Halving BMPR2 levels blunts BMP9/10 signaling in lung artery endothelial cells

Pulmonary arterial hypertension (PAH) is a progressive, life-limiting disease in which small lung arteries narrow and stiffen, eventually causing right heart failure. Loss-of-function mutations in BMPR2, which encodes a type II BMP receptor, underlie most heritable cases and a sizable minority of idiopathic cases. BMPR2 levels are also reduced in some patients without mutations. Yet it has been unclear how a quantitative drop in BMPR2 reshapes endothelial responses to its natural ligands. This matters because BMP9/10 manipulation has produced opposite results in different models, and the Activin-trapping drug sotatercept is clinically beneficial.

The team worked with primary human pulmonary artery endothelial cells (PAECs) from two commercial sources, used at passages 4–8. They modulated BMPR2 expression up and down and compared a panel of TGF-β family ligands (including BMP7, BMP9, BMP10, TGF-β1, GDF11, Activin A and Activin B). Readouts were SMAD1/5/8 signaling (reporter assays and related measures), proliferation by BrdU incorporation, and caspase-3/7 activity by Caspase-Glo. The caspase assay was explicitly treated as a marker of stress-associated caspase engagement, not proof of apoptosis. They also used the Activin type II receptor–blocking antibody bimagrumab, receptor-Fc traps, and the small-molecule BMP inhibitor LDN-193189.

Of the ligands tested, only BMP9 and BMP10 produced robust SMAD1/5/8 activation and proliferation in PAECs under these conditions. A moderate (~50%) reduction in BMPR2 protein weakened BMP9/10-induced SMAD signaling, abolished the proliferative response, and was associated with a modest rise in caspase-3/7 activity, consistent with early stress or injury signaling. When BMPR2 was limiting, BMP9/10 responses became sensitive to bimagrumab, suggesting Activin type II receptors, such as ACVR2A, contribute in this context. Overexpressing BMPR2 enhanced BMP9/10-dependent SMAD signaling and proliferation.

The authors propose a receptor-dosage model in which normal BMPR2 abundance is needed to sustain homeostatic BMP9/10 signaling in the pulmonary endothelium. This may help reconcile the "Janus" effects of BMP9/10 in PAH models, where the same pathway can be protective or harmful depending on receptor context. It may also relate to why Activin-pathway blockade by sotatercept helps some patients, though the paper does not test this directly.

Caveats: the work is in vitro, using cultured primary cells and a ~50% BMPR2 reduction that only approximates haploinsufficiency. Caspase-3/7 activity does not establish cell death, and the model has not been tested in animals or patient-derived cells. The full text supplied was truncated after the methods, so the detailed results, statistics, and discussion could not be reviewed. This summary rests on the abstract, introduction, and methods.