Endothelial-Specific Loss of Sphingosine-1-Phosphate Receptor 1 Increases Vascular Permeability and Exacerbates Bleomycin-induced Pulmonary Fibrosis

Lung Fibrosis Is Linked to Increased Endothelial Cell Activation and Dysfunctional Vascular Barrier Integrity

Pulmonary fibrosis (PF) can be a fatal disease characterized by progressive lung scarring. It is still poorly understood how the pulmonary endothelium is involved in the disease pathogenesis. Differences of the pulmonary vasculature between patients and donors were analyzed using transmission electron microscopy, immunohistochemistry, and single-cell RNA sequencing. Vascular barrier resistance, endothelial-immune cell adhesion, and sensitivity to an inflammatory milieu were studied in vitro. Integrity and activation markers were measured by ELISA in human plasma. Transmission electron microscopy demonstrated abnormally swollen endothelial cells (ECs) in fibrotic lungs compared with donors. A more intense CD31 and von Willebrand Factor (vWF) and patchy vascular endothelial (VE)-Cadherin staining in fibrotic lungs supported the presence of a dysregulated endothelium. Integrity markers CD31, VE-Cadherin, Thrombomodulin, and VEGFR-2 (vascular endothelial growth factor receptor-2) and activation marker vWF gene expression was increased in different endothelial subpopulations (e.g., arterial, venous, general capillary, aerocytes) in PF. This was associated with a heightened sensitivity of fibrotic ECs to TNF-α or IFN-γ and elevated immune cell adhesion. The barrier strength was overall reduced in ECs from fibrotic lungs. vWF and IL-8 were increased in the plasma of patients, whereas VE-Cadherin, Thrombomodulin, and VEGFR-2 were decreased. VE-Cadherin staining was also patchy in biopsy tissue and was decreased in plasma samples of patients with PF 6 months after the initial diagnosis. Our data demonstrate highly abnormal ECs in PF. The vascular compartment is characterized by hyperactivation and increased immune cell adhesion, as well as dysfunctional endothelial barrier function. Reestablishing EC homeostasis and function might represent a new therapeutic option for fibrotic lung diseases.

Cardiopulmonary Complications after Pulmonary Embolism in COVID-19

Although pulmonary embolism (PE) is a frequent complication in COVID-19, its consequences remain unknown. We performed pulmonary function tests, echocardiography and computed tomography pulmonary angiography and identified blood biomarkers in a cohort of consecutive hospitalized COVID-19 patients with pneumonia to describe and compare medium-term outcomes according to the presence of PE, as well as to explore their potential predictors. A total of 141 patients (56 with PE) were followed up during a median of 6 months. Post-COVID-19 radiological lung abnormalities (PCRLA) and impaired diffusing capacity for carbon monoxide (DLCOc) were found in 55.2% and 67.6% cases, respectively. A total of 7.3% had PE, and 6.7% presented an intermediate-high probability of pulmonary hypertension. No significant difference was found between PE and non-PE patients. Univariate analysis showed that age > 65, some clinical severity factors, surfactant protein-D, baseline C-reactive protein, and both peak red cell distribution width and Interleukin (IL)-10 were associated with DLCOc < 80%. A score for PCRLA prediction including age > 65, minimum lymphocyte count, and IL-1β concentration on admission was constructed with excellent overall performance. In conclusion, reduced DLCOc and PCRLA were common in COVID-19 patients after hospital discharge, but PE did not increase the risk. A PCRLA predictive score was developed, which needs further validation.

Delineating excess comorbidities in idiopathic pulmonary fibrosis: an observational study

BACKGROUND

Our study examined whether prevalent and incident comorbidities are increased in idiopathic pulmonary fibrosis (IPF) patients when compared to matched chronic obstructive pulmonary disease (COPD) patients and control subjects without IPF or COPD.

METHODS

IPF and age, gender and smoking matched COPD patients, diagnosed between 01/01/1997 and 01/01/2019 were identified from the Clinical Practice Research Datalink GOLD database multiple registrations cohort at the first date an ICD-10 or read code mentioned IPF/COPD. A control cohort comprised age, gender and pack-year smoking matched subjects without IPF or COPD. Prevalent (prior to IPF/COPD diagnosis) and incident (after IPF/COPD diagnosis) comorbidities were examined. Group differences were estimated using a t-test. Mortality relationships were examined using multivariable Cox proportional hazards adjusted for patient age, gender and smoking status.

