Secretoglobins SCGB3A1 and SCGB3A2 define secretory cell subsets in mouse and human airways

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

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

FastCAR: fast correction for ambient RNA to facilitate differential gene expression analysis in single-cell RNA-sequencing datasets

Cell type-specific differential gene expression analyses based on single-cell transcriptome datasets are sensitive to the presence of cell-free mRNA in the droplets containing single cells. This so-called ambient RNA contamination may differ between samples obtained from patients and healthy controls. Current ambient RNA correction methods were not developed specifically for single-cell differential gene expression (sc-DGE) analyses and might therefore not sufficiently correct for ambient RNA-derived signals. Here, we show that ambient RNA levels are highly sample-specific. We found that without ambient RNA correction, sc-DGE analyses erroneously identify transcripts originating from ambient RNA as cell type-specific disease-associated genes. We therefore developed a computationally lean and intuitive correction method, Fast Correction for Ambient RNA (FastCAR), optimized for sc-DGE analysis of scRNA-Seq datasets generated by droplet-based methods including the 10XGenomics Chromium platform. FastCAR uses the profile of transcripts observed in libraries that likely represent empty droplets to determine the level of ambient RNA in each individual sample, and then corrects for these ambient RNA gene expression values. FastCAR can be applied as part of the data pre-processing and QC in sc-DGE workflows comparing scRNA-Seq data in a health versus disease experimental design. We compared FastCAR with two methods previously developed to remove ambient RNA, SoupX and CellBender. All three methods identified additional genes in sc-DGE analyses that were not identified in the absence of ambient RNA correction. However, we show that FastCAR performs better at correcting gene expression values attributed to ambient RNA, resulting in a lower frequency of false-positive observations. Moreover, the use of FastCAR in a sc-DGE workflow increases the cell-type specificity of sc-DGE analyses across disease conditions.

Transcriptional Regulation of Airway Epithelial Cell Differentiation: Insights into the Notch Pathway and Beyond

The airway epithelium is a critical component of the respiratory system, serving as a barrier against inhaled pathogens and toxins. It is composed of various cell types, each with specific functions essential to proper airway function. Chronic respiratory diseases can disrupt the cellular composition of the airway epithelium, leading to a decrease in multiciliated cells (MCCs) and an increase in secretory cells (SCs). Basal cells (BCs) have been identified as the primary stem cells in the airway epithelium, capable of self-renewal and differentiation into MCCs and SCs. This review emphasizes the role of transcription factors in the differentiation process from BCs to MCCs and SCs. Recent advancements in single-cell RNA sequencing (scRNAseq) techniques have provided insights into the cellular composition of the airway epithelium, revealing specialized and rare cell types, including neuroendocrine cells, tuft cells, and ionocytes. Understanding the cellular composition and differentiation processes within the airway epithelium is crucial for developing targeted therapies for respiratory diseases. Additionally, the maintenance of BC populations and the involvement of Notch signaling in BC self-renewal and differentiation are discussed. Further research in these areas could provide valuable insights into the mechanisms underlying airway epithelial homeostasis and disease pathogenesis.

AT2 cell-derived IgA trapped by the extracellular matrix in silica-induced pulmonary fibrosis

Pulmonary fibrosis is an interstitial lung disease caused by various factors such as exposure to workplace environmental contaminants, drugs, or X-rays. Epithelial cells are among the driving factors of pulmonary fibrosis. Immunoglobulin A (IgA), traditionally thought to be secreted by B cells, is an important immune factor involved in respiratory mucosal immunity. In the current study, we found that lung epithelial cells are involved in IgA secretion, which, in turn, promotes pulmonary fibrosis. Spatial transcriptomics and single-cell sequencing suggest that Igha transcripts were highly expressed in the fibrotic lesion areas of lungs from silica-treated mice. Reconstruction of B-cell receptor (BCR) sequences revealed a new cluster of AT2-like epithelial cells with a shared BCR and high expression of genes related to IgA production. Furthermore, the secretion of IgA by AT2-like cells was trapped by the extracellular matrix and aggravated pulmonary fibrosis by activating fibroblasts. Targeted blockade of IgA secretion by pulmonary epithelial cells may be a potential strategy for treating pulmonary fibrosis.

Targeting ATP12A, a Nongastric Proton Pump α Subunit, for Idiopathic Pulmonary Fibrosis Treatment

Idiopathic pulmonary fibrosis (IPF) is a pathological condition of unknown etiology that results from injury to the lung and an ensuing fibrotic response that leads to the thickening of the alveolar walls and obliteration of the alveolar space. The pathogenesis is not clear, and there are currently no effective therapies for IPF. Small airway disease and mucus accumulation are prominent features in IPF lungs, similar to cystic fibrosis lung disease. The ATP12A gene encodes the α-subunit of the nongastric H+, K+-ATPase, which functions to acidify the airway surface fluid and impairs mucociliary transport function in patients with cystic fibrosis. It is hypothesized that the ATP12A protein may play a role in the pathogenesis of IPF. The authors' studies demonstrate that ATP12A protein is overexpressed in distal small airways from the lungs of patients with IPF compared with normal human lungs. In addition, overexpression of the ATP12A protein in mouse lungs worsened bleomycin induced experimental pulmonary fibrosis. This was prevented by a potassium competitive proton pump blocker, vonoprazan. These data support the concept that the ATP12A protein plays an important role in the pathogenesis of lung fibrosis. Inhibition of the ATP12A protein has potential as a novel therapeutic strategy in IPF treatment.

A Unique Cellular Organization of Human Distal Airways and Its Disarray in Chronic Obstructive Pulmonary Disease

Rationale: Remodeling and loss of distal conducting airways, including preterminal and terminal bronchioles (pre-TBs/TBs), underlie progressive airflow limitation in chronic obstructive pulmonary disease (COPD). The cellular basis of these structural changes remains unknown. Objectives: To identify biological changes in pre-TBs/TBs in COPD at single-cell resolution and determine their cellular origin. Methods: We established a novel method of distal airway dissection and performed single-cell transcriptomic profiling of 111,412 cells isolated from different airway regions of 12 healthy lung donors and pre-TBs of 5 patients with COPD. Imaging CyTOF and immunofluorescence analysis of pre-TBs/TBs from 24 healthy lung donors and 11 subjects with COPD were performed to characterize cellular phenotypes at a tissue level. Region-specific differentiation of basal cells isolated from proximal and distal airways was studied using an air-liquid interface model. Measurements and Main Results: The atlas of cellular heterogeneity along the proximal-distal axis of the human lung was assembled and identified region-specific cellular states, including SCGB3A2+ SFTPB+ terminal airway-enriched secretory cells (TASCs) unique to distal airways. TASCs were lost in COPD pre-TBs/TBs, paralleled by loss of region-specific endothelial capillary cells, increased frequency of CD8+ T cells normally enriched in proximal airways, and augmented IFN-γ signaling. Basal cells residing in pre-TBs/TBs were identified as a cellular origin of TASCs. Regeneration of TASCs by these progenitors was suppressed by IFN-γ. Conclusions: Altered maintenance of the unique cellular organization of pre-TBs/TBs, including loss of the region-specific epithelial differentiation in these bronchioles, represents the cellular manifestation and likely the cellular basis of distal airway remodeling in COPD.

