Generation of lung epithelial-like tissue from human embryonic stem cells

How to use an in vitro approach to characterize the toxicity of airborne compounds

As part of the development of new approach methodologies (NAMs), numerous in vitro methods are being developed to characterize the potential toxicity of inhalable xenobiotics (gases, volatile organic compounds, polycyclic aromatic hydrocarbons, particulate matter, nanoparticles). However, the materials and methods employed are extremely diverse, and no single method is currently in use. Method standardization and validation would raise trust in the results and enable them to be compared. This four-part review lists and compares biological models and exposure methodologies before describing measurable biomarkers of exposure or effect. The first section emphasizes the importance of developing alternative methods to reduce, if not replace, animal testing (3R principle). The biological models presented are mostly to cultures of epithelial cells from the respiratory system, as the lungs are the first organ to come into contact with air pollutants. Monocultures or cocultures of primary cells or cell lines, as well as 3D organotypic cultures such as organoids, spheroids and reconstituted tissues, but also the organ(s) model on a chip are examples. The exposure methods for these biological models applicable to airborne compounds are submerged, intermittent, continuous either static or dynamic. Finally, within the restrictions of these models (i.e. relative tiny quantities, adhering cells), the mechanisms of toxicity and the phenotypic markers most commonly examined in models exposed at the air-liquid interface (ALI) are outlined.

Differentiation of embryonic stem cells into lung-like cells using lung-derived matrix sheets

Various extracellular matrix (ECM) in the lungs regulate tissue development and homeostasis, as well as provide support for cell structures. However, few studies regarding the effects of lung cell differentiation using lung-derived ECM (LM) alone have been reported. The present study investigated the capability of lung-derived matrix sheets (LMSs) to induce lung cell differentiation using mouse embryonic stem (ES) cells. Expressions of lung-related cell markers were significantly upregulated in ES-derived embryoid bodies (EBs) cultured on an LMS for two weeks. Moreover, immunohistochemical analysis of EBs grown on LMSs revealed differentiation of various lung-related cells. These results suggest that an LMS can be used to promote differentiation of stem cells into lung cells.

Air-liquid interface (ALI) impact on different respiratory cell cultures

The intranasal route has been receiving greater attention from the scientific community not only for systemic drug delivery but also for the treatment of pulmonary and neurological diseases. Along with it, drug transport and permeability studies across the nasal mucosa have exponentially increased. Nevertheless, the translation of data from in vitro cell lines to in vivo studies is not always reliable, due to the difficulty in generating an in vitro model that resembles respiratory human physiology. Among all currently available methodologies, the air-liquid interface (ALI) method is advantageous to promote cell differentiation and optimize the morphological and histological characteristics of airway epithelium cells. Cells grown under ALI conditions, in alternative to submerged conditions, appear to provide relevant input for inhalation and pulmonary toxicology and complement in vivo experiments. Different methodologies and a variety of materials have been used to induce ALI conditions in primary cells and numerous cell lines. Until this day, with only exploratory results, no consensus has been reached regarding the validation of the ALI method, hampering data comparison. The present review describes the most adequate cell models of airway epithelium and how these models are differently affected by ALI conditions. It includes the evaluation of cellular features before and after ALI, and the application of the method in primary cell cultures, commercial 3D primary cells, cell lines and stem-cell derived models. A variety of these models have been recently applied for pharmacological studies against severe acute respiratory syndrome-coronavirus(-2) SARS-CoV(-2), namely primary cultures with alveolar type II epithelium cells and organotypic 3D models. The herein compiled data suggest that ALI conditions must be optimized bearing in mind the type of cells (nasal, bronchial, alveolar), their origin and the objective of the study.

Imaging-guided bioreactor for de-epithelialization and long-term cultivation of ex vivo rat trachea

Recent synergistic advances in organ-on-chip and tissue engineering technologies offer opportunities to create in vitro-grown tissue or organ constructs that can faithfully recapitulate their in vivo counterparts. Such in vitro tissue or organ constructs can be utilized in multiple applications, including rapid drug screening, high-fidelity disease modeling, and precision medicine. Here, we report an imaging-guided bioreactor that allows in situ monitoring of the lumen of ex vivo airway tissues during controlled in vitro tissue manipulation and cultivation of isolated rat trachea. Using this platform, we demonstrated partial removal of the rat tracheal epithelium (i.e., de-epithelialization) without disrupting the underlying subepithelial cells and extracellular matrix. Through different tissue evaluation assays, such as immunofluorescent staining, DNA/protein quantification, and electron beam microscopy, we showed that the epithelium of the tracheal lumen can be effectively removed with negligible disruption in the underlying tissue layers, such as cartilage and blood vessel. Notably, using a custom-built micro-optical imaging device integrated with the bioreactor, the trachea lumen was visualized at the cellular level, and removal of the endogenous epithelium and distribution of locally delivered exogenous cells were demonstrated in situ. Moreover, the de-epithelialized trachea supported on the bioreactor allowed attachment and growth of exogenous cells seeded topically on its denuded tissue surface. Collectively, the results suggest that our imaging-enabled rat trachea bioreactor and localized cell replacement method can facilitate creation of bioengineered in vitro airway tissue that can be used in different biomedical applications.

Induced pluripotent stem cell-based disease modeling and prospective immune therapy for coronavirus disease 2019

The emergence of the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic poses a never before seen challenge to human health and the economy. Considering its clinical impact, with no streamlined therapeutic strategies in sight, it is crucial to understand the infection process of SARS-CoV-2. Our limited knowledge of the mechanisms underlying SARS-CoV-2 infection impedes the development of alternative therapeutics to address the pandemic. This aspect can be addressed by modeling SARS-CoV-2 infection in the human context to facilitate drug screening and discovery. Human induced pluripotent stem cell (iPSC)-derived lung epithelial cells and organoids recapitulating the features and functionality of the alveolar cell types can serve as an in vitro human model and screening platform for SARS-CoV-2. Recent studies suggest an immune system asynchrony leading to compromised function and a decreased proportion of specific immune cell types in coronavirus disease 2019 (COVID-19) patients. Replenishing these specific immune cells may serve as useful treatment modality against SARS-CoV-2 infection. Here the authors review protocols for deriving lung epithelial cells, alveolar organoids and specific immune cell types, such as T lymphocytes and natural killer cells, from iPSCs with the aim to aid investigators in making relevant in vitro models of SARS-CoV-2 along with the possibility derive immune cell types to treat COVID-19.

