Discovery of a murine model of clinical PAH: Mission impossible?

Egln1Tie2Cre Mice Exhibit Similar Therapeutic Responses to Sildenafil, Ambrisentan, and Treprostinil as Pulmonary Arterial Hypertension (PAH) Patients, Supporting Egln1Tie2Cre Mice as a Useful PAH Model

Pulmonary arterial hypertension (PAH) is a progressive and inevitably fatal disease characterized by the progressive increase of pulmonary vascular resistance and obliterative pulmonary vascular remodeling, which lead to right-sided heart failure and premature death. Many of the genetically modified mouse models do not develop severe PH and occlusive vascular remodeling. Egln1^Tie2Cre mice with Tie2Cre-mediated deletion of Egln1, which encodes hypoxia-inducible factor (HIF) prolyl hydroxylase 2 (PHD2), is the only mouse model with severe PAH, progressive occlusive pulmonary vascular remodeling, and right-sided heart failure leading to 50-80% mortality from the age of 3-6 months, indicating that the Egln1^Tie2Cre mice model is a long-sought-after murine PAH model. However, it is unknown if Egln1^Tie2Cre mice respond to FDA-approved PAH drugs in a way similar to PAH patients. Here, we tested the therapeutic effects of the three vasodilators: sildenafil (targeting nitric oxide signaling), ambrisentan (endothelin receptor antagonist), and treprostinil (prostacyclin analog) on Egln1^Tie2Cre mice. All of them attenuated right ventricular systolic pressure (RVSP) in Egln1^Tie2Cre mice consistent with their role as vasodilators. However, these drugs have no beneficial effects on pulmonary arterial function. Cardiac output was also markedly improved in Egln1^Tie2Cre mice by any of the drug treatments. They only partially improved RV function and reduced RV hypertrophy and pulmonary vascular remodeling as well as improving short-term survival in a drug-dependent manner. These data demonstrate that Egln1^Tie2Cre mice exhibit similar responses to these drugs as PAH patients seen in clinical trials. Thus, our study provides further evidence that the Egln1^Tie2Cre mouse model of severe PAH is an ideal model of PAH and is potentially useful for enabling identification of drug targets and preclinical testing of novel PAH drug candidates.

Fatty Acid Metabolism in Endothelial Cell

The endothelium is a monolayer of cells lining the inner blood vessels. Endothelial cells (ECs) play indispensable roles in angiogenesis, homeostasis, and immune response under normal physiological conditions, and their dysfunction is closely associated with pathologies such as cardiovascular diseases. Abnormal EC metabolism, especially dysfunctional fatty acid (FA) metabolism, contributes to the development of many diseases including pulmonary hypertension (PH). In this review, we focus on discussing the latest advances in FA metabolism in ECs under normal and pathological conditions with an emphasis on PH. We also highlight areas of research that warrant further investigation.

Endothelial PHD2 deficiency induces nitrative stress via suppression of caveolin-1 in pulmonary hypertension

BACKGROUND

Nitrative stress is a characteristic feature of the pathology of human pulmonary arterial hypertension. However, the role of nitrative stress in the pathogenesis of obliterative vascular remodelling and severe pulmonary arterial hypertension remains largely unclear.

METHOD

Our recently identified novel mouse model (Egln1Tie2Cre, Egln1 encoding prolyl hydroxylase 2 (PHD2)) has obliterative vascular remodelling and right heart failure, making it an excellent model to use in this study to examine the role of nitrative stress in obliterative vascular remodelling.

RESULTS

Nitrative stress was markedly elevated whereas endothelial caveolin-1 (Cav1) expression was suppressed in the lungs of Egln1Tie2Cre mice. Treatment with a superoxide dismutase mimetic, manganese (III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride or endothelial Nos3 knockdown using endothelial cell-targeted nanoparticle delivery of CRISPR-Cas9/guide RNA plasmid DNA inhibited obliterative pulmonary vascular remodelling and attenuated severe pulmonary hypertension in Egln1Tie2Cre mice. Genetic restoration of Cav1 expression in Egln1Tie2Cre mice normalised nitrative stress, reduced pulmonary hypertension and improved right heart function.

