Lung development and repair: contribution of the ciliated lineage

Airway injury induces alveolar epithelial and mesenchymal responses mediated by macrophages

Acute injury in the airways or the lung activates local progenitors and stimulates changes in cell-cell interactions to restore homeostasis, but it is not appreciated how more distant niches are impacted. We utilized mouse models of airway-specific epithelial injury to examine secondary tissue-wide alveolar, immune, and mesenchymal responses. Single-cell transcriptomics and in vivo validation revealed transient, tissue-wide proliferation of alveolar type 2 (AT2) progenitor cells after club cell-specific ablation. The AT2 cell proliferative response was reliant on alveolar macrophages (AMs) via upregulation of Spp1 which encodes the secreted factor Osteopontin. A previously uncharacterized mesenchymal population we termed Mesenchymal Airway/Adventitial Niche Cell 2 (MANC2) also exhibited dynamic changes in abundance and a pro-fibrotic transcriptional signature after club cell ablation in an AM-dependent manner. Overall, these results demonstrate that acute airway damage can trigger distal lung responses including altered cell-cell interactions that may contribute to potential vulnerabilities for further dysregulation and disease.

Repetitive sulfur dioxide exposure in mice models post-deployment respiratory syndrome

Soldiers deployed to Iraq and Afghanistan have a higher prevalence of respiratory symptoms than nondeployed military personnel and some have been shown to have a constellation of findings on lung biopsy termed post-deployment respiratory syndrome (PDRS). Since many of the subjects in this cohort reported exposure to sulfur dioxide (SO2), we developed a model of repetitive exposure to SO2 in mice that phenocopies many aspects of PDRS, including adaptive immune activation, airway wall remodeling, and pulmonary vascular (PV) disease. Although abnormalities in small airways were not sufficient to alter lung mechanics, PV remodeling resulted in the development of pulmonary hypertension and reduced exercise tolerance in SO2-exposed mice. SO2 exposure led to increased formation of isolevuglandins (isoLGs) adducts and superoxide dismutase 2 (SOD2) acetylation in endothelial cells, which were attenuated by treatment with the isoLG scavenger 2-hydroxybenzylamine acetate (2-HOBA). In addition, 2-HOBA treatment or Siruin-3 overexpression in a transgenic mouse model prevented vascular remodeling following SO2 exposure. In summary, our results indicate that repetitive SO2 exposure recapitulates many aspects of PDRS and that oxidative stress appears to mediate PV remodeling in this model. Together, these findings provide new insights regarding the critical mechanisms underlying PDRS.NEW & NOTEWORTHY We developed a mice model of "post-deployment respiratory syndrome" (PDRS), a condition in Veterans with unexplained exertional dyspnea. Our model successfully recapitulates many of the pathological and physiological features of the syndrome, revealing involvement of the ROS-isoLGs-Sirt3-SOD2 pathway in pulmonary vasculature pathology. Our study provides additional knowledge about effects and long-term consequences of sulfur dioxide exposure on the respiratory system, serving as a valuable tool for future PDRS research.

Lung repair and regeneration: Advanced models and insights into human disease

The respiratory system acts as both the primary site of gas exchange and an important sensor and barrier to the external environment. The increase in incidences of respiratory disease over the past decades has highlighted the importance of developing improved therapeutic approaches. This review will summarize recent research on the cellular complexity of the mammalian respiratory system with a focus on gas exchange and immunological defense functions of the lung. Different models of repair and regeneration will be discussed to help interpret human and animal data and spur the investigation of models and assays for future drug development.

Evidence for lung barrier regeneration by differentiation prior to binucleated and stem cell division

With each breath, oxygen diffuses across remarkably thin alveolar type I (AT1) cells into underlying capillaries. Interspersed cuboidal AT2 cells produce surfactant and act as stem cells. Even transient disruption of this delicate barrier can promote capillary leak. Here, we selectively ablated AT1 cells, which uncovered rapid AT2 cell flattening with near-continuous barrier preservation, culminating in AT1 differentiation. Proliferation subsequently restored depleted AT2 cells in two phases, mitosis of binucleated AT2 cells followed by replication of mononucleated AT2 cells. M phase entry of binucleated and S phase entry of mononucleated cells were both triggered by AT1-produced hbEGF signaling via EGFR to Wnt-active AT2 cells. Repeated AT1 cell killing elicited exuberant AT2 proliferation, generating aberrant daughter cells that ceased surfactant function yet failed to achieve AT1 differentiation. This hyperplasia eventually resolved, yielding normal-appearing alveoli. Overall, this specialized regenerative program confers a delicate simple epithelium with functional resiliency on par with the physical durability of thicker, pseudostratified, or stratified epithelia.

Airway epithelial development and function: A key player in asthma pathogenesis?

Though asthma is a common and relatively easy to diagnose disease, attempts at primary or secondary prevention, and cure, have been disappointing. The widespread use of inhaled steroids has dramatically improved asthma control but has offered nothing in terms of altering long-term outcomes or reversing airway remodeling and impairment in lung function. The inability to cure asthma is unsurprising given our limited understanding of the factors that contribute to disease initiation and persistence. New data have focused on the airway epithelium as a potentially key factor orchestrating the different stages of asthma. In this review we summarize for the clinician the current evidence on the central role of the airway epithelium in asthma pathogenesis and the factors that may alter epithelial integrity and functionality.

The lung mesenchyme in development, regeneration, and fibrosis

Mesenchymal cells are uniquely located at the interface between the epithelial lining and the stroma, allowing them to act as a signaling hub among diverse cellular compartments of the lung. During embryonic and postnatal lung development, mesenchyme-derived signals instruct epithelial budding, branching morphogenesis, and subsequent structural and functional maturation. Later during adult life, the mesenchyme plays divergent roles wherein its balanced activation promotes epithelial repair after injury while its aberrant activation can lead to pathological remodeling and fibrosis that are associated with multiple chronic pulmonary diseases, including bronchopulmonary dysplasia, idiopathic pulmonary fibrosis, and chronic obstructive pulmonary disease. In this Review, we discuss the involvement of the lung mesenchyme in various morphogenic, neomorphogenic, and dysmorphogenic aspects of lung biology and health, with special emphasis on lung fibroblast subsets and smooth muscle cells, intercellular communication, and intrinsic mesenchymal mechanisms that drive such physiological and pathophysiological events throughout development, homeostasis, injury repair, regeneration, and aging.

