Aquaporin-3 expression in human fetal airway epithelial progenitor cells

Epithelial cells/progenitor cells in developing human lower respiratory tract: Characterization and transplantation to rat model of pulmonary injury

Introduction

For cell-based therapies of lung injury, several cell sources have been extensively studied. However, the potential of human fetal respiratory cells has not been systematically explored for this purpose. Here, we hypothesize that these cells could be one of the top sources and hence, we extensively updated the definition of their phenotype.

Methods

Human fetal lower respiratory tissues from pseudoglandular and canalicular stages and their isolated epithelial cells were evaluated by immunostaining, electron microscopy, flow cytometry, organoid assay, and gene expression studies. The regenerative potential of the isolated cells has been evaluated in a rat model of bleomycin-induced pulmonary injury by tracheal instillation on days 0 and 14 after injury and harvest of the lungs on day 28.

Results

We determined the relative and temporal, and spatial pattern of expression of markers of basal (KRT5, KRT14, TRP63), non-basal (AQP3 and pro-SFTPC), and early progenitor (NKX2.1, SOX2, SOX9) cells. Also, we showed the potential of respiratory-derived cells to contribute to in vitro formation of alveolar and airway-like structures in organoids. Cell therapy decreased fibrosis formation in rat lungs and improved the alveolar structures. It also upregulated the expression of IL-10 (up to 17.22 folds) and surfactant protein C (up to 2.71 folds) and downregulated the expression of TGF-β (up to 5.89 folds) and AQP5 (up to 3.28 folds).

Conclusion

We provide substantial evidence that human fetal respiratory tract cells can improve the regenerative process after lung injury. Also, our extensive characterization provides an updated phenotypic profile of these cells.

Single-cell RNA sequencing of mouse lower respiratory tract epithelial cells: A meta-analysis

The respiratory system is a vital component of our body, essential for both oxygen uptake and immune defense. Knowledge of cellular composition and function in different parts of the respiratory tract provides the basis for a better understanding of the pathological processes involved in various diseases such as chronic respiratory diseases and cancer. Single-cell RNA sequencing (scRNA-seq) is a proficient approach for the identification and transcriptional characterization of cellular phenotypes. Although the mouse is an essential tool for the study of lung development, regeneration, and disease, a scRNA-seq mouse atlas of the lung in which all epithelial cell types are included and annotated systematically is lacking. Here, we established a single-cell transcriptome landscape of the mouse lower respiratory tract by performing a meta-analysis of seven different studies in which mouse lungs and trachea were analyzed by droplet and/or plate-based scRNA-seq technologies. We provide information on the best markers for each epithelial cell type, propose surface markers for the isolation of viable cells, harmonized the annotation of cell types, and compare the mouse single-cell transcriptomes with human scRNA-seq data of the lung.

Immune Cells Profiles in the Different Sites of COVID-19-Affected Lung Lobes in a Single Patient

Whereas the COVID-19 disease pathophysiology is under investigation, it is important to identify the pathways of viral transmission and inflammation from the pre-illness to the disease-onset stages. We analyzed five lung lobes from a patient with COVID-19 who finally died after prolonged lung protective ventilation. Pathological examination revealed moderate inflammation in upper lung lobes and uneven yet severe inflammation and diffuse alveolar damage in lower lung lobes. SARS-CoV-2 was detected at higher levels not in severely, but rather moderately inflamed middle lung lobes, and immunohistochemistry and bulk RNA-sequencing results showed that immune cells were detected at higher levels in lower lung lobes. The mRNA expression of cytokine families varied. We found an increase in keratin 5- or aquaporin 3-expressing basal cells in the severely inflamed lower lung lobes, and the alveolar stromal tissues were filled with them. Thus, this analysis of lung samples from a patient helps to determine the COVID-19 pathophysiology at a specific time point, and the virus localization and inflammatory responses at each site of the lungs provide various important indications.

Aquaporins implicated in the cell proliferation and the signaling pathways of cell stemness

Aquaporins (AQPs) are water channel proteins facilitating passive transport of water and other small molecules across biomembranes. Regulation of osmotic homeostasis via AQPs is accompanied by dynamic participation of various cellular signaling pathways. Recently emerging evidence reveals that functional roles of AQPs are further extended from the osmotic regulation via water permeation into the cell proliferation and differentiation. In particular, anomalous expression of AQPs has been demonstrated in various types of cancer cells and cancer stem-like cells and it has been proposed as markers for proliferation and progression of cancer cells. Thus, a more comprehensive view on AQPs could bring a great interest in the cell stemness accompanied by the expression of AQPs. AQPs are broadly expressed across tissues and cells in a cell type- and lineage-specific manner during development via spatiotemporal transcriptional regulation. Moreover, AQPs are expressed in various adult stem cells and cells associated with a stem cell niche as well as cancer stem-like cells. However, the expression and regulatory mechanisms of AQP expression in stem cells have not been well understood. This review highlighted the AQPs expression in stem cell niches/stem cells and the involvement of AQPs in the cell proliferation and signaling pathways associated with cell stemness.