The Cellular and Physiological Basis for Lung Repair and Regeneration: Past, Present, and Future

The respiratory system, which includes the trachea, airways, and distal alveoli, is a complex multi-cellular organ that intimately links with the cardiovascular system to accomplish gas exchange. In this review and as members of the NIH/NHLBI-supported Progenitor Cell Translational Consortium, we discuss key aspects of lung repair and regeneration. We focus on the cellular compositions within functional niches, cell-cell signaling in homeostatic health, the responses to injury, and new methods to study lung repair and regeneration. We also provide future directions for an improved understanding of the cell biology of the respiratory system, as well as new therapeutic avenues.

Protocol for Differentiation of Human iPSCs into Pulmonary Neuroendocrine Cells

Pulmonary neuroendocrine cells (PNECs) are sensory cells within the lung airway epithelia. Here, we provide a detailed protocol for generating induced PNECs (iPNECs) from human induced pluripotent stem cells (iPSCs). The cellular and molecular profile of iPNECs resembles primary human PNECs. Primary human PNECs are exceedingly rare, comprising only 1% of the adult lung. Therefore, a self-renewing source of patient-specific iPNECs facilitates the creation of reproducible human cellular models to study lung diseases characterized by PNEC dysfunction.

For complete details on the use and execution of this protocol, please refer to Hor et al. (2020).

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Efficient generation of iPNECs from human iPSCs

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Air-liquid-interface culture augments iPNEC specification

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Continuous Notch inhibition increases iPNEC specification

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iPNEC gene expression signature is highly concordant with primary human fetal PNECs

Efficient Generation and Transcriptomic Profiling of Human iPSC-Derived Pulmonary Neuroendocrine Cells

Expansion of pulmonary neuroendocrine cells (PNECs) is a pathological feature of many human lung diseases. Human PNECs are inherently difficult to study due to their rarity (<1% of total lung cells) and a lack of established protocols for their isolation. We used induced pluripotent stem cells (iPSCs) to generate induced PNECs (iPNECs), which express core PNEC markers, including ROBO receptors, and secrete major neuropeptides, recapitulating known functions of primary PNECs. Furthermore, we demonstrate that differentiation efficiency is increased in the presence of an air-liquid interface and inhibition of Notch signaling. Single-cell RNA sequencing (scRNA-seq) revealed a PNEC-associated gene expression profile that is concordant between iPNECs and human fetal PNECs. In addition, pseudotime analysis of scRNA-seq results suggests a basal cell origin of human iPNECs. In conclusion, our model has the potential to provide an unlimited source of human iPNECs to explore PNEC pathophysiology associated with several lung diseases.

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PNECs can be efficiently generated from induced pluripotent stem cells (iPSCs)

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Induced PNECs express key PNEC markers and express and secrete all major neuropeptides

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Induced PNECs resemble the transcriptomic profile of human primary fetal PNECs

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Basal cells are able to differentiate into PNECs

In vitro, ex vivo and in vivo methods of lung absorption for inhaled drugs

The assessment and prediction of lung absorption and disposition are an increasingly essential preclinical task for successful discovery and product development of inhaled drugs for both local and systemic delivery. Hence, in vitro, ex vivo and in vivo preclinical methods of lung absorption continue to evolve with several technical, methodological and analytical refinements. As in vitro lung epithelial cell monolayer models, the air-liquid interface (ALI)-cultured Calu-3 cells have most frequently been used, but the NCI-H441 and hAELVi cells have now been proposed as the first immortalized human alveolar epithelial cells capable of forming highly-restricted monolayers. The primary ALI-cultured three-dimensional (3D) human lung cell barriers have also become available; efforts to incorporate aerosol drug deposition into the in vitro lung cell models continue; and stem cell-derived lung epithelial cells and "lung-on-a-chip" technology are emerging. The ex vivo isolated perfused rat lung (IPRL) methods have increasing been used, as they enable the kinetic determination of tissue/organ-level diffusive and membrane protein-mediated absorption and competing non-absorptive loss; the assessment of "pre-epithelial" aerosol biopharmaceutical events in the lung, such as dissolution and release; and the ex vivo-to-in vivo extrapolation and prediction. Even so, in vivo small rodent-based methods have been of mainstay use, while large animal-based methods find an additional opportunity to study region-dependent lung absorption and disposition. It is also exciting that human pharmacokinetic (PK) profiles and systemic exposures for inhaled drugs/molecules may be able to be predicted from these in vivo rodent PK data following lung delivery using kinetic modeling approach with allometric scaling. Overall, the value of these preclinical assessments appears to have shifted more to their translational capability of predicting local lung and systemic exposure in humans, in addition to rationalizing optimal inhaled dosage form and delivery system for drugs/molecules in question. It is critically important therefore to make appropriate selection and timely exploitation of the best models at each stage of drug discovery and development program for efficient progress toward product approval and clinical use.

Current strategies and opportunities to manufacture cells for modeling human lungs

Chronic lung diseases remain major healthcare burdens, for which the only curative treatment is lung transplantation. In vitro human models are promising platforms for identifying and testing novel compounds to potentially decrease this burden. Directed differentiation of pluripotent stem cells is an important strategy to generate lung cells to create such models. Current lung directed differentiation protocols are limited as they do not 1) recapitulate the diversity of respiratory epithelium, 2) generate consistent or sufficient cell numbers for drug discovery platforms, and 3) establish the histologic tissue-level organization critical for modeling lung function. In this review, we describe how lung development has formed the basis for directed differentiation protocols, and discuss the utility of available protocols for lung epithelial cell generation and drug development. We further highlight tissue engineering strategies for manipulating biophysical signals during directed differentiation such that future protocols can recapitulate both chemical and physical cues present during lung development.

