Key role of the endothelial TGF-β/ALK1/endoglin signaling pathway in humans and rodents pulmonary hypertension

Immunotherapy for Pulmonary Arterial Hypertension: From the Pathogenesis to Clinical Management

Pulmonary hypertension (PH) is a progressive cardiovascular disease, which may lead to severe cardiopulmonary dysfunction. As one of the main PH disease groups, pulmonary artery hypertension (PAH) is characterized by pulmonary vascular remodeling and right ventricular dysfunction. Increased pulmonary artery resistance consequently causes right heart failure, which is the major reason for morbidity and mortality in this disease. Although various treatment strategies have been available, the poor clinical prognosis of patients with PAH reminds us that further studies of the pathological mechanism of PAH are still needed. Inflammation has been elucidated as relevant to the initiation and progression of PAH, and plays a crucial and functional role in vascular remodeling. Many immune cells and cytokines have been demonstrated to be involved in the pulmonary vascular lesions in PAH patients, with the activation of downstream signaling pathways related to inflammation. Consistently, this influence has been found to correlate with the progression and clinical outcome of PAH, indicating that immunity and inflammation may have significant potential in PAH therapy. Therefore, we reviewed the pathogenesis of inflammation and immunity in PAH development, focusing on the potential targets and clinical application of anti-inflammatory and immunosuppressive therapy.

Endoglin and soluble endoglin in liver sinusoidal endothelial dysfunction in vivo

Liver sinusoidal endothelial cells (LSECs) play a crucial role in regulating the hepatic function. Endoglin (ENG), a transmembrane glycoprotein, was shown to be related to the development of endothelial dysfunction. In this study, we hypothesized the relationship between changes in ENG expression and markers of liver sinusoidal endothelial dysfunction (LSED) during liver impairment. Male C57BL/6J mice aged 9-12 weeks were fed with 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) diet (intrahepatic cholestasis) or choline-deficient l-amino acid defined high-fat diet (CDAA-HFD) (non-alcoholic steatohepatitis (NASH)). Significant increases in liver enzymes, fibrosis, and inflammation biomarkers were observed in both cholestasis and NASH. Decreased p-eNOS/eNOS and VE-cadherin protein expression and a significant increase in VCAM-1 and ICAM-1 expression were detected, indicating LSED in both mouse models of liver damage. A significant reduction of ENG in the DDC-fed mice, while a significant increase of ENG in the CDAA-HFD group was observed. Both DDC and CDAA-HFD-fed mice showed a significant increase in MMP-14 protein expression, which is related to significantly increased levels of soluble endoglin (sENG) in the plasma. In conclusion, we demonstrated that intrahepatic cholestasis and NASH result in an altered ENG expression, predominantly in LSECs, suggesting a critical role of ENG expression for the proper function of liver sinusoids. Both pathologies resulted in elevated sENG levels, cleaved by MMP-14 expressed predominantly from LSECs, indicating sENG as a liver injury biomarker.

Evolution of pulmonary hypertension in interstitial lung disease: a journey through past, present, and future

Interstitial lung diseases (ILD) are a spectrum of disorders often complicated by pulmonary hypertension (PH) in its course. The pathophysiologic mechanism of WHO group 3 PH is different to other forms of PH. The advent of PH is a harbinger for adverse events like mortality and morbidity, implying that the PH component of disease expedites deteriorated clinical outcomes. In fact, WHO group 3 PH due to ILD has the worse prognosis among all groups of PH. Hence, early detection of PH by a comprehensive screening method is paramount. Given considerable overlap in clinical manifestations between ILD and PH, early detection of PH is often elusive. Despite, the treatment of PH due to ILD has been frustrating until recently. Clinical trials utilizing PAH-specific pulmonary vasodilators have been ongoing for years without desired results. Eventually, the INCREASE study (2018) demonstrated beneficial effect of inhaled Treprostinil to treat PH in ILD. In view of this pioneering development, a paradigm shift in clinical approach to this disease phenotype is happening. There is a renewed vigor to develop a well validated screening tool for early detection and management. Currently inhaled Treprostinil is the only FDA approved therapy to treat this phenotype, but emergence of a therapy has opened a plethora of research toward new drug developments. Regardless of all these recent developments, the overall outlook still remains grim in this condition. This review article dwells on the current state of knowledge of pre-capillary PH due to ILD, especially its diagnosis and management, the recent progresses, and future evolutions in this field.

Pharmacological Agents and Potential New Therapies in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by an imbalance between vasoactive mediators, which causes vascular remodeling, increased pulmonary vascular resistance, and right ventricular overload, ultimately leading to heart failure and death. A metabolic theory has been suggested to explain the pathophysiology of PAH whereby abnormalities in mitochondrial biogenesis can trigger a hyperproliferative and apoptosis-resistant phenotype in cardiopulmonary and malignant cells, leading to mitochondrial dysfunction, which in turn causes the Warburg effect. This can culminate in the mitophagy of pulmonary vessels and cardiomyocytes. The present narrative review focuses on the pathophysiology of PAH, the pharmacological agents currently available for its treatment, and promising and challenging areas of therapeutic investigation.

TGF-β/Smads signaling pathway, Hippo-YAP/TAZ signaling pathway, and VEGF: Their mechanisms and roles in vascular remodeling related diseases

Vascular remodeling is a basic pathological process in various diseases characterized by abnormal changes in the morphology, structure, and function of vascular cells, such as migration, proliferation, hypertrophy, and apoptosis. Various growth factors and pathways are involved in the process of vascular remodeling. The transforming growth factor-β (TGF-β) signaling pathway, which is mainly mediated by TGF-β1, is an important factor in vascular wall enhancement during vascular development and regulates the vascular response to injury by promoting the accumulation of intimal tissue. Vascular endothelial growth factor (VEGF) has an important effect on initiating the formation of blood vessels. The Hippo-YAP/TAZ signaling pathway also plays an important role in angiogenesis. In addition, studies have shown that there is a certain interaction between the TGF-β/Smads signaling pathway, Hippo-YAP/TAZ signaling pathway, and VEGF. Many studies have shown that in the development of atherosclerosis, hypertension, aneurysm, vertebrobasilar dolichoectasia, pulmonary hypertension, restenosis after percutaneous transluminal angioplasty, and other diseases, various inflammatory reactions lead to changes in vascular structure and vascular microenvironment, which leads to vascular remodeling. The occurrence of vascular remodeling changes the morphology of blood vessels and thus changes the hemodynamics, which is the cause of further development of the disease process. Vascular remodeling can cause vascular smooth muscle cell dysfunction and vascular homeostasis regulation. This review aims to explore the mechanisms of the TGF-β/Smads signaling pathway, Hippo-YAP/TAZ signaling pathway, and vascular endothelial growth factor in vascular remodeling and related diseases. This paper is expected to provide new ideas for research on the occurrence and development of related diseases and provide a new direction for research on the treatment of related diseases.

Defining the clinical validity of genes reported to cause pulmonary arterial hypertension

PURPOSE

Pulmonary arterial hypertension (PAH) is a rare, progressive vasculopathy with significant cardiopulmonary morbidity and mortality. Genetic testing is currently recommended for adults diagnosed with heritable, idiopathic, anorexigen-, hereditary hemorrhagic telangiectasia-, and congenital heart disease-associated PAH, PAH with overt features of venous/capillary involvement, and all children diagnosed with PAH. Variants in at least 27 genes have putative evidence for PAH causality. Rigorous assessment of the evidence is needed to inform genetic testing.

METHODS

An international panel of experts in PAH applied a semi-quantitative scoring system developed by the NIH Clinical Genome Resource to classify the relative strength of evidence supporting PAH gene-disease relationships based on genetic and experimental evidence.

RESULTS

Twelve genes (BMPR2, ACVRL1, ATP13A3, CAV1, EIF2AK4, ENG, GDF2, KCNK3, KDR, SMAD9, SOX17, and TBX4) were classified as having definitive evidence and 3 genes (ABCC8, GGCX, and TET2) with moderate evidence. Six genes (AQP1, BMP10, FBLN2, KLF2, KLK1, and PDGFD) were classified as having limited evidence for causal effects of variants. TOPBP1 was classified as having no known PAH relationship. Five genes (BMPR1A, BMPR1B, NOTCH3, SMAD1, and SMAD4) were disputed because of a paucity of genetic evidence over time.

CONCLUSION

We recommend that genetic testing includes all genes with definitive evidence and that caution be taken in the interpretation of variants identified in genes with moderate or limited evidence. Genes with no known evidence for PAH or disputed genes should not be included in genetic testing.

