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

Vagal-α7nAChR signaling promotes lung stem cells regeneration via fibroblast growth factor 10 during lung injury repair

BACKGROUND

Proliferation and transdifferentiation of lung stem cells (LSCs) could promote lung injury repair. The distal airways of the lung are innervated by the vagus nerve. Vagal-alpha7 nicotinic acetylcholine receptor (α7nAChR) signaling plays a key role in regulating lung infection and inflammation; however, whether this pathway could regulate LSCs remains unknown.

METHODS

LSCs (Sca1⁺CD45^-CD31^- cells) were isolated and characterized according to a previously published protocol. α7nAChR knockout mice and wild-type littermates were intratracheally challenged with lipopolysaccharide (LPS) to induce lung injury. A cervical vagotomy was performed to study the regulatory effect of the vagus nerve on LSCs-mediated lung repair. α7nAChR agonist or fibroblast growth factor 10 (FGF10) was intratracheally delivered to mice. A single-cell suspension of lung cells was analyzed by flow cytometry. Lung tissues were collected for histology, quantitative real-time polymerase chain reaction (RT-PCR), and immunohistochemistry.

RESULTS

We found that LSCs maintained multilineage differentiation ability and transdifferentiated into alveolar epithelial type II cells (AEC2) following FGF10 stimulation in vitro. Vagotomy or α7nAChR deficiency reduced lung Ki67⁺ LSCs expansion and hampered the resolution of LPS-induced lung injury. Vagotomy or α7nAChR deficiency decreased lung FGF10 expression and the number of AEC2. The α7nAChR agonist-GTS-21 reversed the reduction of FGF10 expression in the lungs, as well as the number of Ki67⁺ cells, LSCs, Ki67⁺ LSCs, and AEC2 in LPS-challenged vagotomized mice. Supplementation with FGF10 counteracted the loss of Ki67⁺ LSCs and AEC2 in LPS-challenged α7nAChR knockout mice.

CONCLUSIONS

The vagus nerve deploys α7nAChR to enhance LSCs proliferation and transdifferentiation and promote lung repair in an FGF10-dependent manner during LPS-induced lung injury.

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 regeneration: a tale of mice and men

The respiratory system is the main site of gas exchange with the external environment in complex terrestrial animals. Within the trachea and lungs are multiple different tissue niches each consisting of a myriad of cells types with critical roles in air conduction, gas exchange, providing important niche specific cell-cell interactions, connection to the cardiovascular system, and immune surveillance. How the respiratory system responds to external insults and executes the appropriate regenerative response remains challenging to study given the plethora of cell and tissue interactions for this to occur properly. This review will examine the various cell types and tissue niches found within the respiratory system and provide a comparison between mouse and human lungs and trachea to highlight important similarities and differences. Defining the critical gaps in knowledge in human lung and tracheal regeneration is critical for future development of therapies directed towards respiratory diseases.

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.

Transcriptional characterisation of human lung cells identifies novel mesenchymal lineage markers

RATIONALE

The lung mesenchyme gives rise to multiple distinct lineages of cells in the mature respiratory system, including smooth muscle cells of the airway and vasculature. However, a thorough understanding of the specification and mesenchymal cell diversity in the human lung is lacking.

METHODS

We completed single-cell RNA sequencing analysis of fetal human lung tissues. Canonical correlation analysis, clustering, cluster marker gene identification and t-distributed stochastic neighbour embedding representation was performed in Seurat. Cell populations were annotated using ToppFun. Immunohistochemistry and in situ hybridisation were used to validate spatiotemporal gene expression patterns for key marker genes.

RESULTS

We identified molecularly distinct populations representing "committed" fetal human lung endothelial cells, pericytes and smooth muscle cells. Early endothelial lineages expressed "classic" endothelial cell markers (platelet endothelial cell adhesion molecule/CD31 and claudin 5), while pericytes expressed platelet-derived growth factor receptor-β, Thy-1 membrane glycoprotein and basement membrane molecules (collagen IV, laminin and proteoglycans). We observed a large population of "nonspecific" human lung mesenchymal progenitor cells characterised by expression of collagen I and multiple elastin fibre genes (ELN, MFAP2 and FBN1). We closely characterised the diversity of mesenchymal lineages defined by α₂-smooth muscle actin (ACTA2) expression. Two cell populations, with the highest levels of ACTA2 transcriptional activity, expressed unique sets of markers associated with airway or vascular smooth muscle cells. Spatiotemporal analysis of these marker genes confirmed early and persistent spatial specification of airway (HHIP, MYLK and IGF1) and vascular (NTRK3 and MEF2C) smooth muscle cells in the developing human lung.