Repetitive Sulfur Dioxide Exposure in Mice Models Post-Deployment Respiratory Syndrome

Soldiers deployed to Iraq and Afghanistan have a higher prevalence of respiratory symptoms than non-deployed military personnel and some have been shown to have a constellation of findings on lung biopsy termed post-deployment respiratory syndrome (PDRS). Since many of the deployers in this cohort reported exposure to sulfur dioxide (SO 2 ), we developed a model of repetitive exposure to SO 2 in mice that phenocopies many aspects of PDRS, including adaptive immune activation, airway wall remodeling, and pulmonary vascular disease (PVD). Although abnormalities in small airways were not sufficient to alter lung mechanics, PVD was associated with the development of pulmonary hypertension and reduced exercise tolerance in SO 2 exposed mice. Further, we used pharmacologic and genetic approaches to demonstrate a critical role for oxidative stress and isolevuglandins in mediating PVD in this model. In summary, our results indicate that repetitive SO 2 exposure recapitulates many aspects of PDRS and that oxidative stress may mediate PVD in this model, which may be helpful for future mechanistic studies examining the relationship between inhaled irritants, PVD, and PDRS.

Lung development and regeneration: newly defined cell types and progenitor status

The lung is the most critical organ of the respiratory system supporting gas exchange. Constant interaction with the external environment makes the lung vulnerable to injury. Thus, a deeper understanding of cellular and molecular processes underlying lung development programs and evaluation of progenitor status within the lung is an essential part of lung regenerative medicine. In this review, we aim to discuss the current understanding of lung development process and regenerative capability. We highlight the advances brought by multi-omics approaches, single-cell transcriptome, in particular, that can help us further dissect the cellular player and molecular signaling underlying those processes.

Orthodenticle homeobox 2 is transported to lysosomes by nuclear budding vesicles

Transcription factors (TFs) are transported from the cytoplasm to the nucleus and disappear from the nucleus after they regulate gene expression. Here, we discover an unconventional nuclear export of the TF, orthodenticle homeobox 2 (OTX2), in nuclear budding vesicles, which transport OTX2 to the lysosome. We further find that torsin1a (Tor1a) is responsible for scission of the inner nuclear vesicle, which captures OTX2 using the LINC complex. Consistent with this, in cells expressing an ATPase-inactive Tor1aΔE mutant and the LINC (linker of nucleoskeleton and cytoskeleton) breaker KASH2, OTX2 accumulated and formed aggregates in the nucleus. Consequently, in the mice expressing Tor1aΔE and KASH2, OTX2 could not be secreted from the choroid plexus for transfer to the visual cortex, leading to failed development of parvalbumin neurons and reduced visual acuity. Together, our results suggest that unconventional nuclear egress and secretion of OTX2 are necessary not only to induce functional changes in recipient cells but also to prevent aggregation in donor cells.

Temporal Notch signaling regulates mucociliary cell fates through Hes-mediated competitive de-repression

Tissue functions are determined by the types and ratios of cells present, but little is known about self-organizing principles establishing correct cell type compositions. Mucociliary airway clearance relies on the correct balance between secretory and ciliated cells, which is regulated by Notch signaling across mucociliary systems. Using the airway-like Xenopus epidermis, we investigate how cell fates depend on signaling, how signaling levels are controlled, and how Hes transcription factors regulate cell fates. We show that four mucociliary cell types each require different Notch levels and that their specification is initiated sequentially by a temporal Notch gradient. We describe a novel role for Foxi1 in the generation of Delta-expressing multipotent progenitors through Hes7.1. Hes7.1 is a weak repressor of mucociliary genes and overcomes maternal repression by the strong repressor Hes2 to initiate mucociliary development. Increasing Notch signaling then inhibits Hes7.1 and activates first Hes4, then Hes5.10, which selectively repress cell fates. We have uncovered a self-organizing mechanism of mucociliary cell type composition by competitive de-repression of cell fates by a set of differentially acting repressors. Furthermore, we present an in silico model of this process with predictive abilities.

A human fetal lung cell atlas uncovers proximal-distal gradients of differentiation and key regulators of epithelial fates

We present a multiomic cell atlas of human lung development that combines single-cell RNA and ATAC sequencing, high-throughput spatial transcriptomics, and single-cell imaging. Coupling single-cell methods with spatial analysis has allowed a comprehensive cellular survey of the epithelial, mesenchymal, endothelial, and erythrocyte/leukocyte compartments from 5-22 post-conception weeks. We identify previously uncharacterized cell states in all compartments. These include developmental-specific secretory progenitors and a subtype of neuroendocrine cell related to human small cell lung cancer. Our datasets are available through our web interface (https://lungcellatlas.org). To illustrate its general utility, we use our cell atlas to generate predictions about cell-cell signaling and transcription factor hierarchies which we rigorously test using organoid models.

Application of second-generation sequencing in congenital pulmonary airway malformations

To investigate the differential expression of genes in whole transcripts of congenital pulmonary airway malformation (CPAM) using second-generation sequencing (also known as next-generation sequencing, NGS) technology. Children with CPAM were strictly screened after setting the criteria, and grouped by taking CPAM parietal tissue and CPAM lesion tissue respectively, and RNA-Seq libraries were established separately using second-generation sequencing technology, followed by differential expression analysis and GO (gene ontology) functional enrichment analysis, KEGG (Kyoto encyclopedia of genes and genomes, a database) pathway analysis and GSEA (Gene Set Enrichment Analysis) analysis. Five cases were screened from 36 children with CPAM, and high-throughput sequencing was performed to obtain 10 whole transcripts of samples with acceptable sequence quality and balanced gene coverage. One aberrantly expressed sample (3b) was found by analysis of principal components, which was excluded and then subjected to differential expression analysis, and 860 up-regulated genes and 203 down-regulated genes. GO functional enrichment analysis of differentially expressed genes demonstrates the functional class and cellular localization of target genes. The whole transcript of CPAM shows obvious gene up and down-regulation, differentially expressed genes are located in specific cells and belong to different functional categories, and NGS can provide an effective means to study the transcriptional regulation of CPAM from the overall transcriptional level.

