Megha Talati

Cytoskeletal defects in Bmpr2-associated pulmonary arterial hypertension

The heritable form of pulmonary arterial hypertension (PAH) is typically caused by a mutation in bone morphogenic protein receptor type 2 (BMPR2), and mice expressing Bmpr2 mutations develop PAH with features similar to human disease. BMPR2 is known to interact with the cytoskeleton, and human array studies in PAH patients confirm alterations in cytoskeletal pathways. The goal of this study was to evaluate cytoskeletal defects in BMPR2-associated PAH. Expression arrays on our Bmpr2 mutant mouse lungs revealed cytoskeletal defects as a prominent molecular consequence of universal expression of a Bmpr2 mutation (Rosa26-Bmpr2(R899X)). Pulmonary microvascular endothelial cells cultured from these mice have histological and functional cytoskeletal defects. Stable transfection of different BMPR2 mutations into pulmonary microvascular endothelial cells revealed that cytoskeletal defects are common to multiple BMPR2 mutations and are associated with activation of the Rho GTPase, Rac1. Rac1 defects are corrected in cell culture and in vivo through administration of exogenous recombinant human angiotensin-converting enzyme 2 (rhACE2). rhACE2 reverses 77% of gene expression changes in Rosa26-Bmpr2(R899X) transgenic mice, in particular, correcting defects in cytoskeletal function. Administration of rhACE2 to Rosa26-Bmpr2(R899X) mice with established PAH normalizes pulmonary pressures. Together, these findings suggest that cytoskeletal function is central to the development of BMPR2-associated PAH and that intervention against cytoskeletal defects may reverse established disease.

Evidence for Right Ventricular Lipotoxicity in Heritable Pulmonary Arterial Hypertension

RATIONALE Shorter survival in heritable pulmonary arterial hypertension (HPAH), often due to BMPR2 mutation, has been described in association with impaired right ventricle (RV) compensation. HPAH animal models are insulin resistant, and cells with BMPR2 mutation have impaired fatty acid oxidation, but whether these findings affect the RV in HPAH is unknown. OBJECTIVES To test the hypothesis that BMPR2 mutation impairs RV hypertrophic responses in association with lipid deposition. METHODS RV hypertrophy was assessed in two models of mutant Bmpr2 expression, smooth muscle-specific (Sm22(R899X)) and universal expression (Rosa26(R899X)). Littermate control mice underwent the same stress using pulmonary artery banding (Low-PAB). Lipid content was assessed in rodent and human HPAH RVs and in Rosa26(R899X) mice after metformin administration. RV microarrays were performed using human HPAH and control subjects. RESULTS RV/(left ventricle + septum) did not rise directly in proportion to RV systolic pressure in Rosa26(R899X) but did in Sm22(R899X) (P < 0.05). Rosa26(R899X) RVs demonstrated intracardiomyocyte triglyceride deposition not present in Low-PAB (P < 0.05). RV lipid deposition was identified in human HPAH RVs but not in controls. Microarray analysis demonstrated defects in fatty acid oxidation in human HPAH RVs. Metformin in Rosa26(R899X) mice resulted in reduced RV lipid deposition. CONCLUSIONS These data demonstrate that Bmpr2 mutation affects RV stress responses in a transgenic rodent model. Impaired RV hypertrophy and triglyceride and ceramide deposition are present as a function of RV mutant Bmpr2 in mice; fatty acid oxidation impairment in human HPAH RVs may underlie this finding. Further study of how BMPR2 mediates RV lipotoxicity is warranted.