RESULTS

Across 3055 IPF patients, 38% had 3 or more prevalent comorbidities versus 32% of COPD patients and 21% of matched control subjects. Survival time reduced as the number of comorbidities in an individual increased (p < 0.0001). In IPF, prevalent heart failure (Hazard ratio [HR] = 1.62, 95% Confidence Interval [CI]: 1.43-1.84, p < 0.001), chronic kidney disease (HR = 1.27, 95%CI: 1.10-1.47, p = 0.001), cerebrovascular disease (HR = 1.18, 95%CI: 1.02-1.35, p = 0.02), abdominal and peripheral vascular disease (HR = 1.29, 95%CI: 1.09-1.50, p = 0.003) independently associated with reduced survival. Key comorbidities showed increased incidence in IPF (versus COPD) 7-10 years prior to IPF diagnosis.

INTERPRETATION

The mortality impact of excessive prevalent comorbidities in IPF versus COPD and smoking matched controls suggests that multiorgan mechanisms of injury need elucidation in patients that develop IPF.

The novel molecular mechanism of pulmonary fibrosis: insight into lipid metabolism from reanalysis of single-cell RNA-seq databases

Pulmonary fibrosis (PF) is a severe pulmonary disease with limited available therapeutic choices. Recent evidence increasingly points to abnormal lipid metabolism as a critical factor in PF pathogenesis. Our latest research identifies the dysregulation of low-density lipoprotein (LDL) is a new risk factor for PF, contributing to alveolar epithelial and endothelial cell damage, and fibroblast activation. In this study, we first integrative summarize the published literature about lipid metabolite changes found in PF, including phospholipids, glycolipids, steroids, fatty acids, triglycerides, and lipoproteins. We then reanalyze two single-cell RNA-sequencing (scRNA-seq) datasets of PF, and the corresponding lipid metabolomic genes responsible for these lipids' biosynthesis, catabolism, transport, and modification processes are uncovered. Intriguingly, we found that macrophage is the most active cell type in lipid metabolism, with almost all lipid metabolic genes being altered in macrophages of PF. In type 2 alveolar epithelial cells, lipid metabolic differentially expressed genes (DEGs) are primarily associated with the cytidine diphosphate diacylglycerol pathway, cholesterol metabolism, and triglyceride synthesis. Endothelial cells are partly responsible for sphingomyelin, phosphatidylcholine, and phosphatidylethanolamines reprogramming as their metabolic genes are dysregulated in PF. Fibroblasts may contribute to abnormal cholesterol, phosphatidylcholine, and phosphatidylethanolamine metabolism in PF. Therefore, the reprogrammed lipid profiles in PF may be attributed to the aberrant expression of lipid metabolic genes in different cell types. Taken together, these insights underscore the potential of targeting lipid metabolism in developing innovative therapeutic strategies, potentially leading to extended overall survival in individuals affected by PF.

First magnetic particle imaging to assess pulmonary vascular leakage in vivo in the acutely injured and fibrotic lung

Increased pulmonary vascular permeability is a characteristic feature of lung injury. However, there are no established methods that allow the three-dimensional visualization and quantification of pulmonary vascular permeability in vivo. Evans blue extravasation test and total protein test of bronchoalveolar lavage fluid (BALF) are permeability assays commonly used in research settings. However, they lack the ability to identify the spatial and temporal heterogeneity of endothelial barrier disruption, which is typical in lung injuries. Magnetic resonance (MR) and near-infrared (NIR) imaging have been proposed to image pulmonary permeability, but suffer from limited sensitivity and penetration depth, respectively. In this study, we report the first use of magnetic particle imaging (MPI) to assess pulmonary vascular leakage noninvasively in vivo in mice. A dextran-coated superparamagnetic iron oxide (SPIO), synomag®, was employed as the imaging tracer, and pulmonary SPIO extravasation was imaged and quantified to evaluate the vascular leakage. Animal models of acute lung injury and pulmonary fibrosis (PF) were used to validate the proposed method. MPI sensitively detected the SPIO extravasation in both acutely injured and fibrotic lungs in vivo, which was confirmed by ex vivo imaging and Prussian blue staining. Moreover, 3D MPI illustrated the spatial heterogeneity of vascular leakage, which correlated well with CT findings. Based on the in vivo 3D MPI images, we defined the SPIO extravasation index (SEI) to quantify the vascular leakage. A significant increase in SEI was observed in the injured lungs, in consistent with the results obtained via ex vivo permeability assays. Overall, our results demonstrate that 3D quantitative MPI serves as a useful tool to examine pulmonary vascular integrity in vivo, which shows promise for future clinical translation.