Xenobiotica-metabolizing enzymes in the lung of experimental animals, man and in human lung models

The xenobiotic metabolism in the lung, an organ of first entry of xenobiotics into the organism, is crucial for inhaled compounds entering this organ intentionally (e.g. drugs) and unintentionally (e.g. work place and environmental compounds). Additionally, local metabolism by enzymes preferentially or exclusively occurring in the lung is important for favorable or toxic effects of xenobiotics entering the organism also by routes other than by inhalation. The data collected in this review show that generally activities of cytochromes P450 are low in the lung of all investigated species and in vitro models. Other oxidoreductases may turn out to be more important, but are largely not investigated. Phase II enzymes are generally much higher with the exception of UGT glucuronosyltransferases which are generally very low. Insofar as data are available the xenobiotic metabolism in the lung of monkeys comes closed to that in the human lung; however, very few data are available for this comparison. Second best rate the mouse and rat lung, followed by the rabbit. Of the human in vitro model primary cells in culture, such as alveolar macrophages and alveolar type II cells as well as the A549 cell line appear quite acceptable. However, (1) this generalization represents a temporary oversimplification born from the lack of more comparable data; (2) the relative suitability of individual species/models is different for different enzymes; (3) when more data become available, the conclusions derived from these comparisons quite possibly may change.

Generation of lung organoids from human pluripotent stem cells in vitro

The lung epithelium is derived from the endodermal germ layer, which undergoes a complex series of endoderm-mesoderm-mediated signaling events to generate the final arborized network of conducting airways (bronchi, bronchioles) and gas-exchanging units (alveoli). These stages include endoderm induction, anterior-posterior and dorsal-ventral patterning, lung specification, lung budding, branching morphogenesis, and, finally, maturation. Here we describe a protocol that recapitulates several of these milestones in order to differentiate human pluripotent stem cells (hPSCs) into ventral-anterior foregut spheroids and further into two distinct types of organoids: human lung organoids and bud tip progenitor organoids. The resulting human lung organoids possess cell types and structures that resemble the bronchi/bronchioles of the developing human airway surrounded by lung mesenchyme and cells expressing alveolar-cell markers. The bud tip progenitor organoids possess a population of highly proliferative multipotent cells with in vitro multilineage differentiation potential and in vivo engraftment potential. Human lung organoids can be generated from hPSCs in 50-85 d, and bud tip progenitor organoids can be generated in 22 d. The two hPSC-derived models presented here have been benchmarked with human fetal tissue and found to be representative of human fetal-like tissue. The bud tip progenitor organoids are thus ideal for exploring epithelial fate decisions, while the human lung organoids can be used to model epithelial-mesenchymal cross-talk during human lung development. In addition to their applications in developmental biology, human lung organoids and bud tip progenitor organoids may be implemented in regenerative medicine, tissue engineering, and pharmaceutical safety and efficacy testing.

Options for modeling the respiratory system: inserts, scaffolds and microfluidic chips

The human respiratory system is continuously exposed to varying levels of hazardous substances ranging from environmental toxins to purposely administered drugs. If the noxious effects exceed the inherent regenerative capacity of the respiratory system, injured tissue undergoes complex remodeling that can significantly affect lung function and lead to various diseases. Advanced near-to-native in vitro lung models are required to understand the mechanisms involved in pulmonary damage and repair and to reliably test the toxicity of compounds to lung tissue. This review is an overview of the development of in vitro respiratory system models used for study of lung diseases. It includes discussion of using these models for environmental toxin assessment and pulmonary toxicity screening.

Human Pluripotent Stem Cells (iPSC) Generation, Culture, and Differentiation to Lung Progenitor Cells

Induced pluripotent stem (iPS) cells are the product of adult somatic cell reprogramming to an embryonic-like state by inducing a "forced" expression of specific genes. They are similar to natural pluripotent stem cells, such as embryonic stem (ES) cells, in many aspects, such as the expression of certain stem cell genes and potency and differentiability. Human iPS cells are invaluable resource for basic research, cell therapy, drug discovery, and human organ tissue engineering. iPS cells can be derived from the patient to be treated and thus are genetically identical cells that may avoid immune rejection. The following protocols offer a general guideline for the induction of iPSCs from fibroblasts, and for culture and expansion to produce lung precursor cells.

Stem cells therapy: a review on approaches that can be used for treatment of respiratory failures in sulfur mustard-injured patients

Sulfur mustard (SM) is a toxic agent which causes severe abnormalities in an airway system such as necrosis and inflammation, oxidative stress, chronic bronchitis, shortness of breath, and chronic obstructive pulmonary disease. Although possible mechanisms of SM toxicity have been extensively considered, there is still need to find an appropriate clinical treatment to decrease chronic lung injuries caused by SM. Due to extensive progresses and achievement in tissue repairing through stem cells therapy, the importance of cell therapy for the treatment of lung injuries has been increased. However, several factors such as types of stem cells, necessary conditions for growth and proliferation of stem cells, and their homing into the target tissues are considered as the most important problems in this issue. Mesenchymal stem cells (MSCs) are a class of multipotent stem cells with proliferative and self-renewal capacity which are able to differentiate into different cell lines such as lung epithelial cells. They have a potential repairing and immune modulatory properties which make them as a good candidate for the regeneration of bronchioles tract in SM-exposed patients. Unlike chemical drugs, the differentiation and high-level safety properties of MSCs can be considered as a new strategy for the treatment of SM-injured patients with pulmonary complications. This review aims to consider the therapeutic effects of MSCs in the treatment of SM-induced pulmonary injuries in both animals and humans.

Stem cell therapy in chronic obstructive pulmonary disease. How far is it to the clinic?

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. However, in the latter stages of the disease, the airways become largely obstructed and lung parenchyma becomes destructed due to underlying inflammation. The inappropriate repair of lung tissue after injury may contribute to the development of disease. Novel regenerative therapeutic approaches have been investigated with the aim of repairing or replacing the injured functional structures of the respiratory system. Endogenous and exogenous sources of stem cells are available for the treatment of many diseases. Stem cell therapy is newly introduced to the field of COPD. Currently the research is in its infancy; however, the field is profoundly growing. Previous studies suggest that cell-based therapies and novel bioengineering approaches may be potential therapeutic strategies for lung repair and remodelling. In this paper, we review the current evidence of stem cell therapy in COPD.

Concise Review: Current Status of Three-Dimensional Organoids as Preclinical Models

Three-dimensional (3D) cultures use the property of some cells to self-organize in matrices and generate structures that can be programmed to represent an organ or a pathology. Organoid cultures are the 3D cultivation of source tissue (ranging from cells to tissue fragments) in a support matrix and specialized media that nearly resembles the physiological environment. Depending on the source tissue, growth factors, and inhibitors provided, organoids can be programmed to recapitulate the biology of a system and progression of pathology. Organoids are genetically stable, and genetically amenable, making them very suitable tools to study tissue homeostasis and cancer. In this Review, we focus on providing recent technical advances from published literature to efficiently use organoids as a tool for disease modeling and therapeutics. Also, we discuss stem cell biology principles used to generate multiple organoids and their characteristics, with a brief description of methodology. A major theme of this review is to expand organoid applications to the study disease progression and drug response in different cancers. We also discuss shortcomings, limitations, and advantages of developed 3D cultures, with the rationale behind the methodology. Stem Cells 2018;36:1329-1340.

