TGF-β signaling in stromal cells acts upstream of FGF-10 to regulate epithelial stem cell growth in the adult lung

Tissue fibrosis associated depletion of lipid-filled cells

Fibrosis is primarily described as the deposition of excessive extracellular matrix, but in many tissues it also involves a loss of lipid or lipid-filled cells. Lipid-filled cells are critical to tissue function and integrity in many tissues including the skin and lungs. Thus, loss or depletion of lipid-filled cells during fibrogenesis, has implications for tissue function. In some contexts, lipid-filled cells can impact ECM composition and stability, highlighting their importance in fibrotic transformation. Recent papers in fibrosis address this newly recognized fibrotic lipodystrophy phenomenon. Even in disparate tissues, common mechanisms are emerging to explain fibrotic lipodystrophy. These findings have implications for fibrosis in tissues composed of fibroblast and lipid-filled cell populations such as skin, lung, and liver. In this review, we will discuss the roles of lipid-containing cells, their reduction/loss during fibrotic transformation, and the mechanisms of that loss in the skin and lungs.

Oncogenic KRAS Drives Lipofibrogenesis to Promote Angiogenesis and Colon Cancer Progression

Oncogenic KRAS (KRAS*) contributes to many cancer hallmarks. In colorectal cancer, KRAS* suppresses antitumor immunity to promote tumor invasion and metastasis. Here, we uncovered that KRAS* transforms the phenotype of carcinoma-associated fibroblasts (CAF) into lipid-laden CAFs, promoting angiogenesis and tumor progression. Mechanistically, KRAS* activates the transcription factor CP2 (TFCP2) that upregulates the expression of the proadipogenic factors BMP4 and WNT5B, triggering the transformation of CAFs into lipid-rich CAFs. These lipid-rich CAFs, in turn, produce VEGFA to spur angiogenesis. In KRAS*-driven colorectal cancer mouse models, genetic or pharmacologic neutralization of TFCP2 reduced lipid-rich CAFs, lessened tumor angiogenesis, and improved overall survival. Correspondingly, in human colorectal cancer, lipid-rich CAF and TFCP2 signatures correlate with worse prognosis. This work unveils a new role for KRAS* in transforming CAFs, driving tumor angiogenesis and disease progression, providing an actionable therapeutic intervention for KRAS*-driven colorectal cancer.

SIGNIFICANCE

This study identified a molecular mechanism contributing to KRAS*-driven colorectal cancer progression via fibroblast transformation in the tumor microenvironment to produce VEGFA driving tumor angiogenesis. In preclinical models, targeting the KRAS*-TFCP2-VEGFA axis impaired tumor progression, revealing a potential novel therapeutic option for patients with KRAS*-driven colorectal cancer. This article is featured in Selected Articles from This Issue, p. 2489.

Alveolar Type 2 Epithelial Cell Organoids: Focus on Culture Methods

Lung diseases rank third in terms of mortality and represent a significant economic burden globally. Scientists have been conducting research to better understand respiratory diseases and find treatments for them. An ideal in vitro model must mimic the in vivo organ structure, physiology, and pathology. Organoids are self-organizing, three-dimensional (3D) structures originating from adult stem cells, embryonic lung bud progenitors, embryonic stem cells (ESCs), and induced pluripotent stem cells (iPSCs). These 3D organoid cultures may provide a platform for exploring tissue development, the regulatory mechanisms related to the repair of lung epithelia, pathophysiological and immunomodulatory responses to different respiratory conditions, and screening compounds for new drugs. To create 3D lung organoids in vitro, both co-culture and feeder-free methods have been used. However, there exists substantial heterogeneity in the organoid culture methods, including the sources of AT2 cells, media composition, and feeder cell origins. This article highlights the currently available methods for growing AT2 organoids and prospective improvements to improve the available culture techniques/conditions. Further, we discuss various applications, particularly those aimed at modeling human distal lung diseases and cell therapy.

R-SPONDIN2+ mesenchymal cells form the bud tip progenitor niche during human lung development

The human respiratory epithelium is derived from a progenitor cell in the distal buds of the developing lung. These "bud tip progenitors" are regulated by reciprocal signaling with surrounding mesenchyme; however, mesenchymal heterogeneity and function in the developing human lung are poorly understood. We interrogated single-cell RNA sequencing data from multiple human lung specimens and identified a mesenchymal cell population present during development that is highly enriched for expression of the WNT agonist RSPO2, and we found that the adjacent bud tip progenitors are enriched for the RSPO2 receptor LGR5. Functional experiments using organoid models, explant cultures, and FACS-isolated RSPO2+ mesenchyme show that RSPO2 is a critical niche cue that potentiates WNT signaling in bud tip progenitors to support their maintenance and multipotency.

Thy-1-Integrin Interactions in cis and Trans Mediate Distinctive Signaling

Thy-1 is a cell surface glycosylphosphatidylinositol (GPI)-anchored glycoprotein that bears a broad mosaic of biological roles across various cell types. Thy-1 displays strong physiological and pathological implications in development, cancer, immunity, and tissue fibrosis. Quite uniquely, Thy-1 is capable of mediating integrin-related signaling through direct trans- and cis-interaction with integrins. Both interaction types have shown distinctive roles, even when interacting with the same type of integrin, where binding in trans or in cis often yields divergent signaling events. In this review, we will revisit recent progress and discoveries of Thy-1–integrin interactions in trans and in cis, highlight their pathophysiological consequences and explore other potential binding partners of Thy-1 within the integrin regulation/signaling paradigm.

Fgfr2b signaling is essential for the maintenance of the alveolar epithelial type 2 lineage during lung homeostasis in mice

Fibroblast growth factor receptor 2b (Fgfr2b) signaling is essential throughout lung development to form the alveolar epithelial lineage. However, its role in alveolar epithelial type 2 cells (AT2s) homeostasis was recently considered dispensable. SftpcCreERT2; Fgfr2bflox/flox; tdTomatoflox/flox mice were used to delete Fgfr2b expression in cells belonging to the AT2 lineage, which contains mature AT2s and a novel SftpcLow lineage-traced population called "injury activated alveolar progenitors" or IAAPs. Upon continuous tamoxifen exposure for either 1 or 2 weeks to delete Fgfr2b, a shrinking of the AT2 population is observed. Mature AT2s exit the cell cycle, undergo apoptosis and fail to form alveolospheres in vitro. However, the lung morphometry appears normal, suggesting the involvement of compensatory mechanisms. In mutant lungs, IAAPs which escaped Fgfr2b deletion expand, display enhanced alveolosphere formation in vitro and increase drastically their AT2 signature, suggesting differentiation towards mature AT2s. Interestingly, a significant increase in AT2s and decrease in IAPPs occurs after a 1-week tamoxifen exposure followed by an 8-week chase period. Although mature AT2s partially recover their alveolosphere formation capabilities, the IAAPs no longer display this property. Single-cell RNA seq analysis confirms that AT2s and IAAPs represent stable and distinct cell populations and recapitulate some of their characteristics observed in vivo. Our results underscore the essential role played by Fgfr2b signaling in the maintenance of the AT2 lineage in the adult lung during homeostasis and suggest that the IAAPs could represent a new population of AT2 progenitors.

Impaired AT2 to AT1 cell transition in PM2.5-induced mouse model of chronic obstructive pulmonary disease

Background

Particular matter 2.5 (PM2.5) is one of the most important air pollutant, and it is positively associated with the development of chronic obstructive pulmonary disease (COPD). However, the precise underlying mechanisms through which PM2.5 promotes the development of COPD remains largely unknown.

Methods

Mouse alveolar destruction were determined by histological analysis of lung tissues and lung function test. Alveolar type II cells (AT2) to alveolar type I cells (AT1) transition in PM2.5-induced COPD mouse model was confirmed via immunofluorescence staining and qPCR analysis. The differentially expressed genes in PM2.5-induced COPD mouse model were identified by RNA-sequencing of alveolar epithelial organoids and generated by bioinformatics analysis.

Results

In this study, we found that 6 months exposure of PM2.5 induced a significantly decreased pulmonary compliance and resulted in pulmonary emphysema in mice. We showed that PM2.5 exposure significantly reduced the AT2 to AT1 cell transition in vitro and in vivo. In addition, we found a reduced expression of the intermediate AT2-AT1 cell process marker claudin 4 (CLDN4) at day 4 of differentiation in mouse alveolar organoids treated with PM2.5, suggesting that PM2.5 exposure inhibited AT2 cells from entering the transdifferentiation process. RNA-sequencing of mouse alveolar organoids showed that several key signaling pathways that involved in the AT2 to AT1 cell transition were significantly altered including the Wnt signaling, MAPK signaling and signaling pathways regulating pluripotency of stem cells following PM2.5 exposure.

