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

KGF impedes TRIM21-enhanced stabilization of keratin 10 mediating differentiation in hypopharyngeal cancer

KGF, also known as FGF7, is a member of the fibroblast growth factor (FGF) family that binds with high affinity to the FGF receptor 2b (FGFR2b) and regulates various cellular processes, including cell proliferation and differentiation in a variety of tumors. However, its potential role in hypopharyngeal cancer (HPC) remains largely unknown. In our study, we observed increased expression of FGFR2b in HPC. KGF treatment inhibited the expression of the differentiation marker keratin 10 (K10) protein at the post-transcriptional level in FaDu cells. Furthermore, treatment with the proteasome inhibitor MG132 was found to attenuate KGF-induced K10 reduction, suggesting the involvement of the ubiquitin-proteasome system. Using mass spectrometry and immunoprecipitation analysis, we identified the E3 ubiquitin ligase TRIM21 as a K10-interacting protein. Unexpectedly, instead of causing degradation, TRIM21 enhanced K10 protein stability through K6-linked ubiquitination of K10 at lysine 163 (K163) in the context of KGF exposure. Meanwhile, KGF treatment decreased TRIM21 protein levels, which were regulated by the p38 MAPK pathway, leading to K48-linked ubiquitination-mediated degradation of TRIM21. Notably, TRIM21 knockdown significantly promoted proliferation, inhibited differentiation and migration of FaDu cells, whereas TRIM21 overexpression had opposite effects in vitro and suppressed xenograft tumor growth in vivo. Our study demonstrates that TRIM21 may act as a tumor suppressor in HPC. However, TRIM21 overexpression decreased the sensitivity of FaDu cells to 5-fluorouracil, whereas TRIM21 knockdown or KGF administration significantly increased 5-fluorouracil sensitivity. Taken together, these findings highlight the intricate balance between protein stabilization and degradation orchestrated by KGF. This ubiquitination-mediated non-degradation mechanism of TRIM21 may provide novel therapeutic strategies for HPC and other cancers.

Lung organoids in COPD: recent advances and future prospects

Chronic obstructive pulmonary disease (COPD) is a chronic inflammatory airway disease that is characterized by progressive airflow limitation, a high prevalence, and a high mortality rate. However, the specific mechanisms remain unclear, partly due to the lack of robust data from in vitro experimental models and animal models that do not adequately represent the structure and pathophysiology of the human lung. The recent advancement of lung organoid culture systems has facilitated new avenues for the investigation of COPD. Lung organoids are in vitro models derived from adult stem cells, human pluripotent stem cells, or embryonic stem cells, established through three-dimensional culture. They exhibit a high degree of homology and genetic consistency with human tissues and can better mimic human lungs in terms of function and structure compared to other traditional models. This review will summarise the generation process of lung organoids from different cell sources and their application in COPD research, and provide suggestions for future research directions.

An Optimized Protocol for the Generation of Alveolospheres from Wild-Type Mice

Organoid models have become an integral part of the research methodology in the lung field. These systems allow for the study of progenitor and stem cell self-renewal, self-organization, and differentiation. Distinct models of lung organoids mimicking various anatomical regions of mature lungs have emerged in parallel to the increased gain of knowledge regarding epithelial stem and progenitor cell populations and the corresponding mesenchymal cells that populate the in vivo niche. In the distal lung, type 2 alveolar epithelial cells (AEC2s) represent a stem cell population that is engaged in regenerative mechanisms in response to various insults. These cells self-renew and give rise to AEC1s that carry out gas exchange. Multiple experimental protocols allowing the generation of alveolar organoids, or alveolospheres, from murine lungs have been described. Among the drawbacks have been the requirement of transgenic mice allowing the isolation of AEC2s with high viability and purity, and the occasional emergence of bronchiolar and bronchioalveolar organoids. Here, we provide a refined gating strategy and an optimized protocol for the generation of alveolospheres from wild-type mice. Our approach not only overcomes the need for transgenic mice to generate such organoids, but also yields a pure culture of alveolospheres that is devoid of bronchiolar and bronchioalveolar organoids. Our protocol contributes to the standardization of this important research tool.

