The best hopes for protection against hypoxic/ischemic brain injury may involve the recruitment of augmentation of any autoprotective systems that evolution has provided. Central nervous system glial cells can recruit autoprotective adaptations with hypoxic preconditioning but the mechanism underlying this is unknown. Induction of genes such as vascular endothelial derived growth factor and erythropoietin by hypoxic astrocytes is a major adaptation for increasing brain oxygen supply. In addition the new recognition of glial derived erythropoietin as a direct neuroprotective factor suggests that gene induction by hypoxic gila may be a major autoprotective response of the brain for preventing neurodegeneration as well. The recent identification of a key transcription factor known as hypoxia inducible factor (HIF-1) has focused attention on a central oxygen sensing pathway which mediates hypoxic adaptations through selective gene expression. Multiple hypoxic responses including adaptation to anaerobic metabolism, erythropoiesis, angiogenesis, vasodilation and breathing all appear to be under the control of this transcription factor. They have recently discovered that an oxygen sensing pathway used by specialized organs such as the carotid body also exists in glial cells and is able to transduce hypoxic signals which activate HIF-1 and gene expression. This signalling pathway uses cellular hydrogen peroxide levels to monitor oxygen supply. Unique hydrogen peroxide sensitive potassium channels transduce changes in the cellular hydrogen peroxide levels into membranae depolarization and protein kinase C activation. Protein kinase C then activates HIF-1 to initiate genetic responses to hypoxia. The proposed work will delineate the components of this pathway in cultured human glioma cells by (1) determining the source of dynamic hydrogen peroxide generation. (2) determining the mechanism and specificity of the hypoxia sensing ion channel and membrane depolarization events, (3) determining the mechanism by which protein kinase C activation links hypoxic membrane depolarization to HIF-1 activation, and (4) determining which components of this signalling pathway are missing in other glioma cell lines which components of this signalling pathway are missing in other glioma cell lines which do not respond to hypoxia with HIF-1 activation. Because hypoxia is a central event in many neurological diseases and insults, the proposed research will have a major impact on the treatment and prevention of neurological illness. In addition this research addresses a fundamental enigma in modern biology: How do cells sense oxygen?
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