Cell competition drives bronchiolization and pulmonary fibrosis.

Idiopathic pulmonary fibrosis (IPF) is a progressive scarring disease arising from the maladaptive differentiation of lung stem cells into bronchial epithelial cells rather than into alveolar type 1 (AT1) cells, which are responsible for gas exchange. Here, we report that healthy lungs maintain their stem cells through tonic Hippo and ß-catenin signaling, which promote Yap/Taz degradation and allow for low level expression of the Wnt target gene Myc. Inactivation of upstream activators of the Hippo pathway in lung stem cells inhibits this tonic ß-catenin signaling and Myc expression and promotes their Taz mediated differentiation into AT1 cells. Vice versa, increased Myc in collaboration with Yap promotes the differentiation of lung stem cells along the basal and myoepithelial like lineages allowing them to invade and bronchiolize the lung parenchyma in a process reminiscent of submucosal gland development. Our findings indicate that stem cells exhibiting the highest Myc levels become supercompetitors that drive remodeling, whereas loser cells with lower Myc levels terminally differentiate into AT1 cells.

Protocol for the use of signal amplification by exchange reaction-fluorescence in situ hybridization on adult formalin-fixed paraffin-embedded mouse lung tissue.

Fluorescence in situ hybridization (FISH) is a useful tool for analyzing RNA expression, but difficulties arise with low-abundance RNA and in tissues that are formalin-fixed paraffin-embedded (FFPE) because reagents can be expensive. In this protocol, we adapt a previously designed FISH amplification protocol (SABER [signal amplification by exchange reaction]) for adult mouse FFPE lung sections by using probes that are extended and branched to amplify the signal. We combine FISH and immunostaining to identify cell-specific RNA. For complete details on the use and execution of this protocol, please refer to Kishi et al.1 and Lyu et al.2.

Hippo signaling impairs alveolar epithelial regeneration in pulmonary fibrosis.

Idiopathic pulmonary fibrosis (IPF) consists of fibrotic alveolar remodeling and progressive loss of pulmonary function. Genetic and experimental evidence indicates that chronic alveolar injury and failure to properly repair the respiratory epithelium are intrinsic to IPF pathogenesis. Loss of alveolar type 2 (AT2) stem cells or mutations that either impair their self-renewal and/or impair their differentiation into AT1 cells can serve as a trigger of pulmonary fibrosis. Recent reports indicate increased YAP activity in respiratory epithelial cells in IPF lungs. Individual IPF epithelial cells with aberrant YAP activation in bronchiolized regions frequently co-express AT1, AT2, conducting airway selective markers and even mesenchymal or EMT markers, demonstrating 'indeterminate' states of differentiation and suggesting that aberrant YAP signaling might promote pulmonary fibrosis. Yet, Yap and Taz have recently also been shown to be important for AT1 cell maintenance and alveolar epithelial regeneration after Streptococcus pneumoniae-induced injury. To investigate how epithelial Yap/Taz might promote pulmonary fibrosis or drive alveolar epithelial regeneration, we inactivated the Hippo pathway in AT2 stem cells resulting in increased nuclear Yap/Taz, and found that this promotes their alveolar regenerative capacity and reduces pulmonary fibrosis following bleomycin injury by pushing them along the AT1 cell lineage. Vice versa, inactivation of both Yap1 and Wwtr1 (encoding Taz) or Wwtr1 alone in AT2 cell stem cells impaired alveolar epithelial regeneration and resulted in increased pulmonary fibrosis upon bleomycin injury. Interestingly, the inactivation of only Yap1 in AT2 stem cells promoted alveolar epithelial regeneration and reduced pulmonary fibrosis. Together, these data suggest that epithelial Yap promotes, and epithelial Taz reduces pulmonary fibrosis suggesting that targeting Yap but not Taz-mediated transcription might help promote AT1 cell regeneration and treat pulmonary fibrosis.

Niche-mediated repair of airways is directed in an occupant-dependent manner.