Induction of lung-like cells from mouse embryonic stem cells by decellularized lung matrix

Decellularization of tissues is a recently developed technique mostly used to provide a 3-dimensional matrix structure of the original organ, including decellularized lung tissues for lung transplantation. Based on the results of the present study, we propose new utilization of decellularized tissues as inducers of stem cell differentiation. Decellularized lung matrix (L-Mat) samples were prepared from mouse lungs by SDS treatment, then the effects of L-Mat on differentiation of ES cells into lung cells were investigated. ES cell derived-embryoid bodies (EBs) were transplanted into L-Mat samples and cultured for 2 weeks. At the end of the culture, expressions of lung cell-related markers, such as TTF-1 and SP-C (alveolar type II cells), AQP5 (alveolar type I cells), and CC10 (club cells), were detected in EB outgrowths in L-Mat, while those were not found in EB outgrowths attached to the dish. Our results demonstrated that L-Mat has an ability to induce differentiation of ES cells into lung-like cells.

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Differentiation of ES cells by decellularized lung matrix (L-Mat) was investigated.

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L-Mat induced differentiation of various lung cell-like cells from ES cells.

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L-Mat plays an important role for inducing differentiation of lung cells.

Stem cells in lung repair and regeneration: Current applications and future promise

Lung diseases are major cause of morbidity and mortality worldwide. The progress in regenerative medicine and stem cell research in the lung are currently a fast-growing research topic that can provide solutions to these major health problems. Under normal conditions, the rate of cellular proliferation is relatively low in the lung in vivo, compared to other major organ systems. Lung injury leads to the activation of stem/progenitor cell populations that re-enter the cell cycle. Yet, little is known about stem cells in the lung, despite common thoughts that these cells could play a critical role in the repair of lung injuries. Nor do we fully understand the cellular and architectural complexity of the respiratory tract, and the diverse stem/progenitor cells that are involved in the lung repair and regeneration. In this review, we discuss the conceptual framework of lung stem/progenitor cell biology, and describe lung diseases, in which stem cell manipulations may be physiologically significant. In addition, we highlight the challenges of lung stem cell-based therapy.

When Is an Alveolar Type 2 Cell an Alveolar Type 2 Cell? A Conundrum for Lung Stem Cell Biology and Regenerative Medicine

Generating mature, differentiated, adult lung cells from pluripotent cells, such as induced pluripotent stem cells and embryonic stem cells, offers the hope of both generating disease-specific in vitro models and creating definitive and personalized therapies for a host of debilitating lung parenchymal and airway diseases. With the goal of advancing lung-regenerative medicine, several groups have developed and reported on protocols using defined media, coculture with mesenchymal components, or sequential treatments mimicking lung development, to obtain distal lung epithelial cells from stem cell precursors. However, there remains significant controversy about the degree of differentiation of these cells compared with their primary counterparts, coupled with a lack of consistency or uniformity in assessing the resultant phenotypes. Given the inevitable, exponential expansion of these approaches and the probable, but yet-to-emerge second and higher generation techniques to create such assets, we were prompted to pose the question, what makes a lung epithelial cell a lung epithelial cell? More specifically for this Perspective, we also posed the question, what are the minimum features that constitute an alveolar type (AT) 2 epithelial cell? In addressing this, we summarize a body of work spanning nearly five decades, amassed by a series of "lung epithelial cell biology pioneers," which carefully describes well characterized molecular, functional, and morphological features critical for discriminately assessing an AT2 phenotype. Armed with this, we propose a series of core criteria to assist the field in confirming that cells obtained following a differentiation protocol are indeed mature and functional AT2 epithelial cells.

Prospective isolation of NKX2-1-expressing human lung progenitors derived from pluripotent stem cells

It has been postulated that during human fetal development, all cells of the lung epithelium derive from embryonic, endodermal, NK2 homeobox 1-expressing (NKX2-1+) precursor cells. However, this hypothesis has not been formally tested owing to an inability to purify or track these progenitors for detailed characterization. Here we have engineered and developmentally differentiated NKX2-1GFP reporter pluripotent stem cells (PSCs) in vitro to generate and isolate human primordial lung progenitors that express NKX2-1 but are initially devoid of differentiated lung lineage markers. After sorting to purity, these primordial lung progenitors exhibited lung epithelial maturation. In the absence of mesenchymal coculture support, this NKX2-1+ population was able to generate epithelial-only spheroids in defined 3D cultures. Alternatively, when recombined with fetal mouse lung mesenchyme, the cells recapitulated epithelial-mesenchymal developing lung interactions. We imaged these progenitors in real time and performed time-series global transcriptomic profiling and single-cell RNA sequencing as they moved through the earliest moments of lung lineage specification. The profiles indicated that evolutionarily conserved, stage-dependent gene signatures of early lung development are expressed in primordial human lung progenitors and revealed a CD47hiCD26lo cell surface phenotype that allows their prospective isolation from untargeted, patient-specific PSCs for further in vitro differentiation and future applications in regenerative medicine.

Lung bioengineering: physical stimuli and stem/progenitor cell biology interplay towards biofabricating a functional organ

A current approach to obtain bioengineered lungs as a future alternative for transplantation is based on seeding stem cells on decellularized lung scaffolds. A fundamental question to be solved in this approach is how to drive stem cell differentiation onto the different lung cell phenotypes. Whereas the use of soluble factors as agents to modulate the fate of stem cells was established from an early stage of the research with this type of cells, it took longer to recognize that the physical microenvironment locally sensed by stem cells (e.g. substrate stiffness, 3D architecture, cyclic stretch, shear stress, air-liquid interface, oxygenation gradient) also contributes to their differentiation. The potential role played by physical stimuli would be particularly relevant in lung bioengineering since cells within the organ are physiologically subjected to two main stimuli required to facilitate efficient gas exchange: air ventilation and blood perfusion across the organ. The present review focuses on describing how the cell mechanical microenvironment can modulate stem cell differentiation and how these stimuli could be incorporated into lung bioreactors for optimizing organ bioengineering.