Conclusions

In summary, these data demonstrate a critical role of AT2 to AT1 cell transition in PM2.5-induced COPD mouse model and reveal the signaling pathways that potentially regulate AT2 to AT1 cell transition during this process. Our findings therefore advance the current knowledge of PM2.5-induced COPD and may lead to a novel therapeutic strategy to treat this disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-022-01996-w.

OUP accepted manuscript

Bronchopulmonary dysplasia (BPD) is a neonatal lung disease developing in premature babies characterized by arrested alveologenesis and associated with decreased Fibroblast growth factor 10 (FGF10) expression. One-week hyperoxia (HYX) exposure of newborn mice leads to a permanent arrest in alveologenesis. To test the role of Fgf10 signaling to promote de novo alveologenesis following hyperoxia, we used transgenic mice allowing inducible expression of Fgf10 and recombinant FGF10 (rFGF10) protein delivered intraperitoneally. We carried out morphometry analysis, and IF on day 45. Alveolospheres assays were performed co-culturing AT2s from normoxia (NOX) with FACS-isolated Sca1^Pos resident mesenchymal cells (rMC) from animals exposed to NOX, HYX-PBS, or HYX-FGF10. scRNAseq between rMC-Sca1^Pos isolated from NOX and HYX-PBS was also carried out. Transgenic overexpression of Fgf10 and rFGF10 administration rescued the alveologenesis defects following HYX. Alveolosphere assays indicate that the activity of rMC-Sca1^Pos is negatively impacted by HYX and partially rescued by rFGF10 treatment. Analysis by IF demonstrates a significant impact of rFGF10 on the activity of resident mesenchymal cells. scRNAseq results identified clusters expressing Fgf10, Fgf7, Pdgfra, and Axin2, which could represent the rMC niche cells for the AT2 stem cells. In conclusion, we demonstrate that rFGF10 administration is able to induce de novo alveologenesis in a BPD mouse model and identified subpopulations of rMC-Sca1^Pos niche cells potentially representing its cellular target.

Characterization in mice of the resident mesenchymal niche maintaining AT2 stem cell proliferation in homeostasis and disease

Resident mesenchymal cells (rMCs defined as Cd31^Neg Cd45^Neg Epcam^Neg ) control the proliferation and differentiation of alveolar epithelial type 2 (AT2) stem cells in vitro. The identity of these rMCs is still elusive. Among them, Axin2^Pos mesenchymal alveolar niche cells (MANCs), which are expressing Fgf7, have been previously described. We propose that an additional population of rMCs, expressing Fgf10 (called rMC-Sca1^Pos Fgf10^Pos ) are equally important to maintain AT2 stem cell proliferation. The alveolosphere model, based on the AT2-rMC co-culture in growth factor-reduced Matrigel, was used to test the efficiency of different rMC subpopulations isolated by FACS from adult murine lung to sustain the proliferation and differentiation of AT2 stem cells. We demonstrate that rMC-Sca1^Pos Fgf10^Pos cells are efficient to promote the proliferation and differentiation of AT2 stem cells. Co-staining of adult lung for Fgf10 mRNA and Sftpc protein respectively, indicate that 28% of Fgf10^Pos cells are located close to AT2 cells. Co-ISH for Fgf7 and Fgf10 indicate that these two populations do not significantly overlap. Gene arrays comparing rMC-Sca1^Pos Axin2^Pos and rMC-Sca1^Pos Fgf10^Pos support that these two cell subsets express differential markers. In addition, rMC function is decreased in obese ob/ob mutant compared to WT mice with a much stronger loss of function in males compared to females. In conclusion, rMC-Sca1^Pos Fgf10^Pos cells play important role in supporting AT2 stem cells proliferation and differentiation. This result sheds a new light on the subpopulations of rMCs contributing to the AT2 stem cell niche in homeostasis and in the context of pre-existing metabolic diseases.

Optimizing the in vitro colony-forming assay for more efficient delineation of the interaction between lung epithelial stem cells and their niche

The use of in vitro 3D organoid/colony forming assay (CFA); which mimics the in vivo environment have provided insight into the mechanisms by which lung stem cells maintain and repair the lung. In recent years, the use of CFA has markedly expanded. However, variations among laboratories in lung cell isolation methods, media used, type, origin, and processing methods of mesenchymal cells used as feeders for the epithelial colonies, and terms utilized to describe and quantify the growing colonies, have caused difficulty in reproducing results among different labs. In this study, we compared several previously described methods for lung cell isolation and culture media, to identify their influence on retrieved cells and growing colonies. We also characterized the effect of freeze/thaw, and propagation of fibroblasts on their ability to support epithelial colonies. Importantly, we suggested markers to identify fibroblast subtypes that offer the best support to alveolar stem cell proliferation. Then, we used our optimized assay to confirm the in vitro identity of recently described epithelial progenitors. We also tested the effect of hyperoxia on lung stem cells, and examined the expression of the receptors for the SARS-COV-2 virus's entry into epithelial cells, on our organoids. In summary, our findings facilitate CFA standardization, help understand how niche cell variations influence growing colonies, and confirm some of the recently described lung stem cells.

The autotaxin-lysophosphatidic acid pathway mediates mesenchymal cell recruitment and fibrotic contraction in lung transplant fibrosis

BACKGROUND

Chronic lung allograft dysfunction (CLAD) is the leading cause of mortality in lung transplant recipients. CLAD is characterized by respiratory failure owing to the accumulation of fibrotic cells in small airways and alveoli, inducing tissue contraction and architectural destruction. However, the source of the fibroblastic cells and the mechanism(s) underlying the accumulation and activation remain unexplained. Mesenchymal stromal cells (MSCs) are multipotent progenitors that are normally located in the lung tissue but can be isolated from the alveolar space in lung transplant recipients, where they have a profibrotic phenotype. Our objective was to identify the mediator(s) inducing migration and contractile differentiation of lung tissue MSCs.

METHODS

Bronchoalveolar lavage (BAL) (7 healthy controls and 21 lung transplant recipients), CCL2, HGF, TGFB, EGF, and PDGF-BB and autotaxin were measured by enzyme-linked immunosorbent assay. BAL (7 healthy controls and 31 lung transplant recipients) lysophosphatidic acid (LPA) (16:0, 18:0, 18:1, 22:4) was measured by liquid chromatography with tandem mass spectrometry. The effect of inhibition of candidate mediators on BAL-mediated chemoattraction of MSCs and contraction of MSC-spiked collagen gel assays was assessed. BAL cells from a lung transplant recipient with CLAD were analyzed by single-cell RNA sequencing.

RESULTS

We first demonstrate that BAL fluid from lung transplant recipients and particularly those with CLAD is potently chemoattractive to human lung tissue‒derived MSCs and induces a contractile phenotype. After excluding several candidate mediators, we show that LPA blockade completely abrogated transplant recipient BAL‒mediated chemoattraction of MSCs and contraction of MSC-spiked collagen gels. Furthermore, LPA levels were enriched in transplant recipient BAL, and LPA replicated the observed in vitro profibrotic effects of transplant recipient BAL. Finally, we identify BAL monocyte‒derived macrophages with autotaxin (ENPP2) and fibrotic transcriptional signature.

CONCLUSIONS

Autotaxin-expressing alveolar macrophages are present in CLAD BAL. These cells potentially provide a local source of autotaxin/LPA that drives MSC recruitment and tissue contraction in CLAD. These cells are analogous to an aberrant macrophage population recently identified in idiopathic pulmonary fibrosis, suggesting an overlap in pathogenesis between CLAD and other forms of lung fibrosis.

A single injection of bleomycin reduces glycosaminoglycan sulfation up to 30 days in the C57BL/6 mouse model of lung fibrosis

Bleomycin is a clinically used anticancer drug, but it induces lung fibrosis in certain cancer patients with unknown mechanism. Glycosaminoglycans (GAGs) are required for lung morphogenesis during animal development. In current study, GAG disaccharides including heparan sulfate (HS) and chondroitin sulfate (CS) from bleomycin-induced and control lung tissues in lung fibrosis mouse model were tagged with 1-phenyl-3-methyl-5-pyrazolone (PMP) and deuterated PMP, respectively. The differentially isotope-tagged disaccharides were quantitatively compared by LC-MS. At day 10, the amount of CS disaccharides (U0a0, U0a6, and U0a4) and non-sulfated HS disaccharide (U0A0) were increased by 1.3-, 1.6-, 11.7-, and 2.2-fold, respectively, whereas the amount of CS disaccharide (U0a2), hyaluranan disaccharide (UβA0), and six HS disaccharides (U0A6, U2A0, U0H6, U0S0, U2S0, and U2S6) were decreased from1.1- to 14.3-fold compared to that of the controls. At day 15, under-sulfation of both HS and CS disaccharides was persisted. At day 30, the CS disaccharide compositions were recovered to that of the control levels whereas the HS were still remarkably under-sulfated. In conclusion, GAGs, especially HS, from fibrotic lungs induced by a single injection of bleomycin were significantly under-sulfated up to 30 days, suggesting GAGs might be another class of defective signaling molecules involved in bleomycin-induced lung fibrosis.

