Astrocytic hypoxia inducible factor 1 alpha mediates neuronal cell death in hypoxia

Thesis (Ph. D.)--University of Rochester. School of Medicine and Dentistry. Dept. of Interdepartmental Graduate Program in Neuroscience, 2008.Following a stroke, rapid brain tissue loss is inevitable in the core of the infarct. However, surrounding regions, termed the penumbra, while metabolically compromised remain in a tenuous balance between life and death. It is therefore important to understand the molecular mechanisms that mediate cell viability in these regions in order to minimize tissue damage following a stroke. The master regulator of hypoxia, Hypoxia Inducible Factor 1 alpha (HIF-1) is the key transcription factor that is up regulated in response to hypoxia. HIF-1 is capable of inducing a wide array of targets with a diverse profile of activities, both pathological and protective. While much effort has been made to delineate the molecular mechanisms following hypoxia in neurons, little work has been done to understand the role and activities of astrocytes in these conditions. As astrocyte function is critical during normal physiology, their function in pathology is likely. The focus of this thesis is to understand how and if astrocytes can impact neuronal viability in hypoxia utilizing in vitro co-culture models of astrocytes and neurons. Specifically we are interested in the role of HIF-1 in astrocytes compared to neurons in hypoxia. We exposed co-cultures of astrocytes and neurons to hypoxia-reperfusion injury, where neurons but not astrocytes died in a delayed manner. We have found that HIF-1 function is cell type specific. That is, in neurons HIF-1 provides neuronal protection and mediates cell-autonomous survival in hypoxia-reperfusion injury, as loss of HIF-1 function specifically within neurons perpetuated neuronal cell death. On the contrary and to our surprise, loss of HIF-1 function in astrocytes was capable of protecting neurons from hypoxia-reperfusion induced cell death. This means that astrocytic HIF-1 was toxic to neurons through some non-cellautonomous mechanism. We have identified both iNOS and CD147 as likely downstream mediators of neuronal pathology whose actions are highly dependent on HIF-1. Most interestingly, CD147 a novel, partial HIF-1 target was highly toxic to neurons, as a blocking antibody towards CD147 completely protected co-cultures from death. CD147’s pathological effects were mediated through its downstream activities of MMP and uPA activation. Thus we have identified in this thesis a novel and unique pathway where astrocytes are capable of mediating neuronal pathology in hypoxic related injuries