CONCLUSION

These data suggest that suppression of Cav1 expression secondary to PHD2 deficiency augments nitrative stress through endothelial nitric oxide synthase activation, which contributes to obliterative vascular remodelling and severe pulmonary hypertension. Thus, a reactive oxygen/nitrogen species scavenger might have therapeutic potential for the inhibition of obliterative vascular remodelling and severe pulmonary arterial hypertension.

Suppression of BMP signaling by PHD2 deficiency in Pulmonary Arterial hypertension

BMP signaling deficiency is evident in the lungs of patients with pulmonary arterial hypertension. We demonstrated that PHD2 deficiency suppresses BMP signaling in the lung endothelial cells, suggesting the novel mechanisms of dysregulated BMP signaling in the development of pulmonary arterial hypertension.

Single-Cell RNA Sequencing Reveals Novel Genes Regulated by Hypoxia in the Lung Vasculature

Pulmonary arterial hypertension (PAH) is a chronic progressive disease with significant morbidity and mortality. The disease is characterized by vascular remodeling that includes increased muscularization of distal blood vessels and vessel stiffening associated with changes in extracellular matrix deposition. In humans, chronic hypoxia causes PAH, and hypoxia-induced rodent models of PAH have been used for years to study the disease. With the development of single-cell RNA sequencing technology, it is now possible to examine hypoxia-dependent transcriptional changes in vivo at a cell-specific level. In this study, we used single-cell RNA sequencing to compare lungs from wild-type (Wt) mice exposed to hypoxia for 28 days to normoxia-treated control mice. We additionally examined mice deficient for Notch3, a smooth muscle-enriched gene linked to PAH. Data analysis revealed that hypoxia promoted cell number changes in immune and endothelial cell types in the lung, activated the innate immunity pathway, and resulted in specific changes in gene expression in vascular cells. Surprisingly, we found limited differences in lungs from mice deficient for Notch3 compared to Wt controls. These findings provide novel insight into the effects of chronic hypoxia exposure on gene expression and cell phenotypes in vivo and identify unique changes to cells of the vasculature.

Endothelial Autocrine Signaling through CXCL12/CXCR4/FoxM1 Axis Contributes to Severe Pulmonary Arterial Hypertension

Endothelial autocrine signaling is essential to maintain vascular homeostasis. There is limited information about the role of endothelial autocrine signaling in regulating severe pulmonary vascular remodeling during the onset of pulmonary arterial hypertension (PAH). In this study, we employed the first severe pulmonary hypertension (PH) mouse model, Egln1Tie2Cre (Tie2Cre-mediated disruption of Egln1) mice, to identify the novel autocrine signaling mediating the pulmonary vascular endothelial cell (PVEC) proliferation and the pathogenesis of PAH. PVECs isolated from Egln1Tie2Cre lung expressed upregulation of many growth factors or angiocrine factors such as CXCL12, and exhibited pro-proliferative phenotype coincident with the upregulation of proliferation-specific transcriptional factor FoxM1. Treatment of CXCL12 on PVECs increased FoxM1 expression, which was blocked by CXCL12 receptor CXCR4 antagonist AMD3100 in cultured human PVECs. The endothelial specific deletion of Cxcl12(Egln1/Cxcl12Tie2Cre) or AMD3100 treatment in Egln1Tie2Cre mice downregulated FoxM1 expression in vivo. We then generated and characterized a novel mouse model with endothelial specific FoxM1 deletion in Egln1Tie2Cre mice (Egln1/Foxm1Tie2Cre), and found that endothelial FoxM1 deletion reduced pulmonary vascular remodeling and right ventricular systolic pressure. Together, our study identified a novel mechanism of endothelial autocrine signaling in regulating PVEC proliferation and pulmonary vascular remodeling in PAH.