Understanding Human Lung Development through In Vitro Model Systems

An abundance of information about lung development in animal models exists; however, comparatively little is known about lung development in humans. Recent advances using primary human lung tissue combined with the use of human in vitro model systems, such as human pluripotent stem cell-derived tissue, have led to a growing understanding of the mechanisms governing human lung development. They have illuminated key differences between animal models and humans, underscoring the need for continued advancements in modeling human lung development and utilizing human tissue. This review discusses the use of human tissue and the use of human in vitro model systems that have been leveraged to better understand key regulators of human lung development and that have identified uniquely human features of development. This review also examines the implementation and challenges of human model systems and discusses how they can be applied to address knowledge gaps.

Lung-resident mesenchymal stromal cells are tissue-specific regulators of lung homeostasis

Until recently, data supporting the tissue-resident status of mesenchymal stromal cells (MSC) has been ambiguous since their discovery in the 1950-60s. These progenitor cells were first discovered as bone marrow-derived adult multipotent cells and believed to migrate to sites of injury, opposing the notion that they are residents of all tissue types. In recent years, however, it has been demonstrated that MSC can be found in all tissues and MSC from different tissues represent distinct populations with differential protein expression unique to each tissue type. Importantly, these cells are efficient mediators of tissue repair, regeneration, and prove to be targets for therapeutics, demonstrated by clinical trials (phase 1-4) for MSC-derived therapies for diseases like graft-versus-host-disease, multiple sclerosis, rheumatoid arthritis, and Crohn's disease. The tissue-resident status of MSC found in the lung is a key feature of their importance in the context of disease and injuries of the respiratory system, since these cells could be instrumental to providing more specific and targeted therapies. Currently, bone marrow-derived MSC have been established in the treatment of disease, including diseases of the lung. However, with lung-resident MSC representing a unique population with a different phenotypic and gene expression pattern than MSC derived from other tissues, their role in remediating lung inflammation and injury could provide enhanced efficacy over bone marrow-derived MSC methods. Through this review, lung-resident MSC will be characterized, using previously published data, by surface markers, gene expression patterns, and compared with bone-marrow MSC to highlight similarities and, importantly, differences in these cell types.

Targeted regulation of fibroblast state by CRISPR-mediated CEBPA expression

Background

Fibroblasts regulate tissue homeostasis and the balance between tissue repair and fibrosis. CCAAT/enhancer-binding protein alpha (CEBPA) is a key transcription factor that regulates adipogenesis. CEBPA has been shown to be essential for lung maturation, and deficiency of CEBPA expression leads to abnormal lung architecture. However, its specific role in lung fibroblast regulation and fibrosis has not yet been elucidated.

Methods

Lung fibroblast CEBPA expression, pro-fibrotic and lipofibroblast gene expression were assessed by qRT-PCR. CEBPA gain and loss of function experiments were carried out to evaluate the role of CEBPA in human lung fibroblast activation with and without TGF-β1 treatment. Adipogenesis assay was used to measure the adiopogenic potential of lung fibroblasts. Finally, CRISPR activation system was used to enhance endogenous CEBPA expression.

Results

We found that CEBPA gene expression is significantly decreased in IPF-derived fibroblasts compared to normal lung fibroblasts. CEBPA knockdown in normal human lung fibroblasts enhanced fibroblast pro-fibrotic activation and ECM production. CEBPA over-expression by transient transfection in IPF-derived fibroblasts significantly reduced pro-fibrotic gene expression, ECM deposition and αSMA expression and promoted the formation of lipid droplets measured by Oil Red O staining and increased lipofibroblast gene expression. Inhibition of the histone methyl transferase G9a enhanced CEBPA expression, and the anti-fibrotic effects of G9a inhibition were partially mediated by CEBPA expression. Finally, targeted CRISPR-mediated activation of CEBPA resulted in fibroblasts switching from fibrogenic to lipofibroblast states.

Conclusions

CEBPA expression is reduced in human IPF fibroblasts and its deficiency reduces adipogenic potential and promotes fibrogenic activation. CEBPA expression can be rescued via an inhibitor of epigenetic repression or by targeted CRISPR activation, leading to reduced fibrogenic activation.

Mesenchymal WNT-5A/5B Signaling Represses Lung Alveolar Epithelial Progenitors

Chronic obstructive pulmonary disease (COPD) represents a worldwide concern with high morbidity and mortality, and is believed to be associated with accelerated ageing of the lung. Alveolar abnormalities leading to emphysema are a key characteristic of COPD. Pulmonary alveolar epithelial type 2 cells (AT2) produce surfactant and function as progenitors for type 1 cells. Increasing evidence shows elevated WNT-5A/B expression in ageing and in COPD that may contribute to the disease process. However, supportive roles for WNT-5A/B in lung regeneration were also reported in different studies. Thus, we explored the role of WNT-5A/B on alveolar epithelial progenitors (AEPs) in more detail. We established a Precision-Cut-Lung Slices (PCLS) model and a lung organoid model by co-culturing epithelial cells (EpCAM⁺/CD45^-/CD31^-) with fibroblasts in matrigel in vitro to study the impact of WNT-5A and WNT-5B. Our results show that WNT-5A and WNT-5B repress the growth of epithelial progenitors with WNT-5B preferentially restraining the growth and differentiation of alveolar epithelial progenitors. We provide evidence that both WNT-5A and WNT-5B negatively regulate the canonical WNT signaling pathway in alveolar epithelium. Taken together, these findings reveal the functional impact of WNT-5A/5B signaling on alveolar epithelial progenitors in the lung, which may contribute to defective alveolar repair in COPD.

Glycosaminoglycans: A Link Between Development and Regeneration in the Lung

What can we learn from embryogenesis to increase our understanding of how regeneration of damaged adult lung tissue could be induced in serious lung diseases such as chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), and asthma? The local tissue niche determines events in both embryogenesis and repair of the adult lung. Important constituents of the niche are extracellular matrix (ECM) molecules, including proteoglycans and glycosaminoglycans (GAGs). GAGs, strategically located in the pericellular and extracellular space, bind developmentally active growth factors (GFs) and morphogens such as fibroblast growth factors (FGFs), transforming growth factor-β (TGF-β), and bone morphogenetic proteins (BMPs) aside from cytokines. These interactions affect activities in many cells, including stem cells, important in development and tissue regeneration. Moreover, it is becoming clear that the "inherent code," such as sulfation of disaccharides of GAGs, is a strong determinant of cellular outcome. Sulfation patterns, deacetylations, and epimerizations of GAG chains function as tuning forks in gradient formation of morphogens, growth factors, and cytokines. Learning to tune these fine instruments, that is, interactions between GFs, chemokines, and cytokines with the specific disaccharide code of GAGs in the adult lung, could become the key to unlock inherent regenerative forces to override pathological remodeling. This review aims to provide an overview of the role GAGs play during development and similar events in regenerative efforts in the adult lung.

TGF-β activation impairs fibroblast ability to support adult lung epithelial progenitor cell organoid formation

Transforming growth factor-β (TGF-β)-induced fibroblast-to-myofibroblast differentiation contributes to remodeling in chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis, but whether this impacts the ability of fibroblasts to support lung epithelial repair remains little explored. We pretreated human lung fibroblasts [primary (phFB) or MRC5 cells] with recombinant human TGF-β to induce myofibroblast differentiation, then cocultured them with adult mouse lung epithelial cell adhesion molecule-positive cells (EpCAM⁺) to investigate their capacity to support epithelial organoid formation in vitro. While control phFB and MRC5 lung fibroblasts supported organoid formation of mouse EpCAM⁺ cells, TGF-β pretreatment of both phFB and MRC5 impaired organoid-supporting ability. We performed RNA sequencing of TGF-β-treated phFB, which revealed altered expression of key Wnt signaling pathway components and Wnt/β-catenin target genes, and modulated expression of secreted factors involved in mesenchymal-epithelial signaling. TGF-β profoundly skewed the transcriptional program induced by the Wnt/β-catenin activator CHIR99021. Supplementing organoid culture media recombinant hepatocyte growth factor or fibroblast growth factor 7 promoted organoid formation when using TGF-β pretreated fibroblasts. In conclusion, TGF-β-induced myofibroblast differentiation results in Wnt/β-catenin pathway skewing and impairs fibroblast ability to support epithelial repair likely through multiple mechanisms, including modulation of secreted growth factors.