Sex-biased TGFβ signalling in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a rare cardiovascular disorder leading to pulmonary hypertension and, often fatal, right heart failure. Sex differences in PAH are evident, which primarily presents with a female predominance and increased male severity. Disturbed signalling of the transforming growth factor-β (TGFβ) family and gene mutations in the bone morphogenetic protein receptor 2 (BMPR2) are risk factors for PAH development, but how sex-specific cues affect the TGFβ family signalling in PAH remains poorly understood. In this review, we aim to explore the sex bias in PAH by examining sex differences in the TGFβ signalling family through mechanistical and translational evidence. Sex hormones including oestrogens, progestogens, and androgens, can determine the expression of receptors (including BMPR2), ligands, and soluble antagonists within the TGFβ family in a tissue-specific manner. Furthermore, sex-related genetic processes, i.e. Y-chromosome expression and X-chromosome inactivation, can influence the TGFβ signalling family at multiple levels. Given the clinical and mechanistical similarities, we expect that the conclusions arising from this review may apply also to hereditary haemorrhagic telangiectasia (HHT), a rare vascular disorder affecting the TGFβ signalling family pathway. In summary, we anticipate that investigating the TGFβ signalling family in a sex-specific manner will contribute to further understand the underlying processes leading to PAH and likely HHT.

Extracellular Vesicles in Pulmonary Hypertension: A Dangerous Liaison?

The term pulmonary hypertension (PH) refers to different conditions, all characterized by increased pressure and resistance in the pulmonary arterial bed. PH has a wide range of causes (essentially, cardiovascular, pulmonary, or connective tissue disorders); however, idiopathic (i.e., without a clear cause) PH exists. This chronic, progressive, and sometimes devastating disease can finally lead to right heart failure and eventually death, through pulmonary vascular remodeling and dysfunction. The exact nature of PH pathophysiology is sometimes still unclear. Extracellular vesicles (EVs), previously known as apoptotic bodies, microvesicles, and exosomes, are small membrane-bound vesicles that are generated by almost all cell types and can be detected in a variety of physiological fluids. EVs are involved in intercellular communication, thus influencing immunological response, inflammation, embryogenesis, aging, and regenerative processes. Indeed, they transport chemokines, cytokines, lipids, RNA and miRNA, and other biologically active molecules. Although the precise functions of EVs are still not fully known, there is mounting evidence that they can play a significant role in the pathophysiology of PH. In this review, after briefly recapping the key stages of PH pathogenesis, we discuss the current evidence on the functions of EVs both as PH biomarkers and potential participants in the distinct pathways of disease progression.

Insights into Endothelin Receptors in Pulmonary Hypertension

Pulmonary hypertension (PH) is a disease which affects the cardiopulmonary system; it is defined as a mean pulmonary artery pressure (mPAP) > 20 mmHg as measured by right heart catheterization at rest, and is caused by complex and diverse mechanisms. In response to stimuli such as hypoxia and ischemia, the expression and synthesis of endothelin (ET) increase, leading to the activation of various signaling pathways downstream of it and producing effects such as the induction of abnormal vascular proliferation during the development of the disease. This paper reviews the regulation of endothelin receptors and their pathways in normal physiological processes and disease processes, and describes the mechanistic roles of ET receptor antagonists that are currently approved and used in clinical studies. Current clinical researches on ET are focused on the development of multi-target combinations and novel delivery methods to improve efficacy and patient compliance while reducing side effects. In this review, future research directions and trends of ET targets are described, including monotherapy and precision medicine.

Pulmonary vascular fibrosis in pulmonary hypertension - The role of the extracellular matrix as a therapeutic target

Pulmonary hypertension (PH) is a condition characterized by changes in the extracellular matrix (ECM) deposition and vascular remodeling of distal pulmonary arteries. These changes result in increased vessel wall thickness and lumen occlusion, leading to a loss of elasticity and vessel stiffening. Clinically, the mechanobiology of the pulmonary vasculature is becoming increasingly recognized for its prognostic and diagnostic value in PH. Specifically, the increased vascular fibrosis and stiffening resulting from ECM accumulation and crosslinking may be a promising target for the development of anti- or reverse-remodeling therapies. Indeed, there is a huge potential in therapeutic interference with mechano-associated pathways in vascular fibrosis and stiffening. The most direct approach is aiming to restore extracellular matrix homeostasis, by interference with its production, deposition, modification and turnover. Besides structural cells, immune cells contribute to the level of ECM maturation and degradation by direct cell-cell contact or the release of mediators and proteases, thereby opening a huge avenue to target vascular fibrosis via immunomodulation approaches. Indirectly, intracellular pathways associated with altered mechanobiology, ECM production, and fibrosis, offer a third option for therapeutic intervention. In PH, a vicious cycle of persistent activation of mechanosensing pathways such as YAP/TAZ initiates and perpetuates vascular stiffening, and is linked to key pathways disturbed in PH, such as TGF-beta/BMPR2/STAT. Together, this complexity of the regulation of vascular fibrosis and stiffening in PH allows the exploration of numerous potential therapeutic interventions. This review discusses connections and turning points of several of these interventions in detail.

Cannabidiol alleviates right ventricular fibrosis by inhibiting the transforming growth factor β pathway in monocrotaline-induced pulmonary hypertension in rats

Cannabidiol (CBD) is a non-intoxicating compound of Cannabis with anti-fibrotic properties. Pulmonary hypertension (PH) is a disease that can lead to right ventricular (RV) failure and premature death. There is evidence that CBD reduces monocrotaline (MCT)-induced PH, including reducing right ventricular systolic pressure (RVSP), vasorelaxant effect on pulmonary arteries, and decreasing expression of profibrotic markers in the lungs. The aim of our study was to investigate the effect of chronic administration of CBD (10 mg/kg daily for 21 days) on profibrotic parameters in the RVs of MCT-induced PH rats. In MCT-induced PH, we found an increase in profibrotic parameters and parameters related to RV dysfunction, i.e. plasma pro-B-type natriuretic peptide (NT-proBNP), cardiomyocyte width, interstitial and perivascular fibrosis area, amount of fibroblasts and fibronectin, as well as overexpression of the transforming growth of factor β1 (TGF-β1), galectin-3 (Gal-3), suppressor of mothers against decapentaplegic 2 (SMAD2), phosphorylated SMAD2 (pSMAD2) and alpha-smooth muscle actin (α-SMA). In contrast, vascular endothelial cadherin (VE-cadherin) levels were decreased in the RVs of MCT-induced PH rats. Administration of CBD reduced the amount of plasma NT-proBNP, the width of cardiomyocytes, the amount of fibrosis area, fibronectin and fibroblast expression, as well as decreased the expression of TGF-β1, Gal-3, SMAD2, pSMAD2, and increased the level of VE-cadherin. Overall, CBD has been found to have the anti-fibrotic potential in MCT-induced PH. As such, CBD may act as an adjuvant therapy for PH, however, further detailed investigations are recommended to confirm our promising results.

Serum Biomarkers in Connective Tissue Disease-Associated Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a life-threatening complication of connective tissue diseases (CTDs) characterised by increased pulmonary arterial pressure and pulmonary vascular resistance. CTD-PAH is the result of a complex interplay among endothelial dysfunction and vascular remodelling, autoimmunity and inflammatory changes, ultimately leading to right heart dysfunction and failure. Due to the non-specific nature of the early symptoms and the lack of consensus on screening strategies-except for systemic sclerosis, with a yearly transthoracic echocardiography as recommended-CTD-PAH is often diagnosed at an advanced stage, when the pulmonary vessels are irreversibly damaged. According to the current guidelines, right heart catheterisation is the gold standard for the diagnosis of PAH; however, this technique is invasive, and may not be available in non-referral centres. Hence, there is a need for non-invasive tools to improve the early diagnosis and disease monitoring of CTD-PAH. Novel serum biomarkers may be an effective solution to this issue, as their detection is non-invasive, has a low cost and is reproducible. Our review aims to describe some of the most promising circulating biomarkers of CTD-PAH, classified according to their role in the pathophysiology of the disease.

Biallelic variants in the calpain regulatory subunit CAPNS1 cause pulmonary arterial hypertension

Purpose

The aim of this study was to identify the monogenic cause of pulmonary arterial hypertension (PAH), a multifactorial and often fatal disease, in 2 unrelated consanguine families.

Methods

We performed exome sequencing and validated variant pathogenicity by whole-blood RNA and protein expression analysis in both families. Further RNA sequencing of preserved lung tissue was performed to investigate the consequences on selected genes that are involved in angiogenesis, proliferation, and apoptosis.

Results

We identified 2 rare biallelic variants in CAPNS1, encoding the regulatory subunit of calpain. The variants cosegregated with PAH in the families. Both variants lead to loss of function (LoF), which is demonstrated by aberrant splicing resulting in the complete absence of the CAPNS1 protein in affected patients. No other LoF CAPNS1 variant was identified in the genome data of more than 1000 patients with unresolved PAH.