Lung epithelium development and airway regeneration

The lung is composed of a highly branched airway structure, which humidifies and warms the inhaled air before entering the alveolar compartment. In the alveoli, a thin layer of epithelium is in close proximity with the capillary endothelium, allowing for an efficient exchange of oxygen and carbon dioxide. During development proliferation and differentiation of progenitor cells generates the lung architecture, and in the adult lung a proper function of progenitor cells is needed to regenerate after injury. Malfunctioning of progenitors during development results in various congenital lung disorders, such as Congenital Diaphragmatic Hernia (CDH) and Congenital Pulmonary Adenomatoid Malformation (CPAM). In addition, many premature neonates experience continuous insults on the lung caused by artificial ventilation and supplemental oxygen, which requires a highly controlled mechanism of airway repair. Malfunctioning of airway progenitors during regeneration can result in reduction of respiratory function or (chronic) airway diseases. Pathways that are active during development are frequently re-activated upon damage. Understanding the basic mechanisms of lung development and the behavior of progenitor cell in the ontogeny and regeneration of the lung may help to better understand the underlying cause of lung diseases, especially those occurring in prenatal development or in the immediate postnatal period of life. This review provides an overview of lung development and the cell types involved in repair of lung damage with a focus on the airway.

Flow blockage disrupts cilia-driven fluid transport in the epileptic brain

A carpet of ependymal motile cilia lines the brain ventricular system, forming a network of flow channels and barriers that pattern cerebrospinal fluid (CSF) flow at the surface. This CSF transport system is evolutionary conserved, but its physiological function remains unknown. Here we investigated its potential role in epilepsy with studies focused on CDKL5 deficiency disorder (CDD), a neurodevelopmental disorder with early-onset epilepsy refractory to seizure medications and the most common cause of infant epilepsy. CDKL5 is a highly conserved X-linked gene suggesting its function in regulating cilia length and motion in the green alga Chlamydomonas might have implication in the etiology of CDD. Examination of the structure and function of airway motile cilia revealed both the CDD patients and the Cdkl5 knockout mice exhibit cilia lengthening and abnormal cilia motion. Similar defects were observed for brain ventricular cilia in the Cdkl5 knockout mice. Mapping ependymal cilia generated flow in the ventral third ventricle (v3V), a brain region with important physiological functions showed altered patterning of flow. Tracing of cilia-mediated inflow into v3V with fluorescent dye revealed the appearance of a flow barrier at the inlet of v3V in Cdkl5 knockout mice. Analysis of mice with a mutation in another epilepsy-associated kinase, Yes1, showed the same disturbance of cilia motion and flow patterning. The flow barrier was also observed in the Foxj1^± and FOXJ1Cre^ERT:Cdkl5^y/fl mice, confirming the contribution of ventricular cilia to the flow disturbances. Importantly, mice exhibiting altered cilia-driven flow also showed increased susceptibility to anesthesia-induced seizure-like activity. The cilia-driven flow disturbance arises from altered cilia beating orientation with the disrupted polarity of the cilia anchoring rootlet meshwork. Together these findings indicate motile cilia disturbances have an essential role in CDD-associated seizures and beyond, suggesting cilia regulating kinases may be a therapeutic target for medication-resistant epilepsy.

Altered Expression of Pulmonary Epithelial Cell Markers in Fetal and Adult Mice Generated by in vitro Embryo Culture and Embryo Transfer

Lung development is impaired in mice generated through transfer of in vitro-derived blastocysts. The main objective of the current study was to determine if the composition of epithelial cells in the fetal and adult lung tissue is altered in mice generated through transfer of in vitro-derived blastocysts. The study comprised two experimental (EGs) and two control (CGs) groups. Fetuses (18.5 d.p.c.) and adult mice (8 weeks old) of the EGs (EGfetus, n = 18; EGadult, n = 15) were produced by the transfer of day 5 F2 blastocysts to pseudo-pregnant females. F2 fetuses and adult mice derived from naturally ovulating females served as the CGs (CGfetus, n = 18; CGadult, n = 15). The expression of Tuba-1a (a marker of ciliated cells), Foxj-1 (a marker of motile ciliated cells), Uch-L1 (a marker of neuroendocrine cells), Cldn-10 (a marker of club cells), Aqp-5 (a marker of type I alveolar cells), and Sp-C (a marker of type II alveolar cells) was determined using Western blot, immunohistochemistry/immunofluorescence, and quantitative RT-PCR analyses. Weight of fetuses as well as adult mice is decreased in mice comprising the EGs. Impaired lung development observed in EGfetus was associated with altered expression of Tuba-1a, Foxj-1, Cldn-10, Uch-L1, Sp-C, and Aqp-5. Morphology of the adult lung tissue was similar between the groups except for a significant increase in the thickness of the epithelia in EGadult. The expression of Cldn-10 and Sp-C was also altered in EGadult. It remains to be determined whether altered expression of these genes has any long-term impact on epithelial cell functions in the adult lung tissue.

Club cells employ regeneration mechanisms during lung tumorigenesis

The high plasticity of lung epithelial cells, has for many years, confounded the correct identification of the cell-of-origin of lung adenocarcinoma (LUAD), one of the deadliest malignancies worldwide. Here, we employ lineage-tracing mouse models to investigate the cell of origin of Eml4-Alk LUAD, and show that Club and Alveolar type 2 (AT2) cells give rise to tumours. We focus on Club cell originated tumours and find that Club cells experience an epigenetic switch by which they lose their lineage fidelity and gain an AT2-like phenotype after oncogenic transformation. Single-cell transcriptomic analyses identified two trajectories of Club cell evolution which are similar to the ones used during lung regeneration, suggesting that lung epithelial cells leverage on their plasticity and intrinsic regeneration mechanisms to give rise to a tumour. Together, this study highlights the role of Club cells in LUAD initiation, identifies the mechanism of Club cell lineage infidelity, confirms the presence of these features in human tumours, and unveils key mechanisms conferring LUAD heterogeneity.