An update in club cell biology and its potential relevance to chronic obstructive pulmonary disease

Club cells are found in human small airways where they play an important role in immune defense, xenobiotic metabolism, and repair after injury. Over the past few years, data from single-cell RNA sequencing (scRNA-seq) studies has generated new insights into club cell heterogeneity and function. In this review, we integrate findings from scRNA-seq experiments with earlier in vitro, in vivo, and microscopy studies and highlight the many ways club cells contribute to airway homeostasis. We then discuss evidence for loss of club cells or club cell products in the airways of patients with chronic obstructive pulmonary disease (COPD) and discuss potential mechanisms through which this might occur.

Secretoglobin 3A2 protects lung from developing cigarette smoke-induced pulmonary emphysema

Secretoglobin (SCGB) 3A2 is a bioactive molecule exhibiting various functions such as improving allergic airway inflammation and pulmonary fibrosis and promoting bronchial branching and proliferation during lung development. To determine if and how SCGB3A2 is involved in chronic obstructive pulmonary disease (COPD), a multifactorial disease with both airway and emphysematous lesions, a COPD mouse model was created by exposing Scgb3a2-deficient (KO), Scgb3a2-lung-specific overexpressing (TG), and wild type (WT) mice to cigarette smoke (CS) for 6 months. The KO mice showed loss of lung structure under control condition, and CS exposure resulted in more expansion of airspace and destruction of alveolar wall than WT mouse lungs. In contrast, TG mouse lungs showed no significant changes after CS exposure. SCGB3A2 increased the expression and phosphorylation of signal transducers and activators of transcription (STAT)1 and STAT3, and the expression of α1-antitrypsin (A1AT) in mouse lung fibroblast-derived MLg cells and mouse lung epithelial-derived MLE-15 cells. In MLg cells, A1AT expression was decreased in Stat3-knockdown cells, and increased upon Stat3 overexpression. STAT3 formed a homodimer when cells were stimulated with SCGB3A2. Chromatin immunoprecipitation and reporter assays demonstrated that STAT3 binds to specific binding sites on the Serpina1a gene encoding A1AT and upregulates its transcription in lung tissues of mice. Furthermore, nuclear localization of phosphorylated STAT3 upon SCGB3A2 stimulation was detected by immunocytochemistry. These findings demonstrate that SCGB3A2 protects the lungs from the development of CS-induced emphysema by regulating A1AT expression through STAT3 signaling.

Sex-biased molecular differences in lung adenocarcinoma are ethnic and smoking specific

BACKGROUND

Sex-related differences in cancer epidemiology, tumor biology, immune system activity, and pharmacogenomics have been suggested to be important considerations for precision cancer control. Here we elucidated systematically sex biases in genetic variants, gene expression profiles, and immunological landscapes of lung adenocarcinoma patients (LUADs) with different ancestry and smoking status.

METHODS

Somatic mutation and mRNA expression data of Asian and Non-Asian LUADs were obtained from public databases. Sex-biased genetic mutations, gene expression, biological pathways, and immune infiltration were identified in the context of smoking status and race.

RESULTS

Among nonsmokers, male-biased mutations were prevalent in Asian LUADs, while few sex-biased mutations were detected in Non-Asian LUADs. EGFR was the only mutation whose frequency was significantly higher in females than males in both Asian and Non-Asian nonsmokers. More genes exhibited sex-biased expression in Non-Asian LUADs compared to Asian LUADs. Moreover, genes distinctly expressed in females were mainly related to immune-related pathways, whereas those in males were more involved in activation of DNA repair, E2F_targets, and MYC_targets pathways. We also detected sex-specific immune infiltration in the context of genetic variation. In EGFR-mutant LUADs, males had a significantly increased infiltration of CD8 + T cells, whereas resting CD4 + memory T cells were more abundant in females. Additionally, in KRAS-mutant LUADs, CD8 + and CD4 + T cells were more abundant in females than males. In addition, we detected all female patients with high SCGB3A2 expression were exclusively sensitive to immunotherapy, while this phenomenon was not observed in male patients.

CONCLUSIONS

Our findings provided evidence that sex-related molecular and cellular components are involved in shaping tumor distinct genetic and immune features, which might have important impact on personalized targeted and immune therapy.

Patient-derived spheroids and patient-derived organoids simulate evolutions of lung cancer

Cancer cells harbor many genetic mutations and gene expression profiles different from normal cells. Patient-derived cancer cells (PDCC) are preferred materials in cancer study. We established patient-derived spheroids (PDSs) and patient-derived organoids (PDOs) from PDCCs isolated from the malignant pleural effusion in 8 patients. The morphologies suggested that PDSs may be a model of local cancer extensions, while PDOs may be a model of distant cancer metastases. The gene expression profiles differed between PDSs and PDOs: Gene sets related to inflammatory responses and EMT were antithetically regulated in PDSs or in PDOs. PDSs demonstrated an attenuation of the pathways that contribute to the enhancement of transforming growth factor beta (TGF-β) induced epithelial mesenchymal transition (EMT), while PDOs demonstrated an attenuation of it. Taken together, PDSs and PDOs have differences in both the interaction to the immune systems and to the stroma. PDSs and PDOs will provide a model system that enable intimate investigation of the behavior of cancer cells in the body.

A topographic atlas defines developmental origins of cell heterogeneity in the human embryonic lung

The lung contains numerous specialized cell types with distinct roles in tissue function and integrity. To clarify the origins and mechanisms generating cell heterogeneity, we created a comprehensive topographic atlas of early human lung development. Here we report 83 cell states and several spatially resolved developmental trajectories and predict cell interactions within defined tissue niches. We integrated single-cell RNA sequencing and spatially resolved transcriptomics into a web-based, open platform for interactive exploration. We show distinct gene expression programmes, accompanying sequential events of cell differentiation and maturation of the secretory and neuroendocrine cell types in proximal epithelium. We define the origin of airway fibroblasts associated with airway smooth muscle in bronchovascular bundles and describe a trajectory of Schwann cell progenitors to intrinsic parasympathetic neurons controlling bronchoconstriction. Our atlas provides a rich resource for further research and a reference for defining deviations from homeostatic and repair mechanisms leading to pulmonary diseases.

