Heme Oxygenase and Spinal Cord Injury

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Functional recovery after spinal cord injury is attributed to secondary pathogenic events that mediate early and delayed cell injury. The proposed studies examine the role of heme oxygenase-1 (HO-1) in both early and delayed injury in the traumatized spinal cord. Heme oxygenases are the only enzymes that metabolize the pro-oxidant heme to bile pigments, iron, and carbon monoxide. Extracellular heme is derived from intraparenchymal hemorrhage and heme proteins released from dying cells. Although both hemorrhage and ongoing cell death are features of the injured spinal cord, little is known about how heme contributes to early and delayed injury. The general hypothesis s that HO-1 is crucial for the detoxification of heme in the traumatized cord. The following experiments will test three major hypothesis: Hypothesis: 1. Microglia provide an important line of defense in the traumatized cord by metabolizing heme and compartmentalizing free iron, a product of heme metabolism. Experiments: (1) We will compare the microglial response in HO-1 knockout (KO) and wildtype (WT) mice and will determine if acute induction of HO-1 and ferritin expression in microglia occur in regions of iron accumulation and hemorrhage. Hypothesis 2. Induction of HO-1 in microglia/macrophages promotes wound healing and locomotor recovery by reducing oxidative stress/injury. Experiments: (1) We will measure indicators of oxidative stress/injury and evaluate revascularization, white matter injury, and locomotor recovery in HO-1 WT and KO animals. (2) Similar outcome measures will be evaluated in spinal cord injured mice, pretreated with intrathecal hemoglobin, a strategy that preferentially induces HO-1 in glia. Hypothesis 3. Induction of HO-1 in spinal cord blood vessels, prior to injury, stabilizes the blood-spinal cord barrier, attenuates the early induction of vascular adhesion molecules and infiltration of inflammatory cells, and promotes functional recovery. Experiments: We will determine if systemic administration of heme prior to injury, a strategy which preferentially induces HO-1 in endothelium, will restrict barrier disruption to proteins, limit the infiltration of neutrophils through the modulation of vascular adhesion molecules, and promote locomotor recovery. In summary, these studies reflect a focused effort to link hemorrhage, oxidative stress and functional outcome after spinal cord injury and will determine if HO-1 protects or harm cells, modulates blood-spinal cord barrier function and wound healing, and influences locomotor recovery.