Pathophysiology of Lung Disease and Wound Repair in Cystic Fibrosis

Cystic fibrosis (CF) is an autosomal recessive, life-threatening condition affecting many organs and tissues, the lung disease being the chief cause of morbidity and mortality. Mutations affecting the CF Transmembrane Conductance Regulator (CFTR) gene determine the expression of a dysfunctional protein that, in turn, triggers a pathophysiological cascade, leading to airway epithelium injury and remodeling. In vitro and in vivo studies point to a dysregulated regeneration and wound repair in CF airways, to be traced back to epithelial CFTR lack/dysfunction. Subsequent altered ion/fluid fluxes and/or signaling result in reduced cell migration and proliferation. Furthermore, the epithelial-mesenchymal transition appears to be partially triggered in CF, contributing to wound closure alteration. Finally, we pose our attention to diverse approaches to tackle this defect, discussing the therapeutic role of protease inhibitors, CFTR modulators and mesenchymal stem cells. Although the pathophysiology of wound repair in CF has been disclosed in some mechanisms, further studies are warranted to understand the cellular and molecular events in more details and to better address therapeutic interventions.

Epithelial Stem and Progenitor Cells in Lung Repair and Regeneration

The mammalian lung epithelium is composed of a wide array of specialized cells that have adapted to survive environmental exposure and perform the tasks necessary for respiration. Although the majority of these cells are remarkably quiescent during adult lung homeostasis, a growing body of literature has demonstrated the capacity of these epithelial lineages to proliferate in response to injury and regenerate lost or damaged cells. In this review, we focus on the regionally distinct lung epithelial cell types that contribute to repair after injury, and we address current controversies regarding whether elite stem cells or frequent facultative progenitors are the predominant participants. We also shed light on the newly emerging approaches for exogenously generating similar lung epithelial lineages from pluripotent stem cells.

Role of Aquaporins in the Physiological Functions of Mesenchymal Stem Cells

Aquaporins (AQPs) are a family of membrane water channel proteins that control osmotically-driven water transport across cell membranes. Recent studies have focused on the assessment of fluid flux regulation in relation to the biological processes that maintain mesenchymal stem cell (MSC) physiology. In particular, AQPs seem to regulate MSC proliferation through rapid regulation of the cell volume. Furthermore, several reports have shown that AQPs play a crucial role in modulating MSC attachment to the extracellular matrix, their spread, and migration. Shedding light on how AQPs are able to regulate MSC physiological functions can increase our knowledge of their biological behaviours and improve their application in regenerative and reparative medicine.

Rapid Expansion of Human Epithelial Stem Cells Suitable for Airway Tissue Engineering

RATIONALE

Stem cell-based tracheal replacement represents an emerging therapeutic option for patients with otherwise untreatable airway diseases including long-segment congenital tracheal stenosis and upper airway tumors. Clinical experience demonstrates that restoration of mucociliary clearance in the lungs after transplantation of tissue-engineered grafts is critical, with preclinical studies showing that seeding scaffolds with autologous mucosa improves regeneration. High epithelial cell-seeding densities are required in regenerative medicine, and existing techniques are inadequate to achieve coverage of clinically suitable grafts.

OBJECTIVES

To define a scalable cell culture system to deliver airway epithelium to clinical grafts.

METHODS

Human respiratory epithelial cells derived from endobronchial biopsies were cultured using a combination of mitotically inactivated fibroblasts and Rho-associated protein kinase (ROCK) inhibition using Y-27632 (3T3+Y). Cells were analyzed by immunofluorescence, quantitative polymerase chain reaction, and flow cytometry to assess airway stem cell marker expression. Karyotyping and multiplex ligation-dependent probe amplification were performed to assess cell safety. Differentiation capacity was tested in three-dimensional tracheospheres, organotypic cultures, air-liquid interface cultures, and an in vivo tracheal xenograft model. Ciliary function was assessed in air-liquid interface cultures.

MEASUREMENTS AND MAIN RESULTS

3T3-J2 feeder cells and ROCK inhibition allowed rapid expansion of airway basal cells. These cells were capable of multipotent differentiation in vitro, generating both ciliated and goblet cell lineages. Cilia were functional with normal beat frequency and pattern. Cultured cells repopulated tracheal scaffolds in a heterotopic transplantation xenograft model.

CONCLUSIONS

Our method generates large numbers of functional airway basal epithelial cells with the efficiency demanded by clinical transplantation, suggesting its suitability for use in tracheal reconstruction.

Human Mesenchymal Stem Cells from Adipose Tissue Differentiated into Neuronal or Glial Phenotype Express Different Aquaporins

Aquaporins (AQPs) are 13 integral membrane proteins that provide selective pores for the rapid movement of water and other uncharged solutes, across cell membranes. Recently, AQPs have been focused for their role in production, circulation, and homeostasis of the cerebrospinal fluid and their importance in several human diseases is becoming clear. This study investigated the time course (0, 14, and 28 days) of AQP1, 4, 7, 8, and 9 during the neural differentiation of human mesenchymal stem cells (MSCs) from adipose tissue (AT). For this purpose, two different media, enriched with serum or B-27 and N1 supplements, were applied to give a stimulus toward neural lineage. After 14 days, the cells were cultured with neuronal or glial differentiating medium for further 14 days. The results confirmed that AT-MSCs could be differentiated into neurons, astrocytes, and oligodendrocytes, expressing not only the typical neural markers but also specific AQPs depending on differentiated cell type. Our data demonstrated that at 28 days, AT-MSCs express only AQP1; astrocytes AQP1, 4, and 7; oligodendrocytes AQP1, 4, and 8; and finally neurons AQP1 and 7. This study provides fundamental insight into the biology of the mesenchymal stem cells and it suggests that AQPs can be potential neural markers.

