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

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

Inhaled vitamin A is more effective than intramuscular dosing in mitigating hyperoxia-induced lung injury in a neonatal rat model of bronchopulmonary dysplasia.

Prevention of bronchopulmonary dysplasia (BPD) in premature-birth babies continues to be an unmet medical need. Intramuscular vitamin A is currently employed in preterm neonates to prevent BPD but requires intramuscular injections in fragile neonates. We hypothesized that noninvasive inhaled delivery of vitamin A, targeted to lung, would be a more effective and tolerable strategy. We employed our well-established hyperoxia-injury neonatal rat model, exposing newborn rats to 7 days of constant extreme (95% O2) hyperoxia, comparing vitamin A dosed every 48 h via either aerosol inhalation or intramuscular injection with normoxic untreated healthy animals and vehicle-inhalation hyperoxia groups as positive and negative controls, respectively. Separately, similar vitamin A dosing of normoxia-dwelling animals was performed. Analyses after day 7 included characterization of alveolar histomorphology and protein biomarkers of alveolar maturation [surfactant protein C (SP-C), peroxisome proliferator-activated receptor (PPAR) γ, cholinephosphate cytidylyl transferase, vascular endothelial growth factor and its receptor, FLK-1, and retinoid X receptors (RXR-α, -ß, and -γ], apoptosis (Bcl2 and Bax) key injury repair pathway data including protein markers (ALK-5 and ß-catenin) and neutrophil infiltration, and serum vitamin A levels. Compared with intramuscular dosing, inhaled vitamin A significantly enhanced biomarkers of alveolar maturation, mitigated hyperoxia-induced lung damage, and enhanced surfactant protein levels, suggesting that it may be more efficacious in preventing BPD in extremely premature infants than the traditionally used IM dosing regimen. We speculate lung-targeted inhaled vitamin A may also be an effective therapy against other lung damaging conditions leading to BPD or, more generally, to acute lung injury.

Plasticity in the lung: making and breaking cell identity.

In contrast to a prior emphasis on the finality of cell fate decisions in developmental systems, cellular plasticity is now emerging as a general theme in the biology of multiple adult organ systems. In the lung, lineage tracing has been used to identify distinct epithelial stem and progenitor cell populations. These cells, together with their differentiated progeny, maintain a stable identity during steady state conditions, but can display remarkable lineage plasticity following injury. This Review summarizes our current understanding of the different cell lineages of the adult mammalian lung and their responses to injury. In the lung, which is constantly exposed to infection and aerosolized toxins, epithelial plasticity might be more of a rule than an exception, and it is likely that different injuries elicit different facultative responses.

Perinatal nicotine vaping exposure induces pro-myofibroblastic phenotype in rat bone marrow-derived mesenchymal stem cells.

Perinatal nicotine exposure via tobacco smoking results in increased proclivity to chronic lung disease (CLD); however, the underlying molecular mechanisms remain incompletely understood. We previously demonstrated that in addition to nicotine's direct effects on the developing lung, there are also adverse molecular alterations in bone marrow-derived mesenchymal stem cells (BMSCs), which are vital to lung injury repair. Whether perinatal nicotine exposure via electronic-cigarette (e-cig) vaping also adversely affects BMSCs is unknown. This is highly relevant due to marked increase in e-cig vaping including by pregnant women. Hypothesizing that perinatal nicotine exposure via e-cig vaping predisposes BMSCs to a pro-myofibroblastic phenotype, pregnant rat dams were exposed to fresh air (control), vehicle (e-cig without nicotine), or e-cig (e-cig with nicotine) daily during pregnancy and lactation. At postnatal day 21, offspring BMSCs were isolated and studied for cell proliferation, migration, wound healing response, and expression of key Wnt and PPARγ signaling intermediates (ß-catenin, LEF-1, PPARγ, ADRP and C/EBPα) and myogenic markers (fibronectin, αSMA, calponin) proteins using immunoblotting. Compared to controls, perinatal e-cig exposure resulted in significant decrease in BMSC proliferation, migration, and wound healing response. The expression of key Wnt signaling intermediates (ß-catenin, LEF-1) and myogenic markers (fibronectin, αSMA, calponin) increased significantly, while PPARγ signaling intermediates (PPARγ, ADRP, and C/EBPα) decreased significantly. Based on these data, we conclude that perinatally e-cig exposed BMSCs demonstrate pro-myofibroblastic phenotype and impaired injury-repair potential, indicating a potentially similar susceptibility to CLD following perinatal nicotine exposure via vaping as seen following parenteral perinatal nicotine exposure.

Effects of short-term high-concentration exposure to PM2.5 on pulmonary tissue damage and repair ability as well as innate immune events.

Short-term heavy air pollution still occurs frequently worldwide, especially during the winter heating period in some developing countries, which is usually accompanied by the temporary explosive growth of PM2.5. The pulmonary damage caused by PM2.5 exposure has been determined, but there have been few studies on the repair ability after the cessation of exposure and the important role of innate immune events. This study established a short-term (30 days) high-concentration (15 mg/kg body weight) PM2.5 exposure and recovery (15 days of exposure cessation) model by intratracheal instillation. The results showed that short-term PM2.5 exposure increased the content of collagen fiber in rat lung tissue, which was significantly repaired after recovery by 15 days of exposure cessation. Meanwhile, exposure to PM2.5 also caused changes in lung epithelial function, macrophage polarization and cell autophagy function. Most of these changes could be restored or reversed to a certain extent after recovery. However, there were also some biomarkers, including CLDN18.1, SP-A, SP-D, iNOS, CD206, Beclin1, p62 and LC3B, that were still significantly different between the exposure and control groups after recovery, suggesting that some toxic effects, especially epithelial function damage, were not completely repaired. In addition, there was a significant correlation between pulmonary fibrosis and innate immunity. The present study demonstrated that short-term high-concentration exposure to PM2.5 could cause temporary lung tissue damage and related innate immune events in rats, and the repair ability existed after the cessation of exposure, but part of the damage that required special attention still persisted.