CONCLUSION

Our data suggest that specification of distinct airway and vascular smooth muscle cell phenotypes is established early in development and can be identified using the markers we provide.

Hippo-YAP/TAZ signalling in organ regeneration and regenerative medicine

The Hippo pathway and its downstream effectors, the transcriptional co-activators Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), regulate organ growth and cell plasticity during animal development and regeneration. Remarkably, experimental activation of YAP/TAZ in the mouse can promote regeneration in organs with poor or compromised regenerative capacity, such as the adult heart and the liver and intestine of old or diseased mice. However, therapeutic YAP/TAZ activation may cause serious side effects. Most notably, YAP/TAZ are hyperactivated in human cancers, and prolonged activation of YAP/TAZ triggers cancer development in mice. Thus, can the power of YAP/TAZ to promote regeneration be harnessed in a safe way? Here, we review the role of Hippo signalling in animal regeneration, examine the promises and risks of YAP/TAZ activation for regenerative medicine and discuss strategies to activate YAP/TAZ for regenerative therapy while minimizing adverse side effects.

Hippo-Yap/Taz signaling: Complex network interactions and impact in epithelial cell behavior

The Hippo pathway has emerged as a crucial integrator of signals in biological events from development to adulthood and in diseases. Although extensively studied in Drosophila and in cell cultures, major gaps of knowledge still remain on how this pathway functions in mammalian systems. The pathway consists of a growing number of components, including core kinases and adaptor proteins, which control the subcellular localization of the transcriptional co-activators Yap and Taz through phosphorylation of serines at key sites. When localized to the nucleus, Yap/Taz interact with TEAD transcription factors to induce transcriptional programs of proliferation, stemness, and growth. In the cytoplasm, Yap/Taz interact with multiple pathways to regulate a variety of cellular functions or are targeted for degradation. The Hippo pathway receives cues from diverse intracellular and extracellular inputs, including growth factor and integrin signaling, polarity complexes, and cell-cell junctions. This review highlights the mechanisms of regulation of Yap/Taz nucleocytoplasmic shuttling and their implications for epithelial cell behavior using the lung as an intriguing example of this paradigm. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Signaling Pathways > Cell Fate Signaling Establishment of Spatial and Temporal Patterns > Cytoplasmic Localization.

Evidence for lung repair and regeneration in humans: key stem cells and therapeutic functions of fibroblast growth factors

Regeneration carries the idea of regrowing partially or completely a missing organ. Repair, on the other hand, allows restoring the function of an existing but failing organ. The recognition that human lungs can both repair and regenerate is quite novel, the concept has not been widely used to treat patients. We present evidence that the human adult lung does repair and regenerate and introduce different ways to harness this power. Various types of lung stem cells are capable of proliferating and differentiating upon injury driving the repair/regeneration process. Injury models, primarily in mice, combined with lineage tracing studies, have allowed the identification of these important cells. Some of these cells, such as basal cells, broncho-alveolar stem cells, and alveolar type 2 cells, rely on fibroblast growth factor (FGF) signaling for their survival, proliferation and/or differentiation. While preclinical studies have shown the therapeutic benefits of FGFs, a recent clinical trial for acute respiratory distress syndrome (ARDS) using intravenous injection of FGF7 did not report the expected beneficial effects. We discuss the potential reasons for these negative results and propose the rationale for new approaches for future clinical trials, such as delivery of FGFs to the damaged lungs through efficient inhalation systems, which may be more promising than systemic exposure to FGFs. While this change in the administration route presents a challenge, the therapeutic promises displayed by FGFs are worth the effort.