Effects of Agricultural Organic Dusts on Human Lung-Resident Mesenchymal Stem (Stromal) Cell Function

Agricultural organic dust exposures trigger harmful airway inflammation, and workers experiencing repetitive dust exposures are at increased risk for lung disease. Mesenchymal stem/stromal cells (MSCs) regulate wound repair processes in the lung, and may contribute to either proresolution or -fibrotic lung responses. It is unknown how organic dust exposures alter lung-resident MSC activation and proinflammatory versus prorepair programs in the lung. To address this gap in knowledge, we isolated human lung-resident MSC from lung tissue. Cells were stimulated with aqueous extracts of organic dusts (DE) derived from swine confinement facilities and were assessed for changes in proliferative and migratory capacities, and production of proinflammatory and prorepair mediators. Through these investigations, we found that DE induces significant release of proinflammatory mediators TNF-α, IL-6, IL-8, and matrix metalloproteases, while also inducing the production of prorepair mediators amphiregulin, FGF-10, and resolvin D1. In addition, DE significantly reduced the growth and migratory capacities of lung-resident MSC. Together, these investigations indicate lung-resident MSC activation and wound repair activities are altered by organic dust exposures. These findings warrant future investigations to assess how organic dusts affect lung-resident mesenchymal stem/stromal cell function and impact airway inflammation, injury, and repair during agricultural aerosol exposures.

Role of Fibroblast Growth Factor 10 in Mesenchymal Cell Differentiation During Lung Development and Disease

During organogenesis and pathogenesis, fibroblast growth factor 10 (Fgf10) regulates mesenchymal cell differentiation in the lung. Different cell types reside in the developing lung mesenchyme. Lineage tracing in vivo was used to characterize these cells during development and disease. Fgf10-positive cells in the early lung mesenchyme differentiate into multiple lineages including smooth muscle cells (SMCs), lipofibroblasts (LIFs) as well as other cells, which still remain to be characterized. Fgf10 signaling has been reported to act both in an autocrine and paracrine fashion. Autocrine Fgf10 signaling is important for the differentiation of LIF progenitors. Interestingly, autocrine Fgf10 signaling also controls the differentiation of pre-adipocytes into mature adipocytes. As the mechanism of action of Fgf10 on adipocyte differentiation via the activation of peroxisome proliferator-activated receptor gamma (Pparγ) signaling is quite well established, this knowledge could be instrumental for identifying drugs capable of sustaining LIF differentiation in the context of lung injury. We propose that enhanced LIF differentiation could be associated with improved repair. On the other hand, paracrine signaling is considered to be critical for the differentiation of alveolar epithelial progenitors during development as well as for the maintenance of the alveolar type 2 (AT2) stem cells during homeostasis. Alveolar myofibroblasts (MYFs), which are another type of mesenchymal cells critical for the process of alveologenesis (the last phase of lung development) express high levels of Fgf10 and are also dependent for their formation on Fgf signaling. The characterization of the progenitors of alveolar MYFs as well the mechanisms involved in their differentiation is paramount as these cells are considered to be critical for lung regeneration. Finally, lineage tracing in the context of lung fibrosis demonstrated a reversible differentiation from LIF to "activated" MYF during fibrosis formation and resolution. FGF10 expression in the lungs of idiopathic pulmonary fibrosis (IPF) vs. donors as well as progressive vs. stable IPF patients supports our conclusion that FGF10 deficiency could be causative for IPF progression. The therapeutic application of recombinant human FGF10 is therefore very promising.

Developmental pathways in the pathogenesis of lung fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive and terminal lung disease with no known cure. IPF is a disease of aging, with median age of diagnosis over 65 years. Median survival is between 3 and 5 years after diagnosis. IPF is characterized primarily by excessive deposition of extracellular matrix (ECM) proteins by activated lung fibroblasts and myofibroblasts, resulting in reduced gas exchange and impaired pulmonary function. Growing evidence supports the concept of a pro-fibrotic environment orchestrated by underlying factors such as genetic predisposition, chronic injury and aging, oxidative stress, and impaired regenerative responses may account for disease development and persistence. Currently, two FDA approved drugs have limited efficacy in the treatment of IPF. Many of the genes and gene networks associated with lung development are induced or activated in IPF. In this review, we analyze current knowledge in the field, gained from both basic and clinical research, to provide new insights into the disease process, and potential approaches to treatment of pulmonary fibrosis.

Stellate Cells in Tissue Repair, Inflammation, and Cancer

Stellate cells are resident lipid-storing cells of the pancreas and liver that transdifferentiate to a myofibroblastic state in the context of tissue injury. Beyond having roles in tissue homeostasis, stellate cells are increasingly implicated in pathological fibrogenic and inflammatory programs that contribute to tissue fibrosis and that constitute a growth-permissive tumor microenvironment. Although the capacity of stellate cells for extracellular matrix production and remodeling has long been appreciated, recent research efforts have demonstrated diverse roles for stellate cells in regulation of epithelial cell fate, immune modulation, and tissue health. Our present understanding of stellate cell biology in health and disease is discussed here, as are emerging means to target these multifaceted cells for therapeutic benefit.

Impaired Angiogenic Supportive Capacity and Altered Gene Expression Profile of Resident CD146+ Mesenchymal Stromal Cells Isolated from Hyperoxia-Injured Neonatal Rat Lungs

Bronchopulmonary dysplasia (BPD), the most common complication of extreme preterm birth, can be caused by oxygen-related lung injury and is characterized by impaired alveolar and vascular development. Mesenchymal stromal cells (MSCs) have lung protective effects. Conversely, BPD is associated with increased MSCs in tracheal aspirates. We hypothesized that endogenous lung (L-)MSCs are perturbed in a well-established oxygen-induced rat model mimicking BPD features. Rat pups were exposed to 21% or 95% oxygen from birth to postnatal day 10. On day 12, CD146⁺ L-MSCs were isolated and characterized according to the International Society for Cellular Therapy criteria. Epithelial and vascular repair potential were tested by scratch assay and endothelial network formation, respectively, immune function by mixed lymphocyte reaction assay. Microarray analysis was performed using the Affymetrix GeneChip and gene set enrichment analysis software. CD146⁺ L-MSCs isolated from rat pups exposed to hyperoxia had decreased CD73 expression and inhibited lung endothelial network formation. CD146⁺ L-MSCs indiscriminately promoted epithelial wound healing and limited T cell proliferation. Expression of potent antiangiogenic genes of the axonal guidance cue and CDC42 pathways was increased after in vivo hyperoxia, whereas genes of the anti-inflammatory Janus kinase (JAK)/signal transducer and activator of transcription (STAT) and lung/vascular growth-promoting fibroblast growth factor (FGF) pathways were decreased. In conclusion, in vivo hyperoxia exposure alters the proangiogenic effects and FGF expression of L-MSCs. In addition, decreased CD73 and JAK/STAT expression suggests decreased immune function. L-MSC function may be perturbed and contribute to BPD pathogenesis. These findings may lead to improvements in manufacturing exogenous MSCs with superior repair capabilities.

The role of TGFβ‑HGF‑Smad4 axis in regulating the proliferation of mouse airway progenitor cells

The interaction between airway epithelial progenitor cells and their microenvironment is critical for maintaining lung homeostasis. This microenvironment includes fibroblast cells, which support the growth of airway progenitor cells. However, the mechanism of this support is not fully understood. In the present study, the authors observed that inhibition of transforming growth factor (TGF)‑β signal with SB431542 promotes the expression of hepatocyte growth factor (HGF) in fibroblast cells. The HGF receptor, c‑Met, is expressed on airway progenitor cells; HGF promotes the colony‑forming ability of airway progenitor cells. The deletion of Smad4 in airway progenitor cells increases the colony‑forming ability, suggesting that Smad4 plays a negative role in the regulating the proliferation of airway progenitor cells. These data demonstrated that the regulation of airway progenitor cells by TGF‑β depends on TGF‑βR1/2 on stromal cells, rather than on epithelial progenitor cells. These data suggested a role for the TGF‑β‑TGF‑βR1/2‑HGF‑Smad4 axis in airway epithelial homeostasis and sheds new light on the interaction between airway progenitor cells and their microenvironment.

Derivation of therapeutic lung spheroid cells from minimally invasive transbronchial pulmonary biopsies

Background

Resident stem and progenitor cells have been identified in the lung over the last decade, but isolation and culture of these cells remains a challenge. Thus, although these lung stem and progenitor cells provide an ideal source for stem-cell based therapy, mesenchymal stem cells (MSCs) remain the most popular cell therapy product for the treatment of lung diseases. Surgical lung biopsies can be the tissue source but such procedures carry a high risk of mortality.