The alveolus: Our current knowledge of how the gas exchange unit of the lung is constructed and repaired

The mammalian lung completes its last step of development, alveologenesis, to generate sufficient surface area for gas exchange. In this process, multiple cell types that include alveolar epithelial cells, endothelial cells, and fibroblasts undergo coordinated cell proliferation, cell migration and/or contraction, cell shape changes, and cell-cell and cell-matrix interactions to produce the gas exchange unit: the alveolus. Full functioning of alveoli also involves immune cells and the lymphatic and autonomic nervous system. With the advent of lineage tracing, conditional gene inactivation, transcriptome analysis, live imaging, and lung organoids, our molecular understanding of alveologenesis has advanced significantly. In this review, we summarize the current knowledge of the constituents of the alveolus and the molecular pathways that control alveolar formation. We also discuss how insight into alveolar formation may inform us of alveolar repair/regeneration mechanisms following lung injury and the pathogenic processes that lead to loss of alveoli or tissue fibrosis.

Recent advances in lung organoid development and applications in disease modeling

Over the last decade, several organoid models have evolved to acquire increasing cellular, structural, and functional complexity. Advanced lung organoid platforms derived from various sources, including adult, fetal, and induced pluripotent stem cells, have now been generated, which more closely mimic the cellular architecture found within the airways and alveoli. In this regard, the establishment of novel protocols with optimized stem cell isolation and culture conditions has given rise to an array of models able to study key cellular and molecular players involved in lung injury and repair. In addition, introduction of other nonepithelial cellular components, such as immune, mesenchymal, and endothelial cells, and employment of novel precision gene editing tools have further broadened the range of applications for these systems by providing a microenvironment and/or phenotype closer to the desired in vivo scenario. Thus, these developments in organoid technology have enhanced our ability to model various aspects of lung biology, including pathogenesis of diseases such as chronic obstructive pulmonary disease, pulmonary fibrosis, cystic fibrosis, and infectious disease and host-microbe interactions, in ways that are often difficult to undertake using only in vivo models. In this Review, we summarize the latest developments in lung organoid technology and their applicability for disease modeling and outline their strengths, drawbacks, and potential avenues for future development.

The lung mesenchyme in development, regeneration, and fibrosis

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

Injury activated alveolar progenitors (IAAPs): the underdog of lung repair

Alveolar epithelial type II cells (AT2s) together with AT1s constitute the epithelial lining of lung alveoli. In contrast to the large flat AT1s, AT2s are cuboidal and smaller. In addition to surfactant production, AT2s also serve as prime alveolar progenitors in homeostasis and play an important role during regeneration/repair. Based on different lineage tracing strategies in mice and single-cell transcriptomic analysis, recent reports highlight the heterogeneous nature of AT2s. These studies present compelling evidence for the presence of stable or transitory AT2 subpopulations with distinct marker expression, signaling pathway activation and functional properties. Despite demonstrated progenitor potentials of AT2s in maintaining homeostasis, through self-renewal and differentiation to AT1s, the exact identity, full progenitor potential and regulation of these progenitor cells, especially in the context of human diseases remain unclear. We recently identified a novel subset of AT2 progenitors named "Injury-Activated Alveolar Progenitors" (IAAPs), which express low levels of Sftpc, Sftpb, Sftpa1, Fgfr2b and Etv5, but are highly enriched for the expression of the surface receptor programmed cell death-ligand 1 (Pd-l1). IAAPs are quiescent during lung homeostasis but activated upon injury with the potential to proliferate and differentiate into AT2s. Significantly, a similar population of PD-L1 positive cells expressing intermediate levels of SFTPC are found to be expanded in human IPF lungs. We summarize here the current understanding of this newly discovered AT2 progenitor subpopulation and also try to reconcile the relationship between different AT2 stem cell subpopulations regarding their progenitor potential, regulation, and relevance to disease pathogenesis and therapeutic interventions.

