following injury. In this paper, we discuss issues related to the study of the lung repair and regeneration including the role of putative stem cells in small- and large-animal models. At the end of this paper, the author discuss the potential for using sheep as a model which can help bridge the gap between small-animal model systems and humans.

NOTCH1 is required for regeneration of Clara cells during repair of airway injury

The airways of the mammalian lung are lined with highly specialized epithelial cell types that are the targets of airborne toxicants and injury. Notch signaling plays an important role in the ontogeny of airway epithelial cells, but its contributions to recruitment, expansion or differentiation of resident progenitor/stem cells, and repair and re-establishment of the normal composition of airway epithelium following injury have not been addressed. In this study, the role of a specific Notch receptor, Notch1, was investigated by targeted inactivation in the embryonic lung epithelium using the epithelial-specific Gata5-Cre driver line. Notch1-deficient mice are viable without discernible defects in pulmonary epithelial cell-fate determination and differentiation. However, in an experimental model of airway injury, activity of Notch1 is found to be required for normal repair of the airway epithelium. Absence of Notch1 reduced the ability of a population of cells distinguished by expression of PGP9.5, otherwise a marker of pulmonary neuroendocrine cells, which appears to serve as a reservoir for regeneration of Clara cells. Hairy/enhancer of split-5 (Hes5) and paired-box-containing gene 6 (Pax6) were found to be downstream targets of Notch1. Both Hes5 and Pax6 expressions were significantly increased in association with Clara cell regeneration in wild-type lungs. Ablation of Notch1 reduced Hes5 and Pax6 and inhibited airway epithelial repair. Thus, although dispensable in developmental ontogeny of airway epithelial cells, normal activity of Notch1 is required for repair of the airway epithelium. The signaling pathway by which Notch1 regulates the repair process includes stimulation of Hes5 and Pax6 gene expression.

Neuroepithelial body microenvironment is a niche for a distinct subset of Clara-like precursors in the developing airways

Clara cells of mammalian airways have multiple functions and are morphologically heterogeneous. Although Notch signaling is essential for the development of these cells, it is unclear how Notch influences Clara cell specification and if diversity is established among Clara cell precursors. Here we identify expression of the secretoglobin Scgb3a2 and Notch activation as early events in a program of secretory cell fate determination in developing murine airways. We show that Scgb3a2 expression in vivo is Notch-dependent at early stages and ectopically induced by constitutive Notch1 activation, and also that in vitro Notch signaling together with the pan-airway transcription factor Ttf1 (Nkx2.1) synergistically regulate secretoglobin gene transcription. Furthermore, we identified a subpopulation of secretory precursors juxtaposed to presumptive neuroepithelial bodies (NEBs), distinguished by their strong Scgb3a2 and uroplakin 3a (Upk3a) signals and reduced Ccsp (Scgb1a1) expression. Genetic ablation of Ascl1 prevented NEB formation and selectively interfered with the formation of this subpopulation of cells. Lineage labeling of Upk3a-expressing cells during development showed that these cells remain largely uncommitted during embryonic development and contribute to Clara and ciliated cells in the adult lung. Together, our findings suggest a role for Notch in the induction of a Clara cell-specific program of gene expression, and reveals that the NEB microenvironment in the developing airways is a niche for a distinct subset of Clara-like precursors.

Signaling networks regulating development of the lower respiratory tract

The lungs serve the primary function of air-blood gas exchange in all mammals and in terrestrial vertebrates. Efficient gas exchange requires a large surface area that provides intimate contact between the atmosphere and the circulatory system. To achieve this, the lung contains a branched conducting system (the bronchial tree) and specialized air-blood gas exchange units (the alveoli). The conducting system brings air from the external environment to the alveoli and functions to protect the lung from debris that could obstruct airways, from entry of pathogens, and from excessive loss of fluids. The distal lung enables efficient exchange of gas between the alveoli and the conducting system and between the alveoli and the circulatory system. In this article, we highlight developmental and physiological mechanisms that specify, pattern, and regulate morphogenesis of this complex and essential organ. Recent advances have begun to define molecular mechanisms that control many of the important processes required for lung organogenesis; however, many questions remain. A deeper understanding of these molecular mechanisms will aid in the diagnosis and treatment of congenital lung disease and in the development of strategies to enhance the reparative response of the lung to injury and eventually permit regeneration of functional lung tissue.