The Challenge to Decide between Pulmonary Hypertension Due to Chronic Lung Disease and PAH with Chronic Lung Disease

Chronic lung diseases are strongly associated with pulmonary hypertension (PH), and even mildly elevated pulmonary arterial pressures are associated with increased mortality. Chronic obstructive pulmonary disease (COPD) is the most common chronic lung disease, but few of these patients develop severe PH. Not all these pulmonary pressure elevations are due to COPD, although patients with severe PH due to COPD may represent the largest subgroup within patients with COPD and severe PH. There are also patients with left heart disease (group 2), chronic thromboembolic disease (group 4, CTEPH) and pulmonary arterial hypertension (group 1, PAH) who suffer from COPD or another chronic lung disease as co-morbidity. Because therapeutic consequences very much depend on the cause of pulmonary hypertension, it is important to complete the diagnostic procedures and to decide on the main cause of PH before any decision on PAH drugs is made. The World Symposia on Pulmonary Hypertension (WSPH) have provided guidance for these important decisions. Group 2 PH or complex developmental diseases with elevated postcapillary pressures are relatively easy to identify by means of elevated pulmonary arterial wedge pressures. Group 4 PH can be identified or excluded by perfusion lung scans in combination with chest CT. Group 1 PAH and Group 3 PH, although having quite different disease profiles, may be difficult to discern sometimes. The sixth WSPH suggests that severe pulmonary hypertension in combination with mild impairment in the pulmonary function test (FEV1 > 60 and FVC > 60%), mild parenchymal abnormalities in the high-resolution CT of the chest, and circulatory limitation in the cardiopulmonary exercise test speak in favor of Group 1 PAH. These patients are candidates for PAH therapy. If the patient suffers from group 3 PH, the only possible indication for PAH therapy is severe pulmonary hypertension (mPAP ≥ 35 mmHg or mPAP between 25 and 35 mmHg together with very low cardiac index (CI) < 2.0 L/min/m²), which can only be derived invasively. Right heart catheter investigation has been established nearly 100 years ago, but there are many important details to consider when reading pulmonary pressures in spontaneously breathing patients with severe lung disease. It is important that such diagnostic procedures and the therapeutic decisions are made in expert centers for both pulmonary hypertension and chronic lung disease.

Tumor Necrosis Factor Induces Obliterative Pulmonary Vascular Disease in a Novel Model of Connective Tissue Disease-Associated Pulmonary Arterial Hypertension

OBJECTIVE

Connective tissue disease (CTD)-associated pulmonary arterial hypertension (PAH) is the second most common etiology of PAH and carries a poor prognosis. Recently, it has been shown that female human tumor necrosis factor (TNF)-transgenic (Tg) mice die of cardiopulmonary disease by 6 months of age. This study was undertaken to characterize this pathophysiology and assess its potential as a novel model of CTD-PAH.

METHODS

Histologic analysis was performed on TNF-Tg and wild-type (WT) mice to characterize pulmonary vascular and right ventricular (RV) pathology (n = 40 [4-5 mice per group per time point]). Mice underwent right-sided heart catheterization (n = 29) and micro-computed tomographic angiography (n = 8) to assess vascular disease. Bone marrow chimeric mice (n = 12), and anti-TNF-treated mice versus placebo-treated mice (n = 12), were assessed. RNA sequencing was performed on mouse lung tissue (n = 6).

RESULTS

TNF-Tg mice displayed a pulmonary vasculopathy marked by collagen deposition (P < 0.001) and vascular occlusion (P < 0.001) with associated RV hypertrophy (P < 0.001) and severely increased RV systolic pressure (mean ± SD 75.1 ± 19.3 mm Hg versus 26.7 ± 1.7 mm Hg in WT animals; P < 0.0001). TNF-Tg mice had increased α-smooth muscle actin (α-SMA) staining, which corresponded to proliferation and loss of von Willebrand factor (vWF)-positive endothelial cells (P < 0.01). There was an increase in α-SMA-positive, vWF-positive cells (P < 0.01), implicating endothelial-mesenchymal transition. Bone marrow chimera experiments revealed that mesenchymal but not bone marrow-derived cells are necessary to drive this process. Treatment with anti-TNF therapy halted the progression of disease. This pathology closely mimics human CTD-PAH, in which patient lungs demonstrate increased TNF signaling and significant similarities in genomic pathway dysregulation.