Conclusion

The calpain holoenzyme was previously linked to pulmonary vascular development and progression of PAH in patients. We demonstrated that biallelic LoF variants in CAPNS1 can cause idiopathic PAH by the complete absence of CAPNS1 protein. Screening of this gene in patients who are affected by PAH, especially with suspected autosomal recessive inheritance, should be considered.

Genes in pediatric pulmonary arterial hypertension and the most promising BMPR2 gene therapy

Pulmonary arterial hypertension (PAH) is a rare but progressive and lethal vascular disease of diverse etiologies, mainly caused by proliferation of endothelial cells, smooth muscle cells in the pulmonary artery, and fibroblasts, which ultimately leads to right-heart hypertrophy and cardiac failure. Recent genetic studies of childhood-onset PAH report that there is a greater genetic burden in children than in adults. Since the first-identified pathogenic gene of PAH, BMPR2, which encodes bone morphogenetic protein receptor 2, a receptor in the transforming growth factor-β superfamily, was discovered, novel causal genes have been identified and substantially sharpened our insights into the molecular genetics of childhood-onset PAH. Currently, some newly identified deleterious genetic variants in additional genes implicated in childhood-onset PAH, such as potassium channels (KCNK3) and transcription factors (TBX4 and SOX17), have been reported and have greatly updated our understanding of the disease mechanism. In this review, we summarized and discussed the advances of genetic variants underlying childhood-onset PAH susceptibility and potential mechanism, and the most promising BMPR2 gene therapy and gene delivery approaches to treat childhood-onset PAH in the future.

Atheroprone fluid shear stress-regulated ALK1-Endoglin-SMAD signaling originates from early endosomes

BACKGROUND

Fluid shear stress enhances endothelial SMAD1/5 signaling via the BMP9-bound ALK1 receptor complex supported by the co-receptor Endoglin. While moderate SMAD1/5 activation is required to maintain endothelial quiescence, excessive SMAD1/5 signaling promotes endothelial dysfunction. Increased BMP signaling participates in endothelial-to-mesenchymal transition and inflammation culminating in vascular diseases such as atherosclerosis. While the function of Endoglin has so far been described under picomolar concentrations of BMP9 and short-term shear application, we investigated Endoglin under physiological BMP9 and long-term pathophysiological shear conditions.

RESULTS

We report here that knock-down of Endoglin leads to exacerbated SMAD1/5 phosphorylation and atheroprone gene expression profile in HUVECs sheared for 24 h. Making use of the ligand-trap ALK1-Fc, we furthermore show that this increase is dependent on BMP9/10. Mechanistically, we reveal that long-term exposure of ECs to low laminar shear stress leads to enhanced Endoglin expression and endocytosis of Endoglin in Caveolin-1-positive early endosomes. In these endosomes, we could localize the ALK1-Endoglin complex, labeled BMP9 as well as SMAD1, highlighting Caveolin-1 vesicles as a SMAD signaling compartment in cells exposed to low atheroprone laminar shear stress.

CONCLUSIONS

We identified Endoglin to be essential in preventing excessive activation of SMAD1/5 under physiological flow conditions and Caveolin-1-positive early endosomes as a new flow-regulated signaling compartment for BMP9-ALK1-Endoglin signaling axis in atheroprone flow conditions.

Group 3 Pulmonary Hypertension: From Bench to Bedside

Pulmonary hypertension (PH) because of chronic lung disease is categorized as Group 3 PH in the most recent classification system. Prevalence of these diseases is increasing over time, creating a growing need for effective therapeutic options. Recent approval of the first pulmonary arterial hypertension therapy for the treatment of Group 3 PH related to interstitial lung disease represents an encouraging advancement. This review focuses on molecular mechanisms contributing to pulmonary vasculopathy in chronic hypoxia, the pathology and epidemiology of Group 3 PH, the right ventricular dysfunction observed in this population and clinical trial data that inform the use of pulmonary vasodilators in Group 3 PH.

New Mutations and Pathogenesis of Pulmonary Hypertension: Progress and Puzzles in Disease Pathogenesis

Pulmonary arterial hypertension (PAH) is a complex multifactorial disease with poor prognosis characterized by functional and structural alterations of the pulmonary circulation causing marked increase in pulmonary vascular resistance, ultimately leading to right heart failure and death. Mutations in the gene encoding BMPRII-a receptor for the TGF-β (transforming growth factor-beta) superfamily-account for over 70% of families with PAH and ≈20% of sporadic cases. In recent years, however, less common or rare mutations in other genes have been identified. This review will consider how these newly discovered PAH genes could help to provide a better understanding of the molecular and cellular bases of the maintenance of the pulmonary vascular integrity, as well as their role in the PAH pathogenesis underlying occlusion of arterioles in the lung. We will also discuss how insights into the genetic contributions of these new PAH-related genes may open up new therapeutic targets for this, currently incurable, cardiopulmonary disorder.

Neuropilin-1 deficiency in vascular smooth muscle cells is associated with hereditary hemorrhagic telangiectasia arteriovenous malformations

Patients with hereditary hemorrhagic telangiectasia (HHT) have arteriovenous malformations (AVMs) with genetic mutations involving the activin-A receptor like type 1 (ACVRL1 or ALK1) and endoglin (ENG). Recent studies have shown that Neuropilin-1 (NRP-1) inhibits ALK1. We investigated the expression of NRP-1 in livers of patients with HHT and found that there was a significant reduction in NRP-1 in perivascular smooth muscle cells (SMCs). We used Nrp1^SM22KO mice (Nrp1 was ablated in SMCs) and found hemorrhage, increased immune cell infiltration with a decrease in SMCs, and pericyte lining in lungs and liver in adult mice. Histologic examination revealed lung arteriovenous fistulas (AVFs) with enlarged liver vessels. Evaluation of the retina vessels at P5 from Nrp1^SM22KO mice demonstrated dilated capillaries with a reduction of pericytes. In inflow artery of surgical AVFs from the Nrp1^SM22KO versus WT mice, there was a significant decrease in Tgfb1, Eng, and Alk1 expression and phosphorylated SMAD1/5/8 (pSMAD1/5/8), with an increase in apoptosis. TGF-β1–stimulated aortic SMCs from Nrp1^SM22KO versus WT mice have decreased pSMAD1/5/8 and increased apoptosis. Coimmunoprecipitation experiments revealed that NRP-1 interacts with ALK1 and ENG in SMCs. In summary, NRP-1 deletion in SMCs leads to reduced ALK1, ENG, and pSMAD1/5/8 signaling and reduced cell death associated with AVM formation.

The role of endoglin and its soluble form in pathogenesis of preeclampsia

Preeclampsia remains till today a leading cause of maternal and fetal morbidity and mortality. Pathophysiology of the disease is not yet fully elucidated, though it is evident that it revolves around placenta. Cellular ischemia in the preeclamptic placenta creates an imbalance between angiogenic and anti-angiogenic factors in maternal circulation. Endoglin, a transmembrane co-receptor of transforming growth factor β (TGF-β) demonstrating angiogenic effects, is involved in a variety of angiogenesis-dependent diseases with endothelial dysfunction, including preeclampsia. Endoglin expression is up-regulated in preeclamptic placentas, through mechanisms mainly induced by hypoxia, oxidative stress and oxysterol-mediated activation of liver X receptors. Overexpression of endoglin results in an increase of its soluble form in maternal circulation. Soluble endoglin represents the extracellular domain of membrane endoglin, cleaved by the action of metalloproteinases, predominantly matrix metalloproteinase-14. Released in circulation, soluble endoglin interferes in TGF-β1 and activin receptor-like kinase 1 signaling pathways and inhibits endothelial nitric oxide synthase activation, consequently deranging angiogenesis and promoting vasoconstriction. Due to these properties, soluble endoglin actively contributes to the impaired placentation observed in preeclampsia, as well as to the pathogenesis and manifestation of its clinical signs and symptoms, especially hypertension and proteinuria. The significant role of endoglin and soluble endoglin in pathophysiology of preeclampsia could have prognostic, diagnostic and therapeutic perspectives. Further research is essential to extensively explore the potential use of these molecules in the management of preeclampsia in clinical settings.