YAP regulates alveolar epithelial cell differentiation and AGER via NFIB/KLF5/NKX2-1

Ventilation is dependent upon pulmonary alveoli lined by two major epithelial cell types, alveolar type-1 (AT1) and 2 (AT2) cells. AT1 cells mediate gas exchange while AT2 cells synthesize and secrete pulmonary surfactants and serve as progenitor cells which repair the alveoli. We developed transgenic mice in which YAP was activated or deleted to determine its roles in alveolar epithelial cell differentiation. Postnatal YAP activation increased epithelial cell proliferation, increased AT1 cell numbers, and caused indeterminate differentiation of subsets of alveolar cells expressing atypical genes normally restricted to airway epithelial cells. YAP deletion increased expression of genes associated with mature AT2 cells. YAP activation enhanced DNA accessibility in promoters of transcription factors and motif enrichment analysis predicted target genes associated with alveolar cell differentiation. YAP participated with KLF5, NFIB, and NKX2-1 to regulate AGER. YAP plays a central role in a transcriptional network that regulates alveolar epithelial differentiation.

Release of Notch activity coordinated by IL-1β signalling confers differentiation plasticity of airway progenitors via Fosl2 during alveolar regeneration

While the acquisition of cellular plasticity in adult stem cells is essential for rapid regeneration after tissue injury, little is known about the underlying mechanisms governing this process. Our data reveal the coordination of airway progenitor differentiation plasticity by inflammatory signals during alveolar regeneration. Upon damage, IL-1β signalling-dependent modulation of Jag1/2 expression in ciliated cells results in the inhibition of Notch signalling in secretory cells, which drives reprogramming and acquisition of differentiation plasticity. We identify a transcription factor Fosl2/Fra2 for secretory cell fate conversion to alveolar type 2 (AT2) cells retaining the distinct genetic and epigenetic signatures of secretory lineages. We furthermore reveal that KDR/FLK-1⁺ human secretory cells display a conserved capacity to generate AT2 cells via Notch inhibition. Our results demonstrate the functional role of a IL-1β-Notch-Fosl2 axis for the fate decision of secretory cells during injury repair, proposing a potential therapeutic target for human lung alveolar regeneration.

Differences and similarities between the upper and lower airway: focusing on innate immunity

The nose is the first respiratory barrier to external pathogens, allergens, pollutants, or cigarette smoke, and vigorous immune responses are triggered when external pathogens come in contact with the nasal epithelium. The mucosal epithelial cells of the nose are essential to the innate immune response against external pathogens and transmit signals that modulate the adaptive immune response. The upper and lower airways share many physiological and immunological features, but there are also numerous differences. It is crucial to understand these differences and their contribution to pathophysiology in order to optimize treatments for inflammatory diseases of the respiratory tract. This review summarizes important differences in the embryological development, histological features, microbiota, immune responses, and cellular subtypes of mucosal epithelial cells of the nose and lungs.

Mammalian tracheal development and reconstruction: insights from in vivo and in vitro studies

The trachea delivers inhaled air into the lungs for gas exchange. Anomalies in tracheal development can result in life-threatening malformations, such as tracheoesophageal fistula and tracheomalacia. Given the limitations of current therapeutic approaches, development of technologies for the reconstitution of a three-dimensional trachea from stem cells is urgently required. Recently, single-cell sequencing technologies and quantitative analyses from cell to tissue scale have been employed to decipher the cellular basis of tracheal morphogenesis. In this Review, recent advances in mammalian tracheal development and the generation of tracheal tissues from pluripotent stem cells are summarized.

Airway basal stem cells reutilize the embryonic proliferation regulator, Tgfβ-Id2 axis, for tissue regeneration

During development, quiescent airway basal stem cells are derived from proliferative primordial progenitors through the cell-cycle slowdown. In contrast, basal cells contribute to adult tissue regeneration by shifting from slow cycling to proliferating and subsequently back to slow cycling. Although sustained proliferation results in tumorigenesis, the molecular mechanisms regulating these transitions remain unknown. Using temporal single-cell transcriptomics of developing murine airway progenitors and genetic validation experiments, we found that TGF-β signaling decelerated cell cycle by inhibiting Id2 and contributed to slow-cycling basal cell specification during development. In adult tissue regeneration, reduced TGF-β signaling restored Id2 expression and initiated regeneration. Id2 overexpression and Tgfbr2 knockout enhanced epithelial proliferation; however, persistent Id2 expression drove basal cell hyperplasia that resembled a precancerous state. Together, the TGF-β-Id2 axis commonly regulates the proliferation transitions in basal cells during development and regeneration, and its fine-tuning is critical for normal regeneration while avoiding basal cell hyperplasia.

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

Objectives

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

Materials and Methods

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

Results

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

Conclusions

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

In Vitro Lung Models and Their Application to Study SARS-CoV-2 Pathogenesis and Disease

SARS-CoV-2 has spread across the globe with an astonishing velocity and lethality that has put scientist and pharmaceutical companies worldwide on the spot to develop novel treatment options and reliable vaccination for billions of people. To combat its associated disease COVID-19 and potentially newly emerging coronaviruses, numerous pre-clinical cell culture techniques have progressively been used, which allow the study of SARS-CoV-2 pathogenesis, basic replication mechanisms, and drug efficiency in the most authentic context. Hence, this review was designed to summarize and discuss currently used in vitro and ex vivo cell culture systems and will illustrate how these systems will help us to face the challenges imposed by the current SARS-CoV-2 pandemic.

The aging lung: Physiology, disease, and immunity

The population is aging at a rate never seen before in human history. As the number of elderly adults grows, it is imperative we expand our understanding of the underpinnings of aging biology. Human lungs are composed of a unique panoply of cell types that face ongoing chemical, mechanical, biological, immunological, and xenobiotic stress over a lifetime. Yet, we do not fully appreciate the mechanistic drivers of lung aging and why age increases the risk of parenchymal lung disease, fatal respiratory infection, and primary lung cancer. Here, we review the molecular and cellular aspects of lung aging, local stress response pathways, and how the aging process predisposes to the pathogenesis of pulmonary disease. We place these insights into context of the COVID-19 pandemic and discuss how innate and adaptive immunity within the lung is altered with age.

