Endothelial SMAD4 Deficiency Promotes Pulmonary Hypertension by Impairing Cell Adhesion and Extracellular Matrix Organization

BACKGROUND: Aberrant BMPR2 (bone morphogenetic protein receptor 2) signaling is associated with the pathogenesis of pulmonary hypertension. SMAD4 is an essential downstream effector of BMPR2 signaling, but whether and how it participates in pulmonary hypertension are unclear. METHODS: Globally and vascular cell-specifically inducible knockout mouse models were used to examine the role of SMAD4 deficiency in differential cell compartments in the development of pulmonary hypertension. Single-cell transcriptomic analysis in combination with in vitro cell function measurements was conducted to provide mechanistic insights into pulmonary hypertension pathogenesis. RESULTS: Adult mice with Smad4 global deletion or endothelial cell-specific deletion spontaneously developed pulmonary hypertension manifestations, characterized by elevated right ventricle systolic pressure and excessive muscularization in pulmonary distal vessels, which were accompanied by other cardiovascular abnormalities. By contrast, mice with smooth muscle cell-specific Smad4 deletion had no pulmonary hypertension but rather displayed evident aortic aneurysm and dissection. At the cellular level, SMAD4 deficiency led to impairment of both endothelial cell-cell and cell-matrix adhesions and disorganized ECM (extracellular matrix), resulting in increased vascular leak and weakened endothelium-matrix attachment. Notably, endothelial Itgb1 deletion mimicked the impact of endothelial Smad4 loss on pulmonary hypertension. Finally, enhancing endothelial cell adhesion and ECM assembly by administrating an MMP (matrix metallopeptidase) inhibitor ilomastat could alleviate the pulmonary hypertension manifestations caused by endothelial SMAD4 deficiency. CONCLUSIONS: SMAD4 deficiency in endothelial cells, but not smooth muscle cells, plays a pathogenic role in pulmonary hypertension, via dampening endothelial cell-cell and cell-matrix adhesions and ECM organization.