Clinical characteristics and prognosis analysis of idiopathic and hereditary pulmonary hypertension patients with ACVRL1 gene mutations

Pulmonary arterial hypertension is a kind of heart and lung vascular disease with low incidence and poor prognosis. Genetic variants are the important factors of pulmonary arterial hypertension. The mutations of activin receptor-like kinase-1 (ACVRL1) could cause pulmonary arteriole obstruction and occlusion in pulmonary arterial hypertension patients. The ACVRL1 gene mutation and clinical characteristics of Chinese idiopathic or hereditary pulmonary hypertension (IPAH/HPAH) patients are still unclear. This study aimed to retrospectively study the mutation characteristics of ACVRL1 gene in Chinese IPAH/HPAH patients and its effect on clinical prognosis. We analyzed the clinical, functional, hemodynamic and mutation characteristics of 12 IPAH/HPAH patients with ACVRL1 mutations and compared with 94 IPAH/HPAH patients (27 patients carried bone morphogenetic protein receptor type 2 (BMPR2) mutations and 67 without mutations). All ACVRL1 mutations of 12 patients were single nucleotide missense mutations. The ratio of male to female in 12 patients was 1:1. The diagnosis age of ACVRL1 mutation patients was younger than that of BMPR2 mutation patients (13.6 ± 11.3 years vs. 16.0 ± 12.9 years) but higher than that of patients without mutations (13.6 ± 11.3 years vs. 8.8 ± 8.5 years, p = 0.006). IPAH/HPAH patients with ACVRL1 mutation have rapid disease progresses, high overall mortality rate (approximately 50%) and no response to the acute pulmonary vasodilation test. In conclusion, this is the first study to analyze the ACVRL1 gene mutation and clinical characteristics of Chinese IPAH/HPAH patients. It is beneficial to screen ACVRL1 gene mutation for IPAH/HPAH patients to facilitate genetic counseling and early prevention and treatment.

The proliferation and migration of atherosclerosis-related HVSMCs were inhibited by downregulation of lncRNA XIST via regulation of the miR-761/BMP9 axis

Atherosclerosis (AS) is a chronic inflammatory disease that can be caused by the proliferation and migration of human vascular smooth muscle cells (HVSMCs). Here, we found that lncRNA XIST was related to the abnormal proliferation and migration of HVSMCs, and thus, the mechanism by which XIST regulated HVSMCs was further investigated. HVSMCs were treated with oxidized low-density lipoprotein (ox-LDL, 100 μg/ml) as AS models. CCK8 assays, flow cytometry, Transwell assays and wound healing assays were applied to evaluate cell viability, cell cycle analysis, and cell migration, respectively. A dual-luciferase reporter assay was employed to verify the binding relationships between XIST and miR-761, miR-761, and BMP9. Ox-LDL induced the proliferation and migration of HVSMCs, upregulated the expression of XIST, downregulated miR-761 expression, and activated the BMP9/ALK1/endoglin pathway. Luciferase assays revealed that XIST sponged miR-761. XIST knockdown ameliorated ox-LDL-mediated effects in HVSMCs, which were largely abolished by miR-761 silencing. BMP9 was targeted-inhibited by miR-761. MiR-761 overexpression alleviated ox-LDL-mediated effects in HVSMCs. However, BMP9 overexpression abolished miR-761-mediated effects in HVSMCs treated with ox-LDL. Our findings suggested that XIST knockdown suppressed the proliferation and migration of HVSMCs by promoting miR-761, which targeted-inhibited the BMP9/ALK1/endoglin pathway.

Endothelial FGF signaling is protective in hypoxia-induced pulmonary hypertension

Hypoxia-induced pulmonary hypertension (PH) is one of the most common and deadliest forms of PH. Fibroblast growth factor receptors 1 and 2 (FGFR1/2) are elevated in patients with PH and in mice exposed to chronic hypoxia. Endothelial FGFR1/2 signaling is important for the adaptive response to several injury types and we hypothesized that endothelial FGFR1/2 signaling would protect against hypoxia-induced PH. Mice lacking endothelial FGFR1/2, mice with activated endothelial FGFR signaling, and human pulmonary artery endothelial cells (HPAECs) were challenged with hypoxia. We assessed the effect of FGFR activation and inhibition on right ventricular pressure, vascular remodeling, and endothelial-mesenchymal transition (EndMT), a known pathologic change seen in patients with PH. Hypoxia-exposed mice lacking endothelial FGFRs developed increased PH, while mice overexpressing a constitutively active FGFR in endothelial cells did not develop PH. Mechanistically, lack of endothelial FGFRs or inhibition of FGFRs in HPAECs led to increased TGF-β signaling and increased EndMT in response to hypoxia. These phenotypes were reversed in mice with activated endothelial FGFR signaling, suggesting that FGFR signaling inhibits TGF-β pathway-mediated EndMT during chronic hypoxia. Consistent with these observations, lung tissue from patients with PH showed activation of FGFR and TGF-β signaling. Collectively, these data suggest that activation of endothelial FGFR signaling could be therapeutic for hypoxia-induced PH.

Phenotypic Diversity of Vascular Smooth Muscle Cells in Pulmonary Arterial Hypertension: Implications for Therapy

Pulmonary arterial hypertension (PAH) is a progressive incurable condition that is characterized by extensive remodelling of the pulmonary circulation, leading to severe right heart failure and death. Similar to other vascular contractile cells, pulmonary arterial smooth muscle cells (PA-SMCs) play central roles in physiological and pathological vascular remodelling due to their remarkable ability to dynamically modulate their phenotype to ensure contractile and synthetic functions. The dysfunction and molecular mechanisms underlying their contribution to the various pulmonary vascular lesions associated with PAH have been a major focus of research. The aim of this review is to describe the medial and non-medial origins of contractile cells in the pulmonary vascular wall and present evidence of how they contribute to the onset and progression of PAH. We also highlight specific potential target molecules and discuss future directions that are being explored to widen the therapeutic options for the treatment of PAH.

Sex-dependent right ventricular hypertrophic gene changes after methamphetamine treatment in mice

Methamphetamine (MA) abuse is associated with the development of pulmonary arterial hypertension (PAH) and subsequent right ventricular failure. A recent clinical study demonstrated that female sex is a major risk factor for MA-induced PAH. The mechanisms associated with increased prevalence and severity of MA-induced PAH in females are still unclear. We hypothesized that MA may promote changes in gene expression in the right ventricle contributing to the development and/or worsening of PAH in females. Male and female C57BL/6 mice were treated with either MA or vehicle. Right and left ventricular systolic pressures (RVSP and LVSP, respectively) were assessed and tissue samples were collected for gene expression and histology. LVSP and RVSP were not affected by MA in either males or females. Right ventricular hypertrophy was significantly increased by MA in females but it was not affected by MA in males. In the female mice, MA-induced right ventricular hypertrophy was associated with increased expression of brain natriuretic peptide gene and members of the TGF-β receptor signaling pathway such as TGF-β receptor-1, smad3 and smad7. In male mice, there were no changes in right ventricular gene expression. Our results suggest that MA caused right ventricular hypertrophy in female mice, but not in males and that this was associated with an increase in hypertrophic genes. The right ventricular hypertrophy was not dependent on increased RVSP suggesting a direct effect of MA on the right ventricle. If this translates to PAH patients, it might explain the poor outcome observed in MA-associated female PAH patients.

The alveolar epithelial cells are involved in pulmonary vascular remodeling and constriction of hypoxic pulmonary hypertension

BACKGROUND

Hypoxic pulmonary hypertension (HPH) is a common type of pulmonary hypertension and characterized by pulmonary vascular remodeling and constriction. Alveolar epithelial cells (AECs) primarily sense alveolar hypoxia, but the role of AECs in HPH remains unclear. In this study, we explored whether AECs are involved in pulmonary vascular remodeling and constriction.

METHODS

In the constructed rat HPH model, hemodynamic and morphological characteristics were measured. By treating AECs with hypoxia, we further detected the levels of superoxide dismutase 2 (SOD2), catalase (CAT), reactive oxygen species (ROS) and hydrogen peroxide (H2O2), respectively. To detect the effects of AECs on pulmonary vascular remodeling and constriction, AECs and pulmonary artery smooth cells (PASMCs) were co-cultured under hypoxia, and PASMCs and isolated pulmonary artery (PA) were treated with AECs hypoxic culture medium. In addition, to explore the mechanism of AECs on pulmonary vascular remodeling and constriction, ROS inhibitor N-acetylcysteine (NAC) was used.

RESULTS

Hypoxia caused pulmonary vascular remodeling and increased pulmonary artery pressure, but had little effect on non-pulmonary vessels in vivo. Meanwhile, in vitro, hypoxia promoted the imbalance of SOD2 and CAT in AECs, leading to increased ROS and hydrogen peroxide (H2O2) production in the AECs culture medium. In addition, AECs caused the proliferation of co-cultured PASMCs under hypoxia, and the hypoxic culture medium of AECs enhanced the constriction of isolated PA. However, treatment with ROS inhibitor NAC effectively alleviated the above effects.

CONCLUSION

The findings of present study demonstrated that AECs were involved in pulmonary vascular remodeling and constriction under hypoxia by paracrine H2O2 into the pulmonary vascular microenvironment.