Regeneration of the lung: Lung stem cells and the development of lung mimicking devices

Inspired by the increasing burden of lung associated diseases in society and an growing demand to accommodate patients, great efforts by the scientific community produce an increasing stream of data that are focused on delineating the basic principles of lung development and growth, as well as understanding the biomechanical properties to build artificial lung devices. In addition, the continuing efforts to better define the disease origin, progression and pathology by basic scientists and clinicians contributes to insights in the basic principles of lung biology. However, the use of different model systems, experimental approaches and readout systems may generate somewhat conflicting or contradictory results. In an effort to summarize the latest developments in the lung epithelial stem cell biology, we provide an overview of the current status of the field. We first describe the different stem cells, or progenitor cells, residing in the homeostatic lung. Next, we focus on the plasticity of the different cell types upon several injury-induced activation or repair models, and highlight the regenerative capacity of lung cells. Lastly, we summarize the generation of lung mimics, such as air-liquid interface cultures, organoids and lung on a chip, that are required to test emerging hypotheses. Moreover, the increasing collaboration between distinct specializations will contribute to the eventual development of an artificial lung device capable of assisting reduced lung function and capacity in human patients.

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.

Organoids as a model system for studying human lung development and disease

The lung is a complex organ comprising multiple cell types that perform a variety of vital processes, including immune defense and gas exchange. Diseases of the lung, such as chronic obstructive pulmonary disease, asthma and lung cancer, together represent one of the largest causes of patient suffering and mortality. Logistical barriers that hamper access to embryonic, normal adult or diseased lung tissue currently hinder the study of lung disease. In vitro lung modeling represents an attractive and accessible avenue for investigating lung development, function and disease pathology, but accurately modeling the lung in vitro requires a system that recapitulates the structural features of the native lung. Organoids are stem cell-derived three-dimensional structures that are supported by an extracellular matrix and contain multiple cell types whose spatial arrangement and interactions mimic those of the native organ. Recently, organoids representative of the respiratory system have been generated from adult lung stem cells and human pluripotent stem cells. Ongoing studies are showing that organoids may be used to model human lung development, and can serve as a platform for interrogating the function of lung-related genes and signalling pathways. In a therapeutic context, organoids may be used for modeling lung diseases, and as a platform for screening for drugs that alleviate respiratory disease. Here, we summarize the organoid-forming capacity of respiratory cells, current lung organoid technologies and their potential use in future therapeutic applications.

Personalized medicine for cystic fibrosis: establishing human model systems

With over 1,500 identifiable mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that result in distinct functional and phenotypical abnormalities, it is virtually impossible to perform randomized clinical trials to identify the best therapeutics for all patients. Therefore, a personalized medicine approach is essential. The only way to realistically accomplish this is through the development of improved in vitro human model systems. The lack of a readily available and infinite supply of human CFTR-expressing airway epithelial cells is a key bottleneck. We propose that a concerted two-pronged approach is necessary for patient-specific cystic fibrosis research to continue to prosper and realize its potential: (1) more effective culture and differentiation conditions for growing primary human airway and nasal epithelial cells and (2) the development of collective protocols for efficiently differentiating disease- and patient-specific induced pluripotent stem cells (iPSC) into pure populations of adult epithelial cells. Ultimately, we need a personalized human model system for cystic fibrosis with the capacity for uncomplicated bankability, widespread availability, and universal applicability for patient-specific disease modeling, novel pharmacotherapy investigation and screening, and readily executable genetic modification.

Development of Lung Epithelium from Induced Pluripotent Stem Cells

Considerable progress has been made in the field of in vitro development of alveolar epithelium from induced pluripotent stem cells. Patient specific derived alveolar cells could potentially populate tissue engineered lungs, provide a cell source for drug testing or function as a model for research into lung diseases. Induced to pluripotency through a variety of techniques, stem cells can be differentiated to alveolar epithelium through exposure to a variety of different culture conditions and growth media. The ultimate success of differentiated cells for translational medicine applications will depend on further advances in the understanding of the human lung developmental pathway, and successful application to in vitro culture. In this review will focus the major signaling pathways and molecules in lung development and the existing protocol for directed different ion of iPSC and hESC to cells resembling respiratory epithelium in vitro.

Wnt and FGF mediated epithelial-mesenchymal crosstalk during lung development

BACKGROUND

The adaptation to terrestrial life required the development of an organ capable of efficient air-blood gas exchange. To meet the metabolic load of cellular respiration, the mammalian respiratory system has evolved from a relatively simple structure, similar to the two-tube amphibian lung, to a highly complex tree-like system of branched epithelial airways connected to a vast network of gas exchanging units called alveoli. The development of such an elaborate organ in a relatively short time window is therefore an extraordinary feat and involves an intimate crosstalk between mesodermal and endodermal cell lineages.

RESULTS

This review describes the molecular processes governing lung development with an emphasis on the current knowledge on the role of Wnt and FGF signaling in lung epithelial differentiation.

CONCLUSIONS

The Wnt and FGF signaling pathways are crucial for the dynamic and reciprocal communication between epithelium and mesenchyme during lung development. In addition, some of this developmental crosstalk is reemployed in the adult lung after injury to drive regeneration, and may, when aberrantly or chronically activated, result in chronic lung diseases. Novel insights into how the Wnt and FGF pathways interact and are integrated into a complex gene regulatory network will not only provide us with essential information about how the lung regenerates itself, but also enhance our understanding of the pathogenesis of chronic lung diseases, as well as improve the controlled differentiation of lung epithelium from pluripotent stem cells.

Modelling the Human Respiratory System: Approaches for in Vitro Safety Testing and Drug Discovery

Respiratory research can be broken down into two main areas: (i) exposure to airborne substances (basic toxicology assessment); and (ii) respiratory diseases (understanding disease mechanisms and development of new therapeutics, including toxicological assessment). Both have suffered from inadequate and inaccurate models used to predict human toxicological end points. A growing need therefore exists for accurate in vitro models of the respiratory system, which accurately reflect the human lung situation in vivo. Advances in cell culture techniques and accessibility of human cells/tissues have resulted in the development of increasingly in vivo-like respiratory models. This chapter will focus on the development, advantages and disadvantages of these models and what the future holds for in vitro lung toxicology.