Enhancement of airway epithelial cell differentiation by pulmonary endothelial cell co-culture

Cross-talk between lung epithelial cells and their microenvironment has an important physiological role in development. Using an in vitro model of differentiation of human induced pluripotent stem cells (iPSCs) to air-liquid interface (ALI)-cultured lung epithelial cells, we investigated the contribution of the microenvironment to maintenance of the lung progenitor cell state. Our protocol modeled in vivo cell-to-matrix and cell-to-cell interactions. These included growth of iPSCs on inserts coated with different basement membrane proteins (collagen I, IV, fibronectin, heparan sulfate or Matrigel plus collagen IV) and co-culture with human pulmonary microvascular endothelial cells (HPMECs). Marker gene expression was measured by RT-qPCR and protein expression and localization was confirmed by immunocytochemistry. The results showed that iPSCs grown on collagen IV had the highest success rate in terms of differentiation to robust ALI-cultured lung epithelial cells, followed by fibronectin, collagen I and heparan sulfate. Coating with Matrigel mixed with collagen IV further increased the success rate to > 97 %. Co-culture of iPSCs with HPMECs enhanced the expression of key airway lineage markers (NKX2.1, KRT5, TP63, MUC5AC, MUC16, FOXJ1, CFTR and SCGB1A1) during ALI culture. Cross-talk between iPSCs and their microenvironment during cell differentiation had a significant effect on lung epithelial cell differentiation in these 3D in vitro models. Both matrix proteins and endothelial cells play critical roles in the differentiation of lung progenitor cells.

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.

Secretory Cells Are the Primary Source of pIgR in Small Airways

Loss of secretory IgA (SIgA) is common in chronic obstructive pulmonary disease (COPD) small airways and likely contributes to disease progression. We hypothesized that loss of SIgA results from reduced expression of pIgR (polymeric immunoglobulin receptor), a chaperone protein needed for SIgA transcytosis, in the COPD small airway epithelium. pIgR-expressing cells were defined and quantified at single-cell resolution in human airways using RNA in situ hybridization, immunostaining, and single-cell RNA sequencing. Complementary studies in mice used immunostaining, primary murine tracheal epithelial cell culture, and transgenic mice with secretory or ciliated cell-specific knockout of pIgR. SIgA degradation by human neutrophil elastase or secreted bacterial proteases from nontypeable Haemophilus influenzae was evaluated in vitro. We found that secretory cells are the predominant cell type responsible for pIgR expression in human and murine airways. Loss of SIgA in small airways was not associated with a reduction in secretory cells but rather a reduction in pIgR protein expression despite intact PIGR mRNA expression. Neutrophil elastase and nontypeable H. influenzae-secreted proteases are both capable of degrading SIgA in vitro and may also contribute to a deficient SIgA immunobarrier in COPD. Loss of the SIgA immunobarrier in small airways of patients with severe COPD is complex and likely results from both pIgR-dependent defects in IgA transcytosis and SIgA degradation.

Utility of bile acids in large airway bronchial wash versus bronchoalveolar lavage as biomarkers of microaspiration in lung transplant recipients: a retrospective cohort study

Background

Bronchoalveolar lavage (BAL) is a key tool in respiratory medicine for sampling the distal airways. BAL bile acids are putative biomarkers of pulmonary microaspiration, which is associated with poor outcomes after lung transplantation. Compared to BAL, large airway bronchial wash (LABW) samples the tracheobronchial space where bile acids may be measurable at more clinically relevant levels. We assessed whether LABW bile acids, compared to BAL bile acids, are more strongly associated with poor clinical outcomes in lung transplant recipients.

Methods

Concurrently obtained BAL and LABW at 3 months post-transplant from a retrospective cohort of 61 lung transplant recipients were analyzed for taurocholic acid (TCA), glycocholic acid (GCA), and cholic acid by mass spectrometry and 10 inflammatory proteins by multiplex immunoassay. Associations between bile acids with inflammatory proteins and acute lung allograft dysfunction were assessed using Spearman correlation and logistic regression, respectively. Time to chronic lung allograft dysfunction and death were evaluated using multivariable Cox proportional hazards and Kaplan–Meier methods.

Results

Most bile acids and inflammatory proteins were higher in LABW than in BAL. LABW bile acids correlated with inflammatory proteins within and between sample type. LABW TCA and GCA were associated with acute lung allograft dysfunction (OR = 1.368; 95%CI = 1.036–1.806; P = 0.027, OR = 1.064; 95%CI = 1.009–1.122; P = 0.022, respectively). No bile acids were associated with chronic lung allograft dysfunction. Adjusted for risk factors, LABW TCA and GCA predicted death (HR = 1.513; 95%CI = 1.014–2.256; P = 0.042, HR = 1.597; 95%CI = 1.078–2.366; P = 0.020, respectively). Patients with LABW TCA in the highest tertile had worse survival compared to all others.

Conclusions

LABW bile acids are more strongly associated than BAL bile acids with inflammation, acute lung allograft dysfunction, and death in lung transplant recipients. Collection of LABW may be useful in the evaluation of microaspiration in lung transplantation and other respiratory diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-022-02131-5.

Promises and Challenges of Cell-Based Therapies to Promote Lung Regeneration in Idiopathic Pulmonary Fibrosis

The lung epithelium is constantly exposed to harmful agents present in the air that we breathe making it highly susceptible to damage. However, in instances of injury to the lung, it exhibits a remarkable capacity to regenerate injured tissue thanks to the presence of distinct stem and progenitor cell populations along the airway and alveolar epithelium. Mechanisms of repair are affected in chronic lung diseases such as idiopathic pulmonary fibrosis (IPF), a progressive life-threatening disorder characterized by the loss of alveolar structures, wherein excessive deposition of extracellular matrix components cause the distortion of tissue architecture that limits lung function and impairs tissue repair. Here, we review the most recent findings of a study of epithelial cells with progenitor behavior that contribute to tissue repair as well as the mechanisms involved in mouse and human lung regeneration. In addition, we describe therapeutic strategies to promote or induce lung regeneration and the cell-based strategies tested in clinical trials for the treatment of IPF. Finally, we discuss the challenges, concerns and limitations of applying these therapies of cell transplantation in IPF patients. Further research is still required to develop successful strategies focused on cell-based therapies to promote lung regeneration to restore lung architecture and function.

