Astrocytes and Ischemic Brain Injury

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One of the most feared disabilities in survivors of cardiac arrest is neurological impairment. Global cerebral schemia, as seen with cardiac arrest, causes delayed loss of CA1 pyramidal neurons in the hippocampus while the nearby dentate gyrus (DG) is relatively resistant. Despite much work on hippocampal neuronal njury, little is known of the effects of ischemia on hippocampal astrocytes. This grant seeks to understand the contribution of astrocytes to hippocampal selective vulnerability. Astrocytes are essential to many brain functions including synaptic transmission, metabolic and ionic homeostasis, antioxidant defense, inflammation, the blood brain barrier, and trophic support. This proposal will study three aspects of hippocampal astrocyte survival and response to ischemia and ischemia-like stress. In aim 1, mitochondrial function will be assessed with ischemia and reperfusion. Mitochondria are important both for energy production and for the regulation of cell death. Hippocampal organotypic slice cultures, which reproduce selective CA1 vulnerability, and astrocytes isolated from these two subregions of the hippocampus will be used for electrophysiological studies and real time imaging. The timecourse of changes in mitochondrial membrane potential, reactive oxygen species, and intracellular calcium during reperfusion in astrocytes in CA1 and DG will be assessed. Comparing the vulnerable and resistant areas will allow discrimination of important changes. Respiratory function and mitochondrial complex activity will also be measured. Aim 2 will focus on changes in astrocyte glutamate uptake with ischemia and reperfusion. Early impairment of glutamate uptake by astrocytes endangers neighboring neurons, and we have found early loss of GLT-1 in CA1 astrocytes. Detailed studies of glutamate uptake and transporter expression will be performed. ATP stores required for maintaining membrane potential and glutamate uptake will be monitored. Aim 3 will use overexpression of the chaperone Hsp70 selectively in astrocytes and induction of GLT-1 as protective strategies and assess changes in astrocyte function as well as changes in neuronal survival. Together these studies will provide a picture of hippocampal astrocyte ischemic response and link astrocyte dysfunction to selective neuronal injury in hippocampus. Maintaining and improving astrocyte function provides a novel target for developing therapies to improve neurological function following cardiac arrest.