Bronchoalveolar sublineage specification of pluripotent stem cells: effect of dexamethasone plus cAMP-elevating agents and keratinocyte growth factor

Respiratory progenitors can be efficiently generated from pluripotent stem cells (PSCs). However, further targeted differentiation into bronchoalveolar sublineages is still in its infancy, and distinct specifying effects of key differentiation factors are not well explored. Focusing on airway epithelial Clara cell generation, we analyzed the effect of the glucocorticoid dexamethasone plus cAMP-elevating agents (DCI) on the differentiation of murine embryonic and induced pluripotent stem cells (iPSCs) into bronchoalveolar epithelial lineages, and whether keratinocyte growth factor (KGF) might further influence lineage decisions. We demonstrate that DCI strongly induce expression of the Clara cell marker Clara cell secretory protein (CCSP). While KGF synergistically supports the inducing effect of DCI on alveolar markers with increased expression of surfactant protein (SP)-C and SP-B, an inhibitory effect on CCSP expression was shown. In contrast, neither KGF nor DCI seem to have an inducing effect on ciliated cell markers. Furthermore, the use of iPSCs from transgenic mice with CCSP promoter-dependent lacZ expression or a knockin of a YFP reporter cassette in the CCSP locus enabled detection of derivatives with Clara cell typical features. Collectively, DCI was shown to support bronchoalveolar specification of mouse PSCs, in particular Clara-like cells, and KGF to inhibit bronchial epithelial differentiation. The targeted in vitro generation of Clara cells with their important function in airway protection and regeneration will enable the evaluation of innovative cellular therapies in animal models of lung diseases.

Concise review: current status of stem cells and regenerative medicine in lung biology and diseases

Lung diseases remain a significant and devastating cause of morbidity and mortality worldwide. In contrast to many other major diseases, lung diseases notably chronic obstructive pulmonary diseases (COPDs), including both asthma and emphysema, are increasing in prevalence and COPD is expected to become the third leading cause of disease mortality worldwide by 2020. New therapeutic options are desperately needed. A rapidly growing number of investigations of stem cells and cell therapies in lung biology and diseases as well as in ex vivo lung bioengineering have offered exciting new avenues for advancing knowledge of lung biology as well as providing novel potential therapeutic approaches for lung diseases. These initial observations have led to a growing exploration of endothelial progenitor cells and mesenchymal stem (stromal) cells in clinical trials of pulmonary hypertension and COPD with other clinical investigations planned. Ex vivo bioengineering of the trachea, larynx, diaphragm, and the lung itself with both biosynthetic constructs as well as decellularized tissues have been used to explore engineering both airway and vascular systems of the lung. Lung is thus a ripe organ for a variety of cell therapy and regenerative medicine approaches. Current state-of-the-art progress for each of the above areas will be presented as will discussion of current considerations for cell therapy-based clinical trials in lung diseases.

Second and third trimester amniotic fluid mesenchymal stem cells can repopulate a de-cellularized lung scaffold and express lung markers

BACKGROUND/PURPOSE

This study examined the potential of amniotic fluid mesenchymal stem cells (AF-MSCs) to generate lung precursor cells in vitro and on a xenologous three-dimensional de-cellularized lung scaffold.

METHODS

AF-MSCs were isolated from human amniotic fluid obtained from 17-37 weeks gestation. Lung differentiation was induced on Matrigel or on de-cellularized rat lungs intra-tracheally injected with AF-MSCs by culturing with a modification of small airway growth medium (mSAGM) lacking retinoic acid (RA) and triodothyronine (T3) with addition of fibroblast growth factor-10 (FGF10). Cells and scaffolds were characterized by immunofluorescence and RT-PCR for markers of viability, proliferation, and lung distal airway differentiation (TTF-1(+) and SPC(+)) in the absence of markers of brain (TuJ1(-)) and thyroid (Pax8(-)).

RESULTS

After culture in mSAGM on either Matrigel or lung scaffolds, there were TTF-1(+)/TuJ1(-)/Pax8(-) cells, indicating a lung precursor phenotype. In addition, SPC(+) cells also evolved suggesting a more mature lung phenotype.

CONCLUSIONS

We demonstrate that mid- to late-trimester AF-MSCs can be induced to develop into lung precursor cells when cultured on the appropriate extracellular matrix (ECM), making them a viable source for use in cell therapy or development of an ex vivo tissue engineered lung.

Hypoxia enhances differentiation of mouse embryonic stem cells into definitive endoderm and distal lung cells

We investigated the effects of hypoxia on spontaneous (SP)- and activin A (AA)-induced definitive endoderm (DE) differentiation of mouse embryonic stem cells (mESCs) and their subsequent differentiation into distal pulmonary epithelial cells. SP differentiation for 6 days of mESCs toward endoderm at hypoxia of 1% O2, but not at 3% or 21% (normoxia), increased the expression of Sox17 and Foxa2 by 31- and 63-fold above maintenance culture, respectively. Treatment of mESCs with 20 ng/mL AA for 6 days under hypoxia further increased the expression of DE marker genes Sox17, Foxa2, and Cxcr4 by 501-, 1,483-, and 126-fold above maintenance cultures, respectively. Transient exposure to hypoxia, as short as 24 h, was sufficient to enhance AA-induced endoderm formation. The involvement of hypoxia-inducible factor (HIF)-1α and reactive oxygen species (ROS) in the AA-induced endoderm enrichment was assessed using HIF-1α(-/-) mESCs and the ROS scavenger N-acetylcysteine (NAC). Under SP conditions, HIF-1α(-/-) mESCs failed to increase the expression of endodermal marker genes but rather shifted toward ectoderm. Hypoxia induced only a marginal potentiation of AA-induced endoderm differentiation in HIF-1α(-/-) mESCs. Treatment of mESCs with AA and NAC led to a dose-dependent decrease in Sox17 and Foxa2 expression. In addition, the duration of exposure to hypoxia in the course of a recently reported lung differentiation protocol resulted in differentially enhanced expression of distal lung epithelial cell marker genes aquaporin 5 (Aqp5), surfactant protein C (Sftpc), and secretoglobin 1a1 (Scgb1a1) for alveolar epithelium type I, type II, and club cells, respectively. Our study is the first to show the effects of in vitro hypoxia on efficient formation of DE and lung lineages. We suggest that the extent of hypoxia and careful timing may be important components of in vitro differentiation bioprocesses for the differential generation of distal lung epithelial cells from pluripotent progenitors.

