Endothelial-derived angiocrine signals induce and sustain regenerative lung alveolarization

The lung microvasculature promotes alveolar type 2 cell differentiation via secreted SPARCL1

Lung endothelial cells (ECs) and pericytes are closely juxtaposed with the respiratory epithelium before birth and thus may have instructive roles during development. To test this hypothesis, we screened EC-secreted proteins for their ability to alter cell differentiation in alveolar organoids. We identified secreted protein acidic and rich in cysteine-like protein 1 (SPARCL1) as an extracellular matrix molecule that can promote alveolar type 2 (AT2) cell differentiation in vitro. SPARCL1-treated organoids display lysozyme upregulation and a doubling in the number of AT2 cells at the expense of intermediate progenitors. SPARCL1 also induces the upregulation of nuclear factor κB (NF-κB) target genes, and suppression of NF-κB activation in lung organoids blocked SPARCL1 effects. NF-κB activation by lipopolysaccharide (LPS) was sufficient to induce AT2 cell differentiation; however, pharmacological inhibition of the pathway alone did not prevent it. These data support a role for SPARCL1 and NF-κB in alveolar cell differentiation and suggest a potential value in targeting this signaling axis to promote alveolar maturation and regeneration.

Physiological Modeling of the Vascularized Human Lung Organoid

Human lung organoids (hLOs) derived from induced pluripotent stem cells (iPSCs) are of great interest, as they inform lung development, such as differentiation of lung epithelial subtypes in the distal alveolar unit. An unaddressed question is whether introducing endothelial cells (ECs) and vascularization provides a better representation of hLOs. Here we describe a method in which vessels become integrated with hLOs. hLOs were generated by combining human iPSC-derived lung progenitor cells (LPs) with ECs at varying LP:EC ratios. At the optimal combination of both cells, we observed vessel infiltration of hLOs compared to without ECs. Red blood cells were seen in hLOs implanted into kidney capsules of NOD/SCID mice. Both human and mouse ECs conjoined to form chimeric vessels in hLOs. The vascularized hLOs showed alveolar type II epithelial (ATII) cells and ATI cells, although there was no difference in 1:1 ATII/ATI ratio. We observed primitive airway sacs with alveolar epithelial cells lining the lumen of vascularized hLOs. Electron microscopy revealed surfactant production in ATII cells of vascularized hLOs in contrast to absence of vessels. The vascularized hLOs also mounted a robust inflammatory response characterized by influx of mouse neutrophils after challenging mice with LPS. Thus, interactions of ECs with LPs generated vascularized hLOs that induced ATII and ATI differentiation, although not reaching to the ratio of 1:9 seen in mature human lungs. hLOs also showed the LPS induced inflammatory response upon transplantation into recipient mice. Our results show the potential of vascularized hLOs for studying human lung development and inflammatory lung injury.

Application and Mechanism of Adipose Tissue-Derived Microvascular Fragments in Tissue Repair and Regeneration

One of the long-standing challenges in the field of tissue repair and regeneration is the rapid establishment of local microvascular circulation and restoration of perfusion at the site of defects or injuries. Recently, adipose tissue-derived microvascular fragments (ad-MVFs) have attracted increasing attention from researchers. Adipose tissue is rich in blood vessels, and significant progress has been made in the extraction and preservation techniques for microvascular fragments within it. Ad-MVFs promote tissue and organ repair and regeneration through three main mechanisms. First, they accelerate rapid and efficient vascularization at the injury site, enabling early vessel perfusion. Second, the stem cell components within ad-MVFs provide a rich source of cells for tissue and organ regeneration. Third, they play a role in immune regulation, facilitating integration with host tissues after implantation. The application methods of ad-MVFs are diverse. They can be directly implanted or pre-cultivated, facilitating their combination with various scaffolds and broadening their application scope. These properties have led to the wide use of ad-MVFs in tissue engineering, with promising prospects. This review demonstrates that ad-MVFs can serve as a reliable and highly feasible unit for tissue regeneration.

Blood flow-induced angiocrine signals promote organ growth and regeneration

Recently, we identified myeloid-derived growth factor (MYDGF) as a blood flow-induced angiocrine signal that promotes human and mouse hepatocyte proliferation and survival. Here, we review literature reporting changes in blood flow after partial organ resection in the liver, lung, and kidney, and we describe the angiocrine signals released by endothelial cells (ECs) upon blood flow alterations in these organs. While hepatocyte growth factor (HGF) and MYDGF are important angiocrine signals for liver regeneration, by now, angiocrine signals have also been reported to stimulate hyperplasia and/or hypertrophy during the regeneration of lungs and kidneys. In addition, angiocrine signals play a critical role in tumor growth. Understanding the mechano-elastic properties and flow-mediated alterations in the organ-specific microvasculature is crucial for therapeutic approaches to maintain organ health and initiate organ renewal.

Vascularization of human islets by adaptable endothelium for durable and functional subcutaneous engraftment

Tissue-specific endothelial cells (ECs) are critical for the homeostasis of pancreatic islets and most other tissues. In vitro recapitulation of islet biology and therapeutic islet transplantation both require adequate vascularization, which remains a challenge. Using human reprogrammed vascular ECs (R-VECs), human islets were functionally vascularized in vitro, demonstrating responsive, dynamic glucose-stimulated insulin secretion and Ca2+ influx. Subcutaneous transplantation of islets with R-VECs reversed hyperglycemia in diabetic mice, with high levels of human insulin detected within recipient serum and relapses of hyperglycemia following graft removal. Examination of retrieved grafts demonstrated that engrafted human islets were mainly vascularized by the cotransplanted R-VECs, which had anastomosed with the host microcirculation. Notably, single-cell RNA-sequencing revealed that R-VECs, when cocultured with islets, acquired islet EC-specific characteristics. Together, R-VECs establish an adaptable vascular niche that supports islet homeostasis both in vitro and in vivo.

Microenvironmental determinants of endothelial cell heterogeneity

During development, endothelial cells (ECs) undergo an extraordinary specialization by which generic capillary microcirculatory networks spanning from arteries to veins transform into patterned organotypic zonated blood vessels. These capillary ECs become specialized to support the cellular and metabolic demands of each specific organ, including supplying tissue-specific angiocrine factors that orchestrate organ development, maintenance of organ-specific functions and regeneration of injured adult organs. Here, we illustrate the mechanisms by which microenvironmental signals emanating from non-vascular niche cells induce generic ECs to acquire specific inter-organ and intra-organ functional attributes. We describe how perivascular, parenchymal and immune cells dictate vascular heterogeneity and capillary zonation, and how this system is maintained through tissue-specific signalling activated by vasculogenic and angiogenic factors and deposition of matrix components. We also discuss how perturbation of organotypic vascular niche cues lead to erasure of EC signatures, contributing to the pathogenesis of disease processes. We also describe approaches that use reconstitution of tissue-specific signatures of ECs to promote regeneration of damaged organs.

