Project Summary: Growing evidence suggests the link between right ventricular (RV) fibrosis, poor function of the pressure-overloaded RV, and mortality in pulmonary arterial hypertension (PAH). PAH patients with decompensated RV failure (RVF) have persistent RV fibrosis even when treated with the conventional therapies for PAH. RVF is the main cause of death in PAH and maintaining RV function in PAH is associated with improved patient survival. However, there are currently no available therapies that specifically target RV fibrosis. Therefore, identifying the molecular mechanisms underlying RV fibrosis in PAH is urgently needed to develop novel therapeutic approaches targeting RVF in PAH. We recently reported the significant roles of o xidative stress-sensitive protein kinase D (PKD) at outer mitochondrial membrane (OMM) and its substrate dynamin-related protein 1 (DRP1), a mitochondrial fission protein, in dysregulating CM functions. We also showed that DRP1-mediated mitochondrial fission limits the size of the matrix cavity, thus causing elevated and sustained mitochondrial Ca2+ (mtCa2+) transient in response to cytosolic Ca2+ elevation. Using a preclinical rat PAH model with RV hypertrophy, failure, and fibrosis that significant , we found PKD activation and DRP1 phosphorylation occurs specifically in cardiac fibroblasts (CFs) in the RV (RV-CFs), but not in CMs under PAH, which subsequently causes an PKD-dependent increase in mitochondrial fission, mitochondrial reactive oxygen species (mROS), and CF proliferation. Moreover, we found that PKD activation is associated with increased phosphorylation of a pro-apoptotic protein Bax, which inhibits apoptotic pore formation in the OMM and potentially contributes to the anti-apoptotic phenotype of RV-CFs in PAH. Lastly, we also found that mtCa2+ uptake via mtCa2+ uniporter (MCU) is required for mROS elevation and subsequent activation of proliferative signaling in CFs. Based on these findings, we hypothesize that 1) PKD-dependent Bax phosphorylation allows RV-CFs to be resistant to apoptosis under PAH; 2) PKD-dependent mitochondrial fission limits mtCa2+ and antioxidant capacity by decreasing the size of the matrix cavity and causing increased mtCa2+ and mROS levels, thus acting as a molecular “switch” for proliferative signaling for RV-CFs in PAH; and 3) CF-specific inhibition of PKD at the OMM in vivo can be leveraged as a novel therapy to attenuate cardiac fibrosis in response to stress/injury such as PAH. In Aim 1, we will establish Bax as a novel PKD substrate in the mitochondria and assess the impact of PKD-dependent Bax phosphorylation on OMM permeability. To specifically inhibit PKD activity only at the OMM, we will use an OMM-targeted dominant-negative PKD1 (mt-PKD- DN) that we have newly validated. In Aim 2, we will test whether PKD-dependent enhancement of mitochondrial fission facilitates RV-CF proliferation via increased mtCa2+ and mROS levels. In Aim 3, we will test the therapeutic pote...