Stroke is a leading cause of death and disability in our country for which effective treatments remain elusive. Defining the adaptive and pathological molecular mechanisms by which the brain responds to stroke should advance the design of stroke therapy. This proposal will examine the adaptive and pathological properties of a transcription factor, hypoxia inducible factor 1 alpha (HIF-1alpha). HIF-1 alpha is a master regulator of the response to hypoxia orchestrating the expression of several target genes including erythropoietin, vascular endothelial growth factor, and glycolytic enzymes. Since these genes likely increase neuronal viability, HIF-1 alpha is postulated to serve an adaptive role during stroke. However, a dominant negative form of HIF-1 alpha is protective to neurons during severe hypoxia suggesting a pathological role for HIF-1 alpha during severe hypoxia. In addition, some targets of HIF-1 alpha have pro-apoptotic functions. We hypothesize that HIF-1alpha enhances neuronal viability during mild hypoxia, by augmenting expression of neuroprotective genes within neurons and glial cells, but induces cell-autonomous pro-death processes within neurons during severe hypoxia. To test this hypothesis, we have obtained mice with conditional loss of HIF-1alpha function. These transgenic mice harbor a germ-line recombinational construct, a floxed HIF-1 alpha (HIF-1 alpha f+/f+) gene. HIF-1 alpha f+/f+ mice develop normally. However, loss of HIF-1 alpha function is achieved by exposure of the floxed gene to an enzyme Cre recombinase (Cre).
In Aim 1 of this proposal we will create transgenic mice that express a RU486-regulated form of Cre, which is expressed under the control of a cell type-specific promoter (crePR mice). By crossing HIF-1 alpha mice with crePR mice, cell type-specific loss of HIF-1 alpha function will be achieved by administration of RU486. Utilizing neuronal cultures, we will examine in Aim 2 the effect of loss of HIF-1 alpha function in neurons or glial cells on neuronal viability during severe hypoxia. The effect of loss of HIF-1 alpha function on stroke volume will also be explored in a focal model of stroke.
In Aim 3, we will examine the cell type-specific expression of HIF-1 alpha targets as it relates to hypoxic severity and p53 function. In this way, we will determine if the expression profiles of HIF-1 alpha targets change with severity of hypoxia, differs between cell types, or is altered by p53, which is known to bind in a complex with HIF-1 alpha. By better defining the dual roles of HIF-1 alpha in stroke, we hope to define molecular mechanisms by which the adaptive and pathological functions of HIF-1 alpha can be manipulated to contribute to treatment of stroke.
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