Cyclin G1 Regulates the Alveolarization in Models of Bronchopulmonary Dysplasia by Inhibiting AT2 Cell Proliferation

Disrupted neonatal lung alveologenesis often leads to bronchopulmonary dysplasia (BPD), the most common chronic lung disease in children. The inhibition of type 2 alveolar (AT2) cell proliferation plays an important role in the arrest of alveologenesis. However, the mechanism of AT2 cell proliferation retardation in BPD is still not fully elucidated. The purpose of the present study was to explore the effects of cyclin G1 (CCNG1) on AT2 cell proliferation in hyperoxia-induced lung injury in neonatal mice. Our findings revealed that hyperoxia significantly reduced the proportion of AT2 cells in the lungs of neonatal mice and coincided with an upregulation of CCNG1 expression. Notably, this upregulation of CCNG1 was accompanied by an increase in Wnt signaling. We observed colocalization of CCNG1 and Wnt3a within AT2 cells in the hyperoxia group. Further analysis showed that inhibiting CCNG1 expression regressed the expression of Wnt signaling and enhanced cell proliferation. These results suggest that CCNG1 plays a pivotal role in suppressing AT2 cell proliferation, at least partly by counteracting the effects of Wnt signaling to modulate AT2 cell growth in the BPD model. Our findings contribute to a better understanding of the mechanisms underlying BPD.

Unlocking lung regeneration: insights into progenitor cell dynamics and metabolic control

Regenerative responses are particularly important in the lungs, which are critical for gas exchange and frequently challenged by environmental insults. The lung progenitor cells play a central role in the lung regeneration response, and their dysfunction is associated with various lung diseases. Understanding the mechanisms regulating lung progenitor cell function is essential for developing new therapeutic approaches to promote lung regeneration. This review summarizes recent advancements in the field of lung regeneration, focusing on the metabolic control of lung progenitor cell function. We discuss cell lineage plasticity and cell-cell signaling under different physiological conditions. Additionally, we highlight the connection between progenitor cell dysfunction and lung diseases, emphasizing the need to develop new therapeutic strategies in regenerative medicine to improve lung regenerative capacity.

Single-cell transcriptome unveils unique transcriptomic signatures of human organ-specific endothelial cells

The heterogeneity of endothelial cells (ECs) across human tissues remains incompletely inventoried. We constructed an atlas of > 210,000 ECs derived from 38 regions across 24 human tissues. Our analysis reveals significant differences in transcriptome, phenotype, metabolism and transcriptional regulation among ECs from various tissues. Notably, arterial, venous, and lymphatic ECs shared more common markers in multiple tissues than capillary ECs, which exhibited higher heterogeneity. This diversity in capillary ECs suggests their greater potential as targets for drug development. ECs from different tissues and vascular beds were found to be associated with specific diseases. Importantly, tissue specificity of EC senescence is more determined by somatic site than by tissue type (e.g. subcutaneus adipose tissue and visceral adipose tissue). Additionally, sex-specific differences in brain EC senescence were observed. Our EC atlas offers valuble resoursce for identifying EC subclusters in single-cell datasets from body tissues or organoids, facilitating the screen of tissue-specific targeted therapies, and serving as a powerful tool for future discoveries.

Knockdown of the long noncoding RNA VSIG2-1:1 promotes the angiogenic ability of human pulmonary microvascular endothelial cells by activating the VEGF/PI3K/AKT pathway

BACKGROUND

Abnormal pulmonary vascular development poses significant clinical challenges for infants with bronchopulmonary dysplasia (BPD). Although numerous factors have been suggested to control the development of pulmonary blood vessels, the mechanisms underlying the role of long noncoding RNAs (lncRNAs) in this process remain unclear.

METHODS

A lncRNA array was used to measure the differential expression of lncRNAs in premature infants with and without BPD. The expression of lncRNA-VSIG2-1:1 in patients with BPD and hyperoxia-induced human pulmonary microvascular endothelial cells (HPMECs) was assessed using quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Fluorescence in situ hybridization (FISH) assay was performed to detect the subcellular localization of lncRNA-VSIG2-1:1. Pulmonary microvascular endothelial cells were stably transfected with adenoviral vectors to silence or overexpress lncRNA-VSIG2-1:1. The effects of lncRNA-VSIG2-1:1 on the proliferation, migration, and tube formation abilities of HPMECs subjected to hyperoxia were examined by performing Cell Counting Kit-8 (CCK-8), cell migration, and tubule formation assays. RNA sequencing (RNA-seq) was performed to determine the correlation between lncRNA-VSIG2-1:1 and phosphoinositide-3-kinase (PI3K)/protein kinase B (AKT). The protein levels of vascular endothelial growth factor (VEGF), p-PI3K, PI3K, p-AKT, and AKT were determined using western blotting.

RESULTS

The expression of lncRNA-VSIG2-1:1 was upregulated in patients with BPD and hyperoxia-treated HPMECs. Inhibiting lncRNA-VSIG2-1:1 expression promoted the proliferation, migration, and tube-formation abilities of HPMECs, while significantly increasing VEGF, p-PI3K, and p-AKT levels.

CONCLUSION

Our findings reveal that the suppression of lncRNA-VSIG2-1:1 expression stimulates angiogenesis in vitro by inducing the initiation of the VEGF/PI3K/AKT signaling pathway. This observation may aid the development of novel therapeutic targets for treating BPD.

Emerging insights into epigenetics and hematopoietic stem cell trafficking in age-related hematological malignancies

BACKGROUND

Hematopoiesis within the bone marrow (BM) is a complex and tightly regulated process predominantly influenced by immune factors. Aging, diabetes, and obesity are significant contributors to BM niche damage, which can alter hematopoiesis and lead to the development of clonal hematopoiesis of intermediate potential (CHIP). Genetic/epigenetic alterations during aging could influence BM niche reorganization for hematopoiesis or clonal hematopoiesis. CHIP is driven by mutations in genes such as Tet2, Dnmt3a, Asxl1, and Jak2, which are associated with age-related hematological malignancies.

OBJECTIVE

This literature review aims to provide an updated exploration of the functional aspects of BM niche cells within the hematopoietic microenvironment in the context of age-related hematological malignancies. The review specifically focuses on how immunological stressors modulate different signaling pathways that impact hematopoiesis.

METHODS

An extensive review of recent studies was conducted, examining the roles of various BM niche cells in hematopoietic stem cell (HSC) trafficking and the development of age-related hematological malignancies. Emphasis was placed on understanding the influence of immunological stressors on these processes.

RESULTS

Recent findings reveal a significant microheterogeneity and temporal stochasticity of niche cells across the BM during hematopoiesis. These studies demonstrate that niche cells, including mesenchymal stem cells, osteoblasts, and endothelial cells, exhibit dynamic interactions with HSCs, significantly influenced by the BM microenvironment as the age increases. Immunosurveillance plays a crucial role in maintaining hematopoietic homeostasis, with alterations in immune signaling pathways contributing to the onset of hematological malignancies. Novel insights into the interaction between niche cells and HSCs under stress/aging conditions highlight the importance of niche plasticity and adaptability.

CONCLUSION

The involvement of age-induced genetic/epigenetic alterations in BM niche cells and immunological stressors in hematopoiesis is crucial for understanding the development of age-related hematological malignancies. This comprehensive review provides new insights into the complex interplay between niche cells and HSCs, emphasizing the potential for novel therapeutic approaches that target niche cell functionality and resilience to improve hematopoietic outcomes in the context of aging and metabolic disorders.

NOVELTY STATEMENT

This review introduces novel concepts regarding the plasticity and adaptability of BM niche cells in response to immunological stressors and epigenetics. It proposes that targeted therapeutic strategies aimed at enhancing niche cell resilience could mitigate the adverse effects of aging, diabetes, and obesity on hematopoiesis and clonal hematopoiesis. Additionally, the review suggests that understanding the precise temporal and spatial dynamics of niche-HSC interactions and epigenetics influence may lead to innovative treatments for age-related hematological malignancies.

