Angiocrine Sphingosine-1-Phosphate Activation of S1PR2-YAP Signaling Axis in Alveolar Type II Cells Is Essential for Lung Repair

YAP1 inhibits the senescence of alveolar epithelial cells by targeting Prdx3 to alleviate pulmonary fibrosis

The senescence of alveolar type II (AT2) cells impedes self-repair of the lung epithelium and contributes to lung injury in the setting of idiopathic pulmonary fibrosis (IPF). Yes-associated protein 1 (YAP1) is essential for cell growth and organ development; however, the role of YAP1 in AT2 cells during pulmonary fibrosis is still unclear. YAP1 expression was found to be downregulated in the AT2 cells of PF patients. Deletion of YAP1 in AT2 cells resulted in lung injury, exacerbated extracellular matrix (ECM) deposition, and worsened lung function. In contrast, overexpression of YAP1 in AT2 cells promoted alveolar regeneration, mitigated pulmonary fibrosis, and improved lung function. In addition, overexpression of YAP1 alleviated bleomycin (BLM) -induced senescence of alveolar epithelial cells both in vivo and in vitro. Moreover, YAP1 promoted the expression of peroxiredoxin 3 (Prdx3) by directly interacting with TEAD1. Forced expression of Prdx3 inhibited senescence and improved mitochondrial dysfunction in BLM-treated MLE-12 cells, whereas depletion of Prdx3 partially abrogated the protective effect of YAP1. Furthermore, overexpression of Prdx3 facilitated self-repair of the injured lung and reduced ECM deposition, while silencing Prdx3 attenuated the antifibrotic effect of YAP1. In conclusion, this study demonstrated that YAP1 alleviates lung injury and pulmonary fibrosis by regulating Prdx3 expression to improve mitochondrial dysfunction and block senescence in AT2 cells, revealing a potential novel therapeutic strategy for pulmonary fibrosis.

Cell Cross-Talk in Alveolar Microenvironment: From Lung Injury to Fibrosis

Alveoli are complex microenvironments composed of various cell types, including epithelial, fibroblast, endothelial, and immune cells, which work together to maintain a delicate balance in the lung environment, ensuring proper growth, development, and an effective response to lung injuries. However, prolonged inflammation or aging can disrupt normal interactions among these cells, leading to impaired repair processes and a substantial decline in lung function. Therefore, it is essential to understand the key mechanisms underlying the interactions among the major cell types within the alveolar microenvironment. We explored the key mechanisms underlying the interactions among the major cell types within the alveolar microenvironment. These interactions occur through the secretion of signaling factors and play crucial roles in the response to injury, repair mechanisms, and the development of fibrosis in the lungs. Specifically, we focused on the regulation of alveolar type 2 cells by fibroblasts, endothelial cells, and macrophages. In addition, we explored the diverse phenotypes of fibroblasts at different stages of life and in response to lung injury, highlighting their impact on matrix production and immune functions. Furthermore, we summarize the various phenotypes of macrophages in lung injury and fibrosis as well as their intricate interplay with other cell types. This interplay can either contribute to the restoration of immune homeostasis in the alveoli or impede the repair process. Through a comprehensive exploration of these cell interactions, we aim to reveal new insights into the molecular mechanisms that drive lung injury toward fibrosis and identify potential targets for therapeutic intervention.

Characterization of perivascular alveolar epithelial stem cells and their niche in lung homeostasis and cancer

Lung alveolar structure and function are maintained by subsets of alveolar type II stem cells (AT2s), but there is a need for characterization of these subsets and their associated niches. Here, we report a CD44high subpopulation of AT2s characterized by increased expression of genes that regulate immune signaling even during steady-state homeostasis. Disruption of one of these immune regulatory transcription factor STAT1 impaired the stem cell function of AT2s. CD44high cells were preferentially located near macro- blood vessels and a supportive niche constituted by LYVE1+ endothelial cells, adventitial fibroblasts, and accumulated hyaluronan. In this microenvironment, CD44high AT2 cells were more responsive to transformation by KRAS than general AT2 cells. Moreover, after bacterial lung injury, there was a significant increase of CD44high AT2s and niche components distributed throughout the lung parenchyma. Taken together, CD44high AT2 cells and their perivascular niche regulate tissue homeostasis and tumor formation.

Novel insights into the potential applications of stem cells in pulmonary hypertension therapy

Pulmonary hypertension (PH) refers to a group of deadly lung diseases characterized by vascular lesions in the microvasculature and a progressive increase in pulmonary vascular resistance. The prevalence of PH has increased over time. Currently, the treatment options available for PH patients have limited efficacy, and none of them can fundamentally reverse pulmonary vascular remodeling. Stem cells represent an ideal seed with proven efficacy in clinical studies focusing on liver, cardiovascular, and nerve diseases. Since the potential therapeutic effect of mesenchymal stem cells (MSCs) on PH was first reported in 2006, many studies have demonstrated the efficacy of stem cells in PH animal models and suggested that stem cells can help slow the deterioration of lung tissue. Existing PH treatment studies basically focus on the paracrine action of stem cells, including protein regulation, exosome pathway, and cell signaling; however, the specific mechanisms have not yet been clarified. Apoptotic and afunctional pulmonary microvascular endothelial cells (PMVECs) and alveolar epithelial cells (AECs) are two fundamental promoters of PH although they have not been extensively studied by researchers. This review mainly focuses on the supportive communication and interaction between PMVECs and AECs as well as the potential restorative effect of stem cells on their injury. In the future, more studies are needed to prove these effects and explore more radical cures for PH.

Arrestin beta 1 Regulates Alveolar Progenitor Renewal and Lung Fibrosis

The molecular mechanisms that regulate progressive pulmonary fibrosis remain poorly understood. Type 2 alveolar epithelial cells (AEC2s) function as adult stem cells in the lung. We previously showed that there is a loss of AEC2s and a failure of AEC2 renewal in the lungs of idiopathic pulmonary fibrosis (IPF) patients. We also reported that beta-arrestins are the key regulators of fibroblast invasion, and beta-arrestin 1 and 2 deficient mice exhibit decreased mortality, decreased matrix deposition, and increased lung function in bleomycin-induced lung fibrosis. However, the role of beta-arrestins in AEC2 regeneration is unclear. In this study, we investigated the role and mechanism of Arrestin beta 1 (ARRB1) in AEC2 renewal and in lung fibrosis. We used conventional deletion as well as cell type-specific deletion of ARRB1 in mice and found that Arrb1 deficiency in fibroblasts protects mice from lung fibrosis, and the knockout mice exhibit enhanced AEC2 regeneration in vivo, suggesting a role of fibroblast-derived ARRB1 in AEC2 renewal. We further found that Arrb1-deficient fibroblasts promotes AEC2 renewal in 3D organoid assays. Mechanistically, we found that CCL7 is among the top downregulated cytokines in Arrb1 deficient fibroblasts and CCL7 inhibits AEC2 regeneration in 3D organoid experiments. Therefore, fibroblast ARRB1 mediates AEC2 renewal, possibly by releasing chemokine CCL7, leading to fibrosis in the lung.