Innate immune response of human pluripotent stem cell-derived airway epithelium

The acquisition of innate immune response is requisite to having bona fide differentiation of airway epithelium. Procedures developed to differentiate lung airway from human pluripotent stem cells (hPSCs) have demonstrated anecdotal evidence for innate immune response, but an in-depth exploration of response levels is lacking. Herein, using an established method of airway epithelial generation from hPSCs, we show that hPSC-derived epithelial cells are able to up-regulate expression of TNFα, IL8 and IL1β in response to challenge with bacterial endotoxin LPS, but lack response from genes associated with innate immune response in other cell types. Further, stimulation of cells with TNF-α resulted in auto-induction of TNFα transcript, as well as cytokine responses of IL8 and IL1β. The demonstration of innate immune induction in hPSC-derived airway epithelia gives further strength to the functionality of in vitro protocols aimed at generating differentiated airway cells that can potentially be used in a translational setting. Finally, we propose that innate immune challenge of airway epithelium from human pluripotent stem cell sources be used as a robust validation of functional in vitro differentiation.

Differentiated type II pneumocytes can be reprogrammed by ectopic Sox2 expression

The adult lung contains several distinct stem cells, although their properties and full potential are still being sorted out. We previously showed that ectopic Sox2 expression in the developing lung manipulated the fate of differentiating cells. Here, we addressed the question whether fully differentiated cells could be redirected towards another cell type. Therefore, we used transgenic mice to express an inducible Sox2 construct in type II pneumocytes, which are situated in the distal, respiratory areas of the lung. Within three days after the induction of the transgene, the type II cells start to proliferate and form clusters of cuboidal cells. Prolonged Sox2 expression resulted in the reversal of the type II cell towards a more embryonic, precursor-like cell, being positive for the stem cell markers Sca1 and Ssea1. Moreover, the cells started to co-express Spc and Cc10, characteristics of bronchioalveolar stem cells. We demonstrated that Sox2 directly regulates the expression of Sca1. Subsequently, these cells expressed Trp63, a marker for basal cells of the trachea. So, we show that the expression of one transcription factor in fully differentiated, distal lung cells changes their fate towards proximal cells through intermediate cell types. This may have implications for regenerative medicine, and repair of diseased and damaged lungs.

CAR expression in human embryos and hESC illustrates its role in pluripotency and tight junctions

Coxsackie virus and adenovirus receptor, CXADR (CAR), is present during embryogenesis and is involved in tissue regeneration, cancer and intercellular adhesion. We investigated the expression of CAR in human preimplantation embryos and embryonic stem cells (hESC) to identify its role in early embryogenesis and differentiation. CAR protein was ubiquitously present during preimplantation development. It was localised in the nucleus of uncommitted cells, from the cleavage stage up to the precursor epiblast, and corresponded with the presence of soluble CXADR3/7 splice variant. CAR was displayed on the membrane, involving in the formation of tight junction at compaction and blastocyst stages in both outer and inner cells, and CAR corresponded with the full-length CAR-containing transmembrane domain. In trophectodermal cells of hatched blastocysts, CAR was reduced in the membrane and concentrated in the nucleus, which correlated with the switch in RNA expression to the CXADR4/7 and CXADR2/7 splice variants. The cells in the outer layer of hESC colonies contained CAR on the membrane and all the cells of the colony had CAR in the nucleus, corresponding with the transmembrane CXADR and CXADR4/7. Upon differentiation of hESC into cells representing the three germ layers and trophoblast lineage, the expression of CXADR was downregulated. We concluded that CXADR is differentially expressed during human preimplantation development. We described various CAR expressions: i) soluble CXADR marking undifferentiated blastomeres; ii) transmembrane CAR related with epithelial-like cell types, such as the trophectoderm (TE) and the outer layer of hESC colonies; and iii) soluble CAR present in TE nuclei after hatching. The functions of these distinct forms remain to be elucidated.

Strategies for whole lung tissue engineering

Recent work has demonstrated the feasibility of using decellularized lung extracellular matrix scaffolds to support the engineering of functional lung tissue in vitro. Rendered acellular through the use of detergents and other reagents, the scaffolds are mounted in organ-specific bioreactors where cells in the scaffold are provided with nutrients and appropriate mechanical stimuli such as ventilation and perfusion. Though initial studies are encouraging, a great deal remains to be done to advance the field and transition from rodent lungs to whole human tissue engineered lungs. To do so, a variety of hurdles must be overcome. In particular, a reliable source of human-sized scaffolds, as well as a method of terminal sterilization of scaffolds, must be identified. Continued research in lung cell and developmental biology will hopefully help identify the number and types of cells that will be required to regenerate functional lung tissue. Finally, bioreactor designs must be improved in order to provide more precise ventilation stimuli and vascular perfusion in order to avoid injury to or death of the cells cultivated within the scaffold. Ultimately, the success of efforts to engineer a functional lung in vitro will critically depend on the ability to create a fully endothelialized vascular network that provides sufficient barrier function and alveolar-capillary surface area to exchange gas at rates compatible with healthy lung function.

Generation of multiciliated cells in functional airway epithelia from human induced pluripotent stem cells

Despite therapeutic advancement, pulmonary disease still remains a major cause of morbidity and mortality around the world. Opportunities to study human lung disease either in vivo or in vitro are currently limited. Using induced pluripotent stem cells (iPSCs), we generated mature multiciliated cells in a functional airway epithelium. Robust multiciliogenesis occurred when notch signaling was inhibited and was confirmed by (i) the assembly of multiple pericentrin-stained centrioles at the apical surface, (ii) expression of transcription factor forkhead box protein J1, and (iii) presence of multiple acetylated tubulin-labeled cilia projections in individual cells. Clara, goblet, and basal cells were all present, confirming the generation of a complete polarized epithelial-cell layer. Additionally, cAMP-activated and cystic fibrosis transmembrane regulator inhibitor 172-sensitive cystic fibrosis transmembrane regulator currents were recorded in isolated epithelial cells. Our report demonstrating the generation of mature multiciliated cells in respiratory epithelium from iPSCs is a significant advance toward modeling a number of human respiratory diseases in vitro.