The Proteomic Profile of Interstitial Lung Abnormalities

Rationale: Knowledge on biomarkers of interstitial lung disease is incomplete. Interstitial lung abnormalities (ILAs) are radiologic changes that may present in its early stages. Objectives: To uncover blood proteins associated with ILAs using large-scale proteomics methods. Methods: Data from two prospective cohort studies, the AGES-Reykjavik (Age, Gene/Environment Susceptibility-Reykjavik) study (N = 5,259) for biomarker discovery and the COPDGene (Genetic Epidemiology of COPD) study (N = 4,899) for replication, were used. Blood proteins were measured using DNA aptamers, targeting more than 4,700 protein analytes. The association of proteins with ILAs and ILA progression was assessed with regression modeling, as were associations with genetic risk factors. Adaptive Least Absolute Shrinkage and Selection Operator models were applied to bootstrap data samples to discover sets of proteins predictive of ILAs and their progression. Measurements and Main Results: Of 287 associations, SFTPB (surfactant protein B) (odds ratio [OR], 3.71 [95% confidence interval (CI), 3.20-4.30]; P = 4.28 × 10^-67), SCGB3A1 (Secretoglobin family 3A member 1) (OR, 2.43 [95% CI, 2.13-2.77]; P = 8.01 × 10^-40), and WFDC2 (WAP four-disulfide core domain protein 2) (OR, 2.42 [95% CI, 2.11-2.78]; P = 4.01 × 10^-36) were most significantly associated with ILA in AGES-Reykjavik and were replicated in COPDGene. In AGES-Reykjavik, concentrations of SFTPB were associated with the rs35705950 MUC5B (mucin 5B) promoter polymorphism, and SFTPB and WFDC2 had the strongest associations with ILA progression. Multivariate models of ILAs in AGES-Reykjavik, ILAs in COPDGene, and ILA progression in AGES-Reykjavik had validated areas under the receiver operating characteristic curve of 0.880, 0.826, and 0.824, respectively. Conclusions: Novel, replicated associations of ILA, its progression, and genetic risk factors with numerous blood proteins are demonstrated as well as machine-learning-based models with favorable predictive potential. Several proteins are revealed as potential markers of early fibrotic lung disease.

Emerging role of an immunomodulatory protein secretoglobin 3A2 in human diseases

Secretoglobin (SCGB) 3A2 was first identified in 2001 as a protein exhibiting similarities in amino acid sequence and gene structure to SCGB1A1, a multi-functional cytokine-like molecule highly expressed in airway epithelial Club cells that was the first identified and extensively studied member of the SCGB gene superfamily. SCGB3A2 is a small secretory protein of ~10 kDa that forms a dimer and a tetramer. SCGB3A2 is predominantly expressed in airway epithelial Club cells, and has anti-inflammatory, growth factor, anti-fibrotic, and anti-cancer activities that influence various lung diseases. This review summarizes the current understanding of SCGB3A2 biological functions and its role in human diseases with emphasis on its mechanisms of actions and signaling pathway.

Effects of obesity on CC16 and their potential role in overweight/obese asthma

Introduction

Club cell secretory protein-16 (CC16) is a major anti-inflammatory protein expressed in the airway; however, the potential role of CC16 on overweight/obese asthma has not been assessed. In this study, we examined whether obesity reduces airway/circulatory CC16 levels using experimental and epidemiological studies. Then, we explored the mediatory role of CC16 in the relationship of overweight/obesity with clinical asthma measures.

Methods

Circulating CC16 levels were assessed by ELISA in three independent human populations, including two groups of healthy and general populations and asthma patients. The percentage of cells expressing club markers in obese vs. non-obese mice and human airways was determined by immunohistochemistry. A causal mediation analysis was conducted to determine whether circulatory CC16 acted as a mediator between overweight/obesity and clinical asthma measures.

Results

BMI was significantly and monotonously associated with reduced circulating CC16 levels in all populations. The percentage of CC16-expressing cells was reduced in the small airways of both mice and humans with obesity. Finally, mediation analysis revealed significant contributions of circulatory CC16 in the association between BMI and clinical asthma measures; 21.8% of its total effect in BMI’s association with airway hyperresponsiveness of healthy subjects (p = 0.09), 26.4% with asthma severity (p = 0.030), and 23% with the required dose of inhaled corticosteroid (p = 0.042). In logistic regression analysis, 1-SD decrease in serum CC16 levels of asthma patients was associated with 87% increased odds for high dose ICS requirement (p < 0.001).

Conclusions

We demonstrate that airway/circulating CC16, which is inversely associated with BMI, may mediate development and severity in overweight/obese asthma.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-022-02038-1.

Uterus globulin associated protein 1 (UGRP1) binds podoplanin (PDPN) to promote a novel inflammation pathway during Streptococcus pneumoniae infection

Background

Streptococcus pneumoniae is the major cause of life‐threatening infections. Toll‐like receptors (TLRs) and NOD‐like receptors (NLRs) could recognise S. pneumoniae and regulate the production of pro‐inflammatory cytokines. UGRP1, highly expressed in lung, is predominantly secreted in airways. However, the function of UGRP1 in pneumonia is mainly unknown.

Methods and results

We showed that upon TLR2/TLR4/NOD2 agonists stimulation or S. pneumoniae infection, treatment with UGRP1 could promote phosphorylation of p65 and enhance IL‐6, IL‐1β and TNFα production in macrophages. We further elucidated that after binding with cell‐surface receptor PDPN, UGRP1 could activate RhoA to enhance interaction of IKKγ and IKKβ, which slightly activated NF‐κB to improve expression of TLR2, MyD88, NOD2 and NLRP3. Deletion of UGRP1 or blocking UGRP1 interaction with PDPN protected mice against S. pneumoniae‐induced severe pneumococcal pneumonia, and activating RhoA with agonist in UGRP1‐deficient mice restored the reduced IL‐6 production.

Conclusion

We demonstrated that UGRP1–PDPN–RhoA signaling could activate NF‐κB to promote expression of TLR2, MyD88, NOD2 and NLRP3, which enhanced inflammatory cytokines secretion during S. pneumoniae infection. Antibodies, which could interrupt interaction of UGRP1 and PDPN, are potential therapeutics against S. pneumoniae.

Critical changes in hypothalamic gene networks in response to pancreatic cancer as found by single-cell RNA sequencing

OBJECTIVE

Cancer cachexia is a devastating chronic condition characterized by involuntary weight loss, muscle wasting, abnormal fat metabolism, anorexia, and fatigue. However, the molecular mechanisms underlying this syndrome remain poorly understood. In particular, the hypothalamus may play a central role in cachexia, given that it has direct access to peripheral signals because of its anatomical location and attenuated blood-brain barrier. Furthermore, this region has a critical role in regulating appetite and metabolism.

METHODS

To provide a detailed analysis of the hypothalamic response to cachexia, we performed single-cell RNA-seq combined with RNA-seq of the medial basal hypothalamus (MBH) in a mouse model for pancreatic cancer.

RESULTS

We found many cell type-specific changes, such as inflamed endothelial cells, stressed oligodendrocyes and both inflammatory and moderating microglia. Lcn2, a newly discovered hunger suppressing hormone, was the highest induced gene. Interestingly, cerebral treatment with LCN2 not only induced many of the observed molecular changes in cachexia but also affected gene expression in food-intake decreasing POMC neurons. In addition, we found that many of the cachexia-induced molecular changes found in the hypothalamus mimic those at the primary tumor site.

CONCLUSION

Our data reveal that multiple cell types in the MBH are affected by tumor-derived factors or host factors that are induced by tumor growth, leading to a marked change in the microenvironment of neurons critical for behavioral, metabolic, and neuroendocrine outputs dysregulated during cachexia. The mechanistic insights provided in this study explain many of the clinical features of cachexia and will be useful for future therapeutic development.