TP63 basal cells are indispensable during endoderm differentiation into proximal airway cells on acellular lung scaffolds

The use of decellularized whole-organ scaffolds for bioengineering of organs is a promising avenue to circumvent the shortage of donor organs for transplantation. However, recellularization of acellular scaffolds from multicellular organs like the lung with a variety of different cell types remains a challenge. Multipotent cells could be an ideal cell source for recellularization. Here we investigated the hierarchical differentiation process of multipotent ES-derived endoderm cells into proximal airway epithelial cells on acellular lung scaffolds. The first cells to emerge on the scaffolds were TP63⁺ cells, followed by TP63⁺/KRT5⁺ basal cells, and finally multi-ciliated and secretory airway epithelial cells. TP63⁺/KRT5⁺ basal cells on the scaffolds simultaneously expressed KRT14, like basal cells involved in airway repair after injury. Removal of TP63 by CRISPR/Cas9 in the ES cells halted basal and airway cell differentiation on the scaffolds. These findings suggest that differentiation of ES-derived endoderm cells into airway cells on decellularized lung scaffolds proceeds via TP63⁺ basal cell progenitors and tracks a regenerative repair pathway. Understanding the process of differentiation is key for choosing the cell source for repopulation of a decellularized organ scaffold. Our data support the use of airway basal cells for repopulating the airway side of an acellular lung scaffold.

From Clones to Buds and Branches: The Use of Lung Organoids to Model Branching Morphogenesis Ex Vivo

Three-dimensional (3D) organoid culture systems have rapidly emerged as powerful tools to study organ development and disease. The lung is a complex and highly specialized organ that comprises more than 40 cell types that offer several region-specific roles. During organogenesis, the lung goes through sequential and morphologically distinctive stages to assume its mature form, both structurally and functionally. As branching takes place, multipotent epithelial progenitors at the distal tips of the growing/bifurcating epithelial tubes progressively become lineage-restricted, giving rise to more differentiated and specialized cell types. Although many cellular and molecular mechanisms leading to branching morphogenesis have been explored, deeper understanding of biological processes governing cell-fate decisions and lung patterning is still needed. Given that these distinct processes cannot be easily analyzed in vivo, 3D culture systems have become a valuable platform to study organogenesis in vitro. This minireview focuses on the current lung organoid systems that recapitulate developmental events occurring before and during branching morphogenesis. In addition, we also discuss their limitations and future directions.

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

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

Serum neuron-specific enolase: A promising biomarker of silicosis

BACKGROUND

Silicosis is a type of chronic pulmonary fibrosis caused by long-term inhalation of silica dust particles. There has been no ideal biomarker for the diagnosis and differential diagnosis of silicosis until now. Studies have found that elevated neuron-specific enolase (NSE) concentration in the serum of silicosis patients is helpful for diagnosis and severity assessment of the disease. However, the number of cases in these studies was not enough to arouse attention.

AIM

To investigate the clinical significance of serum NSE in the diagnosis and staging of silicosis.

METHODS

From January 2017 to June 2019, 326 cases of silicosis confirmed in Quanzhou First Hospital Affiliated to Fujian Medical University were included in the silicosis group. A total of 328 healthy individuals or medical patients without silicosis were included in the control group. Serum NSE concentrations of all subjects were determined by electrochemical luminescence.

RESULTS

There were no significant differences in sex, age, smoking index and complications between the silicosis and control groups. The mean serum NSE concentration was 26.57 ± 20.95 ng/mL in the silicosis group and 12.42 ± 2.68 ng/mL in the control group. The difference between the two groups was significant (U = 15187, P = 0.000). Among the 326 patients with silicosis, 103 had stage I silicosis, and the mean serum NSE concentration was 15.55 ± 6.23 ng/mL. The mean serum NSE concentration was 21.85 ± 12.05 ng/mL in 70 patients with stage II silicosis. The mean serum NSE concentration was 36.14 ± 25.72 ng/mL in 153 patients with stage III silicosis. Kruskal–Wallis H test suggested that the difference in serum NSE concentration in silicosis patients in the three groups was significant (H = 130.196, P = 0.000). Receiver operating characteristic curve analysis indicated that the area under the curve was 0.858 (95% confidence interval: 0.828-0.888; P = 0.000). When the NSE concentration was 15.82 ng/mL, the Jorden index was the largest, the sensitivity was 72%, and the specificity was 90%.

CONCLUSION

Serum NSE concentration may be a promising biomarker for the diagnosis and assessment of severity of silicosis.

Identification of a Repair-Supportive Mesenchymal Cell Population during Airway Epithelial Regeneration

Tissue regeneration requires coordinated and dynamic remodeling of stem and progenitor cells and the surrounding niche. Although the plasticity of epithelial cells has been well explored in many tissues, the dynamic changes occurring in niche cells remain elusive. Here, we show that, during lung repair after naphthalene injury, a population of PDGFRα⁺ cells emerges in the non-cartilaginous conducting airway niche, which is normally populated by airway smooth muscle cells (ASMCs). This cell population, which we term “repair-supportive mesenchymal cells” (RSMCs), is distinct from conventional ASMCs, which have previously been shown to contribute to epithelial repair. Gene expression analysis on sorted lineage-labeled cells shows that RSMCs express low levels of ASMC markers, but high levels of the pro-regenerative marker Fgf10. Organoid co-cultures demonstrate an enhanced ability for RSMCs in supporting club-cell growth. Our study highlights the dynamics of mesenchymal cells in the airway niche and has implications for chronic airway-injury-associated diseases.

Moiseenko et al. explore the dynamics of mesenchymal cells in the peribronchial niche in response to airway injury. They identify a population of mesenchymal cells located in close proximity to airway smooth muscle cells (ASMCs). This population, termed “repair-supportive mesenchymal cells” (RSMCs), is recruited to facilitate airway epithelial regeneration.

First contact: the role of respiratory cilia in host-pathogen interactions in the airways

Respiratory cilia are the driving force of the mucociliary escalator, working in conjunction with secreted airway mucus to clear inhaled debris and pathogens from the conducting airways. Respiratory cilia are also one of the first contact points between host and inhaled pathogens. Impaired ciliary function is a common pathological feature in patients with chronic airway diseases, increasing susceptibility to respiratory infections. Common respiratory pathogens, including viruses, bacteria, and fungi, have been shown to target cilia and/or ciliated airway epithelial cells, resulting in a disruption of mucociliary clearance that may facilitate host infection. Despite being an integral component of airway innate immunity, the role of respiratory cilia and their clinical significance during airway infections are still poorly understood. This review examines the expression, structure, and function of respiratory cilia during pathogenic infection of the airways. This review also discusses specific known points of interaction of bacteria, fungi, and viruses with respiratory cilia function. The emerging biological functions of motile cilia relating to intracellular signaling and their potential immunoregulatory roles during infection will also be discussed.

