Yes / Pulmonary Arterial Hypertension (PAH) is a devastating cardiovascular disorder characterised by the remodelling of pre-capillary pulmonary arteries. The vascular remodelling observed in PAH patients results from excessive proliferation and apoptosis resistance of pulmonary arterial smooth muscle (PASMCs) and endothelial cells (PAECs). We have previously demonstrated that mutations in the type II receptor for bone morphogenetic protein (BMPRII) underlie the majority of the familial and inherited forms of the disease. We have further demonstrated that BMPRII deficiency promotes excessive proliferation and attenuates apoptosis in PASMCs, but the underlying mechanisms remain unclear. The major objective of this study is to investigate how BMPRII deficiency impairs apoptosis in PAH. Using multidisciplinary approaches, we demonstrate that deficiency in the expression of BMPRII impairs apoptosis by modulating the alternative splicing of the apoptotic regulator, Bcl-x (B-cell lymphoma X) transcripts: a finding observed in circulating leukocytes and lungs of PAH subjects, hypoxia-induced PAH rat lungs as well as in PASMCs and PAECs. BMPRII deficiency elicits cell specific effects: promoting the expression of Bcl-xL transcripts in PASMCs whilst inhibiting it in ECs, thus exerting differential apoptotic effects in these cells. The pro-survival effect of BMPRII receptor is mediated through the activin receptor like kinase 1 (ALK1) but not the ALK3 receptor. Finally, we show that BMPRII interacts with the ALK1 receptor and pathogenic mutations in the BMPR2 gene abolish this interaction. Taken together, dysfunctional BMPRII responsiveness impairs apoptosis via the BMPRII-ALK1-Bcl-xL pathway in PAH. We suggest Bcl-xL as a potential biomarker and druggable target. / This work was supported by a fellowship (awarded to MTN) from the Department of Health via the NIHR Comprehensive Biomedical Research Centre award to Guy’s & St Thomas’ NHS Foundation Trust in partnership with King’s College London, Heptagon Life Science Proof of Concept Fund (grant KCL24 to MTN), the Great Britain Sasakawa Foundation (grant B70 to MTN), the Royal Society (grant 43049 to MTN), the Medical Research Council (grant G900865 to RCT, MTN and NWM) and the University of Bradford (grants 003200, 66006/001NAS and DH005 to MTN). NS and MYB were supported by scholarships from the Commonwealth Scholarship Commission, UK and Scientific and Technological Research Council of Turkey (TUBITAK), respectively.