Ataxia telangiectasia mutated is required for efficient proximal airway epithelial cell regeneration following influenza A virus infection

Children and young adults with mutant forms of ataxia telangiectasia mutated (ATM), a kinase involved in DNA damage signaling and mitochondrial homeostasis, suffer from recurrent respiratory infections, immune deficiencies, and obstructive airways disease associated with disorganized airway epithelium. We previously showed in mice how Atm was required to mount a protective immune memory response to influenza A virus [IAV; Hong Kong/X31 (HKx31), H3N2]. Here, Atm wildtype (WT) and knockout (Atm-null) mice were used to investigate how Atm is required to regenerate the injured airway epithelium following IAV infection. When compared with WT mice, naive Atm-null mice had increased airway resistance and reduced lung compliance that worsened during infection before returning to naïve levels by 56 days postinfection (dpi). Although Atm-null lungs appeared pathologically normal before infection by histology, they developed an abnormal proximal airway epithelium after infection that contained E-cadherin+, Sox2+, and Cyp2f2+ cells lacking secretoglobin family 1 A member 1 (Scgb1a1) protein expression. Patchy and low expression of Scgb1a1 were eventually observed by 56 dpi. Genetic lineage tracing in HKx31-infected mice revealed club cells require Atm to rapidly and efficiently restore Scgb1a1 expression in proximal airways. Since Scgb1a1 is an immunomodulatory protein that protects the lung against a multitude of respiratory challenges, failure to efficiently restore its expression may contribute to the respiratory diseases seen in individuals with ataxia telangiectasia.

An essential function for autocrine hedgehog signaling in epithelial proliferation and differentiation in the trachea

The tracheal epithelium is a primary target for pulmonary diseases as it provides a conduit for air flow between the environment and the lung lobes. The cellular and molecular mechanisms underlying airway epithelial cell proliferation and differentiation remain poorly understood. Hedgehog (HH) signaling orchestrates communication between epithelial and mesenchymal cells in the lung, where it modulates stromal cell proliferation, differentiation and signaling back to the epithelium. Here, we reveal a previously unreported autocrine function of HH signaling in airway epithelial cells. Epithelial cell depletion of the ligand sonic hedgehog (SHH) or its effector smoothened (SMO) causes defects in both epithelial cell proliferation and differentiation. In cultured primary human airway epithelial cells, HH signaling inhibition also hampers cell proliferation and differentiation. Epithelial HH function is mediated, at least in part, through transcriptional activation, as HH signaling inhibition leads to downregulation of cell type-specific transcription factor genes in both the mouse trachea and human airway epithelial cells. These results provide new insights into the role of HH signaling in epithelial cell proliferation and differentiation during airway development.

Summary: A conserved autocrine role for HH signaling in tracheal epithelial cell proliferation and differentiation is revealed, suggesting potential new interventions for airway epithelial proliferation and differentiation defects.

Identification of a Gene Prognostic Model of Gastric Cancer Based on Analysis of Tumor Mutation Burden

Introduction: Gastric cancer is one of the most common cancers. Although some progress has been made in the treatment of gastric cancer with the improvement of surgical methods and the application of immunotherapy, the prognosis of gastric cancer patients is still unsatisfactory. In recent years, there has been increasing evidence that tumor mutational load (TMB) is strongly associated with survival outcomes and response to immunotherapy. Given the variable response of patients to immunotherapy, it is important to investigate clinical significance of TMB and explore appropriate biomarkers of prognosis in patients with gastric cancer (GC).

Material and Methods: All data of patients with gastric cancer were obtained from the database of The Cancer Genome Atlas (TCGA). Samples were divided into two groups based on median TMB. Differently expressed genes (DEGs) between the high- and low-TMB groups were identified and further analyzed. We identified TMB-related genes using Lasso, univariate and multivariate Cox regression analysis and validated the survival result of 11 hub genes using Kaplan-Meier Plotter. In addition, “CIBERSORT” package was utilized to estimate the immune infiltration.

Results: Single nucleotide polymorphism (SNP), C > T transition were the most common variant type and single nucleotide variant (SNV), respectively. Patients in the high-TMB group had better survival outcomes than those in the low-TMB group. Besides, eleven TMB-related DEGs were utilized to construct a prognostic model that could be an independent risk factor to predict the prognosis of patients with GC. What’s more, the infiltration levels of CD4⁺ memory-activated T cells, M0 and M1 macrophages were significantly increased in the high-TMB group compared with the low-TMB group.

Conclusions: Herein, we found that patients with high TMB had better survival outcomes in GC. In addition, higher TMB might promote immune infiltration, which could provide new ideas for immunotherapy.

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.

Identification of Mucus-Associated Molecular Subtypes of Chronic Obstructive Pulmonary Disease: A Latent Profile Analysis Based on MUC5B-Associated Genes

BACKGROUND Chronic obstructive pulmonary disease (COPD) is a disease with high heterogeneity, which is a major challenge in clinical individualized treatment. A mucus phenotype is one of the main characteristics of COPD. MATERIAL AND METHODS Gene expression profiles of lung tissue samples were from the Lung Genomics Research Consortium. MUC5B-associated gene signatures were obtained based on a nonlinear feature screening algorithm. These signatures were used to fit a latent profile analysis (LPA) model to identify COPD molecular subtypes and build a subtype classifier to verify the subtypes. Then, we explored the characteristics of cilium assembly and beating signatures, transcriptome features, immune infiltration among the 3 subtypes by xCell, single-sample gene set enrichment analysis, network perturbation amplitude, and weighted gene co-expression network analysis algorithms. An external dataset was used to verify the above COPD subtypes. RESULTS Three subtypes associated with mucus were identified by LPA and verified in an external dataset. Subtype 1 displayed higher T helper type 1 (Th1) and basophil infiltration, higher Th17/regulatory T cells (Tregs) ratio, a higher level of cilium assembly and beating, and lower mast cell and Treg infiltration. The subtypes 2 and 3 demonstrated higher macrophage M2 infiltration in lung tissue, while subtype 3 had higher neutrophil and eosinophil infiltration than subtype 2. CONCLUSIONS Overall, this work identified 3 mucus-associated molecular subtypes related to MUC5B expression, which deepens the understanding of airway mucus secretion in COPD and potentially provides valuable information for precision therapy.

Cigarette Smoke Specifically Affects Small Airway Epithelial Cell Populations and Triggers the Expansion of Inflammatory and Squamous Differentiation Associated Basal Cells

Smoking is a major risk factor for chronic obstructive pulmonary disease (COPD) and causes remodeling of the small airways. However, the exact smoke-induced effects on the different types of small airway epithelial cells (SAECs) are poorly understood. Here, using air–liquid interface (ALI) cultures, single-cell RNA-sequencing reveals previously unrecognized transcriptional heterogeneity within the small airway epithelium and cell type-specific effects upon acute and chronic cigarette smoke exposure. Smoke triggers detoxification and inflammatory responses and aberrantly activates and alters basal cell differentiation. This results in an increase of inflammatory basal-to-secretory cell intermediates and, particularly after chronic smoke exposure, a massive expansion of a rare inflammatory and squamous metaplasia associated KRT6A⁺ basal cell state and an altered secretory cell landscape. ALI cultures originating from healthy non-smokers and COPD smokers show similar responses to cigarette smoke exposure, although an increased pro-inflammatory profile is conserved in the latter. Taken together, the in vitro models provide high-resolution insights into the smoke-induced remodeling of the small airways resembling the pathological processes in COPD airways. The data may also help to better understand other lung diseases including COVID-19, as the data reflect the smoke-dependent variable induction of SARS-CoV-2 entry factors across SAEC populations.