Motile cilia genetics and cell biology: big results from little mice

Our understanding of motile cilia and their role in disease has increased tremendously over the last two decades, with critical information and insight coming from the analysis of mouse models. Motile cilia form on specific epithelial cell types and typically beat in a coordinated, whip-like manner to facilitate the flow and clearance of fluids along the cell surface. Defects in formation and function of motile cilia result in primary ciliary dyskinesia (PCD), a genetically heterogeneous disorder with a well-characterized phenotype but no effective treatment. A number of model systems, ranging from unicellular eukaryotes to mammals, have provided information about the genetics, biochemistry, and structure of motile cilia. However, with remarkable resources available for genetic manipulation and developmental, pathological, and physiological analysis of phenotype, the mouse has risen to the forefront of understanding mammalian motile cilia and modeling PCD. This is evidenced by a large number of relevant mouse lines and an extensive body of genetic and phenotypic data. More recently, application of innovative cell biological techniques to these models has enabled substantial advancement in elucidating the molecular and cellular mechanisms underlying the biogenesis and function of mammalian motile cilia. In this article, we will review genetic and cell biological studies of motile cilia in mouse models and their contributions to our understanding of motile cilia and PCD pathogenesis.

Exposure to Cigarette Smoke Enhances the Stemness of Alveolar Type 2 Cells

Chronic exposure to cigarette smoke (CS) causes chronic inflammation, oxidative stress, and apoptosis of epithelial cells, which results in destruction of the lung matrix. However, the mechanism by which the lung fails to repair the CS-induced damage, thereby succumbing to emphysema, remains unclear. Alveolar type 2 (AT2) cells comprise the stem cells of the alveolar compartments and are responsible for repairing and maintaining lung tissues. In this study, we examined the effect of chronic CS on AT2 stem cells. Adult mice expressing GFP in their AT2 cells were exposed to CS for > 3 months. Histological assessment showed that CS not only induced emphysematous changes but also increased the number of AT2 cells compared with that of air-exposed lungs. Assessment of sorted GFP+/AT2 cells via the stem cell three-dimensional organoid/colony-forming assay revealed that the number and size of the colonies formed by the CS-exposed AT2 stem cells were significantly higher than those of air-exposed control AT2 cells. Although CS-exposed lungs had more apoptotic cells, examination of the surviving AT2 stem cells in two-dimensional in vitro culture revealed that they developed a higher ability to resist apoptosis. Microarray analysis of CS-exposed AT2 stem cells revealed the upregulation of genes related to circadian rhythm and inflammatory pathways. In conclusion, we provide evidence that AT2 stem cells respond to chronic CS exposure by activating their stem cell function, thereby proliferating and differentiating faster and becoming more resistant to apoptosis. Disturbances in expression levels of several circadian rhythm-related genes might be involved in these changes.

Cell-Based Therapeutic Approaches for Cystic Fibrosis

Cystic Fibrosis (CF) is a chronic autosomal recessive disease caused by defects in the cystic fibrosis transmembrane conductance regulator gene (CFTR). Cystic Fibrosis affects multiple organs but progressive remodeling of the airways, mucus accumulation, and chronic inflammation in the lung, result in lung disease as the major cause of morbidity and mortality. While advances in management of CF symptoms have increased the life expectancy of this devastating disease, and there is tremendous excitement about the potential of new agents targeting the CFTR molecule itself, there is still no curative treatment. With the recent advances in the identification of endogenous airway progenitor cells and in directed differentiation of pluripotent cell sources, cell-based therapeutic approaches for CF have become a plausible treatment method with the potential to ultimately cure the disease. In this review, we highlight the current state of cell therapy in the CF field focusing on the relevant autologous and allogeneic cell populations under investigation and the challenges associated with their use. In addition, we present advances in induced pluripotent stem (iPS) cell approaches and emerging new genetic engineering methods, which have the capacity to overcome the current limitations hindering cell therapy approaches.

Wnt/β-catenin signaling is critical for regenerative potential of distal lung epithelial progenitor cells in homeostasis and emphysema

Wnt/β-catenin signaling regulates progenitor cell fate decisions during lung development and in various adult tissues. Ectopic activation of Wnt/β-catenin signaling promotes tissue repair in emphysema, a devastating lung disease with progressive loss of parenchymal lung tissue. The identity of Wnt/β-catenin responsive progenitor cells and the potential impact of Wnt/β-catenin signaling on adult distal lung epithelial progenitor cell function in emphysema are poorly understood. Here, we used TCF/ Lef:H2B/GFP reporter mice to investigate the role of Wnt/β-catenin signaling in lung organoid formation. We identified an organoid-forming adult distal lung epithelial progenitor cell population characterized by a low Wnt/β-catenin activity, which was enriched in club and alveolar epithelial type (AT)II cells. Endogenous Wnt/β-catenin activity was required for the initiation of multiple subtypes of distal lung organoids derived from the Wnt^low epithelial progenitors. Further ectopic Wnt/β-catenin activation specifically led to an increase in alveolar organoid number; however, the subsequent proliferation of alveolar epithelial cells in the organoids did not require constitutive Wnt/β-catenin signaling. Distal lung epithelial progenitor cells derived from the mouse model of elastase-induced emphysema exhibited reduced organoid forming capacity. This was rescued by Wnt/β-catenin signal activation, which largely increased the number of alveolar organoids. Together, our study reveals a novel mechanism of lung epithelial progenitor cell activation in homeostasis and emphysema.

