Human lung development: recent progress and new challenges

Lung Ultrasound Pattern in Healthy Infants During the First 6 Months of Life

OBJECTIVES

Lung ultrasound (LUS) has gained a primary role in the diagnosis and management of pleuropulmonary disorders in pediatric practice. However, normal and pathologic patterns are translated from adult studies and have never been specifically studied in children, particularly in infants. This was a prospective observational pilot study aiming to define the normal LUS pattern in healthy infants during the first 6 months of life.

METHODS

We recruited healthy neonates at 7 to 10 days of life, and these were followed until the sixth month of life (times: 7-10 days, 1 month, 3 months, and 6 months). We excluded neonates with a gestational age before 33 weeks and neonates with cardiac or lung abnormalities or diseases, immune deficiencies, metabolic or genetic conditions, and acute or chronic respiratory diseases. A LUS evaluation was performed by a single certified pediatrician. The chest wall was examined in 18 areas, addressing A-lines, short and long B-lines, pleural abnormalities, and subpleural consolidations.

RESULTS

Thirty-seven neonates were enrolled and followed until the sixth month of life, 27 (73%) of whom were born at term (≥37 weeks) and 10 (27%) of whom were born preterm (33-36 weeks). Most of the patients at 7 to 10 days showed multiple B-lines (long and short) with a progressive normalization toward a normal A pattern at 6 months (P < .00001; 95% confidence interval, 13.75-23.24). No infants showed subpleural consolidations or pleural abnormalities.

CONCLUSIONS

This study has implications for the interpretation of LUS during the first 6 months of life. Most healthy infants show a diffuse pattern of vertical artifacts (B-lines), and the LUS pattern tends to be similar to the physiologic pattern (A-lines) after the sixth month of life. The only pathologic LUS findings were pleural irregularities and effusion and subpleural consolidations, which have never been described in healthy infants.

Human pluripotent stem cell-derived lung organoids: Potential applications in development and disease modeling

The pulmonary system is comprised of two main compartments, airways and alveolar space. Their tissue and cellular complexity ensure lung function and protection from external agents, for example, virus. Two-dimensional (2D) in vitro systems and animal models have been largely employed to elucidate the molecular mechanisms underlying human lung development, physiology, and pathogenesis. However, neither of these models accurately recapitulate the human lung environment and cellular crosstalk. More recently, human-derived three-dimensional (3D) models have been generated allowing for a deeper understanding of cell-to-cell communication. However, the availability and accessibility of primary human cell sources from which generate the 2D and 3D models may be limited. In the past few years, protocols have been developed to successfully employ human pluripotent stem cells (hPSCs) and differentiate them toward pulmonary fate in vitro. In the present review, we discuss the advantages and pitfalls of hPSC-derived lung 2D and 3D models, including the main characteristics and potentials for these models and their current and future applications for modeling development and diseases. Lung organoids currently represent the closest model to the human pulmonary system. We further focus on the applications of lung organoids for the study of human diseases such as pulmonary fibrosis, infectious diseases, and lung cancer. Finally, we discuss the present limitations and potential future applications of 3D lung organoids. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cells and Disease Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cell Differentiation and Reversion.

Human Lung Stem Cell-Based Alveolospheres Provide Insights into SARS-CoV-2-Mediated Interferon Responses and Pneumocyte Dysfunction

Coronavirus infection causes diffuse alveolar damage leading to acute respiratory distress syndrome. The absence of ex vivo models of human alveolar epithelium is hindering an understanding of coronavirus disease 2019 (COVID-19) pathogenesis. Here, we report a feeder-free, scalable, chemically defined, and modular alveolosphere culture system for the propagation and differentiation of human alveolar type 2 cells/pneumocytes derived from primary lung tissue. Cultured pneumocytes express the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) receptor angiotensin-converting enzyme receptor type-2 (ACE2) and can be infected with virus. Transcriptome and histological analysis of infected alveolospheres mirror features of COVID-19 lungs, including emergence of interferon (IFN)-mediated inflammatory responses, loss of surfactant proteins, and apoptosis. Treatment of alveolospheres with IFNs recapitulates features of virus infection, including cell death. In contrast, alveolospheres pretreated with low-dose IFNs show a reduction in viral replication, suggesting the prophylactic effectiveness of IFNs against SARS-CoV-2. Human stem cell-based alveolospheres, thus, provide novel insights into COVID-19 pathogenesis and can serve as a model for understanding human respiratory diseases.