Bronchioalveolar stem cells increase after mesenchymal stromal cell treatment in a mouse model of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) remains a major complication of prematurity resulting in significant morbidity and mortality. The pathology of BPD is multifactorial and leads to alveolar simplification and distal lung injury. Previous studies have shown a beneficial effect of systemic treatment with bone marrow-derived mesenchymal stromal cells (MSCs) and MSC-conditioned media (MSC-CM) leading to amelioration of the lung parenchymal and vascular injury in vivo in the hyperoxia murine model of BPD. It is possible that the beneficial response from the MSCs is at least in part due to activation of endogenous lung epithelial stem cells. Bronchioalveolar stem cells (BASCs) are an adult lung stem cell population capable of self-renewal and differentiation in culture, and BASCs proliferate in response to bronchiolar and alveolar lung injury in vivo. Systemic treatment of neonatal hyperoxia-exposed mice with MSCs or MSC-CM led to a significant increase in BASCs compared with untreated controls. Treatment of BASCs with MSC-CM in culture showed an increase in growth efficiency, indicating a direct effect of MSCs on BASCs. Lineage tracing data in bleomycin-treated adult mice showed that Clara cell secretory protein-expressing cells including BASCs are capable of contributing to alveolar repair after lung injury. MSCs and MSC-derived factors may stimulate BASCs to play a role in the repair of alveolar lung injury found in BPD and in the restoration of distal lung cell epithelia. This work highlights the potential important role of endogenous lung stem cells in the repair of chronic lung diseases.

Therapeutic potential of lung epithelial progenitor cells derived from embryonic and induced pluripotent stem cells

Embryonic stem (ES) cells derived from preimplantation blastocysts and induced pluripotent stem (iPS) cells generated from somatic cell sources are pluripotent and capable of indefinite expansion in vitro. They provide a possible unlimited source of cells that could be differentiated into lung progenitor cells for potential clinical use in pulmonary regenerative medicine. Because of inherent difficulties in deriving endodermal cells from undifferentiated cell cultures, applications using lung epithelial cells derived from ES and iPS cells have lagged behind similar efforts devoted to other tissues, such as the heart and spinal cord. However, during the past several years, significant advances in culture, differentiation, and purification protocols, as well as in bioengineering methodologies, have fueled enthusiasm for the development of stem cell-based lung therapeutics. This article provides an overview of recent research achievements and discusses future technical challenges that must be met before the promise of stem cell applications for lung disease can be realized.

The human airway epithelial basal cell transcriptome

BACKGROUND

The human airway epithelium consists of 4 major cell types: ciliated, secretory, columnar and basal cells. During natural turnover and in response to injury, the airway basal cells function as stem/progenitor cells for the other airway cell types. The objective of this study is to better understand human airway epithelial basal cell biology by defining the gene expression signature of this cell population.

METHODOLOGY/PRINCIPAL FINDINGS

Bronchial brushing was used to obtain airway epithelium from healthy nonsmokers. Microarrays were used to assess the transcriptome of basal cells purified from the airway epithelium in comparison to the transcriptome of the differentiated airway epithelium. This analysis identified the "human airway basal cell signature" as 1,161 unique genes with >5-fold higher expression level in basal cells compared to differentiated epithelium. The basal cell signature was suppressed when the basal cells differentiated into a ciliated airway epithelium in vitro. The basal cell signature displayed overlap with genes expressed in basal-like cells from other human tissues and with that of murine airway basal cells. Consistent with self-modulation as well as signaling to other airway cell types, the human airway basal cell signature was characterized by genes encoding extracellular matrix components, growth factors and growth factor receptors, including genes related to the EGF and VEGF pathways. Interestingly, while the basal cell signature overlaps that of basal-like cells of other organs, the human airway basal cell signature has features not previously associated with this cell type, including a unique pattern of genes encoding extracellular matrix components, G protein-coupled receptors, neuroactive ligands and receptors, and ion channels.