Re-epithelialization: a key element in tracheal tissue engineering

Trachea-tissue engineering is a thriving new field in regenerative medicine that is reaching maturity and yielding numerous promising results. In view of the crucial role that the epithelium plays in the trachea, re-epithelialization of tracheal substitutes has gradually emerged as the focus of studies in tissue-engineered trachea. Recent progress in our understanding of stem cell biology, growth factor interactions and transplantation immunobiology offer the prospect of optimization of a tissue-engineered tracheal epithelium. In addition, advances in cell culture technology and successful applications of clinical transplantation are opening up new avenues for the construction of a tissue-engineered tracheal epithelium. Therefore, this review summarizes current advances, unresolved obstacles and future directions in the reconstruction of a tissue-engineered tracheal epithelium.

Diversity of epithelial stem cell types in adult lung

Lung is a complex organ lined with epithelial cells. In order to maintain its homeostasis and normal functions following injuries caused by varied extraneous and intraneous insults, such as inhaled environmental pollutants and overwhelming inflammatory responses, the respiratory epithelium normally undergoes regenerations by the proliferation and differentiation of region-specific epithelial stem/progenitor cells that resided in distinct niches along the airway tree. The importance of local epithelial stem cell niches in the specification of lung stem/progenitor cells has been recently identified. Studies using cell differentiating and lineage tracing assays, in vitro and/or ex vivo models, and genetically engineered mice have suggested that these local epithelial stem/progenitor cells within spatially distinct regions along the pulmonary tree contribute to the injury repair of epithelium adjacent to their respective niches. This paper reviews recent findings in the identification and isolation of region-specific epithelial stem/progenitor cells and local niches along the airway tree and the potential link of epithelial stem cells for the development of lung cancer.

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.

Cancer stem cells: progress and challenges in lung cancer

The identification of a subpopulation of tumor cells with stem cell-like characteristics first in hematological malignancies and later in solid tumors has emerged into a novel field of cancer research. It has been proposed that this aberrant population of cells now called "cancer stem cells" (CSCs) drives tumor initiation, progression, metastasis, recurrence, and drug resistance. CSCs have been shown to have the capacity of self-renewal and multipotency. Adopting strategies from the field of stem cell research has aided in identification, localization, and targeting of CSCs in many tumors. Despite the huge progress in other solid tumors such as brain, breast, and colon cancers no substantial advancements have been made in lung cancer. This is most likely due to the current rudimentary understanding of lung stem cell hierarchy and heterogeneous nature of lung disease. In this review, we will discuss the most recent findings related to identification of normal lung stem cells and CSCs, pathways involved in regulating the development of CSCs, and the importance of the stem cell niche in development and maintenance of CSCs. Additionally, we will examine the development and feasibility of novel CSC-targeted therapeutic strategies aimed at eradicating lung CSCs.

Looking for the elusive lung stem cell niche

This discourse contains three perspectives on various aspects of Stem Cell Biology and tools available to study and translate into Regenerative Medicine. The lung incessantly faces onslaught of the environment, constantly undergoes oxidative stress, and is an important organ of detoxification. In degenerative diseases and inflammation, the lung undergoes irreversible remodeling that is difficult to therapeutically address and/or transplant a dying tissue. The other difficulty is to study its development and regenerative aspects to best address the aforementioned problems. This perspective therefore addresses- firstly, review of types of stem cells, their pathway of action and models in invertebrate organisms vis-a-vis microenvironment and its dynamics; secondly, stem cells in higher organisms and niche; and lastly data and inference on a novel approach to study stem cell destruction patterns in an injury model and information on putative lung stem cell niche. Stem cells are cryptic cells known to retain certain primitive characteristics making them akin to ancient cells of invertebrates, developmental stages in invertebrates and vertebrates and pliant cells of complex creatures like mammals that demonstrate stimulus-specific behavious, whether to clonally propagate or to remain well protected and hidden within specialized niches, or mobilize and differentiate into mature functionally operative cells to house-keep, repair injury or make new tissues. In lung fibrosis, alveolar epithelium degenerates progressively. In keeping with the goal of regenerative medicine, various models and assays to evaluate long and short term identity of stem cells and their niches is the subject of this perspective. We also report identification and characterization of functional lung stem cells to clarify how stem cell niches counteract this degenerative process. Inferences drawn from this injury model of lung degeneration using a short term assay by tracking side population cells and a long term assay tracking label retaining cells have been presented.

