Alveolar Epithelial Regeneration Research Articles

Research progress of KL-6 in respiratory system diseases.

This article comprehensively elucidates the discovery of Krebs von den Lungen-6 (KL-6), its structural features, functional mechanisms, and the current research status in various respiratory system diseases. Discovered in 1985, KL-6 was initially considered a tumor marker, but its elevated levels in interstitial lung disease (ILD) led to its recognition as a relevant serum marker for ILD. KL-6 is primarily produced by type 2 alveolar epithelial cell regeneration. Over the past 30 years since the discovery of KL-6, the number of related research papers has steadily increased annually. Following the coronavirus disease 2019 (COVID-19) pandemic, there has been a sudden surge in relevant literature. Despite KL-6's potential as a biomarker, its value in the diagnosis, treatment, and prognosis varies across different respiratory diseases, including ILD, idiopathic pulmonary fibrosis (IPF), COVID-19, and lung cancer. Therefore, as an important serum biomarker in respiratory system diseases, the value of KL-6 still requires further investigation.

Glycogen Synthase Kinase-3 Inhibition by CHIR99021 Promotes Alveolar Epithelial Cell Proliferation and Lung Regeneration in the Lipopolysaccharide-Induced Acute Lung Injury Mouse Model.

Acute respiratory distress syndrome (ARDS) is a life-threatening lung injury that currently lacks effective clinical treatments. Evidence highlights the potential role of glycogen synthase kinase-3 (GSK-3) inhibition in mitigating severe inflammation. The inhibition of GSK-3α/β by CHIR99021 promoted fetal lung progenitor proliferation and maturation of alveolar epithelial cells (AECs). The precise impact of CHIR99021 in lung repair and regeneration during acute lung injury (ALI) remains unexplored. This study intends to elucidate the influence of CHIR99021 on AEC behaviour during the peak of the inflammatory phase of ALI and, after its attenuation, during the repair and regeneration stage. Furthermore, a long-term evaluation was conducted post CHIR99021 treatment at a late phase of the disease. Our results disclosed the role of GSK-3α/β inhibition in promoting AECI and AECII proliferation. Later administration of CHIR99021 during ALI progression contributed to the transdifferentiation of AECII into AECI and an AECI/AECII increase, suggesting its contribution to the renewal of the alveolar epithelial population and lung regeneration. This effect was confirmed to be maintained histologically in the long term. These findings underscore the potential of targeted therapies that modulate GSK-3α/β inhibition, offering innovative approaches for managing acute lung diseases, mostly in later stages where no treatment is available.

Airway epithelial cell identity and plasticity are constrained by Sox2 during lung homeostasis, tissue regeneration, and in human disease

Maintenance of the cellular boundary between airway and alveolar compartments during homeostasis and after injury is essential to prohibit pathological plasticity which can reduce respiratory function. Lung injury and disease can induce either functional alveolar epithelial regeneration or dysplastic formation of keratinized epithelium which does not efficiently contribute to gas exchange. Here we show that Sox2 preserves airway cell identity and prevents fate changes into either functional alveolar tissue or pathological keratinization following lung injury. Loss of Sox2 in airway epithelium leads to a loss of airway epithelial identity with a commensurate gain in alveolar and basal cell identity, in part due to activation of Wnt signaling in secretory cells and increased Trp63 expression in intrapulmonary basal-like progenitors. In idiopathic pulmonary fibrosis, loss of SOX2 expression correlates with increased WNT signaling activity in dysplastic keratinized epithelium. SOX2-deficient dysplastic epithelial cells are also observed in COVID-19 damaged lungs. Thus, Sox2 provides a molecular barrier that suppresses airway epithelial plasticity to prevent acquisition of alveolar or basal cell identity after injury and help guide proper epithelial fate and regeneration.

Abstract 17355: Mitochondrial Calcium Uptake Regulates Lung Alveolar Epithelial Regeneration by Connecting Mitochondrial Metabolism With Epigenetic Changes