CONCLUSION/SIGNIFICANCE

The human airway epithelial basal cell signature identified in the present study provides novel insights into the molecular phenotype and biology of the stem/progenitor cells of the human airway epithelium.

Lung adenocarcinoma originates from retrovirus infection of proliferating type 2 pneumocytes during pulmonary post-natal development or tissue repair

Jaagsiekte sheep retrovirus (JSRV) is a unique oncogenic virus with distinctive biological properties. JSRV is the only virus causing a naturally occurring lung cancer (ovine pulmonary adenocarcinoma, OPA) and possessing a major structural protein that functions as a dominant oncoprotein. Lung cancer is the major cause of death among cancer patients. OPA can be an extremely useful animal model in order to identify the cells originating lung adenocarcinoma and to study the early events of pulmonary carcinogenesis. In this study, we demonstrated that lung adenocarcinoma in sheep originates from infection and transformation of proliferating type 2 pneumocytes (termed here lung alveolar proliferating cells, LAPCs). We excluded that OPA originates from a bronchioalveolar stem cell, or from mature post-mitotic type 2 pneumocytes or from either proliferating or non-proliferating Clara cells. We show that young animals possess abundant LAPCs and are highly susceptible to JSRV infection and transformation. On the contrary, healthy adult sheep, which are normally resistant to experimental OPA induction, exhibit a relatively low number of LAPCs and are resistant to JSRV infection of the respiratory epithelium. Importantly, induction of lung injury increased dramatically the number of LAPCs in adult sheep and rendered these animals fully susceptible to JSRV infection and transformation. Furthermore, we show that JSRV preferentially infects actively dividing cell in vitro. Overall, our study provides unique insights into pulmonary biology and carcinogenesis and suggests that JSRV and its host have reached an evolutionary equilibrium in which productive infection (and transformation) can occur only in cells that are scarce for most of the lifespan of the sheep. Our data also indicate that, at least in this model, inflammation can predispose to retroviral infection and cancer.

Cancer stem cells in lung tumorigenesis

Although stem cells were discovered more than 50 years ago, we have only recently begun to understand their potential importance in cancer biology. Recent advances in our ability to describe, isolate, and study lung stem cell populations has led to a growing recognition of the central importance cells with stem cell-like properties may have in lung tumorigenesis. This article reviews the major studies supporting the existence and importance of cancer stem cells in lung tumorigenesis. Continued research in the field of lung cancer stem cell biology is vital, as ongoing efforts promise to yield new prognostic and therapeutic targets.

National Heart, Lung, and Blood Institute perspective: lung progenitor and stem cells--gaps in knowledge and future opportunities

Because the lung stem cell field is so new, there remain many unanswered questions that are being addressed regarding the identification, location, and role of exogenous and endogenous stem and progenitor cell populations in growth, regeneration, and repair of the lung. Advancing lung stem cell biology will require multidisciplinary teams and a long term effort to unravel the biologic processes of stem cells in the lung. While no clinical research in lung stem cell therapies are currently funded by NHLBI, the knowledge gained by understanding the basic biology of the lung stem cell populations will be needed to translate to diagnostic and therapeutic strategies in the future.

Clara cell: progenitor for the bronchiolar epithelium

Clara cells were first described as a morphologically distinct cell type by Kolliker in 1881, but they take their name from the seminal study of human and rabbit bronchioles by Max Clara in 1937. Since their discovery, Clara cells have been identified as central players in protecting the airway from environmental exposures. The diverse functions of Clara cells in lung homeostasis include roles in xenobiotic metabolism, immune system regulation, and progenitor cell activity. Recent identification of a sub-population of Clara cells as a bronchiolar tissue-specific stem cell and a potential tumor initiating cell has focused the attention of cell and molecular biologists on the Clara cell and its behavior under normal and disease conditions.