Notch signaling in the mammalian respiratory system, specifically the trachea and lungs, in development, homeostasis, regeneration, and disease

The respiratory system has ideal tissue structure and cell types for efficient gas exchange to intake oxygen and release carbon dioxide. This complex system develops through orchestrated intercellular signaling among various cell types, such as club, ciliated, basal, neuroendocrine, AT1, AT2, endothelial, and smooth muscle cells. Notch signaling is a highly conserved cell-cell signaling pathway ideally suited for very short-range cellular communication because Notch signals are transmitted by direct contact with an adjacent cell. Enthusiastic efforts by Notch researchers over the last two decades have led to the identification of critical roles of this signaling pathway during development, homeostasis, and regeneration of the respiratory system. The dysregulation of Notch signaling results in a wide range of respiratory diseases such as pulmonary artery hypertension (PAH), chronic obstructive pulmonary disease (COPD), interstitial pulmonary fibrosis (IPF), and lung cancer. Thus, a deep understanding of the biological functions of Notch signaling will help identify novel treatment targets in various respiratory diseases.

Airway epithelial cell isolation techniques affect DNA methylation profiles with consequences for analysis of asthma related perturbations to DNA methylation

The airway epithelium forms the interface between the inhaled environment and the lung. The airway epithelium is dysfunctional in asthma and epigenetic mechanisms are considered a contributory factor. We hypothesised that the DNA methylation profiles of cultured primary airway epithelial cells (AECs) would differ between cells isolated from individuals with asthma (n = 17) versus those without asthma (n = 16). AECs were isolated from patients by two different isolation techniques; pronase digestion (9 non-asthmatic, 8 asthmatic) and bronchial brushings (7 non-asthmatic and 9 asthmatic). DNA methylation was assessed using an Illumina Infinium HumanMethylation450 BeadChip array. DNA methylation of AECs clustered by isolation technique and linear regression identified 111 CpG sites differentially methylated between isolation techniques in healthy individuals. As a consequence, the effect of asthmatic status on DNA methylation was assessed within AEC samples isolated using the same technique. In pronase isolated AECs, 15 DNA regions were differentially methylated between asthmatics and non-asthmatics. In bronchial brush isolated AECs, 849 differentially methylated DNA regions were identified with no overlap to pronase regions. In conclusion, regardless of cell isolation technique, differential DNA methylation was associated with asthmatic status in AECs, providing further evidence for aberrant DNA methylation as a signature of epithelial dysfunction in asthma.

Toxicological effects of ambient fine (PM2.5-0.18) and ultrafine (PM0.18) particles in healthy and diseased 3D organo-typic mucocilary-phenotype models

The knowledge of the underlying mechanisms by which particulate matter (PM) exerts its health effects is still incomplete since it may trigger various symptoms as some persons may be more susceptible than others. Detailed studies realized in more relevant in vitro models are highly needed. Healthy normal human bronchial epithelial (NHBE), asthma-diseased human bronchial epithelial (DHBE), and COPD-DHBE cells, differentiated at the air-liquid interface, were acutely or repeatedly exposed to fine (i.e., PM_2.5-0.18, also called FP) and quasi-ultrafine (i.e., PM_0.18, also called UFP) particles. Immunofluorescence labelling of pan-cytokeratin, MUC5AC, and ZO-1 confirmed their specific cell-types. Baselines of the inflammatory mediators secreted by all the cells were quite similar. Slight changes of TNFα, IL-1β, IL-6, IL-8, GM-CSF, MCP-1, and/or TGFα, and of H3K9 histone acetylation supported a higher inflammatory response of asthma- and especially COPD-DHBE cells, after exposure to FP and especially UFP. At baseline, 35 differentially expressed genes (DEG) in asthma-DHBE, and 23 DEG in COPD-DHBE, compared to NHBE cells, were reported. They were involved in biological processes implicated in the development of asthma and COPD diseases, such as cellular process (e.g., PLA2G4C, NLRP1, S100A5, MUC1), biological regulation (e.g., CCNE1), developmental process (e.g., WNT10B), and cell component organization and synthesis (e.g., KRT34, COL6A1, COL6A2). In all the FP or UFP-exposed cell models, DEG were also functionally annotated to the chemical metabolic process (e.g., CYP1A1, CYP1B1, CYP1A2) and inflammatory response (e.g., EREG). Another DEG, FGF-1, was only down-regulated in asthma and specially COPD-DHBE cells repeatedly exposed. While RAB37 could help to counteract the down-regulation of FGF-1 in asthma-DHBE cells, the deregulation of FGR, WNT7B, VIPR1, and PPARGC1A could dramatically contribute to make it worse in COPD-DHBE cells. Taken together, these data contributed to support the highest effects of UFP versus FP and highest sensitivity of asthma- and notably COPD-DHBE versus NHBE cells.

