BMPR2 Dosage Gates BMP9/10 Signaling Output in Pulmonary Artery Endothelium

Pulmonary arterial hypertension (PAH) is characterized by dysfunction and remodeling of the pulmonary artery endothelium and smooth muscle. In heritable PAH, heterozygous loss-of-function mutations in the type II Bone Morphogenetic Protein (BMP) receptor gene (BMPR2) are the most common genetic cause. However, the mechanisms by which reduced BMPR2 levels alter endothelial signaling to drive PAH pathogenesis remain incompletely understood. To determine how BMPR2 levels govern signaling output and endothelial functional responses, we modulated BMPR2 expression in human pulmonary artery endothelial cells (PAECs) and assessed ligand-dependent SMAD1/5/8 signaling, proliferation, and caspase-3/7 activity. We found that BMP9 and BMP10 robustly activated SMAD1/5/8 signaling and promoted proliferation in PAECs, whereas the other ligands in this panel did not elicit a comparable signaling or proliferative response under these assay conditions. A moderate (~50%) reduction in BMPR2 protein levels (an in vitro approximation of haploinsufficiency) attenuated BMP9/10-induced SMAD1/5/8 activation, abolished proliferative responses, and was associated with a modest increase in caspase-3/7 activity, consistent with caspase pathway activation and early stress/injury signaling. Under BMPR2-limiting conditions, BMP9/10 responses became sensitive to Activin type II receptor blockade by bimagrumab, consistent with a context-dependent contribution of Activin type II receptors. Conversely, BMPR2 overexpression enhanced BMP9/10-dependent SMAD signaling and proliferation. Together, these findings support a receptor-dosage model where physiological BMPR2 expression is required to sustain homeostatic BMP9/10 signaling in pulmonary artery endothelium. This framework provides a basis for interpreting context-dependent pathway effects in PAH.