Human tracheobronchial basal cells. Normal versus remodeling/repairing phenotypes in vivo and in vitro

Human tracheobronchial epithelial (TBE) basal cells (BCs) function as progenitors in normal tissue. However, mechanistic studies are typically performed in vitro and frequently use BCs recovered from patients who die of nonrespiratory disease. It is not known whether the cadaveric epithelium (1) is undergoing homeostatic remodeling and/or repair, or (2) yields BC clones that represent homeostatic processes identified in tissue. We sought to compare the phenotype of TBE-BCs with that of BCs cultured under optimal clone-forming conditions. TBE pathology was evaluated using quantitative histomorphometry. The cultured BC phenotype was determined by fluorescence-activated cell sorter analysis. Clone organization and cell phenotype were determined by immunostaining. The cadaveric TBE is 20% normal. In these regions, BCs are keratin (K)-5(+) and tetraspanin CD151(+), and demonstrate a low mitotic index. In contrast, 80% of the cadaveric TBE exhibits homeostatic remodeling/repair processes. In these regions, BCs are K5(+)/K14(+), and a subset expresses tissue factor (TF). Passage 1 TBE cells are BCs that are K5(+)/TF(+), and half coexpress CD151. Optimal clone formation conditions use an irradiated NIH3T3 fibroblast feeder layer (American Type Culture Collection, Frederick, MD) and serum-supplemented Epicult-B medium (Stemcell Technologies, La Jolla, CA). The TF(+)/CD151(-) BC subpopulation is the most clonogenic BC subtype, and is enriched with K14(+) cells. TF(+)/CD151(-) BCs generate clones containing BCs that are K5(+)/Trp63(+), but K14(-)/CD151(-). TF(+) cells are limited to the clone edge. In conclusion, clonogenic human TBE BCs (1) exhibit a molecular phenotype that is a composite of the normal and remodeling/reparative BC phenotypes observed in tissue, and (2) generate organoid clones that contain phenotypically distinct BC subpopulations.

[Regeneration of airway epithelium]

INTRODUCTION

Epithelial regeneration is a complex process. It can lead to the remodeling of the airway epithelium as in asthma, COPD or cystic fibrosis.

BACKGROUND

The development of in vivo and in vitro models has allowed the analysis of remodeling mechanisms and showed the role of components of extracellular matrix, proteases, cytokines and growth factors. Airway epithelial progenitors and stems cells have been studied in these models. However, their identification remains difficult.

CONCLUSION

Identification and characterization of airway epithelial progenitor/stem-cells, and a better knowledge of the regeneration process may allow the development of new therapeutic strategies for airway epithelial reconstitution.

Building and maintaining the epithelium of the lung

Airspaces of the lung are lined by an epithelium whose cellular composition changes along the proximal-to-distal axis to meet local functional needs for mucociliary clearance, hydration, host defense, and gas exchange. Advances in cell isolation, in vitro culture techniques, and genetic manipulation of animal models have increased our understanding of the development and maintenance of the pulmonary epithelium. This review discusses basic cellular mechanisms that regulate establishment of the conducting airway and gas exchange systems as well as the functional maintenance of the epithelium during postnatal life.

Airway stem cells: review of potential impact on understanding of upper airway diseases

Epithelial remodeling is a part of our natural defense mechanisms, and includes migration, proliferation, and differentiation of epithelial cells, as well as the interactions between epithelial and stromal cells. It is not yet possible to distinguish between cause and effect during epithelium remodeling, and are there no clear roles for the many factors involved in respiratory infectious and inflammatory diseases due to a lack of critical information about epithelial cell responses. Most reported data are from lower airway studies or animal models. Therefore, research based on human nasal epithelial stem/progenitor cells can illuminate the pathophysiology of nasal airway disease from a different, more specific perspective. In this review, we discuss epithelial stem/progenitor cell research throughout the airway, with special attention to phenotypes and characterization of these cells from the nasal airway. Recently, we have isolated and cultured P63-positive human epithelial stem/progenitor cells from turbinate biopsies of healthy volunteers and from inflamed mucosa of patients with chronic rhinosinusitis with and without nasal polyposis. These cells propagate in serum-free, growth factor-supplemented, Dulbecco's modified Eagle's medium/F12 media, on either human fibroblast or 3T3 feeder layers. Self-renewal, proliferation, and differentiation potential at an air-liquid interface are being investigated to understand the molecular pathways underlying nasal inflammation. This in vitro culture system for nasal epithelial regeneration will allow molecular studies of human nasal epithelial cell interactions, differentiation, and repair, as well as responses to both environmental agents and to potential anti-inflammatory treatments.

