Arterial ischemic stroke occurs as frequently in term babies as in the elderly but immaturity affects mechanisms of ischemic injury and recovery. The neonatal brain is preferentially susceptible to hypoxic-ischemic, pure ischemic and excitotoxic injury with widespread neuronal apoptosis. Compared to the extensive knowledge of the molecular events regulating apoptosis, relatively little is known about the processes responsible for clearance of apoptotic neurons and degradation of cellular debris. Failure to rapidly remove apoptotic neurons by macrophages allows dying cells to undergo post-apoptotic necrosis and leads to injury exacerbation. Microglial cells can protect the brain by removing dying cells but can also contribute to injury or even provoke neuronal apoptosis during normal postnatal brain development. In an animal model of neonatal stroke, focal transient middle cerebral artery (MCA) occlusion in immature rodents, microglial activation is rapid, these cells produce a number of toxic species but poorly phagocytose apoptotic neurons. Using our in vivo model and in vitro models of primary microglial cells cultured with apoptotic neurons, we will determine the effects of the scavenger receptor CD36 in mediating removal of apoptotic neurons after neonatal stroke and evaluate signaling mechanisms involved in CD36-mediated recognition, engulfment and phagocytosis of apoptotic neurons by microglial cells. Pharmacological approaches and mice with deleted CD36 and caspase-3 and cells derived from deficient mice will be used. We will test the hypothesis that microglial cells exacerbate acute ischemia-reperfusion injury to the neonatal brain by insufficient clearance of apoptotic neurons. We will determine if ablation of microglia protects the neonatal brain from stroke (Aim 1), if limited phagocytosis of apoptotic neurons by microglial cells after neonatal stroke is modulated by the scavenger receptor CD36 (Aim 2), if inflammatory cytokines adversely affect CD36-medated phagocytosis of neurons dying in a caspase-3 dependent manner (Aim 3), and if removal of apoptotic neurons via CD36 depends on caspase-3 activation (Aim 4).
TO PUBLIC HEALTH: Neonatal stroke is an existing serious and frequent disorder. Understanding the mechanisms of brain injury in the neonatal brain is an important step in the identification of therapeutic targets that protect the newborn from the lifelong consequences of stroke.
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