Since its association with familial pulmonary arterial hypertension (PAH) in 2000, Bone Morphogenetic Protein Receptor II (BMPR2) and its related signaling pathway have become recognized as a key regulator of pulmonary vascular homeostasis. Herein, we define BMPR2 deficiency as either an inactivation of the receptor, decreased receptor expression, or an impairment of the receptor’s downstream signaling pathway. Although traditionally the phenotypic consequences of BMPR2 deficiency in PAH have been thought to be limited to the pulmonary vasculature, there is evidence that abnormalities in BMPR2 signaling may have consequences in many other organ systems and cellular compartments. Revisiting how BMPR2 functions throughout health and disease in cells and organs beyond the lung vasculature may provide insight into the contribution of these organ systems to PAH pathogenesis as well as the potential systemic manifestation of PAH. Here we review our knowledge of the consequences of BMPR2 deficiency across multiple organ systems.

BACKGROUND: Pathogenic concepts of right ventricular (RV) failure in pulmonary arterial hypertension focus on a critical loss of microvasculature. However, the methods underpinning prior studies did not take into account the 3-dimensional (3D) aspects of cardiac tissue, making accurate quantification difficult. We applied deep-tissue imaging to the pressure-overloaded RV to uncover the 3D properties of the microvascular network and determine whether deficient microvascular adaptation contributes to RV failure. METHODS: Heart sections measuring 250-µm-thick were obtained from mice after pulmonary artery banding (PAB) or debanding PAB surgery and properties of the RV microvascular network were assessed using 3D imaging and quantification. Human heart tissues harvested at the time of transplantation from pulmonary arterial hypertension cases were compared with tissues from control cases with normal RV function. RESULTS: Longitudinal 3D assessment of PAB mouse hearts uncovered complex microvascular remodeling characterized by tortuous, shorter, thicker, highly branched vessels, and overall preserved microvascular density. This remodeling process was reversible in debanding PAB mice in which the RV function recovers over time. The remodeled microvasculature tightly wrapped around the hypertrophied cardiomyocytes to maintain a stable contact surface to cardiomyocytes as an adaptation to RV pressure overload, even in end-stage RV failure. However, microvasculature-cardiomyocyte contact was impaired in areas with interstitial fibrosis where cardiomyocytes displayed signs of hypoxia. Similar to PAB animals, microvascular density in the RV was preserved in patients with end-stage pulmonary arterial hypertension, and microvascular architectural changes appeared to vary by etiology, with patients with pulmonary veno-occlusive disease displaying a lack of microvascular complexity with uniformly short segments. CONCLUSIONS: 3D deep tissue imaging of the failing RV in PAB mice, pulmonary hypertension rats, and patients with pulmonary arterial hypertension reveals complex microvascular changes to preserve the microvascular density and maintain a stable microvascular-cardiomyocyte contact. Our studies provide a novel framework to understand microvascular adaptation in the pressure-overloaded RV that focuses on cell-cell interaction and goes beyond the concept of capillary rarefaction.

Protein tyrosine kinases (PTKs) are essential for eukaryotic signaling. By targeting select PTKs, the group of drugs known as Tyrosine kinase inhibitors (TKIs) have proven to be effective for treating multiple diseases ranging from cancer to pulmonary fibrosis. However, some TKIs also paradoxically lead to the development of adverse conditions such as pulmonary arterial hypertension (PAH) by promoting endothelial cell dysfunction (ECD). We hypothesize that (1) subsets of PTKs may disproportionately modulate signaling pathways critical for endothelial homeostasis, such as BMPR2 signaling, and (2) inhibiting those pro-endothelial PTKs can promote the development of ECD. Herein we use an agnostic high-throughput siRNA screen to investigate how PTKs affect the canonical BMPR2 signaling pathway. Our major finding is that within the Src-family of non-receptor PTKs, the Src-B family promotes canonical BMPR2 signaling while the Src-A family suppresses it. We focus on two representative members of each family, Lck (for Src-B) and Fyn (for Src-A) that are the strongest activators or inhibitors of BMPR2 signaling in the screen. We confirm that Lck is expressed in the endothelium of pulmonary arteries and show that Lck knockout (termed si-Lck) in pulmonary artery endothelial cells (PAECs) suppresses canonical BMPR2 signaling while Fyn knockout (termed si-Fyn) promotes canonical BMPR2 signaling. Furthermore, Lck and Fyn are responsible for opposing functional behaviors in PAECs: si-Lck promotes apoptosis and interferes with tube formation while si-Fyn suppresses apoptosis and promotes tube formation. After analyzing the whole-transcriptome signature of si-Lck and si-Fyn PAECs we find that in addition to BMPR2 signaling suppression, si-Lck (and not si-Fyn) increases a broad number of ECD markers and increases canonical NF-κβ signaling. In summary, for the first time we show that Src-A and B Family of PTKs exert differential control over key endothelial cell signaling pathways resulting in direct phenotypic consequences. This knowledge may help to guide the design of more precise TKIs which avoid adverse drug reactions brought about through endothelial cell dysfunction.