Oxidative injury is a common consequence of BMPR2 mutations

Hereditary pulmonary arterial hypertension (PAH) is usually caused by mutations in BMPR2. Mutations are found throughout the gene, and common molecular consequences of different types of mutation are not known. Knowledge of common molecular consequences would provide insight into the molecular etiology of the disease. The objective of this study was to determine the common molecular consequences across classes of BMPR2 mutation. Increased superoxide and peroxide production and alterations in genes associated with oxidative stress were a common consequence of stable transfection of the vascular smooth muscle cells, with three distinct classes of BMPR2 mutation, in the ligand binding domain, the kinase domain and the cytoplasmic tail domain. Measurement of oxidized lipids in whole lung from transgenic mice expressing a mutation in the BMPR2 cytoplasmic tail showed a 50% increase in isoprostanes and a two-fold increase in isofurans, suggesting increased reactive oxygen species (ROS) of mitochondrial origin. Immunohistochemistry on BMPR2 transgenic mouse lung showed that oxidative stress was vascular-specific. Electron microscopy showed decreased mitochondrial size and variability in the pulmonary vessels from BMPR2-mutant mice. Measurement of oxidized lipids in urine from humans with BMPR2 mutations demonstrated increased ROS, regardless of disease status. Immunohistochemistry on hereditary PAH patient lung confirmed oxidative stress specific to the vasculature. Increased oxidative stress, likely of mitochondrial origin, is a common consequence of BMPR2 mutation across mutation types in cell culture, mice and humans.

Estrogen Metabolite 16α-Hydroxyestrone Exacerbates Bone Morphogenetic Protein Receptor Type II–Associated Pulmonary Arterial Hypertension Through MicroRNA-29–Mediated Modulation of Cellular Metabolism

Background— Pulmonary arterial hypertension (PAH) is a proliferative disease of the pulmonary vasculature that preferentially affects women. Estrogens such as the metabolite 16&agr;-hydroxyestrone (16&agr;OHE) may contribute to PAH pathogenesis, and alterations in cellular energy metabolism associate with PAH. We hypothesized that 16&agr;OHE promotes heritable PAH (HPAH) via microRNA-29 (miR-29) family upregulation and that antagonism of miR-29 would attenuate pulmonary hypertension in transgenic mouse models of Bmpr2 mutation. Methods and Results— MicroRNA array profiling of human lung tissue found elevation of microRNAs associated with energy metabolism, including the miR-29 family, among HPAH patients. miR-29 expression was 2-fold higher in Bmpr2 mutant mice lungs at baseline compared with controls and 4 to 8-fold higher in Bmpr2 mice exposed to 16&agr;OHE 1.25 &mgr;g/h for 4 weeks. Blot analyses of Bmpr2 mouse lung protein showed significant reductions in peroxisome proliferator–activated receptor-&ggr; and CD36 in those mice exposed to 16&agr;OHE and protein derived from HPAH lungs compared with controls. Bmpr2 mice treated with anti–miR-29 (20-mg/kg injections for 6 weeks) had improvements in hemodynamic profile, histology, and markers of dysregulated energy metabolism compared with controls. Pulmonary artery smooth muscle cells derived from Bmpr2 murine lungs demonstrated mitochondrial abnormalities, which improved with anti–miR-29 transfection in vitro; endothelial-like cells derived from HPAH patient induced pluripotent stem cell lines were similar and improved with anti–miR-29 treatment. Conclusions— 16&agr;OHE promotes the development of HPAH via upregulation of miR-29, which alters molecular and functional indexes of energy metabolism. Antagonism of miR-29 improves in vivo and in vitro features of HPAH and reveals a possible novel therapeutic target.

Fatty Acid Metabolic Defects and Right Ventricular Lipotoxicity in Human Pulmonary Arterial Hypertension

Background— The mechanisms of right ventricular (RV) failure in pulmonary arterial hypertension (PAH) are poorly understood. Abnormalities in fatty acid (FA) metabolism have been described in experimental models of PAH, but systemic and myocardial FA metabolism has not been studied in human PAH. Methods and Results— We used human blood, RV tissue, and noninvasive imaging to characterize multiple steps in the FA metabolic pathway in PAH subjects and controls. Circulating free FAs and long-chain acylcarnitines were elevated in PAH patients versus controls. Human RV long-chain FAs were increased and long-chain acylcarnitines were markedly reduced in PAH versus controls. With the use of proton magnetic resonance spectroscopy, in vivo myocardial triglyceride content was elevated in human PAH versus controls (1.4±1.3% triglyceride versus 0.22±0.11% triglyceride, P=0.02). Ceramide, a mediator of lipotoxicity, was increased in PAH RVs versus controls. Using an animal model of heritable PAH, we demonstrated reduced FA oxidation via failure of palmitoylcarnitine to stimulate oxygen consumption in the PAH RV. Conclusions— Abnormalities in FA metabolism can be detected in the blood and myocardium in human PAH and are associated with in vivo cardiac steatosis and lipotoxicity. Murine data suggest that lipotoxicity may arise from reduction in FA oxidation.

