Rationale Pulmonary arterial hypertension (PAH) is a lethal syndrome characterized by pulmonary vascular obstruction due in part to pulmonary artery smooth muscle cell (PASMC) hyperproliferation. Mitochondrial fragmentation and normoxic activation of hypoxia-inducible factor-1α (HIF-1α) have been observed in PAH PASMCs, however their relationship and relevance to the development of PAH is unknown. Dynamin-related protein-1 (DRP1) is a GTPase that, when activated by kinases that phosphorylate Serine-616, causes mitochondrial fission. It is however unknown whether mitochondrial fission is a prerequisite for proliferation. Objective We hypothesize that DRP1 activation is responsible for increased mitochondrial fission in PAH PASMCs and that DRP1 inhibition may slow proliferation and have therapeutic potential. Methods and Results Experiments were conducted using human control and PAH lungs (n=5) and PASMCs in culture. Parallel experiments were performed in rat lung sections and PASMCs and in rodent PAH models induced by the HIF-1α activator, cobalt, chronic hypoxia, and monocrotaline. HIF-1α activation in human PAH leads to mitochondrial fission by cyclin B1/CDK1-dependent phosphorylation of DRP1 at Serine-616. In normal PASMC, HIF-1α activation by CoCl2 or desferrioxamine causes DRP1-mediated fission. HIF-1α inhibition reduces DRP1 activation, prevents fission and reduces PASMC proliferation. Both the DRP1 inhibitor Mdivi-1 and siDRP1 prevent mitotic fission and arrest PAH PASMCs at the G2/M interphase. Mdivi-1 is antiproliferative in human PAH PASMC and in rodent models. Mdivi-1 improves exercise capacity, right ventricular function and hemodynamics in experimental PAH. Conclusion DRP-1-mediated mitotic fission is a cell cycle checkpoint that can be therapeutically targeted in hyperproliferative disorders such as PAH.

Background Right ventricular hypertrophy (RVH) and RV failure contribute to morbidity and mortality in pulmonary arterial hypertension (PAH). The cause of RV dysfunction and the feasibility of therapeutically targeting the RV are uncertain. We hypothesized that RV dysfunction and electrical remodeling in RVH result, in part, from a glycolytic-shift in the myocyte, caused by activation of pyruvate dehydrogenase kinase (PDK). Methods and Results We studied 2 complementary rat models: RVH+PAH (induced by monocrotaline) and RVH+without PAH (induced by pulmonary artery banding, PAB). Monocrotaline-RVH reduced RV O2-consumption and enhanced glycolysis. RV 2-fluoro-2-deoxy-glucose uptake, Glut-1 expression and pyruvate dehydrogenase phosphorylation increased in monocrotaline-RVH. The RV monophasic action potential duration and QTc-interval were prolonged due to decreased expression of repolarizing voltage-gated K+ channels (Kv1.5, Kv4.2). In the RV working-heart model, the PDK inhibitor, dichloroacetate, acutely increased glucose oxidation and cardiac work in monocrotaline-RVH. Chronic dichloroacetate therapy improved RV repolarization and RV function in vivo and in the RV Langendorff model. In PAB-induced RVH, a similar reduction in cardiac output and glycolytic shift occurred and it too improved with dichloroacetate. In PAB-RVH the benefit of dichloroacetate on cardiac output was ~1/3 that in monocrotaline-RVH. The larger effects in monocrotaline-RVH likely reflect dichloroacetate’s dual metabolic benefits in that model: regression of vascular disease and direct effects on the RV. Conclusion Reduction in RV function and electrical remodeling in 2 models of RVH relevant to human disease (PAH and pulmonic stenosis) result, in part, from a PDK-mediated glycolytic shift in the RV. PDK inhibition partially restores RV function and regresses RVH by restoring RV repolarization and enhancing glucose oxidation. Recognition that a PDK-mediated metabolic shift contributes to contractile and ionic dysfunction in RVH offers insight into the pathophysiology and treatment of RVH.

Mitochondrial fission, regulated by dynamin-related protein-1 (Drp1), is a newly recognized determinant of mitochondrial function, but its contribution to left ventricular (LV) impairment following ischemia-reperfusion (IR) injury is unknown. We report that Drp1 activation during IR results in LV dysfunction and that Drp1 inhibition is beneficial. In both isolated neonatal murine cardiomyocytes and adult rat hearts (Langendorff preparation) mitochondrial fragmentation and swelling occurred within 30 min of IR. Drp1-S637 (serine 637) dephosphorylation resulted in Drp1 mitochondrial translocation and increased mitochondrial fission. The Drp1 inhibitor Mdivi-1 preserved mitochondrial morphology, reduced cytosolic calcium, and prevented cell death. Drp1 siRNA similarly preserved mitochondrial morphology. In Langendorff hearts, Mdivi-1 reduced mitochondrial reactive oxygen species, improved LV developed pressure (92±5 vs. 28±10 mmHg, P<0.001), and lowered LV end diastolic pressure (10±1 vs. 86±13 mmHg, P<0.001) following IR. Mdivi-1 was protective if administered prior to or following ischemia. Because Drp1-S637 dephosphorylation is calcineurin sensitive, we assessed the effects of a calcineurin inhibitor, FK506. FK506 treatment prior to IR prevented Drp1-S637 dephosphorylation and preserved cardiac function. Likewise, therapeutic hypothermia (30°C) inhibited Drp1-S637 dephosphorylation and preserved mitochondrial morphology and myocardial function. Drp1 inhibition is a novel strategy to improve myocardial function following IR.