DEK terminates diapause by activation of quiescent cells in the crustacean Artemia

To cope with harsh environments, the Artemia shrimp produces gastrula embryos in diapause, a state of obligate dormancy, having cellular quiescence and suppressed metabolism. The mechanism behind these cellular events remains largely unknown. Here, we study the regulation of cell quiescence using diapause embryos of Artemia We found that Artemia DEK (Ar-DEK), a nuclear factor protein, was down-regulated in the quiescent cells of diapause embryos and enriched in the activated cells of post-diapause embryos. Knockdown of Ar-DEK induced the production of diapause embryos whereas the control Artemia released free-swimming nuaplii. Our results indicate that Ar-DEK correlated with the termination of cellular quiescence via the increase in euchromatin and decrease in heterochromatin. The phenomena of quiescence have many implications beyond shrimp ecology. In cancer cells, for example, knockdown of DEK also induced a short period of cellular quiescence and increased resistance to environmental stress in MCF-7 and MKN45 cancer cell lines. Analysis of RNA sequences in Artemia and in MCF-7 revealed that the Wnt and AURKA signaling pathways were all down-regulated and the p53 signaling pathway was up-regulated upon inhibition of DEK expression. Our results provide insight into the functions of Ar-DEK in the activation of cellular quiescence during diapause formation in Artemia.

Cellular crosstalk in the development and regeneration of the respiratory system

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

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.

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.

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Bronchial epithelial stem cells are required for alveolar epithelial regeneration

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Fgf10-Fgfr2b signaling is required for alveolar type 2 stem cell maintenance

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Fgfr2b signaling drives alveolar epithelial regeneration by BESCs

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Fgf10-Fgfr2b promotes basal cell to alveolar type 2 cell differentiation

Yap and its subcellular localization have distinct compartment-specific roles in the developing lung

Although the Hippo-yes-associated protein (Yap) pathway has been implicated in lung development, the specific roles for Yap and its nucleocytoplasmic shuttling in the developing airway and alveolar compartments remain elusive. Moreover, conflicting results from expression studies and differences in the lung phenotypes of Yap and Hippo kinase null mutants caused controversy over the dynamics and significance of Yap subcellular localization in the developing lung. Here, we show that the aberrant morphogenesis of Yap-deficient lungs results from the disruption of developmental events specifically in distal epithelial progenitors. We also show that activation of nuclear Yap is enough to fulfill the Yap requirements to rescue abnormalities in these lungs. Remarkably, we found that Yap nucleocytoplasmic shuttling is largely dispensable in epithelial progenitors for both branching morphogenesis and sacculation. However, if maintained transcriptionally active in airways, nuclear Yap profoundly alters proximal-distal identity and halts epithelial differentiation. Taken together, these observations provide novel insights into the crucial importance of Hippo-Yap signaling in the lung prenatally.

Repairing the lungs one breath at a time: How dedicated or facultative are you?

In this review, Leach and Morrisey focus on lung regeneration to explore the importance of facultative regeneration controlled by functional and differentiated cell lineages as well as how they are positioned and regulated by distinct tissue niches.

Tissue regeneration involves various types of cellular and molecular responses depending on the type of tissue and the injury or disease that is inflicted. While many tissues contain dedicated stem/progenitor cell lineages, many others contain cells that, during homeostasis, are considered physiologically functional and fully differentiated but, after injury or in disease states, exhibit stem/progenitor-like activity. Recent identification of subsets of defined cell types as facultative stem/progenitor cells has led to a re-examination of how certain tissues respond to injury to mount a regenerative response. In this review, we focus on lung regeneration to explore the importance of facultative regeneration controlled by functional and differentiated cell lineages as well as how they are positioned and regulated by distinct tissue niches. Additionally, we discuss the molecular signals to which cells respond in their differentiated state during homeostasis and those signals that promote effective regeneration of damaged or lost cells and structures after injury.