Augmentation of Endothelial S1PR1 Attenuates Postviral Pulmonary Fibrosis

Respiratory viral infections are frequent causes of acute respiratory distress syndrome (ARDS), a disabling condition with a mortality of up to 46%. The pulmonary endothelium plays an important role in the development of ARDS as well as the pathogenesis of pulmonary fibrosis; however, the therapeutic potential to modulate endothelium-dependent signaling to prevent deleterious consequences has not been well explored. Here, we used a clinically relevant influenza A virus infection model, endothelial cell-specific transgenic gain-of-function and loss-of-function mice as well as pharmacologic approaches and in vitro modeling, to define the mechanism by which S1PR1 expression is dampened during influenza virus infection and determine whether therapeutic augmentation of S1PR1 has the potential to reduce long-term postviral fibrotic complications. We found that the influenza virus-induced inflammatory milieu promoted internalization of S1PR1, which was pharmacologically inhibited with paroxetine, an inhibitor of GRK2. Moreover, genetic overexpression or administration of paroxetine days after influenza virus infection was sufficient to reduce postviral pulmonary fibrosis. Taken together, our data suggest that endothelial S1PR1 signaling provides critical protection against long-term fibrotic complications after pulmonary viral infection. These findings support the development of antifibrotic strategies that augment S1PR1 expression in virus-induced ARDS to improve long-term patient outcomes.

Emerging opportunities to treat idiopathic pulmonary fibrosis: Design, discovery, and optimizations of small-molecule drugs targeting fibrogenic pathways

Idiopathic pulmonary fibrosis (IPF) is the most common fibrotic form of idiopathic diffuse lung disease. Due to limited treatment options, IPF patients suffer from poor survival. About ten years ago, Pirfenidone (Shionogi, 2008; InterMune, 2011) and Nintedanib (Boehringer Ingelheim, 2014) were approved, greatly changing the direction of IPF drug design. However, limited efficacy and side effects indicate that neither can reverse the process of IPF. With insights into the occurrence of IPF, novel targets and agents have been proposed, which have fundamentally changed the treatment of IPF. With the next-generation agents, targeting pro-fibrotic pathways in the epithelial-injury model offers a promising approach. Besides, several next-generation IPF drugs have entered phase II/III clinical trials with encouraging results. Due to the rising IPF treatment requirements, there is an urgent need to completely summarize the mechanisms, targets, problems, and drug design strategies over the past ten years. In this review, we summarize known mechanisms, target types, drug design, and novel technologies of IPF drug discovery, aiming to provide insights into the future development and clinical application of next-generation IPF drugs.

Edema and lymphatic clearance: molecular mechanisms and ongoing challenges

Resolution of edema remains a significant clinical challenge. Conditions such as traumatic shock, sepsis, or diabetes often involve microvascular hyperpermeability, which leads to tissue and organ dysfunction. Lymphatic insufficiency due to genetic causes, surgical removal of lymph nodes, or infections, leads to varying degrees of tissue swelling that impair mobility and immune defenses. Treatment options are limited to management of edema as there are no specific therapeutics that have demonstrated significant success for ameliorating microvascular leakage or impaired lymphatic function. This review examines current knowledge about the physiological, cellular, and molecular mechanisms that control microvascular permeability and lymphatic clearance, the respective processes for interstitial fluid formation and removal. Clinical conditions featuring edema, along with potential future directions are discussed.

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.