Repair and regeneration of the alveolar epithelium in lung injury

Considerable progress has been made in understanding the function of alveolar epithelial cells in a quiescent state and regeneration mechanism after lung injury. Lung injury occurs commonly from severe viral and bacterial infections, inhalation lung injury, and indirect injury sepsis. A series of pathological mechanisms caused by excessive injury, such as apoptosis, autophagy, senescence, and ferroptosis, have been studied. Recovery from lung injury requires the integrity of the alveolar epithelial cell barrier and the realization of gas exchange function. Regeneration mechanisms include the participation of epithelial progenitor cells and various niche cells involving several signaling pathways and proteins. While alveoli are damaged, alveolar type II (AT2) cells proliferate and differentiate into alveolar type I (AT1) cells to repair the damaged alveolar epithelial layer. Alveolar epithelial cells are surrounded by various cells, such as fibroblasts, endothelial cells, and various immune cells, which affect the proliferation and differentiation of AT2 cells through paracrine during alveolar regeneration. Besides, airway epithelial cells also contribute to the repair and regeneration process of alveolar epithelium. In this review, we mainly discuss the participation of epithelial progenitor cells and various niche cells involving several signaling pathways and transcription factors.

Epithelial stem cells and niches in lung alveolar regeneration and diseases

Alveoli serve as the functional units of the lungs, responsible for the critical task of blood-gas exchange. Comprising type I (AT1) and type II (AT2) cells, the alveolar epithelium is continuously subject to external aggressors like pathogens and airborne particles. As such, preserving lung function requires both the homeostatic renewal and reparative regeneration of this epithelial layer. Dysfunctions in these processes contribute to various lung diseases. Recent research has pinpointed specific cell subgroups that act as potential stem or progenitor cells for the alveolar epithelium during both homeostasis and regeneration. Additionally, endothelial cells, fibroblasts, and immune cells synergistically establish a nurturing microenvironment-or "niche"-that modulates these epithelial stem cells. This review aims to consolidate the latest findings on the identities of these stem cells and the components of their niche, as well as the molecular mechanisms that govern them. Additionally, this article highlights diseases that arise due to perturbations in stem cell-niche interactions. We also discuss recent technical innovations that have catalyzed these discoveries. Specifically, this review underscores the heterogeneity, plasticity, and dynamic regulation of these stem cell-niche systems. It is our aspiration that a deeper understanding of the fundamental cellular and molecular mechanisms underlying alveolar homeostasis and regeneration will open avenues for identifying novel therapeutic targets for conditions such as chronic obstructive pulmonary disease (COPD), fibrosis, coronavirus disease 2019 (COVID-19), and lung cancer.

Inhibition of sphingosine kinase 1 attenuates LPS-induced acute lung injury by suppressing endothelial cell pyroptosis

Acute lung injury (ALI) is a frequent complication of sepsis, with pyroptosis playing a pivotal role. Analysis of Gene Expression Omnibus (GEO) mouse sepsis datasets revealed the upregulation of sphingosine kinase 1 (SphK1) in septic mouse lung tissues, which was validated in lipopolysaccharide (LPS)-treated mice. Therefore, this study aimed to explore the potential role and underlying mechanisms of SphK1, the primary kinase responsible for catalyzing the formation of the bioactive lipid sphingosine-1-phosphat, in sepsis development. Mice received an intraperitoneal injection of SphK1 inhibitor prior to LPS administration. Mouse lung vascular endothelial cells (MLVECs) were exposed to LPS and SphK1 inhibitor. The SphK1 inhibitor mitigated ALI, as evidenced by hematoxylin and eosin (H&E) staining and the wet-to-dry (W/D) weight ratio and reduced Evans blue dye leakage. Furthermore, the SphK1 inhibitor inhibited the activation of the NOD-like receptor protein 3 inflammasome and the subsequent induction of pyroptosis both in vivo and in vitro. Intriguingly, using co-immunoprecipitation (Co-IP) combined with mass spectrometry, our findings revealed that SphK1 associates with pyruvate kinase M2 (PKM2), facilitating PKM2 phosphorylation and its nuclear translocation. TEPP-46, which has the ability to stabilize PKM2 and inhibit the phosphorylation and nuclear translocation of PKM2, markedly reduced the expression of pyroptosis-associated markers and alleviated lung injury. Concludingly, our results suggest that targeting SphK1 is a promising therapeutic strategy for ALI.

Lung regeneration: diverse cell types and the therapeutic potential

Lung tissue has a certain regenerative ability and triggers repair procedures after injury. Under controllable conditions, lung tissue can restore normal structure and function. Disruptions in this process can lead to respiratory system failure and even death, causing substantial medical burden. The main types of respiratory diseases are chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), and acute respiratory distress syndrome (ARDS). Multiple cells, such as lung epithelial cells, endothelial cells, fibroblasts, and immune cells, are involved in regulating the repair process after lung injury. Although the mechanism that regulates the process of lung repair has not been fully elucidated, clinical trials targeting different cells and signaling pathways have achieved some therapeutic effects in different respiratory diseases. In this review, we provide an overview of the cell type involved in the process of lung regeneration and repair, research models, and summarize molecular mechanisms involved in the regulation of lung regeneration and fibrosis. Moreover, we discuss the current clinical trials of stem cell therapy and pharmacological strategies for COPD, IPF, and ARDS treatment. This review provides a reference for further research on the molecular and cellular mechanisms of lung regeneration, drug development, and clinical trials.

Hydrogen sulfide inhibits alveolar type II cell senescence and limits pulmonary fibrosis via promoting MDM2-mediated p53 degradation

AIM

Senescence of alveolar type II (AT2) cells is an important driver of pulmonary fibrosis. This study aimed to investigate whether and how dysregulation of hydrogen sulfide (H2 S) production affected AT2 cell senescence, and then explored the effect of H2 S on the communication between AT2 and fibroblasts.

METHODS

ICR mice were intratracheally administered with bleomycin (3 mg/kg). Sodium hydrosulfide (NaHS, 28 μmol/kg/d) was intraperitoneally injected for 2 weeks. The H2 S-generating enzyme cystathionine-β-synthase (CBS) knockout heterozygous (CBS+/- ) mice were used as a low H2 S production model.

RESULTS

Analysis of microarray datasets revealed downregulation of H2 S-generating enzymes in lung tissues of patients with pulmonary fibrosis. Decreased H2 S production was correlated with higher levels of cell senescence markers p53 and p21 in bleomycin-induced lung fibrosis. CBS+/- mice exhibited increased levels of p53 and p21. The numbers of AT2 cells positive for p53 and p21 were increased in CBS+/- mice as compared to control mice. H2 S donor NaHS attenuated bleomycin-induced AT2 cell senescence both in vivo and in vitro. H2 S donor suppressed bleomycin-induced senescence-associated secretory phenotype (SASP) of AT2 cells via inhibiting p53/p21 pathway, consequently suppressing proliferation and myofibroblast transdifferentiation of fibroblasts. Mechanically, H2 S suppressed p53 expression by enhancing the mouse double-minute 2 homologue (MDM2)-mediated ubiquitination and degradation of p53.

