The Intrinsic Caspase Death Pathway in Stroke Neurodegeneration
Stroke has been a major source of morbidity and mortality for centuries. Eight-five percent of all strokes are ischemic in nature, meaning they are caused by the occlusion of a major cerebral artery. Despite extensive research to develop effective treatments for ischemic stroke, therapeutic options remain limited. Apoptosis (also termed "programmed cell death") is a process by which a stressed or damaged cell commits "suicide". In stroke, runaway apoptosis contributes to stroke neurodegeneration and neurological decline for days to weeks after disease onset. Cysteine-ASPartic proteASEs (caspases) are key mediators of apoptosis that are activated in distinct molecular pathways, but their impact in stroke is poorly defined. Direct evidence for caspase activation in stroke and the functional relevance of this activity has not been previously characterized. For this dissertation, we developed an unbiased technique for in vivo trapping of active caspases in rodent models of ischemic stroke. We isolated active caspase-9 as a principal contributor to ischemic neurodegeneration in rodents (Rattus norvegicus and Mus musculus). Caspase-9 is the initiator caspase for the intrinsic cell death pathway. Intranasal delivery of a novel, cell membrane-penetrating inhibitor for caspase-9 confirmed the pathogenic relevance of caspase-9 activity in stroke. Caspase-9 inhibition provided neurofunctional protection and established caspase-6 as its downstream target. Caspase-6 is an effector caspase and a member of the intrinsic death pathway that has never been implicated in stroke until now. Coincidentally, we discovered that caspase-6 is specifically activated within the axonal compartment. The temporal and spatial pattern of activation demonstrates that neuronal caspase-9 activity induces caspase-6 activation, which mediates axonal loss in the early stages of stroke (+/- 24 hours). We developed a novel inhibitor for caspase-6, based on a catalytically inactive clone, which demonstrated neuroprotective and axoprotective efficacy against ischemia. Collectively, these results assert that selective inhibition of caspase-9 and caspase-6 is an effective translational strategy for stroke. The impact of caspase activity is not restricted to neuronal death, as caspases can exacerbate inflammation and alter glial function. Thus, caspases are logical therapeutic targets for stroke. However, they have never been clinically evaluated due to a paucity of ideal drug candidates. This dissertation outlines fresh insights into the mechanisms of stroke neurodegeneration and offers novel caspase-based therapeutic strategies for clinical evaluation.
Pathology, Molecular biology, Neurosciences
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