Lung regeneration: implications of the diseased niche and ageing

Most chronic and acute lung diseases have no cure, leaving lung transplantation as the only option. Recent work has improved our understanding of the endogenous regenerative capacity of the lung and has helped identification of different progenitor cell populations, as well as exploration into inducing endogenous regeneration through pharmaceutical or biological therapies. Additionally, alternative approaches that aim at replacing lung progenitor cells and their progeny through cell therapy, or whole lung tissue through bioengineering approaches, have gained increasing attention. Although impressive progress has been made, efforts at regenerating functional lung tissue are still ineffective. Chronic and acute lung diseases are most prevalent in the elderly and alterations in progenitor cells with ageing, along with an increased inflammatory milieu, present major roadblocks for regeneration. Multiple cellular mechanisms, such as cellular senescence and mitochondrial dysfunction, are aberrantly regulated in the aged and diseased lung, which impairs regeneration. Existing as well as new human in vitro models are being developed, improved and adapted in order to study potential mechanisms of lung regeneration in different contexts. This review summarises recent advances in understanding endogenous as well as exogenous regeneration and the development of in vitro models for studying regenerative mechanisms.

Fgf10-CRISPR mosaic mutants demonstrate the gene dose-related loss of the accessory lobe and decrease in the number of alveolar type 2 epithelial cells in mouse lung

CRISPR/Cas9-mediated gene editing often generates founder generation (F0) mice that exhibit somatic mosaicism in the targeted gene(s). It has been known that Fibroblast growth factor 10 (Fgf10)-null mice exhibit limbless and lungless phenotypes, while intermediate limb phenotypes (variable defective limbs) are observed in the Fgf10-CRISPR F0 mice. However, how the lung phenotype in the Fgf10-mosaic mutants is related to the limb phenotype and genotype has not been investigated. In this study, we examined variable lung phenotypes in the Fgf10-targeted F0 mice to determine if the lung phenotype was correlated with percentage of functional Fgf10 genotypes. Firstly, according to a previous report, Fgf10-CRISPR F0 embryos on embryonic day 16.5 (E16.5) were classified into three types: type I, no limb; type II, limb defect; and type III, normal limbs. Cartilage and bone staining showed that limb truncations were observed in the girdle, (type I), stylopodial, or zeugopodial region (type II). Deep sequencing of the Fgf10-mutant genomes revealed that the mean proportion of codons that encode putative functional FGF10 was 8.3 ± 6.2% in type I, 25.3 ± 2.7% in type II, and 54.3 ± 9.5% in type III (mean ± standard error of the mean) mutants at E16.5. Histological studies showed that almost all lung lobes were absent in type I embryos. The accessory lung lobe was often absent in type II embryos with other lobes dysplastic. All lung lobes formed in type III embryos. The number of terminal tubules was significantly lower in type I and II embryos, but unchanged in type III embryos. To identify alveolar type 2 epithelial (AECII) cells, known to be reduced in the Fgf10-heterozygous mutant, immunostaining using anti-surfactant protein C (SPC) antibody was performed: In the E18.5 lungs, the number of AECII was correlated to the percentage of functional Fgf10 genotypes. These data suggest the Fgf10 gene dose-related loss of the accessory lobe and decrease in the number of alveolar type 2 epithelial cells in mouse lung. Since dysfunction of AECII cells has been implicated in the pathogenesis of parenchymal lung diseases, the Fgf10-CRISPR F0 mouse would present an ideal experimental system to explore it.

Capillary cell-type specialization in the alveolus

In the mammalian lung, an apparently homogenous mesh of capillary vessels surrounds each alveolus, forming the vast respiratory surface across which oxygen transfers to the blood1. Here we use single-cell analysis to elucidate the cell types, development, renewal and evolution of the alveolar capillary endothelium. We show that alveolar capillaries are mosaics; similar to the epithelium that lines the alveolus, the alveolar endothelium is made up of two intermingled cell types, with complex 'Swiss-cheese'-like morphologies and distinct functions. The first cell type, which we term the 'aerocyte', is specialized for gas exchange and the trafficking of leukocytes, and is unique to the lung. The other cell type, termed gCap ('general' capillary), is specialized to regulate vasomotor tone, and functions as a stem/progenitor cell in capillary homeostasis and repair. The two cell types develop from bipotent progenitors, mature gradually and are affected differently in disease and during ageing. This cell-type specialization is conserved between mouse and human lungs but is not found in alligator or turtle lungs, suggesting it arose during the evolution of the mammalian lung. The discovery of cell type specialization in alveolar capillaries transforms our understanding of the structure, function, regulation and maintenance of the air-blood barrier and gas exchange in health, disease and evolution.