NOTCH inhibition promotes bronchial stem cell renewal and epithelial barrier integrity after irradiation

Hyperactivity of the NOTCH pathway is associated with tumor growth and radiotherapy resistance in lung cancer, and NOTCH/γ-secretase inhibitors (GSIs) are a potential therapeutic target. The therapeutic outcome, however, is often restricted by the dose-limiting toxicity of combined treatments on the surrounding healthy tissue. The NOTCH signaling pathway is also crucial for homeostasis and repair of the normal airway epithelium. The effects of NOTCH/γ-secretase inhibition on the irradiation of normal lung epithelium are unknown and may counteract antitumor activity. Here we, therefore, investigated whether normal tissue toxicity to radiation is altered upon NOTCH pathway inhibition. We established air-liquid interface pseudostratified and polarized cultures from primary human bronchial epithelial cells and blocked NOTCH signaling alone or after irradiation with small-molecule NOTCH inhibitor/GSI. We found that the reduction in proliferation and viability of bronchial stem cells (TP63+) in response to irradiation is rescued with concomitant NOTCH inhibition. This correlated with reduced activation of the DNA damage response and accelerated repair by 24 hours and 3 days postirradiation. The increase in basal cell proliferation and viability in GSI-treated and irradiated cultures resulted in an improved epithelial barrier function. Comparable results were obtained after in vivo irradiation, where the combination of NOTCH inhibition and irradiation increased the percentage of stem cells and ciliated cells ex vivo. These encourage further use of normal patient tissue for toxicity screening of combination treatments and disclose novel interactions between NOTCH inhibition and radiotherapy and opportunities for tissue repair after radiotherapy.

Chloride channels regulate differentiation and barrier functions of the mammalian airway

The conducting airway forms a protective mucosal barrier and is the primary target of airway disorders. The molecular events required for the formation and function of the airway mucosal barrier, as well as the mechanisms by which barrier dysfunction leads to early onset airway diseases, remain unclear. In this study, we systematically characterized the developmental landscape of the mouse airway using single-cell RNA sequencing and identified remarkably conserved cellular programs operating during human fetal development. We demonstrated that in mouse, genetic inactivation of chloride channel Ano1/Tmem16a compromises airway barrier function, results in early signs of inflammation, and alters the airway cellular landscape by depleting epithelial progenitors. Mouse Ano1-/-mutants exhibited mucus obstruction and abnormal mucociliary clearance that resemble the airway defects associated with cystic fibrosis. The data reveal critical and non-redundant roles for Ano1 in organogenesis, and show that chloride channels are essential for mammalian airway formation and function.

In Vitro and In Vivo Development of the Human Airway at Single-Cell Resolution

Bud tip progenitor cells give rise to all murine lung epithelial lineages and have been described in the developing human lung; however, the mechanisms controlling human bud tip differentiation into specific lineages are unclear. Here, we used homogeneous human bud tip organoid cultures and identified SMAD signaling as a key regulator of the bud tip-to-airway transition. SMAD induction led to the differentiation of airway-like organoids possessing functional basal cells capable of clonal expansion and multilineage differentiation. To benchmark in vitro-derived organoids, we developed a single-cell mRNA sequencing atlas of the human lung from 11.5 to 21 weeks of development, which revealed high degrees of similarity between the in vitro-derived and in vivo airway. Together, this work sheds light on human airway differentiation in vitro and provides a single-cell atlas of the developing human lung.

A comparative study of the turnover of multiciliated cells in the mouse trachea, oviduct, and brain

BACKGROUND

In mammals, multiciliated cells (MCCs) line the lumen of the trachea, oviduct, and brain ventricles, where they drive fluid flow across the epithelium. Each MCC population experiences vastly different local environments that may dictate differences in their lifetime and turnover rates. However, with the exception of MCCs in the trachea, the turnover rates of these multiciliated epithelial populations at extended time scales are not well described.

RESULTS

Here, using genetic lineage-labeling techniques we provide a direct comparison of turnover rates of MCCs in these three different tissues.

CONCLUSION

We find that oviduct turnover is similar to that in the airway (~6 months), while multiciliated ependymal cells turnover more slowly.

Paradigms that define lung epithelial progenitor cell fate in development and regeneration

Purpose of Review

Throughout the lifespan, lung injury impedes the primary critical function essential for life-respiration. To repair quickly and efficiently is critical and is orchestrated by a diverse repertoire of progenitor cells and their niche. This review incorporates knowledge gained from early studies in lung epithelial morphogenesis and cell fate and explores its relevance to more recent findings of lung progenitor and stem cells in development and regeneration.

Recent Findings

Cell fate in the lung is organized into an early specification phase and progressive differentiation phase in lung development. The advent of single cell analysis combined with lineage analysis and projections is uncovering new functional cell types in the lung providing a topographical atlas for progenitor cell lineage commitment during development, homeostasis, and regeneration.

Summary

Lineage commitment of lung progenitor cells is spatiotemporally regulated during development. Single cell sequencing technologies have significantly advanced our understanding of the similarities and differences between developmental and regenerative cell fate trajectories. Subsequent unraveling of the molecular mechanisms underlying these cell fate decisions will be essential to manipulating progenitor cells for regeneration.

Epithelial cell plasticity defines heterogeneity in lung cancer

Lung cancer is the leading cause of cancer death for both men and women and accounts for almost 18.4% of all deaths due to cancer worldwide, with the global incidence increasing by approximately 0.5% per year. Lung cancer is regarded as a devastating type of cancer owing to its high prevalence, reduction in the health-related quality of life, frequently delayed diagnosis, low response rate, high toxicity, and resistance to available therapeutic options. The highly heterogeneous nature of this cancer with a proximal-to-distal distribution throughout the respiratory tract dramatically affects its diagnostic and therapeutic management. The diverse composition and plasticity of lung epithelial cells across the respiratory tract are regarded as significant factors underlying lung cancer heterogeneity. Therefore, definitions of the cells of origin for different types of lung cancer are urgently needed to understand lung cancer biology and to achieve early diagnosis and develop cell-targeted therapies. In the present review, we will discuss the current understanding of the cellular and molecular alterations in distinct lung epithelial cells that result in each type of lung cancer.

Cellular crosstalk in the development and regeneration of the respiratory system

The respiratory system, including the peripheral lungs, large airways and trachea, is one of the most recently evolved adaptations to terrestrial life. To support the exchange of respiratory gases, the respiratory system is interconnected with the cardiovascular system, and this interconnective nature requires a complex interplay between a myriad of cell types. Until recently, this complexity has hampered our understanding of how the respiratory system develops and responds to postnatal injury to maintain homeostasis. The advent of new single-cell sequencing technologies, developments in cellular and tissue imaging and advances in cell lineage tracing have begun to fill this gap. The view that emerges from these studies is that cellular and functional heterogeneity of the respiratory system is even greater than expected and also highly adaptive. In this Review, we explore the cellular crosstalk that coordinates the development and regeneration of the respiratory system. We discuss both the classic cell and developmental biology studies and recent single-cell analysis to provide an integrated understanding of the cellular niches that control how the respiratory system develops, interacts with the external environment and responds to injury.

