A stroke occurs when the blood supply to a portion of the brain is disrupted, depriving brain tissue of oxygen and glucose, and commonly causing cell death. The sole US FDA-approved treatment for ischemic stroke (87 % of all cases), tissue-type plasminogen activator (tPA), dissolves the clot, allowing for reperfusion of the ischemic tissue. However, reperfusion can exacerbate damage to the blood-brain barrier and prompt an abnormal accumulation of fluid in the brain, called edema. Cerebral edema is caused by a loss of vascular integrity of small blood vessels, and is the primary cause of mortality within the firs three days following a stroke. A better understanding of the molecular mechanisms underlying the development of ischemic-induced edema is required for optimal therapeutic treatment. The Troy laboratory has shown that active caspase-9 plays a critical role in the formation of cerebral edema and in neuronal injury. Reversibly inhibiting caspase-9 with a novel, cell-permeant caspase-9 inhibitor provides neuroprotection out to three weeks and abolishes edema in mice. This work proposes a novel mechanism for the development of ischemic induced edema and will utilize in vitro and in vivo approaches to determine caspase-9's role in edema formation. Aim 1 aspires to elucidate the edema-related substrates of caspase-9 to determine the mechanistic details of this medically significant mechanism. Aim 2 seeks to dissect the cell-specific contributions of caspase-9 activation during stroke in endothelial cells and neurons using cell-specific deletion of caspase-9 in mice to identify the location of this mechanism and the order of progression of stroke pathogenesis. This edema-specific mechanism may be broadly applicable to edema in other central nervous system disorders, such as traumatic brain injury and macular edema, and outside the central nervous system in pulmonary edema and angioedema. The intranasal efficacy of this caspase-9 inhibitor affords the potential for developing this therapy a an intervention in human stroke. Columbia University provides an exceptional scientific environment to carry out this research, with high caliber faculty, colleagues, and resources. The training program will provide exposure to seminars, classes in grantsmanship, mentoring opportunities, and interactions with multiple research groups at Columbia and at other universities.