ABCG2pos lung mesenchymal stem cells are a novel pericyte subpopulation that contributes to fibrotic remodeling

Genesis of myofibroblasts is obligatory for the development of pathology in many adult lung diseases. Adult lung tissue contains a population of perivascular ABCG2(pos) mesenchymal stem cells (MSC) that are precursors of myofibroblasts and distinct from NG2 pericytes. We hypothesized that these MSC participate in deleterious remodeling associated with pulmonary fibrosis (PF) and associated hypertension (PH). To test this hypothesis, resident lung MSC were quantified in lung samples from control subjects and PF patients. ABCG2(pos) cell numbers were decreased in human PF and interstitial lung disease compared with control samples. Genetic labeling of lung MSC in mice enabled determination of terminal lineage and localization of ABCG2 cells following intratracheal administration of bleomycin to elicit fibrotic lung injury. Fourteen days following bleomycin injury enhanced green fluorescent protein (eGFP)-labeled lung MSC-derived cells were increased in number and localized to interstitial areas of fibrotic and microvessel remodeling. Finally, gene expression analysis was evaluated to define the response of MSC to bleomycin injury in vivo using ABCG2(pos) MSC isolated during the inflammatory phase postinjury and in vitro bleomycin or transforming growth factor-β1 (TGF-β1)-treated cells. MSC responded to bleomycin treatment in vivo with a profibrotic gene program that was not recapitulated in vitro with bleomycin treatment. However, TGF-β1 treatment induced the appearance of a profibrotic myofibroblast phenotype in vitro. Additionally, when exposed to the profibrotic stimulus, TGF-β1, ABCG2, and NG2 pericytes demonstrated distinct responses. Our data highlight ABCG2(pos) lung MSC as a novel cell population that contributes to detrimental myofibroblast-mediated remodeling during PF.

Endothelial HIF signaling regulates pulmonary fibrosis-associated pulmonary hypertension

Pulmonary hypertension (PH) complicating chronic parenchymal lung disease, such as idiopathic pulmonary fibrosis, results in significant morbidity and mortality. Since the hypoxia-inducible factor (HIF) signaling pathway is important for development of pulmonary hypertension in chronic hypoxia, we investigated whether HIF signaling in vascular endothelium regulates development of PH related to pulmonary fibrosis. We generated a transgenic model in which HIF is deleted within vascular endothelial cells and then exposed these mice to chronic intraperitoneal bleomycin to induce PH associated with lung fibrosis. Although no differences in the degree of fibrotic remodeling were observed, we found that endothelial HIF-deficient mice were protected against development of PH, including right ventricle and pulmonary vessel remodeling. Similarly, endothelial HIF-deficient mice were protected from PH after a 4-wk exposure to normobaric hypoxia. In vitro studies of pulmonary vascular endothelial cells isolated from the HIF-targeted mice and controls revealed that endothelial HIF signaling increases endothelial cell expression of connective tissue growth factor, enhances vascular permeability, and promotes pulmonary artery smooth muscle cell proliferation and wound healing ability, all of which have the potential to impact the development of PH in vivo. Taken together, these studies demonstrate that vascular endothelial cell HIF signaling is necessary for development of hypoxia and pulmonary fibrosis associated PH. As such, HIF and HIF-regulated targets represent a therapeutic target in these conditions.