Lipid metabolism in idiopathic pulmonary fibrosis: From pathogenesis to therapy

Idiopathic pulmonary fibrosis (IPF) is a chronic irreversible interstitial lung disease characterized by a progressive decline in lung function. The etiology of IPF is unknown, which poses a significant challenge to the treatment of IPF. Recent studies have identified a strong association between lipid metabolism and the development of IPF. Qualitative and quantitative analysis of small molecule metabolites using lipidomics reveals that lipid metabolic reprogramming plays a role in the pathogenesis of IPF. Lipids such as fatty acids, cholesterol, arachidonic acid metabolites, and phospholipids are involved in the onset and progression of IPF by inducing endoplasmic reticulum stress, promoting cell apoptosis, and enhancing the expression of pro-fibrotic biomarkers. Therefore, targeting lipid metabolism can provide a promising therapeutic strategy for pulmonary fibrosis. This review focuses on lipid metabolism in the pathogenesis of pulmonary fibrosis.

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 Role of Sphingolipids in Regulating Vascular Permeability in Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a disease that causes scarring and fibrotic transformation of the lung parenchyma, resulting in the progressive loss of respiratory function and, often, death. Current treatments that target profibrotic factors can slow the rate of progression but are unable to ultimately stop it. In the past decade, many studies have shown that increased vascular permeability may be both a predictive and perpetuating factor in fibrogenesis. Consequently, there is a search for therapeutic targets to try and modulate vascular permeability in fibrotic lungs. One such class of targets that show great promise is sphingolipids. Sphingolipids are common in cell membranes and are increasingly recognized as critical to many cell signaling pathways, including those that affect the integrity of the vascular endothelial barrier. In this focused review we look at sphingolipids, particularly the sphingosine-1-phosphate (S1P) axis and its effects on vascular permeability, and how those effects may affect the pathogenesis of IPF. We further examine existing S1P modulators and their potential efficacy as therapeutics for IPF.

S1PR1 attenuates pulmonary fibrosis by inhibiting EndMT and improving endothelial barrier function

BACKGROUND

Idiopathic pulmonary fibrosis (IPF) is a chronic fatal disease of unknown etiology. Its pathological manifestations include excessive proliferation and activation of fibroblasts and deposition of extracellular matrix. Endothelial cell-mesenchymal transformation (EndMT), a novel mechanism that generates fibroblast during IPF, is responsible for fibroblast-like phenotypic changes and activation of fibroblasts into hypersecretory cells. However, the exact mechanism behind EndMT-derived fibroblasts and activation is uncertain. Here, we investigated the role of sphingosine 1-phosphate receptor 1 (S1PR1) in EndMT-driven pulmonary fibrosis.

METHODS

We treated C57BL/6 mice with bleomycin (BLM) in vivo and pulmonary microvascular endothelial cells with TGF-β1 in vitro. Western blot,flow cytometry, and immunofluorescence were used to detect the expression of S1PR1 in endothelial cells. To evaluate the effect of S1PR1 on EndMT and endothelial barrier and its role in lung fibrosis and related signaling pathways, S1PR1 agonist and antagonist were used in vitro and in vivo.

RESULTS

Endothelial S1PR1 protein expression was downregulated in both in vitro and in vivo models of pulmonary fibrosis induced by TGF-β1 and BLM, respectively. Downregulation of S1PR1 resulted in EndMT, indicated by decreased expression of endothelial markers CD31 and VE-cadherin, increased expression of mesenchymal markers α-SMA and nuclear transcription factor Snail, and disruption of the endothelial barrier. Further mechanistic studies found that stimulation of S1PR1 inhibited TGF-β1-mediated activation of the Smad2/3 and RhoA/ROCK1 pathways. Moreover, stimulation of S1PR1 attenuated Smad2/3 and RhoA/ROCK1 pathway-mediated damage to endothelial barrier function.

CONCLUSIONS

Endothelial S1PR1 provides protection against pulmonary fibrosis by inhibiting EndMT and attenuating endothelial barrier damage. Accordingly, S1PR1 may be a potential therapeutic target in progressive IPF.