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

Background

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

Methods

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

Results

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

Conclusions

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

Electronic supplementary material

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

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

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

Lentiviral Vectors for the Treatment and Prevention of Cystic Fibrosis Lung Disease

Despite the continued development of cystic fibrosis transmembrane conductance regulator (CFTR) modulator drugs for the treatment of cystic fibrosis (CF), the need for mutation agnostic treatments remains. In a sub-group of CF individuals with mutations that may not respond to modulators, such as those with nonsense mutations, CFTR gene transfer to airway epithelia offers the potential for an effective treatment. Lentiviral vectors are well-suited for this purpose because they transduce nondividing cells, and provide long-term transgene expression. Studies in primary cultures of human CF airway epithelia and CF animal models demonstrate the long-term correction of CF phenotypes and low immunogenicity using lentiviral vectors. Further development of CF gene therapy requires the investigation of optimal CFTR expression in the airways. Lentiviral vectors with improved safety features have minimized insertional mutagenesis safety concerns raised in early clinical trials for severe combined immunodeficiency using γ-retroviral vectors. Recent clinical trials using improved lentiviral vectors support the feasibility and safety of lentiviral gene therapy for monogenetic diseases. While work remains to be done before CF gene therapy reaches the bedside, recent advances in lentiviral vector development reviewed here are encouraging and suggest it could be tested in clinical studies in the near future.

Fibrillin-2 is a key mediator of smooth muscle extracellular matrix homeostasis during mouse tracheal tubulogenesis

Epithelial tubes, comprised of polarised epithelial cells around a lumen, are crucial for organ function. However, the molecular mechanisms underlying tube formation remain largely unknown. Here, we report on the function of fibrillin (FBN)2, an extracellular matrix (ECM) glycoprotein, as a critical regulator of tracheal tube formation.We performed a large-scale forward genetic screen in mouse to identify regulators of respiratory organ development and disease. We identified Fbn2 mutants which exhibit shorter and narrowed tracheas as well as defects in tracheal smooth muscle cell alignment and polarity.We found that FBN2 is essential for elastic fibre formation and Fibronectin accumulation around tracheal smooth muscle cells. These processes appear to be regulated at least in part through inhibition of p38-mediated upregulation of matrix metalloproteinases (MMPs), as pharmacological decrease of p38 phosphorylation or MMP activity partially attenuated the Fbn2 mutant tracheal phenotypes. Analysis of human tracheal tissues indicates that a decrease in ECM proteins, including FBN2 and Fibronectin, is associated with tracheomalacia.Our findings provide novel insights into the role of ECM homeostasis in mesenchymal cell polarisation during tracheal tubulogenesis.

Non-lytic clearance of influenza B virus from infected cells preserves epithelial barrier function

Influenza B virus (IBV) is an acute, respiratory RNA virus that has been assumed to induce the eventual death of all infected cells. We and others have shown however, that infection with apparently cytopathic viruses does not necessarily lead to cell death; some cells can intrinsically clear the virus and persist in the host long-term. To determine if any cells can survive direct IBV infection, we here generate a recombinant IBV capable of activating a host-cell reporter to permanently label all infected cells. Using this system, we demonstrate that IBV infection leads to the formation of a survivor cell population in the proximal airways that are ciliated-like, but transcriptionally and phenotypically distinct from both actively infected and bystander ciliated cells. We also show that survivor cells are critical to maintain respiratory barrier function. These results highlight a host response pathway that preserves the epithelium to limit the severity of IBV disease.

Infection of a cell with influenza B virus (IBV) often results in cell death and the role of surviving cells in pathogenesis is unclear. Here, Dumm et al. generate a recombinant IBV that activates a host-cell reporter to permanently label infected cells, and show that surviving cells are important to preserve epithelial barrier function.

A Cell/Cilia Cycle Biosensor for Single-Cell Kinetics Reveals Persistence of Cilia after G1/S Transition Is a General Property in Cells and Mice

The cilia and cell cycles are inextricably linked. Centrioles in the basal body of cilia nucleate the ciliary axoneme and sequester pericentriolar matrix (PCM) at the centrosome to organize the mitotic spindle. Cilia themselves respond to growth signals, prompting cilia resorption and cell cycle re-entry. We describe a fluorescent cilia and cell cycle biosensor allowing live imaging of cell cycle progression and cilia assembly and disassembly kinetics in cells and inducible mice. We define assembly and disassembly in relation to cell cycle stage with single-cell resolution and explore the intercellular heterogeneity in cilia kinetics. In all cells and tissues analyzed, we observed cilia that persist through the G1/S transition and into S/G2/M-phase. We conclude that persistence of cilia after the G1/S transition is a general property. This resource will shed light at an individual cell level on the interplay between the cilia and cell cycles in development, regeneration, and disease.

•

Arl13bCerulean-Fucci2a biosensor labels the cell and cilia cycles

•

Analysis of cells and mice reveals persistence of cilia after the G1/S transition

•

Inducible mouse line allows lineage tracing and ex vivo live imaging

•

Organisms can tolerate artificially lengthened cilia without overt phenotypes.

The effect of mesenchymal stem cell-secreted factors on airway epithelial repair

Aim: This study was aimed to investigate the effect of mesenchymal stem cell (MSC)-secreted factors on airway repair. Materials & methods: An indirect in vitro coculture model of injured airway epithelium explant with MSCs was developed. LC–MS/MS analysis was performed to determine factors secreted by MSCs and their involvement in epithelium repair was evaluated by histopathological assessment. Results: The identification of 54 of MSC proteins of which 44 of them were secretory/extracellular proteins. 43 of the secreted proteins were found to be involved in accelerating airway epithelium repair by stimulating the migratory, proliferative and differentiation abilities of the endogenous repair mechanisms. MSC-secreted proteins also initiated epithelial–mesenchymal transition process during early repair. Conclusion: MSC-secreted factors accelerated airway epithelial repair by stimulating the endogenous reparative and regenerative ability of lung cells.