CONCLUSION

H2 S inactivated p53-p21 pathway, consequently suppressing AT2 cell senescence as well as cell communication between senescent AT2 cells and fibroblasts. Aberrant H2 S synthesis may contribute to the development of pulmonary fibrosis through promoting the activation loop involving senescent AT2 cells and activated fibroblasts.

Pancreatic Acinar Cells-Derived Sphingosine-1-Phosphate Contributes to Fibrosis of Chronic Pancreatitis via Inducing Autophagy and Activation of Pancreatic Stellate Cells

BACKGROUND & AIMS

Studies have demonstrated that activated pancreatic stellate cells (PSCs) play a crucial role in pancreatic fibrogenesis in chronic pancreatitis (CP); however, the precise mechanism for PSCs activation has not been fully elucidated. We analyzed the role of injured pancreatic acinar cells (iPACs) in the activation of PSCs of CP.

METHODS

Sphingosine kinase 1 (SPHK1)/sphingosine-1-phosphate (S1P) signaling was evaluated in experimental CP induced by cerulein injection or pancreatic duct ligation, as well as in PACs injured by cholecystokinin. The activation of PSCs and pancreatic fibrosis in CP samples was evaluated by immunohistochemical and immunofluorescence analyses. In vitro coculture assay of iPACs and PSCs was created to evaluate the effect of the SPHK1/S1P pathway and S1P receptor 2 (SIPR2) on autophagy and activation of PSCs. The pathogenesis of CP was assessed in SPHK1-/- mice or PACs-specific SPHK1-knockdown mice with recombinant adeno-associated virus serotypes 9-SPHK1-knockdown, as well as in mice treated with inhibitor of SPHK1 and S1P receptor 2 (S1PR2).

RESULTS

SPHK1/S1P was remarkably increased in iPACs and acinar cells in pancreatic tissues of CP mice. Meanwhile, the pathogenesis, fibrosis, and PSCs activation of CP was significantly prevented in SPHK1-/- mice and recombinant adeno-associated virus serotypes 9-SPHK1-knockdown mice. Meanwhile, iPACs obviously activated PSCs, which was prevented by SPHK1 knockdown in iPACs. Moreover, iPACs-derived S1P specifically combined to S1PR2 of PSCs, by which modulated 5' adenosine monophosphate-activated protein kinase/mechanistic target of rapamycin pathway and consequently induced autophagy and activation of PSCs. Furthermore, hypoxia-inducible factor 1-α and -2α promoted SPHK1 transcription of PACs under hypoxia conditions, which is a distinct characteristic of the CP microenvironment. Coincidently, inhibition of SPHK1 and S1PR2 activity with inhibitor PF-543 and JTE-013 obviously impeded pancreatic fibrogenesis of CP mice.

CONCLUSIONS

The activated SPHK1/S1P pathway in iPACs induces autophagy and activation of PSCs by regulating the S1PR2/5' adenosine monophosphate-activated protein kinase/mammalian target of rapamycin pathway, which promotes fibrogenesis of CP. The hypoxia microenvironment might contribute to the cross talk between PACs and PSCs in pathogenesis of CP.

JTE-013 Alleviates Pulmonary Fibrosis by Affecting the RhoA/YAP Pathway and Mitochondrial Fusion/Fission

Pulmonary fibrosis may be due to the proliferation of fibroblasts and the aggregation of extracellular matrix, resulting in the stimulation of inflammation damage, destroying lung tissue structure, seriously affecting the patient's respiratory function, and even leading to death. We investigated the role and mechanism of JTE-013 in attenuating bleomycin (BLM)-induced pulmonary fibrosis. BLM-induced pulmonary fibrosis was established in mice. Type 2 alveolar epithelial cells (MLE-12) were stimulated with sphingosine monophosphate (S1P) in vitro. JTE-013, an S1PR2 (sphingosine 1-phosphate receptor 2) antagonist, and Verteporfin were administered in vivo and in vitro. IL-4, IL-5, TNF-α, and IFN-γ were measured by ELISA. IL-4 and IFN-γ positive cells were detected by flow cytometry. Inhibition of S1PR2 with JTE-013 significantly ameliorated BLM-induced pathological changes and inflammatory cytokine levels. JTE-013 also significantly reduced the expression of RHOA/YAP pathway proteins and mitochondrial fission protein Drp1, apoptosis, and the colocalization of α-SMA with YAP, Drp1, and Tom20, as detected by immunohistochemistry, immunofluorescence staining, TUNEL, and Western blot. In vitro, S1PR2 and YAP knockdown downregulated RHOA/YAP pathway protein expression, Drp1 phosphorylation, and Drp1 translocation, promoted YAP phosphorylation and phenotypic transformation of MFN2, and inhibited the up-regulation of mitochondrial membrane potential, reactive oxygen species production, and cell apoptosis (7.13% vs. 18.14%), protecting the integrity of the mitochondrial dynamics. JTE-013 also inhibited the expression of fibrosis markers α-SMA, MMP-9, and COL1A1, and alleviated the symptoms of pulmonary fibrosis. Conclusively, JTE-013 has great anti-pulmonary fibrosis potential by regulating RHOA/YAP and mitochondrial fusion/fission.

The endothelium in lung fibrosis: a core signaling hub in disease pathogenesis?

Pulmonary fibrosis (PF) is a progressive chronic lung disease characterized by excessive deposition of extracellular matrix (ECM) and structural destruction, associated with a severe 5-year mortality rate. The onset of the disease is thought to be triggered by chronic damage to the alveolar epithelium. Since the pulmonary endothelium is an important component of the alveolar-capillary niche, it is also affected by the initial injury. In addition to ensuring proper gas exchange, the endothelium has critical functional properties, including regulation of vascular tone, inflammatory responses, coagulation, and maintenance of vascular homeostasis and integrity. Recent single-cell analyses have shown that shifts in endothelial cell (EC) subtypes occur in PF. Furthermore, the increased vascular remodeling associated with PF leads to deteriorated outcomes for patients, underscoring the importance of the vascular bed in PF. To date, the causes and consequences of endothelial and vascular involvement in lung fibrosis are poorly understood. Therefore, it is of great importance to investigate the involvement of EC and the vascular system in the pathogenesis of the disease. In this review, we will outline the current knowledge on the role of the pulmonary vasculature in PF, in terms of abnormal cellular interactions, hyperinflammation, vascular barrier disorders, and an altered basement membrane composition. Finally, we will summarize recent advances in extensive therapeutic research and discuss the significant value of novel therapies targeting the endothelium.

NEW DATA ON HISTOGENESIS AND HISTOLOGICAL STRUCTURE OF LUNG CANCER

BACKGROUND

Lung cancer (LC) is one of the most common malignant neoplasms in men around the world, which poses a number of important challenges for scientists.

AIM

To analyze the histogenesis, features of the histological structure, and growth of LC.

MATERIALS AND METHODS

The surgical material of 81 patients with LC was studied. Histological preparations were stained with hematoxylin and eosin (H&E) using the Papanicolaou method. Immunohistochemical reactions with monoclonals (Ki67, PCNA) were conducted.