Integrin signaling and mechanotransduction in regulation of somatic stem cells

Somatic stem cells are characterized by their capacity for self-renewal and differentiation, making them integral for normal tissue homeostasis. Different stem cell functions are strongly affected by the specialized microenvironment surrounding the cells. Consisting of soluble signaling factors, extracellular matrix (ECM) ligands and other cells, but also biomechanical cues such as the viscoelasticity and topography of the ECM, these factors are collectively known as the niche. Cell-ECM interactions are mediated largely by integrins, a class of heterodimeric cell adhesion molecules. Integrins bind their ligands in the extracellular space and associate with the cytoskeleton inside the cell, forming a direct mechanical link between the cells and their surroundings. Indeed, recent findings have highlighted the importance of integrins in translating biophysical cues into changes in cell signaling and function, a multistep process known as mechanotransduction. The mechanical properties of the stem cell niche are important, yet the underlying molecular details of integrin-mediated mechanotransduction in stem cells, especially the roles of the different integrin heterodimers, remain elusive. Here, we introduce the reader to the concept of integrin-mediated mechanotransduction, summarize current knowledge on the role of integrin signaling and mechanotransduction in regulation of somatic stem cell functions, and discuss open questions in the field.

Wnt/Fgf crosstalk is required for the specification of basal cells in the mouse trachea

Basal progenitor cells are crucial for the establishment and maintenance of the tracheal epithelium. However, it remains unclear how these progenitor cells are specified during foregut development. Here, we found that ablation of the Wnt chaperone protein Gpr177 (also known as Wntless) in mouse tracheal epithelium causes a significant reduction in the number of basal progenitor cells accompanied by cartilage loss in Shh-Cre;Gpr177^loxp/loxp mutants. Consistent with the association between cartilage and basal cell development, Nkx2.1⁺p63⁺ basal cells are co-present with cartilage nodules in Shh-Cre;Ctnnb1^DM/loxp mutants, which maintain partial cell-cell adhesion but not the transcription regulation function of β-catenin. More importantly, deletion of Ctnnb1 in the mesenchyme leads to the loss of basal cells and cartilage, concomitant with reduced transcript levels of Fgf10 in Dermo1-Cre;Ctnnb1^loxp/loxp mutants. Furthermore, deletion of Fgf receptor 2 (Fgfr2) in the epithelium also leads to significantly reduced numbers of basal cells, supporting the importance of Wnt/Fgf crosstalk in early tracheal development.

Building and Regenerating the Lung Cell by Cell

The unique architecture of the mammalian lung is required for adaptation to air breathing at birth and thereafter. Understanding the cellular and molecular mechanisms controlling its morphogenesis provides the framework for understanding the pathogenesis of acute and chronic lung diseases. Recent single-cell RNA sequencing data and high-resolution imaging identify the remarkable heterogeneity of pulmonary cell types and provides cell selective gene expression underlying lung development. We will address fundamental issues related to the diversity of pulmonary cells, to the formation and function of the mammalian lung, and will review recent advances regarding the cellular and molecular pathways involved in lung organogenesis. What cells form the lung in the early embryo? How are cell proliferation, migration, and differentiation regulated during lung morphogenesis? How do cells interact during lung formation and repair? How do signaling and transcriptional programs determine cell-cell interactions necessary for lung morphogenesis and function?

A Comprehensive Analysis of Fibroblast Growth Factor Receptor 2b Signaling on Epithelial Tip Progenitor Cells During Early Mouse Lung Branching Morphogenesis

This study demonstrates that FGF10/FGFR2b signaling on distal epithelial progenitor cells, via ß-catenin/EP300, controls, through a comprehensive set of developmental genes, morphogenesis, and differentiation. Fibroblast growth factor (FGF) 10 signaling through FGF receptor 2b (FGFR2b) is mandatory during early lung development as the deletion of either the ligand or the receptor leads to lung agenesis. However, this drastic phenotype previously hampered characterization of the primary biological activities, immediate downstream targets and mechanisms of action. Through the use of a dominant negative transgenic mouse model (Rosa26rtTA; tet(o)sFgfr2b), we conditionally inhibited FGF10 signaling in vivo in E12.5 embryonic lungs via doxycycline IP injection to pregnant females, and in vitro by culturing control and experimental lungs with doxycycline. The impact on branching morphogenesis 9 h after doxycycline administration was analyzed by morphometry, fluorescence and electron microscopy. Gene arrays at 6 and 9 h following doxycycline administration were carried out. The relationship between FGF10 and ß-catenin signaling was also analyzed through in vitro experiments using IQ1, a pharmacological inhibitor of ß-catenin/EP300 transcriptional activity. Loss of FGF10 signaling did not impact proliferation or survival, but affected both adherens junctions (up-regulation of E-cadherin), and basement membrane organization (increased laminin). Gene arrays identified multiple direct targets of FGF10, including main transcription factors. Immunofluorescence showed a down-regulation of the distal epithelial marker SOX9 and mis-expression distally of the proximal marker SOX2. Staining for the transcriptionally-active form of ß-catenin showed a reduction in experimental vs. control lungs. In vitro experiments using IQ1 phenocopied the impacts of blocking FGF10. This study demonstrates that FGF10/FGFR2b signaling on distal epithelial progenitor cells via ß-catenin/EP300 controls, through a comprehensive set of developmental genes, cell adhesion, and differentiation.

