Pulmonary arterial hypertension (PAH) is lethal syndrome characterized by obstruction of the pulmonary vasculature due, in part, to excessive cell proliferation. We recently discovered that this hyperproliferative phenotype is associated with fragmentation of the mitochondrial network in pulmonary arterial smooth muscle cells (PASMC). Preliminary data indicate that the fragmentation results from an imbalance in mitochondrial fusion and fission, favoring fission. In human and rodent PAH PASMC there is decreased expression of mitofusin-2, a mitochondrial GTPase that mediates fusion, and increased expression of the activated form of dynamin-related protein (DRP-1), a mitochondrial GTPase that mediates fission. Expression of other fusion/fission mediators is minimally perturbed. Increasing mitochondrial fusion, by augmenting mitofusin-2 (using adenoviral gene transfer), or inhibiting DRP-1 (using a small module inhibitor, mdivi-1, or DRP-1 siRNA), yields concordant antiproliferative results. Preliminary data suggest that whereas a decreased fusion/fission ratio in PAH favors proliferation, forced fusion inhibits the mitotic fission requird for cell division and arests cels in G2-M phase. We speculate that mitotic fission is an unrecognized mitotic checkpoint which if violated (by forced fusion) prevents or slows mitosis. Hypothesis: A decreased mitochondrial fusion/fission ratio promotes proliferation of human PAH vascular cells. Corollary: Fusing the mitochondrial network prevents mitotic fission, causing antiproliferative G2-M phase arrest. Definition of the molecular basis for mitochondrial fragmentation in PAH vascular cells and exploration of its role as a therapeutic target has been hampered by the scarcity of human cells and tissues. The 2-year study uses tissues/cells from 15 WHO category 1 PAH patients vs 15 normal subjects/year to determine the molecular basis for the imbalance of mitochondrial fission and fusion in human PAH. We also assess whether therapies targeting mitochondrial fission and fusion can correct the proliferation diathesis in human PAH. Fission and fusion rates in PASMC and endothelial cells are quantified by 2-photon confocal microscopy, using mitochondrial-targeted, photoactivated green fluorescent protein. The effects of enhancing mitofusin-2 or inhibiting DRP-1 on cell cycle progression and proliferation are quantified by flow cytometry. A complete profile of fission and fusion mediators is measured in cells and lungs by immunoblot or immunofluorescence, while histologic compartmentalization of mitochondrial abnormalities in the lung is assessed by laser capture microdissection. The value of mitofusin-2 and DRP-1 in the blood as potential biomarkers of PAH is assessed. Patient mitochondrial fission/fusion abnormalities are correlated with their demographics and hemodynamics to assess their prognostic importance. Innovation and Impact: The discovery that impaired mitochondrial fusion and enhanced fission contributes to a proliferative diathesis in PAH is novel. Therapeuticaly, the observation that mitofusin-2 augmentation or DRP-1 inhibition induces G2-M arrest suggests a new antiproliferative strategy. Access to more human samples from the PHBI will catalyze our efforts to devise new mitochondrial-targeted, antiproliferative PAH therapies.