Aggressive early-stage lung adenocarcinoma is characterized by epithelial cell plasticity with acquirement of stem-like traits and immune evasion phenotype

Lung adenocarcinoma (LUAD) is the main non-small-cell lung cancer diagnosed in ~40-50% of all lung cancer cases. Despite the improvements in early detection and personalized medicine, even a sizable fraction of patients with early-stage LUAD would experience disease relapses and adverse prognosis. Previous reports indicated the existence of LUAD molecular subtypes characterized by specific gene expression and mutational profiles, and correlating with prognosis. However, the biological and molecular features of such subtypes have not been further explored. Consequently, the mechanisms driving the emergence of aggressive LUAD remained unclear. Here, we adopted a multi-tiered approach ranging from molecular to functional characterization of LUAD and used it on multiple cohorts of patients (for a total of 1227 patients) and LUAD cell lines. We investigated the tumor transcriptome and the mutational and immune gene expression profiles, and we used LUAD cell lines for cancer cell phenotypic screening. We found that loss of lung cell lineage and gain of stem cell-like characteristics, along with mutator and immune evasion phenotypes, explain the aggressive behavior of a specific subset of lung adenocarcinoma that we called C1-LUAD, including early-stage disease. This subset can be identified using a 10-gene prognostic signature. Poor prognosis patients appear to have this specific molecular lung adenocarcinoma subtype which is characterized by peculiar molecular and biological features. Our data support the hypothesis that transformed lung stem/progenitor cells and/or reprogrammed epithelial cells with CSC characteristics are hallmarks of this aggressive disease. Such discoveries suggest alternative, more aggressive, therapeutic strategies for early-stage C1-LUAD.

Inhibition of Gabrp reduces the differentiation of airway epithelial progenitor cells into goblet cells

Bronchial asthma is an intractable pulmonary disease that affects millions of individuals worldwide, with the overproduction of mucus contributing to high morbidity and mortality. Gamma-aminobutyric acid (GABA) is associated with goblet cell hyperplasia in the lungs of primate models and Club cells serve as airway epithelial progenitor cells that may differentiate into goblet and ciliated cells. In the present study, it was investigated whether the GABAA receptor pi (Gabrp) is essential for Club cell proliferation and differentiation in mice. Validation of microarray analysis results by reverse transcription-quantitative PCR (RT-qPCR) demonstrated that Gabrp is highly expressed in mouse Club cells. Predominant expression of Gabrp in mouse Club cells was further confirmed based on naphthalene-induced Club cell injury in mice, with organoid cultures indicating significant reductions in the organoid-forming ability of mouse Club cells in the presence of Gabrp antagonist bicuculline methiodide (BMI). Furthermore, the RT-qPCR results indicated that the mRNA levels of chloride channel accessory 3, pseudogene (Clca3p), mucin (Muc)5Ac and Muc5B were significantly decreased in BMI organoid cultures. These results suggested that blocking GABA signaling through Gabrp inhibits mouse Club cell proliferation, as well as differentiation into goblet cells. Therefore, targeting GABA/Gabrp signaling may represent a promising strategy for treating goblet cell hyperplasia in bronchial asthma.

Epithelial Stem and Progenitor Cells in Lung Repair and Regeneration

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

Stem cells in pulmonary alveolar regeneration

The lungs are constantly exposed to the external environment and are therefore vulnerable to insults that can cause infection and injury. Maintaining the integrity and barrier function of the lung epithelium requires complex interactions of multiple cell lineages. Elucidating the cellular players and their regulation mechanisms provides fundamental information to deepen understanding about the responses and contributions of lung stem cells. This Review focuses on advances in our understanding of mammalian alveolar epithelial stem cell subpopulations and discusses insights about the regeneration-specific cell status of alveolar epithelial stem cells. We also consider how these advances can inform our understanding of post-injury lung repair processes and lung diseases.

Derivation of Airway Basal Stem Cells from Human Pluripotent Stem Cells

The derivation of tissue-specific stem cells from human induced pluripotent stem cells (iPSCs) would have broad reaching implications for regenerative medicine. Here, we report the directed differentiation of human iPSCs into airway basal cells ("iBCs"), a population resembling the stem cell of the airway epithelium. Using a dual fluorescent reporter system (NKX2-1GFP;TP63tdTomato), we track and purify these cells as they first emerge as developmentally immature NKX2-1GFP+ lung progenitors and subsequently augment a TP63 program during proximal airway epithelial patterning. In response to primary basal cell medium, NKX2-1GFP+/TP63tdTomato+ cells display the molecular and functional phenotype of airway basal cells, including the capacity to self-renew or undergo multi-lineage differentiation in vitro and in tracheal xenografts in vivo. iBCs and their differentiated progeny model perturbations that characterize acquired and genetic airway diseases, including the mucus metaplasia of asthma, chloride channel dysfunction of cystic fibrosis, and ciliary defects of primary ciliary dyskinesia.

Single-cell RNA sequencing reveals heterogeneous tumor and immune cell populations in early-stage lung adenocarcinomas harboring EGFR mutations

Lung adenocarcinoma (LUAD) harboring EGFR mutations prevails in Asian population. However, the inter-patient and intra-tumor heterogeneity has not been addressed at single-cell resolution. Here we performed single-cell RNA sequencing (scRNA-seq) of total 125,674 cells from seven stage-I/II LUAD samples harboring EGFR mutations and five tumor-adjacent lung tissues. We identified diverse cell types within the tumor microenvironment (TME) in which myeloid cells and T cells were the most abundant stromal cell types in tumors and adjacent lung tissues. Within tumors, accompanied by an increase in CD1C+ dendritic cells, the tumor-associated macrophages (TAMs) showed pro-tumoral functions without signature gene expression of defined M1 or M2 polarization. Tumor-infiltrating T cells mainly displayed exhausted and regulatory T-cell features. The adenocarcinoma cells can be categorized into different subtypes based on their gene expression signatures in distinct pathways such as hypoxia, glycolysis, cell metabolism, translation initiation, cell cycle, and antigen presentation. By performing pseudotime trajectory, we found that ELF3 was among the most upregulated genes in more advanced tumor cells. In response to secretion of inflammatory cytokines (e.g., IL1B) from immune infiltrates, ELF3 in tumor cells was upregulated to trigger the activation of PI3K/Akt/NF-κB pathway and elevated expression of proliferation and anti-apoptosis genes such as BCL2L1 and CCND1. Taken together, our study revealed substantial heterogeneity within early-stage LUAD harboring EGFR mutations, implicating complex interactions among tumor cells, stromal cells and immune infiltrates in the TME.