Developmental programs that orchestrate cell fate and tissue architecture during organogenesis can cause disease when reactivated in adults. Here we identify a population of endothelial cells (ECs) defined by the pioneer factor early B cell factor 1 (EBF1) that controls both pulmonary artery (PA) morphogenesis and pulmonary arterial hypertension (PAH). During embryonic development, the PA emerges from an endothelial niche harbored within the vascular plexus, where Aplnr ⁺ endothelial progenitors give rise to both arterial ECs and EBF1 ⁺ ECs. Rather than incorporating directly into the PA intima, EBF1 ⁺ ECs control the branching and maturation of the PA tree through paracrine vasculotrophic signals that direct plexus expansion, arterialization, and mural cell recruitment. Although essential for development, most EBF1 ⁺ ECs disappear upon completion of PA morphogenesis, leaving only a rare residual population within the adult Aplnr ⁺ gCap compartment. In PAH, vascular injury reactivates the EBF1 program through recruitment of the pre-existing Ebf1 lineage together with an additional contribution consistent with de novo Ebf1 induction in gCaps, reconstituting this artery-forming niche in a maladaptive reprise of PA development. Unlike their transient embryonic counterparts, these EBF1 ⁺ ECs persist within occlusive intimal lesions and express vasculotrophic signals associated with pathological PA remodeling. Combined with endothelial injury, gCap-specific Ebf1 overexpression drives a coordinated program of neo-arterialization, occlusive vascular remodeling, and severe PAH. Conversely, gCap-specific genetic deletion of Ebf1 in mice and endothelial-targeted AAV-mediated Ebf1 knockdown in rats protect against disease, showing that endothelial EBF1 is required for PAH pathogenesis. These findings demonstrate that reactivation and persistence of a transient embryonic artery-forming niche in adulthood drive pathological vascular remodeling and PAH.

Rationale: While therapeutic advances have improved survival in Group 1 Pulmonary Arterial Hypertension (PAH), patients with Group 1.5 “PAH with features of venous/capillary involvement” (formerly Pulmonary Veno-Occlusive Disease or Pulmonary Capillary Hemangiomatosis, now collectively termed PVOD/PCH) remain underrecognized, develop serious complications from usual PAH therapy titrations, and suffer high mortality awaiting necessary lung transplant. Identifying PVOD/PCH early before therapy initiation could aid management and expedite transplant referral. Objective: We aimed to develop a likelihood score distinguishing PVOD/PCH from other forms of PAH using clinical variables. Methods: Due to low PVOD/PCH prevalence and no dedicated registries, we leveraged published case control/series studies to assess the ability of several clinical variables to discriminate PVOD/PCH from PAH. From pooled literature-derived data, we performed Sensitivity/Specificity and simulation-based Receiver Operator Characteristic (ROC) analyses to estimate variable performance. Top-performing variables formed a PVOD/PCH Likelihood score, which had its accuracy tested for distinguishing histopathology-confirmed PVOD/PCH cases (n = 37) from PAH controls (n = 60) in three cohorts of transplant-eligible patients from the United States, Spain, and the Netherlands. Measurements and Main Results: DLCO, six-minute walk desaturation, PaO2, sex, smoking history, CT septal line thickening, and CT lymphadenopathy had the highest sensitivity and specificity performance and were incorporated into the PVOD score. Across test cohorts, the score achieved a ROC area under the curve of 0.97 (95% CI 0.93-1.00) for discriminating PVOD/PCH and it retained accuracy when data were missing. Conclusions: This score could facilitate early PVOD/PCH identification in incident PAH, potentially helping expedite transplant referral and informing therapy initiation/titration decisions.

Tweetable abstract Generally underappreciated, the lung is a site of tremendous metabolism. Transpulmonary profiles obtained in the cardiac cath lab have the potential to aid in biological discovery, inform tailored pharmacotherapy, and monitor for treatment response.https://bit.ly/3Lv0fkl