Human Induced Pluripotent Stem Cell-Derived Lung Epithelial System for SARS-CoV-2 Infection Modeling and Its Potential in Drug Repurposing

The lung is the most vulnerable target for the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection, and respiratory failure causing acute respiratory distress syndrome is its foremost outcome. However, the current primary in vitro models in use for SARS-CoV-2 display apparent limitations for modeling such complex human respiratory disease. Although patient cells can directly model the effects of a drug, their availability and capacity for expansion are limited compared with transformed/immortalized cells or tumor-derived cell lines. An additional caveat is that the latter may harbor genetic and metabolic abnormalities making them unsuitable for drug screening. Therefore, it is important to create physiologically relevant human-cell models that can replicate the pathophysiology of SARS-CoV-2, thus facilitating drug testing. In this study, we show preliminary data on how human induced pluripotent stem cells-derived lung epithelial cell system could emerge as a relevant and sensitive platform for modeling SARS-CoV-2 infection and drug screening.

Cell therapy for the preterm infant: promise and practicalities

Recent decades have seen the rapid progress of neonatal intensive care, and the survival rates of the most preterm infants are improving. This improvement is associated with changing patterns of morbidity and new phenotypes of bronchopulmonary dysplasia and preterm brain injury are recognised. Inflammation and immaturity are known contributors to their pathogenesis. However, a new phenomenon, the exhaustion of progenitor cells is emerging as an important factor. Current therapeutic approaches do not adequately address these new mechanisms of injury. Cell therapy, that is the use of stem and stem-like cells, with its potential to both repair and prevent injury, offers a new approach to these challenging conditions. This review will examine the rationale for cell therapy in the extremely preterm infant, the preclinical and early clinical evidence to support its use in bronchopulmonary dysplasia and preterm brain injury. Finally, it will address the challenges in translating cell therapy from the laboratory to early clinical trials.

It's about time: clocks in the developing lung

The discovery of peripheral intracellular clocks revealed circadian oscillations of clock genes and their targets in all cell types, including those in the lung, sparking exploration of clocks in lung disease pathophysiology. While the focus has been on the role of these clocks in adult airway diseases, clock biology is also likely to be important in perinatal lung development, where it has received far less attention. Historically, fetal circadian rhythms have been considered irrelevant owing to lack of external light exposure, but more recent insights into peripheral clock biology raise questions of clock emergence, its concordance with tissue-specific structure/function, the interdependence of clock synchrony and functionality in perinatal lung development, and the possibility of lung clocks in priming the fetus for postnatal life. Understanding the perinatal molecular clock may unravel mechanistic targets for chronic airway disease across the lifespan. With current research providing more questions than answers, it is about time to investigate clocks in the developing lung.

A functional genomics approach to investigate the differentiation of iPSCs into lung epithelium at air-liquid interface

The availability of robust protocols to differentiate induced pluripotent stem cells (iPSCs) into many human cell lineages has transformed research into the origins of human disease. The efficacy of differentiating iPSCs into specific cellular models is influenced by many factors including both intrinsic and extrinsic features. Among the most challenging models is the generation of human bronchial epithelium at air‐liquid interface (HBE‐ALI), which is the gold standard for many studies of respiratory diseases including cystic fibrosis. Here, we perform open chromatin mapping by ATAC‐seq and transcriptomics by RNA‐seq in parallel, to define the functional genomics of key stages of the iPSC to HBE‐ALI differentiation. Within open chromatin peaks, the overrepresented motifs include the architectural protein CTCF at all stages, while motifs for the FOXA pioneer and GATA factor families are seen more often at early stages, and those regulating key airway epithelial functions, such as EHF, are limited to later stages. The RNA‐seq data illustrate dynamic pathways during the iPSC to HBE‐ALI differentiation, and also the marked functional divergence of different iPSC lines at the ALI stages of differentiation. Moreover, a comparison of iPSC‐derived and lung donor‐derived HBE‐ALI cultures reveals substantial differences between these models.