Fatty acid metabolism in pulmonary arterial hypertension: role in right ventricular dysfunction and hypertrophy

Pulmonary arterial hypertension (PAH) is a complex, multifactorial disease in which an increase in pulmonary vascular resistance leads to increased afterload on the right ventricle (RV), causing right heart failure and death. Our understanding of the pathophysiology of RV dysfunction in PAH is limited but is constantly improving. Increasing evidence suggests that in PAH RV dysfunction is associated with various components of metabolic syndrome, such as insulin resistance, hyperglycemia, and dyslipidemia. The relationship between RV dysfunction and fatty acid/glucose metabolites is multifaceted, and in PAH it is characterized by a shift in utilization of energy sources toward increased glucose utilization and reduced fatty acid consumption. RV dysfunction may be caused by maladaptive fatty acid metabolism resulting from an increase in fatty acid uptake by fatty acid transporter molecule CD36 and an imbalance between glucose and fatty acid oxidation in mitochondria. This leads to lipid accumulation in the form of triglycerides, diacylglycerol, and ceramides in the cytoplasm, hallmarks of lipotoxicity. Current interventions in animal models focus on improving RV dysfunction through altering fatty acid oxidation rates and limiting lipid accumulation, but more specific and effective therapies may be available in the coming years based on current research. In conclusion, a deeper understanding of the complex mechanisms of the metabolic remodeling of the RV will aid in the development of targeted treatments for RV failure in PAH.

Bone Marrow–derived Cells Contribute to the Pathogenesis of Pulmonary Arterial Hypertension

RATIONALE Pulmonary arterial hypertension (PAH) is a progressive lung disease of the pulmonary microvasculature. Studies suggest that bone marrow (BM)-derived circulating cells may play an important role in its pathogenesis. OBJECTIVES We used a genetic model of PAH, the Bmpr2 mutant mouse, to study the role of BM-derived circulating cells in its pathogenesis. METHODS Recipient mice, either Bmpr2(R899X) mutant or controls, were lethally irradiated and transplanted with either control or Bmpr2(R899X) BM cells. Donor cells were traced in female recipient mice by Y chromosome painting. Molecular and function insights were provided by expression and cytokine arrays combined with flow cytometry, colony-forming assays, and competitive transplant assays. MEASUREMENTS AND MAIN RESULTS We found that mutant BM cells caused PAH with remodeling and inflammation when transplanted into control mice, whereas control BM cells had a protective effect against the development of disease, when transplanted into mutant mice. Donor BM-derived cells were present in the lungs of recipient mice. Functional and molecular analysis identified mutant BM cell dysfunction suggestive of a PAH phenotype soon after activation of the transgene and long before the development of lung pathology. CONCLUSIONS Our data show that BM cells played a key role in PAH pathogenesis and that the transplanted BM cells were able to drive the lung phenotype in a myeloablative transplant model. Furthermore, the specific cell types involved were derived from hematopoietic stem cells and exhibit dysfunction long before the development of lung pathology.

Idiopathic and heritable PAH perturb common molecular pathways, correlated with increased MSX1 expression

The majority of pulmonary arterial hypertension (PAH) is not associated with BMPR2 mutation, and major risk factors for idiopathic PAH are not known. The objective of this study was to identify a gene expression signature for IPAH. To accomplish this, we used Affymetrix arrays to probe expression levels in 86 patient samples, including 22 healthy controls, 20 IPAH patients, 20 heritable PAH patients (HPAH), and 24 BMPR2 mutation carriers that were as yet unaffected (UMC). Culturing the patient cells removes the signatures of drug effects and inflammation which have made interpretation of results from freshly isolated lymphocytes problematic. We found that gene expression signatures from IPAH patients clustered either with HPAH patients or in a single distinct group. There were no groups of genes changed in IPAH that were not also changed in HPAH. HPAH, IPAH, and UMC had common changes in metabolism, actin dynamics, adhesion, cytokines, metabolism, channels, differentiation, and transcription factors. Common to IPAH and HPAH but not UMC were an upregulation of vesicle trafficking, oxidative/nitrosative stress, and cell cycle genes. The transcription factor MSX1, which is known to regulate BMP signaling, was the most upregulated gene (4∗) in IPAH patients. These results suggest that IPAH cases have a shared molecular origin, which is closely related to, but distinct from, HPAH. HPAH and IPAH share the majority of altered signaling pathways, suggesting that treatments developed to target the molecular etiology of HPAH will also be effective against IPAH.