Molecular and Genetic Profiling for Precision Medicines in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a rare and chronic lung disease characterized by progressive occlusion of the small pulmonary arteries, which is associated with structural and functional alteration of the smooth muscle cells and endothelial cells within the pulmonary vasculature. Excessive vascular remodeling is, in part, responsible for high pulmonary vascular resistance and the mean pulmonary arterial pressure, increasing the transpulmonary gradient and the right ventricular “pressure overload”, which may result in right ventricular (RV) dysfunction and failure. Current technological advances in multi-omics approaches, high-throughput sequencing, and computational methods have provided valuable tools in molecular profiling and led to the identification of numerous genetic variants in PAH patients. In this review, we summarized the pathogenesis, classification, and current treatments of the PAH disease. Additionally, we outlined the latest next-generation sequencing technologies and the consequences of common genetic variants underlying PAH susceptibility and disease progression. Finally, we discuss the importance of molecular genetic testing for precision medicine in PAH and the future of genomic medicines, including gene-editing technologies and gene therapies, as emerging alternative approaches to overcome genetic disorders in PAH.

Endothelin-1 Upregulates Activin Receptor-Like Kinase-1 Expression via Gi/RhoA/Sp-1/Rho Kinase Pathways in Human Pulmonary Arterial Endothelial Cells

Background: Pulmonary arterial hypertension (PAH) is characterized by pulmonary vasoconstriction and organic stenosis. It has been demonstrated that endothelin-1 (ET-1) induces pulmonary vasoconstriction through the activation of RhoA. In addition, a gene mutation of activin receptor-like kinase (ACVRL)-1 is recognized in PAH patients. However, little is known about the association between ET-1 and ACVRL-1.

Objective: In the present study, we aimed to investigate the effect of ET-1 on ACVRL-1 expression and delineate the involvement of the Gi/RhoA/Rho kinase pathway.

Methods: ET-1 was added to culture medium of human pulmonary arterial endothelial cells (PAECs). Pre-treatment with pertussis toxin (PTX) or exoenzyme C3 transferase (C3T) was performed for inhibition of Gi or RhoA, respectively. Rho kinase was inhibited by Y27632. Mithramycin A was used for inhibition of Sp-1, which is a transcriptional factor of ACVRL-1. The active form of RhoA (GTP-RhoA) was assessed by pull-down assay.

Results: ACVRL-1 expression was increased by ET-1 in the PAECs. Pull-down assay revealed that ET-1 induced GTP-loading of RhoA, which was suppressed by pre-treatment with PTX or C3T. Further, PTX, C3T, and Y27632 suppressed the ET-1-induced ACVRL-1 expression. ET-1 increased the activity of the ACVRL-1 promoter and stabilized the ACVRL-1 mRNA. Sp-1 peaked 15 min after adding ET-1 to the PAECs. PTX and C3T prevented the increase of Sp-1 induced by ET-1. Inhibition of Sp-1 by mithramycin A suppressed ET-1-induced ACVRL-1 upregulation.

Conclusion: The present study demonstrated that ET-1 increases ACVRL-1 expression in human PAECs via the Gi/RhoA/Rho kinase pathway with the involvement of Sp-1.

Targeting transforming growth factor-β receptors in pulmonary hypertension

The transforming growth factor-β (TGF-β) superfamily includes several groups of multifunctional proteins that form two major branches, namely the TGF-β-activin-nodal branch and the bone morphogenetic protein (BMP)-growth differentiation factor (GDF) branch. The response to the activation of these two branches, acting through canonical (small mothers against decapentaplegic (Smad) 2/3 and Smad 1/5/8, respectively) and noncanonical signalling pathways, are diverse and vary for different environmental conditions and cell types. An extensive body of data gathered in recent years has demonstrated a central role for the cross-talk between these two branches in a number of cellular processes, which include the regulation of cell proliferation and differentiation, as well as the transduction of signalling cascades for the development and maintenance of different tissues and organs. Importantly, alterations in these pathways, which include heterozygous germline mutations and/or alterations in the expression of several constitutive members, have been identified in patients with familial/heritable pulmonary arterial hypertension (PAH) or idiopathic PAH (IPAH). Consequently, loss or dysfunction in the delicate, finely-tuned balance between the TGF-β-activin-nodal branch and the BMP-GDF branch are currently viewed as the major molecular defect playing a critical role in PAH predisposition and disease progression. Here we review the role of the TGF-β-activin-nodal branch in PAH and illustrate how this knowledge has not only provided insight into understanding its pathogenesis, but has also paved the way for possible novel therapeutic approaches.

Altered TGFβ/SMAD Signaling in Human and Rat Models of Pulmonary Hypertension: An Old Target Needs Attention

Recent translational studies highlighted the inhibition of transforming growth factor (TGF)-β signaling as a promising target to treat pulmonary arterial hypertension (PAH). However, it remains unclear whether alterations in TGF-β signaling are consistent between PAH patients and animal models. Therefore, we compared TGF-β signaling in the lungs of PAH patients and rats with experimental PAH induced by monocrotaline (MCT) or SU5416+hypoxia (SuHx). In hereditary PAH (hPAH) patients, there was a moderate increase in both TGFβR2 and pSMAD2/3 protein levels, while these were unaltered in idiopathic PAH (iPAH) patients. Protein levels of TGFβR2 and pSMAD2/3 were locally increased in the pulmonary vasculature of PAH rats under both experimental conditions. Conversely, the protein levels of TGFβR2 and pSMAD2/3 were reduced in SuHx while slightly increased in MCT. mRNA levels of plasminogen activator inhibitor (PAI)-1 were increased only in MCT animals and such an increase was not observed in SuHx rats or in iPAH and hPAH patients. In conclusion, our data demonstrate considerable discrepancies in TGFβ-SMAD signaling between iPAH and hPAH patients, as well as between patients and rats with experimental PAH.

BMP9 and BMP10 Act Directly on Vascular Smooth Muscle Cells for Generation and Maintenance of the Contractile State

BACKGROUND

Vascular smooth muscle cells (VSMCs) show a remarkable phenotypic plasticity, allowing acquisition of contractile or synthetic states, but critical information is missing about the physiologic signals, promoting formation, and maintenance of contractile VSMCs in vivo. BMP9 and BMP10 (bone morphogenetic protein) are known to regulate endothelial quiescence after secretion from the liver and right atrium, whereas a direct role in the regulation of VSMCs was not investigated. We studied the role of BMP9 and BMP10 for controlling formation of contractile VSMCs.

METHODS

We generated several cell type-specific loss- and gain-of-function transgenic mouse models to investigate the physiologic role of BMP9, BMP10, ALK1 (activin receptor-like kinase 1), and SMAD7 in vivo. Morphometric assessments, expression analysis, blood pressure measurements, and single molecule fluorescence in situ hybridization were performed together with analysis of isolated pulmonary VSMCs to unravel phenotypic and transcriptomic changes in response to absence or presence of BMP9 and BMP10.

RESULTS

Concomitant genetic inactivation of Bmp9 in the germ line and Bmp10 in the right atrium led to dramatic changes in vascular tone and diminution of the VSMC layer with attenuated contractility and decreased systemic as well as right ventricular systolic pressure. On the contrary, overexpression of Bmp10 in endothelial cells of adult mice dramatically enhanced formation of contractile VSMCs and increased systemic blood pressure as well as right ventricular systolic pressure. Likewise, BMP9/10 treatment induced an ALK1-dependent phenotypic switch from synthetic to contractile in pulmonary VSMCs. Smooth muscle cell-specific overexpression of Smad7 completely suppressed differentiation and proliferation of VSMCs and reiterated defects observed in adult Bmp9/10 double mutants. Deletion of Alk1 in VSMCs recapitulated the Bmp9/10 phenotype in pulmonary but not in aortic and coronary arteries. Bulk expression analysis and single molecule RNA-fluorescence in situ hybridization uncovered vessel bed-specific, heterogeneous expression of BMP type 1 receptors, explaining phenotypic differences in different Alk1 mutant vessel beds.

CONCLUSIONS

Our study demonstrates that BMP9 and BMP10 act directly on VSMCs for induction and maintenance of their contractile state. The effects of BMP9/10 in VSMCs are mediated by different combinations of BMP type 1 receptors in a vessel bed-specific manner, offering new opportunities to manipulate blood pressure in the pulmonary circulation.

'There and Back Again'-Forward Genetics and Reverse Phenotyping in Pulmonary Arterial Hypertension

Although the invention of right heart catheterisation in the 1950s enabled accurate clinical diagnosis of pulmonary arterial hypertension (PAH), it was not until 2000 when the landmark discovery of the causative role of bone morphogenetic protein receptor type II (BMPR2) mutations shed new light on the pathogenesis of PAH. Since then several genes have been discovered, which now account for around 25% of cases with the clinical diagnosis of idiopathic PAH. Despite the ongoing efforts, in the majority of patients the cause of the disease remains elusive, a phenomenon often referred to as "missing heritability". In this review, we discuss research approaches to uncover the genetic architecture of PAH starting with forward phenotyping, which in a research setting should focus on stable intermediate phenotypes, forward and reverse genetics, and finally reverse phenotyping. We then discuss potential sources of "missing heritability" and how functional genomics and multi-omics methods are employed to tackle this problem.