Introduction: Lung disease is the third leading cause of death and expanding. The endogenous regeneration of lung alveolar epithelium is severely compromised with age and chronic lung disease resulting in deaths after infections such as COVID-19. Mammalian lung alveolar epithelium consists of: alveolar epithelial type 1 and type 2 cells (AT1 and AT2). Stem/progenitor AT2 cells trans-differentiate into the gas exchanging AT1 cells in response to injury and regenerate the lost alveolar epithelium. Molecular mechanism of AT2-to-AT1 cell differentiation is unknown and its identification is critical for therapeutic development. Mitochondrial calcium controls fundamental cellular processes such as ATP production, metabolism, cell death etc. Dysregulation of mitochondrial calcium is implicated in diabetes, heart failure and neurodegeneration. Mitochondrial calcium uptake 1 (MICU1) negatively regulates mitochondrial calcium uptake which affects downstream signaling such as metabolite bioavailability and epigenetics. Results: We have identified a remarkable increase in MICU1 expression and a decrease in mitochondrial calcium uptake in AT2 cells during AT2-to-AT1 cell differentiation in adult mice after bacterial lung infection injury. Micu1 knockout in AT2 cells resulted in mitochondrial calcium overload, reduced AT2-to-AT1 cell differentiation and impaired lung regeneration after lung injury in mice. MICU1-dependent mitochondrial calcium regulated the citric acid cycle metabolite, α-ketoglutarate in AT2 cells. The metabolite, α-ketoglutarate, also functions as a signaling molecule and alters cellular epigenetics by activating Jumonji C domain containing histone lysine demethylases. Micu1 knockout AT2 cells exhibited α-ketoglutarate downregulation and the epigenetic change of H3K9 hypermethylation. Pharmacological inhibition of Jumonji histone demethylase in mouse AT2 cells increased H3K9me3 level and completely blocked AT2-to-AT1 cell differentiation in vitro. Conclusions: Overall, the results suggest that MICU1-mediated mitochondrial calcium uptake plays a key role in AT2-to-AT1 cell differentiation and lung alveolar epithelial regeneration by linking mitochondrial metabolism with epigenetics.

Therapeutic effects of fatty acid binding protein 1 in mice with pulmonary fibrosis by regulating alveolar epithelial regeneration

IntroductionIdiopathic pulmonary fibrosis is a progressive fibrotic lung disease with limited therapeutic options and high lethality, related to alveolar type II epithelial (ATII) cell dysregulation, the abnormal repair of alveolar...

CD8 T cell response and its released cytokine IFN-γ are necessary for lung alveolar epithelial repair during bacterial pneumonia.

Alveolar epithelial regeneration depends on the activity of resident quiescent progenitor cells. Alveolar epithelial type II (AT2) cells are known as the alveolar epithelial progenitor cells. They exit quiescent state, proliferate rapidly in response to injury and differentiate into alveolar epithelial type I (AT1) cells to regenerate the damaged alveolar epithelium. Although AT2 cell plasticity has been a very intense field of research, the role of CD8 T cell response and their released cytokine IFN-γ, in regulating AT2 cell plasticity and alveolar epithelial repair and regeneration after injury remains largely unknown. We used flow cytometry to quantify the amount of CD8 T cells in mouse lungs after bacterial pneumonia caused by Streptococcus pneumoniae. To determine whether CD8 T cells and their released cytokine IFN-γ are necessary for AT2 cell activity during alveolar epithelial regeneration, we performed loss of function studies using anti-CD8 or anti-IFN-γ monoclonal antibody (mAb) treatment in vivo. We assessed the effects of CD8 T cells and cytokine IFN-γ on AT2 cell differentiation capacity using the AT2- CD8 T cell co-culture system in vitro. We detected a transient wave of accumulation of CD8 T cells in mouse lungs, which coincided with the burst of AT2 cell proliferation during alveolar epithelial repair and regeneration in mice following bacterial pneumonia caused by Streptococcus pneumoniae. Depletion of CD8 T cells or neutralization of cytokine IFN-γ using anti-CD8 or anti-IFN-γ monoclonal antibody significantly reduced AT2 cell proliferation and differentiation into AT1 cells in mice after bacterial pneumonia. Furthermore, co-culture of CD8 T cells or cytokine IFN-γ with AT2 cells promoted AT2-to-AT1 cell differentiation in both murine and human systems. Conversely, blockade of IFN-γ signaling abrogated the increase in AT2-to-AT1 cell differentiation in the AT2- CD8 T cell co-culture system. Our data demonstrate that CD8 T-cell response and cytokine IFN-γ are necessary for promoting AT2 cell activity during alveolar epithelial repair and regeneration after acute lung injury caused by bacterial pneumonia.

Regulation of cell proliferation and transdifferentiation compensates for ventilator-induced lung injury mediated by NLRP3 inflammasome activation.