Lung stem and progenitor cells in tissue homeostasis and disease

The mammalian lung is a complex organ containing numerous putative stem/progenitor cell populations that contribute to region-specific tissue homeostasis and repair. In this review, we discuss recent advances in identifying and studying these cell populations in the context of lung homeostasis and disease. Genetically engineered mice now allow for lineage tracing of several lung stem and progenitor cell populations in vivo during different types of lung injury repair. Using specific sets of cell surface markers, these cells can also be isolated from murine and human lung and tested in 3D culture systems and in vivo transplant assays. The pathology of devastating lung diseases, including lung cancers, is likely in part due to dysregulation and dysfunction of lung stem cells. More precise characterization of stem cells with identification of new, unique markers; improvement in isolation and transplant techniques; and further development of functional assays will ultimately lead to new therapies for a host of human lung diseases. In particular, lung cancer biology may be greatly informed by findings in normal lung stem cell biology as evidence suggests that lung cancer is a disease that begins in, and may be driven by, neoplastic lung stem cells.

Alveolar epithelial differentiation of human induced pluripotent stem cells in a rotating bioreactor

Traditional stem cell differentiation protocols make use of a variety of cytokines including growth factors (GFs) and inhibitors in an effort to provide appropriate signals for tissue specific differentiation. In this study, iPSC-derived type II pneumocytes (iPSC-ATII) as well as native isolated human type II pneumocytes (hATII) were differentiated toward a type I phenotype using a unique air-liquid interface (ALI) system that relies on a rotating apparatus that mimics in vivo respiratory conditions. A relatively homogenous population of alveolar type II-like cells from iPSC was first generated (iPSC-ATII cells), which had phenotypic properties similar to mature human alveolar type II cells. iPSC-ATII cells were then cultured in a specially designed rotating culture apparatus. The effectiveness of the ALI bioreactor was compared with the effectiveness of small molecule-based differentiation of type II pneumocytes toward type 1 pneumocytes. The dynamics of differentiation were examined by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), flow cytometry and immunocytochemistry. iPSC-ATII and hATII cells cultured in the ALI bioreactor had higher levels of type I markers, including aquaporin-5(AQ5), caveolin-1, and T1α, at both the RNA and protein levels as compared with the flask-grown iPSC-ATII and hATII that had been treated with small molecules to induce differentiation. In summary, this study demonstrates that a rotating bioreactor culture system that provides an air-liquid interface is a potent inducer of type I epithelial differentiation for both iPS-ATII cells and hATII cells, and provides a method for large-scale production of alveolar epithelium for tissue engineering and drug discovery.

[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.

Expansive generation of functional airway epithelium from human embryonic stem cells

Production of human embryonic stem cell (hESC)-derived lung progenitors has broad applicability for drug screening and cell therapy; however, this is complicated by limitations in demarcating phenotypic changes with functional validation of airway cell types. In this paper, we reveal the potential of hESCs to produce multipotent lung progenitors using a combined growth factor and physical culture approach, guided by the use of novel markers LIFRα and NRP1. Lung specification of hESCs was achieved by priming differentiation via matrix-specific support, followed by air-liquid interface to allow generation of lung progenitors capable of in vitro maturation into airway epithelial cell types, resulting in functional characteristics such as secretion of pulmonary surfactant, ciliation, polarization, and acquisition of innate immune activity. This approach provided a robust expansion of lung progenitors, allowing in vivo assessment, which demonstrated that only fully differentiated hESC-derived airway cells were retained in the distal airway, where they aided in physiological recovery in immunocompromised mice receiving airway injury. Our study provides a basis for translational applications of hESCs for lung diseases.

Human iPS cell-derived alveolar epithelium repopulates lung extracellular matrix

The use of induced pluripotent stem cells (iPSCs) has been postulated to be the most effective strategy for developing patient-specific respiratory epithelial cells, which may be valuable for lung-related cell therapy and lung tissue engineering. We generated a relatively homogeneous population of alveolar epithelial type II (AETII) and type I (AETI) cells from human iPSCs that had phenotypic properties similar to those of mature human AETII and AETI cells. We used these cells to explore whether lung tissue can be regenerated in vitro. Consistent with an AETII phenotype, we found that up to 97% of cells were positive for surfactant protein C, 95% for mucin-1, 93% for surfactant protein B, and 89% for the epithelial marker CD54. Additionally, exposing induced AETII to a Wnt/β-catenin inhibitor (IWR-1) changed the iPSC-AETII-like phenotype to a predominantly AETI-like phenotype. We found that of induced AET1 cells, more than 90% were positive for type I markers, T1α, and caveolin-1. Acellular lung matrices were prepared from whole rat or human adult lungs treated with decellularization reagents, followed by seeding these matrices with alveolar cells derived from human iPSCs. Under appropriate culture conditions, these progenitor cells adhered to and proliferated within the 3D lung tissue scaffold and displayed markers of differentiated pulmonary epithelium.

Generation of multipotent foregut stem cells from human pluripotent stem cells

Human pluripotent stem cells (hPSCs) could provide an infinite source of clinically relevant cells with potential applications in regenerative medicine. However, hPSC lines vary in their capacity to generate specialized cells, and the development of universal protocols for the production of tissue-specific cells remains a major challenge. Here, we have addressed this limitation for the endodermal lineage by developing a defined culture system to expand and differentiate human foregut stem cells (hFSCs) derived from hPSCs. hFSCs can self-renew while maintaining their capacity to differentiate into pancreatic and hepatic cells. Furthermore, near-homogenous populations of hFSCs can be obtained from hPSC lines which are normally refractory to endodermal differentiation. Therefore, hFSCs provide a unique approach to bypass variability between pluripotent lines in order to obtain a sustainable source of multipotent endoderm stem cells for basic studies and to produce a diversity of endodermal derivatives with a clinical value.

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Multipotent foregut stem cells are derived from pluripotent cells

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Foregut stem cells can be expanded and retain their differentiation potential

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Foregut stem cells differentiate into liver and pancreatic cells

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Foregut stem cells decrease differentiation variability between cell lines

Emerging in vitro models for safety screening of high-volume production nanomaterials under environmentally relevant exposure conditions

The rising production of nanomaterial-based consumer products has raised safety concerns. Testing these with animal and other direct models is neither ethically nor economically viable, nor quick enough. This review aims to discuss the strength of in vitro testing, including the use of 2D and 3D cultures, stem cells, and tissue constructs, etc., which would give fast and repeatable answers of a highly specific nature, while remaining relevant to in vivo outcomes. These results can then be combined and the overall toxicity predicted with relative accuracy. Such in vitro models can screen potentially toxic nanomaterials which, if required, can undergo further stringent studies in animals. The cyto- and phototoxicity of some high-volume production nanomaterials, using in vitro models, is also reviewed.