Electrophilic nitrated fatty acids are potential therapeutic candidates for inflammatory and fibrotic lung diseases

Several types of exposures can cause acute or chronic inflammatory reactions in the lungs often leading to asthma, pulmonary fibrosis, chronic obstructive pulmonary disease (COPD), acute lung injury, lung cancer, and other deleterious health outcomes. Current therapy, with inhaled or oral glucocorticoids, successfully targets inflammation but also produces adverse effects that limit their enthusiastic use. Accordingly, the need remains for interventions that are safer and more effective. Nitrated fatty acids (NFAs) are highly electrophilic and are produced endogenously by non-enzymatic reactions of nitric oxide with conjugated unsaturated fatty acids. The literature indicates that NFAs are detected in humans at the nanomolar range and are produced more robustly under inflammatory conditions. Recent studies on novel NFAs report antiinflammatory, antioxidant, and antifibrotic effects, while also acting as partial agonists of peroxisome proliferator-activated receptor-gamma (PPAR-γ). Furthermore, these functions of NFAs occur via reversible electrophilic alkylation of cysteine residues and regulation of antiinflammatory, antioxidant signaling through modulation of transcription factors, including nuclear factor E2-related factor 2 (Nrf2), PPAR-γ, and NF-κB. Here, we review and update the role of NFA signaling mechanisms and their therapeutic potential in various lung diseases. As NFAs display strong electrophilic interaction with multimechanistic pathways, they can be considered promising drug candidates for challenging lung diseases.

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.

iPSC-Derived Intestinal Organoids from Cystic Fibrosis Patients Acquire CFTR Activity upon TALEN-Mediated Repair of the p.F508del Mutation

Cystic fibrosis (CF) is the main genetic cause of death among the Caucasian population. The disease is characterized by abnormal fluid and electrolyte mobility across secretory epithelia. The first manifestations occur within hours of birth (meconium ileus), later extending to other organs, generally affecting the respiratory tract. It is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CFTR encodes a cyclic adenosine monophosphate (cAMP)-dependent, phosphorylation-regulated chloride channel required for transport of chloride and other ions through cell membranes. There are more than 2,000 mutations described in the CFTR gene, but one of them, phenylalanine residue at amino acid position 508 (p.F508del), a recessive allele, is responsible for the vast majority of CF cases worldwide. Here, we present the results of the application of genome-editing techniques to the restoration of CFTR activity in p.F508del patient-derived induced pluripotent stem cells (iPSCs). Gene-edited iPSCs were subsequently used to produce intestinal organoids on which the physiological activity of the restored gene was tested in forskolin-induced swelling tests. The seamless restoration of the p.F508del mutation resulted in normal expression of the mature CFTR glycoprotein, full recovery of CFTR activity, and a normal response of the repaired organoids to treatment with two approved CF therapies: VX-770 and VX-809.

Recapitulating idiopathic pulmonary fibrosis related alveolar epithelial dysfunction in a human iPSC-derived air-liquid interface model

Idiopathic pulmonary fibrosis (IPF) is a fatal disease of unknown cause that is characterized by progressive fibrotic lung remodeling. An abnormal emergence of airway epithelial-like cells within the alveolar compartments of the lung, herein termed bronchiolization, is often observed in IPF. However, the origin of this dysfunctional distal lung epithelium remains unknown due to a lack of suitable human model systems. In this study, we established a human induced pluripotent stem cell (iPSC)-derived air-liquid interface (ALI) model of alveolar epithelial type II (ATII)-like cell differentiation that allows us to investigate alveolar epithelial progenitor cell differentiation in vitro. We treated this system with an IPF-relevant cocktail (IPF-RC) to mimic the pro-fibrotic cytokine milieu present in IPF lungs. Stimulation with IPF-RC during differentiation increases secretion of IPF biomarkers and RNA sequencing (RNA-seq) of these cultures reveals significant overlap with human IPF patient data. IPF-RC treatment further impairs ATII differentiation by driving a shift toward an airway epithelial-like expression signature, providing evidence that a pro-fibrotic cytokine environment can influence the proximo-distal differentiation pattern of human lung epithelial cells. In conclusion, we show for the first time, the establishment of a human model system that recapitulates aspects of IPF-associated bronchiolization of the lung epithelium in vitro.