Increased differentiation capacity of bone marrow-derived mesenchymal stem cells in aquaporin-5 deficiency

Mesenchymal stem cells (MSCs) are adult stem cells with a self-renewal and multipotent capability and express extensively in multitudinous tissues. We found that water channel aquaporin-5 (AQP5) is expressed in bone marrow-derived MSCs (BMMSCs) in the plasma membrane pattern. BMMSCs from AQP5(-/-) mice showed significantly lower plasma membrane water permeability than those from AQP5(+/+) mice. In characterizing the cultured BMMSCs from AQP5(-/-) and AQP5(+/+) mice, we found no obvious differences in morphology and proliferation between the 2 genotypes. However, the multiple differentiation capacity was significantly higher in AQP5(-/-) than AQP5(+/+) BMMSCs as revealed by representative staining by Oil Red O (adipogenesis); Alizarin Red S and alkaline phosphatase (ALP; osteogenesis); and type II collagen and Safranin O (chondrogenesis) after directional induction. Relative mRNA expression levels of 3 lineage differentiation markers, including PPARγ2, C/EBPα, adipsin, collagen 1a, osteopontin, ALP, collagen 11a, collagen 2a, and aggrecan, were significantly higher in AQP5(-/-) -differentiating BMMSCs, supporting an increased differentiation capacity of AQP5(-/-) BMMSCs. Furthermore, a bone-healing process was accelerated in AQP5(-/-) mice in a drill-hole injury model. Mechanistic studies indicated a significantly lower apoptosis rate in AQP5(-/-) than AQP5(+/+) BMMSCs. Apoptosis inhibitor Z-VAD-FMK increased the differentiation capacity to a greater extent in AQP5(+/+) than AQP5(-/-) BMMSCs. We conclude that AQP5-mediated high plasma membrane water permeability enhances the apoptosis rate of differentiating BMMSCs, thus decreasing their differentiation capacity. These data implicate AQP5 as a novel determinant of differentiation of BMMSCs and therefore a new molecular target for regulating differentiation of BMMSCs during tissue repair and regeneration.

Stem cells and regenerative medicine in lung biology and diseases

A number of novel approaches for repair and regeneration of injured lung have developed over the past several years. These include a better understanding of endogenous stem and progenitor cells in the lung that can function in reparative capacity as well as extensive exploration of the potential efficacy of administering exogenous stem or progenitor cells to function in lung repair. Recent advances in ex vivo lung engineering have also been increasingly applied to the lung. The current status of these approaches as well as initial clinical trials of cell therapies for lung diseases are reviewed below.

Adult stem cells underlying lung regeneration

Despite the massive toll in human suffering imparted by degenerative lung disease, including COPD, idiopathic pulmonary fibrosis and ARDS, the scientific community has been surprisingly agnostic regarding the potential of lung tissue, and in particular the alveoli, to regenerate. However, there is circumstantial evidence in humans and direct evidence in mice that ARDS triggers robust regeneration of lung tissue rather than irreversible fibrosis. The stem cells responsible for this remarkable regenerative process has garnered tremendous attention, most recently yielding a defined set of cloned human airway stem cells marked by p63 expression but with distinct commitment to differentiated cell types typical of the upper or lower airways, the latter of which include alveoli-like structures in vitro and in vivo. These recent advances in lung regeneration and distal airway stem cells and the potential of associated soluble factors in regeneration must be harnessed for therapeutic options in chronic lung disease.

RNA-Seq quantification of the human small airway epithelium transcriptome

Background

The small airway epithelium (SAE), the cell population that covers the human airway surface from the 6^th generation of airway branching to the alveoli, is the major site of lung disease caused by smoking. The focus of this study is to provide quantitative assessment of the SAE transcriptome in the resting state and in response to chronic cigarette smoking using massive parallel mRNA sequencing (RNA-Seq).

Results

The data demonstrate that 48% of SAE expressed genes are ubiquitous, shared with many tissues, with 52% enriched in this cell population. The most highly expressed gene, SCGB1A1, is characteristic of Clara cells, the cell type unique to the human SAE. Among other genes expressed by the SAE are those related to Clara cell differentiation, secretory mucosal defense, and mucociliary differentiation. The high sensitivity of RNA-Seq permitted quantification of gene expression related to infrequent cell populations such as neuroendocrine cells and epithelial stem/progenitor cells. Quantification of the absolute smoking-induced changes in SAE gene expression revealed that, compared to ubiquitous genes, more SAE-enriched genes responded to smoking with up-regulation, and those with the highest basal expression levels showed most dramatic changes. Smoking had no effect on SAE gene splicing, but was associated with a shift in molecular pattern from Clara cell-associated towards the mucus-secreting cell differentiation pathway with multiple features of cancer-associated molecular phenotype.

Conclusions

These observations provide insights into the unique biology of human SAE by providing quantit-ative assessment of the global transcriptome under physiological conditions and in response to the stress of chronic cigarette smoking.

Glycine decarboxylase activity drives non-small cell lung cancer tumor-initiating cells and tumorigenesis

Identification of the factors critical to the tumor-initiating cell (TIC) state may open new avenues in cancer therapy. Here we show that the metabolic enzyme glycine decarboxylase (GLDC) is critical for TICs in non-small cell lung cancer (NSCLC). TICs from primary NSCLC tumors express high levels of the oncogenic stem cell factor LIN28B and GLDC, which are both required for TIC growth and tumorigenesis. Overexpression of GLDC and other glycine/serine enzymes, but not catalytically inactive GLDC, promotes cellular transformation and tumorigenesis. We found that GLDC induces dramatic changes in glycolysis and glycine/serine metabolism, leading to changes in pyrimidine metabolism to regulate cancer cell proliferation. In the clinic, aberrant activation of GLDC correlates with poorer survival in lung cancer patients, and aberrant GLDC expression is observed in multiple cancer types. This link between glycine metabolism and tumorigenesis may provide novel targets for advancing anticancer therapy.