In injured airways of the adult lung, epithelial progenitors are called upon to repair by nearby mesenchymal cells via signals transmitted through the niche. Currently, it is unclear whether repair is coordinated by the mesenchymal cells that maintain the niche or by the airway epithelial cells that occupy it. Here, we show that the spatiotemporal expression of Fgf10 by the niche is primarily orchestrated by the niche's epithelial occupants-both those that reside prior to, and following, injury. During homeostasis, differentiated airway epithelial cells secrete Sonic hedgehog (Shh) to inhibit Fgf10 expression by Gli1+ peribronchial mesenchymal cells in the niche. After injury, remaining epithelial cells produce Wnt7b to induce Fgf10 expression in airway smooth muscle cells in the niche. We find that this reliance on a common activator of airway epithelial stem cells also allows for the recruitment of remote stem cell populations when local populations have been exhausted.

Mesenchymal stromal cell aging impairs the self-organizing capacity of lung alveolar epithelial stem cells.

Multicellular organisms maintain structure and function of tissues/organs through emergent, self-organizing behavior. In this report, we demonstrate a critical role for lung mesenchymal stromal cell (L-MSC) aging in determining the capacity to form three-dimensional organoids or 'alveolospheres' with type 2 alveolar epithelial cells (AEC2s). In contrast to L-MSCs from aged mice, young L-MSCs support the efficient formation of alveolospheres when co-cultured with young or aged AEC2s. Aged L-MSCs demonstrated features of cellular senescence, altered bioenergetics, and a senescence-associated secretory profile (SASP). The reactive oxygen species generating enzyme, NADPH oxidase 4 (Nox4), was highly activated in aged L-MSCs and Nox4 downregulation was sufficient to, at least partially, reverse this age-related energy deficit, while restoring the self-organizing capacity of alveolospheres. Together, these data indicate a critical role for cellular bioenergetics and redox homeostasis in an organoid model of self-organization and support the concept of thermodynamic entropy in aging biology.

Many tissues in the body are capable of regenerating by replacing defective or worn-out cells with new ones. This process relies heavily on stem cells, which are precursor cells that lack a set role in the body and can develop into different types of cells under the right conditions. Tissues often have their own pool of stem cells that they use to replenish damaged cells. But as we age, this regeneration process becomes less effective. Many of our organs, such as the lungs, are lined with epithelial cells. These cells form a protective barrier, controlling what substances get in and out of the tissue. Alveoli are parts of the lungs that allow oxygen and carbon dioxide to move between the blood and the air in the lungs. And alveoli rely on an effective epithelial cell lining to work properly. To replenish these epithelial cells, alveoli have pockets, in which a type of epithelial cell, known as AEC2, lives. These cells can serve as stem cells, developing into a different type of cell under the right conditions. To work properly, AEC2 cells require close interactions with another type of cell called L-MSC, which supports the maintenance of other cells and also has the ability to differentiate into several other cell types. Both cell types can be found close together in these stem cell pockets. So far, it has been unclear how aging affects how these cells work together to replenish the epithelial lining of the alveoli. To investigate, Chanda et al. probed AEC2s and L-MSCs in the alveoli of young and old mice. The researchers collected both cell types from young (2-3 months) and aged (22-24 months) mice. Various combinations of these cells were grown to form 3D structures, mimicking how the cells grow in the lungs. Young L-MSCs formed normal 3D structures with both young and aged AEC2 cells. But aged L-MSCs developed abnormal, loose structures with AEC2 cells (both young and old cells). Aged L-MSCs were found to have higher levels of an enzyme (called Nox4) that produces oxidants and other 'pro-aging' factors, compared to young L-MSCs. However, reducing Nox4 levels in aged L-MSCs allowed these cells to form normal 3D structures with young AEC2 cells, but not aged AEC2 cells. These findings highlight the varying effects specific stem cells have, and how their behaviour is affected by pro-aging factors. Moreover, the pro-aging enzyme Nox4 shows potential as a therapeutic target ­ downregulating its activity may reverse critical effects of aging in cells.

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

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

Erratum: The WW domains dictate isoform-specific regulation of YAP1 stability and pancreatic cancer cell malignancy: Erratum.

[This corrects the article DOI: 10.7150/thno.42795.].

Resident mesenchymal vascular progenitors modulate adaptive angiogenesis and pulmonary remodeling via regulation of canonical Wnt signaling.