RESULTS

In histological preparations of all LC types (squamous, adenocarcinoma, and small cell), along with solid growth, tumor growth in the alveoli was determined, which started from the basal membrane and grew toward the alveolus center, as evidenced by the morphological features of growth, tumor spread, and development of necrosis in the center.

CONCLUSION

In all the studied histological preparations of LC, tumor growth in the alveoli is noted, which is confirmed by structural and cellular signs and the nature of tumor decay in the alveolus center, which corresponds to the general patterns of development of malignant epithelial tumors.

Upregulation of alveolar fluid clearance is not sufficient for Na+,K+-ATPase β subunit-mediated gene therapy of LPS-induced acute lung injury in mice

Acute Lung Injury/Acute Respiratory Distress Syndrome (ALI/ARDS) is characterized by diffuse alveolar damage and significant edema accumulation, which is associated with impaired alveolar fluid clearance (AFC) and alveolar-capillary barrier disruption, leading to acute respiratory failure. Our previous data showed that electroporation-mediated gene delivery of the Na⁺, K⁺-ATPase β1 subunit not only increased AFC, but also restored alveolar barrier function through upregulation of tight junction proteins, leading to treatment of LPS-induced ALI in mice. More importantly, our recent publication showed that gene delivery of MRCKα, the downstream effector of β1 subunit-mediated signaling towards upregulation of adhesive junctions and epithelial and endothelial barrier integrity, also provided therapeutic potential for ARDS treatment in vivo but without necessarily accelerating AFC, indicating that for ARDS treatment, improving alveolar capillary barrier function may be of more benefit than improving fluid clearance. In the present study, we investigated the therapeutical potential of β2 and β3 subunits, the other two β isoforms of Na⁺, K⁺-ATPase, for LPS-induced ALI. We found that gene transfer of either the β1, β2, or β3 subunits significantly increased AFC compared to the basal level in naïve animals and each gave similar increased AFC to each other. However, unlike that of the β1 subunit, gene transfer of the β2 or β3 subunit into pre-injured animal lungs failed to show the beneficial effects of attenuated histological damage, neutrophil infiltration, overall lung edema, or increased lung permeability, indicating that β2 or β3 gene delivery could not treat LPS induced lung injury. Further, while β1 gene transfer increased levels of key tight junction proteins in the lungs of injured mice, that of either the β2 or β3 subunit had no effect on levels of tight junction proteins. Taken together, this strongly suggests that restoration of alveolar-capillary barrier function alone may be of equal or even more benefit than improving AFC for ALI/ARDS treatment.

The tissue-specific transcriptional landscape underlines the involvement of endothelial cells in health and disease

Endothelial cells (ECs) that line vascular and lymphatic vessels are being increasingly recognized as important to organ function in health and disease. ECs participate not only in the trafficking of gases, metabolites, and cells between the bloodstream and tissues but also in the angiocrine-based induction of heterogeneous parenchymal cells, which are unique to their specific tissue functions. The molecular mechanisms regulating EC heterogeneity between and within different tissues are modeled during embryogenesis and become fully established in adults. Any changes in adult tissue homeostasis induced by aging, stress conditions, and various noxae may reshape EC heterogeneity and induce specific transcriptional features that condition a functional phenotype. Heterogeneity is sustained via specific genetic programs organized through the combinatory effects of a discrete number of transcription factors (TFs) that, at the single tissue-level, constitute dynamic networks that are post-transcriptionally and epigenetically regulated. This review is focused on outlining the TF-based networks involved in EC specialization and physiological and pathological stressors thought to modify their architecture.

Endotyping COPD: hypoxia-inducible factor-2 as a molecular "switch" between the vascular and airway phenotypes?

COPD is a heterogeneous disease with multiple clinical phenotypes. COPD endotypes can be determined by different expressions of hypoxia-inducible factors (HIFs), which, in combination with individual susceptibility and environmental factors, may cause predominant airway or vascular changes in the lung. The pulmonary vascular phenotype is relatively rare among COPD patients and characterised by out-of-proportion pulmonary hypertension (PH) and low diffusing capacity of the lung for carbon monoxide, but only mild-to-moderate airway obstruction. Its histologic feature, severe remodelling of the small pulmonary arteries, can be mediated by HIF-2 overexpression in experimental PH models. HIF-2 is not only involved in the vascular remodelling but also in the parenchyma destruction. Endothelial cells from human emphysema lungs express reduced HIF-2α levels, and the deletion of pulmonary endothelial Hif-2α leads to emphysema in mice. This means that both upregulation and downregulation of HIF-2 have adverse effects and that HIF-2 may represent a molecular "switch" between the development of the vascular and airway phenotypes in COPD. The mechanisms of HIF-2 dysregulation in the lung are only partly understood. HIF-2 levels may be controlled by NAD(P)H oxidases via iron- and redox-dependent mechanisms. A better understanding of these mechanisms may lead to the development of new therapeutic targets.

Crosstalk between Extracellular Matrix Stiffness and ROS Drives Endometrial Repair via the HIF-1α/YAP Axis during Menstruation

Although the menstrual cycle driven by sex steroid hormones is an uncomplicated physiological process, it is important for female health, fertility and regenerative biology. However, our understanding of this unique type of tissue homeostasis remains unclear. Here, we examined the biological effects of mechanical force by evaluating the changing trend of extracellular matrix (ECM) stiffness, and the results suggested that ECM stiffness was reduced and that breaking of mechanotransduction delayed endometrium repair in a mouse model of simulated menses. We constructed an ECM stiffness interference model in vitro to explain the mechanical force conduction mechanism during endometrial regeneration. We discovered that ECM stiffness increased the expression and nuclear transfer of YAP, which improved the creation of a microenvironment, in a manner that induced proliferation and angiogenesis for endometrial repair by activating YAP. In addition, we observed that physiological endometrial hypoxia occurs during the menstrual cycle and that the expression of HIF-1α was increased. Mechanistically, in addition to the classical F-actin/YAP pathway, we also found that the ROS/HIF-1α/YAP axis was involved in the transmission of mechanical signals. This study provides novel insights into the essential menstrual cycle and presents an effective, nonhormonal treatment for menstrual disorders.

