PROJECT ABSTRACT/SUMMARY Paracrine Action of BMP3 in Pulmonary Hypertension Pulmonary arterial hypertension (PAH) is a severe vascular disease characterized by persistent precapillary pulmonary hypertension, leading to right heart failure and death. Despite intense research in the last decades PH remains an incurable disease with a high morbidity and mortality. New directions and therapies to improve understanding and treatment of PAH are desperately needed. Although evidence demonstrates the importance of the Bone Morphogenetic Protein (BMP) signaling pathway in PAH, our current understanding of the exact pathophysiological role of several members of this pathway remains limited. In this R01 we propose detailed functional and mechanistic studies to understand the role of BMP3 in PAH. We hypothesize that PASMC-derived BMP3 inhibits PAEC dysfunction and pathological pulmonary vascular remodeling. Based on robust preliminary studies, we propose testing this central hypothesis with 3 specific aims. Aim 1 will define BMP3 regulation in PAH and its effects in vitro. We will assess the levels of pulmonary BMP3 levels and circulating BMP3 levels in lung and serum of patients with clinical PAH and of PAH-diseased animals. We will also determine whether sex affects pulmonary BMP3 expression in humans and rodents, and we will investigate the effect of PASMC-derived BMP3 on PAEC function in cocultures of PASMCs and PAECs isolated from non-PAH and PAH patients. Aim 2 will characterize the in vivo effects of cell-specific BMP3 deletion and of BMP3 upregulation. We will provide a detailed characterization of the cardiopulmonary phenotypes of SMC- and EC-specific BMP3 KO mice. We will also examine the effects of exogenous recombinant BMP3 on PAH in the Sugen/Hypoxia model in rats. Using a novel approach, which we have demonstrated to be highly specific and efficient for protein overexpression in the lung (i.e. chemically modified mRNAs (modRNAs)), we will assess whether BMP3 modRNA reverses PAH in the Sugen/Hypoxia model in rats. Aim 3 will dissect the molecular mechanisms of BMP3. We will perform a series of in vitro and in vivo studies to determine the mechanism(s) of BMP3 regulation and to define BMP3 mechanism(s) of action. Promoter activity assays, co-immunoprecipitation experiments, GST pull down assays, quantitative PCR measurements and western blot analyses will determine the pathways implicated in BMP3 signals. Cocultures of PASMCs and PAECs and lungs from BMP3-KO mice and from rats overexpressing BMP3 in vivo will determine the pathways implicated in BMP3 signals.