Current advancements and future perspectives of long noncoding RNAs in lipid metabolism and signaling

BACKGROUND

The investigation of lncRNAs has provided a novel perspective for elucidating mechanisms underlying diverse physiological and pathological processes. Compelling evidence has revealed an intrinsic link between lncRNAs and lipid metabolism, demonstrating that lncRNAs-induced disruption of lipid metabolism and signaling contribute to the development of multiple cancers and some other diseases, including obesity, fatty liver disease, and cardiovascular disease.

AIMOF REVIEW

The current review summarizes the recent advances in basic research about lipid metabolism and lipid signaling-related lncRNAs. Meanwhile, the potential and challenges of targeting lncRNA for the therapy of cancers and other lipid metabolism-related diseases are also discussed.

KEY SCIENTIFIC CONCEPT OF REVIEW

Compared with the substantial number of lncRNA loci, we still know little about the role of lncRNAs in metabolism. A more comprehensive understanding of the function and mechanism of lncRNAs may provide a new standpoint for the study of lipid metabolism and signaling. Developing lncRNA-based therapeutic approaches is an effective strategy for lipid metabolism-related diseases.

Global sphingosine-1-phosphate receptor 2 deficiency attenuates neuroinflammation and ischemic-reperfusion injury after neonatal stroke

Arterial ischemic stroke is common in neonates-1 per 2,300-5,000 births-and therapeutic targets remain insufficiently defined. Sphingosine-1-phosphate receptor 2 (S1PR2), a major regulator of the CNS and immune systems, is injurious in adult stroke. Here, we assessed whether S1PR2 contributes to stroke induced by 3 h transient middle cerebral artery occlusion (tMCAO) in S1PR2 heterozygous (HET), knockout (KO), and wild type (WT) postnatal day 9 pups. HET and WT of both sexes displayed functional deficits in Open Field test whereas injured KO at 24 h reperfusion performed similarly to naives. S1PR2 deficiency protected neurons, attenuated infiltration of inflammatory monocytes, and altered vessel-microglia interactions without reducing increased cytokine levels in injured regions at 72 h. Pharmacologic inhibition of S1PR2 after tMCAO by JTE-013 attenuated injury 72 h after tMCAO. Importantly, the lack of S1PR2 alleviated anxiety and brain atrophy during chronic injury. Altogether, we identify S1PR2 as a potential new target for mitigating neonatal stroke.

Apolipoprotein M/sphingosine 1-phosphate protects against diabetic nephropathy

Diabetic nephropathy remains a common cause of end-stage renal failure and its associated mortality around the world. Sphingosine 1-phosphate (S1P) is a multifunctional lipid mediator and binds to HDL via apolipoprotein M (ApoM). Since HDL has been reported to be epidemiologically associated with kidney disease, we attempted to investigate the involvement of the ApoM/S1P axis in the pathogenesis/progression of diabetic nephropathy. In type 2 diabetic patients, the serum ApoM levels were inversely correlated with the clinical stage of diabetic nephropathy. The decline in the eGFR over a 5-year observation period proceeded more rapidly in subjects with lower serum ApoM levels. In a mouse model of streptozotocin-induced diabetes, deletion of ApoM deteriorated the phenotypes of diabetic nephropathy: the urinary albumin and plasma creatinine levels increased, the kidneys enlarged, and renal fibrosis and thickening of the basement membrane progressed. On the other hand, overexpression of ApoM ameliorated these phenotypes. These protective effects of ApoM were partially inhibited by treatment with VPC23019, an antagonist of S1P1 and S1P3, but not by treatment with JTE013, an antagonist of S1P2. ApoM/S1P axis attenuated activation of the Smad3 pathway, while augmented eNOS phosphorylation through the S1P1 pathway. Moreover, ApoM/S1P increased the SIRT1 protein levels and enhanced mitochondrial functions by increasing the S1P content of the cell membrane, which might cause selective activation of S1P1. ApoM might be a useful biomarker for predicting the progression of diabetic nephropathy, and the ApoM/S1P-S1P1 axis might serve as a novel therapeutic target for preventing the development/progression of diabetic nephropathy.