Mechanical ventilation is an important means of respiratory support and treatment for various diseases. However, its use can lead to serious complications, especially ventilator-induced lung injury (VILI). The mechanisms underlying this disease are complex, but activation of inflammatory signalling pathways results in activation of cytokines and inflammatory mediators, which play key roles in VILI. Recent studies have demonstrated that nod-like receptor protein 3 (NLRP3) inflammasome activation mediates VILI and also accompanied by cell proliferation and transdifferentiation to compensate for alveolar membrane damage. Type I alveolar epithelial cells (AECs I), which are involved in the formation of the blood-air barrier, are vulnerable to damage but cannot proliferate by themselves; thus, replacing AECs I relies on type II alveolar epithelial cells (AECs II). The review aims to introduce the mechanisms of NLRP3 inflammasome activation and its inhibitors, as well as the mechanisms that regulate cell proliferation and transdifferentiation. A large number of relevant literature was searched, then the key content was summarized and figures were also made. The mechanism of NLRP3 inflammasome activation has been further explored, including but not limited to pathogenic and aseptic inflammatory signals, such as, pathogenic molecular patterns and host-derived danger-associated molecular patterns activate toll-like receptor 4/nuclear factor-kappaB pathway or reactive oxygen species, cyclic stretch, adenosine triphosphate induce K+ efflux through P2X7, Ca2+ inflow, mitochondrial damage, etc, eventually induce NIMA-related kinase 7/NLRP3 binding and NLRP3 inflammasome activation. Not only that, the review also described in detail the inhibitors of NLRP3 inflammasome. And the mechanisms regulating cell proliferation and transdifferentiation are complex and unclear, including the Wnt/β-catenin, Yap/Taz, BMP/Smad and Notch signalling pathways. NLRP3 inflammasome activation mediated VILI, and VILI is alleviated after interfering with its activation, and inflammation and repair exist simultaneously in VILI. Clarifying these mechanisms is expected to provide theoretical guidance for alleviating VILI by inhibiting the inflammatory response and accelerating alveolar epithelial cell regeneration in the early stage.

PAI-1 regulates AT2-mediated re-alveolarization and ion permeability

BackgroundAcute lung injury is characterized by overwhelmingly elevated PAI-1 in both lung edema fluid and the circulating system. The role of increased PAI-1, encoded by Serpine1 gene, in the regeneration of injured lung epithelium has not been understood completely. This study aimed to investigate the role of Serpine1 in the regulation of alveolar type 2 epithelial cell (AT2) fate in a humanized mouse line carrying diseased mutants (Serpine1Tg).MethodsWild-type (wt) and Serpine1Tg AT2 cells were either cultured as monolayers or 3D alveolospheres. Colony-forming assay and total surface area of organoids were analyzed. AT1 and AT2 cells in organoids were counted by immunohistochemistry and fluorescence-activated cell sorting (FACS). To test the potential effects of elevated PAI-1 on the permeability in the epithelial monolayers, we digitized the biophysical properties of polarized AT2 monolayers grown at the air–liquid interface.ResultsA significant reduction in total AT2 cells harvested in Serpine1Tg mice was observed compared with wt controls. AT2 cells harvested from Serpine1Tg mice reduced significantly over the wt controls. Spheroids formed by Serpine1Tg AT2 cells were lesser than wt control. Similarly, the corresponding surface area, a readout of re-alveolarization of injured epithelium, was markedly reduced in Serpine1Tg organoids. FACS analysis revealed a significant suppression in the number of AT2 cells, in particular, the CD44+ subpopulation, in Serpine1Tg organoids. A lesser ratio of AT1:AT2 cells in Serpine1Tg organoids was observed compared with wt cultures. There was a significant increase in transepithelial resistance but not amiloride inhibition.ConclusionsOur study suggests elevated PAI-1 in injured lungs downregulates alveolar epithelial regeneration by reducing the AT2 self-renewal, particularly in the CD44+ cells.

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 indicate 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 cells stem cells impaired alveolar epithelial regeneration and resulted in increased pulmonary fibrosis upon bleomycin injury. Interestingly, 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.