IL-17RA is required for CCL2 expression, macrophage recruitment, and emphysema in response to cigarette smoke

Chronic Obstructive Pulmonary Disease (COPD) is characterized by airspace enlargement and peribronchial lymphoid follicles; however, the immunological mechanisms leading to these pathologic changes remain undefined. Here we show that cigarette smoke is a selective adjuvant that augments in vitro and in vivo Th17, but not Th1, cell differentiation via the aryl hydrocarbon receptor. Smoke exposed IL-17RA(-/-) mice failed to induce CCL2 and MMP12 compared to WT mice. Remarkably, in contrast to WT mice, IL-17RA(-/-) mice failed to develop emphysema after 6 months of cigarette smoke exposure. Taken together, these data demonstrate that cigarette smoke is a potent Th17 adjuvant and that IL-17RA signaling is required for chemokine expression necessary for MMP12 induction and tissue emphysema.

Mesenchymal stem cells for repair of the airway epithelium in asthma

The airway epithelium is constantly faced with inflammatory and potentially injurious stimuli. Following damage, rapid repair mechanisms involving proliferation and differentiation of resident progenitor and stem cell pools are necessary in order to maintain a protective barrier. In asthma, evidence pointing to a compromised ability of the epithelium to properly repair and regenerate is rapidly accumulating. The consequences of this are presently unknown but are likely to have a significant impact on lung function. Mesenchymal stem cells have the potential to serve as a universal source for replacement of specific cells in several diseases and thus offer hope as a potential therapeutic intervention for the treatment of the chronic remodeling changes that occur in the asthmatic epithelium. However, controversy exists regarding whether these cells can actually home to and engraft within the airways and contribute to tissue function or whether this mechanism is necessary, since they can have potent paracrine immunomodulatory effects. This article focuses on the current knowledge about specific stem cell populations that may contribute to airway epithelial regeneration and discusses the use of mesenchymal stem cells as a potential therapeutic intervention.

Endogenous and exogenous stem cells: a role in lung repair and use in airway tissue engineering and transplantation

Rapid repair of the denuded alveolar surface after injury is a key to survival. The respiratory tract contains several sources of endogenous adult stem cells residing within the basal layer of the upper airways, within or near pulmonary neuroendocrine cell rests, at the bronchoalveolar junction, and within the alveolar epithelial surface, which contribute to the repair of the airway wall. Bone marrow-derived adult mesenchymal stem cells circulating in blood are also involved in tracheal regeneration. However, an organism is frequently incapable of repairing serious damage and defects of the respiratory tract resulting from acute trauma, lung cancers, and chronic pulmonary and airway diseases. Therefore, replacement of the tracheal tissue should be urgently considered. The shortage of donor trachea remains a major obstacle in tracheal transplantation. However, implementation of tissue engineering and stem cell therapy-based approaches helps to successfully solve this problem. To date, huge progress has been achieved in tracheal bioengineering. Several sources of stem cells have been used for transplantation and airway reconstitution in animal models with experimentally induced tracheal defects. Most tracheal tissue engineering approaches use biodegradable three-dimensional scaffolds, which are important for neotracheal formation by promoting cell attachment, cell redifferentiation, and production of the extracellular matrix. The advances in tracheal bioengineering recently resulted in successful transplantation of the world's first bioengineered trachea. Current trends in tracheal transplantation include the use of autologous cells, development of bioactive cell-free scaffolds capable of supporting activation and differentiation of host stem cells on the site of injury, with a future perspective of using human native sites as micro-niche for potentiation of the human body's site-specific response by sequential adding, boosting, permissive, and recruitment impulses.

A single cell functions as a tissue-specific stem cell and the in vitro niche-forming cell

Tissue-specific stem cell (TSC) behavior is determined by the stem cell niche. However, delineation of the TSC-niche interaction requires purification of both entities. We reasoned that the niche could be defined by the location of the TSC. We demonstrate that a single CD49f(bright)/Sca1(+)/ALDH(+) basal cell generates rare label-retaining cells and abundant label-diluting cells. Label-retaining and label-diluting cells were located in the rimmed domain of a unique clone type, the rimmed clone. The TSC property of self-renewal was tested by serial passage at clonal density and analysis of clone-forming cell frequency. A single clone could be passaged up to five times and formed only rimmed clones. Thus, rimmed clone formation was a cell-intrinsic property. Differentiation potential was evaluated in air-liquid interface cultures. Homogenous cultures of rimmed clones were highly mitotic but were refractory to standard differentiation signals. However, rimmed clones that were cocultured with unfractionated tracheal cells generated each of the cell types found in the tracheal epithelium. Thus, the default niche is promitotic: Multipotential differentiation requires adaptation of the niche. Because lung TSCs are typically evaluated after injury, the behavior of CD49f(bright)/Sca1(+)/ALDH(+) cells was tested in normal and naphthalene-treated mice. These cells were mitotically active in the normal and repaired epithelium, their proliferation rate increased in response to injury, and they retained label for 34 days. We conclude that the CD49f(bright)/Sca1(+)/ALDH(+) tracheal basal cell is a TSC, that it generates its own niche in vitro, and that it participates in tracheal epithelial homeostasis and repair.