Adaptive angiogenesis is necessary for tissue repair, however, it may also be associated with the exacerbation of injury and development of chronic disease. In these studies, we demonstrate that lung mesenchymal vascular progenitor cells (MVPC) modulate adaptive angiogenesis via lineage trace, depletion of MVPC, and modulation of ß-catenin expression. Single cell sequencing confirmed MVPC as multipotential vascular progenitors, thus, genetic depletion resulted in alveolar simplification with reduced adaptive angiogenesis. Following vascular endothelial injury, Wnt activation in MVPC was sufficient to elicit an emphysema-like phenotype characterized by increased MLI, fibrosis, and MVPC driven adaptive angiogenesis. Lastly, activation of Wnt/ß-catenin signaling skewed the profile of human and murine MVPC toward an adaptive phenotype. These data suggest that lung MVPC drive angiogenesis in response to injury and regulate the microvascular niche as well as subsequent distal lung tissue architecture via Wnt signaling.

The WW domains dictate isoform-specific regulation of YAP1 stability and pancreatic cancer cell malignancy.

YAP1 is a key mediator of the Hippo pathway capable of exerting a profound effect on organ size as well as tumorigenesis. Alternative mRNA splicing of human YAP1 results in at least 8 protein isoforms that differ within the 2nd WW motif and the transcriptional activation domain. Methods: To investigate the isoform-specific differences in their mRNA expression, transcriptional activity and tumor-promoting function, we cloned cDNA encoding all of the eight YAP1 protein isoforms. Then, we examined their mRNA expression, subcellular localization, transcriptional regulation properties, interactions with key regulatory partners, and protein stability in response to changes in cell density, as well as their effects on pancreatic cancer cell malignancy both in vitro and in vivo. Results: Multiple YAP1 mRNA isoforms are expressed in commonly used pancreatic cancer lines as well as human pancreatic cancer PDX lines. Based on the analysis of heterologous reporter and endogenous target genes, all YAP1 isoforms are capable of activating transcription, albeit to a different extent. Importantly, we unveiled a marked discrepancy between the mRNA and protein expression levels of the YAP1-1 and YAP1-2 isoforms. We further discovered that the YAP1-2 isoform, which contains two tandem WW motifs, is less stable at the protein level, particularly at high cell densities. Mechanistically, we found that the presence of the 2nd WW motif in YAP1-2 facilitates the de novo formation of the YAP1-2/AMOT/LATS1 complex and contributes to a stronger binding of YAP1-2 to LATS1 and subsequently increased YAP1-2 ubiquitination and degradation by ß-TRCP. Conclusion: Our data reveals a potent effect of YAP1-1 on pancreatic cancer malignancy in vitro and in vivo and provides novel mechanistic insight into isoform-specific and cell density-dependent regulation of YAP1 stability, as well as its impact on cancer malignancy.

Temporospatial Expression of Fgfr1 and 2 During Lung Development, Homeostasis, and Regeneration.

Fgfr1 (Fibroblast growth factor receptor 1) and Fgfr2 are dynamically expressed during lung development, homeostasis, and regeneration. Our current analysis indicates that Fgfr2 is expressed in distal epithelial progenitors AT2, AT1, club, and basal cells but not in ciliated or neuroendocrine cells during lung development and homeostasis. However, after injury, Fgfr2 becomes upregulated in neuroendocrine cells and distal club cells. Epithelial Fgfr1 expression is minimal throughout lung development, homeostasis, and regeneration. We further find both Fgfr1 and Fgfr2 strongly expressed in cartilage progenitors and airway smooth muscle cells during lung development, whereas Fgfr1 but not Fgfr2 was expressed in lipofibroblasts and vascular smooth muscle cells. In the adult lung, Fgfr1 and Fgfr2 were mostly downregulated in smooth muscle cells but became upregulated after injury. Fgfr1 remained expressed in mesenchymal alveolar niche cells or lipofibroblasts with lower levels of expression in their descendant (alveolar) myofibroblasts during alveologenesis.

FGF10-FGFR2B Signaling Generates Basal Cells and Drives Alveolar Epithelial Regeneration by Bronchial Epithelial Stem Cells after Lung Injury.