Interleukin-11 disrupts alveolar epithelial progenitor function

Interleukin-11 (IL-11) is linked to the pathogenesis of idiopathic pulmonary fibrosis (IPF), since IL-11 induces myofibroblast differentiation and stimulates their excessive collagen deposition in the lung. In IPF there is disrupted alveolar structural architecture, yet the effect of IL-11 on the dysregulated alveolar repair remains to be elucidated. We hypothesised that epithelial-fibroblast communication associated with lung repair is disrupted by IL-11. Thus, we studied whether IL-11 affects the repair responses of alveolar lung epithelium using mouse lung organoids and precision-cut lung slices (PCLS). Additionally, we assessed the anatomical distribution of IL-11 and IL-11 receptor (IL-11R) in human control and IPF lungs using immunohistochemistry. IL-11 protein was observed in airway epithelium, macrophages and in IPF lungs, also in areas of alveolar type 2 (AT2) cell hyperplasia. IL-11R staining was predominantly present in smooth muscle and macrophages. In mouse organoid co-cultures of epithelial cells with lung fibroblasts, IL-11 decreased organoid number and reduced the fraction of Prosurfactant Protein C-expressing organoids, indicating dysfunctional regeneration initiated by epithelial progenitors. In mouse PCLS exposed to IL-11, ciliated cell markers were increased. The response of primary human fibroblasts to IL-11 on gene expression level was minimal, though bulk RNA-sequencing revealed IL-11 modulated various processes which are associated with IPF, including unfolded protein response, glycolysis and Notch signalling. IL-11 disrupts alveolar epithelial regeneration by inhibiting progenitor activation and suppressing the formation of mature alveolar epithelial cells. Evidence for a contribution of dysregulated fibroblast-epithelial communication to this process is limited.

Wnt3a-Loaded Extracellular Vesicles Promote Alveolar Epithelial Regeneration after Lung Injury.

Compromised regeneration resulting from the deactivation of Wnt/β-catenin signaling contributes to the progression of chronic obstructive pulmonary disease (COPD) with limited therapeutic options. Extracellular cytokine-induced Wnt-based signaling provides an alternative option for COPD treatment. However, the hydrophobic nature of Wnt proteins limits their purification and use. This study devises a strategy to deliver the membrane-bound wingless-type MMTV integration site family, member 3A (Wnt3a) over a long distance by anchoring it to the surface of extracellular vesicles (EVs). The newly engineered Wnt3aWG EVs are generated by co-expressing Wnt3a with two genes encoding the membrane protein, WLS, and an engineered glypican, GPC6ΔGPI -C1C2. The bioactivity of Wnt3aWG EVs is validated using a TOPFlash assay and a mesoderm differentiation model of human pluripotent stem cells. Wnt3aWG EVs activate Wnt signaling and promote cell growth following human alveolar epithelial cell injury. In an elastase-induced emphysema model, impaired pulmonary function and enlarged airspace are greatly restored by the intravenous delivery of Wnt3aWG EVs. Single-cell RNA sequencing-based analyses further highlight that Wnt3aWG EV-activated regenerative programs are responsible for its beneficial effects. These findings suggest that EV-based Wnt3a delivery represents a novel therapeutic strategy for lung repair and regeneration after injury.

Design, synthesis, and biological evaluation of indolin-2-one derivatives as novel cyclin-dependent protein kinase 8 (CDK8) inhibitors

Cyclin-dependent protein kinase 8 (CDK8) plays important roles in regulating fibrotic growth factors and inflammatory signaling pathways. Long-term chronic inflammation of the lungs can lead to idiopathic pulmonary fibrosis (IPF). Abnormal alveolar epithelial regeneration leads to the release of various fibrotic growth factors and the activation of inflammatory cells. CDK8 regulates profibrotic cytokines broadly implicated in the pathogenesis of fibrosis. Therefore, inhibition of CDK8 is considered a promising strategy for treating IPF. Here, CDK8 inhibitors were designed and optimized using a fragment-based drug design strategy. Testing results revealed that 71% of the synthesized compounds inhibited CDK8 activity better than the original compound E966–0530. Of these compounds, compound 4k exhibited the strongest CDK8 enzyme-inhibiting activity (IC50 =129 nM). Notably, it displayed a 13-fold increase in potency when compared to E966–0530. Experiments on toxicity and inhibition of epithelial-mesenchymal transition (EMT) protein expressions showed that compound 4k can inhibit EMT protein expressions, but with no significant cytotoxicity for alveolar epithelial cells. Compound 4k showed a potent inhibitory effect in cell migration assays. Furthermore, compound 4k significantly inhibited the phosphorylation of p-Smad3 and RNA Pol II, which are critical mediators in the fibrotic response signaling pathway. Compound 4k remarkably reduced TGF-β1-induced oxidative stress. The above results reveal optimized CDK8 inhibitors with potential use for IPF therapeutic treatment.

Regulation of the alveolar regenerative niche by amphiregulin-producing regulatory T cells.