Simple Models of Lung Development

Models are essential to further our understanding of lung development and regeneration and to facilitate identification and testing of potential treatments for lung diseases. A wide variety of rodent and human models are available that recapitulate one or more stages of lung development. This chapter describes the existing 'simple' in vitro, in silico and ex vivo models of lung development. We define which stage(s) of development each model recapitulates and highlight their pros and cons.

State of the Art in Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a form of usual interstitial pneumonia (UIP), though its origin is unknown [...]

FGF10 Therapeutic Administration Promotes Mobilization of Injury-Activated Alveolar Progenitors in a Mouse Fibrosis Model

Idiopathic pulmonary fibrosis (IPF) is a devastating interstitial lung disease with dire consequences and in urgent need of improved therapies. Compelling evidence indicates that damage or dysfunction of AT2s is of central importance in the development of IPF. We recently identified a novel AT2 subpopulation characterized by low SFTPC expression but that is enriched for PD-L1 in mice. These cells represent quiescent, immature AT2 cells during normal homeostasis and expand upon pneumonectomy (PNX) and were consequently named injury-activated alveolar progenitors (IAAPs). FGF10 is shown to play critical roles in lung development, homeostasis, and injury repair demonstrated in genetically engineered mice. In an effort to bridge the gap between the promising properties of endogenous Fgf10 manipulation and therapeutic reality, we here investigated whether the administration of exogenous recombinant FGF10 protein (rFGF10) can provide preventive and/or therapeutic benefit in a mouse model of bleomycin-induced pulmonary fibrosis with a focus on its impact on IAAP dynamics. C57BL/6 mice and SftpcCreERT2/+; tdTomatoflox/+ mice aged 8–10 weeks old were used in this study. To induce the bleomycin (BLM) model, mice were intratracheally (i.t.) instilled with BLM (2 μg/g body weight). BLM injury was induced after a 7-day washout period following tamoxifen induction. A single i.t. injection of rFGF10 (0.05 μg/g body weight) was given on days 0, 7, 14, and 21 after BLM injury. Then, the effects of rFGF10 on BLM-induced fibrosis in lung tissues were assessed by H&E, IHC, Masson’s trichrome staining, hydroxyproline and Western blot assays. Immunofluorescence staining and flow cytometry was used to assess the dynamic behavior of AT2 lineage-labeled SftpcPos (IAAPs and mature AT2) during the course of pulmonary fibrosis. We observed that, depending on the timing of administration, rFGF10 exhibited robust preventive or therapeutic efficacy toward BLM-induced fibrosis based on the evaluation of various pathological parameters. Flow cytometric analysis revealed a dynamic expansion of IAAPs for up to 4 weeks following BLM injury while the number of mature AT2s was drastically reduced. Significantly, rFGF10 administration increased both the peak ratio and the duration of IAAPs expansion relative to EpCAMPos cells. Altogether, our results suggest that the administration of rFGF10 exhibits therapeutic potential for IPF most likely by promoting IAAP proliferation and alveolar repair.

FGFR2b signalling restricts lineage-flexible alveolar progenitors during mouse lung development and converges in mature alveolar type 2 cells

The specification, characterization, and fate of alveolar type 1 and type 2 (AT1 and AT2) progenitors during embryonic lung development are poorly defined. Current models of distal epithelial lineage formation fail to capture the heterogeneity and dynamic contribution of progenitor pools present during early development. Furthermore, few studies explore the pathways involved in alveolar progenitor specification and fate. In this paper, we build upon our previously published work on the regulation of airway epithelial progenitors by fibroblast growth factor receptor 2b (FGFR2b) signalling during early (E12.5) and mid (E14.5) pseudoglandular stage lung development. Our results suggest that a significant proportion of AT2 and AT1 progenitors are lineage-flexible during late pseudoglandular stage development, and that lineage commitment is regulated in part by FGFR2b signalling. We have characterized a set of direct FGFR2b targets at E16.5 which are likely involved in alveolar lineage formation. These signature genes converge on a subpopulation of AT2 cells later in development and are downregulated in AT2 cells transitioning to the AT1 lineage during repair after injury in adults. Our findings highlight the extensive heterogeneity of pneumocytes by elucidating the role of FGFR2b signalling in these cells during early airway epithelial lineage formation, as well as during repair after injury.