Idiopathic pulmonary fibrosis is a common form of interstitial lung disease resulting in alveolar remodeling and progressive loss of pulmonary function because of chronic alveolar injury and failure to regenerate the respiratory epithelium. Histologically, fibrotic lesions and honeycomb structures expressing atypical proximal airway epithelial markers replace alveolar structures, the latter normally lined by alveolar type 1 (AT1) and AT2 cells. Bronchial epithelial stem cells (BESCs) can give rise to AT2 and AT1 cells or honeycomb cysts following bleomycin-mediated lung injury. However, little is known about what controls this binary decision or whether this decision can be reversed. Here we report that inactivation of Fgfr2b in BESCs impairs their contribution to both alveolar epithelial regeneration and honeycomb cysts after bleomycin injury. By contrast overexpression of Fgf10 in BESCs enhances fibrosis resolution by favoring the more desirable outcome of alveolar epithelial regeneration over the development of pathologic honeycomb cysts.

Hippo signaling promotes lung epithelial lineage commitment by curbing Fgf10 and ß-catenin signaling.

Organ growth and tissue homeostasis rely on the proliferation and differentiation of progenitor cell populations. In the developing lung, localized Fgf10 expression maintains distal Sox9-expressing epithelial progenitors and promotes basal cell differentiation in the cartilaginous airways. Mesenchymal Fgf10 expression is induced by Wnt signaling but inhibited by Shh signaling, and epithelial Fgf10 signaling activates ß-catenin signaling. The Hippo pathway is a well-conserved signaling cascade that regulates organ size and stem/progenitor cell behavior. Here, we show that Hippo signaling promotes lineage commitment of lung epithelial progenitors by curbing Fgf10 and ß-catenin signaling. Our findings show that both inactivation of the Hippo pathway (nuclear Yap) or ablation of Yap result in increased ß-catenin and Fgf10 signaling, suggesting a cytoplasmic role for Yap in epithelial lineage commitment. We further demonstrate redundant and non-redundant functions for the two nuclear effectors of the Hippo pathway, Yap and Taz, during lung development.

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.

Fgf10 Signaling in Lung Development, Homeostasis, Disease, and Repair After Injury.

The lung is morphologically structured into a complex tree-like network with branched airways ending distally in a large number of alveoli for efficient oxygen exchange. At the cellular level, the adult lung consists of at least 40-60 different cell types which can be broadly classified into epithelial, endothelial, mesenchymal, and immune cells. Fibroblast growth factor 10 (Fgf10) located in the lung mesenchyme is essential to regulate epithelial proliferation and lineage commitment during embryonic development and post-natal life, and to drive epithelial regeneration after injury. The cells that express Fgf10 in the mesenchyme are progenitors for mesenchymal cell lineages during embryonic development. During adult lung homeostasis, Fgf10 is expressed in mesenchymal stromal niches, between cartilage rings in the upper conducting airways where basal cells normally reside, and in the lipofibroblasts adjacent to alveolar type 2 cells. Fgf10 protects and promotes lung epithelial regeneration after different types of lung injuries. An Fgf10-Hippo epithelial-mesenchymal crosstalk ensures maintenance of stemness and quiescence during homeostasis and basal stem cell (BSC) recruitment to further promote regeneration in response to injury. Fgf10 signaling is dysregulated in different human lung diseases including bronchopulmonary dysplasia (BPD), idiopathic pulmonary fibrosis (IPF), and chronic obstructive pulmonary disease (COPD), suggesting that dysregulation of the FGF10 pathway is critical to the pathogenesis of several human lung diseases.

Protein Tyrosine Phosphatase-N13 Promotes Myofibroblast Resistance to Apoptosis in Idiopathic Pulmonary Fibrosis.