Following respiratory viral infection, regeneration of the epithelial barrier is required to preserve lung function and prevent secondary infections. Lung regulatory T (Treg) cells are critical for maintaining blood oxygenation following influenza virus infection through production of the EGFR ligand amphiregulin (Areg); however, how Treg cells engage with progenitors within the alveolar niche is unknown. Here, we describe local interactions between Treg cells and an Areg-responsive population of Col14a1+EGFR+ lung mesenchymal cells that mediate type II alveolar epithelial (AT2) cell-mediated regeneration following influenza virus infection. We propose a mechanism whereby Treg cells are deployed to sites of damage and provide pro-survival cues that support mesenchymal programming of the alveolar niche. In the absence of fibroblast EGFR signaling, we observe impaired AT2 proliferation and disrupted lung remodeling following viral clearance, uncovering a crucial immune/mesenchymal/epithelial network that guides alveolar regeneration.

Contribution of Trp63CreERT2-labeled cells to alveolar regeneration is independent of tuft cells.

Viral infection often causes severe damage to the lungs, leading to the appearance of ectopic basal cells (EBCs) and tuft cells in the lung parenchyma. Thus far, the roles of these ectopic epithelial cells in alveolar regeneration remain controversial. Here, we confirm that the ectopic tuft cells are originated from EBCs in mouse models and COVID-19 lungs. The differentiation of tuft cells from EBCs is promoted by Wnt inhibition while suppressed by Notch inhibition. Although progenitor functions have been suggested in other organs, pulmonary tuft cells don't proliferate or give rise to other cell lineages. Consistent with previous reports, Trp63CreERT2 and KRT5-CreERT2-labeled ectopic EBCs do not exhibit alveolar regeneration potential. Intriguingly, when tamoxifen was administrated post-viral infection, Trp63CreERT2 but not KRT5-CreERT2 labels islands of alveolar epithelial cells that are negative for EBC biomarkers. Furthermore, germline deletion of Trpm5 significantly increases the contribution of Trp63CreERT2-labeled cells to the alveolar epithelium. Although Trpm5 is known to regulate tuft cell development, complete ablation of tuft cell production fails to improve alveolar regeneration in Pou2f3-/- mice, implying that Trpm5 promotes alveolar epithelial regeneration through a mechanism independent of tuft cells.

Transcriptional responses to injury of regenerative lung alveolar epithelium

SummaryThe significance of alveolar epithelial type 2 (AT2) cell proliferation for lung alveolar epithelial homeostasis and regeneration after injury has been widely accepted. However, the heterogeneity of AT2 cell population for cell proliferation capacity remains disputed. By single-cell RNA sequencing and genetic lineage labeling using the Ki67 knock-in mouse model, we map all proliferative AT2 cells in homeostatic and regenerating murine lungs after injury induced by Streptococcus pneumoniae infection. The proliferative AT2 cell population displays a unique transcriptional program, which is regulated by activating transcription factor 3 (ATF3) and thyroid hormone receptor alpha (THRA) transcription factors. Overexpression of these two transcription factors in AT2 cells promoted AT2 cell proliferation and improved lung function after injury. These results indicate that increased expression of ATF3 and THRA at the onset of lung epithelial regeneration is required to permit rapid AT2 cell proliferation and hence progression through the recovery of lung epithelium.

Glutamine Metabolism Is Required for Alveolar Regeneration during Lung Injury.

(1) Background: Abnormal repair after alveolar epithelial injury drives the progression of idiopathic pulmonary fibrosis (IPF). The maintenance of epithelial integrity is based on the self-renewal and differentiation of alveolar type 2 (AT2) cells, which require sufficient energy. However, the role of glutamine metabolism in the maintenance of the alveolar epithelium remains unclear. In this study, we investigated the role of glutamine metabolism in AT2 cells of patients with IPF and in mice with bleomycin-induced fibrosis. (2) Methods: Single-cell RNA sequencing (scRNA-seq), transcriptome, and metabolomics analyses were conducted to investigate the changes in the glutamine metabolic pathway during pulmonary fibrosis. Metabolic inhibitors were used to stimulate AT2 cells to block glutamine metabolism. Regeneration of AT2 cells was detected using bleomycin-induced mouse lung fibrosis and organoid models. (3) Results: Single-cell analysis showed that the expression levels of catalytic enzymes responsible for glutamine catabolism were downregulated (p < 0.001) in AT2 cells of patients with IPF, suggesting the accumulation of unusable glutamine. Combined analysis of the transcriptome (p < 0.05) and metabolome (p < 0.001) revealed similar changes in glutamine metabolism in bleomycin-induced pulmonary fibrosis in mice. Mechanistically, inhibition of the key enzymes involved in glucose metabolism, glutaminase-1 (GLS1) and glutamic-pyruvate transaminase-2 (GPT2) leads to reduced proliferation (p < 0.01) and differentiation (p < 0.01) of AT2 cells. (4) Conclusions: Glutamine metabolism is required for alveolar epithelial regeneration during lung injury.