Controlled mechanical loading improves bone regeneration by regulating type H vessels in a S1Pr1-dependent manner

Despite the best treatment, approximately 10% of fractures still face undesirable repair and result in delayed unions or non-unions. Dynamic mechanical stimulation promotes bone formation, when applied at the correct time frame, with optimal loading magnitude, frequency, and repetition. Controlled mechanical loading significantly increases osteogenic cells during the matrix deposition phase of bone repair. In the bone defect, the blood vessel network guides the initial bone formation activities. A unique blood vessel subtype (Type H) exists in bone, which expresses high levels of CD31 and endomucin, and functions to couple angiogenesis and osteogenesis. However, how this form of controlled mechanical loading regulates the Type H vessels and promotes bone formation is still not clear. Sphingosine 1-phosphate (S1P) participates in the bone anabolic process and is a key regulator of the blood vessel. Its receptor, sphingosine 1-phosphate receptor 1 (S1Pr1), is a mechanosensitive protein that regulates vascular integrity. Therefore, we hypothesis that controlled anabolic mechanical loading promotes bone repair by acting on Type H vessels. To study the effect of S1Pr1 on loading induced-bone repair, we utilized a stabilized tibial defect model, which allows for the application of anabolic mechanical loading. Mechanical loading upregulated S1Pr1 within the entire defect, with up to 80% expressed in blood vessels, as observed by deep tissue imaging. Additionally, S1Pr1 antagonism by W146 inhibited the anabolic effects of mechanical loading. We showed that mechanical loading or activating S1Pr1 could induce YAP nuclear translocation, a key regulator in the cell's mechanical response, in endothelial cells (ECs) in vitro. Inhibition of S1Pr1 in endothelial cells by siRNA reduced loading-induced YAP nuclear translocation and expressions of angiogenic genes. In vivo, YAP nuclear translocation in Type H vessels was up-regulated after mechanical loading but was inhibited by antagonizing S1Pr1. S1Pr1 agonist, FTY720, increased bone volume and Type H vessel volume, similar to that of mechanical stimulation. In conclusion, controlled anabolic mechanical loading enhanced bone formation mainly through Type H vessels in a S1Pr1-dependent manner.

Comprehensive Review of the Vascular Niche in Regulating Organ Regeneration and Fibrosis

The vasculature occupies a large area of the body, and none of the physiological activities can be carried out without blood vessels. Blood vessels are not just passive conduits and barriers for delivering blood and nutrients. Meanwhile, endothelial cells covering the vascular lumen establish vascular niches by deploying some growth factors, known as angiocrine factors, and actively participate in the regulation of a variety of physiological processes, such as organ regeneration and fibrosis and the occurrence and development of cancer. After organ injury, vascular endothelial cells regulate the repair process by secreting various angiocrine factors, triggering the proliferation and differentiation process of stem cells. Therefore, analyzing the vascular niche and exploring the factors that maintain vascular homeostasis can provide strong theoretical support for clinical treatment targeting blood vessels. Here we mainly discuss the regulatory mechanisms of the vascular niche in organ regeneration and fibrosis.

Ex situ-armus: experimental models for combating respiratory dysfunction

Ex situ experimental models have become a main stay in pulmonary research. Organoids and explant systems have uncovered novel stem cell subsets, served as disease models, delineated cell fate transitions, and aided high throughput pre-clinical drug screening. Integration of gene-editing and bioengineering approaches have further generated novel avenues for regenerative medicine and transplantation strategies. In this article, we highlight recent studies, aided by ex situ systems, which have contributed to significant advances in our understanding of the human lower respiratory tract. We present key observations from these studies to gain improved insights into human disease. We conclude this article with a summary of existing challenges and potential technological advances to successfully mirror human tissue physiology.

Impaired Alveolar Re-Epithelialization in Pulmonary Emphysema

Alveolar type II (ATII) cells are progenitors in alveoli and can repair the alveolar epithelium after injury. They are intertwined with the microenvironment for alveolar epithelial cell homeostasis and re-epithelialization. A variety of ATII cell niches, transcription factors, mediators, and signaling pathways constitute a specific environment to regulate ATII cell function. Particularly, WNT/β-catenin, YAP/TAZ, NOTCH, TGF-β, and P53 signaling pathways are dynamically involved in ATII cell proliferation and differentiation, although there are still plenty of unknowns regarding the mechanism. However, an imbalance of alveolar cell death and proliferation was observed in patients with pulmonary emphysema, contributing to alveolar wall destruction and impaired gas exchange. Cigarette smoking causes oxidative stress and is the primary cause of this disease development. Aberrant inflammatory and oxidative stress responses result in loss of cell homeostasis and ATII cell dysfunction in emphysema. Here, we discuss the current understanding of alveolar re-epithelialization and altered reparative responses in the pathophysiology of this disease. Current therapeutics and emerging treatments, including cell therapies in clinical trials, are addressed as well.

Preparation of the sphingolipid fraction from mycelia of Cordyceps sinensis and its immunosuppressive activity

ETHNOPHARMACOLOGICAL RELEVANCE

Cordyceps sinensis (CS) is an herbal tonic in traditional Chinese medicine and is used to treat a wide range of disorders, including immune, kidney, respiratory, lung and cardiovascular diseases, in China. Most studies are focused mainly on nucleotides and polysaccharides from CS and consider them to be the main active ingredients, while other ingredients are often disregarded. Hundreds of sphingolipids have been identified from CS and showed inhibitory effects on mouse splenic lymphocytes.

AIM OF THE STUDY

This study aimed to establish a method for preparing a fraction of sphingolipids from the mycelial powder of CS and evaluate its immunosuppressive activity.

MATERIALS AND METHODS

Fraction of sphingolipids (Fr-SPLs) were prepared by silica gel chromatography and reversed-phase chromatography. Its components were identified and quantified by Quadrupole-Orbitrap UHPLC-MS/MS. PBMCs were prepared from human blood, and splenic lymphocytes, B cells, and T cells were prepared from mouse spleens. The inhibitory effect of Fr-SPLs on cell viability was evaluated by CCK-8 assay. PBMC apoptosis and the ratio of CD4⁺ T cells and CD8⁺ T cells were quantified by flow cytometry analysis. The expression of IL-2, IL-10, and TNF-α in PBMCs was detected by ELISA kits.

RESULTS

A fraction containing 84.83% of sphingolipids (SPLs) was prepared from the mycelia of CS and named Fr-SPLs. 15 SPLs were identified from the Fr-SPLs. Fr-SPLs significantly inhibited the viability of human peripheral blood mononuclear cells (PBMCs) with an IC₅₀ value of 9.82 μg/mL and promoted PBMC apoptosis in a dose-dependent manner. Moreover, Fr-SPLs inhibited the viability of mouse splenocytes, as well as that of B cells and T cells derived from splenocytes. Furthermore, Fr-SPLs reduced the production of IL-2, IL-10, and TNF-α in PBMCs.

CONCLUSIONS

Fr-SPLs show immunosuppressive activity, and this study will be useful for preparing immunosuppressive components from CS and its mycelia for hyperimmune disease.

Pharmacological sphingosine-1 phosphate receptor 1 targeting in cigarette smoke-induced emphysema in mice

Primarily caused by chronic cigarette smoking (CS), emphysema is characterized by loss of alveolar cells comprising lung units involved in gas exchange and inflammation that culminate in airspace enlargement. Dysregulation of sphingolipid metabolism with increases of ceramide relative to sphingosine-1 phosphate (S1P) signaling has been shown to cause lung cell apoptosis and is emerging as a potential therapeutic target in emphysema. We sought to determine the impact of augmenting S1P signaling via S1P receptor 1 (S1P1) in a mouse model of CS-induced emphysema. DBA2 mice were exposed to CS for 4 or 6 mo and treated with pharmacological agonists of S1P1: phosphonated FTY720 (FTY720-1S and 2S analogs; 0.01-1.0 mg/kg) or GSK183303A (10 mg/kg). Pharmacological S1P1 agonists ameliorated CS-induced lung parenchymal apoptosis and airspace enlargement as well as loss of body weight. S1P1 agonists had modest inhibitory effects on CS-induced airspace inflammation and lung functional changes measured by Flexivent, improving lung tissue resistance. S1P1 abundance was reduced in chronic CS-conditions and remained decreased after CS-cessation or treatment with FTY720-1S. These results support an important role for S1P-S1P1 axis in maintaining the structural integrity of alveoli during chronic CS exposure and suggest that increasing both S1P1 signaling and abundance may be beneficial to counteract the effects of chronic CS exposure.