Parasympathetic innervation maintains epithelial progenitor cells during salivary organogenesis

The maintenance of a progenitor cell population as a reservoir of undifferentiated cells is required for organ development and regeneration. However, the mechanisms by which epithelial progenitor cells are maintained during organogenesis are poorly understood. We report that removal of the parasympathetic ganglion in mouse explant organ culture decreased the number and morphogenesis of keratin 5-positive epithelial progenitor cells. These effects were rescued with an acetylcholine analog. We demonstrate that acetylcholine signaling, via the muscarinic M1 receptor and epidermal growth factor receptor, increased epithelial morphogenesis and proliferation of the keratin 5-positive progenitor cells. Parasympathetic innervation maintained the epithelial progenitor cell population in an undifferentiated state, which was required for organogenesis. This mechanism for epithelial progenitor cell maintenance may be targeted for organ repair or regeneration.

Airway basal stem cells: a perspective on their roles in epithelial homeostasis and remodeling

The small airways of the human lung undergo pathological changes in pulmonary disorders, such as chronic obstructive pulmonary disease (COPD), asthma, bronchiolitis obliterans and cystic fibrosis. These clinical problems impose huge personal and societal healthcare burdens. The changes, termed 'pathological airway remodeling', affect the epithelium, the underlying mesenchyme and the reciprocal trophic interactions that occur between these tissues. Most of the normal human airway is lined by a pseudostratified epithelium of ciliated cells, secretory cells and 6-30% basal cells, the proportion of which varies along the proximal-distal axis. Epithelial abnormalities range from hypoplasia (failure to differentiate) to basal- and goblet-cell hyperplasia, squamous- and goblet-cell metaplasia, dysplasia and malignant transformation. Mesenchymal alterations include thickening of the basal lamina, smooth muscle hyperplasia, fibrosis and inflammatory cell accumulation. Paradoxically, given the prevalence and importance of airway remodeling in lung disease, its etiology is poorly understood. This is due, in part, to a lack of basic knowledge of the mechanisms that regulate the differentiation, maintenance and repair of the airway epithelium. Specifically, little is known about the proliferation and differentiation of basal cells, a multipotent stem cell population of the pseudostratified airway epithelium. This Perspective summarizes what we know, and what we need to know, about airway basal cells to evaluate their contributions to normal and abnormal airway remodeling. We contend that exploiting well-described model systems using both human airway epithelial cells and the pseudostratified epithelium of the genetically tractable mouse trachea will enable crucial discoveries regarding the pathogenesis of airway disease.

Basal cells as stem cells of the mouse trachea and human airway epithelium

The pseudostratified epithelium of the mouse trachea and human airways contains a population of basal cells expressing Trp-63 (p63) and cytokeratins 5 (Krt5) and Krt14. Using a KRT5-CreER(T2) transgenic mouse line for lineage tracing, we show that basal cells generate differentiated cells during postnatal growth and in the adult during both steady state and epithelial repair. We have fractionated mouse basal cells by FACS and identified 627 genes preferentially expressed in a basal subpopulation vs. non-BCs. Analysis reveals potential mechanisms regulating basal cells and allows comparison with other epithelial stem cells. To study basal cell behaviors, we describe a simple in vitro clonal sphere-forming assay in which mouse basal cells self-renew and generate luminal cells, including differentiated ciliated cells, in the absence of stroma. The transcriptional profile identified 2 cell-surface markers, ITGA6 and NGFR, which can be used in combination to purify human lung basal cells by FACS. Like those from the mouse trachea, human airway basal cells both self-renew and generate luminal daughters in the sphere-forming assay.

Deletion of Pten expands lung epithelial progenitor pools and confers resistance to airway injury

RATIONALE

Pten is a tumor-suppressor gene involved in stem cell homeostasis and tumorigenesis. In mouse, Pten expression is ubiquitous and begins as early as 7 days of gestation. Pten(-/-) mouse embryos die early during gestation indicating a critical role for Pten in embryonic development.

OBJECTIVES

To test the role of Pten in lung development and injury.

METHODS

We conditionally deleted Pten throughout the lung epithelium by crossing Pten(flox/flox) with Nkx2.1-cre driver mice. The resulting Pten(Nkx2.1-cre) mutants were analyzed for lung defects and response to injury.

MEASUREMENTS AND MAIN RESULTS

Pten(Nkx2.1-cre) embryonic lungs showed airway epithelial hyperplasia with no branching abnormalities. In adult mice, Pten(Nkx2.1-cre) lungs exhibit increased progenitor cell pools composed of basal cells in the trachea, CGRP/CC10 double-positive neuroendocrine cells in the bronchi, and CC10/SPC double-positive cells at the bronchioalveolar duct junctions. Pten deletion affected differentiation of various lung epithelial cell lineages, with a decreased number of terminally differentiated cells. Over time, Pten(Nxk2.1-cre) epithelial cells residing in the bronchioalveolar duct junctions underwent proliferation and formed uniform masses, supporting the concept that the cells residing in this distal niche may also be the source of procarcinogenic stem cells. Finally, increased progenitor cells in all the lung compartments conferred an overall selective advantage to naphthalene injury compared with wild-type control mice.