Inhibition of angiogenesis and regenerative lung growth in Lepob/ob mice through adiponectin-VEGF/VEGFR2 signaling

Introduction

Obesity is associated with impairment of wound healing and tissue regeneration. Angiogenesis, the formation of new blood capillaries, plays a key role in regenerative lung growth after unilateral pneumonectomy (PNX). We have reported that obesity inhibits angiogenesis. The effects of obesity on post-PNX lung vascular and alveolar regeneration remain unclear.

Methods

Unilateral PNX is performed on Lep o b / o b obese mice to examine vascular and alveolar regeneration.

Results

Regenerative lung growth and expression of vascular endothelial growth factor (VEGF) and its receptor VEGFR2 induced after PNX are inhibited in Lep o b / o b obese mice. The levels of adiponectin that exhibits pro-angiogenic and vascular protective properties increase after unilateral PNX, while the effects are attenuated in Lep o b / o b obese mice. Post-PNX regenerative lung growth and increases in the levels of VEGF and VEGFR2 are inhibited in adiponectin knockout mice. Adiponectin stimulates angiogenic activities in human lung endothelial cells (ECs), which is inhibited by decreasing the levels of transcription factor Twist1. Adiponectin agonist, AdipoRon restores post-PNX lung growth and vascular and alveolar regeneration in Lep o b / o b obese mice.

Discussion

These findings suggest that obesity impairs lung vascular and alveolar regeneration and adiponectin is one of the key factors to improve lung regeneration in obese people.

The impact of hormones on lung development and function: an overlooked aspect to consider from early childhood

The impact of hormones on the respiratory system constitutes a multifaceted and intricate facet of human biology. We propose a comprehensive review of recent advancements in understanding the interactions between hormones and pulmonary development and function, focusing on pediatric populations. We explore how hormones can influence ventilation, perfusion, and pulmonary function, from regulating airway muscle tone to modulating the inflammatory response. Hormones play an important role in the growth and development of lung tissues, influencing them from early stages through infancy, childhood, adolescence, and into adulthood. Glucocorticoids, thyroid hormones, insulin, ghrelin, leptin, glucagon-like peptide 1 (GLP-1), retinoids, cholecalciferol sex steroids, hormones derived from adipose tissue, factors like insulin, granulocyte-macrophage colony-stimulating factor (GM-CSF) and glucagon are key players in modulating respiratory mechanics and inflammation. While ample evidence underscores the impact of hormones on lung development and function, along with sex-related differences in the prevalence of respiratory disorders, further research is needed to clarify their specific roles in these conditions. Further research into the mechanisms underlying hormonal effects is essential for the development of customizing therapeutic approaches for respiratory diseases. Understanding the impact of hormones on lung function could be valuable for developing personalized monitoring approaches in both medical and surgical pediatric settings, in order to improve outcomes and the quality of care for pediatric patients.

A systems view of the vascular endothelium in health and disease

The dysfunction of blood-vessel-lining endothelial cells is a major cause of mortality. Although endothelial cells, being present in all organs as a single-cell layer, are often conceived as a rather inert cell population, the vascular endothelium as a whole should be considered a highly dynamic and interactive systemically disseminated organ. We present here a holistic view of the field of vascular research and review the diverse functions of blood-vessel-lining endothelial cells during the life cycle of the vasculature, namely responsive and relaying functions of the vascular endothelium and the responsive roles as instructive gatekeepers of organ function. Emerging translational perspectives in regenerative medicine, preventive medicine, and aging research are developed. Collectively, this review is aimed at promoting disciplinary coherence in the field of angioscience for a broader appreciation of the importance of the vasculature for organ function, systemic health, and healthy aging.

Sustained amphiregulin expression in intermediate alveolar stem cells drives progressive fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal fibrotic disease. Recent studies have highlighted the persistence of an intermediate state of alveolar stem cells in IPF lungs. In this study, we discovered a close correlation between the distribution pattern of intermediate alveolar stem cells and the progression of fibrotic changes. We showed that amphiregulin (AREG) expression is significantly elevated in intermediate alveolar stem cells of mouse fibrotic lungs and IPF patients. High levels of serum AREG correlate significantly with profound deteriorations in lung function in IPF patients. We demonstrated that AREG in alveolar stem cells is both required and sufficient for activating EGFR in fibroblasts, thereby driving lung fibrosis. Moreover, pharmacological inhibition of AREG using a neutralizing antibody effectively blocked the initiation and progression of lung fibrosis in mice. Our study underscores the therapeutic potential of anti-AREG antibodies in attenuating IPF progression, offering a promising strategy for treating fibrotic diseases.

Inhibiting endothelial Rhoj blocks profibrotic vascular intussusception and angiocrine factors to sustain lung regeneration

Lung regeneration after fibrosis requires formation of functional new vasculature, which is essential for gas exchange and cellular cross-talk with other lung cells. It remains unknown how the lung vasculature can be regenerated without fibrosis. Here, we tested the role of N6-methyladenosine (m6A) modification of forkhead box protein O1 (Foxo1) mRNA in lung regeneration after pneumonectomy (PNX) in mice, a model for lung regrowth after surgical resection. Endothelial cell (EC)-specific knockout of methyltransferase-like 3 (Mettl3) and Foxo1 caused nonproductive intussusceptive angiogenesis (IA), which impaired regeneration and enhanced fibrosis. This nonproductive IA was characterized by enhanced endothelial proliferation and increased vascular splitting with increased numbers of pillar ECs. Endothelial-selective knockout of Mettl3 in mice stimulated nonproductive IA and up-regulation of profibrotic factors after PNX, promoting regeneration to fibrotic transition. EC-specific mutation of m6A modification sites in the Foxo1 gene in mice revealed that endothelial Mettl3 modified A504 and A2035 sites in the Foxo1 mRNA to maintain pro-regenerative endothelial glycolysis, ensuring productive IA and lung regeneration without fibrosis. Suppression of Mettl3-Foxo1 signaling stimulated a subset of hyperglycolytic and hyperproliferative 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (Pfkfb3)+, Ras homolog family member J (Rhoj)+, and platelet-derived growth factor subunit B (Pdgfb)+ ECs in both human and mouse lungs with fibrosis. Inhibiting this Pfkfb3+Rhoj+Pdgfb+ EC subset normalized IA, alleviated fibrosis, and restored regeneration in bleomycin (BLM)-injured mouse lungs. We found that m6A modification of Foxo1 in the mouse vasculature promoted lung regeneration over fibrosis after PNX and BLM injury.