Hippo signaling promotes lung epithelial lineage commitment by curbing Fgf10 and β-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 β-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 β-catenin signaling. Our findings show that both inactivation of the Hippo pathway (nuclear Yap) or ablation of Yap result in increased β-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.

Emerging roles of YAP/TAZ in lung physiology and diseases

The YAP and TAZ, as the downstream effectors of Hippo pathway, have emerged as important translational co-activators of a wide variety of biological processes. YAP/TAZ plays a crucial role in the lung development and physiology. Dysregulation of YAP/TAZ signaling pathway contributes to the development and progression of chronic lung diseases, including lung cancer, pulmonary fibrosis, pulmonary hypertension, COPD, asthma, and lung infection. Therefore, owing to its critical functions, delineation of the signaling mechanisms of YAP/TAZ in pathological conditions will shed light on developing strategies for its therapeutic targeting. Currently, the complex regulation of this pathway is under extensive investigation. In this review, we summarize and present recent findings of molecular mechanisms of YAP/TAZ in the lung physiological and pathological conditions, as well as the implications of YAP/TAZ for lung diseases treatment and regeneration.

Lats1/2 inactivation reveals Hippo function in alveolar type I cell differentiation during lung transition to air breathing

Lung growth to its optimal size at birth is driven by reiterative airway branching followed by differentiation and expansion of alveolar cell types. How this elaborate growth is coordinated with the constraint of the chest is poorly understood. Here, we investigate the role of Hippo signaling, a cardinal pathway in organ size control, in mouse lung development. Unexpectedly, we found that epithelial loss of the Hippo kinase genes Lats1 and Lats2 (Lats1/2) leads to a striking reduction of lung size owing to an early arrest of branching morphogenesis. This growth defect is accompanied by abnormalities in epithelial cell polarity, cell division plane and extracellular matrix deposition, as well as precocious and increased expression of markers for type 1 alveolar epithelial cells (AEC1s), an indicator of terminal differentiation. Increased AEC1s were also observed in transgenic mice with overexpression of a constitutive nuclear form of downstream transcriptional effector YAP. Conversely, loss of Yap and Taz led to decreased AEC1s, demonstrating that the canonical Hippo signaling pathway is both sufficient and necessary to drive AEC1 fate. These findings together reveal unique roles of Hippo-LATS-YAP signaling in the developing mouse lung.

FGF10 and Human Lung Disease Across the Life Spectrum

Lung diseases impact patients across the lifespan, from infants in the first minutes of life through the aged population. Congenital abnormalities of lung structure can cause lung disease at birth or make adults more susceptible to chronic disease. Continuous inhalation of atmospheric components also requires the lung to be resilient to cellular injury. Fibroblast growth factor 10 (FGF10) regulates multiple stages of structural lung morphogenesis, cellular differentiation, and the response to injury. As a driver of lung airway branching morphogenesis, FGF10 signaling defects during development lead to neonatal lung disease. Alternatively, congenital airway abnormalities attributed to FGF10 mutations increase the risk of chronic airway disease in adulthood. FGF10 also maintains progenitor cell populations in the airway and promotes alveolar type 2 cell expansion and differentiation following injury. Here we review the cellular and molecular mechanisms linking FGF10 to multiple lung diseases, from bronchopulmonary dysplasia in extremely preterm neonates, cystic fibrosis in children, and chronic adult lung disorders. Understanding the connections between FGF10 and lung diseases may lead to exciting new therapeutic strategies.