Methods

In this study we demonstrate that therapeutic lung cells, termed “lung spheroid cells” (LSCs) can be generated from minimally invasive transbronchial lung biopsies using a three-dimensional culture technique. The cells were then characterized by flow cytometry and immunohistochemistry. Angiogenic potential was tested by in-vitro HUVEC tube formation assay. In-vivo bio- distribution of LSCs was examined in athymic nude mice after intravenous delivery.

Results

From one lung biopsy, we are able to derive >50 million LSC cells at Passage 2. These cells were characterized by flow cytometry and immunohistochemistry and were shown to represent a mixture of lung stem cells and supporting cells. When introduced systemically into nude mice, LSCs were retained primarily in the lungs for up to 21 days.

Conclusion

Here, for the first time, we demonstrated that direct culture and expansion of human lung progenitor cells from pulmonary tissues, acquired through a minimally invasive biopsy, is possible and straightforward with a three-dimensional culture technique. These cells could be utilized in long-term expansion of lung progenitor cells and as part of the development of cell-based therapies for the treatment of lung diseases such as chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF).

Electronic supplementary material

The online version of this article (doi:10.1186/s12931-017-0611-0) contains supplementary material, which is available to authorized users.

The Future of Bronchopulmonary Dysplasia: Emerging Pathophysiological Concepts and Potential New Avenues of Treatment

Yearly more than 15 million babies are born premature (<37 weeks gestational age), accounting for more than 1 in 10 births worldwide. Lung injury caused by maternal chorioamnionitis or preeclampsia, postnatal ventilation, hyperoxia, or inflammation can lead to the development of bronchopulmonary dysplasia (BPD), one of the most common adverse outcomes in these preterm neonates. BPD patients have an arrest in alveolar and microvascular development and more frequently develop asthma and early-onset emphysema as they age. Understanding how the alveoli develop, and repair, and regenerate after injury is critical for the development of therapies, as unfortunately there is still no cure for BPD. In this review, we aim to provide an overview of emerging new concepts in the understanding of perinatal lung development and injury from a molecular and cellular point of view and how this is paving the way for new therapeutic options to prevent or treat BPD, as well as a reflection on current treatment procedures.

Temporal, spatial, and phenotypical changes of PDGFRα expressing fibroblasts during late lung development

Many studies have investigated the source and role of epithelial progenitors during lung development; such information is limited for fibroblast populations and their complex role in the developing lung. In this study, we characterized the spatial location, mRNA expression and Immunophenotyping of PDGFRα⁺ fibroblasts during sacculation and alveolarization. Confocal microscopy identified spatial association of PDGFRα expressing fibroblasts with proximal epithelial cells of the branching bronchioles and the dilating acinar tubules at E16.5; with distal terminal saccules at E18.5; and with alveolar epithelial cells at PN7 and PN28. Immunohistochemistry for alpha smooth muscle actin revealed that PDGFRα⁺ fibroblasts contribute to proximal peribronchiolar smooth muscle at E16.5 and to transient distal alveolar myofibroblasts at PN7. Time series RNA-Seq analyses of PDGFRα⁺ fibroblasts identified differentially expressed genes that, based on gene expression similarity were clustered into 7 major gene expression profile patterns. The presence of myofibroblast and smooth muscle precursors at E16.5 and PN7 was reflected by a two-peak gene expression profile on these days and gene ontology enrichment in muscle contraction. Additional molecular and functional differences between peribronchiolar smooth muscle cells at E16.5 and transient intraseptal myofibroblasts at PN7 were suggested by a single peak in gene expression at PN7 with functional enrichment in cell projection and muscle cell differentiation. Immunophenotyping of subsets of PDGFRα⁺ fibroblasts by flow cytometry confirmed the predicted increase in proliferation at E16.5 and PN7, and identified subsets of CD29⁺ myofibroblasts and CD34⁺ lipofibroblasts. These data can be further mined to develop novel hypotheses and valuable understanding of the molecular and cellular basis of alveolarization.

Two-Way Conversion between Lipogenic and Myogenic Fibroblastic Phenotypes Marks the Progression and Resolution of Lung Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a form of progressive interstitial lung disease with unknown etiology. Due to a lack of effective treatment, IPF is associated with a high mortality rate. The hallmark feature of this disease is the accumulation of activated myofibroblasts that excessively deposit extracellular matrix proteins, thus compromising lung architecture and function and hindering gas exchange. Here we investigated the origin of activated myofibroblasts and the molecular mechanisms governing fibrosis formation and resolution. Genetic engineering in mice enables the time-controlled labeling and monitoring of lipogenic or myogenic populations of lung fibroblasts during fibrosis formation and resolution. Our data demonstrate a lipogenic-to-myogenic switch in fibroblastic phenotype during fibrosis formation. Conversely, we observed a myogenic-to-lipogenic switch during fibrosis resolution. Analysis of human lung tissues and primary human lung fibroblasts indicates that this fate switching is involved in IPF pathogenesis, opening potential therapeutic avenues to treat patients.

Understanding the developmental pathways pulmonary fibroblasts may follow during alveolar regeneration

Although pulmonary alveolar interstitial fibroblasts are less specialized than their epithelial and endothelial neighbors, they play essential roles during development and in response to lung injury. At birth, they must adapt to the sudden mechanical changes imposed by the onset of respiration and to a higher ambient oxygen concentration. In diseases such as bronchopulmonary dysplasia and interstitial fibrosis, their adaptive responses are overwhelmed leading to compromised gas-exchange function. Thus, although fibroblasts do not directly participate in gas-exchange, they are essential for creating and maintaining an optimal environment at the alveolar epithelial-endothelial interface. This review summarizes new information and concepts about the ontogeny differentiation, and function of alveolar fibroblasts. Alveolar development will be emphasized, because the development of strategies to evoke alveolar repair and regeneration hinges on thoroughly understanding the way that resident fibroblasts populate specific locations in which extracellular matrix must be produced and remodeled. Other recent reviews have described the disruption that diseases cause to the fibroblast niche and so my objective is to illustrate how the unique developmental origins and differentiation pathways could be harnessed favorably to augment certain fibroblast subpopulations and to optimize the conditions for alveolar regeneration.

Developmental Reprogramming in Mesenchymal Stromal Cells of Human Subjects with Idiopathic Pulmonary Fibrosis

Cellular plasticity and de-differentiation are hallmarks of tissue/organ regenerative capacity in diverse species. Despite a more restricted capacity for regeneration, humans with age-related chronic diseases, such as cancer and fibrosis, show evidence of a recapitulation of developmental gene programs. We have previously identified a resident population of mesenchymal stromal cells (MSCs) in the terminal airways-alveoli by bronchoalveolar lavage (BAL) of human adult lungs. In this study, we characterized MSCs from BAL of patients with stable and progressive idiopathic pulmonary fibrosis (IPF), defined as <5% and ≥10% decline, respectively, in forced vital capacity over the preceding 6-month period. Gene expression profiles of MSCs from IPF subjects with progressive disease were enriched for genes regulating lung development. Most notably, genes regulating early tissue patterning and branching morphogenesis were differentially regulated. Network interactive modeling of a set of these genes indicated central roles for TGF-β and SHH signaling. Importantly, fibroblast growth factor-10 (FGF-10) was markedly suppressed in IPF subjects with progressive disease, and both TGF-β1 and SHH signaling were identified as critical mediators of this effect in MSCs. These findings support the concept of developmental gene re-activation in IPF, and FGF-10 deficiency as a potentially critical factor in disease progression.

Impaired Capacity of Fibroblasts to Support Airway Epithelial Progenitors in Bronchiolitis Obliterans Syndrome

BACKGROUND

Bronchiolitis obliterans syndrome (BOS) often develops in transplant patients and results in injury to the respiratory and terminal airway epithelium. Owing to its rising incidence, the pathogenesis of BOS is currently an area of intensive research. Studies have shown that injury to the respiratory epithelium results in dysregulation of epithelial repair. Airway epithelial regeneration is supported by stromal cells, including fibroblasts. This study aimed to investigate whether the supportive role of lung fibroblasts is altered in BOS.

METHODS

Suspensions of lung cells were prepared by enzyme digestion. Lung progenitor cells (LPCs) were separated by fluorescence-activated cell sorting. Lung fibroblasts from patients with BOS or healthy controls were mixed with sorted mouse LPCs to compare the colony-forming efficiency of LPCs by counting the number of colonies with a diameter of ≥50 μm in each culture. Statistical analyses were performed using the SPSS 17.0 software (SPSS Inc., USA). The paired Student's t-test was used to test for statistical significance.

RESULTS

LPCs were isolated with the surface phenotype of CD31-CD34-CD45- EpCAM+Sca-1+. The colony-forming efficiency of LPCs was significantly reduced when co-cultured with fibroblasts isolated from patients with BOS. The addition of SB431542 increased the colony-forming efficiency of LPCs to 1.8%; however, it was still significantly less than that in co-culture with healthy control fibroblasts (P < 0.05).