Dopaminylation of endothelial TPI1 suppresses ferroptotic angiocrine signals to promote lung regeneration over fibrosis

Lungs can undergo facultative regeneration, but handicapped regeneration often leads to fibrosis. How microenvironmental cues coordinate lung regeneration via modulating cell death remains unknown. Here, we reveal that the neurotransmitter dopamine modifies the endothelial niche to suppress ferroptosis, promoting lung regeneration over fibrosis. A chemoproteomic approach shows that dopamine blocks ferroptosis in endothelial cells (ECs) via dopaminylating triosephosphate isomerase 1 (TPI1). Suppressing TPI1 dopaminylation in ECs triggers ferroptotic angiocrine signaling to aberrantly activate fibroblasts, leading to a transition from lung regeneration to fibrosis. Mechanistically, dopaminylation of glutamine (Q) 65 residue in TPI1 directionally enhances TPI1's activity to convert dihydroxyacetone phosphate (DHAP) to glyceraldehyde 3-phosphate (GAP), directing ether phospholipid synthesis to glucose metabolism in regenerating lung ECs. This metabolic shift attenuates lipid peroxidation and blocks ferroptosis. Restoring TPI1 Q65 dopaminylation in an injured endothelial niche overturns ferroptosis to normalize pro-regenerative angiocrine function and alleviate lung fibrosis. Overall, dopaminylation of TPI1 balances lipid/glucose metabolism and suppresses pro-fibrotic ferroptosis in regenerating lungs.

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.

Processing of angiocrine alarmin IL-1α in endothelial cells promotes lung and liver fibrosis

BACKGROUND

Fibrosis results from excessive scar formation after tissue injury. Injured cells release alarmins such as interleukin 1 (IL-1) α and β as primary mediators initiating tissue repair. However, how alarmins from different cell types differentially regulate fibrosis remains to be explored.

METHODS

Here, we used tissue specific knockout strategy to illustrate a unique contribution of endothelial cell-derived IL-1α to lung and liver fibrosis. The two fibrotic animal model triggered by bleomycin and CCl4 were used to study the effects of endothelial paracrine/angiocrine IL-1α in fibrotic progression. Human umbilical vein endothelial cells (HUVEC) were performed to explore the production of angiocrine IL-1α at both transcriptional and post-transcriptional levels in vitro.

RESULTS

We found that endothelial paracrine/angiocrine IL-1α primarily promotes lung and liver fibrosis during the early phase of organ repair. By contrast, myeloid cell-specific ablation of IL-1α in mice resulted in little influence on fibrosis, suggesting the specific pro-fibrotic role of IL-1α from endothelial cell but not macrophage. In vitro study revealed a coordinated regulation of IL-1α production in human primary endothelial cells at both transcriptional and post-transcriptional levels. Specifically, the transcription of IL-1α is regulated by RIPK1, and after caspase-8 (CASP8) cleaves the precursor form of IL-1α, its secretion is triggered by ion channel Pannexin 1 upon CASP8 cleavage.

CONCLUSIONS

Endothelial cell-produced IL-1α plays a unique role in promoting organ fibrosis. Furthermore, the release of this angiocrine alarmin relies on a unique molecular mechanism involving RIPK1, CASP8, and ion channel Pannexin 1.

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.

Mechanisms of pulmonary endothelial barrier dysfunction in acute lung injury and acute respiratory distress syndrome

Endothelial cells (ECs) form a semi-permeable barrier between the interior space of blood vessels and the underlying tissues. Pulmonary endothelial barrier integrity is maintained through coordinated cellular processes involving receptors, signaling molecules, junctional complexes, and protein-regulated cytoskeletal reorganization. In acute lung injury (ALI) or its more severe form acute respiratory distress syndrome (ARDS), the loss of endothelial barrier integrity secondary to endothelial dysfunction caused by severe pulmonary inflammation and/or infection leads to pulmonary edema and hypoxemia. Pro-inflammatory agonists such as histamine, thrombin, bradykinin, interleukin 1β, tumor necrosis factor α, vascular endothelial growth factor, angiopoietin-2, and platelet-activating factor, as well as bacterial toxins and reactive oxygen species, cause dynamic changes in cytoskeletal structure, adherens junction disorganization, and detachment of vascular endothelial cadherin (VE-cadherin) from the actin cytoskeleton, leading to an increase in endothelial permeability. Endothelial interactions with leukocytes, platelets, and coagulation enhance the inflammatory response. Moreover, inflammatory infiltration and the associated generation of pro-inflammatory cytokines during infection cause EC death, resulting in further compromise of the structural integrity of lung endothelial barrier. Despite the use of potent antibiotics and aggressive intensive care support, the mortality of ALI is still high, because the mechanisms of pulmonary EC barrier disruption are not fully understood. In this review, we summarized recent advances in the studies of endothelial cytoskeletal reorganization, inter-endothelial junctions, endothelial inflammation, EC death, and endothelial repair in ALI and ARDS, intending to shed some light on the potential diagnostic and therapeutic targets in the clinical management of the disease.

Vascular endothelial-derived SPARCL1 exacerbates viral pneumonia through pro-inflammatory macrophage activation

Inflammation induced by lung infection is a double-edged sword, moderating both anti-viral and immune pathogenesis effects; the mechanism of the latter is not fully understood. Previous studies suggest the vasculature is involved in tissue injury. Here, we report that expression of Sparcl1, a secreted matricellular protein, is upregulated in pulmonary capillary endothelial cells (EC) during influenza-induced lung injury. Endothelial overexpression of SPARCL1 promotes detrimental lung inflammation, with SPARCL1 inducing 'M1-like' macrophages and related pro-inflammatory cytokines, while SPARCL1 deletion alleviates these effects. Mechanistically, SPARCL1 functions through TLR4 on macrophages in vitro, while TLR4 inhibition in vivo ameliorates excessive inflammation caused by endothelial Sparcl1 overexpression. Finally, SPARCL1 expression is increased in lung ECs from COVID-19 patients when compared with healthy donors, while fatal COVID-19 correlates with higher circulating SPARCL1 protein levels in the plasma. Our results thus implicate SPARCL1 as a potential prognosis biomarker for deadly COVID-19 pneumonia and as a therapeutic target for taming hyperinflammation in pneumonia.

Exosomes: A New Hope for Angiogenesis-Mediated Bone Regeneration

Bone is a metabolically dynamic structure that is generally remodeled throughout the lifetime of an individual but often causes problems with increasing age. A key player for bone development and homeostasis, but also under pathological conditions, is the bone vasculature. This complex system of arteries, veins, and capillaries forms distinct structures where each subset of endothelial cells has important functions. Starting with the basic process of angiogenesis and bone-specific blood vessel formation, coupled with initial bone formation, the importance of different vascular structures is highlighted with respect to how these structures are maintained or changed during homeostasis, aging, and pathological conditions. After exemplifying the current knowledge on bone vasculature, this review will move on to exosomes, a novel hotspot of scientific research. Exosomes will be introduced starting from their discovery via current isolation procedures and state-of-the-art characterization to their role in bone vascular development, homeostasis, and bone regeneration and repair while summarizing the underlying signal transduction pathways. With respect to their role in these processes, especially mesenchymal stem cell-derived extracellular vesicles are of interest, which leads to a discussion on patented applications and an update on ongoing clinical trials. Taken together, this review provides an overview of bone vasculature and bone regeneration, with a major focus on how exosomes influence this intricate system, as they might be useful for therapeutic purposes in the near future.

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.