A prenatally disrupted airway epithelium orchestrates the fetal origin of asthma in mice

BACKGROUND

Prenatal challenges such as maternal stress perception increase the risk and severity of asthma during childhood. However, insights into the trajectories and targets underlying the pathogenesis of prenatally triggered asthma are largely unknown. The developing lung and immune system may constitute such targets.

OBJECTIVE

Here we have aimed to identify the differential sex-specific effects of prenatal challenges on lung function, immune response, and asthma severity in mice.

METHODS

We generated bone marrow chimeric (BMC) mice harboring either prenatally stress-exposed lungs or a prenatally stress-exposed immune (hematopoietic) system and induced allergic asthma via ovalbumin. Next-generation sequencing (RNA sequencing) of lungs and assessment of airway epithelial barrier function in ovalbumin-sensitized control and prenatally stressed offspring was also performed.

RESULTS

Profoundly enhanced airway hyperresponsiveness, inflammation, and fibrosis were exclusively present in female BMC mice with prenatally stress-exposed lungs. These effects were significantly perpetuated if both the lungs and the immune system had been exposed to prenatal stress. A prenatally stress-exposed immune system alone did not suffice to increase the severity of these asthma features. RNA sequencing analysis of lungs from prenatally stressed, non-BMC, ovalbumin-sensitized females unveiled a deregulated expression of genes involved in asthma pathogenesis, tissue remodeling, and tight junction formation. It was also possible to independently confirm a tight junction disruption. In line with this, we identified an altered perinatal and/or postnatal expression of genes involved in lung development along with an impaired alveolarization in female prenatally stressed mice.

CONCLUSION

Here we have shown that the fetal origin of asthma is orchestrated by a disrupted airway epithelium and further perpetuated by a predisposed immune system.

Reconstructed Single-Cell Fate Trajectories Define Lineage Plasticity Windows during Differentiation of Human PSC-Derived Distal Lung Progenitors

Alveolar epithelial type 2 cells (AEC2s) are the facultative progenitors responsible for maintaining lung alveoli throughout life but are difficult to isolate from patients. Here, we engineer AEC2s from human pluripotent stem cells (PSCs) in vitro and use time-series single-cell RNA sequencing with lentiviral barcoding to profile the kinetics of their differentiation in comparison to primary fetal and adult AEC2 benchmarks. We observe bifurcating cell-fate trajectories as primordial lung progenitors differentiate in vitro, with some progeny reaching their AEC2 fate target, while others diverge to alternative non-lung endodermal fates. We develop a Continuous State Hidden Markov model to identify the timing and type of signals, such as overexuberant Wnt responses, that induce some early multipotent NKX2-1+ progenitors to lose lung fate. Finally, we find that this initial developmental plasticity is regulatable and subsides over time, ultimately resulting in PSC-derived AEC2s that exhibit a stable phenotype and nearly limitless self-renewal capacity.

Reduced Prenatal Pulmonary Lymphatic Function Is Observed in Clp1 K/K Embryos With Impaired Motor Functions Including Fetal Breathing Movements in Preparation of the Developing Lung for Inflation at Birth

Embryonic lungs must be inflated immediately after birth to establish respiration. In addition to pulmonary surfactant, recently, we have revealed lymphatic function as a previously unknown regulator of prenatal lung compliance that prepares the embryonic lung for inflation at birth. It is well-documented that the late gestation embryo performs episodic breathing-like movements called as fetal breathing movements (FBMs), but the physiological importance of these events is not clear. Here we aimed to study the physiological role of FBMs in preparation for air inflation at birth. Clp1^K/K late gestation embryos develop a progressive loss of spinal motor neurons associated with axonal degeneration and denervation of neuromuscular junctions serving as an ideal genetic model to test the possible role of FBMs. We demonstrated that Clp1^K/K newborns show impaired motor function resulting in fatal respiratory failure after birth. Next, we showed that the alveolar septa are thicker, and the alveolar area is reduced in Clp1^K/K late gestation embryos, while the expression of molecular markers of lung development are not affected. Importantly, pulmonary lymphatic vessels are dilated and the prenatal pulmonary lymphatic function is reduced in Clp1^K/K late gestation embryos. Our results have revealed that Clp1^K/K mice show impaired motor functions including FBMs, and late gestation Clp1^K/K embryos display reduced prenatal lymphatic function and impaired lung expansion represented as thickened alveolar septa and reduced alveolar area in preparation of the developing lung for inflation at birth. These findings suggest a possible mechanism that FBMs, similarly to breathing movements after birth, stimulate prenatal lymphatic function in pulmonary collecting lymphatics lacking smooth muscle coverage to prepare the developing lung for inflation and gas exchange at birth. Moreover, these results raise the possibility that stimulating FBMs during late gestation might be an effective way to reduce the risk of the development of neonatal respiratory failure.