Cellular and Molecular Control of Lipid Metabolism in Idiopathic Pulmonary Fibrosis: Clinical Application of the Lysophosphatidic Acid Pathway

Idiopathic pulmonary fibrosis (IPF) is a representative disease that causes fibrosis of the lungs. Its pathogenesis is thought to be characterized by sustained injury to alveolar epithelial cells and the resultant abnormal tissue repair, but it has not been fully elucidated. IPF is currently difficult to cure and is known to follow a chronic progressive course, with the patient's survival period estimated at about three years. The disease occasionally exacerbates acutely, leading to a fatal outcome. In recent years, it has become evident that lipid metabolism is involved in the fibrosis of lungs, and various reports have been made at the cellular level as well as at the organic level. The balance among eicosanoids, sphingolipids, and lipid composition has been reported to be involved in fibrosis, with particularly close attention being paid to a bioactive lipid "lysophosphatidic acid (LPA)" and its pathway. LPA signals are found in a wide variety of cells, including alveolar epithelial cells, vascular endothelial cells, and fibroblasts, and have been reported to intensify pulmonary fibrosis via LPA receptors. For instance, in alveolar epithelial cells, LPA signals reportedly induce mitochondrial dysfunction, leading to epithelial damage, or induce the transcription of profibrotic cytokines. Based on these mechanisms, LPA receptor inhibitors and the metabolic enzymes involved in LPA formation are now considered targets for developing novel means of IPF treatment. Advances in basic research on the relationships between fibrosis and lipid metabolism are opening the path to new therapies targeting lipid metabolism in the treatment of IPF.

The matricellular protein CCN3 supports lung endothelial homeostasis and function

Aberrant vascular remodeling contributes to the progression of many aging-associated diseases, including idiopathic pulmonary fibrosis (IPF), where heterogeneous capillary density, endothelial transcriptional alterations, and increased vascular permeability correlate with poor disease outcomes. Thus, identifying disease-driving mechanisms in the pulmonary vasculature may be a promising strategy to limit IPF progression. Here, we identified Ccn3 as an endothelial-derived factor that is upregulated in resolving but not in persistent lung fibrosis in mice, and whose function is critical for vascular homeostasis and repair. Loss and gain of function experiments were carried out to test the role of CCN3 in lung microvascular endothelial function in vitro through RNAi and the addition of recombinant human CCN3 protein, respectively. Endothelial migration, permeability, proliferation, and in vitro angiogenesis were tested in cultured human lung microvascular endothelial cells (ECs). Loss of CCN3 in lung ECs resulted in transcriptional alterations along with impaired wound-healing responses, in vitro angiogenesis, barrier integrity as well as an increased profibrotic activity through paracrine signals, whereas the addition of recombinant CCN3 augmented endothelial function. Altogether, our results demonstrate that the matricellular protein CCN3 plays an important role in lung endothelial function and could serve as a promising therapeutic target to facilitate vascular repair and promote lung fibrosis resolution.

Extensive and Persistent Extravascular Dermal Fibrin Deposition Characterizes Systemic Sclerosis

Systemic sclerosis (SSc) is an autoimmune disease characterized by progressive multiorgan fibrosis. While the cause of SSc remains unknown, a perturbed vasculature is considered a critical early step in the pathogenesis. Using fibrinogen as a marker of vascular leakage, we found extensive extravascular fibrinogen deposition in the dermis of both limited and diffuse systemic sclerosis disease, and it was present in both early and late-stage patients. Based on a timed series of excision wounds, retention on the fibrin deposit of the splice variant domain, fibrinogen αEC, indicated a recent event, while fibrin networks lacking the αEC domain were older. Application of this timing tool to SSc revealed considerable heterogeneity in αEC domain distribution providing unique insight into disease activity. Intriguingly, the fibrinogen-αEC domain also accumulated in macrophages. These observations indicate that systemic sclerosis is characterized by ongoing vascular leakage resulting in extensive interstitial fibrin deposition that is either continually replenished and/or there is impaired fibrin clearance. Unresolved fibrin deposition might then incite chronic tissue remodeling.