Endothelial H2S-AMPK dysfunction upregulates the angiocrine factor PAI-1 and contributes to lung fibrosis

Dysfunction of the vascular angiocrine system is critically involved in regenerative defects and fibrosis of injured organs. Previous studies have identified various angiocrine factors and found that risk factors such as aging and metabolic disorders can disturb the vascular angiocrine system in fibrotic organs. One existing key gap is what sense the fibrotic risk to modulate the vascular angiocrine system in organ fibrosis. Here, using human and mouse data, we discovered that the metabolic pathway hydrogen sulfide (H2S)-AMP-activated protein kinase (AMPK) is a sensor of fibrotic stress and serves as a key mechanism upregulating the angiocrine factor plasminogen activator inhibitor-1 (PAI-1) in endothelial cells to participate in lung fibrosis. Activation of the metabolic sensor AMPK was inhibited in endothelial cells of fibrotic lungs, and AMPK inactivation was correlated with enriched fibrotic signature and reduced lung functions in humans. The inactivation of endothelial AMPK accelerated lung fibrosis in mice, while the activation of endothelial AMPK with metformin alleviated lung fibrosis. In fibrotic lungs, endothelial AMPK inactivation led to YAP activation and overexpression of the angiocrine factor PAI-1, which was positively correlated with the fibrotic signature in human fibrotic lungs and inhibition of PAI-1 with Tiplaxtinin mitigated lung fibrosis. Further study identified that the deficiency of the antioxidative gas metabolite H2S accounted for the inactivation of AMPK and activation of YAP-PAI-1 signaling in endothelial cells of fibrotic lungs. H2S deficiency was involved in human lung fibrosis and H2S supplement reversed mouse lung fibrosis in an endothelial AMPK-dependent manner. These findings provide new insight into the mechanism underlying the deregulation of the vascular angiocrine system in fibrotic organs.

Endothelial cells regulate alveolar morphogenesis by constructing basement membranes acting as a scaffold for myofibroblasts

Alveologenesis is a spatially coordinated morphogenetic event, during which alveolar myofibroblasts surround the terminal sacs constructed by epithelial cells and endothelial cells (ECs), then contract to form secondary septa to generate alveoli in the lungs. Recent studies have demonstrated the important role of alveolar ECs in this morphogenetic event. However, the mechanisms underlying EC-mediated alveologenesis remain unknown. Herein, we show that ECs regulate alveologenesis by constructing basement membranes (BMs) acting as a scaffold for myofibroblasts to induce septa formation through activating mechanical signaling. Rap1, a small GTPase of the Ras superfamily, is known to stimulate integrin-mediated cell adhesions. EC-specific Rap1-deficient (Rap1iECKO) mice exhibit impaired septa formation and hypo-alveolarization due to the decreased mechanical signaling in myofibroblasts. In Rap1iECKO mice, ECs fail to stimulate integrin β1 to recruit Collagen type IV (Col-4) into BMs required for myofibroblast-mediated septa formation. Consistently, EC-specific integrin β1-deficient mice show hypo-alveolarization, defective mechanical signaling in myofibroblasts, and disorganized BMs. These data demonstrate that alveolar ECs promote integrin β1-mediated Col-4 recruitment in a Rap1-dependent manner, thereby constructing BMs acting as a scaffold for myofibroblasts to induce mechanical signal-mediated alveologenesis. Thus, this study unveils a mechanism of organ morphogenesis mediated by ECs through intrinsic functions.

mTert induction in p21-positive cells counteracts capillary rarefaction and pulmonary emphysema

Lung diseases develop when telomeres shorten beyond a critical point. We constructed a mouse model in which the catalytic subunit of telomerase (mTert), or its catalytically inactive form (mTertCI), is expressed from the p21Cdkn1a locus. Expression of either TERT or TERTCI reduces global p21 levels in the lungs of aged mice, highlighting TERT non-canonical function. However, only TERT reduces accumulation of very short telomeres, oxidative damage, endothelial cell (ECs) senescence and senile emphysema in aged mice. Single-cell analysis of the lung reveals that p21 (and hence TERT) is expressed mainly in the capillary ECs. We report that a fraction of capillary ECs marked by CD34 and endowed with proliferative capacity declines drastically with age, and this is counteracted by TERT but not TERTCI. Consistently, only TERT counteracts decline of capillary density. Natural aging effects are confirmed using the experimental model of emphysema induced by VEGFR2 inhibition and chronic hypoxia. We conclude that catalytically active TERT prevents exhaustion of the putative CD34 + EC progenitors with age, thus protecting against capillary vessel loss and pulmonary emphysema.

Direct thrombin inhibitors fail to reverse the negative effects of heparin on lung growth and function after murine left pneumonectomy

Neonates with congenital diaphragmatic hernia (CDH) frequently require cardiopulmonary bypass and systemic anticoagulation. We previously demonstrated that even subtherapeutic heparin impairs lung growth and function in a murine model of compensatory lung growth (CLG). The direct thrombin inhibitors (DTIs) bivalirudin and argatroban preserved growth in this model. Although DTIs are increasingly used for systemic anticoagulation clinically, patients with CDH may still receive heparin. In this experiment, lung endothelial cell proliferation was assessed following treatment with heparin-alone or mixed with increasing concentrations of bivalirudin or argatroban. The effects of subtherapeutic heparin with or without DTIs in the CLG model were also investigated. C57BL/6J mice underwent left pneumonectomy and subcutaneous implantation of osmotic pumps. Pumps were preloaded with normal saline, bivalirudin, or argatroban; treated animals received daily intraperitoneal low-dose heparin. In vitro, heparin-alone decreased endothelial cell proliferation and increased apoptosis. The effect of heparin on proliferation, but not apoptosis, was reversed by the addition of bivalirudin and argatroban. In vivo, low-dose heparin decreased lung volume compared with saline-treated controls. All three groups that received heparin demonstrated decreased lung function on pulmonary function testing and impaired exercise performance on treadmill tolerance testing. These findings correlated with decreases in alveolarization, vascularization, angiogenic signaling, and gene expression in the heparin-exposed groups. Together, these data suggest that bivalirudin and argatroban fail to reverse the inhibitory effects of subtherapeutic heparin on lung growth and function. Clinical studies on the impact of low-dose heparin with DTIs on CDH outcomes are warranted.NEW & NOTEWORTHY Infants with pulmonary hypoplasia frequently require cardiopulmonary bypass and systemic anticoagulation. We investigate the effects of simultaneous exposure to heparin and direct thrombin inhibitors (DTIs) on lung growth and pulmonary function in a murine model of compensatory lung growth (CGL). Our data suggest that DTIs fail to reverse the inhibitory effects of subtherapeutic heparin on lung growth and function. Clinical studies on the impact of heparin with DTIs on clinical outcomes are thus warranted.

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.

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.

Longitudinal plasma proteomics reveals biomarkers of alveolar-capillary barrier disruption in critically ill COVID-19 patients

The pathobiology of respiratory failure in COVID-19 consists of a complex interplay between viral cytopathic effects and a dysregulated host immune response. In critically ill patients, imatinib treatment demonstrated potential for reducing invasive ventilation duration and mortality. Here, we perform longitudinal profiling of 6385 plasma proteins in 318 hospitalised patients to investigate the biological processes involved in critical COVID-19, and assess the effects of imatinib treatment. Nine proteins measured at hospital admission accurately predict critical illness development. Next to dysregulation of inflammation, critical illness is characterised by pathways involving cellular adhesion, extracellular matrix turnover and tissue remodelling. Imatinib treatment attenuates protein perturbations associated with inflammation and extracellular matrix turnover. These proteomic alterations are contextualised using external pulmonary RNA-sequencing data of deceased COVID-19 patients and imatinib-treated Syrian hamsters. Together, we show that alveolar capillary barrier disruption in critical COVID-19 is reflected in the plasma proteome, and is attenuated with imatinib treatment. This study comprises a secondary analysis of both clinical data and plasma samples derived from a clinical trial that was registered with the EU Clinical Trials Register (EudraCT 2020-001236-10, https://www.clinicaltrialsregister.eu/ctr-search/trial/2020-001236-10/NL ) and Netherlands Trial Register (NL8491, https://www.trialregister.nl/trial/8491 ).