Cell death and ischemia-reperfusion injury in lung transplantation.

Lung transplantation is the most effective therapy for patients with end-stage lung disease. However, concern of donor lung damage and ischemia-reperfusion induced lung injury limits the use of "marginal" donor lungs. Recent transcriptomic studies have demonstrated that the enrichment of gene-clusters related to cell death and inflammation are the most profound signals during ischemia-reperfusion in human lung transplants. Herein, we focus on the relationship between inflammation and programmed cell death, especially necroptosis, mitochondrial permeability transition-initiated regulated necrosis, pyroptosis, ferroptosis, and autophagic cell death. Cell death-related molecules have been tested as potential biomarkers for donor lung assessment. Inhibitors for various types of cell death have been explored as therapeutics for ischemia reperfusion injury in lung transplantation. A deeper understanding of these mechanisms may help to improve donor management, organ preservation, prevention and treatment of primary graft dysfunction during and post transplantation. Moreover, evaluation and treatment of cell death and inflammation during ex vivo lung perfusion may be a game changer in donor organ management, assessment, repair, and reconditioning.

Improved Differentiation Ability and Therapeutic Effect of miR-23a-3p Expressing Bone Marrow-Derived Mesenchymal Stem Cells in Mice Model with Acute Lung Injury.

BACKGROUND AND OBJECTIVES: Implantation of bone marrow-derived mesenchymal stem cells (BMSCs) has been recognized as an effective therapy for attenuating acute lung injury (ALI). This study aims to discover microRNA (miRNA)-mediated improvement of BMSCs-based therapeutic effects. METHODS AND RESULTS: Mice were treated with lipopolysaccharide (LPS) for induction of ALI. BMSCs with lentivirus- mediated expression of miR-23b-3p or fibroblast growth factor 2 (FGF2) were intratracheally injected into the mice with ALI. The expressions of miR-23b-3p, FGF2, Occludin, and surfactant protein C (SPC) in lung tissues were analyzed by immunoblot or quantitative reverse transcription polymerase chain reaction. Histopathological changes in lung tissues were observed via hematoxylin-eosin staining. Lung edema was assessed by the ratio of lung wet weight/body weight (LWW/BW). The levels of interleukin (IL)-1ß, IL-6, IL-4, and IL-8 in bronchoalveolar lavage fluid (BALF) were assessed by ELISA. LPS injection downregulated the expressions of miR-23b-3p, SPC and Occludin in the lung tissues, increased the LWW/BW ratio and aggravated histopathological abnormalities, while upregulating IL-1ß, IL-6, IL-4, and IL-8 in the BALF. Upregulated miR-23b-3p counteracted LPS-induced effects, whereas downregulated miR-23b-3p intensified LPS-induced effects. FGF2, which was downregulated by miR-23b-3p upregulation, was a target gene of miR-23b-3p. Overexpressing FGF2 downregulated the expressions of miR-23b-3p, SPC and Occludin, increased the LWW/BW ratio and aggravated histopathological abnormalities, while upregulating IL-1ß, IL-6, IL-4, and IL-8, and it offset miR-23b-3p upregulation-caused effects on the ALI mice. CONCLUSIONS: Overexpression of miR-23b-3p in BMSCs strengthened BMSC-mediated protection against LPS-induced mouse acute lung injury via targeting FGF2.

Pediatric Acute Respiratory Distress Syndrome: Fibrosis versus Repair.

Clinical and basic experimental approaches to pediatric acute lung injury (ALI), including acute respiratory distress syndrome (ARDS), have historically focused on acute care and management of the patient. Additional efforts have focused on the etiology of pediatric ALI and ARDS, clinically defined as diffuse, bilateral diseases of the lung that compromise function leading to severe hypoxemia within 7 days of defined insult. Insults can include ancillary events related to prematurity, can follow trauma and/or transfusion, or can present as sequelae of pulmonary infections and cardiovascular disease and/or injury. Pediatric ALI/ARDS remains one of the leading causes of infant and childhood morbidity and mortality, particularly in the developing world. Though incidence is relatively low, ranging from 2.9 to 9.5 cases/100,000 patients/year, mortality remains high, approaching 35% in some studies. However, this is a significant decrease from the historical mortality rate of over 50%. Several decades of advances in acute management and treatment, as well as better understanding of approaches to ventilation, oxygenation, and surfactant regulation have contributed to improvements in patient recovery. As such, there is a burgeoning interest in the long-term impact of pediatric ALI/ARDS. Chronic pulmonary deficiencies in survivors appear to be caused by inappropriate injury repair, with fibrosis and predisposition to emphysema arising as irreversible secondary events that can severely compromise pulmonary development and function, as well as the overall health of the patient. In this chapter, the long-term effectiveness of current treatments will be examined, as will the potential efficacy of novel, acute, and long-term therapies that support repair and delay or even impede the onset of secondary events, including fibrosis.