RATIONALE: Idiopathic pulmonary fibrosis (IPF) is a progressive, fibrotic interstitial lung disease characterized by (myo)fibroblast accumulation and collagen deposition. Resistance to Fas-induced apoptosis is thought to facilitate (myo)fibroblast persistence in fibrotic lung tissues by poorly understood mechanisms. OBJECTIVES: To test the hypothesis that PTPN13 (protein tyrosine phosphatase-N13) is expressed by IPF lung (myo)fibroblasts, promotes their resistance to Fas-induced apoptosis, and contributes to the development of pulmonary fibrosis. METHODS: PTPN13 was localized in lung tissues from patients with IPF and control subjects by immunohistochemical staining. Inhibition of PTPN13 function in primary IPF and normal lung (myo)fibroblasts was accomplished by: 1) downregulation with TNF-α (tumor necrosis factor-α)/IFN-γ, 2) siRNA knockdown, or 3) a cell-permeable Fas/PTPN13 interaction inhibitory peptide. The role of PTPN13 in the development of pulmonary fibrosis was assessed in mice with genetic deficiency of PTP-BL, the murine ortholog of PTPN13. MEASUREMENTS AND MAIN RESULTS: PTPN13 was constitutively expressed by (myo)fibroblasts in the fibroblastic foci of patients with IPF. Human lung (myo)fibroblasts, which are resistant to Fas-induced apoptosis, basally expressed PTPN13 in vitro. TNF-α/IFN-γ or siRNA-mediated PTPN13 downregulation and peptide-mediated inhibition of the Fas/PTPN13 interaction in human lung (myo)fibroblasts promoted Fas-induced apoptosis. Bleomycin-challenged PTP-BL-/- mice, while developing inflammatory lung injury, exhibited reduced pulmonary fibrosis compared with wild-type mice. CONCLUSIONS: These findings suggest that PTPN13 mediates the resistance of human lung (myo)fibroblasts to Fas-induced apoptosis and promotes pulmonary fibrosis in mice. Our results suggest that strategies aimed at interfering with PTPN13 expression or function may represent a novel strategy to reduce fibrosis in IPF.

Fgf10-Hippo Epithelial-Mesenchymal Crosstalk Maintains and Recruits Lung Basal Stem Cells.

The lung harbors its basal stem/progenitor cells (BSCs) in the protected environment of the cartilaginous airways. After major lung injuries, BSCs are activated and recruited to sites of injury. Here, we show that during homeostasis, BSCs in cartilaginous airways maintain their stem cell state by downregulating the Hippo pathway (resulting in increased nuclear Yap), which generates a localized Fgf10-expressing stromal niche; in contrast, differentiated epithelial cells in non-cartilaginous airways maintain quiescence by activating the Hippo pathway and inhibiting Fgf10 expression in airway smooth muscle cells (ASMCs). However, upon injury, surviving differentiated epithelial cells spread to maintain barrier function and recruit integrin-linked kinase to adhesion sites, which leads to Merlin degradation, downregulation of the Hippo pathway, nuclear Yap translocation, and expression and secretion of Wnt7b. Epithelial-derived Wnt7b, then in turn, induces Fgf10 expression in ASMCs, which extends the BSC niche to promote regeneration.

Disruption of lineage specification in adult pulmonary mesenchymal progenitor cells promotes microvascular dysfunction.

Pulmonary vascular disease is characterized by remodeling and loss of microvessels and is typically attributed to pathological responses in vascular endothelium or abnormal smooth muscle cell phenotypes. We have challenged this understanding by defining an adult pulmonary mesenchymal progenitor cell (MPC) that regulates both microvascular function and angiogenesis. The current understanding of adult MPCs and their roles in homeostasis versus disease has been limited by a lack of genetic markers with which to lineage label multipotent mesenchyme and trace the differentiation of these MPCs into vascular lineages. Here, we have shown that lineage-labeled lung MPCs expressing the ATP-binding cassette protein ABCG2 (ABCG2+) are pericyte progenitors that participate in microvascular homeostasis as well as adaptive angiogenesis. Activation of Wnt/ß-catenin signaling, either autonomously or downstream of decreased BMP receptor signaling, enhanced ABCG2+ MPC proliferation but suppressed MPC differentiation into a functional pericyte lineage. Thus, enhanced Wnt/ß-catenin signaling in ABCG2+ MPCs drives a phenotype of persistent microvascular dysfunction, abnormal angiogenesis, and subsequent exacerbation of bleomycin-induced fibrosis. ABCG2+ MPCs may, therefore, account in part for the aberrant microvessel function and remodeling that are associated with chronic lung diseases.

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.

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-ß and SHH signaling. Importantly, fibroblast growth factor-10 (FGF-10) was markedly suppressed in IPF subjects with progressive disease, and both TGF-ß1 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.