The HIF-1α/EGF/EGFR Signaling Pathway Facilitates the Proliferation of Yak Alveolar Type II Epithelial Cells in Hypoxic Conditions

The yak is a unique creature that thrives in low-oxygen environments, showcasing its adaptability to high-altitude settings with limited oxygen availability due to its unique respiratory system. However, the impact of hypoxia on alveolar type II (AT2) epithelial cell proliferation in yaks remains unexplored. In this study, we investigated the effects of different altitudes on 6-month-old yaks and found an increase in alveolar septa thickness and AT2 cell count in a high-altitude environment characterized by hypoxia. This was accompanied by elevated levels of hypoxia-inducible factor-1α (HIF-1α) and epidermal growth factor receptor (EGFR) expression. Additionally, we observed a significant rise in Ki67-positive cells and apoptotic lung epithelial cells among yaks inhabiting higher altitudes. Our in vitro experiments demonstrated that exposure to hypoxia activated HIF-1α, EGF, and EGFR expression leading to increased proliferation rates among yak AT2 cells. Under normal oxygen conditions, activation of HIF-1α enhanced EGF/EGFR expressions which subsequently stimulated AT2 cell proliferation. Furthermore, activation of EGFR expression under normoxic conditions further promoted AT2 cell proliferation while simultaneously suppressing apoptosis. Conversely, inhibition of EGFR expression under hypoxic conditions had contrasting effects. In summary, hypoxia triggers the proliferation of yak AT2 cells via activation facilitated by the HIF-1α/EGF/EGFR signaling cascade.

Discovery of Transcription Factors Involved in the Maintenance of Resident Vascular Endothelial Stem Cell Properties

A resident vascular endothelial stem cell (VESC) population expressing CD157 has been identified recently in mice. Herein, we identified transcription factors (TFs) regulating CD157 expression in endothelial cells (ECs) that were associated with drug resistance, angiogenesis, and EC proliferation. In the first screening, we detected 20 candidate TFs through the CD157 promoter and gene expression analyses. We found that 10 of the 20 TFs induced CD157 expression in ECs. We previously reported that 70% of CD157 VESCs were side population (SP) ECs that abundantly expressed ATP-binding cassette (ABC) transporters. Here, we found that the 10 TFs increased the expression of several ABC transporters in ECs and increased the proportion of SP ECs. Of these 10 TFs, we found that six (Atf3, Bhlhe40, Egr1, Egr2, Elf3, and Klf4) were involved in the manifestation of the SP phenotype. Furthermore, the six TFs enhanced tube formation and proliferation in ECs. Single-cell RNA sequence data in liver ECs suggested that Atf3 and Klf4 contributed to the production of CD157+ VESCs in the postnatal period. We concluded that Klf4 might be important for the development and maintenance of liver VESCs. Our work suggests that a TF network is involved in the differentiation hierarchy of VESCs.

Nitric oxide is required for lung alveolarization revealed by deficiency of argininosuccinate lyase

Inhaled nitric oxide (NO) therapy has been reported to improve lung growth in premature newborns. However, the underlying mechanisms by which NO regulates lung development remain largely unclear. NO is enzymatically produced by three isoforms of nitric oxide synthase (NOS) enzymes. NOS knockout mice are useful tools to investigate NO function in the lung. Each single NOS knockout mouse does not show obvious lung alveolar phenotype, likely due to compensatory mechanisms. While mice lacking all three NOS isoforms display impaired lung alveolarization, implicating NO plays a pivotal role in lung alveolarization. Argininosuccinate lyase (ASL) is the only mammalian enzyme capable of synthesizing L-arginine, the sole precursor for NOS-dependent NO synthesis. ASL is also required for channeling extracellular L-arginine into a NO-synthetic complex. Thus, ASL deficiency (ASLD) is a non-redundant model for cell-autonomous, NOS-dependent NO deficiency. Here, we assessed lung alveolarization in ASL-deficient mice. Hypomorphic deletion of Asl (AslNeo/Neo) results in decreased lung alveolarization, accompanied with reduced level of S-nitrosylation in the lung. Genetic ablation of one copy of Caveolin-1, which is a negative regulator of NO production, restores total S-nitrosylation as well as lung alveolarization in AslNeo/Neo mice. Importantly, NO supplementation could partially rescue lung alveolarization in AslNeo/Neo mice. Furthermore, endothelial-specific knockout mice (VE-Cadherin Cre; Aslflox/flox) exhibit impaired lung alveolarization at 12 weeks old, supporting an essential role of endothelial-derived NO in the enhancement of lung alveolarization. Thus, we propose that ASLD is a model to study NO-mediated lung alveolarization.

Alveolar Type 2 Epithelial Cell Organoids: Focus on Culture Methods

Lung diseases rank third in terms of mortality and represent a significant economic burden globally. Scientists have been conducting research to better understand respiratory diseases and find treatments for them. An ideal in vitro model must mimic the in vivo organ structure, physiology, and pathology. Organoids are self-organizing, three-dimensional (3D) structures originating from adult stem cells, embryonic lung bud progenitors, embryonic stem cells (ESCs), and induced pluripotent stem cells (iPSCs). These 3D organoid cultures may provide a platform for exploring tissue development, the regulatory mechanisms related to the repair of lung epithelia, pathophysiological and immunomodulatory responses to different respiratory conditions, and screening compounds for new drugs. To create 3D lung organoids in vitro, both co-culture and feeder-free methods have been used. However, there exists substantial heterogeneity in the organoid culture methods, including the sources of AT2 cells, media composition, and feeder cell origins. This article highlights the currently available methods for growing AT2 organoids and prospective improvements to improve the available culture techniques/conditions. Further, we discuss various applications, particularly those aimed at modeling human distal lung diseases and cell therapy.

Endothelial Notch Signaling Regulates the Function of the Retinal Pigment Epithelial Barrier via EC Angiocrine Signaling

The outer blood-retina barrier (oBRB), comprises tightly connected retinal pigment epithelium (RPE) cells, Bruch's membrane, and choroid blood vessels, and is essential for retinal health and normal visual function. Disruption of the RPE barrier and its dysfunction can lead to retinal disorders such as age-related macular degeneration (AMD). In the present study, we investigated the essential role of choroid endothelial cells (ECs) in the RPE barrier formation process and its dysfunction. We discovered that ECs promoted RPE barrier formation through angiocrine signaling. Through blocking or activating endothelial Notch signaling and conducting experiments in vitro and in vivo, we confirmed that endothelial Notch signaling regulated the expression of heparin-binding epidermal growth factor (HBEGF) and consequently impacted the expression and activity of matrix metalloproteinases (MMP)-9 in RPE cells. This modulation influenced the RPE extracellular matrix deposition, tight junctions and RPE barrier function. In in vivo experiments, the intravitreal administration of recombinant HBEGF (r-HBEGF) alleviated the RPE barrier disruption induced by subretinal injection (SI) or laser treatment and also rescued RPE barrier disruption in endothelial Notch-deficient mice. Our results showed that the endothelial Notch signaling drove HBEGF expression through angiocrine signaling and effectively improved RPE barrier function by regulating the MMP-9 expression in RPE cells. It suggests that the modulation of Notch signaling in the choroidal endothelium may offer a novel therapeutic strategy for retinal degenerative diseases.

