ABSTRACT ______ The central focus of application is to establish a novel pulmonary hypertension (PH) model in mice by specifically targeting the endothelium, and reveal a novel endothelium-driven signaling network of uncoupled eNOS-ER stress- mitochondrial dysfunction axis in the pathogenesis of PH, targeting of which would result in novel therapeutics for PH. Pulmonary hypertension is a severe human disease characterized by intensive remodeling of small arteries in the lung, resulting in vasoconstriction, elevated vascular resistance and pulmonary arterial pressure, and eventually right heart failure. In preliminary studies, we have generated a novel PH model by directly targeting eNOS to provoke eNOS uncoupling (DAHP to inhibit GTPCHI) and eNOS uncoupling-dependent endothelial dysfunction. Of note, endothelial dysfunction is one of the earliest events and may the initiating step of idiopathic pulmonary artery hypertension (IPAH). Importantly, DAHP-treated mice developed robust PH phenotypes of increased mean pulmonary artery pressure (mPAP) and right ventricular systolic pressure (RVSP), accompanied by extensive vascular remodel characterized by typical human like vascular lesions of medial thickness, neointimal formation, and plexiform features. RNA-sequencing (RNA- seq) data indicated that, comparing to human patients with PH, the DAHP model had more overlappingly and substantially regulated genes vs. the hypoxia model (217 vs. 92). In preliminary studies we have also revealed new molecular mechanisms mediating PH development downstream of uncoupled eNOS, involving ER stress and mitochondrial dysfunction. Additionally, since reversal of eNOS cofactor tetrahydrobiopterin salvage enzyme dihydrofolate reductase (DHFR) deficiency downstream of NADPH oxidase (NOX) activation is robustly effective in preserving eNOS coupling activity, novel genetic strains specifically targeting NOX isoforms and DHFR will be examined for efficacies modulating PH phenotypes (16 novel and unique strains, most of which made in house). In Aim 1 we aim to establish a novel human like murine model of PH by fully characterizing phenotypes of DAHP-treated mice, and by comparing its gene regulation profile to that of human patients with PH and of Sugen5416/Hypoxia (SuHx)-treated mice. Also to further examine roles in PH development of novel candidate genes identified by RNA-seq analyses. In Aim 2, we will examine novel endothelium- driven signaling network of uncoupled eNOS-ER stress-mitochondrial dysfunction axis in the pathogenesis of PH using DAHP, hypoxia and Su/Hx models of PH. In Aim 3, we will examine whether strategies targeting endothelial DHFR, such as endothelium-specific transgenesis of DHFR, or knockout of NOX isoforms specifically in the endothelium to preserve DHFR function, would be of novel therapeutic potential for PH. We will also examine whether global and conditional knockout of DHFR, or endothelium-specific overexpression of NOX isoforms, leads to PH d...