The Vasculature in Pulmonary Fibrosis

Purpose of Review

The current paradigm of idiopathic pulmonary fibrosis (IPF) pathogenesis involves recurrent injury to a sensitive alveolar epithelium followed by impaired repair responses marked by fibroblast activation and deposition of extracellular matrix. Multiple cell types are involved in this response with potential roles suggested by advances in single-cell RNA sequencing and lung developmental biology. Notably, recent work has better characterized the cell types present in the pulmonary endothelium and identified vascular changes in patients with IPF.

Recent Findings

Lung tissue from patients with IPF has been examined at single-cell resolution, revealing reductions in lung capillary cells and expansion of a population of vascular cells expressing markers associated with bronchial endothelium. In addition, pre-clinical models have demonstrated a fundamental role for aging and vascular permeability in the development of pulmonary fibrosis.

Summary

Mounting evidence suggests that the endothelium undergoes changes in the context of fibrosis, and these changes may contribute to the development and/or progression of pulmonary fibrosis. Additional studies will be needed to further define the functional role of these vascular changes.

S1PR1 serves as a viable drug target against pulmonary fibrosis by increasing the integrity of the endothelial barrier of the lung

Idiopathic pulmonary fibrosis (IPF) is a progressive lung disease with unclear etiology and limited treatment options. The median survival time for IPF patients is approximately 2-3 years and there is no effective intervention to treat IPF other than lung transplantation. As important components of lung tissue, endothelial cells (ECs) are associated with pulmonary diseases. However, the role of endothelial dysfunction in pulmonary fibrosis (PF) is incompletely understood. Sphingosine-1-phosphate receptor 1 (S1PR1) is a G protein-coupled receptor highly expressed in lung ECs. Its expression is markedly reduced in patients with IPF. Herein, we generated an endothelial-conditional S1pr1 knockout mouse model which exhibited inflammation and fibrosis with or without bleomycin (BLM) challenge. Selective activation of S1PR1 with an S1PR1 agonist, IMMH002, exerted a potent therapeutic effect in mice with bleomycin-induced fibrosis by protecting the integrity of the endothelial barrier. These results suggest that S1PR1 might be a promising drug target for IPF therapy.

Sphingosine 1-phosphate receptor-targeted therapeutics in rheumatic diseases

Sphingosine 1-phosphate (S1P), which acts via G protein-coupled S1P receptors (S1PRs), is a bioactive lipid essential for vascular integrity and lymphocyte trafficking. The S1P-S1PR signalling axis is a key component of the inflammatory response in autoimmune rheumatic diseases. Several drugs that target S1PRs have been approved for the treatment of multiple sclerosis and inflammatory bowel disease and are under clinical testing for patients with systemic lupus erythematosus (SLE). Preclinical studies support the hypothesis that targeting the S1P-S1PR axis would be beneficial to patients with SLE, rheumatoid arthritis (RA) and systemic sclerosis (SSc) by reducing pathological inflammation. Whereas most preclinical research and development efforts are focused on reducing lymphocyte trafficking, protective effects of circulating S1P on endothelial S1PRs, which maintain the vascular barrier and enable blood circulation while dampening leukocyte extravasation, have been largely overlooked. In this Review, we take a holistic view of S1P-S1PR signalling in lymphocyte and vascular pathobiology. We focus on the potential of S1PR modulators for the treatment of SLE, RA and SSc and summarize the rationale, pathobiology and evidence from preclinical models and clinical studies. Improved understanding of S1P pathobiology in autoimmune rheumatic diseases and S1PR therapeutic modulation is anticipated to lead to efficacious and safer management of these diseases.

Extracellular Lipids in the Lung and Their Role in Pulmonary Fibrosis

Lipids are major actors and regulators of physiological processes within the lung. Initial research has described their critical role in tissue homeostasis and in orchestrating cellular communication to allow respiration. Over the past decades, a growing body of research has also emphasized how lipids and their metabolism may be altered, contributing to the development and progression of chronic lung diseases such as pulmonary fibrosis. In this review, we first describe the current working model of the mechanisms of lung fibrogenesis before introducing lipids and their cellular metabolism. We then summarize the evidence of altered lipid homeostasis during pulmonary fibrosis, focusing on their extracellular forms. Finally, we highlight how lipid targeting may open avenues to develop therapeutic options for patients with lung fibrosis.