A Potent Histone Deacetylase Inhibitor MPT0E028 Mitigates Emphysema Severity via Components of the Hippo Signaling Pathway in an Emphysematous Mouse Model

Background

Chronic obstructive pulmonary disease (COPD) is a major cause of chronic mortality. The objective of this study was to investigate the therapeutic potential of a novel potent histone deacetylase (HDAC) inhibitor MPT0E028 on emphysema.

Materials and Methods

A mouse model of porcine pancreatic elastase (PPE)-induced emphysema was orally administered 0, 25, or 50 mg/kg body weight (BW) of the MPT0E028 five times/week for 3 weeks. Pulmonary function, mean linear intercept (MLI), chest CT, inflammation, yes-associated protein (YAP), transcriptional coactivator with PDZ-binding motif (TAZ), surfactant protein C (SPC), T1-α, p53, and sirtuin 1 (SIRT1) levels were examined.

Results

50 mg/kg BW of the MPT0E028 significantly decreased the tidal volume in emphysematous mice (p < 0.05). Emphysema severity was significantly reduced from 26.65% (PPE only) to 13.83% (50 mg/kg BW of the MPT0E028). Total cell counts, neutrophils, lymphocytes, and eosinophils significantly decreased with both 25 and 50 mg/kg BW of the MPT0E028 (p < 0.05). Also, 50 mg/kg BW of the MPT0E028 significantly decreased the levels of KC, TNF-α, and IL-6 in lung tissues and serum (p < 0.05). Expressions of p-TAZ/TAZ in lung tissues significantly decreased with 50 mg/kg BW of the MPT0E028 (p < 0.05). Expressions of p53 significantly decreased in alveolar regions with 50 mg/kg BW of the MPT0E028 (p < 0.05), and the expression of SPC increased in alveolar regions with 50 mg/kg BW of the MPT0E028 (p < 0.05).

Conclusions

Our study showed that the potent HDAC inhibitor MPT0E028 reduced the severity and inflammation of emphysema with improvement in lung function, which could be regulated by Hippo signaling pathway. The MPT0E028 may have therapeutic potential for emphysema.

Lung Extracellular Matrix Hydrogels Enhance Preservation of Type II Phenotype in Primary Alveolar Epithelial Cells

One of the main limitations of in vitro studies on lung diseases is the difficulty of maintaining the type II phenotype of alveolar epithelial cells in culture. This fact has previously been related to the translocation of the mechanosensing Yes-associated protein (YAP) to the nuclei and Rho signaling pathway. In this work, we aimed to culture and subculture primary alveolar type II cells on extracellular matrix lung-derived hydrogels to assess their suitability for phenotype maintenance. Cells cultured on lung hydrogels formed monolayers and maintained type II phenotype for a longer time as compared with those conventionally cultured. Interestingly, cells successfully grew when they were subsequently cultured on a dish. Moreover, cells cultured on a plate showed the active form of the YAP protein and the formation of stress fibers and focal adhesions. The results of chemically inhibiting the Rho pathway strongly suggest that this is one of the mechanisms by which the hydrogel promotes type II phenotype maintenance. These results regarding protein expression strongly suggest that the chemical and biophysical properties of the hydrogel have a considerable impact on the transition from ATII to ATI phenotypes. In conclusion, culturing primary alveolar epithelial cells on lung ECM-derived hydrogels may facilitate the prolonged culturing of these cells, and thus help in the research on lung diseases.

Function of epithelial stem cell in the repair of alveolar injury

Alveoli are the functional units of blood-gas exchange in the lung and thus are constantly exposed to outside environments and frequently encounter pathogens, particles and other harmful substances. For example, the alveolar epithelium is one of the primary targets of the SARS-CoV-2 virus that causes COVID-19 lung disease. Therefore, it is essential to understand the cellular and molecular mechanisms by which the integrity of alveoli epithelial barrier is maintained. Alveolar epithelium comprises two cell types: alveolar type I cells (AT1) and alveolar type II cells (AT2). AT2s have been shown to function as tissue stem cells that repair the injured alveoli epithelium. Recent studies indicate that AT1s and subgroups of proximal airway epithelial cells can also participate alveolar repair process through their intrinsic plasticity. This review discussed the potential mechanisms that drive the reparative behaviors of AT2, AT1 and some proximal cells in responses to injury and how an abnormal repair contributes to some pathological conditions.

Hippo signaling pathway and respiratory diseases

The hippo signaling pathway is a highly conserved evolutionary signaling pathway that plays an important role in regulating cell proliferation, organ size, tissue development, and regeneration. Increasing evidences consider that the hippo signaling pathway is involved in the process of respiratory diseases. Hippo signaling pathway is mainly composed of mammalian STE20-like kinase 1/2 (MST1/2), large tumor suppressor 1/2 (LATS1/2), WW domain of the Sav family containing protein 1 (SAV1), MOB kinase activator 1 (MOB1), Yes-associated protein (YAP) or transcriptional coactivator with PDZ-binding motif (TAZ), and members of the TEA domain (TEAD) family. YAP is the cascade effector of the hippo signaling pathway. The activation of YAP promotes pulmonary arterial vascular smooth muscle cells (PAVSMCs) proliferation, which leads to pulmonary vascular remodeling; thereby the pulmonary arterial hypertension (PAH) is aggravated. While the loss of YAP leads to high expression of inflammatory genes and the accumulation of inflammatory cells, the pneumonia is consequently exacerbated. In addition, overexpressed YAP promotes the proliferation of lung fibroblasts and collagen deposition; thereby the idiopathic pulmonary fibrosis (IPF) is promoted. Moreover, YAP knockout reduces collagen deposition and the senescence of adult alveolar epithelial cells (AECs); hence the IPF is slowed. In addition, hippo signaling pathway may be involved in the repair of acute lung injury (ALI) by promoting the proliferation and differentiation of lung epithelial progenitor cells and intervening in the repair of pulmonary capillary endothelium. Moreover, the hippo signaling pathway is involved in asthma. In conclusion, the hippo signaling pathway is involved in respiratory diseases. More researches are needed to focus on the molecular mechanisms by which the hippo signaling pathway participates in respiratory diseases.