CONCLUSION

The epithelial-supportive capacity of fibroblasts is impaired in the development of BOS and suggest that inefficient repair of airway epithelium could contribute to persistent airway inflammation in BOS.

Diversity of Interstitial Lung Fibroblasts Is Regulated by Platelet-Derived Growth Factor Receptor α Kinase Activity

Epithelial-mesenchymal cell interactions and factors that control normal lung development are key players in lung injury, repair, and fibrosis. A number of studies have investigated the roles and sources of epithelial progenitors during lung regeneration; such information, however, is limited in lung fibroblasts. Thus, understanding the origin, phenotype, and roles of fibroblast progenitors in lung development, repair, and regeneration helps address these limitations. Using a combination of platelet-derived growth factor receptor α-green fluorescent protein (PDGFRα-GFP) reporter mice, microarray, real-time polymerase chain reaction, flow cytometry, and immunofluorescence, we characterized two distinct interstitial resident fibroblasts, myo- and matrix fibroblasts, and identified a role for PDGFRα kinase activity in regulating their activation during lung regeneration. Transcriptional profiling of the two populations revealed a myo- and matrix fibroblast gene signature. Differences in proliferation, smooth muscle actin induction, and lipid content in the two subpopulations of PDGFRα-expressing fibroblasts during alveolar regeneration were observed. Although CD140α(+)CD29(+) cells behaved as myofibroblasts, CD140α(+)CD34(+) appeared as matrix and/or lipofibroblasts. Gain or loss of PDGFRα kinase activity using the inhibitor nilotinib and a dominant-active PDGFRα-D842V mutation revealed that PDGFRα was important for matrix fibroblast differentiation. We demonstrated that PDGFRα signaling promotes alveolar septation by regulating fibroblast activation and matrix fibroblast differentiation, whereas myofibroblast differentiation was largely PDGFRα independent. These studies provide evidence for the phenotypic and functional diversity as well as the extent of specificity of interstitial resident fibroblasts differentiation during regeneration after partial pneumonectomy.

Characterization of intercellular communication and mitochondrial donation by mesenchymal stromal cells derived from the human lung

Background

Bone marrow-derived mesenchymal stromal cells (BM-MSCs) are capable of repairing wounded lung epithelial cells by donating cytoplasmic material and mitochondria. Recently, we characterized two populations of human lung-derived mesenchymal stromal cells isolated from digested parenchymal lung tissue (LT-MSCs) from healthy individuals or from lung transplant recipients’ bronchoalveolar lavage fluid (BAL-MSCs). The aim of this study was to determine whether LT-MSCs and BAL-MSCs are also capable of donating cytoplasmic content and mitochondria to lung epithelial cells.

Methods

Cytoplasmic and mitochondrial transfer was assessed by co-culturing BEAS2B epithelial cells with Calcein AM or Mitotracker Green FM-labelled MSCs. Transfer was then measured by flow cytometry and validated by fluorescent microscopy. Molecular inhibitors were used to determine the contribution of microtubules/tunnelling nanotubes (TNTs, cytochalasin D), gap junctions (carbenoxolone), connexin-43 (gap26) and microvesicles (dynasore).

Results

F-actin microtubules/TNTs extending from BM-MSCs, LT-MSCs and BAL-MSCs to bronchial epithelial cells formed within 45 minutes of co-culturing cells. Each MSC population transferred a similar volume of cytoplasmic content to epithelial cells. Inhibiting microtubule/TNTs, gap junction formation and microvesicle endocytosis abrogated the transfer of cytoplasmic material from BM-MSCs, LT-MSCs and BAL-MSCs to epithelial cells. In contrast, blocking connexin-43 gap junction formation had no effect on cytoplasmic transfer. All MSC populations donated mitochondria to bronchial epithelial cells with similar efficiency. Mitochondrial transfer was reduced in all co-cultures after microtubule/TNT or endocytosis inhibition. Gap junction formation inhibition reduced mitochondrial transfer in BM-MSC and BAL-MSC co-cultures but had no effect on transfer in LT-MSC co-cultures. Connexin-43 inhibition did not impact mitochondrial transfer. Finally, bronchial epithelial cells were incapable of donating cytoplasmic content or mitochondria to any MSC population.

Conclusion

Similar to their bone marrow counterparts, LT-MSCs and BAL-MSCs can donate cytoplasmic content and mitochondria to bronchial epithelial cells via multiple mechanisms. Given that BM-MSCs utilize these mechanisms to mediate the repair of damaged bronchial epithelial cells, both LT-MSCs and BAL-MSCs will probably function similarly.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0354-8) contains supplementary material, which is available to authorized users.

Mesenchymal Stromal Cells are Readily Recoverable from Lung Tissue, but not the Alveolar Space, in Healthy Humans

Stromal support is critical for lung homeostasis and the maintenance of an effective epithelial barrier. Despite this, previous studies have found a positive association between the number of mesenchymal stromal cells (MSCs) isolated from the alveolar compartment and human lung diseases associated with epithelial dysfunction. We hypothesised that bronchoalveolar lavage derived MSCs (BAL-MSCs) are dysfunctional and distinct from resident lung tissue MSCs (LT-MSCs). In this study, we comprehensively interrogated the phenotype and transcriptome of human BAL-MSCs and LT-MSCs. We found that MSCs were rarely recoverable from the alveolar space in healthy humans, but could be readily isolated from lung transplant recipients by bronchoalveolar lavage. BAL-MSCs exhibited a CD90^Hi , CD73^Hi , CD45^Neg , CD105^Lo immunophenotype and were bipotent, lacking adipogenic potential. In contrast, MSCs were readily recoverable from healthy human lung tissue and were CD90^{Hi or Lo} , CD73^Hi , CD45^Neg , CD105^Int and had full tri-lineage potential. Transcriptional profiling of the two populations confirmed their status as bona fide MSCs and revealed a high degree of similarity between each other and the archetypal bone-marrow MSC. 105 genes were differentially expressed; 76 of which were increased in BAL-MSCs including genes involved in fibroblast activation, extracellular matrix deposition and tissue remodelling. Finally, we found the fibroblast markers collagen 1A1 and α-smooth muscle actin were increased in BAL-MSCs. Our data suggests that in healthy humans, lung MSCs reside within the tissue, but in disease can differentiate to acquire a profibrotic phenotype and migrate from their in-tissue niche into the alveolar space. Stem Cells 2016;34:2548-2558.

Influenza Virus Infects Epithelial Stem/Progenitor Cells of the Distal Lung: Impact on Fgfr2b-Driven Epithelial Repair

Influenza Virus (IV) pneumonia is associated with severe damage of the lung epithelium and respiratory failure. Apart from efficient host defense, structural repair of the injured epithelium is crucial for survival of severe pneumonia. The molecular mechanisms underlying stem/progenitor cell mediated regenerative responses are not well characterized. In particular, the impact of IV infection on lung stem cells and their regenerative responses remains elusive. Our study demonstrates that a highly pathogenic IV infects various cell populations in the murine lung, but displays a strong tropism to an epithelial cell subset with high proliferative capacity, defined by the signature EpCamhighCD24lowintegrin(α6)high. This cell fraction expressed the stem cell antigen-1, highly enriched lung stem/progenitor cells previously characterized by the signature integrin(β4)+CD200+, and upregulated the p63/krt5 regeneration program after IV-induced injury. Using 3-dimensional organoid cultures derived from these epithelial stem/progenitor cells (EpiSPC), and in vivo infection models including transgenic mice, we reveal that their expansion, barrier renewal and outcome after IV-induced injury critically depended on Fgfr2b signaling. Importantly, IV infected EpiSPC exhibited severely impaired renewal capacity due to IV-induced blockade of β-catenin-dependent Fgfr2b signaling, evidenced by loss of alveolar tissue repair capacity after intrapulmonary EpiSPC transplantation in vivo. Intratracheal application of exogenous Fgf10, however, resulted in increased engagement of non-infected EpiSPC for tissue regeneration, demonstrated by improved proliferative potential, restoration of alveolar barrier function and increased survival following IV pneumonia. Together, these data suggest that tropism of IV to distal lung stem cell niches represents an important factor of pathogenicity and highlight impaired Fgfr2b signaling as underlying mechanism. Furthermore, increase of alveolar Fgf10 levels may represent a putative therapy to overcome regeneration failure after IV-induced lung injury.