The TGF-β1 Induces the Endothelial-to-Mesenchymal Transition via the UCA1/miR-455/ZEB1 Regulatory Axis in Human Umbilical Vein Endothelial Cells

Transforming growth factor-beta 1 (TGF-β1) plays important roles in the endothelial-to-mesenchymal transition (EndMT). Recently, long noncoding RNAs (lncRNAs) have been identified to be involved in the physiological and pathological processes of human diseases. However, the role of endothelial lncRNAs in the TGF-β1-mediated control of angiogenesis and its underlying mechanism remains largely unclear. In this study, we first demonstrated that TGF-β1 induced EndMT; promoted cell viability, proliferation, migration, and tube formation of human umbilical vein endothelial cells (HUVECs). Second, our study displayed that TGF-β1 upregulated the lncRNA UCA1 expression in HUVECs, knocked down UCA1 with small interfering RNAs, and inhibited the function of TGF-β1 in HUVECs. Third, our study showed that UCA1 was located in the cytoplasm and absorbed miR-455 in TGF-β1-treated HUVECs. Further, the miR-455 inhibitor restored the role of the inhibited UCA1 in HUVECs treated with TGF-β1. Fourth, our study revealed that miR-455 inhibited ZEB1 expression, and overexpression of ZEB1 restored the role of miR-455 in HUVECs treated with TGF-β1. Finally, our study revealed that UCA1 exerted its role via regulating the UCA1/miR-455/ZEB1 regulatory axis in HUVECs treated with TGF-β1. Collectively, our study identified the role of the UCA1/miR-455/ZEB1 pathway in HUVECs treated with TGF-β1 and indicated the potential therapeutic role of this regulatory axis in angiogenesis.

The BMP Receptor 2 in Pulmonary Arterial Hypertension: When and Where the Animal Model Matches the Patient

Background: Mutations in bone morphogenetic protein receptor type II (BMPR2) are leading to the development of hereditary pulmonary arterial hypertension (PAH). In non-hereditary forms of PAH, perturbations in the transforming growth factor-β (TGF-β)/BMP-axis are believed to cause deficient BMPR2 signaling by changes in receptor expression, the activity of the receptor and/or downstream signaling. To date, BMPR2 expression and its activity in the lungs of patients with non-hereditary PAH is poorly characterized. In recent decades, different animal models have been used to understand the role of BMPR2 signaling in PAH pathophysiology. Specifically, the monocrotaline (MCT) and Sugen-Hypoxia (SuHx) models are extensively used in interventional studies to examine if restoring BMPR2 signaling results in PAH disease reversal. While PAH is assumed to develop in patients over months or years, pulmonary hypertension in experimental animal models develops in days or weeks. It is therefore likely that modifications in BMP and TGF-β signaling in these models do not fully recapitulate those in patients. In order to determine the translational potential of the MCT and SuHx models, we analyzed the BMPR2 expression and activity in the lungs of rats with experimentally induced PAH and compared this to the BMPR2 expression and activity in the lungs of PAH patients. Methods: the BMPR2 expression was analyzed by Western blot analysis and immunofluorescence (IF) microscopy to determine the quantity and localization of the receptor in the lung tissue from normal control subjects and patients with hereditary or idiopathic PAH, as well as in the lungs of control rats and rats with MCT or SuHx-induced PAH. The activation of the BMP pathway was analyzed by determining the level and localization of phosphorylated Smad1/5/8 (pSmad 1/5/8), a downstream mediator of canonical BMPR2 signaling. Results: While BMPR2 and pSmad 1/5/8 expression levels were unaltered in whole lung lysates/homogenates from patients with hereditary and idiopathic PAH, IF analysis showed that BMPR2 and pSmad 1/5/8 levels were markedly decreased in the pulmonary vessels of both PAH patient groups. Whole lung BMPR2 expression was variable in the two PAH rat models, while in both experimental models the expression of BMPR2 in the lung vasculature was increased. However, in the human PAH lungs, the expression of pSmad 1/5/8 was downregulated in the lung vasculature of both experimental models. Conclusion: BMPR2 receptor expression and downstream signaling is reduced in the lung vasculature of patients with idiopathic and hereditary PAH, which cannot be appreciated when using human whole lung lysates. Despite increased BMPR2 expression in the lung vasculature, the MCT and SuHx rat models did develop PAH and impaired downstream BMPR2-Smad signaling similar to our findings in the human lung.

Novel Molecular Mechanisms of Pulmonary Hypertension: A Search for Biomarkers and Novel Drug Targets-From Bench to Bed Site

Pulmonary hypertension (PH) is defined as increased mean pulmonary artery pressure (mPAP) above 25 mmHg, measured at rest by right heart catheterization. The exact global prevalence of PH is difficult to estimate, mainly due to the complex aetiology, and its spread may be underestimated. To date, numerous studies on the aetiology and pathophysiology of PH at molecular level were conducted. Simultaneously, some clinical studies have shown potential usefulness of well-known and widely recognized cardiovascular biomarkers, but their potential clinical usefulness in diagnosis and management of PH is poor due to their low specificity accompanied with numerous other cardiovascular comorbidities of PH subjects. On the other hand, a large body of basic research-based studies provides us with novel molecular pathomechanisms, biomarkers, and drug targets, according to the evidence-based medicine principles. Unfortunately, the simple implementation of these results to clinical practice is impossible due to a large heterogeneity of the PH pathophysiology, where the clinical symptoms constitute only a common denominator and a final result of numerous crosstalking metabolic pathways. Therefore, future studies, based mostly on translational medicine, are needed in order to both organize better the pathophysiological classification of various forms of PH and define precisely the optimal diagnostic markers and therapeutic targets in particular forms of PH. This review paper summarizes the current state of the art regarding the molecular background of PH with respect to its current classification. Novel therapeutic strategies and potential biomarkers are discussed with respect to their limitations in use in common clinical practice.

Transcriptional profiling of lung cell populations in idiopathic pulmonary arterial hypertension

Despite recent improvements in management of idiopathic pulmonary arterial hypertension, mortality remains high. Understanding the alterations in the transcriptome–phenotype of the key lung cells involved could provide insight into the drivers of pathogenesis. In this study, we examined differential gene expression of cell types implicated in idiopathic pulmonary arterial hypertension from lung explants of patients with idiopathic pulmonary arterial hypertension compared to control lungs. After tissue digestion, we analyzed all cells from three idiopathic pulmonary arterial hypertension and six control lungs using droplet-based single cell RNA-sequencing. After dimensional reduction by t-stochastic neighbor embedding, we compared the transcriptomes of endothelial cells, pericyte/smooth muscle cells, fibroblasts, and macrophage clusters, examining differential gene expression and pathways implicated by analysis of Gene Ontology Enrichment. We found that endothelial cells and pericyte/smooth muscle cells had the most differentially expressed gene profile compared to other cell types. Top differentially upregulated genes in endothelial cells included novel genes: ROBO4, APCDD1, NDST1, MMRN2, NOTCH4, and DOCK6, as well as previously reported genes: ENG, ORAI2, TFDP1, KDR, AMOTL2, PDGFB, FGFR1, EDN1, and NOTCH1. Several transcription factors were also found to be upregulated in idiopathic pulmonary arterial hypertension endothelial cells including SOX18, STRA13, LYL1, and ELK, which have known roles in regulating endothelial cell phenotype. In particular, SOX18 was implicated through bioinformatics analyses in regulating the idiopathic pulmonary arterial hypertension endothelial cell transcriptome. Furthermore, idiopathic pulmonary arterial hypertension endothelial cells upregulated expression of FAM60A and HDAC7, potentially affecting epigenetic changes in idiopathic pulmonary arterial hypertension endothelial cells. Pericyte/smooth muscle cells expressed genes implicated in regulation of cellular apoptosis and extracellular matrix organization, and several ligands for genes showing increased expression in endothelial cells. In conclusion, our study represents the first detailed look at the transcriptomic landscape across idiopathic pulmonary arterial hypertension lung cells and provides robust insight into alterations that occur in vivo in idiopathic pulmonary arterial hypertension lungs.