Single-cell transcriptional profiling identifies a cluster of potential metastasis-associated UBE2C+ cells in immature ovarian teratoma

To dissect the disease heterogeneity and identify the underlying cellular and molecular events related to metastasis of immature ovarian teratoma in children, single-cell RNA sequencing was performed for a 2-year-old patient with liver metastases from immature ovarian teratoma. A total of 5976 cells were obtained for further analysis, with a median unique molecular identifier count of 6011 per cell and a median number of 1741 genes detected per cell. Fourteen clusters were recognized, with the main lineages comprising epithelial cells, macrophages, fibroblasts, glial cells, and dendritic cells. Ten subclusters of epithelial cells were further defined, originating from the urinary tract, esophagus, bronchus, lung, skin, and gastrointestinal tract. An undefined UBE2C + population in an active state of proliferation was also identified and its biological processes were related to meiosis and maturation of oocytes. Pseudotime analysis revealed different distributions of epithelial cells in the development trajectory. In conclusion, a cluster of UBE2C + epithelial cells in an active state of proliferation was identified in an immature ovarian teratoma in a child, and may contribute to metastasis by regulating epithelial-mesenchymal transition. These findings help toward understanding the origin of the malignant behaviors, offer a potential biomarker for early determination of the tumor nature, and provide new ideas for the therapy of immature ovarian teratoma in children.

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.

Muc5b-deficient mice develop early histological lung abnormalities

Gel-forming mucins are the main organic component responsible for physical properties of the mucus hydrogels. While numerous biological functions of these mucins are well documented, specific physiological functions of each mucin are largely unknown. To investigate in vivo functions of the gel-forming mucin Muc5b, which is one of the major secreted airway mucins, along with Muc5ac, we generated mice in which Muc5b was disrupted and maintained in the absence of environmental stress. Adult Muc5b-deficient mice displayed bronchial hyperplasia and metaplasia, interstitial thickening, alveolar collapse, immune cell infiltrates, fragmented and disorganized elastin fibers and collagen deposits that were, for approximately one-fifth of the mice, associated with altered pulmonary function leading to respiratory failure. These lung abnormalities start early in life, as demonstrated in one-quarter of 2-day-old Muc5b-deficient pups. Thus, the mouse mucin Muc5b is essential for maintaining normal lung function.

Novel dynamics of human mucociliary differentiation revealed by single-cell RNA sequencing of nasal epithelial cultures

The upper airway epithelium, which is mainly composed of multiciliated, goblet, club and basal cells, ensures proper mucociliary function and can regenerate in response to assaults. In chronic airway diseases, defective repair leads to tissue remodeling. Delineating key drivers of differentiation dynamics can help understand how normal or pathological regeneration occurs. Using single-cell transcriptomics and lineage inference, we have unraveled trajectories from basal to luminal cells, providing novel markers for specific populations. We report that: (1) a precursor subgroup of multiciliated cells, which we have entitled deuterosomal cells, is defined by specific markers, such as DEUP1, FOXN4, YPEL1, HES6 and CDC20B; (2) goblet cells can be precursors of multiciliated cells, thus explaining the presence of hybrid cells that co-express markers of goblet and multiciliated cells; and (3) a repertoire of molecules involved in the regeneration process, such as keratins or components of the Notch, Wnt or BMP/TGFβ pathways, can be identified. Confirmation of our results on fresh human and pig airway samples, and on mouse tracheal cells, extend and confirm our conclusions regarding the molecular and cellular choreography at work during mucociliary epithelial differentiation.

The airway epithelium in asthma

Asthma is a genetically and phenotypically complex disease that has a major impact on global health. Signs and symptoms of asthma are caused by the obstruction of airflow through the airways. The epithelium that lines the airways plays a major role in maintaining airway patency and in host defense. The epithelium initiates responses to inhaled or aspirated substances, including allergens, viruses, and bacteria, and epithelial-derived cytokines are important in the recruitment and activation of immune cells in the airway. Changes in the structure and function of the airway epithelium are a prominent feature of asthma. Approximately half of individuals with asthma have evidence of active type 2 immune responses in the airway. In these individuals, epithelial cytokines promote type 2 responses, and responses to type 2 cytokines result in increased epithelial mucus production and other effects that cause airway obstruction. Recent work also implicates other epithelial responses, including interleukin-17, interferon and ER stress responses, that may contribute to asthma pathogenesis and provide new targets for therapy.

Endogenous lung stem cells for lung regeneration

INTRODUCTION

Lifelong maintenance of a healthy lung requires resident stem cells to proliferate according to tissue requirements. Once thought to be a quiescent tissue, evolving views of the complex differentiation landscape of lung stem and progenitor cells have broad implications for our understanding of how the lung is maintained, as well as the development of new therapies for promoting endogenous regeneration in lung disease.

AREAS COVERED

This review collates a large body of research relating to the hierarchical organization of epithelial stem cells in the adult lung and their role in tissue homeostasis and regeneration after injury. To identify relevant studies, PubMed was queried using one or a combination of the terms 'lung', 'airway', 'alveoli', 'stem cells', 'progenitor', 'repair' and 'regeneration'.

EXPERT OPINION

This review discusses how new technologies and injury models have challenged the demarcations between stem and progenitor cell populations.

Spatiotemporal Expression of Three Secretoglobin Proteins, SCGB1A1, SCGB3A1, and SCGB3A2, in Mouse Airway Epithelia

Secretoglobins (SCGBs) are cytokine-like small molecular weight secreted proteins with largely unknown biological functions. Three SCGB proteins, SCGB1A1, SCGB3A1, and SCGB3A2, are predominantly expressed in lung airways. To gain insight into the possible functional relationships among the SCGBs, their protein and mRNA expression patterns were examined in lungs during gestation and in adult mice, using Scgb3a1-null and Scgb3a2-null mice as negative controls, by immunohistochemistry and by qRT-PCR analysis, respectively. The three SCGBs exhibited unique spatiotemporal expression patterns during embryogenesis. The lack of Scgb3a1 or Scgb3a2 did not affect expression of the other Scgb genes as determined by mRNA measurements. Moreover, the lack of Scgb3a1 or Scgb3a2 did not affect development of the pulmonary neuroepithelial bodies during embryogenesis, while the lack of Scgb3a2 may have resulted in slightly fewer ciliated cells than in the wild-type. These results suggest that SCGB1A1, SCGB3A1, and SCGB3A2 each may possess its own unique biological function.

Human lung development: recent progress and new challenges

Recent studies have revealed biologically significant differences between human and mouse lung development, and have reported new in vitro systems that allow experimental manipulation of human lung models. At the same time, emerging clinical data suggest that the origins of some adult lung diseases are found in embryonic development and childhood. The convergence of these research themes has fuelled a resurgence of interest in human lung developmental biology. In this Review, we discuss our current understanding of human lung development, which has been profoundly influenced by studies in mice and, more recently, by experiments using in vitro human lung developmental models and RNA sequencing of human foetal lung tissue. Together, these approaches are helping to shed light on the mechanisms underlying human lung development and disease, and may help pave the way for new therapies.