CONCLUSIONS

Pten has a pivotal role in lung stem cell homeostasis, cell differentiation, and consequently resistance to lung injury.

Human embryonic stem cells and lung regeneration

Human embryonic stem cells are pluripotent cells derived from the inner cell mass of preimplantation stage embryos. Their unique potential to give rise to all differentiated cell types has generated great interest in stem cell research and the potential that it may have in developmental biology, medicine and pharmacology. The main focus of stem cell research has been on cell therapy for pathological conditions with no current methods of treatment, such as neurodegenerative diseases, cardiac pathology, retinal dysfunction and lung and liver disease. The overall aim is to develop methods of application either of pure cell populations or of whole tissue parts to the diseased organ under investigation. In the field of pulmonary research, studies using human embryonic stem cells have succeeded in generating enriched cultures of type II pneumocytes in vitro. On account of their potential of indefinite proliferation in vitro, embryonic stem cells could be a source of an unlimited supply of cells available for transplantation and for use in gene therapy. Uncovering the ability to generate such cell types will expand our understanding of biological processes to such a degree that disease understanding and management could change dramatically.

Misexpression of ELF5 disrupts lung branching and inhibits epithelial differentiation

ELF5, an Ets family transcription factor found exclusively in epithelial cells, is expressed in the distal lung epithelium during embryogenesis, then becomes restricted to proximal airways at the end of gestation and postnatally. To test the hypothesis that ELF5 represses distal epithelial differentiation, we generated a transgenic mouse model in which a doxycycline inducible HA-tagged mouse Elf5 transgene was placed under the control of the lung epithelium-specific human SFTPC promoter. We found that expressing high levels of ELF5 during early lung development disrupted branching morphogenesis and produced a dilated epithelium. The effects of ELF5 on morphogenesis were stage-dependent, since inducing the transgene on E16.5 had no effect on branching. ELF5 reduced expression of the distal lung epithelial differentiation markers Erm, Napsa and Sftpc, and type II cell ultrastructural differentiation was immature. ELF5 overexpression did not induce the proximal airway epithelial markers Ccsp and Foxj1, but did induce expression of p63, a marker of basal cells in the trachea and esophagus. High ELF5 levels also induced the expression of genes found in other endodermal epithelia but not normally associated with the lung. These results suggest that precise levels of ELF5 regulate the specification and differentiation of epithelial cells in the lung.

Review: ex vivo engineering of living tissues with adult stem cells

Adult stem cells have the potential to revolutionize regenerative medicine with their unique abilities to self-renew and differentiate into various phenotypes. This review examines progress and challenges in ex vivo tissue engineering with adult stem cells. These rare cells are harvested from a variety of tissues, including bone marrow, adipose, skeletal muscle, and placenta, and differentiate into cells of their own lineage and in some cases atypical lineages. Insight into the stem cell niche leads to the identification of matrix components, soluble factors, and physiological conditions that enhance the ex vivo amplification and differentiation of stem cells. Scaffolds composed of metals, naturally occurring materials, and synthetic polymers organize stem cells into complex spatial groupings that mimic native tissue. Cell signals from covalently bound ligands and slowly released regulatory factors in scaffolds direct stem cell fate. Future advances in stem cell biology and scaffold design will ultimately improve the efficacy of tissue substitutes as implants, in research, and as extracorporeal devices.

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.

Basal cells of the human adult airway surface epithelium retain transit-amplifying cell properties

In numerous airway diseases, such as cystic fibrosis, the epithelium is severely damaged and must regenerate to restore its defense functions. Although the human airway epithelial stem cells have not been identified yet, we have suggested recently that epithelial stem/progenitor cells exist among both human fetal basal and suprabasal cell subsets in the tracheal epithelium. In this study, we analyzed the capacity of human adult basal cells isolated from human adult airway tissues to restore a well-differentiated and functional airway epithelium. To this end, we used the human-specific basal cell markers tetraspanin CD151 and tissue factor (TF) to separate positive basal cells from negative columnar cells with a FACSAria cell sorter. Sorted epithelial cells were seeded into epithelium-denuded rat tracheae that were grafted subcutaneously in nude mice and on collagen-coated porous membranes, where they were grown at the air-liquid interface. Sorted basal and columnar populations were also analyzed for their telomerase activity, a specific transit-amplifying cell marker, by the telomeric repeat amplification protocol assay. After cell sorting, the pure and viable CD151/TF-positive basal cell population proliferated on plastic and adhered on epithelium-denuded rat tracheae, as well as on collagen-coated porous membranes, where it was able to restore a fully differentiated mucociliary and functional airway epithelium, whereas viable columnar negative cells did not. Telomerase activity was detected in the CD151/TF-positive basal cell population, but not in CD151/TF-negative columnar cells. These results demonstrate that human adult basal cells are at least airway surface transit-amplifying epithelial 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.