Aberrant Epithelial Cell Proliferation in Peripheral Airways in Bronchiectasis

Dilation of bronchi and bronchioles caused by destruction and excessive epithelial remodeling is a characteristic feature of bronchiectasis. It is not known how epithelial progenitor cells contribute to these pathologic conditions in peripheral airways (bronchioles) in bronchiectasis. We aimed to explore the expression levels of signature airway progenitor cells in the dilated bronchioles in patients with bronchiectasis. We obtained the surgically resected peripheral lung tissues from 43 patients with bronchiectasis and 33 control subjects. Immunostaining was performed to determine the expression patterns of thyroid transcription factor-1 (TTF-1, for labeling progenitor cells in distal airways), P63 (basal cells), club cell 10 kDa protein (CC10, club cells), and surfactant protein C (SPC, alveolar type II epithelial cells) in epithelium or sub-epithelium. Here, we reported significantly lower percentage of TTF-1⁺ cells and CC10⁺ cells, and higher percentage of P63⁺ cells within the epithelium of dilated bronchioles compared with control bronchioles. In airway sub-epithelium of the dilated bronchioles, epithelial hyperplasia with disarrangement of TTF-1⁺ cells yielded cuboidal (100%) and columnar (93.0%) type among bronchiectasis patients. Most progenitor cell markers co-localized with TTF-1. The median (the 1st, 3rd quartile) percentage of P63⁺TTF-1⁺, CC10⁺TTF-1⁺, and SPC⁺TTF-1⁺ cells was 16.0% (8.9, 24.0%), 14.5% (7.1, 20.8%), and 52% (40.3, 64.4%), respectively. For cuboidal epithelial hyperplasia, 91.0% (86.5, 94.0%) of areas co-stained with SPC and TTF-1. Columnar epithelial hyperplasia was characterized by TTF-1 co-staining with P63⁺TTF-1⁺ and CC10⁺TTF-1⁺ cells. Taken together, aberrant proliferation of airway progenitor cells in both epithelium and sub-epithelium are implicated in bronchiectasis.

Advances in Pluripotent Stem Cells: History, Mechanisms, Technologies, and Applications

Over the past 20 years, and particularly in the last decade, significant developmental milestones have driven basic, translational, and clinical advances in the field of stem cell and regenerative medicine. In this article, we provide a systemic overview of the major recent discoveries in this exciting and rapidly developing field. We begin by discussing experimental advances in the generation and differentiation of pluripotent stem cells (PSCs), next moving to the maintenance of stem cells in different culture types, and finishing with a discussion of three-dimensional (3D) cell technology and future stem cell applications. Specifically, we highlight the following crucial domains: 1) sources of pluripotent cells; 2) next-generation in vivo direct reprogramming technology; 3) cell types derived from PSCs and the influence of genetic memory; 4) induction of pluripotency with genomic modifications; 5) construction of vectors with reprogramming factor combinations; 6) enhancing pluripotency with small molecules and genetic signaling pathways; 7) induction of cell reprogramming by RNA signaling; 8) induction and enhancement of pluripotency with chemicals; 9) maintenance of pluripotency and genomic stability in induced pluripotent stem cells (iPSCs); 10) feeder-free and xenon-free culture environments; 11) biomaterial applications in stem cell biology; 12) three-dimensional (3D) cell technology; 13) 3D bioprinting; 14) downstream stem cell applications; and 15) current ethical issues in stem cell and regenerative medicine. This review, encompassing the fundamental concepts of regenerative medicine, is intended to provide a comprehensive portrait of important progress in stem cell research and development. Innovative technologies and real-world applications are emphasized for readers interested in the exciting, promising, and challenging field of stem cells and those seeking guidance in planning future research direction.