Concise Review: The Bystander Effect: Mesenchymal Stem Cell-Mediated Lung Repair

Mesenchymal stem or stromal cells (MSCs), a heterogeneous subset of adult stem/progenitor cells, have surfaced as potential therapeutic units with significant clinical benefit for a wide spectrum of disease conditions, including those affecting the lung. Although MSCs carry both self-renewal and multilineage differentiation abilities, current dogma holds that MSCs mainly contribute to tissue regeneration and repair by modulating the host tissue via secreted cues. Thus, the therapeutic benefit of MSCs is thought to derive from so called bystander effects. The regenerative mechanisms employed by MSCs in the lung include modulation of the immune system as well as promotion of epithelial and endothelial repair. Apart from secreted factors, a number of recent findings suggest that MSCs engage in mitochondrial transfer and shedding of membrane vesicles as a means to enhance tissue repair following injury. Furthermore, it is becoming increasingly clear that MSCs are an integral component of epithelial lung stem cell niches. As such, MSCs play an important role in coupling information from the environment to stem and progenitor populations, such that homeostasis can be ensured even in the face of injury. It is the aim of this review to outline the major mechanisms by which MSCs contribute to lung regeneration, synthesizing recent preclinical findings with data from clinical trials and potential for future therapy. Stem Cells 2016;34:1437-1444.

Role of fibroblast growth factors in organ regeneration and repair

In its broad sense, regeneration refers to the renewal of lost cells, tissues or organs as part of the normal life cycle (skin, hair, endometrium etc.) or as part of an adaptive mechanism that organisms have developed throughout evolution. For example, worms, starfish and amphibians have developed remarkable regenerative capabilities allowing them to voluntarily shed body parts, in a process called autotomy, only to replace the lost parts afterwards. The bizarre myth of the fireproof homicidal salamander that can survive fire and poison apple trees has persisted until the 20th century. Salamanders possess one of the most robust regenerative machineries in vertebrates and attempting to draw lessons from limb regeneration in these animals and extrapolate the knowledge to mammals is a never-ending endeavor. Fibroblast growth factors are potent morphogens and mitogens that are highly conserved among the animal kingdom. These growth factors play key roles in organogenesis during embryonic development as well as homeostatic balance during postnatal life. In this review, we provide a summary about the current knowledge regarding the involvement of fibroblast growth factor signaling in organ regeneration and repair. We also shed light on the use of these growth factors in previous and current clinical trials in a wide array of human diseases.

Evidence for lung epithelial stem cell niches

Recent studies have identified epithelial stem and progenitor cell populations of the lung. We are just beginning to understand the mechanisms that regulate their homeostatic, regenerative and maladaptive behaviors. Here, we discuss evidence of regulatory niches for epithelial stem cells of the lung.

Recent advances in the mechanisms of lung alveolarization and the pathogenesis of bronchopulmonary dysplasia

Alveolarization is the process by which the alveoli, the principal gas exchange units of the lung, are formed. Along with the maturation of the pulmonary vasculature, alveolarization is the objective of late lung development. The terminal airspaces that were formed during early lung development are divided by the process of secondary septation, progressively generating an increasing number of alveoli that are of smaller size, which substantially increases the surface area over which gas exchange can take place. Disturbances to alveolarization occur in bronchopulmonary dysplasia (BPD), which can be complicated by perturbations to the pulmonary vasculature that are associated with the development of pulmonary hypertension. Disturbances to lung development may also occur in persistent pulmonary hypertension of the newborn in term newborn infants, as well as in patients with congenital diaphragmatic hernia. These disturbances can lead to the formation of lungs with fewer and larger alveoli and a dysmorphic pulmonary vasculature. Consequently, affected lungs exhibit a reduced capacity for gas exchange, with important implications for morbidity and mortality in the immediate postnatal period and respiratory health consequences that may persist into adulthood. It is the objective of this Perspectives article to update the reader about recent developments in our understanding of the molecular mechanisms of alveolarization and the pathogenesis of BPD.

Fibroblast growth factor signaling in myofibroblasts differs from lipofibroblasts during alveolar septation in mice

Pulmonary alveolar fibroblasts produce extracellular matrix in a temporally and spatially regulated pattern to yield a durable yet pliable gas-exchange surface. Proliferation ensures a sufficient complement of cells, but they must differentiate into functionally distinct subtypes: contractile myofibroblasts (MF), which generate elastin and regulate air-flow at the alveolar ducts, and, in mice and rats, lipofibroblasts (LF), which store neutral lipids. PDGF-A is required but acts in conjunction with other differentiation factors arising from adjacent epithelia or within fibroblasts. We hypothesized that FGF receptor (FGFR) expression and function vary for MF and LF and contributes to their divergent differentiation. Whereas approximately half of the FGFR3 was extracellular in MF, FGFR2 and FGFR4 were primarily intracellular. Intracellular FGFR3 localized to the multivesicular body, and its abundance may be modified by Sprouty and interaction with heat shock protein-90. FGF18 mRNA is more abundant in MF, whereas FGF10 mRNA predominated in LF, which also express FGFR1 IIIb, a receptor for FGF10. FGF18 diminished fibroblast proliferation and was chemotactic for cultured fibroblasts. Although PDGF receptor-α (PDGFR-α) primarily signals through phosphoinositide 3-kinase and Akt, p42/p44 MAP kinase (Erk1/2), a major signaling pathway for FGFRs, influenced the abundance of cell-surface PDGFR-α. Observing different FGFR and ligand profiles in MF and LF is consistent with their divergent differentiation although both subpopulations express PDGFR-α. These studies also emphasize the importance of particular cellular locations of FGFR3 and PDGFR-α, which may modify their effects during alveolar development or repair.

The LIM-domain only protein 4 contributes to lung epithelial cell proliferation but is not essential for tumor progression

BACKGROUND

The lung is constantly exposed to environmental challenges and must rapidly respond to external insults. Mechanisms involved in the repair of the damaged lung involve expansion of different epithelial cells to repopulate the injured cellular compartment. However, factors regulating cell proliferation following lung injury remain poorly understood. Here we studied the role of the transcriptional regulator Lmo4 during lung development, in the regulation of adult lung epithelial cell proliferation following lung damage and in the context of oncogenic transformation.

METHODS

To study the role of Lmo4 in embryonic lung development, lung repair and tumorigenesis, we used conditional knock-out mice to delete Lmo4 in lung epithelial cells from the first stages of lung development. The role of Lmo4 in lung repair was evaluated using two experimental models of lung damage involving chemical and viral injury. The role of Lmo4 in lung tumorigenesis was measured using a mouse model of lung adenocarcinoma in which the oncogenic K-Ras protein has been knocked into the K-Ras locus. Overall survival difference between genotypes was tested by log rank test. Difference between means was tested using one-way ANOVA after assuring that assumptions of normality and equality of variance were satisfied.

RESULTS

We found that Lmo4 was not required for normal embryonic lung morphogenesis. In the adult lung, loss of Lmo4 reduced epithelial cell proliferation and delayed repair of the lung following naphthalene or flu-mediated injury, suggesting that Lmo4 participates in the regulation of epithelial cell expansion in response to cellular damage. In the context of K-Ras(G12D)-driven lung tumor formation, Lmo4 loss did not alter overall survival but delayed initiation of lung hyperplasia in K-Ras(G12D) mice sensitized by naphthalene injury. Finally, we evaluated the expression of LMO4 in tissue microarrays of early stage non-small cell lung cancer and observed that LMO4 is more highly expressed in lung squamous cell carcinoma compared to adenocarcinoma.

CONCLUSIONS

Together these results show that the transcriptional regulator Lmo4 participates in the regulation of lung epithelial cell proliferation in the context of injury and oncogenic transformation but that Lmo4 depletion is not sufficient to prevent lung repair or tumour formation.

Expression of α-Smooth Muscle Actin Determines the Fate of Mesenchymal Stromal Cells

Pro-fibrotic microenvironments of scars and tumors characterized by increased stiffness stimulate mesenchymal stromal cells (MSCs) to express α-smooth muscle actin (α-SMA). We investigated whether incorporation of α-SMA into contractile stress fibers regulates human MSC fate. Sorted α-SMA-positive MSCs exhibited high contractile activity, low clonogenicity, and differentiation potential limited to osteogenesis. Knockdown of α-SMA was sufficient to restore clonogenicity and adipogenesis in MSCs. Conversely, α-SMA overexpression induced YAP translocation to the nucleus and reduced the high clonogenicity and adipogenic potential of α-SMA-negative MSCs. Inhibition of YAP rescued the decreased adipogenic differentiation potential induced by α-SMA, establishing a mechanistic link between matrix stiffness, α-SMA, YAP, and MSC differentiation. Consistent with in vitro findings, nuclear localization of YAP was positively correlated in α-SMA expressing stromal cells of adiposarcoma and osteosarcoma. We propose that α-SMA mediated contraction plays a critical role in mechanically regulating MSC fate by controlling YAP/TAZ activation.