SARS-CoV-2 Infection and Lung Regeneration

The lung is the primary site of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-induced immunopathology whereby the virus enters the host cells by binding to angiotensin-converting enzyme 2 (ACE2). Sophisticated regeneration and repair programs exist in the lungs to replenish injured cell populations. However, known resident stem/progenitor cells have been demonstrated to express ACE2, raising a substantial concern regarding the long-term consequences of impaired lung regeneration after SARS-CoV-2 infection. Moreover, clinical treatments may also affect lung repair from antiviral drug candidates to mechanical ventilation. In this review, we highlight how SARS-CoV-2 disrupts a program that governs lung homeostasis. We also summarize the current efforts of targeted therapy and supportive treatments for COVID-19 patients. In addition, we discuss the pros and cons of cell therapy with mesenchymal stem cells or resident lung epithelial stem/progenitor cells in preventing post-acute sequelae of COVID-19. We propose that, in addition to symptomatic treatments being developed and applied in the clinic, targeting lung regeneration is also essential to restore lung homeostasis in COVID-19 patients.

Review article: the sphingosine 1 phosphate/sphingosine 1 phosphate receptor axis - a unique therapeutic target in inflammatory bowel disease

BACKGROUND

Ozanimod, a high selective sphingosine 1 phosphate (S1P) receptor (S1PR) 1/5 modulator was approved by the Food and Drug Administration for the treatment of adult patients with moderately to severely active ulcerative colitis. Additional S1PR modulators are being tested in clinical development programmes for both ulcerative colitis and Crohn's disease.

AIM

To provide an overview of advances in understanding S1PRs biology and summarise preclinical and clinical investigations of S1P receptor modulators in chronic inflammatory disease with special emphasis on inflammatory bowel diseases (IBD).

METHODS

We performed a narrative review using PubMed and ClinicalTrials.gov.

RESULTS

Through S1PRs, S1P regulates multiple cellular processes, including proliferation, migration, survival, and vascular barrier integrity. The S1PRs function of regulating lymphocyte trafficking is well known, but new functions of S1PRs expand our knowledge of S1PRs biology. Several S1PR modulators are in clinical development for both ulcerative colitis and Crohn's disease and have shown promise in phase II and III studies with ozanimod now being approved for ulcerative colitis.

CONCLUSIONS

S1P receptor modulators constitute a novel, promising, safe, and convenient strategy for the treatment of IBD.

Dysregulated Cell Signaling in Pulmonary Emphysema

Pulmonary emphysema is characterized by the destruction of alveolar septa and irreversible airflow limitation. Cigarette smoking is the primary cause of this disease development. It induces oxidative stress and disturbs lung physiology and tissue homeostasis. Alveolar type II (ATII) cells have stem cell potential and can repair the denuded epithelium after injury; however, their dysfunction is evident in emphysema. There is no effective treatment available for this disease. Challenges in this field involve the large complexity of lung pathophysiological processes and gaps in our knowledge on the mechanisms of emphysema progression. It implicates dysregulation of various signaling pathways, including aberrant inflammatory and oxidative responses, defective antioxidant defense system, surfactant dysfunction, altered proteostasis, disrupted circadian rhythms, mitochondrial damage, increased cell senescence, apoptosis, and abnormal proliferation and differentiation. Also, genetic predispositions are involved in this disease development. Here, we comprehensively review studies regarding dysregulated cell signaling, especially in ATII cells, and their contribution to alveolar wall destruction in emphysema. Relevant preclinical and clinical interventions are also described.

Comprehensive epigenomic profiling of human alveolar epithelial differentiation identifies key epigenetic states and transcription factor co-regulatory networks for maintenance of distal lung identity

BACKGROUND

Disruption of alveolar epithelial cell (AEC) differentiation is implicated in distal lung diseases such as chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis, and lung adenocarcinoma that impact morbidity and mortality worldwide. Elucidating underlying disease pathogenesis requires a mechanistic molecular understanding of AEC differentiation. Previous studies have focused on changes of individual transcription factors, and to date no study has comprehensively characterized the dynamic, global epigenomic alterations that facilitate this critical differentiation process in humans.

RESULTS

We comprehensively profiled the epigenomic states of human AECs during type 2 to type 1-like cell differentiation, including the methylome and chromatin functional domains, and integrated this with transcriptome-wide RNA expression data. Enhancer regions were drastically altered during AEC differentiation. Transcription factor binding analysis within enhancer regions revealed diverse interactive networks with enrichment for many transcription factors, including NKX2-1 and FOXA family members, as well as transcription factors with less well characterized roles in AEC differentiation, such as members of the MEF2, TEAD, and AP1 families. Additionally, associations among transcription factors changed during differentiation, implicating a complex network of heterotrimeric complex switching in driving differentiation. Integration of AEC enhancer states with the catalog of enhancer elements in the Roadmap Epigenomics Mapping Consortium and Encyclopedia of DNA Elements (ENCODE) revealed that AECs have similar epigenomic structures to other profiled epithelial cell types, including human mammary epithelial cells (HMECs), with NKX2-1 serving as a distinguishing feature of distal lung differentiation.

CONCLUSIONS

Enhancer regions are hotspots of epigenomic alteration that regulate AEC differentiation. Furthermore, the differentiation process is regulated by dynamic networks of transcription factors acting in concert, rather than individually. These findings provide a roadmap for understanding the relationship between disruption of the epigenetic state during AEC differentiation and development of lung diseases that may be therapeutically amenable.

The interplay between fibroblast-like synovial and vascular endothelial cells leads to angiogenesis via the sphingosine-1-phosphate-induced RhoA-F-Actin and Ras-Erk1/2 pathways and the intervention of geniposide

The changes of fibroblast-like synoviocytes (FLSs) and vascular endothelial cells (VECs) biological functions are closely related to angiogenesis in rheumatoid arthritis (RA). Nevertheless, how the crosstalk between FLSs and VECs interferes with RA is far from being clarified. Herein, we studied the effect of the reciprocal interactions between FLSs and VECs on angiogenesis and mechanism of geniposide (GE). After administration of GE, improvement of synovial hyperplasia in adjuvant arthritis rats was accompanied by downregulation of SphK1 and p-Erk1/2. The dynamic interaction between FLSs and VECs triggers the release of S1P by activating p-Erk1/2 and SphK1, then activating RhoA-F-actin and Ras-Erk1/2 pathways. When exposed to the inflammatory microenvironment mediated by FLSs-VECs crosstalk, proliferation, migration, and permeability of VECs were enhanced, the angiogenic factors were imbalanced. Meanwhile, the proliferation and secretory ability of FLSs increased. Interestingly, depletion of S1P or blocking of the activation of SphK1 by GE and PF-543 prevented the changes. In conclusion, S1P released during FLSs-VECs crosstalk changed their biological functions by activating RhoA-F-actin and Ras-Erk1/2 pathways. GE acted on p-Erk1/2 and SphK1, inhibited the secretion of S1P, and blocked the interplay between FLSs and VECs. These results provide new insights into the mechanism of angiogenesis in RA.