Retinoic Acid: A Key Regulator of Lung Development

Retinoic acid (RA) is a key molecular player in embryogenesis and adult tissue homeostasis. In embryo development, RA plays a crucial role in the formation of different organ systems, namely, the respiratory system. During lung development, there is a spatiotemporal regulation of RA levels that assures the formation of a fully functional organ. RA signaling influences lung specification, branching morphogenesis, and alveolarization by regulating the expression of particular target genes. Moreover, cooperation with other developmental pathways is essential to shape lung organogenesis. This review focuses on the events regulated by retinoic acid during lung developmental phases and pulmonary vascular development; also, it aims to provide a snapshot of RA interplay with other well-known regulators of lung development.

Pulmonary Consequences of Prenatal Inflammatory Exposures: Clinical Perspective and Review of Basic Immunological Mechanisms

Chorioamnionitis, a potentially serious inflammatory complication of pregnancy, is associated with the development of an inflammatory milieu within the amniotic fluid surrounding the developing fetus. When chorioamnionitis occurs, the fetal lung finds itself in the unique position of being constantly exposed to the consequent inflammatory meditators and/or microbial products found in the amniotic fluid. This exposure results in significant changes to the fetal lung, such as increased leukocyte infiltration, altered cytokine, and surfactant production, and diminished alveolarization. These alterations can have potentially lasting impacts on lung development and function. However, studies to date have only begun to elucidate the association between such inflammatory exposures and lifelong consequences such as lung dysfunction. In this review, we discuss the pathogenesis of and fetal immune response to chorioamnionitis, detail the consequences of chorioamnionitis exposure on the developing fetal lung, highlighting the various animal models that have contributed to our current understanding and discuss the importance of fetal exposures in regard to the development of chronic respiratory disease. Finally, we focus on the clinical, basic, and therapeutic challenges in fetal inflammatory injury to the lung, and propose next steps and future directions to improve our therapeutic understanding of this important perinatal stress.

Characterization of an immortalized human small airway basal stem/progenitor cell line with airway region-specific differentiation capacity

Background

The pathology of chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF) and most lung cancers involves the small airway epithelium (SAE), the single continuous layer of cells lining the airways ≥ 6th generations. The basal cells (BC) are the stem/progenitor cells of the SAE, responsible for the differentiation into intermediate cells and ciliated, club and mucous cells. To facilitate the study of the biology of the human SAE in health and disease, we immortalized and characterized a normal human SAE basal cell line.

Methods

Small airway basal cells were purified from brushed SAE of a healthy nonsmoker donor with a characteristic normal SAE transcriptome. The BC were immortalized by retrovirus-mediated telomerase reverse transcriptase (TERT) transduction and single cell drug selection. The resulting cell line (hSABCi-NS1.1) was characterized by RNAseq, TaqMan PCR, protein immunofluorescence, differentiation capacity on an air-liquid interface (ALI) culture, transepithelial electrical resistance (TEER), airway region-associated features and response to genetic modification with SPDEF.

Results

The hSABCi-NS1.1 single-clone-derived cell line continued to proliferate for > 200 doubling levels and > 70 passages, continuing to maintain basal cell features (TP63⁺, KRT5⁺). When cultured on ALI, hSABCi-NS1.1 cells consistently formed tight junctions and differentiated into ciliated, club (SCGB1A1⁺), mucous (MUC5AC⁺, MUC5B⁺), neuroendocrine (CHGA⁺), ionocyte (FOXI1⁺) and surfactant protein positive cells (SFTPA⁺, SFTPB⁺, SFTPD⁺), observations confirmed by RNAseq and TaqMan PCR. Annotation enrichment analysis showed that “cilium” and “immunity” were enriched in functions of the top-1500 up-regulated genes. RNAseq reads alignment corroborated expression of CD4, CD74 and MHC-II. Compared to the large airway cell line BCi-NS1.1, differentiated of hSABCi-NS1.1 cells on ALI were enriched with small airway epithelial genes, including surfactant protein genes, LTF and small airway development relevant transcription factors NKX2–1, GATA6, SOX9, HOPX, ID2 and ETV5. Lentivirus-mediated expression of SPDEF in hSABCi-NS1.1 cells induced secretory cell metaplasia, accompanied with characteristic COPD-associated SAE secretory cell changes, including up-regulation of MSMB, CEACAM5 and down-regulation of LTF.

Conclusions

The immortalized hSABCi-NS1.1 cell line has diverse differentiation capacities and retains SAE features, which will be useful for understanding the biology of SAE, the pathogenesis of SAE-related diseases, and testing new pharmacologic agents.