S-endoglin expression is induced in hyperoxia and contributes to altered pulmonary angiogenesis in bronchopulmonary dysplasia development

Altered pulmonary angiogenesis contributes to disrupted alveolarization, which is the main characteristic of bronchopulmonary dysplasia (BPD). Transforming growth factor β (TGFβ) plays an important role during lung vascular development, and recent studies have demonstrated that endoglin is engaged in the modulation of TGFβ downstream signalling. Although there are two different isoforms of endoglin, L- and S-endoglin, little is known about the effect of S-endoglin in developing lungs. We analysed the expression of both L- and S-endoglin in the lung vasculature and its contribution to TGFβ-activin-like kinase (ALK)-Smad signalling with respect to BPD development. Hyperoxia impaired pulmonary angiogenesis accompanied by alveolar simplification in neonatal mouse lungs. S-endoglin, phosphorylated Smad2/3 and connective tissue growth factor levels were significantly increased in hyperoxia-exposed mice, while L-endoglin, phosphor-Smad1/5 and platelet-endothelial cell adhesion molecule-1 levels were significantly decreased. Hyperoxia suppressed the tubular growth of human pulmonary microvascular endothelial cells (ECs), and the selective inhibition of ALK5 signalling restored tubular growth. These results indicate that hyperoxia alters the balance in two isoforms of endoglin towards increased S-endoglin and that S-endoglin attenuates TGFβ-ALK1-Smad1/5 signalling but stimulates TGFβ-ALK5-Smad2/3 signalling in pulmonary ECs, which may lead to impaired pulmonary angiogenesis in developing lungs.

Kinases as potential targets for treatment of pulmonary hypertension and right ventricular dysfunction

Pulmonary hypertension (PH) is a progressive pulmonary vasculopathy that causes chronic right ventricular pressure overload and often leads to right ventricular failure. Various kinase inhibitors have been studied in the setting of PH and either improved or worsened the disease, highlighting the importance of understanding the specific role of the respective kinases in a spatiotemporal cellular context. In this review, we will summarize the knowledge on the role of kinases in PH and focus on druggable targets for which certain criteria are met: (a) deregulation of the kinase in PH; (b) small-molecule inhibitors are available (e.g. from the oncology field); (c) preclinical studies have shown their efficacy in PH models; and (d) when available, therapeutic exploitation in human PH has been initiated. Along this line, clinical considerations such as personalized medicine approaches to predict therapy response and adverse side events such as cardiotoxicity together with their clinical management are discussed. LINKED ARTICLES: This article is part of a themed issue on Risk factors, comorbidities, and comedications in cardioprotection. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.1/issuetoc.

Pulmonary Hypertension Remodels the Genomic Fabrics of Major Functional Pathways

Pulmonary hypertension (PH) is a serious disorder with high morbidity and mortality rate. We analyzed the right-ventricular systolic pressure (RVSP), right-ventricular hypertrophy (RVH), lung histology, and transcriptomes of six-week-old male rats with PH induced by (1) hypoxia (HO), (2) administration of monocrotaline (CM), or (3) administration of monocrotaline and exposure to hypoxia (HM). The results in PH rats were compared to those in control rats (CO). After four weeks exposure, increased RVSP and RVH, pulmonary arterial wall thickening, and alteration of the lung transcriptome were observed in all PH groups. The HM group exhibited the largest alterations, as well as neointimal lesions and obliteration of the lumen in small arteries. We found that PH increased the expression of caveolin1, matrix metallopeptidase 2, and numerous inflammatory and cell proliferation genes. The cell cycle, vascular smooth muscle contraction, and oxidative phosphorylation pathways, as well as their interplay, were largely perturbed. Our results also suggest that the upregulated Rhoa (Ras homolog family member A) mediates its action through expression coordination with several ATPases. The upregulation of antioxidant genes and the extensive mitochondrial damage observed, especially in the HM group, indicate metabolic shift toward aerobic glycolysis.

Alginate Oligosaccharide Alleviates Monocrotaline-Induced Pulmonary Hypertension via Anti-Oxidant and Anti-Inflammation Pathways in Rats

Pulmonary arterial hypertension (PAH) is a serious and fatal cardiovascular disorder characterized by increased pulmonary vascular resistance and progressive pulmonary vascular remodeling. The underlying pathological mechanisms of PAH are multi-factorial and multi-cellular. Alginate oligosaccharide (AOS), which is produced by depolymerizing alginate, shows better pharmacological activities and beneficial effects. The present study was undertaken to investigate the effects and potential mechanisms of AOS-mediated alleviation of pulmonary hypertension. Pulmonary hypertension was induced in Sprague-Dawley rats by a single intraperitoneal injection of monocrotaline (MCT; 60 mg/kg). Five weeks after the injection of MCT, AOS (5, 10, and 20 mg·kg^-1·d^-1) was injected intraperitoneally for another three weeks. The results showed that AOS prevented the development of MCT-induced pulmonary hypertension and right ventricular hypertrophy in a dose-dependent manner. AOS treatment also prevented MCT-induced pulmonary vascular remodeling via inhibition of the TGF-β1/p-Smad2 signaling pathway. Furthermore, AOS treatment downregulated the expression of malondialdehyde, nicotinamide adenine dinucleotide phosphate oxidase, and pro-inflammatory cytokines, decreased macrophage infiltration, and upregulated the expression of anti-inflammatory cytokines. These findings indicate that AOS exerts anti-oxidative and anti-inflammatory effects in pulmonary arteries, which may contribute to the alleviation of pulmonary hypertension and pulmonary vascular remodeling.

BMPR2 acts as a gatekeeper to protect endothelial cells from increased TGFβ responses and altered cell mechanics

Balanced transforming growth factor-beta (TGFβ)/bone morphogenetic protein (BMP)-signaling is essential for tissue formation and homeostasis. While gain in TGFβ signaling is often found in diseases, the underlying cellular mechanisms remain poorly defined. Here we show that the receptor BMP type 2 (BMPR2) serves as a central gatekeeper of this balance, highlighted by its deregulation in diseases such as pulmonary arterial hypertension (PAH). We show that BMPR2 deficiency in endothelial cells (ECs) does not abolish pan-BMP-SMAD1/5 responses but instead favors the formation of mixed-heteromeric receptor complexes comprising BMPR1/TGFβR1/TGFβR2 that enable enhanced cellular responses toward TGFβ. These include canonical TGFβ-SMAD2/3 and lateral TGFβ-SMAD1/5 signaling as well as formation of mixed SMAD complexes. Moreover, BMPR2-deficient cells express genes indicative of altered biophysical properties, including up-regulation of extracellular matrix (ECM) proteins such as fibrillin-1 (FBN1) and of integrins. As such, we identified accumulation of ectopic FBN1 fibers remodeled with fibronectin (FN) in junctions of BMPR2-deficient ECs. Ectopic FBN1 deposits were also found in proximity to contractile intimal cells in pulmonary artery lesions of BMPR2-deficient heritable PAH (HPAH) patients. In BMPR2-deficient cells, we show that ectopic FBN1 is accompanied by active β1-integrin highly abundant in integrin-linked kinase (ILK) mechano-complexes at cell junctions. Increased integrin-dependent adhesion, spreading, and actomyosin-dependent contractility facilitates the retrieval of active TGFβ from its latent fibrillin-bound depots. We propose that loss of BMPR2 favors endothelial-to-mesenchymal transition (EndMT) allowing cells of myo-fibroblastic character to create a vicious feed-forward process leading to hyperactivated TGFβ signaling. In summary, our findings highlight a crucial role for BMPR2 as a gatekeeper of endothelial homeostasis protecting cells from increased TGFβ responses and integrin-mediated mechano-transduction.

The Biological Bases of Group 2 Pulmonary Hypertension

Pulmonary hypertension (PH) is a potentially fatal condition with a prevalence of around 1% in the world population and most commonly caused by left heart disease (PH-LHD). Usually, in PH-LHD, the increase of pulmonary pressure is only conditioned by the retrograde transmission of the left atrial pressure. However, in some cases, the long-term retrograde pressure overload may trigger complex and irreversible biomechanical and biological changes in the pulmonary vasculature. This latter clinical entity, designated as combined pre- and post-capillary PH, is associated with very poor outcomes. The underlying mechanisms of this progression are poorly understood, and most of the current knowledge comes from the field of Group 1-PAH. Treatment is also an unsolved issue in patients with PH-LHD. Targeting the molecular pathways that regulate pulmonary hemodynamics and vascular remodeling has provided excellent results in other forms of PH but has a neutral or detrimental result in patients with PH-LHD. Therefore, a deep and comprehensive biological characterization of PH-LHD is essential to improve the diagnostic and prognostic evaluation of patients and, eventually, identify new therapeutic targets. Ongoing research is aimed at identify candidate genes, variants, non-coding RNAs, and other biomarkers with potential diagnostic and therapeutic implications. In this review, we discuss the state-of-the-art cellular, molecular, genetic, and epigenetic mechanisms potentially involved in PH-LHD. Signaling and effective pathways are particularly emphasized, as well as the current knowledge on -omic biomarkers. Our final aim is to provide readers with the biological foundations on which to ground both clinical and pre-clinical research in the field of PH-LHD.