MICU1-dependent mitochondrial calcium uptake regulates lung alveolar type 2 cell plasticity and lung regeneration.

Lung alveolar type 2 (AT2) cells are progenitors for alveolar type 1 (AT1) cells. Although many factors regulate AT2 cell plasticity, the role of mitochondrial calcium (mCa2+) uptake in controlling AT2 cells remains unclear. We previously identified that the miR-302 family supports lung epithelial progenitor cell proliferation and less differentiated phenotypes during development. Here, we report that a sustained elevation of miR-302 in adult AT2 cells decreases AT2-to-AT1 cell differentiation during the Streptococcus pneumoniae–induced lung injury repair. We identified that miR-302 targets and represses the expression of mitochondrial Ca2+ uptake 1 (MICU1), which regulates mCa2+ uptake through the mCa2+ uniporter channel by acting as a gatekeeper at low cytosolic Ca2+ levels. Our results reveal a marked increase in MICU1 protein expression and decreased mCa2+ uptake during AT2-to-AT1 cell differentiation in the adult lung. Deletion of Micu1 in AT2 cells reduces AT2-to-AT1 cell differentiation during steady-state tissue maintenance and alveolar epithelial regeneration after bacterial pneumonia. These studies indicate that mCa2+ uptake is extensively modulated during AT2-to-AT1 cell differentiation and that MICU1-dependent mCa2+ uniporter channel gating is a prominent mechanism modulating AT2-to-AT1 cell differentiation.

Science, Medicine, and the Anesthesiologist

Science, Medicine, and the Anesthesiologist

VEGF receptor 2 (KDR) protects airways from mucus metaplasia through a Sox9-dependent pathway.

Mucus-secreting goblet cells are the dominant cell type in pulmonary diseases, e.g., asthma and cystic fibrosis (CF), leading to pathologic mucus metaplasia and airway obstruction. Cytokines including IL-13 are the major players in the transdifferentiation of club cells into goblet cells. Unexpectedly, we have uncovered a previously undescribed pathway promoting mucous metaplasia that involves VEGFa and its receptor KDR. Single-cell RNA sequencing analysis coupled with genetic mouse modeling demonstrates that loss of epithelial VEGFa, KDR, or MEK/ERK kinase promotes excessive club-to-goblet transdifferentiation during development and regeneration. Sox9 is required for goblet cell differentiation following Kdr inhibition in both mouse and human club cells. Significantly, airway mucous metaplasia in asthmatic and CF patients is also associated with reduced KDR signaling and increased SOX9 expression. Together, these findings reveal an unexpected role for VEGFa/KDR signaling in the defense against mucous metaplasia, offering a potential therapeutic target for this common airway pathology.

The History and Mystery of Alveolar Epithelial Type II Cells: Focus on Their Physiologic and Pathologic Role in Lung.

Alveolar type II (ATII) cells are a key structure of the distal lung epithelium, where they exert their innate immune response and serve as progenitors of alveolar type I (ATI) cells, contributing to alveolar epithelial repair and regeneration. In the healthy lung, ATII cells coordinate the host defense mechanisms, not only generating a restrictive alveolar epithelial barrier, but also orchestrating host defense mechanisms and secreting surfactant proteins, which are important in lung protection against pathogen exposure. Moreover, surfactant proteins help to maintain homeostasis in the distal lung and reduce surface tension at the pulmonary air–liquid interface, thereby preventing atelectasis and reducing the work of breathing. ATII cells may also contribute to the fibroproliferative reaction by secreting growth factors and proinflammatory molecules after damage. Indeed, various acute and chronic diseases are associated with intensive inflammation. These include oedema, acute respiratory distress syndrome, fibrosis and numerous interstitial lung diseases, and are characterized by hyperplastic ATII cells which are considered an essential part of the epithelialization process and, consequently, wound healing. The aim of this review is that of revising the physiologic and pathologic role ATII cells play in pulmonary diseases, as, despite what has been learnt in the last few decades of research, the origin, phenotypic regulation and crosstalk of these cells still remain, in part, a mystery.