Electronic supplementary material

The online version of this article (10.1186/s12931-019-1140-9) contains supplementary material, which is available to authorized users.

The Human Lung Cell Atlas: A High-Resolution Reference Map of the Human Lung in Health and Disease

Lung disease accounts for every sixth death globally. Profiling the molecular state of all lung cell types in health and disease is currently revolutionizing the identification of disease mechanisms and will aid the design of novel diagnostic and personalized therapeutic regimens. Recent progress in high-throughput techniques for single-cell genomic and transcriptomic analyses has opened up new possibilities to study individual cells within a tissue, classify these into cell types, and characterize variations in their molecular profiles as a function of genetics, environment, cell-cell interactions, developmental processes, aging, or disease. Integration of these cell state definitions with spatial information allows the in-depth molecular description of cellular neighborhoods and tissue microenvironments, including the tissue resident structural and immune cells, the tissue matrix, and the microbiome. The Human Cell Atlas consortium aims to characterize all cells in the healthy human body and has prioritized lung tissue as one of the flagship projects. Here, we present the rationale, the approach, and the expected impact of a Human Lung Cell Atlas.

The Other Side of the Fascia: The Smooth Muscle Part 1

According to current scientific standards, the fascia is a connective tissue derived from two separate germ layers, the mesoderm (trunk and limbs, part of the neck) and the ectoderm (cervical tract and skull). The fascia has the property of maintaining the shape and function of its anatomical district, but it also can adapt to mechanical-metabolic stimuli. Smooth muscle and non-voluntary striated musculature originated from the mesoderm have never been properly considered as a type of fascia. They are some of the viscera present in the mediastinum, in the abdomen and in the pelvic floor. This text represents the first article in the international scientific field that discusses the inclusion of some viscera in the context of what is considered fascia, thanks to the efforts of our committee for the definition and nomenclature of the fascial tissue of the Foundation of Osteopathic Research and Clinical Endorsement (FORCE).

From the pathophysiology of the human lung alveolus to epigenetic editing: Congress 2018 highlights from ERS Assembly 3 "Basic and Translational Science."

The European Respiratory Society (ERS) International Congress is the largest respiratory congress and brings together leading experts in all fields of respiratory medicine and research. ERS Assembly 3 shapes the basic and translational science aspects of this congress, aiming to combine cutting-edge novel developments in basic research with novel clinical findings. In this article, we summarise a selection of the scientific highlights from the perspective of the three groups within Assembly 3. In particular, we discuss new insights into the pathophysiology of the human alveolus, novel tools in organoid development and (epi)genome editing, as well as insights from the presented abstracts on novel therapeutic targets being identified for idiopathic pulmonary fibrosis.

The amount of basic and translational science presented at #ERSCongress is steadily increasing, showing novel cutting-edge technologies and models.http://bit.ly/2GgXIJi

FGF Signaling in Lung Development and Disease: Human Versus Mouse

Fibroblast growth factor 10 (FGF10) plays an important role in mouse lung development, injury, and repair. It is considered the main morphogen driving lung branching morphogenesis in rodents. While many studies have found FGF10 SNPs associated with COPD and branch variants in COPD smokers, there is no evidence of a causative role for FGF10 or these SNPs in human lung development and pediatric lung diseases. We and others have shown divergent roles for FGF10 in mouse lung development and early human lung development. Herein, we only review the existing literature on FGF signaling in human lung development and pediatric human lung diseases, comparing what is known in mouse lung to that in human lung.

Heterogeneity of Pulmonary Stem Cells

Epithelial stem cells reside within multiple regions of the lung where they renew various region-specific cells. In addition, there are multiple routes of regeneration after injury through built-in heterogeneity within stem cell populations and through a capacity for cellular plasticity among differentiated cells. These processes are important facets of respiratory tissue resiliency and organism survival. However, this regenerative capacity is not limitless, and repetitive or chronic injuries, environmental stresses, or underlying factors of disease may ultimately lead to or contribute to tissue remodeling and end-stage lung disease. This chapter will review stem cell heterogeneity among pulmonary epithelia in the lower respiratory system, discuss recent findings that may challenge long-held scientific paradigms, and identify several clinically relevant research opportunities for regenerative medicine.

Human development: recent progress and future prospects

Summary: Our second Special Issue on human development highlights recent progress in this field, new technologies that have contributed to this progress and some of the ethical issues to consider.