Abstract P1075: Elucidate The Mechanism Driving The Imbalance Between BMPs And Activin/Gdfs Signaling In Pulmonary Arterial Hypertension

Abstract

Transforming Growth Factor-beta (TGF-β) family signaling is involved in Pulmonary Arterial Hypertension (PAH) in humans. PAH is a rare, highly morbid disorder characterized by dysregulated pulmonary vascular remodeling, elevated pulmonary vascular resistance, and the progressive obliteration of the pulmonary arteriolar circulation. The progression of PAH is largely influenced by the proliferation and migration of the pulmonary smooth muscle cells (PASMCs) and pulmonary endothelial cells (PAECs). While the exact mechanism is not understood, there is evidence that germline mutations in the BMPRII gene, one of the TGF-β family type II receptors, is present in PAH patients. Due to the functional similarity of BMPRII and type II Activin receptors ACVR2A and ACVR2B, they interact with overlapping TGF-β family ligands to activate either the SMAD 1/5/8 or SMAD 2/3 signaling. In the pulmonary vasculature, the BMP-9 ligand regulates cell proliferation by binding to type I receptors, ALK-1 and BMPRII, to activate SMAD 1/5/8 signaling. In PAH, there is a decrease in SMAD 1/5/8 signaling and an increase in SMAD 2/3 signaling. However, what causes the signaling switch is still unknown. In our lab, we found out that BMPRII and ACVR2A bind TGF-β ligands with nearly identical geometry based on our crystal structures. Both of the receptors contain a conserved hydrophobic hot spot within the ligand binding pocket. We found that the BMPRII ligand binding domain may exhibit greater flexibility than ACVR2A. We also investigated eight soluble BMPRII extracellular domain mutants and expressed them as BMPRII-Fc fusion proteins (BMPRII-Fc variants). Using surface plasmon resonance, we observed four of the BMPRII-Fc variants bind to high affinity ligands, such as BMP9, BMP10, Activin B and Activin A. In reporter gene assays, we found these BMPRII-Fc variants act as a ligand trap and attenuate BMP9 and BMP10-mediated SMAD1/5/8 signaling in HEK293 cells. Interestingly, they did not inhibit Activin B-mediated SMAD2/3 signaling. In conclusion, our findings show gain of function of specific BMPRII mutations and suggest that imbalances between SMAD1/5/8 and SMAD2/3 signaling may lead to PAH progression.