Simultaneous Targeting of IL-1-Signaling and IL-6-Trans-Signaling Preserves Human Pulmonary Endothelial Barrier Function During a Cytokine Storm-Brief Report

BACKGROUND

Systemic inflammatory diseases, such as sepsis and severe COVID-19, provoke acute respiratory distress syndrome in which the pathological hyperpermeability of the microvasculature, induced by uncontrolled inflammatory stimulation, causes pulmonary edema. Identifying the inflammatory mediators that induce human lung microvascular endothelial cell barrier dysfunction is essential to find the best anti-inflammatory treatments for critically ill acute respiratory distress syndrome patients.

METHODS

We have compared the responses of primary human lung microvascular endothelial cells to the main inflammatory mediators involved in cytokine storms induced by sepsis and SARS-CoV2 pulmonary infection and to sera from healthy donors and severely ill patients with sepsis. Endothelial barrier function was measured by electric cell-substrate impedance sensing, quantitative confocal microscopy, and Western blot.

RESULTS

The human lung microvascular endothelial cell barrier was completely disrupted by IL (interleukin)-6 conjugated with soluble IL-6R (IL-6 receptor) and by IL-1β (interleukin-1beta), moderately affected by TNF (tumor necrosis factor)-α and IFN (interferon)-γ and unaffected by other cytokines and chemokines, such as IL-6, IL-8, MCP (monocyte chemoattractant protein)-1 and MCP-3. The inhibition of IL-1 and IL-6R simultaneously, but not separately, significantly reduced endothelial hyperpermeability on exposing human lung microvascular endothelial cells to a cytokine storm consisting of 8 inflammatory mediators or to sera from patients with sepsis. Simultaneous inhibition of IL-1 and JAK (Janus kinase)-STAT (signal transducer and activator of transcription protein), a signaling node downstream IL-6 and IFN-γ, also prevented septic serum-induced endothelial barrier disruption.

CONCLUSIONS

These findings strongly suggest a major role for both IL-6 trans-signaling and IL-1β signaling in the pathological increase in permeability of the human lung microvasculature and reveal combinatorial strategies that enable the gradual control of pulmonary endothelial barrier function in response to a cytokine storm.

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

Introduction

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.

Methods

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.

Results

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.

Discussion

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.

A pneumonectomy model to study flow-induced pulmonary hypertension and compensatory lung growth

In newborns, developmental disorders such as congenital diaphragmatic hernia (CDH) and specific types of congenital heart disease (CHD) can lead to defective alveolarization, pulmonary hypoplasia, and pulmonary arterial hypertension (PAH). Therapeutic options for these patients are limited, emphasizing the need for new animal models representative of disease conditions. In most adult mammals, compensatory lung growth (CLG) occurs after pneumonectomy; however, the underlying relationship between CLG and flow-induced pulmonary hypertension (PH) is not fully understood. We propose a murine model that involves the simultaneous removal of the left lung and right caval lobe (extended pneumonectomy), which results in reduced CLG and exacerbated reproducible PH. Extended pneumonectomy in mice is a promising animal model to study the cellular response and molecular mechanisms contributing to flow-induced PH, with the potential to identify new treatments for patients with CDH or PAH-CHD.

Beyond epithelial damage: vascular and endothelial contributions to idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive scarring disease of the lung with poor survival. The incidence and mortality of IPF are rising, but treatment remains limited. Currently, two drugs can slow the scarring process but often at the expense of intolerable side effects, and without substantially changing overall survival. A better understanding of mechanisms underlying IPF is likely to lead to improved therapies. The current paradigm proposes that repetitive alveolar epithelial injury from noxious stimuli in a genetically primed individual is followed by abnormal wound healing, including aberrant activity of extracellular matrix-secreting cells, with resultant tissue fibrosis and parenchymal damage. However, this may underplay the importance of the vascular contribution to fibrogenesis. The lungs receive 100% of the cardiac output, and vascular abnormalities in IPF include (a) heterogeneous vessel formation throughout fibrotic lung, including the development of abnormal dilated vessels and anastomoses; (b) abnormal spatially distributed populations of endothelial cells (ECs); (c) dysregulation of endothelial protective pathways such as prostacyclin signaling; and (d) an increased frequency of common vascular and metabolic comorbidities. Here, we propose that vascular and EC abnormalities are both causal and consequential in the pathobiology of IPF and that fuller evaluation of dysregulated pathways may lead to effective therapies and a cure for this devastating disease.

Endothelial AHR activity prevents lung barrier disruption in viral infection

Disruption of the lung endothelial-epithelial cell barrier following respiratory virus infection causes cell and fluid accumulation in the air spaces and compromises vital gas exchange function1. Endothelial dysfunction can exacerbate tissue damage2,3, yet it is unclear whether the lung endothelium promotes host resistance against viral pathogens. Here we show that the environmental sensor aryl hydrocarbon receptor (AHR) is highly active in lung endothelial cells and protects against influenza-induced lung vascular leakage. Loss of AHR in endothelia exacerbates lung damage and promotes the infiltration of red blood cells and leukocytes into alveolar air spaces. Moreover, barrier protection is compromised and host susceptibility to secondary bacterial infections is increased when endothelial AHR is missing. AHR engages tissue-protective transcriptional networks in endothelia, including the vasoactive apelin-APJ peptide system4, to prevent a dysplastic and apoptotic response in airway epithelial cells. Finally, we show that protective AHR signalling in lung endothelial cells is dampened by the infection itself. Maintenance of protective AHR function requires a diet enriched in naturally occurring AHR ligands, which activate disease tolerance pathways in lung endothelia to prevent tissue damage. Our findings demonstrate the importance of endothelial function in lung barrier immunity. We identify a gut-lung axis that affects lung damage following encounters with viral pathogens, linking dietary composition and intake to host fitness and inter-individual variations in disease outcome.

Peptide-Functionalized Electrospun Meshes for the Physiological Cultivation of Pulmonary Alveolar Capillary Barrier Models in a 3D-Printed Micro-Bioreactor

In vitro environments that realize biomimetic scaffolds, cellular composition, physiological shear, and strain are integral to developing tissue models of organ-specific functions. In this study, an in vitro pulmonary alveolar capillary barrier model is developed that closely mimics physiological functions by combining a synthetic biofunctionalized nanofibrous membrane system with a novel three-dimensional (3D)-printed bioreactor. The fiber meshes are fabricated from a mixture of polycaprolactone (PCL), 6-armed star-shaped isocyanate-terminated poly(ethylene glycol) (sPEG-NCO), and Arg-Gly-Asp (RGD) peptides by a one-step electrospinning process that offers full control over the fiber surface chemistry. The tunable meshes are mounted within the bioreactor where they support the co-cultivation of pulmonary epithelial (NCI-H441) and endothelial (HPMEC) cell monolayers at air-liquid interface under controlled stimulation by fluid shear stress and cyclic distention. This stimulation, which closely mimics blood circulation and breathing motion, is observed to impact alveolar endothelial cytoskeleton arrangement and improve epithelial tight junction formation as well as surfactant protein B production compared to static models. The results highlight the potential of PCL-sPEG-NCO:RGD nanofibrous scaffolds in combination with a 3D-printed bioreactor system as a platform to reconstruct and enhance in vitro models to bear a close resemblance to in vivo tissues.