CONCLUSION

The TNF-Tg mouse represents a novel model of CTD-PAH, recapitulates key disease features, and can serve as a valuable tool for discovery and assessment of therapeutics.

Therapeutic Targeting of Vascular Remodeling and Right Heart Failure in Pulmonary Arterial Hypertension with a HIF-2α Inhibitor

RATIONALE

Pulmonary arterial hypertension (PAH) is a devastating disease characterized by progressive vasoconstriction and obliterative vascular remodeling that leads to right heart failure (RHF) and death. Current therapies do not target vascular remodeling and RHF, and result in only modest improvement of morbidity and mortality.

OBJECTIVES

To determine whether targeting HIF-2α (hypoxia-inducible factor-2α) with a HIF-2α-selective inhibitor could reverse PAH and RHF in various rodent PAH models.

METHODS

HIF-2α and its downstream genes were evaluated in lung samples and pulmonary arterial endothelial cells and smooth muscle cells from patients with idiopathic PAH as well as various rodent PAH models. A HIF-2α-selective inhibitor was used in human lung microvascular endothelial cells and in Egln1Tie2Cre mice, and in Sugen 5416/hypoxia- or monocrotaline-exposed rats.

MEASUREMENTS AND MAIN RESULTS

Upregulation of HIF-2α and its target genes was observed in lung tissues and isolated pulmonary arterial endothelial cells from patients with idiopathic PAH and three distinct rodent PAH models. Pharmacological inhibition of HIF-2α by the HIF-2α translation inhibitor C76 (compound 76) reduced right ventricular systolic pressure and right ventricular hypertrophy and inhibited RHF and fibrosis as well as obliterative pulmonary vascular remodeling in Egln1Tie2Cre mice and Sugen 5416/hypoxia PAH rats. Treatment of monocrotaline-exposed PAH rats with C76 also reversed right ventricular systolic pressure, right ventricular hypertrophy, and pulmonary vascular remodeling; prevented RHF; and promoted survival.

CONCLUSIONS

These findings demonstrate that pharmacological inhibition of HIF-2α is a promising novel therapeutic strategy for the treatment of severe vascular remodeling and right heart failure in patients with PAH.

Sex differences in pulmonary arterial hypertension: role of infection and autoimmunity in the pathogenesis of disease

Registry data worldwide indicate an overall female predominance for pulmonary arterial hypertension (PAH) of 2–4 over men. Genetic predisposition accounts for only 1–5% of PAH cases, while autoimmune diseases and infections are closely linked to PAH. Idiopathic PAH may include patients with undiagnosed autoimmune diseases based on the relatively high presence of autoantibodies in this group. The two largest PAH registries to date report a sex ratio for autoimmune connective tissue disease-associated PAH of 9:1 female to male, highlighting the need for future studies to analyze subgroup data according to sex. Autoimmune diseases that have been associated with PAH include female-dominant systemic sclerosis, systemic lupus erythematosus, rheumatoid arthritis, Sjögren’s syndrome, and thyroiditis as well as male-dominant autoimmune diseases like myocarditis which has been linked to HIV-associated PAH. The sex-specific association of PAH to certain infections and autoimmune diseases suggests that sex hormones and inflammation may play an important role in driving the pathogenesis of disease. However, there is a paucity of data on sex differences in inflammation in PAH, and more research is needed to better understand the pathogenesis underlying PAH in men and women. This review uses data on sex differences in PAH and PAH-associated autoimmune diseases from registries to provide insight into the pathogenesis of disease.