Activated Endothelial TGFβ1 Signaling Promotes Venous Thrombus Nonresolution in Mice Via Endothelin-1: Potential Role for Chronic Thromboembolic Pulmonary Hypertension

RATIONALE

Chronic thromboembolic pulmonary hypertension (CTEPH) is characterized by defective thrombus resolution, pulmonary artery obstruction, and vasculopathy. TGFβ (transforming growth factor-β) signaling mutations have been implicated in pulmonary arterial hypertension, whereas the role of TGFβ in the pathophysiology of CTEPH is unknown.

OBJECTIVE

To determine whether defective TGFβ signaling in endothelial cells contributes to thrombus nonresolution and fibrosis.

METHODS AND RESULTS

Venous thrombosis was induced by inferior vena cava ligation in mice with genetic deletion of TGFβ1 in platelets (Plt.TGFβ-KO) or TGFβ type II receptors in endothelial cells (End.TGFβRII-KO). Pulmonary endarterectomy specimens from CTEPH patients were analyzed using immunohistochemistry. Primary human and mouse endothelial cells were studied using confocal microscopy, quantitative polymerase chain reaction, and Western blot. Absence of TGFβ1 in platelets did not alter platelet number or function but was associated with faster venous thrombus resolution, whereas endothelial TGFβRII deletion resulted in larger, more fibrotic and higher vascularized venous thrombi. Increased circulating active TGFβ1 levels, endothelial TGFβRI/ALK1 (activin receptor-like kinase), and TGFβRI/ALK5 expression were detected in End.TGFβRII-KO mice, and activated TGFβ signaling was present in vessel-rich areas of CTEPH specimens. CTEPH-endothelial cells and murine endothelial cells lacking TGFβRII simultaneously expressed endothelial and mesenchymal markers and transcription factors regulating endothelial-to-mesenchymal transition, similar to TGFβ1-stimulated endothelial cells. Mechanistically, increased endothelin-1 levels were detected in TGFβRII-KO endothelial cells, murine venous thrombi, or endarterectomy specimens and plasma of CTEPH patients, and endothelin-1 overexpression was prevented by inhibition of ALK5, and to a lesser extent of ALK1. ALK5 inhibition and endothelin receptor antagonization inhibited mesenchymal lineage conversion in TGFβ1-exposed human and murine endothelial cells and improved venous thrombus resolution and pulmonary vaso-occlusions in End.TGFβRII-KO mice.

CONCLUSIONS

Endothelial TGFβ1 signaling via type I receptors and endothelin-1 contribute to mesenchymal lineage transition and thrombofibrosis, which were prevented by blocking endothelin receptors. Our findings may have relevant implications for the prevention and management of CTEPH.

Histological hallmarks and role of Slug/PIP axis in pulmonary hypertension secondary to pulmonary fibrosis

Pulmonary hypertension secondary to pulmonary fibrosis (PF-PH) is one of the most common causes of PH, and there is no approved therapy. The molecular signature of PF-PH and underlying mechanism of why pulmonary hypertension (PH) develops in PF patients remains understudied and poorly understood. We observed significantly increased vascular wall thickness in both fibrotic and non-fibrotic areas of PF-PH patient lungs compared to PF patients. The increased vascular wall thickness in PF-PH patients is concomitant with a significantly increased expression of the transcription factor Slug within the macrophages and its target prolactin-induced protein (PIP), an extracellular matrix protein that induces pulmonary arterial smooth muscle cell proliferation. We developed a novel translational rat model of combined PF-PH that is reproducible and shares similar histological features (fibrosis, pulmonary vascular remodeling) and molecular features (Slug and PIP upregulation) with human PF-PH. We found Slug inhibition decreases PH severity in our animal model of PF-PH. Our study highlights the role of Slug/PIP axis in PF-PH.

Effect of Transcriptional Regulator ID3 on Pulmonary Arterial Hypertension and Hereditary Hemorrhagic Telangiectasia

Pulmonary arterial hypertension (PAH) can be discovered in patients who have a loss of function mutation of activin A receptor-like type 1 (ACVRL1) gene, a bone morphogenetic protein (BMP) type 1 receptor. Additionally, ACVRL1 mutations can lead to hereditary hemorrhagic telangiectasia (HHT), also known as Rendu-Osler-Weber disease, an autosomal dominant inherited disease that results in mucocutaneous telangiectasia and arteriovenous malformations (AVMs). Transcriptional regulator Inhibitor of DNA-Binding/Differentiation-3 (ID3) has been demonstrated to be involved in both PAH and HTT; however, the role of its overlapping molecular mechanistic effects has yet to be seen. This review will focus on the existing understanding of how ID3 may contribute to molecular involvement and perturbations thus altering both PAH and HHT outcomes. Improved understanding of how ID3 mediates these pathways will likely provide knowledge in the inhibition and regulation of these diseases through targeted therapies.

Downregulation of miR‑98 contributes to hypoxic pulmonary hypertension by targeting ALK1

Chronic hypoxia is one of the most common causes of secondary pulmonary hypertension, the mechanisms of which remain unclear. MicroRNAs (miRNAs) are small, noncoding RNAs that inhibit the translation or accelerate the degradation of mRNA. Previous studies have demonstrated that deregulated miRNA expression contributes to various cellular processes including cell apoptosis and proliferation, which are mediated by hypoxia. In the present study, the expression of miR‑98 was identified to be decreased in the lung tissue of a hypoxic pulmonary hypertension (HPH) rat model and pulmonary artery (PA) smooth muscle cells (PASMCs), which was induced by hypoxia. By transfecting miR‑98 mimics into PASMCs, the high expression of miR‑98 inhibited cell proliferation, but upregulated hypoxia‑induced PASMCs apoptosis. However, these effects of miR‑98 mimics on PASMCs were reversed by ALK1 (activin receptor‑like kinase‑1) overexpression. ALK1 was identified as a candidate target of miR‑98. In addition, overexpressing miR‑98 markedly decreased the pulmonary artery wall thickness and the right ventricular systolic pressure in rats induced by hypoxia. These results provided clear evidence that miR‑98 was a direct regulator of ALK1, and that the downregulation of miR‑98 contributed to the pathogenesis of HPH. These results provide a novel potential therapeutic strategy for the treatment of HPH.

Pulmonary hypertension secondary to congenital diaphragmatic hernia: factors and pathways involved in pulmonary vascular remodeling

Congenital diaphragmatic hernia (CDH) is a severe birth defect that is characterized by pulmonary hypoplasia and pulmonary hypertension (PHTN). PHTN secondary to CDH is a result of vascular remodeling, a structural alteration in the pulmonary vessel wall that occurs in the fetus. Factors involved in vascular remodeling have been reported in several studies, but their interactions remain unclear. To help understand PHTN pathophysiology and design novel preventative and treatment strategies, we have conducted a systematic review of the literature and comprehensively analyzed all factors and pathways involved in the pathogenesis of pulmonary vascular remodeling secondary to CDH in the nitrofen model. Moreover, we have linked the dysregulated factors with pathways involved in human CDH. Of the 358 full-text articles screened, 75 studies reported factors that play a critical role in vascular remodeling secondary to CDH. Overall, the impairment of epithelial homeostasis present in pulmonary hypoplasia results in altered signaling to endothelial cells, leading to endothelial dysfunction. This causes an impairment of the crosstalk between endothelial cells and pulmonary artery smooth muscle cells, resulting in increased smooth muscle cell proliferation, resistance to apoptosis, and vasoconstriction, which clinically translate into PHTN.

Diagnosis and Pathophysiological Mechanisms of Group 3 Hypoxia-Induced Pulmonary Hypertension

PURPOSE OF REVIEW

Group 3 hypoxia-induced pulmonary hypertension (PH) is an important and increasingly diagnosed condition in both the pediatric and adult population. The majority of pulmonary hypertension studies to date and all three classes of drug therapies were designed to focus on group 1 PH. There is a clear unmet medical need for understanding the molecular mechanisms of group 3 PH and a need for novel non-invasive methods of assessing PH in neonates.

RECENT FINDINGS

Several growth factors are expressed in patients and in animal models of group 3 PH and are thought to contribute to the pathophysiology of this disease. Here, we review some of the findings on the roles of vascular endothelial growth factor A (VEGFA), platelet-derived growth factor B (PDGFB), transforming growth factor-beta (TGFB1), and fibroblast growth factors (FGF) in PH. Additionally, we discuss novel uses of echocardiographic parameters in assessing right ventricular form and function. FGF2, TGFB, PDGFB, and VEGFA may serve as biomarkers in group 3 PH along with echocardiographic methods to diagnose and follow right ventricle function. FGFs and VEGFs may also function in the pathophysiology of group 3 PH.