Vascular Contribution to Lung Repair and Fibrosis

Lungs are constantly exposed to environmental perturbations and therefore have remarkable capacity to regenerate in response to injury. Sustained lung injuries, aging, and increased genomic instability, however, make lungs particularly susceptible to disrepair and fibrosis. Pulmonary fibrosis constitutes a major cause of morbidity and is often relentlessly progressive, leading to death from respiratory failure. The pulmonary vasculature, which is critical for gas exchanges and plays a key role during lung development, repair, and regeneration, becomes aberrantly remodeled in patients with progressive pulmonary fibrosis. Although capillary rarefaction and increased vascular permeability are recognized as distinctive features of fibrotic lungs, the role of vasculature dysfunction in the pathogenesis of pulmonary fibrosis has only recently emerged as an important contributor to the progression of this disease. This review summarizes current findings related to lung vascular repair and regeneration and provides recent insights into the vascular abnormalities associated with the development of persistent lung fibrosis.

The lung mesenchyme in development, regeneration, and fibrosis

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

Differential roles of regulatory T cells in acute respiratory infections

Acute respiratory infections trigger an inflammatory immune response with the goal of pathogen clearance; however, overexuberant inflammation causes tissue damage and impairs pulmonary function. CD4+FOXP3+ regulatory T cells (Tregs) interact with cells of both the innate and the adaptive immune system to limit acute pulmonary inflammation and promote its resolution. Tregs also provide tissue protection and coordinate lung tissue repair, facilitating a return to homeostatic pulmonary function. Here, we review Treg-mediated modulation of the host response to respiratory pathogens, focusing on mechanisms underlying how Tregs promote resolution of inflammation and repair of acute lung injury. We also discuss potential strategies to harness and optimize Tregs as a cellular therapy for patients with severe acute respiratory infection and discuss open questions in the field.

Flt1 produced by lung endothelial cells impairs ATII cell transdifferentiation and repair in pulmonary fibrosis

Pulmonary fibrosis is a devastating disease, in which fibrotic tissue progressively replaces lung alveolar structure, resulting in chronic respiratory failure. Alveolar type II cells act as epithelial stem cells, being able to transdifferentiate into alveolar type I cells, which mediate gas exchange, thus contributing to lung homeostasis and repair after damage. Impaired epithelial transdifferentiation is emerging as a major pathogenetic mechanism driving both onset and progression of fibrosis in the lung. Here, we show that lung endothelial cells secrete angiocrine factors that regulate alveolar cell differentiation. Specifically, we build on our previous data on the anti-fibrotic microRNA-200c and identify the Vascular Endothelial Growth Factor receptor 1, also named Flt1, as its main functional target in endothelial cells. Endothelial-specific knockout of Flt1 reproduces the anti-fibrotic effect of microRNA-200c against pulmonary fibrosis and results in the secretion of a pool of soluble factors and matrix components able to promote epithelial transdifferentiation in a paracrine manner. Collectively, these data indicate the existence of a complex endothelial-epithelial paracrine crosstalk in vitro and in vivo and position lung endothelial cells as a relevant therapeutic target in the fight against pulmonary fibrosis.

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.

Endothelial-derived angiocrine factors as instructors of embryonic development

Blood vessels are well-known to play roles in organ development and repair, primarily owing to their fundamental function in delivering oxygen and nutrients to tissues to promote their growth and homeostasis. Endothelial cells however are not merely passive conduits for carrying blood. There is now evidence that endothelial cells of the vasculature actively regulate tissue-specific development, morphogenesis and organ function, as well as playing roles in disease and cancer. Angiocrine factors are growth factors, cytokines, signaling molecules or other regulators produced directly from endothelial cells to instruct a diverse range of signaling outcomes in the cellular microenvironment, and are critical mediators of the vascular control of organ function. The roles of angiocrine signaling are only beginning to be uncovered in diverse fields such as homeostasis, regeneration, organogenesis, stem-cell maintenance, cell differentiation and tumour growth. While in some cases the specific angiocrine factor involved in these processes has been identified, in many cases the molecular identity of the angiocrine factor(s) remain to be discovered, even though the importance of angiocrine signaling has been implicated. In this review, we will specifically focus on roles for endothelial-derived angiocrine signaling in instructing tissue morphogenesis and organogenesis during embryonic and perinatal development.

Regulating the cell shift of endothelial cell-like myofibroblasts in pulmonary fibrosis

Pulmonary fibrosis is a common and severe fibrotic lung disease with high morbidity and mortality. Recent studies have reported a large number of unwanted myofibroblasts appearing in pulmonary fibrosis, and shown that the sustained activation of myofibroblasts is essential for unremitting interstitial fibrogenesis. However, the origin of these myofibroblasts remains poorly understood. Here, we create new mouse models of pulmonary fibrosis and identify a previously unknown population of endothelial cell (EC)-like myofibroblasts in normal lung tissue. We show that these EC-like myofibroblasts significantly contribute myofibroblasts to pulmonary fibrosis, which is confirmed by single-cell RNA sequencing of human pulmonary fibrosis. Using the transcriptional profiles, we identified a small molecule that redirects the differentiation of EC-like myofibroblasts and reduces pulmonary fibrosis in our mouse models. Our study reveals the mechanistic underpinnings of the differentiation of EC-like myofibroblasts in pulmonary fibrosis and may provide new strategies for therapeutic interventions.

Vascular Endothelial-derived SPARCL1 Exacerbates Viral Pneumonia Through Pro-Inflammatory Macrophage Activation

Inflammation upon infectious lung injury is a double-edged sword: while tissue-infiltrating immune cells and cytokines are necessary to control infection, these same factors often aggravate injury. Full appreciation of both the sources and targets of inflammatory mediators is required to facilitate strategies to maintain antimicrobial effects while minimizing off-target epithelial and endothelial damage. Recognizing that the vasculature is centrally involved in tissue responses to injury and infection, we observed that pulmonary capillary endothelial cells (ECs) exhibit dramatic transcriptomic changes upon influenza injury punctuated by profound upregulation of Sparcl1 . Endothelial deletion and overexpression of SPARCL1 implicated this secreted matricellular protein in driving key pathophysiologic symptoms of pneumonia, which we demonstrate result from its effects on macrophage polarization. SPARCL1 induces a shift to a pro-inflammatory "M1-like" phenotype (CD86 + CD206 - ), thereby increasing associated cytokine levels. Mechanistically, SPARCL1 acts directly on macrophages in vitro to induce the pro-inflammatory phenotype via activation of TLR4, and TLR4 inhibition in vivo ameliorates inflammatory exacerbations caused by endothelial Sparcl1 overexpression. Finally, we confirmed significant elevation of SPARCL1 in COVID-19 lung ECs in comparison with those from healthy donors. Survival analysis demonstrated that patients with fatal COVID-19 had higher levels of circulating SPARCL1 protein compared to those who recovered, indicating the potential of SPARCL1 as a biomarker for prognosis of pneumonia and suggesting that personalized medicine approaches might be harnessed to block SPARCL1 and improve outcomes in high-expressing patients.