Regulation of FGF10 Signaling in Development and Disease

Fibroblast Growth Factor 10 (FGF10) is a multifunctional mesenchymal-epithelial signaling growth factor, which is essential for multi-organ development and tissue homeostasis in adults. Furthermore, FGF10 deregulation has been associated with human genetic disorders and certain forms of cancer. Upon binding to FGF receptors with heparan sulfate as co-factor, FGF10 activates several intracellular signaling cascades, resulting in cell proliferation, differentiation, and invasion. FGF10 activity is modulated not only by heparan sulfate proteoglycans in the extracellular matrix, but also by hormones and other soluble factors. Despite more than 20 years of research on FGF10 functions, context-dependent regulation of FGF10 signaling specificity remains poorly understood. Emerging modes of FGF10 signaling regulation will be described, focusing on the role of FGF10 trafficking and sub-cellular localization, heparan sulfate proteoglycans, and miRNAs. Systems biology approaches based on quantitative proteomics will be considered for globally investigating FGF10 signaling specificity. Finally, current gaps in our understanding of FGF10 functions, such as the relative contribution of receptor isoforms to signaling activation, will be discussed in the context of genetic disorders and tumorigenesis.

Idiopathic pulmonary fibrosis: Epithelial-mesenchymal interactions and emerging therapeutic targets

Idiopathic pulmonary fibrosis (IPF) is a chronic fibrotic disease of the lung that is marked by progressive decline in pulmonary function and ultimately respiratory failure. Genetic and environmental risk factors have been identified that indicate injury to, and dysfunction of the lung epithelium is central to initiating the pathogenic process. Following injury to the lung epithelium, growth factors, matrikines and extracellular matrix driven signaling together activate a variety of repair pathways that lead to inflammatory cell recruitment, fibroblast proliferation and expansion of the extracellular matrix, culminating in tissue fibrosis. This tissue fibrosis then leads to changes in the biochemical and biomechanical properties of the extracellular matrix, which potentiate profibrotic mechanisms through a "feed-forward cycle." This review provides an overview of the interactions of the pathogenic mechanisms of IPF with a focus on epithelial-mesenchymal crosstalk and the extracellular matrix as a therapeutic target for idiopathic 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.

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.

Human lung development: recent progress and new challenges

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

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

Active epithelial Hippo signaling in idiopathic pulmonary fibrosis

Hippo/YAP signaling plays pleiotropic roles in the regulation of cell proliferation and differentiation during organogenesis and tissue repair. Herein we demonstrate increased YAP activity in respiratory epithelial cells in lungs of patients with idiopathic pulmonary fibrosis (IPF), a common, lethal form of interstitial lung disease (ILD). Immunofluorescence staining in IPF epithelial cells demonstrated increased nuclear YAP and loss of MST1/2. Bioinformatic analyses of epithelial cell RNA profiles predicted increased activity of YAP and increased canonical mTOR/PI3K/AKT signaling in IPF. Phospho-S6 (p-S6) and p-PTEN were increased in IPF epithelial cells, consistent with activation of mTOR signaling. Expression of YAP (S127A), a constitutively active form of YAP, in human bronchial epithelial cells (HBEC3s) increased p-S6 and p-PI3K, cell proliferation and migration, processes that were inhibited by the YAP-TEAD inhibitor verteporfin. Activation of p-S6 was required for enhancing and stabilizing YAP, and the p-S6 inhibitor temsirolimus blocked nuclear YAP localization and suppressed expression of YAP target genes CTGF, AXL, and AJUBA (JUB). YAP and mTOR/p-S6 signaling pathways interact to induce cell proliferation and migration, and inhibit epithelial cell differentiation that may contribute to the pathogenesis of IPF.

Human airway branch variation and chronic obstructive pulmonary disease

Susceptibility to chronic obstructive pulmonary disease (COPD) beyond cigarette smoking is incompletely understood, although several genetic variants associated with COPD are known to regulate airway branch development. We demonstrate that in vivo central airway branch variants are present in 26.5% of the general population, are unchanged over 10 y, and exhibit strong familial aggregation. The most common airway branch variant is associated with COPD in two cohorts (n = 5,054), with greater central airway bifurcation density, and with emphysema throughout the lung. The second most common airway branch variant is associated with COPD among smokers, with narrower airway lumens in all lobes, and with genetic polymorphisms within the FGF10 gene. We conclude that central airway branch variation, readily detected by computed tomography, is a biomarker of widely altered lung structure with a genetic basis and represents a COPD susceptibility factor.