Lipid-Derived Mediators are Pivotal to Leukocyte and Lung Cell Responses in Sepsis and ARDS

Acute inflammation in the lung is essential for host defense against pathogens and other injuries but chronic or excessive inflammation can contribute to several common respiratory diseases. In health, the inflammatory response is controlled by several cellular and molecular mechanisms. In addition to anti-inflammatory processes, there are non-phlogistic pro-resolving mechanisms that are engaged to promote the resolution of inflammation and a return to homeostasis. Defects in the production or actions of specialized pro-resolving mediators are associated with diseases characterized by excess or chronic inflammation. In this article, we review cellular and biochemical mechanisms for specialized pro-resolving mediators in health and in sepsis and the acute respiratory distress syndrome as examples of unrestrained inflammatory responses that result in life-threatening pathology. We are honored to contribute to this special edition of the Journal to help celebrate Professor Viswanathan Natarajan’s contributions to our understanding of lipid-derived mediators and metabolism in lung cell responses to inflammatory, infectious, or mechanical insults; his foundational discoveries in cell biochemistry and biophysics are continuing to catalyze further advances by the field to uncover the mechanistic underpinnings of important human diseases.

Activation of yes-associated protein mediates sphingosine-1-phosphate-induced proliferation and migration of pulmonary artery smooth muscle cells and its potential mechanisms

The aims of the present study were to examine the molecular mechanisms underlying sphingosine-1-phosphate (S1P)-induced rat pulmonary artery smooth muscle cells (PASMCs) proliferation/migration and to determine the effect of yes-associated protein (YAP) activation on S1P-induced PASMCs proliferation/migration and its potential mechanisms. S1P induced YAP dephosphorylation and nuclear translocation, upregulated microRNA-130a/b (miR-130a/b) expression, reduced bone morphogenetic protein receptor 2 (BMPR2), and inhibitor of DNA binding 1(Id1) expression, and promoted PASMCs proliferation and migration. Pretreatment of cells with Rho-associated protein kinase (ROCK) inhibitor Y27632 suppressed S1P-induced YAP activation, miR-130a/b upregulation, BMPR2/Id1 downregulation, and PASMCs proliferation/migration. Knockdown of YAP using small interfering RNA also suppressed S1P-induced alterations of miR-130a/b, BMPR2, Id1, and PASMCs behavior. In addition, luciferase reporter assay indicated that miR-130a/b directly regulated BMPR2 expression in PASMCs. Inhibition of miR-130a/b functions by anti-miRNA oligonucleotides attenuated S1P-induced BMPR2/Id1 downregulation and the proliferation and migration of PASMCs. Taken together, our study indicates that S1P induces activation of YAP through ROCK signaling and subsequently increases miR-130a/b expression, which, in turn, downregulates BMPR2 and Id1 leading to PASMCs proliferation and migration.

Sphingosine 1 Phosphate (S1P) Receptor 1 Is Decreased in Human Lung Microvascular Endothelial Cells of Smokers and Mediates S1P Effect on Autophagy

Destruction of alveoli by apoptosis induced by cigarette smoke (CS) is a major driver of emphysema pathogenesis. However, when compared to cells isolated from non-smokers, primary human lung microvascular endothelial cells (HLMVECs) isolated from chronic smokers are more resilient when exposed to apoptosis-inducing ceramide. Whether this adaptation restores homeostasis is unknown. To better understand the phenotype of HLMVEC in smokers, we interrogated a major pro-survival pathway supported by sphingosine-1-phosphate (S1P) signaling via S1P receptor 1 (S1P1). Primary HLMVECs from lungs of non-smoker or smoker donors were isolated and studied in culture for up to five passages. S1P1 mRNA and protein abundance were significantly decreased in HLMVECs from smokers compared to non-smokers. S1P1 was also decreased in situ in lungs of mice chronically exposed to CS. Levels of S1P1 expression tended to correlate with those of autophagy markers, and increasing S1P (via S1P lyase knockdown with siRNA) stimulated baseline macroautophagy with lysosomal degradation. In turn, loss of S1P1 (siRNA) inhibited these effects of S1P on HLMVECs autophagy. These findings suggest that the anti-apoptotic phenotype of HLMVECs from smokers may be maladaptive, since it is associated with decreased S1P1 expression that may impair their autophagic response to S1P.

The diversity of adult lung epithelial stem cells and their niche in homeostasis and regeneration

The adult lung, a workhorse for gas exchange, is continually subjected to a barrage of assaults from the inhaled particles and pathogens. Hence, homeostatic maintenance is of paramount importance. Epithelial stem cells interact with their particular niche in the adult lung to orchestrate both natural tissue rejuvenation and robust post-injury regeneration. Advances in single-cell sequencing, lineage tracing, and living tissue imaging have deepened our understanding about stem cell heterogeneities, transition states, and specific cell lineage markers. In this review, we provided an overview of the known stem/progenitor cells and their subpopulations in different regions of the adult lung, and explored the regulatory networks in stem cells and their respective niche which collectively coordinated stem cell quiescence and regeneration states. We finally discussed relationships between dysregulated stem cell function and lung disease.

Isolation and culture of mouse alveolar type II cells to study type II to type I cell differentiation

Alveolar type II cells (AT2) are stem cells of lung epithelium. To study these cells, we developed protocols to isolate primary AT2 and test the effects of potential niche factors in regulating the AT2 progenitor functions. AT2 freshly isolated from mouse lungs are grown in 2D or 3D culture. AT2 are able to differentiate into alveolar type I cells (AT1) in these conditions. S1P or an inhibitor of S1P signaling is added in culture to modulate AT2 to AT1 transition.

For complete details on the use and execution of this protocol, please refer to Chen et al. (2020).

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A protocol for isolating mouse lung alveolar type II cells

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Grow type II cells in 2D culture and observe their differentiation to type I cells

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Grow type II cells in 3D culture and generate alveolospheres

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Test the role of sphingosine-1-phosphate signaling in type II-to-I cell transition

Stem cells in pulmonary alveolar regeneration

The lungs are constantly exposed to the external environment and are therefore vulnerable to insults that can cause infection and injury. Maintaining the integrity and barrier function of the lung epithelium requires complex interactions of multiple cell lineages. Elucidating the cellular players and their regulation mechanisms provides fundamental information to deepen understanding about the responses and contributions of lung stem cells. This Review focuses on advances in our understanding of mammalian alveolar epithelial stem cell subpopulations and discusses insights about the regeneration-specific cell status of alveolar epithelial stem cells. We also consider how these advances can inform our understanding of post-injury lung repair processes and lung diseases.

Heterogeneous groups of alveolar type II cells in lung homeostasis and repair

Alveoli are the gas-exchanging units of the lung, and the alveolar barrier is often a key battleground where pathogens, allergens, and other insults from the environment are encountered. This is seen in the current coronavirus disease 2019 (COVID-19) pandemic, as alveolar epithelium is one of the major targets of SARS-COV-2, the virus that causes COVID-19. Thus, it is essential to understand the mechanisms in order to maintain the integrity of alveoli epithelium. Alveolar type II (AT2) cells behave as tissue stem cells that repair alveoli epithelium during steady-state replacement and after injury. However, not all AT2 cells are equal in their ability for self-renewal or differentiation. Through marker gene identification, lineage tracing, and single-cell RNA-sequencing (scRNA-seq), distinct subpopulations of AT2 cells have been identified that play the progenitor role in a different context. The revelation of AT2 heterogeneity has brought new insights into the role of AT2 cells in various lung disease settings and potentiates the finding of more therapeutics targets. In this mini review, we discuss the recently identified subpopulations of AT2 cells and their functions under steady-state, postinjury, and pathological conditions.