Summary: This Review describes how recent technological advances have shed light on the mechanisms underlying human lung development and disease, and outlines the future challenges in this field.

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

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

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

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

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

Another cat and mouse game: Deciphering the evolution of the SCGB superfamily and exploring the molecular similarity of major cat allergen Fel d 1 and mouse ABP using computational approaches

The mammalian secretoglobin (SCGB) superfamily contains functionally diverse members, among which the major cat allergen Fel d 1 and mouse salivary androgen-binding protein (ABP) display similar subunits. We searched for molecular similarities between Fel d 1 and ABP to examine the possibility that they play similar roles. We aimed to i) cluster the evolutionary relationships of the SCGB superfamily; ii) identify divergence patterns, structural overlap, and protein-protein docking between Fel d 1 and ABP dimers; and iii) explore the residual interaction between ABP dimers and steroid binding in chemical communication using computational approaches. We also report that the evolutionary tree of the SCGB superfamily comprises seven unique palm-like clusters, showing the evolutionary pattern and divergence time tree of Fel d 1 with 28 ABP paralogs. Three ABP subunits (A27, BG27, and BG26) share phylogenetic relationships with Fel d 1 chains. The Fel d 1 and ABP subunits show similarities in terms of sequence conservation, identical motifs and binding site clefts. Topologically equivalent positions were visualized through superimposition of ABP A27:BG27 (AB) and ABP A27:BG26 (AG) dimers on a heterodimeric Fel d 1 model. In docking, Fel d 1-ABP dimers exhibit the maximum surface binding ability of AG compared with that of AB dimers and the several polar interactions between ABP dimers with steroids. Hence, cat Fel d 1 is an ABP-like molecule in which monomeric chains 1 and 2 are the equivalent of the ABPA and ABPBG monomers, respectively. These findings suggest that the biological and molecular function of Fel d 1 is similar to that of ABP in chemical communication, possibly via pheromone and/or steroid binding.

Another cat and mouse game: Deciphering the evolution of the SCGB superfamily and exploring the molecular similarity of major cat allergen Fel d 1 and mouse ABP using computational approaches

The mammalian secretoglobin (SCGB) superfamily contains functionally diverse members, among which the major cat allergen Fel d 1 and mouse salivary androgen-binding protein (ABP) display similar subunits. We searched for molecular similarities between Fel d 1 and ABP to examine the possibility that they play similar roles. We aimed to i) cluster the evolutionary relationships of the SCGB superfamily; ii) identify divergence patterns, structural overlap, and protein-protein docking between Fel d 1 and ABP dimers; and iii) explore the residual interaction between ABP dimers and steroid binding in chemical communication using computational approaches. We also report that the evolutionary tree of the SCGB superfamily comprises seven unique palm-like clusters, showing the evolutionary pattern and divergence time tree of Fel d 1 with 28 ABP paralogs. Three ABP subunits (A27, BG27, and BG26) share phylogenetic relationships with Fel d 1 chains. The Fel d 1 and ABP subunits show similarities in terms of sequence conservation, identical motifs and binding site clefts. Topologically equivalent positions were visualized through superimposition of ABP A27:BG27 (AB) and ABP A27:BG26 (AG) dimers on a heterodimeric Fel d 1 model. In docking, Fel d 1-ABP dimers exhibit the maximum surface binding ability of AG compared with that of AB dimers and the several polar interactions between ABP dimers with steroids. Hence, cat Fel d 1 is an ABP-like molecule in which monomeric chains 1 and 2 are the equivalent of the ABPA and ABPBG monomers, respectively. These findings suggest that the biological and molecular function of Fel d 1 is similar to that of ABP in chemical communication, possibly via pheromone and/or steroid binding.

Single-Cell Transcriptomic Profiling of Pluripotent Stem Cell-Derived SCGB3A2+ Airway Epithelium

Lung epithelial lineages have been difficult to maintain in pure form in vitro, and lineage-specific reporters have proven invaluable for monitoring their emergence from cultured pluripotent stem cells (PSCs). However, reporter constructs for tracking proximal airway lineages generated from PSCs have not been previously available, limiting the characterization of these cells. Here, we engineer mouse and human PSC lines carrying airway secretory lineage reporters that facilitate the tracking, purification, and profiling of this lung subtype. Through bulk and single-cell-based global transcriptomic profiling, we find PSC-derived airway secretory cells are susceptible to phenotypic plasticity exemplified by the tendency to co-express both a proximal airway secretory program as well as an alveolar type 2 cell program, which can be minimized by inhibiting endogenous Wnt signaling. Our results provide global profiles of engineered lung cell fates, a guide for improving their directed differentiation, and a human model of the developing airway.

Depletion of club cells attenuates bleomycin-induced lung injury and fibrosis in mice

Background

The role of bronchiolar epithelial cells in the pathogenesis of pulmonary fibrosis has not been clarified. We previously demonstrated DNA damage in murine bronchioles in the early stages of bleomycin-induced pulmonary fibrosis that subsequently extended to alveolar cells at the advanced stages of the disease. Club cells are progenitor cells for bronchioles and are known to play protective roles against lung inflammation and damage. The aim of the present study was to elucidate the role of club cells in the development of pulmonary fibrosis.

Methods

C57BL/6 J mice received naphthalene intraperitoneally on day −2 to deplete club cells and were given intratracheal bleomycin or a vehicle on day 0. Lung tissues were obtained on days 1, 7, and 14, and bronchoalveolar lavage was performed on day 14. Bronchiolar epithelial cells sampled by laser capture microdissection were analyzed by gene expression microarray analysis on day 14.

Results

Club cell depletion induced by naphthalene protected mice from bleomycin-induced lung injury and fibrosis. Bleomycin-triggered bronchiolar TGF-β1 expression was reduced. Gene expression microarray analysis revealed that genes associated with inflammatory response and chemokine activity were downregulated in the bleomycin-injured bronchiolar epithelium with club cell injury compared to that in bronchiolar epithelium without cell injury.

Conclusions

Club cells are involved in the development of lung injury and fibrosis.

Plasticity in the lung: making and breaking cell identity

In contrast to a prior emphasis on the finality of cell fate decisions in developmental systems, cellular plasticity is now emerging as a general theme in the biology of multiple adult organ systems. In the lung, lineage tracing has been used to identify distinct epithelial stem and progenitor cell populations. These cells, together with their differentiated progeny, maintain a stable identity during steady state conditions, but can display remarkable lineage plasticity following injury. This Review summarizes our current understanding of the different cell lineages of the adult mammalian lung and their responses to injury. In the lung, which is constantly exposed to infection and aerosolized toxins, epithelial plasticity might be more of a rule than an exception, and it is likely that different injuries elicit different facultative responses.