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The α-SMA-positive myofibroblast fraction of human MSCs exhibits low clonogenicity

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Formation of α-SMA stress fibers enhances nuclear translocation of YAP/TAZ in MSCs

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α-SMA knockdown favors adipogenesis, while overexpression promotes osteogenesis

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α-SMA-mediated lineage choice of MSCs is YAP dependent

Characterization of the cell of origin and propagation potential of the fibroblast growth factor 9-induced mouse model of lung adenocarcinoma

Fibroblast growth factor 9 (FGF9) is essential for lung development and is highly expressed in a subset of human lung adenocarcinomas. We recently described a mouse model in which FGF9 expression in the lung epithelium caused proliferation of the airway epithelium at the terminal bronchioles and led to rapid development of adenocarcinoma. Here, we used this model to characterize the effects of prolonged FGF9 induction on the proximal and distal lung epithelia, and examined the propagation potential of FGF9-induced lung tumours. We showed that prolonged FGF9 over-expression in the lung resulted in the development of adenocarcinomas arising from both alveolar type II and airway secretory cells in the lung parenchyma and airways, respectively. We found that tumour cells harboured tumour-propagating cells that were able to form secondary tumours in recipient mice, regardless of FGF9 expression. However, the highest degree of tumour propagation was observed when unfractionated tumour cells were co-administered with autologous, tumour-associated mesenchymal cells. Although the initiation of lung adenocarcinomas was dependent on activation of the FGF9-FGF receptor 3 (FGFR3) signalling axis, maintenance and propagation of the tumour was independent of this signalling. Activation of an alternative FGF-FGFR axis and the interaction with tumour stromal cells is likely to be responsible for the development of this independence. This study demonstrates the complex role of FGF-FGFR signalling in the initiation, growth and propagation of lung cancer. Our findings suggest that analysing the expressions of FGF-FGFRs in human lung cancer will be a useful tool for guiding customized therapy.

Biomarkers and signaling pathways of colorectal cancer stem cells

The progression of colorectal cancer is commonly characterized by accumulation of genetic or epigenetic abnormalities, altering regulation of gene expression as well as normal protein structures and functions. Nonetheless, there are some questions that remain to be elucidated, such as the origin of cancer cells and populations of cells initiating and propagating tumor development. Currently, there are two rival theories describing the process of carcinogenesis. One is the stochastic model, arguing that any cell is capable of initiating and triggering the development of cancer. Meanwhile, the cancer stem cell model hypothesizes that only a small fraction of stem cells possesses cancer-promoting properties. Typically, colorectal cancer stem cells (CSCs) share the same molecular signaling profiles with normal stem cells or embryonic stem cells, such as Wnt, Notch, TGF-β, and Hedgehog. Nevertheless, CSCs differ from normal stem cells and the bulk of tumor cells in their tumorigenic potential and susceptibility to chemotherapeutic drugs. This may be a possible explanation of the high percentage of cancer recurrence in patients who underwent chemotherapeutic treatment and surgery. This review article focuses on the colorectal cancer stem cell biomarkers and the role of upregulated signaling pathways implicated in the initiation and progression of colorectal cancer.

Clonal culture of adult mouse lung epithelial stem/progenitor cells

Clonal culture of stem cells is crucial for their identification, and the characterization of the cellular and molecular mechanisms that regulate their proliferation and differentiation. In the adult mouse lung, epithelial stem/progenitor cells are defined by the phenotype CD45(neg) CD31(neg) EpCAM(pos) CD104(pos) CD24(low). Here we describe a tissue dissociation and flow cytometry strategy for the detection and isolation of adult mouse lung epithelial stem/progenitor cells, and a three-dimensional colony-forming assay for their clonal culture in vitro.

Harnessing the potential of lung stem cells for regenerative medicine

In response to recurrent exposure to environmental insults such as allergens, pollution, irritants, smoke and viral/bacterial infection, the epithelium of the lung is continually damaged. Homeostasis of the lung requires a balance between immune regulation and promotion of tissue regeneration, which requires the co-ordinated proliferation and differentiation of stem and progenitor cells. In this review we reflect on the current understanding of lung epithelial stem and progenitor cells and advocate a model hierarchy in which self-renewing multipotent lung epithelial stem cells give rise to lineage restricted progenitor cells that repopulate airway and alveolar epithelial cell lineages during homeostasis and repair. We also discuss the role of mesenchymal progenitor cells in maintaining the structural integrity of the lung and propose a model in which mesenchymal cells act as the quintessential architects of lung regeneration by providing molecular signals, such as FGF-10, to regulate the fate and specificity of epithelial stem and progenitor cells. Moreover, we discuss the current status and future prospects for translating lung stem cell therapies to the clinic to replace, repair, or regenerate diseased lung tissue. This article is part of a directed issue entitled: Regenerative Medicine: the challenge of translation.

Walking along the Fibroblast Growth Factor 10 Route: A Key Pathway to Understand the Control and Regulation of Epithelial and Mesenchymal Cell-Lineage Formation during Lung Development and Repair after Injury

Basic research on embryonic lung development offers unique opportunities to make important discoveries that will impact human health. Developmental biologists interested in the molecular control of branching morphogenesis have intensively studied the developing lung, with its complex and seemingly stereotyped ramified structure. However, it is also an organ that is linked to a vast array of clinical problems in humans such as bronchopulmonary dysplasia in premature babies and emphysema, chronic obstructive pulmonary disease, fibrosis, and cancer in adults. Epithelial stem/progenitor cells reside in niches where they interact with specific extracellular matrices as well as with mesenchymal cells; the latter are still poorly characterized. Interactions of epithelial stem/progenitor cells with their microenvironments are usually instructive, controlling quiescence versus activation, proliferation, differentiation, and migration. During the past 18 years, Fgf10 has emerged not only as a marker for the distal lung mesenchyme during early lung development, but also as a key player in branching morphogenesis and a critical component of the niche for epithelial stem cells. In this paper, we will present the current knowledge regarding the lineage tree in the lung, with special emphasis on cell-lineage decisions in the lung mesenchyme and the role of Fgf10 in this context.

In search of the elusive lipofibroblast in human lungs

Although the pulmonary interstitial lipofibroblast (LF) has been widely recognized in rat and mouse lungs, their presence in human lungs remains controversial. In a recent issue of the Journal, Tahedl and associates (Tahedl D, Wirkes A, Tschanz SA, Ochs M, Mühlfeld C. Am J Physiol Lung Cell Mol Physiol 307: L386-L394, 2014) address this controversy and provide the most detailed stereological analysis of LFs in mammals other than rodents. Strikingly, their observations demonstrate that LFs were only observed in rodents, which contrasts with earlier reports. This editorial reviews the anatomical, physiological, and biochemical characteristics of the LF to better understand the significance of LFs for lung development and disease. Although lipid droplets are a signature of the LF cell type, it remains unclear whether lipid storage is the defining characteristic of LFs, or whether other less overt properties determine the importance of LFs. Are lipid droplets an adaptation to the neonatal environment, or are LFs a surrogate for other properties that promote alveolar development, and do lipid droplets modify physiology or disease in adults?

Regulation of fibroblast lipid storage and myofibroblast phenotypes during alveolar septation in mice

Signaling through platelet-derived growth factor receptor-α (PDGFRα) is required for alveolar septation and participates in alveolar regeneration after pneumonectomy. In both adipose tissue and skeletal muscle, bipotent pdgfrα-expressing progenitors expressing delta-like ligand-1 or sex-determining region Y box 9 (Sox9) may differentiate into either lipid storage cells or myofibroblasts. We analyzed markers of mesenchymal progenitors and differentiation in lung fibroblasts (LF) with different levels (absent, low, or high) of pdgfrα gene expression. A larger proportion of pdgfrα-expressing than nonexpressing LF contained Sox9. Neutral lipids, CD166, and Tcf21 were more abundant in LF with a lower compared with a higher level of pdgfrα gene expression. PDGF-A increased Sox9 in primary LF cultures, suggesting that active signaling through PDGFRα is required to maintain Sox9. As alveolar septation progresses from postnatal day (P) 8 to P12, fewer pdgfrα-expressing LF contain Sox9, whereas more of these LF contain myocardin-like transcription factor-A, showing that Sox9 diminishes as LF become myofibroblasts. At P8, neutral lipid droplets predominate in LF with the lower level of pdgfrα gene expression, whereas transgelin (tagln) was predominantly expressed in LF with higher pdgfrα gene expression. Targeted deletion of pdgfrα in LF, which expressed tagln, reduced Sox9 in α-actin (α-SMA, ACTA2)-containing LF, whereas it increased the abundance of cell surface delta-like protein-1 (as well as peroxisome proliferator-activated receptor-γ and tcf21 mRNA in LF, which also expressed stem cell antigen-1). Thus pdgfrα deletion differentially alters delta-like protein-1 and Sox9, suggesting that targeting different downstream pathways in PDGF-A-responsive LF could identify strategies that promote lung regeneration without initiating fibrosis.