Date of Award

2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Neuroscience

First Advisor

Robert W. Neumar

Second Advisor

Harry Ischiropoulous

Abstract

Brain injury after cardiac arrest is a significant contributor to morbidity and mortality. Selectively vulnerable neuron populations undergo delayed neurodegeneration in the hours to days following reperfusion. Multiple factors influence neuronal death after global brain ischemia, including excitatory synaptic input, mitochondrial dysfunction, oxidative stress, inflammation, and disruption of intracellular Ca2+ homeostasis. In hippocampal CA1 pyramidal neurons, these factors activate cell death via pathologic activity of calpains, the family of Ca2+-dependent proteases, and calpain inhibition protects against neurodegeneration. Therefore, we hypothesized that pathologic calpain activity is necessary and sufficient for neurodegeneration in other selectively vulnerable neuron populations. In this thesis, the mechanism of calpain-mediated neurodegeneration and optimization of current therapeutic approaches were explored using adult rat models of brain ischemia. First, post-ischemic protease activity in mitochondria was characterized, specifically the mechanism of electron transport chain subunit proteolysis. Complex I subunit NDUFB8 and Complex V subunit  were cleaved in a Ca2+-independent manner in vitro by a mitochondrial cysteine protease. In addition, NDUFB8 proteolysis was detected in hippocampal synaptosomes after global and focal brain ischemia, indicating that proteolytic activity occurs in vivo and could play a role in post-ischemic mitochondrial dysfunction. Second, the role of calpain activity in neurodegeneration of cerebellar Purkinje cells was examined using an RNA interference approach to cause functional knockdown of calpain activity. Surprisingly, knockdown of calpain activity did not significantly alter Purkinje cell neurodegeneration 48 hours after cardiac arrest, suggesting that alternative mechanisms may be involved in post-ischemic neurodegeneration in the cerebellum. Finally, the optimal parameters of post-cardiac arrest therapeutic hypothermia to minimize Purkinje cell loss were explored. Based on the parameters tested, therapeutic hypothermia initiated 0-8 hours after return of spontaneous circulation and maintained for 24 or 48 hours resulted in similarly significant neuroprotection compared to normothermia seven days after cardiac arrest in the adult rat. In conclusion, these results paint a more complex picture of post-ischemic neurodegeneration, which may include contributions of multiple pro-cell death stimuli rather than pathologic calpain activity alone, and highlight the effectiveness of therapeutic hypothermia, which has broad effects on post-cardiac arrest pathology that could contribute to neuroprotection.

Share

COinS